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Biomechanical Design(051:083)

Lecturer:

- Tae-Hong Lim, Ph.D.1420 Seamans Center

335-5810 (office); talim@engineering.uiowa.edu (email)

- David Wilder, PhD and Nicole Grossland, PhD

Office Hours:M, W, F: 4:00 – 5:00 PM

Appointment

Pre-requisites:57:007 Statics

57:019 Mechanics of Deformable Bodies

51:050 Biomechanics

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Text and Grading

Text:“The Mechanical Design Process, 3rd Edition”

David G. Ullman

McGraw-Hill (ISBN: 0-07-237338-5)

Grading:15% Homework and Quiz

20% Individual Design Notebook

10% Reverse engineering report

40% Team Project

15% Final Exam– Final exam will be a short answer exam covering the terminology and concepts studied

throughout this course

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Individual Design Notebooks

You are to keep a design notebook for use in this course. This is to be a spiral bound notebook.

– Every page must be numbered at the beginning of the term.– No pages can be removed and each page must be dated and initialed when used.

All work related to this course (homework and design project) will be entered into this note book.

Each notebook will be collected at the end of the term and graded on the number of “quality entries” it contains.

– A quality entry is a significant sketch or drawing of some aspect of design; a listing of functions, ideas or other features; a table such as morphology or decision matrix; or a page of text.

– Unintelligible entries are not quality entries.

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Team Project

Design Team:– Design teams will be organized by the instructor.– Team members will determine the leader (CEO).

Team Project:– Each team will determine a design problem (new invention or modification) related to

biomechanical devices through discussion with the instructor.– The team project will be considered completed by obtaining final product documentation

(drawings, part list with specified materials, and assembly instructions).• No final product in physical form is required.

Each team should record the whole history of the design in the Product Development File (PDF).

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Documents in the PDFProblem Appraisal Phase

Understanding the Problem:1. Description of Customers2. Customer’s Requirements3. Weighting of Customer’s

Requirements4. Competition’s Benchmarks vs.

Customer’s Requirements5. Engineering Requirements6. Competition’s Benchmarks vs.

Engineering Requirements7. Engineering Targets

Planning the Project8. Task Titles9. Objectives of Each Task10. Personnel Required for Each

Task11. Time Required for Each Task12. Schedule of Tasks

Conceptual Design Phase

Concept Generation:13. Function Decomposition14. Literature and Patent Search

Process and Results15. Function-Concept Mapping16. Sketches of Overall Concepts

Concept Evaluation andAssessment of Tech. Readiness:17. Identification of Failure Modes18. Identification of Critical

Parameters

Concept Selection:19. Decision Matrices to Determine

Best Concepts20. Analsysi, Experiments and

Models Supporting Evaluation

Product Design Phase

Product Generation:21. Usable off-the-shelf Products22. Shape Development Driven by

Function23. Materials Selection24. Manufacturing Process

Selection

Product Evaluation:25. Comparison to Engineering

Function26. Functional Changes Noted27. Design for Assembly Evaluation28. Cost Evaluation29. Analysis, Experiments and

Models Supporting Evaluation

Final Product Documentation:30. Layout Drawings31. Detail Drawings of

Manufactured Parts32. Parts List (Bill of Materials)33. Assembly Instructions

* The file is to be maintained by the group in a binder. This PDF, when completed, is effectively a final report. It will be graded on completeness and quality of both the design and the documentation itself.

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Biomechanical Design

Design:– Deliberate purposive planning– A mental project or scheme in which means to an end are laid out– A preliminary sketch or outline showing the main features of something to be executed: DELINEATION– The arrangement of elements or details in a product or work of art– The creative art of executing aesthetic or functional designs

Biomechanical Design:– Design something related to biomechanics, such as:

Biomechanical devices• Medical devices: orthopedic implants, scissors, scalpers, staplers, etc.• Exercising devices: treadmill, weight-lifting, helmets, wrist-guards, etc.• Rehabilitation devices: wheel-chairs, canes, etc.

Biomechanical activities• Exercises for fitness or strengthening body parts

Biomechanical design includes:– Development (or invention) of new biomechanical stuffs; and – Modification of existing biomechanical stuffs

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What will we learn in this class?

Typical design process:– Identification of design problems– Design– Evaluation of the design– Decision making– Final report

Techniques helping generate better quality designs in less time:– Concurrent engineering– Computer aided drawing

Legal and regulation issues:– Safety and liability– Patent, FDA, CE, and UL

Importance of communication of design data:– Records of design data, design process, and final report– Oral presentations

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The Life of A Product*Process of idea development, production, use, and end of product life.

*The whole process must be considered in the design process.

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Design Process What is the design process?

– Design process is the organization and management of people and the information they develop in the evolution of the product.

Why study the design process?– Design process determines the efficiency of new product development.

• 85% of the problems with new products not working as intended, taking too long to bring to market, or costing too much are the result of poor design process.

– The design process needs to improved consistently and executed for developing better products because:

• There is a continuous need for new, cost-effective, high-quality products.

• Most products require a team of people from diverse areas of expertise to develop an idea into hardware.

– We will study the design process to get the tools to develop an efficient design process regardless of the product being developed.

3 types of knowledge used by designers:– Knowledge to generate ideas

• Experience and natural ability

– Knowledge to evaluate ideas• Experience and formal training (focus of most engineering education)

– Knowledge to structure the process• Non-domain-specific knowledge

• What we will study in this class

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One person, with sufficient knowledge of the physics, materials and manufacturing processes to manage all aspects of the design and construction of the project, could design and manufacture an entire product in the past.

By the middle of the 20th century, products and manufacturing processes had become too complex for one person to have sufficient knowledge or time to focus on all aspects of the evolving product.

– Different groups of people for marketing, design, manufacturing and overall management

One-way communication over the wall:– What is manufactured is not often what the customer had in mind.– Inefficient, costly, and greater possibility for making poor-quality products

History of the Design Process

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Simultaneous Engineering (in late 1970s and early 1980s): Simultaneous development of the manufacturing process with the evolution of the product by assigning

manufacturing representatives to be members of design team

Concurrent Engineering (in late 1980s): Integrated Product and Process Design (IPPD) in the 1990s A greater refinement in thought about what it takes to efficiently develop a product Primarily focusing on the integration of teams of people, design tools and techniques, and information about

the product and the processes used to develop and manufacture it. 10 Key Features of Concurrent Engineering

Focus on the entire product life (chap 1) Use and support of design team (chaps 3 and 5) Realization that the processes are as important as the product (chaps 4 and 5) Attention to planning for information-centered tasks (chap 5) Careful product requirements development (chap 6) Encouragement of multiple concept generation and evaluation (chaps 7 and 8) Awareness of the decision-making process (chap 8) Attention to designing in quality during every phase of the design process (throughout) Concurrent development of product and manufacturing process (chaps 9-13) Emphasis on communication of the right information to the right people at the right time (throughout)

– A key point of concurrent engineering is a concern for information.– Drawings, plans, concept sketches, meeting notes, etc.

History of the Design Process

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Controllable Variables in Concurrent Engineering

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Overview of the Design Process

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Design Problems

What size SAE grade 5 bolt should be used to fasten together two pieces of 1045 sheet steel (4 mm thick and 6 cm wide) which are lapped over each other and loaded with 100 N?

- Well defined analysis problem finding the diameter of the bolt

Design a joint to fasten together two pieces of 1045 sheet steel (4 mm thick and 6 cm wide), which are lapped over each other and loaded with 100 N?

Ill-defined design problem with number of potential problems-How to connect the sheets? (Bolted, glued, welded, etc.?)-Disassembly required later?-What working environment?-etc.

HWK#1: Change a problem from one of your engineering science classes into a design problem by changing as few words as possible. Do your home work in your design notebook. Due is one week.

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Design problems have many satisfactory solutions and no clear best solution.

Design problems - are ill-defined;- have no correct answer;- have no clear best answer.

Design process knowledge is based upon the domain knowledge.

Mechanical design problems begin with an ill-defined need and result in a piece of machinery that behaves in a certain way.

PARADOX:A designer must develop a machine that has the capabilities to meet some need that is not fully defined.

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Basic Actions of Design Problem Solving ESTABLISH the need or realize that there is a problem to be solved.

– New needs also can be established throughout the design effort because new design problems arise as the product evolves. Design of these details poses new subproblems.

PLAN how to solve the problem.– Planning occurs mainly at the beginning of a project. Plans are always updated because

understanding is improved as the process progresses. UNDERSTAND the problem by developing requirements and uncovering existing

solutions for similar problems.– Formal efforts to understand new design problems continue throughout the process. Each

new subproblem requires new understanding. GENERATE alternative solutions.

– Concept Generation vs. Product Generation EVALUATE the alternatives by comparing them to the design requirements and to

each other.– Evaluation techniques also depend on the design phase; there are differences between the

evaluation techniques used for concepts and those used for products. DECIDE and acceptable solutions

– Decision making requires a commitment based upon incomplete evaluation. – Decision requires a consensus of team members.

COMMUNICATE the results– Communication of the information developed to others on the design team and to

management is an essential part of concurrent design.

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Basic Terminologies used to describe the Design Process

“Communication” as a one key feature of concurrent engineering– Communication depends on a shared understanding of terminology.

Function:– What a product or a system is supposed to do;– Described using action verbs and a noun describing the object on which the

action occurs:• Record images; quantify the blood pressure; fix an unstable spine segment; etc.

System:– A grouping of objects that perform a specific function;

• Shutter system; timer system; CD-R system; cooling system; etc.– A system can be decomposed into another subsystems or further into individual

components (or parts).– Multiple systems can be assembled into a higher level system or further into a

final product. Feature: the important form and function aspects of mechanical devices

• dimensions, material properties, shapes, or functional details (speed of opening and closing for shutter system)

Decomposition of design disciplines

In general, during the design process, the function of the system and its decomposition are considered first. After the function has been decomposed to the finest subsystems possible, assemblies and components are developed to provide these functions.

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Function, Behavior, and Performance Function:

– describes what a device does.– But, function provides no information about how a device accomplishes the function.

Form:– The term “form” relates to any aspect of physical shape, geometry, construction, material,

or size.– provides some information on how a device accomplishes the function.

Behavior and Performance in association with Function.

- Function is the desired output from a system yet to be designed.

- Behavior is the actual output, the response of the system’s physical properties to the input energy or control.

- Performance is the measure of function and behavior – how well the device does what it is designed to do.

- A clear picture of desired performance should developed in the beginning of the design process.

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Types of Mechanical Design Problems Selection Design:

– choosing one item (or more) from a list of similar items– choosing a bearing, bolt, motor, etc. from a catalog

Configuration Design:– How to assemble all the components into the completed product

Parametric Design:– Finding values for the features that characterize the object being studied or that meet the

requirements

– Design a cylindrical tank: V = r2l, determine r and l for known V Original Design:

– Design a process, assembly or component not previously in existence Redesign:

– Redesign of an existing product– Most design problems are redesign problems since they are based on prior,

similar solutions. Conversely, most design problems are original as they contain something new that makes prior solutions inadequate.

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Languages of Mechanical Design

A mechanical object can be described by: Semantic language:

– Verbal or textual representation of the object– “bolt” or “The shear stress is equal to the shear forces on the bolt divided by the

x-sectional area.” Graphical language:

– drawing of the object– Sketches, scaled representations of orthogonal drawings, or artistic renderings

Analytical Language:– Equation, rules, or procedures representing the form of function of the object = F/A

Physical Language:– Hardware or physical model of the project

- In most cases, the initial need is expressed in a semantic language as a written specification or a verbal request by a customer or supervisor, and the final result of the design process is a physical product.

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Design, State, Constraints and Decision Design State:

– Collection of all the knowledge, drawings, models, analyses and notes thus far generated

– In the beginning, design state is just the problem statement Design Constraints:

– Factors limiting the design process• Examples: size, strength of material, corrosion properties, anatomy, etc.

– In the beginning, the design requirements effectively constrains the possible solutions to a subset of all possible product designs.

– Two sources of constraints added during the design process:• Designer’s knowledge of mechanical devices and the specific problem being solved• Result of design decisions

Design Decision:– Continuous comparison between design state and the goal (requirements for the

product given in the problem statement)• The difference controls the process.

– Design is the successive development and application of constraints until only one unique products remains.

– Each design decision changes the design state. The design progresses in increment punctuated by design decisions.

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The Value of Information

*The most valuable information is the decisions that are communicated to others.

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Design as Refinement of Abstract Representations

Graphical Refinement

See Table 2.2 for levels of abstraction in other languages.

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Information-processing Model of Human Problem Solving

Information-Processing System used by the human mental system in solving any type of problem

25Chunks of Information

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Information-processing Model of Human Problem Solving

Types of Knowledge that might be in a chunk of information:– General knowledge

• Information that most people know and apply without regard to a specific domain– “red is a color.” “4 is bigger than 3.”

• Gained through everyday experiences and basic schooling

– Domain Specific Knowledge:• Information on the form or function of an individual object or a class of objects

– Bolts are used to carry shear or axial stress

– The proof stress of a grade 5 bolt is 85 kpsi.

• Gained from study and experience in the specific domain– It may take about 10 years to gain enough specific knowledge to be considered an expert in a domain

– Procedural Knowledge:• The knowledge of what to do next

– If there is no answer to problem X, then decompose X into two independent subproblems of x1 and x2 that are easier to solve.

• Gained mostly from experience

• Required for solving mechanical design problems

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The size of STM is a major limiting factor in the ability to solve problem.– To accommodate this limitation, breakdown problems into finer and finer subproblems

until we can “get our mind around it”– in other word, manage the info in our STM

– Human designers are quite limited although our expertise about the constraints and potential solutions increases and our configuration of chunks becomes more efficient as we solve problems.

– These limitations would preclude our ability to solve complex problems.

Implications of the Information-processing Model

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Mental Processes that Occur during Design

Understanding the problem:– A problem is understood by comparing the requirements on the desired function to information in the

long-term memory.– Every designer’s understanding of the problem is different, we need to develop a method to ensure that

the problem is fully understood with minimal bias from the designer’s own knowledge.

Generating a solution:– Use the information stored in LTM that meets the design requirements.– If no solution found from LTM, then use a three step approach

• Decompose the problem into subproblems

• Try to find partial solutions to the subproblems

• Recombine the subsolutions to fashio a total solution

– Creative part of this approach is in knowing how to decompose and recombine cognitive chunks

Evaluating the solution:– Evaluation requires comparison between generated ideas and the laws of nature, the capability of

technology and the requirements of the design problem itself.– Evaluation requires modeling the concept to see how it performs.– The ability to model is usually a function of knowledge in the domain.

Deciding:– A decision is made at the end of each problem-solving activity to accept the generated and evaluated

idea or to address another topic that is related to the problem.

Controlling the design process:– Path from initial problem to solution seemed random.

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Problem-Solving Behavior A person’s problem-solving behavior affects how problems are solved individually and has a significant

impact on team effectiveness. Four Personal-Problem Solving Dimensions (or styles):

– Individual Problem-solving Style:• Introvert:

– Solve problem internally (reflective); a good listener; think and speak; enjoy having time alone for problem solving

• Extrovert:

– Sociable; tend to speak and think

– About 75% Americans and 48% of engineering students and executives

– Individual Preference to work with Facts or Possibilities:• Facts oriented people:

– literal, practical, and realistic

– 75 % of Americans, 66% of top executives, 34% of all engineering students

• Possibility oriented people:

– Like concepts and theories and look for relationship between pieces of information and meaning of the information

– Objectivity with which decisions are made:• Objective:

– Logical, detached and analytical

– Taking objective approach to make decisions

– 51% of Americans, 68% of engineering students, 95% of top executives

• Subjective:

– Make decisions based on an interpersonal involvement, circumstances, and the “right thing to do”

– Need to Make Decisions:• Decisive:

– Tend to make decisions with a minimum of stress and like an ordered, scheduled, controlled and deliberate environment

– 50% of Americans, 64 % of engineering students, and 88% of top executives

• Flexible:

– People goes with flow is flexible, adaptive, and spontaneous, and finds making and sticking with decisions difficult

• Please make sure to read section 3.3.6 carefully for better design team activities.

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Characteristics of a Creative Designer Problem solving involves:

– Understanding the problem, generating solutions, evaluating the solutions, deciding on the best one, and determining what to do next

Criteria of Creative Solution:– It must solve the design problem.– It must be original.

• Originality and creativity are assessed by society.

Creativity in relation to other Attributes– Intelligence: no correlation with creativity– Visualization Ability:

• Creative engineers have good ability to visualize, to generate and manipulate visual images in their head.• The ability to manipulate complex images can be improved with practice and experience.

– Knowledge:• A person must have knowledge of existing products to be a creative designer• A firm foundation in bioengineering science is essential to being a creative biomechanical designer.

– Partial Solution Manipulation: important attribute– Risk Taking: certainly required– Conformity: Creative people tend to be nonconformists.

• Constructive nonconformists take a stand because they think they are right and might generate a good idea.• Obstructive nonconformists take a stand just to have an opposing view and will slow down the design progress.

– Technique:• Creative designers have more than one approach to problem solving.

– Environment:• Higher creativity when the work environment allows risk taking and nonconformity and encourages new ideas.

– Practice:• Creativity comes with practice.• Practice enhances the number and quality of ideas.

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Creative Designer A creative designer is a:

– Visualizer;– Hard worker; and– Constructive nonconformist with knowledge about the domain and

ability to dissect things in his or her head

Good News:– Designers with no strong natural ability can develop creative methods

by using good problem-solving techniques to help decompose the problems in ways that maximize the potential for understanding it, for generating good solutions, for evaluating the solutions, for deciding which solution is best and for deciding what to do next

– A design project requires:• much attention to detail and convention;

• demands strong analytical skills; and thus

• People with a variety of skills.

– There are many good designers who are not particularly creative individuals.

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Engineering Design Team A team is a group of people working toward a common understanding. Team vs. Individual Problem Solving

– There are social aspects of team work.– Each team member may have different understanding of the problem, different alternatives for solving

it, and different knowledge for evaluating it. (more solutions but also more confusion)

Team Goals:– A small number of people with complementary skills who are committed to a common purpose,

common performance goals and a common approach for which team members hold themselves mutually accountable are required for an effective team.

– Team members must:• learn how to collaborate with each other, i.e., to get the most out of other team members.

• Comprise to reach decisions through consensus rather than by authority.

• Establish communications.

• Be committed to the good of the team.

Team Roles:– Organizer; Creator; Resource-investigator; Motivator; Evaluator; Team worker; Solver; Completer

(finisher or pusher)

Building Team Performance:– For developing productive teams;

• Keep the team productive

• Select team members on the basis of skills in both primary and secondary roles

• Establish clear rule of behavior

• Set and seize upon a few immediate performance-oriented goals.

• Spend time together.

• Develop a common understanding

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Overview of the Design Process

An Ideal Flow Chart of Activities During Design Process

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What initiates a Design Project? Need for a New Design:

– Market:• About 80% of new product development is market-driven.

• Assessment of the market is most important in understanding the design problem because there is no way recover the costs of design and manufacture without market demand.

• Incorporation of the latest technology can improve its perception as a high quality product.

– New product idea without market demand• To use new technologies whose development requires an extensive amount of capital

investment and possibly years of scientific and engineering time– High financial risk but greater profit due to uniqueness

• Examples of successful products: sticky notes; Walkman

Need for Redesign– By market demand for a new model– Desire to include a new technology– Fix a problem with an existing product– Redesign process can be applied to the subproblems that result from the

decomposition of a higher-level system.

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Project Planning:– to allocate the resources of money, people, and equipment to accomplish the design activities:– Planning should precede any commitment of resources although requiring speculations about the

unknowns• Easier to plan a project similar to earlier projects than to plan a totally new one

– Plans are often updated whenever unknown demands become certain with the progress of design project Specifications Definition:

– Goal is to understand the problem and to lay the foundation for the remainder of the design project.• Identify the customers: Generate the customer’s requirements: Evaluate the competition: Generate engineering

specifications: Set targets for its performance– Design Review:

• formal meeting for progress report and design-decision making

Conceptual Design:– To generate and evaluate the concepts for the product

• Generate concepts based on the defined specifications for developing a functional model of the product • Evaluate concepts by comparing the concepts generated to the targets for its performance

– Design Review Product Development:

– Evaluate the product for performance, cost, and production– Make product decisions– Documentation and Communication

• BOM (Bill of Materials), Drawings, etc.

Product Support:– Support for vendors, maintenance of engineering change, customer, manufacturing and assembly, and

retirement of the product

Overview of the Design Process

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Why Do We Have to Follow The Design Process Techniques?

Paradox:– Techniques in the design process may imply “RIGIDITY” whereas the creativity implies

“FREEDOM.”

Following the techniques in the design process helps the designers develop a quality product that meets the needs of the customer by several ways:

– Eliminating expensive changes later– Developing creative solutions to design problems systematically

• Creativity does not spring from randomness.

• “Genius is 1 percent inspiration and 99 percent perspiration.”

• The inspiration for creativity can only occur if the perspiration occur early is properly directed and focused.

• The techniques that make up the the design process are only an attempt to organize the perspiration.

– Forcing documentation of the progress of the design (record of the design’s evolution) that will be useful later in the design process.

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Design Process Examples

Simple Process

Complex Process

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Communication during the Design Process

Design Records:– Importance of documents in design file:

• To demonstrate the state-of-the-art design practices• To prove originality in case of patent application• To demonstrate professional design procedures in case of a lawsuit

– Design Notebook:• A diary of the design tracking the ideas development and the decision made in a design notebook

– Name; Affiliation; Title of the problem; Problem Statement; and all sketches, notes and calculations that concerns the design

• A design notebook sequentially numbered, signed and dated pages is considered good documentation whereas random bits of information scrawled on bits of papers are not.

– Good evidences for legal purposes (patent or lawsuit) as well as a reference to the history of the designer’s own work

Documents Communicating with Management:– Needed for periodic presentations to managers, customers, and other team members for design review– Regardless of its form (oral or written);

• Make it understandable (consider the recipients’ level of knowledge about the design problem)• Carefully consider the order of presentation (whole – parts – whole: 3-step approach: gradual

introduction of new ideas)• Be prepared with quality material (good visual and written documentation; following the agenda;

being ready for questions) Documents Communicating the Final Design:

– Material describing the final design, e.g, Drawings (or data files) of individual components and of assemblies

– Written documentation to guide• manufacture, assembly, inspection, installation, maintenance, recruitment, and quality control

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Team Project

Select an original design problem to solve throughout the remainder of the course.

– The problem should concern biomechanical devices in which some of the design team members have some knowledge or training.

– The final product will be data from analyses and evaluation and final drawings, not an actual hardware.

Design Team Activity :– Each student should start gathering the design ideas immediately and

recording the ideas in the design notebook.– Start the team meeting ASAP for:

• Organization of the team

• Planning the design process to finish the project by the end of April.

– Each team will present their design project in May.Any discussion about the design project with the instructor is welcome.

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Project Definition and Planning

• Concurrent engineering encourages involvement through out the entire product life cycle from the project definition to product retirement.

• Project definition and planning is the first phase of the mechanical design process.

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Project Definition Why developing new products?

– To fill some market need• Mostly driven by the customer

– To exploit a technological development• Driven by new technologies and what is learned during the design

Project Definition:– the challenges of choosing from the many suggestions as to which products to

spend time and money on to develop or refine– “Fuzzy front end” of dealing with vague design ideas

Specific Questions in Project Definition Phase:– Is there a good potential return on investment (ROI)?– Does the new product or improvement fit the company image?– Does it fit the distribution channels?– Is there sufficient production capacity in-house or with known vendors?– What will the project cost?

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Project Planning Planning is like trying to measure the smile of the Cheshire cat; you are

trying to quantify something that isn’t there.

Planning is the process used to develop a scheme for scheduling and committing the resources of time, money, and people.

– Producing a map showing how product design process activities are scheduled.– The whole activities of specification definition, conceptual design, and product

development must be scheduled and have resources committed to them– Planning generates a procedure for developing needed information and

distributing it to the correct people at the correct time.– Important information: product requirements, concept sketches, system

functional diagrams, component drawings, assembly drawings, material selections, and any other representation of decisions made during the development of the product.

Typical Master Plan (a generic process) of a Company for Specific Products:

– A blue print for a process: • product development process; delivery process; new product development plan; or

product realization plan, etc. We will refer to this generic process as the product development process

(PDP).

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ISO-9000 A quality management system of the International Standard Organization

– First issued in 1987 and now adopted by over 150 countries– Over 350,000 companies worldwide and 8500 U.S. companies have the ISO-9000

certification ISO-9000 registration means that the company has a quality system that:

– Standardizes, organizes, and controls operations.– Provides for consistent dissemination of information.– Improves various aspects of the business-based use of statistical data and analysis.– Enhances customer responsiveness to products and services.– Encourage improvement.

To receive certification,– One should develop a process that describes how to develop products, handle product

problems, and interact with customers and vendors.• Required written procedures that:

– Describe how most work in the organization gets carried out (i.g., the design of new products, the manufacture of products, and the retirement of products).

– Control distribution and reissue of documents.– Design and implement a corrective and protective action system to prevent problems from recurring.

– Evaluation of the effectiveness of the process by an accreditted external auditor– Certification expires in 3 years and audits at 6-month intervals to maintain the currency of

the certificate. ISO-9000 requires a company to have a documented development process on which

the plan for a particular product can be based.

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Background for Developing a Design Project Plan A plan tells how a project will be initiated, organized, coordinated, and monitored, e.g.,

managerial activities. Types of Design Projects:

– Variation of existing product:– Improvement of existing product:

• Redesign of some features of an existing product due to:– Customers request; no longer supply of materials or components from the vendor; needed improvement in

manufacturing; or New technology or new understanding of an existing technology– Development of a new product for a single (or small) run or for mass production

Members of the Design Team:– Product design engineer:– Product manager (product marketing engineer):– Manufacturing engineer; Detailer; Drafter; Technician; Materials specialist; QC/QA specialist;

Analyst; Industrial engineer; Assembly manager; Vendor’s or supplier’s representatives Structure of Design Teams:

– Functional Organization (13 %): • Each project is assigned to a relevant functional area, focusing a single discipline

– Functional Matrix (26 %): • project manager with limited authority is designated to coordinate the project across different functional areas

– Balanced Matrix (16 %):• A project manager is assigned to oversee the project and shares the responsibility and authority with functional

managers.– Project Matrix (28 %):

• A project manager oversees the whole project and functional managers assign personnel as needed.– Project Team (16 %)– Organize the talent around the project whenever possible.

• Structures focus on the project are more successful than those built around the functional areas in the company.

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Planning for Deliverables

Deliverables:– All models of the product, such as drawings, prototypes, bills of materials, analysis results, test results,

and other representations of the information generated in the project– Measure of the progress in design project

Models vs. Prototypes:– Models are analytical and/or physical representations of design information.– Prototypes are physical models. Solid models in CAD can replace the physical models these days.

4 Purposes of Prototypes:– Proof-of-concept:

• Developing function of the product to compare with the goals

• Learning tool

– Proof-of-product:• Refine the components and assemblies

• Geometry, materials and manufacturing processes are as important as functions

• Rapid prototyping and CAD models have greatly improved the time and cost efficiency in building prototypes.

– Proof-of-process:• Verify both the geometry and manufacturing process.

• Exact materials and manufacturing processes are used to build sample for functional testing.

– Proof-of-production• Verify the entire production product.

• The result of preproduction run.

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Types of Models

Phase

Physical

(Form and Function)

Medium

Analytical

(mainly Function)Graphical

(mainly Form)

Concept

Final Product

Proof-of-concept prototype

Proof-of-product prototype

Proof-of-process and proof-of-production prototype

Back-of-the envelope analysis

Engineering science analysis

Finite element analysis; detailed simulation

Sketches

Layout drawings

Detail and assembly drawings; solid models

An important decision in planning the project: -How many models and prototypes should be scheduled in the design process?

Because of cost effectiveness, there is a strong move toward replacing physical prototypes with computer models. But not always right.

Be sure to set realistic goals for the time required and the information learned.

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Five Steps in Planning Step 1: Identify the Tasks

– Tasks in terms of the activities that need to be performed (generate concepts, producing prototypes, etc.)

– Make the tasks as specific as possible.

Step 2: State the Objective for Each Task– Each task must be characterized by a clearly stated objective

• The results of the tasks (or activities) should be the stated objectives.– Task objectives should be:

• Defined as information to be refined or developed and communicated to others.– This information should be contained in deliverables

• Easily understood by all in the design team.• Specific in terms of exactly what information is to be developed. If concepts are

required, then tell how many are sufficient.• Feasible, given the personnel, equipment, and time available

Step 3: Estimate the Personnel, Time, and Other Resources Needed to meet the Objectives

Step 4: Develop a sequence for the tasks Step 5: Estimate the Product Development Costs

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Step 3: Estimate the Personnel, Time, and Other Resources Needed to Meet the Objective

Necessary Identification for each Task:– Who on the design team will be responsible for meeting the objectives?– What percentage of their time will be required?– Over what period of time they will be needed?

Time (in hours) = A x PC x D0.85

– A = a constant based on past projects • A = 30 for a small company with good communication

• A = 150 for a large company with average communication

– PC = product complexity based on function PC = j x Fj (j = the level in the functional diagram; Fj = the number of functions at

that level)

– D = project difficulty• D = 1, not too difficult; D = 2, difficult; D = 3, extremely difficult

Time estimation = (o + 4m + p)/6– O = optimistic estimate; m = most-likely estimate; p = pessimistic estimate

Time Distribution across the Phases of the Design Process– Project Planning (3 –5 %); Specifications Definition (10 – 15%); Conceptual Design (15 –

35%); Product Development (50 – 70%); Product Support (5 – 10%)

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Step 4: Develop a Sequence for the Tasks

The goal is to have each task accomplished before its result is needed and to make use of all of the personnel, all of the time.

– For each task, it is essential to identify its precessors and successors.– Tasks are often interdependent – two tasks need decisions from each other in order to be

completed.• Sequential vs. Parallel (uncoupled and coupled) tasks

Bar Chart (or Gantt Chart) – best way to develop a schedule for a fairly simple project

Design Structure Matrix (DSM) – for a complex project with coupled tasks– Showing the relationship (or inter-dependence) among tasks (example: see page 104 – Useful tool for to help sequence the tasks (Page 104)

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Step 5: Estimate the Product Development Cost

The planning document can serve as a basis for estimating the cost of designing the new product in terms of:

– Personnel cost– Resources (supplies and equipment)

Team Project:- Planning must be done and written in the PDF.- Read examples in pages from 105 – 109 for planning. - Gantt chart or DSM should be good entries.

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Understanding the Problem and the Development of Engineering Specifications

Importance of finding the right problem to be solved:– Unnecessary effort to design a retarder (dampener) determining the final

position of the solar panels in the Mariner IV satellite– Finding the right problem to be solved is often not easy although it may seem a

simple task. Creeping Specifications:

– Specifications changing during the design process• More features can be added as more is learned during the process• New technologies or competitive products introduced during the design (ignore,

incorporate or start all over?)• Changes in any spec. affecting the previous decisions depedent upon that spec

Engineering Specifications (requirements) should be:– Discriminatory:

• Reveal the difference between alternatives.– Measurable (most important and major topic of chap 6)– Orthogonal

• Each specification should identify a unique feature of the alternative.– “Product must give smooth ride over rough road.” vs. “Product should reduce shocks from bumps.”

– Universal• Characterizing an important attribute of all the proposed alternatives

– External• Only external features are observable.

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Quality Function Deployment (QFD)

Most popular technique used to generate engineering specifications in an organized manner

– Developed in Japan in the mid-1970s and introduced to the US in the late 1980s

• 69% of the US companys use the QFD method recently

Important Points– Employ QFD no matter how well the design team thinks it understands a

problem.– QFD takes time to complete, but time spent for QFD saves time later.– QFD can be applied to the entire problem and also any subproblems.– QFD helps overcome our cognitive limitation.

• We tend to try to assimilate the customer’s functional requirement (what is to be designed) in terms of form (how it will look).

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Quality Function Deployment (QFD)

House of Quality

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Example of QFDStep 1: identify the customers

Step 2: determine the requirements

Step 3: determine the relative importance of requirements

Step 4: identify and evaluate the competition

Step 5: generate engineering specification

Step 6: Relate customers’ requirements to engineering specifications

Step 7: Set engineering targets

Step 8: identify the relationships between engineering requirements

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QFD Step 1: Identify the Customers: Who are they?

For general products:

Who are the customers?-Consumers-Designers’ management-Manufacturing personnel-Sales staff-Service personnel-Standard organizations-Etc.

For many products, there are 5 or more classes of customers whose voices need to be heard

For a spinal implant system:

Who are the customers?-Orthopaedic surgeons-Neurosurgeons-Nurses-Hospitals-Distributors-Sales Reps-Patients ?

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QFD Step 2: Determine the Customer’s Requirements: What do the customers want

Consumers:- works as it should, - lasts long,- is easy to maintain, - looks attractive, - incorporates the latest technology,- and has many features.

Production Customer: - is easy to produce (both manufacture and assemble),- uses available resources (human skills, equipment, and raw materials),- uses standard parts and methods, - uses existing facilities,- produces a minimum scraps and rejected parts.

Marketing/Sales Customer:- easy to package, store, and transport,- attractive and suitable for display

Types of Requirements:-Basic features: neutral satisfaction with existence

-basic assumed functions; not included in QFD- Performance Features: good satisfaction with existence

- verbalized in the form that the better the performance, major part of QFD- Excitement (or WOW) Features: high satisfaction with existence

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How to collect customer’s requirements:– Observation of customers– Surveys: mail, telephone, face-to-face– Focus-group technique

• A group of surgeons for orthopaedic implants

Steps for developing useful data for requirements:– Specify the information needed:

• Reduce the problem to a single statement. If impossible, more than one data collecting effort may be warranted.– Determine the type of data-collection method to be used:

• Depending on the use of data collection methods– Determine the content of individual questions:

• Write a clear goal for the results expected from each question.– Design the questions:

• Each question should seek unbiased, unambiguous, clear and brief information.• Do not: assume that the customers have more than common knowledge; use jargon; lead the customer toward the answer

you want; tangle two questions together. • Do use complete sentences

– Order the questions:• Order them to give context

– Take data:• Any set of questions should be considered a test or verification. • Repeated application is required to generate usable information.

– Reduce the data:• Make a list of customer’s requirements in the customer’s own word (easy; fast; other abstract terms).• The list should be in positive terms, i.e., wanted, not unwanted

QFD Step 2: Determine the Customer’s Requirements: What do the customers want?

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Types of Customers’ Requirements: Functional Performance

– Performance about the product’s desired behavior– Flow of energy, information, or materials; Operational steps; operation sequence

Human Factors– Required in any products that is seen, touched, heard, tasted, smelled or controlled by a human– Appearance; Force and motion control; Ease of controlling and sensing state

Physical Requirements– Available spatial envelope; Physical properties

Reliability– Mean time between failures; Safety (hazard assessment)

Life-Cycle-Concerns– Distribution (shipping); Maintainability; Diagnosability; Repairability; Testability; Cleanability; Installability;

Retirement Resources Concerns

– Time; Cost; Capital; Unit; Equipment; Standards; Environment Manufacturing Requirements

– Materials; Quantity; Company capabilities

**For Spinal Implants:- Functional performance (flow of energy, operational steps and operation sequence)- Human Factors - Physical Requirements- Reliability (mechanical failure, corrosion, biocompatibility, and complications due to the failure)- Other requirements are not as critical as for other common products

QFD Step 2: Determine the Customer’s Requirements: What do the customers want?

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Resource Concerns Time requirements:

– Timing to introduce a new product Cost Requirements:

– Capital Cost:– Cost per Unit:– Cost estimation will be covered in Chap 12.

Standards (Codes):– Types of Standards:

• Performance: seat-belt strength, helmet durability– The Product Standards Index lists US standards that apply to various products.– American National Standards Institute (ANSI) does not write standards but is a clearing house for

standards written by other organizations

• Test Methods:– American Society for Testing and Materials (ASTM) publishes over 4000 individual standards covering the

properties of materials, specifying equipment test the properties and outlining the procedures for testing.– Underwriters Laboratories (UL) testing standards.

• Codes of Practice:– Give parameterized design methods for standard mechanical components, such as pressure vessels, welds,

elevators, piping and heat exchangers

– Knowledge of which standards apply to the current situation are important to requirements and must be noted from the beginning of the project

Environment Concerns:

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Evaluate the importance of each of the customers’ requirements

– Generate a weighting factor for each requirement considering• To whom is the requirement important

• How is a measure of importance developed for this diverse group of requirements

– How to determine the weight factor• Customer’s rating from 1 (unimportant) to 10 (important)

• Fixed Sum Method:– Distribute the importance on all the listed requirements

QFD Step 3: Determine the Relative Importance of the Requirements: Who vs. What

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Determine how the customer perceives the competition’s ability to meet each of the requirements.

– 1 The product does not meet the requirement at all.– 2 The product meets the requirement slightly.– 3 The product meets the requirement somewhat.– 4 The product meets the requirement mostly.– 5 The product meets the requirement completely.

Why studying existing products?– It creates an awareness of what already exists.– It reveals opportunities to improve on what already exists.

This process is called “Competition Benchmarking.”

QFD Step 4: Identify and Evaluate the Competition: How satisfied is the customer now?

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Engineering Specifications:– Restatement of the design problems in terms of parameters that can be measured and have target

values.• Measurable behaviors of the product-to-be

– If units for an engineering parameter can not be found, the parameter is not measurable and must be readdressed.

Examples:– Easy to attach

• The number of steps; time to attach; number of parts; number of tools used

Every effort must be made to find as many ways as possible to measure customer’s requirements.

Carefully check each entry to see what nouns are or noun phrases have been used because each noun refers to an object that is part of the product or its environment and should be considered to see if new objects are being assumed.

– “easy to adjust suspension system …” then “an adjustable suspension system” has been assumed as part of the solution.

QFD Step 5: Generate Engineering Specifications: How will the customers’ requirements be met?

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QFD Step 6: Relate Customers’ Requirements to Engineering Specifications: How to measure what?

Strong, medium, weak, and no relationship

QFD Step 7: Set Engineering Targets: How much is good enough?

1. Ascertain how the competition meets the engineering goal.2. Establish targets for the new product.Remember:

- Set the target early.- Too tight target may eliminate new ideas.- If a target is much different than the values achieved by the competition, it should be questioned.

Strong negative; negative; Positive; Strong positive

QFD Step 8: Identify Relationships between Engineering Requirements: How are the “HOWS” dependent to each

other?

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Example of QFDStep 1: identify the customers

Step 2: determine the requirements

Step 3: determine the relative importance of requirements

Step 4: identify and evaluate the competition

Step 5: generate engineering specification

Step 6: Relate customers’ requirements to engineering specifications

Step 7: Set engineering targets

Step 8: identify the relationships between engineering requirements

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Basic Methods for Idea Generation Brainstorming:

– Record all the ideas generated.– Generate as many ideas as possible, then visualize them.– Think wild.– Do not allow evaluation of the ideas

6-3-5 Method (Brainwriting):– Brainwriting to force equal participation by all.– 6 (optimal number of members); 3 (number of ideas); 5 (minute interval)– No verbal communication allowed until the end.

Use of Analogies in Design:– Consider needed function and then ask, What else provides this function?

Use of Extremes and Inverses:– Transform current concepts into others by taking them to extremes or considering

inverses Finding Ideas in Reference Books and Trade Journals: Using Experts to Help Generate Concepts:

– Needed to design in a new domain

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Guideline for Team Project

Use Brainstorming Method to determine a design item.– Make sure to write your own ideas in an individual notebook and finalized

team project item in PDF with minutes of brainwriting.– Do not try to make a fancy and complete product. Any improvement in a

small component can be a product for the design team project as long as the techniques introduced in this class are well executed.

Make sure to follow all the steps suggested in this book and produce a good design records.

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Concept Generation

Concept:– An idea that is sufficiently developed to evaluate the

physical principles that govern its behavior.– Goal of concept generation is to confirm that the proposed

product will operate as anticipated and that, with reasonable further development, it will meet the targets set.

• Concepts must be refined enough to evaluate the technologies needed to realize them, to evaluate the basic architecture (form) of them, and to evaluate the manufacturability to some extent

• Concepts can be represented in deliverables (sketch, flow diagram, a set of calculations, prototypes, etc)

– Examples of weak methodology:• Start design with a concept to be developed into a

product.• There is a tendency for designers to take their first

idea and start to refine it toward a product.– If you generate one idea, it is probably a poor one. If you

generate twenty ideas, you may have a good one. (or, alternatively, he who spends too much time developing a single concept realizes only that concept.

Main goal of this chapter is to learn techniques for the generation of many concepts.

– Functional decomposition – Concept variant generation

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Understanding the Function of Existing Devices

Bench marking (review of existing devices) is always a good practice because:– There is nothing so new that ideas for it can not be borrowed from other devices.– Lots of engineering hours have been spent developing the features of existing products (it is

foolish to ignore it). Defining Function:

– Remember that function tells what the product must do, whereas its form conveys how the product will do it.

• Develop the what and then map the how as we mapped what the customer required into how the requirements were to be measured in QFD.

– Function is the logical flow of energy (including static forces), material, or information between objects or the change of state of an object caused by one or more of the flows.

• Functions required to attach any component to another are GRASP, POSITION, ATTACH. In undertaking these actions, human provides information and energy in controlling movement and in applying force to it. (Flow of energy, material, and information)

– Functions associated with flow of energy:• Types of energy: mechanical, electrical, fluid and thermal• Its action: transformed, stored, transferred (conducted), supplied, and dissipated

– Functions associated with flow of material:• Through-flow: position, lift, hold, support, move, translate, rotate and guide• Diverging flow: disassemble, separate• Converging flow: assembling or joining materials (mix, attach, and position relative to)

– Functions associated with flow of information:• In the form of mechanical signals, electrical signals or software• Generally used as part of automatic control or to interface with a human operator

– Function associated with the change of state of an object:• Changes in energy storage, kinetic or potential energy, material properties, form or information content

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Using Product Decomposition to understand the Function of Existing Product

Step 1: For the whole device, examine interfaces with other objects.– Examine the flows of energy, information and material into and out of the device

Step 2: Remove a component for more detailed study.– Carefully note how it was fastened to the rest of the device and also any relationship it

has to other parts that may not contact.• It may have a clearance with some other parts in order to function.

Step 3: Examine each interface of the component to find the flow of energy– Understand:

• How the functions identified in step 1 are transformed by the component;• How the parts are fastened together;• How forces are transformed and flow from one component to anther; and • The purpose of each feature of component

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Patents as a Source of Ideas

Patent Literature:– Good source of ideas although hard

to read– Sources for Patent Searches

• http://www.uspto.gov/patft/index.html

• http://www.delphion.com/home

• http://gb.espacenet.com - source for European and other foreign patents

Types of Patents:– Utility Patents:

• Claiming how an idea operates or is used

– Design Patents:• Covering only the look or form of

the idea

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Technique for Designing with Function

Goal of Functional Modeling:– To decompose the problem in terms of flow of energy, material, and

information• Decomposition forces a detailed understanding of what the product-to-be is to

do.

4 Basic Steps:– Find the overall function that needs to be accomplished.

• The goal is to generate a single statement of the overall function on the basis of the customer requirements.

– Create subfunction descriptions.• The goal is to decompose the overall function.

– Order the subfunctions.– Refine subfunctions.

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Step 1: Find the overall function Most important functions must be

reduced to a simple clause and put it in a black box.

– Inputs into and outputs out of this black box are all flows of the energy, material and information.

Guidelines:– Energy must be conserved.– Material must be conserved– All interfacing objects and known, fixed

parts of the system must be identified.• List all the objects (all features,

components, assemblies, humans, etc) that interact, or interface, with the system.

– Ask how will the customer know if the system performing?

• Answers to this question will help identify information flows that are important.

– Use action verbs to convey flow.• Typical mechanical design functions:

– See Table 7.1

The BikeE suspension Example: Overall function:

– transfer and absorb• To transfer forces between wheel,

chain, and frame and absorb peak loads between wheel and frame

– Goal: alter the energy flow

Make sure to state the overall function of your design in individual design notebook (personal ideas) and finalized overall function in the PDF.

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Step 2: Create subfunction description Goal is to decompose the overall function into subfunctions. Reasons for Decomposition:

– The resulting decomposition controls the search for solution to the design problem.• Since concepts follow function and products follow concepts, we must fully understand the function

before wasting time generating products that solve the wrong problem.– Decomposition into functional detail leads to a better understanding of the problem.

• Most good ideas come from fully understanding the functional needs of the design problem.• It is useful to begin function decomposition before the QFD and use the functional development to help

determine the engineering specifications.– Decomposition may lead to the realization that there are some already existing components that

can provide some of the functionality required.

Guidelines:– Consider what needs to happen (the function) not how.

• Detailed, structured-oriented how consideration must be suppressed as they add detail too soon, which limit the number of possible concepts too early.

– Use only objects described in the problem specification or overall function.• Use only “nouns” previously used to describe the material flow or interfacing objects to avoid the new

components creeping into the product.– Break the function down as finely as possible.

• Let each function represent a change or transformation in the flow of material, energy, or information.– Use standard notation when possible.

• Whenever available, use common notations, such as block diagrams used to represent elec. circuits, piping system, or transfer functions in systems dynamics and control, although there is no standard notation for general mechanical product design.

– Consider all operational sequences.• Think of each function in terms of its preparation, use, and conclusion.

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Functional Decomposition for the Space Shuttle aft Field Joint

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Step 3: Order the Subfunctions

Order the functions found in step 2 to accomplish the overall function in step 1.

Guidelines:– The flows must be in logical or temporal order.

• Arrange the subfunctions in independent group (preparation, use, and conclusion).

• In each group, arrange them so that the output of one function is the input of another.

– Redundant functions must be identified and combined.• Similar subfunctions must be combined into one.

– Functions not within the system boundary must be eliminated.• This step helps the team come to mutual agreement on the exact system boundary; it is

often not as simple as it sounds.

– Energy and material must be conserved as they flow through the system.• Inputs to each function must match the outputs of the previous function.

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Step 4: Refine Subfunctions Examine each subfunction to see if it can be further divided

into sub-subfunctions. This step should be continued until:

– ‘atomic’ functions are developed; or• “atomic” implies that the function can be fulfilled by existing objects.

– new objects are needed for further refinement.

It is a struggle to develop the suggested diagram of function decompositions.

– It is a fact that the design can be only as good as the understanding of the functions required by the problem.

– This exercise is both the first step in developing ideas for solutions and another step in understanding the problem.

The functional decomposition diagrams are intended to be updated and refined as the design progresses.

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Concept Generation

Concepts are the means for providing function.– Any form that gives an indication how the function can be achieved.– What to do Function vs. How to do Concepts (forms)– Remember that the idea is often not original.

Many methods for concept generation are available, but no single method is best.

– A good designer is familiar with these methods and uses them, or a combination of them, as needed.

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Basic Methods for Concept Generation

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Morphological Method This technique uses the functions identified

to foster ideas.– Powerful method that can be used formally or

informally as part of everyday thinking– 2 step approach

Step 1 - Developing Concepts for Each Function:

– Goal is to find as many concepts as possible that can provide each function identified in the decomposition.

• How to store mechanical energy: springs, elastomers (rubbers or plastics), etc.

• If there is a function with only one conceptual idea, this function must be re-examined.

– Situations explaining the lack of more concepts.

• The designer has made a fundamental assumption.

• The function is directed at how, not what. - e.g. “store energy in coil spring” rather than “store energy”

• Domain knowledge is limited.– Keep the concepts as abstract as possible and at

the same level of abstraction for better comparison of developed concepts.

• The force required for moving an object can be provided by a hydraulic piston, a linear electric motor, the impact of another object, or magnetic repulsion.

– Refined mechanical components vs. basic physical principles.

Morphology for BikeE Suspension System

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Morphological Method Step 2 - Combining Concepts:

– to combine these individual concepts into overall concepts to meet all the functional requirements.

• Select one concept for each function and combine those into a single design.

– Pitfalls:• This method may generate too many ideas.• It erroneously assumes that each function of

the design is independent and that each function satisfies only one function.

• The results may not make any sense.

Concept generation process is the time that back-of-the-envelope sketches begin useful.

– We remember functions by their forms– Only way to design an object with any

complexity is to use sketches to extend the short-term memory.

– Sketches made in the design notebook provide a clear record of the development of the concept and the product. Combined Concepts for BikeE Suspension System

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Logical Methods for Concept Generation The Theory of Inventive Machines, TRIZ:

– Developed by Genrikh Altshuller (a ME engineer, inventor, and Soviet patent investigator) in Soviet Union in the 1950s based on patterns found in patented ideas

– Goal of TRIZ:• Find the major contradiction that is making the problem hard to solve, then• Use TRIZ’s 40 inventive ideas for overcoming the contraindication

– With TRIZ, we can systematically innovate; we don’t have to wait for an inspiration or use the trial and error common to other methods.

Axiomatic Design:– Evolved in MIT by Prof. Nam Suh in an effort to make the design process

logical.– 1st Axiom:

• Maintain the independence.– Then, a change in a specific design parameter should have an effect only on a single function.

– 2nd Axiom:• Minimize the information content of the design.

– The simplest design has the highest probability of success and is the best alternative.

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Concept Evaluation How to choose the best of the concepts generated for

development into a quality product?– Goal is to expend the least amount of resources on deciding which concepts

have the highest potential for becoming a quality product.– It is difficult to evaluate concepts, or to choose which concepts to spend

time, particularly when we still have very limited knowledge and data on which to base this selection.

• Design is learning, and resources are limited.

Techniques for systematic evaluation of rough concepts.– Evaluation implies both “comparison” and “decision making.”

• It is the comparisons between alternative concepts and the requirements that they must meet that gives the information necessary to make decisions.

– For all design decisions:• Itemizing the alternatives and the criteria for their evaluation• Comparing the alternatives to the criteria to each other

– For comparisons:• Alternatives and criteria must be in the same language and they must exist at the

same level of abstraction

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Concept Evaluation TechniquesBe ready during concept evaluation to abandon your favorite idea, if you can not defend it in a rational way.

Abandon if necessary “the way things have always have been done around here”.

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Information Presentation in Concept Evaluation

Design Evaluation Cycles

85

Evaluation based on Feasibility Judgment Three Immediate Reactions of a Designer as a concept is generated based on

designer’s “gut feel”:– It is not Feasible.– It might work if something else happens.– It is worth considering.

Implications of Each of these Reactions:– It Is Not Feasible.

• Before discarding an idea, ask “Why is it not feasible?”• Make sure not to discard an idea because:

– a concept is similar to ones that are already established, or– a concept is not invented here (less ego-satisfying).

– It is Conditional.• To judge a concept workable if something else happens.• Factors are the readiness of technology, the possibility of obtaining currently unavailable

information, or the development of some other part of the product.– It is Worth Considering.

• The hardest concept to evaluate is one that is not obviously a good idea or a bad one, but looks worth considering.

• Such a concept requires engineering knowledge and experience. If sufficient knowledge is not immediately available, it must be developed using models or prototypes that are easily evaluated.

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Evaluation based on GO/NO-GO Screening

Measures for deciding to go or no-go:– Criteria defined by the customer requirements:

• Absolute evaluation by comparing each alternative concept with the customer requirements.

• A concept with a few no-go responses may be worth modifying rather than eliminating

• This type of evaluation not only weeds out designs that should not be considered further, but also helps generates new ideas.

– Readiness of the technologies used:• This technique refines the evaluation by forcing an absolute comparison with

state-of-the-art capabilities.

• 6 Measures for a Technology’s Maturity:– Are the critical parameters that control the function identified?

– Are the safe operating latitude and sensitivity of the parameters known?

– Have the failure modes been identified?

– Can the technology be manufactured with known process?

– Does hardware exist that demonstrates positive answers to the preceding four questions?

– Is the technology controllable throught the product’s life cycle?

• If these questions are not answered in the positive, a consultant or vendor is added to the team.

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Evaluation based on a Basic Decision Matrix

Decision-Matrix Method (or Pugh’s Method):

– effective comparison of alternative concepts (basic form Table 8.2)

– Iteratively test the completeness and understanding of requirements, rapidly identifies the strongest

Step 1: Choose the criteria for Comparison.– Criteria are the functional requirements and

engineering specification determined in QFD.– The concepts must be refined enough to

compare with the engineering targets for evaluation (mismatch in the level of abstraction).

Step 2: Develop Relative Importance Weightings.

– Step 3 in QFD should provide the data for relative importance.

Step 3: Select the Alternatives to be Compared.

Step 4: Evaluate Alternatives.– Relative evaluation among alternatives

Step 5: Compute the Satisfaction.

**This method is most effective if each member performs it independently and the individual results are then compared.

- have BDM in individual notebook.

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Decision Matrix for Energy Management System

S indicates “Same as datum”

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Robust Decision Making

Robust decision refers to make decisions that are as insensitive as possible to the uncertainty, incompleteness, and evolution of the information that they are based on.

For robust decision making, we need to improve the method used to evaluate the alternatives (step 4 in decision-matrix method).

Word Equations used for Robust Decision Making– Satisfaction = belief that an alternative meets the criteria– Belief = knowledge + confidence

• Belief is the confidence placed on an alternative’s ability to meet a target set by a criterion, requirement, or specification, based on current knowledge.

• Belief (virtual sum of knowledge and confidence) can be expressed on a “belief map.”

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Belief Map

Knowledge = a measure of the information held by a decision maker about a feature of an alternative defined by a criterion

Mea

sure

of

Co

nfi

den

ce:

Belief = p(k) x p(c) + (1 – p(k)) x (1 – p(c))

91

Belief Map showing the ProbabilitiesBelief Model:

Belief = p(k) x p(c) + (1 – p(k)) x (1 – p(c))

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Extreme Cases of Belief Map

p(k) = 1.0p(c) = 1.0

Certainly work

p(k) = 1.0p(c) = 0.0

Certainly not work

p(k) = 0.5p(c) = 0.0

Pessimist corner

No more info. For evaluation

p(k) = 0.5p(c) = 1.0

Optimist corner

Not clear

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Belief Map Example

Point A: p(k) = 0.8 (informed knowledge) and p(c) = 0.65 (high confidence); belief = 0.59 He/she has a belief of 59% that the concept will meet the functional requirements.

Point B: p(k) = 0.9, p(c)=0.25, Belief = 0.30

Arrow shows the evolution (With increasing knowledge, confidence and belief decrease.).

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Belief Map to Choose Where to Eat

Criteria: delighted (p(c) = 1.0) if < $5.00 disgusted (p(c)= 0.0) if > $10.00 fast food = $6.00 (sure) Family diner = $9 (most likely) Chinese = $7 (little knowledge)

Fast food should be chosen for the best probability for satisfaction.

* Choice can be altered with inclusion of other criteria, such as food quality, taste, nutrition, etc.

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Advanced Decision Matrix Steps 1 through 3: same as the Decision Matrix Method Step 4: Evaluate Alternatives

– Use a belief map for comparison– If little is known or the evaluation result is that the alternative possibly meets the criterion, then belief

= 0.5

Step 5: Compute Satisfaction– Satisfaction = (belief x importance weighting)

• Max satisfaction = 100 (evaluator is 100% satisfied.)

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Product Safety and Liability

Safety is best thought of early in the design process.– Product safety and liability are often overlooked until late in the project.

Product Safety:– Design for safety means ensuring that the product will not injury or loss.– 2 Safety Issues:

• Who or what is protected from injury or loss?

• How is the protection actually implemented in the product?

– 3 Ways to Institute the Safety:• Design safety directly into the product

– device poses no inherent danger during normal operation

• Add protective devices to the product (automatic cut-off switch).

• Warning of the dangers inherent in the use of the product.

Products Liability:– Special branch of law dealing with alleged personal injury or property or

environmental damage resulting from a defect in a product.

97

Products Liability

3 Different Charges of Negligence against Designers:– The product was defectively designed.

• Failure to use state-of-the-art design considerations.

• Improper analyses, use of poor materials

– The design did not include proper safety devices.– The designer did not foresee possible alternative uses of the product.

What to do for protecting ourselves as a designer?– Keep good records to show all that was considered during the design process.– Use commonly accepted standards when available.– Use state-of-the-art evaluation techniques for proving the quality of the design

before it goes into production.– Follow a rational design process so that the reasoning behind design decisions can

be defended.

98

Hazard AssessmentWhen a potential hazard is identified,1. Estimate the frequency of occurrence

(Table 8.6);2. Estimate the consequence of occurrence

(Table 8.7);3. Predict the hazard index using the

hazard-assessment matrix (Table 8.8);

If the hazard index is low (undesired or unacceptable), further consideration must be given on the design.

99

The Product Design Phase

Goal:- Refine the concepts already generated into

quality products.- Giving flesh to what was the skeleton of an

idea (hardware design, shape design, or embodiment design)

Steps:- Generate Product (chapter 10)- Evaluate the product (chaps 11, 12)- Decision Making

*Drawings, Bills of Materials, and other preliminary records of the product design effort

100

Drawings Produced during Product Design

Layout Drawings:

Detail Drawings:

Assembly Drawings:

Graphical Models produced in CAD Systems:

101

Layout Drawings- A layout drawing is a working document that supports the development of the

major components and their relationship.

- Characteristics:- Frequently changed during the design process. Care should be taken not to lose the records of

changes.- A layout drawing is made to scale.- Only the important dimensions are shown.- Tolerances are usually not shown, unless they are critical.- Notes on the layout drawing are used to explain in a design feature or the function of the

product.- A layout drawing often becomes obsolete as detail drawings and assembly drawings are

developed.

102

Detail Drawings The detail of individual

components, developed as the product evolves on the layout drawing, are documented on detail drawings.

Characteristics:– All dimensions must be toleranced.

No tolerance indicates the use of standard tolerances used in a company.

– Materials and manufacturing detail must be in clear and specific language.

– Drawing standards (ANSI Y 14.5M-1994, Dimensions and Tolerancing, and DOD-STD-100, Engineering Drawing Practices, or company standards) should be followed.

– Detail drawings are a final presentation of the design effort and will be used to communicate the product to manufacturing. Thus, a signature block for management approval is a standard part of a detail drawing.

103

Assembly Drawings Assembly drawings are made to

show how the component fit together.

Characteristics:– Each component is identified with a

number or letter keyed to the bill of materials (BOM).

– References can be made to other drawings and specific assembly instructions for additional needed information.

– Necessary detailed views are included to convey information not clear in the major views.

– Assembly drawings require a signature block.

104

Graphical Models produced in CAD Systems

Positive Aspects: Rapid representation of concepts and

the ability to see how they assemble and operate without the need for hardware.

Improves the design process because features, dimensions, and tolerances are developed and recorded only once (less potential error)

Easy to ensure that mating components fit together.

Detail and assembly drawings are produced semi-automatically, reducing the need to have expert knowledge of drafting methods and drawing standards.

Files created are useful for making prototypes and developing figures for any other purposes.

Negative Aspects: There is a tendency to abandon

sketching although sketches are a rapid way to develop a high number of ideas. (Longer time for solid modeling)

Too much time is often spent on details too soon because solid model systems require details in order to even make a rough drawing.

Often valuable design time is spent just using the tool.

Many solid modeling systems require the components and assemblies to be planned out ahead of time. These systems are more like an automated drafting system than a design aid.

105

Bills of Materials (Part Lists) An index to the product

– It is a good practice to generate the BOM as the product evolves on a spreadsheet, which is easy to update.

– To keep lists to a reasonable length, a separate list is made for each assembly. Minimum Pieces of Information on BOM

– The item number or letter– The part number– The quantity needed in the assembly– Name or Description of the Component– Material from which the component is made– Source of the component (if the component is purchased from other companies)

106

Product Data Management A major undertaking in a company is to manage design

information, a company’s most valuable asset.

Product Data Management Systems (PDMs)– Database programs used to support the management of documents and files,

product structure and processes for better management of both product and process information

– Records stored in PDMS• CAD files (all kinds of drawings and solid models)• Text documents (meeting notes, contracts, etc.)• Spread sheets (QFD, decision matrices, and other analysis)• Database reports• Parts libraries, vendor data, engineering change orders

– PDMs allow easy search and organization of the data.– PDMs allow management of the way people create and modify data.– PDMs help support the task schedule by the tools to support the development

and maintenance of Gantt charts, worker allocation, and task definitions.

107

Product Generation (Chaps 10, 11, 12)

The goal of product generation is to transform the design concepts into products that perform the desired functions.

Such refinement requires work on all the elements in the figure.

108

Form Generation Form development is the evolution of

components, how they are configured relative to each other and how they are connected to each other within the constraints.

– Understand the spatial constraints:– Configure components:– Develop Connections: Create and refine

interfaces for functions:– Develop Components:

Understand the Spatial Constraints:– Spatial constraints are the walls or

envelope for the product.– Some spatial constraints are for

functionally needed space. • For medical implants, human body limits

the space significantly.

Kyphoplasty: The balloon size is constrained by the vertebral body size.

109

Form Generation

Configure Components:– Configuration is the architecture, structure, or arrangement of the components and

assemblies of components in the product.– Developing the configuration involves decisions that divide the product into

individual components and develop the location and orientation of them.– 6 Reasons to decompose a product or assembly into separate components:

components must be separate if:• They need to move relative to each other;

• They need to be of different materials for functional purposes;

• They need to be moved for accessibility;

• They need to accommodate material or production limitations;

• There are available standard components that can be considered for the product;

• Separate components would minimize costs.

– Location and Orientation of the Components with each other:• Location: Measure of relative position in a space (x, y, z)

• Orientation: angular relationship of the components

110

Form Generation

Develop Connections (Create and refine interfaces for functions):– A key step when embodying a concept because the connections or interfaces between

components support their function and determine their relative positions and locations.

– Guidelines to help develop and refine the interfaces between components:• Interfaces must always reflect force equilibrium and consistent flow of energy, material,

and information. – These flows are means through which the product will be designed to meet the functional

requirements.

• After developing interfaces with external objects, consider the interfaces that carry the most critical functions.

– In general, most critical functions are those that seem hardest to achieve or those described as most important in the customers’ requirements.

• Try to maintain functional independence in the design of an assembly or component.– This means that the variation in each critical dimension in the assembly or component should

affect only one function.

• Exercise care when separating the product into separate components.– Complexity arises since one function often occurs across many components or assemblies and

since one component may play a role in many functions.

• Creating and refining interfaces may force decompositions that result in new functions or may encourage the refinement of the functional breakdown.

111

Form Generation Develop Connections (Create and refine interfaces for

functions):– Types of Connections:

• Fixed, nonadjustable connection• Adjustable connection

– Should allow at least 1-DOF that can be locked.

• Separable connection• Locator connection

– The interface determines the location or orientation of one of the components relative to another.– Care must be taken in these connections to account for errors that can accumulate in joints.

• Hinged or pivoting connection– Many connections have one or more degrees of freedom.– The ability to transmit energy and information is usually key to the function of the device

– Degrees of Freedom (DOF):• Number of directions required for complete description of location and

orientation of a component moving relative to another.• Fundamentally, every connection between 2 components has 6 DOF – 3

translations and 3 rotations.• It is the design of connections that determine how many DOF the final

product will have.

112

Examples of Connections with Varying DOF

3 DOF Situation

A single pin or short wall was inserted into B for positioning A. The effect will be to only limit the position of A relative to B in the +x direction.

Efforts to fully constrain along the x-axis.Putting a second support on the x-axis to limit motion in –x direction can have unintended consequences.

Most joints need to position parts relative to each other and transmitted forces. Thus, it is worthwhile to think in terms of positioning and then force transmission.

113

Examples of Connections with Varying DOF

Block A restriction in x-direction and z-rotation

Block A is fully constrained under the force F.

Other fully constrained blocks.

114

Form Generation Develop Components:

– Determine adequate shape and size of each component.• Critical dimensions on most components are found in functional interfaces.• In designing the bodies of components, be aware that stiffness determines the adequate size more frequently than

stress.– Use force flow visualization to estimate the required stiffness and strength of each component– Use standard shapes when possible.

Major functions are to transfer forces and clear (not interfere with) other components.

115

Form Generation Material between interfaces generally serves 3 main purposes.

– To carry forces or other forms of energy between interfaces with sufficient strength and rigidity

– To act as an enclosure or guide for other components– To provide appearance surfaces

When the body of a component provides the function (e.g. needed mass, stiffness, or strength), shape of the component can be as important as the interface.

– It is best to connect interfaces with strong structural shape.– Size and shape (area an polar moment of inertia) of the component should be

considered.• Rod to resist tension or compression

• Hollow cylinder to resist torque

• I-beam to carry bending loads in the most efficient way possible

116

Form Generation Force Flow Visualization

– A good method to visualize how forces are transmitted through components and assemblies

– Method• Treat forces like a fluid that flows in and out of the interfaces

and through the component.

• The fluid takes the path of least resistance through the component.

• Sketch multiple flow lines. The direction of each flow line will represent the maximum principal stress at the location.

• Label the flow lines for the major types of stress occurring at the location: tension (T), Compression (C), shear (S), or bending (B).

– B can be decomposed into T and C.

– S must occur between T and C on a flow line.

• Remember that force is transmitted at interfaces primarily by compression. Shear only occurs in adhesive, welded and friction surfaces.

– Advantages:• Force flow helps us visualize the stresses in a component or

assembly.

• It is best if the force paths are short and direct.– The more indirect the path, the more potential failure points and stress

concentrations.

117

Form Generation

Refine and Patch– Refine is to make an object less abstract (or more concrete).– Patching is to change a design without changing its level of abstraction.

• The goal is to make things work and to make them simpler.

Types of Patching– Combining is making one component serve multiple functions or replace multiple components.

• strongly encouraged when the product is evaluated for its ease of assembly.– Decomposing is breaking a component into multiple components or assemblies.

• It is worthwhile to consider returning to the beginning of the design process.– Magnifying means making a component or some feature of it bigger relative to adjacent items.

• Exaggerating the size or number of a feature often increases one’s understanding.– Minifying means making a component or some feature of it smaller.

• Eliminating, streamlining, or condensing a feature may improve the design.– Rearranging means reconfiguring the components or their features.

• This often leads to new ideas because the reconfigured shapes force to rethinking of how the component fulfills the functions.

– Reversing means transposing or changing the view of the component or feature.– Substituting means identifying other concepts, components, or features that will work in place of

the current idea.• Care must be taken because new ideas sometimes carry with them new functions.

118

Example of Patching

119

Materials and Process Selection

Material and production processes selected must evolve as the shape of the product evolves in concurrent engineering.

– Constraints in determining the material:• Mass (weight); Environment (corrosion); Strength and stiffness

• Biocompatibility

Information influencing the embodiment of the product:– Quantity of the product to be manufactured– Prior-use knowledge

• When studying existing devices, get into the habit of determining what kind of materials were used for what types of functions.

– Knowledge and experience– Availability of a material

120

Vendor Development

Mechanical designers seldom design basic mechanical components (such as bolts, nuts, gears, or bearings) for each new product since such components are readily available.

– This is not true in designing an orthopaedic implant.

Advantages of using a components available through vendors.– Vendors have history of designing and manufacturing the product, so they already have the

expertise and machinery to produce a quality product.– They already know what can go wrong during design and production.– They specialize in the design and manufacture of the component, so they can make it in

volumes high enough to keep the cost below what can be achieved through an in-house effort.

After using concurrent engineering which involves a small number of vendors in the design process from the beginning and includes them in the decisions that affect what they will be supplying, many companies have been able to reduce the number of vendors.

– These tight relationships lead to improved product quality.

121

Product Evaluation for Performance and the Effects of Variation

to compare the performance of the product to the engineering specifications (or targets) developed earlier in the design project

The process of Product Evaluation– Monitoring functional change– Goals of performance evaluation– Accuracy, variation, and noise– Modeling for performance evalution– Tolerance analysis– Sensitivity analysis– Robust design– Design for cost (DFC)– Value Engineering– Design for manufacture (DFM)– Design for assembly (DFA)– Design for reliability (DFR)– Design for test and maintenance– Design for the environment

122

Functional Evaluation

Benefits in refining the function model as the form is evolving– The functions that the product must accomplish can be kept very clear by updating

the functional breakdown.• Nearly every decision about the form of an object adds something, either desirable or

undesirable to the function of the object.– Tracking the evolution of function means continuously updating the flow models of

energy, information, and materials.• These flows determine the performance of the product.

In earlier design process:

Design Problems

Function models

Potential Concepts

Product Generation

Changes in functions and concepts

123

Goals of Performance Evaluation

To evaluate the product design relative to targets set previously. Factors must be supported by the evaluation of product performance:

– Evaluation must result in numerical measures of the product for comparison with the engineering requirement targets developed during the problem understanding.

• Measurements must be of sufficient accuracy and precision for valid comparison.– Evaluation should give some indication of which features of the product design to modify;

and by how much in order to bring the performance on target.– Evaluation procedures must include the influence of variations due to manufacturing, aging,

and environmental changes.• Insensitivity to these “noises” while meeting the targets results in a robust, quality product.

Additional concepts for better design:– Optimization; trade studies, accuracy, tolerances, sensitivity analysis and robust design

P-Diagram

124

How to Evaluate Performance? Performance can be evaluated using a

model.– Graphical model for form evaluation

• Sketches and layout drawings – Analytical model for function evaluation

• Back-of-the-envelope analysis,• engineering science analysis, • or detailed computer simulation

(optimization, FEM, etc.)– Physical model for function and form

evaluation• Prototypes for proof-of-concept, proof-of

product and proof-of-production

Example: Design of a Tank to hold liquid:

– A customer’s requirement is to design the “best” tank to hold “exactly” 4 m3 of liquid.

– Assume that conceptual design of the tank resulted in a cylindrical shape with an internal radius r and an internal length l.

– Analytical model: V = r2l 1.27 m3 = r2l

*More thoughts required:1. Other quality measures that may limit the

potential r and l values.- weight, size targets, manufacturability,

environment, etc.

2. What is meant by the terms “best” and “exactly”?- accuracy, variation, and noise

125

Accuracy, Variation, and Noise The purpose of modeling is to find the easiest method by which to evaluate the product for

comparison with the engineering targets using available resources. Two types of errors in any model

– Errors due to inaccuracy– Errors due to variation

Accuracy– The correctness or truth of the model’s estimate– In case of distributed results, the best estimate (mean) will be a good predictor of product performance.– The variation in the results obtained from the model refers to statistical variation of the results about the mean

value.• Precision, resolution, range and deviation are also used to refer to the distribution of the evaluation.

– The obvious goal in modeling is to develop an accurate model with a small variation.– Accuracy tells “how much” whereas distribution tells “how sure”.

Why concern the variation?– Each parameter that defines the product or process has variation and so each may vary greatly from the desired

mean.

126

Examples of Variation

Remember that, during production, not all samples of the product:

– are exactly the same size;– are made of exactly same material; or– behave in exactly the same way.

We have to consider how such variations affect the performance in the design process.

– Deterministic analytic models– Non-deterministic (or stochastic)

analytical models that account for both the mean and the variation by using methods from probability and statistics

127

Effect of Variation on Product Quality

A product is considered to be of high quality if its quality measures stay on target regardless of parameter variation due to manufacturing, aging, or environment.

Control parameters vs. Noise as a source of variation

– Control parameters:• parameters controllable by the designer,

such as working environment, geometry, etc.

– Noise:• Uncontrollable parameters

• Noises affecting the design parameters

– Manufacturing, or unit-to-unit variations

– Aging, or deterioration, effects, including etching, corrosion, wear and other surface effects

– Environmental, or external, conditions including all effects of the operating environment.

128

How to Deal With Noises

Noises that affect the strength are often accounted for by using a “safety factor (or factor of safety).”

– FS = Sal/ap (Sal = allowable strength; ap = applied stress)– Rule-of-Thumb Factor of Safety (see appendix C)

Keep noises small by tightening manufacturing variations (generally expensive) Add active controls that compensate for the variations (generally complex and

expensive). Shield the product from aging and environmental effects (sometimes difficult

and may be impossible). Make the product insensitive to the noises (robust design).

– Key Philosophy of Robust Design:• Determine values for the parameters based on easy-to-manufacture tolerances and default

protection from aging and environmental effects so that the best performance is achieved. The term, best performance, implies that the engineering targets are met and the product is insensitive to noise. If noise-insensitivity cannot be met by adjusting the parameters, then tolerances must be tightened or the product shielded from the effects of aging and environment.

129

Modeling for Performance Evaluation

Steps to give order to the considerations taken into account during evaluation:1. Identify the output responses (i.e., critical or quality parameters) that need

to be measured.

2. Note how accurate the output needs to be.

3. Identify the input signal, the control parameters and their limits, and noises.

4. Understand analytical modeling capabilities.

5. Understand the physical modeling capabilities

6. Select the most appropriate modeling method.

7. Perform the analysis or experiments.

8. Verify the results.

130

Tolerance Analysis

Theoretically, tolerance is assumed to represent 3% standard deviations about the mean value, implying that 99.68% of all the samples should fall within the tolerance.

Focus of tolerance design is the concern about tolerances on dimensions and other variables (i.e., material properties) that affect the product.

– It is shown that only a fraction of the tolerances on a typical component actually affect its function.

131

Effect of Tighter Tolerances on the Manufacturing Cost

*Specification of tighter tolerances will increase the manufacturing cost.- use nominal tolerance whenever possible.

Meaning of the Tolerances Specified on the Drawings:

1. It communicates information to manufacturing that is essential in helping to determine the manufacturing processes that will be used.

2. Tolerance information is used to establish quality-control guide-line. (conformance quality)

132

Additive Tolerance Stack-up Most common form of tolerance

analysis.

*Example of Air shock-swingarm:When the joint is assembled,

lg = ls – (lb + 2 x lw)

lg = gap lengthls = distance between fingerslb = bushing lengthlw = washer thickness

Worst-case Analysis:If lb = 19.97 (min), lw = 1.95 (min), ls = 24.1 (max), thenlg = 0.23 mm.If lb = 20.03 (max), lw = 2.05 (max), ls = 23.9 (min), thenlg = -0.23 mm (interference).

If you want assembly to be easy, no interference, thenYou should specify ls = 24.33 0.1 mm so that the narrowest possible distance between the fingers will still fit the widest components.

133

Statistical Stack-Up Analysis A more accurate estimate of the gap can be found statistically, in a form of statistical analysis. Consider a stack-up problem composed of n components, each with mean length li and tolerance ti (assumed

symmetric about the mean).

In general, a length of the dependent parameter is,

l = l1 l2 l3 ….. ln

the sign on each term depends on the structure of the device.

The standard deviation is

s = (s12

+ s22

+ s32

+ ….. + sn2)1/2

Since s = t / 3,

t = (t12

+ t22

+ t32

+ ….. + tn2)1/2

For the example,

lg = ls – (lb + 2 x lw), tg = (ts2

+ tb2

+ 2 x tw2) ½

For ls = 24.00 0.1, lb = 20.00 0.03, lw = 2.00 0.05;

lg = 24 – (20 + 2 x 2) = 0.0 and tg = (0.102 + 0.032 + 2 x 0.052) 1/2 = 0.126 mm

On the average, there is no gap and the tolerance on it is 0.126 mm.

134

Example of Statistical Stack-Up Analysis For the example of Air shock-swingarm,

lg = ls – (lb + 2 x lw), tg = (ts2

+ tb2

+ 2 x tw2) ½

For ls = 24.00 0.1, lb = 20.00 0.03, lw = 2.00 0.05;

lg = 24 – (20 + 2 x 2) = 0.0 and tg = (0.102 + 0.032 + 2 x 0.052) 1/2 = 0.126 mm

On the average, there is no gap and the tolerance on it is 0.126 mm.

In this problem, let’s make further assumptions:

1) When bolted, the fingers can flex up to 0.07 mm inward without undo stress on the welds to compensate for any clearance.

2) The assembly personnel can get the parts in between the fingers even if there is a 0.03 mm interference.

Then, what percentage of the assemblies will meet these requirements?

Figure shows that the probability for problems to occur during assembly is 29% (24 + 5).

How can we readjust the tolerance values?1) Inspect each part and reworking on the

numbers.2) Determine which tolerance is most sensitive to

the results using sensitivity analysis and repeat the tolerance analysis.

135

Sensitivity Analysis Technique for evaluating the statistical relationship of control

parameters and their tolerances in a design problem.– Sensitivity analysis allows the contribution of each parameter to the variation

to be easily found

For 1-dimensional problem (air shock-swingarm):s = (s1

2 + s2

2 + s3

2 + ….. + sn

2)1/2

For Pi = si2 / s2, where Pi is the contribution of the i-th term to the tolerance (or variance) of the dependent

variable

1 = P1 + P2 + ….. + Pn

For air shock-swingarm problem;Ps = (0.1 x 0.1)/(0.126 x 0.126) = 0.63 = 63%

Pb = (0.03 x 0.03)/(0.126 x 0.126) = 0.05 = 5%

Pw = (0.05 x 0.05)/(0.126 x 0.126) = 0.16 = 16% 0.63 + 0.05 + 2 x 0.16 = 1.00

- The tolerance on the spacing has the greatest effect on the gap. Thus, the tolerance on the spacing is the most likely candidate for change.

136

Multi-dimensional Sensitivity Analysis

)......,,,( ,321 nxxxxfF

),.......,,,( 321 nxxxxfF 2/1

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)(......)(

nn

sx

Fs

x

Fs

Consider a general function:

F = a dependent parameter (length, volume, stress or energy) andxi = the control parameters (usually dimensions and material properties)

For means and standard deviations (si),

If F/ xi = 1, this SD equation becomes a linear equation

137

Tank Problem

lrV 21416.3

For the independent parameters of r and l, the mean volume is:

The tolerance on these parameters can be based on what is easy to achieve with nominal manufacturing processes.

Let tr = 0.03 m (sr = 0.01) and tl = 0.15 m (sl = 0.05), then SD on this volume is:

2

2/1

22

22

1416.3

2830.6

rl

V

and

rlr

V

where

sr

Vs

l

Vs rlv

For point A, V/ r = 6.61 and V/ l = 4.60, so sv = [6.612 x 0.052 + 4.602 x 0.032]1/2 = 0.239

- 99.68% (3 SD) of all the vessels built will have volumes within 0.717 m3(3 x 0.239) of the target 4 m3.

For point B, V/ r = 16 and V/ l = 0.78, so sv = [0.782 x 0.052 + 162 x 0.032]1/2 = 0.166- 99.68% (3 SD) of all the vessels built will have volumes within 0.498 m3of

the target 4 m3.

*Reduction in variation can be achieved not by changing the tolerances on the parameters but by changing only their nominal values.

*If we can find the values of r and l that give the smallest variance on the volume, then we are employing the philosophy of robust design.

138

Robust Design by Analysis

In the previous tank example, the tank with greater length had less sensitivity to the large tolerance on the length, so the tank volume varies less.

What are the most robust values for the parameters?– It is impossible to have V = 4 m3, exactly due to random variations in r and l.– The best we can do is to minimize the difference between V and 4 m3.

)(2

....

var

222222

2

2

21

2

1

TlrsrsrlC

TFsx

Fs

x

FC

biasianceC

lr

nn

The objective function to be minimized is:T = target

For the tank,

For known SDs on r and l, and known target T

40

220

24220

2

222

22322

lrC

rsrll

C

rlsrslrr

C

r

lr

3

12

2

414.1

r

l

l

r

s

sl

s

slr

For sr=0.01, sl=0.05;

r = 0.71 m; l = 2.52 m; sv = 0.138 m3

Improvement in volume variation!*If this SD is not small enough, we need to tighten the tolerances of r and/or l.

139

Summary: Robust Design

Step 1: Establish the relationship between quality characteristics and the control parameters. Also define a target for the quality characteristics.

Step 2: Based on known tolerances (SDs) on the control variables, generate the equation for the standard deviation of the quality characteristics.

Step 3: Solve the equation for the minimum SD of the quality characteristic subject to this variable being kept on target.

)......,,,( ,321 nxxxxfF

2/1

2221

2

1

)(......)(

nn

sx

Fs

x

Fs

Limitations on this method:1. It is only good for design problems that can be represented by an equation.2. The objective function used in the previous example does not allow for the inclusion

of constraints in the problem. For example, if the radius had to be less than 1.0 m because of space limitations, the previous cost function would need additional terms to include.

140

Robust Design Through Testing

Used when the quality characteristics cannot be represented in an equation.– V = f(r,l), i.e., analytically in-deterministic relationship, as compared with V = r2l,

analytically deterministic relationship– Begin by building a tank with some best-guess dimensions and measure the volume.

Repeat building a tank until we can find the right dimensions. Drawbacks:

– Repetitive model building is not efficient.– There is no guarantee that the final design will be the most robust.

Steps to overcome such drawbacks.1. Identify signals, noise, control, and quality factors (i.e., independent parameters.2. For each quality measure (i.e., output response) to be evaluated, recall or determine

its target value and the nature of the quality loss function.3. Design the experiment.4. Take and reduce data.5. Analyze the results, and select new test conditions if needed.

141

Step 1: Identify signals, noise, control, and quality factors

Step 2: For each quality measure (output response), determine its target value and the nature of the quality loss function.

Quality loss is proportional to the mean square deviation (MSD), average difference between the output response and the target. This difference is often referred to as signal-to-noise (S/N) ratio.

n

iiy

n 1

21

n

i iyn 12

11 22

1

1myyy

n

n

ii

n

iiy

n 1

21log10

n

i iyn 12

11log10

n

ii yy

n 1

21log10

Quality Loss Function: smaller-is-better larger-is-better Nominal-is-best

MSD

S/N Ratio

142

Step 3: Design the experiment.– The experiment should be designed so that the results give a clear understanding

of 1) the effects on the output response of changing control parameters; and 2) an understanding of the effects of noise.

• An ideal experiment will show how to adjust the control parameter to meet the target and show which one to choose so that the resulting system is insensitive to noise.

– Goals:• Control factors can be changed to represent the options available.

• Noises can be controlled over the expected range.

• The output responses can be measured accurately.

143

Step 4: Take and Reduce Data Step 5: Analyze the Results, and Select New Test Conditions if Needed

Is 4.34 m3 is close enough?

If not, there can be 2 ways for further refinement:1) r can be estimated to bring the output to 4 because l=5.5 resulted in better S/N. (How

much to change r is not clear from the table)2) Perform experiments by setting new values for r and l around the values.

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Product Evaluation for Cost, Manufacture, Assembly, and Other Measures

Cost Estimating in Design– Most difficult and yet important tasks

• A rough estimate should be generated in the conceptual phase or at the beginning of the embodiment phase; and

• Cost estimate is refined as the product is refined.

– DFC (Design For Cost): keeping an evolving cost estimate current as the product is refined.

Determining the Cost of a Product:– Direct Cost

• All costs that can be directly traced to a specific component, assembly, or product

– Indirect Cost

It is the responsibility of the designer to know the manufacturing cost of components designed.

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Cost of Machined Components

Machining is to remove portions of the material not wanted

7 significant control factors for the machining cost:1. From what material is the component to

be machined?

2. What type of machine is used to manufacture the component?

3. What are the major dimensions of the component?

4. How many machined surfaces are there, and how much material is to be removed?

5. How many components are made?

6. What tolerance and surface finishes are required?

7. What is the labor rate for machinists?

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Cost of Injection-Molded Components

Most popular method for making high-volume products with less precision requirements

Factors for Cost:– All factors for machined component– Cost for manufacturing the mold

• Wall thickness

• component complexity

– Molding time (cooling time)– Number of components

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Value Engineering

Developed by GE in the 1940s and evolved into the 1980s.

How to determine the value of a function in relation to the required cost?

– Value = Worth of a feature, component, or assembly / Cost of it– Value = function provided per dollar of cost

The worth of the function to the customer must be well identified.

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Design For Manufacture (DFM)

DFM is widely used but poorly defined. DFM is establishing the shape of components to allow for efficient,

high-quality manufacture.– Key concern: Specification of the best manufacturing process– How to hold the components for machining?– How to release from the molds?– How to move components between the processes?

The concurrent engineering philosophy, with manufacturing engineers as members of the design team, help incorporate the DFM.

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Design –for-assembly (DFA) Evaluation DFA is the best practice used to measure the ease with which a product can be assembled in

terms of efficiency.– Assembling a product means that a person must 1) retrieve components from storage, 2) handle the

components to orient them relative to each other, and 3) mate them.– A product with high assembly efficiency has a few components that are easy to handle and virtually fall

together during assembly.

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Guidelines for better DFA Evaluation of the overall assembly:

1. Overall component count should be minimized.2. Make minimum use of separate fasteners.3. Design the product with a base component for locating other components.4. Do not require the base to be repositioned during assembly.5. Make the assembly sequence efficient.

Evaluation of component retrieval:6. Avoid component characteristics that complicate retrieval.7. Design components for a specific type of retrieval handling, and mating.

Evaluation of component handling:8. Design all components for end-to-end symmetry.9. Design all components for symmetry about their axes.10. Design components that are not symmetric about their axes of insertion to be clearly

asymmetric. Evaluation of component mating:

11. Design components to mate through straight line assembly.12. Make use of chamfers, leads and compliance to facilitate insertion and alignment.13. Maximize component accessibility.

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DFA for Orthopaedic Implants

DFA is critical in designing a product for mass production.

For orthopaedic implants:– Actual assembly is performed during operation by surgeons.– Easy assembly is one of the major features that surgeons are looking for

from an implant.– Make sure to minimize the number of components, assembling

procedures and instruments for assembly.– It is always better to consider the removal of implants.

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Design For Reliability (DFA)

Reliability is a measure of how the quality of a product is maintained over time.– Quality = satisfactory performance under a stated set of operating conditions.

• Unsatisfactory performance = failure• Mechanical failure = any change or error that renders a component, assembly or system

incapable of performing its intended function.• Typical source of mechanical failure: wear, fatigue, yielding, jamming, bonding

weakness, property change, buckling and imbalance– Failure Modes and Effects Analysis (FMEA):

• Technique for identifying failure potential used in calculating the reliability of a product. Failure-Potential Analysis

1. Identify the function affected.2. Identify the effect of failure on other parts of the system.3. Identify the failure modes affecting the function.4. Identify the corrective action.

Reliability (R(t)) = exp(-Lt), L = failure rate or mean time between failures (MTBF) determined from the experiments– R(t) is the probability that the component has not failed.– R(8760 hrs) = 0.892 implies that it would be expected that 89.2 out of 100 would

still be operating after a year within specifications.

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Design for Test and Maintenance (DFTM):– Testability refers to the ease with which the performance of critical functions

is measured.– To design products that are easy to diagnose, disassemble, and repair at any

level of function.– Practice following the design process suggested in this class increases the

testability.

Design for the Environment:– Green design, environmentally conscious design, life cycle design, or design

for recyclability.– Guidelines:

• Be aware of the environmental effects of the materials used in products.

• Design the product with high separability

• Design components that can be reused to be recycled.

• Be aware of the environmental effect of the material not reused.

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Launching and Supporting the Product

Documentation and communication– Quality assurance and quality control– Manufacturing instructions– Assembly instructions– Installation instructions– Operating instructions– Maintenance instructions– Retirement instructions

Support– Vendor relationships– Customer relations– Support for manufacturing and assembly

Engineering changes Patent applications