Reliability ModuleBy: Alex Miller and Mark Robinson

Material Summarized fromReliability Module

Derived from original material on the NASA Space Systems Engineering Website (http://spacese.spacegrant.org/).

Key Concepts● Understanding the importance of reliability

as an engineering discipline● Definitions and uses for important

engineering concepts○ Constant failure rate, MTBF, 'bathtub' curve, etc.

● The meaning and practice of redundancy (fault tolerance, functional redundancy, fault avoidance)

● Ways to calculate reliability ○ Mathematically○ Visually (block diagrams)

Source: Exploration Systems Engineering Version 1.0, Reliability Module

Definition of Reliability 1● Reliability: "The probability that the

system-of-interest will not fail for a given period of time under specified operating conditions."○ Reliability is an inherent system design

characteristic○ Reliability plays a key role in determining the

system's cost effectiveness

Source: Exploration Systems Engineering Version 1.0, Reliability Module

NASA's Systems Engineering Handbook definition (1995 edition)

Definition of Reliability 2● Reliability Engineering: "Specialty

discipline within the systems engineering process."○ Design - including design features that ensure the system can perform

in the predicted physical environment throughout the mission.○ Trade studies - reliability as a figure of merit. Often traded with cost.○ Modeling - reliability prediction models, reflecting environmental

considerations and applicable experience from previous projects.○ Test - making independent predictions of system reliability for test

planning/program; sets environmental test requirements and specifications for hardware qualification.

NASA's Systems Engineering Handbook definition (1995 edition)

Constant Failure RateProbability Distribution is usually described as an exponential function: R=et/M

R = reliabilityt = timeM = Mean time between failures

Source: Exploration Systems Engineering Version 1.0, Reliability Module

For systems that must operate continuously, it is common to express their reliability in terms of the Mean Time Between Failure (MTBF)

"Bathtub" Failure Rate CurveThree phases:

1. Burn in period - higher failure rate due to infant mortality

2. Useful life period - low failure rate due to random events

3. Old age - increase failure rate due to parts wearing out

Source: Exploration Systems Engineering Version 1.0, Reliability Module

"Bathtub" Failure Rate Curve

Redundancy● Redundancy: "Exceeding what is

necessary...characterized by or containing an excess." (source: Merriam-Webster Dictionary)

● Two key ideas for systems engineering:○ Fault Tolerance - The ability of a system to continue

operating after a component failure has occurred■ Implemented through the use of redundant

systems○ Functional Redundancy - Design is robust enough to

respond to component failures through work-arounds and the use of systems in ways that they weren't originally intended for■ Examples: Apollo 13, MER, NEAR Shoemaker

Source: Exploration Systems Engineering Version 1.0, Reliability Module

Fault Avoidance● Ways to achieve reliability in space:

○ Generous environmental and design margins○ Use of high-quality design components

■ Class S (space qualified) ■ Commercial-Off-The-Shelf

● Must be careful that OTS parts work in intended environment○ Highly controlled manufacturing, assembly, and

documentation○ Acceptance testing or inspections on all parts if

possible

Source: Exploration Systems Engineering Version 1.0, Reliability Module

Example Calculations

Source: Exploration Systems Engineering Version 1.0, Reliability Module

Example Calculations

Component 1

Reliability: 0.90

Component 2

Reliability: 0.90

Input

Input

Component 1

Reliability: 0.90

Component 2

Reliability: 0.90

Series: A x B = Total Reliability

(0.9) x (0.9) =

0.81 reliability

Parallel: (A + B) - (A x B) = Total Reliability

(0.9 + 0.9) - (0.9 x 0.9) =

(1.80 - 0.81) =

0.99 reliability

Both need to work

Only one needs to work

Tie-In to Rover project● All subsystem groups have selected

components based on reliability (OTS, heritage, etc.)○ Fault avoidance

● When practical, redundancy has been added to all key mission components○ We have created designs, made trade-studies, put

together models, and intend to lab test components● We strive to make the rover 'smart' so that it

can enter safe-mode or question the commands from ground when threatened○ Increases reliability by decreasing the chance for

critical mistakesSource: Exploration Systems Engineering Version 1.0, Reliability Module

Summary-Reliability is a key attribute of space systems, influencing systems engineering activities such as design, trade studies, modeling, and test.-The reliability function, R(t), is determined from the probability that a system will be successful for at least some specified time.-The Bathtub curve expresses the failure rate as it depends on the age of the system. Early and late in life of the system (similar to the human body) significantly higher failure rates occur called “infant mortality” and “old age” regions. Between these regions normally lies an extended period of approximately constant failure rate. The reliability of systems operating in this region can be simply characterized by an exponential function.-Ways to achieve reliability include fault tolerance, functional redundancy and fault avoidance.-Block diagrams and event trees are useful tools in calculating reliability. An understanding of probability basics is required.

Source: Exploration Systems Engineering Version 1.0, Reliability Module