UNIVERSITY OF NEBRASKA-LINCOLN DEPARTMENT OF MECHANICAL AND MATERIALS ENGINEERING MECH 447- DESIGN PROJECT

Modify Tyler-Bot 2 (TB2) to add 1 Degree of Freedom

(DOF)

Prepared for:

William Bill Dick

Instructor of Mechanical Engineering

University of Nebraska-Lincoln

Dr. Jeff Hawks

Research Assistant Professor Mechanical & Material Engineering

University of Nebraska-Lincoln

By:

MohdAsri, MuhammadAlifSophian

Palnitkar, Aditya

Paul, Rajkrishna

i

Executive Summary

In this report Tyler Bot2 (TB2) is modified to achieve one additional degree of freedom (DOF).

In Minimally Invasive Surgery (MIS), a small incision is made in patient’s body rather than a big

incision in open incision surgeries. MIS is in great demand due to its biggest advantage of

relatively less trauma to the patient and hence faster recovery.

TB2 has 4 DOF and this report explains in detail the addition of yawing to the forearm in y-z

plane. To achieve this, many different approaches were studied in detail. According to the Pugh

matrix, the best solution was to use pulley system. This system will use stainless steel for driver

and driven pulleys, similar to a conveyor belt. The driven pulley will be on the forearm to

achieve yawing (as the pulley is free to rotate) and the driving pulley will be attached to a motor.

The belt is cylindrical in shape and is made of Nitinol, which is a biocompatible material. The

coefficient of friction between Nitinol and steel is enough, so the belt will not slip. Between the

two pulleys, the belt runs through idlers also made of stainless steel. Robot arm moves and

hence the length of the belt changes. To take it into consideration andmaintain the tension in the

belt, tensioning device is used. Tensioning device consists of a spring which expands and

contracts depending on the motion of the arm where it willmaintain27.78N tension in the wire.

Most of the parts will be made using rapid prototyping machine while some will be standard

parts. The body housing used will be same as used in TB2 i.e. Accura60, a water resistant

stereolithography resin. For the assembly of the parts, JB weld will be used as it has a tensile

strength of about 30MPa. The cost for this modification to the TB2 will be $1121.04, which

includes all the parts, labor and assembling cost. Detailed bill of material is discussed in

Appendix B.

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Table of Content

Introduction……………………………………………………………………………………………………………………………………1

Background…………………………………………………………………………………………………………………………………….1

Analysis………………………………………………………………………………………………………………………………………….4

Torque and Force Analysis…………………………………………………………………………………………………..5

Workspace………………………………………………………………………………………………………………………….7

Design…………………………………………………………………………………………………………………………………………….8

T-Joint……………………………………………………………………………………………………………………………….10

Cable Wire………………………………………………………………………………………………………………………..12

Upper Arm………………………………………………………………………………………………………………………..15

Shoulder Guide………………………………………………………………………………………………………………….16

Pulleys………………………………………………………………………………………………………………………………17

Tensioning Device …………………………………………………………………………………………………………….21

Motor Selection………………………………………………………………………………………………………………….24

Tubing………………………………………………………………………………………………………………………………….25

JB Weld……………………………………………………………………………………………………………………………….25

Result and Conclusion…………………………………………………………………………………………………………………26

References…………………………………………………………………………………………………………………………………..27

Appendix A: Quality Function Deployment (QFD)…………………………………………………………………………..2

Functional Decomposition………………………………………………………………………………………………..3

Pugh Matrix……………………………………………………………………………………………………………………..4

Failure Mode Effect Analysis…………………………………………………………………………………………….5

Material Selection…………………………………………………………………………………………………………….6

Gantt Chart……………………………………………………………………………………………………………...9

Appendix B: Bill of Materials……………………………………………………………………………………………………….10

Appendix C:Calculations……………………………………………………………………………………………………………..12 Cable Extension ………………………………………………………………………………………………………..12 Spring Selection..………………………………………………………………………………………………………….16 Safety Factor Analysis………………………………………………………………………………………………..17

Appendix D:

Drawings……………………………………………………………………………………………………………..………….(attached)

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List of Figures Figure 1:Tyler Bot (TB 2.0)

Figure 2: Force Body Diagram (FBD) in the Pulley

Figure3: Labeled Components and Joints Tyler Bot (TB 2.0)

Figure 4: Entire Assembly

Figure 5: T-Joint Model Top View

Figure 6: T-Joint Model Side View

Figure 7: T-Joint Bracket

Figure 8: Shoulder Guide Model

Figure 9: Systems of Idlers

Figure 10: Side View of Cable Alignment

Figure 11: Prospective View of Cable Alignment

Figure 12: Upper arm Model

Figure 13 Complete Upper arm Model

Figure 14: Shoulder Guide Model

Figure 15: Pulley Schematic

Figure 16: Assembly of Tensioning Pulley

Figure 17: System of Idlers

Figure 18: System of Idler with Cable wires in 2 different

Figure 19: Tensioning Device

Figure 20: Spring Attached to tension Lid

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List of Tables

Table 1: Joint limit and ranges of the Tyler Bot 2.0

Table 2 : Properties of the selected spring

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Introduction Since the time when surgical operations began, they have been performed through a large

incision. Large incision surgeries causes trauma to the patient. New techniques have been in

great demand where the incision is very small; these procedures are called minimally invasive

surgeries (MIS). MIS is replacing the open procedures to the patients due to the significant

benefits like fast recovery, less tissue trauma and cheaper cost of operation. This technology will

be in high demand, as in future surgeons can perform surgeries from remote locations. This

report will discuss in detail about modification of TylerBot 2 (4 degree of freedom) to add one

more degree of freedom.

Background Minimally Invasive Surgery

Laparoscopic Surgery

With the development of medical surgeries, minor operations on localized body parts

were done using special instrumentations to change the concept of surgery from

traditional open surgery. Laparoscopic surgery although causes various difficulties while

operating, due to the various invasiveness of its surgical procedures, but several research

and continuous improvement is focusing to reduce its invasions and reduce the size of

incision. It has some extremely beneficial effect on operations for the patients, such as

less post-operative discomfort, quicker recovery times, shorter hospital stay, earlier return

to full activities and much smaller scars. Laparoscopic surgeries are performed by robots

for specialized operations.

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Robotic Surgery

In Vivo Robots

Researchers have designed a new way to allow individuals with non-medical

backgrounds to perform minimally invasive surgery by assembling a millimeter-sized

camera robot to record the minor and minute operations taking place. Unlike room-size

and expensive surgical robots, mini in vivo robots are extremely mobile and inexpensive.

The robotic operations can be performed almost anywhere, right from the most remote

area(outer space) to a commercial hospital.

Surgical Robots

These robots are one of

theinnovations in medical devices

with some improved mechanism,

which can help surgeons to assist in

surgeries, increase their dexterity and

assist them with more reliable

operations to the patients. The current

concern for this project is on Tyler

Bot surgical robot (TB 2.0). TB 2.0 is one of the most successful robots in TB family. It

was designed by Tyler Wortman, a graduate student from the University of Nebraska-

Lincoln Mechanical Engineering as an innovation to develop minimally invasive surgery.

Figure 1: Tyler Bot 2.0 (TB2.0)

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Tyler Bot

TB 2.0 consists of two 4-DOF arms which can be individually inserted into a single 4cm

incision. The main objective of the robot was developed for less procedure with colon

resections. TB2 was designed to maximize the joint range of motion. Each arm of TB2.0

consists of a torso, upper arm, and forearm. A shoulder joint with 2 DOF located between

the torso and upper arm, provides the yawing and pitch. The end effector located at the

forearm of the robot provides rotational degree of freedom along with open and close

actuation. Tyler Bot has a 4 DOF robot arm .Workspace was calculated by finding the

maximum and minimum reach and then revolving about the joint axes. The workspace of

TB2 has a 95mm square revolved around its torso, the minimum reach of the robot is

50.8mm and maximum reach is 132.2mm. Average forces across the workspace in the X,

Y, and Z directions include 10.3N, 14.8N and 25.5N. These were taken from the

information provided by the blue dragon data. The torques provided at Joint 1, Joint 2

and Joint 3 of TB2.9 are 264.17Nm, 264.17Nm and 1220.61Nm respectively (refer to

Figure 3). The aim of the project was to modify the design of TB2, to add another DOF

near its forearm, so that the forearm can yaw in the y-z plane of motion. To perform the

objective, the Joint 1 of TB2.0 was primarily modified, including the other parts too. The

torque of 264.17mNm produced by the motors at Joint 1 was taken as a reference torque

to make the forearm yaw in the y-z plane of motion. This was the first approach of

calculation in modifying the TB2.0. Motor selection and inserting other components in

the design for its improvement were based on this assumption.

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Analysis Once the project topic was chosen, first step was to do background research on the related

resources. Tyler Wortman’s thesis, BLUE Dragon and many other patents were analyzed.

Functional decomposition of the task was done to find the function requirements. A QFD

chart filled by a surgeon was used to determine the constraints and prioritize the different

functions performed by the robot (Appendix A1: Chart1). After intense brainstorming and

reviewing patents documents and understanding the problem, three different ideas were

considered. Some of the ideas, to get an additional degree of freedom were:

o Adding a motor to actuate the forearm

o Using shape metal alloy as to actuate the yawing motion in forearm

o Using pulley system to perform yawing

Main constraints in selecting the best design were the size (length and width) of the arm, the

weight, kinematic design and biocompatibility. Pugh matrix was used to compare these three

different ideas and finalize the best(Appendix A3: Table I and Table II). Adding an extra

motor was rejected as it would increase the forearm size by almost one third. This will reduce

the work space and maneuverability of the robot. Shape Memory Alloy (Nitinol wire) can be

used for actuation and it talks almost no space but it operates in on or off mode i.e. the

surgeon will not be able to yaw the arm intermediately. Furthermore this SMA works the

best only in a controlled environment. At the end it was decided to use the pulley system

because it will be more compact, easier to control and more robust.

5

Tyler bot was sketched in Computer Aided Design (CAD) using SolidWork® according to the

diagram descriptions in Tyler Wortman’s thesis [1]and good engineering estimate. Then the

other components were added on the existing sketch. Once getting the initial idea it was

important to dive into detailed drawings and calculations. According to Tyler Wortman’s thesis

the torque required for elbow yaw was 264.17mNm. Hence the same torque was taken to yaw

the elbow in y-z plane. This torque was used to calculate the force required on the joints and the

dimensions of the components.

Torque and Force Analysis

The maximum torque required at Joint J2 and Joint J3 (refer

Figure 3)is 264.17mNm to yaw the arms in different planes.

Thus the motor installed in those joints were capable of

producing that much torque. The pulley diameter at joint J3 is

9mm. From the torque equation the tangential forces acting at

the top and the bottom of the pulleys are calculated and its free

body diagram is shown in figure 2. The tangential forces are

calculated as 27.78N each side. The tangential forces acting on

the pulley becomes the tension of the cable, as the movement of the cable wires causes the

pulleys to rotate. The tension in the cable wire is considered constant throughout the system and

applied continuously to prevent it from slacking and slipping out of its path.

To ensure the pulley system operates as it should, slippage of cable wire at certain areas should

be avoided. The critical areas are identified to be at the driving and driven pulley, whereas other

Figure 2: Force Body Diagram

(FBD) in the Pulley

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component like various idlers, the cable wire can just slide on its surface to ensure smooth

motion. For recall, driving pulley is connected to the motor assembly while the driven pulley is

attached to the joint of the TB2 forearm. If the slippage occurs at the two identified areas, the

cable wire will just sliding over the groove surface of the pulley without driving the both pulley.

As a result, yawing motion couldn’t be achieved. As the intended design system is using Nitinol

wire, the material for the driving and driven pulley should possessed high coefficient of friction

(COF). Below is the correlation to use in order to select the material for the two pulleys.

F cable driven < Ffriction ; where Ffriction = µsN

From the force body diagram at the critical area (driving/driven pulley), the normal force, N

acting on the surface is identified to be 55.56N and the driven force of the cable wire is 27.78N.

The product of the normal force with the COF (also known as µs) should be higher than 27.78N.

The table of COF is retrieved from the Engineeringtoolbox.com. From the intended design, the

material for the pulley is chosen to be steel that fulfilling the biocompatibility features. Hence,

the closest available data for the COF is between mild steel and nickel which is 0.64. As nickel is

one of the main components in Nitinol, the assumption of taking this COF value is valid as the

material chooses for the pulleys is steel. In conclusion, the steel ASTM F2229 for the pulley will

have enough coefficient of friction to prevent from the slippage of cable wire. For better

recommendation, performing surface of modification by increasing the roughness on the surface

of the pulley helps to ensure the stability of the pulley system

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Workspace The preexisting joint limit of the Tyler Bot 2.0 is set to be same. Table 1 below,has list of all the

specified joint limit and ranges with addition of the 1 more DOF:

Table 1: Joint limit and ranges of the TylerBot 2.0

The joint J3 can perform two motions, one in x-y plane and one in y-z plane. The robot arm is

designed to yaw ± 90 degrees in the y-z plane, but it has limitations. The new yawing supports the

shoulder rotation for ± 180 degrees and shoulder yaw for -45 to +65 degree. When it comes to joint J3,

the pulley system will only work when the arm is not yawed in x-z plane. The pulley on joint J3 is

designed with deeper groove so that it will allow some bending of the arm (in x-z plane) without slipping

of the belt. But to be on the safer side, pulley system will not work when the arm yaws. The addition of

one more degree of freedom will increase the work space of the system and make the process faster.

Joint (J) θ Limit

1 -90˚ to +90˚

2 -45˚ to+65˚

3 0˚ to +125˚

3 -90o to 90

o

4 -180˚ to +180˚

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Design

This section explains in detail about the design of the pulley system to modify Tyler Bot (TB2)

and add another degree of freedom (DOF) to its motion (refer figure 3). TB2 has 2 arms with

each arm consisting of a torso, upper arm and a forearm with end effector (grasper).

Figure 3:Labeled Components and Joints Tyler Bot (TB 2.0)

A 2DOF shoulders joint located between the torso and the upper arm provides yaw and pitch.

The elbow provides 1 DOF causes yawing in x-y plane. The end effector located at the each arm

also provides rotational degrees of freedom with open and close actuation for grasping.

Figure 3 is labeled with joint J3 connecting the forearm with upper arm and joint J2 connecting

the upper and torso. The joint J3 is modified to add one more degree of freedom to the robot. The

designing was implemented such that addition of another DOF does not hinder the workspace of

the existing TB2. Previous design of the forearm for TB2 can only yaw in one plane i.e. in the x-

y plane. Modification of Joint J3, and addition of a T-joint, along with various components like

X

Y

Upper Arm

Forearm

Joint J1

Joint J2

Joint J3

Joint J4

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pulleys and tensioning device, allows it to yaw, in the y-z plane also i.e. (out of the plane of

paper). The T-joint has a pulley attached to it which helps to yaw the forearm using a Nitinol

cable wire. The wire runs from the pulley on Joint J3 to the driving pulley shown in figure 4.

Other components added to design the system are listed and described below (refer figure 4). The

materials selected for manufacturing the components of the entire system are biocompatible and

details are provided in Appendix A-5. The pulleys, idlers, joints, shafts and rollers are

manufactured by a biocompatible stainless steel ASTM F2229 with an ultimate tensile strength

of (Sut) of 1340MPa and yield strength (Sy) of 1180MPa.

Figure 4 Entire Assembly of Design System

Driving

Pulley

Double

Grooved Idler

Forearm

Bracket (T-joint)

Casing

Idler

Metal Plate

Driven Pulley

Idler Pin

Shaft

#1(Bracket)

Spring Upper Arm

Shoulder Guide

Shoulder Guide

Roller

Tensioner

Lid

Connector

Rod

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T-Joint

Joint J3 controls the yawing motion of the forearm in the x-y plane on the plane of paper and the

y-z plane coming out of the plane of paper as shown in figure 5. Previous TB2 design supported

yawing only in the x-y plane. However the modification of Joint J3 has allowed an addition of

another DOF to the entire system. A maximum torque limit of 264.17mNm[1 ]

is applied at joint

J3 to yaw the forearm.

As the name suggests the bracket is in form of a T joint which is connected to a pulley and when

the pulley rotates, it yaw’s the forearm accordingly. The design is kept very simple but with a

high factor of safety and effectiveness. The bracket has a space for 2 bearings on each side to

reduce friction and support any type of radial or axial load while surgery. Between the forearm

and the T joint there are washers to reduce stress concentration and friction between the rotating

parts. A schematic of entire joint is shown in Figure 5 and Figure 6. All the part will be glued

together with the strong steel epoxy known as JB Weld (Appendix A-5 Catalog 3) and the

bracket is made of machined rolled steel ASTM F2229.

The critical thing while designing the T joint was to maintain the alignment of the pulleys with

the system so that the belt can run through, without slipping or cutting. Also it can be noticed in

the figure 5 and 6, that the joint between forearm and upper arm is a little off center, this is to

increase the workspace between the two arms. The design of the bracket allows forearm to yaw ±

90 degrees in y-z plane.

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Figure 5 T-Joint Model Top View

Figure 6 T-Joint Model Side view

T joint consists of bracket (figure 7) and a detailed drawing of the components can be found in

Appendix D drawing number1. The 3mm bearing is attached to the each side of the T-joint

bracket and the specification of the bearing can be referred in the Appendix D-Catalog 3

T-Joint

3mm Bearing

Driving Pulley

Shaft #1 Bracket

Linkage 1 (Forearm)

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Figure 7:T-Joint Bracket

Cable Wire

Material chosen for the belt was biocompatible Nitinol (Ni-Al) wire. This wire is shape memory

alloys with high flexibility, and kink resistance crucial to medical device applications, including

tensioning wires and guide wires. Since the cable goes through several turns, it is necessary to be

kink resistance. Another important reason for Nitinol was the high frictional coefficient of 0.64

with stainless steel, so that the belt may not slip over the pulleys. It has Young’s modulus of 28

GPa, yield strength and ultimate tensile strength 70MPa and 895MPa respectively. When the

joints and the arms are in motion the entire cable wire will have to be in tension of 27.78N to

remain in contact with the surfaces of all the pulleys and idlers. Other specifications of Nitinol

wire are listed in Appendix A Catalog 2.

13

Figure 8: Cable Wire and Assembly Model

Figure 9: Cable wire passing through double grooved idler

The shape of the cable wire was chosen to be round shaped as it will have twists and turns. A

schematic of the entire run of the belt is shown in Figure 8. The belt is completely aligned and is

in the same plane with the entire arm assembly. The cable wire runs from joint J3 to the driving

pulley as shown in figure 8. From joint J3 the pulley goes through the guide bracket and then

through the double grooved idler which guide the wire into variable angle as shown in figure 9.

The cable wire then runs through shoulder guide to enter in the tensioning device which is

14

outside the body of the patient. Inside the tensioning device the belt passes through 4 idlers(refer

to figure 8), a tensioner pulley for keeping the wire under tension and driving pulley to drive the

cable wire.

Figure 10 Side View of Cable Wire Alignment

Figure 11 Perspective View of Cable Wire Alignment

The cable wire has a total length of 294mm with diameter of 1mm. This was chosen so that the

cable wire is strong enough and has enough contact with the pulley to make it easy to control.

The ends of the wire have to be attached due to the small size, as it was tough to weld them. It

was decided to attach the wire ends using a small Nitinol tube. Both the ends will be inserted in

15

the tube and then the tube will be compressed. Now, this part being rigid cannot go through the

pulleys. It has to be placed in a location where it will not go over the pulley even when the arm is

moved to the maximum or minimum length. It was decided to place the joint in the center of the

shoulder rolling guide as it has a distance of 20mm from the pulleys on each side.

Modification of upper arm

The upper arm is exactly the same as in TB2.0 except two changes. There is a bracket guide in

front of the upper arm and a double groove idler near the end which can be seen in Figure 12.

The guide in the front has designed so that the wire may not lock when the forearm yaws in the

x-y plane. The guide is made of stainless steel ASTM F2229 and has two 2mm holes. Near the

other end of the arm double grooved idler made of the same materials attached to guide the wire

on the shoulder. It can be noted in the figure 13, that the double grooved idler is in the same

plane as the guide between the upper arm and shoulder. This will reduce the slipping of the belt

and also wearing due to miss alignment as the TB 2.0 arm in motion.

Figure 12: Upper arm Model Figure 13: Complete Upper arm Model

Bracket Guide

Doubled Grooved Idler

Linkage 2

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The linkage 2connects to joints J2 and J3 as shown in figure 9. The bracket guide near joint J3is

glued upper and is elevated a little above the upper arm as it had to be aligned with the driven

pulley in the J3 (refer figure 10). Double grooved idler of diameter 6mm is mounted on the shaft,

so that the belt can pass as well as return from the same pulley. It is attached with the arm using a

3mm diameter shaft. For exact dimensions of the upper arm is shown in Appendix D -20.

Shoulder Guide

The shoulder guide has a bracket and a roller assembly which were designed to allow the arm to

rotate along the shoulder joint as shown in figure 14. The shoulder roller guide roller (Appendix

D-16) inserted in the slot of the bracket guide is ±90 which is equal to the motion of the

shoulder. It prevents the wire from getting locked due to the extreme angle when robot arm is in

motion. On both the sides of the guide there is a curved pipe which is called shoulder guide angle

(Appendix D-14 and 15). It is aligned with the wire and shoulder guide roller for smooth motion.

it can be noted that the shoulder guide angle pushes the wires to maintain the alignment. The

bracket will be glued on the torso of the robot to make the design compact and simple.

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Figure 14: Shoulder Guide Model

Pulleys

Pulley at Joint 3(Driven Pulley)

Driven pulley (Appendix D-10) has a 9mm diameter with a 3mm bore. It is assembled on a

2.98mm shaft, connecting the linkage L1 to the bracket assembled on the T-joint at J3 as shown

in figure 5 The groove of the pulley is selected to be circular so that the cable wire can pass over

it and prevent from having sharp turns and even a twist, as the arm yaws in and out. Based on the

torque exerted at the joint J3, the pulley has load of 55.54N.

Shoulder Guide Angle # 1

Shoulder Guide Angle # 2

Shoulder Guide Bracket

Shoulder Guide Rollers

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Figure 15: Pulleys Schematic

The bending and the shear stress of the pulley are calculated by the forces acting on the shaft

passing through it. The force acts at the point where the tension of the cable wire passing over

the driven pulley. It causes a normal force to act in the opposite direction to balance the tension

produced by the wire (refer to Figure 2). The bending moment about the shaft is calculated as

0.682Nm and hence the bending stress is 258MPa by equation 1,

σ=

(1)

where d is the diameter of the shaft, and M is the bending moment.

The shear stress (τ) is taken into consideration while the shaft is rotating, due to the tension in the

wires. It is calculated by equation 2

τ =

(2)

where T is the shear moment.

Driven Pulley

Driving Pulley

Doubled Grooved Idler

Tensioning Pulley

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Since the stress is acting on the shaft in different planes, the von Misses theory is applied to find

the plane stress by equation 3

σ'=

(3)

The plane stress is calculated as 258.38MPa. Finally, the safety factor of the driven pulley

connected to the shaft is calculated as 4.56. For the bending stress we are using the equation 4

nf =

(4)

The safety factor is very high, which shows that the driving pulley and the shaft will be able to

resist all the forces acting on it, while the system is working, and long lasting.

Pulley connected to the hook of the spring (Tensioning Pulley)

The tensioning pulley (Appendix D-19) acts as a tensioning device other than functioning

exactly like a pulley. While the wire is in tension due to the motion of the arms, the entire tension

of the wire acts on the pulley from the top of its rim. The force of tension gets counterbalanced

by the tensioning pulley on each side. It is assembled on the spring, so that the force of tension

due to wire acts on the pulley is transferred to the spring. Thus the tensioning pulley also moves

up and down along with the spring extension. It stabilizes and prevents the wire from slacking

between the driver pulley, driven pulley and the idlers. The tensioning pulley of 9mm in

diameter and 2mm bore diameter is assembled on the end point of the spring (refer Figure 16). It

is also mounted on a 3mm bearing in order to allow it free to rotate.

20

Figure 16: Assembly of Tensioning Pulley

As the tensioner pulley is attached at ending part of the protrusion of the spring, determining the

static failure crucial. Bending stress is calculated by taking the straight extension of the spring

passing through the tensioning pulley. The force acts at the point where the tension of the cable

wire passing over the tensioning pulley causes a normal force to act in the opposite direction to

balance the tension. Since it is acting as a component of a tensioner, it is not rotating, thus the

shear stress in neglected while calculating. The bending stress is calculated as 452 MPa with a

safety factor of 2.61.

Pulley connected to the motor (Driving Pulley)

It is the final pulley which connects the whole system to the motor placed above in the casing.

The driving pulley was designed by considering the torque required to rotate the arms. The

pulley diameter was tested with various values to meet the difference of the forces acting on the

sides of this pulley due to the tension in the cable wire. The diameter of the driver pulley was

21

also chosen by taking the appropriate angular velocity for the forearm to meet the requirement

for an optimum speed and torque for yawing.

It has a diameter of 9mm and a bore of 3 mm, which will be glued straight to the output shaft of

the motor using JB Weld.

An exact approach from the previous pulley calculation was taken for calculating the bending

stress, shear stress and safety factor for the driving pulley. The bending and shear stress

calculated were 877MPa and 77.38MPa respectively. The force of tension from the cable wire

causes a normal force to act upwards at the point on the motor shaft, where it is in contact with

the inside bore of the driver pulley. Bending and shear moment are calculated as 0.23618Nm and

0.0416Nm respectively. The Plane stress calculated by the Von Misses Theory is 890 MPa. The

safety factor is calculated as 1.32. The safety factor has a reasonable value. It shows that the

material will be resistive to high forces and long lasting.

Tensioning Device

System of Idlers

There are four idlers attached to the metal plate and later glued with JB Weld to the casing. This

system of idler is designed to guide the wire and maintain it under tension. The individual idler

set consist of a idler ( refer Appendix D-8) and its pin (Appendix D-7) which is glued (using JB

Weld) to the wall of the metal plate as shown in figure17. The two parts is made from ASTM

F2229where it will deliver a high safety factor to compensate its smaller dimension. The bending

and shear stress acting on the idlers are611.91MPa and 31.45MPa respectively. With plane stress

as 612.71MPa, the static safety factor is found out to be1.92

22

Figure 17:Systems of Idlers

Figure 18: System of Idlers with Cable Wire in Two Different Views

Spring

While designing a spring and choosing its material, certain conditions were prioritized. The main

concern of spring design is to miniaturize its dimensions. The preferred range of spring index is

recommended from 4 to12 to prevent surface cracking and tangle of the spring. Thus a spring

23

index of 10 is used as an optimal value. Number of coils in a spring depends upon the modulus

of rigidity, diameter of the wire, the maximum extension of the spring, diameter of the coiled

spring and the maximum force (Fmax), which is 6.79N for the system. The spring extends or

contracts to put the whole system along with the cable wire under required tension. When the

arm assembly is at the position of maximum extension ,the spring expands 16mm. When the

system is in its minimum position, the spring contracts to its free length. Spring details are given

in Table 2 below.

Table 2: Properties of the selected spring

In the overall design, the spring functions as a tensioning device

to make the whole design as a contained system. The spring is

placed inside a cylindrical cavity of the casing (refer Appendix

D-2),having dimension of 70.6mm in height with 25mm in

diameter. There is a 4.5mm slot throughout the cylinder where

the spring end comes out to join the tensioning pulley. The

expansion or contraction of pulley changes the length of the belt

and hence helps in maintaining the tension. The upper part of the

spring is made flat and circular, so that it can be glued with the

JB Weld from inside of the tensioner lid (Appendix D-3).

D

(mm)

C d

(mm)

(MPa)

(MPa)

Active

Coils

Spring

Rate

(N/m)

(N)

Deflection

(mm)

Total

Coils

Solid

Length

( mm)

Free

Length

(mm)

23 10 2 1784.6

0

803.07 18 0.435 1129.

19

15.6 20 38.26 53.86

Figure 19: Tensioning Device

24

Figure 20: The Spring attached to Tensioner Lid

Motor Selection

This additional yawing motion will be actuated with the 0515B brushless DC Micromotor with

attachment of Micro-Planetary gearhead from 06A series, provided by the Faulhaber Micromo

(refer to the Appendix D-Catalog 1 and 2). This brushless motor will provide the 250mNm

torque needed to drive the pulley at the Joint J3 so that it can yaw. Mounting of the 06A Micro-

planetary gearhead in the motor will help to amplify the maximum stall torque of the motor from

0.2mNm to 250mNm by using gear reduction of 625:1. The specified motor is electronically

reversible, so it will suit the double input of yawing motion and hence it can yaw up or down

depending on the surgeon or operator. In selecting the motor, the concern of how much current

that will be needed to power it up is not in the constraint. Any motors can be chosen for the

design as long as it can provide the needed torque, but the main constraint is to find the smallest

motor as possible. Choosing a brushless motor is one of the options of miniaturizing the design

components and makes it more compact. With the mounted gearhead, the specified ratio can be

achieved without having to choose larger dimension of motor.

25

The motor will be placed outside the body of the patient. The motor along with the gearhead will

be mounted on a slot next to the tensioning device. It will be in a motor casing inside the cavity

to reduce vibration.

Tubing

The belt and the pulleys system stick out of the arm and moving of the belt inside the patient’s

body is not acceptable. So the belt will be covered with tubing which will protect the patient’s

body. Teflon tubing was chosen for this application as it is biocompatible and flexible. The

tubing will cover the belt from the driven pulley to the shoulder guide. The tubing will have slits

in it so that it can slide over different pulleys and brackets then it will be sealed with glue.

Diameter of the tube is 10mm and its length is 30mm. As the tube will bend when the arm is

bending it might interfere with the belt but it is not a problem as the yawing motion is not

designed to work for that range. With self-lubricating property of Teflon and its very low

coefficient of friction, it will not prevents the cable wire from moving if there is any potential of

interference between the tubing . Refer to the Appendix A-5 for more details about its

dimensions and specification.

JB- Weld

Most of the components will be assembled using JB-Weld, which is epoxy based glue. It can be

used on many different types of surfaces like PVC, glass, fabric and metal. The glue sets in

around 20minutes and takes 15-24 hours to cure. Once cured it has a tensile strength of 3960psi

(30MPa) which is adequate for the use in some of the robot joints. An extra spacing is reserved

between the joints or related components in order to apply the glue. While applying the glue on

the surfaces, they should be dry and oil free. Keep it away from eyes and skin. MSDS sheet for

JG-Weld is attached in the Appendix A-5 Catalog 3.

26

Results and Conclusion

In this report, modification of TylerBot2 (TB2) from 4DOF to 5DOF was discussed. Using the

required torque of 264.17mNm (from TB thesis), the calculations were performed to find the

tension in the belt and the dimensions of the pulley. Tensioning device was designed on the basis

of maximum and minimum length resulted from the position of the TB2 arm. The extension in

the belt was calculated as 16mm and hence the spring was designed for that extension. All the

pulleys are stainless steel ASTM F229; the cable wire is made of Nitinol and the tubing which

covers the belt and pulleys is made of Teflon-FEP. All the materials used are biocompatible.

The pulley system will be working simultaneously with the shoulder yaw and shoulder rotation.

But as the elbow yaws, the pulley loses contact with the belt and hence the pulley system stops

working. All the motion of TB2.0 is still performed but the yawing in y-z plane only works when

elbow yaw is at 0 degrees. Addition of one more degree of freedom will make the operation

procedure faster and easy.

The cost of the addition of the system will be $1121.04 which includes the cost of all the parts,

labor and assembling. This cost is not so expensive considering the relief to the patient and

doctors. The patient has to stay in the hospital for 1-2 days as compared to 4-6 days with open

surgical procedure. The doctor is more comfortable with the motion as it is similar to that of the

human hand.

In future the pulley system can be made more compact and the tensioning device can be removed

if length of the belt can be kept constant. Other avenue to explore might be to use one actuation

device to get two DOF, using a clutch mechanism.

27

References Book and Journals

1. Tyler Wortman. “Design Analysis and Testing of In-Vivo Surgical Robots”.

Mechanical and Materials Engineering Department, the University of Nebraska-

Lincoln. December 1, 2011.

2. Richard G. B., J. K. Nisbett. “Shigley’s Mechanical Engineering Design 9th

Edition”.

New York: McGraw Hill, 2011

3. M. Ashby, H. Shercliff, D. Cebon. Materials, Engineering, Science, Processing and

Design 2nd

Edition, Burlington: Elsevier,2010

4. Carl A. N. Introduction to Biocompatibility. A Prime on Engineering Design of

Biomedical Device. University of Nebraska-Lincoln, 2009

5. Yunhui L. , Dong S. Biologically Inspired Robotics. London: CRC Press

6. Y. Bellouard. Micro Robotics Methods and Application.Florida: CRC Press, 2010

7. H. Asada, J.-J. E. Slotine. Robot Analysis and Control. Canada: John Wiley & Sons,

Inc. , 1986

8. H. Asada, K. Y. Toumi. Direct Drive Robots Theory and Practice. Massachusetts:

The MIT Press, 1987

Online sources

1. J.D. Brown – “Quantifying Surgeon Grasping Mechanics in Laparoscopy Studies

in Health Technology and Informatics”.Medicine Meets Virtual Reality. Jan,

2004.Pdf format. Oct. 10, 2012

2. “Sheave Design Manual”. Groove design. Nylatech.com. Web. Nov.27,2012

28

3. “Blue Dragon”. Bionics Lab University of California Santa Cruz. Web. Oct. 3,

2012

4. “How to Select a DC Micromotor”. MicromoElectronics, Inc. 2007. Web. Oct 3,

2012

5. “Robot Dynamics”. Society of Robots. Web. Oct 14, 2012

6. Michael J. M. , Constantinos M. , Charles P. , “Design and Dynamic of A Shape

Memory Alloy Wire Bundle Actuator”. Department of Mechanical and Aerospace

Engineering Rutgers University. Web. Sep. 2, 2012.

7. “Friction and Coefficient of Friction”.EngineeringToolbox.com. Web. Nov. 15,

2012

1

Appendix A 1. Functional Decomposition……………………………………………………………………………Chart 1

2. Quality Function Deployment……………………………………………………………………..Chart 2

3. Pugh Matrix

a. Pugh matrix first iteration ……………………………………………………………….Table I

b. Pugh matrix second iteration…………………………………………………………. Table II

4. Finite Mode Effect Analysis………………………………………………………………………….Chart 3

5. Material Selection

a. Teflon tubing……………………………………………………………………………….Catalog 1

b. Nitinol wire………………………………………………………………………………….Catalog 2

c. MSDS JB Welds…………………………………………………………………………….Catalog 3

6. Gant Chart……………………………………………………………………………………………………Chart 4

2

Appendix A-1: Functional Decomposition

Chart 1: Functional decomposition for Minimally Invasive Surgery (MIS)

Minimally Invasive Surgery

(MIS)

Vision

Ultrasonic S.E.M Camera

Mechanism

Maneuvarability Size Controlability 3 D.O.F.

Yaw Rotation Grasp

Data Feedback

Haptics

Biocompactible

Shape Material selection

3

Appendix A-2: Quality Function Deployment

Chart 2: QFD for better In-Vivo Robot

4

Appendix A-3: Pugh Decision Matrix

Pugh decision matrix was used to evaluate the best design among the three chosen designs. This will

allow for evaluation of all the view points and comparison between each of them. The first step is to list

all critical functional requirements and then out of the three concepts choose a datum which will be

compared to the remaining two. The first iteration, with Motor as a datum is shown in table 1.

From the first, it was noted that pulley system had more advantage over motor. SO for the second

iteration Pulley system was chosen as datum. Table 2, below shows the result.

Pulley system again proved to be better that the other two, and hence it was chosen as the best solution

for the problem.

Motor (Datum) Shape memory Alloy Pulley System

Length of arm 0 +1 +1

Weight of the arm 0 +1 +1

Degree of yaw 0 -1 0

Controllability 0 -1 -1

Diameter of the arm 0 +1 +1

Total 0 +1 +2

Motor Shape memory Alloy Pulley System (Datum)

Length of arm -1 -1 0

Weight of the arm -1 +1 0

Degree of yaw 0 -1 0

Controllability +1 -1 0

Diameter of the arm -1 +1 0

Total -2 -1 0

Table I: Pugh Matrix (First Iteration)

Table II: Pugh Matrix (Second Iteration)

5

Appendix A-4: Failure Mode Effect Analysis

This analysis helps to list out all the possible failures with weighting each of the potentials

severity regarding the design system. It also help to point out the occurrence and detection of

those listed failures so that the user is aware of the precaution, solution and planning ahead of time

Table III: Pulley Design system mode failure analysis

6

Appendix A-5: Material Selection

Selection of Biocompatibility Material and its Manufacturing Method

The selection of materials for the design process must be biocompatible which mean the

capacity of the material to function in the proximity to tissue without evoking an adverse

reaction while the tissue function is restored or enhanced by promoting the desirable

interaction between it and the material (which mean biomaterial). There are two ways that

influence on this selection of biomaterial which are either by choosing the bulk material that

have properties defined as biomaterials or using surface modification to increase

biocompatibility. Here, the design opts to use the material that already is biocompatible rather

than coating with chemical modification to non-biocompatible. Hence this can reduce the

intriguing of producing or manufacturing the design material, and hassle in applying the surface

coating for every surgery procedure. American Society for Testing and Materials (ASTM), United

States Pharmacopeia (USP) and International Organization for Standardization (ISO) had

outlining the medical grade standard that fulfilling the regulations governing biocompatibility

must be USP class VI or ISO 10993. Hence the selection materials for the design are chose from

that class.

As most of the design parts contain complex 3D shape with mostly the dimensions are between

25mm to 0.5mm, the best ways for manufacturing is by CNC (Computer Numerical Control)

Machining due to its precise machining with high tolerance.

7

Catalog 1: Teflon® Tubing 10mm Specification

Catalog 2: Nitinol Wire Specification

Catalog3: JB Weld MSDS Specification sheet

8

9

Appendix A- 6: Gant Chart

10

Appendix B Bill of Materials

Item Name Drawing# Tota

l

Part # Material Source Estimated

Cost ,$

Brushless DC motor-15mm

(mounted MicroPlanetary

Gearhead reduction)

1 0515A/B

(with 06A

Gearhead)

Nickel Alloy Faulhaber Micromo 311.30

Flanged-Double shield

bearing

2 A7Y

5MFSS0603

G

440C Stainless Steel SDP/SI 14.12

Washer-1.5mm 5 93475A179 Stainless steel Type 316 McMaster Carr 3.36

Washer-2.0mm 3 90965A110 Stainless steel Type 316 McMaster Carr 1.48

Spring 018 1 Customized Beryllium-copper W.B. Jones Spring

Co.

11.61

Driving pulley 004 1 Customized Stainless Steel ASTM F2229 Firstcut® 25.01

Driven pulley 010 1 Customized Stainless Steel ASTM F2229 Firstcut® 25.00

Double grooved idler 006 1 Customized Stainless Steel ASTM F2229 Firstcut® 35.00

Idler 008 5 Customized Stainless Steel ASTM F2229 Firstcut® 62.50

Shoulder Guide – angle#2 014 1 Customized Stainless Steel ASTM F2229 Firstcut® 43.75

Shoulder Guide - angle#1 015 1 Customized Stainless Steel ASTM F2229 Firstcut® 47.50

Shoulder Guide 017 1 Customized Stainless Steel ASTM F2229 Firstcut® 75.56

Shoulder Guide - roller 016 1 Customized Stainless Steel ASTM F2229 Firstcut® 37.51

Casing 002 1 Customized Accura60 Stereolitography Rapid prototyping

machine

35.00

Tensioner lid 023 1 Customized Accura60 Stereolitography Rapid prototyping

machine 9.50

Tensioning pulley 019 1 Customized Stainless Steel ASTM F2229 Firstcut® 37.50

Idler pin 007 5 Customized Stainless Steel ASTM F2229 Firstcut® 11.25

11

Shaft (Double Groove

Idler)

013 1 Customized Stainless Steel ASTM F2229 Firstcut® 10.00

Shaft#1 (T-Joint) 011 1 Customized Stainless Steel ASTM F2229 Firstcut® 5.00

Bracket(T-Joint) 001 1 Customized Stainless Steel ASTM F2229 Firstcut® 56.26

Metal Plate 009 1 Customized Stainless Steel ASTM F2229 Firstcut® 12.52

Driven Pulley cover 021 1 Customized Stainless Steel ASTM F2229 Firstcut® 43.75

Connector rod 003 1 Customized Stainless Steel ASTM F2229 Firstcut® 15.03

Motor Casing 022 1 Customized Stainless Steel ASTM F2229 Firstcut® 10.02

Teflon tubing-8mm 1 5557K35 Teflon ® FEP McMaster Carr 4.01

Nitinol wire 1 Customized Nitinol J.M. Medical

Components

22.74

J-B Weld Professional Size

Steel Reinforced Epoxy

1 8280- 10 oz. NA Amazon® 14.76

Assembling cost *7 NA NA NA **140.00

Total Cost 1121.04

Remarks: Price range of relatively same group material with the ASTM F2229 is Ti 6Al-4V, which has range between $28.28 to

$47.13 per kg (based on the CES Granta Design Software). Hence, the mean price is chosen ($37.71/kg) with CNC machining cost

reflecting on the current market is $75.00 per hour.

*Estimated time in assembling the designed system

** Based on hourly wage of $20.00/hour

12

Appendix C: Calculations and Approach

Cable Wire extension The cable wire is connected close to the arm linkages of the robot. It is attached to the torso,

upper arm and the forearm at close proximity with the help of idlers and pulleys. The position of

the arms with maximum and minimum yawing from each joints into and out of the workspace

are taken into consideration, to find the extension of the cable wire.

The figure 7 CB’ is the length of the cable wire connected close to the forearm linkage, when

there is no yawing. The forearm yaws by 125o inside the workspace. AC is the length of the

cable wire connected close to the upper arm. Thus AB is the minimum distance and ACB’ is the

maximum distance. Distance AB is calculated by applying cosine law.

cosϴ =

(1)

The angle ϴ between AC and BC was calculated as 75.72o, by taking consideration of the

alignment of the arms.

The maximum length ACB’ is calculated by finding the total length of the arms.

13

Figure I12FBD of Cable Wire Aligned to Forearm and Upper Arm minimum extension [XY plane]

The following figure I shows the FBD diagram of the cable wire aligned with the

upper arm AD, connected to the Idler located on the upper arm at point D. The upper

arm can yaw inside the workspace by 45oand can yaw outside the workspace by 65

o.

Thus considering the same X-Y plane, the cable wire gets extended to its maximum

length when it yaws out by 65o and minimally extended when it yaws in by 45

o. The

imaginary line A”O is the maximum extended length of the cable wire, and the

imaginary line A’O is the minimum contracted length of the cable wire. The equation

1 in the section is used to calculate the length. OD is the length of the cable wire

which is aligned with the shoulder of the robot.

All these measurement for the cable wire aligned with arms are considered for the X-

Y plane.

14

Figure II FBD of Cable Wire Aligned to Upper arm and Shoulder maximum extension [XY Plane]

Taking the Y-Z plane in consideration, the upper arm can yaw up and down by 90o

from the shoulder. Thus it was found that when the upper arm, yaws up by 90o in the

Y-Z plane and yaws in the X-Y plane by 45 o inside, the minimum extension of the

cable wire occurs. At the same time, when the upper arm yaws down by 90o in the

Y-Z plane and yaws out of the workspace by 65o in the X-Y plane, it causes the

maximum extension of the cable wires. The following figure II and III shows the

maximum and minimum extension of the cable wires on Y-Z plane.

The extension taking place at every segment of the cable wire, aligned with every arm

is taken into consideration. All these extensions are added up to find the final

extension of the entire cable wire assembled throughout the robot.

15

Figure III FBD of Cable Wire Aligned to Upper arm and Shoulder maximum extension [YZ Plane]

Figure IV FBD of Cable Wire Aligned to Upper arm and Shoulder minimum extension [YZ Plane]

16

Spring selection

1. Spring index C=

(2)

C=10, D=20mm, d=2mm

D=diameter of the loop of the spring coil

d= diameter of the spring wire

2. Kb =

(3)

Kb = 1.135

3. F=Fs (when the spring is compressed to its maximum limit)

Fmax=

Fs= (1 +ζ)

(4)

ζ = 0.15 (Fraction overrun to closure)

4. Sut =

( 5)

A is measured in kpsi, and m is exponential factor.

Sut =1784.602MPa

5. Ssy = 0.45 Sut (6)

Ssy=803.07 MPa.

The Force of the spring is calculated by equation 6

6. F=

(7)

F =111.06N

7. The number of coils (Na) in the spring is calculated by equation 7.

17

Na=

( 8)

Na = 25 turns

G is the modulus of rigidity of the material used in the spring. For this design

Beryllium–copper wire B197 is used with a modulus of rigidity of 44.8Gpa

8. Total number of coils (Nt)

Nt = Na+1 =26 turns (9)

9. Deflection in the spring (y)

y=

(10)

the extension of the spring is calculated as 15.6mm.

10. The solid length ( Ls )

Ls = (Nt+1)d (11)

The solid length is calculated as Ls = 52.18mm

11. The free length of the spring (Lo )

Lo = y + Ls (12)

It comes out as 67.78 mm.

Safety Factor Analysis FEA analysis is performed on Shafts connected to Pulley P1 at Joint J1, Pulley P4,

Pulley P3, and idlers.

1. Bending Moment

M=FN. L (13)

FN – Normal force acting on the shaft from the pulley and the cable wire system

18

L- length of the shaft from the point of action of the normal force

M-Bending Moment

2. Shear Moment

T= FT.ds (14)

T- shear moment

FT- Force of tension acting on the pulley

ds- diameter of the shaft

3. Bending Stress

It is calculated at the critical points of the all shaft at which the normal force from

the pulley is acting.

σ=

(15)

4. Shear Stress

It is calculated at the critical points of the shafts where the force of tension of the

cable wire causes it rotate or produces a shear.

τ =

(16)

5. Von Misses Theory

Since the bending stress and the shear stress are acting on different planes, a plane

stress acts as a combination of the two stresses, which shows the overall stress

acting on the shaft. Plane stress is produced when both the bending as well shear

stress, acts at the same time. It is calculated by an equation developed from Von

Misses Theory

σ'=

(17)

19

6.Coulomb Mohr’s theory

(18)

6. Safety Factor

nf=

(19)

7. Vibration of the Pulley on the shaft

The pulley deflection on the shaft was also considered to ensure the vibration

taking place on the shaft and the critical regions at joints. The following equation

ymax=

(20)

was considered. The force F was assumed as the normal force acting on the shaft

as a reaction to the force of tension by the cable wires. E is the Elastic modulus of

ASTM F2229. I is the moment of inertia of the shaft. l is the length of the shaft.

It was found from the calculation that the vibrations of the pulleys are negligible,

so for the rest of the calculation, it was neglected.

20

Appendix D List of Technical Drawings

1. Bracket (T-Joint)

2. Casing

3. Connector rod

4. Driving pulley

5. Forearm

6. Double grooved Idler

7. Idler pin

8. Idler

9. Metal plate

10. Driven pulley

11. Shaft#1 (Bracket)

12. Shaft#2 T-Joint

13. Shaft (Double Groove Idler)

14. Shoulder guide angle #2

15. Shoulder guide angle #1

16. Shoulder guide roller

17. Shoulder guide

18. Spring

19. Tensioning pulley

20. Upper arm

21. Driven pulley cover

22. Motor casing

23. Tensioner lid

List of Catalogs Drawing

Catalog 1: Brushless DC Micromotor 0515B Catalog 2: Micro Planetary Gearheads 06A

Catalog 3: Washers for 2.2mm and 1.5mm Catalog 4: Flanged double shield bearing 3mm

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

Catalog 1: Brushless DC Micromotor 0515B Specification

45

Catalog 2: Micro Planetary Gearheads 06A Specification (mounted on 0515B)

46

Catalog 3: Washers Specification for Both 2.2mm and 1.5mm

Catalog 4: Flanged Double Shield 3mm Bearing Specification

47