ME 350 Final Report Mobile Backpack Carriers3images.coroflot.com/user_files/individual_files/...The main purpose of this report is to present the different components of our overall

ME 350 Final Report

Mobile Backpack Carrier

April 17th, 2012

Section 2 Team 27

Steve Hwang

Melvyn Pard

Theera Pornphaithoonsakun

Nan Zhong

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TABLE OF CONTENTS Contents Page

Introduction and specifications…………………………………………………………………...…...

Functional decomposition……………………………………………………………….……..............

Motion generation………………………………………………………………………….………......

Design process……………………………………………………………………...................

Final design diagram………………………………………………………………………….

Loading and defection analysis………… …………………………………………………...

Energy conversion……………………………………………………………………………….…....

Power…………………………………………………………………………………………

Travel speed……………………………………………………………………………..…....

Design…………………………………………………………………………………………

Testing Result ……………………………………………………………………………...…

Transmission selection………………………………………………………………………..

Mounts and joints…………………………………………………………………………......

Strength analysis at torque transmitting joints………………………………………………..

Pulley teeth……………………………………………………………………………….…...

Deflection On the Motor Mount………………………………………………………………

Motion generation design revisions…………………………………………………………..

Safety and motor control………………………………………………………………………………

Calculations and tuning of code values……………………………………………………….

Control codes…………………………………………………………………….…………...

Design modification………………………………………………………………………….………..

Linkage clearance……………………………………………………………………..………

Code Adjustment……………………………………………………………….…..………...

Design critique and evaluation………………………………………………………….…………..…

Appendix A……………………………………………………………………………………………

Alternative design 1………………………………………………………………………..…

Alternative design 2…………………………………………………………………………..

Alternative design 3…………………………………………………………………………..

Appendix B……………………………………………………………………………………………

Motion generator design drawings and manufacturing plan………………………………….

Assembling instruction……………………………………………………………….…….....

Bill of materials…………………………………………………………..………….………..

Appendix C…………………………………………………………………………………….……...

Energy conversion design drawings and manufacturing plan………………………………...

Assembling instruction………………………………………………………………………..

Bill of materials………………………………………………………...………………….….

Appendix D………………………………………………………………………………………....…

Control and safety system wiring……………………………………………………………..

Control code……………………………………………………………………………….…

Bill of materials…………………………………………………………………………….…

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INTRODUCTION AND SPECIFICATIONS

Many university students in a wheelchair find it inconvenient to reach their textbooks and laptops in the

backpack on the back of the wheelchair. Therefore the Mechanical Engineering Department of the

University of Michigan assigned us to design, build and test a battery-driven mechanism to carry a

secured backpack from the back of a wheelchair to the side. There are two merits for this mechanism.

First, when the backpack is on the back of the wheelchair, the wheelchair could easily travel through

narrow corridors and doorways. Second, when classes begin and students need to put out their textbooks,

this mechanism moves the backpack to the side for easy access. To design such device, safety

requirements must be met. The device should move smoothly and stop quickly once someone has his

hand in the way of operation. The device should also have good craftsmanship. This is important because

it can affect the performance, safety as well as impression of our mobile backpack carrier.

We designed a four-bar linkage and the transmission, capable of transferring the backpack (12 lb) from

the back to the side of the wheelchair and return it without hitting obstacles created by the wheel, handles,

and other parts of the wheelchair. It is important to have the full device be compact on the back of the

wheel chair. The volume, which is the smallest rectangular box to encase our device, should be

minimized. The backpack end position has to be accessible. This can be done by minimizing the

horizontal distance, which is the distance from the outside edge of the armrest to the far edge of the

backpack holder. The final offset angle, which is the angle of the backpack holder at final position

relative to the arm rest of the wheelchair, should ideally be 90 degrees. Moreover, the forward offset,

which is the perpendicular distance from the centerline to the farthest point on the backpack holder,

should be maximized. Power consumption should be minimized. The mechanism needs to deliver the

backpack close to the wheelchair in nine seconds and the speed should be less than 10 degrees per second

for the last 30 degrees of travel. Safety is another major requirement. Our system has to include proximity

sensors to detect human proximity, and hard stop and limit switches to stop the backpack from moving

out of the desired range.

The main purpose of this report is to present the different components of our overall design: motion

generator, energy conversion/transmission components, and safety and motor controls. Initial design,

revisions, and final testing results are included and explained. The report ends with the critique and

evaluation of our overall design.

EXECUTIVE SUMMARY

We designed our backpack holder device’s linkage mechanism with the goal of having minimum volume

and linkage lengths to minimize power consumption, linkage deflection, and friction. We chose 6 : 1

timing belt and pulley and an electric motor for our transmission, and we used a combination of Arduino,

H bridge, and various sensors and switches to control and ensure the safety of our device. Modeling tools

were used to CAD and model the mechanics, dynamics, and loading of our design. Our backpack holder

device was manufactured, iteratively redesigned at certain parts, and tested. Our final testing results show

that our device fulfilled all requirements listed in the ME 350 project description document except not

being able to have a decelerated speed of less than 10 degrees per second. This is mainly because of the

speed and torque limitation of our motor. Other reasons could be that the redesigned holder has increased

weight and therefore extra deflection and friction at the joints. A large limitation to our device is on the

infrared sensors. The two sensors cannot sense anything expect those directly in front of them, therefore

creating blind spots for objects to be hit by the device, creating a safety hazard. We can improve our

design by using elastic material on the edges of moving parts to absorb shock when collision happens,

making our device safer. Another improvement is to implement feedback control in the controller coding,

so steady speeds can be maintained. In conclusion, we do not recommend the usage of our device to

wheelchair-bound students in the meantime, as explained our device is not safe enough, and further

adjustments on parts, change or increase in sensors, and a more powerful motor are needed before usage.

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FUNCTIONAL DECOMPOSITION

The functional decomposition of our backpack holder is shown in Figure 1 below. It helps us break down

our project tasks into individual, manageable components while revealing the functional relationships

between them. The transmission of energy, information, and mass are shown as thin solid lines, dashed

lines, and thick solid lines, respectively.

Figure 1: Functional decomposition of mobile backpack carrier

MOTION GENERATION

Lincage software was utilized to design 3 point motion for our four-bar linkage mechanism. Solidworks

was used to CAD out our design and make sure that the dimensions fit. The CAD model was then loaded

into ADAMS for force/kinetic analysis.

Design Process The preliminary linkage system below was designed to have short links. Short links result in smaller

volume and also lower deflection of links, which in turns lowers joint friction and power output.

Transmission angle graph and linkage info has been provided below. The transmission angle is above the

required 15 degrees as can be seen in Figure 2 below, having a range of approximately 40 to 90 degrees.

The range is acceptable as a low angle results in inefficient power transmission from the output link to the

coupler link. To maximize forward offset both linkages are places near to the adapter.

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Figure 2: Top view of mechanism path from Lincage, showing short links and acceptable

transmission angle

In order to pick the above linkage concept, each team member designed a linkage system concept (a total

of four concepts for the team). Based on these design concepts, we made a decision matrix focusing on

the scoring criterions in the final competition. Table 1 below shows the design matrix used to choose our

team’s final design. The initial length from the adaptor was used to quantify initial volume. Final angle

offset, horizontal offset, as well as forward offset were criteria defined in the project description. As

shown in the design matrix, Melvyn’s design scored highest and thus concurred to be the viable choice.

Table 1: Based on the decision matrix Melvyn’s design was the viable choice

Melvyn Nan Steve Theera

Value Score Value Score Value Score Value Score

Initial length from the adapter 9.94 in 2 10.40 in 2 8.47 in 3 20.24 in 1

Final Angle Offset 0 deg 3 10 deg 1 0 deg 3 0 deg 3

Horizontal Offset(Min) 7.14 in 2 7.74 in 2 7.30 in 2 8.10 in 1

Forward Offset (Max) 11.65 in 3 15.46 in 3 12.43 in 3 17.50 in 3

Transmission Angle Range 40 deg 3 25 deg 2 20 deg 1 50 deg 3

Total Score

13

10

12

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Final Design Diagrams

In our final design, we decreased the weight of the coupler and both the bottom and top supporter to

decrease stress and deflection on the links as this would increase friction. More than this our coupler and

backpack holder were heavily modified to allow the backpack to travel without hitting the wheel of the

wheelchair. In addition flanges and angle bracket were included to ensure proper power transmission

throughout the experiment.

Figure 3: Right view of all the parts of the linkage system with labels

Figure 4: Isometric view of final design

Ground-input

Joint

Motion transmission

bolt

Input link

Input-coupler

Joint

Ground link

Coupler

Coupler-output

Joint Output link

Output-Ground

Joint

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Figure 5: Back view of final design

Figure 6: Isometric exploded view of final design

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Figure 7: Exploded View 1 (Left)

Figure 8: Exploded View 2 (Right)

Figure 9: Top view of final position

Figure 10: Top view of initial position

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Figure 11: Section Drawing of Ground-input joint Figure 12: Section Drawing of Input-coupler joint

ground

link

Washer

Locknut

Bushing

Bearing

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Table 2 below shows the combination of components used in the four joints in our mechanism. A

combination of sleeve bearings, needle thrust bearings and thrust washers are used to ensure low friction

Table 2: Materials needed for each joint

Ground-input joint Input-coupler joint Coupler-output joint Output-ground joint

1 - 5”Shoulder screw

7 - 1 3/8” thrust Washer

1 - 3/8” Sleeve Bearing

2 - Needle bearing

1 - 5/16” LockNut

1 - 1.75”Shoulder

screw 5 - 3/8” thrust

Washer


2 - Needle bearing

1 - 5/16” LockNut

1 - 2.25”Shoulder

screw

6 - 3/8” thrust Washer


2 - Needle bearing

1 - 5/16” LockNut

1 - 1.75”Shoulder screw

5 - 3/8” thrust Washer


1 - Needle bearing

1 - 5/16” LockNut

Figure 13: Section Drawing of Output-ground joint

Counter bore is applied here to lower the surface

level of the input link so that the whole system will

not interfere with the handle bar when moving.

Figure 14: Section Drawing of coupler-output joint

All the washers, bushings and bearings are used to

reduce the overall friction within the joints.

Shoulder

screew

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Loading and Deflection Analysis Load magnitudes were calculated using ADAMS dynamic simulation software. Data from ADAMS were

then used to calculate the worse scenario deflections to ensure the viability of our overall design. The

loads data were also compared with load ratings to evaluate the quality of our joints design. Our joints

design are acceptable as the maximum loads the joint components could endure are much higher than the

maximum loads experienced in ADAMS simulation.

Loading analysis: Loading analysis for each joint was calculated via ADAMS dynamic simulation

software. Table 3 below shows the loads in 3 directions for each joint. Figure 16 is a screenshot of the

CAD model in ADAMS

Table 3: Loading and torque on each joint are within acceptable values.

Ground-

input

Ground-

output

Input-

coupler

Output-

coupler

Max Force (lb) X 0 0 0 0

Y 0 0 0 0

Z 46 43 21 20

Max Torque (lb-in) X 200 160 65 7

Y 0 23 158 113

Z 0 0 0 0

Figure 15: ADAMS Figure of Final Design

Deflection analysis: To estimate the total vertical deflection of the mechanism, deflection of each linkage

part were calculated separately then added up. To simplify the calculation of vertical displacement and to

estimate the worst scenario, each of the components was simplified as cantilever beams simply supported

at one side, shown in Figure 16 below. Vertical displacement was calculated using Equation 1

below. The parameters include vertical force, , length, , Young’s modulus of aluminum, , and area

moment of inertia, , of the cross sectional area. Moment of inertia is calculated using Equation 2 below.

Parameters include cross sectional area width, b, and height, h.

Eq. 1

Eq. 2

Table 4 p. 12 shows each links length, area moment of inertia, force, and deflection as well as total

deflection. Force used for input and output links were from joint load analysis done in ADAMS (Table 3),

and force used for coupler link was from weight of backpack. Moment of inertia of the coupler link was

calculated using the smallest cross sectional area to consider worse scenario.

Sleeve

bearing

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Table 4: Deflection and loading analysis of links.

Link Length (in) Area Moment (in2) Force (lb) Deflection (in)

Input 9.85 0.034 21 0.019

Output 9.68 0.034 20 0.018

Coupler 7.50 0.031 12 0.005

Total 0.042

Figure 16: Free Body Diagram to calculate the deflection of each pivot

Load ratings: Maximum load rating for each joint component is shown in Table 5 below. The maximum

forces, in deflected coordinate axis, as shown in Figure 17 below, of all the joints together, are also shown

in the table as comparisons. Note that Figure 17 only shows the vertical force and its components as an

example, the original planar forces are not shown. The forces in deflected coordinate axis were calculated

using simple trigonometry from the forces in the original coordinate axis. The X’-component as shown in

Figure 17 below, is essentially the maximum planar force. The data in Table 5 shows that all forces are

below the maximum load ratings of the components, therefore our joint design is able to handle the loads

of our linkage mechanism.

Table 5: Load on each respected bearing, washer, and screw

Shoulder Screw Sleeve Bearing Needle Thrust Bearing Thrust Washer

Max Load (lb) 19000 750 1664 47

Max Z’ force (lb) 21 21 21 21

Max X’/Y’ force (lb) Negligible Negligible Negligible Negligible

Figure 17: Vertical force on a joint

Beam outline

Bearing Horizontal Datum

Vertical force and its

components

X’/Y’-component

Z’-component

Length

Vertical Deflection

Force

Horizontal datum

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ENERGY CONVERSION

Transmission is necessary for minimizing power draw of the motor while meeting specified speed and

torque requirements for the designed mechanism. While good transmission design is important, it is also

important to minimize required torque, which relates back to minimizing friction in the linkage and joint

design. Timing belt and pulley combination of 6:1 ratio was chosen as the transmission type. Loading and

deflection analysis were performed to ensure the validity of the transmission design.

A transmission ratio of 6:1 is chosen. To validate this, past data of torque required to drive the input link

were used to derive required motor output speed, motor output power, and input power, making sure that

all fit in plausible ranges. All calculations were done under the assumption of 9 V input on the motor.

Therefore, stall torque stall current , and no-load speed, = 60 rpm; all

were calculated proportionally from the 12 V data given on the manufacturer Pololu’s website.

Required motor output torque values were derived using the transmission ratio of 6:1, then required motor

input speeds were calculated using the torque-speed relationship shown in Figure 18 below. The torque

speed relationship was estimated to be linear using the stall torque and the no-load speed .

Figure 18: Output torque-speed relationship of the motor is estimated to be linear. Max power is shown.

Power Calculations

The required output power values were calculated using respective torque and speed values. The

maximum output power is simply the maximum area under the torque-speed curve in Figure 18. The

required motor input power can be calculated by first using motor constant

to

calculate required input current, then multiplying the current and the constant 9V. The efficiency of the

motor is output power over input power.

The results of the calculations are summarized in Table 6. When required output torque is high, motor

input power is high and output speed and overall efficiency is low. It is due to the fact that the motor is

operating at the bad high torque and small output power area on the torque-speed curve. Therefore, high

input link torque is unfavorable. Low input torque is favorable and would minimize required power.

Table 6: High required input link torque is unfavorable. Power is minimized with low torque.

Required values Low Medium High

Input link torque (oz-in) 290 560 1120

Motor output torque (oz-in) 48 93 187

Motor output speed (rpm) 45.5 32 0.3

Motor output power (W) 1.6 2.1 0.03

Motor input power (W) 8.7 16.8 33.6

Efficiency (%) 18 12 0.1

0

25

50

75

100

125

150

175

200

0 10 20 30 40 50 60 70

To

rqu

e (o

z-i

n)

Speed (rpm)

Max Output Power

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Travel Speed Calculations

It is given that the total travel time of the mechanism should be 9 seconds and that during the last of

travel, the speed should be decelerated with a maximum of 10 degrees per second. Given that our

mechanism’s total travel angle is , the equation below shows the relationship between the first speed,

and the decelerated second speed,

In the worst case scenario from past data, where input link torque is 1120 oz-in, the mechanism simply

cannot achieve 9 seconds travel time with the maximum output speed shown in Table 6. It is obvious

when plugging the values into Eq. 3. The medium torque range, however, provides enough space for

speed adjustment. At 1.7 rpm, =4.3 rpm. This again shows the

importance of minimizing input link torque.

Design

Results from both power and speed/travel time calculations show that a high required input link torque is

undesirable. Friction at the joints could contribute greatly to the required torque, therefore our design was

geared toward minimizing friction. Thrust washers, bronze bushings, and thrust bearings were used to

ensure smooth rotation at the joints. Deflections at the joints could increase friction, therefore to minimize

joint deflection the input and output link lengths were designed to be short, and double support were

added to both joints connecting ground and input/output links.

Testing Results

Final testing was performed using 9V from a power source, matching the theoretical calculations. Three

tests were performed and we made speed adjustments to our machine between the tests. Table 7 below

shows the result of three tests. Run 2 had the best result. While we fulfilled the total time requirement of

close to 9 seconds and stay in a good range (Table 6 in Motor input power calculations) of max power, we

could not fulfill the decelerated speed requirement. The main reason for failing to fulfill that

requirement is due to the limitation of the torque and speed of the motor. This will be further discussed in

the concluding discussion section of this report.

Table 7: Out of 3 tests, the 2nd

trial was chosen to be the best.

Test Total Time (sec) Low Speed (deg/sec) Max Power (W)

1 8.36 13 21

2 8.55 15 18

3 8.17 16 18

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Figure 19 below shows the angle vs. forward speed. In the last 30 degrees the speed was too high,

approximately 15 degrees per second, and did not match up the 10 degrees per second requirement.

Figure 19: Best test results shows that forward speed during the last 30 degrees was too high.

Power is close to the calculated power for medium torque range (~16W). However, that does not actually

reveal much because pulse width modulation (PWM) is used to control the motor. Power usage is

constantly fluctuated from zero to a definite value as shown in Figure 20 below. Therefore, the real power

that we want to know and want to compare with theoretical calculations is not apparent from this data.

We can however note that the power does peak at the end, showing that the ending few seconds require

highest power and therefore highest torque.

Figure 20: Best test, max power fits calculated range

0

5

10

15

20

25

30

35

40

45

0 50 100 150 200 250

Fo

rwa

rd S

pee

d (

deg

/s)

Angle (degrees)

0

2

4

6

8

10

12

14

16

18

20

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9

Pow

er (

Watt

)

Time (sec)

Max approximately 18 W

Decelerated speed 15deg/s

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Transmission Selection

The choice of transmission came down between gear and belt and pulley. Timing belt was chosen as the

choice of belt because it needs least tension of all belts and is less likely to slip than flat belt. Our team

ultimately chose the timing belt and pulley combination using the criterion shown below in Table 8. The

scoring of each criterion is given on 3 points maximum and 1 point minimum scale. The facts that belt is

less sensitive to deflection and that it is more adjustable were important reasons for our final decision.

Table 8: Timing belt was chosen for transmission type

Criterion Gears Points Timing belt Points

Volume Compact 3 Spaced 1

Manufacturability Require precision 1 Require less precision 2

Allowable deflection Little, teeth require good fit 1 Much more, belt is flexible 3

Adjustability Little, position requires

precision

1 More, belt direction does

not matter as much

3

Total 6 9

The components which we chose to use are pulleys 3.800"OD XL and 0.870"OD XL and timing belt 14"

XL from Mcmaster-Carr.

Mounts and Joints

Pulleys of 6:1 ratio and a timing belt were utilized for power transmission.

Figure 21: Whole back view of mechanism and transmission

The motor mount consists of a piece of angle stock, U-bolt, and a hole on the ground plate for the spacing

of the motor. A shoulder bolt goes through the right side pulley and through the input link for motion

transfer. In the final stages of design and manufacturing the angle bracket that came with the motor was

added an emplaced into the angles stock using bolts, to further secure the motor.

Angle bracket

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Figure 22: Back view of joints and mountings for transmission system

The slot on the angle stock mount with the screws and also a slot on the plate below together allow

adjustment of the mount to tension the belt.

Figure 23: Front view

Figure 24: Exploded view of joint

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Figure 25: Cross-sectional view from back.

Strength analysis for timing belt

The strength analysis for the timing belt design starts with the calculation of the theoretical allowable

force, then the estimation of the force on the belt. We used safety factor criterion to determine the best

belt model. Finally the theoretical torque results are compared with the experimental ones. The results for

these analysis processes can be seen as follows.

For all the calculations mention in this section, we use following assumptions:

1. All forces are acting on the horizontal surface.

2. No slip between the belt and the pulley.

3. No external force acting on the pulley.

4. All objects are rigid.

Theoretical allowable force: To determine whether the belt can withstand the torque, we first calculated

the maximum allowable force, , for the timing belt using Eq. 4; where b, , and , are the

allowable tension [lbf], belt width [in], pulley correction factor, and velocity correction factor

respectively. We used the correction factor of 0.5 to assume the worst case scenario for the 0.5 in wide

belt and assumed the velocity correction factor for Urethane belt to be 1. The manufacturer’s allowed

tension, Fa, was assumed using Eq. 5 where the UTS is the ultimate tensile strength of Urethane (5511.43

psi), and the cross section area of belt is based of 0.5 in wide and 0.01 in thick belt.

The Fa was calculated to be 13.77 lbf. Using this value, the maximum allowable force was determined to

be 13.78 lbf.

Theoretical force on belt: We calculated the timing belt force for three applied torques at input link

which are 1120 (high), 560 (average), and 290 (low) oz-in. These torques values were collected from last

year’s backpack holder design statistics and converted to the torque applied at the motor multiplying a

mechanical advantage factor of 1/6. The free body diagram used for this application is show in Figure 26.

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The tension forces in the belt T1, and T2 were calculated by Eq. 6 and 7, respectively; where Ft, Fi, and Fc

note tension forces due to the transmitted torque, initial preloading, and centrifugal force, respectively. T

and d represent transmitted torque, and diameter of pulley (0.637 in), respectively. We assume the initial

load to be negligible since no initial tension is needed for the timing belt; while the centrifugal force was

also determined to be negligible.

Figure 26: Free body diagram for the motor driven pulley

Eq. 6

Eq. 7

The maximum forces on the belt based on T2 were calculated to be 18.31, 9.16, and 4.74 lbf for high,

average, and low applied torque, respectively.

Safety factor: The theoretical applied forces were compared using the safety factor, SF, (calculated by

theoretical allowable force divided by theoretical force on belt). The safety factor less than one will

demonstrate failure. In this case, the high torque (SF = 0.75) is not practical while average (SF = 1.50)

and low torque (SF = 2.91) falls within the safety factor criterion.

Comparison between : Experiment was performed during lab on Monday 2/20/12 to obtain

the torque load data shown in Table 9 below. The results were compared with the historical torque values

from last year and allowable loads using the safety factor criterion. The experimental torque requirements

were at the average range for the historical, showing that we have a good linkage design. The safety factor

for the maximum force recorded in the experiment was calculated to be more than one, and this confirms

that our design will not fail on the timing belt power transmission system.

In conclusion, the results for the strength analysis are shown in Table 9 below. The high belt tension from

the highest input link torque results in the risk of the belt breaking. This again shows the importance of

having low input torque, justifying our design efforts to minimize friction and deflection at the joints.

Table 9: Summary of strength analysis on timing belt

Applied input torque (oz-in)

Historical Experimental

High(1120) Average (560) Low (290) Max torque (726.4) Min torque (240)

Belt tension [lbf] 18.31 9.16 4.74 11.89 3.91

Safety factor <1 1.50 2.91 1.16 3.53

Driving pulley

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Figure 27: Top view of driven pulley Figure 28: Front view of driving pulley

Assuming the pulley is static; the force acting on the shoulder bolt should be the vector sum of all the

other force acting on the driven pulley

We choose the worst case scenario at maximum torque = 1125 oz-in for the driven pulley and 187.5 oz-in

for the driving pulley. Applying Eq. 8—12 above, we calculate the normal & shear stress under the worst

case scenario to see if it exceeds max allowable normal& shear stress. The maximum allowable normal

stress data is derived from each component’s ultimate tensile strength. The maximum shear stress was

given from each manufacture’s website.

Pulley Teeth

To determine whether the stress acting on the teeth of input and output pulleys exceeds the material limit,

we assume the force is acting uniformly onto each contacted tooth (Figure 29 p. 21). By measuring the arc

of the timing belt contacting the pulleys in CAD, we determined that 4 teeth of the input pulley and 36

teeth of the output pulley can hold up the transmission force . We got the dimension of the tooth with

average thickness t and width D from the manufacture’s website. We then applied Eq. 11 to calculate the

shear stress acting on the cross section of those teeth.

Inp

ut

lin

k

driven pulley

Off-axis pin

Shoulder bolt

torque

Driving pulley

DC motor

Set screw

21

Figure 29: Pulley Teeth Free Body Diagram

Table 10. The comparison of stress and maximum allowable stress for torque transmitting joints

Force (lb) Normal

stress (ksi)

Maximum allowable

normal stress (ksi)

Shear

stress (ksi)

Maximum allowable

shear stress (ksi)

Shoulder bolt 123.8 0.87 180 1.12 84

Off-axis pin 50.22 0.74 180 0.45 84

Set screw 46.83 0.54 212 23.83 143

Input teeth 4.58 n/a n/a 0.179 9

Output teeth 0.51 n/a n/a 0.020 9

As can be seen in Table 10, none of the stresses exceed the corresponding maximum allowable values

Deflection on the Motor Mount

We mounted our motor on the angle stock, which is a 90 degree aluminum stock with dimension 2.5” x

2” x ¼” thick. To simplify the problem to a cantilever beam and to calculate worst case scenario, we

ignore the ¼” thickness on the bottom, treated as a 2.5” x ¼”rectangle with a force exerting on it. Figure

10 below illustrates the scenario. Total loading on the pulley, , which is the combination of two tensions

from the belt, is simplified to be a load on the top of the mount . It is calculated from the maximum motor

output torque of 187 oz-in from Table 6, p. 13 and the pulley diameter of 0.367 in, to be = 65 lb.

Figure 30: Fix angle stock mount

.

22

Then applying the beam deflection formula on the cantilever beam, we have:

Where E is the Young’s modulus of aluminum, , I is the inertia of the angle stock, height

b = 2.25 in, thickness t = 0.25 in, and length L = 2.8in. The resulting deflection is .

The mount deflection is negligible as belt permits small deflections, and will not affect the functionality

of our transmission design.

Motion Generation Design Revisions

The original supporter plate (ground plate) and the input link were changed minutely while designing

transmission to accommodate for the addition of the new transmission components. There was a new hole

on the supporter plate for the motor to go through, and there was a new through hole on the input link for

the motion transferring shoulder bolt to go through.

Figure 31: Changes made to old parts are pointed out by arrows.

Revisions of the drawings and manufacturing plans for the supporter plate and the input link can be found

in APPENDIX C after the information for transmission components.

SAFETY & MOTOR CONTROLS

The safety and motor controls system for our backpack holder system consist of an Arduino, an H-bridge,

a 400-point breadboard for wiring connections, and a variety of sensors/switches,. This section of the

report explains the capability and limitation of the important components, particularly the sensors, and the

physical integration of components to our system, the electrical mechanism, as well as the control coding.

Arduino: An Arduino Uno microcontroller board is used to control the entire electrical system. C code

from the computer is loaded onto the Arduino through its USB connection. All the sensors/switches,

motor, and H-bridge are connected into different pins on it.

23

H-Bridge: The H-bridge (Solarbotics Compact L298 Motor Driver) is used to drive the motor. It is

connected to an external power source (Pyramid Power supply #PS26KX) because the power from the

Arduino is not sufficient to drive the motor. The H-bridge allows voltage applied to the motor to go in

different directions and thus drives the motor in different directions. Speed of the motor is controlled

through Pulse Width Modulation (PWM) written in the Arduino code.

Encoder: The halleffect encoder is mounted to the motor axis at the bottom of the motor. It comes with

the motor and is used for counting the number of motor rotation. The encoder reading values determine

when the motor will be run in high speed or low speed (last 30 degrees of input link motion) for the

forward direction. The thresholds for switching to decelerated speed are 1980 for returning and 11800 for

forward last 30 degrees, as mentioned in the coding section in Appendix D. The calculation for the

encoder count can also be found in Appendix D. A major limitation to this encoder is its resolution, 64

counts per revolution of the motor shaft. Also the fact that the encoder reads rotations of the motor shaft

means that errors from slipping in the pulley can build up over time, resulting in error of actual angle

count. One solution to this problem is addressed later in the introduction of the limit switches.

Rocker switch: The rocker switch is a two way switch. It can either be activated to move the motor

forward or backward. The switch can be taped anywhere on the handle bars for user accessibility.

Infrared proximity sensor: The purpose of the Sharp GP2D120XJ00F infrared proximity sensor is to

stop the motor when hand is in the way of the linkage motion. Two infrared sensors are mounted by

double sided tape to the input linkage in both normal to forward and normal to backward directions as

shown in Figure 31 below. Limitations to this sensor including having the effective range of 4 to 30cm

and being only able to sense objects directly in front of it, not at an angle. Other limitations include the

response time of 39ms and the startup delay of 44ms. For the backpack holder application, we set the

stopping threshold for the sensors at 80 and 120 for forward and backward direction, respectively. To

mount the sensors affectively to our design, we have chosen to include an aluminum angle stock onto the

input link where the sensors will be placed on either side of the angle stock. Sensors will be mounted and

placed with double sided tape. Mounting the sensors on the angle bracket does not have a significant

effect on our overall volume.

Figure 32 (Left) Figure 33 (Right): The infrared proximity sensors are mounted on angle stock on the

top of input link facing forward and backwards for safety feature.

Sensor mount

Infrared Proximity Sensor

24

Limit switch (snap action switch): The Jameco 187733 limit switch stops the motor when the linkage

touches activates it. This switches are place on the ground link where the input link will contact at the

start and end positions. For mounting, the switches are double-sided-taped to the ground link next to the

hard stopper as shown in Figure 34 and 35 below. This is important because the switch is restricted from

absorbing impact from the linkage by itself. Another function of the switch at the initial position is to

reset the encoder position counting to zero. This is important because of discrepancy between encoder

count and actual traveled error from possible belt slip builds up over time. Resetting corrects for the error.

Figure 34 (Left), Figure 35 (Right): Limit switches are mounted to the ground link next to the hard

stoppers (left and right picture present forward and reverse hard stops respectively, and black box

represents limit switches.

Calculation and Tuning of Code Values

As mentioned in the prior sensors and switches descriptions, the sensors that require the calibration for

the threshold values are the encoder and infrared proximity sensors. Motor speed also has to be adjusted

to achieve desired travel time and decelerated speed. The procedures and derivations of values are

described as follows.

Encoder : The lower count threshold is the value of the encoder count when the input link is 30 degrees

from the starting position, where the mechanism is moving in a reverse direction, where the upper count

threshold is the value of the encoder count for the forward direction prior to the last thirty degrees of

travel. These thresholds are used to determine when to use decelerated speed.

To calculate both of these values the initial and final angle of the mechanism were measured.

Where, (

) (

) (

) (

) where for a lower

count threshold we used 30 degrees for , resulting in a calculation of 2096 counts, on the other hand we

used 190 degrees after subtracting by 30 degrees for the upper count threshold, resulting in a value of

13274 counts. This method was proved to be a viable ballpark reference, where we fine-tuned the counts

by experimentally checking the counts at the specific positions of lower and upper counts and placed this

numbers in place of the calculated values.

Infrared Proximity Sensor: The threshold values of the two infrared proximity sensors were simply

realized through placing an object just outside the distance where our mechanism could potentially

contact and hurt someone. Trial and error method was also used to determine the safety position. The

threshold for the sensor pointing in forward travelling direction was set to 120, while the one in backward

direction was set to 80.

Limit Switches

25

Circuit: The wiring diagram of our sensor and control system is shown in Figure D-1 in Appendix D is

the wiring diagram for the sensors and control system.

For packaging and safety, all the wires were wrapped and heated shrank to have a complete seal from the

outside environment. The zip ties were also used to hold the proximity sensor wires to the input link and

prevent the wires from tangling with other linkage components.

Control Codes To implement the feedback system, the C code (Appendix D) was used to couple with the Arduino

control platform. To ensure consistency of outcome for each testing and eliminate errors that could

accumulate in the encoder count, the code was modified where the count was reset to zero every time the

backward limit switch (at initial position) was toggled. The extra code which we added into the given

code is seen bolded below.

if (limitSwitchReverse==HIGH){ // if the back limit switch is made

analogWrite (pwmPin,0); // stop the motor

encoderCount == 0;

Serial.print("The back limit switch is made. Count:"); // print this to the serial monitor

Serial.println(encoderCount); // print the encoder count to the serial monitor

}

DESIGN MODIFICATION

After the manufacturing process of the linkage, powertrain, and transmission mounting, we encountered a

few clearance and motion timing issues. These can be categorized to three problems: linkage clearance,

powertrain/ drivetrain mounting, and coding for timing.

Linkage Clearance

There were two main issues with the linkage clearance during the initial testing motion generator testing.

First, the backpack was hitting the wheel. And second, the input link was rubbing against a bolt on the

ground link. The problems and solutions for these issues are explained below.

Backpack Hitting Wheel: We found out that the links length in the preliminary design is not adequate to

carry the backpack over the wheel constraint. This was because we did not predict in our CAD model, the

slant down angle of the actual backpack when hanged on a backpack holder bar. To solve this clearance

problem, we accommodated our GSI’s advice by strapping the backpack around an additional support bar

to shorten the vertical distance from the wheel. We first physically measure the minimum clearance

required for the backpack to the wheel during the linkage motion to be about 6 inches. We then

manufactured two extension links and cut down to the optimal (minimal) length according to the physical

mounting. Grooves were also added to the coupler link to adjust the horizontal and forward offset during

the load testing. The comparison of the designs before and after adding the extension links is shown in

Figure 36 p.26.

Figure 36: Comparison of the linkage design before (left) and after (right) adjustment for the wheel

clearance

26

Input Link Rubbing Against Spacer Bolt: Another clearance problem that we encounter during the

motion generator test was that the real backpack weighted more than the mass modeled in our ADAMS

model. Because we did not take a high enough safety factor against the input link defection into account,

the input interfered with the bolt head in the counter bore hole in the ground link. To solve this problem,

we added two additional washers between the ground link and the input link to raise the clearance

between the input link and the ground link.

Code adjustment

A slight code change to the original code given to our team, listed in Appendix D, was made to reset

encoder count value to 0 when the input link hits the backward limit switch (thus returning to initial

position). This is essential due to the fact that the encoder count is prone to error from slippage of timing

belt. Without resetting the count, the error could build up over time and undermine our setting of

decelerated speed thresholds. The code modification is highlighted in the control codes section on p.28.

DESIGN CRITIQUE & EVALUATION

This section includes the results and conclusion to our linkage system design project. The final testing

results, safety consideration, comparison of the theoretical and experimental testing, and future

improvement to our system are discussed.

From the preliminary stages of our design to the final stages many changes were made, mainly minor

modifications of tolerance and cutting off weight, but one modification, the redesign of the coupler and

backpack holder, was especially significant. With our preliminary mechanism the backpack would travel

and hit the wheel of the wheelchair, where an impact of about 5 inches of backpack to wheel was

observed. To account for this collision the team devised a two-bar backpack holder design, and a new

method to strap the backpack was used, where we hang it on the first backpack holder and let it rest on the

second one (the one closer to the backpack). It can be seen in Figure 4, p. 7. By using this method, a

clearance of eight inches from the wheel were observed, as the backpack was raised up and moved out

due to the new mechanism. While, our two backpack holder mechanism cleared the wheel affectively it

did increase our volume considerably. In addition to this, we ultimately chose to only use one U-bolt and

the motor’s angle bracket to secure the motor onto our manufactured mount. Where we found that using

two U-bolts, our earlier design, is not as effective as the previous mentioned method in preventing

vibration and tilting of the motor. The vibration and tilting usually resulted in low pulley tension and

slipping of pulley

Moving on, in comparison to our CAD model, our device were accurate in most categories except angle

offset where it was designed to be zero degrees whereas it was tested to be four degrees. This was due to

the setting on the coupler as a slight adjustment on the bolts could have decreased the offset angle. Our

device also matches the various loading and deflection analysis performed during the design process.

Deflections on the device were insignificant and all components survived loadings.

We were not able to decelerate the speed to the required 10 degrees per second. At that speed our

mechanism would not have enough momentum to move the backpack to the final position. This is due to

a combination of reasons. The redesigned backpack holder added not only volume but also extra weight

to the mechanism. Deflections were not apparent but minor ones could have occurred in the joints and

increased friction. One possibility is that due to the five degrees incline of the wheelchair, loading is non-

vertical and imbalanced on the joint thrust bearing and washers, therefore friction is higher. In an attempt

to remedy the problem, lubricants were sprayed in the joints to lower friction. The largest problem,

however, is the torque and speed limitation of the motor. Due to the limitation, the motor was not able to

provide enough torque at the desired speed.

27

In reference to the control algorithm we changed the code by setting encoder count to zero each time the

mechanism hit the back rocker, as this will reset the count and ensure a constant outcome in every test.

Another way that our control algorithm could be improved is to use feedback control to maintain steady

speed. It is clear in Figure 19, p. 16 that speed is not maintained steadily at both high and decelerated. It is

most likely due to disturbances and the fact that loading on the mechanism from the backpack changes

over the entire travel because the wheel chair is tilted at 5 degrees.

On the other hand, the infrared sensors are mounted on an angle stock above all other materials. These

high positions prevent them from detecting the mechanism itself. However, they failed to detect the

objects that are in the way when at a lower/upper vertical position than the sensors or just on the sides.

Therefore, the infrared sensors have huge blind spots and therefore buried safety hazard of this design to

bystanders. To accomplish similar objective many more infrared sensors or alternative electrostatic

sensors could be used as it utilizes the reflection of high frequency sound waves to detect parts or distance

to parts. This device would be able to increase angle of detection, where we found this to be a problem in

our sensor. In addition to this, load cells may also be used on frequent accidental contact points on the

device where contact to an object will stop the mechanism.

Apart from the safety problem of the infrared sensor to bystanders, another large problem of our design is

that at initial position all the weight is attached to the back of the wheelchair. When the person in the

wheelchair stands up the wheelchair would most likely fall backwards. Counter weight is definitely

required but that would increase the overall weight of the wheelchair dramatically. With the current

design on a normal, non-electrically powered wheelchair, our device is impractical and unsafe. We would

give the safety of our device 2 out of 5 because of this issue and the limitation of the infrared sensor

mentioned in the previous paragraph.

In hindsight, we would have started building our mechanism sooner and tested with the backpack sooner,

as we would have enough time to change our design and fine tune for problems that were not taken into

account in the preliminary stages of design. Being time crunched in the final days of design and

manufacturing we decided to emplace washers where in places a larger clearance is needed, looking

forward, this might not be the best solution as this will increase further probability of failure as the

addition of several washers are not designed for a larger clearance.. To increase the safety factor

additional sensors and adjustment to parts are needed where plastics or other more elastic material are to

be used to decrease impact force. A particular dangerous part is the backpack holder where it may be a bit

sharp on the edges; putting plastic reinforcements here will solve the issue.

In conclusion, we do not recommend the usage of our device to wheelchair-bound students in the

meantime, as explained our device is not safe enough for the wheelchair occupant and bystander, where

further adjustments on parts and an increase in sensors are needed before usage. Looking forward, we

would design the mechanism differently where one major problem is that the wheelchair user is not able

to view the device when it is in its starting position or while it is in motion, where this is because the user

is facing forward. The inclusion of plastic plates would have been useful to produce flanges, and the

inclusion of a more powerful motor would help eliminate the problem of not reaching the desired

decelerated speed.

28

APPENDIX A

This Appendix section shows the alternative preliminary design as mentioned in the decision matrix on

page xx. For each design, brief information and analyses on CAD design, initial volume, final position

offsets, and transmission angle are included.

Alternative design 1 (Steve’s design) The linkage mechanism below was designed to have short links so that torque loads on the ground joints

could be minimized. The ground joints were also placed as closed to the adaptor as possible to minimize

the length of the ground link extending out of the adaptor, also to minimize moment on the design.

The graph of the transmission angle is shown below on the right. The low minimum of 20 degrees means

that the energy through the coupler would not be transmitted efficiently to the output arm over a long

range of motion. This could result in very slow motion of the mechanism and high power consumption of

the motor.

Figure A-1: Design 1 lincage analysis

Figure A-2: Design 1, top view of final position

29

Figure A-3: Design 1 Isometric View Figure A-4: Design 1 Isometric Back View

Figure A-5: Design 1 ADAMS Picture

The high loads at the ground-input joint could lead to bending, which would increase friction and cause

other problems such as damaging the bearing. The problems could be approached by having two side

supports on joints on the ground link. Thrust needle bearings could be added between linkages to lower

friction, and if there is too much deflection, aluminum tubing could be used to replace the plate material

of the input and output links.

Table A-1: Design 1 force analysis in ADAMS

Ground-input Ground-output Input-coupler Output-coupler


Y 0 0 0 0

Z 30 41 27 37


Y 0 0 0 0

Z 300 419 0 32

30

Alternative design 2 (Theera’s design) This design created motion path using two point motion criterions. The motivation behind this is for

freedom of designing the travelling path. The designer still found that the design still pass the geometric

constraint requirements (the linkage path has to be away from the wheel). The design focuses on making

the transmission angle to be as close to 90 degrees as possible. As shown in the transmission angle graph

generated from the Lincages software, the transmission angle swept from 50° to 90°. The designer

believed that this concept will significantly help reduce the power required to operate the system.

However, the trade-off for the high transmission angle is the additional linkages length which lowers the

design packaging scores for initial volume as well as the higher deflection in the bending bar might cause

significant amount of friction on the joints. Therefore this design was not selected as our final design.

Figure A-6: Design 2 lincage analysis and info

Figure A-7: design 2 top view at initial position

Figure A-8: design 2 top view at final position

Figure A-9: design 2 top angled view

31

According to the ADAMS analysis, there seems to be high load at the input coupler joint (about 20 lbs

which is higher than the weight of the backpack). This is expected because the linkage may cause the

mechanical advantage on force acting on the joint. Therefore for manufacturing consideration, some

bushings will need to be placed to increase the strength of the joint.


Ground-input Ground_output Input_coupler Output-coupler


Y 0 0 0 0

Z 10 13 20 7


Y 47 57 0 0

Z 0 0 0 78

Alternative design 3 (Nan’s design) Transmission angle is the angle between the coupler and the output. Ideally, the range of transmission

angle should be between 30 to 150 degrees. If transmission is poor, the power input needed to drive the

output linkage would be too large to be provided by a motor and corresponds to no motion results or even

causes joints fail. In our project design requirement, it should be above 20 degrees through the useful

range of motion of the linkage, which is met by this design.

Figure A-10: Design 3 lincage analysis

All parts of the links including ground link, input link, output link, coupler and the backpack holder are

all machining from the 1/2”x 12” x 12” Aluminum Plate (Model #9246K33) from Mcmaster-Carr.

32

Figure A-11: Design 3 top view at initial

position

Figure A-12: Design 3, top view at final position

Figure A-13: Design 3, Isometric View

Since the loads are heavy, to prevent major joint bending, double shear interfaces (clevis) are used to

mount the linkage. We can use the bronze thrust washers to minimize aluminum to aluminum contact to

decrease friction.


Ground-input Ground-output Input-coupler Output-coupler


Y 15 10 9 14

Z 50 35 36 43

Max Torque (lb-in) X 350 320 322 330

Y 83 0 45 42

Z 0 0 0 0

33

APPENDIX B

Appendix B provides detailed information the final motion generator system. This includes detailed

design drawing of each part on the system, assembling instruction of the parts, and bill of materials

(BOM).

Motion generator system drawings and manufacturing plan

The detailed drawings and manufacturing procedures table are attached in the following pages

34

Backpack Holder x 2

Part Material Machine Process Tool(s) Speed (rpm)

Backpack

Holder Al 6061

Vertical

Band Saw

Cut down square tubing to

10.80”

- 400

Mill

Climb mill one end flat and

smooth

1/2” end mill 650

Mill other end to 10.50” 1/2” end mill 650

Center drill holes #4 center drill 1500

Drill holes 1/4” drill bit 1500

-

File edges File -

Deburr holes Deburr -

35

Input Link


Input

Link

Al 6061

Vertical

Band Saw

Cut down square tubing

to 10.00”

- 400

Mill

Climb mill one end flat

and smooth

1/2” end mill 650



Drill 1/2” holes 31/64” drill bit 750

Drill 3/8” hole 3/8” drill bit 1000

Ream 1/2” holes 1/2” ream 200

-

File edges File -


Al 6061 +

Bearing (x 2)

Arbor

Press

Press fit bearings into

holes

- -

36

Output Link


Output

Link

Al 6061

Vertical

Band Saw

Cut down square tubing

to 10.10”

- 400

Mill

Climb mill one end flat

and smooth

1/2” end mill 650


Center drill 2 holes

0.50” from each side

#4 center drill 1500


Ream holes 1/2” ream 200

-

File edges File -


Al 6061 +

Bearing (x 2)

Arbor

Press


holes

- -

37

Coupler Link The manufacturing plan for this part is attached on next page.

38


Coupler

Link

Al 6061

Water Jet Cut outside geometry - -

- File outside edges File -

Mill

Face mill outside

geometries

1/2” mill 650


Drill 1/4” diameter holes

for locating grooves

1/4” drill bit 1500

Drill 1/2” diameter holes 31/64” drill bit 750

Ream 1/2” diameter holes 1/2” ream 200

Mill the grooves 1/4” mill 1300

-

File outside edges File -

Deburr holes and grooves Deburr -

Al 6061 +

Bearing (x 2)

Arbor

Press


holes

- -

39

Coupler Extension x 2


Coupler

Extension Al 6061

Vertical

Band Saw

Cut down square tubing to 7.00” - 400

Mill

Climb mill one end flat and

smooth

1/2” end mill 650




-

File edges File -


40

Ground Link The manufacturing plan for this part is attached on next page.

41


Ground

Link

Al 6061


File Deburr outside geometry - -

Mill

Face mill outside geometries 1/2” mill 650

Center drill holes and groove

end positions

#4 center drill 1500

Drill 1/4” diameter holes and

1/4” wide groove end

positions

1/4” drill bit 1500

Drill 3/8” diameter holes U drill bit 1000


Step drill 1.4” wide groove

end positions

1/4” drill bit

31/64” drill bit

3/4” drill bit

1” dril bit

1.25” drill bit

1.40” drill bit

1500

750

500

375

200

150

Mill grooves 1/4” mill 1300

Mill 3/8” counter bores 3/8” mill 850

Mill 3/4” counter bores 3/4” mill 400



-

File outside edges File -

Deburr holes and grooves Deburr -

Al 6061 +

Bearing (x 2)

Arbor

Press

Press fit bearings into holes - -

42

Top Supporter


Top

Supporter

Al 6061


- File outside geometry File -

Mill

Face mill outside

geometries

1/2” mill 650





- Deburr holes Deburr -

Al 6061 +

Bearing

Arbor

Press

Press fit a bearing into

1/2” hole

- -

43

Bottom Supporter

Part Material Machine Process Tool(s) Speed

(rpm)

Bottom

Supporter

Al 6061

Water

Jet

Cut outside geometry - -

File Deburr outside geometry - -

Mill

Face mill outside geometries 1/2” mill 650




Ream 1/2” diameter holes 1/2" ream 200

- Deburr holes Deburr -

Al 6061 +

Bearing

Arbor

Press

Press fit a bearing into 1/2”

hole

- -

44

Top spacer x 2


(rpm)

Top

Spacer Al 6061

Vertical

Band

Saw

Cut down aluminum round stock to

2.5”

- 400

Lathe

Face out one end flat and smooth

and set the reference

Lathe tool 1500

Turn out diameter by a bit then

measure diameter

Lathe tool (point

cutting)

Caliper (measure

length)

1500

Turn the outside diameter to 0.500” Lathe tool 1500

Center drill #4 center drill 1500

Drill a through 1/4” hole 1/4” drill bit 1500

Face the piece to length of 1.400” Lathe tool 1500

Deburr outside edges on lathe on

both side

File 200

Shamfer the holes on both side Shamfer tool 400

45

Bottom Spacer x 2


Bottom

Spacer Al 6061

Vertical

Band Saw


2.5”

- 400

Lathe



Lathe tool 1500


measure diameter

Lathe tool (point

cutting)

Caliper

(measure length)

1500


Center drill #4 center drill 1500

Drill a through 1/4” hole 1/4” drill bit 1500



both side

File 200

Shamfer the holes on both side Shamfer tool 400

The through hole is tapped.

46

Hard Stopper x 2


Bottom

Spacer Al 6061

Vertical

Band Saw


2.0”

- 400

Lathe



Lathe tool 1500


measure diameter

Lathe tool (point

cutting)

Caliper

(measure length)

1500




both side

File 200

47

Motion Generation Assembly Instructions

Linkage mechanism is generally assembled before assembly the transmission components. At joints

connecting linkages, and pulleys, we first place sandwiches of two thrust washers plus one thrust bearing.

And at the contact surface of a bolt and any surface we also have one or two thrust washers. The specific

number of components needed at each joint can be seen the exploded view pictures. Two examples of

joint configurations are seen in the pictures below. The order of assembling the linkages are first putting

the connections at the right spots, placing the thrust washers and bearings in between contact surfaces,

and placing the bolt through, and then locking the nut with a wrench. For joints with double supports, the

double supports and their spacers are placed and secured with their respective bolts and nuts before

putting the joint shoulder bolt in. For the joint connecting the input link and the ground link, make sure to

place the pulley at the top before putting the bolt in. A shoulder bolt meant for transferring motion from

the pulley to the input link must also be put in and secured with a nut. Finally, our backpack holder is

bolted and nut-locked onto the coupler link with four bolts. The configurations can be seen clearly in the

figures provided below.

48

Input link to ground joint cross section Input link to coupler joint cross section

49

Front Explode

Back Explode

APPENDIX C

Appendix C provides detailed information for preliminary final design’s energy conversion and

transmission system. This includes detailed design drawing of each part on the system, assembling

instruction of the parts, and bill of materials (BOM).

Energy conversion system drawings and manufacturing plan

The detailed drawings and manufacturing procedures table are attached in the following pages

2

Motor Mount The manufacturing plan for this part is attached on next page.

3

Pulley Flange x 2


Pulley

Flange Al 6061


- File outside geometry File -

Mill





4

Motion Generation Transmission Assembly Instructions

For the transmission, first secure the motor onto the angle bracket that came with the motor. Secure the

small pulley onto the motor shaft with a set screw, making sure that the screw sets on a flat surface of the

shaft. Secure the angle bracket onto the motor mount, and place the U-bolt at a middle-lower section of

the motor, and secure with nuts. Afterwards, put the timing belt onto the two pulleys. Push the motor

mount in the direction away from the big pulley, tensioning the belt, then secure the bolts which fasten the

motor mount to the ground link. The configurations can be seen in the figures below

Top view

5

APPENDIX D

This Appendix section explains the final design’s control and safety system and its integration to the

physical linkage system. The wiring diagram, controller code with brief comments, design drawing for

proximity sensors mount, and bill of materials (BOM) are included below.

Control and safety system wiring

Figure D-1: Wiring diagram

2

Controller Code

// ME350 Project Sketch_Red H-bridge

// sketch for mobile backpack carrier

// set the values of variables that affect the performance

int lowerCountThreshold=1980; // count was fine-tuned in lab. On the way back (backward)

int upperCountThreshold=11800; // count was fine-tuned in lab. On the way there (forward)

int proxSwitch1Threshold=80; // a value between 0 and 1024

int proxSwitch2Threshold=120; // a value between 0 and 1024

int motorSpeedHigh=240; // a value between 0 and 255

int motorSpeedLow=220; // a value between 0 and 255

//set the initial values of variables

int proxSwitch1=0;

int proxSwitch2=0;

int rockerSwitchForward=0;

int rockerSwitchReverse=0;

int limitSwitchForward=0;

int limitSwitchReverse=0;

volatile long encoderCount = 0;

//set the values of variables that represent pin numbers

const int proxSwitch1Pin = 3;

const int proxSwitch2Pin = 2;

const int rockerSwitchForwardPin = 5;

const int rockerSwitchReversePin = 6;

const int limitSwitchForwardPin = 7;

const int limitSwitchReversePin = 8;

const int pwmPin = 9;

const int i1Pin = 10;

const int i2Pin = 11;

const int encoderPinA = 2;

const int encoderPinB = 4;

void setup(){

//declare which digital pins are inputs and outputs

pinMode(rockerSwitchForwardPin,INPUT);

pinMode(rockerSwitchReversePin,INPUT);

pinMode(limitSwitchForwardPin,INPUT);

pinMode(limitSwitchReversePin,INPUT);

pinMode(encoderPinA,INPUT);

pinMode(encoderPinB,INPUT);

pinMode(pwmPin,OUTPUT);

pinMode(i1Pin,OUTPUT);

pinMode(i2Pin,OUTPUT);

//turn on pullup resistors

digitalWrite(encoderPinA,HIGH);

digitalWrite(encoderPinB,HIGH);

3

// (the other sensors already have physical resistors on the breadboard)

//set interrupt for encoder pins

attachInterrupt(0,doEncoder,CHANGE);

//set initial speed of motor to 0

analogWrite(pwmPin,0);

//begin serial communication for display of variable states

Serial.begin(9600);

Serial.println("start");

}

void loop(){

proxSwitch1=analogRead(proxSwitch1Pin);

proxSwitch2=analogRead(proxSwitch2Pin);

rockerSwitchForward=digitalRead(rockerSwitchForwardPin);

rockerSwitchReverse=digitalRead(rockerSwitchReversePin);

limitSwitchForward=digitalRead(limitSwitchForwardPin);

limitSwitchReverse=digitalRead(limitSwitchReversePin);

if ((proxSwitch1<proxSwitch1Threshold)&&(proxSwitch2<proxSwitch2Threshold)) { // if both prox

switches are not made

if (rockerSwitchForward==HIGH){ // if rocker switch is being pressed forward

digitalWrite (i1Pin,LOW); // set direction of motor to be forward (i1 pin)

digitalWrite (i2Pin,HIGH); // set direction of motor to be forward (i2 pin)

if (limitSwitchForward==HIGH){ // if the forward limit switch is made


Serial.print("The forward limit switch is made. Count:"); // print this to the serial monitor


}

else if (encoderCount < upperCountThreshold){ // if the encoder count is below the upper count

threshold

analogWrite (pwmPin,motorSpeedHigh); // run motor at high speed

Serial.print("Direction: Forward Speed: High Count:"); // print this to the serial monitor


}

else{ // otherwise

analogWrite (pwmPin,motorSpeedLow); // run the motor at low speed

Serial.print("Direction: Forward Speed: Low Count:"); // print this to the serial monitor


}

}

else if (rockerSwitchReverse==HIGH){ // if rocker switch is being pressed backward

digitalWrite (i1Pin,HIGH); // set direction of motor to be reverse (i1Pin)

digitalWrite (i2Pin,LOW); // set direction of motor to be reverse (i2Pin)

if (limitSwitchReverse==HIGH){ // if the back limit switch is made


encoderCount == 0; // setting encoder count to 0 everytime it hits the back rocker switch.

Serial.print("The back limit switch is made. Count:"); // print this to the serial monitor

4


}

else if (encoderCount>lowerCountThreshold){ // if the encoder count is above the lower count

threshold

analogWrite (pwmPin,motorSpeedHigh); // run motor at high speed

Serial.print("Direction: Reverse Speed: High Count:"); // print this to the serial monitor


}

else{ // otherwise

analogWrite (pwmPin,motorSpeedLow); // run the motor at low speed

Serial.print("Direction: Reverse Speed: Low Count:"); // print this to the serial monitor


}

}

else {

analogWrite(pwmPin,0); // stop the motor

Serial.println("Rocker switch is not being pressed."); // print this to the serial monitor

}

}

else{ // if at least one of the prox switches is made

analogWrite(pwmPin,0); // stop the motor

Serial.println("There is an object in the way."); // print this to the serial monitor

}

}

void doEncoder(){

if( digitalRead(encoderPinA) == digitalRead(encoderPinB) ){

encoderCount--;

}

else{

encoderCount++;

}

}

Threshold Calculations

Lower count threshold (

) (

) (

) (

)

=

Upper count threshold = (

) (

) (

) (

)

=

5

Sensor Mount


(rpm)

Sensor

Mount Al 6061

Vertical

Band

Saw

Cut down aluminum angle stock to

the approximate geometry as shown

above

- 400

File File the surface for surface finish File -

Sand

paper

Sand the filed surface to improve

surface finish

Sand paper #200 -

Documents

ME 350 Final Report Mobile Backpack Carriers3images.coroflot.com/user_files/individual_files/...The main purpose of this report is to present the different components of our overall