Arm Rehabilitation Medical Device Design Report Volume II · PDF fileArm Rehabilitation...

Arm Rehabilitation Medical Device

Design Report—Volume II

Team Members

Anne Halverson Brandon Peterson

Bruce Weldon Lawrence Formosa

Ludong Sun Yashovardhan Sand

Industry Sponsor

Sister Kenny Rehabilitation Institute/Sister Kenny Research Center

ARM REHABILITATION 1| P a g e

Table of Contents

1. PROBLEM DEFINITION SUPPORTING DOCUMENTS ............................................................... 2

1.1 ANNOTATED BIBLIOGRAPHY ..................................................................................................... 2

1.1.1 SUMMARY ................................................................................................................................. 2

1.1.2 WORKS CITED .......................................................................................................................... 2

1.2 PATENT SEARCH ............................................................................................................................ 4

1.2.1 OBJECTIVE ................................................................................................................................ 4

1.2.2 SEARCH CRITERIA .................................................................................................................. 4

1.2.3 FINDINGS ................................................................................................................................... 4

1.3 USER NEED RESEARCH ................................................................................................................. 7

1.4 CONCEPT ALTERNATIVES ........................................................................................................... 9

1.4.1 CRANK-SLIDER ........................................................................................................................ 9

1.4.2 FOUR BAR MECHANISM ........................................................................................................ 9

1.4.3 GRAVITY ASSISTED DEVICE .............................................................................................. 10

1.5 CONCEPT SELECTION .................................................................................................................. 11

2. DESIGN DESCRIPTION SUPPORTING DOCUMENTS .............................................................. 12

2.1 MANUFACTURING PLAN ............................................................................................................ 12

2.1.1 MANUFACTURING OVERVIEW .......................................................................................... 12

2.1.2 PARTS DRAWINGS ................................................................................................................ 13

2.1.3 BILL OF MATERIALS ............................................................................................................. 29

2.1.4 MANUFACTURING PROCEDURE ........................................................................................ 29

3. EVALUATION SUPPORTING DOCUMENTS ............................................................................... 32

3.1 EVALUATION REPORTS .............................................................................................................. 32

3.1.1 SAFETY .................................................................................................................................... 32

3.1.2 ASSISTIVE FORCE .................................................................................................................. 34

3.1.3 STRUCTURAL INTREGITY ................................................................................................... 36

3.1.4 VARIABILITY .......................................................................................................................... 45

3.1.5 REPEATABILITY..................................................................................................................... 46

3.2 COST ANALYSIS ............................................................................................................................ 47

3.3 REGULATORY AND SAFETY CONSIDERATIONS .................................................................. 47

4. WORKS CITED ........................................................................................ Error! Bookmark not defined.


1. PROBLEM DEFINITION SUPPORTING DOCUMENTS

1.1 ANNOTATED BIBLIOGRAPHY

1.1.1 SUMMARY

With the growing elderly population and the predicted rise in strokes [1][2], much effort has been put into

post-stroke rehabilitation. Robotic assistance devices [3], [5], [6], [7], and [8] are very popular among the

different devices available. The major drawback of robotics devices is price and the footprint, which may

be large and prevent patients buying the device and practicing at home. Relatively cheap and simple

devices [4], [9] have been developed recently, though they will not provide assistance. Our design

involves developing a mechanism [10] for arms to move forward and backward, and we have studied

mechanisms in currently available devices to aid in designing our own. Simultaneously our device needs

to be able to store energy initially to provide force assistance; several energy storage methods, including

constant force spring [11], magnet and fluid power, were considered.

1.1.2 WORKS CITED

[1] Zieve, David, 2012. “Stroke”, from the internet:

http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001740/

This website gave background information on strokes.

[2] National Stroke Association, (no date). “Stroke 101 Fact Sheet”, from the internet:

http://www.stroke.org/site/DocServer/STROKE_101_Fact_Sheet.pdf?docID=4541

This fact sheet, put out by the National Stroke Association, describes all the types of strokes and different

symptoms to look for in the event of a stroke.

[3] Interactive Motion Technologies, 2012. “InMotion Arm”, from the internet:

http://interactive-motion.com/clinical-solutions/upper-extremity-rehabilitiation/inmotion-arm/

This website gives information on the inMotion Arm robotic rehabilitation device. This product has a

very extensive list of possible exercises to perform.

[4] Development of the Tailwind stroke rehabilitation device, from the internet:

http://www.tailwindtherapy.com/pdf/White-Paper-2-Engineering-Development-Jan09.doc

The Tailwind is one of the most basic arm rehabilitation devices currently on the market. It has an

adjustable range of motion, and also can be set up for the patient to reach upwards and outwards. There is

no assistance involved in the device, so the patient is providing all forces of motion.


[5]Y. Ren, H-S Park, L.Q. Zhang, "Developing a whole-arm exoskeleton robot with hand opening and

closing mechanism for upper limb stroke rehabilitation," Rehabilitation Robotics, 2009. ICORR 2009.

IEEE International Conference, vol., no., pp.761,765, 23-26 June 2009

The device introduced in this paper is a robotics device. When robots implement assistive training,

patients are often asked to grip a handle tightly, which may induce strong hand muscle contractions with

the hand at an abnormal posture.

[6]L.Q. Zhang, Hyung-Soon Park, Y. Ren, "Developing an Intelligent Robotic Arm for Stroke

Rehabilitation," Rehabilitation Robotics, 2007. ICORR 2007. IEEE 10th International Conference, vol.,

no., pp.984,993, 13-15 June 2007

This is a robotics device which helps stroke patients with shoulder, elbow and wrist motion

simultaneously.

[7] P.S. Lum, C. G. Burgar, H.F. Van der Loos, “The use of a robotic device for post-stroke movement

therapy,” presented at International Conference on Rehabilitation Robotics, Bath, U.K.

A robot for patient-initiated, therapist-supervised movement therapy

[8]Armeo therapy concept, Internet: http://www.hocoma.com/products/armeo/,[March.5, 2013]

This is a commercial available robotics assistance arm rehabilitation device.

~~~

The following references are related to assistance-free and mechanism-only arm rehabilitation device.

The mechanisms involved in these designs had new ideas to pursue.

[9]Mobile arm support Jaeco, Internet:

http://www.rehabmart.com/category/Mobile_Arm_Support_Jaeco.htm, [March. 5, 2013]

[10]A. Erdman., G. Sandor, and S.Kota, Mechanism Design: Analysis and Synthesis, Volume I, Fourth

Edition. Upper Saddle River, NJ: Prentice-Hall, 2001.

This textbook covers the basics about design, analysis and synthesis of many mechanical linkages. Our

process of mechanical design may refer to this book.

[11] Wikipedia contributors, Internet: http://en.wikipedia.org/wiki/Constant-force_spring, [MARCH.5

2013]

This webpage introduces the concept of a constant force spring.

[12] G. Syring, FDA Regulation of Medical Devices, Internet:

http://www.qrasupport.com/FDA_MED_DEVICE.html [March. 5.2013]

http://www.hocoma.com/products/armeo/,%5bMarch.5

http://www.rehabmart.com/category/Mobile_Arm_Support_Jaeco.htm

http://en.wikipedia.org/wiki/Constant-force_spring

http://www.qrasupport.com/FDA_MED_DEVICE.html


This webpage gives an overview of medical device classification by the FDA

[13] Shoulder Complex, Internet: http://www.pt.ntu.edu.tw/hmchai/Kines04/KINupper/Shoulder.ht,

[March. 5 2013]

This webpage has a large overview of how the shoulder functions and what muscle groups perform

different motions. It is necessary to understand these functions to correctly design the rehab device

1.2 PATENT SEARCH

1.2.1 OBJECTIVE

The first objective of a patent search is to get ideas about what solutions already available for our problem

and to understand the motivation of the proposed device(s). After designing a new device, a patent search

is then used to find if a new idea or design is patentable; also, there is the benefit of gaining new ideas

from previous patents and integrating those in our own design. By doing a comprehensive patent search,

there is less risk of mistakenly duplicating current others' intelligent property.

1.2.2 SEARCH CRITERIA

In order to find as much information as possible, both the Google and USPTO patent search engines were

used. The keywords used were "arm rehabilitation device" and "arm rehab". By using "arm rehabilitation

device” any previous rehabilitation devices were found directly. By using "arm rehab", the results gave

other related information. Use of the word rehabilitation is not used explicitly, due to high number of

devices which would have patents and used for some form of rehabilitation, though this search was also

performed.

1.2.3 FINDINGS

From the patent search, there are two major types of arm rehabilitation devices; assistance-free and

assisted. Most devices which provide assistance utilize robotics. For example, there is a recently patented

device called “robotics rehabilitation apparatus.” The device is an 8+2 degrees of freedom, intelligent

rehabilitation root, capable of controlling the shoulder, wrist, and fingers individually, and allowing

functional arm movements with accompanying drunk and scapular motions. This device is the most

recent and most developed of those found. Although we were not allowed to use robotics in our design,

the idea that the device can not only help with shoulder flexion and elbow extension but also help with

rehabilitation of hands and wrist was also integrated for consideration.

The other type of patent was assistance-free, but all patents of assistance-free devices were filed around a

decade ago. Most new patents all involve robotics, since they are very convenient and controllable. The

device introduced below was patented in 1989, and is similar to the tailwind arm rehabilitation device.

However, this device involves a feature to measure the angle of motion, and there is a clock to count time.

http://www.pt.ntu.edu.tw/hmchai/Kines04/KINupper/Shoulder.ht


For this device, the forearm is cradled in a movable lever on a mounting plate hinged to a base plate in

such a manner that arm movement at various angles between horizontal and vertical exercise all the

muscles in the shoulder. The angle(s) that may be prescribed by a therapist results in exercising part of the

muscles in the shoulder in each position as well as muscles in the arm. One specification of our device is

that ideally the range of motion would be adjustable, which is very similar to this feature of this patent.

Based on the patent search shown here, our proposed device will not risk duplicating any current patent

by mistake, primarily because our proposed device can provide assistance, but does not have electrical

components. The biggest concern of our device is energy storage.

This section contains the front page of all relevant patents found during our patent search.


1.3 USER NEED RESEARCH

To determine the user needs of our design, we consulted our project advisors, a rehabilitation specialist

and an engineer with experience designing medical devices. Through multiple interviews we were able to

establish a list of user needs, which we then ranked based on their relative importance. Table 1 shows the

results of this ranking process, with needs of high importance ranked as a nine, needs of moderate

importance ranked as a five, and needs of lower importance ranked as a one.

Table 1- Design Needs with Importance Rankings

# Need Importance

1 Rehab device should be low cost 9

2 Rehab device should require minimal setup 9

3 Rehab device should be safe to use 9

4 Rehab device should allow for continuous repetition 9

5 Rehab device should be small 5

6 Rehab device should be able to be clamped to table and/or wheelchair 5

7 Rehab device should be completely electronics free 9

8 Rehab device path of motion is appropriate 9

9 Rehab device is reliable/consistent 9

10 Rehab device can be used by either right or left arm 5

11 Rehab device is easy to manufacture 5

12 Rehab device assists motion going forward (flexion) 9

13 Rehab device assists motion going backward (extension) 1

14 Rehab device is lightweight 5

15 Rehab device requires little training 5

16 Rehab device allows user to adjust level of assistance 9

17 Arm rest of device should be able to be used by any arm 9

18 Arm rest of device should be comfortable 9

19 Rehab device reduces weight of arm during motion 5

20 Rehab device should be easy to store 5

21 Rehab device should be easy to transport 5

22 Arm rest should have arm straps and grip handle 9

23 Range of motion should be adjustable 5

The next step was to convert these user needs into very specific and measureable design specifications, as

shown in Table 2. To ensure only necessary design requirements are included, each requirement

corresponds to at least one user need, as shown in the left column of the table. Upon completion of this


table we had specific requirements from which to base our design on. After the design had been finalized

and a prototype was built, these metrics were used to evaluate the design.

Table 2- Design Specifications

Need #’s Metric Importance Units Marginal Value Ideal Value

5, 14, 21 Device weight 5 lbs 10-40 10-25

5, 20, 21 Device geometry: height 5 in 0-12 6-10

5, 20, 21 Device geometry: footprint 5 In*in 10*10-25*25 14*14-17*17

2 Initial set up time 9 min 30 10

2 Set up time (each exercise) 5 min 5 <1

15 Training time 5 min 60 30

4, 9 Min. cycles per exercise 9 integer 50 200

12 Assistance on flexion 9 lbs 3-5 4-5

13 Assistance on extension 1 lbs 0-1 0

12 Force required: flexion 9 lbs 1-5 1-2

13 Force required: extension 9 lbs 3-5 3-4

1 Cost to produce 9 US $ 10-50 10-30

8 Full flexion distance 9 in 18 12

17, 18 Arm rest: width 9 in 2-4 3

17, 18 Arm rest: length 9 in 12-18 16

17, 18 Arm rest: grip circumference 9 in 2-4 3

17, 18 Arm rest: strap length 5 in 4-8 7

17, 18 Arm rest: strap width 5 in 2-3 3

6 Table mount max span 5 in 3-6 5

5, 14, 21 Overall device volume 5 in3 1500-3500 2600

11, 12, 7 Maximum frictional coef. 5 integer 0.5 0.3

4, 9 Product lifetime 9 years 5-10 10

17 Used by both arms 5 binary yes/no yes

7, 12, 13 Efficiency (Fin, Fout) 5 % 25% 40%

3 Class 1 medical device 9 binary yes yes

16 Assistance is adjustable 9 binary yes yes

23 Range of motion is adjustable 5 binary yes yes


1.4 CONCEPT ALTERNATIVES

1.4.1 CRANK-SLIDER

This possible design is a crank-slider mechanism [7] driven by a constant force torsional spring [8]. The

spring would be wound up and used to drive the crank, while the arm would rest on the slider link and be

driven by the crank. A simple crank-slider is shown in Figure 5.

Figure 5 – An example of a crank-slider mechanism which would be used in this design.

Roller bearings would be used under the arm rest in order to reduce the force(s) of friction. The benefits

of this design are the linear motion obtained, the consistency of the input force, and the fact that both

motions, forward and backward, are assisted. The possible setbacks are the size of the crank that would

be necessary to get the full range of motion, and the cost of the constant force spring, the bearings, and

materials necessary would most likely drive the cost up above $100 retail.

1.4.2 FOUR BAR MECHANISM

One way to assist the arm in achieving shoulder flexion would be to use a four bar mechanism, as shown

in Figure 3. This mechanism would be clamped to a side of table. The user would sit next to the table

with the edge of the table parallel to the user’s arm. The armrest would start at a ten degree angle from

the horizontal. As the arm is moved forward, the armrest would rotate through the middle position and to

the final armrest position, as shown in the figure.


Figure 6- Four Bar Mechanism for shoulder flexion assistance

Although this design would remove a large amount of the weight of the arm, it would not provide direct

assistance to the user in either direction of motion. Springs or some other element would need to be

added to create the necessary assistance in moving the arm.

The upside of this design is that it would be very efficient. There would be very little friction to take

energy out of the system. If coupled with an energy storing element, it could potentially be a solution to

the problem, and was a concept used for the final design.

1.4.3 GRAVITY ASSISTED DEVICE

Gravity was considered as the energy input for the system. As shown in Figure 7, the downward path of a

weighted cable (weight is distributed throughout cord, not concentrated at end) would provide an

increasing level of assistance.

Figure 7- Simple design of the Gravity Assisted Device


1.5 CONCEPT SELECTION

In order to select the ideal design concept, a series of comparisons were done with the other alternatives.

The most important design specifications were selected with the input of industry experts. For each

concept alternative, an estimated score was given for that design’s performance for that specification; this

is shown in Table 3.

Table 3- Selection Criteria and Process for Final Design

Design Specification Four Bar Mechanism Crank-Slider Gravity Assisted Device

Assistance on Flexion 5 4 5

Low Resistance on

Extension

5 4 2

Class I Medical Device 5 5 5

Easily Repeatable 5 2 2

Adjustable Assistance 4 2 3

Ideal Path of Motion 4 5 5

Approximate Cost 5 2 5

Total Score 33 27 24

As the scoring of the design concepts show, the four bar mechanism is the highest scoring, and so was

then taken onward as the final design concept.

A large factor in selecting a design was the ability to provide the correct level of assistance throughout the

motion of the arm. When a counter weight is added to the four bar mechanism design, the potential

energy of all links, the counter weight, and the patient’s arm can be brought very close to zero at any

given time through the motion, as shown in Plot 1. This was a key indication that the four bar mechanism

design would be able to provide an adequate level of assistance.


Plot 1- Energy analysis versus angular position

2. DESIGN DESCRIPTION SUPPORTING DOCUMENTS

2.1 MANUFACTURING PLAN

2.1.1 MANUFACTURING OVERVIEW

A major constraint on our model and final product is cost. Since this arm rehabilitation device

classifies as a medical device, the mark-up on the product (if it were to go into full-scale

production) would be of the order of five to seven. Our goal is to make a product cheap enough

to be purchased without straining the customer buying power.

To optimize the cost and reduce labor time, a significant number of the parts to be used were

purchased off the shelf. The rest are steel or aluminum parts which were machined and all

plastic parts were also machined from plastic rods.

This design is a modified four bar mechanism with an additional fifth link; a hanging support

was used to mount the model to a table. The coupler assembly consisted of multiple components.

The vertical fasteners were machined Al 6061 cylindrical rods. The arm rest and bottom support

link were machined Al 6063-T52 blocks. All nuts, washers, bolts, screws, bearings, slider

carriage, and slider track were pre-made ordered parts from McMaster Carr. The other

mechanism links were machined Al 6061. The counter weight used was an Al 6061 tab inserted

in the fifth link. An Al 6061 frame was used to cover the links below the table, to add to the

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Ener

gy (

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les)

Angle of Theta (degrees)

Energy Analysis

Energy of Arm and Links Couter Weight Energy Total Energy


aesthetics, and give the device a clean look, all while adding to the structural stability of the

device. Any attachments that need to be made between components will be done using a 3M

product, Scotch Weld Epoxy Adhesive. This two-part compound provides structural strengths

that are better than welding, and it is less labor intensive when compared with welding.

The manufacturing and assembling process is relatively simple due to the simplicity of the

model. The presence of rods, bearings, nuts, and bolts makes it very easy and intuitive to

assemble. The detailed part list given below will make the assembly easier to understand.

2.1.2 PARTS DRAWINGS

The parts drawings are included in the next 15 pages of the document. A summary of the parts

is shown below in Table 4, followed by the entire device assembly, and the subassemblies or

parts.

Table 4- Summary of Parts in Device


2.1.3 BILL OF MATERIALS

Table 5- Bill of Materials for Device Prototype

ITEM NO. Description Qty. Price/unit Total Price

1 frame for mechanism (12”x16”, .125” thick Al 6061)

1 n/a 32.79

2 6383K11 (3/16” Bearing) 3 3.06 9.18

3 slider piece and guide rail 1 n/a 48.21

4 counterweight link (2”x4”x14”, 1/8” thick AL)

1 n/a 12.97

5 6383K49 (3/4” Bearing) 3 6.93 20.79

6 Connecting rod for arm, green and counterweight (3/4”x1’, Steel)

2 10.63 21.26

7 green link (3/4”x10.78, .065” thick Al) 1 n/a 6.33

8 link piece for end of all but special link

2 1.17 2.34

9 pin for top bearings 2 3.59 7.18

10 link piece for bottom supporting rod 2 7.89 15.78

11 Red link (3/4”x10.97, .065” thick Al) 1 n/a 7.53

12 bottom link 1 n/a 11.93

13 arm rest support 1 n/a 7.66

14 Vertical links for fastening 2 5.54 11.08

15 90490A033 nut 8 .08 0.64

16 91083A033 washer 8 .05 0.40

17 92198A722 SS bolt 12 1.13 13.56

18 Hanging Mount Rods 2 4.76 9.52

19 U mount 1 n/a 39.82

20 mount for top pins 2 12.11 24.22

21 91465A110 Bolt 2 0.30 0.60

22 93827A211 Nut 2 0.07 0.14

23 91083A029 SS Washer 2 0.12 0.24

24 under table mounts 4 4.76 19.04

25 under table levelers 3 8.72 26.16

Total 349.37

2.1.4 MANUFACTURING PROCEDURE

A. Coupler Assembly

1- Arm rest support, and bottom link are machined from Al 6063-T52 according to


part drawings.

2- Vertical supports (x2) made of Al 6061-T6 are purchased.

3- Nuts (x4), washers (x4), and screws (x4) are purchased (bill of materials shows

part numbers).

4- Holes are drilled into vertical supports, arm rest support, and bottom link as shown in

part drawings.

5- The vertical supports are fastened to the arm rest support and bottom link using the

screw, washer, and bolt, as shown in the assembly drawing.

B. 4-Bar Mechanism

1- The red and green links, as we have called them in this report, are to be purchased

from a manufacturer (dimensions are shown in product specs), and are made of Al

6061-T6.

2- The connecting rod and pins are also purchased, and are made of steel.

C. Slider assembly

1- The slider carriage (part number 9728K31) and slider track (part number 9728K6)

are purchased from a manufacturer.

D. Counter Weight

1- The counterweight link is machined from Al 6061-T6 according to part drawings.

E. Hanging Mount

1- The U mount is machined from Al 6061 according to parts drawings.

2- Under table threaded rod mounts are purchased from McMaster, these are bolted

onto the Hanging Mount Rods as prescribed in the parts drawings.

3- Levelers are used as pads for under the table, and should be inserted into the threaded

rod mounts.

F. Device Frame

1- The device frame is machined from Al 6061-T6 and is adhered to the hanging

mount in the position as shown in the part drawings.

2- The slider track is attached to the frame with epoxy adhesive.

G. End-Link Pieces


1- Item number 8, the end-link for the top joints of the 4-bar mechanism, is machined

from ABS rods according to parts drawings.

2- Item number 10, the end-link for the bottom joints of the 4-bar mechanism, is

machined from ABS rods according to parts drawings.

H. Device Assembly

1- Two of the smaller bearings, part number 6383K11, are inserted into the

large mounting holes of the mechanism frame and held in place with epoxy

adhesive.

2- One each of item number 8 is slid onto the ends of the red and green links, and

epoxy adhesives are used to assemble.

3- One each of item number 10 is slid onto the other end of the red and green links, and

epoxy adhesive is used to assemble.

4- Two of the larger bearings, part number 6383K52, are attached to the holes in both of

item number 10, the end-links, using epoxy adhesive.

5- Two of the larger bearings, part number 6383K52, are attached to the two large holes

on the arm rest support, by using epoxy adhesive.

6- The final large bearing, part number 6383K52, is placed in the hole of

the counterweight link, and are held in place using epoxy adhesive.

7- One of item 6, the connecting rod for the 4-bar mechanism, is inserted through the

large bearing on the end of the green link, the bearing of the counterweight link, and

one of the bearings of the arm rest support, as shown in the assembly drawing. The

ends are attached to the inner bearing surface using epoxy adhesive.

8- The second item 6 is inserted through the large bearing on the end of the red link,

and through the second bearing of the arm rest support, and is attached to the inner

bearing surfaces using epoxy adhesive.

9- The third small bearing, part number 6383K11, is inserted onto the pin which

rests on the slider carriage, and attached using epoxy adhesive.

10- The outer surface of the third small bearing is then attached to the large hole on the

back of the counterweight link, and assembled using epoxy adhesive.

11- The end-links still free on the red and green links (part number 8), is placed over the

two small bearings on the device frame.

12- Two small pins, item number 9, are inserted through the end-pieces and also the


bearings and the end of the pins are fastened using epoxy adhesive.

13- The entire device is now assembled, and the adhesive should dry for 24 hours

before the device can be considered ready to use.

3. EVALUATION SUPPORTING DOCUMENTS

3.1 EVALUATION REPORTS

3.1.1 SAFETY

INTRODUCTION

The safety and comfort of a medical device to be used in therapy is critical. This device is built for

numerous repetitions and possibly to be used on a daily basis, too. Since this device is a potential for

stroke patients, the comfort and safety, although binary evaluation criteria, can be deciding factors for the

purchase of this device by patients, hospitals, and other concerned parties. Hence a comprehensive

evaluation plan is set up to qualitatively quantify these criterions. The evaluation plan is a coupling of a

questionnaire and prototype testing by a diverse sample of participants. This will give us a better gage of

the usability of the device; this evaluation is to check the safety and hence aid in the identification of

potential problems that may be experienced by patients. Based on our test results, we can take proper

actions such as modifying and or removing unsafe parts.

METHODS

The method of our test is to invite a diverse group of participants composed of patients, advisors,

therapists, fellow classmates, but most importantly people who are not aware of the functioning of the

device (i.e. people for whom the device functionality may not be intuitive). The last category is the most

essential as their testing of the device will not biased; other participants might be slightly biased in the

sense that they might try to overcompensate during a repetition to safeguard the device since they would

probably know the basic structure of the device. All engineers and advisors testing the device would be

able to gage pinch points and potential locations of higher stress. Stroke patients and people unaware of

the device would go through a learning curve to adapt to the device. This would be a real test of the

robustness and resilience of the device. The target number of people would be 20.

The test would involve 200 repetitions per participant. A post-test questionnaire will be handed to each

participant to rate their experience. The questionnaire may also help with evaluating other specifications

such as assistance force.

QUESTIONNAIRE

1. Is this device intuitive to use?


2. On a scale of 1 to 5, where 1 means "poor", 2 means "fair", 3 means "average", 4 means "good",

and 5 means "excellent", how would you rate your feeling of comfort about this device?

1 2 3 4 5

3. Any specific discomfort? Please elaborate.

4. (If you are a patient) do you think force assistance provided by the device large enough to aid

with the shoulder flexion?

5. What feature(s) of the device would you change?

6. What feature(s) add(s) to the comfort and safety of the device?

7. Are there any pinch points which hinder the arm motion?

8. Does the device pose a safety hazard?

9. Do you feel at any point during a repetition that the device might not be stable?

10. Are there any sharp edges which pose potential safety issues?

We will collect and compile the responses from the above questionnaire and evaluate the mean score for

question 1. Only a mean score of four or is acceptable to us. For any lower score than four, we will make

the changes suggested by participants and advisors. The questionnaire is designed to get a high and low

level perspective response from the participants to ensure customer safety, comfort, and satisfaction.

RESULTS.

Average score for question 2: 4.5

Important advice and suggestions:

It was mentioned in multiple questionnaires that the device is rather hard to lift up (for transporting). The

handle could be optimized for comfort, as could the padding on the arm rest. Participants did not notice

an added assistance at any point in the device’s motion, but they did feel that the exercise was easy to

perform. Many felt that the motion was easier than just moving an arm forward. This implies that the

device does provide some assistance or at least compensates for a portion of the arm weight and makes

the motion easier.

DISCUSSION

6 test subjects were utilized for our analysis. Although more would have been ideal, the results from

these five people indicate that the comfort and safety of the device are not concerns. The intuitive nature


of the device is also good because it is clearly an arm device, and after placing an arm on the device, it

was clear to participants the motion and exercise that should be performed.

This type of analysis would be more telling if there had been more participants of varying ages. The

results are important because they show that there is not any risk involved in the simple act of the device.

The people tested are all of good health and strength, however, and it would be good to have a person

with reduced strength attempt to use the device and see how they find the motion to be

3.1.2 ASSISTIVE FORCE

INTRODUCTION

The assistive force provided to the user of the device is one of the most important design specifications,

as the entire purpose of the device is to allow the patients without a lot of physical strength to practice

arm flexion. The dimensions of the final design were chosen with the goal of minimizing the amount of

force the patient has to input while the arm is being lifted away from the body. The purpose of this test

was to measure the actual force assistance of the device and verify that an appropriate amount of

assistance is provided to the user throughout the entire path of motion.

METHODS

The method of testing the force assistance provided by the device was similar to the method used to create

the energy analysis used in design selection and analysis. Similarly to the last analysis, the device was

analyzed at 11 positions of the arm that are equally spaced throughout the path of motion. The key

difference between the two analyses is that in this experiment we were no longer interested in the height

of the links and counter weight at each position. What we were interested in was the assistive force

provided to the user at each position.

To measure the force provided to the user at each position, we first had a person of average size rest their

arm on the device to provide a load similar to that of an average patient. Without the user applying any

force to support their arm or the move the device, we then used a force gauge to pull the mechanism into

static equilibrium in at the desired angle of theta, as defined in Figure 8. The force gauge was held

parallel to the armrest at all times. The force measured by the force gauge will be equal and opposite to

the force assistance of the device at any given location.

This method of static analysis depends on a few key assumptions. The first assumption is that the inertia

of the device in negligible. The second major assumption is that the static assistive force is equivalent to

the true assistive force when the mechanism is in motion.


RESULTS

The assistive force at each position is shown in Plot 2.

Plot 2- The Force Assistance Provided at Each Angular Position

-3

-2

-1

0

1

2

3

556065707580859095100105

Forc

e A

ssis

tan

ce (

lbs)

Angle Between Link 1 and Horizontal (degrees)

Force Assistance Curve

Figure 8- A diagram of the setup used to find the assistive force at the starting position.


DISCUSSION

The force curve found through this evaluation shows unexpected results. The energy analysis suggested

that the most assistance would be provided near the end of the motion when the arm was nearly fully

extended, but the force analysis curve actually shows that the end of the motion requires the largest input

force. The friction associated with the transmission angle of the sliding joint has a large effect on the

force assistance at the beginning of the motion. This additional friction may actually be beneficial to the

user since it requires a small input of force to initiate the motion, which will help teach the patient how to

activate the correct muscles.

The maximum force required to move the device at any position is very important due to the limited

strength of the user. This experiment showed that the largest force input required was about 2.5 pounds,

compared to an ideal specification of less than 2 pounds and an acceptable specification of less than 5

pounds. Therefore, the design has provided an adequate level of assistance to the user.

3.1.3 STRUCTURAL INTREGITY

INTRODUCTION

To determine an overall picture of how the device responds when stressed, a simplified model was run

through an ANSYS (version 14.0) Static Structural [1] analysis, modeled near the natural resting position

of the device where the patient would be able to apply the most load on the device.

METHODS

To run this analysis, we were limited to the student version of ANSYS, and this limited the refinement of

the mesh used in the finite element analysis to less than 256000 nodes, or approximately 200000

elements. To meet this limitation, all screws, washers, and nuts were removed from the model. Because of

how ANSYS treats static structures, removal of screws used to fix parts together in reality has little effect

on the results, as the software assumed a rigid connection between parts designated as having surface

connections and linked together as was done in the analysis. This may change how stresses are

transmitted between parts in the simulation versus reality, but this should be a minimal difference

considering the proximity of screws in reality to these new stress routes through the material in the

simulation, and will only amplify the stresses in these parts, thus providing an added safety factor.

Next, bearings in the model were replaced with titanium disks to best approximate the effects of the

bearings on their surrounding parts, while minimizing the number of elements required in modeling them.

Finally, much of the supporting structure used to balance the model under a table was removed, primarily

to lower the node count. To minimize the effects of this, the rectangular aluminum supports, which the

support structure connects to, were modeled as having fixed top surfaces, meaning the simulation would

not allow these surfaces to move. In reality these struts may have some non-negligible bending moments

associated with the tested loads, and some of the hardware used to fix the device to a table will have

associated stress concentrations that may not be negligible. Again, due to element limitations, this was

deemed the best option for simulating the stresses in the device.


For maximum loading, a downward force of 35 pounds being input by the patent on the arm rest and a

counterweight force of 15 pounds were used, or approximately twice the expected maximum loads. The

patient input force was centered on the top surface of the armrest, and the counterweight force was

centered inside the round counterweight mounting tube.

Figure 10- Simplified Model

RESULTS

Starting with the overall deformation of the device, the largest deviation of any point on the device from

its unloaded position is 2.3mm, represented in red in Figure 11.


Figure 11- Model Deformation

From Figure 11, we can see that the overall weakness of the device in terms of bending lies with the

position of the red link. Because the green link is connected to the blue link on the same bar as the

counterweight beam, it is supported and prevented somewhat from bending, whereas the red link has no

such additional support. To counter this, the red link could be moved in closer to the protective cover

provided it doesn’t interfere with the counterweight’s rotational path, but because the deformation is only

2.3 millimeters this does not represent a fatal flaw in the design and should not provide any safety or

failure risk.

A second reason for this particular deformation is the choice of material, ABS, used to connect the red

link to its bearing. Focusing on this in Figure 12, a noticeable bending of the material does occur.


Figure 12- Red-Link ABS Deformation

Though the deformation shown appears significant, the resulting stress is negligible. As shown in Figure

13, a Von-Mises stress in the ABS is comparatively negligible, and was found to be less than even 1 MPa,

which is far less than the listed 48 MPa yield strength [2].

Figure 13- Red-Link ABS Equivalent Stress


Despite this low stress, creep failure could not be immediately dismissed. Using a standard creep

modeling equation [3], and published values for ABS [4], creep was found to be negligible at both one

hour and 1000 hours, with resulting magnitudes of strain at a stress of 1 MPa.

( ) ( )

Though creep failure was numerically found to be negligible, should any imperfection such as a surface

crack or stress concentration factor such as a sharp corner cut be exacerbated, the ABS could still suffer

unexpected failure. To correct this potential, the sharp cut edges of the connecting ANS part should either

be rounded-off (outwards), or the thickness in which the bearing sits should be thickened.

Figure 14- Red-Link ABS Equivalent Strain

Fortunately, Figure 14 shows that the natural strains from the load, not from creep, would not occur in the

same location as creep strains. Based on the strain pattern shown, the strain is resulting from bending

tension between the bearing and the larger section of ABS where the red link sits (above the red creep line

in Figure 14).

Next looking at an overall picture of the Von-Mises stresses in the model in Figure 15, it is apparent that

around the counterweight link more stress occurs despite the smaller deformation it experiences.

Primary

Creep Strain

Location


Figure 15- Equivalent Stresses in Simplified Model

Zooming in on this link in Figure 16, there are two notable stress concentration areas. The first along the

steel rod has a maximum value of 35.1 MPa, and because this is far below any common steel yield

strengths [5], it was deemed safe.

Figure 15- Equivalent Stress around Counterweight Link


The second is the concentrations on the aluminum counterweight link, with maximum values shown to be

49.1 MPa, which is roughly half of common aluminum yield strengths [5]. Given the lesser bending this

link exhibits comparatively, this was again deemed safe, with a built in safety factor of at least 4.

Next the slider attaching the counterweight link to the protective cover plate was examined, as shown in

Figure 16 on the counterweight link and in Figure 17 by itself.

Figure 16- Counterweight Slider

Figure 16 clearly shows the highest stresses yet seen in the model, at just over 70 MPa; again this is still

far below any common published steel yield strengths [5]. Given how ANSYS modeling of the problem

works though, this value may be elevated. Because the all the parts were constrained to only deform in the

simulation and the contact surface couldn’t change, and thus not slide as could happen in reality, these

values are suspect. Regardless, a safety factor of over 8 results and this part was deemed safe in this

analysis.

Next, to be complete shear stress was investigated at the slider, shown in Figure 18.

Figure 17- Slider Equivalent Stress


Figure 18- Slider Shear Stress

Again an elevation of stress is shown, but in this case the value was 16.7 MPa. For both the steel pin and

the aluminum guide frame, this is below published yield stresses [5], and is safe.

Finally the Von-Mises stress in the pin connecting the red link to the rectangular support structure was

examined, shown in Figure 19.

Figure 19- Red Link Pin

Because the red link was previously found to have larger deformation, the green link pin will have lower

stress values and examination of just the red link pin was sufficient. With maximum stresses of 35 MPa, a

large safety factor results and the pins are safe for extended use.


Because the slider exhibited the most stressed state, care should be taken especially there if new loading

schemes are later developed.

DISCUSSION

The analysis of the structural integrity of this device showed a lot of potential deformation due to the

bending load. To account for this, an improvement to the device prototype could be made. By adding

two additional links (another red and green link) on the opposite side of the device frame, as shown in

Figure 20, the device would no longer be under such extreme bending conditions.

Figure 20- Diagram of Plane for Additional Links


The plane indicated by the red rectangle in Figure 20 is where the additional links are added. The

addition of these new links constrains the device so that the bending cannot occur. The working

prototype which was constructed contained these two extra links.

3.1.4 VARIABILITY

INTRODUCTION

This device should be able to easily be adjusted for any patient. Although adjustability was not able to be

incorporated into this design, an evaluation on the effect of variables such as patient weight and height

was desired. In order to determine the effect of changing these variables, a static force analysis was

performed similarly to section 3.1.2. An “average” person was included for comparison, as well as two

people above both height and weight average, and two below.

METHODS

To measure the force provided to the user at each position, we had the person rest their arm on the device

to provide a load similar to that of a patient their size. Without the user applying any force to support

their arm or the move the device, we then used a force gauge to pull the mechanism into static equilibrium

in at the desired angle of theta, as defined in Figure 8. The force gauge was held parallel to the armrest at

all times. The force measured by the force gauge will be equal and opposite to the force assistance of the

device at any given location.

This method of static analysis depends on a few key assumptions. The first assumption is that the inertia

of the device in negligible. The second major assumption is that the static assistive force is equivalent to

the true assistive force when the mechanism is in motion.

RESULTS

Figure 21- Force Assistance of Varied Patient Weight/Height

-3

-2

-1

0

1

2

5060708090100110

Stat

ic A

ssis

tive

Fo

rce

(lb

s)

Angle of Deflection (degrees)

Force Assistance- Varied User Weight and Height

125 lbs, 5' 3''

150 lbs, 5' 7''

AVERAGE (165 lbs, 5' 10'')

185 lbs, 5' 11''

200 lbs, 6' 2''


DISCUSSION

As is shown in Figure 21, there is a relationship between the patient size and the force assistance

provided. As patients get larger, the assistance provided is greater. This could be due to the increased

weight of their arm, and also a difference in the energy of their arm, because the initial height and height

change will be greater throughout the motion.

Most significantly, the results show that there is not a big difference between the assistance for people of

different sizes. The implications of this are that there may not be a necessity of having adjustable

assistance. The users of extreme size (either small or large) did not complain about the path of motion

seeming unnatural.

3.1.5 REPEATABILITY

INTRODUCTION

In order for this device to be effective, a patient must be able to consistently perform repetitions of the

exercise. Reconnecting neurological pathways requires a high number of reiterations, so this was an

important design specification to evaluate.

METHODS

To evaluate the repeatability of this device, multiple people (5) were asked to use the device for a

minimum of 200 repetitions. After the user completed the repetitions, they will be asked if at any point

they felt the device resist, or lock. Through their comments and observations of the user during their

repetitions, the repeatability and the reliability of the device can be assessed.

RESULTS

There were 5 people who were asked to participate in this part of the evaluation. Table 6 summarizes the

results of their trials.

Table 6- Repeatability Study Results

User Did device lock? Comments

1 no Initiation of motion required force input

2 no The motion was extremely easy to repeat

3 no

4 no No problem at any point

5 no

DISCUSSION

As the results section shows, there was no instance of device locking for any user. This means that all the

people tested were able to repeat the exercise for at least 200 repetitions. For the purposes of this

evaluation, this number of successive repetitions is sufficient. It is reasonable to assume that a patient


will not be using the device for much more than 200 repetitions. If they do, it is also reasonable to

assume that the risk of the device locking up will not increase as the repetitions increases.

This evaluation was effective in showing that there are no points of excess friction or of extreme

transmission angles, both of which would cause the device to either lock up or resist motion.

3.2 COST ANALYSIS

The cost of this device will be significantly lower than the prototype cost. The prototype cost about $400

in materials; however, this cost will be greatly reduced by bulk purchasing of the raw materials and

choosing valuable suppliers. This device can be expected to have a production run of a few hundred to a

few thousand due to the significant price decrease compared to competitors. The material cost of the

bolts, nuts and screws will drop significantly as well as the non-machined metals. By choosing quality

suppliers, the raw material price of this device should be closer to $100.

Since this device has many parts, assembly and machining will likely bump up the cost. By assuming a

$50/hour machining/assembly cost, the device will most likely take 3 to 4 hours to machine and assemble.

If 4 hours is assumed, the total price will increase to $300. Although this is more expensive than desired,

it is still $1,000 under the market price of the cheapest competitor.

The actual market price of this device will be significantly cheaper than the closest competitor making it

desirable in the stroke rehabilitation market. This device can begin to start bringing more affordable

options for patient’s and rehab centers. It will likely be a popular solution for patients and rehab centers

because it provides much desired cost savings. By entering the market at a lower price than competitors

and serving the same function, it can pressure other companies to start making more affordable solutions.

This will help take some financial stress off the families of stroke patients and drive innovation towards

cheaper solutions.

3.3 REGULATORY AND SAFETY CONSIDERATIONS

As defined by the FDA, a medical device is an instrument, apparatus, implement, machine,

contrivance, implant, in vitro reagent, or other similar or related article, including a component

part, or accessory which is intended for use in the diagnosis of disease or other conditions, or in

the cure, mitigation, treatment, or prevention of disease, in man or other animals, or intended to

affect the structure or any function of the body of man or other animals, and which does not

achieve its primary intended purposes through chemical action within or on the body of man or

other animals and which is not dependent upon being metabolized for the achievement of any of

its primary intended purposes [6]. Based on this definition medical devices are further classified

into three sub-categories, namely, Class I, Class II, and Class III. Our device is classified as a

Class I medical device since it is simple in design, easy to manufacture, and is safe to use as

there are no sharp or potential heavy and harmful parts. This device is non-life sustaining and

does not have contain any electronics which is evident form the prototype.


By theory this device does fall under a Class I category but a simple test plan is needed to

confirm this classification. Once a prototype is built it will be put in use by multiple test subjects

including patients, therapists, and engineers. Each subject will perform a minimum of 200

repetitions to check the robustness of the design and if there is a potential safety hazard. They

will focus on the placement of parts and observe if any parts come in the way of the arm forward

or reverse motion. Testing the prototype with multiple subjects will give us an idea of the wear in

the components from which we can understand if there might be a potential risk of injury. Since

energy is supplied into this device by a patient and due to the presence of friction between

bearings, links, and joints, this is not a perpetual motion device is not life sustaining since the

primary function is of arm rehabilitation, and does not deal with cardiac effects.

If there are no significant problems experienced or observed in the device after the testing phase

is complete, and there are no traces of electronic components, this device can be classified as a

Class I medical device.

4. WORKS CITED

[1] "Structural Mechanics." Structural Mechanics Solutions. ANSYS, n.d. Web. 04 May 2013.

<http://www.ansys.com/Products/Simulation Technology/Structural Mechanics>.

[2] "Thermoplastics - Physical Properties." Thermoplastics - Physical Properties. The Engineering

Toolbox, n.d. Web. 04 May 2013. <http://www.engineeringtoolbox.com/physical-properties-

thermoplastics-d_808.html>.

[3] Callister, William D., Jr., and David G. Rethwisch. "Chapter 15, Viscoelastic Creep." Materials

Science and Engineering: An Introduction 8th. New York: Wiley, n.d. 578. Print.

[4] "ABS MOLDED." ABS MOLDED. 3D CAM, n.d. Web. 04 May 2013. <http://3d-

cam.com/ABS_MOLDED.html>.

[5] "Elastic Properties and Young Modulus for Some Materials." Elastic Properties and Young

Modulus for Some Materials. Enginnering Toolbox, n.d. Web. 04 May 2013.

<http://www.engineeringtoolbox.com/young-modulus-d_417.html>.

[6] “Medical Devices.” Is The Product a Medical Device, n.d. Web. 05 May 2013.

<http://www.fda.gov/medicaldevices/deviceregulationandguidance/overview/classifyyourdevice/

ucm051512.htm>

http://www.fda.gov/medicaldevices/deviceregulationandguidance/overview/classifyyourdevice/ucm051512.htm

http://www.fda.gov/medicaldevices/deviceregulationandguidance/overview/classifyyourdevice/ucm051512.htm

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