Final Report :: Team Touchtech Final Project

8/9/2019 Final Report :: Team Touchtech Final Project

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This report is the final document detailing Team Touchtech’s design of an

intuitive, wearable skin-stretch feedback device with two degrees of freedom,

which helps restore the sense of touch to prosthesis users.

Submitted to Rice University Engineering Capstone Design Course 2014-2015

on May 1, 2015 by

Zach Bielak

Hannah Chen

Kensey King

Ehren Murray

Melissa Yuan

on behalf of

Team Touchtech

Mechanical Engineering Department, Rice University

Financial Sponsor:


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List of Figures ...................................................................................................................................... 4

List of Tables ........................................................................................................................................ 6

Executive Summary ............................................................................................................................. 7

1 | Introduction ..................................................................................................................................... 8

1.1 Prosthesis Usage without Haptic Feedback ............................................................................. 8

1.2 Current Haptic Technology Solutions ..................................................................................... 9

1.3 Theories Pertaining to Skin-Stretch ....................................................................................... 11

Level of Perception ...................................................................................................................... 11

Level of Intuitiveness ................................................................................................................... 13

Degree of Comfort ....................................................................................................................... 13

1.4 Market Analysis ....................................................................................................................... 14

Market Opportunity of Prosthesis Users ...................................................................................... 14

Other Potential Market Opportunities .......................................................................................... 16

Market Analysis Conclusions ...................................................................................................... 17

1.5 Customer Needs ....................................................................................................................... 17

User Needs ................................................................................................................................... 18

Payer Needs .................................................................................................................................. 18

Regulations and Standards ........................................................................................................... 18

Customer Needs Conclusions ...................................................................................................... 19

1.6 Original Design Specifications ................................................................................................ 19

Functionality Specifications ......................................................................................................... 19

Safety and Comfort Specifications .............................................................................................. 20

Ease of Use Specifications ........................................................................................................... 20

Cost Specifications ....................................................................................................................... 21

1.7 Conclusion and Final Report .................................................................................................. 22

2 | Design Strategy ............................................................................................................................. 23

2.1 Chosen Prototype and Implementation Plan ........................................................................ 23

2.2 Meeting Design Criteria .......................................................................................................... 30

2.3 Problem Decomposition .......................................................................................................... 32

2.4 Design Criteria for Decision Analysis .................................................................................... 32

Design Criteria for Feedback Device ........................................................................................... 32Design Criteria for User Input Mechanism .................................................................................. 33

2.5 Decision Analysis for Feedback Device .................................................................................. 35

Brainstorming Methodology ........................................................................................................ 35

Perceptibility in 2 DOFs .............................................................................................................. 36

Comfort ........................................................................................................................................ 37

Size ............................................................................................................................................... 37


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Design Simplicity ......................................................................................................................... 37

Processing Time ........................................................................................................................... 38

Intuitiveness ................................................................................................................................. 38

Pugh Matrix Analysis for Feedback Device ................................................................................ 38

2.6 Decision Analysis for User Input Mechanism ....................................................................... 39

Brainstorming Methodology ........................................................................................................ 39

Disconnect between input and gripper motion ............................................................................ 39

Disconnect between input and feedback motion .......................................................................... 40

Required magnitude of input motion ........................................................................................... 40

Pugh Matrix Analysis for User Input Mechanism ....................................................................... 40

2.7 Project Schedule and Team Responsibilities ......................................................................... 41

3 | Final Design ................................................................................................................................... 42

3.1 Subsystems ................................................................................................................................ 42

Housing ........................................................................................................................................ 42

Contact Piece ................................................................................................................................ 44

Electronics .................................................................................................................................... 44

3.2 Pilot Testing .............................................................................................................................. 47

Degrees of Freedom ..................................................................................................................... 47

Magnitudes ................................................................................................................................... 48

Comfort ........................................................................................................................................ 48

Durability ..................................................................................................................................... 49

3.3 Implementation ........................................................................................................................ 50

Demonstration .............................................................................................................................. 50

Regulations ................................................................................................................................... 50

Applicable Standards ................................................................................................................... 51

4 | Testing and Results ....................................................................................................................... 52

4.1 DOF Testing ............................................................................................................................. 52

4.2 Object Testing .......................................................................................................................... 54

4.3 Desensitization Testing ............................................................................................................ 56

4.4 Comfort Testing ....................................................................................................................... 56

5 | Summary and Recommendations................................................................................................ 58

5.1 Current Status .......................................................................................................................... 58

5.2 Design Features ........................................................................................................................ 58

5.3 Potential Improvements .......................................................................................................... 59

5.4 Next Steps ................................................................................................................................. 59

References ........................................................................................................................................... 61

Document References .................................................................................................................... 61

Figure References .......................................................................................................................... 62

Appendices.......................................................................................................................................... 63


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Figure 1: Prosthesis users are forced to constantly watch their prosthesis to control them ........... 9

Figure 2: Vibration pads, shown on the user’s arm, provide vibrational feedback ...................... 10

Figure 3: Skin-stretch visualization; Neutral position, no stretch applied ................................... 10

Figure 4: Skin-stretch visualization; linear skin-stretch at 9.3mm displacement ......................... 11

Figure 5: The amount of free nerve endings in hairy (nonglabrous) skin make it a good

candidate for sensing the tangential forces of skin stretch feedback ..................................... 12

Figure 6: A sample application of multi-DOF skin stretch feedback, in this case applied to the

finger with 2 degrees of freedom ........................................................................................... 12

Figure 7: Various sizes and shapes of skin stretch applicators .................................................... 13

Figure 8: Example of a complex and expensive prosthetic arm created by the Applied Physics

Laboratory at Johns Hopkins University ................................................................................ 15

Figure 9: A picture of the 1992 Teleoperations Control Station for NASA’s Jet PropulsionLaboratory .............................................................................................................................. 16

Figure 10: Da Vinci Surgical System ........................................................................................... 17

Figure 11: The haptic feedback device must be wearable, comfortable, and safe in addition to

achieving its functional goals ................................................................................................ 20

Figure 12: Design intention of gripper claw with position encoder and force pads..................... 23

Figure 13: Actual gripper claw with integrated slide potentiometer controller, position encoder,

and force pad resistor ............................................................................................................. 24

Figure 14: Preliminary design intention of rocker with gripping contact pad ............................. 24

Figure 15: Rotational motion of contact pad from bottom view, corresponding to position ....... 25

Figure 16: Linear stretch using rocking motion, corresponding to force applied to gripper ........ 26

Figure 17: Preliminary wooden housing prototype and assembly ............................................... 27

Figure 18: Preliminary design intention for integrated housing and casing ................................. 28

Figure 19: Intended design of integrated gripper/slide potentiometer device .............................. 28

Figure 20: Sparkfun Inventor RedBoard with prototyping wires attached .................................. 29

Figure 21: Functional decomposition of system .......................................................................... 32

Figure 22: Signal flow between user input, gripper motion, and haptic feedback ....................... 34

Figure 23: Brainstorming sketches of final design candidates ..................................................... 35

Figure 24: Design candidate ideas shown in Figure 23a and 23c ................................................ 36

Figure 25: Housing (isometric view) ............................................................................................ 42Figure 26: Underside of prototype housing and contact piece ..................................................... 43

Figure 27: Contact piece. .............................................................................................................. 44

Figure 28: RedBoard microcontroller and associated circuitry. ................................................... 45

Figure 29: The slide potentiometer that controls the aperture of the gripper servo ..................... 46

Figure 30: The gripper servo assembly. ....................................................................................... 46

Figure 31: A sample drop test from 1 meter ................................................................................ 49


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Figure 32: DOF Testing being performed by team member ........................................................ 53

Figure 33: DOF testing results ..................................................................................................... 54

Figure 34: Objects used for Object Testing .................................................................................. 54

Figure 35: Object testing setup ..................................................................................................... 55

Figure 36: Object (active) testing results ..................................................................................... 56

Figure 37: The final prototype of Team Touchtech’s design ....................................................... 59


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Table 1: Summary of User and Payers Needs .............................................................................. 18

Table 2: Original Design Specifications (for Final Target Specs, see Table 3) ........................... 21

Table 3: Cycle II Target Specifications (as modified from the Original Design Specs) .............. 31

Table 4: Pugh Matrix for Feedback Design .................................................................................. 39

Table 5: Pugh Matrix for User Input Mechanism ......................................................................... 40

Table 6: Degree of Freedom Testing (n=5) .................................................................................. 47

Table 7: Magnitude Testing for Rocking, 4 Signals (n=3) ........................................................... 48

Table 8: Magnitude Testing for Rotation, 5 Signals (n=3) ........................................................... 48

Table 9: Comfort Results throughout Pilot Testing (n=6) ............................................................ 49

Table 10: Target Specifications to Secure FDA Approval ........................................................... 50

Table 11: User-defined Comfort Scale ......................................................................................... 57


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34&51+.6& 7188#29Team Touchtech is a group of five senior Mechanical Engineering students at Rice University whohave developed a skin-stretch feedback device that provides haptic feedback from a prosthetic hand

system with two degrees of freedom (DOFs). This device will facilitate proprioceptive feedback to prosthesis users, allowing for improved dexterity and motor control of the user’s prosthesis. Theneeds for this device include:

• Currently, when an upper-body limb amputee controls a prosthetic device, feedback to the

user is primarily provided by sight, inhibiting intuitive motor control.•

Research has tested vibratory and auditory signals to serve as feedback for the user, but

neither method is effective in practical usage. Users complain that vibratory feedback isirritating and desensitizing, while audio feedback is intrusive to their daily lives.

• Research has also begun to explore skin-stretch as a method of haptic feedback, but currently

only single-degree of freedom solutions exist. Because these systems only exploit one DOF,the amount of useful information they provide to a user is severely limited.

• Due to the high degree of focus and attention required to use their prosthesis, about 44% of

prosthesis owners ultimately choose not to use their prosthetic limb at all.

After consulting with the director of the Orthotics and Prosthetics Clinic of the Baylor College ofMedicine, Mr. Jared Howell, the Team identified several additional constraints. These include:

• The device must provide noninvasive feedback to the user• The device must be able to be worn comfortably with an existing prosthetic limb• The device must be relatively small and lightweight, yet durable enough to endure daily use• The device must fit a full range of users of different sizes

This device will ultimately be used by MAHI research lab at Rice University to conduct further

research on the applications of 2-DOF skin-stretch haptic feedback. Dr. Amy Blank of the MAHI labhas tasked the team with several additional design objectives listed below in order of importance:

•

Ability to discriminate between two distinct DOFs• Ability to distinguish between different degrees of signals in each DOF•

Ability to accurately and consistently determine feedback signals (at least 70% accuracy)• Employ feedback signals that are intuitive and easy to learn• Keep the design low cost

After a year of prototyping and testing, the Team developed a wearable and intuitive 2-DOF hapticfeedback device that relays feedback using only noninvasive skin-stretch on the user’s upper or lower

arm. A circular and highly frictional contact piece provides the two distinct DOFs—rotational and

linear skin-stretch—with a high degree of perceptibility and user comfort. Through these DOFs, prosthesis users are able to accurately determine both the aperture of their prosthetic hand (via therotation of the contact piece) and the amount of force applied to gripping an external object (via thelinear motion of the contact piece). The user can be trained to understand the extent of the rotation orlinear motion produced by the device, and map that feedback to the operation of their prosthetic limb.

At this point, the final device prototype has been constructed and rigorously tested on 20 subjects.The results of this testing indicate the device has met all of the original design objectives and

constraints identified. Due to its success, the final device prototype will be a featured demonstrationat the 2015 World Haptics Conference in Chicago.

As the formal conclusion of this project, this Final Report summarizes the efforts of the team over

the course of the past year. It presents an extended problem context, the resulting design criteria, afinal design overview, the results of extensive testing, and concluding recommendations for the

project (including its transition to the MAHI lab for further research and development.


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et. al., 2012). Such a high dependency on visual cues requires focused attention to apply the

appropriate muscle force in order to accomplish tasks. Individuals who lack proprioception use

“slow, ponderous movements...and tend to use excessive force to hold objects.” Furthermore,

because even simple tasks require a high level of focus from the individual, complex tasks

involving both significant cognitive attention as well as fine motor control (eg. writing notes

while listening) are often beyond the ability of prosthesis users (Robles-de-la-Torre, 2006).

Figure 1: Oftentimes, without any other form of sensory feedback

prosthesis users are forced to constantly watch their

prosthesis to control them effectively

Without either position or force feedback, prosthesis users suffer a major impairment in

motor control, which extends to a variety of skills that have become significantly more difficultand must be relearned. These difficulties are further exacerbated if visual information is

unavailable (eg. dark environments).

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Sensory information can be artificially relayed to prosthesis users with position and force

feedback device systems worn on the body. Current systems vary in both the feedback

mechanism and in the type of information they convey.

Most systems that have undergone testing to date are still in development; commerciallyavailable solutions are typically limited to vibrational feedback devices that provide information

about gripped objects (O’Malley, 2011). However, prolonged use of vibrational feedback can

lead to desensitization, and vibrations become less perceptible when the user is in motion (Bark

et. al., 2010). Therefore vibrational feedback is well-suited for short-term cues, but not for the

continuous feedback that is often required for proprioception and gripping. Electrocutaneous

feedback, which evocates tactile sensation with electric current flowing through the skin of the


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user, has also been explored for upper limb prosthesis users, but a major concern with this

mechanism is the discomfort or pain it causes for the wearer (Gurari et. al., 2012).

Figure 2: Vibration pads, shown on the user’s arm, provide

vibrational feedback, which can often lead to user

desensitization in the arm

In an attempt to resolve these shortcomings, various feedback mechanisms involving the

controlled stretching of skin (“skin-stretch”) have been studied (Figures 3 and 4). Skin-stretch is

an attractive concept for research in haptic feedback because the receptors in skin are sensitive to

tangential forces. Additionally, skin-stretch feedback allows for much greater flexibility in the

design for feedback mechanisms than either vibrational or electrocutaneous feedback. The mostcommon specific means of skin-stretch feedback are linear stretching (pulling/pushing) and

rotational stretching (twisting), and these methods have been studied in previous research (Liang

et. al., 2014 & Bark et. al., 2010).

Figure 3: Skin-stretch visualization; Neutral position, no stretch applied

Source: Bark “Rotational Skin Stretch Feedback”


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Figure 4: Skin-stretch visualization; linear skin-stretch at 9.3mm displacement

Source: Bark “Rotational Skin Stretch Feedback”

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Level of Perception

As this academic interest in skin-stretch interfaces continues to grow, research groups

have begun devising theories regarding the most effective applications of skin-stretch devices.

Researchers have found that, under optimal conditions, skin-stretch feedback provides a high

level of perception when compared to other modes of feedback. Using skin-stretch devices, users

can accurately sense both linear and rotational motion. Depending on the location of the device

on the user’s body, high spatial resolutions are possible—as small as 0.13 mm linearly and 15

degrees rotationally (Robles-de-la-Torre, 2006). Specifically, researchers have found that the

forearm is ideal for perceiving such fine movements, as its nonglabrous (hairy) skin contains

receptors that are highly sensitive to both tangential forces and displacement (Robles-de-la-

Torre, 2006). Due to this high perceptibility, skin-stretch devices can employ small motorswithout compromising accurate feedback, allowing for the fabrication of “compact, low power,

wearable” devices (Robles-de-la-Torre, 2006).


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Figure 5: The amount of free nerve endings in hairy (nonglabrous) skin make it a good candidate for

sensing the tangential forces of skin stretch feedback

Source: Mark Cutkosky, Stanford

Furthermore, skin-stretch feedback can exploit both static and moving components to

provide richer information to the user. If the feedback device utilizes two or more contact areas,

both the relative and absolute motions/positions of the contact areas can be perceived as well.

Specifically concerning the perception of velocity, it has been found that feedback is most

accurately perceived at low speeds (Robles-de-la-Torre, 2006).Skin-stretch devices also allow for multiple DOFs to be employed. For prosthetic users,

adding more DOFs to feedback “significantly enhances [the] speed and accuracy” of performing

tasks (Robles-de-la-Torre, 2006). To complement these increased spatial DOFs, designers have

also been able to optimize a variety elements of skin-stretch feedback, including the location of

stretch, size of contact point(s), normal force applied, method of attaching applicator, and

number of contact points employed (Robles-de-la-Torre, 2006).

Figure 6: A sample application of multi-DOF skin stretch feedback, in

this case applied to the finger with 2 degrees of freedom


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Figure 7: Various sizes and shapes of skin stretch applicators

used by researchers at Stanford

However, research has highlighted that this high level of perception begins to decrease

when skin conditions change during use. As the body moves, skin may stiffen or release sweat,

increasing the minimum stretch necessary for a user to accurately detect feedback. Additionally,

too many different signals may interfere with one another, causing feedback confusion for the

user.

Level of Intuitiveness

Although skin-stretch feedback is often initially perceived as “more difficult” during

tests, it can become intuitive with training, as demonstrated within the span of short research

experiments (Bark et. al., 2010). Skin-stretch is a natural physiological mode of proprioception,

helping one to perceive “the location of one’s limbs in space and how they are moving,” and has

been found to be more intuitive for users “even if the feedback is not physiologically accurate”.

Skin-stretch is especially intuitive when it is performed near the joint of interest and when it

responds to an effort actively performed by the user. However, studies show that visual feedback

will still “dominate over kinesthetic cues when visual-haptic mismatch exists” (Bark et. al.,

2010).

Degree of Comfort

Skin-stretch haptic feedback is noninvasive, allowing for comfortable operation.

However, in choosing the placement of the skin-stretch device on the user, a position must be

found that is both comfortable and adequately sensitive. Additionally, recent research has

investigated three different modes of interacting with the skin: bare contact (i.e., with a roller

bearing), frictional grip (i.e., with a rubber pad), and adhesion. Although contact points of the

latter category provide a stronger “sensation of intensity,” they can become uncomfortable after

prolonged or vigorous usage (Robles-de-la-Torre, 2006). Furthermore, some studies highlight the

disadvantages of continuous haptic feedback, which often becomes uncomfortable for the user.

Skin-stretch feedback is prone to causing this type of discomfort if used excessively, although to

a lesser extent than vibratory feedback (Robles-de-la-Torre, 2006).


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Figure 8: Example of a complex and expensive prosthetic arm created

by the Applied Physics Laboratory at Johns Hopkins University

Many amputees and limb-loss patients find their prosthetic limbs cumbersome andrelatively useless due to lack of controllability and feedback. In fact, many patients stop using

their prosthesis altogether due to these issues, despite the high cost of fitting and purchasing a

prosthetic limb (Schweitzer, 2010). Currently, very few commercial solutions are available to

prosthesis users. The majority of these only employ vibrational or auditory feedback, both of

which prompt complaints from users due to discomfort and rapid desensitization (Bark et. al,

2008). Based upon this information, our team identified a strong “willingness to pay” within the

prosthesis patient community for an intuitive haptic feedback technology. Based on the richer

information that our feedback device would provide and the corresponding increase in intuitive

motor control, we estimate that around 5% of prosthetics patients would purchase an additional

skin-stretch feedback component. This estimation is based on existing market data of the numberof prosthesis users who buy accessories for their prosthesis. Therefore, in total, we evaluated the

potential market size of prosthetics users to be $202.5 million in the U.S..

A number of informed assumptions were made in this analysis. First, both upper- and

lower-limb loss patients were considered, despite that we plan for our device to be primarily used

in conjunction with upper-limb prosthetics. This assumption was made because both upper- and

lower-limb prosthesis users face the same challenge of inadequate motor control, and both would

be willing to purchase an extra feedback device to increase the intuitive controllability of their

prostheses. Furthermore, only amputees were considered (not those born without limbs) as these

are the patients most likely to struggle with rehabilitation and re-learning motor skills (Robles-

de-la-Torre, 2006). Finally, the market analysis was limited to only the U.S. primarily because

prosthetics information is not uniformly tracked internationally, complicating the accurate

estimation of a global prosthesis audience size (Gailey, 2008). Furthermore, due to the high costs

of prostheses, our team believes the U.S. prosthesis community would be a substantially-sized

and willing market to begin with, before expanding globally.


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Other Potential Market Opportunities

Aside from prosthesis users, the team has considered several other market opportunities

that may find use for haptic feedback. One of these is physical therapy patients who have either

suffered nerve loss in the hands or are rehabilitating motor control. In the U.S., the populations

of these two types of patients are approximately 7,350 and 374,000 people respectively, ascalculated from Bureau of Labor Statistics data (“Physical Therapists”, 2014). As the haptic

feedback devices for these patients would similarly cost around $3,000, this segment represents a

market size of $34.4 million.

Companies and institutions pursuing explorative teleoperations also demonstrate a need

for haptic feedback to improve motor control of remote robotic devices. Precise manipulation is

vital for missions into space, underwater habitats, and toxic waste cleanup sites, and our device

could help enhance this manipulation. Currently there are 75 major teleoperations companies

across the globe, and our team assumed each would be willing to pay around $357,000 (akin to

the amount NASA is spending) to collaborate with our team and further develop the feedback

device (Brown, 2014). In total, this amounts to a $1.33 million market size.

Figure 9: A picture of the 1992 Teleoperations Control Station for

NASA’s Jet Propulsion Laboratory. Teleoperations systems like this

could tremendously benefit from improved sensory feedback to the operator

Additionally, haptic feedback has a high potential to be integrated into virtual reality

environments. Integrating rich sensory cues such as touch creates a more immersive experience

the user. “Innovators/Hard gamers” and “Early adopters/Light gamers” are forecasted to account

for up to 15% of the U.S. general population in the next three years, amounting to 30 million

users (“Consumer Virtual Reality”, 2013). We believe we can reach a 1% market share with a


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$200 haptic feedback device that would enhance these virtual interactions, creating a market

segment size of $90 million.

Finally, the integration of accurate haptic feedback has been highly desired by the 1500

hospitals currently employing robot-assisted surgery across the U.S. (Brown, 2014 and Kolata,

2010). By providing surgeons with more intuitive and accurate tactile confirmation of their

actions, haptics could improve both the safety and success of these operations. Assuming that

such a haptic feedback mechanism would cost 15% of the price of a da Vinci Surgical System

(the de facto industry standard), this amounts to a market segment size of $15.64 million (Lee,

2014).

Figure 10: Da Vinci Surgical System. Robot-assisted surgery

is one of several areas outside of prosthetics which could

benefit from sophisticated tactile feedback

Market Analysis ConclusionsIn total, all of these potential markets indicate a total market size of $343.9 million.

However, Team Touchtech has decided to focus mostly on the market segment of prosthesis

users, due to its relatively large size of $202.5 million.


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User Needs

The user needs for this device are functionality, wearability, and ease of use.

Functionality is rated 4 because by definition it is a necessary condition for success. The

components of wearability are safety and comfort. The need for safety is crucial in any medical

device, and discomfort is cited as one of the most common sources of dissatisfaction with prosthetics, which leads to reduced usage of the device (Webster et. al., 2012). Wearability is

therefore rated 3, indicating that it is crucial for success as a supporting feature, but is not a part

of the feedback functionality. Intuitiveness is rated 2 because learning difficulty could deter

some users, but would not limit the long-term effectiveness of the device. Table 1 summarizes

the user needs and importance ratings determined by the team.

Table 1: Summary of User and Payers Needs

Need Description Importance

Rating

Functional Provides feedback signals with multiple DOF andmagnitudes; each signal is clearly recognizable

4

Wearability Comfortable and safe to attach to body 3

Intuitiveness Quickly learnable 2

Affordability Inexpensive and durable 1

Payer Needs

Commercial and government insurances only cover 20-50% of the cost of prostheses, and

prostheses often require replacement due to wear and constantly improving technology (“Your

Medicare Coverage”, 2014). Furthermore, prices of prostheses have been called “horrendously

expensive” by some users (Schweitzer, 2014). Therefore, the attached feedback device should

further increase costs as little as possible. Low manufacturing cost is the most direct method of

achieving this need, but durability is also desirable, as it will cut long-term replacement and

repair costs.

Because feedback is such a significant enhancement to prostheses, the device will likely

will be successful in the market of people who can afford prostheses; relative to the large cost of

a prosthetic, the device will almost certainly be inexpensive. Therefore, the combined needs of

affordability are rated 1 (Table 1), including that, while desirable, these features are not a priority

for the feedback device.

Regulations and Standards

Because worn feedback devices are currently in the research phase, there are very few

industry standards or regulations applied to the device. Once commercialized, the device is likely

to be classified as a type I device, which faces minimum regulatory requirements such as 510(k)


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submission, FDA approval, and general control requirements (Resnik et. al., 2010). These

requirements are not likely to influence the device design because they are mainly for

commercial listing purposes. Also, existing industry standards, such as ISO 22523:2006

“External limb prostheses and external orthoses”, should be consulted when applicable (“ISO/TC

168”, 2014). Meeting these requirements is not rated for our design purpose; however, the design

should still meet safety requirements from the FDA, and therefore the team has incorporated

these requirements into the user needs.

Customer Needs Conclusions

Customers, including the users, payers, and the regulators, pose unique needs and

requirements on the designed device. Our skin-stretch feedback device will aim to target

patients’ desires to receive easy, reliable, and accurate tactile feedback. The device also will be

cost-efficient because patients pay for the majority of the cost of prostheses. Regulations are less

of a concern during design phase, but should be carefully considered and met in the future.

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Our team has selected 12 design specifications to target in developing a multi-DOF skin-

stretch haptic feedback prototype (Table 2). Based on customer needs analysis, we have rated the

importance of these specifications on a scale of 1-4, where in order of decreasing importance we

consider functionality (4), user safety and comfort (3), intuitiveness (2), and cost of the device

(1).

Functionality SpecificationsThe functionality specifications are critical by definition to the success of the project.

They are defined as: number of degrees of freedom and distinguishability of signals. Although as

many degrees of freedom as possible would be theoretically ideal, this project will strive for two

distinguishable DOFs, which is an improvement over the MAHI lab’s 1-DOF skin stretch device

and other commercially available haptic feedback devices.

Furthermore, in order to provide rich information using multi-DOF, the user must be able

to clearly distinguish between distinct signals within each feedback dimension. The current

MAHI lab skin-stretch device that we aim to improve upon can generate 5 distinguishable

signals in 1-DOF. Depending on the selected mechanism, a 2-DOF device could achieve this

number in both of its dimensions. Therefore, the target number of distinguishable signals for a 2-

DOF device is 25 (five times five, assuming totally independent mechanisms).

Additionally, haptic feedback users have reported that desensitization to signals may

occur over time, particularly while using vibrational feedback. The skin-stretch device should

overcome this problem, and produce signals that can be perceived accurately for up to an hour.

Lastly, the device should be consistent enough that a user can identify the same signal

correctly 75% of the time.


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Safety and Comfort Specifications

Users will frequently wear the final device directly on the skin. It is therefore essential

that the device poses no health risk to the user. The device should also impose no strain or

discomfort on the user, and no pain or discomfort due to stimuli at the contact surface should be

reported. Comfort will be quantified on a user-defined scale (see “Testing and Results”).Because the device will be worn on the arm, mass of the device should therefore be comparable

to a heavy wristwatch, which is generally no more than 200g. Higher masses up to 300g would

also be acceptable, especially in the prototyping phase where the strain of long-term use is not

relevant. No pain or discomfort due to stimuli at the contact surface should be reported.

Additionally, the device should have a small profile as not to constrain the user’s motions, and

should be able to fit the 25th-percentile-sized female as scaling up in size for such technology up

is simpler than to scale down in size.

Figure 11: The haptic feedback device must be wearable,

comfortable, and safe in addition to achieving

its functional goals

Ease of Use Specifications

Also of importance is that the device must be easy to learn and easy to use in day-to-day

applications. Excessively complicated devices would likely deter use; therefore basic

competence with the device should be learnable in no more than 30 minutes. Additionally, iffeedback actuation is too slow, the device will be less helpful in real applications. Thus, the

device should have a delay of no more than 2 milliseconds between signal detection and

actuation of the feedback mechanism.


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Cost Specifications

In customer needs analysis, we’ve found that the patients pay for the majority of the

prosthesis cost. Consequently, the device needs to be affordable at the first purchase and in long

term. Market analysis anticipates that devices could be sold in the range of $900-$9000.

Therefore the ideal manufacturing cost would be $900 per device, with the marginally acceptablecost matching our design budget of $2500. To lower the device’s long-term cost, we need to

design the device to be durable. Because the device will likely be exposed to physical impacts

over the course of the user’s daily life, we aim to make the device strong enough to endure 50

drop tests from 1 meter.

Table 2: Original Design Specifications (for Final Target Specs, see Table 3)

Specification Description Ideal Goal Marginal Goal

Degrees of Freedom Independent, distinguishable

feedback methods

2 2

DistinguishableSignals

Total number of distinguishablesignals

25 16

Desensitization

Duration of perceptibility

60 minutes of perception

30 minutes

Consistency Average percentage of signalidentification within one user

75% consistent signal identification

60%

Time Delay Time between signal detection

and feedback completion

0.002 seconds of

processing

0.05 seconds

Learning Curve

Time required to associate all

signals with stimulus

30 minutes of

training

60 minutes

Comfort Percentage of users reportingdiscomfort within 10 minutes

No user discomfortafter 10 min

5%

Mass Mass of device and housing 200 g 300g

Size

Size of device and housing

Fits 25th percentile

female

Fits 50th

percentile female

Safety Number of health risks to users 0 health risks to

wearer

0 health risks to

wearer

Durability

Number of 1m drop testendured

Endures 50 drop testfrom 1 m

Endures 30 droptests from 1m

Manufacturing Cost Cost per unit $900 $1480


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Team Touchtech will construct a skin-stretch feedback device that combines linear and

rotational motion within a ring-shaped contact pad. The ring-shaped contact pad will rotate about

its center axis in one degree of freedom, and will also rock along the latitudinal axis of the user’s

upper arm in another degree of freedom. The basis for this decision was a process consisting of

brainstorming all feasible ideas, performing preliminary tests on perceptibility, conducting

research on design necessities, eliminating designs deemed undesirable based on testing and

research, and finally constructing a Pugh Matrix to rate the final candidate options. This

document specifies the exact details of how the final product will operate and how its

subfunctions will interact with each other.

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The team has elected to create a device with two distinct feedback signals (rotational and

linear) to relay two degrees of freedom to the user. A gripper claw will be used as the sensory

input to the feedback device: a servo motor will be integrated into the gripper claw to control the

gripper’s open-close position, and a force pad will be wired at one of the fingers of the claw to

relay the force of external objects on the claw. To control this gripper, the user will move a linear

slide potentiometer to open and close the gripper claw. The open-close position of the claw will

control the rotational feedback motion, while the force sensed at the gripper fingers will control

the linear feedback motion. The original design intent of this gripper claw assembly can be seen

in Figure 12, and the actual design implementation of the gripper claw in the most recent

prototype can be seen in Figure 13.

Figure 12: Design intention of gripper claw with position encoder and force pads from Cycle I


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Figure 13: Actual gripper claw with integrated slide potentiometer controller (held in hand), position

encoder (within servo), and force pad resistor (attached to the gripper claw)

The haptic feedback will be provided via a contact interface to be contained within a 3D

printed structural housing. This device as a whole will then be strapped to the user’s upper arm

using velcro straps to provide skin-stretch feedback for the interactions of the gripper claw with

external objects. A soft gripping rubber material will be used as the contact interface between the

device and the user’s skin to provide both high perceptibility as well as user comfort (Figure 14).

Additionally, the use of a rubber material ensures that the contact pad will not slip against the

user’s skin even through large rotations.

Figure 14: Preliminary design intention of rocker with gripping contact pad

The contact pad will be rotated by a servo to relate the open-close position of the gripper

claw as illustrated in Figure 15. The pad will rest at the neutral “0” position while the claw is

open halfway between its maximum open and closed position. As the claw fingers open wider,


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the contact pad will rotate and stretch the user’s skin clockwise. As the claw fingers close

together, the contact pad will rotate and stretch the user’s skin counterclockwise. The rotation of

the contact pad will be achieved through a micro-servo motor. The open-close position of the

gripper claw will be sensed by the divided-voltage output of the rotary encoder inside of the

servo itself.

Figure 15: Rotational motion of contact pad from bottom view, corresponding to position

(a) Gripper in closed position, contact pad rotated counterclockwise

(b) Gripper in neutral position, no contact pad rotation

(c) Gripper in open position, contact pad rotated clockwise

Additionally, the contact pad will rock linearly along the latitudinal axis of the user’s

upper arm. This movement will correspond with any force feedback felt at the fingers of the

gripper claw through a single force pad resistor on one of the gripper fingers. When no force is

applied to the force pad resistors, the contact pad will be at a neutral position. As force is

applied, the contact pad will rock linearly along the axis of the user’s upper arm and stretch the

user’s skin to one side. As the force is released, the contact pad will rock back toward the neutral

position until no force is perceived (Figure 16). The rocking motion of the contact pad will be

achieved through a second, independent micro-servo motor.


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Figure 16: Linear stretch using rocking motion, corresponding to force applied to gripper

(a) No force applied to gripper, contact pad in neutral position

(b) Some force applied to gripper, small linear stretch applied

(c) More force applied to gripper, greater linear stretch applied

The two independent micro-servo motors which control the two degrees of freedom of

the contact pad will be integrated within the housing of the device. The housing will be

constructed as a base plate that fits the curvature of the user’s arm, with two short columns oneither end of the base plate which support the servo/axle subassembly (Figure 17). The main axle

between the Servo 1 and Servo 2 will be hexagonal to best transmit torque. The supporting axle

between Servo 2 and the second column extending from the base plate will be circular to allow

for rotational freedom. The contact pad will be directly attached to Servo 2. When actuated,

Servo 1 will rotate the hexagonal axle and thereby rock Servo 2, causing a linear stretch on the

user’s skin. Meanwhile, Servo 2 controls rotational stretch and, when actuated, will rotate the

contact pad around its own central axis causing rotational stretch on the user’s skin. This

assembly is shown in an initial prototype made of laser cut wood in Figure 17.


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Figure 17: Preliminary wooden housing prototype and assembly

Ultimately, the housing will be 3D printed from ABS plastic and will be no more than 4

cm in height. The two base plates shown in Figure 17 will be built as a single plate with a slight

curvature to fit snugly against the user’s arm. This not only allows for improved comfort for the

user, but also increases stability of the device when strapped to the user’s arm. This curvature

will be calculated by averaging the upper arm curvature of a 50th percentile female and a 50th

percentile male. Additionally, building the base plate as a single piece rather than two separate

pieces prevents the device from moving relative to itself and causing interfering signals to theuser. In this way, the only significant movement the user can discern should be the deliberate

feedback signals from the contact surface. As shown in Figure 17, the base plate will also have

slits cut into either side to allow for the attachment of velcro straps, which will be used to attach

the device to the user’s upper arm. Velcro straps were chosen because they are easily adjustable

to a wide range of arm diameters and can be tightly, but comfortably, secured around a user’s

arm. Finally, the device as a whole will be enclosed within a 3D printed casing to provide

protection for the device’s moving parts. This casing will be able to be easily removed such that

the user may still access the contact surface and servo motors for maintenance and repair. The

preliminary design intention for this housing and casing (excluding the servos, axle, and contact

surface) is shown in Figure 18. The entire feedback structure will not exceed the dimensions of a

25th percentile female’s upper arm, as stated in the 2008 CDC National Health Statistics Report

(McDowell 2008). This will ensure that, for the vast majority of users, the haptic feedback

device will not extend past the length or width of the upper arm.


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Figure 18: Preliminary design intention for integrated housing and casing(a) isometric view, (b) bottom view

Figure 19: Intended design of integrated gripper/slide potentiometer device

For demonstration purposes, the gripper claw and slide potentiometer will be integratedinto a separate device that an able-bodied user will be able to strap to their upper arm. The

gripper will be screwed down to a long base plate with the slide potentiometer integrated through

a slit into the base plate. Users will be able to strap their arm down to the base plate and control

the open-close position of the gripper claw using the slide potentiometer at their fingertips.

While controlling the gripper claw using the slide potentiometer, the user will simultaneously

receive feedback signals mapped from the gripper claw to the skin-stretch feedback device on


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their upper arm as previously described. In this way, able-bodied users may use the gripper claw

to manipulate objects with the same sensory feedback disconnect that a prosthesis user would

encounter with their own prosthetic arm or hand. The intended design for this integrated gripper

claw/slide potentiometer assembly is shown in Figure 19.

In addition to the physical and mechanical subsystems of the device, the electricalfeatures of the system have been thoroughly analyzed. As aforementioned, there are three input

signals into the electronic subsystem, and three corresponding pulse width modulation (PWM)

output signals that control three servos. The first input is that of the slide potentiometer, which

controls the aperture of the gripper claw. The second input is from the rotary encoder located

inside of the gripper servo, which controls the rotational motion of the haptic feedback device. In

order to read the open-close position of the gripper claw, a wire was soldered onto the gripper

servo’s internal position encoder, providing a divided voltage proportional to the aperture of the

claw. Finally, the third input is from the force pad at the finger of the gripper claw, which

controls the linear rocking motion of the haptic feedback device. The team has chosen to use a

Sparkfun Inventor RedBoard to transform the three divided voltage input signals into threeseparate PWM output signals. The RedBoard is shown in Figure 20.

Figure 20: Sparkfun Inventor RedBoard with prototyping wires attached

Due to the nature of servo motors and the signal inputs, it was necessary to filter some of

the high-frequency signal noise. The presence of noise might cause unintended motions at the

haptic feedback interface with the skin, which is highly undesirable as it would give the userfalse signals. To decrease this noise, the team analyzed and installed simple RC low-pass filters

on both the slide potentiometer and the rotary encoder of the gripper claw. The combination of

the resistive and the capacitive values were optimized to minimize time delay of the system

while also effectively cutting out all high frequency noise.

The various wires connecting these resistors and capacitors will be condensed into a

single printed circuit board (PCB) to be produced in January 2015. This PCB, along with the


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RedBoard, will be installed in the integrated gripper claw/slide potentiometer demonstration

device shown in Figure 19. One bundle of wires will then be run from this device to the haptic

feedback interface on the upper arm to control the two servos within the device.

The entire system will be powered by a single cord that plugs into a regular 120VAC wall

outlet. The team at first considered using integrated batteries, but testing proved that it was not possible to effectively draw enough current from conventional batteries to power both the

RedBoard and the three servos of the system. Additionally, the use of a wall plug-in reduces the

cost of testing and demonstration, as the team will not have to continually purchase batteries to

operate the system.

Once the system is powered on, the user first moves the control input mechanism (a slide

potentiometer) to control the position of the gripper claw fingers: pushing the slider up closes the

gripper, while pulling the slider down opens the gripper. The external object of interest then

interacts with the gripper fingers. Depending on the malleability of the object, the gripper fingers

will be impeded by the object and eventually stop moving when fully closed upon the object.

Additionally, depending on the object, the force experienced at the fingertips of the gripper claw

will increase as the gripper continues to close on the object. These two signals in turn control the

feedback interface (the ring-shaped contact pad) and provide haptic feedback.

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This final design of this prototype has been developed to meet the twelve design criteria

(Table 3).


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Table 3: Final Target Specifications (as modified from the Original Design Specs in Table 2)

Of these criteria, achieving a two-degree of freedom device was first in importance. By

using two independently controlled servo motors, the device is able to relay feedback signals to

the user in two degrees of freedom. Because the device utilizes two distinct feedback motions for

the movement of the gripper claw, preliminary testing shows the two types of signals to be both

distinguishable from one another and intuitive to learn. Rotational motion using a soft rubber

ring was demonstrated to have consistently accurate perceptibility, thus it is used as the position

feedback mechanism. Previous research also demonstrated linear rocking to be easily perceptible

for simple signals, thus the team has chosen it to be used as the force feedback mechanism as it

requires less precision than the position feedback. During preliminary testing by the team, these

motions did not interfere with each other and the user was capable of distinguishing each of the

signals separately. Finally, the criteria for comfort and size were also considered during design.

Though the device requires two micro-servo motors, the overall design is low-profile and uses

only one major point of contact with the user’s upper arm. Additionally, the device housing will

be 3D printed with ABS plastic, which is lightweight and allows for custom modification to

adjust the size of the device. Soft rubber will be used as the interface between the device anduser to retain high feedback perceptibility while maintaining comfort for the user.


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In order to arrive at this chosen prototype, the team considered the original project

mission statement: to develop an intuitive and wearable skin-stretch device that provides

accurate and reliable tactile feedback in two degrees of freedom. This mission statement was

then decomposed into functional blocks and subsystems, as shown in Figure 21. The systeminputs consist of (1) the user actions, (2) the external object the user interacts with, and (3)

electrical power to the system. The ultimate output of the system is the haptic feedback the user

receives in order to improve control over the gripper claw. To close the loop, the user then

mentally processes the haptic feedback and responds to correct the input provided to control the

gripper. The major subsystems are divided into: gripper control, environment sensing, processing

the signals, and feedback output.

Figure 21: Functional decomposition of system

The focus of this design project is on the creation of the feedback device itself, and thus

the following design analysis will focus mostly on this feedback device. The other three

subsystems—gripper control, environment sensing, and signal processing—are discussed to a

lesser degree.

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Design Criteria for Feedback Device

In developing the design for the haptic feedback device, the team focused on six

particular design criteria stemming from the quantitative specifications. These criteria are: (1) the


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user’s ability to distinguish between the two degrees of freedom, (2) device comfort, (3) device

size and profile, (4) the design simplicity, (5) processing time of the microcontroller, and (6) the

training required to effectively use the device.

Because the feedback device will incorporate two degrees of freedom, the user will

receive two distinct signals corresponding to each degree of freedom. For the user to successfullyuse this feedback to control the motion of the gripper claw, they must be able to accurately

distinguish between the two signals. Thus, in designing the mechanism of the device, it is vital

the two feedback signals do not interfere with one another. Therefore, the user’s ability to

distinguish the two distinct signals is weighted most heavily.

Device comfort and device size were the next two most important criteria, weighted

equally. After discussion with Mr. Howell at the Baylor College of Medicine, the team

determined that high comfort and small size was essential to the adoption of the feedback device

for prosthesis users. Thus these considerations were also weighed heavily in brainstorming and

developing possible designs. Similarly, design simplicity was also taken into consideration

(though weighted below comfort and size) in effort to align with user needs. Should the design

be too complex (ie. many moving parts), users will also be reluctant to adopt the device.

Finally, processing time of the microcontroller and intuitiveness of the design were also

considered, though weighed much less than the previous criteria. Processing time is taken into

account in considering methods of actuation (ie. linear actuators, servo motors) while designing

for the desired feedback motions. Intuitiveness of the device is considered in each design relative

to each other. Though the team expects training time to be necessary for users to effectively use

any feedback device, a more intuitive design which allows for a quicker rehabilitation period is

most ideal. As such, intuitiveness is considered, but weighted lowest in the Pugh Scoring Matrix

(Table 4).

Design Criteria for User Input Mechanism

In addition to examining the design criteria of the feedback device, the team also

considered the method through which the user would control the gripper claw. Although this

analysis is not relevant to the design of the feedback device itself, it is very important in testing

the feedback device once it is prototyped. As the feedback device will be tested on able-bodied

users, it is important to choose an input mechanism that closely simulates the most important

elements of a prosthetic user input mechanism. These important elements are: (1) disconnect between input mechanism and gripper motion, (2) disconnect between input mechanism and

feedback motion, and (3) required magnitude of user input.

In this design project, the team is primarily concerned with the level of intuition between

the gripper motion and the feedback being provided (shown as Link 2 in Figure 22). Therefore,

in order to isolate that segment of the process, the team is limiting the amount of natural intuition

between user input and gripper motion (Link 1) and between the haptic feedback and the user


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input (Link 3), as ultimately the prosthetics users will not have native intuition between either of

those links. These disconnects will provide a truer simulation of the experience of using the

feedback device, and will provide more legitimate testing data on the intuitiveness of using and

understanding the feedback device’s signals.

Figure 22: Signal flow between user input, gripper motion, and haptic feedback

Between the disconnects of Link 1 and Link 3 in Figure 22, the most significant is the

perceptual disconnect between the input mechanism and the motion of the gripper (Link 1).

Prosthesis users very rarely control their prosthetics using intuitive motions: often, the twitching

of a chest or upper arm muscle controls the opening of a prosthetic hand. To most accurately

simulate this experience, the team wanted to provide a similar disconnect for the able-bodied testsubjects, and thus weighted this particular disconnect as most important.

A disconnect between input mechanism and feedback mechanism (Link 3) is also

considered desirable, but to a lesser extent. In most cases, motion made by the user to control the

gripper claw differs from the motion of the feedback mechanism on the arm; however, this is not

always the case. For example, the prosthetic user input of a muscle twitch is very similar to the

recently researched haptic feedback motion of tapping. Therefore, the disconnect between

feedback and input motions is not functionally imperative to testing the prototype, but would

help in further reflecting the unintuitive control that a real prosthesis user might have over a

prosthetic device. Lastly, the required magnitude of the input is important, as the ultimate testing device

should not be too large, nor should it require larger movements than an able-bodied test subject

could provide through his/her fingers. However, this consideration is markedly less important

than the disconnect between the input motion and the gripper motions (Link 1), about on par

with the importance of the disconnect between the input mechanism and feedback mechanism

(Link 3).


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Team Touchtech brainstormed ideas for both the feedback device and the user control

input. However, as noted above, the scope of this design project focuses primarily on the

feedback device rather than the user control. Therefore, the majority of the brainstorming,

testing, analysis, and design will be focused on the feedback device. The team will use the usercontrol input mechanism to test the feedback device with able-bodied test subjects.

Brainstorming Methodology

After compiling a list of brainstormed solutions, Team Touchtech selected the feedback

device design discussed in the “Chosen Prototype” section after testing perceptibility of different

motion, shapes, and materials, researching relevant design factors, and rating the final candidate

solutions against the design criteria in a Pugh Matrix (Table 4).

Brainstorming was conducted as a group during two separate brainstorming sessions.

Each member first spent 15 minutes brainstorming feedback signals individually on notecards,writing one idea per notecard. Next, group members passed the ideas around the table, either

further developing another member’s idea or using the idea as inspiration for a new idea entirely.

During these brainstorming sessions, group members also began developing feedback

mechanisms for selected feedback signals using the same notecard brainstorming method.

Ideas were then narrowed down using the previously determined design criteria and

preliminary testing with simple materials such as K’NEX or model clay. The four final candidate

design solutions that Team Touchtech considered in further detail are shown in Figure 23 and are

explained below:

a.

A hemisphere trackball moving in two perpendicular axes on the skinb. One contact point moving linearly along the axis of the arm and rotating about the center

axis of the contact point

c. Two points of contact that pinch longitudinally and rotate around their center axis

d. Two points of contact that pinch longitudinally and move linearly to either side

Figure 23: Brainstorming sketches of final design candidates. Refer to paragraph above for descriptions

of each motion


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For the sake of clarity, the design candidates illustrated in Figure 23a and 23c above are

shown on a human arm in Figure 24. If they had been chosen as the final design, Figure 24

provides a rough idea of how they would be implemented on human skin.

Figure 24: Design candidate ideas shown in Figure 23a and 23c as implemented

on a human arm

Perceptibility in 2 DOFs

Perceptibility testing revealed that mechanisms with two contact points which pinch the

skin and rotate about a common center did not provide easily distinguishable degrees of freedom.

The rotational motion, when used in conjunction with linear motion, also feels like pinching and

creates confusion between signals. Additionally, pinching provides limited range of signals

because the skin can only comfortably be stretched apart or push together by approximately 1

inch. Thus, ideas with two points of contact are not favorable.

The team also explored two possible ways to give linear signals: rocker over the skin and

linear 2D motion on the skin. The comparison showed that rocker gives a larger signal, while the

regular linear motion is easier to construct. Rockers, however, require smaller torque to maintain

the pressure on the skin surface during motion because a portion of the force is in the normal

direction of skin surface, while a similar torque given to linear motion yields smaller pressure onthe skin.

Additionally, contact surface testing with different shapes revealed that a ring-shaped

contact pad provided higher perceptibility that a solid circle. Furthermore, contact surface testing

confirmed that a high friction material such as rubber is better for skin stretch than a smooth

material such as plastic, which is overly prone to slip.


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Comfort

In testing, the curved surface of a rocker allows greater range of comfortable motion. The

rocker motion is preferable to movement purely in a line tangential the contact surface because

the rocker would more easily slip across the skin before causing discomfort. Furthermore, the

rocker was found to allow a greater range of motion prior to discomfort.

Size

The team sketched possible mechanisms for each of the brainstorming ideas and judged

how well each idea would be expected to fit within the size criterion. Based on this, it was seen

that two independent points of contact would cover a larger area on the skin due to the necessity

of distinct control systems for each contact point. Furthermore, because each contact point would

require a full range of motion for accurate perception, the device footprint for linear movement

of two contact points would be double that of a single contact point. Similarly, control of a

trackball requires a full sphere (rather than a hemisphere) which increases the height of the

feedback mechanism and housing. This significantly limits the user’s range of motion as they

must be aware of a device protruding from their arm, making it much more prone to damage.

The linear/rotational contact ring design could be built while keeping a low-profile and allowing

the servos to be stationary relative to the mechanism housing.

Design Simplicity

The control system for the trackball would be difficult to implement as it requires a round

object to be rotated independently about two axes. Although this seems to be a trivial task,research into how to control a trackball in two degrees of freedom reveals it is deceptively hard.

In the team’s design, it is necessary to have both degrees of freedom actuating simultaneously;

for this to happen for a trackball, there must be “some slip...at the interface” between the

trackball and the servos spinning it (Webster 2005). Constructing such an interface that provides

high amounts of friction in one direction and allows free slipping in another direction is very

difficult at small scales, as the contact between the servos and the trackball represent “an area

rather than an idealized point contact” (Webster 2005). Many research groups have addressed

this issue by employing rubber O-rings as interfaces; however, these would require frequent

replacement as the constant slipping would wear them down, which adds to the complexity of

this concept. Thus, due to the complex nature of controlling such a trackball, this concept was

markedly not simple.

Likewise, controlling two independent contact points to move in coordination with each

other clearly is a more complex mechanism than controlling a single contact point. Such designs

would require more moving parts, increasing the possibility that some part of the mechanism


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might break during normal use. For these reasons, a single contact featuring a combination of

linear and rotational motion was judged to be the simplest to build, control, and maintain.

Processing Time

There is little to distinguish designs with regard to processing delay. While the

computing speed depends mostly on the complexity of the algorithm, computing speed is much

faster than the speed of human response. Therefore, there will not have a significant difference

among the four brainstorming ideas. However, in terms of actuation, both designs involving two

points of contact were conceived as employing linear actuators, which were found to move at

relatively slow speeds: the fastest micro-linear actuator discovered reaches speeds of no more

than 20 mm/s. Therefore, the designs with two contact points were considered relatively less

desirable.

Intuitiveness

Testing of different motions and combination of motions on skin revealed that a single

contact point generally works better than having two contact points when the test subjects learn

to associate activity at gripper to the feedback motion. During testing, motion from two contacts

was perceived as more complicated and necessitated more training time for the users to learn the

motion combinations. Additionally, learning to distinguish between two degrees of freedom is

more difficult when the types of feedback motion are not clearly distinct from one another. For

example, a combination of linear and rotational feedback is more intuitive than rotational

feedback in two directions (i.e., trackball).

Pugh Matrix Analysis for Feedback Device

The above tests and research results are quantified in the Pugh Matrix (Table 4). This

matrix compared the final concepts based on how well they satisfied the most important factors

among the design specifications, which were weighted according to their anticipated importance

to the completed prototype. The expected performance on each factor is rated on a scale of 1-5,

with 5 being the highest score and 3 being the baseline benchmark. From the Pugh Matrix, a

single point of contact with a hollow circle that used linear-rotational feedback scored highest

among all alternatives.


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Table 4: Pugh Matrix for Feedback Design

Factors Weight (a) Hemisphere

Trackball

moving indifferent

directions

(rocker)

(b) One point of

contact/hollow circle

that moves linearlyand rotates around

center

(c) Two points of

contact that pinch

longitudinally androtate around

center

(d) Two points of

contact that pinch

longitudinally andmove linearly to

either side

Distinguish between two

DOF

30% 5 5 3 4

Comfort 20% 4 3 3 3

Size 20% 3 4 2 2

Design

Simplicity

15% 3 4 3 3

Processing

Time

10% 4 4 4 2

Intuitiveness 5% 2 5 3 3

Total Score 100% 3.85 4.15 2.9 3

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Brainstorming Methodology

Similar to the design analysis for the feedback device, a brainstorming session was held

to identify possible user input mechanisms. However, this brainstorming session was much

shorter, and the team limited itself to devices readily available on the market. This brief session

produced three possible input mechanisms: (1) a rotating potentiometer, (2) a sliding

potentiometer, and (3) a joystick. These were then entered into the Pugh Matrix (shown in Table

3) and judged against the design criteria previously explained.

Disconnect between input and gripper motion

None of the proposed input mechanisms were significantly similar to the action of

opening and closing a gripper, and thus all ideas scored highly in this category.


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Disconnect between input and feedback motion

For the majority of its operation in everyday situations, the feedback device described in

the “Chosen Strategy” section will rotate on the skin whenever the gipper claw moves, and will

occasionally move in a linear motion whenever the grippers come into contact with an object.

With this in mind, the input of a rotating potentiometer highly resembles the rotating contactinterface of the feedback device, and thus was undesirable for this category. The other two input

mechanisms do resemble the linear motion the feedback device would perform when the gripper

fingers experience force, but since this linear feedback is not employed as often as the rotational

feedback, these mechanisms were slightly more desirable.

Required magnitude of input motion

The rotating potentiometer provides the smallest magnitude of user input, as the knob

itself would not be translated in space, only rotated. On the other hand, the slide potentiometer

requires translation in order to receive user input, and thus was less desirable. However, thevolume of space that the slider sweeps through in its range of motion is still relatively small.

Least desirable of all is the joystick, which translates in space and requires the most volume as

the joystick sweeps through its range of motion when compared to the other two concepts.

Pugh Matrix Analysis for User Input Mechanism

The above test and research results are quantified in the Pugh Matrix below (Table 5).

Ultimately, the slide potentiometer was chosen as the ideal user input mechanism.

Table 5: Pugh Matrix for User Input Mechanism

Factors WeightRotating

Potentiometer

Slide

PotentiometerJoystick

Disconnect between input

and gripper motion 50% 4 4 4

Disconnect between input

and feedback motion 25% 1 4 4

Required magnitude of

input motion 25% 5 4 2

Total score 3.5 4 3.5


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The project schedule and team responsibilities are outlined in form of Gantt chart in

Appendix A.


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The design of this feedback device is an integration of three subsystems: the structural

housing, the contact piece, and the electronic components.

Housing

Housing refers to the mechanical components of the design that hold the motors and

contact piece together and facilitate attachment of the device to the human arm. The housing

itself consists of the base plate, the axle, and the casing.

Figure 25: Housing (isometric view). The total housing consists of the base plate (white plastic piece),

casing (plexiglass enclosing base plate). The baseplate shown here has four slits for Velcro straps. In the

final design, there will be only one large slit on each half, connecting a single, thick elastic strap for

comfort and stability purposes.

Base Plate: This 3D printed plastic piece contains two protruding columns (see Figure 25). One

contains a rectangular opening matching the dimensions of the rocker servo (Servo 1 in Figure

17). The other column serves as a grounding point for the axle connecting the rocker servo to therotating servo (Servo 2 and associated axle in Figure 17). The height of these columns dictates

the height of the axle and therefore the height of the contact piece relative to the base plate; in

essence, these columns therefore dictate how much pressure the contact piece will exert on the

skin when the device is strapped on. The contact interface itself is able to touch the skin because

of a large square aperture in the base plate.


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The bottom of the base plate is curved to roughly match the curvature of a human arm,

increasing comfort. The entire feedback structure does not exceed the dimensions of a 25th

percentile female’s upper arm, as stated in the 2008 CDC National Health Statistics Report

(McDowell 2008). This will ensure that, for the vast majority of users, the haptic feedback

device will not extend past the length or width of the upper arm. Additionally, the base plate will

ultimately contain one large slit on each side to connect a wide elastic strap, instead of the two

small slits on each side shown in Figure 26. The use of a single strap instead of two straps will

improve user comfort by distributing the pressure in the strap over a larger area, as well as

contributing to functionality by pressing the contact interface into the skin evenly. The bott

Documents

Final Report :: Team Touchtech Final Project