Piezoelectric Knee Brace

ME – 423 Engineering Design VII

Phase III Report

Group 5

December 5, 2013

Authors

Andy Continisio

Anthony Worthington

Liem Nguyen

Advisor

Prof. Robert Chang

STEVENS INSTITUTE OF TECHNOLOGY

Castle Point on Hudson

Hoboken, NJ 07030

October 1st, 2013

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Abstract A piezoelectric knee brace is an orthotic device designed to support the human knee

while simultaneously providing electro-stimulation therapy to the surrounding muscles and

ligaments. The device will utilize electricity generated from human movement through the use

of piezoelectric materials integrated into the brace. The therapeutic benefits of the device are

two-fold: 1) maintain proper knee alignment to aid in rehabilitation, and 2) pain management and

muscle activation through the electro-stimulation treatment. This type of all in one device will

be the first of its kind. This paper highlights the requirements for such a device, current state of

the art designs, and further analysis needed to make the piezoelectric knee brace an effective and

financially successful endeavor.

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Table of Contents Abstract ......................................................................................................................................................... 2

Table of Contents .......................................................................................................................................... 3

Project Objective ........................................................................................................................................... 4

Project Background ....................................................................................................................................... 4

State of the Art Review ................................................................................................................................. 5

Knee Brace Design ................................................................................................................................... 5

Piezoelectric Material ............................................................................................................................... 7

Electro-Stimulation Technology ............................................................................................................... 9

Current Products and Best Practices ........................................................................................................ 9

Conceptual Designs .................................................................................................................................... 11

Plan of Technical Analysis ......................................................................................................................... 18

Technical Analysis ....................................................................................................................................... 19

System Specifications ................................................................................................................................. 23

Budget of Expected Expenses ..................................................................................................................... 23

Project Schedule ......................................................................................................................................... 24

Impact of Design ......................................................................................................................................... 25

Conclusion ................................................................................................................................................... 25

Works Cited ................................................................................................................................................ 26

Appendix A .................................................................................................................................................. 26

Appendix B .................................................................................................................................................. 27

Appendix C .................................................................................................................................................. 27

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Project Objective The purpose of this project is to design a knee brace that harvests electrical energy from

human movement through the use of piezoelectrics. This electricity will then be used to provide

electro-stimulation therapy to help the user manage chronic knee pain. The main focus of the

design will be on the energy harvesting and delivery mechanism.

From a business standpoint, research will be conducted to determine if the product

warrants securing a patent. Designing a product with a potential for commercial success is also a

primary goal. The design team recognizes the potential applications for a successfully designed

energy mechanism outside of this and other medical applications: namely, providing power for

everyday electronic devices.

Using the basic product design process outline in ME – 322, Design VI, the piezoelectric

knee brace will undergo all major phases of development, including product specifications,

concept generation and selection, detailed technical analysis, and culminating in prototyping and

successful testing. The process will draw heavily from mechanical engineering principles in the

design of the energy harvesting system. Electrical and biomedical engineering disciplines will

play a role in the design as well with respect to the power storage and delivery system and the

knee brace design itself.

Project Background Since 2010, over 650,000 knee replacement surgeries have been performed annually in

the United States. Approximately 5,000,000 Americans are living with at least one total knee

replacement. The actual number of knee injuries that do not result in replacement, or any

surgery at all, is nearly impossible to track. From high school athletes, to weekend warriors, to

the elderly, the knee is one of the most common causes of pain and injury across all ages,

genders, and fitness levels. Further, recovery and rehabilitation is highly variable. Total knee

replacements require a minimum of 12-14 weeks before 90% range of motion is typically

achieved. More severe ligament damage can take up to two years of recovery.

In addition, the knee joint is an especially vital part of the human body and undergoes a

great deal of stress, wear, and tear. Osteoarthritis, tendinitis, torn meniscus, and many other

causes of knee pain contribute to patients suffering from knee pain. Each year 5 million

Americans seek medical help for painful knees. Even more self-diagnose and treat themselves

with pills and home remedies. Furthermore, treatments associate with the knee represent 26% of

orthopedic revenue annually. Nearly 20,000,000 doctor’s visits are conducted annually

concerning the knee.

Further, lingering pain and discomfort is common and has few viable long term treatment

options. Physical therapy is expensive and usually not covered by insurance after a short post-

surgery window of time. The possibility of re-injury is especially high among recovering

athletes and others. Current, non-surgical treatments for pain include prescription pain killers,

demanding physical rehab programs, knee braces, and electro-stimulation therapy. All options

come with a large price to match the inconvenience they can impose on the patient.

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Knee braces offer a relatively inexpensive way to safeguard against re-injury and provide

some comfort and pain management. When combined with periodic electro-stimulation therapy,

recovery is boosted, in part to the muscle stimulation, as well as a reduction in pain that would

otherwise inhibit physical exercise. Current limitations of electro-stimulation therapy are its cost

to the patient, inconvenience in use, short term healing effects, and lack of availability in general.

The piezoelectric knee brace aims to solve these key issues of cost and availability, while

still maintaining overall effectiveness in its application.

State of the Art Review Medical research and technology is constantly advancing. Current devices and practices

serve to meet many of the individual issues outlined above, but lack the proper configuration to

be truly effective. In an effort to best determine what is on the market today, four main

categories of study were determined: knee brace design, piezoelectric material, electro-

stimulation technology, and current products and best practices.

Knee Brace Design Knee braces fall into five main categories: prophylactic, functional, rehabilitative,

unloader, and knee sleeves. They vary in function, cost, level and type of support provided, and

duration of use.

The prophylactic knee brace is used a preventative measure before any injury has

occurred. The prophylactic brace is typically used by athletes in high impact sports such as

football. The brace strengthens the knee by supporting the ligaments, particularly the MCL. The

ligament strength can be boosted by as much as 30%, which dramatically reduces the potential

for injury from impact.

Figure 1 - Prophylactic Knee Brace

The functional knee brace is used after injury and corrective action have occurred. It

supports the knee during the second phase of healing, after the patient has regained much of their

range of motion. Like the prophylactic brace, it supports the main ligaments of the knee. It is

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used for 6-12 months after the initial healing period, and can be used during semi-strenuous

physical activity.

Figure 2 - Functional Knee Brace

The rehabilitative knee brace is used immediately after knee surgery for 1-2 months. It

restricts nearly all movement of the knee. Typically, the patient is using crutches and any impact

or load on the knee could cause re-injury.

Figure 3 - Rehabilitative Knee Brace

The unloader knee brace usually requires a prescription and is used to treat knee

osteoarthritis. Its unique design forces the knee to maintain proper alignment. It also forces

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much of the normal knee load to the thigh. This aids in pain management as well as injury

prevention.

Figure 4 - Unloader Knee Brace

Finally, the knee sleeve is the most recognizable and easily obtained knee brace. It

provides minimal support and alignment through compression of the knee joint. It is sold at

pharmacies and sporting goods stores alike, and typically can be bought for $20-$40.

Figure 5 - Knee Sleeve

Overall, the market for knee braces is heavily covered with the various configurations

shown. A wide variety in price and quality is observed, with high end prophylactic braces

nearing $2000. The research conducted suggests there is little room for innovation in this sector

with regards to knee brace support design.

Piezoelectric Material A piezoelectric material is one that exhibits an electrical response to a mechanical stress,

or a mechanical response to an electrical stimulus. In plain words, when the material is under

load, it produces a small electric charge; conversely, if current is passed through it, the material

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will move. They have been used in sensor applications, as well as small power sources and

timekeeping applications. Fig. XX illustrates the piezoelectric effect.

Figure 6 - The piezoelectric effect

Piezoelectrics can be categorized into two broad areas: ceramics and polymers. Ceramics

have rigid structures and were the first type to be used in various applications. They typically

have higher electrical outputs. Polymers have, until recently, been under-utilized as piezoelectric

devices. Recent improvements to their electric potential have caused them to be used more.

Their flexibility gives them greater variability in application compared to ceramics.

Piezoelectrics are modeled based on their configuration. The two most basic

configurations are the cantilever beam model and the tensile model. The cantilever beam model

dictates that the piezoelectric have a fixed and free end, with a load applied perpendicular to the

material axis. The deflection causes the electric response. Oscillatory vibration maximizes the

output from this set up.

Figure 7 - Cantilever Beam Configuration

The tensile load model has the material under axial load and the resultant

elongation/compression causes the electrical response. This configuration produces less power

than the cantilever beam, but has more applications in typical products.

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Figure 8 - Tensile Model

More piezoelectric configuration models can be found in appendix A.

Electro-Stimulation Technology

Electro-stimulation therapy is a medical procedure, in which electric current is passed

through tissue, muscle, and bone in order to provide therapeutic benefits to the patient. These

benefits range from general pain management, to improved joint mobility, tissue repair, and

blood flow improvement. Electro-stimulation machines can be stationary units or portable kits.

The current is passed through two or four electrodes placed on the skin of the patient.

There are several modalities, or delivery methods, by which the current is transmitted.

The most common are High Voltage Pulsed Current (HVPC), Russian, and Transcutaneous

Electrical Nerve Stimulation (TENS). TENS is primarily used for pain management, and is

relatively simple compared to the other modalities. This makes it an ideal modality for the

Piezoelectric Knee Brace.

TENS is characterized by the following specifications:

Category Minimum Maximum

Current .10 mA 10 mA

Voltage 10 V 110 V

Pulse Duration 40 microseconds 250 microseconds

Pulse Frequency 2 pulses per second 250 pulses per second

These criteria represent the average capabilities of portable and stationary TENS units.

Typically, the units have varying levels of intensity controlled by the user. These specifications

will serve as a target benchmark for the Piezoelectric Knee Brace.

Current Products and Best Practices

State of the art research uncovered three products and concepts that indicate high

feasibility for the Piezoelectric Knee Brace. They do not meet all design goals, but collectively

show that such a design is possible.

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The first is a product called from BioniCare. The BioniCare Knee System is a knee brace

with integrated electro-stimulation therapy, powered by an external battery pack. Essentially it

acts as an unloader brace, it also requires a prescription and is used to treat knee osteoarthritis.

Studies show that it may in fact be a viable alternative to knee replacement surgery. It carries a

hefty cost of $2000-2500, depending on the model and specific configuration options.

Figure 9 - BioniCare Knee System

Next, a piezoelectric energy harvesting concept is being developed for military

applications. The device, called the Pizaccato, currently designed to be mounted to the knee,

would generate electricity to power specific equipment carried by soldiers on the battlefield. It

consists of a four piezoelectric ceramics in a cantilever beam configuration. They rotate and

strike nubs along an outer ring as the wearer walks. Current estimates put power generation in

the range of 2-30 milliwatts. Furthermore, full scale production cost estimates suggest units

selling for between $10 and $20.

Figure 10 - Piezoelectric Energy Harvester

Last is a prototype developed by Georgia Institute of Technology research students. It is

essentially a polymer piezoelectric material mounted to the elbow. Modeled by the tensile

loading configuration, the polymer stretches and contracts as the elbow bends. Electrical output

estimates are in the range of 2 microwatts/cc of piezoelectric material. While this seems low, the

knee would provide more room to scale the design up, and well as utilization of all 360 degrees

surrounding the joint.

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Figure 11 - Georgia Tech Concept

Conceptual Designs

The next part of product development is the generation of various concepts. To be

considered, concepts need to meet the most basic design objective. The first of the five concepts

is the cantilever hub concept (CHC). The CHC uses a ceramic piezoelectric cantilever beam on a

free-spinning hub. This cantilever hub is then combined with a fixed outer ring containing nubs.

When said free-spinning hub oscillates as the user flexes the knee, it strikes the nubs lining the

fixed outer ring. This will cause a deflection, which will then produce energy through the

piezoelectric. The energy developed will then be stored or sent back into the desired anatomy via

electrodes. This method is hypothesized to develop a high potential for power generation.

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Figure 12 - Cantilever Hub Conceptual Drawing

The next conceptual design is the tensile concept (TC), modeled after the Georgia

Technical Institute research project. The TC is a series of polymer piezoelectric tensile strips that

run parallel to the length of the leg. When the knee is fully extended, the strips are in a neutral

position. When the knee bends, a tension is developed on the exterior side of the strips. This

tension allows the polymer piezoelectric to develop energy. This energy will then be gathered

and stored or sent back into the knee anatomy via electrodes. The polymer strips could be easily

integrated with various braces.

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Figure 13 - Tensile Conceptual Drawing

The third conceptual design is the zipper concept (ZC). The ZC uses a series of ceramic

piezoelectric cantilever beams to harvest energy. The configuration is set such that a fixed

exterior sheath is attached to the posterior tibial part of the lower leg. The cantilevers are

arranged on the inside the sheath facing each other in pairs. A series of agitators is then set on a

mobile rod inside the sheath. The mobile rod oscillates with the extension and contraction of the

knee. While oscillating, the agitators strike the cantilever beams inside the fixed sheath. These

strikes from the agitator allow the cantilevers to develop energy that is either stored or released

back into the knee via electrodes.

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Figure 14 - Zipper Conceptual Drawing

The fourth concept is the gear crank concept. This concept utilizes the mechanical

advantage of a gear train to increase the frequency of oscillations of the piezoelectric material

during each step. In the gear crank concept, a driving gear is fixed to the axis of rotation of the

knee. The driving gear drives a smaller gear which shares a shaft with an “agitation gear”. The

agitation gear rotates in at the same speed as the driven gear and has a series of prongs extending

out of it. Surrounding the agitation gear is series of piezoelectric cantilever strips. As the

agitation gear spins, it strikes the piezoelectric cantilevers and thus generates energy from the

mechanical movement.

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Figure 15 - Gear Crank Concept Model

The fifth concept is the impact concept. The impact concept has a series of piezoelectric

distally weighted cantilever beams attached to the knee brace. When the user takes a step, the

impact of the step is absorbed and amplified by the distal weights and the energy is harvested

through the piezoelectric cantilevers. The energy is then stored in a battery and then transferred

to the electrodes in TENS pulses.

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Figure 16 - Impact Concept Design

Concept Evaluation and Comparison

The five conceptual designs have different benefits and drawbacks. The first step in

analyzing the concepts was a general concept screening. Various criteria were developed based

on the product requirements. The goal was to determine which concepts to proceed with by

eliminating those concepts deemed not worthy of continued development. The result of this

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initial screening eliminated the tensile concept due to its low potential to meet the energy

generation requirement. The cantilever hub concept was also removed from consideration, as the

gear crank concept is similar, and superior, concept.

With the field narrowed, a more in depth analysis was needed to select the final concept.

This required more quantifiable criteria, based on the requirements. Relative weights were

assigned to each criterion based on customer needs and input. A table showing the criteria,

weights, and baseline requirements and design goals is shown below.

Figure 17 - Criteria and Requirements

This table was the basis for scoring the remaining three concepts. Each concept received a score

between one and three for each criterion. The score was based on a direct comparison of the

concepts with each other. The results of this analysis are shown in the concept scoring matrix

below.

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Figure 18 - Concept Scoring Matrix

Based on the scoring, the Gear Crank concept was selected for continued development.

This was mainly due to the high weight assigned to the energy generation criterion. If there is

not enough power produced, then the product will not fulfill its most basic and crucial

requirements.

Plan of Technical Analysis

Our project is highly dependent on several technical aspects. These aspects will be

considered, tested, and analyzed to determine the final development of our designs. It is

important to determine the parameters of each technical aspect, as well as an analytical model for

analysis, finally resulting in a physical model to test and verify results. The following four high

level objectives were created to show the focus on the technical analysis: determining an

optimum piezoelectric configuration, designing the energy storage system, designing an electro-

stimulation delivery circuit, and integrating the energy mechanism into a knee brace.

The first priority of technical analysis is to determine the optimum piezoelectric

configuration. The main considerations will be the piezoelectric material, configuration and

loading conditions, and ratio of kinetic movement to electricity produced. Ceramics are better for

a rigid structure while a polymer would better suit a flexible design. Cantilever and tensile

loading conditions will also be explored. Following the conceptual configuration, an analytical

model will be derived through equations relating human movement and electricity generation.

Once derived, these equations will be optimized to develop the most energy from human

generated kinetic movement. Eventually, a physical model will be developed that can simulate

repetitive movement constrained by the range of motion from a human knee. Appendix B

contains various measurements of range of motion associated with different physical activities.

This will give validity to the results of the analytical model.

After energy conversion, the piezoelectric knee brace will need to store the captured

energy. The first piece will be to determine the storage parameters. These will largely depend on

the configuration selected, but also include basic electrical circuit component parameters.

Ideally, an analytical model will be developed that can predict energy losses of various storage

systems based on the highlighted parameters. Again, the final step will be to validate the model

with a physical test. This requires that the energy storage circuit be fabricated and tested in the

lab.

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The electro-stimulation delivery circuit is the second priority in technical analysis, after

the harvesting configuration. The primary design parameters will be constrained by the TENS

specifications to comply with FDA requirements. Significant research of electrical control

components will need to be conducted to deliver the electricity according to these TENS

specifications. An analytical model consisting mainly of a circuit diagram and expected outputs

will be developed. Once complete, the physical circuit will be fabricated and testing in the lab.

Finally, consideration needs to be given throughout with respect to integrating the energy

mechanism with the knee brace. This will be affected primarily by the piezoelectric

configuration. The ideal design situation is a mechanism that can work with any of the various

types of knee braces, however, specific consideration will be given to the functional knee brace

and the knee sleeve, as these are the most common braces.

Technical Analysis

The first step in our technical analysis was to determine the most effective configuration

of piezoelectric strips for maximum energy harvesting. The TENS electro stimulation therapy

modality selected requires delivery of 20V and 5mA. Assumptions for the scenario with the

highest demand were made:

TENS delivery circuit is run for 60 minutes

User is walking constantly for 60 minutes

User is walking with an ideal human gait

Torque developed in knee from each step was 40Nm

No losses through circuitry

Running the device for 60 minutes while delivering 5mA at 20V requires 360 joules.

The goal is to determine the configuration needed to meet the requirements of this most

demanding scenario. There were several design variables that would affect the energy harvested,

the main ones being: physical configuration and number of piezoelectric strips, size of

piezoelectric strips, force applied to the piezoelectric, and frequency of piezoelectric oscillations.

These represent the essential factors to determine.

The COMSOL multi-physics MEMS software module was used to analyze a

piezoelectric in the cantilever configuration. The cantilever was modeled with one end fixed and

a load applied to the distal free end. The piezoelectric material chosen was lead zirconium

titanate (PZT-5H also known as NAVY Type VI) since it is both readily available and a ceramic

piezoelectric, thus has a higher power potential. Polyetheretherketone (PEEK) material was

chosen to model the linear elastic properties of a cantilever beam in the specified configuration.

PEEK was chosen due to its lack of distortion the intended properties of the model. The

cantilever was placed in a d33 configuration, the most effective energy harvesting configuration

for cantilever beams. By determining the magnitude of the force applied to the PZT, the model

will output the electric potential generated by the beam. This data is then used in calculations

determining the maximum energy generation of the gear crank concept. The design would be an

iterative process, as the force applied to each strip would change with the number of strips.

With the torque produced at the knee during each step known, the force applied to each

piezoelectric strip could be determined. Utilizing data from a study published in the Journal of

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Rehabilitation Research and Design, the average maximum torque generated by the knee was

determined to be about 40 Nm (Martinez). The data represented normal walking on level-ground

at an average speed of 1.31 m/s. The data was then used to represent the maximum possible

torque that could be applied to the device while walking. The torque was assumed to be

generated about the knee’s center of rotation. Thus, the group chose a gear size of 0.05m in

diameter to rotate about the same axis to determine the amount of torque the gear can generate.

At a radial distance of 0.025m from the axis of rotation (tip of the driver gear), the potential

torque is 1 Nm. The group chose a gear ratio of 2:1 to increase the angle of rotation of the driven

gear so that the piezoelectric beams could be actuated more frequently. The calculated torque of

the driven gear is 0.5 Nm. To find the deflection and thus output of the piezoelectric beams, the

tangential force of the Plucker gear (fixed to the driven gear and thus has the same gear

ration/speed) was calculated to be 40 N.

This 40 N tangential force will be applied to each of the piezoelectric beams. The group

chose to use 10 beams to apply a 4 N force per beam to reduce the amount of force and prevent

failure of the piezoelectric beams. Using the previously mentioned COMSOL Multi-physics

MEMS model developed, the 4 N load was applied to the piezoelectric beams to determine the

output. The maximum voltage output reading from the COMSOL model was 120 V.

Based on the limited research and applicable mathematical models for determining exact

piezoelectric current outputs, experimental results are much more reliable. The Pizaccato

concept, explained in the state of the art review, is a similar loading condition with applicable

experimental results. Therefore, it was compared with the loading conditions of the gear crank

design. To determine the amount of energy the device needs to generate to meet the

requirements, the most demanding scenario was considered, as outlined above. In Figure 19, the

upper table represents a list of possible voltage and current outputs that were considered,

however, to maintain a reasonably conservative stimulation to the end user of the product, the

team chose 20 V and 5 mA as our maximum. This ensures the output will not the threshold of

danger which occurs at about 10 mA. At 20 V and 5 mA, the energy usage over an hour would

be 360 J. Through comparison to the energy production from the Pizaccato design (8.136 J over

an hour), the calculated multiplication factor specify that 44x the output of the Pizaccato knee

brace was needed to obtain the required energy generation. The gear crank concept’s inherent

features provide enhancements over the Pizaccato design. In order to reach the 44x output of the

Pizaccato concept, several multiplication factors were identified within the design. First was a

gear ratio of 2:1, followed by 2.5x more piezoelectric strips, and 8x higher maximum voltage

output. This technical analysis allowed the group to determine the designs necessary for the

desired energy generation of 360 J. By comparison, our concept is within 1.1x the amount of

energy needed to be generated in the worst case scenario.

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Figure 19 - Calculation Spreadsheet

Now that the energy harvesting configuration is complete, the next step is to determine

the energy storage system. This consists of the required circuitry and storage device for the

electricity. This required considerable research. The most important considerations were to

determine the specific device that would store the energy, and subsequently the requirements for

charging such a device. The group consulted with Prof. Fisher of the Mechanical Engineering

Department and with Prof. McNair of the Electrical Engineering Department. Prior senior

design projects were also reviewed, specifically one 2008, by Arturo Dizon, Michael Ivey, Neil

Patel, and Mark Vizthum. Their project looked into energy harvesting using piezoelectric

materials as well. They addressed one of our main concerns with the impedance of the materials

we were using. Piezoelectric material has a very high impedance value and impedance matching

is performed to maximize the power transfer from the piezoelectric strips to the energy

harvesting circuit. To provide the impedance matching required, energy harvesting circuits are

available which maximize the power attainable from various energy sources. The previous

project utilized an EH300 energy harvesting circuit and according to their sponsor “Advanced

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Cerametrics, the energy harvesting PCB addresses the impedance matching issue”. After through

research of possible energy harvesting circuits, the team chose to use the same energy harvesting

circuit due to its impedance matching ability and its overall dimensions. The team chose to

charge a battery rather than a capacitor for the device because a capacitor does not have a

constant output current over a long period of time. The output of a battery is more stable for a

longer period of time, thus providing a consistent level of therapy. Thus we chose to use AA Ni-

MH batteries that can be trickle charged by the energy harvesting circuit.

With the energy stored, the next phase of technical analysis required specification as to

how it would be delivered to the user. By reviewing the circuits used in various portable TENS

devices, several standard circuits were observed. The chosen circuit diagram schematic is shown

below.

Figure 20 - TENS Circuit Diagram

The main component is the 555 IC timer chip, which allows the pulsed output of the

square waveform to be generated. Potentiometers are used to adjust various aspects of the

output, including intensity and frequency of the pulses. A transformer is used to increase the

voltage just before the pulse is delivered to the electrodes to provide the maximum 20 V

originally specified.

The last major goal of technical analysis was to determine the integration considerations

in attaching the device to a knee brace. The ideal device would be compatible with knee braces

of various types. The design decisions made included the component housings and fasteners

along with the mounting mechanism. The actual housing would be secured to the brace by

utilizing the existing knee brace straps. By developing SolidWorks models of each designed

component and assembling them, the ideal housing shape was determined. These drawings are

detailed in the next section of this report.

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System Specifications The piezoelectric material will be from American Piezo, model 40-1055 PZT. They will

be 25mm long with a 4mm x 2mm cross section. All other non-electrical components will be

made of ABS plastic. ABS was chosen for several reasons. Firstly, it is compatible with 3-D

printers, which may be an ideal manufacturing method for the plastic parts. Secondly, the

material properties of ABS will allow it to meet all loading conditions.

The part most susceptible to failure will be the small agitators of the plucker. They will

be subject to loads of 4 Newtons. The cross section of the piece is 2.5mm x 5mm. Modeling a

plucker as a cantilever beam, SolidWorks Simulation was used to evaluate the stress developed

in this critical part. The results are shown below.

Figure 21 - Results of SolidWorks Stress Analysis of Plucker

For detailed component and assembly drawings of all major parts, please refer to

Appendix C.

Budget of Expected Expenses

With the components of the product specified, a more detailed budget was drawn up.

This included the piezoelectric material, electrical components, and some testing equipment.

Items that are accessible at no cost are still included in the budget for thoroughness. The total

budget comes in at under $500, considerably less than the initial estimate of nearly $800. This is

due in part to the reduced cost of piezoelectric material obtained after some research. The

tabulated budget is shown below.

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It is important to note that the only aspect missing from the budget is manufacturing costs

due to 3-D printing. This is not something that can be counted as trivial, but cost estimations

were not obtained at the time of this report.

Figure 22 - Tentative Project Budget

Project Schedule

The Gantt Chart established in phase I was adhered to throughout the semester. Key

deadlines were updated as announced, but no major alterations occurred.

Figure 23 - Project Gantt Chart

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Impact of Design

With the design nearly finalized, it is now important to consider non-technical aspects,

including ethical, environmental, social, political, safety, sustainability, and standardization

concerns. Because this is a medical application, ethical and safety concerns are especially

important. In this vein, it becomes our duty as engineers to do our due diligence in designing a

safe product. Applying electricity to the body, even in small currents, can be lethal. Currents as

low as .10 mA applied to the chest can stop the heart. When delivered to the lower extremities,

currents above 10 mA are considered in the extremely painful to lethal range.

The design calls for 5 mA applied to the knee at maximum intensity settings, which is

within the safe range. The electrodes will be located on the inner portion of the knee brace,

which will prevent their accidental application to other parts of the body. Furthermore, the

chance of accidental operation of the device will be greatly reduced thanks to the on/off

mechanism. All electrical components will be contained in the housing, preventing any

unintentional augmentation and arcing. Furthermore, by using standard electrical components

versus a microcontroller, unintended outputs due to software malfunction are impossible.

The product is subject to FDA standards. Utilizing the 510K approval procedure, the

device must be deemed safe and effective for it to pass the FDA review. This procedure can

make use of previous devices of similar design or intended use.

Environmentally, the product makes use of a ‘green’ energy harvesting mechanism in the

sense that it does not require any power from the grid. Additionally, ABS plastic is a recyclable

material. Many of the electrical components will require special considerations for recycling.

There is a general push in society for environmentally friendly products, which gives the product

an initial advantage from a business standpoint. There is opportunity in creating a channel

through which consumers could recycle the product at the end of its lifecycle.

Conclusion

With the semester and first three phases of senior design coming to a close, the group is

pleased with the progress it has made. Phases II and III were more difficult than originally

expected. The considerations for the electrical systems of the product were definitely

underestimated at the start and required a substantial research and learning effort in order to be

understood enough to make educated design decisions.

Ultimately, the goal of the first semester of senior design is to provide a feasible and

detailed design such that prototype development is justified. The group is still waiting on quotes

from piezoelectric vendors, and these should be obtained within the next two weeks. With all the

parts identified, this milestone will be realized and parts can be ordered before the semester

closes for the Winter break. The group plans to hit the ground running in the Spring semester.

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Works Cited 1. Fawzi, Natalie: http://www.healthline.com/health/total-knee-replacement-

surgery/statistics-infographic

2. Ernesto C. Martinez-Villalpando, SM, "Agonist-antagonist active knee prosthesis: A

preliminary study in level-ground walking", Journal of Rehabilitation Researc &

Development 46.3 (2009); 361-374.

Appendix A

The diagrams shown display different piezoelectric configurations.

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Appendix B

This graph highlights the range of motion associated with various physical activities. It

will play a key role in technical analysis.

Appendix C

Technical Drawings

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