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PORTABLE KNEE FORCE SENSOR
Michael BurnsJason Atwood
Krizia PliskinEric RederscheidWyatt Dickerson
Ian SchlieperCasey Webster
ME Senior Design Team 6Client: Dr. Troy FlanaganDirector: Prof. Jack Zable
TABL
E OF C
ONTE
NTS Background
Specifications
How it Works
Design Evolution
Final Product - The JOEY
Mechanical Features
Electrical Features
Software Features
Testing
Challenges & Conclusion
Team Careers & Special Thanks
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Over time, NBA players can experience small micro tears in their patellar tendon. These microtearscompromise the tendon's strength and can result in substantial pain. Research has shown that performingspecific isometric exercises everyday can improve the function of the patellar tendon. Tracking playersperformance in these exercises can give key insight to recovery and injury prevention. The purpose of thisproject is to provide the Milwaukee Bucks NBA team with a portable device that can accurately measure andtrack a variety of isometric exercises while the team travels during the season.
BACKGROUND
The Milwaukee Bucks' primary solution for monitoring knee force is the Kanga-Tech KT360. However, this device is not portable therefore creating gaps inplayer data while they are on the road during the NBA season. The primaryfunction of the KT360 is to analyze the force output of a athlete by having theuser hold an isometric exercise against the sensor heads. This data is displayedin real time which allows the player the ability to make small adjustments whiletraining. Once the data is recorded, it is then sent to be stored in the Bucks'database for further analysis.
70%of NBA players report significant knee pain from patellar tendonitis
Alternative solutions are handheld dynamometers shown to the right. Whilethese devices are portable they do not read high enough force values and due tofailure to lock onto a rigid surface, their accuracy is questionable.
There is no device on the market that meets the requirements of the Milwaukee Bucks. Inresponse, the design team developed a product that meets the following three main objectives
PORTABILITY ACCURACY VERSATITILITY
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SPECIFICATIONSTo ensure the functionality and effectiveness of this device, the team developed engineering requirements andspecifications that were tracked throughout the course of the project. All of which were centered around theportability, accuracy, and versatility of the product. The table below showcases the 6 most critical specifications that the team focused on throughout the project. Intotal, the team operated within an additional 13 specifications and requirements not listed in the table below.
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HOW IT WORKS
From very early on in this project's development, its core functionality was set in place. Above, one can see across-sectional view of the interaction between the player, device, and roller. When the player extends their leg(shown on the top left), the exerted force is transmitted to a Comfort Pad (A). This pad conforms to the playersleg while also compressing the rest of the assembly. Below the Comfort Pad is the Top Plate (B). Thiscomponent ensures that the applied force gets distributed equally among the sensors (C). The sensors arespecially configured strain gauges. The top most part of the sensor, the button, creates a single point ofcontact with the Top Plate. When deflected, an analog signal proportional to the measured displacement in thestrain gauge is converted via a calibration factor into a force reading. The last component in the generalassembly is the Back Plate (D). This foundational component acts as the housing for the sensors and interfaceto the workout equipment roller as show in the figure above. Throughout the project, each of thesecomponents became more complicated while still achieving their core functionality.
FORCE
A
B
CCD
ROLLER
Note: This detailed view is rotated 180 degrees
DESIGN EVOLUTION
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V0. INITIAL PRETOTYPEDefined Major Systems; Force Pack, Brain, & AttachmentDetermine Physical SizeSimple & Intuitive UseModular Focus
V1. PROOF OF CONCEPTBluetooth ConnectionTethered to, and powered by a ComputerSensor AccuracyTPU Pad EffectivenessMaterial Strength
V2. FULLY FUNCTIONALSent to ClientExternal Brain Unit Provided PowerLarger Pad Profile to Accommodate NBA Player SizeClumsy Setup
V3. ALL IN ONE SOLUTIONIntegrated Electronics in to Force PackFormal Test QualificationsResized Battery & Force Pad
V4. USER IMPROVEMENTSLED DimmingIn-Unit Battery ChargingBattery Removal LeverElectronics Lid Pad
V5. FINAL PRODUCT - "JOEY"Carbon Fiber Composite FrameHorizontal Strap Slots AddedFinalized On-Board Software
Spring Semester
Fall Semester
The JOEY is a Bluetooth Capable, Portable Force Measurement System that attaches to various pieces of workout equipment. This product gives the NBA’s Milwaukee Bucks the capability to
track various strength metrics while traveling.
GENERAL USEThe device is easy to use and can be fully set up to record data in under 5 minutes. The iterative design processused in this project allowed for the team to engineer the final product with users in mind. The JOEY attaches toexercise equipment via two buckled straps. When the blue LED light on the top of the device turns solid, there isstrong Bluetooth connection with the device's web app and data is ready to be recorded.
Strap the JOEY onto exercise machine
Navigate to www.JOEYforce.com
Connect your device via Bluetooth
Begin Workout
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The JOEY consists of six manufacturedcomponents, which can be seen to the left.The Back Plate (A) is the base to the JOEY, itholds the sensors, electronics, battery, andstraps in place. The Snap (B) holds thesensors from above, eliminating the possibilityfor the wire interference. The Force Plate (C)normalizes the force from the athlete to thesensors. A Comfort Pad (D) utilizes a GyroidInfill (E) to distribute the applied load evenly.Through several comfort tests, the teamdetermined that the Gyroid Infill provided avery compressible pad that could easily springback to form after use. The Lid (F) protectsthe electronics from external factors andrestricts battery motion. For safety, allexposed corners are rounded. The Lid also hasan additional TPU pad (G) on top of it in casean athlete comes in contact with it.
MECHANICAL FEATURES
Interchangeable Pads
Accessible Battery Replacement
Restricted Vertical Motion
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(A)
(B)
(C)
(D)
(F)
(G)
(E)
ELECTRICAL FEATURES
The JOEY’s electronics consist of four 50 kg load cell sensors, an HX711, Raspberry Pi Zero W, LED,and battery. The load cells are strain gauges configured into a Wheatstone Bridge (pictured below)that provide a differential voltage proportional to the applied force. The HX711 then amplifies thissignal and converts the voltage measured into a digital signal. The Raspberry Pi reads this digital
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Load Cell
signal, translates it to a floating point number, and then sends thevalue ten times a second to the web app via Bluetooth. All of this isdone through on-board software in the Raspberry Pi. The Bluetoothconnection between the JOEY and the web app is indicated by thestate of the LED. The LED’s light flashing indicates the JOEY is inpairing mode and ready to connect. A solid light indicates the JOEYand web app have connected successfully. The battery; an AnkerPowerCore+ Mini, provides up to 16 hours of power to the JOEY witha USB-A to micro-USB cable connected to the Raspberry Pi whilealso allowing for in-unit charging and easy replacement.
HX711 Raspberry Pi Zero W Web App
SOFTWARE FEATURESFor the software portion of the project, the team hired a contractor to fill in the lack of Computer Scienceexpertise. Casey Webster can be reached at [email protected] with more information at kc-visual.com.
There are two components to the Software running this product: the internal software to the JOEY device, andthe software in the form of a web application for user interaction. The JOEY software acts as a passivefunction that reads the digitally amplified message and updates the shared force variable with the web app.
Key specifications while designing the software platform were: device accessibility, ease of use, and cloudconnection. The use of a web app allows users to interact with the product on any device capable of aninternet connection. Common iPhone apps were used as inspiration to give users a sense of familiarity and isconnected to Google Firebase.
The Milwaukee Bucks Training Staff has access to an administrative console within the web app. This tool isused to create custom workouts for the athletes and to access all previous recordings and key statistics suchas the max output force and average output force.
USER INTERFACE
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All hardware for the JOEY was fabricated using 3D printers allowingrapid prototyping and multiple design iterations. The first fiveprototypes were printed out a plastic PLA material. For robustness,the internal structure was made of triangles, which is the strongestshape in nature. To ensure the device was strong enough towithstand the maximum expected load of 2000N (450 lbs), the teamexecuted a 3-point bending test on the Universal Testing Machine,shown on the left. The two fixtures on the bottom were separated ata distance simulating the sensors. A third fixture applied anincreasing force from the top in the middle of the plate, until theplate failed. This test was repeated multiple times resulting withfailure at 3000 Newtons (675 lbs) providing a safety factor of 1.5.
TESTING
A critical aspect of this project was ease of use. The teamsgoal was for a new user to pick up the device with no priorknowledge and successfully record data. During the V3, V4,and V5 iterations, the team conducted multiple tests wherethe new user was given the JOEY alongside the user manualand left to their own intuition to setup the device and recorddata. One of these user tests is shown on the left. Throughout this process the team made note of certain areasof the user experience that seemed unclear and made therequisite adjustments in the user manual to provide moreclarity. Of the 12 total user tests conducted across threeprototypes, all users were successful in operating the JOEY.
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The Universal Testing Machine was implemented to qualifymany different specifications of the project, one of whichwas to validate 5% accuracy in force readings. One of themany tests conducted can be seen in the graph on the right.This particular test consisted of force being applied to theJOEY in 250 Newton increments, pausing for ten seconds ateach load, then continuing. The team then compared thedata recorded by the JOEY to the applied load. The JOEYpassed this test by being within the 5% allotment for error.Repeating this test multiple times then gave the teamconfidence the precision of the JOEY.
CHALLENGES AND LESSONS LEARNEDThroughout the 9 month process that went into developing the JOEY and the corresponding web app, the teamovercame many unique challenges. The first challenge was clear and concise communication. This unique NBAseason made meeting times limited. It was then consequently important to deliver all necessary information asefficiently as possible. To over come this, the team used tool such as: Trello to track action items, Slack forconsistent communication, and weekly email updates for our Client and Director. The second challenge was manufacturing several prototypes in the COVID environment. Social distancingdictated that the team needed to be flexible and cohesive in order to “pass off” different components of theproject to one another during manufacturing. Through this process, the team learned the value of collaborationand trust in one another. Distribution of work was also a main focus of the team to ensure everyone wascontributing a fair amount of work. The final challenge the team overcame was balancing the size and functionality of the device to find the mosteffective configuration. It can be seen throughout the design evolution that the size and shape of the devicewas in constant flux. There was a fine balance between too large and cumbersome and too small anduncomfortable to use, or underpowered electronically. The final product displays how the team learned to workthrough an iterative process to build on each prototype leading them to their best iteration, being delivered atthe end of the project.
CONCLUSION
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The final state of the JOEY system is accurate, portable, and versatile. It is able to repeatedly take data within5% of the actual applied load. This allows the Milwaukee Bucks to be able to depend on the product. When thetraveling case is fully packed, the entire package only weighs 3.2 lbs and can fit into a standard backpack. TheJOEY can record strength metrics on a variety of different workout equipment. Even in the absence of thesemachines, the system is able to attach to a rigid structure and maintain its core functionality. The MilwaukeeBucks will receive four copies of the final product to be able to implement this technology throughout theirteam and have hardware for years to come.
Wyatt Dickerson • Ian Schlieper • Krizia Pliskin • Jason Atwood • Michael Burns • Eric Rederscheid
Michael BurnsProject Manager
Michael will be working as a Mechanical Engineer at In-Situ, a Water Quality Device Company in Fort Collins, CO
Jason AtwoodLogistics Manager
Jason has been selected to attend Euro NATO Joint Jet Pilot Training and is awaiting active duty Air Force order to begin training at Sheppard Air Force Base in Texas.
Krizia PliskinFinancial Manager
Krizia will be working as an Analysis & Test Engineer at Roccor, a Small-Satellite Aerospace Company in Longmont, CO
Eric RederscheidCAD Engineer
Wyatt DickersonTest Engineer
Eric will be working as a Product Development Engineer at REP Fitness, a Fitness Company in Denver, CO
Ian Schlieper Systems Engineer
Wyatt is currently working at Spark in Boulder, CO while he continues his search for a career in Design, Manufacturing, or R&D.
Special Thanks
Casey Webster - App Developer
Dr. Steinbrenner and Dr. Kotys-Schwartz - Mechanical Senior Design Instructors
Dr. Flanagan and the Milwaukee Bucks Organization - Client
Professor Jack Zable - Project Director
CU Idea Forge & ITLL
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Ian will be working as a Flight Test/Electronic Warfare Engineer at MTSI, an Aerospace Defense Contractor in Colorado Springs, CO
