Muscle Strength Testing Device Capstone Project

Project Description:

For my Senior Capstone Engineering Project, my engineering team and I collaborated with a client who was a Doctor of Physical Therapy. Since we were the first engineering team to take on this project, our goal was to create a first-iteration muscle strength testing device for various isometric strength tests. 

Our Design goals were to: 

  • Construct device capable of accurately measuring isometric shoulder flexion muscle strength muscle force output
  • Construct device capable of measuring force to a magnitude of at least 100 lbf with a margin of error within 1 lbf 
  • Device height adjustable

Work Completed:

The mechanical design of the device was first modeled and simulated with SolidWorks. Static stress analysis in Solidworks was also applied to test if the material is capable of sustaining patient forces, from this analysis we were able to decide on cheaper material with adequate yield stress. 

Afterward, we ordered 80/20 metal parts for our frame assembly which we then mounted onto a plywood base. We chose to use 80/20 metal parts since it allows for rapid prototyping, easy customization, and is strong enough to withstand the forces of our application.

As shown above you can see the device being used for strength testing the shoulder flexion movement. You will also notice we designed the testing pad in Solidworks which we then 3D printed to hold the load cell inside. The testing pad is integrated into the frame which is the point of contact for where the force is being applied.

We constructed our device to be vertically adjustable, we used linear bearings with locks to traverse along the two 80/20 beams. This allows for patients to apply forces in a variety of movements and also accounts for different patient heights.

For the electrical portion of our design, we used a load cell sensor hooked up to an amplifier and an Arduino. Our load cell circuit is set up in a Wheatstone bridge configuration, which is used to measure an unknown resistance. When a mechanical force is applied to the sensor, the sensor undergoes elastic deformation, which changes the resistance of the loadcell, this altered value of resistance causes voltage fluctuation, this electrical signal is then amplified and received by the computer which it then converts it into a force measurement.

At first, we used a breadboard to hook up our electrical circuit but we found that it took up too much space and also did not support strong wire connections. To solve this, we implemented an IO Shield and soldered all the connections there. This resulted in a more compact and robust electrical design. In addition, we modeled housing for both the load cell and the whole circuit in Solidworks and then 3D-printed them. 


Lessons Learned:

This project gave me valuable experience in building up my own engineering skills while working in a team and collaborating with a client all while taking on a real engineering project from the ground up. I got to build up my experience with using SolidWorks, 3D printing, programming, and electronics. I learned more about how load cell sensors work and how to make them work. I learned how to implement an automatic tare function as well as new conditional statements for ease of gathering accurate and consistent data