MECHANICAL ENGINEER
Robotic Exercise Device Lower Joint (IHMC Robotics)
About the Project
The project was done during my summer internship at IHMC robotics. The grasshopper (or hopper) is a robotic exercise device that uses electric actuators rather than weights to resist motion. The device is being developed for NASA's Mars habitat to allow for working out in space, as well as by Cobalt to eventually produce a commercial version. My role on the project was to design and prototype the new lower joint component of the hopper, which contained a counter balance and a centering system.
Design Requirements
The lower joint of the hopper had to contain a counterbalance system that counteracted the torque caused by the weight of the actuator at any point during its rotation. It also needed to be able to rotate left and right (perpendicular to the rotation joint) by 10 degrees in each direction, and center itself when not under load
Overall Design
I used a spring cam system as the counterbalance. This consisted of a stainless steel cable attached to the moving joint on one end and a plunger on the end of a large compression spring on the other. As the joint rotated, the compression spring was compressed, providing a resistive force to counter the weight of the arm. To account for the changing torque needed to balance the arm at different points in its rotation as well as the changing force applied by the spring the more it was compressed, the cable wrapped around a cam profile with a changing radius. This profile was calculated using Matlab.
The left right centering was accomplished by inserting the counter balance assembly into a bushing block with compression springs in it, that allowed 10 degrees rotation in either direction while naturally forcing the assembly to the center. It also included a tapered locking mechanism that could prevent rotation when engaged.
Results
By the conclusion of my internship, I was able to finalize the design of the lower joint and construct a working prototype. The lower joint was later put into production, and made into a fully-functional robotic exercise device, which was tested by NASA.
