Gait Simulator, Part II

This is a retrospective post so there aren't as many pictures and I won't go into as much detail (if you're really going to try and construct a gait simulator, let me know and I'll gladly fill you in on the details).

Stryker was sufficiently satisfied with my Senior Design Project that they asked me to expand on the project to make it more capable. Like Part I, this was a fairly complex project so I'll just list out the expansions and additional capabilities below in an attempt to keep it brief:

1. The major update was the computerization of the simulator. Using an Arduino Duemilanove and Arduino programming language (C++ derivative that I had to teach myself), I was able to juggle multiple inputs and outputs at the same time. This was a significant improvement over the finicky potentiometer in the previous design. 

2. The board above the Arduino was an add-on with upgraded reed-type relays and 4 IC's to help with the control of 4 new components. Two new electronic valves allowed each piston to be actuated independently of each other (the previous 4-way valve was actually a binary valve. It only had two states: OpenA/CloseB or CloseA/OpenB) and two new electronic pressure regulators allowed the air pressure in each line to be adjusted dynamically, instantly, and independently. For instance, if the force platform determined that the "heel strike" wasn't receiving force indicative of a 200lb person, it would let more air into the first piston on the next cycle to increase the force exerted on the "heel". The programming here employed some recursive formulas to take into account the previous two cycles' data so the adjustment by the air pressure regulators wouldn't ping-pong back and forth (overshoot and undershoot the target force).

3. The foot platform was updated with a force sensors to supply real-time force data that the Arduino would use to dynamically adjust the amount of air going into each piston. This is to ensure that the same force curve is applied during every cycle of gait, which creates a more biomechanically relevant simulation (as people generally exert the same amount of force on their feet step after step regardless of foot orientation).

4. An auto shut-off program would cut in after 10 cycles of significantly decreased force (implant failure). A physical on and off switch was also integrated into the board.

5. A counter was integrated into the program so you wouldn't have to manually count the curves in the Instron data (yes, we had to do this on the previous design).

The final product (pictured above), was a truly self-contained, complete system. A technician could input the desired weight of the simulation (anywhere from 100-500lbs) and hit the start button and then the machine would do its thing until implant failure. There's a video below showing the machine in motion:


This was probably the most challenging project I've ever taken on. I had never dealt with PIC's before this and the force sensors were pretty finicky (took about 3 weeks to calibrate them). I also didn't have the appropriate plugs to interface with the electronic pressure regulators so I had to fabricate my own plugs out of old molex connectors I had laying around and hot glue. Even getting the simulator to sit statically on the Instron was a headache (500lbs is a lot of force!)

In the end, it was the magnitude of the challenge that made the victory so sweet. I would totally do this again if I had to.