January 24, 2015
It was a smaller than usual build session today. Joseph, Sammy, and Cole were off at an awesome astro photography field trip, and Ed, Ian, and Conner were busy with The Bay School Play.
Thelonious built the right foot:
Gabriel and Tony worked on a promising new sensor system. We’ve previous tried Hall Effect Sensors, but they weren’t very reliable, so then we tried slide potentiometers which worked better. However, last week it became apparent that the slide potentiometers worked best when the wearer was strapped firmly to the armature. The problem is, if the wearer is attached to the armature they can’t move the sliders. That observation caused us to consider using something called a force resistive sensor. These are devices whose resistance drops as a force is applied. The flat square pad shown at left here is one such device:
After some experimentation we realized that the voltage reported by the force resistors had a very non-linear relationship to the applied force. To compensate, we measured the actual relationship between force and voltage by placing loads of known weight on the sensor, then measuring the voltage:
This data allows us to convert voltages measured by the Arduino into forces. That’s important because the wearer feels the force they apply to the armature, and they expect the suit to respond accordingly. Here’s the strap we built to wrap around the wearers shin:
It has two force resistive sensors; one positioned on the shin, and one on the calf. The idea is that the Arduino measures the forces applied at these sensors, and uses the force difference to determine which direction to move the joint. The data we collected looks good, so the next step is to test it on the suit. To that end, we built a breadboard version of a control system to do that:
We are hopeful that this approach will be robust enough to allow us to walk reliably. Once we’re walking we can finalize our budget and launch the Kickstarter campaign.