Week 10: Input Devices
My objective this week was to make a force sensor which could be used as a basis for a power measuring system for cycling. Typically such systems involve attaching strain gauges to drive train components on a bike such as the rear hub or pedal crank arms. Systems normally cost in the region of £1500 ($2400). I am interested in the idea of measuring the motive force as it is transmitted through a cycling shoe creating a cheaper system that is easily transferred between bikes (in the shoe). My force sensor relies on measuring the change in capacitance as two plates separated by an elastomer pad move closer or further apart as force varies.
The first step was to manufacture the step response transmit/receive board. This was straight forward except for a short, which was discovered later, between the Rx and ground. This was due to a burr on the track beneath resistor R3 which was subsequently removed. 
The capacitance plates comprised two small areas of copper sheet from vinyl with soldered leads stuck to thin plywood backing.
The C program was embedded and the python script ran on Ubuntu. The system functioned correctly. I decided to use art foam for the elastomer material and initially produced a response curve using a single thickness. I calculated I wanted to read forces in the range 0-500N or 0-50kg force (approx.) and found that a single thickness of the foam was not linear over this range. A double thickness did not reach its elastic limit. I plotted force vs. reading over this range and determine the slope and intercept for the linear relationship. I used these data to modify the python script to produce a direct reading of kgf.
Next steps will be to find a thinner force sensor arrangement which is suitable to be inserted in a shoe or under the cleat. I also want to compare its performance to that of a strain gauge fitted in or attached to the shoe.