Rev2 Circuit Complete

A project log for Human Power Output Measurement Device For Cycling

A training tool that provides real-time human power output and form feedback

valgusvalgus 10/18/2014 at 22:070 Comments

The Rev2 circuit was built and tested [awhile ago]. The design intent was to create a signal conditioner and amplifier for a full wheatstone bridge, single-rail supply (6-18VDC), and jumper settable gain options of 10, 50, 100, 500, and 1000. By design, the output range is 0-5VDC, centered at 2.5VDC. However a potentiometer could easily replace the voltage divider that provides the 2.5VDC reference voltage for the instrumentation amplifier if a different reference is desired.

In the photo above, you can see the LM7805 5VDC voltage regulator. In the next prototype, a batter charging circuit, boost converter (to get 5VDC from 3.7VDC LiPo battery), ATmega328, LE bluetooth module, and gyro will be added. The boost converter will replace the inefficient voltage regulator in circuit Rev2, but was suitable for early prototyping and sensor validation. The Analog Device's instrumentation amplifier is top middle, surrounded by polypropylene filtering capacitors. And finally, five 0.1%, 50PPM metal film resistors (one on bottom of board) provide the 5 different gain options depending on the position of the white jumper. This turned out to be a fortuitous design addition, whereby I can easily increase the gain to ridiculous levels to facilitate balancing the bridge, and then quickly change to a lower gain for measurement use. Noise levels were similar to the Rev1 design, approximately 10mV Pk-Pk. As previous results have shown, this is suitable for my purposes, and combined with simple digital filterring techniques, measurement uncertainty is approximately 1-2lbf at 340lbf.

Right now the largest hurdle moving forward is developing an Android application that will read the sensor data, and display it in a meaningful way. For prototyping, I purchased a LE bluetooth module (Bluefruit LE - nRF8001 Breakout - v1.0) and will continue working towards this end. If anyone is interested in helping/collaborating, please contact me.