Gyro calibration table

Finally getting close to finishing the gyro calibration table. This was made from a re-purposed record player. I gutted the electronics, the tone arm, and even the motor. Replaced the motor with an electric starter motor for a nitro RC car.

I had problem with the speed control because the controller I first got was good for up to 1.5 amps. I had measured the motor resistance and 1.5 seemed enough. But when running the motor draws closer to 2.5 amps and the controller went into thermal shutdown. So I replaced the H-Bridge with a simple FET and a mechanical reversing switch. The gate of the FET went to the PWM output of the Arduino. Now I can control the speed for rather slow to ridiculously fast.

The next step was feedback on the speed. For this I painted on 96 black strips on the outside edge of the polished aluminum platter. I then used a reflective IR sensor to watch the stripes go by. And this is where I hit my second snag.

I could not get a single interrupt per stripe. I would get as many as twenty or thirty. After significant time investigating, I learned that the Arduino digital input pins have a Schmidt trigger with only 60mV of hysteresis. As the black stripe would swing into view, the voltage on the output would ramp up and not snap on. This allowed the noisy signal to be in the hysteresis band for enough time to trigger a gazillion interrupts. I was forced to condition the signal with a comparator wired for 1 volt of hysteresis before I could get good clean interrupts.

Next I needed to insure that the rotation speed calculations being done on the Arduino were correct. For this I turned to a trick my son taught me (one good reason to send your munchkins to college!). Place a contracting dot on the item that is spinning then take a picture of it with your SLR. Using ImageJ, measure the amount the dot was smeared. Now you can multiply the smear angle by the shutter speed and get the angular rate. My two values were very similar and I think I can now trust the Arduino calculations.

Progress is now on hold while I replace the master hard disk drive in my file server. SMART warned me it was going bad so out I went to by a replacement. Glad this happened now and not during the Taiwan flood when drives were horribly expensive.