FRC971 Spartan Robotics 2016 Release Video

[AustinSchuh]

Originally Posted by AdamHeard

The one on the left is a potentiometer (and do to limited # of turns has to be near the output).

The one on the right is an encoder. I know 971 goes out of their way to eliminate backlash as much as possible (like custom sized hex shafts), so putting it earlier in the reduction yields higher resolution (traded off against backlash, which in this case is minimized).

Correct. The backlash on each of the joints is on the order of 1/16" at the end of the long arm. We have found more value in having high resolution for the encoder than having low backlash. Ask me again in a couple years, and we may have a different opinion...

We hook up the index pulse on the encoder so we get a very accurate pulse once per revolution. That pulse triggers DMA to capture the encoder value at that point in time. This lets us figure out the encoder value very accurately. Unfortunately, there will be something like 30 of these pulses through the range of motion on the arm. A slow moving filter estimates which one of these 30 pulses we saw, and uses that to zero within 1 encoder tick.

We started doing this in 2015, and haven't looked back. This takes all the hard work out of zeroing joints with hall effect sensors on our robot. We wrote the class to support this in 2015, and have been steadily improving it and adding features. All we need to do is to have the software exercise the joint until it passes an index pulse, or have a human move it past an index pulse while disabled, and we are then fully calibrated.