paper: Shooter Wheel Speed Control

[Chris Hibner]

We switched to this type of speed control (thanks for the post, Martin).

One thing that we found that is important: do NOT filter the encoder signal. Let it come in without any averaging or filtering. Here are some reasons why:

1) The inertia of the wheel and the on/off method of controlling the motor will filter out the noise naturally (mechanically), so why bother. Your control will be precise as long as the noise is zero-mean. If the noise isn't zero-mean, then if you put in a filter your filter output will be offset by the noise mean anyway, so the filter doesn't buy you anything.

2) The noise on the encoder signal serves to give you more resolution on your output and gives you more precise speed control. That sounds like it's impossible, but it's true. What you ideally want is as many on/off transitions as possible. The noise helps this to happen. The effect is similar to the theory behind sigma delta modulators.

3) Unless your filter and your mechanical system are phase matched, your shooter speed will oscillate, much like what was described by John a few posts above. We experienced this first hand when we first tried this approach. I noticed the oscillation and said "duh! I forgot to remove the filter." We removed the filtering and voila - perfect speed control.


One important note: if you use your shooter speed as an input to some sort of shooting logic (for example: don't allow the robot to shoot unless the speed is within a certain tolerance of the commanded speed), then you should definitely filter the signal going into the shooting logic. Just bypass the filter for your speed controller.