[Jared Russell]

Our 2011 KOP Allen-Bradley sensors are being used for speed measurement, as Dustin noted. Here is our exact arrangement:

4.875" BaneBot wheels (black plastic), with the sensor looking at the side (e.g. visual axis of the sensor is parallel to the axle) of the wheel about 2/3 of the way between the axle and the outer diameter. We used a ~1.5" x 1.5" patch of retroreflective tape from an optical tachometer kit (very similar to the tape on the goals) at one point on each wheel. The sensor is about .5" from the plane of the wheel/tape. We used the trim knob on the sensors to ensure we got a strong, unambiguous 0/1 signal from the tape and the background. The sensors are powered by 24VDC from a solenoid breakout, and are designated as WPIlib 1x Counters (count rising edges only) in the code. We use getPeriod() on the Counters to measure the time between consecutive measurements.

We nominally spin our first wheel at 3000RPM, and the second at 4700RPM. But I have measured speeds up to about 6000RPM without issue (haven't tried faster than that). We do no filtering of the speed signal whatsoever.

We do speed control with a Feedforward + P controller, using Victor 888s in coast mode (RS550 motors with 3:1 reduction). We run the following algorithm at 100Hz:

Output = Max(0, [Kv * desired_rpm + Kp * (desired_rpm - actual_rpm)])

We are using a Kv that maps to the free speed spec of the motor (e.g. to spin an RS550 with free speed ~19000RPM at 3:1 reduction at 3000RPM, you want about 50% power).

We use a Kp of 0.01. This means if we are below our setpoint by >=100RPMs, we apply full power to the motor (regardless of Kv, so we actually go full tilt on the motor even closer to our setpoint using a nonzero Kv).

It is worth noting that our control scheme is exactly equal to bang-bang control if you set Kv = 0 and Kp = (infinity). We did not have time to do a lot of experimentation on the various control regimes; we simply found one that worked (similar to what we used in 2012) and ran with it.

Our software only lets the autonomous program or driver fire discs if both wheel speeds are within 120RPM of the specified value.