Velocity Control - Battery Compensation Term

[Brian Selle]

Originally Posted by Ether

I've seen the .8 number in various places for drivetrains, which include rolling friction, carpet deformation, multi-stage gearboxes, and gears/chains or belts/pulleys.

It would be very interesting to see test data for a quality 2:1 single-stage gearbox (properly assembled and lubed) connected directly to a properly balanced spokeless shooter flywheel, when the flywheel is spinning unloaded (i.e. not firing balls) at its operating speed. Volunteers?

I wrote down some numbers last night from one of our prototypes. The setup is not exactly as we discussed but a data point nonetheless.

(2) 775pros through 4:1 VersaPlanetary gearboxes driving a set of shooter wheels via 24T HTD pulleys (1:1) and belts. The shaft is nicely supported running on Thunderhex bearings. CTR Mag encoder on the VP.

A vbus command of 1.0 yielded roughly 4400-4500 RPM. The battery checked in around 13V before we started the test but I didn't record the exact voltage while it was running.

775pro free speed is 18730, so a perfectly efficient 4:1 gearbox would give 18730/4 = 4683 RPM. The efficiency factor for this free running system is 4400/4683 = 0.94 to 4500/4683 = 0.96

I forgot to check the motor currents but IIRC from the previous night it was on the order of 3-6 amps per motor. With the motor is running at 17600-18000 RPM this roughly matches the motor curve of 6-8.5 amps.

0.95 factor seems like a better number for a free running system. However, in the case of a shooter or intake, using a 0.8 factor may get you closer to your desired vbus because of slippage at the wheel/ball interface. For the prototype mentioned above, we calculated the theoretical RPM required to achieve our desired exit velocity based on a trajectory simulation. We wanted gear ratio to run the motors around 60-65% vbus. The closest fit just under the target was a 4:1 gearbox that put it at 50% vbus. In testing, the motors ran at 60-65% vbus. So 0.5/0.625 = 0.8 magic. A bit sketchy, based on a theoretical trajectory simulation, but my guess is that using a 0.8 factor for shooter/intakes will get you close.

[ozrien]

Originally Posted by Tom Line

When doing velocity control with the talon srx's, the velocity you get from your feed-forward term is going to be affected by battery voltage. Has anyone experimented or used a battery offset term to correct for this? Is it worth it?

Originally Posted by Paul Copioli

Tom,
The I gain is for things like that. We use I and P for velocity control and as long as you design your gearbox / motor combo with enough headroom then the I and P can compensate...

I totally agree with Paul.

And yet... we just added a form of battery compensation in the last CTRE installer. Check out section 10.8 in the latest Talon SRX Software Reference Manual. When enabled, the output of PIDF represents a portion of a specified voltage, which is compared against the measured battery voltage, yielding a voltage-compensated duty cycle.