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Unread 04-17-2012, 06:26 PM
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Re: paper: Shooter Wheel Speed Control

Originally Posted by Ether View Post
I should have been clearer:

The "low_command" is a tuning constant, not the target speed, and was not intended to be so low that it tops out at a speed below the "spinup_speed". "low_command" should be high enough that it briskly accelerates up to (and beyond) spinup_speed, at which point the full voltage is applied.

Many shooter wheel setups can handle application of full voltage to the motor even at zero RPM, but some can't. It's a function of the gear ratio and the wheel inertia and the motor being used. For such setups, simply set spinup_speed=0.0

The code I posted was for those shooter setups that can't handle a full 12 volts being applied at zero RPM, but could, for example, handle, say, 8 volts at zero RPM and 12 volts at 1000 RPM. For that example, you would set spinup_speed to 1000 RPM and low_command to 8 volts. Simple, and gets the job done.

With the code I posted, the minimum continuous RPM is simply commanded as the target speed by each team's software to whatever they want it to be, and the code I posted will bang-bang regulate at that speed. That target speed can be zero if the team so desires.

For really finicky setups (e.g. high inertia wheel, not much gear ratio), you could always add a third "else" statement:

if (measured_speed >= target_speed) motor_command = 0.0;

else if (measured_speed >= spinup_speed) motor_command = 1.0;

else if (measured_speed >= medium_speed) motor_command = medium_command;

else motor_command = low_command;

Got it. So, in essence, you want the absolute maximum acceleration possible, for the particular shooter design, that it can handle without tripping the over-current.

With the version I added today, you can set the trip point for low to high acceleration as well as the rate of slew rate limiter. By tuning these two, you can achieve almost exactly what you are describing. The only difference is the start up voltage will be a ramp, with an adjustable slope, instead of a step from 0v. Granted, it is not the absolute fastest acceleration you can get, but I doubt the differences will be noticeable in actual performance. You will see it on a scope, but that's pretty much it. I'm guessing a only few millisecond difference between the two designs from 0 RPM to target RPM in the 2500-4000 range.
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