Percent Output and Duty Cycle Control

The simplest possible way to command a motor is to just say what fraction of full power to use, 0.5 for "half power," -1.0 for "full power in reverse," 0.0 for "stop." In WPILib, this is what a plain motor.set(speed) call does, and it's almost always the very first way any FRC programmer commands a motor.

What "Percent" Actually Means

Under the hood, this is a duty cycle: the motor controller switches the full battery voltage on and off very quickly, and "50% output" just means the switch is on for half of every cycle. A brushless motor controller running trapezoidal or sinusoidal commutation does the equivalent thing across its phases, but the idea is identical, percent output scales how much of the available battery voltage actually reaches the motor, moment to moment. This same switching trick shows up again from two different angles later on, as the actual PWM voltage-control mechanism inside a brushed motor, and as a general-purpose signal for talking to hardware in the Hardware Communication addendum.

$$V_{motor} \approx \text{percent} \times V_{battery}$$

Why "Percent" Is a Deceptive Name

Here's the catch this whole section exists to fix: that equation has $V_{battery}$ in it, and the battery's voltage is not constant. A fully charged battery might sit near 13 volts; a battery that's been driving four motors hard for two minutes might sag to 9 volts or lower under load. Command 0.5 on a fresh battery and a nearly dead one, and the voltage the motor actually receives, and therefore its actual speed, is different both times, even though the code asked for the exact same thing.

That's not a bug, it's simply what "percent of whatever's currently available" means. Percent output is perfectly fine for anything a human is actively correcting in real time, like driving with a joystick, since a driver naturally compensates for a robot that feels a little sluggish. It's a poor choice for anything that needs to behave consistently, run after run, battery after battery, which is exactly the problem Voltage Control solves.