Voltage Control

Percent output commands a fraction of whatever the battery happens to be providing right now. Voltage control commands an actual, specific number of volts instead, and lets the motor controller figure out what duty cycle is needed to deliver it.

Compensating for Battery Sag

A modern FRC motor controller (a TalonFX or SPARK) constantly measures its own input voltage. If it's told to produce 6 volts and the battery is sitting at 12 volts, it uses a 50% duty cycle; if the battery has sagged to 9 volts under load, it automatically raises the duty cycle so 6 volts still comes out the other side. Commanding 0.5 produces a different result depending on the battery; commanding 6 volts produces the same result, regardless of the battery, right up until the battery sags below the commanded voltage entirely (at which point 6 volts simply isn't available to give, no matter the duty cycle).

$$\text{duty cycle} = \frac{V_{commanded}}{V_{battery}}$$

Why This Matters

Consistency is the entire point. A shooter flywheel commanded to "70% power" spins at a different speed on a fresh battery than a drained one, right when consistent shot speed matters most, late in a match. The same flywheel commanded to "10 volts" holds much closer to the same behavior throughout the whole match, since the controller is actively compensating for exactly the voltage swings that made percent output unreliable in the first place.

Voltage control is still open loop, nothing is measuring whether 10 volts is actually the right number to produce the outcome that's wanted, it's just a more consistent, more predictable version of "guess and hope." Getting from "a consistent voltage" to "the correct voltage for a specific desired speed or acceleration" is exactly what Feedforward Models are for.