[kamocat]

Ether is correct, PID-based speed control is your best option. Know that you must test and tune this until you can trust it to work how you want.


Now, about how Jaguars control motors: This is my understanding; please correct me if I'm wrong.
Say you have a DC brushed motor that draws 6A at 12V DC under normal load. That's 72W.
Now say you're driving it at 50% duty cycle, at 15khz. (I think that's the chop rate of a Jaguar). That motor coils have enough impedance that the motor effectively gets the RMS voltage - the average voltage. At 50% duty cycle, that would be 6v. Using ohm's law, we say that the motor would draw 3A. That makes 18W; a quarter of full power.

Now let's compare that to a 2 ohm resistor, with no significant impedance. The 50% duty cycle is still 12v, but only half the time. That means we take 60W / 2 = 30W. In this case, the Jaguar is better described as controlling current, as opposed to the voltage control in the previous example.

Victor 884's have a chop rate of 150hz, if I remember correctly. I know it's very poor control, and very audible at low speeds. It's clear that the motor impedance is not enough to make up for the low chop rate, and so the Victor effectively controls current. (I've looked at the waveform of a Victor 884 controlling a Globe motor. At 50% duty cycle, the inductive ring almost fades completely before the MOSFETs switch.)
Could this be why the control resembles a square root function? The Victor has a square root function internally because P=R*I^2, when (because of the low chop rate) the equation is actually P=V*I? (The only manipulated variable here is I; V and R are constants.)