Modeling motor control

[EricVanWyk]

Marshall -

To account for motor heating, consider re-ordering your tests pseudo-randomly. This will spread the time dependent effects across your sample space. Hopefully, this will result in a curve that makes more sense (but will likely be "wider").


Calculating effective motor voltage from Vbus * Duty Cycle is actually a pretty good method as long as the motor current is continuous (doesn't go to zero), and your measurement of Vbus is good enough.

Measuring Vbus half way through the pulse is a reasonably standard method. Assume that the applied voltage waveform looks roughly like a trapezoid: It starts at Vbus nominal when the switch starts, linearly(ish) droops due to increasing IR loss, and then goes to zero when the switch stops. The measured Vbus looks just like this, but with some ugly ringing around the transitions. By measuring halfway through, we kill two birds:
1) We are far away from the ringing.
2) We effectively take the average of the Vbus across the cycle (if it droops linearly).
Pretty sweet, eh? Simple way to get a darn good estimation. If you think about 2) a bit, you'll see why there usually isn't a filter in place - it would mess up the reading! If there is one, it usually has a very small time constant compared to the pulse width.

Continuous current is absolutely key to using effective motor voltage calculations - if the current stops, the underlying assumptions fall apart entirely. This is "The Big Difference"(TM) between Jaguars and Victors, and is why they have such different responses.
In a continuous current control mode, the effective motor voltage is (nearly) equivalent to what would happen with a pure DC source of the same voltage.

The reason for this gap is that when current is stops flowing, the motor is effectively floating with no applied voltage. If current is always flowing, the motor controller is always in control of the applied voltage.

Continuous current occurs whenever the off period is shorter than the amount of time it takes the current to decay to zero. Mathematically, this puts a relationship between switch frequency (on and off periods, actually) and minimum average current. That is to say that for a given frequency and duty cycle, there is a minimum average current to establish continuous current control. For a Jaguar, this is a few amps. For a Victor, this is a few hundred.