Modeling motor control

[Kevin Sevcik]

Alright, I'm transferring a post from the current sensing thread over here to keep things better organized.

Originally Posted by kamocat

No, they can't modulate the PWM pin. If they did, that would be locked antiphase, because the MOSFET driver would alternate between driving the low side and driving the high side.

Noooo, the Jags aren't operating in locked anti phase, and the tan wouldn't if the PWM pin was modulated.

Locked anti-phase required reserving the entire h-bridge to go from forward bias to reverse bias in respective PWM phases. It's a Forward-Reverse phase modulation. That's definitely not what either Jag is doing.

Both Jags hold one half of the h-bridge constant. So in forward motion, the M- half is held high-OFF, low-ON for the entire PWM cycle. In the Tan Jags, the M+ high side is selected via the PWM pin, and the output disable is modulated, so the M+ low side would never turn on. So that's a Forward-Open modulation. The Black Jags do things differently and alternately drive the M+ high and low sides. That's a Forward-Shorted modulation, because we never turn on the M- high side.

So I was suggesting that the Tan hardware seems perfectly capable of the same Forward-Shorted operation as the Black if they modulated the PWM pin instead of the Output Disable pin.

[kamocat]

Oh, you're right, my mistake.
If they modulated the PWM pin, it would have the same control as the Black Jaguar.
Isn't that interesting? I wonder why they haven't changed that yet.

[Kevin Sevcik]

They very well may have changed it for the FRC firmware. Again with my caveats that this is all based off the reference source code released by Luminary Micro, which may not be identical to the FRC firmware. I'm not much interested in hunting up a disassembler for pick apart the FRC firmware just to double check things though. Anyways, the only reason I can think of for not doing it is if the don't trust the dead time protection in their FET drivers.

FETs all have some delay between turning off the gate supply and when conduction actually stops. So if you're simply turning off the high side and turning on the low side at the same instant, you can get "shoot-through" where you've actually shorted V+ to V- for some small amount of time. This is obviously bad, so most integrated FET drivers have provisions for adding "dead time" between turning one side off and the other on. The Tan Jag's driver says it has built in dead time, but Luminary Micro may not have trusted it, or may have discovered a problem with their implementation of it. Or they may have just not wanted bother with the possibility of a problem.

[Al Skierkiewicz]

It may well have been to help the bootstrap work.

[Ether]

Originally Posted by Kevin Sevcik

Both Jags hold one half of the h-bridge constant. So in forward motion, the M- half is held high-OFF, low-ON for the entire PWM cycle. In the Tan Jags, the M+ high side is selected via the PWM pin, and the output disable is modulated, so the M+ low side would never turn on. So that's a Forward-Open modulation. The Black Jags do things differently and alternately drive the M+ high and low sides. That's a Forward-Shorted modulation, because we never turn on the M- high side.

Just to clarify, even with "Forward-Open", the Tan Jag still has a path for the current through the Q4,5,&6 Zener diodes.

[Kevin Sevcik]

Originally Posted by Ether

Just to clarify, even with "Forward-Open", the Tan Jag still has a path for the current through the Q4,5,&6 Zener diodes.

Yes, but with a 1V drop, which will drain the current a fair bit quicker. And the more important difference that if your current decays quick enough that you'd start regenerative braking, the diodes will stop that. So the "Forward-Open" mode has an advantage in the (rare) case that a motor is turning fast enough with low enough current that you'd encounter reverse torque.

Come to think of it, this explains Marshal's report of different operating characteristics between the tan and black jags. If you're running the motor at high/full speed and drop to a lower PWM command, the current will decay quicker in the tan jag, but you'll never get reverse torque, so you'll "coast" during the off cycle. In the Black Jag, there's a path for reverse current during the off cycle, so it's possible for the back emf to build up a reverse current and create reverse torque to slow the motor down quicker.

I think this makes sense, but I don't have any simulations set up at the moment to test it.

[Ether]

Originally Posted by Kevin Sevcik

Come to think of it, this explains Marshal's report of different operating characteristics between the tan and black jags. If you're running the motor at high/full speed and drop to a lower PWM command, the current will decay quicker in the tan jag, but you'll never get reverse torque, so you'll "coast" during the off cycle. In the Black Jag, there's a path for reverse current during the off cycle, so it's possible for the back emf to build up a reverse current and create reverse torque to slow the motor down quicker.

I think this makes sense, but I don't have any simulations set up at the moment to test it.

That is a reasonable hypothesis, and makes sense intuitively, but the analysis doesn't support it (unless my math or assumptions are wrong - which is certainly possible).

With a CIM and a Black Jag: even if the CIM is going at full throttle no-load, it takes almost 45 microseconds for the 2.7amp no-load current to decay to zero if you short the motor. The Jag's PWM period is 33 microseconds.

[Ether]

Originally Posted by Kevin Sevcik

Yes, but with a 1V drop, which will drain the current a fair bit quicker.

I plugged an additional 1 volt into the analysis mentioned in my earlier post; it makes very little difference with a CIM.

I don't have L values for other motors.

And the more important difference that if your current decays quick enough that you'd start regenerative braking, the diodes will stop that. So the "Forward-Open" mode has an advantage in the (rare) case that a motor is turning fast enough with low enough current that you'd encounter reverse torque.

Good point. That may (or may not) happen with other motors.