Dual PID loops with motor encoder + scale encoder per axis

That's what I meant with:

"the inherent velocity feedback from the BEMF makes it so"

You do have velocity feedback (and damping) in a open loop PWM system
The gain in the loop is inversely proportional to the motor resistance.

If you slow the motor so its BEMF is less the the applied voltage, the current increases.
If you speed the motor so its BEMF is greater than the applied voltage, the current reverses
to oppose the motors motion.


 

And between the drive and the motor, you only have power cables, no tachometer feedback, right? Only two wires (thermal sensor notwithstanding)?

This is sounding quite bizarre. Negative feedback needs to be calibrated which is either a tach signal (potentiometer) or a D-term.

Craig

Correct, just the +/- wires between the drive and the motor. There's no analog tach on the motors, just a quadrature encoder that has no connection to the drive electronics.

PCW wrote:

That's what I meant with:

"the inherent velocity feedback from the BEMF makes it so"

You do have velocity feedback (and damping) in a open loop PWM system
The gain in the loop is inversely proportional to the motor resistance.

If you slow the motor so its BEMF is less the the applied voltage, the current increases.
If you speed the motor so its BEMF is greater than the applied voltage, the current reverses
to oppose the motors motion.




 

Heck, literally live with this stuff and surrounded by it as I type (I'm weird image missing /media/kunena/emoticons/wink.png) but none of this jibes at all. Been exclusively torque-mode and dual-loop for 3 decades image missing /media/kunena/emoticons/dizzy.png

And yet, with zero command, you can rotate the motor by hand, right?

Craig

PCW wrote:

That's what I meant with:

"the inherent velocity feedback from the BEMF makes it so"

You do have velocity feedback (and damping) in a open loop PWM system
The gain in the loop is inversely proportional to the motor resistance.

If you slow the motor so its BEMF is less the the applied voltage, the current increases.
If you speed the motor so its BEMF is greater than the applied voltage, the current reverses
to oppose the motors motion.



 

A velocity loop maintains commanded velocity which can be anything including zero velocity. The drive responds to disturbances to maintain commanded velocity including zero velocity. The actual RPM makes no difference. Turning by hand when commanded velocity is zero, constitutes a load disturbance. IOW the velocity loop won't allow it.

Craig