Dual PID loops with motor encoder + scale encoder per axis

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19 Apr 2024 14:39 #298620 by PCW
Not sure its worth the effort for dual loops since you are starting
out close to velocity mode. I would see where you get with single
loop tuning first.





 

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19 Apr 2024 16:43 #298625 by Tinine
Sorry but he has nothing close to velocity mode.

Example: A fixed command of 50% and a velocity drive, within its capacity will maintain 50% velocity as the load increases. It would increase the current to the motor.
The drive that the OP has, a fixed command of 50% will mean 50% current output, no matter what. Increase the load and the motor will slow down.

Craig

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19 Apr 2024 17:14 #298630 by TangentAudio

Sorry but he has nothing close to velocity mode.

Example: A fixed command of 50% and a velocity drive, within its capacity will maintain 50% velocity as the load increases. It would increase the current to the motor.
The drive that the OP has, a fixed command of 50% will mean 50% current output, no matter what. Increase the load and the motor will slow down.

 

Hi Craig,
To be clear this is actually not what I observed.  If I command 50% output, it seems to output a fixed voltage roughly 1/2 of the max voltage of the drive supply.  If I increase physical load on the motor, current to the motor will increase dramatically as it tries to maintain speed.  Voltage at the motor does drop slightly (just a few volts, but keep in mind it's at the end of 10 feet of cable). Current will increase by a large factor depending on the load.  For example at 50% the motor draws 0.5A when unloaded.  That will jump up to several amps as I apply a load.

I measured RPM, voltage and current at the motor for the full range of the PWM.  That was all measured with an unloaded motor.  I've reattached the graphs since that post was a couple pages back.

I did not have a way to apply a consistent load to the motor (grabbing the pulley with a rag) so I couldn't really produce the same graphs with varying constant loads.  But my observation was that it behaved more like a bench supply in CV mode rather than in CC mode.  I think that's a fair analogy to what the drive is doing?

I really do appreciate the discussion.

cheers,
Steve
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19 Apr 2024 17:26 #298631 by Tinine
Hi Steve,

Is this an open-loop command or under PID. IOW, you're saying that as the load increases, your motor command remains the same?

Craig

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19 Apr 2024 17:28 - 19 Apr 2024 17:29 #298632 by TangentAudio
That graph was done in open loop. I disconnected all of the PID signals and was just controlling the PWM output value directly.  And yes, the observation about current increasing with load - that's with the same PWM value and also open loop.
Last edit: 19 Apr 2024 17:29 by TangentAudio.

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19 Apr 2024 17:37 - 19 Apr 2024 17:41 #298633 by PCW
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.


 
Last edit: 19 Apr 2024 17:41 by PCW.

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19 Apr 2024 17:39 #298634 by Tinine
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

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19 Apr 2024 17:44 #298637 by TangentAudio
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.

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19 Apr 2024 17:46 #298638 by Tinine

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  ) but none of this jibes at all. Been exclusively torque-mode and dual-loop for 3 decades 

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