Dual PID loops with motor encoder + scale encoder per axis

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02 May 2024 08:43 #299534 by rodw
I don't think your home switch will work. Home switches need to stay triggered until the end of travel. This lets Linuxcnc which direction to travel to find the triggering edge. I predict some big crashes under the current config.

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02 May 2024 12:40 - 02 May 2024 13:12 #299546 by TangentAudio
Hi Rod,
Unless I'm missing something, I'm not sure I follow your concern. The switches are located at the end of travel on each end of the table. They stay active for at least 10mm of table travel beyond their initial trigger point. They're positioned as limit switches really, but I should be able to home off of the left one as well.

I know it looks like there's more travel on my Bridgeport table beyond the switches, but it's not actually usable travel. The motor mount interferes with the saddle on the left side which makes a physical limit. On the right side, the bearing casting does clear into the saddle, but this travel isn't really usable as the X axis gage wheel sensor runs off the end of the table, meaning the X axis DRO stops working. The usable travel at each end is essentially where the edge of the table lines up with the edge of the saddle on the respective side, ~24 inches total.
Last edit: 02 May 2024 13:12 by TangentAudio.
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02 May 2024 13:39 - 02 May 2024 13:46 #299551 by Tinine
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Last edit: 02 May 2024 13:46 by Tinine.

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02 May 2024 14:46 #299559 by TangentAudio
I must admit that I'm no longer sure what we're debating.  But here's a reflection of my current understanding.

The original motors do not have an analog tacho, but they do have a digital incremental encoder.  The drive hardware appears to be an open-loop H bridge that is PWM controlled.  The motor encoder does not connect to the drive hardware, but it did connect to the original microcontroller-based control unit.

Other than when stopped, the open loop drive hardware seems to act like a velocity drive.  Commanded PWM value correlates to speed, not torque.  It is not commanding the drive to output a current, it's commanding the drive to output a voltage. 

When stopped with a 0 PWM command, it does appear to be a bit like a torque drive in that the shaft can be rotated (though there is some resistance as you try to spin it quickly). 

However, in the original system, there was a control loop running on the microcontroller that took take the place of the tacho feedback in your drawing, and if it "saw" motor encoder movement when stopped, it would command a PWM value to try to maintain the zero position.  And likewise when moving, it could more closely control the rotational speed to the commanded value by monitoring the encoder in the control loop.

If my understanding is correct, as a system, the original inner loop probably acted like a proper velocity drive, but the feedback was implemented with a digital encoder and a control loop in firmware instead of with analog tachos and op amps.  If we focus on just the hardware portion of the drive as we have been, it's really not exactly a velocity drive (but it is similar), nor is it a torque drive (except it kind of looks like one when commanded to stop).

 

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02 May 2024 15:25 #299566 by Tinine

  It is not commanding the drive to output a current, it's commanding the drive to output a voltage. 


 

The voltage is what produces the current. No voltage - no current.

A velocity mode drive rotates the shaft at an RPM that is proportional to the command signal based on the velocity-gain setting.
They are rate devices.

With zero command, the negative-feedback will fight any attempt to rotate the shaft by hand.

If you attempt to drive this motor with only the P-term (I=0,D=0), it will overshoot.

Craig
 

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02 May 2024 15:43 #299569 by TangentAudio
But if it was a torque mode drive as you seem to suggest, for a given commanded PWM value the motor current should NOT increase when the motor is mechanically loaded, and it very demonstrably does, by significant amounts.

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02 May 2024 17:22 #299570 by TangentAudio

I don't think your home switch will work. Home switches need to stay triggered until the end of travel. This lets Linuxcnc which direction to travel to find the triggering edge. I predict some big crashes under the current config.

I had some time at lunch today, so I wired up the switches to my temporary control breadboard.  The left switch is configured as both X- limit and home, and the "disable limits while homing" option is set in my ini.

When not homing, both switches act as limits and will e-stop LinuxCNC, which I have tested but did not demo in the video.  I have the soft limits set correctly so it stops before tripping the limit when jogging.



cheers,
Steve

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02 May 2024 17:28 #299572 by Tinine
Well to take it to an extreme, as an example. What happens when to lock a rotor of a motor and switch on the power? You are slowing the motor down and therefore reducing the back-EMF.

Craig

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02 May 2024 18:00 #299579 by rodw

Hi Rod,
Unless I'm missing something, I'm not sure I follow your concern. The switches are located at the end of travel on each end of the table. They stay active for at least 10mm of table travel beyond their initial trigger point. They're positioned as limit switches really, but I should be able to home off of the left one as well.

Forget useable travel.
For homing, The machine cannot be allowed to be positioned on either side of the home switch and the switch be off in both these positions. It must be set up so it is on on one side and off on the other side of the home trigger point. This lets Linuxcnc know where it is in relation to the home switch. I learnt the hard way.

Please read homing configuration in the docs until you understand this. linuxcnc.org/docs/stable/html/config/ini-homing.html

If using a separate homing switch, it is possible to start homing on the wrong side of the home switch, which combined with HOME_IGNORE_LIMITS option will lead to a hard crash.
 

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02 May 2024 18:11 #299581 by TangentAudio

Forget useable travel.
For homing, The machine cannot be allowed to be positioned on either side of the home switch and the switch be off in both these positions. It must be set up so it is on on one side and off on the other side of the home trigger point. This lets Linuxcnc know where it is in relation to the home switch. I learnt the hard way.

Please read homing configuration in the docs until you understand this. linuxcnc.org/docs/stable/html/config/ini-homing.html

If using a separate homing switch, it is possible to start homing on the wrong side of the home switch, which combined with HOME_IGNORE_LIMITS option will lead to a hard crash.

Rod, it is not possible to start homing on the wrong side of the home/limit switch in my setup.  The limit switch (left switch) is at the physical limit of where the table can travel due to a mechanical interference between the motor mount and saddle.  This is what I mean by usable travel.  In the photos I posted, it may look like the table can travel more in the X- direction beyond the switch, but it can't.

I really do appreciate the heads up (I'm not immune to dumb mistakes by any means), but just for the record, I have been using LinuxCNC for about 15 years and I'm not a stranger to how it handles homing.

cheers,
Steve
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