Help to ID and find manuals for this Gould Gettys Analog Servo

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30 Sep 2024 12:11 #311061 by rmavalente
Hello Mates,

I'm in the process of retrofitting this INDUMA Knee Milling Machine from NUM750 to linux CNC, right now the machine is almost running.

It happens that I have no data at all on the servo drives. All I know they're from Gould Gettys. The Y axis has some drift I'd like to adjust, but I do not know which pot adjusts it. 

Googling the driver look similar to the A121 N360 from Gettys. 

Another doubt on analog servos: they tend to drift and slow move, even with 0V voltage reference. What is the best strategy to hold axis in position? Internal PID corrections or to disable the servo output?

Greetings from Brazil!
 
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30 Sep 2024 12:34 #311064 by Todd Zuercher
It is completely normal for an analog velocity mode servo to drift when the position loop is not being closed. Yes you must have some form of position feedback to the control system commanding the servo. Usually that would be an encoder connected to Linuxcnc with a PID loop controlling the velocity command to the servo. Normally the servo should be disabled unless Linuxcnc is in control of if and sending commands to correct for any drift. (In other words that enable signal you have jumpered on the drive's input pannel needs to be connected to a digital output from Linuxcnc.) That will stop the drive from drifting when Linuxcnc is not enabled (once you've tuned the position loop in Linuxcnc.)

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30 Sep 2024 16:21 - 30 Sep 2024 16:22 #311075 by rmavalente
Hello,

tks for the reply. I`m using MESA 5i25 + 7i77, with glass scales.

This enable setting its original from Induma, would you suggest to have individual axis enable (three outputs) or just one enabling all axis is enough? What would be the output name in PNCConf?
Also, I've one output to energize the axis controllers, dos PNCConf can handle it or should I use classic ladder?

I was reading some topics here about axis PID calibration and found interesting that the controller runs heavy on P (20..40) and almost no I (0) or very very little D values. Is this usual? Kinda different from other PIDs I've used in industry.

Sorry for the amount of questions, LCNC is new to me and Im very excited with it!
Last edit: 30 Sep 2024 16:22 by rmavalente.

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30 Sep 2024 20:34 #311096 by Todd Zuercher
Just one enabling all of the drives is enough, While the 7i77 has individual outputs for each analog output, I think they are all switched together by the same hal enable pin.

On my Mesa 5i25+7i77 machine it is this line of hal code that enables/disables all of the servo drives.
net x-enable <= joint.0.amp-enable-out => hm2_5i25.0.7i77.0.1.analogena

As to the balance of PID constants in Linuxcnc vs other control systems I can't really say. It may have something to do with Linuxcnc's relatively slow servo thread rate making large amounts of I-term unstable? I do know that my machine which uses a torque commanded servo does use quite a lot of I-term. I have to be careful and use the error limit functions in the PID component to prevent excessive windup and overshoot. But most Linuxcnc's velocity commanded position loops are served best by mainly using P, and some FF1 (and maybe FF2)
The following user(s) said Thank You: tommylight, rmavalente

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