Mesa 7i77 and linear encoder tuning PID.

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17 Aug 2012 16:09 #23349 by ROG
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What’s would you expect delay to be for a motor to show rotation with no load ... 2 Ms ? .... better than 2Ms?

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17 Aug 2012 17:21 #23353 by PCW
Depends on the drive and drive tuning. In torque mode, most motors cannot reach full torque at 0 RPM in less than 1-5 ms due to motor inductance (and are slower to reach max torque at higher speeds due to back EMF). Since the velocity loop does not seem to be working very well I expect that the inner torque loop on the drive is not being commanded a very high torque until the analog velocity command is quite high.

I would try and tune the drives velocity loop first since I think this is the root of the problem. To do this I would add the siggen component and wire its square wave output to one of the analog outputs, set the frequency to maybe 5 Hz and the
amplitude to say .5 (so +-5% of full speed) Then tune the drives loop for best performance (no oscillation, perhaps some small rapidly damped overshoot after the step.) HALScope should be set to monitor and trigger on the siggen square
and monitor the rotary encoder velocity.

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17 Aug 2012 17:41 #23355 by andypugh
ROG wrote:

Servo sitting on bench, no belt / no load.


What are the units of pid.error? 2m on a mm scale (0.002mm) might be good.

Do you have any deadband?

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17 Aug 2012 18:03 #23359 by ROG
Units are mm ....
Deadband was 0.0001, legacy from linear encoder.
Just looking at the response of the drive / motor with no mechanical load really with those plots, not really gone anywhere near tuning anything as such, that’s all over the place now

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17 Aug 2012 19:04 #23364 by ROG
I would try and tune the drives velocity loop first since I think this is the root of the problem. To do this I would add the siggen component and wire its square wave output to one of the analog outputs, set the frequency to maybe 5 Hz and the
amplitude to say .5 (so +-5% of full speed) Then tune the drives loop for best performance (no oscillation, perhaps some small rapidly damped overshoot after the step.) HALScope should be set to monitor and trigger on the siggen square
and monitor the rotary encoder velocity


With servo attached or not attached to machine .... motor only ?
I’ll have a go with this in the morning .....

Thanks everybody.

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19 Aug 2012 17:15 - 19 Aug 2012 17:20 #23398 by ROG
After 30 hours of tweaking … over the weekend, I’ve drawn the conclusion that the drives are going in the bin.
In my opinion, they are un-tunable if the servo is attached to any kind of load.
I set the velocity loop in the drive as PCW suggestion.
Initially with the servo off the machine and no load. Worst following error including acceleration phase, 0.3 microns.
Motor then put back on machine, and tried with same drive settings, …. massive instability / oscillation.
Set FF1=1 ….. And re-tune drive velocity loop with servo on machine.
Speed loop gain Cn-04 (Hz) – this affects the acceleration ramp.

Speed loop integration constant Cn-05 (ms) – this needs to be set to the minimum value, otherwise there is a delay before acceleration commences.

Cn-17 Torque command filter time constant (ms) - any amount of this adds significant delay before any movement of the servo. Has to be set to zero to get anything like any kind of immediate response.

So, if the last two parameters are set to minimal values, then speed loop CN04 – Gain, has to be such a low value that although the acceleration ramp begins quickly, with the minimal settings of CN05 and CN17, but is not steep enough to keep up with the demand.

Any increase, then causes instability, unless CN17 and or CN05 are increased, which adds a delay before the acceleration ramp commences..

Viscous circle.

50mm / Min Zoomed out

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19 Aug 2012 17:16 - 19 Aug 2012 17:17 #23399 by ROG
50mm / min Zoomed in

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Last edit: 19 Aug 2012 17:17 by ROG.

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19 Aug 2012 17:18 #23400 by ROG
2000 mm / min Zoomed out

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19 Aug 2012 17:19 #23401 by ROG
2000 mm / min Zoomed In

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20 Aug 2012 11:36 #23410 by andypugh
How bad is it? Does the axis wobble by an appreciable amount, or is it mainly a noise problem?

Have you tried talking to Omron?

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