Step or velocity control for servo gantry

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12 years 1 month ago #51662 by DaBit
Replied by DaBit on topic Step or velocity control for servo gantry
- Setup all the scalings so that units make sense. If PID output is 10, then the motor should turn 10 revs/sec. This is not necessary from the computer point of view, but it helps the poor guy watching halscope and entering parameters a lot.

- Assuming the PID does position in, velocity out: set FF0 to 0.0, FF1 to 1.0, FF2 to 0.0, P to some small value (1.0 or so), I and D to 0.0.
- Make sure you set the 'error-previous-target' pin of the PID controller to 1, otherwise you have a velocity-dependant following error.
- Set FERROR and MAX_FERROR to high values. Make them 1000 or so.
- Do some jogs, watch over/undershoots on f-error at the beginning and ending of the jog. Try to compensate them by adding a little FF2. Compensate a linear increase/decrease in f-error by adjusting FF1 until the f-error is approximately flat during cruise.
- Now increase P until the system becomes unstable, then backoff.

This should give you very decent tracking already. Try to arrive at this point first.

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12 years 1 month ago #51693 by crisiacuf
Replied by crisiacuf on topic Step or velocity control for servo gantry
It seems I have the PID setup corectly but when I make FF1 = 1 this happens:

Attachment not found





Here is my HAL for X axis:
Code:
#******************* # AXIS X #******************* setp pid.x.Pgain [AXIS_0]P setp pid.x.Igain [AXIS_0]I setp pid.x.Dgain [AXIS_0]D setp pid.x.bias [AXIS_0]BIAS setp pid.x.FF0 [AXIS_0]FF0 setp pid.x.FF1 [AXIS_0]FF1 setp pid.x.FF2 [AXIS_0]FF2 setp pid.x.deadband [AXIS_0]DEADBAND setp pid.x.maxoutput [AXIS_0]MAX_OUTPUT net x-index-enable <=> pid.x.index-enable net x-enable => pid.x.enable net x-output => pid.x.output net x-pos-cmd => pid.x.command net x-vel-fb => pid.x.feedback-deriv net x-pos-fb => pid.x.feedback #my insert setp pid.x.error-previous-target 1 # ---PWM Generator signals/setup--- setp hm2_5i25.0.7i77.0.1.analogout0-scalemax [AXIS_0]OUTPUT_SCALE setp hm2_5i25.0.7i77.0.1.analogout0-minlim [AXIS_0]OUTPUT_MIN_LIMIT setp hm2_5i25.0.7i77.0.1.analogout0-maxlim [AXIS_0]OUTPUT_MAX_LIMIT net x-output => hm2_5i25.0.7i77.0.1.analogout0 net x-pos-cmd axis.0.motor-pos-cmd net x-enable axis.0.amp-enable-out # enable _all_ sserial pwmgens net x-enable hm2_5i25.0.7i77.0.1.analogena # ---Encoder feedback signals/setup--- setp hm2_5i25.0.encoder.00.counter-mode 0 setp hm2_5i25.0.encoder.00.filter 1 setp hm2_5i25.0.encoder.00.index-invert 0 setp hm2_5i25.0.encoder.00.index-mask 0 setp hm2_5i25.0.encoder.00.index-mask-invert 0 setp hm2_5i25.0.encoder.00.scale [AXIS_0]ENCODER_SCALE net x-pos-fb <= hm2_5i25.0.encoder.00.position net x-vel-fb <= hm2_5i25.0.encoder.00.velocity net x-pos-fb => axis.0.motor-pos-fb net x-index-enable axis.0.index-enable <=> hm2_5i25.0.encoder.00.index-enable net x-pos-rawcounts <= hm2_5i25.0.encoder.00.rawcounts # ---setup home / limit switch signals--- net home-x => axis.0.home-sw-in net min-x => axis.0.neg-lim-sw-in net max-x => axis.0.pos-lim-sw-in

What linuxcnc version do you have? My head hurts trying to compare my hal with yours :)

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12 years 1 month ago #51694 by PCW
Replied by PCW on topic Step or velocity control for servo gantry
FF1 =1

only makes sense if your output scale is in machine units per second at full scale

So for analog velocity mode drives the output scale (= scalemax and maxlim and -minlim for 7I77)
= how fast your axis move in mm/s at 10V velocity command

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12 years 4 weeks ago #51700 by DaBit
Replied by DaBit on topic Step or velocity control for servo gantry
What PCW said. And you seem to be using only the scale as position feedback. This is going to be pretty hard and will result in a sluggish loop at best since you only have 200 pulses per millimeter to work with and any form of nonlinear mechanics will bite hard. And since metal is not infinitely stiff and nothing can move without play the nonlinearity is there. Maybe small, maybe not, but it is there.
On my mill the encoder on the motor and lead of the screw is the primary positioning method. The scale is only used to correct residual errors and it does so only 'slowly'.

Forget about the scale at this point and make the servo work flawless without it first. Thinking about adding the scale to the positioning loop before that makes no sense.

I am running 2.7.0~pre BTW, but that does not matter. It would work the same with 2.5 or 2.6, especially if you set pid.x.error-previous-target=1

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12 years 4 weeks ago #51703 by crisiacuf@yahoo.com
Replied by crisiacuf@yahoo.com on topic Step or velocity control for servo gantry
My axis moves at 25 mm/s @ 1V measured
My encoder velocity corectly shows 25.xxxx @ 1V
Max speed is 250mm/s @10V

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12 years 4 weeks ago #51704 by DaBit
Replied by DaBit on topic Step or velocity control for servo gantry
Then make:

[AXIS_0]OUTPUT_SCALE -> 250
[AXIS_0]OUTPUT_MIN_LIMIT -> -250
[AXIS_0]OUTPUT_MAX_LIMIT -> 250

and make sure that the axis moves with 10mm/sec in one direction when you set hm2_5i25.0.7i77.0.1.analogout0 to 10 and 10mm/sc in the other direction when you set hm2_5i25.0.7i77.0.1.analogout0 to -10

Did you also connect the rotary encoder outputs of your servo drive to the 7i77? If not you should do so. Once again: forget about the scale for now, and get that servo running properly first.

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12 years 4 weeks ago #51705 by crisiacuf@yahoo.com
Replied by crisiacuf@yahoo.com on topic Step or velocity control for servo gantry
Got it!

Why you use the rotary encoder was my next question but you answered it allready.
What other mesa board beside 7i77 do you use for the rest of the encoders? XYYZ takes 8 encoder inputs and 7i77 hax only 6.

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12 years 4 weeks ago #51706 by DaBit
Replied by DaBit on topic Step or velocity control for servo gantry
My Z-axis is driven by a stepper, so I need only 7 encoders (Z only needs 1 encoder-input for the scale).
I loaded the 7i77+7i76 firmware on the 6i25 and used a small breadboard with 3x SN75176 and some extra electronics on it for the stepper channels and extra RS422-capable encoder channels.

With 7i77+7i76 firmware on a 5i25/6i25 you can do up to 8 encoders. More than that and you probably need custom firmware, but it can be done. I tried building custom firmware once, and it worked. But so far I have no need for it.

I am sure that it is possible to buy another piece of Mesa hardware for the extra encoder inputs (7i48 maybe?), but I'll let PCW elaborate on that.

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12 years 4 weeks ago #51710 by alan_3301
Replied by alan_3301 on topic Step or velocity control for servo gantry

setp pid.x.Pgain [AXIS_0]P


shouldnt this be pid.0.Pgain?

in the docs <loopnum> is a number, don't know if a letter works?

Ignore if I am wrong.

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12 years 4 weeks ago #51717 by PCW
Replied by PCW on topic Step or velocity control for servo gantry
To add additional differential encoders, I would use either a 7I85 or 7I85S.
Either of these options will add 4 encoder channels that can be TTL or differential
The 7I85S may be more suitable if you anticipate needing any step/dir or PWM outputs

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