Mesa 7i96s PID Spindle Setup

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1 year 7 months ago #323685 by scda
Mesa 7i96s PID Spindle Setup was created by scda
Hi all,

I am working on a weiler cnc lathe retrofit. So far it works well, the only thing left ist the PID tuning of the spindle speed. 
The spindle also works without PID but the offset to the actual speed varies quite a bit (not linear), I was hoping to correct this with a PID loop.

VFD: Huanyang with analog voltage input (0-10V).

Currently I am trying to figure out how to implenent the PID setup for this.

How can I scale the PID output to 0-10V (or 0-4600 rpm) ? Currently my PID output can be negative and so the pwmgen.value of the Mesa7i96s spindle is negative which does not work for my setup. I feed the PID ouput to the scaled pwmgen input.

I think the system has to adjust the current value sent to the pwmgen.value just below and above the current setpoint in order to adjust for velocity offset. I was thinking of using the sum function (which kind of works but also does not really correct the non-linearity) but I think there must be an easier way...

Cheers,
David

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1 year 7 months ago #323686 by PCW
Replied by PCW on topic Mesa 7i96s PID Spindle Setup
Normally this is just done with normal PID,
with FF0 set to 1 and PWM scale set to RPM at 10V

PID would mainly be for Keeping constant RPM vs load.
If you have major non-linearities, these can be corrected with
the lincurve component.

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1 year 7 months ago #323689 by scda
Replied by scda on topic Mesa 7i96s PID Spindle Setup
Thanks. Yeah I think the main problem was that I have set the P value too high (above 2 is too much --> runaway). So the system seems to be working now.
  • I set a non-zero value to the DEADBAND setting (10)
  • I am using a low pass filter on the encoder feedback signal before feeding it to the PID loop
  • I use a low P value to stop the system "from running away"
Code:
#====================================================================================== #  Spindle0 #====================================================================================== setp   pid.s.Pgain     [SPINDLE_0]P setp   pid.s.Igain     [SPINDLE_0]I setp   pid.s.Dgain     [SPINDLE_0]D setp   pid.s.bias      [SPINDLE_0]BIAS setp   pid.s.FF0       [SPINDLE_0]FF0 setp   pid.s.FF1       [SPINDLE_0]FF1 setp   pid.s.FF2       [SPINDLE_0]FF2 setp   pid.s.deadband  [SPINDLE_0]DEADBAND setp   pid.s.maxoutput [SPINDLE_0]MAX_OUTPUT setp   pid.s.error-previous-target true setp hm2_7i96s.0.pwmgen.00.enable 1 setp hm2_7i96s.0.pwmgen.00.scale [SPINDLE_0]MAX_RPM setp lowpass.0.gain [SPINDLE_0]LOWPASS_GAIN setp    hm2_7i96s.0.encoder.04.counter-mode 0 setp    hm2_7i96s.0.encoder.04.filter 1 setp    hm2_7i96s.0.encoder.04.index-invert 0 setp    hm2_7i96s.0.encoder.04.index-mask 0 setp    hm2_7i96s.0.encoder.04.index-mask-invert 0 setp    hm2_7i96s.0.encoder.04.scale  [SPINDLE_0]ENCODER_SCALE net spindle-index-enable      =>    pid.s.index-enable net spindle-enable            =>    pid.s.enable  net spindle-vel-cmd-rpm         =>    pid.s.command         net spindle-vel-fb-rpm          =>    pid.s.feedback net spindle-output            <=    pid.s.output        => hm2_7i96s.0.pwmgen.00.value     net spindle-vel-cmd-rps        <=    spindle.0.speed-out-rps     net spindle-vel-cmd-rps-abs    <=    spindle.0.speed-out-rps-abs net spindle-vel-cmd-rpm        <=    spindle.0.speed-out         net spindle-vel-cmd-rpm-abs    <=    spindle.0.speed-out-abs     net spindle-enable        <=    spindle.0.on             # =>    hm2_7i96s.0.pwmgen.00.enable net spindle-cw            <=     spindle.0.forward        =>    hm2_7i96s.0.outm.00.out-04  net spindle-ccw            <=    spindle.0.reverse         =>    hm2_7i96s.0.outm.00.out-05 net spindle-revs            =>    spindle.0.revs     net spindle-at-speed        =>    spindle.0.at-speed net spindle-vel-fb-rps      =>    spindle.0.speed-in            <=     hm2_7i96s.0.encoder.04.velocity net spindle-vel-fb-rpm_raw    =>    hm2_7i96s.0.encoder.04.velocity-rpm    =>     lowpass.0.in     net spindle-vel-fb-rpm        <=    lowpass.0.out net spindle-index-enable    <=>    spindle.0.index-enable

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1 year 7 months ago #323692 by PCW
Replied by PCW on topic Mesa 7i96s PID Spindle Setup
You should never get a runaway, but you will get oscillations with a too high PID value

To avoid a runaway around 0 speed where a negative PID output may occur might require
some hal magic, say using the PID output sign to run the VFD direction signal or forcing the
PWM value to 0 when the PID output is negative.
 

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