Re: stepper-servo system

Graham,

 I'm interested in your EDM work.

 What is the instruction cycle time on the propeller?
 My work with 1656/7 pics as edm osc's were choked by slow clocks ( 20mHz ) and clumsy loops.
 
 How many cycles to 'normal' instructions?
 This determines the max oscillator freq.
 But you may want 2 cpu's clocking each other ( one for on, one for off )

 Or you may be looking at a dynamic offtime based on gap condition..
 Something like Iso-energetic pulses where the on time is preceded by a process dependant voltage
( the sequence is: wait for the gap to allow a high-ish voltage (say 90 to 120 V=)
  wait for the high-ish voltage to drop into the discharge zone ( say 35 to 28V=)
  then begin decrementing the 'on-time'
  then begin decrementing the off time,
  repeat 4ever )

Now for your 'stepper in a servo loop',
I think you mean to control a stepper motor by the process, not a velocity of position loop.
I dont know of public algorithms, but there has been work done.
A hardware system I've used had 2 thresholds, which evaluated average gap voltage into 3 classes,
open, cutting, bad. ( A window comparator )

Open conditions allowed forward pulses ( changed state of amplifier's DIR signal, and enable the Step input )
 The 'velocity' was proportional to the difference between a desired target voltage ( say 30V )
 and the actual average gap voltage ( say 28V ) , this gives a +2 error.
 The proportion is determined by (Err / Range) * Gain

Good conditions were a null point in the control system.
The Step line is disabled ( or just not en-abled).
No motion is desired when the cut has no error.
 Just wait a few pulses and it'll shift off this nullpunkt :-)

Bad conditions changed the sense of the amplifier's DIR line.
 A separate gain can be used for 'running away'

I wrote up an Eagle circuit for the EMC group
http://wiki.linuxcnc.org/cgi-bin/emcinfo.pl/emcinfo.pl?ElectricalDischargeMachining
it's a gap sensor with gap-error out, with 0 to 5 V signal (where 2.5 is null, 0 is runaway & 5 is charge! )
So it's all analog, you'd have to add the window comaprator to get the 3 state output ( fwd,wait,bwd).
and some oscillator that output STEP pulses porportional to the ERROR size would be needed.

I used a higher gain on runaway than for the infeed ( safer )
You may even add another rate device for a 'jump'

keep in touch with your progress.

oh, and the guys at Heidenhain suggested a low pass filter at 500Hz,
they said just chuck anything over that!

If you use a dual trace scope with 1 on the sensor output
and the other on the process, you can evaluate the circuit on a working system.
At this time the circuit is merely evaluating, not commanding any motors.
It's leeching off the working system.

By hooking the steppers up with no electrode, but driven by a running system,
and you previously tested sense circuit...
you can evaluate the motor response ( I like to stick a junk screwdriver into the gap
at this point to see how quick the system can react! Very effect and very practical.)
These motor's are just 'pupetting' ,just reacting.

Last to close the loop, attach the electrode to you system, move the power leads
over to your tool and try cutting. :-)
Leave the motor to leadscrew connection exposed ( belt or coupler)
It's motion is more informative than a scope!
Nervous but progressing is good. Sluggish is bad. Buzzing is bad.

regards
TomP
tAG