On Monday 17 December 2012 21:12:02 andy pugh did opine:
> On 17 December 2012 23:39, Gene Heskett <[email protected]> wrote:
> > I am at my wits end with this controller, and am now ready to throw
> > more money at it.
>
> What is the problem? What are you trying to achieve?
Stable, suitably stiff control where if I ask it for 5 rps I get 4 to 6, 10
rps gets me 9 to 11, 15 rps gets me about 15, and 20 should be about tapped
out when the head gear is in low range.
With the rube goldburg feedback rig I have right now, I'm getting within 5
rps only, and while its stable at whatever it is below 5 rps, by the time
it gets to 12 rps, the gain is so high its oscillating 3 rps either side of
it at about 2 hertz/second. This is with all the control non-linearity in
the feedback circuit between encoder.velocity and pid.feedback.
> Does it now work better or worse than it did before you swapped the
> capacitor in the PWM to Voltage stage?
With it in, it speeds up hard, then coasts back down at about 1/2 hertz. A
sawtooth shape out of the encoder.velocity, sweeping over a 20% range.
Swapping the 10 uf cap there, for a .2 u-f raises the feedback pole to
about 2, or 3 hertz. Neither is solidly controlled because the mid band
gain in the controller is at least 20x what it is at 1 rps. At 12 rps, it
takes a Pgain of less than 1 to be stable, whereas at 1 rps, a Pgain of 20
is usable and quite stiff when loaded.
> How many fuses can you buy for the price of the new drive?
About 100. I've blown at least half that many already in the last decade.
That drive, at $125, looks like a heck of a buy for a line powered, full H-
Bridge PWM input controller capable of 20 Amps when attached to a decent
heat sink. I need 4 amps max with this motor.
There seems to be plenty of room in the existing box to hold it as its not
that different from the JT250 that is in there now.
But the fuses are almost an afterthought since the shutdown trip on this
driver can be set to trip off when the drive is at 90% load and wouldn't
ever need the 5 amp fuse I'd put in the holder. This JT250 is supposed to
have an overcurrent shutdown too, but it has yet to trip that I know of.
I could probably stabilize it much better if the feedback gain was a
straight line near unity, and the compression to bend the straight line
motion-rps out to compensate for the near exponential rise in gain in the
motor circuit was in the path to pid.0.command.
Humm, this gives me yet another idea to try. Send the velocity to
feedback, but drive pid.0.command like this:
net src.cmd motion-out-rps>>scale>>invert>>mult2(both ins)>>sum2.0.in0
|
--------------------------------->sum2.0.in1
sum2.out>pid.command
Then I could use the scale and scale offset to control how much compression
was exerted to turn down the pid.command input at the higher speeds.
Whether that would stabilize the P and I gain is TBD. At least I am
assuming that sum2 can subtract the increasingly negative output of
mult2.in0 as the requested speed rises.
another equally nasty thought, clip out all the filtering, both in the C41
and in the JT250's pot arm circuit, go back to PWM mode in pwmgen and just
bang the JT250 with the pwmgen output. But I've no clue how fast the optos
are in the JT250, and the opamps they drive aren't megahertz speed daemons
either.
But its a thought.
Cheers, Gene
--
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
My web page: <http://coyoteden.dyndns-free.com:85/gene> is up!
Roses are red;
Violets are blue.
I'm schizophrenic,
And so am I.
I was taught to respect my elders, but its getting
harder and harder to find any...
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