On 01/10/2021 12:27 PM, Gene Heskett wrote:
On Sunday 10 January 2021 11:38:42 Jon Elson wrote:
On 01/10/2021 01:36 AM, Gene Heskett wrote:
On Saturday 09 January 2021 20:57:24 Jon Elson wrote:
On 01/09/2021 06:39 PM, Gene Heskett wrote:
Yes so idb drives it closer by overdriving it an "amount" so that
the friction locked point is closer to the desired point without
going over. The trick is to null the error without changing the
sign of the error. If the sign changes, then it oscillates, hard.
So you can narrow the friction caused deadband by a small amount
of overdrive, but you also cannot change the sign of the error.
Yes, that's the tricky part. Any time you introduce a
discontinuity into the transfer function,
it makes things unstable at that point.
Jon
Thinking along a different line, I am running the pwmgen at 4khz,
mainly because my ears have a "carhart notch" at least 120 db deep
so theres not a chance in hell I'd ever hear it. What would happen
if I dropped the pwmgen frequency to 400 hz. that same percentage of
a pulse coming into the controller would have 10x the length of time
to actually move the motor, which may drive it closer to a null than
tickling it with a 1% pulse 4k times a second. The trouble with that
is that both pwmgens run at the same frequency, and Jon's servo,
driving the spindle, may trip its current limits with the longer
pulses.
Jon? Can you opine on that? Is there a minimum frequency for those
toroids?
Well, at 50 KHz, which is what it was designed for, the
motor current is always continuous, but could look like a
sine wave with low inductance motors. At 4 KHz, it is
likely to already be discontinuous except maybe at high duty
cycles. So, it really won't make a difference to the
toroids. I suspect the
motors will "sing", maybe pretty loudly, with 400 Hz PWM,
but maybe you don't care.
Yes, a long pulse with 400 Hz rep rate could allow current
to build a lot. But, in a PROPERLY tuned PID system, there
should not be sudden excursions in pulse width. Now, you
have to watch out for
excessive acceleration parameters in the .ini file. My PWM
servo amp has TWO current limits.
The first (adjustable by resistor) limit just cuts off the
PWM pulse early, and resets on the next PWM pulse. The
second limit (not adjustable) trips around 20 A and latches
the servo amp into E-stop.
Requiring the pushbutton reset. With a nominally 17 amp limit set
resistively, the other. triggering the shutdown can be counted on one,
maybe 2 hands in all these years of using it for a spindle controller.
Oh, now one issue is with the PWM frequency being LOWER than
the servo period, that might make for some delay in updating
the PWM generator. Is this with a Pico controller or a
Mesa? I know my controller will immediately update the end
of the PWM pulse if the width is changed in the middle of a
cycle. I can't say for Mesa.
mesa, and that possibility had not occurred to me, my bad. So I'd
consider 2 khz as being the lowest workable minimum, and when the 17 amp
kicks in I do hear a chirp from the iron in the motor so it obviously
slower than the 4 khz pwm. And its possible its going to 100%. But IIRC
theres a .98 max output set. But it is not set there. PWM_SCALE is 46 in
this case. How does one figure that value to be 98% mathematically?
I have a max duty cycle parameter in my ppmc driver for the
PWM generator. I have no idea
whether the Mesa has a similar function or not.
Jon
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