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? > Jon > > > _______________________________________________ > Emc-users mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/emc-users Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) If we desire respect for the law, we must first make the law respectable. - Louis D. Brandeis Genes Web page <http://geneslinuxbox.net:6309/gene> _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
