On Tuesday 08 December 2020 19:56:13 Chris Albertson wrote: > If the output of the PID controller is force ad the measured feedback > is the position, a P only controller will always oscillate. Friction > will damp it eventually. A negative "D" will reduce the velocity and > help with overshoot and "I" is required if the final result has an > offset. > > The root cause is the force is a couple of steps removed from > the position. > > If you want the system to go full speed then slam on the brakes at the > last minute and hit the marks then you need a controller that has a > physical model of the system and use that to predict the future and > plan ahead. PID is always reactive. It can't look ahead. > > But PIS can come close. After you get P tuned "close" try "D" and > depending on how it is wired may be negative > I'll give that a try tomorrow. Or maybe right now since I wasn't sleeping well. And with a Dgain of -2.5 got a 22mv error at 101 degrees a minute with little, maybe 10% ringing on the rising edge, and a less than 10% peak "effort" in reverse to stop it, That is a major improvement and doesn't trip off the psu getting stopped. What does that 22mv mean in folllowing error since I have that set for about half an inch at the moment?
This I think might be good enough to go ahead and finish the mount machining and get it installed on the bs-1. With all the excitement and sadness here, I've only been working on easily interruptible things. Thanks Chris. > On Tue, Dec 8, 2020 at 4:42 PM Gene Heskett <ghesk...@shentel.net> wrote: > > On Tuesday 08 December 2020 19:05:52 Chris Albertson wrote: > > > What next? You have set P-gain but what about I and D gains? > > > > That has been tried, gently, but doesn't seem to have the desired > > effect. > > > > > On Tue, Dec 8, 2020 at 12:49 PM Gene Heskett > > > <ghesk...@shentel.net> > > > > wrote: > > > > Lo all; > > > > > > > > I'm back to playing with the servo again, following John > > > > Thorntons instructs from the wiki article. And I note a couple > > > > things. > > > > > > > > 1. The pid_a.FF1 value has little or no effect on the amplitude > > > > of the error. So that probably needs addressed by educating me > > > > on how its calculated. > > > > > > > > 2.pid_a.FF2 isn't working as that article says it should, a > > > > little effect on the shape but zero effect that would lead to a > > > > zero error when cruising > > > > > > > > 3. pid_a.Pgain effects the amplitude of the error signal, with > > > > about 350000 being the onset of a low level oscillation, about > > > > 10% when cruising. But its also the lowest error at 15 to 20 > > > > millivolts. > > > > > > > > 4. I am getting some improvement. but nothing like the pix on > > > > that wiki article. > > > > > > > > It explains that pid_a.FF1 should be 10/velocity@10V (velocity > > > > is in machine units per second). Since I do not yet have the > > > > motor actually moving the BS-1 clone, nor have I managed to get > > > > a home switch set up so I can actually measure the scale, the > > > > scale temporarily set is likely wrong. Since I've 2 worms in > > > > series, for playing I've a high value set for scale, 5000. With > > > > the encoder on the back of the motor it could well be higher > > > > than that. > > > > > > > > But no combination of numbers results in a cruising error of > > > > zero, its alway plus at cruise speed. > > > > > > > > And if I go at more than about 30% high pwm, motor inertia > > > > causes the servo to use reverse to stop if I ask for more than > > > > 60 degrees a second. This use of reverse causes the supply, a > > > > 350 watt 24 volt switcher, to do a shutdown because it thinks is > > > > been crowbarred, and the powerdown to recover is around 3 > > > > minutes. > > > > > > > > I think I might have a way that will fix that as this driver can > > > > do crowbar the motor braking if the signals are re-arranged and > > > > fast enough. If I setup a hardware timer in the form of a a > > > > retriggerable ooneshot drive from the pwm signal, which is > > > > running at 4 kilohertz, or 250u-secs pulse is a 100% signal, but > > > > the timer timesout in 200 microseconds, it will time out and > > > > apply the brakes for whats left of the 250 microsecond pwm cycle > > > > if the pwn drops below 20%. This should slow the motor fast > > > > enough it will never actually use reverse to stop. And that > > > > mode will never effect the psu. > > > > > > > > But first, what can I do about the ineffectiveness of setting > > > > FF1 anyplace between .5 and 25. I think its effecting the motors > > > > cruising speed but has zero effect on the amplitude or shape of > > > > the error if enough Pgain is used to make it follow well. > > > > > > > > With Pgain at 250000 it cruises stably, with a reasonably flat > > > > top to the error, about 20 millivolts of error, but FF2 seems to > > > > have little effect on the initial overshoot, or the overshoot > > > > when stopping, and its the overshoot when stopping that is > > > > shutting down the supply unless I set max jog to below 60 so > > > > friction stops it. That 60 is about 1/3rd of what the motor can > > > > do in terms of output shaft rpms if fed its normal 24 volts. > > > > > > > > Anybody know what I should look at next? > > > > 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 Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
