John Kasunich wrote: > No, for two reasons: > > 1) A lowpass filter will not solve PID of instability - if anything it > will make it worse since it introduces a lag. > > 2) ilowpass is an _integer_ filter, it can't be used for floating point > signals (which include the command, feedback, and output of PID loops). > > The correct solution is to tune your PID loops (using D and deadband as > needed) until they are stable. You must use halscope to view the > resulting waveforms and decide what to adjust. > > Once you have the axes tuned as well as possible, halscope will tell you > the maximum error you have during a rapid. Assuming that is acceptable, > you set the following error a bit higher so it doesn't trip. > > Yes, agreed, any lag introduced is a major problem in a control loop. But, the quantizing nature of the encoder and counter cause there to be a lot of power right at 1/2 f. It seems to me, a real neophyte in control theory, that that power is NON-information, almost entirely a product of the sampling process, ie. noise. Both the P term, but most especially the D term, magnify these velocity fluctuations. I normally run with a D term setting around 2, with a P term of 1500, on one machine. Make the D much higher, and the system breaks into oscillation. D should be damping, of course, but it creates so much amplification of the velocity "jitter" that it excites some modes in the system. (In my way of thinking, too much D should just make the response greatly underdamped, but instead, it becomes unstable. This is likely due to excessive mass & spring elsewhere, possibly mechanical.)
Maybe the whole problem is the system is trying to have too much bandwidth for an already sluggish mechanical system. With a 1 KHz servo-thread, it is trying for a 500 Hz bandwidth, and everything should just move to a lower bandpass. Especially on desktop machines with little motors, they definitely can have some real mechanical response at 500 Hz. On the other hand, I have had some improvement by increasing the servo-thread rate, moving the 1/2 f noise up to a higher frequency. I'll be glad to demonstrate all of this behavior at the Fest, we can instrument to your heart's content, and see what you think. I agree that a gradual lowpass filter would introduce too much lag, but there are a wide range of digital filter algorithms to choose from. Jon ------------------------------------------------------------------------------ This SF.net email is sponsored by: High Quality Requirements in a Collaborative Environment. Download a free trial of Rational Requirements Composer Now! http://p.sf.net/sfu/www-ibm-com _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
