Tom wrote: > > Thanks Jon > I get the feeling you were responding to John K's comments more than my > own... > Yes, at least mostly. But, I really have found this excessive high-frequency effect on the D term to be a real problem when tuning my own servo systems. I am pretty sure that most servos that use EMC's PID as is have to suffer from the same thing to some extent. My own analog velocity servo drives on my Bridgeport have a softer rolloff in the velocity loop, so they mostly handle it without great trouble, but it is not real hard to set up oscillations. My PWM drives are basically PWM-in voltage amplifiers with no filtering at all, and I have had a LOT of trouble getting stable response under all conditions. With insanely high encoder resolution I can make them work pretty well, but I really can't tell somebody else how to tune them, because I don't really know how **I** do it - I just kind of feel my way and eventually get a response I can live with, but I know it would be better with a bit more damping. But, if I turn D up any higher, it becomes unstable. There obviously is a phase shift somewhere in the system, I have a good idea it is from electrical through the motor to mechanical, and then back from the encoder. But, I don't really have a tool to quantify it. (For analog servo amps, I have a really fancy Schlumberger Dynamic Signal Analyzer that can crank out the tabular form of a Bode plot from any control loop system. But, that instrument won't work on a system where everything is digital.) It would be REALLY cool to work up a little HAL component that could excite a servo loop at various frequencies and examine the response, in amplitude and phase. > still I will take that as an invitation to attend the Fest in May ;-) > Oh, if you can make it, it is a MUST!!! for anyone interested in EMC.
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
