Bruce Klawiter wrote: > > > The > problems I guess I was hoping would go away is the dithering or > > oscillation > when the machine is at rest. > Yes, this is the universal problem with servos. There is no way for the dithering to be less than one encoder count. So, in your case, the resolution is ~ .0004", and so it will bounce back and forth that amount. You can try to suppress it with some deadband, but that is cutting into your accuracy with such a coarse scale. > With > a P=.1 the axis drifts, at P=1 it seems to hold steady but the PID > > error > is terrible. P=11 the PID error is the smallest I can get, P=13 the > > oscillation > about tears the machine apart. > Well, I'd try some more with the D term, and see if it helps any. > Adjusting > anything else I can get the PID error tiny amounts better but the > > oscillation > never seems to get better > > > > I > would also like to get rid of the jerking it sometimes does when moving. > > It > did not do this with the old controller > > > > Here > are some halscope images of the dithering and jerking: > > https://sites.google.com/site/bmklawt/home/pid-tuning > > First picture shows error jumping +1 / -1 encoder count, exactly as expected from the 10 milli -mm encoder resolution. Frequency about 2 Hz.
Second picture shows spikes. Have you run a long-term test to see if the axis is creeping in position? I am thinking these may be electrical noise getting into the encoder signals and causing erroneous counts to be detected. At such low resolution (means a single count = a lot of movement) then a singe extra count could seriously upset the servo loop. Calibrate the axis to a dial indicator, then run the axis back and forth a while, and then re-check the calibration to the indicator. If it has crept even a few counts worth, then that is likely the situation. If the encoder has differential output, have you set the PPMC encoder jumpers for differential? Otherwise, have you run the commtest option of the ppmcdiags program for a while to make sure the communication between PC and PPMC is good? > Adjusting > D doesn’t do much. > Well, it OUGHT to do something. But, the way EMC's PID works, too much D can cause oscillations, as well. So, there is a valley where it helps the most, and either side it can cause instability. Jon ------------------------------------------------------------------------------ All the data continuously generated in your IT infrastructure contains a definitive record of customers, application performance, security threats, fraudulent activity and more. Splunk takes this data and makes sense of it. Business sense. IT sense. Common sense. http://p.sf.net/sfu/splunk-d2dcopy1 _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
