Bruce Klawiter wrote: > OK it appears I have my servos tuned as good as they are going to get. > See images 5, 6 and 7: https://sites.google.com/site/bmklawt/home/pid-tuning > Adding any I or D to any axis make things worse to violent. > > > Anyway I have terrible oscillation or dithering on the Z axis and if I move > the axis a bit the dithering changes, how can I stop this. > Well, you should use some D. You may have to start with values as low as .01 and work your way up. There will be a valley that gives the best results. Definitely too much D causes oscillation. Also a little deadband can help the system settle when idle.
BUT, there is a limit to how well it can do. Any servo needs to hunt. If you have an encoder with 50,000 counts/inch, then a jitter of +/- one encoder count will be almost undetectable. On your machine, the encoder count is 10 um is approximately .0004" which is a lot. The only advice I might give is to reduce P so you can put a little D on it, and then tighten up the following error with FF1 and FF2, if needed. But, your situation is why I always tell people who want to use cheap linear scales with DRO-class resolution that they will run into exactly this problem. You say the old Anilam control did not have this problem, but I will BET that it also did not achieve as tight a following error as you are now achieving. That is the downfall, if you want tight position control, you get dithering. If you turn the P way down, the dithering becomes a sluggish crawl back and forth that is undetectable, but the following error is not kept small. Jon ------------------------------------------------------------------------------ The demand for IT networking professionals continues to grow, and the demand for specialized networking skills is growing even more rapidly. Take a complimentary Learning@Cisco Self-Assessment and learn about Cisco certifications, training, and career opportunities. http://p.sf.net/sfu/cisco-dev2dev _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
