On Fri, May 29, 2015, at 09:23 PM, Mark Johnsen wrote: > I have made huge strides on my Supermax conversion from an Anilam Crusador > M to a 5i25/7i77. I was very happy to have the encoder resolution and > motor/encoder directions setup so that I can start tuning. I was able to > follow John Thornton's Velocity tuning instructions, but I have the > almighty dither...
What exactly do you mean by dither? Axis moving back and forth when the commanded position is constant? Have you tried deadband equal to one or two counts of the scale? > Specs for the system are West Amp Servo's w/ tach feedback and 0.01mm > resolution glass scales connected to the 7i77. > > Has anyone been able to tune this system and remove the dither without an > additional high-res encoder added to the system? > > I've read as much as I can, mostly on the forum and here are the options > I've found. > > 1. Try to tune out the dither. PCW mentioned maybe add a touch of > pid-bias or FF3 (need to add FF3 to PID comp). NOTE: I need to go and > work this a little more, but not optimistic. > 2. 7i87 for tach feedback, but PCW says you're still dead reckoning and w/ > time you'll drift. (ie no velocity when idle). And, I didn't see any more > about this after about January 2013. I wish this would work because it > would be rather inexpensive for hardware. Is the 7i87 an analog input module? With only velocity feedback from the motors, tuning is likely to be tricky. It might work though. And if the tachs are already there it is probably the least expensive solution. > 3. External encoders on ball screws or back of motors. I've heard people > mention it, but haven't actually read about someone being successful with > it. I think the best approach if you have linear scales is to add rotary encoders on the motors and use both. Use dual PID loops, one loop getting its feedback from the scale, and the other getting its feedback from the motor encoder. Sum the two PID outputs together. The scale PID should have I-gain only, for steady-state accuracy in spite of things like screw lash and thermal expansion of the screw. The encoder PID should have P-gain and D-gain for good dynamic response and stability. FF gains can be applied to either PID loop, since the same command goes to both. This has been done successfully. I used to have a blog posting describing how we did it on Stuart Stevenson's big horizontal boring mill. (Ten foot long X axis.) But my blog has been down since I upgraded my server. Sorry about that. > 4. Higher resolution linear scales. I don't think that is the answer. Any lash at all between motor and scale will make it very difficult to tune. > Ok - I'm going for the jugular here, I found on Ebay some 1um renishaw > linear encoders and scales from the same guy. I'm thinking I can get a > discount and get 3 axes of 1um linear scales for $400. It's more than > adding rotary encoders, but I think the futzing around and simplicity of > the final solution is compelling. Also, I can remove (and sell) the glass > scales, put the renishaw's in place and wire in. > > Suggestions? I would add motor encoders before buying higher grade linear scales. > Thank you, > Mark John Kasunich [email protected] ------------------------------------------------------------------------------ _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
