I have had a couple reports of strange things happening with the CUI AMT encoders, that are so nicely affordable from Digi-Key. Well, I was fiddling with a Keling brushless motor fitted with one of those encoders, and couldn't seem to get it to run smoothly, and started thinking about those reports and the fact that the CUI encoder is by definition an interpolating device. So, I decided to fit a HEDS optical encoder to the same motor AT THE SAME TIME and compare the results. I was hoping for a smoking gun, but did not find anything completely outstanding. However, the two encoders definitely show significant differences, and I believe that is the cause of the tuning difficulty.
So, I have 4 HalScope shots if anyone is interested in looking at them. In http://pico-systems.com/images/compare_encoders.png the velocity derived from timestamp estimation is compared. ppmc.0.encoder.00.velocity (red) is the CUI encoder, and ppmc.0.encoder.01.velocity (white) is the HEDS. There are a number of jagged features in the CUI trace that are absent in the HEDS one. Also, one momentary deviation seen by the HEDS encoder is totally missed by the CUI! The jagged artifacts may be due to time jitter when movement is reported by the CUI. in http://pico-systems.com/images/compare_encoder2.png I show the servo cycle-by-cycle velocity in raw encoder count units, not helped at all by velocity estimation. The CUI encoder seems to suppress some of the typical dithering seen in optical encoders at near zero speed. But, it also seems to flatten out places where velocity is not changing rapidly. In both of these plots, EMC was controlling motion using the CUI encoder. In http://pico-systems.com/images/heds.png I show a well-tuned servo response with the HEDS encoder providing position feedback. Note following error is easily held within an equivalent 200 u-In band. In http://pico-systems.com/images/CUI.png I show the same setup with the CUI encoder providing feedback, and the best tuning I could accomplish. Peak error is at least 3 times worse, and the velocity trace also shows lots of velocity fluctuation. Does anyone see a smoking gun that I missed? I THINK I can see a phase lag in the CUI encoder in the encoder2 (raw delta) plot. There are some places where this delay could be up to a couple millisecond, that would be enough to really foul up the servo loop. On the encoders plot (velocity estimate) it seems to be kind of one-sided, delay when velocity is rising, early when velocity is falling. That seems even more perverse. I will bring this stuff to the Workshop if anyone wants to see it and play with it live. Thanks, Jon ------------------------------------------------------------------------------ EditLive Enterprise is the world's most technically advanced content authoring tool. Experience the power of Track Changes, Inline Image Editing and ensure content is compliant with Accessibility Checking. http://p.sf.net/sfu/ephox-dev2dev _______________________________________________ Emc-developers mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-developers
