Tom Easterday wrote: > On Dec 12, 2011, at 9:38 PM, Jon Elson wrote: > >> Watch out for the CUI AMT-10x encoders at Digi-Key, they have a lag >> responding to acceleration, and >> may not be well-suited to CNC systems such as EMC2. >> > > Jon, > We are using these encoders on a gantry machine and are currently trying to > debug a small spike (0.001) of error we see on start and stop of the motor. > I wonder if this is related to the lag you refer to. How and why does this > present itself in these encoders? How would be see it? > Yes, that is EXACTLY the kind of problem I saw. This encoder is highly interpolated, so it has a tracking counter that is incremented or decremented by an estimate of the current velocity. If you don't do these computations correctly, there is a lag in responding to changes in velocity. As far as I can tell, the interpolator in such devices as the Analog Devices AD2S1200 has a really fine 2nd or 3rd order filter to make this work well. Obviously, CUI did not go to the trouble of such mathematics.
After never being happy with these encoders and having a bit of customer feedback, I decided to compare to a standard HP optical encoder (with no interpolation.) So, I put the optical encoder on the other end of the motor, and fed both to encoder inputs on my PWM controller board. Most useful was to plot velocity derived from both encoders at the same time. This comes up often enough, I have the plots permanently on my web site. See http://pico-systems.com/images/compare_encoder2.png The red trace is the CUI encoder, which was controlling the motor (badly). The white trace is the HP encoder. Both are providing 4000 counts/rev. You can clearly see the velocity peaks of the CUI encoder are higher, and if you look closely at the slopes, you can see the CUI is slow to react to velocity changes, and then has to overshoot significantly so the position can catch up. This of course accounts for the bad servo behavior. As the Halscope is set for 20 ms/div, the velocity lag is easily 3-5 ms in duration! If you have a plain optical encoder handy, you could easily set up the same arrangement to compare the encoders. Jon ------------------------------------------------------------------------------ Systems Optimization Self Assessment Improve efficiency and utilization of IT resources. Drive out cost and improve service delivery. Take 5 minutes to use this Systems Optimization Self Assessment. http://www.accelacomm.com/jaw/sdnl/114/51450054/ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
