Kirk Wallace wrote: > On Thu, 2007-10-11 at 20:53 -0500, Jon Elson wrote: > >>Kirk Wallace wrote: >> >>>What is the status of PID_MAX_VEL and PWM_OUTPUT_SCALE? My understanding >>>is that they were created to fix persistence issues. I am using EMC >>>2.1.7 so should I still use them? >>> >> >>Arrghhh! I don't actually know. PID_MAX_VEL is a standard part >>of the modern ini file, and should still be used. It is to >>allow the servo algorithm to have a little extra headroom for >>acceleration. > > > So PID_MAX_VEL is not unique to the UPC? No, any version that uses the servo mode of operation will have it. Only setups that use stepgen don't use PID, I think. > >> >>You can just change these lines in hnc_motion-3a.hal >> >>and remove the PWM_ from the 3 lines above. Then, remove the >>now redundant lines with PWM_OUTPUT_SCALE from your ini file. > > > For some reason that I can't recall, the above did not work for me right > away, so I put PWM_OUTPUT_SCALE back the way it was, set it to -1, > zeroed out the pid/ff settings and started over. Now I have P set to ~50 > and FF1 set to .25 and everything else set to 0. My FERROR is now .0050 > and MIN_FERROR is .0013. Typically, I am getting .0005" difference > between the commanded position and the actual position, which is better > than a couple of days ago, but I am hoping to do better. I just need to > put in more effort. > Hmm, I don't know why that didn't work. (I see I still have it this way on my downloadable configs directories on my web page, too!) But, I have definitely done it on my development systems. Anyway, to get the error down, you want to have the P gain as high as possible. You need to add a small amount of D to keep it stable as you raise P. You might have P set somewhere between 80 and 250, depending on all the other factors (motor, supply voltage, friction, etc.) As the encoder resolution rises, then the linear dimension of each encoder count is smaller. So, with a high res encoder, the P term and all others needs to be larger.
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