On Friday 30 June 2017 09:27:29 Peter C. Wallace wrote: > On Fri, 30 Jun 2017, Les Newell wrote: > > Date: Fri, 30 Jun 2017 12:11:09 +0100 > > From: Les Newell <les.new...@fastmail.co.uk> > > Reply-To: "Enhanced Machine Controller (EMC)" > > <emc-users@lists.sourceforge.net> > > To: emc-users@lists.sourceforge.net > > Subject: Re: [Emc-users] Rough motion with servos > > > >> I think it is time to put a scope on the encoder. it sounds like > >> there may be a big duty cycle or phase angle error in that encoder. > >> In other words, at constant speed, the 4 quadrature transitions are > >> not evenly spaced, but at least one of them is out of time. > > > > I can see the phase angle errors in halscope by looking at the > > measured velocity at low speed. There is a regular 4 step cycle. I > > can see the same effect on the other encoders, though it is not as > > pronounced. All encoders have some phase angle error but I have to > > admit this one is worse than I would like. The problem is compounded > > by it being a fairly low resolution encoder. However this does not > > explain the occasional big spikes, especially the example where I > > had a spike of zero velocity for one count. The machine was moving > > at low speed so I could see individual counts in Halscope. To read > > zero velocity that count should have been MUCH wider than the others > > and it was not. There has to be something wrong with the the way the > > velocity is calculated. > > > > Les > > I dont think so, the velocity calculation in the driver has not > changed for many years, and electrical noise can simulate a reversal > which will cause a 0 velocity reading... > This has been a very consistent error for years Peter. And I've been sitting here, with halscope running remotely on this machine for the last hour watching it, and just had a thought. This consistency could be explained if there was a difference in the gain as applied to the velocity averager of about 2/1 between the a and b inputs. The timing, and the resultant error I am seeing right now, with the spindle turning around 95 rpms, based on hall effect devices on the 60 tooth bull gear, is telling me the velocity is stepping over a 2 division (400mv p-p) range centered on nominally 1.2 volts. The shape of this 4 step pattern can be explained by a gain difference in the a vs b swings. Is this something we've been ignoring for a decade or more? I can bend the opto's a tad on TLM, getting the phasing as perfect as I can, and adjusting the leds for a 50% duty cycle. But the velocity output on TLM, triggered on the index, looks very similar regardless of which of the 3 machines I can look at. The velocity is consistently low while the b input is high, and consistently high while the b input is low.
And of course if I crank it up to 1000 rpms, quantization noise is the dominant noise in the a/b pattern, but the general shape of the velocity error only changes because of the rise time of the halscope. It may be Peter, that my encoders are junk, but why are they so consistently junk in the same repeatable direction pattern? Optical or hall effect? > > -------------------------------------------------------------------- > >---------- Check out the vibrant tech community on one of the world's > > most engaging tech sites, Slashdot.org! http://sdm.link/slashdot > > _______________________________________________ > > Emc-users mailing list > > Emc-users@lists.sourceforge.net > > https://lists.sourceforge.net/lists/listinfo/emc-users > > Peter Wallace > Mesa Electronics > > (\__/) > (='.'=) This is Bunny. Copy and paste bunny into your > (")_(") signature to help him gain world domination. > > > ---------------------------------------------------------------------- >-------- Check out the vibrant tech community on one of the world's > most engaging tech sites, Slashdot.org! http://sdm.link/slashdot > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Genes Web page <http://geneslinuxbox.net:6309/gene> ------------------------------------------------------------------------------ Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
