John while we are on this subject got a quick question. Just want to check my maths.
My encoder card on vfd max count rate is 300Khz. Max rpm is 10000rpm So a 1000 ppr encoder should give me 18000 rpm at 300khz. I just need 10000 rpm so that should be well within spec Regards Andrew On Wed, May 6, 2020, 6:45 PM John Dammeyer <jo...@autoartisans.com> wrote: > > > > -----Original Message----- > > From: Nicklas Karlsson [mailto:nicklas.karlsso...@gmail.com] > > Sent: May-05-20 9:13 PM > > To: Enhanced Machine Controller (EMC) > > Subject: Re: [Emc-users] Fluctuating RPM using CUI ATM 10 encoder > > > > > Do you have a dual trace 'scope? Spin the spindle and look at the > input > > > and output of the opto-isolator. See if there really is a lag. If > you > > > have a new scope would you have it capture the screen and post it. > I'm > > > really skeptical that those isolators are THAT bad. > > > > > > Here is the datasheet for the opto. > > > https://www.mouser.com/datasheet/2/143/EL817-G-26528.pdf > > > They claim an 18 uS rise and fall time. A 1KHz square wave will > have a > > > 1000 uS period and a 500 uS pulse width at 2 KHz the pulse width is > still > > > more than 10X the raise time of the opto. But maybe the opto is a > fake > > > counterfeit part with very poor performance. > > > > Opto couplers are rather slow. With a high resolution encoder frequency > could be maybe up to a few hundred kHz, > > 2000*period/revolution*3600rpm=120000period/second=120kHz, 200PPR will > be 12kHz at 3600rpm, 1/(2*18µs) = about 27778Hz = > > about a little bit below 28kHz if mad no misstake. > > > > I think your math is out. Most of the hi res encoders are running 2500 > lines per rev. Many are less. But we're only looking at one encoder > channel and that works out to twice the number of edges but the actual > frequency is still lines per rev per second. > > So how many revs per second is that? You have to divide RPM by 60 to > get revs per second so 3600 RPM is 60 RPS. > > Now you have 60 * 2500 Hz = 150kHz. Still way too high for that > opto-isolator. > > Now if you do like Sam did for his hex milling you update the encoder. > But initially he was milling with about 60 teeth and Linux is more than > fine for that when power tapping etc. So 60 * 60 is 3600 Hz and the opto > would be fine for that. Even 200 lines at 60 rps (3600 RPM) is 12,000 > hz. > > Don't understand how you got 27778Hz > > John > > > > > _______________________________________________ > > Emc-users mailing list > > Emc-users@lists.sourceforge.net > > https://lists.sourceforge.net/lists/listinfo/emc-users > > > > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users > _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
