1000 PPR encoder turning 300 RPS (18000 RPM) will produce 300,000 Hz output. At 10000 RPM it's 167 RPS or 167Khz.
I've been reading through the LinuxCNC source code for the last hour or two. If one was just using a parallel port and not sophisticated external hardware the hal_parport.c code looks like it reads the input at the base_period rate; say 25kHz => 40 uS. If you figure a standard mill running max 3000 RPM (50 RPS) then a you probably need to look at the spindle encoder at least twice per encoder period. At 25kHz we're seeing a ParPort task run time of 40 uS. So I'd guess the max pulse size from the spindle encoder has to be at least 80 uS. The spindle at 50 RPS uses up 20mS. Divide that by the 80uS implies the largest usable encoder is 250 lines. Have I got that right? I've not found the spindle support code yet. Only the HAL install code. John > -----Original Message----- > From: andrew beck [mailto:andrewbeck0...@gmail.com] > Sent: May-06-20 12:31 AM > To: Enhanced Machine Controller (EMC) > Subject: Re: [Emc-users] Fluctuating RPM using CUI ATM 10 encoder > > 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*18�s) = 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 _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
