Okuma machine builders really don’t like quantization noise so they use 40 million CPR encoders on the axis motors.
> On Jun 21, 2021, at 11:34 AM, Chris Albertson <albertson.ch...@gmail.com> > wrote: > > Quantization noise is fundamental and present in all digital systems. > There is absolutely no way to avoid it. All you can do is minimize it. > > To know how fast a shaft is spinning or how far it moves, you have to count > edges and if you get 100 counts you just can not know if you are closer to > 100 or to 101 counts. Basically the counter always rounds down even if at > 100.99999 it tells you "100". So the cound is "wrong" 50% of the time. > This applies not only to encoders, but A/D converts and every other kind of > measurement. > > This is the nature of digital data, of representing the world with a fixed > number of bits. > >> On Sat, Jun 19, 2021 at 4:00 PM John Dammeyer <jo...@autoartisans.com> >> wrote: >> >> But doesn't the 7i92 FPGA deal with quadrature encoders and therefore >> doesn't really need to deal with a sampling process but instead looks at >> edges? And because of the way quadrature works, with the A/B phasing you >> don't get the same types of errors compared to polling a bit level X times >> per second and trying to decide when it's high/low. >> >> John >> >> >>> -----Original Message----- >>> From: Chris Albertson [mailto:albertson.ch...@gmail.com] >>> Sent: June-19-21 3:21 PM >>> To: Enhanced Machine Controller (EMC) >>> Subject: Re: [Emc-users] Machining question >>> >>> There are two kinds of noise, >>> 1) electrical noise superimposed on the signal. >>> 2) quantization noise from the sampling process. What happens here is >> that >>> the computer counts the number of line crossings during a given time. it >>> is just random luck when the time starts so on average there is a plus or >>> minus one count error. >>> >>> So a one count error sounds small but lets say we have a 600 count >>> encoder and we are running at 500 RPM and sampling 100 times per second. >>> This works out to 50 counts per period. a one count error is a 2% >> error. >>> You will see a larger percent error at slower speeds or with a lower >>> resolution encoder. >>> >>> So even with perfect wires and perfect grounding and zero electrical >> noise >>> the RPM speed is going to jump around randomly over a 4 RPM range because >>> of unavoidable quantization error. >>> >>> The solutions either the use a larger sample time apply a low-pass >> DIGITAL >>> filter to the signal. No amount of analog filters on the wires will >> work. >>> >>> On Sat, Jun 19, 2021 at 2:55 PM Gene Heskett <ghesk...@shentel.net> >> wrote: >>> >>>> On Saturday 19 June 2021 12:59:19 John Dammeyer wrote: >>>> >>>>> I need to do a couple of things. For one the AC Servo makes a lot of >>>>> electrical noise. The frame of the motor is connected to earth >>>>> through power line ground. But my bench setup has the control side >> 5V >>>>> not isolated from the 'PC' side (optos are kind of useless here) and >>>>> although the Pi4 doesn't appear to have any trouble the scope shows a >>>>> pretty noisy encoder signal. >>>> >>>> You are about to learn the star ground system I think. >>>> >>>> Thats where all grounds go to a single bolt, and no other grounds are >>>> allowed anyplace. >>>> >>>> Shielding in motor cables ends without touching the motor, only >> connected >>>> to this same single bolt which is grounded. The common line of any >> power >>>> supply is connected only to this bolt, and the common grounds to every >>>> piece of pcb in the system comes from that bolt. The used to be green >>>> static ground wire in any power cord is rerouted to this bolt. And >>>> static grounds on a power supply are fed from this single bolt. Because >>>> the psu case is usually bolted down wherever its at, you may have to >>>> uncover the supply and remove any connection from the earth labeled >>>> terminal, and the psu's case. It may be a screw in the corner of a pcb, >>>> but it needs to be removed. >>>> >>>> Anything connected to ground at some other point in addition to that >> bolt >>>> becomes an antenna, is called a ground loop, picking up both magnetic >>>> and electrostatic noises. I had to learn that the hard way while >>>> building the Sheldon, blowing two 7i90HD boards out by picking up 80 >>>> volts p-p of switching noise that reached up to the bandwidth limits of >>>> both of my 100mhz scopes. Hell on an fpga with a 3 volt limit. I >> killed >>>> the power and redid it to the single bolt model, and that 80 volts of >>>> noise was reduced to under 100 millivolts. Took me about a week to find >>>> all the connections that should not have been made. Even a small bypass >>>> capacitor to a local ground instead of back to that bolt can become a >>>> lethal weapon to the elecronics. Its a noise injector in that case. >>>> >>>> The whole idea is to make the ground, if it bounces from a lightning >>>> strike on the can on the pole that powers your place, which may make >> the >>>> whole system bounce 100+ kilovolts, and if you are in that circuit at >>>> that instant it WILL get your attention. BUT, everything tied to that >>>> bolt should bounce in unison, meaning an individual board input will >>>> only see the difference signal its being fed with, and it should >> survive >>>> that lightning strike with no damage. >>>> >>>> My soap box as a CET just collapsed, so I'll do an Andy Capp and >> shudup. >>>> >>>> >>>>> ===================== HAL code =================== >>>>> Second is some of the HAL parameters and reducing to 0.003 makes the >>>>> display more stable. # Use ACTUAL spindle velocity from spindle >>>>> encoder >>>>> # spindle-velocity bounces around so we filter it with lowpass >>>>> # spindle-velocity is signed so we use absolute component to remove >>>>> sign # ACTUAL velocity is in RPS not RPM so we scale it. >>>>> >>>>> setp scale.spindle.gain 60 >>>>> setp lowpass.spindle.gain 0.003 >>>>> net spindle-vel-fb-rps => scale.spindle.in >>>>> net spindle-fb-rpm scale.spindle.out => >>>>> abs.spindle.in net spindle-fb-rpm-abs abs.spindle.out >>>>> => lowpass.spindle.in net spindle-fb-rpm-abs-filtered >>>>> lowpass.spindle.out >>>>> ===================== end HAL code =================== >>>>> >>>>> But I'm still using the original signal for at speed and the LED >> never >>>>> comes on nor does it know it's at speed. I should probably be >>>>> filtering the RPS rather than RPM I think? >>>>> >>>>> ===================== HAL code =================== >>>>> net spindle-vel-fb-rps => spindle-near-speed.in1 >>>>> net spindle-ramped => spindle-near-speed.in2 >>>>> >>>>> # ---Setup spindle at speed signals--- >>>>> # the output from spindle-near-speed 'near' component is sent to >>>>> spindle.0.at-speed # and when this is true motion will start. The LED >>>>> in the pyvcp-panel.xml is renamed to be a LED. net >>>>> spindle-near-speed-led <= spindle-near-speed.out => >> spindle.0.at-speed >>>>> ===================== end HAL code =================== >>>>> >>>>> John >>>>> >>>>>> -----Original Message----- >>>>>> From: andy pugh [mailto:bodge...@gmail.com] >>>>>> Sent: June-19-21 2:48 AM >>>>>> To: Enhanced Machine Controller (EMC) >>>>>> Subject: Re: [Emc-users] Machining question >>>>>> >>>>>> On Sat, 19 Jun 2021 at 07:11, John Dammeyer < >> jo...@autoartisans.com> >>>> wrote: >>>>>>> I'm finding the AXIS Spindle speed oscillates about +/- 5 RPM. >> I >>>>>>> thought that I was filtering it. >>>>>> >>>>>> It generally needs to be filtered, but it is possible you are >>>>>> filtering it, but not enough to get a steady reading. >>>>>> >>>>>> For whatever reason encoder velocity does always seem to be a bit >>>>>> noisy, despite the counters using timetamped edges and all the >> other >>>>>> tricks that might help. >>>>>> >>>>>> -- >>>>>> atp >>>>>> "A motorcycle is a bicycle with a pandemonium attachment and is >>>>>> designed for the especial use of mechanical geniuses, daredevils >> and >>>>>> lunatics." >>>>>> ? George Fitch, Atlanta Constitution Newspaper, 1912 >>>>>> >>>>>> >>>>>> _______________________________________________ >>>>>> 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 >>>> >>>> >>>> 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) >>>> If we desire respect for the law, we must first make the law >> respectable. >>>> - Louis D. Brandeis >>>> Genes Web page <http://geneslinuxbox.net:6309/gene> >>>> >>>> >>>> _______________________________________________ >>>> Emc-users mailing list >>>> Emc-users@lists.sourceforge.net >>>> https://lists.sourceforge.net/lists/listinfo/emc-users >>>> >>> >>> >>> -- >>> >>> Chris Albertson >>> Redondo Beach, California >>> >>> _______________________________________________ >>> 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 >> > > > -- > > Chris Albertson > Redondo Beach, California > > _______________________________________________ > 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
