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
