Yes, and an easier way to verify is to think the 60 RPM = 1 Rev. per second. So you need 60 slots per rev to give 60 Hz at 1 rev per second.
But why bother making the math easy for a computer? Use however many slots gives the best data for the PID controller. That said, 60 is enough unless you are going really slow. Thinking about the non-symetric square wave. I doubt it is possible to make it perfect and I don't think it needs to be. Say the slots are parallel cuts. Then they are only 50% at one exact diameter but the IR light beam overs a spot, not a point. So half the beam is being interrupted with a less then 50% duty cycle and the other have more then 50%. You would need to make wedge shape slots to be perfect 50% over the entire sensor aperture Worse is that the Schmidt trigger is asymmetric. The on-point is 1.5 times higher than the off-point. That 1.5 factor is relatively large But does it matter if the controller only looks at leading edges? OK, you have some fraction of a degree error with a direction change. It might matter if the motor was driving a lead screw. It makes the wave square I think the solution is to add a radial adjustment All that said, I cheated and just bought a $15 600 line optical encoder, an Omron clone. On Fri, Jul 2, 2021 at 8:56 PM John Dammeyer <jo...@autoartisans.com> wrote: > Oh and if my math is correct, the 60 slots/tabs when reported as frequency > by the scope is the spindle RPM. > > 1055 pulses/second divided by 60 pulses/rev = 17.5833333 rev/sec. (pulses > cancel out). > > And 17.5833333 rev/sec multiplied by 60 sec/minute = 1055 revs/minute > (seconds cancel out) > > Here's the test setup with the protoboard that took me all afternoon to > wire up. When I'm ready I have a TTL to RS422 driver on it in case > electrical noise is an issue. At the moment it's not populated. > > > -----Original Message----- > > From: John Dammeyer [mailto:jo...@autoartisans.com] > > Sent: July-02-21 8:30 PM > > To: 'Enhanced Machine Controller (EMC)' > > Subject: Re: [Emc-users] Mounting spindle sensors. > > > > > I am prepared to state categorically that it must be a coincidence as > > > "math" was not a word in Douglas Adams's lexicon. As a speaker of > > > British English he would have always used "maths" as the shorter > > > version of "mathematics" > > > > > Hi Andy, > > That's the same comments I've heard. However, nothing is to say that > someone didn't tell him to use that number and may have even > > explained that to him. The chance has to be very very very small that > someone would pick a number that by luck that worked out to > > spell MATH which is required for an understanding of the universe. > > > > In either case MATH is also required to understand what the scope is > showing and why the waveforms aren't symmetrical. > > > > A couple of caveats here so don't judge me too harshly. > > 1. The drawing of the encoder disk was done with the idea of using a 3mm > tool to cut 3.32mm slots which would result in 3.32 mm > > tabs at the expected diameter. > > 2. Using a 4mm tool to cut 3.32 mm slots was a bit of a mistake. Ever > so slightly. Well. OK. A big mistake. > > > > However it was a good exercise, using LinuxCNC and a number of different > G-Code files for the hub, the perimeter, the mounting holes > > and the slots. > > > > And I'll have to draw up a new larger disk and do this all over again > because although the sensor bodies do a great job of being clear of > > the lower pulley the actual sensor mounting tabs stick up too high. > Can't get the belt on the lower pulley without removing a sensor > > bracket mounting screw and pivoting it out of the way. > > > > Finally, and that would only show up in a video, the ever so slightly > larger bore of the self-made cast aluminium motor pulley gives > > that pulley an ever so slight wobble which through the V belt is > transferred to the spindle pulley (also self-made casting with inner > > cone to fit on cone spindle drive). > > > > It shows up as jitter on the scope. > > > > Didn't really care at the time (wasn't going to do it over) since the > intention is to change to toothed belts anyway. Once the full CNC > > system is operational with coolant, mister and splash shields, power > draw bar. And I haven't even started on ball screws yet. > > > > I'll post the schematic of the interface board to the PhotoLogic OPB990 > series slotted optical switches in another posting. > > > > John Dammeyer > > > > > > > > > > _______________________________________________ > 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
