> From: Gene Heskett [mailto:ghesk...@shentel.net]
> On Sunday 13 June 2021 14:43:00 John Dammeyer wrote: > > > At the moment I still have the stepgen 5 on the second parallel port > > so I can measure both PWM and STEPPING. > > > > Raspberry Pi4 + 1GB + LinuxCNC + MESA_7i92H + ProbotixBoB + custom > > VHDL for the MESA board thanks to Peter Wallace at MESA. > > > > AXIS display shows 1500 RPM, PWM is 50% and if I zoom in on the step > > pulses my scope tells me it's 250kHz which is 50% of the max step rate > > as currently set. > > BBBuuutt, 250 khz is right at the ragged edge of what the opto-isolators > in the input of most stepper drivers can pass. If you want maximum > speed, cut the drivers divisor in half at the dipswitches, and cut that > axis's scale in half for the same motor speed at half the step rate... Hey Gene, You're probably using cheap far east parts when you talk about optos. This one that I use is good to 10Mbps. https://www.onsemi.com/products/optoelectronics/high-performance-optocouplers/high-speed-logic-gate-optocouplers/hcpl2631v And the AM26LS31 differential line driver is also way up there in speed. And the spec on the Bergerda AC Servo is that it also can handle that speed. I've attached the schematic of my interface board for the spindle. Note I do use the slow 4N27's for the slow signals like ENABLE and FAULT. Even the PWM DIR signal which is separate from the STEP/DIR signals is only buffered with a 4N27 since again the speed for direction change isn't a big deal. > Well above what the average bob can pass. And 3k rpms is pushing a > stepper too fast for anything but laying loose on the table. > if you want real, usable torque, stay below 1500 revs. IMHO steppers are really useless above about 750 RPM unless they are so oversized that the torque drop off above that speed still runs the machine. But I'm _not_ using steppers. AC and DC Servo motors. Don't suffer from the issues that steppers suffer and wonderfully quiet. Where before with the GECKO and 1000 oz-in stepper on the knee I couldn't hear the bevel gears rattling. Now the motor is so quiet the other noises become annoying. Same with the spindle. The large cast iron single phase AC 2HP 220VAC motor had quite the hum when it was turning. The much smaller 1.8kW AC Servo is so much quieter and variable speed. I looked into getting a three phase motor for it and a VFD. The Bergerda combination ended up being cheaper. But I did have to cast and machine my own pulleys. Now the noisiest part of the spindle is the rattling of the spline inside the drive part of the quill. John
PWMSpindle.pdf
Description: Adobe PDF document
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