Raspberry Pi3 GIPO & Single axis

With software stepping, if you have 50us jitter your step timing will also have 50us jitter.

After having spent some time with the pi in software stepping, I feel I can now answer my own question: how much jitter is acceptable?

The answer is that it depends on the step rate, as I suspected. Any jitter that is more than 1 or 2 percent of the step period will be audible. This is not noticeable at low speeds, say up to around 50 full steps per second, probably because the actual motor position is not tightly controlled between full steps due to the nature of stepper motors. At higher speed where the chopper keeps hitting full steps with different speed demands due to the jitter, the motor makes a metallic grating sound, as if it had a bad bearing. This quickly becomes strong enough to throw the motor out of sync and stall. A 5% jitter visibly shakes the motor.

Do those Trinamic drivers help? Actually, they do, somewhat. They make those slow step rates quite usable. When configured for full step mode, a single step is interpolated to a quadrature sine wave, causing the motor to gently advance to the next full step. Starting at about 12hz, it begins using the previous step spacing to smooth the steps into a continuous motion.

The TMC2209 that I have also do something at higher speeds that is should take care of the jitter problem, but I haven’t seen it work successfully. At around 750 full steps per second, the controller should switch over to an internal step timer, thereby causing the motor to run jitter free at higher speeds. Perhaps the amount of jitter in my signal is too high for it to work? More testing is needed here.

I had that metallic grating sound on my RPi3, too, and could not figure out where it came from :/

What I see on my setup: without OpenGL, the jutter is ~ 1/5th, but that eleminates axis as a GUI.