How would a border line too high back EMF manifest itself?
-
My new printer will be using remote drive pulleys (8mm shafts and bigger bearings than stepper motors have). This also gives me the opportunity to play around with different pulley diameters. I'm considering using 20 tooth pulleys on the stepper and 40 tooth on the bottom of the drive pulley shaft with 20 tooth as the main belt drive pulleys, effectively giving me 10 full steps per mm (160 @16x micro-stepping) as opposed to the more usual 5 full steps per mm (80 micro steps @16x). Of course, this would mean that the motor spins twice as fast for any given linear move rate. For the non - print travel speed that I'd like to use, the motor data sheet tells me the torque would drop to 70% of it's maximum but the gearing effect of the pulleys will halve the motor torque required, so it should (in theory) work well.
But, the online calculator for back EMF tells me that this might be a problem at the intended travel speed. So the question is, if the back EMF was border-line becoming a problem, how would I know? I'll run some speed tests and then M122 diagnostics but how would problematic back EMF manifest itself? High hiccup count? If not that, then what?
-
@deckingman high back EMF leads to a drop in torque and you'd therefore lose/skip steps
-
@jay_s_uk said in How would a border line too high back EMF manifest itself?:
@deckingman high back EMF leads to a drop in torque and you'd therefore lose/skip steps
Sure. But is there anything in the M122 diagnostic report which would tell me when I'm getting close to (or at) the point where it's skipping steps? I guess I could command a longish move at increasing speeds and physically measure the head position each time, but that seems a bit crude. So I was wondering if there was a more elegant way to determine the point (speed) where back EMF is becoming problematic. I seem to recall looking at hiccup count some time back in the early days of RRF3 when step pulse frequency for expansion boards was a bit of an issue, but I'm not sure if hiccup count would increase if the back EMF was causing skipped steps.
-
@deckingman other symptoms of reaching the back emf limit are:
- The motor becomes noisier. But resonances that get excited at high speeds can have the same effect.
- If the back emf limit is reached at low speed then you can get spurious "phase may be open circuit" errors.
If it's borderline then I suggest you don't worry about it. Or you can reduce the motor current a little, which will reduce the inductive back emf.
-
OK. So I get the impression that there is nothing in any diagnostic reports, nor any messages that will tell me if back EMF due to rotational speed is problematic. I suspected that might be the case but it was worth asking the question.
-
@deckingman that's correct, the driver doesn't provide any report that it is unable to maintain the requested current, unless the speed is low.
-
@deckingman I just wanted to mention that it is best practice to use the lowest possible PSU voltage for the EMF calculator, when all motors are running and heaters/fans are on. That's the real back-EMF scenario to figure out borderline values...
