Back EMF Calculator - inconsistent calc?
achrn last edited by
I think there's an inconsistent treatment of the 'Requested max speed' parameter in the back EMF calculator at https://www.reprapfirmware.org/
Looking at the back EMF values (top two results), it looks as if the 'requested max speed' input is print head speed, and for delta geometry it's assuming a mimumum rods angle of about 30 degrees (and therefore a ratio of 1.7 between head speed and belt speed).
For my motor and drive properties, if I set cartesian printer geometry at 500 mm/s I get EMFs of 14.2V & 27.7V, I get those same EMFs for CoreXY at 354 mm/s (noting that 354 x 1.414 = 500) and for Delta at 294 mm/s (294 x 1.7 = 500).
But then the third line of the results block seems to be treating 'requested max speed' as being the motor speed value. It reports that step pulse frequency is 80 kHz for a cartesian at 500 mm/s (that's OK), but for the coreXY at 354 mm/s it says 56.6 kHz, and for the delta at 294 mm/s it's 47.0 kHz. That is, it's now treating 'requested max speed' not as print head speed, but as belt speed.
I think the bottom two lines are back to treating speed as head speed - from cartesian to CoreXY there's a factor of 1.414 and from cartesian to delta a factor of 1.700, i.e. as expected from the geometry.
So I think the 'step pulse frequency' result needs a multiplier adding in according to selected geometry. Ideally, there would also be notes or footnotes saying that 'Requested max speed' is print head speed, and that for the delta it assumes a rod angle of 30 degrees at the extremes of travel.