CNC Spindle Speed Monitoring and Control



  • Many Variable Frequency Drives (VFD) used to control CNC spindles have a spindle speed output giving one pulse per revolution.

    I’d like to propose a firmware option that would configure one of the micro’s timers to measure the frequency of this signal and hence the actual RPM of the spindle.

    Why would that be useful?

    Well, it could be used to do things like:

    a) ensuring the spindle is fully up to speed after an M3/M4 (this is most important as it avoids the risk of trying to cut metal with a too low spindle speed);

    b) closing the loop around the spindle speed, to improve the loss of accuracy one usually gets using PWM/analogue speed control with low cost VFD; and

    c) enabling DWC to display the actual spindle RPM.

    To be fair, point a) could equally be addressed by enabling an M3/M4 dwell time in the M453 CNC mode setup command, but this wouldn’t give you b) and c)....



  • definitely RS485 (modbus) support would be good to be able to get feedback from the spindle.


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