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DVR8840 speed measurement with back EMF

Lukasz Nowak
Prodigy 40 points
Other Parts Discussed in Thread: DRV8840

Hi,

I am going to use DVR8840 for my 40W DC motor drive. I would like to implement also speed measurement basing on back EMF of the motor. I see that by setting DECAY = 1 (fast decay mode) the outputs go high Z when ENBL=0, so the motor winding is floating and back EMF could be measured at that time (using differential amplifier directly  on the motor).

Is that correct? Has anybody done speed measurement with DRV8840? Any other concerns?

Thanks in advance for answering,

Lukasz

  • 0
    TI__Mastermind 21320 points

    Hi Lukasz,

    Are you just curious of the speed while the motor is slowing? But yes, in fast decay you could monitor voltage to determine a speed. Most of the time we don't see people interested in this sort of feedback for a DC motor.

    -Nick

  • 0 in reply to Nicholas Oborny
    Prodigy 40 points

    Hi Nick,

    Thanks for answer.

    I am interested in speed especially during ramp up of the motor. There might be different loads on the motor and i want to start with the same speed over time ratio in all cases (but not very precise thou). There might be also an obstacle during the start, so if the motor's current increases while the speed doesn't - then i can detect that. I need to mention that i don't have other speed sensor or encoder etc. in the system, so i think that is the only way to get the speed. I will drive the motor with ~20kHz PWM and think to check the speed 100 times per sec by switching to fast decay, wait till the winding has discharged and measure back EMF then. Yes, the motor wil be slowing then, but only for like few ms I suppose.

    Lukasz

  • 0 in reply to Lukasz Nowak
    TI__Mastermind 21320 points

    This sounds reasonable. Let me know how it goes! I will be curious to see what kind of control you can achieve.