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DRV8825: Current irregularities in Fast, Mixed and Slow Decay modes

David Boyle1
Prodigy 150 points
Part Number: DRV8825
Other Parts Discussed in Thread: DRV8846, DRV8880

While running a DRV8825 in 1/32 steps with Fast Decay mode, I have noticed departures from a smooth sine wave which I would like to eliminate.  We have measured the displacement of the motor during the cycle and see a reduction in displacement at the same step locations... so, it seems there may be a correlation.  Here is a plot showing DRV8825 sine table, the measured phase current and measured displacement (on two different pieces of hardware) over a 1/2 cycle:

Can you help me understand the cause of the departure from sine table?  Is this inherient in the IC design, or indicative of a problem in my circuit/load?

Also, I tried switching to the Slow and Mixed decay modes thinking they might improve current accuracy.  They do, for the most part, however, I see some unexplained behavior in these modes too.  This is a bit more puzzling as in each mode, the currents appear to (what I will call) 1/2 step (?) when within about 16 usteps of null.  In the case of Slow mode, the current 1/2 step as current decreases too or rises from null.  In the case of Mixed mode, the 1/2 step only occurs when rising from null.  I don't see any reason for this behavior in the datasheet, or any mention of it on the support site... so, I suspect there is something wrong in my circuit.  I just am at a loss as to what.  Here is a plot of the rms phase current over 1/2 cycle (about 70 usteps) in each of the three modes.  I am single stepping at a very slow rate while taking this data (> 1 sec).

I also spun the motor in each decay mode and see similar waveforms, step rate is 2msec:

SLOW:

Mixed:

Fast:

I then repeated the spinning in each mode but at a faster step rate of 62.usec/step:

Slow:

Mixed:

Fast:

Do you have any ideas as to why the current waveforms in Slow and Mixed modes are showing this odd 1/2 step behavior at low step rates?

Do you have any idea as to why the current waveform in Slow is mostly good at the higher step rate, except for the whoop-dee-doo when current is decreasing?  And, why Mixed now looks pretty good?

I looked at the timing, logic levels and edge quality of STEP... and see nothing out of the ordinary or out of spec range.

Any clues would help!!

Thanks,

DaveB

  • 0
    TI__Expert 4435 points

    Hello Dave,

    Your graphs show you are losing current regulation, this happens because of multiple factors; kind of motor you are using, supplied voltage, speed, etc.

    As you discovered, your motor is better tuned at higher speed, again, this is dependent on motor inductance, voltage supplied, etc.

    The slow decay at fast speed is better, the "whoop-dee-doo" is caused by the back EMF of the motor.

    The mixed decay for this device only works for 75% of the PWM cycle while deacreasing, the remaining of the cycle (increasing) is slow decay as stated in Section 8.3.3 of the DRV8825 datasheet (last paragraph).

    I recommend the short Engineer It video which talk about losing current regulation here. There are devices with adaptive decay or autotune like the DRV8846 or DRV8880 that can take care of your problems. I hope this helps, thank you.