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DRV8825: My step motor has some undesirable noise after I stop pulsing the STEP pin.

Lu Feiyue
Intellectual 660 points
Part Number: DRV8825


I  pulse the STEP pin to  drive a  step motor to  rotate half a circle  in forward and reverse direction.

But  after I stop pulsing the STEP  pin， the motor makes  some noise.

However, if the motor rotates in the forward direction  before I  stop pulsing, the noise usually occurs. 

if the motor rotates in the reverse direction  before I  stop pulsing, I didn't hear the noise.  So strange.

My configuration of DRV8825 is  as follows：

8 mocro-step,1,4A full-scale chopping current, mixed decay mode.

Could you tell me  what may cause the noise?

  • 0
    TI__Guru 56650 points

    Feiyue，

    We need to check few points.

    1. DRV8825 PWM frequency is 30kHz. If the motor winding time constant (L/R) too high, the inductor current cannot reach the setting in one switching cycle, we may see the inductor current repeats every 2 cycles and the inductor current has 15kHz component which could give us an audible noise. In this case, you can check the winding current to confirm the issue. And then, you can increase the input voltage and add a series resistor to check the difference.

    2. The magnetic force change is related to the winding ripple current. If you change the decay mode to slow decay or fast decay, does the noise change?

    3. Forward has noise; reverse don’t. Rotate half circle.

    What is the winding current difference between forward direction and reverse direction?

    4. To prevent input voltage ripple or other noise injection to the board or motor, would you add more input capacitor near the DRV8825?

    Overall, if we can monitor the winding current on a scope at normal mode and noise mode, it can speed up the debugging process.

  • 0 in reply to Wang5577
    Intellectual 660 points
    What does "current chopping threshold" mean?

    Does each micro-step has a "current chopping threshold"?

    or the "current chopping threshold" is the relative current of a micro-step?

    So, each micro-step has a decay period using already setted decay-mode?
  • 0 in reply to Lu Feiyue
    TI__Guru 56650 points
    Feiyue,

    "During stepping, IFS defines the current chopping threshold (ITRIP) for the maximum current step." ITRIP=IFS at winding maximum current. Other ITRIP value in each micro-step shows on table 2. Also, figure 8,9 and 10 gives the test waveform.
    Decay mode and micro-step setting are independent. The decay period is applied to each PWM cycle after the current reaches the chopping threshold.

    Do you find the root cause of the noise? Is it related to "current chopping threshold" or "decay mode" setting?
  • 0 in reply to Wang5577
    Intellectual 660 points
    Yeah, when I use slow decay mode, the noise disappears. Using mixed decay mode and fast decay mode causes some noise for my step motor.