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DRV10970 - Sinusoidal output & speed variability

Other Parts Discussed in Thread: DRV10970, TLC555

I am working to develop a BLDC motor driver. The selected motor is a Faulhaber 2622, 6V version, datasheet attached. 

I'm using the DRV10970 eval board and have configured the board properly such that the motor is spinning. The configuration is 30deg Hall sensor IC mode, but the Hall sensor logic is inverted from what the 10970 expects, so the inputs are driving the negative side while the positive is referenced to 2.5V, per the datasheet. I am probing on the phases of the motor during drive and not seeing sinusoidal control, despite the settings on the evaluation board but I cannot understand why. 

Eval Board Configuration: 

DAA - 10degrees (2-3)

VM - 5V

CMTMOD -  Sinusoidal operation @ 30degree placement (1-2)  

LED2 is not indicating a rotor lock, obviously, and I seem to be well under the current limit threshold set by R16 on the eval board, however the behavior suggests that it's in trapezoidal mode due to an error condition. Scope capture of voltage & current on Phase W are also attached. 

Would appreciate if someone could help me understand what's going on, as I do not have a lot of experience with motor control. Is this functioning as expected? 

NEW MOTOR - 2622_B_DFF.pdf

  • Hi Dave,

    Our experts have been notified and should reply soon.
  • Hi Dave,

    Can you make sure that changes highlighted in section 9 Changes in EVM for Inverted Hall Signals in EVM userguide is followed 

    Current waveform doesn't look right, it is neither sine nor trapezoidal, what is input duty cycle ?

    Can you also try increasing bus voltage to 6 V  

    Please share BEMF v/s Hall signals waveform with us and while measuring BEMF can you connect phase signals to external Star/delta connected resistor network 

    Regards,

    Krushal 

  • Changes in section 9 have been made, yes. Hall sensors are connected to negative side inputs, and positive side inputs are referenced to 2.5V. Changing bus voltage of the motor does not affect the waveforms aside from changing the motor speed. 

    Are you asking about the duty cycle of the PWM frequency from the TLC555 timer circuit? That is approximately 40%.

    BEMF/Hall waveforms are here, with labels. 

  • Could I please get an update?

  • Hi Dave,

    The Hall signals look good. DRV10970 is spinning motor in sine mode, but due to low inductance of this motor there is distortion in waveform.

    Can you please explain what makes you think it is trapezoidal waveform? Typically the shape of waveform is monitored for whole electrical cycle and not PWM cycle 

    Can you try spinning motor at higher speed and see whether it improves shape of current waveform

    Regards,

    Krushal 

  • High speed waveforms attached. I am still a little confused. My understanding was that in sinusoidal drive mode, the voltage and current on the individual phases would be sinusoidal, but I clearly see a pulse width modulated square wave on the phase (Ch2), and while the current (Ch4) begins to look like something sinusoidal, it is only during the pulses. Is this normal? 

  • Hi Dave,

    DRV10970 induces third harmonics to voltage waveform and it outputs pulse width modulated voltage waveform. Please refer section 8.4.1.2
    Sinusoidal Pulse Wide Modulation (SPWM) Control Mode in datasheet for details.
    Output of driver is pulse modulated and motor act as first order filter. One technique to see sine wave is to process signals on scope by using average/filter operation with smaller sampling rate and then subtracting two phase voltages

    Phase current looks somewhat sinusoidal over one electrical cycle (cycle of hall signals). The smaller inductance in motor is causing distortion in phase current. Can you please try spinning motor in trap mode (CMTMOD pin floating) and compare that waveform with this one.

    Let me know how it goes

    Regards,
    Krushal
  • Hi Krushal,

    I finally see a waveform that is similar to the datasheet. Thank you very much for your patience. I did a dumb thing and forgot the change the scope acquisition mode from sample to average. See below, I'm satisfied with this now. 

    I do have one final question regarding speed variability. For my application, stability and consistency of speed for the motor is critical. It seems that I could do this in two ways, chiefly: 

    1) Control the duty cyle into the PWM input pin

    2) Adjust the motor voltage, VM

    Which is likely to have better results? If controlling the duty cycle, I would expect that a very low drift oscillator chip would be my best option. Do you agree? 

  • Hi Dave,

    I am glad that things are working out at your end.

    Both the mode of controlling speed should work, we have customers using them without problem. It basically depends on application and your overall system. We use 555 based timer on EVM for PWM control and it works just fine.

    And yes oscillator chip should also work as long as you are able to control the duty cycle and frequency is within spec for DRV10970 (15kHz - 100kHz)

    Let me know if you need any further clarification 

    Regards,

    Krushal 

  • Hi Krushal,

    I'm now trying to finalize my design for some evaluation testing. I'm trying to calculate duty cycle required for a given motor speed. Based upon the motor datasheet per above, and the 8:1 gear ratio included in the motor (Vm = 5V), and a desired speed of 252 rpm, I'm calculating that the duty cycle required on the PWM input is ~37.1%:

    DesiredSpeed * GearRatio / (Kn * Vm)

    Based upon a 4 pole motor topology (i.e. two hall sensor periods = 1 rpm), that says that I should expect a hall sensor period of 14.88 ms to achieve 252 rpm. 

    I am measuring a duty cycle of 38.6% per the TLC555 timer on the evaluation board and the resistors I selected on hand, but the Hall sensor period varies drastically based upon Trapezoidal or Sinusoidal mode. Can you help me understand why this would be? Figures below.

    Ch3 is the hall sensor period, and in both cases DAA is configured for 10 degrees. The only difference is CMTMOD configuration. Floating for Trapezoidal mode, HIGH for Sinusoidal mode. The hall sensor period for trapezoidal mode ends up being pretty close, and the speed is roughly what I expect, considering that I am neglecting friction losses (no real load), but the sinusoidal speed is not correct and doesn't seem to match the duty cycle being fed into the DRV10970. 

  • Hi Dave,

    Based on the picture, amplitude of current out to motor seems to be about the same whether you are in trap or sine mode and period of hall signals looks about 15mS. I also see that U-V voltage is about 5V in both cases.

    What is the input duty coming into PWM pin for both cases?

    Thanks,

    Sei

  • Duty Cycle on PWM pin is 38.6%
  • Dave,

    I am scratching my head now. if input duty, amplitude of current, applied voltage and RPM are same. Can you explain me what is the concern?
  • I think you are misunderstanding. RPM is NOT the same. If you observe the measurements and the period on Channel3, Hall sensor period varies between the two from 15.41ms to 14.85ms. This corresponds to a difference in motor speed of almost 10 rpm. 

    With PWM duty cycle and Vm being the same, the only difference in configuration of the DRV10970 is trapezoidal vs sinusoidal mode. So my question is why does this result in different motor speed?

  • Thanks for the clarification Dave.

    DRV10970 controls the peak of the applied sine voltage or peak of trapezoidal voltage. Yes peak is same between two drive scheme but RMS voltage is not same. Trapezoidal would be bigger, hence trapezoidal voltage should result in higher RPM as you are seeing.

    Thanks,
    Seil
  • Got it - thank you.