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DRV8889-Q1: Inquiries about EVM and device

Yuto Kitamura
Expert 6535 points
Part Number: DRV8889-Q1

Tool/software:

Dear team,

Would you please help me to understand below queries?

1. I learned we need to change STALL_TH when we change the motor speed. In general, which should it be changed to increase or decrease the STALL_TH when speed-up.

2. On EVM, we can adjust the motor speed(pps) and acceleration rate(pps/s). It should be MCU processing issue, but on EVM how much is the time-wise resolution of acceleration rate?

3. Does IC or ambient temperature affect to output current accuracy? In other words, can we understand below specification is for all temperature range or only room temp? 

4. How long does it take to be ready to operate normally after VM is greater than 4.5V?

Best regards,
Yuto

  • 0
    TI__Mastermind 44685 points

    Hi Kitamura-san,

    Thanks for your post.

    Yuto Kitamura said:
    1. I learned we need to change STALL_TH when we change the motor speed. In general, which should it be changed to increase or decrease the STALL_TH when speed-up.

    While this may depend on motor parameters, L, R and BEMF constant as well as VM, IFS current, and microstep settings, the general trend assuming everything else was fixed would be the torque count may decrease with increase in speed from a peak torque count point. This means the STALL_TH may have to be reduced to avoid a false stall detections. This also would depend on current speed. If the current speed is low and is increased, the torque count may increase. The plot may resemble an upside down parabola like below, y-axis would be relative torque count and x-axis relative speed. This is from observed general trend. There is no easy mathematical way to plot this for any given stepper. It becomes complex while VM, IFS changes during speed change keeping microstep mode changed. With changes in microstep mode, the apex of parabola speed would change as well. 

    Yuto Kitamura said:
    2. On EVM, we can adjust the motor speed(pps) and acceleration rate(pps/s). It should be MCU processing issue, but on EVM how much is the time-wise resolution of acceleration rate?

    The acceleration input widget allows input with resolution of 1 PPSPS (pulse per second per second). The MCU, in its firmware design, increases or decreases speed every 32 ms, the rate of change calculated from the PPSPS input value.

    Yuto Kitamura said:
    3. Does IC or ambient temperature affect to output current accuracy? In other words, can we understand below specification is for all temperature range or only room temp? 

    This specification covers entire operating conditions. 

    Yuto Kitamura said:
    4. How long does it take to be ready to operate normally after VM is greater than 4.5V?

    See below specification in the datasheet. Thank you.

    Regards, Murugavel 

  • 0 in reply to Murugavel4637
    TI__Expert 6535 points

    Hi Murugavel,

    Thanks for comment. I understood overall except Q1.

    I think TRQ_COUNT stands for BEMF torque or voltage. If so, higher speed leads higher BEMF = higher TRQ_COUNT.

    It is relative to Q2, but which does TRQ_COUNT indicates the value of BEMF calculated by 1/Toff or the change of BEMF calculated by (1/Toff1 - 1/Toff2)?

    best regards,
    Yuto

  • +1 in reply to Yuto Kitamura
    TI__Mastermind 44685 points

    Hi Kitamura-san,

    Yuto Kitamura said:
    I think TRQ_COUNT stands for BEMF torque or voltage. If so, higher speed leads higher BEMF = higher TRQ_COUNT.

    This is true for direct measure of BEMF voltage which would be proportional to speed like you mentioned. The plot I showed is the behavior of the device and the microstepping at higher speeds. At higher microstepping the step size is small and number of samples available for calculation is reduced. Additionally as the speed increases the tOFF cycles get distorted at higher speeds and peaks of the sine wave current. This can be observed using an oscilloscope and current probe. See this application report about the stall feature in this device. Thank you.

    Regards, Murugavel