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DRV8308: Accuracy of FGOUT

ye ye
Prodigy 10 points
Part Number: DRV8308

Using 8308 to drive the motor and AEDR-9930 encoder to collect speed values with an error of 0.08%, while using FGOUT signal to calculate speed error of 1.5%. What is the reason and what is the accuracy of FGOUT?

  • 0
    TI__Mastermind 18981 points

    Hi Ye, 

    Our team is currently out of office for a US holiday, so responses may be delayed.

    Please give us further time to review & reply to your question, with a new reply next week on debug comments and next steps 

    Best Regards, 
    Andrew 

  • 0 in reply to Andrew Liu
    TI__Expert 5578 points

    Hi Ye,

    When properly tuned DRV8308 can drive motors with <0.1% cycle jitter and fast torque compensation for varying loads. 

    DRV8308 can only support hall sensors and cannot support encoder signal inputs to commutate the motor. 

    Best,

    Robert

  • 0 in reply to Robert Wedan
    Prodigy 10 points

    Hi Robert,

    the current speed of error with FGout is 1.5%  ,and we want to improve the motor of speeds accuracy，what some methods can be taken for improving the  accuracy. Thanks

  • 0 in reply to ye ye
    TI__Expert 5578 points

    Hi Ye,

    I will aim to provide an answer on this by Wednesday of next week!

    Best,

    Robert

  • 0 in reply to Robert Wedan
    TI__Expert 5578 points

    Hi Ye,

    I believe FGOUT accuracy is related to the accuracy of the CLKIN and LOOP GAIN. Motor speed in rpm = (frequency of CLKIN) * 60/pole pairs. 

    What is SPEEDTH configured to? This parameter sets how much speed variation is allowed across Hall_U periods. Can you change the SPEEDTH to zero (0.2%) and see if this helps?

    For further inquiries please reference the DRV8308EVM User's Guide, specifically the tuning guide section found here: http://www.ti.com/lit/pdf/slvua41

    Best,

    Robert

  • 0
    Genius 5040 points
    ye ye said:
    Using 8308 to drive the motor and AEDR-9930 encoder to collect speed values with an error of 0.08%, while using FGOUT signal to calculate speed error of 1.5%. What is the reason and what is the accuracy of FGOUT?

    Hi Ye,

    I don't think using encoder is more accurate than using the FGOUT as the speed feedback, because both signals are generated by the motor rotational speed, in pulses.  So they should have the same absolute speed accuracy. 

    However, encoder might have higher resolution than FG signal, and the high resolution can affect the closed loop speed gain. 

    what is the resolution of the encoder (pulses per turn)? Do you have FG generated from Hall sensors or copper trace on the pcb? What is the FGSEL value? I think using FGSEL=2 gives the highest resolution for FG.

    Brian

  • 0 in reply to ye ye
    TI__Mastermind 18981 points

    Hi Ye,

    Please let us know if you require further assistance on this debug. 

    If not, please help mark thread as resolved. Thanks 

    Best Regards, 
    Andrew 

  • +1 in reply to Andrew Liu
    TI__Expert 5578 points

    Hi Ye,

    If there are no more questions, could you please help mark this thread as resolved?

    I will close the thread for now, but feel free to open it again if you have an additional question around this topic.

     

    Best,

    Robert

  • 0 in reply to Robert Wedan
    Prodigy 10 points

    how mark this thread as resolved?