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Original question:

DRV10987: About the Open loop current

DRV10987: Question about the parameter

NAOKI AOYAMA
Guru 11215 points
Part Number: DRV10987

Hello,

Our customer ask us again.

1) What is the difference between the calculations of the Start up current and the Software current limit?

   For the calculations above, are the parameter of RPH_CT ( = Rm) and Kt same?

2) Are there any ways to check if the value of Rm and Kt are correct?

  Is it OK as long as the set value and the measured value match in the Software current limit?

Best Regards,

Naoki Aoyama

  • 0
    TI__Mastermind 24405 points

    Hi Naoki,

    Thanks for posting your question in MD forum.

    1) What is the difference between the calculations of the Start up current and the Software current limit?

    Startup current is not calculated by the device. Startup current depends on the load acting on the motor during startup. Software current limit is configured by writing the SWiLimitThr[3:0] bits. Device calculates the voltage limit based on programmed resistance, BEMF constant and the software current limit. Refer to section 8.4.7.1 in the datasheet for more information. 

    2) Are there any ways to check if the value of Rm and Kt are correct?

    Values of Rm and Kt should be available in the motor datasheet. These parameters can also be measured using multi-meter and oscilloscope. Please check the Video on how to measure Rm and Kt. These measured values can be programmed in the device. Since software current limit works based on programmed Rm and Kt, it is important to program the accurate values for this function to work accurately. 

    Regards,

    Vishnu

  • 0 in reply to Vishnu Balaraj
    Guru 11215 points

    Hello Vishnu,

    Thank you for your reply.

    Does the DRV10987 use these parameters for another calculations? (In addition to calculation of Software current limit)

    Regards,

    Naoki Aoyama

  • 0 in reply to NAOKI AOYAMA
    TI__Mastermind 24405 points

    Hi Naoki san,

    Programmed Rm and Kt are also used in below functions:

    1. Abnormal speed detection

    Programmed Rm and Kt are used for the BEMF estimation in the Abnormal speed detection. If the estimated motor BEMF is much higher than the output amplitude for a certain period of time, it means the estimated speed is wrong, and the device triggers Abnormal speed fault. See section 8.4.8.2 in the datasheet for more details. 

    2. Abnormal Kt detection

    Also, programmed Kt is used for Abnormal Kt detection. Based on the programmed Kt value, a hysteresis from Kt_low to Kt_high is created. If the estimated Kt goes beyond the hysteresis bandwidth, abnormal Kt gets detected and the device triggers Abnormal Kt fault. See section 8.4.8.3 in the datasheet for more details.  

    Regards,

    Vishnu

  • 0 in reply to Vishnu Balaraj
    Guru 11215 points

    Hi Vishnu,

    Thank you for your reply.

    I understood about the parameter.

    I will feedback the customer the information.

    Thanks,

    Naoki Aoyama