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Motor current volume while sttoping

user4530078
Prodigy 120 points
Other Parts Discussed in Thread: DRV8880

 Dear Motor Drive Support Team,

 In order to suppress the raising heat on the motor and the driver, I have the torque control by using  TRQ1/2 signals of DRV8880.

I set the torque 100% while the motor is rotating, mean while, the torque is reduced to be 25 % while it is stopped.

Under this condition, I measured motor current at the motor is rotating and stooped. 

I confirmed the current volume as I expected while the motor is rotation, however the volume is much lower while it is stooped. 

e.x. At Full step mode with 2.0A maximal current and 25% torque setting at 24V power supply, the volume is as lower as 18.5mA.

Even at 100%  torque setting, the volume is only increased to 20.5mA. 

Could you tell me the basic idea of the motor driver current control while a motor is stopped.

Yohjiro Kawai,

 

  • 0
    TI__Guru** 114905 points

    Hi Kawai-san,

    Can you provide scope captures of both windings when the motor is stopped at 100% and 25% torque?

  • 0 in reply to Rick Duncan
    Prodigy 120 points

     Hi Rick,

    I measured the current again and waked that my previous measurement was something wrong.

    Please confirm the attached file that provide captures of both cases on TRQ 100% and 25% while motor is stopping.

    Do you think my measurement volume are reasonable?

    Can I determine the current volume by (max current setting volume / root(2))*TRQ%? 

    Yohjiro,

    Current measurement while stopping the motor.pdf

     

  • 0 in reply to user4530078
    TI__Guru** 114905 points
    Hi Kawai-san,

    The measurements seem reasonable. What is the VREF and sense resistor values?

    Please note the 1.41A is a target trip point for the comparator. The current can exceed this during the blanking time. Also, once the current enters regulation, the current typically decays until the next drive cycle begins.

    In the first image (page 1), the current is above the desired 1.41A set. This can be due to the current exceeding the target during the blanking time. Tolerances in the system such as resistor tolerances, comparator, and noise in the system can also affect the trip point.

    The RMS current during hold will be less than the target (calculated) current because the device enters current regulation once the target current is reached. The current typically decays some until the next drive cycle begins. The amount of ripple during each current regulation cycle determines the RMS current.
  • 0 in reply to Rick Duncan
    Prodigy 120 points

     Hi Rick,

     Thanks for your response.

      a1) VREF is 3.3V ( connected to V3P3 pin directly) 

      a2) SEN_resistors are 0.25 ohm respectively.

    I found the formula to be determined the IFS as below. 

       IFS = ((3.3*torque%)/(6.6*0.25)).

    Do I how to determined IFS by IFS/root(2) at full step mode and during hold (stopping the motor)?

    Yohjiro,

     

  • 0 in reply to user4530078
    TI__Guru** 114905 points
    Hi Kawai-san,

    Do I how to determined IFS by IFS/root(2) at full step mode and during hold (stopping the motor)?

    I am not sure I understand your question.

    When using full step mode, the current trip value is set at 71% of the calculated value (IFS/root(2)). You can see this in the indexer table of the datasheet.