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LAUNCHXL-F28379D: Fast current loop - Torque PI control equations and their relation with Ld and Rs.

Arun Kumar
Expert 1030 points
Part Number: LAUNCHXL-F28379D

Hello, 

With reference to this document 

https://www.ti.com/lit/an/spraco3/spraco3.pdf

I would like to know some details about the PI control loop employed in the fast current loop library.

In fcl_cpu_code.dm in the PI wrap function FCL_runPICtrlWrap_M1(),

why are we using invZbase and how does it relate to the rotor resistance and rotor inductance,

I see that :

float32_t Vbase = pMotor->FCL_params.Vdcbus * 1.15 / 2;

float32_t invZbase = pMotor->FCL_params.Ibase / Vbase;

I understand that FCL_params.Vdcbus is the actual dclink voltage(PPBresult*74.1/4096) , but why are we multiplying by 1.15/2?

And how is this related to the rotor resistance and inductance?

fclVars[0].pi_iq.Kp = pMotor->FCL_params.Lq * invZbase *
pMotor->FCL_params.wccQ;
fclVars[0].pi_iq.Ki = pMotor->FCL_params.Rq * invZbase *
pMotor->FCL_params.wccQ * pMotor->FCL_params.tSamp;

It would be helpful if this was explained in layman's terms :)

  • 0
    Expert 1030 points

    PS: does this have something to do with the voltage decoupler?

  • 0
    TI__Guru** 114520 points
    Arun Kumar said:

    I understand that FCL_params.Vdcbus is the actual dclink voltage(PPBresult*74.1/4096) , but why are we multiplying by 1.15/2?

    And how is this related to the rotor resistance and inductance?

    The coefficient used in invZbase variable to convert the SI value of the R and L to the PU format according to the model of the motor and implemented SVM. 

    Arun Kumar said:
    PS: does this have something to do with the voltage decoupler?

    Yes. Calculates the coupling voltages to eliminate the d-q axis coupling that causes nonlinearity of the control.

  • 0 in reply to Yanming Luo
    Expert 1030 points

    Thanks for your reply Yanming.

    The coefficient used in invZbase variable to convert the SI value of the R and L to the PU format according to the model of the motor and implemented SVM. 

    So does this mean that the Kp and Ki would change during runtime? invZbase  would change because there would be slight variations in the dclink?

    So Kp is basically, ( rotor inductance * bandwidth)?

    And Ki is rotor resistance * bandwidth * sampling time? WHy is sampling time added in KI value when the bandwidth already takes care of the sampling time?

  • +1 in reply to Arun Kumar
    TI__Guru** 114520 points
    Arun Kumar said:
    So does this mean that the Kp and Ki would change during runtime? invZbase  would change because there would be slight variations in the dclink?

    Yes.

    Arun Kumar said:
    So Kp is basically, ( rotor inductance * bandwidth)?

    Right.

    Arun Kumar said:
    And Ki is rotor resistance * bandwidth * sampling time? WHy is sampling time added in KI value when the bandwidth already takes care of the sampling time?

    The Ki is transformed into the discrete time domain. You may find this theory introduction in the related digital control or PI/PID textbook.

  • 0 in reply to Yanming Luo
    Intellectual 440 points

    I'm having a hard time understanding your response to Arun's questions.

    "float32_t Vbase = pMotor->FCL_params.Vdcbus * 1.15 / 2;  but why are we multiplying by 1.15/2?

    Why the 1.15/2?  Is this because this is the maximum modulation index in the linear region with SVPWM?

    Thanks,

  • 0 in reply to Niloc2004
    TI__Guru** 114520 points

    Correct, it's related to the usage of DC bus voltage with SVPWM. You don't need to change this coefficient for any hardware and device if you want to implement the example code.