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what is the relation between the Ki，Kp and Sample Frequency?

Eli_jia
Intellectual 530 points
Other Parts Discussed in Thread: MOTORWARE

Hi everyone,

I found the section 12.2 of  spruhj1c.pdf explains how to calculate the value of Kp,Ki in a PI regulator, Ki = R/L, Kp=L*Bandwidth, equation 37 and 38 on page 305.

But what is the relation between the Ki，Kp and Sample Frequency?

Could you give me any hints? Thanks!

Eli

  • 0 in reply to Igor Gorbachev
    TI__Guru** 119380 points

    for the current controllers, the equations for how we calculate the default gains are 

    Kp: = [IQ_Current / IQ_Voltage] * Ls * Frequency * 0.25

    [the 0.25 just reduces the gain from ideal stiffness to enable wider initial stability across more motors; for high performance you must increase this back to the full limit and we recommend doing step response testing to tune for your system[

    Ki= Rs / Ls / Frequency

    [during the motor ID process we use the initial RoverL high frequency estimation to set this value]

  • 0 in reply to Igor Gorbachev
    Intellectual 530 points

    Hi Igor,

    Thanks. your hint is helpful.

    Eli

  • 0 in reply to ChrisClearman
    Intellectual 530 points

    Hi ChrisClearman ,

         Sorry! I have another question about this. If I want to change the PI structure to the incremental form, do the equations to calculate the Kp, Ki  need to be modified? Thanks!

     

  • 0 in reply to Eli_jia
    TI__Guru** 119380 points

    what do you mean the incremental form?

    are you using a different PID controller than the one provided in MotorWare?

     

  • 0 in reply to ChrisClearman
    Intellectual 530 points

    Hi,

        The incremental form of PID means the output of the PID regulator is not the final output U(k),  just the increment deta_U referes to the last time output U(k-1), so the U(k) = U(k-1)+deta_U.

         If you don't mind, we can communicate with E-mail, then I can give you more detailed information.

         Thanks!

    Eli

  • 0 in reply to ChrisClearman
    Prodigy 10 points

    I know it might be a stupid question, but what is IQ_current and IQ_voltage?

    /Jens

  • 0 in reply to Jens Overby
    TI__Guru** 119380 points
    beyond the comments that are in the user.h? Essentially they are used to create a per-unit value for all voltages/currents in the system.

    //! \brief Defines the full scale current for the IQ variables, A
    //! \brief All currents are converted into (pu) based on the ratio to this value
    //! \brief WARNING: this value MUST be larger than the maximum current readings that you are expecting from the motor or the reading will roll over to 0, creating a control issue
    #define USER_IQ_FULL_SCALE_CURRENT_A (10.0) // 10.0 Example for hvkit_rev1p1 typical usage

    //! \brief Defines full scale value for the IQ30 variable of Voltage inside the system
    //! \brief All voltages are converted into (pu) based on the ratio to this value
    //! \brief WARNING: this value MUST meet the following condition: USER_IQ_FULL_SCALE_VOLTAGE_V > 0.5 * USER_MOTOR_MAX_CURRENT * USER_MOTOR_Ls_d * USER_VOLTAGE_FILTER_POLE_rps,
    //! \brief WARNING: otherwise the value can saturate and roll-over, causing an inaccurate value
    //! \brief WARNING: this value is OFTEN greater than the maximum measured ADC value, especially with high Bemf motors operating at higher than rated speeds
    //! \brief WARNING: if you know the value of your Bemf constant, and you know you are operating at a multiple speed due to field weakening, be sure to set this value higher than the expected Bemf voltage
    //! \brief It is recommended to start with a value ~3x greater than the USER_ADC_FULL_SCALE_VOLTAGE_V and increase to 4-5x if scenarios where a Bemf calculation may exceed these limits
    //! \brief This value is also used to calculate the minimum flux value: USER_IQ_FULL_SCALE_VOLTAGE_V/USER_EST_FREQ_Hz/0.7
    #define USER_IQ_FULL_SCALE_VOLTAGE_V (450.0) // 450.0 Example for hvkit_rev1p1 typical usage