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TMS320F28069: Integrator Method For the Ideal Form PI Controller in DCL Library V3.3

İlker
Intellectual 290 points
Part Number: TMS320F28069


Hello 

I am using DCL Library V3.3, DCL_PI_CLA structure with DCL_Run_PI_L3 macro.

Can you tell me which method does that library use to implement integration ? (Euler,Trapezoid etc.)

Also, Can you also share the Z-Domain Transfer Function for the Ideal Form Controller

What it seems to me is ; Integration method uses Forward Euler Methode and the Transfer Function is " Kp*(1 + Ki * (1/(z-1)))."  Is that accurate ?

Thanks and Regards, İlker.

  • 0
    TI__Expert 6965 points

    In linear operation range, your observation is correct. Keep in mind the real implementation has saturation and anti-windup implemented.

  • 0 in reply to Han Zhang6
    Intellectual 290 points

    Hello Han, Thank you for the quick response.

    After posting to the forum, I realized that the DCL V3.3 documentation says that the L5 uses Trapezoidal Methode, and L1-L4 uses Backward Euler. And now you agree on the Forward Euler for the L3.

    In this case Can I say documentation has a flaw ? I am asking this because after running through the documentation I decided to go with Trapezoid Method for its constant phase shift in bode below the nyquist frequency.  So Can you also Confirm that the L5 uses Trapezoidal as it is presented in the documentation.

    Regards,İlker.

  • 0 in reply to İlker
    TI__Expert 6965 points

    The transfer function you showed in the original post aligns with the implementation.

    I cannot find the keyword backward Euler in documentation. Can you please point out where you found it? Thanks.

    For L5, it uses Tustin method as shown in documentation.

  • 0 in reply to Han Zhang6
    Intellectual 290 points

    I am sorry, It was not in the DCL Documentation, but in the DCL Training Video 7.2: Controllers, at minute 12:42


    https://training.ti.com/dcl-training-video-controllers?context=1137766-1140335-1140338

    @12:42::

    By way of contrast, here we see an example of a series PI controller with a tustin integrator. Most PID controllers and DCL use a backwards Euler type of integrator to minimize the cycle count. The backwards Euler is a perfectly satisfactory method for tuning transient response. However, better phase characteristics are available with the tustin integrator at the expense of an additional delay element. This type of controller would be better suited to situations where a user wanted to load controller gains from a 0-  plus gain definition. The C6 and C7 controllers have tustin integrators run on the FPU32. The L5 is the equivalent controller for the CLA.

  • 0 in reply to İlker
    TI__Expert 6965 points

    OK. Got you. Thanks for pointing that out.

    Actually it is not essential to focus on forward vs backward Euler. If you look at the difference equation of the implementation (which is standard in industry), it is a forward Euler. However, if you take into account the you take into account that you typically update control signal one step after you take the sample, it can be seen as backward Euler.

    If you are targeting a higher controller bandwidth, Tustin is normally considered a better choice.

    Han