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Discrete PI for motor control applications

Rushi Jariwala
Expert 1225 points
Other Parts Discussed in Thread: MOTORWARE

Hi,

I saw the PI libraries used in control suite and motorware. Both use a backward approximation method rather than trapezoidal/tustin implementation. However, according to theory and literature, tustin method is better. 

I want to understand which is better for the application of motor control where we want to control speed and currents. I want to know the pros and cons and why TI has chosen backward approximation. 

Thanks. 

  • 0
    TI__Mastermind 27200 points

    I presume you mean the PID & PI controllers in the DCL.

    Backwards Euler was chosen because we wanted low computational latency and fast transient response. The forward Euler has a delay element in line with the integral path, so the newest input doesn't affect the output for one cycle. The Tustin integrator has two delays so needs two storage elements and takes a few instruction cycles longer to compute. The backwards Euler method has performed satisfactorily in all our testing.

    It is true that the backwards Euler method does not maintain perfect -90 deg. phase at higher frequencies, but this was not an objective and has not been a problem. If phase response is critical, a PI controller with Tustin integrator will be included in the next DCL version, but I cannot way when that will be. Since the library is supplied as source you could implement such a thing yourself if you were curious to try.

    Hope this helps.

    Regards,

    Richard

  • 0 in reply to Richard Poley
    Expert 1225 points
    Hi Richard,

    Thanks for the nice explanation. It helps.