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IDDK_PM_Servo_F2837x MAIN main isr timing

MARK PESACH
Intellectual 850 points

Hi

I am using the IDDK_PM_Servo_F2837x code example, and every thing is working very well.

i whould be very grateful is some one can explain the next issue:

the main motor ISR is tied to the  PWM  period staring point , if i use the shunt resistor current sensing method  , the current sampling is also tied to the pwm period stating point. from  what i understand this can lead to big  phase delay , because every time the main isr is executed the current pi controller may use old sample ? 

best regards

Mark

  • 0
    TI__Mastermind 43730 points
    Mark,

    The shunt current value can only be sensed at the PWM starting point, and hence phase delay for a conventional PWM modulation in unavoidable.

    However with the phase current sense some alternative schemes like dual sampling etc can be used to reduce this phase delay by half.

    I would probably rephrase your comment as, every time the PI controller is executed the PWM update is delayed by a PWM period, it is not using an old sample, it has the latest but there is a phase delay.

    If you have enough modulation index left you can actually use the current sample and update immediately as well by disabling the shadow load, but with this you should understand the PWM generation very correctly for your application.

    Alternatively people use overmodulation like DC Clamp PWM to avoid such regions.

    Hope this helps.