output rate vs step responce

Hi All, can anbody explain why in PLC or DCS analog input modules the output rate is often many times the step responce. eg one +/-10V input module had an output update time of 10mS but a step responce time of

100mS. Similarly I have seen modules with a 3dB point set to 17Hz and the output rate at 1Khz.

Is it something to do with PID control requirements?

These are analog input modules so output rate is the conversion rate of the analog to digital conversion signal sent to the controller.


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If you have a high-gain loop and noisy feedback you can saturate your control output with the feedback noise and completely loose the linearity of your loop. So it is often a good idea when you have low speed requirements from your loop and noisy feedback to low-pass filter the feedback.

You keep the 3dB point of the low-pass filter sufficiently high (somewhere between 2x and 10x your closed-loop bandwidth), and balance your need for speed with the quality of your feedback.

All of my control loops have involved custom circuits and software, so I don't know for sure but I'd hope that the PLC lets you set the 3dB point of the lowpass, or eliminate it entirely.

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Tim Wescott

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