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Need help answering a question

thread830-424292

Need help answering a question

Need help answering a question

(OP)
Hi Guys,

I've been given some revision material for a control exam, but the one of the answers to the question is incomplete. Would someone help me answer it so i can start remembering it.

Q. For a general Direct Digital Control (DDC) system, discuss the practical hardware and software aspects that influence the selection of sampling increment, T.


This is the incomplete answer that was given:

A. Discussion should include computing speed, its relationship to sampling rate and characteristics of process quantity changes with time.

I must admit, I'm rubbish at control theory.. but can rely on my maths skills for the other questions. Could someone help answer this for me?

RE: Need help answering a question

(OP)
Maybe its way the question is written, but my understanding from electronics is this.. with a microcontroller if you are storing data you're limited to the amount of available bits. So there is a compromise between the resolution and sampling rate. If you increase the resolution the sample rate must decrease so you dont use too many bits.. and vice versa.

And, the microcontrollers clock speed will put a cap on the sampling rate anyway.

Do you think this is close?

RE: Need help answering a question

Seems to me that you crippled your answer by making a rash assumption. The sampling period needs to be high enough that the its functioning is able to affect the plant. Typically, that's on the order of 5x to 10x the desired control bandwidth. Then, the control algorithm must be able to execute within that sampling period, so that dictates the required throughput of the controller.

If you assume a controller to start with, you might wind up with something that's completely useless. A control algorithm that's too laggy might was well not be there at all, since the lag will result in loop instability.

TTFN (ta ta for now)
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