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# taking value of variable after some periods, or after few zero crossing

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nikola simanic on 4 May 2015
Closed: MATLAB Answer Bot on 20 Aug 2021
Hello everyone. I need help with some relay experiment for process identification, i.e to make appropriate scheme in Simulink. I have to put a relay in front of process, and process should start to oscillate(system with negative feedback). After two or three periods of oscillations, I have to find a frequency of oscillations w. Then, I have to "turn off" relay, and continue activation of process with sine wave sin(w*t), w is a frequency which is determined earlier. After few periods I should find new frequency w2 and lead that value to block sin(w*t), and so on until frequency show certain convergence. Every help is welcome. I know how to find frequency, i.e. how to make a scheme which calculates frequency all time. I know how to do it manually with sine wave generator, but it is not problem. All process should be automatic and my main problem is how to take value of that frequency after some periods of time, or after few cycling periods, few zero crossing of process or relay etc. Thank you in advance for any help.

Purushottama Rao on 4 May 2015
As i understood from your problem decsription, basically you require a frequency divider. Pls find the attachement for a simple freq divider ( even for varibale frequency application).
Create a counter whih counts for every zero crossing of the plant process.Compare the counter with a required value to again adjuts the frequency and make the counter value zero to make it ready for next iteration