I am in the process of designing a digital controller for level control of a coupled tank system but I am not sure how to select the sampling interval for the control signal. I believe there is deep relationship between the sampling interval and the time constant of the system under consideration. I also came across a website where it was stated that " Sample time should be 10 times per process time constant or faster " but I am not being able to find any good journals on that.
Can anyone please help?
The sampling interval depends with the dynamics of your system to be controlled. In fact the rule of "10 times" refers exactly to this point.
1.Let assume we have a tank where the content will vary between a minimum and a maximum level. We should first know (or estimate) how long will take to get a maximum level when level is minimum, and opposite. Once you know the duration, then you will have the sampling interval you are looking for.
2.You should design then a range for your sampling interval. You can use the rule of duration +/- 3StDev to describe the interval you need in sampling.
3.Have a difference between sampling interval and shipping of that signal you have from sensors. A 1:1 relation can be assign if the shipping cost is not to concern, or you can design different rate of sampling:shipping rate, depending with your actual application.
Dr. Dragos, thank you for your elaborate answer. However, I have one doubt. What to do when the time taken to fill up (from minimum to maximum) is different from the time taken to empty (from maximum to minimum) the tanks ? Should I consider the average of the two values then ?
In the case the two are different, apply an ANOVA and determine if the two are different in statistical sense (or just have an estimation based on expected StDev). If yes, you should consider two sampling intervals. If ANOVA will not shows a significant interval, then apply for the mean value as duration.
Hamit Can The link in your answer is not alive anymore, could you please offer the title of the article? Thanks~
Iliescu Dragos Dr. Dragos, I have the same question with the OP, can you provide a paper that I can refer to? Thanks~
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