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How to use rate limiter with square signal and variable step size?

Asked by Jonathan on 5 Aug 2014
Latest activity Answered by Jonathan on 6 Aug 2014
Hi
Here's a screenshot of the model I'd like to set up.
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I feed a customized rectangular signal to a rate limiter to avoid vertical slopes. The input signal is basically like this:
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Unfortunately that doesn't seem to work. I'm using ode15s, it's a requirement. Here's the error message Simulink throws:
Error: Input signals to Rate Limiter '.../Rate Limiter' are neither discrete nor continuous sample time signals. Only discrete or continuous input signals are supported
Quite surprisingly I found a workaroud by adding an integrator directly followed by a derivative. This works:
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But it's ugly and I'm getting some very annoying stability issues in some cases. And I doubt very much that it is considered "good practice".
So how is one supposed to use this rate limiter block in such a situation?
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John

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2 Answers

Answer by Fernando
on 5 Aug 2014
 Accepted Answer

Hi John,
The problem you have is all about sample times, and sample times propagation. In your first example, there is probably no definition of sample time in any block, so that in the sample time propagation phase Simulink cannot determine the appropriate sample time for the block.
The workaround works because you've inserted an integrator, which has a defined continuous sample time. This is then back-propagated to the rate limiter block and to other blocks.
The easiest way to solve the problem in your first implementation is to select Sample time mode: continuous in the Rate Limiter Dialog Box.

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Answer by Jonathan on 6 Aug 2014

Your solution seems to work extremely well! I thought that was going to be much more complicated. Thank you very much!

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