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Rate Transition in If-action Subsystems

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I am trying to setup a control system with different controller operating modes. I want to handle the selectionm of the operating mode with a if-action subsystem structure. I'm getting the following error that I am not completly sure how to handle:
'ctrlActiveHandler_0_4_sim/Axis_Control/AbsPosMode/Horizontal_Axis_Control/Rate Transition' is a rate transition block. A rate transition block is not allowed inside the if action subsystem 'ctrlActiveHandler_0_4_sim/Axis_Control/AbsPosMode' unless its input or output port sample time is asynchronous sample time. Consider placing the rate transition block outside the if action subsystem.
I can't take the rate transition outside the subsystem as it ensures integrity between to different blocks. And I am not sure about the asynchrounus sample time. Can someone explain this error and its fix?
Thank you very much.

Accepted Answer

Fangjun Jiang
Fangjun Jiang on 20 Oct 2020
Edited: Fangjun Jiang on 20 Oct 2020
Let's say your whole model is executed at every 0.1s. At every 0.1 second, the condition is checked and the "If-action" subsystem is executed or not executed. So the execution of the subsystem is asynchronous or triggered. I can't think of a scenerio where inside the subsystem, a rate transition is meaningful, although your error message says there might be such a case.
Move the rate transiton block outside of the "if-action" subsystem. I can't imagine why you can't.
Added below:
Now I think I might understand such a case indicated in the error message. When the "If-action" subsystem is executed, all its inputs must be available. The "If-action" subsystem is only executed at most once per step size, so there is no need to do rate transition inside. However, if one of its input is asynchronous or triggered which means it is not available at the time when the "If-action" subsystem is executed, then you might need to do rate transition inside (for the purpose of holding last available value). Still, even in that case, I think it is possible or better to do the rate transition outside.
Fangjun Jiang
Fangjun Jiang on 21 Oct 2020
Yes. It certainly looks like the differences are there. I never had the need to worry about this subtle differences.

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