conditionally executed subsystem introduces direct feedthrough

i have to run a subsystem conditionally with a fixed step size and generate code for it. I descided to use an enabled subsystem. The subsystem has several inputs and outputs. The problem i am stucking with is that this nonvirtual subsystem now defines a direct feethrough for all outputs (they need all inputs of the same timestep), even if states are introduced inside the subsystem. So what happens is that I introduce an algebraic loop in the parent subsystem - this would not be the case without an enabled subystem.

Adding a unit delay block for the outputs of the enabled subsystem is not acceptable for me because my fixed-step simulation can get instable an inaccurate.

Has anybody any suggestion how to deal with this problem? Or an explanation why the subsystem has to behaves this way?

Answer 1

[Fangjun Jiang]

Did you use a Merge block? That's typically needed when you have a conditionally executed subsystem.

Answer 2

[Markus]

Hi Fangjun & thank you for answering!

Yes, in fact i have several enbaled subsystems running in parallel. Each of them has the same bus as output and afterwards i merge them.

I do not know for sure but i would suggest that this is a problem with the enabled subsystem itself, not with the following merge block. As far as I can see the information which signal has direct feedthough and which not seems to be generally lost and therefore the enabled subsystem declares all outputs as direct feedthough...

Answer 3

[Pan Zhao]

Hi Markus,

I was wondering whether you figured out your problem. I met a similar problem and solved it in the way explained here https://www.mathworks.com/matlabcentral/answers/345895-why-does-putting-a-s-function-inside-an-enabled-subsystem-cause-an-algebraic-loop.

Regards Pan