MATLAB simulink error

Question

I am trying to simulate a block diagram in simulink. The diagram is as follows :

When I change the gain to any other value other than 1 I get the following error :

I have tried using unit delay, z inverse delay.When I use them I don't get the error but the output changes.i.e does not match with the output of '>>step(sys)'

Please help.

Thank you.

Answer 1

The output of your gain block feeds back to its input over the summation and trandfer function block. That is the algebraic loop that is causing trouble. You should introduce a unit delay block in the feedback signal, i.e. on the signal line just before the summation block.