Hello,
To my understanding, in Simulink, by default, if you have an operation (such as sqrt()) on a value such that Simulink cannot prove that the result will be real-valued, then the output signal type will be set to "complex" instead of "real". The signal type does not vary with the inputs. It could be that with the particular inputs and logic that Simulink's reasoning engine is inadequate to come up with the proof, but if Simulink thinks that it might turn out complex then Simulink marks it as complex. For any step that it turns out to be real-valued, the complex component would of course be 0.
You can find the line that is being set as complex (you might have to hunt back a few blocks) and tell Simulink to mark the line as real. If you do that and Simulink encounters a value for which the output needs to be complex, Simulink will throw an error at run-time. You can insert a conversion block to remove the complex part.
Hope it helps
