In the example you mentioned, the robstub function is used to compute the robust stability margin of a closed-loop system with uncertain parameters. The robust stability margin is a measure of how much the uncertain parameters can vary before the closed-loop system becomes unstable.
If the bounds on the uncertain parameters are greater than 1, it means that no combination of these uncertain parameters will lead to instability of the closed-loop system. This means that the robust stability margin is infinite, indicating that the closed-loop system is robustly stable.
However, even though the closed-loop system is robustly stable, it is still possible for the nominal plant (the plant without any uncertainty) to be unstable. This is because the robstub function only considers the worst-case uncertainty, and the worst-case uncertainty may not be the same as the actual uncertainty in the system.
For example, consider a closed-loop system with uncertain parameters that have bounds greater than 1. If the actual uncertainty in the system is such that the nominal plant is stable, then the closed-loop system will be stable as well. However, if the actual uncertainty is such that the nominal plant is unstable, then the robstub function will still report that the robust stability margin is infinite, even though the closed-loop system is actually unstable.
In other words, the robstub function only provides a conservative estimate of the robust stability margin, and it does not guarantee that the closed-loop system will always be stable. It is still possible for the closed-loop system to be unstable if the actual uncertainty in the system is different from the worst-case uncertainty considered by the robstub function.
I hope this helps clarify the behavior of the robstub function.
