Glossary of Control Engineering Terms - U

Under-Damped A linear system that has overshoot is said to be under-damped. This applies to 2nd-order or higher-order systems.

Unmodelled Dynamics: The real world is not simple or linear. Models will only ever be an approximation and be of a lower order than the real processes. It is important to model the essential dynamics and to some have some knowledge (or bound) of what was not modelled such that the controller will be able to cope with the unknown.

Unstable: A system is unstable when oscillations persist or grow. There are rigorous definitions of stability for linear systems - if any pole of a system has a positive real part then the system will be unstable, which leads to features on the frequency response plots (Bode, Nyquist and Nichols). For nonlinear systems it is much more difficult to define instability rigorously.