<P> The early methods of Bode and others were fairly robust; the state - space methods invented in the 1960s and 1970s were sometimes found to lack robustness, prompting research to improve them . This was the start of the theory of robust control, which took shape in the 1980s and 1990s and is still active today . </P> <P> In contrast with an adaptive control policy, a robust control policy is static; rather than adapting to measurements of variations, the controller is designed to work assuming that certain variables will be unknown but bounded . </P> <P> Informally, a controller designed for a particular set of parameters is said to be robust if it also works well under a different set of assumptions . High - gain feedback is a simple example of a robust control method; with sufficiently high gain, the effect of any parameter variations will be negligible . From the closed loop transfer function perspective, high open loop gain leads to substantial disturbance rejection in the face of system parameter uncertainty . </P> <P> The major obstacle to achieving high loop gains is the need to maintain system closed loop stability . Loop shaping which allows stable closed loop operation can be a technical challenge . </P>

When is a control system said to be robust