Favorable effect of time delays on tracking performance of type-I control systems

Time delays in a control system are usually considered to degrade system performance. This is true when stability margins (gain and/or phase margins) of the control system are taken as the performance criteria. However, there are many instances when performance criteria other than stability margins are used for control system design and controllers comparison. For example, tracking error is one such performance criterion that is widely used for designing many real control systems. When tracking error is considered as the performance criterion, it is shown that consistent time delays in the feedback path can actually reduce the steady-state tracking error of a control system to polynomial reference inputs (such as ramps).