An explicit formula for a robust controller for SISO systems with unknown delays

An explicit formula is presented for a robust controller for a SISO plant with unknown time delay and a nonminimum-phase transmission zero. For a known transmission zero, the controller has a remarkable property of robust stability and zero steady-state error for any perturbation of the time delay in the plant transfer function. Such property never occurs for PID and other traditional controllers due to infinite or large zero frequency gain. In case of unknown zero, the robust stability is maintained for arbitrary delay perturbations and the steady-state error can be estimated from bounds on the location of that zero