On periodic control of nonminimum phase systems

The author reviews some results reported for the problem of control of linear time invariant (LTI) nonminimum phase plants, i.e., plants with zeros on the complex right half plane (RHP), via linear periodically time varying controllers. To highlight the invariance property of RHP zeros, a simple proof that no internally stabilizing (possibly nonlinear time varying) controller exists that will make a nonminimum phase plant become minimum phase is given. Two periodic controllers that, applied to a nonstably invertible continuous-time, minimal LTI plant, yield a discrete-time LTI plant with arbitrary zeros are analyzed