TWO KINDS OF HARMONIC PROBLEMS IN CONTROL SYSTEMS

This paper discusses the harmonic problems in control systems from two aspects:Oneis the harmonic control among different subsystems,and the other is the harmonic control amongmultiple inputs.Some intrinsic problems in such systems are discussed.It is pointed out that somesubsystems must be unstable to stabilize the whole interconnected system by an example.Especiallyfor discrete-time multi-input systems,a necessary and sufficient condition is presented for the strictdecrease of the quadratic optimal performance index with the control input extensions.This showsan essential difference between single-input and multi-input control systems.Finally,some futureresearch directions are discussed in harmonic control of interconnected systems,allocation of multi-control inputs,fault-tolerant control,and fault-diagnosis.