Passive interconnection interpretation of PID control

In this paper the application of PID control to passive systems via passive interconnection is explored. The stability properties of PID control applied to linear systems is well understood, but the application to nonlinear system is more complex. In this work we exploit the passive structure of a PID controller by interconnecting with passive plants to ensure closed-loop stability. In many cases, however, the regulation output does not coincide with the passive output of the plant and the integral of the passive output is the regulation output of interest. Two methods for applying PID to the integral of the passive output via passive interconnection are introduced.