LQ-optimal control by spectral factorization of extended semigroup boundary control systems with approximate boundary observation

A model of boundary control system with boundary observation is described and analyzed, which involves no unbounded operator except for the dynamics generator. The resolution of a LQ-optimal control problem for this model provides a stabilizing feedback for a nominal system with unbounded operators. The model consists of an extended abstract differential equation whose state components are the boundary input, the state (up to an affine transformation) and a Yosida-type approximation of the output of the nominal system. It is shown that, under suitable conditions, the model is well-posed and, in particular, that the dynamics operator is the generator of an analytic C₀- semigroup and the model is observable. A LQ-optimal control problem is posed for the model, and a general method of resolution based on the problem of spectral factorization of a multi-dimensional operator-valued spectral density is described. It is expected that this approach will lead hopefully to a good trade-off between the cost of modelling and the efficiency of methods of resolution of control problems for such systems.