Nonlinear multivariable control system

There is disclosed an apparatus for and method of controlling a plant such as a turbojet engine wherein a model of the plant is created and plant performance request signals are applied to both the plant and the model of the plant. The performance of the model of the plant is used to modify the performance request signals supplied to the plant. Plant and plant model responses may also be compared to monitor degradation in performance of the plant. To adapt the scheme to nonlinear plants, the creation of a plurality of linear plant models each mimicking plant operation over a different limited portion of the toal range of plant performance is employed and there is a plant model for each of several different plant operation points about each of which, the actual plant operation is approximately linear. The technique may further include the creating of a model of the inverse of the plant, in which case, the step of applying plant performance request signals to both the plant and the model of the plant includes passing the performance request signals through the model of the inverse of the plant and subsequently to both the plant and the model of the plant. A preliminary step of screening the plant performance requests and modifying any signals requesting an excessively abrupt change in plant performance is also disclosed.