1. A computer-implemented system for controlling the operation of a processing apparatus having at least one independently controlled manipulated variable and at least one controlled variable responsive to said manipulated variable, comprising: a set of at least two models for defining an expected variation in magnitude for each controlled variable as a respective function of each manipulated variable, said model set having a dynamic response inertial characteristic; and means for implementing a change defined from said model set to modify said manipulated variable in said processing apparatus.