Modular multivariable control apparatus and method

A method of controlling a process including at least two process variables and at least two constrained control efforts, with each control effort having an influence on at least one such process variable, comprising the steps of:

• a) modelling the influence that a change in each such control effort has on each such process variable;
• b) describing the desired trajectory which each such process variable will follow towards its desired value in response to a system change;
• c) ranking each such control effort relative to each such process variable according to the priority in which it will be changed;
• d) coupling the control of each such process variable;
• e) changing an unsaturated control effort of highest rank of a first set of at least one control effort to compensate for a disturbance to a process variable until such disturbance is compensated for or until such highest ranked control effort is saturated, then changing a next highest ranked control effort of the first set until such disturbance is compensated for;
• and,
• f) changing at least one control effort of a second set of at least one control effort so as to compensate for changes in said first set of at least one control efforts such that substantially no influence is exhibited on other process variables in the system in which no change is desired.

An apparatus for controlling a process including at least two process variables ranked in accordance with the relative importance of their control and at least two control efforts for controlling said process variables, comprising:

• means for measuring the values of each process variable;
• means for effecting a change in the value of at least one control effort of a first set of control efforts to compensate for a disturbance to a first process variable;
• and
• compensating means for compensating for the effects that a change in said first set of control efforts has on other process variables by effecting a change in at least one control effort of a second set of control efforts and for delaying or prohibiting changes to said first set of control efforts until compensation for any higher ranked process variables can be initiated.