1. In a cascade control system wherein a primary process is controlled by a primary controller responding to a first error signal indicative of the deviation of a primary controlled variable from a first set point to modify a second set point, and a secondary process is controlled by a secondary controller responding to a second error signal indicative of the deviation of a secondary variable from said second set point, the improvement comprising: means responsive to said second error signal for modifying said first error signal by an amount sufficient to reduce the effects on said first error signal of changes in said primary variable resulting from disturbances affecting said secondary process.