Feedforward as a supplement to feedback adjustment in allowing for feedstock changes

Many industrial processes must be adjusted from time to time to maintain their mean continuously close to the target value. Compensations for deviations of the process mean from the target may be accomplished by feedback and/or by feedforward adjustment. Feedback adjustments are made in reaction to errors at the output; feedforward adjustments are made to compensate anticipated changes. This article considers the complementary use of feedback and feedforward adjustments to compensate for anticipated step changes in the process mean as may be necessary in a manufacturing process each time a new batch of feedstock material is introduced. We consider and compare five alternative control schemes: (1) feedforward adjustment alone, (2) feedback adjustment alone, (3) feedback- feedforward adjustment, (4) feedback and indirect feedforward to increase the sensitivity of the feedback scheme, and (5) feedback with both direct and indirect feedforward.