Improvements and Extensions to Recent Electroheat Control Contributions

Abstract

The increasing use of electroheat in industrial processes necessitates a thorough investigation into the current state of control techniques for electric process heating. A new unified approach to the assessment of stability of continuously and discontinuously controlled electroheat processes has been derived. It utilizes a newly defined critical frequency locus in conjunction with the describing function. An exponential describing function is proposed for determining the conditions for a critically damped response to step-inputs in discontinuously controlled electroheat processes. The results obtained closely correlate with actual test results. The improvements obtainable by incorporating derivative feedback in discontinuously controlled processes have been analytically investigated. Facile displays have been developed that determine the optimum derivative content in the feedback loop. The computer control of electroheat processes has been thoroughly examined. A design of model-referenced integrating controller has been established from first principles. It eliminates offset errors due to disturbance inputs. A time-optimal control strategy has been established. It combines the advantages of discontinuous control with the advantages of the new model-referenced integrating control. It results in a performance that is greatly superior to that obtained with conventional control techniques for these processes