Quasi-Continuous Temperature Control in the Electric Space Heating Process

Abstract
Quasi-continuous temperature control is needed when electric space heating is applied to temperature-dependent processes. In such applications, multiposition systems cannot supply the accurate control often required, and quasi-continuous systems have economic and operating advantages over many continuous systems. The reference input of a quasi-continuous system can be a trimmed, optimized, open-loop command derived from the incoming and outgoing quality levels of the process. Feedforward correction for process lag, and overriding commands, can easily be incorporated into such systems. Quasi-continuous characteristics contain a number of discontinuities. Time can be saved by representing these characteristics by a continuous profile. A multitude of intermediate values of manipulated variable can be obtained by new and inexpensive methods. When the heating load must remain intact, phase-discontinuity methods can be used which approximate level-discontinuity control. For fast response processes, phase-controlled SCR's (silicon-controlled rectifiers) can be used. The T-T (temperature-time) diagrams have limitations as representations of quasi-continuously controlled processes. Time domain information can be obtained from the phase plane where quasi-continuous performance resembles a spiral. Nonlinearities can be advantageously introduced into quasi-continuous profiles. Performance is improved by derivative feedback. It is possible to predict quasi-continuous system behavior by means of a 3-dimensional actuating-signal display.

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