Improved Control Strategies for Electric Space Heating Processes

This paper describes three new control strategies for the discontinuous control of electric space heating processes. Discontinuous controllers are invariably used for such processes in order to reduce the cost of control. The disadvantages of such systems are the large overshoots and undershoots, large periods of oscillation, and offset errors. The first strategy utilizing an ON-OFF controller with derivative feedback is shown to minimize the magnitude of oscillations, the period of oscillation, and offset errors. The second strategy utilizes a three-position controller with derivative feedback. Besides eliminating oscillations and offset errors, the controller enables a time optimal start-up of the process. By minimizing the deadzone in the controller, sensitivity to disturbances is increased. A simple method of designing such a controller is also indicated. The third strategy utilizes a pulsewidth modulated (PWM) controller for processes where power is available either at its maximum rating or no power at all. By a proper choice of the parameters, it is shown that the magnitude of oscillations and the period of oscillation can be controlled by the designer. Offset error is completely eliminated, indicating a more effective utilization of power. All the three strategies described require only a minor modification of the existing controllers; the resulting improvement in performance is expected to compensate the slight increase in cost of control. The mathematical derivations involved in the design and analysis of such controllers have deliberately avoided, since the presentation is specifically intended for practicing engineers.