New PID Setting Resistor Using CdS Photoconductive Cells

Abstract

This paper describes a contactless variable resistor using CdS photoconductive cells, which has been developed to be applied to PID function generator of electronic process controller. Proportional, integral and derivative functions, control functions of electronic controller, are generated by resistor-capacitor networks and sensitivity of PID functions is set by adjusting the value of resistors. As a PID setting resistor, carbon slide resistor and multi-position switch with calibrated resistors have been used. Either of them, however, is not perfect for use as a variable resistor in controller, because it is difficult for the former to produce logarithmic resistance variation characteristics and on the other hand, it is impossible for the latter to continuously vary the resistance. Pointed out as a defect common to both of them is a poor contact between brush and contacts. The authors have developed a new variable resistor which is suitable for application to PID setting resistor. Its principle comes from the fact that the inner resistance of CdS photoconductive cell is inversely proportional to the intensity of illumination exposed to the cell surface. The new PID setting resistor consists of three units, each of which is composed of a lamp, CdS photoconductive cell and a shadow mask. The shadow mask is placed between the lamp and the CdS photoconductive cell. The cell resistance can be controlled by adjusting either the position of the shadow mask or the lamp voltage, in other words, by regulating the illumination intensity exposed to the cell surface. The photo-variable resistor may be used in place of ordinary resistors not only for usual controllers, but also for adaptive controllers in which PID functions are remotely set by the output signal of the computers.