An inexpensive solar pointing control

Abstract

The theory, design and performance of an inexpensive, biaxial solar pointing control are presented and discussed. An altazimuth mount driven by fractional horsepower d.c. motors is used as the stable platform. Two identical error-sensing circuits are employed to drive the motors. A dual element photoconductive cell is used as the sensor in each circuit. The two elements form the variable arms of a simple a.c. bridge which is balanced when the two elements are equally illuminated. This condition prevails when the sensor is pointing at the sun. The off-balance voltage which results when the two are unequally illuminated is used to forward-bias the emitter junction of a unijunction transistor in a relaxation oscillator circuit which triggers a silicon controlled rectifier switching circuit. This circuit changes the polarity of the supply voltage to the d.c. motor which positions the altazimuth mount to restore the balance. Tracking accuracies of ?0? 30' of arc both in elevation and azimuth are easily attainable with loads up to 30 1b.