GB1143739A - Improvement in servo-mechanism - Google Patents

Abstract

1,143,739. Automatic exposure control. KOPARU K.K. Aug. 15, 1966 [Nov. 6, 1965; May 27, 1966; July 13, 1966; July 14, 1966], No. 36401/66. Headings G2A and G3R. In a system for automatically controlling light intensity, a photo-resistor is included in a bridge circuit the output of which controls PNP and NPN transistors energizing a moving coil mechanism adjusting a diaphragm which determines the amount of light falling on the photo-resistor, and an e. m.f. induced in an auxiliary winding of the moving coil mechanism, during movement, is applied in opposition to the output of the bridge circuit. As applied to automatic control of the exposure of a photographic camera, light from the camera lens passes through an adjustable diaphragm 27, Fig. 1, and is deflected by a half mirror (not shown) on to a photo-resistor 1. Variations in light intensity from a desired value upsets the balance of a bridge circuit in which the photo-resistor is connected so that one or other of transistors 8, 9 is rendered conductive according to the sense of variation. The state of further transistors then changes so that an armature winding L 1 is energized in such a sense that its movement relative to a permanent magnet 25 causes appropriate adjustment of diaphragm 27. An e. m.f. induced in a second armature winding L 2 by movement of the armature applies rate feedback in opposition to the output of the bridge circuit to improve stability, and capacity such as 23 is provided to eliminate high frequency components of current in winding L 1 . Thermistors such as 12 are provided to compensate for temperature variations and to increase sensitivity of the system when the deviation is small. A suitable pin and slot lever coupling between armature 26 and diaphragm 27 is described.