GB1328981A - Focussing system for a projector - Google Patents

Abstract

1328981 Acoustic range-finding; phase detectors ELMO CO Ltd 1 Sept 1970 31813/70 Headings H3A and H4D [Also in Division G3] Displacement of the image carrier from its focused position in a projector (e.g. due tohe at distortion or variations in slide mount thicknesses) is compensated for by an automatic system which detects changes in the phase of an ultrasonic beam reflected from the carrier and applies a phase error signal to control a focusadjustment mechanism. In the Fig. 4 embodiment, transducer 1 driven by generator 6 directs an ultrasonic beam on to slide or film 3 for reflection to transducer 2, whose output is phase-compared at 9 with the generator signal. The two compared signals are in phase quadrature when the projector is focused, giving zero average output from phase detector 9; but if the film or slide is displaced the phase detector output goes positive or negative and controls the forward or backward drive of motor 12 as appropriate to change the distance between projection lens 4 and the film or slide to restore focus. Phase detection, Fig. 3B:-One oscillatory input signal, at 20, switches both transistors 22, 23 on in one half-cycle and both off in the next half-cycle. When the transistors are on, a loop current I 1 will flow through transistor 24 base or a loop current I 2 will flow through transistor 25 base, according to the instantaneous polarity of the sesond input signal, from source 21. Transistors 24 and 25 are switched on by I 1 and I 2 , respectively, to supply power to motor 12 from source 38 or 39.