Servo control arrangement utilizing alternately energized light sources and a single photodetector

A servo control arrangement is provided that accurately controls the positioning of a movable member. The movable member is controlled by a motor through a drive train. A coded member is carried either by the drive train or the controlled movable member so as to move in accordance with the controlled movable member. The coded member includes an array of apertures with each of the apertures corresponding to a controlled position of the controlled movable member. In a preferred arrangement, the controlled movable member is a rotary turntable that carries a slide tray for projection apparatus. In that arrangement, each of the apertures corresponds to one of the slide positions of the rotary turntable and slide tray. Feedback information representing the position of the coded member is provided by alternately energized light sources and a single photodetector device. The light sources are disposed on one side of the coded member and the single photodetector is disposed on the opposite side of the coded member such that light from the illumination sources passes through each aperture of the coded member and impinges upon the photodetector device as each aperture moves to within a predetermined positional range of a detecting station formed at the point of alignment of the illumination sources and the photodetector. The output of the photodetector represents the illumination received by the photodetector during the energized time of each respective illumination source. Thus, the output of the photodetector provides differential position feedback signals during the respective energized time period of the illumination sources; the differential position feedback signals being utilized to control operation of the motor.