GB1419111A - Photoelectric transducer - Google Patents

Abstract

1419111 Photo-electrically monitoring and controlling movement of an instrument membrane NATIONAL RESEARCH DEVELOPMENT CORP 21 Feb 1973 [24 Feb 1972 26 Oct 1972 24 Nov 1972] 8501/72 49379/72 and 54351/72 Heading G1A In a device for photo-electrically monitoring the displacement of a membrane 1 due to sound or voltage or pressure forces, radiation reflected (or passed through) the membrane is used to provide a signal representing movement of the membrane from its null position, the signal activating electrostatic means which tend to return the membrane to its null position. As shown in Fig.1 (a microphone) light from a solid state device is reflected off the surface of an electrically conductive optically reflective membrane 1 to be received by two photo-detectors 11 and 12 which are axially displaced on either side of the image plane. Thus, if the membrane oscillates the detectors will produce a periodic signal of the same frequency but 180 degrees out of phase with respect to one another. The amplified outputs of the detectors are taken via phase detecting means 15 which control electrostatic elements 17 and 18 via a gain control 16, to try and maintain the membrane 1 in its null position and the feed back signal required to do this is also fed via amplifying equipment to activate a loud speaker or recording equipment. In a modification of Fig. 1 a voltage comparator Fig. 2, (not shown), the outside electrostatic electrode 17 (which receives the voltage to be tested) is positioned further away from the membrane and the feedback signal is supplied to the inside electrostatic electrode 18. In Fig 3 an interferometric system senses the membrane's movement wherein a reference signal formed from the light source 7, mirror 25 and detector 27 is compared for phase difference with a measurement signal formed from the light source 7, membrane 1 and detector 23. The phase difference signal then being processed as before. In a further embodiment Fig. 4 (not shown), a source with an associated lens system and a detector with an associated lens system are placed equidistantly on either side of the membrane. The central area of the face adjacent the membrane of each lens system is arranged to be fully reflecting and the source and detector are arranged to be optically conjugate so that there is a zeropath difference for the two associated systems when the membrane is in the null position. The membrane in this and other embodiments may be oscillated by applying a sinusoidal voltage to the electrostatic electrodes and any exterior forces acting on the membrane will amplitude modulate the oscillations of the membrane, the signal from them being demodulated by conventional circuitry or by providing the bias of the detector with an identical sinusoidal voltage variation so the output of the detector will then be a signal directly representing the magnitude and direction of the membrane under the action of the force to be measured.