Patent GB-973466-A (GB)
Title: Device for detecting the position of a reflecting surface
Inventor: PASSE JEAN
IPC: G21C17/035
Filed: 1963-01-31
Abstract: 973, 466. Detecting the position of a reflecting surface. COMMISSARIAT A L'ENERGIE ATOMIQUE. Jan. 31,1963 [Feb. 7, 1962], No. 4045/63. Heading G1A. [Also in Division G3] A device for detecting or controlling the position of a reflecting surface of constant orientation, for example the level of the free surface of a liquid, comprises a light source 14<SP>1</SP> having a pair of substantially identical objects 16<SP>1</SP>, 18<SP>1</SP> and from which light beams at a predetermined frequency are alternately directed towards an optical device which directs said beams on to the reflecting surface and directs the reflected beams to a sensing device 22<SP>1</SP> through a slit of a screen 24<SP>1</SP> when the reflecting surface is in a predetermined position with respect to the source, said sensing device being selectively sensitive to light modulation at the predetermined frequency and said slit being so located that the screen attenuates one of the beams upon departure of the reflecting surface from the predetermined position whereby said sensing device indicates displacement of the reflecting surface from the said predetermined position. In the arrangement shown, the source 14<SP>1</SP> consists of a double electrode neon tube fed with constant frequency e.g. 50 c/s, alternating current so that its electrodes 16<SP>1</SP>, 18<SP>1</SP> are alternately illuminated each at a frequency of 50 c/s. Images of the electrodes are directed through a lens 20<SP>1</SP> by a mirror 30 on to the reflecting surface under investigation and pass again through the lens 20<SP>1</SP> which brings them to a focus at a slit in a screen 24<SP>1</SP> placed in front of a photomultiplier 22<SP>1</SP>. The slit is so positioned that the images of the electrodes are symmetrically formed in relation to the centre of the slit when the surface under investigation is in a predetermined position and thus the beams from the electrodes reach the photomultiplier 22<SP>1</SP> with the same intensity and its output signal therefore has a fundamental frequency of 100 c/s (Fig. 3b not shown). Any displacement of the investigated surface causes a lateral displacement of the images at the slit so that one of them is masked by a screen 24<SP>1</SP> with a consequent appearance of a fundamental component of 50 c/s in the output signal of the photomultiplier 22<SP>1</SP> (Figs. 3c and 3d not shown). This signal may be fed to a circuit having a selective response for a frequency of 50 c/s such as, the narrow pass band amplifier 36 the phase of the signal giving the direction in which the surface is displaced, and the signal may be used to either adjust the level of the surface to a given value (i.e. with the measuring components stationary) or in accordance with a given time function law (i.e. the measuring components enclosed as a unit 28 and displaced in accordance with this law). In the arrangement shown the amplifier 36 feeds one of the windings of a three phase motor 42 which turns a winch 44 having a cable 45 on which is suspended the measuring unit 28 so that the unit 28 is returned to a position where the beams from the electrodes 16<SP>1</SP>,18<SP>1</SP> reach the photomultiplier with the same intensity. The movements of the motor 42 and winch 44 may be directly displayed or transmitted to a telemetering receiver 46 by a system such as a selsyn system. The neon tube 14<SP>1</SP> may be replaced by a continuously light emitting lamp which emits two beams which are alternately masked by a rotating screen.