Patent GB-780024-A (GB)
Title: Improvements in or relating to weighing apparatus
IPC: G01G7/04
Filed: 1955-06-08
Abstract: 780,024. Photo-electric measuring arrangements. SCRIBBANS-KEMP, Ltd., HENDON, A. L., and ROGERS, P. O. June 8, 1955, No. 16428/55. Class 40 (3). [Also in Groups XVIII and XXXVIII] In a weighing-apparatus, a weight-carrying platform is mounted on a conducting coil located in a magnetic field by anti-friction bearings, the platform and coil being floatingly supported by passing current through the coil, means being provided for detecting movement of the coil when the weight is placed on the platform, and means, responsive to the detection means, for increasing the current through the coil to restore the position of the coil and also including means for indicating the increase in current. In the diagrammatic sketch illustrated in Fig. 5, the function of the basic instrument is to provide a voltage output which is linearly proportional to the weight applied to the instrument, the instrument comprises a shaft 1 carrying a coil 2 which is positioned freely in the annular gap of a magnet 3. The shaft 1, provided at the top with a weighing platform 9, is supported by two bearing systems in which a triangular section of the shaft is at each face in contact with bearings which allow free vertical movement of the shaft but prevent rotation of same. Also secured to the shaft are two axially-spaced shutter vanes 4 which control the amount of light from a lamp 1 reaching two photo-cells 5 which are connected to a D.C. amplifier. Application of the weight to the weighing platform 9 upsets the equilibrium of the system and displaces the shaft 1, shutter vanes 4 and coil 2 downwards. This allows more light to fall on one photo-cell 5 and less on the other and the light change causes current through the coil to increase and hence produce a force opposing that of the applied weight; re-establishing equilibrium. The D.C. amplifier is shown in Fig. 6, wherein the photo-cells 5 are connected in opposition to the grid of cathode follower valve V1 which has series-connected cathode resistances R1 and R2, the centre-point of which is connected over resistance R3 to the grid. The cathode of valve V1 is connected over resistance R4 to the grid of the valve V2 which has an anode resistance R5, the anode of valve V2 being connected to the grid of valve V3 in the anode circuit of which is connected the moving coil. The output current of valve V3 is arranged to flow through resistor R6, producing a voltage V, which is directly proportional to the applied weight. By introducing rate-of-thechange feed-back into the amplifier, by means of capacitor C1 and resistance R7 connected in series between the cathode of valve V1 and the grid of valve V2, the damping co-efficient can be adjusted so that the system is critically damped. A metal coil former helps in this respect in that it produces eddy current damping, and hence generates a retarding force proportional to the velocity of the coil former. A low-amplitude, low-frequency sine wave is permanently applied to the moving coil, conveniently by application over a condenser C2 to the cathode of valve V2, so as to produce slight vibration of the moving system, to break down the friction effects in the bearing system and to increase the sensitivity. Specification 727,164, [Group XXXVIII], is referred to.