Patent GB-662230-A (GB)
Title: Automatic weighing scales
IPC: G01G13/32
Filed: 1947-06-20
Abstract: 662,230. Electric control systems. WILLIAMS, L. S., YEASTING, M. C., and TOLEDO SCALE CO. June 20, 1947. No. 1685/51. Divided out of 662,137. Class 40 (i). [Also in Groups XVIII and XL (b)] In a weighing machine in which the load acts on a pivotal lever and is counterbalanced by driving a poise along the lever, means for sensing and limiting the displacement of the lever from its balanced position controls the power flow to the poise driving motor proportionally to the sensed displacement and a part of the sensing and limiting means is adapted to be driven proportionally to the poise movement to effect control of the power flow. As shown, Figs. 1, 2, the load applied at 191 to a pivotal lever 188 is counterbalanced by driving a poise 193 along the lever. Displacement of the lever from its balance position is sensed e.g. by a photo-electric device 203 in which the amount of light passing from a source 207 to a photo-cell 210 through an aperture 209 is determined by a focussing lens 208 on an arm 202 carried by the lever 188. The cell output controls an electric motor 197 which drives the poise in the appropriate direction and at the same time drives the arm 202 circumferentially relative to the lever 188 acting as a radius arm through a pinion and rack, to restore the lens. 208 to the neutral position as the poise reaches the balance position. In a modification, however, the lens 208 is fixed to the lever and the light source 207 and photo-cell 210 are mounted on a carriage which is driven by the poise driving motor to follow-up the lens movements, Figs. 12-15 (not shown). The sensing device may alternatively, be electromagnetic or electrostatic etc. The motor also drives a counter 206 which may be graduated to read the weight. The motor 197, Figs. 1, 3, drives through gearing a drum 196 on which two metal tapes 194, 200 are wound in opposite directions. The tape 194 passes over a pulley 195 through the line joining the pivot points of the lever 188 and is connected to the poise 193. The tape 200 passes over pulley 195 in the reverse direction and over a further pulley 201 mounted on the lever 188 to a drum 199 at the other end of the lever. A further tape 198 is resiliently secured to the poise 193 and is wound on the drum 199 in the opposite direction to the tape 200 so that for either direction of rotation of the motor the poise is driven along the lever 188 and the drum 199 is rotated correspondingly to drive through a pinion and rack, Figs. 8, 9 (not shown) the arm 202 carrying the lens 208. The arm 202 carries a projection at its lower end engaging between fixed limit stops and the lever 188 is connected to dash-pot device, 204, the arrangement being such that normally the delaying action produced by the dash-pot enables the motor to drive the poise and the arm 202 to a new follow-up position before the lever is deflected far enough to be limited by the stops. Control circuit. In the "balanced" position of the lever 188 one half of the light focussed by the lens 208 is received by the photo-cell 210 which through a cathode loaded amplifier 281 produces equal output voltages to ground from a push-pull stage 287, 288. These outputs are fed to the grids of rectifier pairs 308, 309 and 312, 313 respectively and are slightly negative with respect to the rectifier cathodes fed from line 318. The rectifier anodes are A.C..fed by transformer action from the shaded pole pairs of the induction motor 269 to which they are respectively connected and normally pass equal currents through the respective pairs of shaded poles 325, 329. A movement of the lens 208 upsets the balance of the push-pull stage outputs resulting in a differential current flow through the shaded poles which causes the motor to run in the corrective sense giving a torque proportional to the deviation of the lens 208 from the neutral position.