Patent GB-857691-A (GB)
Title: Improvements in electrical weighing scales
Inventor: LYONS LAWRENCE J
IPC: G01G23/16
Filed: 1957-03-07
Abstract: 857,691. Weight indicating apparatus. FAIRBANKS, MORSE & CO. March 7, 1957, No. 7595/57. Class 40 (1). Electrical weighing equipment in which a signal proportional to the load is compared with a variable voltage in a self-balancing follow-up system comprises a second variable voltage source arranged progressively to eliminate the effect on indicating and recording apparatus of previous loads, thus enabling the net weights of a succession of items to be ascertained without the removal of previous items. As shown (Fig. 1), four series-connected bridge-type strain gauge load cells 11-14, energized by secondaries 21-24 of a transformer 16, support the four corners of a load receiving platform (not shown) to develop an output signal which is fed to an amplifier 62 through two voltage producing networks 20, 30, energized by transformer secondaries 25, 26, respectively. Network 20 constitutes a tare weight balancing circuit comprising series resistors 42-44, resistor 43 being tapped at its connection with the load cells to provide zero adjustment, in parallel with series resistors 45, 46, the latter forming a voltage divider with a tap 52 mechanically linked for movement with a servomoter 64 which is connectable to the amplifier 62. Network 30 constitutes a net weight balancing circuit comprising resistors 58, 59, which form an indicator deflection adjusting device, and a voltage divider 55, the tap 54 of which is mechanically linked with a servomotor 65 normally connected to the amplifier 62, rotation of motor 65 also operating the pointer 67 of a net weight indicator 66 which in turn controls a series of weight indicating cams for net weight record printing. The reference windings of motors 64, 65 are connected to a control circuit 71 which comprises. a printing circuit and a switching circuit. Any quadrature component of the load signal is eliminated by the effect of a capacitor 41 connected to a voltage divider 39 across secondary 24. Operation.-A load placed on load cells 11-14 generates a signal which is fed via amplifier 62 to the motor 65 to move tap 54 until the output from voltage divider 55 cancels the load signal. Manual operation of a switch (not shown) actuates switching mechanism within control unit 71 to initiate a printing cycle to record the weight of the load. Simultaneously, or subsequently, manual operation of a second switch (not shown) causes the output of the amplifier 62 to be transferred to motor 64 and closes contacts 72<SP>1</SP>, thus shorting out network 30, whereupon tap 52 is moved to a position of balance on voltage divider 46; an alternative second switch may be kept closed during a batch weighing operation, in which case operation of the printing cycle switch simultaneously initiates this second balancing step. A null detector (not shown) operates to release contacts 72<SP>1</SP> and to restore the amplifier output to motor 65, which responds to the unbalance created by the reinclusion of network 30 in the circuit by restoring the tap 54, the indicator 66, and associated printing cams to their zero positions, thus preparing the equipment for the next weighing operation. A reduction of the load causes motor 65 to attempt to move tap 54 back beyond the zero point, thus closing a switch 117 which operates to close contacts 72<SP>1</SP> and to connect the amplifier output to the motor 64, which moves tap 52 to equate the opposing voltage output from network 20 with the reduced load signal. Fig. 2 (not shown) depicts the switching and printing circuit apparatus of the control circuit 71 in detail.