GB1362770A - Digital force balance - Google Patents

Abstract

1362770 Digital force balance STANDARD TELEPHONES & CABLES Ltd 14 Dec 1972 57689/72 Heading G4H In a digital force balance, a deflection indicator allows clock pulses to pass to a multistage counter while the balance is out of equilibrium and the output of each counter stage drives a separate digital to analogue converter which converters provide via electromagnets a balance restoring force proportional to the counter reading. As shown balance beam 1 is pivoted at 2 and the load is placed in pan 3. The deflection sensor 6 is a l.v.d.t. and carries a dashpot 10. The l.v.d.t. output is rectified and smoothed at 11 and compared at 12 with a reference voltage 13 which can be varied for tare allowances. When the l.v.d.t. voltage is greater than the reference gate 14 is enabled and pulses from clock 15 are fed to counter 16. The output of each counter stage is fed direct to the corresponding digital to analogue converter 17 and the correct output of these drives the coils of electromagnet 18 providing the restoring force. The windings of electromagnet 18 are such that the maximum current from each digital to analogue converter 17 is about the same. The counter is periodically reset by a slow (0À2 Hz) clock 22. The deflection sensor may also comprise either a shaped vane capacity sensor or a lamp and photosensor. In an alternative embodiment an up/down counter is used and the l.v.d.t. is coupled so that deflections in one direction are positive and in the other negative. Tare correction is provided by an additional winding on the electromagnet 18 operated by a manually adjustable direct current supply.