GB919771A - Method and device for measuring forces, applicable especially to automatic weighing - Google Patents

Abstract

919,771. Measuring weight electrically; electric selective signalling systems. PESAGE AUTOMATIQUE. June 17, 1960 [June 18, 1959], No. 21356/60. Class 40 (1). A weight is measured by opposing it by the force produced by a current-carrying coil in a magnetic field, balance being ascertained by a differential capacitor with movable plates attached to a support carrying the weight. The current through the coil at balance is proportional to the weight. Construction (Fig. 1).-A rod 7, supported by two resilient plates 10, 10a, carries the object 9 to be weighed at one end and, at the other end, a coil 6 located in the circular gap 5 of a permanent magnet 2. Vertical displacement of the rod 7 is indicated by the change in capacitance between fixed plates 14, 14a, and movable plates 15, 15a mounted on the rod 7, as measured in a normally balanced A.C. bridge circuit, whose output signal indicates by its magnitude and phase sense the extent and the direction of displacement of rod 7. Circuitry (Fig. 5).-The current through coil 6 is obtained from a power amplifier 21, and is automatically adjusted to such a value that the p.d. across a resistor Ro also carrying this current is substantially equal to a series opposing stable E.M.F. V, the difference between these voltages being converted to A.C. by vibrator 24, amplified at 22 and demodulated at 25 before amplification at 21. If this current is too small to centralize the moving plates 15 of differential capacitor 14-15, an output signal is obtained from its bridge circuit which is fed to a stepping device 27, comprising a relaxation oscillator which successively energizes a series of thyratrons each controlling a relay Re 1 ... Re n introducing a resistor R 1 ... R n in parallel with Ro. When the moving plates 15 are recentralized the stepping device 27 stops, and the current through coil 6 is indicative of the weight of the object 9. In another embodiment (Fig. 6, not shown), the H.F. output from the capacitor bridge is use to supply the current through coil 6.