Patent GB-1487559-A (GB)
Title: Weighing balances employing moire fringe techniques
IPC: G01G23/375
Filed: 1973-12-05
Abstract: 1487559 Weighing by interferometry OERTLING Ltd 4 March 1975 [5 Dec 1973] 56346/73 Heading G1A A weighing balance comprises a beam displaceable in response to a load placed on a carrier pivoted thereon, a pair of gratings arranged to produce moire fringes, one of said gratings being movable with said beam relative to the other grating, means 22 for scanning the moire fringes at a known frequency and for producing a signal the phase of which is indicative of the spatial phase of the fringes, means 23 for comparing the phase of this signal with that of a second signal derived from the signals controlling said scanning to produce a signal indicative of the phase difference of the compared signals and means responsive to the said phase difference signal to provide an indication of the load on the beam. Pulses from an oscillator 20 are divided by N by divider 21 and fed by line 76 to the C inputs of D type flip-flops 77, 78 of ring counter 75. Photodiodes 50 to 53 are positioned so that each receives light from a different part of one of the fringes and their outputs are connected two at a time to the emitter follower 70 by MOS switches 60 to 63 controlled by ring counter 75. Each complete cycle of operation of the MOS switches corresponds to four pulses from the divider 21. Differences in sensitivity of the diodes may be corrected by potentiometers 80, 81. The signal at the emitter follower 70 is amplified by transistors 86 and 88, filtered by transistors 92, 93, and the square wave signal output of amplifier 107 is fed to input 23A of comparator 23. A second input 23B is derived from oscillator 20 via gating circuit 24 and a divide by 2N circuit 25. If inputs 23A, 23B are out-ofphase due to movement of the fringe pattern the comparator produces an input to terminals 24C or 24E, depending on the sense of the phase difference and the gating circuit 24 passes oscillator pulses along one of two lines 26A or 26D to add or subtract pulse registered on a counter depending on the sense of the phase difference, a correspondingly fewer or greater number of pulses are passed to the divider 25. The circuit 24 continues to pass pulses to the counter until the beam reaches its equilibrium position and the number of pulses passed is proportional to the angular movement of the beam and hence is a function of the load on the pan. In a circuit providing non-overlapping switching an arrangement of NOR gates (120 to 123), Fig.3 (not shown) is connected between the ring counter 70 and the control terminals 71 to 74. If it is necessary to amplify the photodiode currents the switching operation may be performed on a combined amplifier signal by means of a phase modulator circuit, Fig.4 (not shown) in which the outputs of photodiodes 50 and 52 are combined in one current differencing amplifier (150) and those of diodes 51, 53 in a similar amplifier (151). The outputs of these amplifiers are fed to a modulator stage comprising an amplifier (159) the gain of which is switched positive or negative by four switching transistors (180 to 188) controlled by a ring counter (174). The output from the modulator stage amplifier (159) is smoothed by a capacitor (171) and fed to a filter circuit and a squaring circuit to produce a spatial phase signal as described with reference to Fig.2.