Patent GB-971787-A (GB)
Abstract: 971, 787. Measuring electrically; voltage and current measurement. LEEDS & NORTHRUP CO. March 26, 1962 [April 10, 1961], No. 11448/62. Headings G1N and G1U. [Also in Divisions G3 and H3] In a transducer having a magnetic circuit and input means for producing an M M F to establish a variable flux level in the circuit, bias means are provided for producing an opposing flux in the magnetic circuits and means responsive to a net flux level other than zero produce a D.C. output proportional to the flux level, and a compensating winding fed by the D.C. output produces a compensating flux which, together with the opposing flux balances the input flux and reduces the net flux level substantially to zero. The transducer is described as applied to measuring linear displacement and measuring voltage; also to automatic current regulation (see Division G3). Measuring linear displacement. As shown, Fig. 1, the D.C. flux set up in a magnetic circuit comprising a saturable core 11 and a pair of poles 13, 14 of ferrite &c. by bias permanent magnets 32, 33, is nullified by the effect of a displaceable permanent magnet probe 31 when the latter is in its reference position. When the probe is displaced from its reference position a net D.C. flux appears in the magnetic circuit. The core 11 is periodically saturated by the passage of current from an oscillator 27 through a driving winding 12, thus varying the reluctance of the core and hence the D.C. flux level This induces an A.C. signal in an output winding 17 which is fed via an amplifier and synchronous rectifier unit 20 to a compensating winding 23 which operates to maintain the net flux level at substantially zero. The feedback current, which determines the load output at 24, is thus representative of the extent of displacement of the probe 31, which may be mechanically linked to a Bourdon tube, diaphragm, motor &c. To avoid coupling between the output winding 17 and the driving winding 12, the latter is wound on an auxiliary core 11a so oriented that the saturating flux is perpendicular to the D.C. flux. Alternatively, the driving winding 12 may be symmetrically wound on a saturable core ring 11<SP>1</SP>, Fig. 2, poles 13, 14 projecting normal to the plane of the ring 11<SP>1</SP>. The frequency of oscillator 27 is stabilized by a piezo-electric device 36. Transistor 35 operates as a unidirectional current device, supplying saturating current to winding 12 every other half-cycle; in this way the frequency of the output signal is kept equal to the oscillator frequency. Transistor amplifier 20a comprises three stages, coupling between the first and second stages being by way of a piezo-electric device 72 having a common grounded terminal 72a, a high impedance terminal 72b connected to the output of the first stage, and a low impedance terminal 72c connected to the input of the second stage to form a tuned circuit coupling. In the synchronous rectifier 20b, a bridge network 89 provides a low-impedance unidirectional path between the amplifier 20a and comperisating winding 23 during one half of each cycle; during the other half-cycle the bridge is biased in the non-conducting direction. The bridge is controlled by the oscillator 27, the output of which is supplied via a transformer 94 and a phasing network 95, 96, 97 to opposite points of the bridge. Transducer. Figs. 4, 5. Permanent magnet probe 131, which may be provided with soft iron polepieces 131b, is located in a well 165 in transducer housing 150, between poles 113, 114 which are supported by disc members 154, 157; the saturable core ring 111, which carries the driving winding 112, is held against the lower ends of the poles by means of a supporting member 159. D.C. bias flux is provided by helical high-permeability extensions 113b, 114b on the upper ends of the poles, which form a magnetic bridge with the permanent magnet 131, (see Fig. 3A). Output winding 117 and compensating winding 123a, 123b, 123c, 123d, are wound on the poles. Discs 154, 157 are suspended from a top plate 152 by.rods 155, 156. The reference position of the probe 131 is adjusted by means of a movable permanent magnet 177. Measuring voltage Fig. 3. An unknown D.C. voltage E x  is converted to A.C. by a vibrator 141, amplified at 143, and supplied to the control winding of a 2-phase motor 144 which operates to position the pen and index 146a of a recorder 146 and also to displace permanent magnet probe 131 between limits imposed by stops 134. The feedback current to windings 123a . . . 123d is dropped across a resistor 124 in opposition to the unknown signal E x , and when equilibrium is reached the position of pen and index 146a indicates the strength of E x . Potentiometer 116 in the feedback circuit is adjusted to determine the span of the arrangement, and a temperature sensitive resistor 115, shunted across potentiometer 116, compensates for the effects of variation of temperature on the system.