Patent GB-921313-A (GB)
Title: Magnetic flowmeter apparatus
IPC: G01F1/60
Filed: 1961-11-30
Abstract: 921,313. Semi - conductor circuits. FOXBORO CO. Nov. 30, 1961 [Dec. 28, 1960], No. 42976/61. Class 40 (6). [Also in Group XXXVI] In a flowmeter of the kind in which an A.C. signal proportional to the rate of flow of a fluid is generated by the passage of the fluid through an alternating magnetic field, the said signal is amplified in an arrangement which produces a D.C. output, and an A.C. feed-back signal proportional to said D.C. output is produced and supplied to the amplifier in opposition to the A.C. flow signal. As shown, a flow head of the type disclosed in Specification 816,107 comprises a stainless steel pipe section 12 through which the test liquid flows, a pair of coils 14, 16, supplied from an A.C. source 20, and a pair of electrodes 22 located in contact with the liquid. The A.C. signal developed in the head is passed via balanced secondary windings 30, 32 of a transformer 34 to an arrangement having a high input impedance and comprising transistors 90, 92 connected in push-pull and having their emitters connected to the bases of a second pair of transistors 98, 100. The output from this arrangement is transformer coupled to the input of a multi-stage amplifier comprising transistors 126, 132, 134, 136, 138 in common emitter configurations, the first, second, and third stages being coupled through diodes 140, 142, the third and fourth stages being directcoupled, and the fourth and fifth stages being coupled through a phase-correcting potentiometer 174. Negative feed-back is provided from the output of transistor 134 via a feed-back circuit comprising resistors 162, 128, and capacitors 130, 164; a second negative feedback circuit, from the output to the input of transistor 132, comprises a capacitor 168 and resistor 170. The output from the multi-stage amplifier is transformer-coupled to a phasesense detector 42 comprising transistors 192, 194 having their collectors connected via diodes 196, 198 to opposite ends of a secondary winding 200 of a power transformer 202, so that a D.C. output is furnished through a resistor 210 when the inputs to transistors 192, 194 are in phase with the reference voltage in winding 200. This D.C. output, smoothed by a capacitor 212, is used to actuate a recording device 48; it is also used to control a Hall-generator 44, being impressed on the control electrodes 56, 58 of a semi-conductor wafer 52, e.g. of indium arsenide, which is traversed by an alternating magnetic field produced by windings 54 located one adjacent each main face of the wafer 52, the phase of the field being adjustable by trimming potentiometer 74, so that the output of the generator at electrodes 62, 64 is an A.C. of magnitude proportional to the D.C. input, which is supplied to primary windings 38, 40 of transformer 34 as negative feed-back. The feed-back circuit includes a level-set potentiometer 70 which determines the amplitude of a constant regenerative component of the primary input to transformer 34, a span-adjusting potentiometer 80 (which is ganged with a gain-adjusting potentiometer 128 in the amplifier), and a humbucking circuit comprising a coil 76 coupled to Hall-generator windings 54 and a potentiometer 78.