Patent GB-889450-A (GB)
Title: Electrical measuring circuit
IPC: G07C3/14
Filed: 1959-03-05
Abstract: 889,450. Bridge measurements; determining physical dimensions by electric tests. NEDERLANDSE ORGANISATIE VOOR TOEGEPAST - NATUURWETENSCHAPPELIJK ONDERZOEK TEN BEHOEVE VAN NIJVERHEID, HANDEL EN VERKEER. March 5, 1959 [March 6, 1958; March 8, 1958], No. 7714/59. Class 37 [Also in Group XL (c)] A measuring circuit for a variable capacitance whose value may depend on a quantity to be measured, e.g. the thickness of a textile thread passing between capacitance plates, comprises a capacitance bridge with one arm containing the variable capacitance, and a pair of vacuum tubes the anodes of which are connected to one bridge diagonal and to a current source through corresponding anode impedances, the grids of which are connected to the remaining diagonal and to the cathodes through corresponding grid impedances, while the circuit as a whole operates as a retroactive generator of oscillations whose amplitude varies as a measure of the value of the variable capacitance, and which may be applied directly or through a rectifier, e.g. to a registering device or to a relay operating a blade for cutting or a clamping device for engaging a textile thread under test. In Fig. 2 a capacitance bridge comprising reference capacitance Cm and an oppositelydisposed similar variable capacitance is completed by capacitances Cag, respective diagonals are connected to the anodes and grids of triodes 1, 2; the anode/grid capacitances of which may constitute Cag while the anodes are fed through equal resistances R1, and the cathodes and grids are returned through equal resistances Rk and Rg respectively. The amplitude of oscillations developed at either anode is a measure of the value of variable capacitance Cm, and the anode signal may be rectified, filtered, and applied as a bias to the grids over resistances Rg to give an automatic gain-control effect, and a consequent non- linear response. The rectified control signal may be applied over a further resistance capacitance filter to a thyratron energizing a relay through a normally closed restoring switch (Fig. 3, not shown), whereby inhomogeneties in the cross-section of a textile thread passing through the test capacitance may strike the thyratron and operate the relay to engage a thread catcher device and to momentarily open the restoring switch to extinguish the thyratron. Variations of voltage between the triode anodes may be used to measure or register the variable capacitance value, and for this purpose a bridge rectifier may be diagonally connected between symmetrical taps on the anode impedances to develop a capacitance smoothed unidirectional output voltage (Fig. 4, not shown). Fig. 5 shows a complete slub-catching circuit for textile processing wherein the capacitance bridge comprises capacitances 41, 44 in parallel with the anode/grid capacitances of tubes 1, 2; a reference capacitance 42 in parallel with a trimming capacitance 43, and a test capacitance through which a thread passes in series with capacitance 40; the variable cathode resistance 31 enabling the slopes of the two tubes to be matched. The anode of tube 2 is directly coupled through a capacitance resistance low-pass filter to thyratron 10, and on passage of a slub in the thread the capacitance Cm, the amplitude of oscillation and the anode voltage of tube 2 all increase to fire the thyratron, which energizes relay 12 to operate a thread-cutting device 75 and/or a thread catching device (not shown). One or more circuits are supplied with anode current from power packs of the kind described in Specification 771,499, wherein on firing of a thyratron in response to a slub, relay 68 operates to open contact 69 to de-energize the thyratron supply; the contact reclosing after an interval determined by capacitance 70. A resistance may be connected between the relay and the thyratron anode supply with the junction returned through a capacitance to the slider of a potentiometer between earth and one side of the thyratron heater (Fig. 6, not shown), whereby a small adjustable alternating voltage is superimposed on the voltage of the capacitance ; the latter commencing to discharge on operation of the thyratron until after a predetermined interval its voltage reaches a level at which the net thyratron anode voltage becomes momentarily insufficient to maintain the tube in operation and extinction occurs automatically. Fig. 7 shows a modified capacitance sensitive circuit wherein the reference arm comprises two series capacitances 52, 53 of which the latter is shunted by a neon tube 54; the junction being connected to a positive voltage source over resistance 55. The anodes of tubes 1, 2 are connected to their supply over balanced windings 50, 51 of a relay normally inoperative due to equality of anode currents. On arrival of a slub in the test capacitance Cm, the amplitude of oscillations increases and the increased voltage across 53 strikes the neon tube, causing an unbalance of the capacitance bridge, whereby the current in 50 increases and that in 51 decreases to operate the relay and the cutting device 75. At the same time, capacitance Cm is discharged, and on removal of the slub is again recharged to its normal level after an interval of time to equalize the anode currents, extinguish the neon to restore bridge balance, and de-energize the relay. Fig. 8 shows a further modification wherein an automatic gain control rectifier 62 is coupled to the anode of tube 2 to produce a control voltage which is filtered and applied to the grids of the tubes 1, 2, which have a common feedback cathode resistance Rk. The neon tube 54 is connected between a positive voltage source over a high resistance 55 to the point at which the control voltage is applied to grid resistance Rg of tube 2; shunted to earth over capacitance 56. A capacitance 57 couples the live side of the neon to the anode of tube 1 so that when the amplitude of the oscillations increases above a predetermined level the neon fires, the bridge becomes asymmetrical, the anode currents are unbalanced and the anode relay operates a cutting or clamping device as in Fig. 7.