Patent GB-804435-A (GB)
Title: Improvements in electrical measuring instruments
Filed: 1955-03-07
Abstract: 804,435. Phase-sensitive circuits. MINEAPOLIS-HONEYWELL REGULATOR CO. March 7, 1955 [March 16, 1954], No. 6620/55. Class 38 (4). [Also in Groups XXXVI and XXXVIII] An electrical measuring circuit comprises amplifier means for deriving an A.C. signal having a phase sense and magnitude such that it constitutes an analogue of a variable signal input, and an electro-mechanical vibrator device the operation of which is controlled by said A.C. signal and which chops a separate A.C. power source to derive from said source a D.C. current output to an operating circuit, which D.C. current varies in accordance with the magnitude of the A.C. signal and has its polarity determined by the phase sense of said signal. The lead 3 from a thermocouple A is coupled to a converter amplifier B comprising (Fig. 2) the reed 4 of a vibrator VG, the reed being energized by an A.C. current coil 5 to alternately make contact with opposed contacts 6 and 7. These are connected to the respective halves 8 and 9 of a primary winding on a transformer J. The centre tap 10 is connected through resistor R and one arm h of a feed-back network H to the other side of thermocouple A. The output frequency of secondary winding 11 of transformer J is determined by the frequency of A.C. current that energizes coil 5 which is the same as that of a transformer T and source g (Fig. 1 to be described) and is derived from a secondary winding 03 of a transformer O, the other secondary winding 02 supplying power through a rectifier N to an R.C.-coupled amplifier K, L, M. The A.C. output 12, 13 is connected through an R.C network C, A (Fig. 1) to control its phase. relative to the phase of the output of an A.C. source g and to make its waveform approximate to a sine wave and it is then impressed on the control grid of a valve D. This is fed from a D.C. supply E and its output is connected through a condenser 14 to the operating coil f of a chopper type inverter F from which is derived the feed-back voltage in network H. Coilf is connected through a phase-shift network G to the A.C. current source g so that when output from valve D is zero, the current in coil f leads by 90 degrees the voltage of source g. The chopper blade fa is vibrated between contacts P and PI connected to opposite ends of the secondary winding t1 t2 of the transformer T through resistances R7 and R<SP>1</SP>7. Further resistances R8 and R<SP>1</SP>8 and condensers C6 and C<SP>1</SP>6 connect the ends of coils t1 and t2 to a lead 18 which connects blade fa to an indicating instrument 20. The instrument is connected also to centre tap 19 through a reservoir condenser C7. Coil f of chopper circuit F is primarily energized from source g and secondarily by the output of valve D so that with the input and feedback voltages at the thermocouple balanced and therefore no signal fed to amplifier B, the output from valve D will be zero. When a change of measured condition produces a difference signal at A an A.C. voltage is applied to coilf by valve D of a phase, displaced approximately 90 degrees with respect to the phase of the voltage applied to the coil by generator g. Fig. 3 shows curves 21 and 22 which respectively represent the output from transformer T and the current in chopper coil f in a given relationship where the lines 24, 25 and 26 represent the instants of reversal of blade fa which occur at zero values on the coil current curve 22. With the output from valve D zero, the areas I and II beneath the quarter waves of both sections a and b are identical so that no resultant unidirectional current passes through network H and instrument 20. A difference signal at A causes the phase of blade fa to shift with respect to the voltage from transformer T so that positive and negative areas under the curves become unequal and a resultant unidirectional current flow occurs through network H and instrument 20. In a further embodiment (Fig. 4, not shown) contacts P, P1 are connected directly to windings t1, t2 and a resistance is shown to represent instrument 20. U.S.A. Specification 2,423,540 is referred to.