Patent GB-647651-A (GB)
Title: Improvements in or relating to electrical indicating devices
Inventor: MOSS ERIC BEECROFT
IPC: G08C19/06
Filed: 1947-10-31
Abstract: 647,651. Electric indicating systems for pressure and the like. SMITH & SONS (ENGLAND), Ltd. Oct. 31, 1947, No. 29079. [Class 40 (i)] An electrical indicating device comprises an armature assembly laterally and linearly movable relative to an adjacent pair of inductors, which are energized in parallel from an A.C. supply, so that the ratio of the inductor impedances and the currents therein varies in accordance with the displacement of the armature by the quantity to be indicated; the armature being so shaped that rotation about its axis of displacement moves its periphery to or from both of the inductors, to vary their impedances differentially. Fig. 2 shows the transmitter of an electrical remote indicating system operable by a pressure-sensitive member, in which a non-magnetic housing 12 has an end -plate 11 carrying a bellows 10 to the inside of which the unknown pressure is applied through nozzle 16. The free end of the bellows is secured to a non-magnetic rod 13 cupped at 14 to receive a spring 15 resisting expansion of the bellows and bearing against a shoulder in the bore of a cylindrical element 20 screwed into a sleeve 17 fitting within the housing. The rod 13 carries spaced laminated circular ferromagnetic armatures 71, 21, from each of which a segment is cut, and which are arranged so that their corresponding diametral axes of symmetry are at right angles. Each armature is associated with a pair of coils 25, 26 and 27, 28 wound tangentially to laminated annular cores 29, 30 encircling the armatures, which are spaced apart more widely than the coils. The axes of the pairs of coils are mutually perpendicular, to reduce coupling, and the coils of each pair are connected in series to form a single variable inductor such that axial movement of rod 13 increases the inductance of one inductor and decreases that of the other. The spring tension is adjustable by rotating the cylindrical element 20, threaded in the sleeve 17, and locking it thereto by tab 38 and nut 39. A grubscrew 40 limits the armature travel on excessive pressure, and for balancing the inductances of the coils when the armature is symmetrically disposed, the sleeve 17 may be loosened within the housing 12 by slackening nuts 47, when the stators may be rotated about the segmented armatures until the inductances balance, after which the nuts are retightened. The two variable inductors 61, 62 (Fig. 5) are connected through a plug 36 to a circuit comprising a ratiometer of the kind described in Specification 561,866, [Group XXXVI], wherein they are each connected in series with an adjustable inductance 68, a half-wave rectifier 69, and one of the ratiometer coils 64, 65, across an A.C. supply 67. A second oppositely-poled rectifier 70 is connected across each rectifier 69 and its ratiometer coil to remove any D.C. component. Inductances 68 are adjusted so that the ratiometer indicates accurately the ratio of the currents through the two variable inductors, and the pressure-sensitive transmitter is set up by applying half the full scale pressure, adjusting the spring tension to give half full scale reading, releasing the pressure, and rotating the stator until zero reading is obtained. The coils have low power factors to render the accuracy independent of frequency, and the device is not limited to indicating the movement of pressure displaced members.