Patent GB-682544-A (GB)
Title: Improvements in or relating to electric measuring and indicating apparatus
IPC: G01R17/00
Filed: 1950-04-25
Abstract: 682,544. Bridge circuits; locating conductors. CLAESSON, P. H. E., and JOHANSSON, K. L. April 25, 1950 [Oct. 22, 1949], No. 10218/50. Class 37. [Also in Group XXXV] Apparatus for measuring impedance variations in an antenna is provided with means for effecting compensation of undesired current variations which are slow by comparison with current variations which may occur at instants of measurement, which means are arranged to respond to the slow current variations at a substantially lower rate than to current variations to be measured or indicated. The slow variations concerned may be caused by variations of temperature, humidity, ageing of components, &c. In the bridge circuit shown in Fig. 3 for detecting the change of capacitance of an antenna A<SP>1</SP> caused by approach of a person or object, two arms are formed by inductances L<SP>1</SP>, L<SP>2</SP> and a third by a resistance R<SP>1</SP> and condenser C<SP>1</SP> in parallel. The fourth arm is formed by the capacitance C3 between the antenna A<SP>1</SP> and earth. In one diagonal is a valve E<SP>1</SP> which oscillates and feeds the antenna if the capacitance C<SP>1</SP> is less than C<SP>3</SP>. The degree of oscillation is adjustable by varying the condenser C<SP>1</SP>. When an object approaches the antenna, the capacitance C<SP>3</SP> between the grid and earth increases so that the degree of oscillation of the valve decreases, the grid voltage decreases, and the anode current increases. The condenser C<SP>1</SP> is adjusted to compensate for slow variations by a motor M, Fig. 4, which is driven in one direction or the other as determined by a relay RE', the windings of which are in the anode circuit of the valve E<SP>1</SP>. The motor M consists of a rotor r<SP>1</SP> and a stator having three windings S<SP>1</SP>, S<SP>2</SP>, S<SP>3</SP>, which are successively supplied with current by sequentially acting relays RE<SP>2</SP>, RE3, RE<SP>4</SP>. The speed of operation of these relays is increased by means of condensers C<SP>2</SP>, C<SP>3</SP>, C<SP>4</SP>, so that the rotor r<SP>1</SP> rotates slowly and causes a slow adjustment of the condenser C<SP>1</SP>. If rapid correction is required, for example if the antenna A<SP>1</SP> is altered, a switch K<SP>1</SP> is operated and the condensers C<SP>2</SP>, C<SP>3</SP>, C<SP>4</SP> are disconnected. Fig. 5 shows a modified form of motor having four poles P<SP>6</SP> ... P<SP>9</SP> wound with coils S<SP>6</SP>... S<SP>9</SP>, opposite pairs S<SP>6</SP>, S<SP>8</SP> and S<SP>7</SP>, S<SP>9</SP> of which are supplied in succession with current pulses by means of relays RE<SP>3</SP>, RE<SP>4</SP>. The direction of rotation is determined by a polarizing field produced by two windings S<SP>10</SP>, S<SP>11</SP>, wound on a common core, corresponding to the windings of the relay RE<SP>1</SP>, Fig. 4. According to the direction of the polarizing field produced by these windings, the field is strengthened in one airgap and weakened in the opposite one thus determining the direction of rotation of the rotor. In a modification, Fig. 6, the bridge is similar to that shown in Fig. 3, but the valve is replaced by a transistor T<SP>1</SP> having high and and low potential electrodes 2, 1 and a common electrode 3. The output circuit of the transistor may be connected to a relay or motor as shown in Figs. 4 and 5. As shown, however, the adjusting device consists of a heating element R<SP>2</SP> and a condenser comprising a plate V<SP>2</SP> and a bi-metal spring V<SP>1</SP>. When signals are in progress the adjusting device is disconnected by a relay in series with the valve El, Fig. 3, which cuts off the current to the motor. This relay also has contacts for operating lamps or bells.