Patent GB-924869-A (GB)
Title: Apparatus for detecting displacement of a metallic part
IPC: B61L23/12
Filed: 1960-03-02
Abstract: 924,869. Transformers. SOC. INDUSTRIELLE DE LIAISONS ELECTRIQUE. March 2, 1960 [March 6, 1959; Feb. 22, 1960], No. 7426/60. Class 38 (2). [Also in Groups XXX and XXXVI] An E-shaped magnetic core carrying coils 1, 2, 3, Fig. 1, is mounted at the side of a railway line 6 to respond to the passage of a train wheel, coil 1 being energized from an oscillator and the difference in induced voltages in coils 2 and 3, which are arranged to differ in the absence of a wheel, being changed by the wheel presence to effect an indication of the train location. The output signal which drops to a minimum as the wheel covers the first gap rises to a maximum when the wheel covers the second, the intermediate normal level being achieved by physical asymmetry in the coil construction. A voltagewheel-position graph is described with reference to Fig. 4 (not shown), and measurement of the time intervals for the purpose of measuring train speed is discussed. To prevent actuation by each wheel the indicator (a relay) may stay locked in after the passage of the first wheel until such times as it is tripped by the first wheel reaching a point further along the track where another similar device or a mechanical trip switching arrangement may be located. In the circuit, Fig. 3, a relay R1 energized by the normal unbalance current is released when the voltage decreases at the approach of a wheel from X to energize a bell 14 and relays R2 and R5, the current path including contacts of relay R5 and an amplifier 12. The input circuit is thus broken as soon as relay R1 is operated. Relay R4 responds to a train moving in the opposite direction to energize a self-holding relay R3 which may operate a second indicator (not shown). An alternative four-coil arrangement embedded in a plastic polyester resin coated with an impregnated fibre glass 112, Fig. 11, whereby it can be secured adjacent a rail, has two pairs of coils mounted on separate U-shaped cores, one of which has extended limbs to provide the asymmetry. The coils of the transducers are energized by oscillators 127, 128, Fig. 12, and the outputs are applied via transistor pre-amplifier level-detectors 129, 130, and a common power amplifier 131, to control the energization of a bell 134 by a source 133. Two outputs A, B at different level from the preamplifier are applied to low level indicator 156, Fig. 14, and high level indicator 157, respectively, the former feeding a power amplifier through winding L3 and transformer T4, Fig. 15, and including in circuit a feed-back path through winding L3 and rectifier 174, to ensure a sharp cut-off when the normal unbalance signal is removed by the presence of a wheel. This cut-off results in the release of relay 132 to ring bell 134 and the removal of feed-back from a gating transistor 181 in the power amplifier input so that resumption of the normal current does not reoperate relay 132. Operation of the high-level indicator 157 by a train in the reverse direction store a voltage on condenser 177 to prevent the response of 156. This may be reoperated by a manually controlled switch 182, or automatically by a further similar unit along the track which provides an input to transformer T5 from its high level indicator 157. The two units can form a symmetrical arrangement for dealing with trains in both directions and in this case a second power amplifier and alarm bell will be necessary. The disappearance of unbalance current provides a foil-safe facility. In a modification, balanced transducers are used but their outputs are unbalanced by adjustment of their individual couplings with a transformer secondary coil. An enclosed concentrically wound transformer described with reference to Fig. 17 has a pair of usually adjustable cores to set the degree of asymmetry. The length of a metal body being passed through such an area may also be measured by measuring signal time intervals.