GB1473306A - Inductive loop detection devices - Google Patents

Abstract

1473306 Detecting metal objects DEFENCE SECRETARY OF STATE FOR 1 Aug 1974 [11 Sept 1973] 42656/73 Heading G1N An inductive loop detection apparatus includes an inductive testing loop inductively coupled to the detecting loop and which simulates an object to be tested e.g. a motor vehicle or aircraft travelling along a road or runway. A twin core cable buried under the road or runway and arranged in the form of a loop has one core 6 coupled to a conventional impedance detection apparatus 7 and the other core 5 forming a testing loop connected to a test circuit including impedances Z1-Z6 and a 2- pole twelve way electromagnetically operated switch 1, 2, 3, 4. When the wipers 3, 4 are in the position shown loop 5 is in open circuit and has very little effect on the normal operation of detection loop 6. To test the detection circuit 7 push switch 8 is pressed to cause current to flow through magnet coil 10 of the switch and move wipers 3, 4 to contacts C 2 , C 2 ' and cause impedance Z 1 to complete a circuit with coil 5. This has a similar effect on the coil 6 and circuit 7 to that of a metallic object close to detecting loop 6. Due to the connection of the circuit shown current continues to flow in coil 10 to move wipers 3, 4 to the next position C 3 , C 3 ' to open circuit testing coil 5 so that the effect at detection loop 6 is of an object moving over the detecting loop. Each time button 8 is pressed the switch moves round two positions to connect a successive one of the impedances Z1- Z6 into circuit with the testing loop 5, the impedances decreasing in impedance so that the effect is of successively larger objects. Thus the sensitivity of the apparatus can be measured.