| 摘要 |
1,130,194. Detecting the presence of a metal body. COMPAGNIE GENERALE D'ELECTRONIQUE INDUSTRIELLE LEPAUTE. Oct. 12, 1966 [Oct. 12, 1965], No.45699/66. Heading G1N. [Also in Division G4] Apparatus for detecting the presence of a metal object in a given place or its passage therethrough comprises an A. C. bridge circuit incorporating an electromagnetic detecting loop 46 and a reference coil 43 and is characterised by having one or more insulated conductors (16), whose shape and dimensions are such that they undergo the same capacitance variations in relation to the surrounding medium as the detecting loop, connected to that arm of the bridge which contains the reference coil to provide compensation for temperature changes and penetration of moisture. Connection between the detecting loop and the bridge circiut may be through a two conductor screened cable (14), Fig. 1 (not shown), or a three conductor cable. As shown in Fig. 2, the bridge E is energized by an oscillator D and any out of balance signal is freed from parasitic r.f. by a capicitor 47 and passes through an amplifier F, having a negative feedback loop incorporating a variable rejector circuit, to a rectifier circuit, (diodes 67, 68). A switch 88 enables either the "passage" or the "presence" mode to be selected. Connection between c and d selects the "passage" mode; limiting diodes 74, 75 convert the varying signal (K) due to the irregular shape of the object (J) into a uniform square pulse (M) which is then differentiated by a capacitor 81, (N), amplified and passed to a Schmitt trigger circuit to control a relay 102. Connection between c and e selects the "presence" mode in which the rectified signal is fed directly to the Schmitt trigger circuit. A tuning indicator 78 is provided. |
