GB1295460A - - Google Patents

Abstract

1295460 Detecting conductive bodies electrically; weighing apparatus A A BLACKERBY 24 Dec 1969 [27 Nov 1968] 57298/69 Headings G1N, G1W [Also in Division B8] In an arrangement for determining the presence, proximity or type of a conductive body, a sensing device comprises a main coil 12 and a compensating coil 13 in a conductive housing 11, the end of which is provided with a stainless steel membrane 15 and non-ferrous metallic members 16, 17 defining a slit 18 to collimate the emergent flux, and these coils are arranged as two arms of a bridge circuit which is unbalanced by the presence of a conductive body within the flux of the coil 12. As shown the coils 12, 13 are fed from secondary windings 36, 37 of a transformer 35 fed by a generator 31, and the mid points are connected by a transformer 39. The output from winding 40 is compared in a phase sensitive demodulator 41 with the output of a phase shifting circuit 42, 43, 44. The output of demodulator 41 may be calibrated to represent either the distance of a conductive body or the conductivity (or material) of a body at a fixed distance. The coils 12 and 13 are essentially similar and their arrangement is such as substantially to compensate for temperature and other extraneous variations. The narrow slit 18 minimizes the effect of re-radiated flux. The flux limiting effect of the membrane 15 is produced by heat treatment, a 5mil membrane being heated for 10-20 minutes at 1430‹F followed by slow cooling. The sensing device may be used in an automatic weighing arrangement comprising an endless belt conveyer supported at one station by rollers 55 supported on a member 58 which is mounted between rubber springs 59, 60. The member 58 carries aluminium discs 65 which are mounted adjacent sensing devices 10, the outputs of these devices being displayed on a suitably calibrated meter 70. The downward travel of the conveyer is limited to protect the devices 10. The sensing device may be used for determining the thickness of paint or oxides, conductivity changes in materials due to variations in temperature, humidity and pressure, and for flaw detection .