DETECTING DEVICE FOR LONGITUDINAL CRACKON BELT

PURPOSE:To prevent longitudinal cracks on a belt from being erroneously detected at a position where no loop coil is buried by providing a phase rotating circuit in the captioned device in which the longitudinal cracks is are detected by an output of a detecting coil electromagnetically coupled with the loop coil curried in a conveyor belt along its width. CONSTITUTION:The captioned device includes a detection coil 24 disposed in a confronting relation to a belt 10 so as to be electromagnetically coupled with plural loop coils 14 extending in the width direction of the belt 10 and buried in an equidistant interval. When a constant amplitude AC current is supplied to the aforesaid detecting coil 24 from a constant-current circuit 28, constant and varying fractions of voltage appear at both ends of the coil 24. Here, in order to remove the constant fraction described above, a voltage corresponding thereto is generated from a cancellation voltage generating circuit 34, and applied to a cancelleration circuit 36. In addition, only the aforesaid varying fraction is taken out, amplified 38, phase-detected by detectors 40, 42, and the resultant resistance and clearance variation components are outputted. Phase rotation is effected by a phase rotation circuit 44 based on both variation components.