Eddy current flaw detector utilizing a field coil and pick-up coils in approximatelyparallel relation to the test piece with the pick-up coils located within the field coil

摘要

1,005,815. Detecting flaws. G. K. N. GROUP SERVICES Ltd. Oct. 13,1961 [Oct. 14, 1960; Feb. 24,1961], Nos. 35381/60 and 6777/61. Heading G1N. In an arrangement for detecting flaws in a workpiece wherein electro-magnetic coupling is set up between a surface layer of the workpiece and a detector and the detection circuit includes an amplifier, the gain of the amplifier is controlled by an electrical signal which varies as a function of the distance between the head and the surface. A single turn coil 13 having a wrap around surface parallel to the surface of the workpiece 10 is energized by alternating current of from 1-5 Kc/s from an oscillator 15, and two pick-up coils 16 are provided laying within the coil 13 and having their surface in its surface. A printed circuit may be used. The coils are connected in series opposition and an unbalance signal arising from the presence of a flaw is amplified in a carrier amplifier 21, detected in a detector 22 and applied by way of a band-pass filter 23 to an amplifier 25, the gain of which is controlled in dependence on the distance of the coil from the workpiece by a signal on lines 26, 27, which varies with the distance as indicated by the coil loading. The output of amplifier 25 may be applied to an oscilloscope to control a visual or audible indicator, or to actuate a marking device. A variable gain transistor amplifier T 1 , Fig. 3,has the detected signal from filter 23 coupled to its base b by capacitor C2 and the gain is controlled by the auxiliary signal applied also to the base via resistor R 1 , the impedance of zener diode d varying as a function of the current passing through it and thus varying the amplifier gain. The output is taken via capacitor C3, and manual control of the working point is provided by a variable resistor R3. The detecting head 69 Fig. 7, is mounted on a rotatable sleeve 66 and is supported to be urged into a recess 68 in the sleeve by centrifugal force acting on a weight 74 in opposition to a spring 71. The depth of penetration being adjustable by a screw 76 and a stop 75. The rotating sleeve has the electronic apparatus mounted thereon and connections are made via slip rings. Chuck mounted guide rollers centre the work piece for its passage through the sleeve. The detector head assembly is rotated by a motor driven belt which is microswitch controlled by the presence of the work piece in the centring chuck. An air pressure operated flaw marking scriber in which the rise of voltage from amplifier 25 above a predetermined value causes a Schmidt trigger to actuate a monostable circuit to produce a pulse of definite period for controlling a solenoid operated air valve is described with reference to Fig. 10 (not shown). In a delayed marking arrangement wherein the flaw is marked at a point remote from the measuring point, a pulse developed in a level comparator 120, Fig. 11, in response to a flaw in the detector head is stepped through N shift register stages 133 by pulses from a pulse generator 131 which are timed by the speed of the workpiece as detected by a roller 136 having a conductive sector 138 which periodically closes contacts 137. Specification 1, 005, 816 is referred to.