GB2141234A

GB2141234A - Eddy-current flaw detector

Info

Publication number: GB2141234A
Application number: GB08315591A
Authority: GB
Inventors: Vyacheslav Mikhailovich Malov; Alexei Dmitrievich Pokrovsky; Alexandr Illarionovic Khvostov
Original assignee: Moskovsky Energetichesky Institut
Current assignee: Moskovsky Energetichesky Institut
Priority date: 1983-06-07
Filing date: 1983-06-07
Publication date: 1984-12-12
Also published as: GB8315591D0; GB2141234B

Abstract

An eddy-current flaw detector comprises an eddy-current transducer (1) whose inductance coil (2) is inserted in the oscillatory circuit of a self-excited oscillator (3) electrically connected to the input of a threshold detector (14). The output of the detector (14) is connected to a flaw indicator (15) and to the input of an autotuning channel (16) composed of such series-connected components as a controllable pulse generator (17), a control switch (18), a pulse counter (19) and a digital-to-analog converter (20). The output of the channel (16) is connected to the control input of the self-excited generator (3). The autotuning channel (16) enables the detector to work consistently in testing diverse materials. In a setting mode, the transducer (1) is placed on a flawless surface, and the autotuning channel (16) is activated to bring the oscillator (3) to a condition of oscillation close to quenching. The autotuning channel (16) is then locked, and subsequent passage of the transducer over a flaw will quench the oscillation. Advantageously the output of the autotuning channel (16) may also be used to control the gain of an amplifier (21) in the detection channel. <IMAGE>

Description

SPECIFICATION Eddy-current flaw detector The present invention relates to nondestructive testing in quality control of various articles, more particularly, to an eddy-current flaw detector.

The eddy-current flaw detector according to the invention is designed to reveal surface defects in articles made of electrically conductive materials and operating in conditions characterized by great specific loads. The best field of application is aviation engineering in which materials are used under extreme mechanical conditions to make structures lighter. Furthermore, the eddy-current flaw detector forming the subject of the invention may be used for testing vital parts of the energy-producing equipment, for example, turbine blades and automobile parts subjected to great loads.

The object of the invention is to create an eddycurrent flaw detector, which would permit increasing efficiency in testing articles and reliablility of test results.

There is provided an eddy-current detector wherein an inductance coil of an eddy-current transducer is inserted into an oscillatory circuit of a controllable self-excited oscillator electrically coupled to the input of a threshold detector having its output connected to a flaw detector and to the input of an autotuning channel composed of such seriesconnected components as a controllable pulse generator, a control gate, a pulse counter and a digital-to-analog converter, the output of the autotuning channel being connected to the control input of the self-excited oscillator.

Advantageously the input of a controllable AC amplifier in the eddy-current detector according to the invention is connected to the output of the self-excited oscillator, while the control input thereof is connected to the output of the digital-to-analog converter in the autotuning channel, the output of said amplifier being connected to the input of the threshold detector.

The eddy-current flaw detector in compliance with the invention permits increasing efficiency in testing articles due to automation of a tuning process, which prevents operator's tuning errors, particularly when the material of said tested articles changes. Moreover, the proposed eddy-cuurent flaw detector provides a means for automatically equalizing flaw sensitivity in testing articles fabricated from different materials.

The invention will now be described further with reference to a specific embodiment thereof, taken in conjunction with the accompanying drawing which is a block diagram of an eddy-current flaw detector according to the invention.

Referring to the drawing the eddy-current flaw detector forming the subject of the invention comprises an eddy-current transducer 1 having its inductance coil 2 inserted in the oscillatory circuit of a controllable self-excited oscillator 3 formed by an inductance coil and a capacitor 5.

In the preferred embodiment of the invention the controllable self-excited oscillator 3 uses a capacitive positive feedback circuit and comprises an oscillatory circuit formed by such series-connected apparatus as the inductance coil 4 and the capacitors connected to the collector of a transducer 6 whose base is connected to a voltage divider made up of series-connected resistors 7 and 8 and coupled to a common bus by a capacitor 9. The emitter of the transistor 6 is connected to a resistor 10 and a capacitor 11 which are placed in parallel, second leads thereof being connected to a common bus.

Connected between the collector and emitter of said transistor 6 is a positive feedback circuit incorporating a blocking capacitor 12 and a varicap 13 which are placed in series, the capacitance of said varicap determining the amount of positive feedback.

The output of the self-excited oscillator 3 is electrically coupled to the input of a threshold detector 14. The output of the threshold detector 14 is connected to the input of a flaw indicator 15 and to the input of an autotuning channel 16 comprising such series-connected components as a controllable pulse generator 17, a control gate 18, a pulse counter 19 and a digital-to-analog converter 20. The output of the autotuning channel 16 is connected to the control input of the self-excited oscillator 3.

The control input of the gate 18 is connected to the reset input of the pulse counter 19.

The sensitivity of the eddy-current flaw detector according to the invention varies with particular materials. To enhance testing reliability, said sensitivity must be balanced, which is done by providing a controllable AC amplifier 21 having its input connected to the output of the self-excited oscillator 3 and its output coupled to the input of the threshold detector 14, while the contol input thereof is connected to the output of the digital-to-analog conver ter20.

The eddy-current detector in accordance with the invention operates in the following manner.

The eddy-current transducer 1 with the inductance coil 1 is placed a flawless area on the surface of an article to be tested. Thereafter the pulse counter 19 is reset and the control gate 18 is enabled. A numeric code corresponding to the reset state of the pulse counter 19 is supplied to the digital inputs of the digital-to-analog converter 20 wherein it is converted into DC voltage, the magnitude of which ensures, in a given case, the quenching of oscillations in the controllable self-ixcited oscillator 3. The absence of an r-f signal at the output of the self-exciting oscillator 3 results in that the output of the threshold detector 14 electrically coupled thereto develops a signal enabling the controllable pulse generator 17 in the autotuning channel 16.The output pulses of the controllable pulse generator 17 are applied through the conducting control gate 18 to the input of the pulse counter 19, which changes the numeric code at its outputs. Changing the numeric code at the digital inputs of the digital-to-analog converter 20 varies its output voltage in accordance with the input code change. The process continues until the voltage at the control input of the self-excited oscillator 3 is sufficient to cause occurrence of oscillations. In this case r-f voltage is fed to the input of the threshold detector 14 which assumes state in which the controllable pulse generator 17 in the autotuning channel is disabled.No pulses will then come to the input of the pulse counter 19 through the conducting gate 18 whereby the output code of the counter 19 will no longer be changed and no variations will occur in the output voltage of the digital-to-analog converter 20 and in the operating conditions of the self-excited oscillator 3 which is found in a state close to oscillation quenching. Next, the operating conditions of the self-excited oscillator are maintained by disabling the control gate 18 which provides for the invariable code of the pulse counter 19 and constant output voltage of the digital-to-analog converter 20. This ends the tuning of the eddy-current flaw detector.

As the eddy-current transducer 1 scans the surfaces of an article under test, the presence of surface cracks causes the quenching of oscillations in the self-excited oscillator 3. As a result, there occurs the switching of the threshold detector 14 whose input is electrically coupled to the output of the self-excited oscillator 3, and the flaw indicator 15 operates.

The output voltage of the digital-to-analog converter 20 depends on the type of material being tested since oscillations begin at different control voltages for different materials. Similar testing sensitivity for different types of material may be obtained by inserting an adjustable amplifier 21 in the circuitry of the eddy-current flaw detector, a transfer ratio of said amplifier being dependent upon a DC control voltage (the output voltage of the digital-to-analog converter 20). The above effect is attained by changing the transfer ratio of the amplifer 21.

The proposed eddy-current flaw detector allows increasing efficiency due to the fact that a tuning operation is made automatic and the operator does not, consequently, need much time to perform it.

Furthermore, testing reliability is increased due to the fact that the effect of a subjective factor (skill and nervous state of the operator) upon accuracy and correctness of tuning is essentially eliminated. The above advantage is also attributable to the fact that flaw sensitivity of the device is essentially equalized in operation with different materials.

Claims

1. An eddy-current flaw detector wherein an inductance coil of an eddy-current transducer is inserted in an oscillatory circuit of a controllable self-excited oscillator electrically coupled to the input of a threshold detector having its output connected to a flaw indicator and to the input of an autotuning channel composed of such seriesconnected components as a controllable pulse generator, a control gate, a pulse counter, and a digital-to-analog converter, the output of the autotuning channel being connected to the control input of the self-excited oscillator.

2. An eddy-current flaw detector as claimed in Claim 1, wherein a controllable AC amplifier is connected to the output of the self-excited oscillator via its input and to the output of the digital-to-analog converter via its control input, while the output thereof is connected to the input of the threshold detector.

3. An eddy-current flaw detector substantially as hereinabove described with reference to and as shown in the accompanying drawing.