DE2044331A1 - Device for non-destructive testing of materials - Google Patents

Description

DIPL-ING. HORST ROSE DIPL-ING. PETER KOSEL PATENTANWÄLTE Z U A A «5 J I

3353 Bad Gand.rsheim, September 7, 1970

Hohenhöfen 5

Telephone: (05382) 2842

Telegram address: Siedpatent Gandereheim

Charles ileviLle Owston

Our file no .: 2668/1

Patent dated Sept. 7, 1970

Charles Neville Owston

3 iJchool Lane, .jherington, Newport Pagnell , Buckinghamshire,

England

Device for non-destructive testing of materials

The invention relates to a high-frequency device for non-destructive testing of materials Using eddy currents, with an inductive sensor, which is placed at a distance from the material to be tested; it relates in particular to a device of this type which is used for scanning a surface with an irregular topography is suitable without touching it mechanically.

Methods for testing materials with the aid of eddy currents are known up to frequencies of about 1 MHz. For improvement The resolution and detection of surface imperfections such as pillows or inclusions is an effort has been made to work at higher frequencies too

work.

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In normal non-destructive material testing with eddy currents, an inductive sensor, for example a coil wound on a ferrite rod, is placed near the material to be tested. In the absence of the material to be tested, the impedance of the sensor has two components: an ohmic component R ^, and a reactive component ω JC ₁ The presence of the material to be tested causes a change in the impedance of the sensor, namely the combination of sensor and material is electrical roughly equivalent to a weakly coupled transmitter with an ohmic load connected to its secondary winding. In the presence of the material, the impedance of the sensor

Ohmic component »R ^ + H ₀
Reactive component » 0 * -Ζ < - w

Here R ₁ and L ^ are the resistance and inductance of the sensor, R ₂ and L _{2 are} the resistance and inductance associated with the material to be tested, and K is the coupling factor and is equal to M , where

M is the mutual inductance between the sensor and the material. The coupling factor K is determined by the shape of the sensor and the distance between the sensor and the to testing material.

At frequencies of the order of magnitude of 1 MHz and greater and with a suitable diameter of the probe, one can theoretically and experimentally prove that R _{2 is} much smaller

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is called (*> · Lp. With this assumption, the Impedance of the sensor to:

2 ¹ M Ohmic component «R ,. + R, -, * ^ _r .— ... (1)

^d ^ 2

Reactive component « u) -L ₁ (I - K) ... (2)

With a given sensor, the reactive component is only influenced by the coupling factor K between the sensor and the material, that is, by the distance between them, and it is independent of the specific resistance of the material. In the same way, the ohmic component is influenced by the specific resistance Rp of the ^{material to be} tested and the coupling factor K.

It is therefore an object of the invention to improve the device mentioned at the outset and, in particular, the Measurement of the specific resistance of the material to be tested independent of the size of the coupling factor close.

According to the invention, this is achieved in a device mentioned at the outset in that an oscillating circuit having oscillator is provided, the inductive sensor connected as the inductance of this resonant circuit is, so that the presence of a material to be tested causes a change in the frequency and amplitude of the vibrations of the oscillator, and that a monitoring device responsive to frequency changes of the oscillator is provided, which is a characteristic of the deviation of the oscillator frequency from a predetermined frequency Emits signal. A change in the reactive component of the sensor therefore leads to a change in the oscillator frequency, and this change in frequency is a direct measure of the coupling factor; with the device according to the invention can

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this change is very easily detected and by changing the distance between the sensor and the material to a desired one A value can be set which corresponds to a certain coupling factor; after that, the resistivity can be directly be measured.

In a further development of the invention, the invention is Permitted device designed so that an actuator is assigned to the sensor, which the latter on the of the Uberwachungs = device responds and is designed to adjust the sensor relative to the tested material, to bring the oscillator frequency back to the specified frequency mentioned. This makes the coupling factor through automatic control of the distance between the sensor and the material is always kept at a constant value and you can measure the specific resistance directly. Such a device also allows continuous measurements on work pieces. with irregular surface topography, as the sensor always automatically adjusts itself to the correct distance.

The device is also advantageously designed so that the actuator has a vibrator with a movable coil, which is mechanically connected to the sensor. Such an actuator works quickly and without disruptive play.

The monitoring device advantageously has a frequency discriminator; the output of this discriminator is preferably used to regulate the position of the sensor.

The desired information with regard to the specific resistance of the material to be tested depends as shown above » detracts from the resistance of the probe and the device according to the invention therefore preferably comprises a measuring device, e.g. a voltmeter to display the sensor resistance.

With a device according to the invention, complex surfaces can be scanned automatically in order to obtain information

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with regard to the following points: a) Changes to the Surface topography; b) Changes to the specific upper = » surface resistance; c) mechanical defects such as ice or Inclusions on or near the surface.

Since the itihler automatically moves to the correct distance from the to test material is set and is also relatively insensitive to tilting, the! "ühler be mounted on a cart and scan relatively complex surfaces. The results of such scans can e.g. on an ink pen, a paksimile pen or recorded and displayed by another measuring recorder.

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_6_ 2044131

Further details and advantageous developments of the invention emerge from what is described below and Exemplary embodiment of a device according to the invention shown in the drawing.

Show it

1 shows a block diagram of a device according to the invention for materials testing, and

FIG. 2 shows a detailed circuit diagram of the device according to FIG. 1.

In FIG. 1, a metal object to be examined is designated by 10 and a sensor by 12 . In a certain experimental form, the sensor 12 had a ferrite rod with a diameter of 4.8 mm and an overall length of 9 »5 now. at one end it tapered at 45 to a diameter of about 3.45 mm (3/22 inches). The winding consisted of four turns of enamelled copper wire, which are wound onto the cylindrical part of the ferrite rod. The sensor 12 is mechanically connected via a rod 14 to an electromechanical transducer 16 which can have a vibrator with a movable coil.

At the high frequencies for which the inventive method » Direction is designed, the stray capacitances are more disruptive than at lower frequencies, and the sensor 12 is therefore made part of an oscillating circuit. Therefore an ab » Voice capacitor 18 connected in parallel to it, and the resonant circuit is excited by an oscillator 20. The regulation the position of the sensor is effected by a control circuit which has a frequency discriminator 22, which with the Oscillator circuit is connected, and which also has an amplifier 24 connected to the converter 16.

In order to obtain the desired information regarding the surface resistivity of the metal 10, has the device on a demodulator 26, which is also on

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connected to the oscillator circuit and designed so that it emits an output signal which the amplitude the vibrations of the oscillator 20 is proportional.

Pig. 2 shows the device according to the invention in more detail. The oscillator is an amplitude limited oscillator and is designed by two each as integrated circuits * Deten amplifiers IC 1 and IC 2 and the sensor 12 are formed. Each amplifier stage, e.g. the integrated circuit IC 1 and the resonant circuit assigned to it have a voltage gain of around 12 dB at 26 MHz and over a band » width of 3 MHz. Since the input resistance of the amplifier unit IC 1 can have a value as low as 1 kOhm, and since this could lead to excessive attenuation if the sensor 12 is connected directly to the amplifier input would, the tuning capacitor of the sensor circuit is divided into three capacitors 18a, 18b and 18c and as impedance = converter used. The amplifier unit IC 2 is the amplitude limiter and supplies a constant alternating voltage a feedback resistor 28. This voltage is measured via a diode 30 and monitored. The feedback = Resistor 28 and the sensor 12 form an AC & voltage voltage divider for the feedback signal and give an input voltage to the amplifier unit IC 1, which represent The impedance of the sensor 12 at resonance is essentially proportional.

The amplifier unit IC 1 works as an isolating amplifier, which works at a sufficiently low signal level to a To prevent limiting of its output signal, however, this level is high enough to allow effective demodulation through the diode 36 to enable. Another amplifier unit IC 3 is connected to the output of the amplifier unit IC 1 connected to the output of which in turn the frequency discriminator 22 is connected, the latter from Foster Seeley - type is. The amplifier unit IC 3 and the trains » subordinate discriminator 22 are tuned so that at 26 MHz her

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Exit the same. Is zero. The output of the discriminator 22 is by means of an amplifier 32 designed as an integrated circuit and a transistor amplifier stage designated by 34 reinforced. The output of stage 34 is a direct voltage whose amplitude and sign (polarity) are proportional to the deviation of the oscillator frequency are of the selected standard frequency; this output signal is sent to the vibrator 16, which has a moving coil. leads.

The one in Pig. 1 with 26 designated demodulator and its output circuit from the following switching elements of Fig.2 educated; A diode 36 is connected to the output of the amplifier = unit IC 1 and connected via an adjustable resistor 38, e.g. a potentiometer, to the input of a Amplifier 40, at the output of which a suitable measuring instrument 42 is connected, e.g. a voltmeter or a measuring recorder.

The apparatus described operates as follows: When the sensor 12 is far from any metal objects, the oscillator is tuned to a standard frequency of about 25 MHz. At this frequency, the discriminator 22 emits a large output voltage and the sensor 12 is advanced to the end of the adjustment range of the vibrator 16. If the test item 10 is now brought close to the sensor 12, the oscillator frequency increases as a result of the change in the reactive component of the sensor and gradually approaches 26 MHz, the center frequency of the frequency discriminator 22. When this frequency of 26 MHz is approached, the output voltage of the discriminator drops 22, the vibrator 16 is supplied with a lower voltage, and the feeler 12 moves away from the test object 10. Provided that the gain of the amplifier 32 is large and the movements of the

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Metal object 10 in the adjustment range of vibrator 16 the vibrator places the feeler. 12 on one Distance from metal 10, that the oscillator frequency is quite, is exactly 26 MHz. It follows that the sensor 12 is held at a certain distance from the test object 10 and that the The coupling coefficient K between the sensor 12 and the metal object Ί0 remains constant.

The output voltage of the diode 36 is a puncture of the input » output voltage of the amplifier unit IO 1, hence a puncture the impedance of the sensor 12, and thus also the specific resistance of the metal object 10, which is close to the sensor 12 is arranged, however, the output voltage of the diode depends also on the output voltage of the limiter IC 2, and the Greatest stability is therefore achieved by comparing the two output voltages from diodes 36 and 30 and not by measuring the absolute value of the output = signals of the diode 36. The two output signals from the Diode 36 and diode 30 have an opposite polarity = did so that by setting the potentiometer ^ 38 the on = output signal of the amplifier 4-0 can be set to zero can. The vortex st:? Omin strum ent can either be calibrated according to which setting of the potentiometer 38 is required is in order to ohmic in metal objects with different Components, d. H. specific resistances, in each case to emit the output voltage zero, or according to how the Output voltage at the measuring device 42 related to one at the potentio » meter 38 changes the preset level.

In the above, the special application of the device according to the invention for studying the change in specific surface resistances has been described} However, this device can also be used to great advantage for the detection of cracks or inclusions on surfaces. The nature of a bite changes the path of the eddy currents and thus also the resistance (R ₂ ) and the inductance (L ₂ ).

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It has been observed experimentally that the presence of a crack tends to cause the feeler 12 to move closer to the Moves metal object towards and also increases the effective resistivity measured by the measuring device 42. the Movement of the i ^ hlers 12 in the direction of the metal object 10 is associated with a change in the vibrator 16 supplied th voltage, and this voltage can be recorded as well as the voltage on Meter 4-2.

The device according to the invention can therefore be used for who = to indicate changes in surface topography, surface resistivity and surface cracks, and as follows:

a) Change in voltage applied to vibrator 16 - change in surface topography.

b) Change in display of output meter 42 - change in surface resistivity.

c) Change in voltage on vibrator 16 and on measuring device 42 - Surface crack or inclusion.

Patent attorneys Dipl.-Ing. Horst Rose Dipl.-Ing. Peter Kosel

September 2, 1970 Ra / R

ORiGIMAL! NSPECTED

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Claims (6)

3353 Bad Gaiidersheim, 7. September 1 Hohenhöfen 5 Telephone: (05382) 2842 Telegram address: Siedpatent Ganderaheim Patent application dated Sept. 7, 1970 Pat en tan claims rl Λ High-frequency device for non-destructive Material testing with the help of eddy currents, with an inductive grinder spaced from the to material to be tested is arranged, characterized in that an oscillating circuit (12, 18) having an oscillator (20) is provided, the inductive sensor (12) as the inductance of this Resonant circuit is switched, so that the presence of a material to be tested (10) a change in the Frequency and the amplitude of the oscillations of the oscillator (20) causes, and that one on Itequipment changes of the oscillator (20) responsive monitoring device (22) is provided, which is a for the Deviation of the oscillator frequency from a predetermined frequency emits characterizing signal. 10981 2/1236 Ra / r 2. Apparatus according to claim 1, characterized in that the sensor (12) is assigned an actuator (16) which the latter responds to the signal emitted by the monitoring device (22) and is designed to the To adjust the sensor (12) relative to the tested material (10) in order to bring the oscillator frequency back to the above Bring predetermined frequency. 3. Apparatus according to claim 2, characterized in that the actuator (16) comprises a movable coil vibrator mechanically connected (14) to the sensor (12) is. 4. Device according to one of the preceding claims, characterized in that the monitoring device has a frequency discriminator (22). 5. Device according to one of the preceding claims, characterized in that a demodulator (26, 36, 38, 40) is connected to the resonant circuit (12, 18), whose Output signal (42) of the oscillation amplitude of the oscillator " tors (20) is roughly proportional. 6. Device according to one of the preceding claims, characterized in that the predetermined frequency of the Oscillator (20) is greater than 1 MHz. Patent attorneys Dipl.-Ing. Horst Rose Dipl.-Ing. Peter Kosel 10981 2/1236 ORSGiNAL INSPECTED Read 11 e