RU2085931C1 - Flaw detector electromagnetic transducer - Google Patents

Abstract

FIELD: nondestructive flaw inspection; flaw inspection of electricity conducting equipment. SUBSTANCE: flaw detector transducer has excitation system in the form of U-shaped magnetic core 1 with winding 2 and metering element 3 arranged symmetrically to effective ends of magnetic core 1 and built up of two differentially connected inductance coils 4,5 and magnetodielectric core 6; inductance coils 4,5 are arranged coaxially and have common effective end. Metering element core 6 is placed coaxially relative to coils 4,5. Core size and its permeability are chosen according to condition ΔL_в/2 < ΔL < 1,5ΔL_в, where ΔL is maximum permissible change in inductance of metering element 3 due to core 6 mounted on its axis; ΔL_в is variation in metering element inductance due to change in its external coil turns by one turn. Size T of effective end of magnetic core 1 in direction perpendicular to line crossing both ends is recommended to be chosen from condition D_н<T, where D_н is outer diameter of metering element 3. Winding 2 of magnetic core 1 is recommended to be uniformly distributed part spaced P > 0,5 D_н from its both ends. Transducer is provided with nonmagnetic metal strip used to protect metering element 3 and magnetic core 1. EFFECT: improved validity of inspection. 5 cl, 2 dwg

Description

 The invention relates to non-destructive testing and can be used for inspection of electrically conductive objects.

Known electromagnetic transducer to the flaw detector, containing at least one measuring element in the form of two nested in each other and differentially connected inductance coils, an exciting system consisting of an electrically closed current path with a flat active section, and a closed magnetic circuit with a current winding inductively coupled to an electrically closed loop formed by the current lead, and the measuring elements are located in the zone of the active section of the current lead [1]
A disadvantage of the known electromagnetic transducer is that it does not allow to detect cracks oriented along the excited eddy currents, and when interacting with cracks perpendicular to the initial eddy current circuits, signals are recorded only in the end zones of the cracks. In addition, when eddy currents are excited by a flat bus with a current, the control depth is significantly less than potentially achievable.

Closest to the proposed technical essence, adopted as a prototype, an electromagnetic transducer to a flaw detector, containing an exciting system in the form of a U-shaped or C-shaped magnetic circuit with a winding and a measuring element located symmetrically relative to the working ends of the magnetic circuit and consisting of two differentially included invoices Rod cores [2]
The known electromagnetic transducer does not have a potentially achievable level of balancing, maintained during the interaction of electromagnetic material with defect-free objects, regardless of their specific electrical conductivity, magnetic permeability and variation of the working gap between the surface of the controlled object and the working end of the measuring element. This leads to the need to set the zero before control by introducing a compensating voltage and maintaining a stable working gap, with a variation of which the transmitter is unbalanced. For the same reason, the known electromagnetic transducer has a sensitivity to defects lower than potentially achievable. When scanning a controlled surface by a known transducer, it is possible to skip local defects passing between the coils of the measuring element.

 The objective of the invention is to increase the sensitivity to defects and performance monitoring while detuning from variations in the electromagnetic properties of the material and variations in the working gap.

 The task in an electromagnetic converter to a flaw detector containing an excitation system in the form of a U-shaped or C-shaped magnetic circuit with a winding and a balanced measuring element located symmetrically with respect to the working ends of the magnetic circuit and consisting of two differentially connected inductors and core is achieved by According to the invention, the inductors are made embedded in each other, and the core of the measuring element is placed coaxially with them.

Additionally, the goal is achieved due to the fact that the core is made of magnetodielectric, and its parameters are selected from the condition:
ΔLв / 2 <ΔL <1,5ΔLв,
where ΔL is the maximum possible change in the inductance of the measuring element due to the placement of a core on its axis;
ΔLin the change in the inductance of the measuring element when changing the number of turns of its external coil by one turn.

This goal is achieved due to the fact that the size T of the working end of the magnetic circuit in the direction perpendicular to the line passing through both ends is selected from the condition D _h <T, where D _{h is the} outer diameter of the measuring element.

This goal is achieved due to the fact that the winding of the magnetic circuit is evenly placed on its part, spaced from both ends by a value of P> 0.5D _h .

It is recommended to protect the measuring element and the magnetic circuit from the figurative action of the surface of the controlled objects, to equip the transducer with a protective plate made of non-magnetic metal that completely covers the working end of the transducer, and choose a thickness T and electrical conductivity σ of the plate material from the ratios
T <0.2 mm, σ <4 mS / M.
Figure 1 shows the design of the proposed electromagnetic transducer to a flaw detector with a U-shaped magnetic circuit; figure 2 separately shows the excitation system with a C-shaped magnetic circuit.

 The electromagnetic transducer to the flaw detector contains an excitation system in the form of a U-shaped (Fig. 1) or C-shaped (Fig. 2) magnetic circuit 1 with a winding 2 and a measuring element 3 located symmetrically relative to the working ends of the magnetic circuit 1 and consisting of two differentially connected the inductors 4 and 5 and the magnetodielectric core 6, the inductors 4 and 5 are made embedded in each other. The core 6 of the measuring element is placed on a common axis for coils 4 and 5.

The dimensions of the core 6 and the magnetic permeability of its material are selected from the condition:
ΔLв / 2 <ΔL <1,5ΔLв,
where ΔL is the maximum possible change in the inductance of the measuring element 3 due to the placement of the core 6 on its axis;
ΔLin the change in the inductance of the measuring element when changing the number of turns of its external coil by one turn.

The size T of the working end of the magnetic circuit 1 in the direction perpendicular to the line passing through both ends is recommended to be selected from the condition D _h <T, where D _{h is the} outer diameter of the measuring element 3.

The winding 2 of the magnetic circuit 1 is recommended to be evenly placed on its part, spaced from both ends by a value of P> 0.5D _h .

To protect the measuring element 5 and the magnetic circuit 1 from the abrasive action of the surface of the controlled objects, the transducer is equipped with a protective plate 7 of non-magnetic material that completely covers the working end of the transducer. The thickness T and the electrical conductivity σ of the plate material are recommended to be selected from the relations
T <0.2 mm, σ <4 mS / M.
An electromagnetic converter operates as follows.

 In the manufacture of the Converter balance its measuring element 5 by selecting the number of turns of the coils 4 and 5. This balancing can be carried out with an accuracy of one turn. The number of turns of the external coil 5 is chosen so that the voltage introduced into it by the excitation system exceeds the voltage introduced into the internal coil 4. Then, the exciting winding 2 is connected to the harmonic voltage generator, and the output of the measuring element 5 to the measuring device, for example, to a millivoltmeter. Enter the core 6, smoothly moving it along the common axis of the coils 4 and 5 and registering the unbalance voltage. The core 6 is fixed in the frame of the measuring element 3 when the minimum of the detected unbalance voltage is reached. The core 6 must not violate the symmetry of the magnetic field, which will lead to the appearance of an imbalance in the electromagnetic interaction of the transducer with a defect-free object. Therefore, it should be located coaxially with coils 4 and 5. The presence of the core 6 leads to alignment of the magnetic coupling of both coils 4 and 5 with local inhomogeneities (defects). Therefore, the electromagnetic effect of the core should be minimal, which is achieved by performing the recommended ratio. For the convenience of selecting the dimensions of the core, which is a ferrite chip, it is advisable to make it from a material with a relative magnetic permeability of not more than 5.

 Before monitoring, the winding 2 is connected to a harmonic voltage generator of a flaw detector (not shown), and the output of the measuring element 3 to its measuring unit (not shown). In the simplest case, the measuring unit can be a harmonic voltage amplitude meter. The eddy currents excited in the controlled area form a system of circuits that are closed around each of the ends of the magnetic circuit. The magnetic field they create is symmetrical if there are no continuity violations in the control object. If this symmetry is violated due to a defect, the eddy current circuits are deformed. This deformation leads to a difference in the voltages introduced into the coils 4 and 5. As a result, a voltage appears at the output of the measuring element 3. Symmetry breaking occurs in the presence of any local defect, as well as when interacting with a long crack that is not parallel to the side surface of the magnetic circuit 1. If the crack is parallel to the side surface of the magnetic circuit 1, then during the scan voltage pulses will be recorded that have maxima in the region of the ends of the crack, and there will be no signal strictly at its center.

 The asymmetry of the magnetic field of the exciting system leads to the fact that the level of balancing when interacting with a defect-free object depends on its electromagnetic properties and the magnitude of the working gap between the end of the transducer and the surface of the controlled area. The degree of asymmetry decreases with increasing width of the magnetic circuit and the removal of the turns of winding 2 from the ends of the magnetic circuit 1. When the recommended ratios are achieved, an acceptable degree of symmetry of the exciting system can be achieved.

 A high degree of balancing of the converter, without violating the degree of balancing in the presence of the control object, can significantly increase the sensitivity to defects and provide the ability to detect defects with increased gaps and their variation over a wide range.

 To protect the end of the converter from abrasive action, a protective plate 7 is used, which weakens the magnetic interaction with the controlled area. With the recommended parameters, this attenuation does not exceed 10% of the useful signal, which is quite acceptable. Moreover, the resulting advantages due to the reliability of the protection of the end face of the converter, manufacturability and ease of manufacture significantly exceed the disadvantages due to a slight decrease in sensitivity.

Compared with the known converters, the proposed electromagnetic converter has the following advantages:
higher sensitivity to defects due to the increased level of balancing;
higher productivity, due to the fact that the width of the control zone is approximately equal to the width of the ends of the magnetic circuit;
almost complete suppression of variations in the working gap and electromagnetic properties, by maintaining the level of balancing when exposed to any metal objects without heterogeneities.

Claims (5)

 1. An electromagnetic transducer to a flaw detector, containing an exciting system in the form of a U-shaped or C-shaped magnetic circuit with a winding and a balanced measuring element located symmetrically relative to the working ends of the magnetic circuit and consisting of two differentially connected inductors, characterized in that the inductors are coaxially and have a common working end. 2. The Converter according to claim 1, characterized in that it is equipped with a magnetodielectric core placed coaxially with the inductors, and its parameters are selected from the condition
ΔL _in / 2 <-L <1,5 -L _in ,
where L is the maximum possible change in the inductance of the measuring element due to the placement of a core on its axis;
L _{in the} change in the inductance of the measuring element when changing the number of turns of its external coil by one turn. 3. The Converter according to claim 1, characterized in that the size T of the working end of the magnetic circuit in the direction perpendicular to the line passing through both ends is selected from the condition D _n <T, where D _{n is the} outer diameter of the measuring element. 4. The Converter according to claim 1, characterized in that the winding of the magnetic circuit is evenly placed on its part, spaced from both ends by a value of P> 0.5 D _n 0. 5. The Converter according to claim 1, characterized in that it is equipped with a protective plate of non-magnetic metal covering the end of the entire Converter, and the thickness and electrical conductivity of the plate material are selected from the ratio
T <0.2 mm, σ <2 mS / m

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