Measuring and testing – Vibration – By mechanical waves
Reexamination Certificate
2003-01-06
2004-07-27
Chapman, John E. (Department: 2856)
Measuring and testing
Vibration
By mechanical waves
Reexamination Certificate
active
06766694
ABSTRACT:
The present invention relates to an electromagnetic ultrasonic transducer for impinging ultrasonic waves upon a work piece which is made of an electrically-conductive material or for receiving ultrasonic waves. The transducer comprises a coil support, which is made of a material having good magnetic-conductivity and which has a comb-like structure having parallel running channels and fins as well as HF coils, which are electrically interconnected in series and which are wound onto at least some of the fins at a distance from the radiation side of the transducer formed by the faces of the fins and in such a manner that the adjacent coils have an alternating direction of wind. Furthermore, the ultrasonic transducer has means for generating a static magnetic field.
In using such a transducer, for example, for nondestructive testing of electrically conductive or ferromagnetic work pieces for internal and surface faults, the transducer is placed with its radiation side in the vicinity of the surface of the to-be-tested work piece. Applying a high-frequency current to the HF coils of the transducer induces eddy currents in the work piece. These eddy current lead, under the influence of the magnetic field, to propagation of the ultrasonic waves in the work piece. In the same manner, such a transducer can receive ultrasonic waves reflected at faults in the work piece and convert them into high-frequency current pulses. This can occur using the same HF coils or using a separate set of coils. Due to the noncontact generation of ultrasonic waves in the work piece, such transducers offer particular advantages in testing work pieces in motion.
STATE OF THE ART
In the prior art embodiments of electromagnetic ultrasonic transducers, there is a difference between transducers that generate polarized transverse waves, so-called SV waves (shear vertical waves), in the plane of incidence and transducers that generate horizontally polarized transverse waves, so-called shear horizontal waves (SH waves), in the work piece or receive such shear horizontal waves from the work piece. The type of generation differs in both cases essentially in the kind and direction of the magnetic field which acts on the eddy current induced by the HF coils.
For instance, DE 36 37 366 A1 discloses an electromagnetic ultrasonic transducer for generating vertically polarized transverse waves (SV waves) provided with a coil support having a comb-like structure with parallel running channels and fins on which a plurality of HF coils, which are interconnected in series, are wound in alternating wind direction. By maintaining a minimum distance between the coil windings and the fin faces forming the radiation side of the transducer, the HF coils are protected from damage due to coming into contact with the to-be-tested work piece. In order to generate the required magnetic field, a permanent magnet or an electromagnet is disposed over the comb-like coil support. This magnet supplies a homogeneous static magnetic field in the region of the surface of the work piece.
The transducer periodicity of such an electro-magnetic ultrasonic transducer is determined by the distance between the single fins of the comb structure and the type of winding of the HF coil. Half the transducer periodicity corresponds to the distance between adjacent fins. Sound radiation occurs perpendicular to the fins of the coil support.
DE 36 37 366 A1 also discloses a preferred embodiment of an ultrasonic transducer in which the generated magnetic field is directed parallel to the longitudinal side of the fin and parallel to the surface of the work piece in order to generate transverse waves (SH waves) polarized perpendicular to the plane of incidence. In such a case, generating the ultrasonic waves in the work piece occurs via a purely magnetostriction mechanism. In this case, the magnetic field is aligned parallel to the eddy currents induced in the work piece. The disadvantage of this method of generation is, however, that generation effectiveness and reception amplitude are strongly dependent on the mechanical and metallurgical properties of the surface of the work piece. Furthermore, this type of generation requires that the work piece be made of a ferromagnetic material.
In order to avoid these problems, transducers with a periodic permanent magnet arrangement, in which the magnetic field is essentially directed perpendicular to the surface of the work piece, are used for ultrasonic generation of horizontally polarized transverse waves (SH waves). Prior art transducers comprise two rows of permanent magnets, on the radiation side of which a rectangular coil is wound. Such an assembly, as is shown by way of example schematically in
FIG. 1
, generates particle excursions in electrically conductive work piece materials when HF currents are applied to the coils due to the Lorentz force acting on the eddy currents induced in the surface of the work piece as a result of the magnetic field. These particle excursions lead to the generation of SH waves.
A further improvement of this type of transducer for generating polarized transverse waves perpendicular to the plane of incidence is disclosed in EP 0 609 754 A2. In this embodiment, at least four rows of alternating permanent magnet segments are provided, with two adjacent rows staggered a quarter of the periodicity of the single permanent magnets of each row respectively along their longitudinal axis. Each adjacent permanent magnet arrangement is provided with its own high-frequency coil. These high-frequency coils can be impinged with a HF signal shifted 90° between the two high-frequency coils. In this manner, it is achieved that this ultrasonic transducer, both as transmitter transducer and receiver transducer, is provided with a one-sided direction characteristic with a single main radiation direction so that an improved signal
oise ratio is yielded in the nondestructive testing of the work piece.
However, when using such electromagnetic ultrasonic transducers, the sensitive coil wires lie between the permanent magnets and the surface of the work piece. As the coils must be placed very closely to the surface of the work piece in testing of the work piece in order to generate the ultrasonic amplitudes required for sufficient signal
oise ratio, there is greater risk of damaging and wearing the HF coils. Effective protection against wearing these coils is very difficult to realize in these transducers, because a protective covering would lead to undesired dampening of the amplitudes.
DE 42 23 470 discloses another electromagnetic ultrasonic transducer for generating high-frequency SH waves. This transducer comprises a toroidal strip core on which the windings of the HF coil are disposed at a distance from the faces, i.e. at a distance from the surface of the work piece, which reduces the risk of damaging or wearing the windings when using the transducer. This principle using toroidal strip cores is, however, not suited for setting up low-frequency SH wave transducers with large aperture widths, because only little generation effectiveness can be achieved.
Based on this state of the art, the object of the present invention is to provide an electromagnetic ultrasonic transducer for generating horizontally polarized transverse waves (SH waves), which are in particular also suited for the low-frequency SH wave range and permits testing work pieces made of electrically conducting materials with little risk of wearing for the HF coils.
DESCRIPTION OF THE INVENTION
The object of the present invention is solved using the electromagnetic ultrasonic transducer according to the claims. Advantageous embodiments of the electromagnetic ultrasonic transducer are the subject matter of the subclaims.
The present ultrasonic transducer comprises a coil support made of a material having good magnetic conductivity, i.e. a highly permeable material, which is provided with a comb-like structure having at least almost parallel running channels and fins as well as a plurality of HF coils which are electrically interconnected i
Breiner & Breiner L.L.C.
Chapman John E.
Fraunhofer-Gesellschaft zur Forderung der ange-wandten Forschung
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