Patent Description:
Ultrasonic waves can be used for inspecting a structure, such as a pipe, to identify defects and flaws within the structure. Examples of ultrasonic inspection devices can be found in <CIT>, <CIT> and <CIT>.

<CIT> describes apparatus for inspecting elongate members, such as pipes. The apparatus includes a ring of angularly-spaced transducers (or "exciters") clamped to the outside wall of a pipe. Each transducer includes a piezoelectric element, a metal block adhesively bonded to the piezoelectric element, and a thin faceplate shim secured to the face of the piezoelectric element to provide a wear plate.

<CIT> describes an improved ultrasonic transducer which includes a wear plate, a piezoelectric element arranged rearwards of the wear plate and a rigid block arranged rearwards of the piezoelectric element and which is configured to provide a backing mass for the piezoelectric element. The wear plate extends across the piezoelectric element and rearwards so as to provide a cap over the piezoelectric element and sides of at least a forward portion of the rigid block.

<CIT> describes an electromagnetic acoustic transducer <NUM> has one or a plurality of magnets <NUM> for applying a DC magnetic field to a material under test, and an electrical coil <NUM>, <NUM> supplied by an alternating current source for providing an AC magnetic flux within the material under test. A wear plate <NUM> engages with and slides along the surface of the material under test. The wear plate <NUM> is of electrically conductive ferromagnetic material and has slot apertures <NUM> therein. Thus, both the DC field and the AC flux can penetrate the material under test and create ultrasonic vibration of that material.

<CIT> describes a protective cover or grid for an ultrasound transducer comprises a series of vertically spaced members separated from one another by a predetermined distance, each member being of substantially uniform width and arranged in a cylindrical shape. The protection grid includes a cavity for receiving a cylindrical transducer. The protection grid operates as both a physical protection mechanism for protecting the cylindrical transducer housed therein as well as operating as an impedance matching device.

<CIT> describes an ultrasonic transducer includes a wear plate <NUM>, an active element <NUM> arranged rearwards of the wear plate and a rigid block <NUM> arranged rearwards of the active element and configured to provide a non-resonant backing mass for the active element. The wear plate extends across the active element and rearwards so as to provide a cap over the active element and sides <NUM> of at least a forward portion <NUM> of the rigid block. A heat extracting strip <NUM> may be provided between the plate <NUM> and other components. The arrangement increases the efficiency and operating temperature range of the transducer.

<CIT> describes an ultrasonic nondestructuve testing system and a search unit are disclosed herein. The search unit includes a transducer crystal for transmitting and receiving ultrasonic energy together with an acoustical backing structure which is disposed on the backside of the transducer crystal for dampening spurious ultrasonic energy radiated from its back-side. The backing structure includes compacted or pressed fibers of a dense, non-resonant material.

According to a first aspect of the present invention there is provided an ultrasonic transducer. The transducer includes a wear cap in contact with a piezoelectric element (such as a piezoelectric element). The wear cap includes at least two slots arranged to define a strip and the strip is configured to be in vibrational communication with the piezoelectric element. The strip runs in a direction within <NUM>° of the axis of polarisation of the piezoelectric element.

Thus, the wear cap can be displaced further for a given force and so result in a greater excitation for a given input signal and in a greater output signal for a given received.

The wear cap may include a single 'U'-shaped slot such that the strip is cantilevered.

An outward facing surface of the strip may be flat between the slots. Alternatively, a portion of an outward facing surface of the strip may project outwardly. For example, the outward facing surface of the strip may project outwardly to provide a blunt ridge, or to provide a knife edge or sharp ridge.

The strip may run parallel or substantially parallel to the axis of polarisation of the active element.

The ultrasonic transducer may further comprise a rigid block, wherein the piezoelectric element is interposed between the wear cap and the rigid block, and the rigid block is configured to provide a backing mass for the piezoelectric element.

The rigid block has a top face, a bottom face and side faces. The rigid block may have chamfered edges between the side faces. The rigid block may have chamfered edges between each respective side face and the bottom face. The width of chamfering may vary along the edge(s). For example, the chamfering may taper. The chamfering may become narrower along the edge from one end to the other.

Referring to <FIG>, an ultrasonic transducer assembly <NUM> (herein simply referred to as an "ultrasonic transducer") is shown.

The ultrasonic transducer <NUM> includes a wear cap <NUM> (which may also be referred to as a "wear plate", "face plate" or "contact head") having a front surface <NUM> (or "outward facing surface") which, in use, is pressed into contact with an object or structure (not shown) under inspection, and a rear surface <NUM> (<FIG>). The wear cap <NUM> is made from a ceramic, such as zirconium dioxide (ZiO<NUM>) or aluminium oxide (Al<NUM>O<NUM>), or other suitable material.

The front surface <NUM> of the wear cap <NUM> generally has the shape of a frusto-triangular prism having a centre line <NUM> and comprising a central, flat surface portion <NUM> running between first and second ends <NUM>, <NUM> and having first and second sides <NUM>, <NUM>, and first and second side, sloping surface portions <NUM>, <NUM>.

The wear cap <NUM> includes first and second notches <NUM>, <NUM> (or "slots") running along the first and second sides <NUM>, <NUM> of the flat surface <NUM> which stop short of the ends <NUM>, <NUM>. The slots <NUM>, <NUM> can be formed by a suitable machining process, such as laser machining. The slots <NUM>, <NUM> pass through the wear cap <NUM>, i.e. between the front and rear surfaces <NUM>, <NUM>. The slots <NUM>, <NUM> define a strip <NUM> (shown shaded to aid clarity) which is suspended between the ends <NUM>, <NUM> and is afforded greater freedom to move, i.e. the slots <NUM>, <NUM> increases the flexibility (or "compliance") of the portion of the wear cap <NUM> through which excitations pass. Expressed differently, the wear cap <NUM> can be displaced further for a given applied force. The strip <NUM> generally runs in the same direction as the polarity of an active layer.

The ultrasonic transducer <NUM> includes a transducer stack <NUM> which comprises an earth electrode <NUM> which is closest to the wear cap <NUM>, an active layer <NUM> in the form of a shear polarized piezoelectric layer and a signal electrode <NUM>. The active layer <NUM> is sandwiched between the electrodes <NUM>, <NUM>. A second transducer stack may be included, as described, for example, in <CIT> ibid. which is incorporated herein by reference.

An electrically-insulating layer <NUM> separates the transducer stack <NUM> from a rigid block <NUM> which provides a non-resonant backing mass. The rigid block <NUM> is preferably made of steel or other dense material so as to provide a high mass. The material may be chosen so as to have a low coefficient of thermal expansion or one which is matched to the wear plate <NUM>.

Referring also to <FIG>, the rigid block <NUM> (herein referred to a "first rigid block" or "rigid block with uniform chamfers") includes a recessed edge <NUM> around the top <NUM> of the block <NUM> thereby forming a mesa <NUM>. The rigid block <NUM> includes chamfered side edges <NUM>, i.e. chamfered edges between adjacent side faces <NUM>. The block <NUM> includes chamfered bottom edges <NUM>, i.e. chamfered edges between an adjacent side face <NUM> and the bottom face <NUM>. As will be explained in more detail later, this can help to alter the resonant frequency of the backing mass <NUM>.

Referring also to <FIG>, another rigid block <NUM>' (herein referred to a "second rigid block" or "rigid block with tapered chamfers") is shown. The second rigid block <NUM>' can be used instead of the first rigid block <NUM> shown in <FIG>.

The second rigid block <NUM>' is similar to the first rigid block <NUM> except that the widths of the chamfered side edges <NUM>' and/or the chamfered bottom edges <NUM>' vary between the top <NUM>' and bottom <NUM>' of the block <NUM>'. In particular, the width of the side chamfer becomes smaller towards the bottom <NUM>' of the block <NUM>'.

A miniature coaxial cable <NUM> provides an electrical connection to the signal electrode <NUM>. The cable <NUM> sits in a semi-circular recess <NUM> running down the middle of one of the sides <NUM> of the rigid block <NUM>.

The wear cap <NUM> is shaped to provide a space or recess <NUM> in which the transducer stack <NUM>, the insulating plate <NUM> and a top part of the rigid block <NUM> sit. The wear cap <NUM> may be machined or moulded.

The wear cap <NUM> may have a thickness, t, of material equal to or greater than <NUM> or equal to or greater than <NUM>. As shown in <FIG>, the thickness, t, is behind the point (or region) which comes into contact with the structure under test (not shown). In this example, the thickness t corresponds to the thickness in the middle of the wear cap <NUM>.

In the example hereinbefore described, the wear cap <NUM> has a flat profile across the width and length of the strip <NUM> (i.e. along the x-axis and y-axis) and the strip <NUM> has a uniform thickness across the width and length. However, wear cap <NUM> can have other profiles.

Referring also to <FIG>, a wear cap <NUM>' is shown having a reduced area-of-contact profile, for example, the strip <NUM>' has the shape of a frusto-triangular prism. For example, if the width w between the slots <NUM>', <NUM>' is w, then the width of a contact portion of the wear cap <NUM>' has a width o. w, where α < <NUM>, for example, <NUM>. A wear cap having a knife edge (not shown), such as that described in <CIT> ibid. can be used. In the reduced area of contact, the wear cap <NUM> can be made thicker and so help to reduce heat transfer from a hot structure under test and the active element.

Referring in particular to <FIG>, the rest of the space <NUM> (<FIG>) is filled with a flexible filler (not shown), for example an epoxy adhesive or potting compound. Referring still to <FIG>, the transducer <NUM> can be used in an apparatus or method for inspecting pipes as described in <CIT> and <CIT> which are incorporated herein by reference.

The effect of the notches can be simulated using finite-element static analysis.

Referring to <FIG>, wear caps <NUM>, <NUM> with and without notches <NUM>, <NUM> are simulated for applying a given force, in this case 1N, to the rear surface of a cap over an area that is normally in contact with the active element. A lower surface of the cap (i.e. a rim) is fixed to represent the cap being in fixed contact with the backing mass.

<FIG> shows a computer simulation of displacement of a wear cap <NUM> not having notches.

<FIG> shows a computer simulation of a displacement of a wear cap <NUM> with first and second notches <NUM>, <NUM>.

A central part of the wear cap (e.g. central part <NUM>) having notches can produce larger displacements. For the same applied force acting on the rear surface of a wear cap, a wear cap having notches can be displaced up to four times that of the same wear cap without notches. Consequently, using notches as herein described can help to improve signal-to-noise of received signals.

A transducer operates at frequencies below the first body resonance frequency. This usually depends on the longest dimension of the backing mass, namely diagonal distance between opposite edges. By introducing chamfering, this distance is reduced, thereby resulting in an increase in body resonance frequencies without significantly reducing the weight of the backing mass. To excite a resonance (strongly), the excitation by an active element matches the mode shape of the resonance to some extent.

Varying chamfering, for example as show in <FIG>, so as to produce a backing mass having a non-symmetric shape can help to make it harder to excite a resonant mode due to mismatching of mode shape.

Thus, if the transducer is used close to resonance frequencies, it becomes less likely that these resonances will be excited, if at all.

It will be appreciated that various modifications may be made to the embodiments hereinbefore described. Such modifications may involve equivalent and other features which are already known in the design, manufacture and use of ultrasonic transducers and component parts thereof and which may be used instead of or in addition to features already described herein. Features of one embodiment may be replaced or supplemented by features of another embodiment.

The slots need not be straight or even follow the same path. For example, each slot may be irregularly shaped.

There may be one slot having a 'U'-shape such that it defines a cantilevered slot respectively. There may be one slot having a 'H'-shape such that it defines two cantilevered slots.

There may be three or more slots, for example, defining two or more adjacent strips.

The slots may be filled with a filler (or "encapsulant") for example an adhesive (such as epoxy) or potting compound.

The wear plate may be coated with a protective film or layer, or have a cover formed from a plastics material.

The transducer may be operable outside ultrasonic frequencies. For example, the transducer may operate below <NUM>, for instance as low as <NUM> to <NUM>. The transducer may operate up to <NUM> or more.

The rigid block may be omitted. Instead, the transducer may be held by a metallic band (not shown) which urges the transducer against the object or structure (not shown) being inspected.

Claim 1:
An ultrasonic transducer (<NUM>) including:
a wear cap (<NUM>) characterized in that the wear cap (<NUM>) is in contact with a piezoelectric element;
wherein the wear cap includes at least two slots (<NUM>, <NUM>) arranged to define a strip (<NUM>) and the strip is arranged to be in vibrational communication with the piezoelectric element; and
wherein the strip runs in a direction within <NUM>° of the axis of polarisation of the piezoelectric element.