Patent Description:
Sensors incorporating a piezoelectric element can require a substrate to support the piezoelectric element and an adhesive to attach the piezoelectric element to the substrate. To construct the sensor, the piezoelectric element is positioned on the substrate and the adhesive is often deposited over an entirety of the piezoelectric element, surrounding the piezoelectric element on all sides.

The adhesive is prone to shrinking when cured and, due to the amount of the adhesive attaching the piezoelectric element to the substrate and the position of the adhesive surrounding the piezoelectric element, shrinkage of the adhesive bends the piezoelectric element around a bending point of the substrate. The bending stresses the piezoelectric element and, in subsequent thermal cycling during use, leads to the propagation of cracks and failure of the piezoelectric element, resulting in unreliable sensor performance and limited longevity.

<CIT> discloses a vibration limit switch system with a piezoelectric transmitting unit which is in operable connection with a membrane that can be put in oscillation. The piezoelectric transmitting unit is adhered to the membrane.

<CIT> relates to an assembly for sensing and/or monitoring the level of a material in a vessel. The assembly includes a level sensor. In one embodiment, a piezoelectric transducer is secured inside a hollow tube by bonding, for which an adhesive is used.

<CIT> shows a device for detecting the level of a fluid inside a vessel. The device comprises a piezoelectric transducer, a metal housing, an adhesive and a plastic housing. The piezoelectric transducer is connected to the housing via the adhesive, so that mechanical signals can be transmitted between the transducer and the housing.

<CIT> relates to a monitoring arrangement of construction components in mechanical systems. The arrangement comprises a transducer that is connected to a construction component via a couplant.

A sensor includes a substrate having a curved surface, a piezoelectric element, and an adhesive disposed between the piezoelectric element and the curved surface along a vertical direction. The adhesive attaches the piezoelectric element to the substrate. The adhesive has an exterior bond surface that has a tapered shape along the vertical direction from the piezoelectric element to the curved surface. The adhesive has a planar side and a curved side opposite the planar side, the planar side abuts an inner surface of the piezoelectric element and the curved side abuts the curved surface, wherein the exterior bond surface extends between the planar side and the curved side and defines a lateral extent of the adhesive in a width direction extending perpendicular to the vertical direction. The lateral extent of the adhesive is greater at the planar side attached to the piezoelectric element than at the curved side attached to the curved surface.

The invention will now be described by way of example with reference to the accompanying Figures, of which:.

Exemplary embodiments of the present disclosure will be described hereinafter in detail with reference to the attached drawings, wherein like reference numerals refer to like elements. The present disclosure may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, these embodiments are provided so that the present disclosure will convey the concept of the disclosure to those skilled in the art. In addition, in the following detailed description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the disclosed embodiments, the invention being defined by the appended claims.

Throughout the specification, directional descriptors are used such as "width", "vertical", and "longitudinal". These descriptors are merely for clarity of the description and for differentiation of the various directions. These directional descriptors do not imply or require any particular orientation of the disclosed elements.

A sensor <NUM> according to an embodiment is shown in <FIG>. The sensor <NUM> includes a piezoelectric element <NUM>, a substrate <NUM>, and an adhesive <NUM> attaching the piezoelectric element <NUM> to the substrate <NUM>.

The piezoelectric element <NUM>, as shown in the embodiment of <FIG>, has an outer surface <NUM> and an inner surface <NUM> opposite the outer surface <NUM> in a vertical direction V. A plurality of side surfaces <NUM> extend between the inner surface <NUM> and the outer surface <NUM> along the vertical direction V and define a perimeter <NUM> of the piezoelectric element <NUM> in a width direction W perpendicular to the vertical direction V. The piezoelectric element <NUM> has a piezoelectric width <NUM> in the width direction W. In the embodiment shown in <FIG>, the piezoelectric element <NUM> is a planar element extending in a plane normal to the vertical direction V.

The piezoelectric element <NUM> may be any type of element that vibrates when an extemal voltage is applied to the element. The piezoelectric element <NUM>, in various embodiments, may be a film, a composite, a ceramic, or a crystal. In an embodiment, the piezoelectric element <NUM> is a brittle piezoelectric element, such as a hard ceramic piezoelectric or a single crystal piezoelectric.

As shown in <FIG>, the substrate <NUM> has a curved surface <NUM> facing the piezoelectric element <NUM> along the vertical direction V. In an embodiment, the substrate <NUM> has a cylindrical shape <NUM>, such as a cylinder or a tube, extending perpendicular to the vertical direction V and the width direction W and having a circular cross-section forming the curved surface <NUM>. In other embodiments, the substrate <NUM> could have shapes other than the cylindrical shape <NUM> and cross-sections other than the circular cross-section shown in <FIG>, provided that the substrate <NUM> has the curved surface <NUM> facing the piezoelectric element <NUM> as described herein. In an embodiment, the substrate <NUM> is formed of a metal material. In other embodiments, the substrate <NUM> can alternatively be formed of a plastic material.

The cylindrical shape <NUM> of the substrate <NUM> in the embodiment shown in <FIG> has an inner diameter <NUM>. In an embodiment, a ratio of the piezoelectric width <NUM> to the inner diameter <NUM> is greater than or equal to <NUM> and less than or equal to <NUM>. In another embodiment, the ratio of the piezoelectric width <NUM> to the inner diameter <NUM> is greater than or equal to <NUM> and less than or equal to <NUM>.

The adhesive <NUM> is disposed between the piezoelectric element <NUM> and the substrate <NUM> along the vertical direction V. In an embodiment, the adhesive <NUM> is a thermoset adhesive that cures at a temperature greater than room temperature. In other embodiments, the adhesive <NUM> may be an epoxy, a polyurethane, an acrylic, or any other type of adhesive material, and may be cured by the application of heat greater than room temperature, may be cured at room temperature, may be cured by mixing with a chemical catalyst, or may be cured by the application of radiation, such as ultraviolet light. As used throughout the present specification, the term "room temperature" is intended to be a range of greater than or equal to <NUM> and less than or equal to <NUM>.

The adhesive <NUM> is shown in a cured state C in <FIG>, in which the adhesive <NUM> is adhered to both the piezoelectric element <NUM> and the substrate <NUM> and attaches the piezoelectric element <NUM> to the curved surface <NUM> of the substrate <NUM>. As shown in <FIG>, in the cured state C, the adhesive <NUM> has a planar side <NUM> and a curved side <NUM> opposite the planar side <NUM> in the vertical direction V. The planar side <NUM> corresponds to a shape of the inner surface <NUM> of the piezoelectric element <NUM> and, in the shown embodiment, extends fully across the inner surface <NUM> in the width direction W. The planar side <NUM> abuts the inner surface <NUM> and is adhered to the inner surface <NUM> in the cured state C. The curved side <NUM> corresponds to a shape of the curved surface <NUM> of the substrate <NUM> and extends across a portion of the curved surface <NUM>. The curved side <NUM> abuts the curved surface <NUM> and is adhered to the curved surface <NUM> in the cured state C.

As shown in <FIG>, the adhesive <NUM> has an exterior bond surface <NUM> extending between the planar side <NUM> and the curved side <NUM>. In the cured state C, the exterior bond surface <NUM> has a tapered shape <NUM> along the vertical direction V from the piezoelectric element <NUM> to the curved surface <NUM>. The tapered shape <NUM> is curved inward or concavely in the cured state C in the embodiment shown in <FIG>. In other embodiments, in the cured state C, the tapered shape <NUM> may have an outward or convex curve, may extend in a linear manner, or may extend in an irregular manner from the piezoelectric element <NUM> to the curved surface <NUM>.

The exterior bond surface <NUM> defines a lateral extent <NUM> of the adhesive <NUM> in the width direction W, as shown in <FIG>. Due to the tapered shape <NUM> of the exterior bond surface <NUM>, the lateral extent <NUM> of the adhesive <NUM> is greater at the planar side <NUM> attached to the piezoelectric element <NUM> than at the curved side <NUM> attached to the curved surface <NUM>. <FIG> shows one exemplary lateral extent <NUM> approximately halfway along the exterior bond surface <NUM>, but the lateral extent <NUM> follows the tapered shape <NUM> and is a different quantity in the width direction W at different positions along the exterior bond surface <NUM>.

<FIG> shows a vertical projection <NUM> of the perimeter <NUM> of the piezoelectric element <NUM> in the vertical direction V toward the curved surface <NUM>. In the cured state C, the lateral extent <NUM> of the adhesive <NUM> is disposed within the vertical projection <NUM> of the perimeter <NUM>. The planar side <NUM> of the adhesive <NUM> is disposed along either an entirety of the inner surface <NUM> or less than an entirety of the inner surface <NUM> in the width direction W, and the lateral extent <NUM> of the adhesive <NUM> at the planar side <NUM> does not extend beyond the vertical projection <NUM> of the perimeter <NUM>. The curved side <NUM> has a narrower lateral extent <NUM> in the width direction W than the planar side <NUM>, due to the tapered shape <NUM>, and is also within the vertical projection <NUM>.

In the embodiment shown in <FIG>, the adhesive <NUM> is a matching layer <NUM> that matches a mechanical impedance between the piezoelectric element <NUM> and the substrate <NUM>. To act as the matching layer <NUM>, in some embodiments, the mechanical impedance of the adhesive <NUM> either equals the mechanical impedances of the piezoelectric element <NUM> and the substrate <NUM> or transitions between the mechanical impedances of the piezoelectric element <NUM> and the substrate <NUM>, for example having a mechanical impedance that is approximately an average of the mechanical impedances of the piezoelectric <NUM> and the substrate <NUM>. In other embodiments, in which the impedance of the adhesive <NUM> is not physically able to be between the impedance of the piezoelectric element <NUM> and the impedance of the substrate <NUM>, the impedance of the matching layer <NUM> is selected to optimize signal efficiency; in this embodiment, the impedance of the matching layer <NUM> may be below the impedances of both the piezoelectric element <NUM> and the substrate <NUM>.

An exemplary method not forming part of the present invention shown in <FIG> of constructing the sensor <NUM> shown in <FIG> will now be described in greater detail.

In a first step shown in <FIG>, the piezoelectric element <NUM> is positioned on the outer surface <NUM> with the inner surface <NUM> exposed and facing up in the vertical direction V. The adhesive <NUM> is deposited in an uncured state U on the inner surface <NUM>. A predetermined volume <NUM> of the adhesive <NUM> in the uncured state U is deposited in this step. The predetermined volume <NUM> of the adhesive <NUM> is liquid or semi-solid in the uncured state U; the adhesive <NUM> is capable of changing shape in the uncured state U under the external forces described herein.

With the adhesive <NUM> in the uncured state U on the inner surface <NUM> of the piezoelectric element <NUM>, the substrate <NUM> is moved in the vertical direction V toward the inner surface <NUM> of the piezoelectric element <NUM> and into contact with the adhesive <NUM>. As shown in <FIG>, the curved surface <NUM> of the substrate <NUM> contacts the adhesive <NUM> and spreads the adhesive <NUM> in the uncured state U between the inner surface <NUM> and the curved surface <NUM>.

Due to the predetermined volume <NUM> of the adhesive <NUM>, the adhesive <NUM> spreads along the inner surface <NUM> of the piezoelectric element <NUM> and covers the inner surface <NUM>, and spreads along and covers a portion of the curved surface <NUM> of the substrate <NUM>, as shown in <FIG>. The predetermined volume <NUM> limits the spread of the adhesive <NUM> and prevents the adhesive <NUM> from extending or flowing beyond the perimeter <NUM> at the side surfaces <NUM> of the piezoelectric element <NUM> in the width direction W. The predetermined volume <NUM> also creates the tapered shape <NUM> of the exterior bond surface <NUM> in the uncured state U, as shown in <FIG>.

With the substrate <NUM> positioned with respect to the piezoelectric element <NUM> as shown in <FIG>, and the adhesive <NUM> in the uncured state U between the piezoelectric element <NUM> and the substrate <NUM>, the adhesive <NUM> is cured to the cured state C to attach the piezoelectric element <NUM> to the substrate <NUM> and form the sensor <NUM> shown in <FIG> and <FIG>. As described above, depending on the type of the adhesive <NUM>, the adhesive <NUM> can be cured by the application of a temperature greater than room temperature, or can be cured at other temperatures, by mixing with a chemical catalyst, or by the application of radiation.

During curing, the adhesive <NUM> may shrink from the predetermined volume <NUM> in the cured state C shown in <FIG> and <FIG>, forming the tapered shape <NUM> of the exterior bond surface <NUM> that has the lateral extent <NUM> within the vertical projection <NUM>. The tapered shape <NUM> of the exterior bond surface <NUM> may also have the lateral extent <NUM> within the vertical projection <NUM> prior to curing in the uncured state U.

The adhesive <NUM> formed with the tapered shape <NUM> of the exterior bond surface <NUM>, which is disposed within the vertical projection <NUM>, limits or prevents damage to the piezoelectric element <NUM> during curing to the cured state C. Because the adhesive <NUM> has the tapered shape <NUM> and does not extend beyond the vertical projection <NUM>, if the adhesive <NUM> shrinks during curing, only minimal bending stress is applied to the piezoelectric element <NUM> on the side surfaces <NUM> toward the substrate <NUM>. After curing and during use of the sensor <NUM>, thermal cycles also impose bending stresses on the piezoelectric element <NUM> due to the difference in thermal expansion between the adhesive <NUM> and the piezoelectric element <NUM>. The arrangement of the adhesive <NUM> applies minimal bending stress especially on the portions of the inner surface <NUM> of the piezoelectric element <NUM> adjacent to the side surfaces <NUM> during thermal cycling, minimizing cyclic fatigue. The limiting of bending stress due to the adhesive <NUM> arrangement prevents cracking or other bending damage to the piezoelectric element <NUM>, ensuring greater reliability of the sensor <NUM>.

A method of constructing the sensor <NUM> according to an embodiment of the present invention is shown in <FIG>. Like reference numbers refer to like elements, and only the differences from the embodiment shown in <FIG> will be described in detail.

In the embodiment of the method shown in <FIG>, a plurality of wedges <NUM> are used to create the tapered shape <NUM> of the exterior bond surface <NUM>. The wedges <NUM> are each formed of a resiliently compressible material, such as a foam or a soft potting.

In the embodiment shown in <FIG>, with the adhesive <NUM> in the uncured state U pressed by the substrate <NUM> and spread between the inner surface <NUM> of the piezoelectric element <NUM> and the curved surface <NUM> of the substrate <NUM>, similarly to the state of the sensor <NUM> in the other embodiment shown in <FIG>, the wedges <NUM> are moved along the width direction W and positioned between the piezoelectric element <NUM> and the curved surface <NUM> in contact with the exterior bond surface <NUM>. The wedges <NUM> each have a wedge surface <NUM> that corresponds to the tapered shape <NUM> of the exterior bond surface <NUM>. The wedges <NUM> form the tapered shape <NUM> by constraining the adhesive <NUM> with the wedge surfaces <NUM> as the adhesive <NUM> is cured from the uncured state U into the cured state C.

When the adhesive <NUM> has reached the cured state C, in an embodiment, the wedges <NUM> are removed and the sensor <NUM> is formed as shown in <FIG> and <FIG>. In another embodiment, the wedges <NUM> may remain with the sensor <NUM> as shown in <FIG>. If the wedges <NUM> remain with the sensor <NUM>, the wedges <NUM> do not transfer a load between the piezoelectric element <NUM> and the substrate <NUM> due to a compressibility of the material of the wedges <NUM> or due to a non-stick coating, such as Teflon, that is applied to the wedges <NUM> and prevents adherence to the piezoelectric element <NUM> and the substrate <NUM>.

The wedge surfaces <NUM>, in the embodiment shown in <FIG>, each have a convex shape forming a concave tapered shape <NUM> of the exterior bond surface <NUM>. In other embodiments, the wedge surfaces <NUM> could have a concave shape to form a convex tapered shape <NUM>, could have a linear shape to form a linear tapered shape <NUM>, or could have an irregular shape to form an irregular tapered shape <NUM>.

As shown in <FIG>, the sensor <NUM> according to the embodiments described above can be integrated into a sensor assembly <NUM>. The sensor assembly <NUM> includes a vessel <NUM> and the sensor <NUM> disposed in the vessel <NUM>. The sensor assembly <NUM> shown in <FIG> is only an exemplary application of the sensor <NUM>. <FIG> shows a section of the sensor assembly <NUM> taken along a plane in the vertical direction V and a longitudinal direction L perpendicular to the vertical direction V and the width direction W.

The vessel <NUM>, as shown in <FIG>, has a receiving space <NUM>. The sensor <NUM> formed as shown in <FIG> is disposed in the receiving space <NUM> of the vessel <NUM>, as shown in <FIG>. In the shown embodiment, the vessel <NUM> is a hollow cylindrical member and the receiving space <NUM> has a circular cross-section. In other embodiments, the vessel <NUM> may be any other type of member and the receiving space <NUM> may have a cross-section of any shape.

As shown in <FIG>, the sensor assembly <NUM> contains a fluid <NUM> that is disposed in an interior space <NUM> of the substrate <NUM> of the sensor <NUM>. The fluid <NUM> has a level <NUM> within the interior space <NUM> along the longitudinal direction L. The receiving space <NUM> between the substrate <NUM> and the vessel <NUM> is sealed to an outside environment and is not exposed to any fluid.

In an exemplary application, the sensor <NUM> is used to sense the level <NUM> of the fluid <NUM> in the interior space <NUM>. An external voltage is applied to the piezoelectric element <NUM> of the sensor <NUM>. The piezoelectric element <NUM> vibrates under application of the external voltage, producing ultrasonic wave echoes that pass through the substrate <NUM> and into the interior space <NUM>. The ultrasonic wave echoes are detected and processed to determine the level <NUM> of the fluid <NUM> in the interior space <NUM>. When the level <NUM> of the fluid <NUM> has reached the piezoelectric element <NUM> in the interior space <NUM> along the longitudinal direction L, the ultrasonic wave echoes reverberate and are detected for a first ringdown period. When the level <NUM> of the fluid <NUM> in the interior space <NUM> has not reached the piezoelectric element <NUM>, the ultrasonic wave echoes do not reverberate as efficiently, and the wave echoes are detected for a second ringdown period that is shorter than the first ringdown period.

Claim 1:
A sensor (<NUM>), comprising:
a substrate (<NUM>) having a curved surface (<NUM>);
a piezoelectric element (<NUM>); and
an adhesive (<NUM>) disposed between the piezoelectric element (<NUM>) and the curved surface (<NUM>) along a vertical direction (V) and attaching the piezoelectric element (<NUM>) to the substrate (<NUM>), the adhesive (<NUM>) has an exterior bond surface (<NUM>) that has a tapered shape (<NUM>) along the vertical direction (V) from the piezoelectric element (<NUM>) to the curved surface (<NUM>), wherein the adhesive (<NUM>) has a planar side (<NUM>) and a curved side (<NUM>) opposite the planar side (<NUM>), the planar side (<NUM>) abuts an inner surface (<NUM>) of the piezoelectric element (<NUM>) and the curved side (<NUM>) abuts the curved surface (<NUM>), and wherein the exterior bond surface (<NUM>) extends between the planar side (<NUM>) and the curved side (<NUM>) and defines a lateral extent (<NUM>) of the adhesive (<NUM>) in a width direction (W) extending perpendicular to the vertical direction (V), characterized in that
the lateral extent (<NUM>) of the adhesive (<NUM>) is greater at the planar side (<NUM>) attached to the piezoelectric element (<NUM>) than at the curved side (<NUM>) attached to the curved surface (<NUM>).