Patent Description:
Sensors are of growing importance and become more and more ubiquitous in every-day life. Microelectromechanical systems (MEMS) are an attractive option to answer the demand for increased performance of sensors along with decreased sizes and costs. Surface acoustic wave (SAW) sensors, and to a lower extent bulk acoustic wave (BAW) sensors or Lamb wave or Love wave or shear-plate mode acoustic sensors, offer particularly advantageous options due to a wide variety of measurable ambient parameters including temperature, pressure, strain and torque for example.

Acoustic wave sensors utilize the piezoelectric effect to transduce an electrical signal into a mechanical / acoustic wave. SAW-based sensors are built on single-crystal piezoelectric materials like quartz (SiO2), lithium niobate (LiNbO3), lithium tantalate (LiTaO3), langasite (LGS) and aluminum nitride (AIN) or zinc oxide (ZnO) deposited on silicon. An inter-digitated transducer (IDT) converts the electrical energy of an incident electrical signal into acoustic wave energy. The acoustic wave travels across the surface (or bulk) of a device substrate via the so-called delay line to another IDT that converts the acoustic wave back to an electrical signal that can be detected. In some devices, mechanical absorbers and/or reflectors are provided in order to prevent interference patterns and reduce insertion loss. In some devices, the other (output) IDT is replaced by a reflector that reflects the generated acoustic wave back to the (input) IDT that can be coupled to an antenna for remote interrogation of the sensor device.

Examples of acoustic sensors are shown in <CIT>, <CIT>, <CIT> or <NPL>. A particular class of acoustic sensors comprises resonators exhibiting resonator frequencies that vary according to varying ambient conditions. A conventional surface wave resonator, for example, comprises an electroacoustic transducer with inter-digitated combs arranged between Bragg mirrors. At the resonance frequency, the condition of synchronism between the reflectors is satisfied making it possible to obtain a coherent addition of the different reflections which occur under the reflectors. A maximum of acoustic energy is then observed within the resonant cavity and, from an electrical point of view, a maximum of amplitude of the current admitted by the transducer is observed.

Differential acoustic wave sensors comprise two or more resonators exhibiting different resonance frequencies wherein differences in the measured frequencies reflect variations in the parameter to be measured as, for example, strain.

The differential sensor must be capable to segregate the origin of the perturbation and to reduce or suppress contributions from other parameters, like e.g. vibrations or temperature. This requires the development of a differential sensor for which temperature and vibration sensitivity must be as small as possible or rigorously equal from one resonator another to allow for rejection by signal substraction.

<FIG> shows such a surface acoustic wave differential sensor according to the state of the art. This sensor is configured to measure stress, e.g. on a rotating object. The surface acoustic wave differential sensor <NUM> comprises two surface acoustic wave resonators <NUM>, <NUM> provided on a piezoelectric substrate <NUM>. Each surface acoustic wave resonator <NUM>, <NUM> comprises an inter-digitated transducer structure <NUM>, <NUM> and a pair of reflecting structures <NUM>, <NUM>, <NUM>, <NUM>. The reflecting structures <NUM>, <NUM> are arranged on each side of the inter-digitated transducer structure <NUM> and the reflecting structures <NUM> and <NUM> on each side of the interdigitated transducer structure <NUM>, in both cases with respect to the direction of propagation of the acoustic wave, see arrows <NUM>, <NUM>, of the corresponding transducer structure <NUM>, <NUM>. The two resonators <NUM> and <NUM> are electrically connected to each other in a differential way by two conductive lines <NUM> and <NUM>.

Both resonators <NUM>, <NUM> are positioned on the piezoelectric substrate <NUM> with an angle Ψ of ±<NUM>° in regards with the crystallographic axis X of a singly rotated Quartz substrate <NUM>, corresponding to the usual propagation direction of a Rayleigh wave. Thus, the two resonators are perpendicular to each other.

Each resonator <NUM>, <NUM> exhibits a resonance peak at a frequency f1, f2 respectively.

The resonators <NUM>, <NUM> are connected in parallel and then connected to an antenna to be wirelessly interrogated, the differential measure resulting for the difference of the resonance frequencies measured either simultaneously or sequentially.

By aligning one resonator <NUM>, <NUM> in parallel with the radial direction of a rotating object, the differential sensor <NUM> is sensitive to radial stress occurring on the object. On the occurrence of radial stress, deformations occur in the sensor leading to extension in the one resonator and contraction in the other. This leads to changes with opposite signs and typically the same absolute value, in the resonant frequencies. Thus, the difference in the resonant frequencies changes by the sum of the two absolute values. By measuring the variation of the difference Δf between the two resonant frequencies, one can determine the applied force, as the difference Δf is linearly proportional to the torque M. Unwanted temperature variation effects, do, however, cancel out, as they will affect both resonators in the same way.

However, in the differential sensor <NUM> according to the state of the art, the stress state is not measured at the same location by the two resonators <NUM>, <NUM>, nor the temperature. Consequently, the measurement might be negatively affected by inhomogeneities in the material of the object, leading to errors in the stress determination.

The object of the invention is therefore to overcome the drawback cited above resulting in an improved sensing device.

The object of the invention is achieved by a resonator device according to claim <NUM>. Thus, the two resonators of the device measure at the same location and the measurement is therefore less influenced by inhomogeneities in the material on which the resonators are attached. This is in contrary to the state of the art device, where each resonator measures at a different location.

According to the invention, the each of the at least two parts of the at least two resonators comprises at least one reflecting structure and a part of the inter-digitated transducer structure of the corresponding resonator. The device as described enables to manage parasites due to directivity effects.

According to a variant of the invention, the inter-digitated transducer structure of the resonator can comprise inter-digitated comb electrodes, and wherein for at least one transducer structure of the at least two resonators, said inter-digitated comb electrodes are defined by the Bragg condition given by p = λ/<NUM>, λ being the operating acoustic wavelength of said transducer structure and p being the electrode pitch of said transducer structure. The device as described enables to manage parasites due to directivity effects.

According to a variant, the two different wave propagation directions of the at least two resonators can form an angle Θ with each other, Θ being equal to ± <NUM>° or smaller.

According to a variant of the invention, the electrodes of the inter-digitated transducer structure of the one resonator can be electrically connected with the electrodes of the transducer structure of the other resonator in a differential way. The connection between the electrodes of the at least two resonators can be either in parallel, or in series, depending on their operating conditions. Thus, the device according to the invention can operate either on resonance or anti-resonance, depending on the design choices, in contrast to the state of the art device.

According to a variant of the invention, at least one of the resonators can be arranged and positioned such that its wave propagation direction is parallel to one of the crystalline axis of the piezoelectric substrate.

According to a variant of the invention, at least one of the resonators can be arranged and positioned such that its wave propagation direction makes an angle Ψ to one of the crystalline axis of the piezoelectric substrate, in particular an angle Ψ equal to ±<NUM>°.

According to a variant of the invention, at least a part of the surface of the cavity can be metalized. The device as described enables to filter or select the possible modes of the structure, and even allows for operating in a coupled mode configuration.

According to a variant of the invention, the metalization of the cavity can comprise at least one or more grating. When more than one grating is present, the gratings are superimposed to each other. The device as described enables to filter or select the possible modes of the structure, and even allows for operating in a coupled mode configuration.

According to a variant of the invention, each one of the reflecting structures of the resonators can comprise one or more metallic strips, said metallic strips being connected to each other or connected to ground. Thus, the resonators can also be tag devices. Furthermore, the connection of the metallic strips to each other or to ground results in an improvement of the reflection coefficient of the reflecting structures at the Bragg condition. At the Bragg condition, the reflected waves due to electrical and mechanical loading are in phase so that an improved reflection coefficient of the reflector at the Bragg condition results in a better detection of the reflected waves by the corresponding transducer structure.

According to a variant of the invention, the resonator can be a surface acoustic wave resonator (SAW), a bulk acoustic wave resonator (BAW), a Lamb wave, a Love wave or shear-plate mode acoustic resonator.

The object of the invention is also achieved by a differential sensing device, said sensing device can comprise at least one resonator device as described previously. The differential sensing device enables to measure both the radial and tangential forces in a differential manner, namely the sensor system enables to measure the stress by segregating the origin of the perturbation and to be immune to other stimuli such as temperature, vibrations or pressure.

According to a variant of the invention, the propagation direction of one of the resonators can be parallel or perpendicular to a radial direction to sense a radial force. The resonator enable to measure the radial forces in a differential manner, namely the sensing device enables to measure the stress by segregating the origin of the perturbation and to be immune to other stimuli such as temperature, vibrations or pressure.

According to a variant of the invention, the propagation direction of one of the resonators is at an angle Ψ, in particular at <NUM>° with respect to a radial direction to sense a tangential force. The resonator enable to measure the tangential forces in a differential manner, namely the sensing device enables to measure the stress by segregating the origin of the perturbation and to be immune to other stimuli such as temperature, vibrations or pressure.

According to a variant of the invention, one resonator device can be arranged so that that its wave propagation direction is parallel to one of the crystalline axis of the piezoelectric substrate and one resonator device can be arranged so that its wave propagation direction makes an angle Ψ to one of the crystalline axis of the piezoelectric substrate, in particular an angle Ψ equal to ±<NUM>°. The differential sensing device enables to measure both the radial and tangential forces in a differential manner, namely the sensing device enables to measure the stress by segregating the origin of the perturbation and to be immune to other stimuli such as temperature, vibrations or pressure.

According to a variant of the invention, the differential sensing device can further comprise an antenna connected to the at least one resonator device.

According to a variant of the invention, at least two differential resonator devices can be provided on the same piezoelectric substrate. Therefore, the fabrication process will be simplified and faster compared to the state of the art device for which each differential sensor is fabricated on a separate substrate, as both differential sensors share the same structural characteristics and dimensions.

The invention may be understood by reference to the following description taken in conjunction with the accompanying figures, in which reference numerals identify features of the invention.

The invention will now be described in more detail using advantageous embodiments in an exemplary manner and with reference to the drawings. The described embodiments are merely possible configurations and it should be kept in mind that the individual characteristics as described above can be provided independently of one another or can be omitted altogether during the implementation of the present invention.

<FIG> shows a resonator device according to a first example. In the following, the resonator device will be described as a surface acoustic wave resonator device (SAW). According to variants, bulk acoustic wave (BAW) resonators, Lamb wave or Love wave or shear-plate mode resonators could be used in the same way according to the invention.

In <FIG>, the surface acoustic wave device <NUM> comprises two surface acoustic wave resonators <NUM>, <NUM> provided over or in a surface acoustic wave propagating substrate <NUM>. Each surface acoustic wave resonator <NUM>, <NUM> comprises an inter-digitated transducer structure 208a, 208b and 210a, 210b, each sandwiched by a couple of reflecting structures <NUM>, <NUM> and <NUM>, <NUM>. The reflecting structures <NUM>, <NUM>, <NUM>, <NUM> comprise a reflector with one or more metallic strips <NUM>, and are configured to reflect the surface acoustic wave generated by the inter-digitated transducer structures.

Here, the reflecting structures <NUM>, <NUM>, <NUM>, <NUM> are arranged with a gap from the inter-digitated transducer structures 208a, 208b, 210a, 210b. In a variant of the invention, no gap can be present between the reflecting structures and the transducer structure, so that the reflecting structure can be considered as continuing the inter-digitated transducer periodic structure in a synchronous, i. e with the same period and same metalization ratio, or non synchronous way.

In another variant, the at least one of the reflecting structures <NUM>, <NUM>, <NUM>, <NUM> comprises more than one reflector, wherein the reflectors can have the same number of metallic strips <NUM> or not. In a variant, the metallic strips <NUM> of the reflecting structures <NUM>, <NUM>, <NUM>, <NUM> can be connected to each other and/or shortened. This can result in an improvement of the reflection coefficient of the reflecting structures at the Bragg condition compared to electrically isolated reflecting structures. At the Bragg condition, the reflected waves due to electrical and mechanical loading are in phase so that an improved reflection coefficient of the reflector at the Bragg condition results in a better detection of the reflected waves by the corresponding transducer structure.

The transducer structures 208a, 208b, and the transducer structures 210a, 210b each comprise two inter-digitated comb electrodes 224a, 226a, 224b, 224b and 240a, 242a, 240b, 242b. The electrodes 224a, 226a, 224b, 224b and 240a, 242a, 240b, 242b are formed of any suitable conductive metal, for example Aluminium or Aluminium alloy. Nevertheless, other material may be used which generates stronger reflection coefficient for smaller electrode relative thickness. In that matter, the preferred electrode materials are Copper (Cu), Molybdenum (Mo), Nickel (Ni), Platinum (Pt) or Gold (Au) with an adhesion layer such as Titanium (Ti) or Tantalum (Ta) or Chromium (Cr), Zirconium (Zr), Palladium (Pd), Iridium (Ir), Tungsten (W), etc. In <FIG>, the electrodes comprise fingers. In a variant of the embodiment, they could also have spilt fingers comprising each two or more directly adjacent electrode fingers belonging to the same comb electrode. In another variant, the electrode fingers can be slanted enabling a beam-steering compensation.

The transducer structures 208a, 208b and 210a, 210b are also defined by the electrode pitch p (not shown), corresponding to the edge-to-edge distance between two neighbouring electrode fingers from opposite comb electrodes 224a, b and 226a, b and 240a,b and 242a,b. In a variant of the invention, the electrode pitch p is defined by the Bragg condition given by p = λ/<NUM>, λ being the operating acoustic wavelength of said transducer structures <NUM>, <NUM>. By operating acoustic wavelength λ, one understands λ being the acoustic wavelength following λ = V / f with f the predetermined central frequency of the resonator structure and V the phase velocity of the utilized mode. Such transducer structure, as shown in <FIG>, is also said to be a <NUM>-finger-per-wavelength inter-digitated transducer (IDT).

In a variant of the invention, the inter-digitated transducer <NUM>, <NUM> can operate out of the Bragg conditions, for instance, using a <NUM> or <NUM>-finger-per-wavelength excitation structure or <NUM> -finger-per-two-wavelength transducers or <NUM> or <NUM> finger-per-three wavelength.

The transducer structures 208a, 208b and 210a, 210b can be symmetrical, namely they have the same number of electrode fingers with the same characteristics. However, in a variant of the invention, they can also be different, in particular they can have a different number of electrode fingers and/or a different pitch p.

In a variant, the inter-digitated transducer structures 208a, 208b and 210a, 210b can be tapered to reduce transverse modes.

The substrate <NUM> over or in which the resonators <NUM>, <NUM> are provided is a piezoelectric bulk material, with crystallographic axis X, Y and Z as shown in <FIG>. The piezoelectric bulk material <NUM> herein described by way of example may be Quartz, in particular AT-cut Quartz. According to a variant, the acoustic wave propagating substrate <NUM> on which the resonators <NUM>, <NUM> and hence the transducer structures 208a, 208b and 210a, 210b and the reflectings structures <NUM>, <NUM>, <NUM>, <NUM> are provided can be a composite substrate <NUM>. The composite substrate <NUM> comprises a layer of piezoelectric material of a certain thickness, formed on top of a base substrate. The piezoelectric layer by way of example may be Lithium Tantalate (LiTaO3) or Lithium Niobate (LiNbO3).

According to the invention, the resonators <NUM>, <NUM> are positioned on the substrate <NUM> so that they have two different surface acoustic wave propagation directions but due to cross like arrangement of the two resonators <NUM>, <NUM>, they are sharing at least partially the same area on the substrate <NUM>.

In this embodiment, the first resonator <NUM> is positioned so that its direction of propagation of acoustic wave is in the crystallographic direction X of the acoustic wave propagating substrate <NUM>. In <FIG>, the direction of propagation of the acoustic wave of the second resonator <NUM> is in the crystallographic direction Z of the piezoelectric substrate <NUM>. Thus, the propagation direction of the acoustic wave for the resonator <NUM> is rotated by an angle Θ=<NUM>° compared to the crystallographic direction X of the acoustic wave propagating substrate <NUM>, and compared to the surface acoustic wave propagation direction of the first resonator device <NUM>. In Figure 1a, the angle Θ has a value of <NUM>°, but in a variant of the embodiment, the angle could be different. In a variant, the angle Θ can be lower than <NUM>°, for example with a variation of ±<NUM>°, which enables to correct effects such as beam streering. In these variants, however, the symmetry with the crystallographic X still remains in order to keep equal properties along the two propagation directions.

In this embodiment, besides their wave propagation direction, the resonators <NUM>, <NUM> have the same geometrical structure, meaning that their transducer structure 208a, 208b and 210a, 210b respectively and the reflecting structures <NUM>, <NUM>, <NUM>, <NUM> have the same designs and/or dimensions. In a variant of the embodiment, they can have a different design, e.g. different dimensions and/or different geometry. For example, the reflecting structures <NUM>, <NUM>, <NUM>, <NUM> can be different but the transducer structures 208a, 208b and 210a, 210b are the same or vice-versa or both the reflecting structures <NUM>, <NUM>, <NUM>, <NUM> and the transducer structures 208a, 208b and 210a, 210b can be different.

In this embodiment, the resonators <NUM>, <NUM> are split into two parts, each part of a resonator being separated from the other part by a certain distance d1, d2 respectively.

The region <NUM> located in between the split parts 202a, 202b and 204a, 204b of the two resonators <NUM>, <NUM>, with its dimensions defined by the distances d1 and d2, corresponds to an acoustic cavity <NUM>, in particular a resonant acoustic cavity <NUM>. In <FIG>, the distances d1 and d2 are identical, but in a variant of the embodiment, they can be different.

In this embodiment, the two split parts 202a, 202b of the resonator <NUM> are symmetrical in regards to the cavity <NUM> and identical to each other so that the cavity <NUM> is actually located in the center part of the resonator <NUM>. In a variant of the embodiment, the two split parts 202a, 202b of the resonator <NUM> are not identical and/or symmetric in regards of the cavity <NUM>.

In this embodiment, furthermore, the two split parts 204a, 204b of the resonator <NUM> are also symmetric in regards to the cavity <NUM> and identical to each other so that the cavity <NUM> is actually located in the center part of the resonator <NUM>. Thus, in Figure 1a, the cavity <NUM> is a central cavity common to both resonators <NUM>, <NUM>.

In this embodiment, furthermore, the split parts 202a, 202b and 204a, 204b of both resonators <NUM>, <NUM> are symmetric in regards to the cavity <NUM> and identical. In a variant of the embodiment, the split parts 202a, 202b and 204a, 204b of the resonators <NUM>, <NUM> are not identical and/or symmetric in regards of the cavity <NUM>.

The resonators <NUM>, <NUM> are split in a manner so that actually, the transducer structure of the resonator is split into two parts 208a and 208b and 210a and 210b. Thus, each split part 202a, 202b, 204a, 204b of the resonators <NUM>, <NUM> actually comprises a reflecting structure and a split part of the transducer structure of the respective resonator. Hence, the split part 202a of the resonator <NUM> comprises the reflecting structure <NUM> and the split part 208a of the transducer structure. The split part 202b of the resonator <NUM> comprises the reflecting structure <NUM> and the split part 208b of the transducer structure. The split part 204a of the resonator <NUM> comprises the reflecting structure <NUM> and the split part 210a of the transducer structure. The split part 204b of the resonator <NUM> comprises the reflecting structure <NUM> and the split part 210b of the transducer structure.

In a variant of the embodiment, the resonator is split in between one reflecting structure and the transducer structure. Thus, one split part of the two split parts of the resonator comprises the entire transducer structure with one reflecting structure and the other part the other reflecting structure.

<FIG> shows a surface acoustic wave device according to a second embodiment. Elements carrying the same reference numeral as in <FIG> will not be described again in detail, as they correspond to the ones already described above.

Unlike in the first embodiment, both resonators <NUM>, <NUM> of the surface acoustic wave sensor <NUM> are now positioned at an angle Ψ to the acoustic propagation direction X of the piezoelectric substrate <NUM> in comparison with the surface acoustic wave sensor <NUM> of the first embodiment. This is the only difference with respect to the first embodiment.

Thus, the propagation direction of the acoustic wave for the resonator <NUM> is rotated by an angle Ψ compared to the crystallographic direction X of the acoustic wave propagating substrate <NUM>. The resonator <NUM> is still positioned on the acoustic propagating substrate <NUM> at an angle Θ = <NUM>°. In a variant of the embodiment, another value of Θ different to <NUM>°, for example smaller than <NUM>°, could be used. which would enable to correct effects such as beam streering.

<FIG> shows a third embodiment based on the sensor <NUM> according to the first embodiment. In this embodiment, the two resonators are electrically connected in a differential manner, thereby forming a differential resonator device <NUM>. This configuration can be used to measure stress, e.g. due to presence of radial forces. Elements carrying the same reference numeral as in <FIG> will not be described again in detail, as they correspond to the ones already described above.

In this embodiment, the comb electrodes 224a, 242a, 226b and 240b are electrically connected by the conductive line <NUM> and the comb electrodes 224b, 242b, 226a and 240a are electrically connected by the conductive line <NUM> to form a differential arrangement. The resonators <NUM>, <NUM> are here connected in parallel and the resonator device <NUM> operate at resonance.

In a variant of the invention, the two resonators can be connected in series and the resonator device would operate at anti-resonance operation.

The resonator device <NUM> according to the third embodiment allows to position a test area in the central cavity shared by both resonators and to conduct a measure at the same location by the two resonators, yielding an improvement in the measurement quality and also a better immunity to parasitic stress effects compared to the state of the art device described in <FIG>.

In <FIG>, the two resonator propagation directions, shown as arrows <NUM>, <NUM>, are respectively parallel and perpendicular to the direction of an applied radial force Fr yielding equal and opposed stresses for the two resonators <NUM>, <NUM>. Here, the two principal strain components in the substrate <NUM> are aligned to the two principal strain components in the object due to external radial stress. The propagation directions of the surface acoustic waves propagating through the respective resonators <NUM> and <NUM> may be respectively aligned with each of the two principal strain components of the substrate <NUM> and the two principal strain components of the object due to external stress. Thus, when stress is applied, one of the resonators will be in tension and the other one will be in compression. As a result, their resonant frequencies f1, f2 will change in opposite directions. By sensing the change of the difference Δf between the two resonant frequencies, one can find the applied torque M, as the difference Δf is linearly proportional to the torque M.

Sensing of the change of the difference frequency Δf permits suppression of a number of common-mode interference factors and, reduce variations due to a temperature, which should cancel out in the differential sensing arrangement.

<FIG> shows the fourth embodiment wherein the SAW sensor <NUM> according to the second embodiment is configured to measure the stress due to tangential forces. In this embodiment, the two resonators <NUM> and <NUM> are electrically connected in a differential manner, thereby forming a differential resonator device <NUM>. This configuration can be used to measure stress, e.g. due to presence of tangential forces. Elements carrying the same reference numeral as in <FIG> and <FIG> will not be described again in detail, as they correspond to the ones already described above.

The resonator device <NUM> according to the fourth embodiment allows to position a test area in the central cavity shared by both resonators and to conduct a measure at the same location by the two resonators, yielding an improvement in the measurement quality and also a better immunity to parasitic stress effects compared to the state of the art device described in <FIG>.

In <FIG>, the two sensor propagation directions are shown in <FIG> as arrows <NUM> and <NUM>. In the tangential mode, the stress is orthogonal to the radial direction. It must be considered that the tangential force is exerted on the edge of the object, its central part being blocked. Therefore, due to the reaction of the fixed part, everything happens like in the case of torque. Projecting the resulting force on the two resonators yield one compressional effect for one resonator and one extensional effect for the other one resonator, thus yielding a differential mode. The resonators <NUM>, <NUM> of the SAW device <NUM> are laid down on a piezoelectric substrate206 so that the surface acoustic waves propagate at an angle Ψ of ±<NUM>° relative to the crystallographic X axis of the piezoelectric substrate <NUM>. At this angle, the contribution of temperature variations of third order elastic constants of the substrate <NUM> to the temperature variation of the Force sensitivity, is substantially equal and opposite to the sum total of variations in linear temperature coefficient of expansion, non-zero third order elastic constants, temperature variation of contributions caused by first order elastic constants, and temperature variations of substrate density. Thus, the resonator device <NUM> achieves a reduction of tangential force sensitivity variation with temperature.

The resonator device according to the invention thus operates as a differential sensor in differential mode to segregate the two considered mechanical effects of radial, see <FIG>, and tangential force, see <FIG>.

<FIG> shows the electrical admittance simulation of a surface acoustic wave device according to the second example, as illustrated in <FIG>. For this simulation, a.

Quartz substrate with a (YXIt)/39o/±<NUM>° cut was used. The aperture of both resonators is <NUM>, the cavity length is <NUM>, the pitch in the inter-digitated transducer structure of the first resonator is <NUM> and <NUM> in the mirror, for the second resonator, these values are <NUM> and <NUM> respectively. To reduce spurious resonance on the resonator spectrum signature, small gaps (<NUM> and <NUM>) between the tranducer structures and the associated mirrors were introduced. The metal thickness (AlCu) is <NUM>. All the gratings operate at Bragg conditions. Mirrors are composed of <NUM> strips and the resonators of <NUM> (<NUM>×<NUM>) and <NUM> (<NUM>×<NUM>) finger pair for impedance matching and the resonators are slanted to compensate the beam steering (<NUM>°). The two resonators of the surface acoustic wave device are positioned at an angle of <NUM>° to each other, thus in a cross-type formation. Furthermore, the two resonators are positioned at an angle of ±<NUM>° to the propagation direction X of the quartz substrate. The resonators are identically split into two parts, both parts being symmetrical to each other and a central acoustic cavity shared by both resonators can be seen as described in <FIG> and <FIG>.

The electrical admittance graph plots the conductance (S) and the susceptance (S) on the right and left Y axis respectively in function of the frequency (MHz) on the X axis. As two resonators are present, two resonance peaks are visible in the electrical admittance graph, slightly above <NUM> and slightly above <NUM> respectively, for both the real part of the admittance (conductance G) and the imaginary part of the admittance (susceptance B). The two resonance peaks are balanced to approach a 50Ω matching within the <NUM> centered ISM-band.

When a radial force is applied to the device, as in the third embodiment of the invention as shown in <FIG>, the applied radial force Fr yields equal and opposed stresses for the two resonators <NUM>, <NUM>. Here, the two principal strain components in the substrate <NUM> are aligned to the two principal strain components in the object due to the applied stress. The propagation directions of the surface acoustic waves propagating through the respective resonators <NUM> and <NUM> are respectively aligned with each of the two principal strain components of the substrate <NUM> and the two principal strain components of the object due to stress. Thus, when radial stress is applied, one of the resonators will be in tension and the other one will be in compression. As a result, their resonant frequencies f1, f2 will change in opposite directions, as shown in Figure 1b with the arrows of case a) and b). The change in Δf between the two resonant peaks is then proportional to the applied force F.

In contrary to the state of the art, the resonators <NUM> and <NUM> have a common cavity <NUM>, which corresponds to the location where the measuremnt is made for both resonators <NUM>, <NUM>. Thus, both resonators will measure at the same location and a more accurate value of the applied force will be obtained, compared to the value obtained with a state of the art device as shown in Figure 1a.

<FIG> shows a sensing device according to a fifth example.

In <FIG>, the sensing device <NUM> comprises two differential sensors <NUM> and <NUM> according to the third and fourth embodiment of the invention respectively.

In the embodiment of <FIG>, the differential sensors <NUM> and <NUM> are each located on a quartz dice <NUM>, <NUM>. In a variant of the embodiment, the same quartz dice could comprise both differential sensors <NUM> and <NUM>. They are not described again in detail but reference is made to their description above.

Both quartz dices <NUM>, <NUM> are positioned on an object <NUM>, in order to measure e.g. the stress generated by tangential and radial forces on the object <NUM>. In <FIG>, the object is a wheel. The quartz dice <NUM> and <NUM> are positioned on the same radial line <NUM>, the quartz dice <NUM> closer to the center of the object <NUM> than the quartz dice <NUM>. The quartz dice position on the object <NUM> could also be swapped so that the quartz dice <NUM> is the one closer to the center of the object <NUM>.

The quartz dices <NUM>, <NUM> are glued onto the object <NUM>, which comprises a steel plate at that position, with cyano-acrylate glue (M-bond <NUM>) but any other glue or solid state attachment techniques could be used.

The resonators <NUM>, <NUM> are split into two parts, as described in the third and fourth embodiment, so that the differential sensors <NUM> and <NUM> each comprises a central cavity <NUM>, shared by the two resonators <NUM>, <NUM> of each sensor <NUM>, <NUM>.

In this embodiment, the differential sensor <NUM> is configured to measure the stress on the object <NUM> due to the tangential forces while the other differential sensor <NUM> is configured to measure the stress on the object <NUM> due to radial forces as explained above.

Both differential sensors <NUM>, <NUM> are connected to an antenna <NUM>, to transmit the measurements. In a variant of the embodiment, each differential sensor can have its own antenna. According to the invention, the stress resulting from the forces applied to the object <NUM> and sensed by the sensors <NUM>, <NUM> is measured at the same location at the central cavity <NUM> for each sensor <NUM>, <NUM>, yielding an improvement in the measurement quality and a better immunity to parasitic stress effects.

In a variant, the sensing device <NUM> can comprise more than two differential sensors according to the invention.

In another variant of the invention, the sensing device <NUM> can be applied to any other object, and not only a wheel, in order to measure concommitently the stress due by the radial and tangential forces experienced by the object. Other physical parameters, outside of stress, can also be measured with the sensing device <NUM>. For example, torsional effects and torque can also be measured or any other physical parameter not related to stress.

In another variant of the invention, the sensing device <NUM> can measure the stress due by the radial and tangential forces experienced by the object at the same location. The four resonators of the sensing device would share the same resonant cavity.

<FIG> shows the electrical admittance simulation of the sensing device according to the fifth embodiment of the invention. The sensing device <NUM> as shown in <FIG> comprises two differential sensors <NUM>, <NUM>, each comprising two resonators <NUM>, <NUM>. The differential sensor <NUM> is according to the third embodiment as described in <FIG> and the differential sensor <NUM> is according to the fourth embodiment as described in <FIG>.

The electrical admittance graph plots the conductance (in Siemens - S) and the susceptance (in S) on the right and left Y axis respectively in function of the frequency (MHz) on the X axis. As two differential sensors are present, each comprising two resonators, four resonance peaks are visible in the electrical admittance graph, slightly above and below <NUM>, for both the real part of the admittance (conductance G) and the imaginary part of the admittance (susceptance B). The resonance peaks of each resonator are balanced to approach a 50Ω matching within the <NUM> centered ISM-band.

<FIG> and <FIG>, <FIG> and <FIG> show multiple variants of the resonator device according to the invention.

The basic structure corresponds to the one of the first embodiments and only the differences with respect to that one will be described. Thus, the features common with the first embodiment of <FIG> will not be described in detail again but reference is made to their description above. Furthermore, the variants will be shown based on the structure of the first embodiment but they can be applied to the structure of the second, third or fourth embodiments as well.

<FIG> shows a variant of the first embodiment of <FIG>, where the reflecting structures <NUM>, <NUM>, <NUM> and <NUM> of the resonators <NUM>, <NUM> comprises metallic strips <NUM> which are connected to each other and/or shortened. This results in an improvement of the reflection coefficient of the reflecting structures at the Bragg condition. At the Bragg condition, the reflected waves due to electrical and mechanical loading are in phase so that an improved reflection coefficient of the reflecting structures <NUM>, <NUM>, <NUM> and <NUM> or at the Bragg condition results in a better detection of the reflected waves by the corresponding transducer structure 208a, 208b and 210a, 210b.

The resonator device <NUM> as described in this variant enables to manage parasites due to directivity effects.

<FIG> shows a variant of the first embodiment where the split parts of the resonators <NUM>, <NUM> are not identical or symmetrical with respect to the cavity <NUM>. In <FIG>, the split occurs between the transducer structure <NUM>, <NUM> and one of the reflecting structures <NUM>, <NUM> of the resonator <NUM>, <NUM> respectively. The split part 602a of the resonator <NUM> comprises only the reflecting structure <NUM>. The split part 602b of the resonator <NUM> comprises the entire transducer structure <NUM> and the reflecting structure <NUM>. Correspondingly, the split part 604a of the resonator <NUM> comprises only the reflecting structure <NUM>. The split part 604b of the resonator <NUM> comprises the entire transducer structure <NUM> and the reflecting structure <NUM>. The cavity <NUM> is not perfectly in the center of the two resonators structures <NUM>, <NUM> but is still shared by both resonators <NUM>, <NUM>.

<FIG> shows a further variant of <FIG>, thus, of the first embodiment, where the transducer structure <NUM>, <NUM> of the resonators <NUM>,<NUM> comprises split fingers as electrode means <NUM>. The split fingers <NUM> comprise each two directly adjacent electrode fingers <NUM>, <NUM> belonging to the same comb electrode <NUM>. Thus, the transducer structures <NUM>, <NUM> do not operate at the Bragg conditions.

Furthermore, the split parts of the resonators <NUM>, <NUM> are also different and not symmetric with regards to the cavity <NUM>, as the reflecting structures <NUM>, <NUM> and <NUM>, <NUM> are not identical within a resonator <NUM>, <NUM> respectively. For the resonator <NUM>, the reflecting structure <NUM> comprises more metallic strips <NUM> as the reflecting structure <NUM> (same thing for the resonator <NUM>). The metallic strips <NUM> are also connected to each other and/or shortened. In a variant, they can also not be connected to each other.

Here, like in the second variant of the first embodiment, the split part of a resonator comprises a reflecting structure alone and the other split part of the resonator comprises the full transducer structure and the other reflecting structure adjacent the transducer structure. Again, the cavity <NUM> is not central within the resonators <NUM>, <NUM>, but is still shared by the two resonators <NUM>, <NUM>.

<FIG> shows a further variant of <FIG>, where the transducer structures <NUM>, <NUM> of the resonators <NUM>, <NUM> are different. The resonator <NUM> corresponds to the resonator of <FIG>, wherein the transducer structure <NUM> comprised split fingers <NUM> as electrode means and thus, does not work at the Bragg condition. In the contrary, the resonator <NUM> is the same as in figure1a and the transducer structure <NUM> works at the Bragg condition and is a <NUM>-finger-per-wavelength inter-digitated transducer (IDT). Again, like in figure 1a, the split parts 804a and 804b of the resonator <NUM> comprises each a reflecting structure <NUM>, <NUM> and a part of the transducer structure 810a, 810b respectively. While for the resonator <NUM>, one split part 702a comprises the reflecting structure <NUM> and the entire transducer structure <NUM> and the other split part 702b comprises only a reflecting structure <NUM>.

The metallic strips <NUM> of the reflecting structures <NUM>, <NUM> and <NUM>, <NUM> are also connected to each other and/or shortened. In a variant, they can also not be connected to each other.

<FIG> shows a variant of the first embodiment where the cavity <NUM> of the differential sensor <NUM> is metalized. The cavity <NUM> is a central cavity, as shown in Figure 1a. The metalization of the cavity <NUM> can be done on the whole surface as shown in <FIG>, but it can also be done only on part of the surface of the cavity <NUM>. Thus, the surface of the cavity <NUM> can be fully metalized or partially metalized.

Again, the metallic strips <NUM> of the reflecting structures <NUM>, <NUM> and <NUM>, <NUM> are also connected to each other and/or shortened. In a variant, they can also not be connected to each other.

The resonator device <NUM> as described in this variant enables to filter or select the possible modes of the structure or even allow to operate in a coupled mode configuration.

<FIG> shows the variant of the first embodiment where the cavity <NUM> of the differential sensor <NUM> comprises one or more gratings <NUM>, <NUM>. The grating <NUM>, <NUM> can be a metal grating, deposited on top of the surface of the cavity <NUM>, or it can also be an etched grating. When a single grating is present, it can be a one direction grating. When more than one grating are present, the gratings <NUM>, <NUM> can be superimposed within the surface of the cavity <NUM>, as shown in <FIG>. In a variant, the gratings <NUM>, <NUM> can be located within the full surface of the cavity <NUM> or only partially within the surface of the cavity <NUM>.

The surface acoustic wave device <NUM> as described in this variant enables to filter or select the modes of the structure or even enables to operate in a coupled mode configuration.

<FIG> shows the surface acoustic wave differential sensor according to a seventh variant of the first embodiment of the invention.

In this variant, the reflecting structures of the resonators <NUM>, <NUM> comprises a plurality of reflectors, each comprising more or less metallic strips <NUM>. In this variant, the resonators <NUM>, <NUM> are SAW tag devices. SAW tag devices are sensors, which can be remotely interrogated, providing a wireless measurement of a physical quantity. Whatever this physical quantity is, it is better to put in place differential measurement to guarantee the measurement of an absolute physical quantity or to suppress correlated external perturbations affecting the sensor.

Two SAW-tags are used in a way that only the two first echoes are used to determine the stress value, the other echoes may be used as identification marks and/or as other physical effect markers (for instance temperature).

The SAW tag device <NUM> comprises a transducer structure <NUM> in particular only one transducer structure, and a set of reflectors <NUM>, <NUM> and <NUM>, positioned at various delays on one side of the transducer structure <NUM> in the direction of propagation X as shown in <FIG>. These reflectors <NUM>, <NUM> and <NUM> usually comprise one or more metallic strips <NUM>, e.g. aluminium strips. The SAW tag device <NUM>, <NUM> also comprises an antenna (not shown) connected to the transducer structure <NUM>, <NUM>.

The SAW tag device <NUM> is the same as the SAW tag device <NUM> but its set of reflectors <NUM>, <NUM> and <NUM>, positioned at various delays on one side of the transducer structure <NUM> in the direction of propagation Y as shown in <FIG>.

The SAW tag <NUM>, <NUM> is actually split in two parts 1102a and 1102b, 1104a and b, between the inter-digitated structures <NUM>, <NUM> and the first reflector <NUM>, so that one part of the split SAW tag 1102a, 1104a comprises the set of reflectors <NUM>, <NUM> and <NUM> or delay line and the other part 1102b, 1104b of the SAW tag <NUM>, <NUM> comprises only the inter-digitated transducer structure <NUM>, <NUM>.

The inter-digitated transducer structures <NUM>, <NUM> are operating at Bragg conditions but could operate out of this condition, the reflectors <NUM>, <NUM> and <NUM> are in open circuit mode. The distances L11, L12, L13 and L21, L22 and L23 between the reflectors <NUM>, <NUM> and <NUM> and the transducer structure <NUM>, <NUM> are chosen in such a way the corresponding echoes are not overlapping on the whole measurement range. The cavity <NUM> is shared by both resonators <NUM>, <NUM> although not being centrally located in between the two resonators <NUM>, <NUM>, since the split parts 1102a and 1102b and the split parts 1104a and 1104b of both resonators <NUM>, <NUM> are not the same and not symmetric to each other.

In the variant shown in <FIG>, the split part 1102b and 1104b comprises a reflecting structure <NUM> and the inter-digitated structure <NUM>, <NUM> respectively. The inter-digitated transducer structures <NUM>, <NUM> are operating at Bragg conditions but could operate out of this condition, they are associated here with a reflecting structure <NUM> to reflect and launch all the energy towards the obstacle, the mirrors on which the waves partially reflects are shorten and both delay lines are identical. The distances L11, L12, L13 and L21, L22 and L23 between the reflectors <NUM>, <NUM>, <NUM> and the transducers <NUM> and <NUM> are chosen in such a way the corresponding echoes are not overlapping on the whole measurement range.

In the variant in <FIG>, the surface acoustic wave device <NUM> is a variant of the device <NUM> of <FIG>, where the split part 1204a of the resonator <NUM> only comprises one reflector <NUM> and the split part 1202a of the resonator <NUM> comprises more reflectors than the split part 1102a of the device <NUM>. In this variant, both resonators <NUM> and <NUM> are different although being both SAW tag devices. The resonator <NUM> or SAW tag <NUM> comprises a lot more reflectors as the SAW tag <NUM>.

Again, the cavity <NUM> is shared by both resonators <NUM>, <NUM> although not being centrally located in between the two resonators <NUM>, <NUM>, since the split parts 1202a and b and the split parts 1204a and b of both resonators <NUM>, <NUM> are not the same and not symmetric to each other.

The metallic strips <NUM> of the reflectors <NUM>, <NUM>, <NUM> and <NUM> are also connected to each other and/or shortened. In a variant, they can also not be connected to each other.

The resonator device <NUM> as described in this variant measures the stress at the cavity <NUM>, which is located within the first transducer-reflector gap of the longest SAW-tag <NUM>. The cavity <NUM> is defined by the gap L11 and L21. In another variant, one SAW tag can share more than one cavity with the other SAW tags or resonators. This would enable to measure a distribution of stress.

<FIG> is a variant of the resonator device <NUM> of <FIG>, where the cavity <NUM> is metallized. The metalization of the cavity <NUM> can be done on the whole surface as shown in <FIG>, but it can also be done only on part of the surface of the cavity <NUM>. Thus, the surface of the cavity <NUM> can be fully metalized or partially metalized. In a variant, the cavity <NUM> can also comprise a metallic grating or more than one metallic grating superimposed to each other. In another variant, the cavity surface can be partially or fully covered by an active layer. For example, the active layer could be sensitive to magnetic field. Therefore, by magnetostriction, the film may experience stresses that can be detected according to the invention. The active layer could also be a layer which changes its properties when exposed to gas, e.g. Palladium and hydrogen.

The resonator device <NUM> as described in this variant enables to increase the sensor sensitivity or more generally to optimize the sensor operation.

<FIG> is a variant of the resonator device <NUM> of <FIG>, where the splitting of the resonator <NUM> is done in between the delay lines or resonators <NUM>, <NUM> and <NUM>, such that the split part 1202b of the resonator <NUM> comprises now some reflectors <NUM>, <NUM> and <NUM> on one side of the transducer structure <NUM> with the transducer structure <NUM> and the reflecting structure <NUM> on the other side of the transducer structure <NUM>. The resonator <NUM> is the same as in <FIG>.

The resonator device <NUM> as described in this variant measures the stress at the cavity <NUM>, which is located anywhere else on the delay line of the resonators but between the first transducer-reflector gap, defined by the distance L11 and L21,of the longest SAW-tag <NUM>.

Claim 1:
A resonator device (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) for measuring stress comprising: at least two resonators (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>), each resonator (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) comprising an inter-digitated transducer structure (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) and reflecting structures (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) on or in a piezoelectric substrate (<NUM>, <NUM>, <NUM>),
wherein
the at least two resonators (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) are arranged and positioned such that they have two different wave propagation directions, and
each resonator (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) comprises two parts with the area between the two parts of the at least two resonators (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) forming a cavity (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>), wherein the cavity (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) is shared by the at least two resonators (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>), and
characterized in that one of the two parts of the at least two resonators (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>) comprises at least a subset of the reflecting structures (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) comprising at least two of the reflecting structures (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) positioned at various delays and the other one of the two parts of the at least two resonators (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>) comprises the inter-digitated transducer structure (<NUM>, <NUM>, <NUM>) of the corresponding resonator (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>) and the cavity (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) is not centrally located in between the two resonators.