Wide area monitoring system

Abstract

An ultrasonic device for monitoring comprising flexible ultrasound transducer 100, the ultrasound transducer comprising a substrate 10, a layer of piezoelectric material 20 deposited on the substrate,

Description

FIELD The present disclosure relates to an ultrasonic device for a monitoring system. More particularly the disclosure relates to a monitoring system comprising one or more ultrasonic devices, a method of manufacturing the ultrasonic devices and a method of operating the monitoring system. BACKGROUND Ultrasound spans a range of sound frequencies that are higher than the range that can be heard by humans, and generally have frequencies of greater than 20kHz. Typical ranges of operation extend from 100kHz upto several Gigahertz. Due to the much higher frequencies involved, ultrasonic devices are typically very different from those generally used for audible applications. Analysis using ultrasound waves shows great promise in a range of applications, particularly in imaging such as medical imaging but also in fields such as non-destructive testing (NDT), particularly in industrial NDT. In typical NDT applications an ultrasound transducer attached to an object can be used to generate a measurable signal for the detection of mechanical structures within the object. However, ultrasound has a wide range of uses that are not limited to these examples. The ultrasound transducer is operable to produce ultrasonic waves that are transmitted into the sample and detect reflections of the ultrasonic waves that are reflected from the interfaces between the layers of the sample. By using techniques such as time of flight and other analyses, it is possible to image the layers of the sample and thereby characterise the sample. However, the area inspected is limited to the field of view of the ultrasonic transducer and multiple measurement and / or transducers may be required to test or monitor a large or complex sample effectively or continuously. Conventional ultrasonic transducers are generally formed from bulk ceramic materials, which can be high cost, bulky and difficult to manufacture, particularly with the shapes and properties desired for many applications. Furthermore, traditional ultrasound transducers are not easy to manufacture using automated techniques and often require a high degree of manual operation. Therefore, although well used in hospitals and other controlled healthcare settings, ultrasound systems have traditionally been overly expensive and complex for many other applications. In industrial markets such as power, energy production, oil and gas, renewables or 5 aerospace, the health or characteristic status of an object must be repeatedly (e.g. continuously) measured to detect and monitor signs of corrosion, erosion, cracks and other defects. Improved ultrasonic transducers, associated monitoring systems and methods for operating such systems are therefore desirable. SUMMARY Various aspects of the present invention are defined in the independent claims. Some preferred features are defined in the dependent claims. According to a first aspect of the present disclosure is an ultrasonic device for a monitoring system comprising: at least one flexible ultrasound transducer, the ultrasound transducer comprising: a substrate; a layer of piezoelectric material arranged or deposited on at least part of the substrate; and a plurality of transmit electrodes or electrical contacts; wherein the plurality of transmit electrodes are arranged such that one or more ultrasonic signals is transmitted substantially parallel to the surface of the piezoelectric material. The layer of piezoelectric material may be, comprise or be comprised in a film of piezoelectric material. The layer of piezoelectric material may be configured and / or operable to produce ultrasound, i.e. the layer of piezoelectric material may be or comprise an ultrasound production layer. The ultrasound may be sound waves having a frequency greater than 20kHz, e.g. from 100kHz up to 2 or 5 or 10 Megahertz. The plurality of transmit electrodes may be arranged such that a majority, at least 75%, at least 90%, at least 95% or substantially all of the ultrasonic radiation emitted by the ultrasonic transducer is transmitted substantially parallel to the surface of the piezoelectric material. The piezoelectric material may be or comprise an inorganic material. The piezoelectric material may be a crystalline, e.g. monocrystalline or polycrystalline, piezoelectric material. The layer of piezoelectric material may be non-polymeric, e.g. it may not contain any polymeric material. However, depending on the application, the piezoelectric material may be or comprise a thin film, ceramic, MEMS, polymeric or ceramic piezoelectric material. The piezoelectric material may be or comprise a continuous layer of material having piezoelectric properties, e.g. the piezoelectric material may not comprise discrete domains of piezoelectric material having piezoelectric properties within a matrix of non-piezoelectric material. The layer of piezoelectric material may have a thickness in the range of 2 to 50pm. The layer of piezoelectric material may be the same thickness or thinner than the substrate, e.g. at least 2, 5,6 or 10 times thinner than the substrate. However, the disclosure is not limited to thin film flexible transducers having the composition above and, in other examples, the ultrasonic transducers may be or comprise other types of transducers, such as ceramic, PZT, polymeric or other transducers. The ultrasonic transducer(s) of the ultrasonic device may comprise a plurality of transmit electrodes on the layer of piezoelectric material, which may be comprised in an electrode array on the layer of piezoelectric material. The electrodes may be working electrodes. Each electrode may be metallic, and may comprise metal deposited or otherwise coated on a surface of the layer of piezoelectric material. The substrate may be electrically conductive, i.e. it may be an electrical conductor. The substrate may be planar. The substrate may be a film or sheet. The substrate may be metallic, e.g. a metal film. The substrate may be or comprise a metal or metallic foil such as aluminium foil. The substrate may be or comprise a thin foil. The substrate may have a thickness in the range of 20 to 200pm. The substrate may be substantially the same thickness as the layer of piezoelectric material or thicker than the layer of piezoelectric material, e.g. by at least a factor of 6 or by a factor of 10 or more. The substrate may be, comprise, or be comprised in an electrical ground electrode. The substrate may be, comprise, or be comprised in a counter electrode to the working electrode(s). The counter or ground electrode may form an electrode pair with the at least one working electrode (e.g. the working electrodes of the electrode array), which may be provided on an opposing side of the piezoelectric material to the counter or ground electrode. The one or more ultrasonic signals may be transmitted using a transverse wave mode i.e. the induced sound waves may propagate in a direction perpendicular to the plane in which the direction of displacement of the layer of piezoelectric material lies. The one or more ultrasonic signals may be transmitted using one or each of: a surface wave mode, a longitudinal wave mode, a Rayleigh wave mode and / or a direction perpendicular to the normal of a surface, such as the substrate surface, of the ultrasound transducer. The ultrasonic device may be an ultrasonic device for testing, such as non-destructive testing. The ultrasonic device may be an ultrasonic device for imaging, such as ultrasound imaging. The transmit electrodes of the plurality of transmit electrodes may be distributed linearly and / or radially over at least part of a surface of the layer of piezoelectric material. Two or more or each of the transmit electrodes or respective tangents of two or more or each of the electrodes may be parallel with each other. The plurality of transmit electrodes may be arranged as concentric or eccentric polygons or ellipses around the centre of, or another common point on, the piezoelectric material. At least one of the polygons or ellipses may be circumferentially segmented. The transmit electrodes may be linear, curved, spiral, arcuate or arbitrarily shaped. The plurality of transmit electrodes may be elongate and / or may be arranged in one or more arrays arranged around the centre of, or another common point on, the layer of piezoelectric material. The respective transmit electrodes of each array may be distributed over a direction perpendicular to a longitudinal axis of the transmit electrodes of the respective array. By forming the plurality of transmit electrodes in a particular size, shape and / or orientation and by arranging the plurality of transmit electrodes in a predetermined pattern, array and / or sequence, the directionality of the one or more transmitted ultrasonic signals may be precisely controlled by varying the amplitude, frequency and duration of the ultrasonic signal and / or by only driving individual transmit electrodes or subsets of transmit electrodes at any one time. This may allow precise control over the monitoring parameters which may be tailored to a particular entity to be monitored or a particular monitoring application. The ultrasonic device may further comprise a receive electrode configured to receive an ultrasonic signal. The receive electrode may be configured to produce an electrical signal responsive to, and indicative of, the received ultrasonic signal. The ultrasonic signal may comprise or be derived from the one or more transmitted ultrasonic signals emitted using the plurality of transmit electrodes. The plurality of transmit electrodes may be provided around the receive electrode and / or on opposing sides of the receive electrode. The transmit transducers may be collocated or dispersed on the piezoelectric material. The receive transducers may be collocated or dispersed on the piezoelectric material. The receive electrode may be collocated with, and / or on the same surface of the piezoelectric material as, the plurality of transmit electrodes. The receive electrode may be disposed at, on and / or around the centre of, or other common point on, the layer of piezoelectric material. Collocating a receive electrode on the same ultrasound transducer / piezoelectric layer, produces an ultrasound transducer with both Rx and Tx capability which may provide flexibility in use and installation. Positioning the receive electrode in the middle of the plurality of transmit electrodes may be further beneficial as it may minimise any interference which may be introduced by unintentional interaction between the receive electrode and the one or more transmitted ultrasonic signals. This may provide for a more accurate and reliable monitoring system. The ultrasonic device may further comprise a plurality of ultrasound transducers. The substrate may be a common substrate and the plurality of ultrasound transducers may be disposed on the common substrate. The common substrate may comprise a rigid-flex printed circuit board (PCB), a rigid PCB, a flex PCB or the like. Having a common substrate may allow for a fixed and repeatable distance to be defined between the plurality of ultrasound transducers which can be determined prior to installation. This may significantly simplify the processing of the resulting data as the expected effect of multiple ultrasound transducers on the emitted ultrasonic signal may be pre-calculated. The plurality of transmit electrodes may be configured to be addressable individually, simultaneously or in predetermined groups, patterns or arrays. The plurality of transmit electrodes in at least one or each array of transmit electrodes may be arranged in a comb arrangement, e.g. at least one or each array may be a comb array of transmit electrodes. The transmit electrodes in at least one or each array may be interdigitated. The ultrasonic device may comprise at least one electrical conduction track and / or at least one electrical connector, e.g. on the surface of the piezoelectric material or on an electrically resistive layer disposed thereon. Respective electrical conduction tracks may electrically connect a respective electrode to a respective electrical connector. However, other arrangements for providing electrodes and / or electrically connecting the electrodes and / or electrically coupling the layer of piezoelectric material may be used. The ultrasonic device may further comprise a control system configured to address the plurality of transmit electrodes individually, simultaneously or in predetermined groups, patterns or arrays so as to control the transmission of the one or more ultrasonic signals. By allowing the plurality of transmit electrodes to be addressed in this way to emit the one or more ultrasonic signals specific patterns of ultrasonic waves e.g. using constructive or destructive interference, dispersion or diffraction can be transmitted, which may be customised to a particular entity to which the ultrasonic device is attached or to the particular property or parameter of the entity which is to be monitored and / or imaged. The control system may comprise a digital to analog converter (DAC) that may be configured to convert digital signals into analogue electrical signals which may be applied to the plurality of transmit electrodes in order to induce the layer of piezoelectric material to emit the one or more ultrasonic signals. The ultrasonic device may further comprise an on-board processing system for processing the electrical signal to determine data therefrom. The ultrasonic device may further comprise an on-board communications system for communicating the data with an external processing system. The on-board processing system may comprise at least one digital data processing module, such as at least one processor (which may include one or more different types of processor such as one or more of a central processing unit (CPU), Graphics Processing Unit (GPU), maths co-processor, a tensor processing unit, neural processing unit or other type of artificial intelligence (Al) accelerator, a physics processing unit, a field programmable gate array (FPGA), application specific integrated circuit (ASIC), a digital signal processor (DSP), and / or the like. The digital data processing module may be configured to receive the digitized electrical signal from the at least one ultrasonic transducer or values thereof and may be configured to process the digitized electrical signal from the at least one ultrasonic transducer or values thereof to determine the data therefrom. The digital data processing module may be configured to determine the data from the digitized electronic signal after digitization of the signal but before communication of the data to an external processing system using the on-board communications system of the ultrasonic device. The on-board processing system may comprise a digitizer (e.g. an analog to digital converter (ADC)) or may otherwise be configured to digitize the electrical signal from the at least one ultrasonic transducer or values thereof. The processing of the electrical signal(s) generated by the at least one ultrasonic transducers by the on-board processing system may comprise digital processing of the digitized electrical signal or data derived therefrom. The communication system may preferably comprise a wireless communications system such as a Wi-FiR™, BluetoothR™ or Bluetooth low energy (BLE), ZigBeeR™, long range wide area network (LoRaWAn), near field communications (NFC), infra-red (IR), optical wireless communications (OWC) or Li-Fi, and / or cellular telephone communications system such as a GSM, 2G, 3G, 4G, 5G, LTE communications system or any successors or further generations thereof, and / or the like. However, in certain applications, a wired communications system may be used. The communication system may be configured to communicate over the internet, which may comprise using a secure communications method such as secure sockets layer (SSL), transport layer security (TLS) or the like. The communication system may be configured to communicate the data with the external processing system directly or indirectly, e.g. via one or more relay or intermediate communications devices such as routers, mobile base stations, switching stations and / or the like. The on-board processing system may be configured to process the electrical signal or a digitized version of the electrical signal or one or more properties derived from the electrical signal, e.g. by performing one or more of: quantitative analysis, spectral analysis, statistical analysis, application of machine learning or artificial intelligence techniques to determine the data. The artificial intelligence techniques may comprise applying a neural network, such as a regressive neural network, or other learning algorithm. The neural network or other learning algorithm may be trained using training data. The training data may be derived from historical data, data generated by a model and / or data that has been classified by one of the other processing operations, e.g. quantitative analysis, spectral analysis, statistical analysis or by human classification. The neural network or other learning algorithm may map one or more inputs to one or more outputs. The inputs may comprise a digitized version of the electrical signal(s) output by the at least one ultrasonic transducer and / or data derived from the ultrasonic signal, such as amplitude, time of flight, time since emission, attenuation, frequency, frequency shift, Doppler shift, and / or the like, which may be data for one or more or a continuum of times. The outputs may comprise the data that represents a measure or value of the state, property, parameter or condition of the entity. One or both of the on-board processing system and / or the external processing system may be configured to reconstruct the ultrasonic signal and / or sound field at least in part from the received ultrasonic signal. One or both of the on-board processing system and / or the external processing system may be configured to construct or reconstruct an image of the sample at least in part from the received ultrasonic signal. One or both of the on-board processing system and / or the external processing system may be configured to determine at least one property or parameter or change in property or parameter at least in part from the received ultrasonic signal and / or the reconstruction of the ultrasonic signal and / or sound field. The at least one property or parameter may be or comprise a defect, a crack, a thickness change, corrosion, erosion, pitting, delamination, disbonding, hydrogen attack or other change in the entity, and / or the like. One or both of the on-board processing system and / or the external processing system may be configured to provide a visual indication of the determined at least one identified property or parameter or change in property or parameter, e.g. a visual indication of the at least one identified property or parameter or change in property or parameter provided in or superimposed on an image of the sample. One or both of the on-board processing system and / or the external processing system may be configured to raise an alarm, flag or alert based on the determined at least one identified property or parameter or change in property or parameter meeting an alarm, alert or flag condition, which may specify at least one of: presence, scale, change, rate of change and / or type of property or parameter. The ultrasonic device may further comprise on-board data storage configured to store the data. The ultrasonic device may comprise a power source for powering the at least one ultrasonic transducer, the on-board processing system, the communications system, and / or the data storage. The power source may comprise power storage such as a battery, a fuel cell, a flow cell or other electrochemical power storage device, a capacitor, super-capacitor or other electrostatic storage device, and / or or the like. The power source may comprise an inductive or other wireless charging or power receiving system. The power source may comprise a mechanical power source, such as a kinetic power source. The ultrasonic device may be configured or configurable for mounting or fixing to an entity. The ultrasonic device may be configured to remain attached to the entity, e.g. to collect ultrasonic measurements of the entity over time, e.g. to determine changes and / or evolution of the data and / or one or more of the properties or parameters of the entity with time. According to a second aspect of the present disclosure is a monitoring system comprising one or more ultrasonic devices of the first aspect. The monitoring system may further comprise an external processing system. The external processing system may be configured to process the data communicated from the on-board communications system of each of the ultrasonic devices to produce an image of the entity and / or to identify at least one property or change in a property of an entity to which the monitoring system is attached or mounted. At least one or each of the ultrasonic devices (e.g. the respective on-board processing system of the respective ultrasonic device) may be configured to process the electrical signal collected by the at least one ultrasonic transducer of the respective ultrasonic device to determine data therefrom. At least one or each of the ultrasonic devices (e.g. the respective communications system of the respective ultrasonic device) may be configured to communicate the data (e.g. digital data derived from an analogue ultrasound signal) determined by the on-board processing system to the external processing system. The external processing system may be configured to further process the data received from the ultrasonic device, e.g. to derive further data from the data received from the ultrasonic device. In some examples, the external processing system may be a remote data processing system. The external processing system may be in communication with the ultrasonic device over a network, such as a wide area network or the internet. The external processing system may be located in a different location and / or a different room or building and / or in the order of tens or hundreds of metres away from the ultrasonic device, or several miles away, and / or the like, from the ultrasonic device. In some examples, the external processing system may be local to the ultrasonic device, e.g. at the same location, within the same building, or in the order of cm’s or metres, or in single cable range. The external processing system may be or comprise a server or group of servers, a cloud computing resource, a dedicated or bespoke computing resource, a workstation, a personal computer, a computer system that runs Unix or Linux, or other high capacity computing system. It would be appreciated that the identified property or parameter or change in property or parameter may be the identification of a defect, a crack, a thickness change, corrosion, erosion, pitting, delamination, disbonding, hydrogen attack or other change in the entity. The monitoring system may further comprise external data storage configured to store at least one of: the data communicated from the on-board communications system; the image of the entity; and the at least one property or change in property of the entity. The external processing system may be configured to store the data generated by the on-board processing system and / or the digitized version of the electrical signals produced by the ultrasonic transducers in external data storage, e.g. in centralized data storage, cloud data storage or the like. The external processing system may be configured to process the data by performing one or more of: quantitative analysis, spectral analysis, statistical analysis, application of machine learning or artificial intelligence techniques. One or both of the on-board processing system and / or the external processing system may be configured to reconstruct the ultrasonic signal and / or sound field at least in part from the received ultrasonic signal. One or both of the on-board processing system and / or the external processing system may be configured to construct or reconstruct an image of the sample at least in part from the received ultrasonic signal. One or both of the on-board processing system and / or the external processing system may be configured to determine at least one property or parameter or change in property or parameter at least in part from the received ultrasonic signal and / or the reconstruction of the ultrasonic signal and / or sound field. The at least one property or parameter may be or comprise a defect, a crack, a thickness change, corrosion, erosion, pitting, delamination, disbonding, hydrogen attack or other change in the entity, and / or the like. One or both of the on-board processing system and / or the external processing system may be configured to provide a visual indication of the determined at least one identified property or parameter or change in property or parameter, e.g. a visual indication of the at least one identified property or parameter or change in property or parameter provided in or superimposed on an image of the sample. One or both of the on-board processing system and / or the external processing system may be configured to raise an alarm, flag or alert based on the determined at least one identified property or parameter or change in property or parameter meeting an alarm, alert or flag condition, which may specify at least one of: presence, scale, change, rate of change and / or type of property or parameter. According to a third aspect of the present disclosure is a method of determining and / or monitoring properties of an entity using at least one of the ultrasonic devices of the first aspect or the system of the second aspect. The method may comprise: providing at least one of the ultrasonic devices of the first aspect into or onto an entity; transmitting one or more ultrasonic signals substantially parallel to the surface of the piezoelectric material; receiving one or more ultrasonic signals and producing one or more electrical signals responsive to, and indicative of, the one or more received ultrasonic signals; processing the one or more electrical signals to determine data therefrom; and processing the data to produce an image of the entity and / or to identify at least one property or change in a property of the entity; wherein the one or more received ultrasonic signals comprises or is derived from the one or more transmitted ultrasonic signals. The method may comprise emitting one or more ultrasonic signals from the ultrasonic device(s) (e.g. from one or more or each of the at least one ultrasonic transducer of the ultrasonic device(s)) into or onto the entity. The method may comprise receiving an ultrasonic signal using the ultrasonic device (e.g. at one or more or each of the at least one ultrasonic transducer of the ultrasonic device(s)), wherein the received signal may comprise or be derived from the signal emitted into or onto the entity, e.g. a reflection of the signal emitted into or onto the entity. The received signal may comprise a reflection of the signal emitted into or onto the entity by one or more interfaces on or in the entity. The received signal may comprise a signal derived from the emitted signal or a combination of emitted signals which has / have been subject to diffraction or interference due to a property or parameter or change in a property or parameter of the entity or an edge or discontinuity of the entity. The processing may be performed by the on-board and / or external processing system and may comprise processing the data by performing one or more of: quantitative analysis, spectral analysis, statistical analysis, application of machine learning or artificial intelligence techniques. According to a fourth aspect of the present disclosure is a computer program product that, when implemented on a controller or the on-board processing system of the ultrasonic device of the first aspect or a / the ultrasonic device(s) of the monitoring system of the second aspect, causes the controller or on-board processing system to control the ultrasonic device to perform the method of the third aspect. According to a fifth aspect of the present disclosure is a method of manufacturing the ultrasonic device of the first aspect, the method comprising: providing or depositing a layer of piezoelectric material on at least a part of a substrate; and providing a plurality of transmit electrodes or electrical contacts; wherein the plurality of transmit electrodes are arranged such that one or more ultrasonic signals is transmitted substantially parallel to the surface of the piezoelectric material. The deposition of the layer of piezoelectric material may be by sputter coating. The layer of piezoelectric material may be provided or deposited only on one surface, e.g. one planar surface, of the substrate. The layer of piezoelectric material may be provided or deposited on part or all of the surface, e.g. one planar surface, of the substrate. The piezoelectric material may be or comprise a doped or alloyed piezoelectric material. The sputter coating may comprise using a sputtering target that is formed from, comprises, or has the same constituents as, the piezoelectric material that forms the layer of piezoelectric material in the final piezoelectric device. The piezoelectric material and / or sputtering target may be or comprise a primary piezoelectric material such as a metal oxide or metal nitride, such as zinc oxide or aluminium nitride, or a doped or alloyed metal oxide or metal nitride. The piezoelectric material and / or sputtering target may comprise a dopant or further material (such as an alloying material or a co-deposited material), which may be or comprise a transition metal or compound thereof. The dopant or further material may be vanadium, for example. The dopant or further material may be present in the piezoelectric material and / or sputtering target at a level up to 10% with respect to weight, e.g. from 0.01 to 10% w / w. The primary piezoelectric material, e.g. the metal oxide or metal nitride, may be present in the layer of piezoelectric material in levels from 90% w / w up to 99.99% w / w. The dopant or other material may be integrated, co-deposited or reacted into the primary piezoelectric material, e.g. alloyed with or doped into the primary piezoelectric material, and may not be mixed with or coated onto or in discrete domains with the primary piezoelectric material. The method may comprise depositing the piezoelectric coating using magnetron sputter deposition, e.g. direct current (DC), pulsed DC, radio-frequency, closed field magnetron (CFM) sputtering and / or high power impulse magnetron sputtering (HIPIMS). Further enhancement may be obtained using substrate biasing (e.g. DC and / or RF), which may optimise sputter plasma ion energy during film growth. These particular techniques may provide beneficial film growth morphology and / or enhanced piezoelectric properties of the piezoelectric layer. The dopant or further material may be incorporated with the primary piezoelectric material such as the metal, metal oxide or metal nitride during deposition of the layer of piezoelectric material. The dopant or further material may be incorporated within the sputtering target, e.g. by doping or alloying with the primary piezoelectric material such as the metal oxide or metal nitride or through co-deposition, e.g. from multiple sputter magnetrons. The dopant or further material may not be coated, adhered or provided separately to the primary piezoelectric material in the sputtering target and / or in the layer of piezoelectric material. The dopant or further material may be co-deposited with the primary piezoelectric material (e.g. the metal, metal oxide or metal nitride). For example, the dopant or further material may be provided by a target or sputtering arrangement and the primary piezoelectric material (e.g. the metal, metal oxide or metal nitride) may be provided by a different target or sputtering arrangement. Providing the layer of piezoelectric material by co-depositing the primary piezoelectric material and the dopant or other material may allow the stoichiometry to be more easily adjusted or otherwise varied, e.g. on the fly. The method may comprise depositing the layer of piezoelectric material on the substrate using a rotating drum arrangement, e.g. the substrate may be provided on a rotating drum whilst the piezoelectric layer is deposited onto the substrate. The arrangement may facilitate a higher throughput. In this way, a more uniform and / or consistent piezoelectric layer may be provided. Furthermore, the deposition process may be accelerated and / or thicker layers of piezoelectric material achieved in a given processing time. In addition, the amount of pinholes and other defects may be reduced. The layer of piezoelectric material may be, comprise or be comprised in a film of piezoelectric material. The layer of piezoelectric material may be configured and / or operable to produce ultrasound, i.e. the layer of piezoelectric material may be or comprise an ultrasound production layer. The piezoelectric material may be or comprise an inorganic material. The piezoelectric material may be a crystalline, e.g. polycrystalline or columnar piezoelectric material. The layer of piezoelectric material may be a layer of non-polymeric piezoelectric material. The piezoelectric material may be or comprise a continuous layer of material having piezoelectric properties, e.g. the piezoelectric material may not comprise discrete domains of piezoelectric material having piezoelectric properties within a matrix of non-piezoelectric material. The layer of piezoelectric material may have a thickness in the range of 2 to 50pm. The layer of piezoelectric material may be substantially the same thickness as the substrate or the layer of piezoelectric material may be thinner than the substrate, e.g. at least 2, 5, 6 or 10 times thinner than the substrate. The method may comprise providing at least one electrode on the layer of piezoelectric material. The method may comprise providing a plurality of the electrodes in an electrode array on the layer of piezoelectric material. The method may comprise providing at least one electrical conduction track and / or at least one electrical connector, e.g. on the surface of the piezoelectric material or on an electrically resistive layer disposed thereon. Respective electrical conduction tracks may electrically connect a respective electrode to a respective electrical connector. However, other arrangements for providing electrodes and / or electrically connecting the electrodes and / or electrically coupling the layer of piezoelectric material may be used. The individual features and / or combinations of features defined above in accordance with any aspect of the present invention or below in relation to any specific embodiment of the invention may be utilised, either separately and individually, alone or in combination with any other defined feature, in any other aspect or embodiment of the invention. Furthermore, the present invention is intended to cover apparatus configured to perform any feature described herein in relation to a method and / or a method of using, producing, repairing or manufacturing any apparatus feature described herein. For any of the apparatus features described above as performing a function, the present invention also 5 covers a method comprising performing that function. BRIEF DESCRIPTION OF THE DRAWINGS These and other aspects of the present disclosure will now be described, by way of example only, with reference to the accompanying Figures, in which: Figure 1a is a schematic planar view of an ultrasonic device according to an example embodiment of the present disclosure. Figure 1b is a cross sectional view of the ultrasonic device of Figure 1a along line A-A’. Figures 2a to 2f are schematic planar views of ultrasonic devices according to example embodiments of the present disclosure. Figure 3a is a schematic planar view of an ultrasonic device comprising a plurality of ultrasound transducers according to an example embodiment of the present disclosure. Figure 3b is a schematic planar view of a plurality of ultrasonic devices according to an example embodiment of the present disclosure. Figures 4a and 4b are schematic planar views of example layouts of a plurality of ultrasonic devices according to an example embodiment of the present disclosure. Figures 5a to 5c are schematic illustrations of example connection schemes for transmit electrodes of an ultrasonic device according to an example embodiment of the present disclosure. Figure 6 is a schematic illustration of a monitoring system comprising a plurality of ultrasonic devices according to an example embodiment of the present disclosure. Figure 7 is a schematic planar view of the monitoring system of Figure 6 attached to an entity to be monitored according to an example embodiment of the present disclosure. Figures 8a to 8c are perspective views of an illustration of a monitoring system applied to an entity to be monitored according to an example embodiment of the present disclosure. Figure 9 is a flowchart showing a method of monitoring an entity using a monitoring system according to an example embodiment of the present disclosure. Figure 10 is a flowchart showing a method of producing an ultrasonic device according 5 to an example embodiment of the present disclosure. In the figures, like parts are denoted by like reference numerals. It will be appreciated that the drawings are for illustration purposes only and are not 10 drawn to scale. DETAILED DESCRIPTION OF THE DRAWINGS Figure 1a shows a schematic planar view of an example ultrasound transducer 100 according to the present disclosure and Figure 1b shows an ultrasonic device 1000 including a cross sectional view of the ultrasound transducer 100 along line A-A’ attached to an entity 60 to be monitored. The ultrasound transducer 100 is provided only as an example of an ultrasound transducer which may be comprised in the example ultrasonic device 1000. Ultrasound transducer 100 comprises an electrically conductive substrate 10 in the form of a metal foil, in this case an aluminium foil, and a layer of crystalline piezoelectric material 20 disposed on one planar surface of the substrate 10. The substrate 10 acts to support the layer of piezoelectric material 20 and also functions as a ground electrode. In this example, the piezoelectric material 20 is vanadium doped ZnO but it will be appreciated that other suitable piezoelectric materials such as AIN and / or other dopants, particularly other transition metal dopants, could be used. A plurality of transmit electrodes 30 are provided on a surface of the layer of piezoelectric material 20 that is on an opposite side of the layer of piezoelectric material 20 to the substrate 10. Each of the transmit electrodes 30 is connected to a corresponding electrically conductive track 38, that is in turn electrically connected to a control system 50. The transmit electrodes 30 in this example are thin metallic electrodes and could be provided by deposition or coating of a metallic layer onto the piezoelectric material 20, for example. In the example ultrasound transducer 100, as shown in Figure 1a the transmit electrodes are arranged into four transmit electrode arrays 35, each array comprising six elongate transmit electrodes 30 distributed over a radial direction perpendicular to a longitudinal axis of the transmit electrodes 30. Each of the transmit electrode arrays 35 are circumferentially arranged in regular intervals around a common point (marked X in Figure 1) on the ultrasound transducer 100. Alternative example transmit electrode arrangements are shown in Figures 2a to 2f and described in more detail below. In the example ultrasound transducer 100, as shown in Figure 1b the transmit electrodes 30 are electrically connected to a common electrically conductive track 38, that is in turn electrically connected to the control system 50. Alternative example transmit electrode connection arrangements are shown in Figures 5a to 5c and described in more detail below. The ultrasonic transducer 100 can beneficially be a flexible thin film transducer based on a non-polymeric, polycrystalline piezoelectric material, such as ZnO or AIN, deposited onto a conductive, flexible substrate, such as a metallic foil. Examples of suitable ultrasonic transducers 100 are described in WO 2019 / 166805, WO 2019 / 166815 and PCT / GB2020 / 050468 in the name of the present applicant, the contents of which are incorporated by reference as if set out in full herein. The ultrasound transducer 100 further comprises a receive electrode 40 collocated on the same surface layer of the piezoelectric material 20 as the plurality of transmit electrodes 30. The receive electrode 40 is disposed on the common point X and is surrounded by the transmit electrode arrays 35. The receive electrode 40 is a thin metallic electrode, and could also be formed by deposition or coating of a thin metal layer onto the surface of the piezoelectric material 20. The receive electrode 40 is electrically connected to a conductive track 48 that is in turn electrically connected to the control system 50. The control system 50 comprises a digitizer in the form of an ADC 52 that is operable to convert the analogue electrical signal produced by the ultrasound transducers 100 into a digital signal that can be processed by an on-board processing system 55. The analogue electrical signal is representative of a received ultrasound signal received by the receive electrode. The control system 50 further comprises a DAC 54 that is operable to covert digital signals produced by the on-board processing system 55 into an analogue electrical signal applied to the plurality of electrodes 30. In order to generate the ultrasonic signals 32, the control system 50 applies an alternating electrical driving current to the plurality of transmit electrodes 30 via electrically conductive tracks 38. The deposited layer of piezoelectric material 20 generates ultrasonic signals 32 responsive to, and representative of, the electrical signal generated by the control system 50 e.g. dependent on the frequency, amplitude and / or duration of the electrical signal. The ultrasonic signals 32 are emitted from the ultrasound transducer 100 by periodic displacement of the piezoelectric material 20 as transverse ultrasonic waves i.e. sound waves propagating in a direction perpendicular to the plane in which the direction of displacement of the piezoelectric material 20 lies. The ultrasonic signals 32 are emitted in a direction away from the common point X generally in line with a direction perpendicular to the longitudinal axis of the transmit electrode arrays 35. Due to the common connection arrangement of the electrodes 30, the electrical signal is applied to each of the transmit electrodes 30 at the same time, resulting in the emission of the ultrasonic signals 32 at substantially the same time. Any reflections of the ultrasonic signals 32 and / or any ultrasonic signals emitted by other ultrasound transducers 100 comprised in the ultrasonic device 1000 (and / or other ultrasonic devices 1000 in a monitoring system, or the like) which are directed towards the ultrasound transducer 100 are received by the piezoelectric material 20. The piezoelectric material 20 generates an electrical signal responsive to, and representative of, the received ultrasonic signal 42. The electrical signal is transmitted by the receive electrode 40 along the electrically conductive tracks 48 to the control system 50. The on-board processing system 55 receives digitized versions of the electrical signal from the ADC 52 and processes the digitized electrical signal to determine properties of the entity or object 60 to which it is attached. It will be appreciated that the electrical signal produced by the ultrasound transducer 100 is representative of properties of the received ultrasonic signal 42, such as amplitude and frequency over time. The on-board processing system is configured to derive parameters from the signal, such as time of flight I time between transmission of the emitted ultrasonic signals 32 from the ultrasound transducer 100 and / or from other interconnected ultrasound transducers 100 attached to the entity or object 60 and reception of the receive ultrasonic signal 42, Doppler shift, and / or the like. The on-board processing system is configured to produce an image of the entity or object 60 and / or determine properties of, or changes in properties of, the entity or object 60 from the received ultrasonic signal 42. Non-limiting examples of such properties could include identifying at least one of a defect, a crack, a thickness change, corrosion, erosion, pitting, delamination, disbonding, hydrogen attack or other change in the entity or object 60 to which the monitoring system is attached, among a number of possible properties. The values of the properties of the entity or object determined by the on-board processing system 55 can be temporarily or persistently stored in on-board data storage 56 that is comprised in the ultrasonic device 1000. Additionally or alternatively, the values of the properties of the entity or object determined by the on-board processing system 55 can be transmitted, preferably wirelessly transmitted, directly or indirectly by a communications system 58 comprised in the ultrasonic device 1000 to an external processing system (such as the external processing system 1050 of Figure 6) for further storage, processing, distribution, display and / or for raising an alarm if the values of the properties of the entity or object are indicative of an alarm-worthy condition. In this way, one or more ultrasonic devices 1000 can be mounted on the entity or object 60 to perform ultrasound measurements over time on the entity or object 60, determine the corresponding values of one or more properties of the entity or object 60 on board the ultrasonic device 1000 and then transmit the values of the one or more properties of the entity or object 60 to an external processing system. Figures 2a to 2f illustrate alternative example transmit and receive electrode arrangements which may be used in an ultrasonic device such as the ultrasonic device 1000 of Figure 1. Figure 2a illustrates an ultrasound transducer 100a in which the transmit electrodes 30 are arranged as an array of similar concentric circles (i.e. ellipses where both foci are at the same point) centred on a common point X which lies at the centre of the layer of piezoelectric material 20. A receive electrode 40 is collocated with the transmit electrodes on the layer of piezoelectric material 20 and is disposed at the common point X. By arranging the plurality of transmit electrodes 30 in this way, the emitted ultrasonic signal is transmitted omnidirectionally in a plane defined by the surface of the piezoelectric material 20 i.e. radially in all directions away from the common point X. Having the receive electrode 40 disposed in the centre of the transmit electrodes 30 allows the receive electrode 40 to receive ultrasonic signals omnidirectionally, and ensures that there is minimal or no interference or diffraction of the emitted ultrasonic signal by the receive electrode 40. By collocating the receive electrode 40 with the plurality of transmit electrodes 30 on the layer of piezoelectric material 20 an ultrasound transducer 100a is produced which beneficially comprises both transmit and receive capabilities, reducing the number of ultrasonic devices required to monitor a large entity. Alternatively, this can be used to increase the capability of a monitoring system which only has capacity for a fixed number of ultrasonic devices or ultrasound transducers. Each of the concentric transmit electrodes 30 may be addressed individually, simultaneously or in predetermined or arbitrarily generated sequences to emit an ultrasonic signal. This allows specific patterns of ultrasonic waves e.g. using constructive or destructive interference, dispersion or diffraction to be transmitted, which may be customised to a particular entity to which the ultrasonic device or monitoring system is attached or to the particular property or parameter which is to be monitored and / or imaged. Figure 2b illustrates an ultrasound transducer 100b in which the transmit electrodes 30 are arranged into two transmit electrode arrays 35, each transmit electrode array 35 comprising six elongate linear transmit electrodes 30 distributed over a radial direction perpendicular to a longitudinal axis of the transmit electrodes 30. The two transmit electrode arrays 35 are arranged opposite each other around a common point X. A receive electrode 40 is collocated with the transmit electrodes on the layer of piezoelectric material 20 and is disposed at the common point X. Having the transmit electrodes 30 arranged into transmit electrode arrays 35 allows more control over the directionality of the emitted ultrasonic signals 32 as each transmit electrode array 35 extends in a different radial direction and can be individually addressed. Additionally, each of the elongate transmit electrodes 30 may be addressed individually, simultaneously or in predetermined or arbitrarily generated sequences. This permits significantly more complex patterns and / or sequences of ultrasonic signals 32 to be emitted from the ultrasound transducer 100b. Figure 2c illustrates an ultrasound transducer 100c which is comparable to the ultrasound transducer 100a illustrated in Figure 2a. In the ultrasound transducer 100c each of the similar concentric circular transmit electrodes 30 have been circumferentially segmented into four equal segments forming four transmit electrode arrays, each transmit electrode array comprising six transmit electrodes 30. A receive electrode 40 is collocated with the transmit electrodes on the layer of piezoelectric material 20 and is disposed at the common point X. Figure 2d illustrates an ultrasound transducer 100d which is comparable to the ultrasound transducer 100c illustrated in Figure 2c. The ultrasound transducer 100d does not comprise a receive electrode collocated on the piezoelectric material 20. By dispersing the plurality of transmit electrodes 30 and corresponding receive electrodes between ultrasound transducers, the coverage and pattern of emitted ultrasonic signals 32 can be tailored to the monitoring requirements as more complex monitoring systems can be composed. For example, for monitoring an awkwardly shaped entity or focussing monitoring capability on a particular area of an entity where a particular change in property or parameter is expected or would be safety critical to detect. Figure 2e illustrates an ultrasound transducer 100e which is comparable to the ultrasound transducer 100 illustrated in Figure 1a. The ultrasound transducer 100d does not comprise a receive electrode collocated on the piezoelectric material 20. Figure 2f illustrates an ultrasound transducer 10Of which is comparable to the ultrasound transducer 100a illustrated in Figure 2a. In the ultrasound transducer 10Of the transmit electrodes 30 are arranged as a regularly distributed array of eccentric similar ellipses arranged around a common point X on an elliptical layer of piezoelectric material 20. The common point X is laterally offset from the centre of the layer of piezoelectric material 20. A receive electrode 40 is collocated with the transmit electrodes on the layer of piezoelectric material 20 and is disposed at the common point X. By arranging the plurality of transmit electrodes 30 in this way the directionality of the emitted ultrasonic signal can be determined by varying the frequency of the ultrasonic signal emitted by the transmit electrodes 30. For example, as the pitch of the transmit electrodes 30 is different in each radial direction around the common point X, the constructive and destructive interference patterns created by the ultrasonic waves of the ultrasonic signal emitted from the plurality of transmit electrodes 30 in each direction will result in an ultrasonic signal emitted transmitted from the ultrasound transducer 100f in a limited circumferential range. This arrangement allows highly granular directionality of emitted ultrasonic signals using a reduced number of transmit electrodes 30 simplifying the design, layout and manufacture of the ultrasonic transducer 10Of. Although a number of example ultrasound transducer arrangements have been described as set forth above, it should be understood that these examples are illustrative only and that the invention is no limited to these examples. Those skilled in the art will be able to make modifications and alternatives in view of the disclosure which are contemplated as falling within the scope of appended claims. For example, each ultrasound transducer may comprise two or more receive electrodes, ellipsoid transmit electrodes may be circumferentially segmented into 2 or 3 or 5 or more segments, or the ultrasound transducer may comprise a combination of elongate and ellipsoid transmit and / or receive electrodes. It should also be understood that the arrangement of the transmit and receive electrodes of each ultrasound transducers in a single ultrasonic device or a plurality of ultrasonic devices in a monitoring system can be tailored to the monitoring application. Each ultrasound transducer may have the same or different or similar or complimentary arrangement of electrodes. Figures 3a and 3b illustrate alternative example ultrasound transducer 100 arrangements which may be comprised in an ultrasonic device such as the ultrasonic device 1000 of Figure 1. Figure 3a illustrates a plurality of ultrasound transducers 100 disposed on a single common substrate 20. Each ultrasound transducer comprises collocated transmit and receive electrodes configured to emit and receive ultrasonic signals. In this example the common substrate 20 is a rigid-flex PCB which provides both a means of communication between ultrasound transducers 100 (e.g. along electrically conductive tracks printed onto the common substrate 20) in addition to spacing each of the ultrasound transducers 100 at a fixed distance from each of the other ultrasound transducers 100. A common control cable 108 provides a means of connecting the plurality of ultrasound transducers 100 to a control system such as the control system 50 of ultrasonic device 1000 illustrated in Figure 1b. By fixing the distance between each of the ultrasound transducers 100 before installation, the processing of the electrical signals derived from the received ultrasonic signals to produce an image of the monitored entity and / or to identify at least one property or change in a property of the entity is significantly simplified as the arrangement of ultrasound transducers 100 on an entity is repeatable and comparable from one entity and or application to another. Figure 3b illustrates a plurality of discrete ultrasound transducers 100. Each ultrasound transducer comprises collocated transmit and receive electrodes configured to emit and receive ultrasonic signals. Each ultrasound transducer 100 is controlled using one of a plurality of control cables 108 which provide a means of connecting each of the plurality of ultrasound transducers 100 to a control system such as the control system 50 of ultrasonic device 1000 illustrated in Figure 1b. Alternatively each of the plurality of ultrasound transducers 100 may form a part of an ultrasonic device, with each ultrasonic transducer 100 connected to a control system to form a multi-device monitoring system. Discretely wiring the ultrasound transducers 100 as part of a single ultrasonic device, or plurality of ultrasonic devices, allows flexibility in the position and orientation of the ultrasound transducers 100 when mounting or fixing the ultrasound transducers 100 onto an entity to be monitored. For example, the ultrasound transducers 100 can be mounted around protuberances in the surface of the entity and / or the same ultrasound transducers 100 can be re-used to monitor different entities with different physical characteristics or different monitoring requirements. Figures 4a and 4b illustrate alternative example layouts of ultrasound transducers 100 which may be comprised in an ultrasonic device such as the ultrasonic device 1000 of Figure 1. The ultrasound transducers 100 may be disposed on a common substrate or comprise discretely wired sensors. Figure 4a illustrates a plurality of ultrasound transducers 100 in a regular layout whereby the ultrasound transducers 100 are mounted or fixed at regular intervals across the surface of an entity. Figure 4b illustrates a plurality of ultrasound transducers 100 in an irregular layout or pattern whereby the ultrasound transducers 100 are mounted or fixed onto the surface of an entity in a pattern based on the shape or other physical characteristics of the entity or area of the entity to be monitored or the nature of the property or change of property of the entity to be monitored. Figures 5a to 5c illustrate alternative example means for connecting and controlling a plurality of transmit electrodes which may be the plurality of transmit electrodes 30 of any one of the ultrasound transducers illustrated in Figures 1a, 2a to 2f or any other suitable arrangement. Figure 5a illustrates a transmit electrode array 35a comprising six elongate linear transmit electrodes 30 distributed over a direction perpendicular to a longitudinal axis of the plurality of transmit electrodes 30. Each of the transmit electrodes 30 are electrically connected to a common electrically conductive track 38, that is in turn ultimately electrically connected to a control system such as the control system 50 of Figure 1b. By connecting the plurality of transmit electrodes 30 in this way each of the transmit electrodes 30 can be addressed simultaneously with a single electrically conductive track 38 (or other electrical connection). This simplifies the layout and manufacture of the ultrasound transducer. Even with common electrical control, as described above with reference to Figure 2f, depending on the arrangement of transmit electrodes 30 the directionality of the ultrasonic signal emitted from an ultrasound transducer can be controlled by changing the frequency of the alternating electrical driving current transmitted to the plurality of transmit electrodes 30 via electrically conductive track 38. Figure 5b illustrates a transmit electrode array 35b comparable to the transmit electrode array 35a illustrated in Figure 5a. The plurality of transmit electrodes 30 are electrically connected to form two interdigitated sub-arrays of transmit electrodes 30 with each subarray electrically connected to an electrically conductive track 38. Each electrically conductive track 38 is ultimately connected to a control system. By connecting the plurality of transmit electrodes 30 in this way more complex patterns and / or sequences of ultrasonic signals can be emitted from the plurality of transmit electrode arrays 35b. For example, constructive and destructive interference patterns can be created across the surface of the entity to be monitored to include or exclude particular features or areas of interest or disinterest on or in the entity. Figure 5c illustrates a transmit electrode array 35c comparable to the transmit electrode array 35b illustrated in Figure 5b. Each of the plurality of transmit electrodes 30 is electrically connected to an electrically conductive track 38. Each electrically conductive track 38 is ultimately connected to a control system. By connecting the plurality of transmit electrodes 30 in this way yet further complex patterns and / or sequences of ultrasonic signals can be emitted from the plurality of transmit electrode arrays 35c. It would be appreciated that in an ultrasound transducer comprising a plurality of transmit electrode arrays 35a, 35b, 35c electrically connected as illustrated in Figures 5a to 5c or an alternative or combination thereof, each transmit electrode array may be commonly connected. Alternatively, each transmit electrode array may be individually addressable using individual electrically conductive tracks, cables or connectors. Figure 6 illustrates an example monitoring system 2000 comprising a single ultrasonic device 1000. The ultrasonic device 1000 comprises four ultrasound transducers 100 fixed to an entity to be monitored, and a control system 50 to control the ultrasound transducers 100, digitise the ultrasonic signal(s) received by the ultrasound transducers 100, and process, store and / or communicate the digitised ultrasound signals or data produced therefrom. In examples, the control system 50 can be configured to digitise the received ultrasonic signal(s) separately, multiplexed or simultaneously. The control system may be the control system 50 of the ultrasonic device 1000 illustrated in Figure 1b. It would be understood that the monitoring system 2000 may comprise a plurality of ultrasonic devices 1000 and each ultrasonic device 1000 may be fixed or mounted to the same or different entities to be monitored. The monitoring system 2000 further comprises an external processing system 1050 configured to process the data communicated from the control system 50. The data may be representative of the digitised version of the received ultrasonic signal(s) and / or comprise values of properties or changes in properties of the entity. The monitoring system 2000 further comprises external data storage 1060 configured to store the data communicated from the control system 50, output data from the processing system 1050 or the like. The monitoring system 2000 further comprises I / O devices 1070 which may be used to display a status of the monitoring process, raise an alert or alarm based on the monitoring process and / or allow input from a user to control the monitoring process. The external processing system 1050 can receive the data and / or the values of the properties of the entity determined by the control system 50 or each ultrasound transducer 100. In this way, the external processing system 1050 can add additional benefits to the processing performed on the ultrasonic device 1000. For example, the external processing system 1050 can act as centralized remote storage for the data and / or the values of the properties of multiple entities being monitored by one or more ultrasonic devices 1000. In this way, multiple entities / ultrasonic devices 1000 can be monitored by simply accessing the external processing system 1050 via a single remote user interface. Furthermore, the external processing system 1050 can perform further processing on the data and / or the values of the properties of the entity or entities, e.g. to determine data, trends and parameters that depend on multiple entities or depend on the correlation between different entities. In addition, the external processing system 1050 is configured to raise an alert or alarm in response to the determined values of the properties of the entity or entities meeting an alarm condition. This may allow remote workers or monitoring services and the like to be made aware of determined conditions that may require action. In examples, the alert may comprise a message, an email, a pop-up notification, operation of a light or other visual indicator, an audible indicator, a haptic indicator and / or the like in, on or using one of the I / O devices 1070. The external processing system may raise the alert by automatically electronically signalling a user device over a network or internet when the determined data meets the alert or alarm conditions. The data and / or the values of the properties of the entity or entities being monitored is optionally stored on external storage 1060. One or more different types of analysis can be used the external processing system 1050 to determine the values of the properties of the monitored entity or entities, as would be apparent to one skilled in the art. For example, the values of the properties of the entity may be determined qualitatively e.g. using techniques such as Doppler shift analysis or by using time of flight analysis to determine the distance travelled by the ultrasound through the entity before being reflected, diffracted or the like to thereby determine the location of interfaces between regions of different acoustic impedance. In another example, the values of the properties of the entity can be determined by applying a spectral analysis to determine the values of the properties of the entity, for example, by analysing changes in frequency or frequency distribution in the ultrasound during transit through or across the entity. In another example, the values of the properties of the entity may be determined by applying a statistical analysis to determine the values of the properties of the entity, e.g. based on probabilistic techniques. In other examples, neural networks or artificial intelligence techniques such as deep learning can be used to determine the values of the properties of the entity. In these cases, training data can be used to train the neural network to correctly determine the values of the properties of the entity (as an output of the neural network) from the digitized and processed ultrasound signals (as an input of the neural network. Examples of training data include data from historical measurements or from models or simulation. Other examples include training data derived from the output on one of the other analyses such as the quantitative analysis, the spectral analysis and / or the statistical analysis. Figure 7 illustrates an example arrangement of an ultrasonic device mounted to an entity 60. The example entity 60 has an area of corrosion 62 and a crack 64 which represent example properties / defects of the entity 60 which may be determined, tracked and / or imaged by the ultrasonic device. The ultrasonic device comprises five ultrasound transducers 100 disposed in a regular pattern across the surface of the entity 60. Each ultrasound transducer may be the ultrasound transducer 100 illustrated in Figure 1a. Each ultrasound transducer 100 comprises a single receive electrode disposed at the centre of each ultrasound transducer 100 and a plurality of transmit electrodes collocated on each ultrasound transducer 100 and disposed around the receive electrode in transmit electrode arrays. The ultrasonic device further comprises four ultrasound transducers 101 interposed between ultrasound transducers 100 in a regular pattern. Each ultrasound transducer 101 comprises a single receive electrode disposed at the centre of each ultrasound transducer 101. In use, one or more ultrasonic signals are emitted from one or more or all of the ultrasound transducers 100 sequentially, simultaneously or in predetermined sequences or patterns in predetermined directions relative to the ultrasound transducers 100. The ultrasonic signals will be subject to reflections, dispersion or other interference due to the corrosion 62, crack 64, other ultrasound transducers 100, 101, the edge of the entity 60 and / or other non-conformities in or on the entity 60. The ultrasonic signals will be received by the receive electrode of the ultrasound transducers 100, 101 and processed as described throughout this disclosure to produce an image of the entity 60 and / or to identify the corrosion 62, crack 64 or other property or change in a property of the entity 60. Figures 8a to 8c illustrate example entities (pipes 60a, 60b, 60c) which may be monitored using the monitoring system disclosed herein. A plurality of ultrasound transducers 100 are mounted onto the surface of the entities in a regular pattern. The plurality of ultrasound transducers 100 may be connected so as to form a single ultrasonic device or alternatively may be comprised of a plurality of ultrasonic devices to form a monitoring system. The pipes 60a, 60b, 60c may form part of a pipe system which is monitored using a plurality of ultrasonic devices comprised in a monitoring system. By distributing the plurality of ultrasound transducers 100 across the surface of each entity, the entity can be monitored for changes in properties that may affect the safety and / or integrity of the entity even with complex non-planar entities such as pipes 60a, 60b and 60c. In these examples, the ultrasound transducers 100 are mounted to pipes 60a, 60b and 60c. However, the applications are not limited to this and the ultrasound transducers 100 could equally be mounted to and monitor a support, beam, a part of a building or other architectural structure, a piece of plant or machinery, a wind turbine or tower, a naval or sea based structure, or a part of a vehicle, amongst a number of other suitable possibilities. Figure 9 is a flowchart summarizing a method 3000 of monitoring an entity using a monitoring system comprising an ultrasonic device as described throughout this disclosure. The method comprises, at step S100, providing an ultrasonic device onto the surface of an entity to be monitored. The ultrasonic device comprises a plurality of ultrasound transducers which are mounted or fixed onto the entity in a regular pattern. At step S200 the plurality of ultrasound transducers emit one or more ultrasonic signals in a direction generally parallel to the surface of the entity. The ultrasonic signal is subject to reflections, dispersion or other interference due to non-conformities (e.g. defects, edges or other ultrasound transducers) as it propagates across the surface of the entity. At step S300 the one or more ultrasonic signals are received by one or more of the ultrasound transducers and an electrical signal based on and representative of the received ultrasonic signal is transmitted to a control system of the ultrasonic device. At step S400 the electrical signal(s) are processed to produce data therefrom and at step S500 the data is processed to identify a property or change in a property of the entity. Steps S400 and S500 are optional i.e. the electrical signals and / or data may be processed in other ways, stored in on-board storage or communicated to an external processing system to be processed as part of a wider monitoring system or application. 5 Figure 10 is a flowchart summarizing a method 4000 of manufacturing an ultrasonic device as described throughout this disclosure. The method comprises, at step S600, depositing a piezoelectric layer onto a substrate and then, at step S700, disposing a plurality of transmit electrodes onto the piezoelectric layer. The plurality of transmit electrodes are arranged such that they are operable to transmit one or more ultrasonic 10 signals substantially parallel to the surface of the piezoelectric material. The above detailed description of the drawings is provided in order to give an example of how the concepts described herein may be implemented. However, the scope of protection is defined by the claims and alternatives to the specific examples provided 15 above would be apparent to a person skilled in the art and falling within the scope of the claims are intended to fall within the scope of the present disclosure.

Claims

1. An ultrasonic device for a monitoring system comprising:at least one flexible ultrasound transducer, the ultrasound transducer comprising:a substrate;a layer of piezoelectric material arranged or deposited on at least part of the substrate; anda plurality of transmit electrodes or electrical contacts;wherein the plurality of transmit electrodes are arranged such that one or more ultrasonic signals is transmitted substantially parallel to the surface of the piezoelectric material.

2. The ultrasonic device of claim 1 wherein the transmit electrodes of the plurality of transmit electrodes are distributed radially over at least part of a surface of the layer of piezoelectric material.

3. The ultrasonic device of any preceding claim wherein two or more or each of the transmit electrodes or respective tangents of two or more or each of the electrodes are parallel with each other.

4. The ultrasonic device of any one of claims 1 to 3 wherein the plurality of transmit electrodes are arranged as concentric or eccentric polygons or ellipses around the centre of, or another common point on, the piezoelectric material.

5. The ultrasonic device of claim 3 wherein at least one of the polygons or ellipses is circumferentially segmented.

6. The ultrasonic device of any one of claims 1 to 3 wherein the plurality of transmit electrodes are elongate and are arranged in one or more arrays arranged around the centre of, or another common point on, the layer of piezoelectric material, the respective transmit electrodes of each array being distributed over a direction perpendicular to a longitudinal axis of the transmit electrodes of the respective array.

7. The ultrasonic device of any preceding claim further comprising a receive electrode configured to receive an ultrasonic signal and produce an electrical signal responsive to, and indicative of, the received ultrasonic signal;wherein optionally the ultrasonic signal comprises or is derived from the one or more transmitted ultrasonic signals emitted using the plurality of transmit electrodes.

8. The ultrasonic device of claim 7 wherein the plurality of transmit electrodes are provided around the receive electrode or on opposing sides of the receive electrode.

9. The ultrasonic device of claim 7 or claim 8 wherein the receive electrode is collocated with, and / or on the same surface of the piezoelectric material as, the plurality of transmit electrodes.

10. The ultrasonic device of any one of claims 7 to 9 wherein the receive electrode is disposed at, on and / or around the centre of, or other common point on, the layer of piezoelectric material.

11. The ultrasonic device of any preceding claim comprising a plurality of ultrasound transducers, wherein the substrate is a common substrate and the plurality of ultrasound transducers are disposed on the common substrate.

12. The ultrasonic device of any preceding claim wherein the plurality of transmit electrodes are configured to be addressable individually, simultaneously or in predetermined groups, patterns or arrays.

13. The ultrasonic device of claim 12 further comprising:a control system configured to address the plurality of transmit electrodes individually, simultaneously or in predetermined groups, patterns or arrays so as to control the transmission of the one or more ultrasonic signals.

14. The ultrasonic device of any one of claims 7 to 13 further comprising:an on-board processing system for processing the electrical signal to determine data therefrom; andan on-board communications system for communicating the data with an external processing system.

15. The ultrasonic device of claim 14 wherein the on-board processing system is configured to process the electrical signal by performing one or more of: quantitative analysis, spectral analysis, statistical analysis, application of machine learning or artificial intelligence techniques to determine the data.

16. The ultrasonic device of claim 14 or claim 15 further comprising on-board data storage configured to store the data.

17. The ultrasonic device of claim 16 further comprising a power source for powering the at least one ultrasound transducer, the on-board processing system, the communications system and / or the on-board data storage.

18. The ultrasonic device of any preceding claim, configured or configurable for mounting or fixing to an entity.

19. A monitoring system comprising one or more ultrasonic devices according to any of claims 1 to 1820. The monitoring system of claim 19 when dependent on any one of claims 14 to 18 further comprising:an external processing system;wherein the external processing system is configured to process the data communicated from the on-board communications system to produce an image of the entity and / or to identify at least one property or change in a property of an entity to which the monitoring system is attached or mounted.

21. The monitoring system of claim 20 further comprising external data storage configured to store at least one of:the data communicated from the on-board communications system;the image of the entity; andthe at least one property or change in property of the entity.

22. The monitoring system of claim 20 or claim 21 wherein the external processing system is configured to process the data by performing one or more of: quantitativeanalysis, spectral analysis, statistical analysis, application of machine learning or artificial intelligence techniques.

23. A method of monitoring an entity using the monitoring system of any one of claims 19 to 22, the method comprising:providing at least one of the ultrasonic devices of claims 1 to 18 into or onto the entity;transmitting one or more ultrasonic signals substantially parallel to the surface of the piezoelectric material;receiving one or more ultrasonic signals and producing one or more electrical signals responsive to, and indicative of, the one or more received ultrasonic signals;processing the one or more electrical signals to determine data therefrom; and processing the data to produce an image of the entity and / or to identify at least one property or change in a property of the entity;wherein the one or more received ultrasonic signals comprises or is derived from the one or more transmitted ultrasonic signals.

24. The method of claim 23 wherein the processing is performed by the on-board and / or external processing system and comprises processing the data by performing one or more of: quantitative analysis, spectral analysis, statistical analysis, application of machine learning or artificial intelligence techniques.

25. A computer program product that, when implemented on a controller or the on-board processing system of the ultrasonic device(s) of any of claims 1 to 22 or the external processing system of any one of claim 20 to 22, causes the controller or on-board processing system to control the ultrasonic device(s) to perform the method of claim 23 or claim 24.

26. A method of manufacturing the ultrasonic device of claim 1, the method comprising: providing or depositing a layer of piezoelectric material on at least a part of a substrate; andproviding a plurality of transmit electrodes or electrical contacts;wherein the plurality of transmit electrodes are arranged such that the plurality of transmit electrodes are operable to transmit one or more ultrasonic signals substantially parallel to the surface of the piezoelectric material.5

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