Ultrasonic Transducer Housing

Abstract

A piezoelectric micromachined ultrasonic transducer (PMUT) (302) interfaces with an amorphous medium such that, in use, an ultrasonic signal to or from the PMUT (302) passes through the amorphous medium. The amorphous medium has a plurality of discrete reflectors (110a, 110b, 111a, 111b) distributed therein. A method of imaging using a PMUT or an array of PMUTs (302) includes determining an acoustic transfer function corresponding to the effect of the plurality of discrete reflectors (110a, 110b, 111a, 111b) positioned between the PMUT or PMUT array (302) and a scene to be imaged, imaging the scene using the PMUT or PMUT array (302) by transmitting an ultrasonic signal and receiving the ultrasonic signal at the PMUT or PMUT array (302) after receiving one or more reflections, and processing the received ultrasonic signal to generate an image by applying an inverse of the acoustic transfer function to the received ultrasonic signal.

Description

[Technical field]

[0001] The present invention relates to a housing for an ultrasonic transducer. [Background technology]

[0002] Ultrasonic transducers are devices that generate and receive sound waves at frequencies higher than those audible to humans. They can be used in many applications, from simple ranging applications where the distance to an object can be estimated by measuring the time between sending an ultrasonic signal and receiving the reflected echo signal, to complex medical imaging applications.

[0003] In many applications, it is important to make the transducer as small as possible, either because it is to be mounted in a small device or to allow large arrays to be used. One technology that has been developed for this purpose is that of the piezoelectric micromachined ultrasonic transducer (PMUT), with PMUT elements typically being able to function as both transmitters and receivers when coupled to appropriate circuitry.

[0004] A housing for the ultrasonic transducer can be used to protect the PMUT from external wear and damage. Summary of the Invention

[0005] It is an object of the present invention to provide improvements in certain aspects over currently available PMUTs.

[0006] Viewed from a first aspect, the present invention provides a piezoelectric micromachined ultrasonic transducer (PMUT) arranged to interface with an acoustically resonant cavity, the acoustically resonant cavity having at least one acoustic property that is adjustable in use in response to a signal transmitted or received by the PMUT.

[0007] The present invention extends to a method of imaging using the PMUT described above.

[0008] Thus, those skilled in the art will appreciate that, in accordance with the present invention, the acoustic properties of the acoustically resonant cavity with which the PMUT interfaces can be adjusted. By adjusting the properties of the acoustically resonant cavity, the outgoing or incoming ultrasonic impulse or signal can be modified. Applicant has recognized that this can be beneficial, for example, to improve ultrasonic imaging by improving the efficiency with which the PMUT produces an image.

[0009] In one set of embodiments, the adjustable characteristic is the acoustic resonant volume of the acoustic resonant cavity. For example, the acoustic resonant volume of the acoustic resonant cavity can be adjusted to match its resonant frequency to the transmission frequency of the transmitted ultrasonic signal. This allows the cavity to be optimized even when the frequency used is not constant, as is the case in typical applications. By matching the resonant frequency of the cavity to the transmission frequency, the effective signal strength can be maximized for a given drive power. Similarly, by matching the resonant frequency to that of the incoming signal, the effective received signal strength of the ultrasonic receiver can be maximized.

[0010] The acoustically resonant volume can be adjusted in a number of ways, in one set of embodiments the acoustically resonant cavity comprises a deformable diaphragm, the position and / or shape of which is adjustable to adjust the acoustically resonant volume of the cavity, hi some embodiments the PMUT is mounted on a deformable diaphragm such that the PMUT itself moves with the diaphragm to adjust the volume within the acoustically resonant cavity.

[0011] Alternatively, to adjust the characteristics of the acoustically resonant cavity, in one set of embodiments, the acoustically resonant cavity includes an amorphous medium through which the ultrasonic signal passes in use, the amorphous medium including a plurality of discrete reflectors distributed therein, the discrete reflectors having positions that are adjustable in use to adjust the acoustic characteristics of the acoustically resonant cavity.

[0012] The applicant has recognised that having a plurality of discrete reflectors within an amorphous medium is in itself novel and inventive and therefore according to a second aspect of the present invention there is provided a piezoelectric micromachined ultrasonic transducer (PMUT) arranged to interface with an amorphous medium such that, in use, an ultrasonic signal to or from the PMUT passes through said amorphous medium, said amorphous medium comprising a plurality of discrete reflectors dispersed therein.

[0013] The present invention extends to a method of imaging using the PMUT described above.

[0014] The discrete reflectors may be a plurality of physically separate reflectors, e.g. completely physically separate reflectors, e.g. reflectors dispersed in an amorphous medium. However, the discrete reflectors may be separate parts of a structure. One or more discrete reflectors may be in contact with at least one other discrete reflector, e.g. one or more discrete reflectors may be touching or physically connected. For example, the discrete reflectors may form a structure with a plurality of holes. Thus, in one set of embodiments, the discrete reflectors are elements of a structure with a plurality of holes.

[0015] Thus, in some embodiments, the discrete reflectors form a mesh, e.g., a fiber mesh, such that the plurality of discrete reflectors are elongated, e.g., fiber. The mesh may include overlapping discrete reflectors. The mesh may be a woven mesh.

[0016] In other embodiments, the discrete reflectors form a porous membrane, with the plurality of discrete reflectors being material between the pores of the membrane. The porous membrane may be synthetic or natural and may be formed by perforation.

[0017] The discrete reflectors may be rigid. Alternatively, the discrete reflectors may be deformable, e.g. flexible or elastic.

[0018] The structures may be in the form of a relatively thin layer such that the reflectors are distributed in two dimensions across a surface. Such a surface may be, but is not necessarily, substantially flat. In some embodiments, however, the discrete reflectors are distributed in three dimensions.

[0019] The pores need not be uniform, but may for example be non-uniformly spaced and non-uniformly distributed throughout the structure.

[0020] The structure, such as a mesh or porous membrane, can introduce sources of diffraction and / or reflection that affect the ultrasonic signals transmitted from and / or received by the PMUT. The structure, especially if the structure is a mesh or porous membrane, can also advantageously provide environmental protection.

[0021] When the PMUT is used for imaging, as described above, the structure having holes is preferably positioned between the PMUT and the scene to be imaged, e.g., in the path of a signal transmitted or received by the PMUT. Those skilled in the art will appreciate that the structure may serve to effectively widen the field of view of the PMUT due to perturbations caused by discrete reflectors in the structure.

[0022] This can be accomplished by the beam spreading effect that discrete reflectors have on the ultrasound signal transmitted through the amorphous medium. As the ultrasound beam propagates through the amorphous medium, reflection and diffraction at the discrete reflectors expand the direction of the ultrasound signal, resulting in an overall increase in the divergence of the ultrasound beam as it travels through the amorphous medium. This effectively spreads out the ultrasound signal to provide a more omnidirectional output, widening the field of view. This effect is also frequency dependent, as components of the ultrasound signal having different frequencies are reflected at different angles. Additionally, knowing the effect of the structure on the ultrasound signal, for example by knowing its transfer function, beneficially provides more information to be used in signal processing when using PMUTs for imaging.

[0023] The amorphous medium may be acoustically reflective or non-reflective, and in one set of embodiments the amorphous medium comprises a gel. The amorphous medium may alternatively comprise a liquid or a gas, for example within an acoustically resonant cavity. For example, the amorphous medium may comprise air.

[0024] In one set of embodiments of the second aspect of the invention, the discrete reflectors have positions that are adjustable in use to adjust the above-mentioned acoustic properties of the acoustically resonant cavity, which can provide the advantages described according to the first aspect of the invention.

[0025] Furthermore, by adjusting the position of the reflector, the direction of any output signal, as well as the density of the medium in front of the PMUT, can be adjusted to create local sound speed changes, which can be used, for example, to create a lens effect in front of the PMUT.

[0026] The position of the discrete reflectors can be adjusted in a number of ways. In one set of embodiments, the discrete reflectors are metallic and the position of the discrete reflectors is adjusted using a magnetic field. In a further set of embodiments, piezoelectric contraction elements are arranged around the acoustically resonant cavity. When a current is applied to such piezoelectric contraction elements, the elements contract, thus acting to squeeze the acoustically resonant cavity and adjust the position of the reflectors within the penetration force transmitted through the amorphous medium from the contraction force.

[0027] In one set of embodiments, the PMUT comprises an ultrasonic transmitter, and the acoustically resonant volume is adjusted to match the transmitted signal. For example, if the PMUT is used to transmit signals having widely varying frequencies, the acoustically resonant volume can be adjusted so that the resonant frequency of the cavity matches the resonant frequency of the current transmitted signal to optimize transmission. As the frequency of the transmitted signal changes, the volume of the acoustically resonant cavity can be changed to match. In contrast to the non-tunable cavities of the prior art, high power variable frequency signals can be more easily achieved using the cavities according to the invention.

[0028] Typically, prior art methods of generating broadband signals using PMUTs have been to change the output energy of the PMUT when adjusting the frequency of the signal emitted by the PMUT. However, to emit a chirp from frequency F0 to F1, this is not energy efficient because the resonant peak may be (F1+F0) / 2. To achieve a flat output spectrum, it may be necessary to output less energy at this resonant frequency, which is not efficient. Thus, the overall output energy is reduced, which may reduce the effectiveness of the PMUT for high sensitivity imaging applications.

[0029] In contrast, according to the present invention, the acoustically resonant volume of the cavity can be adjusted as the frequency of the signal radiated by the PMUT during chirp transmission is changed. This can reduce the extent to which the output energy needs to be reduced, allowing a relatively flatter output spectrum to be achieved at a higher overall output energy compared to the prior art methods described above. Thus, the output energy may not require the same degree of adjustment as the frequency is changed to achieve a flat broadband output signal, and thus the PMUT can be used for applications requiring high output energy, such as long-range imaging, or imaging requiring good SNR (such as super-resolution imaging). The transmitted output signal is preferably pre-calculated and stored in memory prior to transmission, rather than adjusted during transmission.

[0030] However, other parameters can be adjusted to lower the output signal energy, for example the PMUT itself and the "shape-changing" housing.

[0031] In one set of embodiments, the PMUT comprises an ultrasonic receiver, and the acoustically resonant volume is adjusted to match the received signal. If the PMUT is used to receive a reflected chirp, any output frequency can be received at any time as a result of echoes from objects at different distances resulting from the transmitted chirp signal. Once the inexact distance to the reflecting object of interest is known, the approximate expected time of the reflected echo is also known. Thus, in accordance with the present invention, the volume of the acoustically resonant cavity can be adjusted at this time to amplify the incoming signal at the time it is expected to arrive, thus resulting in better signal reception.

[0032] In one set of embodiments, the PMUT comprises a dedicated ultrasonic transmitter and at least one separate ultrasonic receiver on a single common semiconductor die. This allows for simultaneous transmission and reception of signals, and therefore no switching electronics are required to switch between "transmit mode" and "receive mode". This reduces the complexity of the system electronics. Further advantages of having a dedicated ultrasonic transmitter and at least one separate ultrasonic receiver on a single common semiconductor die are described in WO 2021 / 079160.

[0033] In one set of embodiments, the cavity comprises an opening for outputting and / or receiving ultrasonic signals. In such embodiments, the acoustically resonant cavity may form a Helmholtz resonator.

[0034] In one set of embodiments, the PMUT is mounted so as to be movable by an actuator, e.g., a motor, which may be used to move the PMUT to radiate signals in different directions, for example, when studying the effect of an acoustically resonant cavity on any transmitted or received signal.

[0035] The present invention also provides a system for transmitting and receiving ultrasonic signals comprising at least a first PMUT and a second PMUT as described in any of the above aspects or embodiments, a transmitter circuit configured to drive the first PMUT as an ultrasonic transmitter, and a receiver circuit configured to detect signals from the second PMUT as an ultrasonic receiver.

[0036] In one set of embodiments of the first or second aspect of the invention, there is provided an arrangement comprising a plurality of the PMUTs described above. In one such set of embodiments, the plurality of PMUTs are arranged in a mosaic array.

[0037] It will be appreciated that while the PMUTs in a mosaic array may be separated by any length, in one set of embodiments, the PMUTs are spaced apart by λ / 2, where λ is the wavelength of the center frequency over the range of frequencies that the PMUTs transmit or receive. As will be appreciated by those skilled in the art of array signal processing, this is the optimal spacing for performing beamforming and the like. Thus, the array can be used as a "one common sensor", i.e., by adding or averaging the signals coming from them, or by using them to alternatively transmit the same signal, but their inputs or outputs can be used individually as part of an array processing method that processes each of the PMUT elements individually. This has certain advantages, such as the ability to better focus on or cancel sounds coming from a particular direction.

[0038] In one set of embodiments, an array of PMUTs is arranged to interface with an acoustically resonant cavity comprising an acoustically non-reflective amorphous medium comprising a plurality of discrete acoustic reflectors.

[0039] The characteristics of the acoustically resonant cavities of an array of PMUTs can be tailored in the same manner as the acoustically resonant cavities described above for PMUTs.

[0040] The determined phase adjustments may be applied to signals from each transmitter or receiver to enable them to function as a coherent array for beamforming, for example. Beam steering may be used on the transmitted ultrasonic signal, the reflected ultrasonic signal, or both. To steer the transmitted ultrasonic signal, the determined phase adjustments may be applied to the signal transmitted by each PMUT in the array such that the resulting transmitted ultrasonic signal is interfered with resulting in an overall signal that is transmitted in the desired direction. Received and reflected ultrasonic signals may be steered in a similar manner. The determined phase adjustments may be applied to received signals from all directions to determine reflected signals from a single direction in the surrounding structure. As discussed above, applying the determined phase adjustments to the transmitted / received signals further enables steering of the transmitted / received signals, as opposed to using an array as a "one common sensor" that transmits in only one direction, although the signal is enhanced by the use of multiple PMUTs.

[0041] Changing the position of the acoustic reflector can be used to "steer" the transmitted or received ultrasonic signal through the cavity. To achieve high quality signal transmission and reception, it is often desirable to move the position of the PMUT during transmission and reception, but this is clearly impractical. By adjusting the position of the acoustic reflector to adjust the properties of the medium through which the signal is transmitted or received, it may be possible to achieve a net effect similar to adjusting the position of the PMUT itself.

[0042] According to the present invention, when transmitting a chirp signal from F0 to F1, the position of the acoustic reflector can be adjusted within the medium such that at any given time, the PMUTs are "spaced" half a wavelength apart for the current frequency. Thus, by adjusting the acoustic reflector, better imaging capabilities can be achieved at all frequencies. By changing the position of the acoustic reflector in front of the array of PMUTs, a lens effect can be created that steers the ultrasound signal.

[0043] In one set of embodiments, one or more housing layers are disposed above an acoustically resonant cavity that includes the PMUT array. One or more of the housing layers may be compressed, for example by piezoelectric contraction elements surrounding them, thus deforming the or each such housing layer. This changes the pressure within each layer, and the change in pressure creates an acoustic gradient that can provide a "lensing effect."

[0044] According to a further aspect of the invention, a plurality of acoustically resonant cavities are provided, each comprising an array of PMUTs arranged to interface with the acoustically resonant cavity, further comprising an amorphous medium having discrete acoustic reflectors. Such an "array of arrays" provides an additional advantage since individual driver electronics can be provided for each "mini-array", e.g., as described above, in an acoustically resonant cavity comprising an amorphous medium having discrete acoustic reflectors. Thus, the only information that can be provided for each array is the direction and intensity of energy, which is generally less information than the signal waveform provided for each element in the array.

[0045] In one set of embodiments, each cavity in the plurality of cavities is coupled to a solid substrate. In an alternative set of embodiments, each cavity in the plurality of cavities is arranged on a common damping medium. In a further alternative set of embodiments, each cavity in the plurality of cavities is arranged in a common damping medium. The damping medium may be an amorphous material, a semi-solid material, an acoustically dielectric material, or a gel, among others.

[0046] An "array of arrays" also reduces the density of PMUTs in the entire array, so that each "mini-array" can be isolated from the others, and crosstalk can also be more easily reduced. By having an "array of arrays," the overall PMUT density is reduced because the number of PMUTs is reduced compared to a single array, and the arrays do not need to be spaced as closely together, with all PMUTs spaced apart by λ / 2. For example, arrays bonded to a solid substrate can each be mounted on a "pillar" that is isolated from the solid substrate that connects them, and thus energy does not propagate between the arrays. Alternatively, the medium on which the PMUTs can be arranged can be a foam with minimal acoustic transmission capabilities.

[0047] According to a further aspect of the invention, there is provided an arrangement comprising a plurality of PMUTs arranged to interface with respective acoustic tubes, the PMUTs being arranged in a mosaic array.

[0048] In any of the PMUT arrays described above, in one set of embodiments the array is mounted so that it is movable by an actuator, e.g., a motor, which moves the array. Similar to the actuators described above that move the PMUTs, this actuator can be used to change the position of the array to learn directional impulse responses in different directions.

[0049] In any of the above arrays, the array may be 3D, i.e., not flat. A 3D array structure may allow for more acoustic output energy on a surface, since each PMUT transmitter produces a signal that combines with signals from other transmitters.

[0050] Additionally, different PMUTs in the array may be different sizes and therefore each have a different individual resonant frequency.

[0051] For example, if the properties of an acoustically resonant cavity or medium interfacing with the PMUT array are adjusted, the motor can be used to "learn" how this shaping affects the array acoustic response, and this information can be further used during transmission for better focusing of the ultrasonic signal, as the effect of the adjusted cavity on the transmitted signal will be learned.

[0052] In one set of embodiments, an input power signal is provided to each array, and further information such as signal direction / pattern, strength and calibration information is provided wirelessly, such that multiple arrays provide a sensor network.

[0053] The reflected signals received at the PMUT array(s) may undergo signal processing, for example, using any suitable image reconstruction technique to generate an image. The signal processing preferably includes beamforming and may be performed in the digital domain. The signal processing may be performed using any suitable processor(s).

[0054] According to a third aspect of the invention there is provided a method of imaging using a PMUT or PMUT array comprising the steps of: determining an acoustic transfer function corresponding to the acoustic effect of a plurality of discrete reflectors positioned between the PMUT or PMUT array and the scene to be imaged; imaging the scene using the PMUT or PMUT array by transmitting an ultrasonic signal and receiving the ultrasonic signal at the PMUT or PMUT array after receiving one or more reflections; processing the received ultrasound signals to generate an image by applying an inverse of an acoustic transfer function to the received ultrasound signals; A method is provided, comprising:

[0055] In some embodiments the method is applied to a PMUT array comprising a plurality of PMUTs according to the second aspect of the invention.In one set of embodiments the plurality of PMUTs are arranged in a mosaic array.

[0056] Those skilled in the art will appreciate that the acoustic transfer function is a mathematical function of the effect of multiple discrete reflectors on an image generated by a PMUT or PMUT array. By using the determined acoustic transfer function when processing the received signal, a clearer image can be formed. As described above, a PMUT or PMUT array can have a wider field of view, for example, due to the presence of discrete reflectors in front of the PMUT. Thus, the method provides imaging that achieves a wider field of view without sacrificing image quality.

[0057] The acoustic transfer function can be determined in a variety of ways. The acoustic transfer function may be pre-determined and stored in the memory of the processor. In one set of embodiments, the acoustic transfer function is determined, for example, using an iterative algorithm and stored in the memory of the processor.

[0058] The acoustic transfer function may include a directional filter, for example a directional impulse response.

[0059] In one set of embodiments, the determining step of the third aspect comprises: (a) selecting an acoustic transfer function to apply to received signals to generate an image; (b) imaging using the PMUT or PMUT array by transmitting an ultrasonic signal and receiving the ultrasonic signal at the PMUT or PMUT array after one or more reflections; (c) using the inverse of the acoustic transfer function to process the received signal to generate an image; (d) determining a sharpness parameter for the image, the determined sharpness parameter being: (i) adjusting the transfer function before repeating steps (b)-(d) when the predetermined threshold is not exceeded; (i) terminating the method when a predetermined threshold is exceeded.

[0060] According to a further aspect, the present invention provides a method for determining an acoustic transfer function for imaging using a PMUT or PMUT array, comprising the steps of: (a) selecting an acoustic transfer function to apply to received signals to generate an image; (b) imaging using the PMUT or PMUT array by transmitting an ultrasonic signal and receiving the ultrasonic signal at the PMUT or PMUT array after one or more reflections; (c) using the inverse of the acoustic transfer function to process the received signal to generate an image; (d) determining a sharpness parameter for the image, the determined sharpness parameter being: (i) adjusting the transfer function before repeating steps (b)-(d) when the predetermined threshold is not exceeded; (i) terminating the method when a predetermined threshold is exceeded.

[0061] Thus, the method can effectively allow for "tuning" the acoustic transfer function to improve the quality of the image obtained using multiple discrete reflectors. For example, if the "wrong" acoustic transfer function is selected, the image obtained from ultrasound imaging will almost always be "blurry". Thus, the sharpness of the image can be used as a criterion for updating the acoustic transfer function selected to obtain the image.

[0062] The sharpness parameter may be a measure such as image sharpness (see https: / / ieeexplore.ieee.org / document / 6783859). Alternatively, the ratio of low reflectance values ​​(close to 0) to high reflectance values ​​may be used to calculate the sharpness parameter.

[0063] The acoustic transfer function may be adjusted using past acoustic transfer functions stored in a memory, for example a server.

[0064] Features of any aspect or embodiment described herein may be applied to other aspects or embodiments described herein, where appropriate. When reference is made to different embodiments or sets of embodiments, it should be understood that these are not necessarily separate and may overlap.

[0065] Specific embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [Brief description of the drawings]

[0066] [Figure 1] 1 is a diagram of a PMUT according to a first embodiment of the present invention; [Figure 2A] FIG. 2 is a diagram of a PMUT according to a second embodiment of the present invention. [Figure 2B] FIG. 4 is a further diagram of a PMUT according to a second embodiment of the present invention. [Diagram 3] FIG. 11 is a diagram of a PMUT according to a third embodiment of the present invention. [Figure 4A] FIG. 2 illustrates the PMUT of FIG. 1 having a transmit chirp signal with a first cavity volume. [Figure 4B] FIG. 2 illustrates the PMUT of FIG. 1 having a transmit chirp signal with a second cavity volume. [Figure 5A] FIG. 1 shows a number of PMUTs arranged in a mosaic array and interfacing with an amorphous medium comprising a random reflector. [Figure 5B] FIG. 1 shows a number of PMUTs arranged in a mosaic array and interfacing with an amorphous medium with regularly spaced reflectors. [Figure 6A] FIG. 1 illustrates multiple PMUTs arranged in a mosaic array within an acoustically resonant cavity and interfacing with an amorphous medium with random reflectors. [Figure 6B] FIG. 1 illustrates multiple PMUTs arranged in a tessellated array within an acoustically resonant cavity and interfacing with an amorphous medium with regularly spaced reflectors. [Figure 7A]FIG. 1 shows multiple PMUTs arranged in a mosaic array and interfacing with an amorphous medium with randomly moving reflectors. [Figure 7B] FIG. 1 shows multiple PMUTs arranged in a mosaic array interfacing with an amorphous medium with regularly spaced reflectors that are moved to tailor the properties of the medium. [Figure 8A] FIG. 6C shows the PMUT array and cavity of FIG. 6B along with a diagram of the acoustic transfer function used to model the effect of the reflector. [Figure 8B] FIG. 8C shows the PMUT array of FIG. 8B with a diagram of the corrected acoustic transfer function. [Figure 9] FIG. 6C shows an array of the PMUTs of FIG. 6B being used to image a known object. [Figure 10] FIG. 1 shows a single transmitter and a single receiver being used to image a single reflector. [Figure 11] FIG. 1 illustrates a PMUT array interfacing with a cavity that is larger than the PMUT array. [Figure 12A] FIG. 1 shows an array of arrays attached to a solid substrate. [Figure 12B] FIG. 1 shows an array of arrays arranged on a medium. [Figure 12C] FIG. 1 shows an array of arrays arranged in a medium. [Figure 13] FIG. 12B shows the array of the array of FIG. 12A in use for imaging an object. [Figure 14] FIG. 1 illustrates a non-planar PMUT array. [Figure 15] FIG. 1 shows an array of arrays, each array interfacing with an acoustic port. [Figure 16A] FIG. 13 illustrates a PMUT array arranged in a stack with multiple layers having different impedances arranged in front of the PMUT array. [Figure 16B] FIG. 16B is an exploded view of FIG. 16A. [Figure 16C] FIG. 16B illustrates the force exerted by the piezoelectric strip shown in FIG. 16A. [Figure 16D] FIG. 16E illustrates the deformation of the layer caused by the forces shown in FIG. 16D. [Figure 17] 1 is a flow chart illustrating a method for updating a directional impulse response to improve the quality of a captured image. [Figure 18] FIG. 5B is a three-dimensional view of the embodiment shown in FIG. 5A. [Figure 19] FIG. 5B is a three-dimensional view of a variation of the embodiment shown in FIG. 5A. [Figure 20] FIG. 13 is a series of plots illustrating how the frequency response of the PMUT and housing can affect the emitted signal. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0067] 1 shows a highly schematic diagram of a PMUT 2 arranged within an acoustically resonant cavity 4, according to one embodiment of the present invention. The acoustically resonant cavity 4 has two openings 6a, 6b through which ultrasonic signals can be transmitted or received. The acoustically resonant volume 7 of the cavity 4 is adjustable using a deformable diaphragm 8.

[0068] The deformable diaphragm 8 can be made from any suitable material, such as a piezoelectric material. The piezoelectric material deforms when a voltage is applied, with the degree of deformation varying depending on the voltage applied and the material used. Common piezoelectric materials include PZT (lead zirconate titanate), KNN ((K,Na)NbO3), ZnO (zinc oxide), BaTiO3 (barium titanate), PMN-PT (Pb(Mg1 / 3Nb2 / 3)O3-PbTiO3), and aluminum nitride (AlN). The deformable diaphragm is attached inside the top of the cavity 4 and bonded to the edges of the cavity 4 such that when a voltage is applied, the edges of the diaphragm 8 do not move, but the unfastened center of the diaphragm 8 deforms, reducing the volume 7 within the cavity. Thus, a controllable driving voltage is supplied to the diaphragm 8 so that the volume 7 of the cavity 4 can be adjusted.

[0069] The position and / or shape of the deformable diaphragm 8 can be adjusted and, thus, the resonant volume of the cavity 4. The acoustic resonant cavity may be a Helmholtz resonator.

[0070] The PMUT2 may be an ultrasonic transmitter, an ultrasonic receiver, or both.

[0071] 2A and 2B show highly schematic diagrams of a PMUT 102 arranged within an acoustically resonant cavity 104 in accordance with a second embodiment of the present invention. The embodiment shown in Figures 2A and 2B has four openings 106a-106d through which ultrasonic signals can be transmitted or received. As with the embodiment of Figure 1, the PMUT 102 may be a transmitter, a receiver, or both.

[0072] The volume of the acoustically resonant cavity 104 is adjustable. The acoustically resonant cavity 104 comprises a deformable diaphragm 108 on which the PMUT 102 is disposed. The position or shape of the diaphragm 108, and thus the PMUT 102, can be adjusted to change the volume of the acoustically resonant cavity 104.

[0073] In Figure 2A, the diaphragm 108 is shown in a deformed position, reducing the volume within the cavity 104. In Figure 2B, the diaphragm 108 is shown in a relaxed position, with the arrow indicating the possible upward deformation of the diaphragm 108 and PMUT 102 as shown in Figure 2A.

[0074] When the PMUT 2, 102 shown in Figures 1, 2A and 2B is used to transmit an ultrasonic signal, the acoustically resonant volume of the cavity 4, 104 can be adjusted to match the emission frequency of the transmitted ultrasonic signal. This is particularly relevant when the PMUT 2, 102 is used to generate a variable frequency signal. If the PMUT 2, 102 is transmitting at a frequency different from the resonant frequency of the cavity 4, 104, a variable frequency transmit signal can be achieved more easily than from a non-tunable cavity. The system can transmit a chirp (e.g., a continuously increasing and decreasing frequency transmission) from frequency F0 to F1 by adjusting the volume of the acoustically resonant cavity 4, 104. Thus, a flatter output spectrum can be achieved across the frequency range F0 to F1 without any adjustment of the input energy, and as a result, the transmit signal can have a high output energy and can be used, for example, for applications requiring a wide range or good SNR.

[0075] When the PMUT 2, 102 shown in Figures 1, 2A and 2B is used to receive reflected chirps, any frequency can be received at any time as a result of echoes from objects at different distances resulting from the transmitted chirp signal. When the inexact distance to the reflecting object of interest is known, the approximate expected time of the reflected echo is also known. Therefore, the volume of the cavity 4, 104 can be adjusted at this point to amplify the incoming signal at the time it is expected to arrive, thus resulting in better signal reception.

[0076] 3 shows a highly schematic diagram of a PMUT 202 arranged within an acoustically resonant cavity 204 in accordance with a third embodiment of the present invention. Similar to the embodiment shown in FIGS. 2A and 2B, the acoustically resonant cavity 204 has four openings 206a-d. Also within the acoustically resonant cavity 204 is an amorphous medium comprising a plurality of discrete reflectors 10. The reflectors 10 can be adjusted during use, for example the positions of the reflectors 10 can be adjusted to adjust the acoustic properties of the acoustically resonant cavity 204.

[0077] For example, the amorphous medium may be a gel, and the reflector 10 includes small magnetized metal balls embedded in the gel. A controllable magnetic field (not shown) can then be applied to the cavity 204, resulting in a change in the position of the reflector 10. Additionally, the density of the medium may be altered by the movement of the metal balls due to the application of the magnetic field. In this way, the gel may act like a spring, compressing when the reflector is subjected to a force due to the magnetic field and expanding when the force is removed. For example, if a magnetic field is used to pack the metal balls very tightly in front of the PMUT, the density will increase, whereas if the metal balls are spread evenly throughout the medium, the density of the medium "in front" of the PMUT will decrease. The change in density of the medium may alter the local speed of sound, which may further be used to create a lens effect on the transmitted ultrasound signal.

[0078] 4A and 4B show the PMUT 2 and acoustically resonant cavity 4 of FIG. 1 together with a transmitted chirp signal 12. In FIG. 4A, the diaphragm 8 is only slightly deformed and therefore the volume of the acoustically resonant cavity 4 is relatively large. As a result, the resonant frequency 14a of the cavity 4 is a low frequency, as shown at a frequency similar to the onset frequency of the chirp signal 12.

[0079] In contrast, in Figure 4B, the diaphragm 8 has been further deformed, thus reducing the volume of the acoustic resonant cavity 4 compared to that of Figure 4A. As a result, the resonant frequency 14b of the cavity 4 is at a higher frequency than the resonant frequency 14a shown in Figure 4A due to the reduced volume of the resonant cavity 4 in Figure 4B.

[0080] 4A and 4B thus show how the resonant frequency of the acoustic resonant cavity 4 can also be adjusted by adjusting the volume of the acoustic resonant cavity 4. Thus, when the PMUT 2 is transmitting a chirp signal, the volume of the acoustic resonant cavity 4 can be continuously adjusted using the adjustable diaphragm 8 during the transmission of the chirp signal 12. Thus, the resonant frequency of the cavity 4 can be matched to the transmission frequency such that a relatively flat spectrum is output with high total energy, so that the chirp signal can be used for imaging applications that require range or good SNR. For example, super-resolution imaging methods generally rely on a high SNR. See, for example, Christensen-Jeffries, K. et al., "Super-resolution ultrasound imaging," Ultrasound in Medicine & Biology, 2020, 46(4), 865-891. A better SNR results in better ultrasound detection and more effective beamforming in array beamforming applications. In addition to this, a sufficiently sensitive ultrasound receiver with a good SNR reduces the need for excessive output power (i.e., less need for a strong signal to improve the SNR), reducing the need for excessive power usage within the device.

[0081] 5A and 5B show an embodiment of another aspect of the invention comprising a plurality of PMUTs 302 arranged in a mosaic array. The array of PMUTs 302 is arranged to interface with a cavity 304. The cavity 304 contains a non-reflective amorphous medium 16 that includes a plurality of discrete reflectors 110. The array of PMUTs 302 is bonded to a solid substrate 18, with the amorphous medium 16 and reflectors 110a, 110b extending between each of the PMUTs 302 in the array.

[0082] The reflectors 110a, 110b may be randomly spaced throughout the amorphous medium 16, as shown by reflector 110a in FIG. 5A, or may be arranged in a regular pattern, as shown by reflector 110b in FIG. 5B.

[0083] A three-dimensional representation of the embodiment shown in Figure 5A is shown in Figure 18. Figure 18 shows a PMUT array 302, a substrate 18, and an amorphous medium 16 comprising a random reflector 110a.

[0084] FIG. 19 shows a variation of the embodiment shown in FIG. 18. FIG. 19 shows a PMUT array 302, substrate 18 and amorphous medium 16 as shown in FIG. 18, with an irregular structure, i.e., random reflectors in the form of overlapping fibers 111a, 111b forming a network or "mesh" of discrete fiber reflectors 111a, 111b. As can be seen in FIG. 19, the mesh has some holes that allow the signal to pass through the amorphous medium 16. The beam spreading effect provided by the mesh in FIG. 19 helps to increase the field of view of the PMUT array 302, which allows more information to be captured when imaging using the PMUT array 302.

[0085] 6A and 6B show an array of PMUTs 302 similar to that shown in FIGS. 5A and 5B, but with a cavity 404 extending over the amorphous medium 16 and reflectors 110a, 110b and having an acoustically transparent opening 306 through which ultrasonic signals can be transmitted or received.

[0086] 7A and 7B are similar to those shown in FIGS. 5A-6B and show an array of PMUTs 302 arranged to interface with a cavity 304. FIG.

[0087] 7A shows discrete reflectors 210a that are free to move throughout the amorphous medium 16. For example, the discrete reflectors 210a, if made of metal, can be moved by mutual induction from an external magnetic field.

[0088] FIG. 7B shows discrete reflectors 210b arranged in a regular pattern throughout the amorphous medium 16. The cavity 304 may be circular, and an external inward force may be applied to the outer edge of the cavity 304 to compress it. As shown in FIG. 7B, this may cause the peripheral reflectors 210b to move inwards. This may have the effect of "shaping" the volume of the cavity 304. For example, a piezoelectric material (not shown) may surround the cavity 304. When a voltage is applied, the piezoelectric material, for example in the form of strips, contracts, resulting in the cavity 304 being compressed, and therefore the medium 16 being compressed. This has the effect of moving the reflectors 210b inwards, creating a "lens" effect.

[0089] In the array shown in Figures 5A-7B, the individual PMUTs may be spaced apart such that during operation, the ultrasonic array of PMUTs 302 emits a steered ultrasonic beam. The determined phase adjustments may be applied to the signals from the respective transmitters or receivers to enable them to function as a coherent array for beamforming, for example. Beam steering may be used on the transmitted ultrasonic signal, the reflected ultrasonic signal, or both. To steer the transmitted ultrasonic signal, the determined phase adjustments may be applied to the signals transmitted by each PMUT 302 in the array such that the resulting transmitted ultrasonic signal is interfered with resulting in an overall signal that is transmitted in the desired direction. Received and reflected ultrasonic signals may be steered in a similar manner. The determined phase adjustments may be applied to the received signals from all directions to determine the reflected signals from a single direction in the surrounding structure.

[0090] Most standard beamforming algorithms benefit from the half-wavelength spacing of the PMUT 302, as it allows each incident wavefront to be distinguished from other incident wavefronts with different angles or wavenumbers, preventing the problem of "grating lobes". Classical beamforming methods that benefit from half-wavelength (or narrower) spacing include (weighted) delay-and-sum beamformers, adaptive beamformers such as MVDR / Capon, direction finding methods such as MUSIC and ESPRIT, and blind source estimation techniques such as DUET, as well as wireless communication methods, ultrasound imaging methods, with additional constraints such as entropy or information maximization.

[0091] Changing the position of the reflectors 210a, 210b can be used to "steer" the transmitted or received ultrasonic signal through the cavity 304. To achieve high quality signal transmission and recording, it is often desirable to move the position of the PMUT 302 while transmitting or receiving, but this is often impractical. Adjusting the properties of the medium 16 through which the signal is transmitted or received by adjusting the position of the reflectors 210a, 210b has the same net effect as adjusting the position of the PMUT 302 itself.

[0092] When transmitting various frequency signals, a typical drawback in array design is that the location of the PMUT elements 302 is optimal for only one frequency, typically the center frequency of the broadband signal, such that the PMUTs 302 have half-wavelength spacing of the center frequency. Thus, the PMUT spacing may not be optimal for other frequencies in the broadband signal. According to the present invention, when transmitting a chirp signal from F0 to F1, the location of the reflectors 210a, 210b can be adjusted in the medium such that at any given time, the PMUTs 302 are "spaced" half-wavelength to the current frequency. Thus, by adjusting the reflectors 210a, 210b, better imaging capabilities can be achieved at all frequencies. By changing the location of the reflectors 210a, 210b in front of the array of PMUTs 302, a lens effect can be created. By having half-wavelength spacing, a relatively simple beamforming method can be easily used. On the transmit side, having half-wavelength spacing is beneficial because it directly determines how well the focused beam can be prevented from "leaking" into other unintended directions. On the receive side, the problem is typically presented slightly better, especially when working in air where there is typically a lot of "space" and some sharp reflectors (as opposed to, e.g., medical imaging, which is all on a continuous spectrum).

[0093] If the elements are not spaced λ / 2 apart at the receive array processing side, a combination of two aspects can be exploited: (a) the scene is typically relatively sparse, and (b) so-called "grating lobes" occur at different angles for different frequencies. Thus, using a wideband signal can help reduce the need for λ / 2 spacing.

[0094] A wideband signal can be used to align the frequencies and use them as a kind of "mutual error protection". Specifically, assume that an array is sampled at a particular wavelength λ1, less than λ1 / 2. The array has a grating lobe at a particular angle β with a main beam at angle α. This means that when observing an object, one cannot say for sure whether the reflector is at angle α or angle β, because the incoming waves from these two angles look exactly the same to the array sensor.

[0095] This is a result of spatial undersampling, a phenomenon similar to aliasing in time-domain signal processing. However, if we also consider another wavelength λ2 and the same main beam at angle α, the associated grating lobes will now typically be at a different angle Ω.

[0096] If the object is at angle β, then for the second wavelength λ2, the response in both α and / or Ω directions (the two cannot be disambiguated) will be low. If not, the object is likely located at angle α. The same logic can be used to discriminate multiple objects at multiple angles up to a particular point when there are points virtually everywhere and the scene is no longer sparse enough to exploit this wideband inheritance potential. In practice, compressive sensing methods take advantage of these wideband capabilities in the presence of sparse scenes with undersampled arrays and typically do not need to rely on search or cancel-one-out techniques as outlined above.

[0097] The operation of the embodiments shown in Figures 5B and 6B will now be described with reference to Figures 8B and 8A, respectively. Figure 8A shows the PMUT array 302 and cavity 404 of Figure 6B along with diagrams of transfer functions F(θ,ω) and F(γ,ω) used to model the predicted effect of the reflector 110 on the transmitted ultrasonic signal, such as a chirp. A transfer function is a mathematical function that theoretically models the output of the PMUT array 302 as a function of the position of the reflector 110. The PMUT array 302 and cavity 304 of Figure 8B are shown along with diagrams of modified transfer functions F'(θ,ω) and F'(γ,ω), which have been modified due to the motion of the reflector 210b.

[0098] 9 shows an array of PMUTs 302 being used to image a known object 20 in order to accurately model the impulse response of the transmitted signal. Instead of a passive reflector, the object 20 may also be an active element, such as a microphone, so that it emits its own signal 22.

[0099] The following equations provide further details about the processing performed for imaging using the PMUT described above. This processing can be performed using any suitable processor.

[0100] First, consider the hypothetical and simplified scenario shown in Figure 10, where there is a single reflector, transmitter, and receiver. Then, assuming a band-limited Dirac pulse transmitted from the transmitter ∂(t), the received signal is

[0101]

number

[0102] x i (t) is the transmitted signal, and r k ∂(t-τ ijk ) is the reflected signal, and f i (α,t) is the filter applied to the transmitted signal, and g j (β,t) is the filter applied to the received signal.

[0103] Then, for a single transmitter, single receiver, and multiple reflectors, the received signal is:

[0104]

number

[0105] Here, the received signal r k is summed over all reflectors k.

[0106] For multiple transmitters, a single receiver, and multiple reflectors, the received signal is:

[0107]

number

[0108] Here, the transmitted signal x i (t) is summed over all transmitters i and the received signal r k is summed over all reflectors k.

[0109] When an array of PMUTs is used for both transmitting and receiving ultrasonic signals, the received signal r k is minimized, and the actual data y j (t s ) is the estimated image data

[0110]

number

[0111] are input to the processor together with

[0112]

number

[0113] The reflections are computed to train directional filters F, G with a focus on image quality.

[0114]

number

[0115] This is then further improved by adding additional constraints on the transfer functions F and G, as shown below.

number

[0116] Then, a number of scenes R={r k To learn F and G over}, the following is computed:

[0117]

number

[0118] This is therefore a blind learning of the array impulse responses across multiple scenes.

[0119] Contrast (r p) can be calculated as the L1 norm of the vector r, or alternatively as the L1 norm of some transformed domain Br, where B can include codebook vectors representing information related to a particular type of information related to the acoustic scene, such as depth or angular edge filters. It can also be related to the distribution of coefficients in r, such as compensating for many zeros and few positive coefficients. Alternatively, it can be the L0 norm of r, or any other compressive transmission-like approach, or sparsity-based approaches, Bayesian, linear programming-based approaches, etc.

[0120] The continuity of a component (F) can be a measure of the rate of change between acoustic transfer functions representing closely spaced angles. f i , f j If is a vector of matrix F representing the directional impulse response in direction I, j, then:

[0121]

number

[0122] In the formula, d(i,j) is f i , f j is a measure of the distance in angles between the impulse responses represented by S. Let S be the set of all pairs of relevant angles to be considered.

[0123] The above calculations pertain to standard ultrasound imaging with known directional filters F, G and a known reflector r.

[0124] The following calculations include modifications to the calculations presented above when there are changes to the "medium" in front of the PMUT being used for imaging. Examples of these changes would be a change in the volume of an acoustically resonant cavity, or a change in the position of a reflector that changes the density of the medium in front of the PMUT.

[0125] For multiple transmitters, a single receiver, and multiple reflectors, the received signal is now:

[0126]

number

[0127] Here, the transmitted signal x i (t) is summed over all transmitters i and the received signal r k is summed over all reflectors k. The directional filter f i , g j {tilde over (w)} further depends on the state vector w and the angles α, β, and the time dependence t.

[0128] Received signal r k is minimized, and the actual data y j (t s ) is the estimated image data

[0129]

number

[0130] and the estimated image data now also depends on the state vector w.

[0131]

number

[0132] As above, the reflections are computed to train directional filters F, G with a focus on image quality.

[0133]

number

[0134] As shown in the equations below, the transfer functions F and G also depend on a state vector w, which provides information about changes in the medium in front of the PMUT being used to transmit or receive the ultrasonic signal. r={r k} F={f i (αa ,β b ,w)} G={g i (α a ,β b ,w)} This is then further improved by adding additional constraints on the transfer functions F and G, which now also depend on the state vector w, as shown below, making use of the knowledge that the transfer functions F and G change progressively with changes in angle.

[0135]

number

[0136] Then, a number of scenes R={r k To learn F and G over}, the following is computed:

[0137]

number

[0138] Returning to the received signal equation,

[0139]

number

[0140] The angle (α, β) is the reflection intensity r k It is not needed as an explicit parameter since it can be calculated based on knowledge of the location of the reflectors, where

[0141] Therefore, the formula can be simplified to:

[0142]

number

[0143] During the ceremony,

[0144]

number

[0145] is a family of driver signal functions.

[0146] This can be further simplified using the driver signal matrix.

[0147]

number

[0148] Therefore, the received signal equation becomes:

[0149]

number

[0150] A vector of reflection coefficients that provides information about the "image" produced during the imaging process can be determined as follows:

[0151]

number

[0152] Therefore, the received signal equation can be further simplified as shown below:

[0153]

number

[0154] As the number N increases, the system of equations becomes better conditioned and therefore the results (images) become better.

[0155] Increasing N increases the number of ways in which the signal is transmitted (X) as well as the ways in which the aperture is changed (w), such as by adjusting the acoustic resonant cavity volume or by adjusting the position of a reflector in the medium in front of the PMUT.

[0156] By transmitting ultrasonic signals through various media, such as an amorphous medium with reflectors, the above equations can be used to "learn" how the sound is affected by the medium and then used to steer the sound during both transmission and reception. The output signal at each array element may be preconditioned and the received signals may then be processed and used to obtain an image of the surroundings.

[0157] Therefore, better images can be obtained by arranging the reflectors so that they are randomly positioned relative to each other. This is possible, for example, when the reflectors comprise randomly positioned metal balls in a gel, as shown in FIG. 3. This has a similar effect to capturing images with a camera from many different angles. By randomly positioning the reflectors and / or moving them (randomly) in many different directions, more information can be determined by the PMUT array 302 as if more camera angles were being viewed.

[0158] The array of PMUTs 302 may also be mounted on a motor (not shown) and moved during observation of a single (or multiple) reflectors. Thus, the directional impulse response may be sampled in all directions. This sampling may be done before the PMUT array 302 is used for imaging, for example in a test room setting, or during use, such as when the array 302 is mounted to a robot or other device that can control the physical position and angle of the array 302. This allows the PMUT array 302 to obtain multiple views of its surroundings.

[0159] By modeling the impulse response of the transmitted signal with a known reflector, the effect of adjusting the position of the reflector 210b to improve imaging can be modeled.

[0160] 11, there is shown a PMUT array 302 interfaced with a cavity 504. The amorphous medium 16 and reflector 110b are arranged in the cavity in front of the PMUT array 302. Unlike the cavities and arrays shown in the previous figures, it is clear that the cavity 504 is much larger than the array 302. The larger cavity 504 provides a "larger aperture" for imaging using the PMUT array 302, while also reducing the cost, which would be much higher if the PMUT array 302 filled the entire cavity 504. Having a "larger aperture" ensures that the PMUT array 302 has better near-field focusing capabilities.

[0161] In practice, not all frequencies have optimal beam patterns or focusing, but by utilizing all frequencies together (i.e., imaging with multiple frequencies) and with the knowledge that (a) all frequencies reflect similarly to some extent, and (b) the acoustic scene in air is often sparse (minimal reflection), good focusing can be achieved at all frequencies.

[0162] 12A-12C show various configurations of multiple arrays, each similar to that of FIG. 6B, so that an "array of arrays" is formed.

[0163] 12A shows each array 402 disposed within a respective cavity 604. Each cavity 604 is coupled to a solid substrate 24 by a respective pillar 25.

[0164] In FIG. 12B, a similar cavity 604' is disposed on the damping medium 26, which may be any suitable damping medium, such as a semi-solid or amorphous medium, a gel, or an acoustic dielectric.

[0165] Each array 402 in FIG. 12C is disposed within a cavity 604″ that is disposed within the attenuating medium 28, which again may be any suitable attenuating medium, for example an amorphous medium similar to that within cavity 604″.

[0166] FIG. 13 shows a system formed from an array of arrays 40 of FIG. 12A being used to image an object 30. The steered ultrasonic signals 32 transmitted from each of the arrays 402a, 402b, 402c are shown with an indication of the width of the emitted ultrasonic signal 34, and also show grating lobes 36. The grating lobes 36 are secondary main lobes that occur when using phased arrays (the main lobe is the ultrasonic signal 32). The grating lobes 36 occur when the array spacing is greater than λ / 2, where λ is the wavelength of the transmitted ultrasonic signal. The grating lobes 36 extend from the arrays 402a, 402b, 402c at angles other than the primary path shown by the steered ultrasonic signal 32. The ultrasonic signals 32 are steered and shaped by applying determined phase adjustments to the signals transmitted by each PMUT in each array 402a, 402b, 402c such that the resulting transmitted ultrasonic signals from each PMUT in each PMUT array 402 are interfered with, resulting in an overall steering signal 32 that is transmitted in a desired direction. As seen in Figure 13, the transmit signals 32 from each of the arrays 402a, 402b, 402c are steered in different directions towards the object 30 being imaged. In this manner, the object 30 may be imaged from multiple directions due to the steering signals 32 from the arrays.

[0167] In some imaging situations, such as high intensity near-field acoustics (such as object levitation or haptic feedback), it is desirable to have a wide overall array baseline and to bring the object of interest close to the surface. This is because it allows the energy to be sharply focused to one "point" and not just in a general direction. It is beneficial to have the energy focused at that point and "fade out" from that point. This requires a long array baseline. It is expensive and difficult to manufacture large arrays of PMUTs because the driving electronics require large FPGAs with many ports.

[0168] 13 adds the added benefit of providing individual driver electronics for each array 402 in the system 40. Each array 402 can be provided with parameters indicative of the direction of energy and the strength of the transmitted signal. Alternatively, in some situations (e.g., multi-touch) a "directional chart" with multiple directions may be included and the processing required to create this chart may be performed in the ASIC of each array 402.

[0169] Providing multiple arrays 402 in the system 40 also "thins out" the arrays overall due to the reduced number of PMUTs, which reduces the appearance of crosstalk. Each array 402 may be insulated from the other arrays, i.e., so that they do not physically contact each other and thus energy does not propagate between them. Each array 402 may be mounted as in FIG. 12A and elevated above the substrate 24 using rods 25, so that they are separated from the base substrate 24 that connects them. Alternatively, the arrays 402 may be placed on a foam medium with minimal acoustic transmission capabilities.

[0170] It is also easier to fabricate smaller arrays 402 in a single cavity 604, as opposed to larger arrays which are more difficult to operate. Larger PMUT arrays may require wire bonding to individual elements which are difficult to fabricate.

[0171] The spacing between arrays 402 in system 40 may also be used to position additional sensors, such as 2D or 3D cameras, to provide additional lines of sight to the object being imaged and focused.

[0172] In some embodiments, only power is provided to each array 402. Signal direction / pattern, signal strength, and other information for calibration may be provided wirelessly to the arrays 402. Thus, the entire system 40 is a sensor network of arrays 402 with an input power source.

[0173] Alternatively, in some embodiments, the additional information may be provided by a wired connection to system 40 .

[0174] Each PMUT array 402 interfaces with a cavity 604. If signal direction / pattern, signal strength, and other information for calibration are provided wirelessly to the array 402, D / A and / or A / D converters may also be provided as needed, along with the ASIC and radio chip.

[0175] It will be understood that the "array of arrays" referred to above can include any of the arrays or PMUTs shown in any of the figures, for example, the medium may not include a reflector. Additionally, the array may be non-planar, such as the non-planar array of PMUTs 502 in the cavity 704 shown in FIG. 14. Alternatively, as shown in FIG. 15, the system 140 may use an acoustic tube / port 42, with no amorphous medium in the cavity 804 in which the PMUT array 602 resides. However, the acoustic tube 42 may become clogged with dust, adversely affecting the transmitted ultrasonic signal.

[0176] Figure 16A shows a PMUT array 702 arranged in a stack 44, with multiple layers having different impedances arranged in front of the PMUT array 702. Figure 16B is an exploded view of Figure 16A.

[0177] A housing layer 46 is placed over the PMUT array 702. The housing layer 46 is made from a gel and has a good impedance match with the PMUT array 702. A second layer 48 is placed over the housing layer 46 and has a different impedance than the housing layer 46. Finally, a third layer 50 is placed on top to form an exterior layer. This exterior layer 50 may be fairly stiff and light to protect the PMUT stack 44.

[0178] Piezoelectric strips 52 are fixed around the exterior of the stack 44 (one for each of the layers 46, 48, 50). When a current is applied to the piezoelectric strips 52, their thickness changes and a different force can be applied to each layer in the stack, as shown by the arrows 54 in FIG. 16C. This causes deformation of the layers in the stack, causing pressure variations between each layer 46, 48, 50, resulting in a wavefront 56, shown in FIG. 16D, that can be controlled and directed using the pressure variations. The pressure variations create an acoustic gradient that can provide a "lensing effect" that can have the same effect as changing the position of the PMUT element itself, as explained above.

[0179] The layers 46, 48, 50 are harder from the PMUT outwards. The layer 46 above the PMUT is typically something relatively soft, such as gel, or even air. The hardness scale transition ensures a good impedance matching strategy. If a hard surface is laminated above the gel or air layer 46, all transmitted energy from the PMUT array 702 is reflected.

[0180] If the outer layer 50 is strong, it can protect the PMUT array 702. The piezoelectric strips 52 can move and bend the PMUT array 702. This can be used to create a "crack-free" layer of dirt and debris, and also to "check" the health of the array when imaging under various conditions.

[0181] The layers 46, 48, 50 may wear over time, changing the acoustic transfer function. However, according to the present invention, the acoustic transfer function may be modified "in situ". For example, an image may be acquired using a previous acoustic transfer function. This stored acoustic transfer function may then be modified to obtain a sharper image. Compared to using the acoustic port shown in FIG. 15, imaging is less susceptible to changes in the shape of the housing surrounding the PMUT due to the ability to correct the acoustic transfer function by placing multiple reflectors around the array, for example on one of the layers 46, 48, 50.

[0182] FIG. 17 is a flow chart illustrating a method for updating the directional impulse response to improve the quality of an acquired image using any of the above embodiments having a reflector in an acoustic resonant cavity. i ,ω j ) is selected, the images obtained from ultrasound imaging will almost always be "blurry". Therefore, the sharpness of the image can be used as a criterion for updating the filter selected to obtain the image.

[0183] In step 58, the original filter F(θ i ,ω j ) are selected and then in step 60 the scene is imaged using these original selected filters. The scene is imaged with a PMUT or PMUT array that transmits ultrasound signals which are reflected and received, and the received signals are processed by a processor locally in the PMUT or in a cloud server.

[0184] In step 62, the sharpness of the image is assessed. A measure such as image sharpness (see https: / / ieeexplore.ieee.org / document / 6783859) or the ratio of low reflectance values ​​(close to 0) to high reflectance values ​​can be used to calculate such sharpness.

[0185] If, in step 64 , the sharpness is above a predetermined threshold, the process ends in step 66 .

[0186] However, if the sharpness does not exceed this threshold, the filter is adjusted in step 68. The filter may be adjusted using a previous filter 70 stored in memory, for example in a server. The adjusted filter is then used to image the scene, and the sharpness of the scene is evaluated using the new filter.

[0187] FIG. 20 shows how the signal output from the transducer is modified by the PMUT and the housing. The top plot shows the original chirp signal s(t) 900 that would be output from the PMUT and ideally amplified into air. The frequency response 901, A(ω), of the PMUT element itself is shown in the second graph, where ω is the wavelength. The third graph shows the signal 902 resulting from driving the chirp s(t) through the PMUT. It can be seen that the center frequency is amplified. The dotted lines 903, 904 show the level of energy (window) that may be achieved for a flat spectrum, i.e. if the signal s(t) is modified to lower the power at the center frequency, or in other words if we replace s(t) with some modified signal s'(t) such that S(ω)A(ω)=K for some positive number K and all ω. A fourth graph 905 shows the effect of driving a chirp signal s(t) through a housing that has a frequency response B(ω) (not shown) that has a different resonant frequency than A(ω) (the PMUT element itself). The bottom graph 906 shows the effect of driving s(t) through both the PMUT and the transducer, resulting in both an amplification of the PMUT frequency and the housing frequency.

[0188] Adjustments can be made to help achieve a flatter output spectrum compared to spectrum 906 shown in Figure 20. We will now describe how this can be achieved.

[0189] Let s(t) be the output signal, typically a chirp. In the ideal case, this would be output as is (with some noise added).

[0190] y(t)=s(t)+n(t) In practice, the results are also affected by the PMUT element itself, which has a transfer function f(t).

[0191] y(t)=s(t)*f(t)+n(t) Typically, the filter f(t) is such that its frequency response F(ω) has one or more resonant peaks. The effect of the housing must also be included as g(t). If there is no adaptation of the housing, then

[0192] y(t)=s(t)*f(t)*g(t)+n(t) A design goal may be to generate an effective output signal y(t) that supports two criteria or a particular combination of two criteria.

[0193] The first (a) is that |Y(ω)| is as large as possible, which indirectly means that the term s(t)*f(t)*g(t)+n(t) also has as large a magnitude as possible, assuming that the expected noise magnitude is constant, i.e., E|N(ω)|=K. Thus, (a) is essentially an SNR maximization criterion.

[0194] The second criterion (b) is that |Y(ω)| be as constant as possible for all values ​​of ω, i.e., the flat output spectrum criterion. Certain parameters can be varied to try to optimize each criterion or to create some useful compromise between the two.

[0195] The following formula can be rewritten:

[0196] y(t)=s(t)*f(t)*g(t)+n(t) =z(t)*g(t)+n(t) Note that s(t)*f(t)=z(t).

[0197] This can be written in the time domain as follows:

[0198]

number

[0199] Recall that tuning the housing characteristics effectively changes the housing filter or housing transfer function g(t) so that, for each sample of y(t),

[0200]

number

[0201] Variable Filter i (. ) is not fixed and can be designed to change shape by adjusting the electrical parameters associated with the housing change. Thus, g i (. )=f(α(t i ) where α(t i ) is some physical parameter (e.g., current) related to the housing, which determines g i The shape of (.) is defined. Assuming we have a functional model of how this happens, at this point there are two tools that can be used to meet two goals:

[0202] One is to adjust z(k) = s(t) * f(t), which can be done indirectly by changing the drive signal s(t), but not by changing the PMUT response f(t).

[0203] The other method is to adjust the housing characteristics and thereby the time-specific housing transfer function: i (. )=f(α(t i )).

[0204] As an example, consider the situation where s(t) is a linear chirp from frequency F0 to F1. There is a resonant peak (F0+F1) / 2 in the middle, which has three times as much energy as any other typical frequency.

[0205] Time-specific filter g i (.) can also be tuned to amplify certain frequencies by a factor of three, where i is the time window index.

[0206] Then, two things can be done: First, s(t) can be modified to become z(t), where the middle frequency of z(t) is attenuated at frequency (F0+F1) / 2 to 1 / 3 the level of the other frequencies.

[0207] Second, while chirping upward from the frequency F0, the time-specific filter g i The (. ) continues to track the frequency of the chirp, resulting in all frequencies above being amplified by a factor of three until they approach the center frequency (F0+F1) / 2. The filter is then unchanged and resumes tracking the chirp signal until the chirp moves beyond the center peak. Recall that near the center frequency, the PMUT itself amplifies the amplitude by a factor of three, so the housing does not need to do any work.

[0208] The net result is a flat frequency power spectrum of the signal y(t) amplified by a factor of three across the entire spectrum.

[0209] In practice, it may be difficult to reach both criteria (flat spectrum and maximum SNR) perfectly, so various tolerance regions and optimization strategies may be employed: gradient descent, linear programming, nonlinear programming, or neural networks may be used to provide tolerance regions or penalties for deviations beyond certain out-of-target values.

[0210] Although the present invention has been illustrated by describing one or more specific embodiments thereof, it will be understood by those skilled in the art that the present invention is not limited to these embodiments and that many variations and modifications are possible within the scope of the appended claims.

Claims

1. A piezoelectric microfabrication ultrasonic transducer (PMUT) is configured to interface with an amorphous medium, and as a result, during use, ultrasonic signals to or from the PMUT pass through the amorphous medium, the amorphous medium includes a plurality of discrete reflectors dispersed inside. A piezoelectric microfabricated ultrasonic transducer (PMUT) in which the discrete reflectors form a mesh or porous film.

2. The PMUT according to claim 1, wherein it is configured to interface with an acoustic resonant cavity, and the acoustic resonant cavity contains the amorphous medium.

3. The PMUT according to claim 1 or 2, wherein the discrete reflectors are dispersed in two dimensions.

4. The PMUT according to claim 1 or 2, wherein the discrete reflectors have positions that can be adjusted during use to adjust the acoustic properties of the acoustic resonance cavity.

5. The PMUT according to claim 1 or 2, wherein the discrete reflectors are made of metal, and the positions of the discrete reflectors are adjusted using a magnetic field.

6. The PMUT according to claim 2, wherein the piezoelectric contraction element is arranged around the acoustic resonant cavity.

7. The PMUT according to claim 1 or 2, wherein the characteristics of the cavity are configured to adjust when the frequency of the signal emitted by the PMUT changes during chirp transmission.

8. The PMUT according to claim 1 or 2, wherein the PMUT comprises a dedicated ultrasonic transmitter and at least one separate ultrasonic receiver on a single common semiconductor die.

9. The PMUT according to claim 1 or 2, wherein the PMUT is mounted to be movable by an actuator.

10. A method of imaging using a PMUT according to claim 1 or 2 or a PMUT array comprising a plurality of PMUTs according to claim 1 or 2, The steps include determining an acoustic transfer function corresponding to the effect of a plurality of discrete reflectors positioned between the PMUT or PMUT array and the scene being imaged, The steps of imaging the scene using the PMUT or PMUT array by transmitting an ultrasonic signal after receiving one or more reflections and receiving the ultrasonic signal in the PMUT or PMUT array, The steps include: processing the received ultrasonic signal to generate an image by applying the inverse function of the acoustic transfer function to the received ultrasonic signal; and Methods that include...

11. The aforementioned decision-making step is: (a) the step of selecting an acoustic transfer function to apply to the received signal in order to generate an image, (b) The step of imaging using the PMUT or PMUT array by transmitting an ultrasonic signal and receiving the ultrasonic signal in the PMUT or PMUT array after receiving one or more reflections, (c) The step of using the inverse function of the acoustic transfer function to process the received signal and generate an image, (d) Determine the sharpness parameters of the image, and the determined sharpness parameters are (i) When the predetermined threshold is not exceeded, adjust the transfer function before repeating steps (b) to (d), (i) When a predetermined threshold is exceeded, the method is terminated. The method according to claim 10, including the method described in claim 10.

12. The method according to claim 10, wherein the acoustic transfer function includes a directional impulse response.

13. A method of imaging using a piezoelectric microfabrication ultrasonic transducer (PMUT) or PMUT array, The steps include determining an acoustic transfer function corresponding to the effect of a plurality of discrete reflectors positioned between the PMUT or PMUT array and the scene being imaged, The steps of imaging the scene using the PMUT or PMUT array by transmitting an ultrasonic signal after receiving one or more reflections and receiving the ultrasonic signal in the PMUT or PMUT array, The steps include: processing the received ultrasonic signal to generate an image by applying the inverse function of the acoustic transfer function to the received ultrasonic signal; and Methods that include...

14. A system for transmitting and receiving ultrasonic signals, A first PMUT and a second PMUT according to claim 1 or 2, A transmitter circuit configured to drive the first PMUT as an ultrasonic transmitter, A receiver circuit configured to detect signals from the second PMUT as an ultrasonic receiver, A system that includes these features.

15. A configuration comprising a plurality of PMUTs as described in claim 1 or 2.

16. The configuration according to claim 15, wherein the plurality of PMUTs are arranged in an array.

17. The configuration according to claim 15, wherein the PMUTs are spaced apart by λ / 2, where λ is the wavelength of the center frequency over the range of frequencies transmitted or received by the PMUTs.

18. The configuration according to claim 16, wherein the array is mounted to be movable by an actuator.

19. The configuration according to claim 16, wherein the array is not flat.

20. The configuration according to claim 16, wherein different PMUTs within the array are of different sizes.

21. A method of imaging using PMUT according to claim 1 or 2.

22. A plurality of acoustic resonant cavities comprising an array of PMUTs according to claim 1 or 2, wherein each acoustic resonant cavity is configured to interface with the acoustic resonant cavities.

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