Ultrasonic transducer with stacked membranes

Abstract

The ultrasonic transducer (100) includes a stack of at least two membranes (10, 20) attached to a substrate (50). A circuit (30) is coupled to the electrodes, the circuit having a controller configured to apply a first electrical signal (S11) to the first electrode (11) on the first membrane (10) and a different second electrical signal (S21) to the second electrode (21) on the second membrane (20). The first and second electrical signals (S11, S21) are configured to apply a varying voltage (ΔV1, ΔV2) between the first electrode (11) and the second electrode (12) during respective vibration cycles (T1, T2) of the membranes (10, 20). The first electrode (11) on the first membrane (10) is configured to interact with the second electrode (21) on the second membrane (20) through an electrostatic force (Fe) that varies according to the varying voltage (ΔV1, ΔV2) during respective vibration cycles (T1, T2).

Description

Technical Field

[0001] The present disclosure relates to an ultrasonic transducer and a control method. Background Art

[0002] Ultrasonic transducers (e.g., sources and / or receivers) have various applications, such as medical imaging, flow meters, etc. The membrane can be actuated by various mechanisms. For example, actuation can be achieved by a piezoelectric transducer coupled to the membrane. Depending on the electrical signal, the piezoelectric material can expand or contract, which can cause the membrane to vibrate. In order to increase the vibration amplitude, the actuation signal can be matched to the resonant frequency of the membrane. Alternatively or additionally, the amplitude of the electrical signal can be increased. However, there may be limitations, such as a maximum voltage, before damage to the actuation mechanism occurs. In addition, the transmission of the force affecting the vibration may not be optimal.

[0003] There remains a need to improve the power and efficiency of membrane-based transducers. Summary of the Invention

[0004] Aspects of the present disclosure relate to an ultrasonic transducer and a control method. The ultrasonic transducer comprises a stack of at least two membranes attached to a (common) substrate. A circuit is coupled to the electrodes, the circuit being configured to apply a first electrical signal to a first electrode on the first membrane and a different second electrical signal to a second electrode on the second membrane. The first electrical signal and the second electrical signal are configured to apply a varying voltage between the first electrode and the second electrode during corresponding vibration cycles of the membrane at ultrasonic frequencies. The first electrode on the first membrane is configured to interact with the second electrode on the second membrane through an electrostatic force that varies according to the varying voltage during corresponding vibration cycles. As described herein, the dynamic variation of the voltage / electrostatic force between the membranes during corresponding vibration cycles can be used to enhance vibrations in at least one of the membranes, resulting in higher amplitude or power. For example, power can be increased in a stack of piezoelectric membranes. In some embodiments, asymmetry in displacement can also increase bandwidth. BRIEF DESCRIPTION OF THE DRAWINGS

[0005] These and other features, aspects, and advantages of the apparatus, systems, and methods of the present disclosure will become better understood through the following description, appended claims, and accompanying drawings, in which:

[0006] Figure 1A and Figure 1B An ultrasonic transducer is shown;

[0007] Figure 2 Examples of corresponding electrical signals and vibrations are shown;

[0008] Figure 3A and Figure 3B An ultrasonic transducer is shown in which some of the electrodes are combined;

[0009] Figure 4 Examples of corresponding electrical signals and vibrations are shown;

[0010] Figure 5A and Figure 5B An ultrasonic transducer is shown in which the membranes vibrate in anti-phase;

[0011] Figure 6 Examples of corresponding electrical signals and vibrations are shown;

[0012] Figure 7A shows a perspective cross-sectional view of an ultrasound transducer, wherein electrostatic electrodes and piezoelectric electrodes (electropads) are arranged adjacently on respective membranes;

[0013] Figure 7B Shown in Figure 7A A bottom view of the first film indicated by VIIB in FIG;

[0014] Figure 7C The relative displacement at resonance is shown as a function of the relative electrode radius of the piezoelectric layer. DETAILED DESCRIPTION

[0015] The terms used to describe specific embodiments are not intended to limit the present invention. As used herein, unless the context clearly indicates otherwise, the singular forms "a", "an" and "the" are intended to also include the plural forms. The term "and / or" includes any one or all combinations of one or more of the listed related items. It should be understood that the terms "include" and / or "comprise" indicate the presence of the features described, but do not exclude the presence or addition of one or more other features. It should also be understood that, unless otherwise stated, when a specific step of a method is described as following another step, the specific step can directly follow the other step, or one or more intermediate steps can be performed before performing the specific step. Similarly, it should be understood that when a connection between structures or components is described, unless otherwise stated, the connection can be established directly or through an intermediate structure or component.

[0016] The present invention is described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. In the drawings, the absolute and relative sizes of systems, components, layers, and regions may be exaggerated for clarity. The embodiments will be described with reference to schematic and / or cross-sectional views of possible ideal embodiments and intermediate structures of the invention. In the specification and drawings, like reference numerals always refer to like elements. Related terms and related term derivatives should be understood to have the meaning as described at the time or shown in the drawings in question. Unless otherwise noted, these related terms are for convenience of description and do not require the system to be constructed or operated in a specific direction.

[0017] Figure 1A and Figure 1B An ultrasonic transducer 100 is shown. In some embodiments, the ultrasonic transducer 100 comprises a stack of at least two membranes 10, 20 attached to a substrate 50. In a preferred embodiment, the membranes 10, 20 are separated by a spacer 5 (e.g., a pocket) therebetween. For example, the substrate 50 surrounds the stack in the membrane planes X, Y. In one embodiment, the first membrane 10 in the stack is parallel to and faces the second membrane 20 in the stack. Preferably, the membranes 10, 20 are configured to vibrate at an ultrasonic frequency U to transmit and receive ultrasonic waves W.

[0018] In some embodiments, each membrane 10, 20 includes a set of electrodes 11-13; 21-23. A circuit 30 can be coupled to the electrodes, the circuit configured to apply electrical signals. For example, the circuit 30 includes a controller or control circuit configured to apply the electrical signals. In one embodiment, a first electrical signal S11 is applied to the first electrode 11 on the first membrane 10, and a different second electrical signal S21 is applied to the second electrode 21 on the second membrane 20. Preferably, the first and second electrical signals S11, S21 are configured to apply (or cause) varying voltages ΔV1, ΔV2 between the first and second electrodes 11, 12 during respective vibration periods T1, T2 of the membranes 10, 20 at, for example, an ultrasonic frequency U or another frequency preferably having some phase relationship with the vibration period. In a preferred embodiment, the first electrode 11 on the first membrane 10 is configured to interact with the second electrode 21 on the second membrane 20 via an electrostatic force Fe that varies according to the varying voltages ΔV1, ΔV2 during respective vibration periods T1, T2.

[0019] For example, during corresponding sub-intervals of periods T1, T2, the value of the voltage between the electrodes varies between ΔV1 and ΔV2. The varying voltage or potential difference between the electrodes can correspond to the varying difference in the respective voltages of electrical signals S1, S2 (e.g., relative to ground) during different portions of the vibration cycle. As shown in the figure, the varying voltage can result in a corresponding change in the charge on at least one of the electrodes + / - relative to the other electrode. While this figure only illustrates the change in charge on first electrode 11, the charge on second electrode 12, or on both electrodes (e.g., if the changes are different), can also vary.

[0020] Typically, the opposite charges - / + on the respective (opposing) electrodes 11, 21 can result in an attractive electrostatic force Fe as shown (e.g., during the second half period T2 of the vibration cycle). In principle, similar charges (+ / +; - / -) on the respective electrodes can repel each other, for example, during the first half period T1 of the vibration cycle (e.g., half of the separated vibration period). However, in practice, the repulsion may be less prominent or negligible. In some embodiments, the charges on the respective electrodes can also be zero during the first half period T1 of the vibration cycle. For example, the electrostatic force is more affected by the charge difference or voltage between the electrodes.

[0021] Without being bound by theory, the (attractive) electrostatic force Fe between charges on a pair of parallel plates can be described as Fe = ε0·A·ΔV 2 / 2d 2 , where “ε0” is the dielectric constant of vacuum (~8.85·10 -12 F·m -1 Farad per meter), "A" is the surface area of ​​the plates, "ΔV" is the voltage or potential difference between the plates, and "d" is the distance between the plates. For example, the surface area of ​​each of the first electrode 11 and the second electrode 12 is at least one hundred square microns, at least two hundred square microns, at least five hundred square microns, at least one square millimeter, at least two square millimeters, at least five square millimeters, at least ten square millimeters, at least twenty square millimeters, at least fifty square millimeters, at least one square centimeter. The larger the surface area of ​​the first electrode 11 and the second electrode 12, the greater the electrostatic force (using the same voltage). For example, the distance between the membranes 10, 20 and / or between the first electrode 11 and the second electrode 12 is less than one centimeter, less than half a centimeter, less than two millimeters, less than one millimeter, less than half a millimeter, less than two hundred microns, less than one hundred microns, or even less than fifty microns. The smaller the distance, the greater the electrostatic force.

[0022] Although the electrodes described herein may, for example, deviate geometrically from an ideal parallel plate pair, the overall trend may be similar. For example, when the voltage ΔV is large, the electrostatic force Fe is generally large, and when the voltage is small or there is no voltage (ΔV=0), the electrostatic force is small or even zero. For example, when the (effective) area of ​​the electrode (electrode disk) is large and / or the effective distance between the electrodes is small, the electrostatic force Fe is generally large. Preferably, the corresponding first electrode and the second electrode include corresponding electrode disks, which cover at least a portion of the corresponding inner surface of the membrane facing each other. Preferably, the distance "d" between the membrane (for example, in a static state) or the electrode disk is relatively small, for example, at least one, two, three, four, five, ten or more times smaller than the diameter of the membrane. Some embodiments described later with reference to Figure 7 may, for example, improve the geometry (for example, the layout of the electrodes and / or the protrusions of the membrane) to keep the distance between the electrodes more uniform at different distances from the center of the membrane.

[0023] In some embodiments, circuit 30 is configured to apply a first voltage ΔV1 between first electrode 11 and second electrode 12 during a first half-period T1 of the vibration cycle, and to apply a second voltage ΔV2 between first electrode 11 and second electrode 12 during a second half-period T2 of the vibration cycle, wherein the second voltage ΔV2 is greater than the first voltage ΔV1. For example, the second voltage ΔV2 is greater than the first voltage ΔV1 by at least one volt, preferably at least two volts, at least five volts, at least ten volts, at least twenty volts, at least fifty volts, at least one hundred volts, or more. The greater the variation in voltages ΔV1 and ΔV2 during different portions of the vibration cycle, the greater the variation in the electrostatic force. For example, the first voltage ΔV1 can be relatively low or preferably zero, i.e., there is no voltage between electrodes 11 and 12. For example, the second voltage ΔV2 can be between one volt and one thousand volts, preferably between ten volts and five hundred volts, and most preferably between fifty volts and two hundred volts.

[0024] In some embodiments, the circuit 30 is configured to apply the attractive electrostatic force Fe between the first electrode 11 and the second electrode 12 only during the second half period T2 of the vibration period by varying the voltages ΔV1 and ΔV2. In other or additional embodiments, when the first membrane 10 (e.g., the center of the first membrane) moves in one direction -Z, the attractive electrostatic force Fe is applied only (or primarily) during a sub-period (e.g., the second half period T2) of the vibration period, and when the first membrane 10 moves in the opposite direction +Z, the attractive electrostatic force Fe is not applied during another sub-period (e.g., the first half period T1) of the vibration period.

[0025] It should be noted that for illustrative purposes, in the figures the membrane is shown at the end of the respective half-periods T1, T2 of the vibration cycle, i.e. when the deviation is maximum at the respective amplitudes A1, A2. In reality, the movement of the membrane (Z position) may lag in phase with the applied force; vice versa, the force may be applied in phase before the movement (see for example Figure 2 This may also depend on the frequency of the applied force relative to the resonant frequency of the membranes. For example, the membranes 10, 20 are configured to exhibit corresponding vibrations at or near their resonant frequencies to transmit and / or receive ultrasonic waves W that resonantly interact with one or both of the membranes.

[0026] Typically, a force that increases the first amplitude A1 is most effective during a period when the membrane is moving in the same direction as the force, for example, during the half-period between when the first membrane 10 moves from a downward position to an upward position. Conversely, the second amplitude A2 can be reduced during the same half-period by a force in the opposite direction of movement. In effect, the first amplitude A1 can be increased at the expense of the second amplitude A2, for example, by pulling on the second membrane.

[0027] In some embodiments, as shown herein, the circuit 30 is configured to apply multiple sets of electrical signals to corresponding multiple sets of electrodes, causing the membranes 10, 20 to vibrate in unison in the same direction +Z or -Z at the same time. In other words, the vibrations of the first membrane and the second membrane are approximately in phase, for example where the phase difference is less than forty-five degrees, preferably less than twenty degrees, less than ten degrees, and most preferably zero degrees. For example, the phase of the vibration is controlled by the corresponding phase of one or more of the electrical (drive) signals. By causing the membranes to move in unison, the second membrane 20 can also increase the amplitude of the first membrane 10, for example by pulling the first membrane when the first membrane is already moving toward the second membrane 20. Although this embodiment shows three electrodes per membrane, there can also be two electrodes, such as shown in Figures 3 to 7. Instead of membranes vibrating in phase, it is also conceivable to cause the membranes to vibrate in anti-phase. For example, referring to Figures 5 to 7 Figure 6 Let me explain this point again.

[0028] The in-phase (or anti-phase) movement of the membranes 10, 20 can be achieved by applying an additional force to at least one of the membranes. In principle, the additional force can also be an electrostatic force, applied, for example, to another electrode (not shown); or caused, for example, by an actuator (not shown) in close proximity to the membranes, or by any other interaction. In a preferred embodiment, as described herein, the additional force is applied by a piezoelectric interaction. Most preferably, a piezoelectric layer 14, 24 is applied to one or both membranes 10, 20.

[0029] In some embodiments, at least one of the membranes 10 and 20 includes a corresponding piezoelectric layer 14 or 24 sandwiched by a corresponding set of electrodes 12, 13; 22, 23 to transmit and receive piezoelectric signals S12, S13; S22, S23 according to a vibration cycle. In other or additional embodiments, the piezoelectric signals S12, S13; S22, S23 are configured to generate corresponding piezoelectric forces F1 or F2 on the corresponding at least one of the membranes 10 and 20. For example, the piezoelectric force can be applied to the first membrane 10 and / or the second membrane 20, preferably both, by applying the same or similar signals, the same or similar signals resulting in synchronized movement.

[0030] In one embodiment, a first group of piezoelectric signals S12 and S13 are applied to a first group of electrodes 12 and 13 on a first membrane 10 to cause a changing voltage ΔV3 and ΔV4 on a first piezoelectric layer 14 on the first membrane 10, and a second group of piezoelectric signals S22 and S23 are applied to a second group of electrodes 22 and 23 on a second membrane 20 to cause a changing voltage DV5 and DV6 on a second piezoelectric layer 24 on the second membrane 20, wherein the changing voltage ΔV3 and ΔV4 on the first piezoelectric layer 14 are in phase or antiphase (180 degrees out of phase) with the changing voltage ΔV5 and ΔV6 on the second piezoelectric layer 24.

[0031] By applying the same or similar phase to the piezoelectric signals on each of the membranes, the membranes can be actuated in unison by the respective piezoelectric forces. By applying signals in opposite phases (or by reversing the polarization of the piezoelectric layers), the membranes can be moved in opposite phases, meeting each other in the center, as will be seen in Figures 5 and Figure 6 . For example, the second varying voltage includes a first piezoelectric voltage ΔV3 between signals S12 and S13 during the first half period T1 of the vibration cycle, and a second piezoelectric voltage ΔV5 between signals S12 and S13 during the second half period T2 of the vibration cycle. For example, the third varying voltage includes a third piezoelectric voltage ΔV4 between signals S22 and S23 during the first half period T1 of the vibration cycle, and a second piezoelectric voltage ΔV6 between signals S22 and S23 during the second half period T2 of the vibration cycle. While in some embodiments it may be advantageous for the piezoelectric signals between the membranes to be consistent (e.g., S12 = S22 and / or S13 = S23), the piezoelectric signals between the membranes may also be offset (e.g., having the same difference S12 - S13 = S22 - S23). The relative amplitudes may also be different (e.g., ΔV3 = c·ΔV5, where "c" may also be negative, depending on the polarization of the respective piezoelectric layers 13 and 24).

[0032] In some embodiments, the first set of piezoelectric signals S12, S13 to the first piezoelectric layer 14 on the first membrane 10 is configured to generate a first piezoelectric force F1 in the same direction -Z as the direction -Z of the electrostatic force Fe during the second half period T2 of the vibration cycle. Figure 4 In other or additional embodiments shown in , the combined first piezoelectric force F1 and electrostatic force Fe result in an increased first amplitude A1 of the first membrane 10 compared to the second amplitude A2 of the second membrane 20. For example, as shown in the figure, the arrows representing the respective directions of the piezoelectric force F1 and the electrostatic force Fe of the first membrane 10 point in the same direction during different portions of the vibration cycle, such as during time T2 in the cycle when both membranes are moving downward.

[0033] In other or additional embodiments, the second set of piezoelectric signals S22, S23 to the second piezoelectric layer 24 on the second membrane 20 is configured to generate a second piezoelectric force F2 in a direction -Z opposite to the direction +Z of the electrostatic force Fe during the second half T2 of the vibration cycle, wherein the combined piezoelectric force Fp and the electrostatic force Fe result in a reduced second amplitude A2 of the second membrane 20. For example, as shown in the figure, during different portions T1, T2 of the vibration cycle, such as times T1 and T2 during which the membranes are both moving upward or downward in the cycle, the arrows representing the respective directions of the piezoelectric force Fp and the electrostatic force Fe of the second membrane 20 point in opposite directions. Instead of increasing one of the vibration amplitudes A1 at the expense of the other amplitude A2, it is also conceivable to increase both amplitudes, as will be described later with reference to Figures 5 and 6. Figure 6 Shown.

[0034] Aspects of the present disclosure can be implemented as a method of controlling an ultrasonic transducer 100 comprising a stack of at least two membranes 10, 20 as described herein. For example, the method includes one or more of the following steps: applying a first electrical signal S11 to a first electrode 11 on the first membrane 10, and applying a different second electrical signal S21 to a second electrode 21 on the second membrane 20 to cause a varying voltage ΔV1, ΔV2 between the first electrode 11 and the second electrode 12 during respective vibration periods T1, T2 of the membranes 10, 20. Correspondingly, the first electrode 11 on the first membrane 10 can interact with the second electrode 21 on the second membrane 20 via an electrostatic force Fe that varies according to the varying voltage ΔV1, ΔV2 during respective vibration periods T1, T2.

[0035] In some embodiments, the circuit 30 includes a signal generator (not shown) configured to generate an electrical signal comprising one or more frequencies at or near the resonant frequency of the first membrane 10 and / or the second membrane 20. In other or additional embodiments, the circuit 30 includes a signal detector (not shown) configured to detect the electrical signal comprising one or more frequencies at or near the resonant frequency of the first membrane 10.

[0036] Although in principle the membrane can support different resonant vibrations, preferably the fundamental mode with the lowest resonant frequency (e.g. denoted as u 01 or 1s) is used to effectively generate or receive sound waves. For example, the resonant frequency is determined by, for example, the diameter of the acoustic membrane and one or more of the membrane material properties. Other or additional parameters may also be used, such as density, Poisson's ratio, and Young's modulus. In some embodiments, the fundamental frequency (Hz) may be determined using parameters such as membrane tension (N / m), density (kg / m 2), diameter (m). Other or additional parameters such as film thickness, elastic modulus, etc. may also be used. Alternatively or additionally, the fundamental frequency of the membrane may be determined by any other analytical or numerical modeling. In one embodiment, the specific resonant frequency is determined by setting a specific diameter that is related to the tension and density of the membrane. For example, the diameter may correspond to half a wavelength at the resonant frequency of a wave propagating in the membrane to generate a standing wave.

[0037] In a preferred embodiment, a piezoelectric transducer is used to actuate the membrane. Most preferably, the piezoelectric material is arranged as a layer on the flexible membrane. Additional layers may also be provided, such as an electrode layer for applying a corresponding electrical signal to the piezoelectric layer. As described herein, capacitive layers and / or conductive layers for applying an electrostatic charge are also contemplated. These layers may be charged by other or additional electrical signals, such as applying an electrostatic charge during a portion of a corresponding vibration cycle or dynamically applying a charge.

[0038] Performance can be improved by driving the transducer with a carrier frequency at or near the corresponding resonant frequency of the transducer. For example, the first resonance or ground resonance of the membrane is used. The resonant frequency of the transducer may be relatively high, for example greater than one kilohertz, greater than ten kilohertz, greater than 100 kilohertz or even greater than one megahertz. Such high frequencies may not be suitable for all applications. For example, for tactile applications, frequencies above 800 Hz may be difficult to feel. For example, the optimal frequency for tactile feedback may be between 50 Hz and 500 Hz, preferably between 100 Hz and 300 Hz.

[0039] In some embodiments, the electrical signal comprises multiple frequencies, including a carrier frequency that corresponds as closely as possible to the resonant frequency of the transducers, and an envelope or modulation frequency depending on the application. For example, a haptic feedback device may use a carrier frequency of 40 kHz, which is amplitude modulated by a modulation frequency of 200 Hz. It is also contemplated to use more than two frequencies, particularly frequencies with a wide bandwidth, for example, including the resonant frequencies of the individual transducers.

[0040] In some embodiments, an acoustic device is formed that includes an array of multiple acoustic transducers as described herein. For example, the transducers can be formed from a patterned stack of layers on a flexible substrate. In one embodiment, the stack of layers includes a piezoelectric layer sandwiched between corresponding bottom and top electrode layers. In some embodiments, the actuating surface of the acoustic transducer includes a portion of the flexible substrate at the contact region. In other or additional embodiments, the membrane can be separately attached to the surrounding substrate.

[0041] In some embodiments, each membrane 10, 20 includes a corresponding flexible foil 15, 25 or other flexible substrate. For example, the flexible foil 15, 25 can serve as a support for other layers (such as electrode layers and / or piezoelectric layers). In some embodiments such as shown, the flexible foil 11 is fixed (such as laminated) or otherwise arranged on a relatively rigid support substrate 50. For example, the flexible foil 11 can be laminated and / or otherwise adhered to the support substrate 50. The fixation between the flexible substrate and the support substrate or other rigid structure can be provided, for example, during and / or after manufacture. Compared with the flexible foil 11, the support substrate can be relatively flat, similar or thicker. Preferably, the bending stiffness of the support substrate 50 is relatively higher than the flexible foil 11, for example, at least twice, three times, five times, ten times or more. Therefore, the support substrate 50 can provide additional structural integrity. Preferably, the support substrate 50 is arranged with an opening at a position corresponding to one or more ultrasonic transducers in the ultrasonic transducer. The opening can enable the surface of the transducer to move relatively freely while still providing rigid support.

[0042] In some embodiments (not shown), one of the first or second films is flush with the surrounding substrate 50. This may be advantageous, for example, to contact a target object. In other or additional embodiments, the corresponding film that contacts the target object may be relatively thick, for example to protect the other film, which may be relatively thin.

[0043] Figure 2 Examples of electrical signals S11 to S13, S21 to S23, which can for example be applied to respective electrodes 11 to 13, 21 to 23 of the ultrasonic transducer 100 shown in the previous figures, and corresponding vibrations Z10, Z20 of the respective membranes 10, 20 are shown.

[0044] At the top of the figure, piezoelectric signals S13, S12 are applied to electrodes 13, 12, which sandwich a first piezoelectric layer 14 therebetween. In some embodiments, such as those shown, the voltage ΔV3, ΔV5 between the signals S12, S13 applied to the electrodes 12, 13, which sandwich the piezoelectric material 14, varies over time. The arrow passing through the piezoelectric material 14 indicates the direction of the first piezoelectric force F1, which depends on the varying voltage ΔV3, ΔV5 between the signals applied to the electrodes and the polarization P1 of the first piezoelectric layer 14.

[0045] The upper middle portion of the figure shows, for example, a first set of electrostatic signals S11 and S21 applied to the first electrode 11 and the second electrode 12. The arrow between the signals S11 and S21 represents the electrostatic force Fe between the electrodes. For example, when a certain voltage ΔV2 exists between the signals S11 and S21, such as when the charges (- / +) on the electrodes 11 and 12 are opposite, or when one electrode is more charged than the other, the electrostatic force Fe is attractive. Conversely, when the voltage ΔV1 between the electrodes is low, such as when the voltage ΔV1 between the electrodes is zero when the charges are the same (the electrodes are the same charge, or the electrodes are uncharged), the electrostatic force Fe can also be relatively low or zero (or repulsive).

[0046] The lower middle portion of the figure shows, for example, a second set of piezoelectric signals S22 and S23 applied to electrodes 22 and 23, which sandwich a second piezoelectric layer 24 therebetween. Arrows passing through the piezoelectric material 14 indicate the direction of the second piezoelectric force F2, which depends on the varying voltages ΔV4 and ΔV6 between the signals applied to the electrodes and the polarization P2 of the second piezoelectric layer 24. In some embodiments, such as those shown here, the second set of piezoelectric signals S22 and S23 applied across the second piezoelectric layer 24 is identical to the first set of piezoelectric signals S12 and S13 applied across the first piezoelectric layer 14. This can provide relatively easy control of the signals; for example, identical signals can be connected to corresponding electrodes.

[0047] The bottom of the figure shows the vibration of the membranes 10, 20. Here, this is represented by, for example, the changing position Z10, Z20 of a point on the respective membrane. It should be noted that the movement of the membrane can follow (lag behind) the applied piezoelectric force and / or electrostatic force. In general, the phase of the vibration can depend on the phase of the electrical signal (e.g. piezoelectric signal S12, S13; S22, S23) (voltage) applied, for example, through the piezoelectric material 14, 24 of one or both of the membranes. The phase of the vibration can also depend on other factors, such as the polarization P1, P2 of the piezoelectric material 14, 24. In addition to the electrical phase caused by the transfer function of the voltage / current source and the complex electrical impedance of the piezoelectric material, there is also an acoustic phase. For a weakly damped resonator, this phase difference varies, for example, from -90 degrees to +90 degrees when the frequency is swept through the resonant frequency. The frequency width of the region where the phase changes can be a measure of the bandwidth for the device. This can also be related to the polarization direction. For example, polarizing the material in opposite directions may result in the vibration being actuated in opposite directions (e.g. in opposite directions). Figure 6 ). In addition to or in lieu of piezoelectric actuation, other forms of actuating the respective membranes are also contemplated. Additionally, other or additional factors (such as a common substrate or pocket for interconnection between the membranes) may play a role in keeping the vibrations approximately in phase.

[0048] In some embodiments, such as the embodiment shown in FIG1 , the respective multiple groups of electrodes 12 , 13 ; 22 , 23 are separate and independent from the first electrode 11 and the second electrode 12 used to apply the electric (electrostatic) signals S11 and S21 , respectively. The respective multiple groups of electrodes are used to apply the piezoelectric electrical signals S12 , S13 ; S22 , S23 to the respective piezoelectric layers 14 , 24 . This can have the advantage of providing independent control over the electrostatic and piezoelectric interactions. As will be discussed further below, one or more of the electrodes can also be combined.

[0049] While this embodiment shows a constant voltage applied to one of the electrostatic electrodes, it is also contemplated that the voltage on both electrostatic electrodes may be varied. For example, the applied signal may include pulse shaping (applying time-dependent pulses) on both membranes to optimize the vibration of the membranes (e.g., making the vibration more or less linear or increasing the amplitude).

[0050] Figure 3A and Figure 3B An ultrasonic transducer 100 is shown in which some of the electrodes 11, 12; 21, 22 are combined. Figure 1A and Figure 1B In contrast, the piezoelectric layers 14, 24 are located on the inside, and some of the electrodes are combined. Another or additional arrangement is also conceivable, in which one of the corresponding electrodes is on the outside. For example, a carrier layer can act as a further capacitor to increase the capacitance between the membranes.

[0051] In some embodiments, the first electrode 11 on the first membrane 10 is also one of the first group of piezoelectric electrodes 12 and 13 on the first membrane 10. The first electrode on the first membrane is configured to interact with the second electrode 21 on the second membrane 20 via a varying electrostatic force Fe, and the first group of piezoelectric electrodes sandwiches the first piezoelectric layer 14. In other or additional embodiments, the second electrode 21 (= 22) on the second membrane 20 is also one of the second group of piezoelectric electrodes 22 and 23 on the second membrane 20. The second electrode on the second membrane is configured to interact with the first electrode 11 (= 12) on the first membrane 10 via a varying electrostatic force Fe, and the second group of piezoelectric electrodes sandwiches the second piezoelectric layer 24. By using the same electrode on at least one membrane for both electrostatic and piezoelectric interactions, the number of electrodes can be reduced.

[0052] Figure 4 The corresponding signals and vibrations are shown, which are applied to the transducer of the previous figure for example. Here, since the corresponding first electrode and second electrode also serve as one of the piezoelectric electrodes, the signal S11 = S12 and the signal S21 = S22. Figure 2In contrast, signals S22 (= S21) and S23 are offset by a voltage of +V22. This is a way of providing a changing electrostatic interaction between the first electrode and the second electrode and a changing piezoelectric interaction of the two piezoelectric layers 14, 24 at the same time. Of course, other signals that provide the same or similar simultaneous interactions can also be envisaged. For example, one or both directions of the polarizations P1, P2 can be flipped, and the corresponding signals are also flipped. For example, a fixed or variable offset can be added to the signal that maintains the relative voltage difference. For example, the piezoelectric signal can be adapted to maintain a similar voltage difference on the corresponding piezoelectric layer, based on any set of electrostatic signals in the electrostatic signal. Although the present figure shows a sinusoidal signal, other signals, such as a block wave or any other shape, can also be envisaged to improve the electrostatic and / or piezoelectric interaction. In addition, the phases can be offset to improve the interaction.

[0053] Figure 5A and Figure 5B An ultrasonic transducer 100 is shown in which the membranes vibrate in anti-phase. Figure 3A and Figure 3B Similar piezoelectric layers on the inside, but this can also be implemented in other embodiments described herein. For example, the signal or polarization of one of the piezoelectric layers can be flipped. In some embodiments, the distance "d" between the membranes can be increased to prevent the membranes from contacting each other when the membranes move toward each other. In other or additional embodiments, the distance "d" of physical contact of the membranes can provide the desired interaction. It is also conceivable that the pressure in the pockets between the membranes prevents contact. One advantage of membranes moving in anti-phase may be that the asymmetry of the membrane movement can increase the bandwidth. One advantage of membranes moving in unison may be that the electrostatic forces on the membranes are not offset by the optional pressure in the pockets (if the membranes move toward each other), so the ultrasound can be more intense.

[0054] Figure 6 The corresponding signals and vibrations are shown, which are applied to the transducer of the previous figure for example. Figure 4In principle, the signals can be similar or identical, while the polarization of one of the piezoelectric layers (here, P2) is reversed, resulting in an opposite piezoelectric force (here, F2 in the second piezoelectric layer 24). Alternatively, for example, the piezoelectric signal can be reversed to one of the piezoelectric layers, while being compensated by the other signal to maintain the relative voltage difference. The corresponding vibrations Z10 and Z20 of the membranes are shown here as being in anti-phase. In some embodiments, this can cause, for example, an asymmetric displacement of the first membrane 10 to one side compared to the opposite side during vibration. For example, the first membrane 10 has a first amplitude between the equilibrium position of the first membrane 10 and the maximum extension position of the first membrane 10 to one side, and a second amplitude between the equilibrium position and the maximum extension position of the first membrane 10 to the other side, wherein the second membrane 20 is configured to influence the movement of the first membrane 10 through electrostatic interaction to cause a difference between the amplitudes, for example, a difference of at least 5 percent, or at least 10 percent. These and other embodiments that cause asymmetric displacements can help increase the effective bandwidth in membrane-based transducers.

[0055] Figure 7A A perspective sectional view of an ultrasound transducer 100 is shown, wherein electrostatic electrodes and piezoelectric electrodes (electric discs) are arranged adjacently on the respective membrane 10 , 20 .

[0056] In some embodiments, each of the membranes 10, 20 comprises a flexible foil 15, 25 having a stack of layers including respective electrodes 11, 12, wherein the membranes 10, 20 are adhered to respective substrate layers 51 to 53 of a substrate 50, wherein an intermediate substrate layer 52 is arranged between the flexible foils 15, 25 to form a spacer between the flexible foils, wherein the ultrasound transducer 100 is formed at an opening in the substrate layers 51 to 53. For example, the spacing or distance between the membranes 10, 20 is determined by the thickness of the intermediate substrate layer 52.

[0057] In some embodiments, at least one of the membranes 10, 20 (and preferably both membranes) has a relatively thicker and / or stiffer portion at the center of the membrane compared to the (radial) edges of the membrane, e.g., the membrane is convex at the center. This can have the effect of increasing the total displacement compared to an out-of-plane, peak-like displacement of the membrane, for example. For example, the relatively thicker or stiffer central portion can have less curvature during deflection (e.g., more square than a Gaussian distribution), so that the inward contraction effect can be extended over a larger area than just the central peak.

[0058] In some embodiments, the center of the membrane is thicker than the edge, for example, by at least 1.1 times, 1.2 times, 1.5 times, 2 times or a greater multiple. In other or additional embodiments, the material at the center of the membrane is harder than at the edge, for example, by at least 1.1 times, 1.2 times, 1.5 times, 2 times or a greater multiple. Preferably, the thickened and / or hardened area extends over a sub-portion of the total area, for example, between 20 percent and 90 percent of the coverage area, preferably between 40 percent and 80 percent, or between 50 percent and 70 percent. In some embodiments, the membrane is provided with an additional layer or projection on at least one side, preferably on the first side 10a pointing inwardly. It should be understood that having additional material off-center relative to the membrane center plane on one side can also contribute to asymmetric displacement.

[0059] Figure 7B Shown in Figure 7A VIIB shows a bottom view of the first membrane 10. In some embodiments, the first membrane 10 includes: a first region composed of a conductive material, the first region forming a first electrode 11, the first electrode being configured to electrostatically interact with a second electrode on the second membrane 20; and a second region composed of a conductive material, the second region being adjacent to the first region, the second region forming a piezoelectric electrode 12 in a set of piezoelectric electrodes 12, 13 on the first membrane 10, the set of piezoelectric electrodes sandwiching a corresponding first piezoelectric layer 14. In other or additional embodiments, the first electrode 11 covers a central region r / R between 0 and r1 of the first membrane 10, and the piezoelectric electrode 12 covers a peripheral region r / R between r1 and r2 of the first membrane 10 surrounding the central region. For example, when two piezoelectric electrodes 12, 13 are separately arranged to sandwich a peripheral region of the first piezoelectric layer 14, the first piezoelectric layer (not shown) can be confined to the peripheral region, or can be a continuous layer.

[0060] Figure 7CThe relative displacement v / V at resonance is shown as a function of the relative electrode radius r / R of the piezoelectric layer. As shown, the more significant displacement is caused by the following parts of the piezoelectric layer and / or piezoelectric electrode, which are arranged in a band around the center, for example, between 0.4 times and 0.9 times the radius R. Advantageously, the center can be covered by the first electrode 11 for electrostatic interaction without losing most of the piezoelectric function. In a preferred embodiment, the central area covered by the first electrode 11 is arranged between the center of the first membrane 10 and a first radial distance r1 from the center, and the peripheral area covered by the piezoelectric electrode 12 is arranged outside the first radial distance r1, for example, up to a second radial distance r2, wherein the first radial distance r1 is between 0.2 times and 0.7 times the radius R of the membrane, preferably between 0.3 times and 0.6 times, and most preferably less than half the radius <0.5R, for example 0.4R, as shown. In another or additional embodiment, the second radial distance r2 is between 0.7 and 1 times the radius R of the membrane, for example up to 0.9 R. In some embodiments, the central region covered by the first electrode 11 is relatively stiff and / or thicker than the surrounding regions. For example, electrostatic interactions can be improved (applied over the entire region) if the central region remains relatively flat during vibration.

[0061] In interpreting the appended claims, it should be understood that the word "comprising" does not exclude the presence of other elements or actions in addition to the elements or actions listed in a given claim; the word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements; any figure marks in a claim do not limit the scope of the claim; multiple "means" may represent the same or different items or implemented structures or functions; unless otherwise specifically stated, any of the means or parts disclosed in a claim may be combined together or separated into further parts. In fact, the fact that some measures are listed in different claims does not mean that a combination of these measures cannot be used to provide benefits. Therefore, the present embodiment may include all valid combinations of claims, wherein each claim can, in principle, refer to any previous claim unless expressly excluded by the context.

Claims

1. An ultrasonic transducer (100), comprising: - a stack of at least two membranes (10, 20) attached to a substrate (50), wherein the membranes (10, 20) are separated by a spacer (5) between the membranes, wherein the substrate (50) surrounds the stack in a plane (X, Y) of the membranes; wherein a first membrane (10) in the stack is parallel to and faces a second membrane (20) in the stack, wherein the membranes (10, 20) are configured to vibrate at an ultrasonic frequency (U) to transmit and receive ultrasonic waves (W), wherein each membrane (10, 20) comprises a set of electrodes (11-13; 21-23); - a circuit (30) coupled to the electrodes, the circuit having a controller configured to apply a first electrical signal (S11) to the first electrode (11) on the first membrane (10), and to apply a different second electrical signal (S21) to the second electrode (21) on the second membrane (20); wherein the first electrical signal and the second electrical signal (S11, S21) are configured to apply a varying voltage (ΔV1, ΔV2) between the first electrode (11) and the second electrode (12) during respective vibration periods (T1, T2) of the membranes (10, 20) at the ultrasonic frequency (U); wherein the first electrode (11) on the first membrane (10) is configured to interact with the second electrode (21) on the second membrane (20) by an electrostatic force (Fe) that varies according to the varying voltage (ΔV1, ΔV2) during the respective vibration periods (T1, T2).

2. The ultrasonic transducer (100) according to claim 1, wherein The controller is configured to apply a first voltage (ΔV1) between the first electrode and the second electrode (11, 12) during a first half period (T1) of the vibration period, and to apply a second voltage (ΔV2) between the first electrode and the second electrode (11, 12) during a second half period (T2) of the vibration period, wherein the second voltage (ΔV2) is greater than the first voltage (ΔV1).

3. The ultrasonic transducer (100) according to claim 2, wherein: The controller is configured to apply an attractive electrostatic force (Fe) between the first and second electrodes (11, 12) only during a second half (T2) of the vibration cycle by varying a voltage (ΔV1, ΔV2).

4. The ultrasonic transducer (100) according to claim 3, wherein: When the first membrane (10) moves in one direction (-Z), the attractive electrostatic force (Fe) is applied only during the second half period (T2) of the vibration period, and when the first membrane (10) moves in the opposite direction (+Z), the attractive electrostatic force is not applied during the first half period (T1).

5. The ultrasonic transducer (100) according to claim 1, wherein At least one of the membranes (10, 20) includes a corresponding piezoelectric layer (14, 24) sandwiched by a corresponding set of electrodes (12, 13; 22, 23) to transmit and receive piezoelectric signals (S12, S13; S22, S23) according to the vibration period, wherein the piezoelectric signals (S12, S13; S22, S23) are configured to generate corresponding piezoelectric forces (F1, F2) on the corresponding at least one of the membranes (10, 20).

6. The ultrasonic transducer (100) according to claim 1, wherein The circuit (30) is configured to apply multiple sets of electrical signals (S12, S13; S22, S23) to corresponding multiple sets of electrodes (12, 13; 22, 23), causing the membranes (10, 20) to move in the same direction (+Z, -Z) simultaneously and vibrate in unison.

7. The ultrasonic transducer (100) according to claim 1, wherein A first set of piezoelectric signals (S12, S13) is applied to a first set of electrodes (12, 13) on the first membrane (10) to cause a varying voltage (ΔV3, ΔV4) across a first piezoelectric layer (14) on the first membrane (10), and a second set of piezoelectric signals (S22, S23) is applied to a second set of electrodes (22, 23) on the second membrane (20) to cause a varying voltage (DV5, DV6) across a second piezoelectric layer (24) on the second membrane (20), wherein the varying voltage (ΔV3, ΔV4) across the first piezoelectric layer (14) is in phase with or in antiphase with the varying voltage (ΔV5, ΔV6) across the second piezoelectric layer (24).

8. The ultrasonic transducer (100) according to claim 1, wherein A first set of piezoelectric signals (S12, S13) to a first piezoelectric layer (14) on the first membrane (10) is configured to generate a first piezoelectric force (F1) in the same direction (-Z, -Z) as the electrostatic force (Fe) during a second half period (T2) of the vibration cycle.

9. The ultrasonic transducer (100) according to claim 8, wherein: The combined first piezoelectric and electrostatic forces (F1, Fe) result in an increased first amplitude (A1) of vibration of the first membrane (10) compared to a second amplitude (A2) of vibration of the second membrane (20).

10. The ultrasonic transducer (100) according to claim 1, wherein The first electrode (11) on the first membrane (10) is also one of a first group of piezoelectric electrodes (12, 13) on the first membrane (10), and the first electrode on the first membrane is configured to interact with the second electrode (21) on the second membrane (20) through the changing electrostatic force (Fe), and the first group of piezoelectric electrodes sandwiches the first piezoelectric layer (14).

11. The ultrasonic transducer (100) according to claim 1, wherein: Each of the membranes (10, 20) comprises a flexible foil (15, 25) having a stack of layers comprising respective first and second electrodes (11, 12), wherein the membranes (10, 20) are adhered to respective substrate layers (51-53) of the substrate (50), wherein an intermediate substrate layer (52) is arranged between the flexible foils (15, 25) to form the spacer (5) between the flexible foils, and wherein the ultrasonic transducer (100) is formed at an opening in the substrate layers (51-53).

12. The ultrasonic transducer (100) according to claim 1, wherein The membranes (10, 20) have a relatively thicker and / or stiffer portion at the center of the respective membrane compared to the edges of the membrane.

13. The ultrasonic transducer (100) according to claim 1, wherein The first membrane (10) includes: a first region composed of a conductive material, the first region forming the first electrode (11), the first electrode being used to electrostatically interact with the second electrode on the second membrane (20); and a second region composed of a conductive material, the second region being adjacent to the first region, the second region forming a piezoelectric electrode (12) in a group of piezoelectric electrodes (12, 13) on the first membrane (10), the group of piezoelectric electrodes sandwiching a corresponding first piezoelectric layer (14).

14. The ultrasonic transducer (100) according to claim 13, wherein: The first electrode (11) covers a central area of ​​the first membrane (10), and the piezoelectric electrode (12) covers a peripheral area of ​​the first membrane (10) surrounding the central area, wherein the central area covered by the first electrode (11) is arranged between the center of the first membrane (10) and a first radial distance (r1) from the center, and the peripheral area covered by the piezoelectric electrode (12) is arranged outside the first radial distance (r1), wherein the first radial distance (r1) is less than half of the radius of the first membrane (10).

15. A method of controlling an ultrasonic transducer (100) comprising a stack of at least two membranes (10, 20) attached to a substrate (50), wherein: The membranes (10, 20) are separated by spacers (5) between the membranes, wherein the substrate (50) surrounds the stack in the plane (X, Y) of the membranes; wherein a first membrane (10) in the stack is parallel to and faces a second membrane (20) in the stack, wherein the membranes (10, 20) are configured to vibrate at an ultrasonic frequency (U) to transmit and receive ultrasonic waves (W), wherein each membrane (10, 20) includes a set of electrodes (11-13; 21-23), and the method comprises: applying a first electrical signal (S11) to a first electrode (11) on the first membrane (10), and applying a different second electrical signal (S21) to the second membrane (10). A second electrode (21) on a membrane (20); wherein the first electrical signal and the second electrical signal (S11, S21) are configured to apply a varying voltage (ΔV1, ΔV2) between the first electrode (11) and the second electrode (12) during corresponding vibration periods (T1, T2) of the membrane (10, 20) at the ultrasonic frequency (U); wherein the first electrode (11) on the first membrane (10) is configured to interact with the second electrode (21) on the second membrane (20) through an electrostatic force (Fe) that varies according to the varying voltage (ΔV1, ΔV2) during the corresponding vibration periods (T1, T2).

16. The method according to claim 15, wherein The method comprises applying a first voltage (ΔV1) between the first electrode and the second electrode (11, 12) during a first half period (T1) of the vibration period, and applying a second voltage (ΔV2) between the first electrode and the second electrode (11, 12) during a second half period (T2) of the vibration period, wherein the second voltage (ΔV2) is greater than the first voltage (ΔV1).

17. The method according to claim 16, wherein The method comprises applying an attractive electrostatic force (Fe) between the first electrode and the second electrode (11, 12) only during a second half (T2) of the vibration cycle by varying a voltage (ΔV1, ΔV2).

18. The method according to claim 17, wherein When the first membrane (10) moves in one direction (-Z), the attractive electrostatic force (Fe) is applied only during the second half period (T2) of the vibration period, and when the first membrane (10) moves in the opposite direction (+Z), the attractive electrostatic force is not applied during the first half period (T1).

19. The method according to claim 15, wherein Multiple sets of electrical signals (S12, S13; S22, S23) are applied to corresponding sets of electrodes (12, 13; 22, 23), causing the membranes (10, 20) to move simultaneously in the same direction (+Z, -Z) and vibrate in unison.

20. The method according to claim 15, wherein A first set of piezoelectric signals (S12, S13) is applied to a first piezoelectric layer (14) on the first membrane (10) to generate a first piezoelectric force (F1) in the same direction (-Z, -Z) as the electrostatic force (Fe) during a second half period (T2) of the vibration cycle, wherein the combined first piezoelectric force and electrostatic force (F1, Fe) result in an increased first amplitude (A1) of the first membrane (10) compared to a second amplitude (A2) of the second membrane (20).

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