Ultrasonic transducer with laminated membrane

Abstract

The ultrasonic transducer (100) comprises a stack of at least two membranes (10, 20) bonded to a substrate (50). An electric circuit (30) connected to the electrodes and having a controller is configured to apply a first electric signal (S11) to a first electrode (11) of the first membrane (10) and a second electric signal (S21) different from the first electric signal to a second electrode (21) of the second membrane (20). The first electric signal and the second electric signal (S11, S21) are configured to apply a varying voltage (ΔV1, ΔV2) between the first electrode (11) and the second electrode (12) during a vibration period (T1, T2) of each of the membranes (10, 20). A first electrode (11) on the first membrane (10) is configured to interact with the second electrode (21) on the second membrane 20 with an electrostatic force (Fe) that varies during each oscillation period (T1, T2) in response to a varying voltage (ΔV1, ΔV2).

Description

[Technical Field]

[0001] The present disclosure relates to an ultrasonic transducer and a method for controlling the same. [Background technology]

[0002] Ultrasound transducers are used in a variety of applications, including medical imaging and flow meters. Membranes can be actuated by a variety of mechanisms. For example, a piezoelectric transducer can be coupled to the membrane. The piezoelectric material expands or contracts in response to an electrical signal, causing the membrane to vibrate. To increase the amplitude of the vibration, the actuation signal can be tuned to the resonant frequency of the membrane. Alternatively, or in addition, the amplitude of the electrical signal can be increased. However, limitations exist, such as a maximum voltage, to prevent damage to the actuation mechanism. Also, the force transmission required to generate the vibration may not be optimal.

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

[0004] The present disclosure relates to an ultrasonic transducer and a control method thereof. The transducer includes a stack of at least two membranes bonded to a (common) substrate. An electrical circuit is connected to the electrodes to apply a first electrical signal to a first electrode on a first membrane and a second electrical signal, different from the first electrical signal, to a second electrode on a second membrane. The first and second electrical signals are configured to apply a varying voltage between the first and second electrodes during each vibration period of the membranes. The first electrode on the first membrane is configured to interact with the second electrode on the second membrane 20 with a varying electrostatic force during each vibration period in response to the varying voltage. As described herein, dynamic changes in the voltage / electrostatic force between the membranes during each vibration period can be utilized to enhance the vibration of at least one of the membranes to achieve higher amplitude or power. For example, power can be enhanced in a stack of piezoelectric membranes. In some embodiments, the asymmetry of the displacement can improve bandwidth. [Brief explanation of the drawings]

[0005] The features, aspects, and advantages of the apparatus, systems, and methods of the present disclosure will become better understood from the following description, appended claims, and accompanying drawings.

[0006] [Figure 1A] 1 illustrates an ultrasonic transducer. [Figure 1B] 1 illustrates an ultrasonic transducer. [Figure 2] FIG. 10 illustrates an example of corresponding electrical signals and vibrations. [Figure 3A] FIG. 1 shows an ultrasonic transducer with some of the electrodes interlocked together. [Figure 3B] FIG. 1 shows an ultrasonic transducer with some of the electrodes interlocked together. [Figure 4] FIG. 10 illustrates an example of corresponding electrical signals and vibrations. [Figure 5A] FIG. 1 is a diagram showing an ultrasonic transducer in which membranes vibrate in opposite phases. [Figure 5B] FIG. 1 is a diagram showing an ultrasonic transducer in which membranes vibrate in opposite phases. [Figure 6] FIG. 10 illustrates an example of corresponding electrical signals and vibrations. [Figure 7A] FIG. 1 is a perspective cutaway view of an ultrasonic transducer in which electrostatic and piezoelectric electrodes (electropads) are adjacently arranged on respective membranes. [Figure 7B] FIG. 7B is a bottom view of the first membrane designated VIIB in FIG. 7A. [Figure 7C] FIG. 10 shows the relative volume displacement at resonance of a piezoelectric layer as a function of relative electrode radius. DETAILED DESCRIPTION OF THE INVENTION

[0007] The terms used to describe particular embodiments are not intended to limit the invention. As used herein, the singular forms "a," "an," and "the" are intended to include the plural, unless the context clearly dictates otherwise. The term "and / or" includes various combinations of one or more of the associated listed elements. The terms "comprising" and / or "having" imply the presence of the stated feature but do not exclude the presence or addition of one or more other features. When a step of a method is described as being performed after another step, unless otherwise specified, it means that the step directly follows the other step or that one or more intermediate steps may be performed before performing the step. Similarly, when a connection between structures or components is described, it means that the connection is direct or can be established through an intermediate structure or component, unless otherwise specified.

[0008] The present invention will now be described with reference to the accompanying drawings, in which embodiments of the invention are shown. For clarity of illustration, the absolute and relative sizes of systems, components, layers, and regions in the drawings may be exaggerated. Embodiments will be described with reference to schematic and / or cross-sectional illustrations of idealized embodiments and intermediate structures of the invention. In the following description and drawings, like numbers refer to like elements. Relative terms and their derivatives should be construed to refer to the orientation currently being described or shown in the drawings being described. These relative terms are for convenience of description only, and do not require the system to be constructed or operated in a particular orientation, unless specifically stated otherwise.

[0009] 1A and 1B show an ultrasonic transducer 100. In some embodiments, the ultrasonic transducer 100 comprises a stack of at least two membranes 10, 20 bonded to a substrate 50. In preferred embodiments, the membranes 10, 20 are separated from one another by a gap 5 therebetween, e.g., by providing a pocket. For example, the substrate 50 surrounds the stack in the planes X, Y of the membranes. In one embodiment, a first membrane 10 of the stack extends parallel to and faces a second membrane 20 of the stack. Preferably, the membranes 10, 20 are configured to vibrate at an ultrasonic frequency U to transmit ultrasonic waves W.

[0010] In some embodiments, each of the membranes 10 and 20 comprises a set of electrodes 11-13 and electrodes 21-23. An electrical circuit 30 may be connected to these electrodes and configured to apply electrical signals. For example, the electrical circuit 30 may include 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 of the first membrane 10, and a different second electrical signal S21 is applied to the second electrode 21 of the second membrane 20. Preferably, the first electrical signal S11 and the second electrical signal S21 are configured to apply (produce) varying voltages ΔV1 and ΔV2 between the first electrode 11 and the second electrode 12 during the vibration periods T1 and T2 of the membranes 10 and 20, respectively, at, for example, an ultrasonic frequency U or another frequency preferably having some phase relationship to the vibration periods. 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 with an electrostatic force Fe that varies during each oscillation period T1, T2 in response to the varying voltages ΔV1, ΔV2.

[0011] For example, the voltage value between the electrodes varies between ΔV1 and ΔV2 during each sub-period of periods T1 and T2. The varying voltage, i.e., the potential difference between the electrodes, may correspond to varying differential values ​​(e.g., compared to ground potential) of the voltages of the respective electrical signals S1 and S2 during different portions of the oscillation period. As shown in the figure, the varying voltage causes a corresponding + or - change in the charge on at least one electrode relative to the other. While only the change in charge on the first electrode 11 is shown in the figure, it is conceivable that a change in charge also occurs on the second electrode 12 or on both electrodes (e.g., if the change is not the same).

[0012] Typically, when the (opposing) electrodes 11, 21 have opposite negative or positive charges, they will create an attractive electrostatic force Fe as shown (e.g., during the second half of the oscillation period T2). In principle, when the same polarity charges (+ / +; - / -) are present on each electrode, they may repel each other, for example, during the first half of the oscillation period T1 (e.g., half of the oscillation period). However, in practice, the repulsive force is not very large or negligible. In some embodiments, during the first half of the oscillation period T1, the charges on each electrode may both be zero. For example, the electrostatic force may be affected by a charge difference or potential difference between the electrodes.

[0013] Without being bound by theory, the (attractive) electrostatic force Fe between charges on a pair of parallel plates is Fe=ε0·A·ΔV 2 / 2d 2 where "ε0" is the dielectric constant of a vacuum (~8.85 10 -12 F·m−1 (Farad / meter), "A" is the surface area of ​​the parallel plates, "ΔV" is the voltage or potential difference between the parallel plates, and "d" is the distance between the parallel plates. For example, the surface area of ​​each of the first electrode 11 and the second electrode 12 is at least 100 square micrometers, at least 200 square micrometers, at least 500 square micrometers, at least 1 square millimeter, at least 2 square millimeters, at least 5 square millimeters, at least 10 square millimeters, at least 20 square millimeters, at least 50 square millimeters, or at least 1 square centimeter. The larger the surface area of ​​the first electrode 11 and the second electrode 12, the greater the electrostatic force (when the same voltage is used). For example, the distance between the membranes 10, 20 and / or between the first electrode 11 and the second electrode 12 is less than 1 centimeter, less than 0.5 centimeters, less than 2 millimeters, less than 1 millimeter, less than 0.5 millimeters, less than 200 micrometers, less than 100 micrometers, or less than 50 micrometers. The shorter the distance, the stronger the electrostatic force.

[0014] The electrodes described herein may deviate slightly from an ideal pair of parallel plates, for example, in terms of their geometry, but generally tend to be similarly shaped. For example, when the voltage ΔV is high, the electrostatic force F is generally high, and when the voltage is low or no voltage (ΔV=0), the electrostatic force is low or zero. For example, the larger the (effective) area of ​​the electrode (electrode pad) and / or the smaller the effective distance between the electrodes, the greater the electrostatic force F. Preferably, the first electrode 11 and the second electrode 12 each include an electrode pad that covers at least a portion of the inner surface of the opposing membrane. Preferably, the distance "d" between the membrane or electrode pads (e.g., in a resting state) is relatively small, at least 1, 2, 3, 4, 5, 10, or more times smaller than the diameter of the membrane. Some embodiments described below with reference to FIG. 7 incorporate improvements to the electrode layout and / or the geometry, such as embossed portions of the membrane, to maintain a constant distance between the electrodes at various distances from the center of the membrane.

[0015] In some embodiments, the electrical circuit 30 is configured to apply a first voltage ΔV1 between the first electrode 11 and the second electrode 12 during the first half T1 of the oscillation period and a second voltage ΔV2 between the first electrode 11 and the second electrode 12 during the second half T2 of the oscillation period. The second voltage ΔV2 is higher than the first voltage ΔV1. For example, the second voltage ΔV2 is at least 1 volt higher than the first voltage ΔV1, preferably at least 2 volts, at least 5 volts, at least 10 volts, at least 20 volts, at least 50 volts, or at least 100 volts higher. The greater the change in the voltages ΔV1 and ΔV2 between different portions of the oscillation period, the greater the corresponding change in the electrostatic force. For example, the first voltage ΔV1 can be relatively low or preferably zero, i.e., no voltage is present between the electrodes 11 and 12. For example, the second voltage ΔV2 may range between 1 and 1000 volts, preferably between 10 and 500 volts, and most preferably between 50 and 200 volts.

[0016] In some embodiments, the electric circuit 30 is configured to apply an attractive electrostatic force Fe between the first electrode 11 and the second electrode 12 only during the second half T2 of the vibration period by using voltage changes ΔV1 and ΔV2. In other or further embodiments, the attractive electrostatic force Fe is applied only (or predominantly) during one subperiod (e.g., the second half T2) of the vibration period in which the first membrane 10 (e.g., its center) moves in one direction, the -Z direction, and is not applied during another subperiod (e.g., the first half T1) of the vibration period in which the first membrane 10 moves in the opposite direction, the +Z direction.

[0017] Note that for purposes of illustration, the membranes are shown at the end of each half-period T1 and T2 of their vibration cycle, i.e., when the membrane sags at their maximum amplitudes A1 and A2. In reality, the membrane motion (Z position) may lag behind the actual applied force, or conversely, may lead ahead of the applied force (see, e.g., FIG. 2). This may also depend on the frequency of the applied force relative to the membrane's resonant frequency. For example, membranes 10 and 20 may vibrate at or near their resonant frequency to transmit and / or receive ultrasonic waves W that resonantly interact with one or both of the membranes.

[0018] Typically, it is most effective to apply a force that increases the first amplitude A1 during the period when the membrane moves in the same direction as the force, e.g., during the half cycle when the first membrane 10 moves from the lower position to the upper position. Conversely, the second amplitude A2 can be decreased by applying a force in the opposite direction during the same half cycle. In effect, for example, the first amplitude A1 can be increased at the expense of the second amplitude A2 by tensioning this second membrane.

[0019] In some embodiments, as shown here, the electrical circuit 30 is configured to apply a set of electrical signals to each set of electrodes that causes the membranes 10, 20 to move simultaneously in the same direction, +Z or -Z, vibrating together. That is, the vibrations of the first and second membranes are substantially in phase, e.g., with a phase difference of less than 45 degrees, preferably less than 20 degrees, less than 10 degrees, and most preferably 0 degrees. For example, the phase of the vibrations is controlled by the respective phases of one or more of the electrical (drive) signals. By moving the two membranes together, the second membrane 20 can further increase the amplitude of the first membrane 10, for example, by pulling on the first membrane 10 while the first membrane 10 is already moving toward the second membrane 20. While three electrodes per membrane are shown in this embodiment, two electrodes may be used, for example, as shown in Figures 3-7. Instead of vibrating the membranes in phase, it is also possible to vibrate the membranes in antiphase. This is described below, for example, with reference to Figures 5-6.

[0020] In-phase (or anti-phase) motion of the membranes 10, 20 may be achieved by applying an additional force to at least one of the membranes. The additional force may in principle be, for example, an electrostatic force applied to another electrode (not shown), or may be caused, for example, by an actuator (not shown) next to the membrane, or any other interaction. In a preferred embodiment, the additional force may be applied by a piezoelectric interaction, as described herein. Most preferably, one or both of the membranes 10, 20 are provided with a piezoelectric layer 14, 24.

[0021] In some embodiments, at least one of membranes 10 and 20 includes a piezoelectric layer 14, 24 sandwiched by respective sets of electrodes 12, 13 and electrodes 22, 23 to transmit piezoelectric signals S12, S13 and S22, S23 according to a vibration period. In other or further embodiments, piezoelectric signals S12, S13 and S22, S23 are configured to generate piezoelectric forces F1, F2 in at least one corresponding membrane 10, 20. For example, piezoelectric forces may be applied to first membrane 10 and / or second membrane 20, preferably both, by applying the same or similar signals that cause synchronized motion.

[0022] In one embodiment, a first set of piezoelectric signals S12, S13 are applied to a first set of electrodes 12, 13 on the first film 10, producing changing voltages ΔV3, ΔV4 across the first piezoelectric layer 14 on the first film 10. A second set of piezoelectric signals S22, S23 are applied to a second set of electrodes 22, 23 on the second film 20, producing changing voltages DV5, DV6 across the second piezoelectric layer 24 on the second film 20. The changing voltages ΔV3, ΔV4 across the first piezoelectric layer 14 are in phase or out of phase (180 degrees out of phase) with the changing voltages ΔV5, ΔV6 across the second piezoelectric layer 24.

[0023] By applying the same or similar phases to the piezoelectric signals of each membrane, the two membranes can be actuated to move in unison by their respective piezoelectric forces. Alternatively, by applying signals of opposite phase (or by reversing the polarity of the piezoelectric layers), the two membranes can be actuated in antiphase, meeting each other in the center, as shown in Figures 5-6. For example, the second varying voltage is composed of a first piezoelectric voltage ΔV3 between signals S12 and S13 during the first half T1 of the vibration cycle, and a second piezoelectric voltage ΔV5 between signals S12 and S13 during the second half T2 of the vibration cycle. For example, the third varying voltage is composed of a third piezoelectric voltage ΔV4 between signals S22 and S23 during the first half T1 of the vibration cycle, and a second piezoelectric voltage ΔV6 between signals S22 and S23 during the second half T2 of the vibration cycle. In some embodiments, it may be beneficial for the piezoelectric signals between the membranes to be matched, e.g., S12 = S22 and / or S13 = S23, although these voltage signals may be offset (e.g., have the same difference S12-S13 = S22-S23) or have different relative amplitudes (e.g., ΔV3 = c·ΔV5, where "c" may be negative depending on the polarity of the piezoelectric layers 13, 24, respectively).

[0024] In some embodiments, the first set of piezoelectric signals S12, S13 supplied to the first piezoelectric layer 14 of the first membrane 10 is configured to generate a first piezoelectric force F1 in the same direction -Z, -Z as the electrostatic force F1 during the second half T2 of the vibration period. In other or further embodiments, the first piezoelectric force F1 and the electrostatic force F1 can be combined to increase the first amplitude A1 of the first membrane 10 compared to the second amplitude A2 of the second membrane 20, as shown, for example, in FIGS. 1-4. For example, as shown in the figures, the arrows indicating the directions of the piezoelectric force F1 and the electrostatic force F1 of the first membrane 10 point in the same direction during different portions of the vibration period, e.g., at time T2 during which the membranes are both descending.

[0025] In other or further embodiments, the second set of piezoelectric signals S22, S23 to the second piezoelectric layer 24 of the second membrane 20 is configured to generate a second piezoelectric force F2 in a direction −Z, +Z opposite to the electrostatic force F2 during the second half of the vibration period T2, such that the piezoelectric force Fp and the electrostatic force F2 together reduce the second amplitude A2 of the second membrane 20. For example, as shown in the figure, during different portions of the vibration period T1, T2, e.g., during time T1 in the period when both membranes are up and time T2 in the period when both membranes are down, the arrows indicating the respective directions of the piezoelectric force Fp and the electrostatic force F2 of the second membrane 20 point in opposite directions. Instead of increasing one amplitude A1 at the expense of the other amplitude A2, it is also possible to increase both amplitudes, as will be described below with reference to FIGS. 5-6.

[0026] Aspects of the present disclosure may be embodied 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 may include one or more steps of applying a first electrical signal S11 to a first electrode 11 of a first membrane 10 and a different second electrical signal S21 to a second electrode 21 of a second membrane 20, to produce varying voltages ΔV1, ΔV2 between the first electrode 11 and the second electrode 12 during vibration periods T1, T2 of the membranes 10, 20, respectively. In this case, the first electrode 11 of the first membrane 10 may interact with the second electrode 21 of the second membrane 20 with a varying electrostatic force F during each vibration period T1, T2 in response to the varying voltages ΔV1, ΔV2.

[0027] In some embodiments, the electronic circuit 30 comprises 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 further embodiments, the electronic circuit 30 comprises a signal detector (not shown) configured to detect an electrical signal comprising one or more frequencies at or near the resonant frequency of the first membrane 10.

[0028] In principle, the membrane can vibrate at several different resonant frequencies, but preferably, in order to efficiently generate or receive acoustic waves, it vibrates at the fundamental mode with the lowest resonant frequency (e.g., u 01 or 1s) is used. For example, the resonant frequency is determined by, for example, one or both of the membrane material properties and the diameter of the acoustic membrane. Other or additional parameters can also be used, such as density, Poisson's ratio, Young's modulus, etc. In some embodiments, the fundamental frequency (Hz) is determined by the membrane tension (N / m), density (kg / m 2 The fundamental frequency of the membrane can be expressed using parameters such as the wavelength (m), the diameter (m), etc. Other or additional parameters, such as the membrane's thickness, elastic modulus, etc., can also be used. Alternatively or additionally, the fundamental frequency of the membrane can be determined by any other analytical or numerical modeling. In one embodiment, a specific diameter, which is related to the membrane's tension and density, determines a specific resonant frequency. For example, the diameter can be set to match half the wavelength of the resonant frequency of a wave propagating within the membrane to generate a standing wave.

[0029] In a preferred embodiment, a piezoelectric transducer is used to actuate the membrane. Most preferably, the piezoelectric material is provided as a layer on the flexible membrane. Other layers may also be provided, such as electrode layers used to apply respective electrical signals to the piezoelectric layer. As described herein, capacitive and / or conductive layers for applying static electricity are also contemplated. These layers may be charged by other or additional electrical signals, for example, by application of a static charge or by dynamic application of a charge during a portion of the respective vibration cycle.

[0030] Driving the transducer with a carrier frequency near its resonant frequency can improve performance. For example, the first or ground resonance of the membrane can be used. The resonant frequency of the transducer can be relatively high, for example, above 1 kHz, 10 kHz, 100 kHz, or even above 1 MHz. Such high frequencies may not be suitable for all applications. For example, frequencies above 800 Hz may be difficult to sense for haptic applications. For example, the optimal frequency for haptic feedback may be between 50 and 500 Hz, preferably between 100 and 300 Hz.

[0031] In some embodiments, the electrical signal is composed of multiple frequencies, including a carrier frequency as close as possible to the resonant frequency of the transducer, and an envelope or modulation frequency depending on the application (for example, a haptic feedback device may use a 40 kHz carrier frequency amplitude modulated by a 200 Hz modulation frequency. It is also contemplated to use more than one frequency, particularly a band that includes multiple frequencies, for example, including the resonant frequencies of multiple transducers).

[0032] In some embodiments, an acoustic device is formed comprising an array of multiple acoustic transducers as described herein. For example, the transducers can be formed by a patterned stack on a flexible substrate. In one embodiment, the stack comprises a piezoelectric layer sandwiched between a bottom electrode layer and a top electrode layer. In some embodiments, the active surface of the acoustic transducer comprises a portion of the flexible substrate at the contact region. In other or further embodiments, multiple membranes may be separately bonded to a surrounding substrate.

[0033] In some embodiments, membranes 10 and 20 each include a flexible foil 15 and 25 or other flexible substrate. For example, flexible foil 15 and 25 may function as a support for other layers, such as electrodes and / or piezoelectric layers. In some embodiments, as shown, flexible foil 11 may be secured, e.g., stacked or otherwise disposed, on a relatively rigid support substrate 50. For example, flexible foil 11 may be laminated and / or otherwise affixed to support substrate 50. Securing between the flexible substrate and the support substrate or other rigid structure may occur, for example, during and / or after manufacturing. The support substrate may have a thickness similar to or greater than that of relatively flat flexible foil 11. Preferably, support substrate 50 has a relatively high bending stiffness compared to flexible foil 11, e.g., by at least a factor of 2, 3, 5, 10, or more. In this case, support substrate 50 may provide additional structural integrity. Preferably, the support substrate 50 includes openings at locations corresponding to one or more of the ultrasound transducers, which allow relatively free movement of the transducer surface while still providing rigid support.

[0034] In some embodiments (not shown), one of the first or second films is positioned flush with the surrounding substrate 50. This can be advantageous, for example, for contacting the object. In other or further embodiments, each of the films that contacts the object can be relatively thick, for example to protect the other film, which is formed relatively thin.

[0035] FIG. 2 shows an example of electrical signals S11 to S13, S21 to S23 applied to the electrodes 11 to 13, 21 to 23, respectively, of the ultrasonic transducer 100 shown in the previous figure, and vibrations Z10, Z20 corresponding to the membranes 10, 20.

[0036] The top of the figure shows piezoelectric signals S13 and S12 applied to electrodes 13 and 12 that sandwich a first piezoelectric layer 14. In some embodiments, as shown, for example, voltages ΔV3 and ΔV5 between signals S12 and S13 applied to electrodes 12 and 13 that sandwich the piezoelectric material 14 vary as a function of time. Arrows through the piezoelectric material 14 indicate the direction of a first piezoelectric force F1, which may depend on the changing voltages ΔV3 and ΔV5 between the signals applied to the electrodes and the polarity P1 of the first piezoelectric layer 14.

[0037] The top center of the figure shows a first set of electrostatic signals S11 and S21 applied to, for example, the first electrode 11 and the second electrode 12. The arrow between the signals S11 and S21 indicates the electrostatic force Fe between the electrodes. When a certain voltage ΔV2 exists between the signals S11 and S21, for example, when the charges (- / +) of the electrodes 11 and 12 are opposite or when one electrode is more charged than the other, the electrostatic force Fe becomes attractive. Conversely, when the voltage ΔV1 between the electrodes is low, for example, when the charges are similar (both are similarly charged or both are uncharged), the electrostatic force Fe may also be relatively small or zero (or repulsive).

[0038] The bottom center of the figure shows a second set of piezoelectric signals S22, S23 applied to electrodes 22, 23, sandwiching second piezoelectric layer 24 between them. The arrows through piezoelectric material 14 indicate the direction of a second piezoelectric force F2, which may depend on the varying voltages ΔV4, ΔV6 between the signals applied to the electrodes and the polarity P2 of second piezoelectric layer 24. In some embodiments, for example, as shown, the second set of piezoelectric signals S22, S23 applied to second piezoelectric layer 24 are identical to the first set of piezoelectric signals S12, S13 applied to first piezoelectric layer 14. This allows for relatively easy control of the signals, e.g., by providing the same signals to each electrode.

[0039] The bottom of the figure shows the vibration of the membranes 10 and 20. Here, the vibration is represented by the varying positions Z10 and Z20 of a point on each membrane, for example. The membrane motion can track (with a delay) the applied piezoelectric and / or electrostatic forces. Typically, the phase of the vibration can depend on the phase of the electrical signal (voltage) applied to one or both of the membrane's piezoelectric materials 14 and 24, e.g., piezoelectric signals S12, S13; S22, S23. The phase of the vibration can also depend on other factors, such as the polarity P1 and P2 of the piezoelectric materials 14 and 24. In addition to the electrical phase induced by the transfer function of a voltage or current source to the complex electrical impedance of the piezoelectric body, there is also an acoustic phase. For a lightly damped harmonic oscillator, this phase difference can range, for example, from -90 degrees to +90 degrees as the frequency is swept around the resonant frequency. The frequency width of the phase-changing region can be an indicator of the device's bandwidth, which is also related to the polarity direction. For example, oppositely polarizing the materials may actuate the vibrations in opposite directions (e.g., as shown in Figure 6). Other forms of actuation of each membrane may be envisioned in addition to or in place of piezoelectric actuation. Also, other or additional factors may serve to keep the vibrations substantially in phase, such as a common interconnected substrate or pocket between the membranes.

[0040] In some embodiments, for example as shown in Figure 1, the respective sets of electrodes 12, 13; 22, 23 for applying the piezoelectric electric signals S12, S13; S22, S23 to the piezoelectric layers 14, 24 are separate and independent from the first electrode 11 and the second electrode 12 for applying the electric (electrostatic) signals S11, S21. This embodiment may have the advantage of providing independent control over the electrostatic and piezoelectric interactions. Also, one or more electrodes may be combined, as further described below.

[0041] Although the above illustrates an embodiment in which a constant voltage is applied to one of the electrostatic electrodes, it is also contemplated that the voltages on both electrostatic electrodes may be varied. For example, the applied signal may include pulse shaping (applying time-dependent pulses) to both membranes to optimize the membrane vibration (e.g., making the vibration more linear or increasing the amplitude).

[0042] 3A and 3B show an ultrasonic transducer 100 in which some of the electrodes 11, 12; 21, 22 are combined. Compared to FIGS. 1A and 1B, in this example the piezoelectric layers 14, 24 are arranged on the inside, and some of the electrodes are combined. Alternative or additional arrangements are also possible, such as one of the electrodes being arranged on the outside. For example, the carrier layer can function as an additional capacitor to add capacitance between the membranes.

[0043] In some embodiments, the first electrode 11 of the first membrane 10, configured to interact with the second electrode 21 of the second membrane 20 by a varying electrostatic force F, is also one of a first set of piezoelectric electrodes 12, 13 that sandwich the first piezoelectric layer 14 of the first membrane 10. In other or further embodiments, the second electrode 21 (=22) of the second membrane 20, configured to interact with the first electrode 11 (=12) of the first membrane 10 by a varying electrostatic force F, is also one of a second set of piezoelectric electrodes 22, 23 that sandwich the second piezoelectric layer 24 of the second membrane 20. By using the same electrode in at least one membrane for both the electrostatic and piezoelectric interactions, the number of electrodes can be reduced.

[0044] FIG. 4, for example, illustrates the signals and vibrations applied to the transducer of the previous figure. Here, the first and second electrodes also function as piezoelectric electrodes, so signal S11 = S12 and signal S21 = S22. Compared to FIG. 2, signal S22 (= S21) and signal S23 are offset by a voltage +V22. This is one way to simultaneously provide a varying electrostatic interaction between the first and second electrodes and a varying piezoelectric interaction in both piezoelectric layers 14 and 24. Of course, other signals providing the same or similar simultaneous interactions are also contemplated. For example, the direction of one or both of the polarities P1 and P2 can be reversed, and the corresponding signals can also be reversed. For example, a fixed or variable offset can be added to the signals that maintain the relative voltage difference. For example, the piezoelectric signals can be adapted to maintain a similar voltage difference in each piezoelectric layer in response to any set of electrostatic signals. Although the diagram shows a sinusoidal signal, other signals are possible, such as block waves or other shapes that improve electrostatic and / or piezoelectric interactions, and the phase can be offset to improve the interaction.

[0045] Figures 5A and 5B show ultrasound transducers in which the membranes vibrate in antiphase. While these figures show the inner piezoelectric layer, as in Figures 3A and 3B, other embodiments described herein may be implemented in this manner. For example, the signal may be inverted or the polarity of one of the piezoelectric layers may be reversed. In some embodiments, the distance "d" between the membranes may be increased to prevent the membranes from contacting each other as they move toward each other. In other or further embodiments, adjusting the distance "d" may provide the desired interaction by forcing the membranes into physical contact. Pressure within a pocket between the membranes may also prevent contact. One advantage of membranes moving in antiphase with each other is that the asymmetry of the membrane motion may increase bandwidth. An advantage of membranes moving in unison is that the electrostatic forces of the membranes (when moving toward each other) are not counteracted by any pressure within the pocket, potentially resulting in stronger ultrasound waves.

[0046] FIG. 6, for example, illustrates the signal and vibration applied to the transducer of the previous figure. Compared to FIG. 4, for example, the signal can be essentially similar or the same, while the polarity of one of the piezoelectric layers 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 for one piezoelectric layer while being compensated by the other signal to maintain the relative voltage difference. The vibrations Z10 and Z20 of each membrane are shown in antiphase in this example. In some embodiments, for example, the movement displacement of the first membrane 10 to one side during vibration can be asymmetric compared to the movement to the other side. For example, the first membrane 10 has a first amplitude measured between the equilibrium position of the first membrane 10 and the position of maximum extension of the first membrane 10 in one direction, and a second amplitude measured between the equilibrium position of the first membrane 10 and the position of maximum extension of the first membrane 10 in the other direction. The second membrane 20 is configured to influence the motion of the first membrane 10 by electrostatic interaction to induce a difference between their amplitudes, for example, at least a 5% difference or at least a 10% difference. These and other embodiments that induce such displacement asymmetry may contribute to increasing the effective bandwidth of membrane-based transducers.

[0047] FIG. 7A is a cutaway perspective view of an ultrasound transducer 100 with electrostatic and piezoelectric electrodes (electropads) adjacently positioned on respective membranes 10, 20. FIG.

[0048] In some embodiments, the membranes 10, 20 comprise flexible foils 15, 25 having a laminate including respective electrodes 11, 12. The membranes 10, 20 are bonded to corresponding substrate layers 51-53 of a substrate 50 with an intermediate substrate layer 52 disposed between the flexible foils 15, 25 to form a space therebetween. The ultrasonic transducers 100 are formed in openings in the substrate layers 51-53. For example, the space or distance between the membranes 10, 20 is determined by the thickness of the intermediate substrate layer 52.

[0049] In some embodiments, at least one, and preferably both, of the membranes 10, 20 have a relatively thicker and / or stiffer portion in the center of the membrane compared to the (radial) edges of the membrane, e.g., the membrane has a central convex shape. This has the effect of increasing the total displacement volume compared to the membrane's peak out-of-plane displacement. For example, a relatively thicker or stiffer central portion may have less curvature during membrane deflection (e.g., blocky rather than Gaussian), so the effect of the inward contraction may be spread over a larger area than the peak at the center.

[0050] In some embodiments, the center of the membrane is thicker than the edges, e.g., at least 1.1, 1.2, 1.5, 2, or more times thicker. In other or further embodiments, the material in the center of the membrane is stiffer than the edges, e.g., at least 1.1, 1.2, 1.5, 2, or more times thicker. Preferably, the thickened and / or stiffened regions extend over a portion of the total area, e.g., between 20 and 90%, preferably between 40 and 80%, or 50 and 70% of the area. In some embodiments, the membrane comprises an additional layer or convexity on at least one side, preferably the inward-facing first side 10a. As will be appreciated, having additional material on one side of the membrane that is off-center contributes to displacement asymmetry.

[0051] 7B is a bottom view of the first membrane 10 designated as VIIB in FIG. 7A. In some embodiments, the first membrane 10 includes a first region of conductive material forming a first electrode 11 that electrostatically interacts with a second electrode in the second membrane 20, and a second region of conductive material forming a piezoelectric electrode 12 of a set of piezoelectric electrodes 12, 13 that sandwich the first piezoelectric layer 14 of the first membrane 10, the second region adjacent to the first region. In other or further embodiments, the first electrode 11 covers a central region r / R of the first membrane 10 between 0 and r1, and the piezoelectric electrode 12 covers a peripheral region r / R of the first membrane 10 between r1 and r2 that surrounds the central region. The first piezoelectric layer (not shown) may be limited to the peripheral region, or may be a continuous layer, for example, with both piezoelectric electrodes 12 and 13 positioned only in the peripheral region of the first piezoelectric layer 14 so as to sandwich the peripheral region.

[0052] FIG. 7C shows the relative volumetric displacement v / V at the resonance of the piezoelectric layer as a function of the relative electrode radius r / R. As shown, larger displacements are induced by portions of the piezoelectric layer and / or piezoelectric electrodes arranged in bands around the center, e.g., between 0.4 and 0.9 times the radius R. Advantageously, the center may be covered by the first electrode 11 for electrostatic interaction without significantly losing piezoelectric function. In a preferred embodiment, the central region covered by the first electrode 11 is located a first radial distance r1 from the center of the first membrane 10. The peripheral region covered by the piezoelectric electrode 12 is located outside the first radial distance r1, e.g., a second radial distance r2. The first radial distance r1 is 0.2 to 0.7 times the membrane radius R, preferably 0.3 to 0.6 times, and most preferably less than half the radius, e.g., <0.5R, e.g., 0.4R, as shown. In another or further embodiment, the second radial distance r2 is between 0.7 and 1 times the radius R of the membrane, e.g., at most 0.9R. In some embodiments, the central region covered by the first electrode 11 is relatively stiff and / or thick compared to the surrounding regions. For example, if the central region remains relatively flat during vibration, the electrostatic interaction may be improved (over the entire region).

[0053] When interpreting the appended claims, it should be understood that the word "comprising" does not exclude the presence of other elements or acts than those recited in the claims. The words "a" or "an" do not exclude the presence of a plurality of such elements. Furthermore, reference numerals in the claims do not limit their scope. Several "means" may be represented by the same or different items, implementation structures, or functions. Unless expressly stated otherwise, any of the disclosed devices or parts thereof can be combined together or separated into further separate parts. However, the fact that certain means are recited in mutually different claims does not mean that a combination of these means cannot be used to advantage. Therefore, the present embodiments encompass all practical combinations of the claims, and each claim can in principle refer to any preceding claim unless the context clearly dictates otherwise.

Claims

1. An ultrasonic transducer (100) comprising a stack of at least two membranes (10, 20) bonded to a substrate (50), The membranes (10, 20) are separated by a gap (5), the substrate (50) surrounds the stack in the plane (X, Y) of the membranes, a first membrane (10) of the membranes in the stack being arranged parallel to and facing a second membrane (20) of the membranes in the stack, the membranes (10, 20) being configured to vibrate at an ultrasonic frequency (U) to transmit ultrasonic waves (W), and each of the membranes (10, 20) comprising a set of electrodes (11-13; 21-23); The ultrasonic transducer (100) further comprises an electrical circuit (30) connected to the electrodes and having a controller; the electric circuit (30) is configured to apply a first electric signal (S11) to a first electrode (11) of the first film (10) and to apply a second electric signal (S21) different from the first electric signal to a second electrode (21) of the second film (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 (21) during each oscillation period (T1, T2) of the membranes (10, 20) at the ultrasonic frequency (U), and the first electrode (11) of the first membrane (10) is configured to interact with the second electrode (21) of the second membrane (20) with a varying electrostatic force (Fe) during each oscillation period (T1, T2) in response to the varying voltage (ΔV1, ΔV2); the controller is configured to apply a first voltage (ΔV1) between the first electrode (11) and the second electrode (21) in a first half (T1) of the vibration period, and to apply a second voltage (ΔV2) between the first electrode (11) and the second electrode (21) in a second half (T2) of the vibration period, the second voltage (ΔV2) being higher than the first voltage (ΔV1); the controller is configured to apply an electrostatic force (Fe) between the first electrode (11) and the second electrode (21) in a direction of attraction to each other only during the second half (T2) of the vibration period by the changing voltage (ΔV1, ΔV2). An ultrasonic transducer (100).

2. 2. The ultrasonic transducer (100) of claim 1, wherein the attractive electrostatic force (Fe) is applied only in the second half (T2) of the vibration cycle when the first membrane (10) moves in one direction (-Z), and is not applied in the first half (T1) of the vibration cycle when the first membrane (10) moves in the opposite direction (+Z).

3. 3. The ultrasonic transducer (100) of claim 1 or 2, wherein at least one of the at least two membranes (10, 20) has a piezoelectric layer (14, 24) sandwiched by a respective set of electrodes (12, 13; 22, 23) to transmit a piezoelectric signal (S12, S13; S22, S23) according to the vibration period, and the piezoelectric signal (S12, S13; S22, S23) is configured to generate a corresponding piezoelectric force (F1, F2) in at least one of the corresponding membranes (10, 20).

4. The ultrasonic transducer (100) of any one of claims 1 to 3, wherein the electrical circuit (30) is configured to apply a set of electrical signals (S12, S13; S22, S23) to a set of corresponding electrodes (12, 13; 22, 23) that causes the membranes (10, 20) to move simultaneously in the same direction (+Z, -Z) and vibrate together.

5. a first set of piezoelectric signals (S12, S13) is applied to a first set of electrodes (12, 13) on the first film (10) to produce a varying voltage (ΔV3, ΔV4) across a first piezoelectric layer (14) on the first film (10); a second set of piezoelectric signals (S22, S23) is applied to a second set of electrodes (22, 23) on the second membrane (20) to produce a varying voltage (ΔV5, ΔV6) across a second piezoelectric layer (24) on the second membrane (20); 5. The ultrasonic transducer (100) of claim 1, wherein the varying voltages (ΔV3, ΔV4) of the first piezoelectric layer (14) are in phase or opposite to the varying voltages (ΔV5, ΔV6) of the second piezoelectric layer (24).

6. 6. The ultrasonic transducer (100) of claim 5, wherein a first set of piezoelectric signals (S12, S13) supplied to the first piezoelectric layer (14) of 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 the second half (T2) of the vibration period.

7. 7. The ultrasonic transducer (100) of claim 6, wherein the first piezoelectric force (F1) combined with the electrostatic force (Fe) increases the first amplitude (A1) of the first membrane (10) compared to the second amplitude (A2) of the second membrane (20).

8. 8. An ultrasonic transducer (100) according to any one of claims 5 to 7, wherein the first electrode (11) of the first membrane (10), which is configured to interact with the second electrode (21) of the second membrane (20) by the changing electrostatic force (Fe), is one of a first set of piezoelectric electrodes (12, 13) that sandwich the first piezoelectric layer (14) of the first membrane (10).

9. each of said membranes (10, 20) having a flexible foil (15, 25) with a laminate including said corresponding first electrode (11) and said second electrode (21); the membranes (10, 20) are adhered to corresponding substrate layers (51-53) of the substrate (50) with an intermediate substrate layer (52) disposed between the flexible foils (15, 25) to form a space therebetween; The ultrasonic transducer (100) according to any one of claims 1 to 8, wherein the ultrasonic transducer (100) is formed in an opening in the substrate layer (51 to 53).

10. 10. The ultrasonic transducer (100) of any one of claims 1 to 9, wherein the membrane (10, 20) has a relatively thicker and / or harder portion in the center of the membrane compared to the edges of the membrane.

11. 11. An ultrasonic transducer (100) according to any one of claims 1 to 10, wherein the first film (10) includes a first region of conductive material forming the first electrode (11) that electrostatically interacts with the second electrode in the second film (20), and a second region of conductive material forming one piezoelectric electrode (12) of a set of piezoelectric electrodes (12, 13) that sandwich the first piezoelectric layer (14) of the first film (10), the second region being adjacent to the first region.

12. The first electrode (11) covers the central region of the first membrane (10) between 0 and r1; the piezoelectric electrode (12) covers a peripheral area of ​​the first membrane (10) surrounding the central area; the central region covered by the first electrode (11) is located at a first radial distance (r1) from the center of the first membrane (10); the peripheral area covered by the piezoelectric electrode (12) is located outside the first radial distance (r1); 12. The ultrasonic transducer (100) of claim 11, wherein the first radial distance (r1) is less than half the radius of the first membrane (10).

13. A method for controlling an ultrasonic transducer (100) comprising a stack of at least two membranes (10, 20) bonded to a substrate (50), comprising: the membranes (10, 20) are separated by a gap (5), the substrate (50) surrounds the stack in the plane (X, Y) of the membranes, a first membrane (10) of the membranes in the stack is arranged parallel to and facing a second membrane (20) of the membranes in the stack, the membranes (10, 20) are configured to vibrate at an ultrasonic frequency (U) to transmit ultrasonic waves (W), and the membranes (10, 20) each include a set of electrodes (11-13; 21-23), The method comprises the steps of applying a first electrical signal (S11) to a first electrode (11) of the first film (10) and applying a different second electrical signal (S21) to a second electrode (21) of the second film (20); the first and second electrical signals (S11, S21) are configured to apply varying voltages (ΔV1, ΔV2) between the first electrode (11) and the second electrode (21) during a respective vibration period (T1, T2) of the membrane (10, 20) at the ultrasonic frequency (U); the first electrode (11) of the first membrane (10) is configured to interact with the second electrode (21) of the second membrane (20) with an electrostatic force (Fe) that varies during each of the oscillation periods (T1, T2) in response to the varying voltages (ΔV1, ΔV2); the controller is configured to apply a first voltage (ΔV1) between the first electrode (11) and the second electrode (21) in the first half (T1) of the oscillation period, and to apply a second voltage (ΔV2) between the first electrode (11) and the second electrode (21) in the second half (T2) of the oscillation period, the second voltage (ΔV2) being higher than the first voltage (ΔV1); the controller is configured to apply an electrostatic force (Fe) between the first electrode (11) and the second electrode (21) in a direction of attraction to each other only during the second half (T2) of the vibration period by the changing voltage (ΔV1, ΔV2). method.

Images