PMUT system

Abstract

A system (10) for emitting an acoustic signal in a frequency band between a lower threshold frequency and an upper threshold frequency, comprising a piezoelectric micromachined ultrasonic transducer (PMUT) (100) having a resonant frequency below the lower threshold frequency, a drive system (110) configured to apply a drive signal to the PMUT (100) so as to drive a membrane (103) of the PMUT (100) to oscillate, and a controller (120). The controller (120) is configured to control the drive system to drive the PMUT (100) at each of a succession of times, at a respective frequency of a succession of frequencies, starting with a first frequency below the lower threshold frequency. A frequency of the drive signal increases at each successive time, and an amplitude of oscillation of the membrane (103) increases at each successive time such that the resonant frequency of the PMUT (100) is caused to increase from an initial resonant frequency below the lower threshold frequency until it is above the lower threshold frequency. The PMUT (100) is subsequently driven at one or more frequencies within the frequency band so as to emit an acoustic signal within the frequency band.

Description

[0001] PMUT System

[0002] TECHNICAL FIELD

[0003] This disclosure relates generally to methods of, and systems for, driving piezoelectric micromachined ultrasonic transducers (PMUTs) to emit acoustic signals.

[0004] BACKGROUND OF THE INVENTION

[0005] It is known to use piezoelectric microelectronic ultrasonic transducers (PMUTs) to emit and receive acoustic signals. PMUTs have a membrane that can be driven, by a periodic electrical signal, applied to an electrode of the PMUT, to vibrate at or close to a fundamental resonant frequency of the PMUT, thereby efficiently emitting an acoustic signal.

[0006] The resonant frequency of a PMUT is largely determined by the geometry and material properties of the membrane. For a membrane of a given material and shape (e.g. rectangular), a PMUT can thus be fabricated to support a desired resonant frequency through appropriate selection of the dimensions of the membrane.

[0007] If it is desired to emit acoustic signals over a wider range of frequencies, this can be achieved by combining multiple PMUTs having different resonant frequencies in an array. However, this can be complex and costly.

[0008] Embodiments of the present invention aim to provide an alternative approach to emitting acoustic signals in a desired frequency band.

[0009] SUMMARY OF THE INVENTION

[0010] According to a first aspect, there is provided a system for emitting an acoustic signal in a frequency band between a lower threshold frequency and an upper threshold frequency, the system comprising: a piezoelectric micromachined ultrasonic transducer (PMUT) having a resonant frequency below the lower threshold frequency of the frequency band; a drive system configured to apply a drive signal to the PMUT so as to drive a membrane of the PMUT to oscillate; and a controller configured to control the drive system to: drive the PMLIT, at each of a succession of times, at a respective frequency of a succession of frequencies, starting, at a first time, with a first frequency that is below the lower threshold frequency, wherein a frequency of the drive signal increases at each successive time, and wherein an amplitude of oscillation of the membrane increases at each successive time, such that the resonant frequency of the PMLIT is caused to increase over the succession of times from an initial resonant frequency below the lower threshold frequency, at the first time, until it is above the lower threshold frequency; and subsequently drive the PMLIT at one or more frequencies within the frequency band so as to emit an acoustic signal within the frequency band.

[0011] According to a second aspect, there is provided a method for emitting an acoustic signal in a frequency band between a lower threshold frequency and an upper threshold frequency, the method comprising: applying a drive signal to a piezoelectric micromachined ultrasonic transducer (PMLIT) that has a resonant frequency below the lower threshold frequency of the frequency band, so as to drive a membrane of the PMLIT to oscillate; driving the PMLIT, at each of a succession of times, at a respective frequency of a succession of frequencies, starting, at a first time, with a first frequency that is below the lower threshold frequency, wherein a frequency of the drive signal increases at each successive time, and wherein an amplitude of oscillation of the membrane increases at each successive time, such that the resonant frequency of the PMLIT is caused to increase over the succession of times from an initial resonant frequency below the lower threshold frequency, at the first time, until it is above the lower threshold frequency; and subsequently driving the PMLIT at one or more frequencies within the frequency band so as to emit an acoustic signal within the frequency band.

[0012] Thus it will be seen that, in accordance with at least some embodiments of the invention, a PMLIT may be caused to emit an acoustic signal efficiently at one or more frequencies within a frequency band above the initial resonant frequency of the PMLIT by applying appropriate drive signals to the PMLIT using the drive system. In contrast to conventionally driven PMUTs, which are made to oscillate at or close to a single resonant frequency, the PMLIT of the disclosed system can be driven to oscillate within a range of frequencies above the initial resonant frequency of the PMLIT by causing the resonant frequency of the PMLIT to increase over time. This is achieved by driving the PMLIT at progressively increasing frequencies and amplitude over time.

[0013] The present inventors have recognised that, when the membrane of a PMLIT is made to oscillate with a sufficiently high amplitude, the resonant frequency of the PMLIT increases due to the movement of the membrane becoming non-linear with respect to the drive signal at large displacements. For some PMUTs comprising a membrane of uniform thickness this may occur when the maximum displacement of the membrane from a rest position is greater than 10% or 15% of the membrane thickness. Once the oscillation of the membrane exceeds a threshold amplitude, the resonant frequency of the PMUT may increase in proportional to the amplitude of oscillation of the membrane (i.e. proportional to the maximum displacement of the membrane from its rest position).

[0014] When driven such that the resonant frequency of the PMUT increases with the amplitude of oscillation of the membrane, the resonant frequency can be caused to increase over time by progressively increasing the frequency at which the PMUT is driven while also causing the amplitude of oscillation of the membrane to increase. This allows the system to emit an acoustic signal at frequencies within the frequency band, i.e. above the small-signal resonant frequency of the PMUT, by first progressively increasing the energy of the oscillations over time. The system can therefore advantageously, in some embodiments, comprise a drive system that is not sufficiently powerful to start driving the PMUT instantaneously, from stationary, at a frequency within the frequency band. It may therefore support a smaller and / or cheaper drive system.

[0015] The acoustic signal emitted by the PMUT within the frequency band may be a constant (i.e. fixed) frequency signal, or the frequency of the emitted acoustic signal may vary within the frequency band over time, e.g. it may be an upward or downward chirp signal. It may have a duration of at least 1 , 10 or 100 milliseconds. The acoustic signal may remain within the frequency band until it ceases. It may at least momentarily be equal to the lower threshold frequency. It may at least momentarily be equal to the upper threshold frequency.

[0016] As the amplitude of the oscillation increases over the succession of times, the acoustic pressure generated by the PMLIT also increases over the succession of times. This can allow embodiments to emit signals with greater acoustic pressure than would be possible if emitting a signal only at or near the initial “small-signal” resonant frequency. In some embodiments the acoustic signal may be emitted with a sound pressure of 100 dB or more.

[0017] The initial resonant frequency of the PMLIT (below the lower threshold frequency) may be a fundamental (i.e. first order) resonant frequency of the PMLIT; it may be a lowest resonant frequency of the PMLIT. It may be a frequency at which the membrane moves linearly with respect to the drive signal (i.e. a small-signal or linear resonant frequency). The initial resonant frequency of the PMLIT may be between 1 kHz and 200 kHz in some embodiments. In preferred embodiments, the acoustic signal may be an ultrasound signal having a frequency greater than 20 kHz, e.g. between 40 and 80 kHz. The lower threshold frequency may be at least 20 kHz or at least 40 kHz.

[0018] The PMLIT may be driven at (which may mean within + / -10% or + / -5% or + / 1% or less of) the (increasing) resonant frequency of the PMLIT at each successive time to maintain oscillation of the PMLIT at the resonant frequency at each successive time. That is to say that the frequency at which the PMLIT is driven increases over time as the resonant frequency of the PMLIT increases, and remains close to the increasing resonant frequency of the PMLIT over this time. By driving the PMLIT at its resonant frequency while the resonant frequency increases, the energy required to maintain oscillations at the increasing resonant frequency of the PMLIT may be reduced.

[0019] The PMLIT may be driven by a drive signal of continuously rising frequency over the succession of times starting from the first frequency (e.g. a rising chirp). The PMLIT may be driven by a drive signal that comprises a succession of rising frequency steps over the succession of times. The steps may be continuous (i.e. not containing any other frequencies, or gaps, over a span of time covering the succession of times).

[0020] The lower threshold frequency may be significantly greater than the initial resonant frequency in some embodiments. In particular, the lower threshold frequency may be greater than the initial resonant frequency by at least twice a full width half maximum (FWHM) of a resonance peak of the PMLIT at the initial resonant frequency. In some embodiments, the lower threshold frequency may be between 75 kHz and 85 kHz, e.g., 80 kHz. The upper threshold frequency may be between 85 and 95 kHz, e.g., 90 kHz. The frequency band may be between 75 and 95 kHz in some embodiments. In some preferred embodiments, the frequency band is between 80 and 90 kHz.

[0021] In some embodiments, the frequency band may be selected such that it corresponds to a reception band of a specific acoustic receiver, which may be a PMUT-based received, e.g. the emitted signal may have a centre frequency selected based on a frequency response of the specific receiver. It may be selected to match an optimum frequency response range of the specific receiver.

[0022] In some embodiments, the PMLIT may comprise a substrate comprising (i.e. defining) a cavity, and the membrane may be arranged across an opening of the cavity. In some embodiments the cavity may be a vented cavity. The membrane may be edge-clamped around a perimeter of the membrane. The membrane may be of any shape but in some embodiments it is rectangular. It may have a uniform thickness. The substrate may be formed from silicon in some embodiments. The membrane may comprise a piezoelectric material layer. The piezoelectric material layer may be formed of any suitable piezoelectric material, e.g. Aluminium Nitride (AIN), Scandium-doped Aluminium Nitride (AIScN), Lead Zirconate Titanate (PZT), etc. The piezoelectric material layer may have a thickness of up to 10 pm, e.g. in the range of 0.5-4 pm. In some embodiments, the piezoelectric material layer is sandwiched between a first electrode (e.g. an upper electrode on top of the piezoelectric material layer) and a second electrode (e.g. a lower electrode beneath the piezoelectric material layer). The drive system may be arranged to apply the drive signal as an analogue electrical signal through the first and second electrodes. In some embodiments, the drive system may comprise a microcontroller and / or drive electronics configured for providing drive signals to the PMLIT. The microcontroller may be configured to provide a digital periodic signal to the drive electronics, and the drive electronics may be configured to convert the digital signal into an analogue drive signal to drive the PMLIT, e.g. by applying a time-varying voltage to an electrode of the PMLIT so as to drive the membrane of the PMLIT to oscillate. The drive signal may be periodic. The drive system may be configured to drive the PMLIT with a simple periodic drive signal (i.e. not a composite signal), during the succession of time intervals and while emitting the acoustic signal.

[0023] In some embodiments, the succession of frequencies may cause the resonant frequency of the PMLIT to increase over the succession of times at a first rate. The first rate may be, for example, a linear rate, a polynomial rate or an exponential rate. The resonant frequency of the PMLIT may be controlled to increase at the first rate through the application of suitable drive signals. In some embodiments, a maximum rate of increase of the resonant frequency of the PMLIT may be set based on one or more properties of the drive system, e.g., based on a maximum voltage of the drive signals that can be applied to the PMLIT by the drive system.

[0024] Driving the PMLIT at one or more frequencies within the frequency band may comprise driving the PMLIT at a second succession of frequencies within the frequency band at a respective second succession of times (e.g. such that the emitted acoustic signal comprises a rising or falling chirp, which may have a linear rate different from a linear rate at which the resonance increased over the succession of times). A frequency of the drive signal may be increased at each successive time of the second succession of times.

[0025] In some embodiments, the PMLIT may be driven at each of the second succession of frequencies within the frequency band so as to cause the resonant frequency of the PMLIT to increase over the second succession of times at a second rate. The second rate may be, for example a linear rate, a polynomial rate or an exponential rate. The resonant frequency of the PMLIT may be controlled to increase within the frequency band at the second rate through the application of suitable drive signals, as described above in relation to the first rate. The second rate may be set based on one or more properties of the drive system, for example, based on the maximum voltage that can be applied by the drive system. In some embodiments, the second rate may be lower than the first rate. In this way, the system may increase the resonant frequency relatively rapidly until the frequency band is reached, in order to minimise the time before starting to emit the acoustic signal, and may then emit a rising chirp (or any other signal) within the frequency band that may have a lower rate of increase in order to provide a more useful signal for signalling purposes. The second rate may be selected based on a desired length of the acoustic signal to be emitted.

[0026] The emitted signal may comprise or consist of a chirp signal in some embodiments. The chirp signal may comprise or consist of one or more linear, exponential or hyperbolic chirps. For example, the chirp signal may comprise an upward chirp of increasing frequency, e.g. increasing from the lower threshold frequency to a predetermined frequency within the frequency band which may be the upper threshold frequency. In addition or alternatively, the chirp signal may comprise a downward chirp of decreasing frequency, e.g. decreasing from the upper threshold frequency to a predetermined frequency within the frequency band. It may comprise any sequence of rising and / or falling and / or constant tones. In some embodiments the emitted signal may comprise a chirp signal comprising a succession of two or more upward and / or downward chirps. For example, the chirp signal may comprise an upward chirp increasing from the lower threshold frequency to the upper threshold frequency, followed by a downward chirp from the upper threshold frequency to a predetermined frequency within the frequency band. It will be appreciated that that any combination of upward and downward chirps may be used in practice, from any start and end frequencies within or outside of the frequency band, provided that at least a portion of the emitted acoustic signal is at a frequency within the frequency band. In some embodiments, emitting the succession of chirps may comprise employing time division multiplexing to include one or more gaps (i.e. periods of silence) in the acoustic signal. This may be particularly beneficial in the case that multiple PMUTs are emitting acoustic signals in proximity to one another, so as to prevent emitted signals from different PMUTs from overlapping in time.

[0027] In some embodiments, the acoustic signal may be modulated to encode information. For example, the acoustic signal may comprise a succession of one or more chirps or carrier frequencies modulated using a modulation scheme, which may be phase-based or frequency-based, such as binary phase shift keying (BPSK) or quadrature phase shift keying (QPSK).

[0028] The drive signal may be a time-varying voltage signal. It may be or comprise a square wave signal in some embodiments. However in some embodiments the drive signal may be or comprise a sinusoidal signal, e.g. a cosine signal. In some embodiments, the drive signal may be or comprise a pulsed signal. It may comprise series of negative pulses, a series of positive pulses, or a series of alternating positive and negative pulses, applied to an electrode of the PMLIT. A second electrode of the PMLIT may be maintained at a constant reference voltage, e.g. ground. The plurality of pulses may be positive and / or negative relative to this reference voltage, or they may be positive and / or negative relative to a positive bias voltage — i.e. being overlaid on a constant positive bias value in some embodiments. The use of a positive voltage bias may be beneficial in embodiments in which the membrane of the PMLIT comprises PZT, in order to avoid any, or excessive, negative voltages that may depolarise the piezoelectric material of the PZT membrane, e.g. when the two electrodes are asymmetric.

[0029] In some embodiments, the amount of energy provided to the PMLIT by the drive signal per unit time may be constant over the succession of times. However, in some embodiments, the amount of energy provided to the PMLIT by the drive signal per unit time may be increased at each successive time as the resonant frequency of the PMLIT increases. This may be achieved by increasing the amplitude of the drive signal at each successive time in some embodiments. In addition or alternatively, in embodiments in which the drive signal comprises a pulsed signal, this may be achieved by increasing the pulse width of the drive signal at each successive time.

[0030] In some embodiments, an amplitude of the drive signal may be determined at least partly based on the upper threshold frequency of the frequency band. For example, it may be selected so as to ensure that sufficient energy can be applied to the PMLIT to cause the PMLIT to oscillate at the upper threshold frequency. In some embodiments, a voltage of the drive signal applied at an electrode of the PMLIT by the drive system may always be less than 20 V, or less than 10 V or less than 5 V (e.g. relative to a ground potential applied to a second electrode). For example, the magnitude of the voltage applied by the drive system may always be between 1 V and 20 V, for example between 3 V and 5V. Limiting the voltage of the drive signal in this way allows the power requirements of the system to be significantly reduced while still allowing the system to emit acoustic signals at frequencies that are significantly greater than the initial (e.g. linear) resonant frequency of the PMLIT.

[0031] In some embodiments, the system may comprise an array of two or more PMUTs, each having a respective resonant frequency below the lower threshold frequency of the frequency band. This may differ slightly due to manufacturing differences. Each PMLIT of the array may have a respective drive system, controlled by the or a respective controller, and configured to apply a drive signal to the PMLIT so as to drive a membrane of the PMLIT to oscillate. The controller or each controller may control each drive system to drive its respective PMLIT, at each of a respective succession of times, at a respective frequency of a respective succession of frequencies, starting, at a respective first time, with a respective first frequency that is below the lower threshold frequency, wherein a frequency of the respective drive signal increases at each successive time, and wherein an amplitude of oscillation of the membrane of its respective PMLIT increases at each successive time such that the resonant frequency of the PMLIT is caused to increase over the succession of times from the initial resonant frequency below the lower threshold frequency, at the respective first time, until it is above the lower threshold frequency; and to subsequently drive the PMLIT at a common frequency within the frequency band so as to cause an acoustic signal to be emitted within the frequency band from the array of PMUTs.

[0032] Each PMUT of the array of PMUTs may be driven independently based on drive signals provided by the controller to its respective drive system. When PMUTs of the array are driven to oscillate at a common frequency within the frequency band (i.e. greater than the resonant frequency of the PMUT), the phase of each PMUT is significantly less sensitive to the inherent small-signal resonant frequency of the particular PMUT, such that the phases of the signals emitted by the individual PMUTs of the array can be precisely controlled and coordinated. This may advantageously allow for more precise beamforming of the acoustic signals emitted from the array to be performed than would be possible if operating the PMUTs at their inherent small-signal resonant frequencies. In some embodiments, the controller or each controller of the array may control the phase of each PMUT of the array so as to cause a beamformed acoustic signal to be emitted within the frequency band — i.e. through appropriate control of the drive systems.

[0033] Features of any aspect or embodiment described herein may, wherever appropriate, be applied to any other aspect or embodiment described herein. Where reference is made to different embodiments or sets of embodiments, it should be understood that these are not necessarily distinct but may overlap.

[0034] BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Certain preferred embodiments of this disclosure will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0036] FIG. 1 is a schematic diagram of a system for emitting acoustic signals according to an exemplary embodiment of the present invention;

[0037] FIGs. 2A, 2B, 2C are plots of drive signals for controlling a system for emitting an acoustic signal according to an exemplary embodiment of the present invention; FIGs. 3A, 3B, 3C are plots of, respectively: the actuation force used to generate a first acoustic signal using a system according to an exemplary embodiment of the present invention, the frequency of the resulting acoustic signal, and the resulting sound pressure;

[0038] FIGs. 4A, 4B, 40 are plots of, respectively: the actuation force used to generate a second acoustic signal using a system according to an exemplary embodiment of the present invention, the frequency of the resulting acoustic signal, and the resulting sound pressure;

[0039] FIGs. 5A,5B, 5C are plots of, respectively: the actuation force used to generate a third acoustic signal using a system according to an exemplary embodiment of the present invention, the frequency of the resulting acoustic signal, and the resulting sound pressure;

[0040] FIGs. 6A, 6B, 60 are plots of, respectively: the actuation force used to generate a fourth acoustic signal using a system according to an exemplary embodiment of the present invention, the frequency of the resulting acoustic signal, and the resulting sound pressure; and

[0041] FIG. 7 is a schematic diagram of an array-based system for emitting an acoustic signals according to an exemplary embodiment of the present invention.

[0042] DETAILED DESCRIPTION

[0043] Figure 1 schematically illustrates a system 10 for emitting an acoustic signal according to an embodiment of the present invention. The system 10 comprises a piezoelectric micromachined ultrasonic transducer (PMLIT) 100, drive electronics 110 for applying analogue drive signals to the PMLIT 100 and a microcontroller 120 configured to control the drive electronics 110.

[0044] The PMLIT 100 comprises a silicon substrate 101 housing a cavity 102, and a composite membrane 103 positioned above the cavity 102 that can be driven to oscillate by the drive electronics 110 so as to generate an acoustic signal 130 as will be explained in the following. In the embodiment shown in Figure 1, the cavity 102 is open on the lower surface of the PMLIT 100 opposite the membrane 103. However, it will be appreciated that this is not essential and in other embodiments, the cavity 102 could be sealed or vented through a smaller opening.

[0045] The membrane 103 comprises a piezoelectric material layer 104 sandwiched between an upper electrode, 105 formed on top of the piezoelectric material layer 104, and a lower electrode 106, formed below the piezoelectric material layer 104, on top of the silicon substrate 101. The membrane 103 of the PMLIT 100 can be driven to oscillate by generating an alternating electric field between the upper electrode 105 and the lower electrode 106. In the system 10, the microcontroller 120 provides digital drive signals to the drive electronics 110, as will be explained in the following. Based on the digital drive signals, the drive electronics output an analogue voltage at the upper electrode 105 and the lower electrode 106 to generate an electric field across the piezoelectric material layer 104.

[0046] The presence of an alternating electric field across the piezoelectric material layer 104 generates mechanical stress in the piezoelectric material layer 104, which causes the membrane 103 to deflect in the direction of the cavity 102. The amount of deflection of the membrane 103 into the cavity depends on the voltages applied at the upper electrode 105 and the lower electrode 106 by drive electronics 110 based on drive signals received from the microcontroller 120. This is shown in Figure 1 by the broken lines in the cavity 102, which schematically illustrate the deflection of the membrane 103 into the cavity 102 to two different extents, i.e. with two different potential differences applied across the electrodes 105, 106. It will be appreciated that the range of deflections of the membrane 103 shown in Figure 1 are purely schematic and the extent of deflection of the membrane 103 into the cavity 102 may be greater or smaller than this is in practice.

[0047] By applying a signal with a voltage that varies periodically across the electrodes 105, 106, the membrane 103 can be made to oscillate at a resonant frequency of the PMLIT 100, causing an acoustic signal 130 at the resonant frequency to be generated in the medium surrounding the membrane 103, e.g. in air surrounding the PMUT 100.

[0048] Conventionally, PMUTs are driven at low amplitudes at or close to the fundamental resonant frequency of the PMUT, such that the maximum deflection of the membrane of the PMUT (e.g. at the centre of the membrane) is a small fraction, e.g. at most 10%, of the membrane thickness. A conventional PMUT may have a membrane thickness of around 4 pm, and the extent of deflection of the membrane is generally less than 0.5 pm. When driven at low amplitudes, as is conventional, the PMUT operates in the linear regime, such that the deflection of the PMUT varies linearly with the amplitude of the drive voltage applied at the electrodes. For this reason, this inherent resonant frequency is referred to herein as the smallsignal resonant frequency. When operating in the linear regime, with small deflection of the membrane 103 of the PMUT 100, the membrane 103 deforms by bending. Driven in this way, the deflection of the membrane 103 is proportional to the force applied, i.e. to the amplitude of the drive voltage applied at the electrodes 105, 106.

[0049] The present inventors have recognised that, for a membrane 103 having appropriate thickness and lateral dimensions, deflection of the membrane 103 exceeding 10%, 50% or 100% of the membrane thickness is possible, without damaging the membrane 103. Suitable membrane dimensions may be determined by experimentation, or through the use finite element modelling. This can allow for greater sound pressures (e.g. 100 dB or more) to be generated than would otherwise be possible for a given membrane material. For example, in some embodiments, the membrane 103 may deflect by 10 pm or more when emitting a signal within a target frequency band, as described below. When subject to such high deflection, the membrane 103 is caused to stretch, in addition to the bending that occurs when the PMLIT 100 is driven in the linear regime. This stretching requires a greater amount of force than bending of the membrane 103 alone, such that the force required to cause deflection of the membrane 103 (and hence the amplitude of the drive voltage) increases progressively with increasing deflection. As a result, the PMLIT 100 behaves non-linearly, such that the amplitude of the deflection of the membrane 103 of the PMLIT 100 varies non-linearly with the amplitude of the applied drive signal.

[0050] The present inventors have also recognised that, when driven to oscillate in the non-linear regime, e.g., with a maximum deflection greater than 10%, 50% or 100% the thickness of the membrane 103, the resonant frequency of the PMLIT 100 becomes dependent on the amplitude of oscillation of the membrane 103. The resonant frequency of the PMLIT 100 can therefore be made to increase from its initial, small-signal value by increasing the amplitude of deflection of the membrane 103. This is a result of the effective spring constant of the PMLIT 100 increasing as the deflection of the membrane 103 of the PMLIT 100 increases.

[0051] When driven to oscillate at frequencies significantly higher than the small-signal resonant frequency, the amplitude of deflection of the membrane 103, and hence the sound pressure produced by the PMLIT 100, is significantly increased. However, to sustain higher amplitude (and hence frequency) oscillations of the membrane 103, higher voltages are required, as damping effects increase with increasing frequency of oscillation.

[0052] The amplitude of oscillation of the membrane 103 that can be attained for a drive voltage at a particular frequency depends on how far the frequency of the applied drive voltage is from the resonant frequency of the PMLIT 100 at the time of actuation and the quality factor of the PMLIT. If the membrane is driven using an applied voltage close to or at the resonant frequency at the time of oscillation, the amplitude of oscillation of the membrane will be at its maximum, however if driven at a frequency far from the resonant frequency at the time of actuation, with the same applied voltage, the amplitude of oscillation of the membrane 103 will be significantly lower.

[0053] If a large actuation force (i.e. drive voltage) is available, it is possible to reach a high amplitude and frequency operating point by applying a high drive voltage at a high frequency for sustained period. This would result in large amplitude oscillation of the membrane 103, causing the resonant frequency to raise towards the frequency of the applied drive signal. The amplitude of the drive voltage could then be gradually reduced as the resonant frequency approaches the actuation frequency. In practice, however, the drive voltage required to achieve this effect is significantly greater than that which is desirable in affordable, practical implementations. For example, in order to cause the membrane 103 to oscillate at 90kHz from a static position, a drive voltage of ~ 300V would be required.

[0054] An alternative approach, implemented in embodiments of the present invention, is to start at (e.g. within + / -10% or + / -5% or less of) the small-signal resonance frequency of the PMLIT 100 and to build up the amplitude of oscillation of the membrane 103 gradually. As the amplitude of oscillation of the membrane 103 increases, the frequency at which the drive voltage is applied can be increased so as to follow the resonant frequency of the PMLIT upwards (e.g. staying within + / - 10% or + / -5% of less of the increasing resonance frequency). Thus, by initially driving the PMLIT 100 at its small-signal resonant frequency, and successively increasing the frequency and optionally the amplitude of the drive voltage applied to the electrodes 105, 106, the deflection of the membrane 103 and thus the resonant frequency of the PMLIT 100 can be caused to increase over time. In practice, the frequency of the applied voltage does not need to exactly match the increasing resonant frequency of the PMLIT 100 at all times, provided that the frequency of the drive voltage is increased gradually and remains close to (e.g. within 10%) the increasing resonant frequency of the PMLIT 100 over time.

[0055] The membrane 103 of the PMLIT 100 can therefore be driven to oscillate at successively increasing resonant frequencies that are greater than the small-signal frequency of the PMLIT 100 using appropriate drive signals. Examples of the digital drive signals provided by the microcontroller 120 to the drive electronics 110 and used to generate analogue drive voltages for application across the upper electrode 105 and the lower electrode 106 of the PMLIT 100 are shown in Figs. 2A-2C.

[0056] The drive signals shown in Figs. 2A-2C are made up of alternating positive and negative pulses overlaid on a constant bias voltage Vb. The width (i.e. duration) and amplitude of the alternating pulses can be adjusted to control the amplitude of deflection of the membrane 103 of the PMLIT 100. By increasing either or both of the pulse width and pulse amplitude, deflection of the membrane 103 can be increased. For example, the narrow pulses shown in Fig. 2A will result in a smaller amount of deflection of the membrane 103 than the wider pulses shown in Fig. 2B. Similarly, the pulses in Fig. 2B will result in a smaller amount of deflection of the membrane 103 than the wider pulses shown in 2C. While shown in Figs 2A-2C as alternating pulses overlaid on (i.e. offset by) a positive bias voltage, a bias voltage is not required in all embodiments.

[0057] Although pulse voltage signals are shown in FIGs 2A-2C, it will be appreciated that the drive voltage could also be a square-wave or sinusoidal or other signal in some embodiments, with or without an overlaid bias voltage.

[0058] As described above, the membrane 103 of the PMLIT 100 can be driven to oscillate at successively increasing resonant frequencies that are greater than the smallsignal resonant frequency of the PMLIT 100 using drive voltages that increase the amplitude of deflection of the membrane 103 over time. Thus, by adjusting the frequency and amplitude at which the drive voltages are applied to the electrodes 105, 106, the PMLIT 100 can be caused to emit an acoustic signal, e.g. an exponential chirp of increasing frequency. By reducing the frequency of the drive signal once the membrane 103 is oscillating at a frequency greater than its smallsignal resonant frequency, it is also possible to emit a downward chirp of decreasing frequency.

[0059] By changing the rate at which the frequency and amplitude of the drive signals are applied to the electrodes 105, 106, the properties of the acoustic signal emitted by the PMLIT 100 can be controlled such that the properties of the acoustic signal 130 can be controlled. For example, it is possible to control the drive voltages applied to the electrodes 105, 106 to cause the PMLIT to emit an acoustic signal 130 at one or more frequencies within a predetermined frequency band.

[0060] An example of a low frequency acoustic signal emitted by a PMLIT according to an embodiment the present invention is shown in Figs. 3A-3C. Fig. 3A shows the frequency of the emitted signal from the PMLIT in response to an actuation signal (i.e. drive voltage) as shown in Fig. 3B, and resulting in the acoustic pressure shown in Fig. 3C.

[0061] More specifically, Fig. 3A shows the frequency of the acoustic signal emitted by a PMLIT from a first time t=0 at which the PMLIT is caused to emit an acoustic signal at its small-signal resonant frequency fo (~2 kHz), and continues through a time period in which the PMLIT is caused to emit an acoustic signal in a frequency band defined by a lower threshold frequency f T (~7 kHz) and an upper threshold frequency T (~10 kHz). As shown in FIG. 3A, the frequency of oscillation of the PMLIT is increased from fo at t=0 to the lower threshold frequency f T of the frequency band at a first rate, n, and is subsequently increased from f T to the upper threshold frequency T of the frequency band at a second rate, r2, that is lower than the first rate. In this way, the oscillation frequency of the PMLIT is brought up to the lower threshold frequency f T of the frequency band quickly, and remains in the frequency band for a sustained time period, so as to emit a chirp signal within the frequency band.

[0062] It can be seen in FIG. 3B that in order to cause the frequency of the emitted acoustic signal to increase from fo into the frequency band defined by f T and firr, the amplitude and the frequency of the applied actuation signal are increased over time in order to sustain oscillations of the membrane of the PMLIT at the increasing frequency. As described above, the increase in resonant frequency of the PMLIT is proportional to the amplitude of oscillations of the membrane of the PMLIT, and hence the sound pressure produced by the PMLIT increases with increasing resonant frequency. This can be seen in FIG. 3C.

[0063] FIGs 4A-4C show a second example of acoustic signal emitted by a PMLIT according to an embodiment the present invention, in which the oscillation frequency of the PMLIT is significantly higher (-70-100 kHz) than in the example shown in Figs. 3A-3C.

[0064] Fig. 4A shows the frequency of the emitted signal from the PMLIT in response to an actuation signal (i.e. drive voltage) as shown in Fig. 4B, and resulting in the sound pressure shown in Fig. 4C. As in the example shown in FIGs 3A-3C, the frequency of the acoustic signal shown in FIG. 4A covers a time period between emission of an acoustic signal at the small-signal resonant frequency fo of the PMLIT at a first time t=0, and continues through a time period in which the PMLIT is caused to emit an acoustic signal in a frequency band defined by a lower threshold frequency f T and an upper threshold frequency fu-r.

[0065] It can be seen in FIGs 4A-4C that the oscillation frequency of the PMLIT is brought up to the lower threshold frequency f T of the frequency band in a first time period, and remains in the frequency band for a second, longer time period, to emit an upward chirp signal within the frequency band in the same way as described above in relation to FIGs 3A-3C.

[0066] FIGs 5A-5C show a third example of acoustic signal emitted by a PMLIT according to an embodiment the present invention, in which the PMLIT is caused to emit a downward chirp with decreasing frequency. Fig. 5A shows the frequency of the emitted signal from the PMLIT in response to an actuation signal (i.e. drive voltage) as shown in Fig. 5B, and resulting in the sound pressure shown in Fig. 5C.

[0067] In the example shown in Fig. 5A, the frequency of the acoustic signal emitted by the PMLIT can be seen to increase, at a first rate, from the linear resonant frequency fo of the PMLIT at a first time t=0, to the upper threshold frequency T of the frequency band at a time t=ti . After this time, the PMLIT is caused to emit a downward chirp signal in which the frequency decreases, at a second rate, lower than the first rate, from the upper threshold frequency T to the lower threshold frequency fLT. In this way, the PMLIT is caused to emit an acoustic signal in a frequency band defined by the upper and lower threshold frequencies between t=h and t=t2. It can be seen in FIG. 5B that in order to cause the frequency of the emitted acoustic signal to increase from fo to the upper threshold frequency firr, the amplitude and the frequency of the applied actuation signal are increased between t=0 and t=h in order to sustain oscillations of the membrane of the PMLIT at increasing frequency. The amplitude and frequency of the applied actuation signal is then decreased, between t=h and t=t2 to allow the amplitude of oscillations of the membrane (and hence the acoustic signal) to reduce during this time period. The sound pressure produced by the PMLIT, shown in FIG. 50, can be seen to vary in line with the frequency of the emitted acoustic signal, such that it increases at a first rate between t=0 and t=ti as the resonant frequency of the PMLIT increases and decreases at a second rate between t=ti and t=t2 as the resonant frequency of the PMLIT is reduced.

[0068] FIGs 6A-6C show a fourth example of acoustic signal emitted by a PMLIT according to an embodiment the present invention. More particularly, FIGs 6A-6C show an example in which the PMLIT is caused to emit a ‘double chirp’ within a frequency band defined by a lower threshold frequency f T and an upper threshold frequency fuB. The ‘double chirp’ shown in FIGs 6A-6C is emitted after the resonant frequency is increased, at a first rate, from the linear resonant frequency fo of the PMLIT at a first time t=0 to the lower threshold frequency f T of a frequency band defined by f T and an upper threshold frequency firr.

[0069] Fig. 6A shows the frequency of the emitted signal from the PMLIT in response to an actuation signal (i.e. drive voltage) as shown in Fig. 6B, and resulting in the sound pressure shown in Fig. 60. In the example shown in Fig. 6A, the frequency of the acoustic signal emitted by the PMLIT can be seen to increase, at a first rate from the linear resonant frequency fo of the PMLIT at a first time t=0, to the lower threshold frequency f T at a time t=ti . After this time, the PMLIT is caused to emit the double chirp described above, i.e. an upward chirp between t=h and t=t2 and a downward chirp between t=t2 and t=ts. In this way, the PMLIT is caused to emit two acoustic chirp signals in the frequency band defined by the upper and lower threshold frequencies.

[0070] As shown in FIGs 3-6, the PMLIT 100 of the system 10 can be used to emit acoustic chirp signals in a range of forms, at frequencies significantly higher than the linear resonant frequency of the PMLIT 100 by applying appropriate drive signals in accordance with embodiments of the present invention.

[0071] The method described herein can also be applied to PMUTs of a phased array, as described in the following in relation to Figure 7.

[0072] Figure 7 shows a system 70 comprising an array of PMUTs 700a, 700b, 700c that are each controlled by a microcontroller 720 and respective drive electronics 710a, 710b, 710c to emit acoustic signals 710a, 710b, 710c respectively.

[0073] Each of PMUTS 700a-700c has the same structure as the PMUT 100 described in relation to Figure 1, i.e. comprising a piezoelectric material layer that can be driven to oscillate in response to voltages applied at electrodes of the PMUTs 700a-700c by their respective drive electronics 710a-710c. As such, a detailed description of the structure of the PMUTs 700a-700c is omitted.

[0074] By driving the PMUTs of the system 70 at frequencies above their small-signal resonant frequencies, as described above in relation to Figures 1-6, significant advantages in terms of the phase response of the PMUTs 700a-700c can be achieved in response to conventional PMUT arrays.

[0075] Typically, in an array of PMUTs, each PMUT is driven around a common smallsignal resonance frequency. In such an array, the phase of each array element of the array depends on the individual small-signal resonant frequencies of the PMUTs of the respective array element, which may vary slightly due to process variations. This causes challenges for operating a phased array of PMUTs to provide accurate beamforming.

[0076] However, by driving the PMUTs 700a-700c of the system 70 in the non-linear regime, at frequencies significantly above the small-signal resonant frequency in accordance with embodiments of the present invention, the phase of each PMUT is practically insensitive to its small-signal eigenfrequency. This allows the phase of individual elements of the array to be precisely controlled, allowing for accurate beamforming of signals emitted from the array of PMUTs 700a-700c. In this way, the array of PMUTs 700a-700c of the system 70 can be made to emit a beamformed acoustic signal through appropriate control of the phase by the microcontroller 720.

[0077] It will be appreciated by those skilled in the art that the present disclosure has been illustrated by describing one or more specific examples thereof, but is not limited to these examples; many variations and modifications are possible, within the scope of the accompanying claims.

Claims

Claims1. A system for emitting an acoustic signal in a frequency band between a lower threshold frequency and an upper threshold frequency, the system comprising: a piezoelectric micromachined ultrasonic transducer (PMLIT) having a resonant frequency below the lower threshold frequency of the frequency band; a drive system configured to apply a drive signal to the PMLIT so as to drive a membrane of the PMLIT to oscillate; and a controller configured to control the drive system to: drive the PMLIT, at each of a succession of times, at a respective frequency of a succession of frequencies, starting, at a first time, with a first frequency that is below the lower threshold frequency, wherein a frequency of the drive signal increases at each successive time, and wherein an amplitude of oscillation of the membrane increases at each successive time such that the resonant frequency of the PMLIT is caused to increase over the succession of times from an initial resonant frequency below the lower threshold frequency, at the first time, until it is above the lower threshold frequency; and subsequently drive the PMLIT at one or more frequencies within the frequency band so as to emit an acoustic signal within the frequency band.

2. The system of claim 1 , wherein the initial resonant frequency is the small signal resonant frequency of the PMLIT.

3. The system of claim 1 or 2, wherein the lower threshold frequency is greater than the initial resonant frequency by at least twice the full width half maximum (FWHM) of the initial resonant frequency of the PMLIT.

4. The system of any of claims 1 to 3, wherein the PMLIT is driven at the resonant frequency of the PMLIT at each of the succession of times.

5. The system of any preceding claim, wherein driving the PMLIT at each of the succession of frequencies causes the resonant frequency of the PMLIT to increase over the succession of times at a first rate.

6. The system of claim 5, wherein the first rate is set based on a maximum voltage of the drive signal that can be applied by the drive system.

7. The system of any preceding claim, wherein driving the PMLIT at one or more frequencies within the frequency band comprises driving the PMLIT at a second succession of frequencies within the frequency band at a respective second succession of times.

8. The system of claim 7, wherein a frequency of the drive signal increases at each successive time of the second succession of times, and wherein an amplitude of oscillation of the membrane increases at each successive time of the second succession of times, such that the resonant frequency of the PMLIT is caused to increase over the second succession of times.

9. The system of claim 8, wherein the resonant frequency of the PMLIT is caused to increase over the second succession of times at a second rate.

10. The system of claim 9, wherein the second rate is lower than the first rate.

11. The system of claim 9 or 10, wherein the second rate is set based on the length of the emitted acoustic signal to be emitted.

12. The system of any preceding claim, wherein the emitted acoustic signal comprises a chirp signal.

13. The system of claim 12, wherein the emitted acoustic signal comprises a succession of two or more linear chirps, and wherein emitting the succession of linear chirps optionally comprises employing time division multiplexing.

14. The system of any preceding claim, wherein the emitted acoustic signal is modulated using binary phase shift keying or quadrature phase shift keying.

15. The system of any preceding claim, wherein the acoustic pressure of the emitted signal increases over the succession of times.

16. The system of any preceding claim wherein the frequency band corresponds to a reception band of a specific acoustic receiver.

17. The system of any preceding claim, wherein the emitted acoustic signal has a centre frequency set based on a frequency response of a specific acoustic receiver.

18. The system of any preceding claim, wherein the drive signal is a pulsed signal.

19. The system of claim 18, wherein an amplitude of the pulsed signal is increased over the succession of times.

20. The system of claim 18 or 19 wherein a pulse width of the pulsed signal is increased at each successive time.

21. The system of any preceding claim, wherein the amplitude of the drive signal depends at least partly on the upper threshold frequency of the frequency band.

22. The system of any preceding claim, wherein the magnitude of a voltage of the drive signal applied by the drive system is always less than 20 volts.

23. The system of any preceding claim, wherein the energy provided to the PMLIT by the drive signal per unit time is constant over the succession of times.

24. The system of any preceding claim, wherein the energy provided to the PMLIT by the drive signal per unit time is increased at each successive time.

25. The system of any preceding claim, comprising an array of two or more PMUTs, wherein each PMLIT of the two or more PMUTs has a respective resonant frequency below the lower threshold frequency of the frequency band; wherein each PMUT of the two or more PMUTs has a respective drive system configured to apply a drive signal to the PMUT so as to drive a membrane of the PMUT to oscillate; and wherein the controller is configured to control each drive system to: drive the respective PMUT, at each of a respective succession of times, at a respective frequency of a respective succession of frequencies, starting, at a respective first time, with a first frequency that is below the lower threshold frequency, wherein a frequency of the respective drive signal increases at each successive time, and wherein an amplitude of oscillation of the membrane of the respective PMUT increases at each successive time such that the resonant frequency of the PMUT is caused to increase over the succession of times from the initial resonant frequency below the lower threshold frequency, at the first time, until it is above the lower threshold frequency; and subsequently drive the respective PMUT at a common frequency within the frequency band so as to emit an acoustic signal within the frequency band from the array of PMUTs.

26. A method for emitting an acoustic signal in a frequency band between a lower threshold frequency and an upper threshold frequency, the method comprising: applying a drive signal to a piezoelectric micromachined ultrasonic transducer (PMUT) that has a resonant frequency below the lower threshold frequency of the frequency band, so as to drive a membrane of the PMUT to oscillate; driving the PMUT, at each of a succession of times, at a respective frequency of a succession of frequencies, starting, at a first time, with a first frequency that is below the lower threshold frequency, wherein a frequency of the drive signal increases at each successive time, and wherein an amplitude of oscillation of the membrane increases at each successive time such that a resonant frequency of the PMUT is caused to increase over the succession of times from aninitial resonant frequency below the lower threshold frequency, at the first time, until it is above the lower threshold frequency; and subsequently driving the PMLIT at one or more frequencies within the frequency band so as to emit an acoustic signal within the frequency band.

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