WO2023184305A1

WO2023184305A1 - Piezomotor and controlling method thereof

Info

Publication number: WO2023184305A1
Application number: PCT/CN2022/084294
Authority: WO
Inventors: Hiroshi Ariga; Kazuki SAKAE; Kazuhiro Hattori; Yasuhide Nihei; Takumi Matsui; Fuwei ZHANG
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2022-03-31
Filing date: 2022-03-31
Publication date: 2023-10-05

Abstract

For a piezo-actuator for linearly driving a driven element by causing an elliptical movement in a vibration element by vibration of piezoelectric elements, a driving device that improves control at very low velocities and/or that allows use of a smaller booster circuit for providing a driving voltage of a piezo-actuator is provided. According to an embodiment, a driving device for driving a driven element in an axial direction is provided. The driving device comprises a piezo-actuator comprising a vibration element to be in contact with a driven element and one or more sets of piezoelectric elements attached to the vibration element; and a controller, wherein each set of piezoelectric elements comprises one or more piezoelectric elements, wherein in each set of piezoelectric elements, a first electrode for expanding or contracting a first area of the piezoelectric elements of the set and a second electrode for expanding or contracting a second area of the piezoelectric elements of the set are arranged along the axial direction, wherein in each set of piezoelectric elements, a common third electrode or separate third electrodes paired with the first electrode and the second electrode are arranged, so that the vibration element of the piezo-actuator has a bending mode in which a portion in the middle along the axial direction becomes convex or concave, whereby at both ends in the axial direction, end portions of the vibration element move in the axial direction; an expansion/contraction mode in which one end portion of both ends in the axial direction expands/contracts, while the other end portion expands/contracts over time in opposite phase, whereby the end portions of the vibration element move in a direction perpendicular to the axial direction. The controller is configured to cause the vibration element to vibrate by applying oscillating voltage signals of the same frequency to the first electrode and the second electrode, so that the driven element in contact with the vibration element is driven in the axial direction. When the oscillating voltage signals applied to the first electrode and the second electrode are in phase, vibration of a first mode based on the bending mode occurs in the vibration element, and when the oscillating voltage signals applied to the first electrode and the second electrode are in opposite phase, vibration of a second mode based on the expansion/contraction mode occurs in the vibration element, wherein the controller is configured to control a velocity of the driven element by controlling a phase difference between the voltage signal applied to the first electrode and the voltage signal applied the second electrode. Another aspect of the present disclosure provides a small boost circuit for providing a driving voltage for a piezo-actuator.

Description

PIEZOMOTOR AND CONTROLLING METHOD THEREOF

TECHNICAL FIELD

The present application relates to a technical field of piezo-actuators, and in particular, to a driving device (also called a piezo-motor) for driving a driven element by using a piezo-actuator and a method of controlling such a driving device.

BACKGROUND

Today, typical cameras are provided with an autofocusing mechanism. The autofocusing mechanism is for focusing on a subject by adjusting an optical distance from a lens unit to an image sensor. The lens unit is typically driven by an actuator.

In recent years, there is an increased demand for telephoto imaging or motion picture imaging with a camera installed on a portable device such as a smartphone. Telephoto imaging requires longer focal lengths, while autofocusing requires a lens unit actuator to have a stroke longer than 1 mm. Since movable parts are weightier than before, a high thrust force is required to move such weighty parts. For still imaging, a high power is required to quickly move the movable parts. On the other hand, for motion picture imaging, control at very low velocities is required of an actuator, whereby a lens unit smoothly starts moving from a state of being at rest or smoothly stops its movement.

An actuator typically used in a portable device such as a smartphone is a linear VCM (voice coil motor) , while a piezo-actuator used for a larger single lens reflex camera has an advantage of high energy density and high power despite its small size.

SUMMARY

A piezo-actuator has been proposed for linearly driving a driven element (e.g., a shaft or some other body) by causing an elliptical movement in a vibration element by vibration of piezoelectric elements. However, with some of such piezo-actuators, control at very low velocities is difficult. Further, since a piezo-actuator requires a high voltage (e.g., 40-100 V) , in order to incorporate one in a portable device such as a smartphone (which typically operates at a voltage of about 3 V) , a boost device (a device for enhancing voltage) is required, and a smaller size of such a boost device is also desired.

The present application addresses the above described issues.

A first implementation of a first aspect of the present application provides a driving device for driving a driven element in an axial direction, the driving device comprising: a piezo-actuator comprising a vibration element to be in contact with a driven element and one or more sets of piezoelectric elements attached to the vibration element; and a controller, wherein each set of piezoelectric elements comprises one or more piezoelectric elements, wherein in each set of piezoelectric elements, a first electrode for expanding or contracting a first area of the piezoelectric elements of the set and a second electrode for expanding or contracting a second area of the piezoelectric elements of the set are arranged along the axial direction, and wherein in each set of piezoelectric elements, a common third electrode or separate third electrodes paired with the first electrode and the second electrode are arranged, so that the vibration element of the piezo-actuator has a bending mode in which a portion in the middle along the axial direction becomes convex or concave, whereby at both ends in the axial direction, end portions of the vibration element move in the axial direction; and an expansion/contraction mode in which one end portion of both ends in the axial direction expands/contracts, while the other end portion expands/contracts over time in opposite phase, whereby the end portions of the vibration element move in a direction perpendicular to the axial direction, wherein the controller is configured to cause the vibration element to vibrate by applying oscillating voltage signals of the same frequency to the first electrode and the second electrode, so that the driven element in contact with the vibration element is driven in the axial direction, wherein when the oscillating voltage signals applied to the first electrode and the second electrode are in phase, vibration of a first mode based on the bending mode occurs in the vibration element, and when the oscillating voltage signals applied to the first electrode and the second electrode are in opposite phase, vibration of a second mode based on the expansion/contraction mode occurs in the vibration element, and wherein the controller is configured to control a velocity of the driven element by controlling a phase difference between the voltage signal applied to the first electrode and the voltage signal applied to the second electrode.

According to the features of this implementation of driving a driven element by using vibration based on the bending mode and vibration based on the expansion/contraction mode and controlling a velocity of the driven element by controlling a phase difference between the voltage signal applied to the first electrode and the voltage signal applied to the second electrode, the vibration element vibrates in a direction perpendicular to the thrust force even when the phase difference is substantially at π and there is no thrust force. Thus, the friction arising between the vibration element and the driven element is kinetic friction. This may avoid problematic behavior where when starting to move the driven element, switching from static friction to kinetic friction makes the driven element suddenly start moving, or the driven element suddenly stops because of switching from kinetic friction to static friction. Thus, linearity of movement with respect to control signals is improved.

According to a second implementation of the first aspect of the present application based on the first implementation of the first aspect of the present application, the application of the voltage signals to the first electrode and the second electrode causes the end portion of the vibration element to make an elliptical movement that is a composition of a vibration component in the axial direction and a vibration component perpendicular to the axial direction, wherein the controller is configured to control vibration amplitudes of the vibration component in the axial direction and the vibration component perpendicular to the axial direction by controlling the phase difference.

According to this implementation, vibration of the vibration element gives rise to a thrust force on the driven element in an advantageous manner.

According to a third implementation of the first aspect of the present application based on the first or second implementation of the first aspect of the present application, the vibration element comprises a hole, wherein the driven element comprises a pillar shape, and the driven element, inserted into the hole, is driven in the axial direction.

According to this implementation, the axis of the driven element may be stabilized.

According to a fourth implementation of the first aspect of the present application based on any of the first to third implementations of the first aspect of the present application, the one or more sets of piezoelectric elements attached to the vibration element comprise two or more sets of piezoelectric elements attached in positions symmetrical about the vibration element, and wherein the same voltage signal is applied to the first electrode of each piezoelectric element, and the same voltage signal is applied to the second electrode of each piezoelectric element.

According to this implementation, the driven element may be driven by a force balanced around the axis.

According to a fifth implementation of the first aspect of the present application based on any of the first to fourth implementations of the first aspect of the present application, the controller is configured to cause the phase difference to be substantially π to let a velocity of the driven element be zero.

According to this implementation, since the vibration elements vibrate in a direction perpendicular to the thrust force even when the phase difference is π and there is no thrust force, the friction between the vibration element and the driven element is kinetic friction while the driven element is substantially at rest (makes no axial movement) .

According to a sixth implementation of the first aspect of the present application based on any of the first to fifth implementations of the first aspect of the present application, the controller is configured to gradually decrease or increase the phase difference from being substantially π when starting to drive the driven element from a velocity of zero, and/or the controller is configured to gradually bring the phase difference closer to being substantially π when the velocity of the driven element is to be made zero.

According to this implementation, the velocity of the driven element can be adjusted by gradually changing the phase difference.

According to a seventh implementation of the first aspect of the present application based on the second implementations of the first aspect of the present application, the controller is further configured to control amplitudes of the voltage signals applied to the first electrode and the second electrode so as to compensate a variation in amplitude of the vibration component perpendicular to the axial direction due to the control of the phase difference.

According to this implementation, the amplitude of the vibration component perpendicular to the axial direction may remain constant when the phase difference between the voltage signals applied to the first electrode and the second electrode is changed to control the velocity of the driven element.

According to an eighth implementation of the first aspect of the present application based on any of the first to seventh implementations of the first aspect of the present application, the controller is configured to control the velocity of the driven element by further controlling frequency and/or amplitude of the voltage signals applied to the first electrode and the second electrode.

According to this implementation, larger variation of the velocity of the driven element can be achieved than when only the phase difference between the voltage signals applied to the first electrode and the second electrode is changed.

According to a ninth implementation of the first aspect of the present application based on any of the first to seventh implementations of the first aspect of the present application, the controller is configured to control the velocity of the driven element by controlling frequency and/or amplitude of the voltage signals rather than phase of the voltage signals applied to the first electrode and the second electrode when the velocity of the driven element is equal to or greater than a predetermined value.

According to this implementation, when velocity control at low velocities is not required, the velocity of the driven element may be adjusted efficiently through control of frequency and/or amplitude of the voltage signals applied to the first electrode and the second electrode.

According to a tenth implementation of the first aspect of the present application based on any of the first to ninth implementations of the first aspect of the present application, the voltage signals applied to the first electrode and the second electrode are square waves, triangular waves, sawtooth waves, or sinusoidal waves.

According to at least some options of this implementation, digital control of the voltage signals applied to the first electrode and the second electrode may be facilitated.

According to an eleventh implementation of the first aspect of the present application based on any of the first to tenth implementations of the first aspect of the present application, the driving device further comprises a boost circuit configured to convert a voltage of a power supply to a voltage for driving the piezo-actuator.

According to this implementation, the piezo-actuator can be driven even when the supply voltage (e.g., the voltage of the power supply of a portable device) is lower than a voltage required to drive the piezo-actuator.

According to a twelfth implementation of the first aspect of the present application based on the eleventh implementation of the first aspect of the present application, the boost circuit comprises a driver for supplying a voltage to the piezo-actuator via an external inductor.

According to a thirteenth implementation of the first aspect of the present application based on the twelfth implementation of the first aspect of the present application, the piezo-actuator is represented by an equivalent circuit comprising an internal R-L-C (resistor-inductor-capacitor) series resonance circuit and a capacitance component connected in parallel, wherein a resonance frequency of a R-L-C series resonance circuit composed of an output impedance of the driver, the external inductor, and the capacitance component of the piezo-actuator substantially matches a resonance frequency of the internal R-L-C series resonance circuit of the piezo-actuator.

According to this implementation, energy of the oscillating voltage signals is efficiently transferred to vibration of the vibration element of the piezo-actuator.

According to a fourteenth implementation of the first aspect of the present application based on any of the first to thirteenth implementations of the first aspect of the present application, parameters of the internal R-L-C series resonance circuit of the piezo-actuator are tuned so that an inductance of the external inductor is 30 μH or lower.

According to this implementation, the size of the external inductor is relatively small, which makes it suitable for incorporation into a portable device. Moreover, the small inductance can be achieved by a design of circuit parameters within the piezo-actuator without introducing additional elements.

According to a fifteenth implementation of the first aspect of the present application based on any of the first to thirteenth implementations of the first aspect of the present application, the boost circuit has an external capacitor in parallel with the capacitance component of the piezo-actuator so that an inductance of the external inductor is 30 μH or lower.

According to this implementation, the size of the external inductor is relatively small, which makes it suitable for incorporation into a portable device. Moreover, the small inductance can be achieved by adding an external element without changing a design of circuit parameters within the piezo-actuator.

According to a second aspect of the present application, a driving device comprising: a piezo-actuator comprising a vibration element and one or more sets of piezoelectric elements attached to the vibration element; and a controller is provided. Each set of piezoelectric elements comprises one or more piezoelectric elements, wherein in each set of piezoelectric elements, a first electrode for expanding or contracting a first area of the piezoelectric elements of the set and a second electrode for expanding or contracting a second area of the piezoelectric elements of the set are arranged along the axial direction, and wherein in each set of piezoelectric elements, a common third electrode or separate third electrodes paired with the first electrode and the second electrode are arranged, so that the vibration element of the piezo-actuator has a bending mode in which a portion in the middle along the axial direction becomes convex or concave, whereby at both ends in the axial direction, end portions of the vibration element move in the axial direction; and an expansion/contraction mode in which one end portion of both ends in the axial direction expands/contracts, while the other end portion expands/contracts over time in opposite phase, whereby the end portions of the vibration element move in a direction perpendicular to the axial direction, wherein the controller is configured to cause the end portion of the vibration element to make an elliptical movement that is a composition of a vibration component in the axial direction and a vibration component perpendicular to the axial direction by applying oscillating voltage signals of the same frequency to the first electrode and the second electrode, wherein the controller is configured to control vibration amplitudes of the vibration component in the axial direction and the vibration component perpendicular to the axial direction by controlling a phase difference between the voltage signal applied to the first electrode and the voltage signal applied to the second electrode, and wherein when the oscillating voltage signals applied to the first electrode and the second electrode are in phase, vibration of a first mode based on the bending mode occurs in the vibration element, and when the oscillating voltage signals applied to the first electrode and the second electrode are in opposite phase, vibration of a second mode based on the expansion/contraction mode occurs in the vibration element.

According to a first implementation of a third aspect of the present application, a driving device for driving a driven element is provided. The driving device comprises a piezo-actuator comprising a vibration element to be in contact with a driven element and one or more sets of piezoelectric elements attached to the vibration element; a controller configured to control voltage signals applied to the piezoelectric elements to vibrate the vibration element of the piezo-actuator so as to drive the driven element; and a boost circuit configured to convert a voltage of a power supply to a voltage for driving the piezo-actuator, wherein the boost circuit comprises a driver for supplying a voltage to the piezo-actuator via an external inductor, wherein the piezo-actuator is represented by an equivalent circuit comprising an internal R-L-C (resistor-inductor-capacitor) series resonance circuit and a capacitance component connected in parallel, wherein a resonance frequency of a R-L-C series resonance circuit composed of an output impedance of the driver, the external inductor, and the capacitance component of the piezo-actuator substantially matches a resonance frequency of the internal R-L-C series resonance circuit of the piezo-actuator, and wherein (i) parameters of the internal R-L-C series resonance circuit of the piezo-actuator are tuned, and/or (ii) the boost circuit has an external capacitor in parallel with the capacitance component of the piezo-actuator, so that an inductance of the external inductor is 30 μH or lower.

According to this implementation, the piezo-actuator can be driven even when the supply voltage (e.g., the voltage of the power supply of a portable device) is lower than a voltage required to drive the piezo-actuator. Further, the size of the external inductor is relatively small, which makes it suitable for incorporation into a portable device. Moreover, the small inductance can be achieved by changing a design of circuit parameters within the piezo-actuator and/or adding an external element.

According to a second implementation of the third aspect of the present application based on the first implementation of the third aspect of the present application, each set of piezoelectric elements comprises one or more piezoelectric elements, wherein in each set of piezoelectric elements, a first electrode for expanding or contracting a first area of the piezoelectric elements of the set and a second electrode for expanding or contracting a second area of the piezoelectric elements of the set are arranged along the axial direction, and wherein in each set of piezoelectric elements, a common third electrode or separate third electrodes paired with the first electrode and the second electrode are arranged, so that the vibration element of the piezo-actuator has a bending mode in which a portion in the middle along the axial direction becomes convex or concave, whereby at both ends in the axial direction, end portions of the vibration element move in the axial direction; and an expansion/contraction mode in which one end portion of both ends in the axial direction expands/contracts, while the other end portion expands/contracts over time in opposite phase, whereby the end portions of the vibration element move in a direction perpendicular to the axial direction, wherein the controller is configured to cause the vibration element to vibrate by applying oscillating voltage signals of the same frequency to the first electrode and the second electrode, so that the driven element in contact with the vibration element is driven in the axial direction, wherein when the oscillating voltage signals applied to the first electrode and the second electrode are in phase, vibration of a first mode based on the bending mode occurs in the vibration element, and when the oscillating voltage signals applied to the first electrode and the second electrode are in opposite phase, vibration of a second mode based on the expansion/contraction mode occurs in the vibration element, and wherein the controller is configured to control a velocity of the driven element by controlling a phase difference between the voltage signal applied to the first electrode and the voltage signal applied to the second electrode.

According to a third implementation of the third aspect of the present application based on the second implementation of the third aspect, the application of the voltage signals to the first electrode and the second electrode causes the end portion of the vibration element to make an elliptical movement that is a composition of a vibration component in the axial direction and a vibration component perpendicular to the axial direction, and wherein the controller is configured to control vibration amplitudes of the vibration component in the axial direction and the vibration component perpendicular to the axial direction by controlling the phase difference.

According to this implementation, vibration of the vibration elements gives rise to a thrust force on the driven element in an advantageous manner.

According to a fourth aspect of the present application, an electronic device comprising the driving device of any implementation of the first to third aspect of the present application is provided.

According to a fifth aspect of the present application, an electronic device comprising a camera is provided, wherein the camera comprises the driving device of any implementation of the first to third aspect of the present application, and wherein the driving device is configured to drive a lens unit of the camera attached to the driven element for autofocusing.

According to a sixth aspect of the present application, the electronic device of the fourth or fifth aspect is provided, wherein the electronic device is a mobile phone or a smart phone.

According to a first implementation of a seventh aspect of the present application, a method for controlling, by a driver, a piezo-actuator for driving a driven element in an axial direction is provided. The piezo-actuator comprises: a vibration element to be in contact with a driven element and one or more sets of piezoelectric elements attached to the vibration element; and wherein each set of piezoelectric elements comprises one or more piezoelectric elements, wherein in each set of piezoelectric elements, a first electrode for expanding or contracting a first area of the piezoelectric elements of the set and a second electrode for expanding or contracting a second area of the piezoelectric elements of the set are arranged along the axial direction, and wherein in each set of piezoelectric elements, a common third electrode or separate third electrodes paired with the first electrode and the second electrode are arranged, so that the vibration element of the piezo-actuator has a bending mode in which a portion in the middle along the axial direction becomes convex or concave, whereby at both ends in the axial direction, end portions of the vibration element move in the axial direction; and an expansion/contraction mode in which one end portion of both ends in the axial direction expands/contracts, while the other end portion expands/contracts over time in opposite phase, whereby the end portions of the vibration element move in a direction perpendicular to the axial direction, wherein the method comprises: causing the vibration element to vibrate by applying oscillating voltage signals of the same frequency to the first electrode and the second electrode, so that the driven element in contact with the vibration element is driven in the axial direction, and controlling a velocity of the driven element by controlling a phase difference between the voltage signal applied to the first electrode and the voltage signal applied to the second electrode, wherein when the oscillating voltage signals applied to the first electrode and the second electrode are in phase, vibration of a first mode based on the bending mode occurs in the vibration element, and when the oscillating voltage signals applied to the first electrode and the second electrode are in opposite phase, vibration of a second mode based on the expansion/contraction mode occurs in the vibration element.

According to a second implementation of the seventh aspect of the present application based on the first implementation of the seventh aspect of the present application, the application of the voltage signals to the first electrode and the second electrode causes the end portion of the vibration element to make an elliptical movement that is a composition of a vibration component in the axial direction and a vibration component perpendicular to the axial direction, and wherein the controlling of a velocity of the driven element by controlling a phase difference comprises controlling vibration amplitudes of the vibration component in the axial direction and the vibration component perpendicular to the axial direction by controlling the phase difference.

Advantageous effects that may be achieved by each dependent implementation based on the first implementation of the seventh aspect are the same as or similar to those achieved by a corresponding implementation for the first aspect, and thus are not repeated.

According to a third implementation of the seventh aspect of the present application based on the first or second implementation of the seventh aspect of the present application, the method further comprises causing the phase difference to be substantially π to let a velocity of the driven element be zero.

According to a fourth implementation of the seventh aspect of the present application based on the first or second implementation of the seventh aspect of the present application, the method further comprises gradually decreasing or increasing the phase difference from being substantially π when starting to drive the driven element from a velocity of zero, and/or gradually bringing the phase difference closer to being substantially π when the velocity of the driven element is to be made zero.

According to a fifth implementation of the seventh aspect of the present application based on any of the first to third implementations of the seventh aspect of the present application, the method further comprises controlling amplitudes of the voltage signals applied to the first electrode and the second electrode so as to compensate a variation in amplitude of the vibration component perpendicular to the axial direction due to the control of the phase difference.

According to a sixth implementation of the seventh aspect of the present application based on any of the first to fifth implementations of the seventh aspect of the present application, the method further comprises controlling the velocity of the driven element by further controlling frequency and/or amplitude of the voltage signals applied to the first electrode and the second electrode.

According to a seventh implementation of the seventh aspect of the present application based on any of the first to fifth implementations of the seventh aspect of the present application, the method further comprises controlling the velocity of the driven element by controlling frequency and/or amplitude of the voltage signals rather than phase of the voltage signals applied to the first electrode and the second electrode when the velocity of the driven element is equal to or greater than a predetermined value.

According to an eighth aspect of the present application, a computer program is provided for causing a driver to perform the method of any of the first to seventh implementations of the seventh aspect of the present application.

According to a ninth aspect of the present application, a computer-readable storage medium having stored thereon the computer program of the eighth aspect of the present application is provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates driving of a driven element in an axial direction by a piezo-actuator controlled by a controller, according to an embodiment of the present application;

FIG. 2 illustrates a piezo-actuator according to an embodiment of the present application;

FIG. 3 illustrates principles of a piezo-actuator according to an embodiment of the present application;

FIG. 4 illustrates an elliptical movement in a piezo-actuator according to an embodiment of the present application;

FIG. 5 illustrates two-phase signals applied to a piezo-actuator according to an embodiment of the present application;

FIG. 6 illustrates a relation between a phase difference and a thrust force in an elliptical movement in a piezo-actuator according to an embodiment of the present application;

FIG. 7 is a flow diagram of a method for driving a piezo-actuator in order to control movement of a driven element in an axial direction, according to an embodiment of the present application;

FIG. 8 illustrates parameters for an equivalent circuit of a piezo-actuator and a boost circuit, according to an embodiment of the present application;

FIG. 9 illustrates parameters for an equivalent circuit of a piezo-actuator and a boost circuit, according to an embodiment of the present application; and

FIG. 10 illustrates an electronic device comprising a driving device including a piezo-actuator, according to an embodiment of the present application.

DESCRIPTION OF EMBODIMENTS

While the following describes embodiments of the present application with reference to drawings, the present invention is not limited to the embodiments illustrated or described.

A piezo-actuator (also referred to as a piezoelectric actuator) provides high energy density and high power despite its small size and has long been used for a single lens reflex camera and other applications. While a device that uses a piezo-actuator for driving a driven element may be called a piezo-motor, the terms piezo-actuator and piezo-motor may be used synonymously. A piezo-motor for linearly driving a driven element along an axis by using vibration of piezoelectric elements in an ultrasonic range may be called a linear ultrasonic motor (linear USM) .

FIG. 1 illustrates driving of a driven element 120 in an axial direction by a piezo-actuator 100 controlled by a controller 110, according to an embodiment of the present application. The controller as used herein generally refers to one or more components comprising a circuit for applying voltage signals (also referred to as driving signals) for operating the piezo-actuator and a circuit for controlling voltages. The control of voltage signals may be performed according to a computer program stored on a storage medium.

Fig. 2 (a) illustrates a piezo-actuator 200 for driving a driven element in an axial direction, according to an embodiment of the present application. According to this embodiment, one or more sets of piezoelectric elements 220 are attached around the vibration element (vibrator) 210. As described below, in the illustrated example, the vibration element has a hole in which a pillar-shaped driven element is inserted. However, the shape of the vibration element is not limited to the illustrated example. As described below, in the illustrated example, each set of piezoelectric elements comprises one piezoelectric element 220 having

electrodes

230 and 240. However, each set of piezoelectric elements may comprise a piezoelectric element having the electrode 230 and a separate piezoelectric element having the electrode 240. The piezoelectric elements of each set of piezoelectric elements comprise a first electrode 230 for expanding or contracting a first area of the piezoelectric elements of the set and a second electrode 240 for expanding or contracting a second area of the piezoelectric elements of the set. The first electrode 230 and the second electrode 240 are arranged along the axial direction. In the illustrated example, the piezoelectric elements in each set comprise one piezoelectric element 220, and the first area and the second area are two areas of the same piezoelectric element 220, but the present application is not limited to this. For example, the piezoelectric elements of each set may comprise two piezoelectric elements, and the first area and the second area may belong to separate piezoelectric elements. By controlled vibration of the vibration element 210, the pillar-shaped driven element 250, inserted in a hole of the vibration element, is driven in an axial direction. The first area and the second area of the piezoelectric elements are arranged along a direction of the axis of the insertion hole. One or more electrodes paired with the first electrode and the second electrode are connected to ground (GND) , as described below in conjunction with Fig. 2 (b) .

The present invention is not limited to the embodiments illustrated. These piezoelectric elements may be attached at positions symmetrical around the vibration elements. The shape of the vibration element is not limited, in particular, not limited to one having a hole in which a pillar-shaped driven element is inserted, provided that the vibration element is in contact with and exerts a force on the driven element. The piezoelectric element may be, but is not limited to, a PZT (lead zirconate titanate) element. Further, while a piezoelectric element is used as an example herein, any suitable electro-mechanical transducer may be used. Further, while an expression such as an actuator driving a driven element is used herein, it should be obvious for a skilled person that this is an expression from the perspective of the actuator (or some component to which the actuator is fixed) , and, in general, a relative movement is caused between the actuator and the driven element. The vibration element may be called a stator. The driven element may be called a slider, a shaft, or the like. Use of such terminology does not limit which of the actuator or the driven element is fixed. Moreover, more than one of a piezo-actuator as illustrated may be provided along an axial direction of the driven element.

Fig. 2 (b) illustrates driving voltages applied to the piezo-actuator as illustrated in Fig. 2 (a) . A first voltage signal Ea is applied to the first electrode 230, while a second voltage signal Eb is applied to the second electrode 240. These voltage signals may be two-phase voltage signals, and, in an embodiment, may be defined as:

Ea = a *sin ωt (1)

Eb = a *sin (ωt + ps) (2)

where a is a signal amplitude, ps is a phase difference between Ea and Eb, ω is an angular frequency, and t is time. The signal amplitude and the angular frequency are the same between the first voltage signal and the second voltage signal.

In an embodiment, the vibration element is made of a conductor such as a metal. In Fig. 2 (b) , GND denotes ground. In an embodiment, there is a third electrode paired with the first electrode on a side of the piezoelectric element opposite to the first electrode, and the first voltage signal Ea is applied between the first electrode and the third electrode. The third electrode may be placed between the piezoelectric element and the vibration element made of a conductor. Similarly, there is a fourth electrode paired with the second electrode on a side of the piezoelectric element opposite to the second electrode, and the second voltage signal Eb is applied between the second electrode and the fourth electrode. The fourth electrode may be placed between the piezoelectric element and the vibration element made of a conductor. The third electrode and the fourth electrode may be separate electrodes or a common electrode. The third electrode and the fourth electrode are connected to ground (GND) , which provides a reference of voltage signals. In the embodiment illustrated in Fig. 2 (b) , the vibration element is made of a conductor, and the conductor is connected to ground (GND) .

Fig. 3 is a schematic diagram of a cross section of the piezo-actuator illustrated in Fig. 2 for illustrating principles of the piezo-actuator of the present application.

Fig. 3 (a) illustrates vibration of the vibration element when ps = 0 (i.e., when Ea and Eb are in phase) (this is referred to as a bending mode) . While the middle portion vibrates up and down, the end portions (e.g., the point indicated by an open circle) moves laterally. Thus, the bending mode produces vibration in the axial direction at the end portion of the vibration element. This results in a thrust in the axial direction as described below.

Fig. 3 (b) illustrates vibration of the vibration element when ps = π (i.e., when Ea and Eb are in opposite phase) . This is a vibration mode in which one of the end portions expands/contracts, while the other end portion expands/contracts in opposite phase, and is referred to as an expansion/contraction vibration mode or an expansion/contraction mode. As the wall of the vibration elements inclines, the end portions (e.g., the point indicated by an open circle) moves up and down. Thus, the expansion/contraction mode produces vibration in a direction perpendicular to the axis (in a direction perpendicular to the plane of the piezoelectric element) .

For a generic phase ps, the end portion of the vibration element makes a movement that is a composition of vibration in the axial direction and vibration in the up/down direction (in the direction perpendicular to the axis) . The magnitude of the vibration in the axial direction may be considered to be in proportion to Ea + Eb (this may be called a first mode and is similar to the bending mode described above) . The magnitude of the vibration in the direction perpendicular to the axis may be considered to be in proportion to Ea–Eb (this may be called a second mode and is similar to the expansion/contraction mode described above) . It should be noted that this does not mean that addition or subtraction as electric signals is made on Ea and Eb, which are applied to different electrodes. Vibrations caused by these voltages at the end portion of the vibration elements are combined.

The vibration amplitudes of the vibration component in the axial direction and the vibration component in the direction perpendicular to the axis caused by the piezo-actuator at the end portion of the vibration element can be estimated to be in proportion to Eq. (3) and Eq. (4) , respectively, as follows:

(in-phase) Ea + Eb = a * (sin (ωt + ps) + sin ωt) = 2 *a cos (ps/2) *sin (ωt + ps/2) (3)

(opposite phase) Ea–Eb = a * (sin (ωt + ps) –sin ωt) = 2 *a sin (ps/2) *cos (ωt + ps/2) (4)

In an embodiment of the present application, the phase difference ps between the voltage signals Ea and Eb applied to the first and second electrodes of the piezo-actuator is set equal to π/2. (It should be noted that the value of ps may be controlled by a controller, as described below. ) In this case, as illustrated in Fig. 3 (c) , the end portion of the vibration element (e.g., the point indicated by an open circle) makes an elliptical movement. Such an elliptical movement translates (pushes, drives, or thrusts) the driven element inserted in a hole of the vibration element in the axial direction.

Fig. 4 illustrates an elliptical movement that is a combination of the vibration amplitudes of the vibration component in the axial direction (the direction of thrust, the direction of translational movement of the driven element) and the vibration component in the direction perpendicular to the axis caused by the vibration of the vibration element at the end portion of the vibration element. The direction of the movement of the driven element is also shown.

Fig. 5 illustrates an example of two-phase signals for the case of ps = π/2, which may cause an elliptical movement as illustrated in Fig. 3 (c) . At time A, when the two curves of these voltage signals intersect, Ea and Eb are of the same sign and of the same magnitude, which corresponds to the situation as in (i) in Fig. 3 (c) . Thereafter, time B, when the two signals are of the same magnitude and of the opposite signs, corresponds to (ii) in Fig. 3 (c) . At time C, when the two curves intersect again, Ea and Eb are again of the same sign and of the same magnitude, which corresponds to (iii) in Fig. 3 (c) . Time D, when the two signals are of the same magnitude and of the opposite signs, corresponds to (iv) in Fig. 3 (d) .

It should be noted that while the voltage signals Ea and Eb are illustrated as sinusoidal waves here to facilitate understanding, it may be advantageous to use other waveforms such as square waves, triangular waves, sawtooth waves, or the like. For example, waveforms that facilitate digital control of the voltage signals applied to the first electrode and the second electrode may be employed.

The vibration element needs to press down the driven element with a relatively large force so that the driven element may be driven by the piezo-actuator. In conventional piezo-motors, when starting to drive the driven element, a relatively large force is required to overcome the static friction that occurs when the driven element is at rest relative to the vibration element. The moment the driven element starts moving, static friction changes to kinetic friction, resulting in a sudden decrease of frictional force. This is problematic because the driven element suddenly starts moving. On the other hand, when decreasing the speed of the driven element, change of kinetic friction to static friction results in a sudden increase of frictional force, which may cause sudden stop of the driven element. With techniques controlling the frequency or amplitude of the voltage signals for driving the piezo-actuator, control at low velocities is difficult because it requires accurate control of voltages or feedforward control (e.g., to apply a larger force to overcome static friction only when starting movement) .

The present inventors recognized from Eq. (3) and Eq. (4) presented above that, in the elliptical movement of the end portion of the vibration element that drives the driven element, the ratio between magnitudes of the component in the axial direction and the component perpendicular to the axial direction can be controlled by changing the value of ps, and that the overall amplitude can be changed by varying the value of a.

Fig. 6 illustrates the relation between the magnitude of the vibration in the axial direction (the horizontal axis) and the magnitude of the vibration perpendicular to the axial direction (the vertical axis) for the cases of the phase difference ps = π/2, (3/4) π, (11/12) π, and π. The case of ps = π/2 produces thrust most efficiently. With the phase difference p= (3/4) π, the force pressing down the driven element is larger, while the thrust is smaller. With the phase difference ps = π, the thrust is zero.

Thus, by changing the value of ps instead of controlling the frequency or amplitude of the voltage signals applied to the piezo-actuator, the controller of the driving device may control the thrust force on the driven element without being affected by switching between static friction and kinetic friction. When starting to move, the thrust may be gradually increased from zero by gradually decreasing the value of ps from π. The maximum efficiency in producing thrust is achieved at ps = π /2. Alternatively, the thrust may be gradually increased from zero in the opposite direction by gradually increasing the value of ps from π. The maximum efficiency in producing the opposite thrust is achieved at ps = (3/2) π. When stopping, the thrust may be gradually brought to zero by gradually bringing the value of ps closer to π. Even when ps = π and there is no thrust, the vibration element vibrates in a direction perpendicular to the thrust force. Thus, the friction between the vibration element and the driven element is kinetic friction. This may avoid problematic behavior in which, when starting to move the driven element, switching from static friction to kinetic friction makes the driven element suddenly start moving, or the driven element suddenly stops because of switching from kinetic friction to static friction. Thus, linearity of movement with respect to control signals is improved.

Thus, the method of controlling a piezo-actuator according to the present application may improve the control at low velocities because changing the phase difference allows the thrust on the driven element to be freely controlled without being affected by switching between static friction and kinetic friction.

According to an embodiment of the present application, in addition to or as an alternative to controlling the phase difference between the voltage signals applied to the first electrode and the second electrode, the frequency and/or amplitude of the voltage signals applied to the first electrode and the second electrode may be controlled.

As can be seen from Eq. (3) and Eq. (4) presented above, when the amplitude a of the voltage signals is kept constant as the thrust force produced by the vibration component in the axial direction of the elliptical movement is decreased by changing the phase difference ps from π/2 to π, the vibration amplitude in a direction perpendicular to the axis increases. According to an embodiment of the present application, when the phase difference ps is varied, the signal amplitude a may be also changed in order to compensate for such an increase of the vibration amplitude in the direction perpendicular to the axis.

Fig. 7 is a flow diagram illustrating an outline of a method of driving a piezo-actuator for a case in which the driven element at rest is moved in the axial direction, and is brought to rest again. In an embodiment, the driving may be performed by the controller 110 of Fig. 1. In an embodiment, a desired velocity of the driven element in the axial direction is provided by a higher-layer controller or a user. In a non-limiting embodiment for driving a lens unit for autofocusing in motion picture imaging, the desired velocity is a velocity for moving the lens unit for moving the lens unit to a desired position for focusing, and may be dynamically determined by the higher-layer controller. It should be noted that the terms "controller 110" or "higher-layer controller" refer to a logical distinction. The controller 110 and the higher-layer controller may be physically embodied by one and the same controller. Embodiments are not limited in this regard.

In an embodiment, the higher-layer controller determines a non-zero velocity indication value based on the current position of the driven element (e.g., a lens unit) and a desired position. When the driven element arrives at the desired position, the velocity indication value of zero is determined. In an embodiment, the higher-layer controller may gradually increase the velocity indication value from zero and gradually decrease it to zero again so that the driven element smoothly starts to move and then smoothly stops. In an embodiment, a position sensor for detecting the position of the driven element is provided, which allows the higher-layer controller to know the position of the driven element. In an embodiment, the position of the driven element is periodically detected, and the velocity of the driven element can be determined based on a difference from the previous detected value. Thus, it can be determined whether the desired velocity is reached.

Referring to the flow diagram of Fig. 7, at step 710, the driven element is stationary, and the voltage signals applied to the first electrode and the second electrode of the piezoelectric element of the piezo-actuator are off.

At step 720, the controller 110 receives from the higher-layer controller a velocity indication value (other than zero) for the driven element.

At step 730, the controller 110 begins applying oscillating voltage signals of the same frequency on the first and second electrodes of the piezo-actuator. Initially, the phase difference between the voltage signals applied to the first and second electrodes are substantially π. In this case, the end portion of the vibration element of the piezo-actuator vibrates only in a direction perpendicular to the axis, and does not drive the driven element in the axial direction. Since power is lost while the driven element is not moved, it is desirable to begin applying voltage signals to the first and second electrodes after the velocity indication value (other than zero) for the driven element is received, though the present application is not limited in this respect.

At step 740, the controller 110 sets the phase difference between the voltage signals applied to the first and second electrodes in a range of π to (3/2) π or π to π/2 according to the velocity indication value received. The direction (e.g., a positive direction) in which the driven element is moved when the phase difference is in the range of π to (3/2) π is opposite to the direction (e.g., a negative direction) in which the driven element is moved when the phase difference is in the range of π to π/2. In an embodiment, the controller 110 may set the phase difference according to a lookup table (LUT) that indicates correspondence between desired velocity indication values and phase differences, though the present application is not limited in this regard.

The controller 110 dynamically adjusts the phase difference between the voltage signals applied to the first and second electrodes according to the velocity indication value received from the higher-layer controller. The phase difference of π/2 (or (3/2) π) corresponds to the most efficient driving in this control of thrust through phase difference. It should be noted that in order to achieve a greater velocity, the controller 110 may also control the amplitude and frequency of the voltage signals applied to the first signal and the second signal in addition of the phase difference between the voltage signals applied to the first and second electrodes.

At step 750, the controller 110 receives the velocity indication value of zero from the higher-layer controller. This may occur when the driven element arrives at the desired position, for example. It should be noted that, in some embodiments, the higher-layer controller may take into account inertia of the driven element, and determine the velocity indication value of zero shortly before the driven element arrives at the desired position.

At step 760, the controller 110 sets the phase difference between the voltage signals applied to the first and second electrodes substantially equal to π. Again, the end portion of the vibration element of the piezo-actuator vibrates only in a direction perpendicular to the axis and does not move the driven element in the axial direction.

At step 770, the controller 110 turns off the voltage signals applied on the first and second electrodes. This eliminates loss of power while the driven element is not moved.

Step 780 is a final state, in which the driven element is stationary, and the voltage signals applied to the first and second electrodes are off.

It should be noted that when the velocity of the driven element is equal to or greater than a predetermined value, the velocity of the driven element may be controlled by controlling the frequency and/or amplitude rather than the phase difference between the voltage signals applied to the first and second electrodes. In a large-velocity regime, control through frequency and/or amplitude may be effective because there is no problem due to switching between static friction and kinetic friction, which may cause sudden onset of movement or sudden stop.

Another aspect of the present disclosure relates to a boost device or boost circuit (a device or circuit for enhancing voltage) for providing a driving voltage for a piezo-actuator.

Since a piezo-actuator requires a high voltage (e.g., 40-100 V) , in order to incorporate one in a portable device such as a smartphone (which typically operates at a voltage of about 3 V) , a boost device (a device for enhancing voltage) is required. The piezo-actuator referred to in this aspect of the present disclosure may be, but is not limited to, a piezo-actuator as described with reference to Figs. 2-7 above.

The piezo-actuator may be represented by an equivalent circuit comprising an internal R-L-C (resistor-inductor-capacitor) series resonance circuit and a capacitance component Cd connected in parallel. In Fig. 8, the equivalent circuit corresponds to the portion surrounded by the dotted line.

In an embodiment, the boost circuit comprises a driver for supplying voltage to the piezo-actuator via an external inductor. The output voltage Vout and the resonance frequency f ₀out of this boost circuit may be represented as

where Vin is an input voltage from a power supply, Rout is an internal resistance of the driver, Lext is an inductance of the external inductor, and Cd is the capacitance component of the piezo-actuator.

The resonance frequency (referred to herein as a driver-side resonance frequency) of an R-L-C (resistor-inductor-capacitor) series resonance circuit determined by the output impedance Rout of the driver, the inductance Lext of the external inductor, and the capacitance component Cd of the piezo-actuator is substantially matched to the resonance frequency of the internal R-L-C series resonance circuit of the piezo-actuator. This matching may maximize the driving amplitude. Strictly speaking, the driver-side resonance frequency further involves a contribution from the internal R-L-C series resonance circuit of the piezo-actuator, but such a contribution may be ignored because the output impedance Rout of the driver is much smaller than the resistance R of the internal R-L-C series resonance circuit of the piezo-actuator (R >> Rout) . As a non-limiting example, a typical value for Rout is 1 ohm, while a typical value for R is 5 kilo-ohms.

However, components inside a portable electronic device such as a smartphone are very small. When the inductance Lext of the external inductor is several tens μH, the component height may exceed 1 mm, which makes it unsuitable for incorporation in a portable electronic device. Thus, Lext should be as small as possible.

Generally, various kinds of inductors are known, including coil inductors with a coiled conductor line, laminated inductors with a stack of sheets having a conductive trace printed thereon, and film inductors in which a coil-shaped metal film pattern is formed by using sputtering or vapor deposition known from semiconductor manufacturing technologies. Laminated inductors and film inductors are generally smaller than coil inductors and thus are suitable for mounting in a portable device such as a smartphone, but they generally have a smaller inductance than coil inductors.

The present application proposes two manners for reducing the inductance of the external inductor Lext. Embodiments of the present application based on these manners allow use of an inductor with a smaller inductance, which makes it feasible to use a laminated inductor or a film inductor as the external inductor.

According to a first manner, a desired or allowable value Lext is first determined based on requirements of component height. Based on the determined value of Lext, the resonance frequency of the piezo-actuator is designed so as to satisfy the resonance condition above. For example, parameters of the R-L-C series resonance circuit in the piezo-actuator are tuned so that the inductance of the external inductor is less than 30 μH (or preferably less than 20 μH or most preferably less than 10 μH) . This is in contrast to a conventional design technique in which parameters of a motor are fixed and the value of Lext is determined accordingly. It should be noted, however, that in the present manner, the design generally results in a high resonance frequency, which tends to increase switching loss.

In a second manner, as illustrated in Fig. 9, the two resonance frequencies are substantially matched by adding an external capacitor with a capacitance Cadd in parallel with the capacitance component Cd of the piezo-actuator. The output voltage Vout and the resonance frequency f ₀out of this boost circuit may be represented as

where Cadd is a capacitance of the additional parallel capacitor. The other parameters are defined in the above.

This addition of the parallel capacitor increases an output current of the driver because of the additional current flowing through the additional capacitor, but it does not waste energy because it is a reactive power that does not consume energy. The larger output current requires an increase in the size of driver elements, but such an increase in size is within a range usually tolerable in typical implementations as in smartphones.

This example embodiment of this aspect of the present disclosure may reduce the inductance of the external inductor of the driver in a boost device for providing a driving voltage for a piezo-actuator. It facilitates incorporation of the boost circuit in a small portable electronic device.

Fig. 10 illustrates a portable electronic device, for example, a smartphone, that incorporates any of the embodiments of any of the aspects of the present application. Embodiments of the present application may be used for autofocusing of a portable electronic device such as a smartphone, but use of embodiments of the present application is not limited to this.

While various embodiments are described above and illustrated in the drawings, the present invention is not limited to the specific embodiment described or illustrated.

The unit division disclosed in embodiments of the present application is not limiting, and embodiments may be configured with other divisions of components.

Where appropriate, some functions may be implemented in a form of a computer program for causing a processor or a computing device to perform one or more functions. For example, various signal processing and control functions may be implemented as a computer program. The computer program may be embodied on a non-transitory computer-readable storage medium. The storage medium may be any medium that can store a computer program and may be a solid-state memory such as a USB drive, a flash drive, a read-only memory (ROM) , and a random-access memory (RAM) ; a magnetic storage medium such as a removable or non-removable hard disk; or an optical storage medium such as an optical disc.

The foregoing descriptions merely illustrate various embodiments of the present application, and are not intended to limit the scope of the invention. Any variation that would readily occur to a person skilled in the art in view of the present disclosure shall fall within the scope of this application. For example, measures separately disclosed may be combined in a single embodiment as appropriate, as long as such measures are not mutually exclusive.

Claims

A driving device for driving a driven element in an axial direction, the driving device comprising:

a piezo-actuator comprising a vibration element to be in contact with a driven element and one or more sets of piezoelectric elements attached to the vibration element; and

a controller,

wherein each set of piezoelectric elements comprises one or more piezoelectric elements, wherein in each set of piezoelectric elements, a first electrode for expanding or contracting a first area of the piezoelectric elements of the set and a second electrode for expanding or contracting a second area of the piezoelectric elements of the set are arranged along the axial direction, and wherein in each set of piezoelectric elements, a common third electrode or separate third electrodes paired with the first electrode and the second electrode are arranged, so that the vibration element of the piezo-actuator has

a bending mode in which a portion in the middle along the axial direction becomes convex or concave, whereby at both ends in the axial direction, end portions of the vibration element move in the axial direction; and

an expansion/contraction mode in which one end portion of both ends in the axial direction expands/contracts, while the other end portion expands/contracts over time in opposite phase, whereby the end portions of the vibration element move in a direction perpendicular to the axial direction,

wherein the controller is configured to cause the vibration element to vibrate by applying oscillating voltage signals of the same frequency to the first electrode and the second electrode, so that the driven element in contact with the vibration element is driven in the axial direction,

wherein when the oscillating voltage signals applied to the first electrode and the second electrode are in phase, vibration of a first mode based on the bending mode occurs in the vibration element, and when the oscillating voltage signals applied to the first electrode and the second electrode are in opposite phase, vibration of a second mode based on the expansion/contraction mode occurs in the vibration element, and

wherein the controller is configured to control a velocity of the driven element by controlling a phase difference between the voltage signal applied to the first electrode and the voltage signal applied to the second electrode.
The driving device of claim 1, wherein the application of the voltage signals to the first electrode and the second electrode causes the end portion of the vibration element to make an elliptical movement that is a composition of a vibration component in the axial direction and a vibration component perpendicular to the axial direction, and

wherein the controller is configured to control vibration amplitudes of the vibration component in the axial direction and the vibration component perpendicular to the axial direction by controlling the phase difference.
The driving device of claim 1 or 2, wherein the vibration element comprises a hole, wherein the driven element comprises a pillar shape, and the driven element, inserted into the hole, is driven in the axial direction.
The driving device of any one of claims 1 to 3, wherein the one or more sets of piezoelectric elements attached to the vibration element comprise two or more sets of piezoelectric elements attached in positions symmetrical about the vibration element, and wherein a first same voltage signal is applied to the first electrode of each piezoelectric element, and a second same voltage signal is applied to the second electrode of each piezoelectric element.
The driving device of any one of claims 1 to 4, wherein the controller is configured to cause the phase difference to be substantially π to let a velocity of the driven element be zero.
The driving device of any one of claims 1 to 5,

wherein the controller is configured to gradually decrease or increase the phase difference from being substantially π when starting to drive the driven element from a velocity of zero, and/or

wherein the controller is configured to gradually bring the phase difference closer to being substantially π when the velocity of the driven element is to be made zero.
The driving device of claim 2, wherein the controller is further configured to control amplitudes of the voltage signals applied to the first electrode and the second electrode so as to compensate a variation in amplitude of the vibration component perpendicular to the axial direction due to the control of the phase difference.
The driving device of any one of claims 1 to 7, wherein the controller is configured to control the velocity of the driven element by further controlling frequency and/or amplitude of the voltage signals applied to the first electrode and the second electrode.
The driving device of any one of claims 1 to 7, wherein the controller is configured to control the velocity of the driven element by controlling frequency and/or amplitude of the voltage signals rather than phase of the voltage signals applied to the first electrode and the second electrode when the velocity of the driven element is equal to or greater than a predetermined value.
The driving device of any one of claims 1 to 9, wherein the voltage signals applied to the first electrode and the second electrode are square waves, triangular waves, sawtooth waves, or sinusoidal waves.
The driving device of any one of claims 1 to 10, further comprising a boost circuit configured to convert a voltage of a power supply to a voltage for driving the piezo-actuator.
The driving device of claim 11, wherein the boost circuit comprises a driver for supplying a voltage to the piezo-actuator via an external inductor.
The driving device of claim 12, wherein the piezo-actuator is represented by an equivalent circuit comprising an internal R-L-C (resistor-inductor-capacitor) series resonance circuit and a capacitance component connected in parallel,

wherein a resonance frequency of a R-L-C series resonance circuit composed of an output impedance of the driver, the external inductor, and the capacitance component of the piezo-actuator substantially matches a resonance frequency of the internal R-L-C series resonance circuit of the piezo-actuator.
The driving device of claim 13, wherein parameters of the internal R-L-C series resonance circuit of the piezo-actuator are tuned so that an inductance of the external inductor is 30 μH or lower.
The driving device of claim 13, wherein the boost circuit has an external capacitor in parallel with the capacitance component of the piezo-actuator so that an inductance of the external inductor is 30 μH or lower.
A driving device comprising:

a piezo-actuator comprising a vibration element and one or more sets of piezoelectric elements attached to the vibration element; and

a controller,

wherein each set of piezoelectric elements comprises one or more piezoelectric elements, wherein in each set of piezoelectric elements, a first electrode for expanding or contracting a first area of the piezoelectric elements of the set and a second electrode for expanding or contracting a second area of the piezoelectric elements of the set are arranged along the axial direction, and wherein in each set of piezoelectric elements, a common third electrode or separate third electrodes paired with the first electrode and the second electrode are arranged, so that the vibration element of the piezo-actuator has

a bending mode in which a portion in the middle along the axial direction becomes convex or concave, whereby at both ends in the axial direction, end portions of the vibration element move in the axial direction; and

an expansion/contraction mode in which one end portion of both ends in the axial direction expands/contracts, while the other end portion expands/contracts over time in opposite phase, whereby the end portions of the vibration element move in a direction perpendicular to the axial direction,

wherein the controller is configured to cause the end portion of the vibration element to make an elliptical movement that is a composition of a vibration component in the axial direction and a vibration component perpendicular to the axial direction by applying oscillating voltage signals of the same frequency to the first electrode and the second electrode,

wherein the controller is configured to control vibration amplitudes of the vibration component in the axial direction and the vibration component perpendicular to the axial direction by controlling a phase difference between the voltage signal applied to the first electrode and the voltage signal applied to the second electrode, and

wherein when the oscillating voltage signals applied to the first electrode and the second electrode are in phase, vibration of a first mode based on the bending mode occurs in the vibration element, and when the oscillating voltage signals applied to the first electrode and the second electrode are in opposite phase, vibration of a second mode based on the expansion/contraction mode occurs in the vibration element.
A driving device for driving a driven element, the driving device comprising:

a piezo-actuator comprising a vibration element to be in contact with a driven element and one or more sets of piezoelectric elements attached to the vibration element;

a controller configured to control voltage signals applied to the piezoelectric elements to vibrate the vibration element of the piezo-actuator so as to drive the driven element; and

a boost circuit configured to convert a voltage of a power supply to a voltage for driving the piezo-actuator,

wherein the boost circuit comprises a driver for supplying a voltage to the piezo-actuator via an external inductor,

wherein the piezo-actuator is represented by an equivalent circuit comprising an internal R-L-C (resistor-inductor-capacitor) series resonance circuit and a capacitance component connected in parallel,

wherein a resonance frequency of a R-L-C series resonance circuit composed of an output impedance of the driver, the external inductor, and the capacitance component of the piezo-actuator substantially matches a resonance frequency of the internal R-L-C series resonance circuit of the piezo-actuator, and

wherein

(i) parameters of the internal R-L-C series resonance circuit of the piezo-actuator are tuned, and/or

(ii) the boost circuit has an external capacitor in parallel with the capacitance component of the piezo-actuator,

so that an inductance of the external inductor is 30 μH or lower.
The driving device of claim 17,

wherein each set of piezoelectric elements comprises one or more piezoelectric elements, wherein in each set of piezoelectric elements, a first electrode for expanding or contracting a first area of the piezoelectric elements of the set and a second electrode for expanding or contracting a second area of the piezoelectric elements of the set are arranged along the axial direction, and wherein in each set of piezoelectric elements, a common third electrode or separate third electrodes paired with the first electrode and the second electrode are arranged, so that the vibration element of the piezo-actuator has

a bending mode in which a portion in the middle along the axial direction becomes convex or concave, whereby at both ends in the axial direction, end portions of the vibration element move in the axial direction; and

an expansion/contraction mode in which one end portion of both ends in the axial direction expands/contracts, while the other end portion expands/contracts over time in opposite phase, whereby the end portions of the vibration element move in a direction perpendicular to the axial direction,

wherein the controller is configured to cause the vibration element to vibrate by applying oscillating voltage signals of the same frequency to the first electrode and the second electrode, so that the driven element in contact with the vibration element is driven in the axial direction,

wherein when the oscillating voltage signals applied to the first electrode and the second electrode are in phase, vibration of a first mode based on the bending mode occurs in the vibration element, and when the oscillating voltage signals applied to the first electrode and the second electrode are in opposite phase, vibration of a second mode based on the expansion/contraction mode occurs in the vibration element, and

wherein the controller is configured to control a velocity of the driven element by controlling a phase difference between the voltage signal applied to the first electrode and the voltage signal applied to the second electrode.
The driving device of claim 18, wherein the application of the voltage signals to the first electrode and the second electrode causes the end portion of the vibration element to make an elliptical movement that is a composition of a vibration component in the axial direction and a vibration component perpendicular to the axial direction, and

wherein the controller is configured to control vibration amplitudes of the vibration component in the axial direction and the vibration component perpendicular to the axial direction by controlling the phase difference.
An electronic device comprising the driving device of any one of claims 1 to 19.
An electronic device comprising a camera,

wherein the camera comprises the driving device of any one of claims 1 to 19, and

wherein the driving device is configured to drive a lens unit of the camera attached to the driven element for autofocusing.
The electronic device of claim 20 or 21, wherein the electronic device is a mobile phone or a smart phone.
A method for controlling, by a driver, a piezo-actuator for driving a driven element in an axial direction, the piezo-actuator comprising:

a vibration element to be in contact with a driven element and one or more sets of piezoelectric elements attached to the vibration element; and

wherein each set of piezoelectric elements comprises one or more piezoelectric elements, wherein in each set of piezoelectric elements, a first electrode for expanding or contracting a first area of the piezoelectric elements of the set and a second electrode for expanding or contracting a second area of the piezoelectric elements of the set are arranged along the axial direction, and wherein in each set of piezoelectric elements, a common third electrode or separate third electrodes paired with the first electrode and the second electrode are arranged, so that the vibration element of the piezo-actuator has

a bending mode in which a portion in the middle along the axial direction becomes convex or concave, whereby at both ends in the axial direction, end portions of the vibration element move in the axial direction; and

an expansion/contraction mode in which one end portion of both ends in the axial direction expands/contracts, while the other end portion expands/contracts over time in opposite phase, whereby the end portions of the vibration element move in a direction perpendicular to the axial direction,

wherein the method comprises:

causing the vibration element to vibrate by applying oscillating voltage signals of the same frequency to the first electrode and the second electrode, so that the driven element in contact with the vibration element is driven in the axial direction, and

controlling a velocity of the driven element by controlling a phase difference between the voltage signal applied to the first electrode and the voltage signal applied to the second electrode,

wherein when the oscillating voltage signals applied to the first electrode and the second electrode are in phase, vibration of a first mode based on the bending mode occurs in the vibration element, and when the oscillating voltage signals applied to the first electrode and the second electrode are in opposite phase, vibration of a second mode based on the expansion/contraction mode occurs in the vibration element.
The method of claim 23, wherein the application of the voltage signals to the first electrode and the second electrode causes the end portion of the vibration element to make an elliptical movement that is a composition of a vibration component in the axial direction and a vibration component perpendicular to the axial direction, and

wherein the controlling of a velocity of the driven element by controlling a phase difference comprises controlling vibration amplitudes of the vibration component in the axial direction and the vibration component perpendicular to the axial direction by controlling the phase difference.
The method of claim 23 or 24, further comprising causing the phase difference to be substantially π to let a velocity of the driven element be zero.
The method of any one of claims 23 to 25, further comprising

gradually decreasing or increasing the phase difference from being substantially π when starting to drive the driven element from a velocity of zero, and/or

gradually bringing the phase difference closer to being substantially π when the velocity of the driven element is to be made zero.
The method of claim 24, further comprising controlling amplitudes of the voltage signals applied to the first electrode and the second electrode so as to compensate a variation in amplitude of the vibration component perpendicular to the axial direction due to the control of the phase difference.
The method of any one of claims 23 to 27, further comprising controlling the velocity of the driven element by further controlling frequency and/or amplitude of the voltage signals applied to the first electrode and the second electrode.
The method of any one of claims 23 to 27, further comprising controlling the velocity of the driven element by controlling frequency and/or amplitude of the voltage signals rather than phase of the voltage signals applied to the first electrode and the second electrode when the velocity of the driven element is equal to or greater than a predetermined value.
A computer program for causing a driver to perform the method of any one of claims 23 to 29.
A computer-readable storage medium having stored thereon the computer program of claim 30.