Description

The invention relates to a piezoelectric actuator, capable of moving a movable unit relative to a stator due to frictional coupling when said actuator is excited by an electric driving power, said actuator comprising a piezoelectric vibrating portion, a friction portion projecting from a main surface of the actuator and fixing portions, at which the actuator is to be fixed to the stator and with said fixing portions being located on opposite sides of said actuator.

In this context, the term 'fixed' means a boundary condition where the actuator at least in the region of the fixing portions has no degree of freedom, and said term 'fixed' stands in contrast to the term 'supporting', describing a state where at least one degree of freedom is existing.

A piezoelectric actuator as mentioned before is known, for example, from the published European patent application EP 0 536 832 A1. According to Fig. 5 of said document, the actuator 1" is fixed at its two end portions to support portions Si, S ₂ . In its middle position, a spherical segment K is attached to the actuator acting as the friction portion. Due to excitation by a corresponding electric driving power, the actuator representing a bimorph performs a deformation which generates a circular or elliptical movement of the friction portion. During the corresponding circular or elliptic movement, the friction portion temporarily comes into contact with the movable unit 9, thereby moving the movable unit into a desired direction. Subsequently, the friction portion moves back to its original position without having contact with the movable unit.

The circular or elliptical movement of the friction portion for driving the movable unit, however, is disadvantageous for several reasons. The change between contact and non- contact regarding the friction portion and the movable unit leads to an unsmooth and noise-afflicted movement of the movable unit. Moreover, the circular or elliptical movement of the friction portion generates a relatively high wear at the surface of the friction portion as well as the surface of the movable unit. A high wear results in debris material which can further negatively influence the driving conditions; especially, a reduced reliability of the actuator can be the consequence. Besides, in some applications, a large amount of debris material is not desired, for example in high-vacuum applications or in food industry applications. The object of the invention is therefore to provide a piezoelectric actuator with a low wear regarding the friction portion and a movable unit to be driven by the piezoelectric actuator, which is very reliable and enables a smooth and defined movement of the movable unit.

In order to solve the above defined object, the invention provides a piezoelectric actuator according to that mentioned at the beginning, which is further developed such that the piezoelectric actuator is a stick-slip piezoelectric actuator with the friction portion during operation being moved along a plane which is parallel to the actuator's main surface.

In this context, the term "stick-slip" contributes to a motion principle which is mainly divided into two phases, namely a driving phase, and a back moving phase. During said driving phase, the frictional coupling between the excited and thus deformed or moved actuator and a movable unit results in a movement or a movement step, respectively, of a movable unit due to static friction. In order not to leave the static friction behavior, the actuator is electrically excited such that the corresponding deformation or movement is relatively slow. During said back moving phase, the actuator is excited such that the corresponding deformation or movement is relatively fast. Thereby, the static friction behavior between the actuator and the movable unit is overcome, and dynamic friction results. Due to the existence of dynamic friction, the actuator can move relatively to the movable unit to a starting position without substantially no (back-)movement of the movable unit.

The in-plane movement of the friction portion allows a permanent contact between the friction portion and a movable unit to be driven by the actuator. Said permanent contact results in a very smooth and thus quiet movement of a movable unit, which besides is very reliable. Moreover, the permanent contact leads to a low wear coming from the friction contact between the actuator and the movable unit.

Preferred embodiments are claimed in the subclaims.

It may prove useful if the actuator is excited by an electric driving power having a frequency substantially equal to any even multiple of the actuator's longitudinal resonance frequency, wherein the frequency is preferably in an ultrasonic range. The excitation at a resonance frequency of the actuator leads to a very effective deformation or movement, respectively. Moreover, the excitation of the actuator's longitudinal resonance frequency results in an oscillation or vibration lying within the plane of the actuator. Thus, the actuator substantially performs no thickness change during operation. A friction element, which is preferably arranged at the actuator for friction coupling between the actuator and a movable unit, thus also is substantially moved within a plane, which is parallel to the actuator's main surface. There is substantially no movement of the friction element out of the corresponding plane, which leads to very liable contact conditions between the friction element and the corresponding counter-surface of the movable unit. In this context, the static friction conditions are very stable during the driving phase, resulting in a very precise movement of the movable unit. The quasi in-plane motion of the friction element allows a very smooth and quiet operation of a corresponding motor.

Due to the excitation principle using a driving power frequency substantially equal to any even multiple of the actuator's longitudinal resonance frequency, it is not needed that an applied friction element is exactly placed in a middle position of the actuator, which significantly facilitates the corresponding manufacturing process of the actuator and the motor.

While exciting the actuator at its second longitudinal resonance frequency or at any even multiple of the actuator's longitudinal resonance frequency, it is not needed to adhere to narrow dimension tolerances of the piezoelectric actuator. Operating the piezoelectric actuator in an ultrasonic range at any higher even multiple of the longitudinal resonance frequency requires small structures, so that the present invention is particularly applicable to miniaturization applications.

It may prove useful if the actuator is shaped as a bar or plate, wherein said fixing portions are located on opposite edges of said bar or plate, or if said actuator is shaped as a polygonal plate, preferably a square plate, wherein said fixing portions are located on the corners of said plate. In order not to suppress the useful vibration, the piezoelectric vibrating element or portion, respectively, is held fixed from its nodal position at fixing portions, which are located on opposite ends thereof. Fixing the actuator to a stator at fixing portions, which are located on opposite ends of the actuator, makes a corresponding piezoelectric motor mechanically more stable. It is to be understood that in case the actuator having more than two fixing portions, two fixing portions are arranged at opposite ends in each case. In addition, realization of a small actuator is possible due to fact that the actuator is held from its end positions. The vibrating piezoelectric element is preferably a piezoelectric ceramic plate in bulk or multilayered form, and may have rectangular, square or other shapes. Conductive electrodes divide the ceramic into two equal symmetric segments/sections. Besides, it may prove useful if the friction portion is located between said fixing portions, preferably in a centered position between said fixing portions. The center of the actuator shows the largest vibration, whereby the vibration is used to move a movable unit due to frictional coupling with the actuator.

Furthermore, it may prove useful if the friction portion is shaped as a sphere. Using a sphere as for the friction portion, a substantially constant and definite contact geometry between the friction element and a movable unit is existent. Preferably, the spherical ball is made of hard ceramic such as aluminum oxide (AI2O3) or Silicon Nitride (Si3N4) or hard metal alloy (or carbides) such as Tungsten Carbide in order to provide a high friction low wear contact with a movable unit.

It may prove advantageous if the actuator has a through-hole into which the friction portion is inserted. This allows a very easy manufacturing process regarding the placement and fixation of the friction portion. In this context, it is preferable if the diameter of the through-hole is marginally larger than the diameter of a sphere-shaped friction portion. Thus, the sphere-shaped friction portion can be easily inserted into the through- hole and also easily be fixed within the through-hole. Furthermore, the corresponding fixation process, which is preferably a gluing process using an adhesive, for example epoxy resin, can be performed without changing the position of the friction portion within the through-hole due to the small difference regarding the diameter of the friction portion and the diameter of the through-hole.

It also may prove advantageous if the friction portion projects from one of the side surfaces of the actuator. Here, it may prove helpful if the half diameter of the friction portion is smaller than the thickness of the actuator. In such case, it is easy to bring the friction portion in contact with a movable unit, which is preferably arranged in parallel with the surface, from which the friction portion projects. According to this embodiment, a very compact motor can be provided.

In addition, it may prove advantageous if the friction portion is adhesively fixed to the actuator. The usage of an adhesive allows a very practicable, easy and cheap fixation of the friction element to the actuator. Preferably, an epoxy resin is used as for the adhesive.

It may prove helpful if the piezoelectric actuator is provided with an arrangement of conductive electrodes, such that, upon application said of said electric driving power to said arrangement, said friction portion is displaced along an axis. More particularly, the actuator is deformed under the impact of the electric driving power, so that the friction portion is displaced with regard to the fixing portions. In such case, the actuator can be operated with high efficiency and high accuracy, as the displacement of the friction portion can be controlled by the amount of electric driving power applied to the arrangement of conductive electrodes. Said movement of the friction portion is along a plane which is parallel to the actuator's main surface.

It may prove beneficial if the piezoelectric actuator is provided with first and second arrangements of conductive electrodes, such that, upon application said electric driving power to said first and second arrangements, said friction portion is displaced along first and second axes, respectively, wherein said first axis is substantially perpendicular to said second axis. Again, the actuator is deformed under the impact of the electric driving power, so that the friction portion is displaced with regard to the fixing portions. Here, the fixing portions are preferably arranged in line with the first and second axes, where in each case two fixing portions are arranged in line with one axis at opposite ends of the piezoelectric actuator. The actuator according to this embodiment allows the friction portion being displaced and a movable unit being moved in a plane. The displacement of the friction portion in the plane can be controlled by the amounts of electric driving power applied to the respective arrangements of conductive electrodes.

It may prove profitable if said first and/or said second axis substantially extends in parallel with the largest diameter of said actuator. In a square-plate shaped actuator, the largest diameters are the diagonals connecting opposite corners. As will be understood, the maximum vibration occurs in the center of the vibrating element, so that the maximum of vibration of the vibrating element can be used to impart motion to a movable unit.

It may prove useful if the piezoelectric actuator is of a multilayer type, wherein conductive electrodes are embedded in said piezoelectric actuator in several layers, preferably in parallel fashion. The actuator according to this embodiment can be configured even more compact, as the piezoelectric actuator of a multilayer type can be made smaller and requires less electric driving power or less electric driving voltage, respectively. The embedded conductive electrodes are preferably electrically connected to outer conductive electrodes being arranged on a surface or on surfaces of the piezoelectric element.

The invention also relates to a piezoelectric motor with the piezoelectric actuator according to any one of the preceding preferred embodiments, with the piezoelectric motor additionally comprising a stator and a movable unit, wherein the movable unit is frictionally coupled over the friction portion with the actuator so as to move relative to the stator when the actuator is excited by an electric driving power, with the actuator being fixed to the stator at the fixing portions.

Here, it may prove useful if said actuator is fixed to the stator at said fixing portions by fixing means. A preferred fixing means is an adhesive so that the fixing is realized by adhesive bonding or gluing, respectively. Preferably, an epoxy resin is used as an adhesive. The usage of an adhesive allows a very practicable, easy and cheap fixation of the actuator to the stator. However, it is also possible to use other fixing means according to which the actuator is soldered or welded to the stator. Furthermore, it is possible to use screws or a screwing structure, respectively, as for the fixing means.

It may prove advantageous if the movable unit is rotatable about a rotation axis with regard to the stator, wherein said actuator is frictionally coupled with said movable unit in an eccentric position with regard to said rotation axis. In such arrangement, a complex rotary motor may be operated by means of a simple linear piezoelectric actuator. The larger the eccentricity, the larger is the torque applied by the actuator.

It may prove helpful if said axis is tangentially to a circle around said rotation axis. In such arrangement, a rotary motor can be operated with high efficiency by the linear actuator.

It may prove advantageous if said actuator is preloaded in the direction of said movable unit by means of a spring, so as to enhance the frictional coupling between said actuator and said movable unit. In such arrangement, the actuator can impart large frictional forces to the movable unit.

It may prove useful if said stator comprises a hole, from which the spring projects in order to preload said actuator. The elements of the motor according to this embodiment can be compactly arranged in miniaturized applications, such as cameras, mobile phones and the like. The spring is preferably a leaf spring.

It also may prove useful if a friction plate attached to a sliding element of the movable unit touches a friction tip at the center of the spherical ball, and two spherical balls together with the friction tip on the actuator guide the movable unit, wherein a preload is maintained by magnetic force obtained by two magnets attached to the sliding element and attracted by the stator, which is preferably a ferrous base plate made from steel. The piezoelectric motor according to the invention can be realized at low cost and can be miniaturized easily. In particular, the piezoelectric actuator/vibrating element according to the invention can be manufactured easily without considering any tight dimension tolerances. Furthermore, holding the piezoelectric actuator/vibrating element from two end positions does not degrade a vibration performance.

Of course, any combination of the before mentioned preferred embodiments or parts of them are also possible or thinkable.

The invention will now be described with reference to the appended drawings:

Brief description of drawings

Fig. 1 is an exploded perspective view of a piezoelectric linear motor containing an inventive actuator being shaped as a rectangular plate.

Fig. 2 is a perspective view of a piezoelectric actuator according to the invention being shaped as a rectangular plate and comprising a piezoelectric vibrating portion on which conductive electrodes are seen on the front surface.

Fig. 3 is a sectional view on line Ill-Ill of the piezoelectric actuator according to Fig. 2.

Fig. 4 is a schematic view of an inventive piezoelectric actuator as seen from the side, wherein the piezoelectric vibrating element or portion has the same polarization directions in two different sections.

Fig. 5 is a schematic view of an inventive piezoelectric actuator as seen from the side, wherein the piezoelectric vibrating element or portion has opposite polarization directions in two different sections.

Fig. 6 shows perspective views of a computer model of an inventive piezoelectric actuator under different load conditions, wherein Fig. 6a shows a rest condition of the actuator, Fig. 6b shows a condition of a deformation of the actuator upon excitation by an electric driving power in the second longitudinal mode of the piezoelectric vibrating element or portion at the end of a first cycle, and Fig. 6c shows a condition of a deformation of the actuator upon excitation by the electric driving power in the second longitudinal mode of the piezoelectric vibrating element or portion at the end of a second cycle. Fig. 7 is a front view of a piezoelectric motor containing an actuator according to the invention in an assembled condition.

Fig. 8 shows schematic side views of a piezoelectric motor with an inventive actuator under the load conditions similar to Figs. 6a to 6c, wherein Fig. 8a shows the motor at a rest condition of the actuator, Fig. 6b shows the motor in a condition of a deformation of the actuator upon excitation by a electric driving power in the second longitudinal mode of the piezoelectric vibrating element at the end of a first cycle, and Fig. 8c shows a condition of a deformation of the actuator upon excitation by the electric driving power in the second longitudinal mode of the piezoelectric vibrating element at the end of a second cycle.

Fig. 9 shows graphs illustrating an electric driving power, wherein Fig. 9a shows a graph voltage vs. time, Fig. 9b shows a graph current vs. time, and wherein Fig. 9c shows a simplified current waveform.

Fig. 10 shows a cross-sectional view of an actuator according to the invention, wherein Fig. 10a is a cross-sectional view of the actuator, wherein the piezoelectric vibrating element has multilayer structure, and wherein Fig 10b is a schematic view of a piezoelectric motor comprising the inventive piezoelectric actuator having a multilayer structure.

Fig. 11 is a perspective view of an inventive actuator, which is substantially embodied as a square plate, wherein the piezoelectric vibrating element is provided with a centered through-hole and has two substantially triangular electrodes on the side surfaces, which are separated by an area extending diagonally between opposite corners of the plate.

Fig. 12 is a perspective view of an inventive piezoelectric actuator, wherein the piezoelectric vibrating element is substantially embodied as a square plate and a friction element is inserted into and fixed in the centered through-hole, wherein four triangular electrodes, separated by areas extending diagonally between opposite corners of the plate and crossing the through-hole, respectively, are arranged on the side surfaces of the plate.

Fig. 13 is a schematic view of a further piezoelectric motor containing an actuator according to the invention. Fig. 14 shows views of an autofocus module comprising a piezoelectric motor with an actuator according to the invention, wherein Fig. 14a shows the autofocus module without shutter, Fig. 14b shows the shutter and Fig. 14c shows an exploded view of the autofocus module.

Fig. 15 is an exploded perspective view of elements of a further piezoelectric motor containing an inventive actuator, wherein the motor is embodied as rotary motor.

Fig. 16 are exploded perspective views of a piezoelectric motor containing an inventive actuator, wherein Fig. 16a is a view from the bottom and Fig. 16b is a view from the top.

Detailed description of preferred embodiments

Fig. 1 is an exploded perspective view of a stick-slip piezoelectric motor 1 containing an inventive piezoelectric actuator 3, wherein the motor 1 is embodied as linear motor. The motor comprises in addition to the piezoelectric actuator 3 a stator 2 and a movable unit 4. The movable unit 4 comprises a friction plate 5, a slider 6 and magnets 7.

The stator 2 is embodied as substantially rectangular ferrous base plate, which is preferably made from steel. Linear guiding grooves 21 for guiding bearing balls 22 are formed in parallel with an axis x.

The piezoelectric actuator 3 is capable of moving the movable unit 4 relative to the stator 2 due to frictional coupling when the actuator 3 is excited by an electric driving power. The actuator 3 comprises a piezoelectric vibrating portion 31 , a sphere-shaped friction portion 32 and fixing portions 33, at which the actuator 3 is to be fixed to the stator 2, wherein the fixing portions 33 are located on opposite sides of the actuator 3. The actuator 3 is further provided with an arrangement of conductive electrodes 34, such that, upon application said of the electric driving power to the arrangement of electrodes 34, the friction portion 32 is displaced along an axis x lying within a plane which is parallel to the actuator's main surface 302. The friction portion 32 is inserted into and fixed in a centered through-hole 306.

Fig. 2 is a perspective view of an inventive actuator 3. The actuator 3 is shaped as a rectangular plate, wherein the fixing portions 33 are located on the opposite shorter edges of the plate. Conductive electrodes 34 are seen on the front surface of piezoelectric vibrating element 31. The electrodes 34 on the back surface have also the same pattern.

Each electrode 34 has a rectangular shape and is arranged in a surface section of the piezoelectric vibrating portion 31 between the friction portion 32 and one of the fixing portions 33. The sphere-shaped friction portion 32 is positioned in the center of the actuator, and is fixed there in a through-hole.

Fig. 3 is a sectional view of the piezoelectric actuator 3 along line Ill-Ill of Fig. 2. As can be seen from this view, the friction portion 32 is a sphere-shaped friction element, which is placed in the center through-hole 306 of the piezoelectric vibrating element 31 and attached within the through-hole 306 with epoxy resin. The friction portion 32 projects from the main surface 302 of the actuator, wherein the part of the friction portion projecting from the main surface 302 is significantly smaller than the half of the diameter of the sphere-shaped friction portion 32. The electrodes 34 are symmetrically arranged on the main surfaces 302, 304 of the piezoelectric vibrating element 31. The arrows indicate the polarization directions of the piezoelectric vibrating element 31 in the sections between the friction portion 32 and the fixing portions 33. In the present case, the polarization directions are identical in the sections right and left of the friction portion 32.

Fig. 4 is a schematic view of an inventive piezoelectric actuator 3 as seen from the side, wherein the piezoelectric vibrating element 31 has the same polarization directions in two sections 31a, 31b between the friction element (not shown) and the fixing portions 33. In this configuration, a first electric line connects the upper left side electrode 34a and the bottom right side electrode 34a. Similarly, a second electric line connects upper right side electrode 34b and the lower left side electrode 34b. The ends of the piezoelectric plate 31 are fixed at the fixing portions 33 by adhesive bonding, i.e. gluing.

Fig. 5 is a schematic view of an inventive piezoelectric actuator 3 as seen from the side, wherein the piezoelectric vibrating element 31 has opposite polarization directions in two sections 31a, 31 b between the friction element (not shown) and the fixing portions 33. In this configuration, a first electric line connects the upper electrodes 34a and a second electric line connects the lower electrodes 34b. Again, the ends of the piezoelectric plate 31 are fixed at the fixing portions 33 by adhesive bonding, i.e. gluing.

Fig. 6 shows perspective views of a computer model of an inventive piezoelectric actuator 3, as used in structural analyses, under different load conditions. Fig. 6a represents a rest condition of the actuator 3, wherein the friction element 32 is in the position x = 0. Fig. 6b represents a condition of a deformation of the actuator 3 upon excitation by an electric driving power in the second longitudinal mode of the piezoelectric vibrating element 31 at the end of a first cycle. As can be seen, the friction element 32 has been displaced along the axis x by an amount +Δx as compared to the rest condition of the actuator 3 shown in Fig. 6a. Furthermore, it can be seen that the actuator's deformations are lying within the actuator's plane, and are perpendicular to the actuator's longitudinal direction.

Consequently, the friction portion 32 or its tip, respectively, is moved within a plane, which is parallel to the actuator's main surface 302. Fig. 6c represents a condition of a deformation of said actuator 3 upon excitation by said electric driving power in the second longitudinal mode of the piezoelectric vibrating element 31 at the end of a second cycle. As can be seen, the friction element 32 has been displaced along the axis x by an amount -Δx as compared to the rest condition of the actuator 3 shown in Fig. 6a, and by an amount -2Δx as compared to the condition of a deformation shown in Fig. 6b. Again, it can be seen that the actuator's deformations are lying within the actuator's plane, and are perpendicular to the actuator's longitudinal direction. Consequently, the friction portion 32 or its tip, respectively, is moved within a plane, which is parallel to the actuator's main surface 302. Under the two different driving and poling configurations explained in context with Figs. 4 and 5, an excitation of the piezoelectric plate 31 , i.e. the actuator 3, in the second longitudinal mode is very efficient. At the second longitudinal mode resonance frequency as can be seen in Figs. 6a to 6c, the center of the piezoelectric plate 31 , where the friction element 32 is located, shows the largest deformation in length direction, i.e. the x-axis.

Fig. 7 is a front view of a piezoelectric motor 1 in an assembled condition containing an inventive actuator. In this assembled condition, the movable unit 4 is coupled with the actuator so as to move relative to the stator 2 due to frictional coupling when the actuator is excited by the electric driving power. The actuator is fixed to the stator 2 at fixing portions, which are located on opposite ends of the actuator. The movable unit 4 is preloaded in the direction of the stator 2 by the magnets 7, which are attached to the slider 6, whereby the stator 2 attracts the magnets 7. The friction plate 5, which is also attached to the slider 6, has a longitudinal groove 51 for guiding the sphere-shaped friction portion 32 of the actuator in order to increase the stability of linear motion of the movable unit 4. A tip of the sphere-shaped friction portion 32 being inserted into a through-hole within the actuator (not seen) is in contact with the friction plate 5. Two spherical balls 22 that are moving in the grooves 21 of the stator 2 and grooves 61 of the slider 6 guide the slider 6. In order to maintain a stable movement of the movable unit 4, the slider 6 is supported in three positions. Fig. 8 shows schematic side views of a piezoelectric motor 1 with an inventive actuator representing the load conditions of the actuator 3 similar to the Figs. 6a to 6c. In this case, the piezoelectric vibrating element 31 has the same polarization directions in two sections 31a, 31b between the friction element 32 and the fixing portions 33. Fig. 8a represents the rest condition of the actuator 3 according to Fig. 6a, wherein the friction portion 32, which is inserted into the through-hole 306, is in a position x = 0 and the stator 2 and the movable unit 4 are in neutral positions. Fig. 8b represents the condition of a deformation of the actuator 3 similar to Fig. 6b, wherein the actuator 3 is excited at the second longitudinal mode resonance frequency at positive cycle of a wave with 80 % duty ratio. As one half 31a of the actuator expands and the other half 31b shrinks, the friction element 32 is displaced along the x-axis by an amount +Δx as compared to the condition represented by Fig. 8a. Said displacement of the friction element is parallel to the actuator's main surface 302, 304. Due to frictional coupling between the friction element 32 and the movable unit 4, the movable unit 4 moves in the same direction with the friction element 32 and is displaced along the x-axis by an amount similar to +Δx. Fig. 8c represents the condition of a deformation of the actuator 3 similar to Fig. 6c, wherein the actuator 3 is excited at the second longitudinal mode resonance frequency at negative cycle of a wave with 80 % duty ratio. In this cases, as the half 31a of the actuator contracts and the other half 31b expands, the friction element 32 is displaced along the x- axis by an amount -Δx as compared to the condition represented by Fig. 8a, and by an amount -2Δx as compared to the condition represented by Fig. 8b. Again, the displacement of the friction element 32 is parallel to the actuator's main surface 302, 304. At the negative cycle of the wave, contracted and expanded sides of the piezoelectric vibrating element are changing suddenly and the friction element 32 moves in the opposite direction. Due to inertia of the movable unit 4 and the high acceleration of the friction element 32 in the opposite direction, the movable unit 4 cannot follow the friction element 32 so slippage takes place. At the end of one period, a microscopic motion of the movable unit 4 is obtained.

Fig. 9 shows graphs illustrating an electric driving power, wherein Fig. 9a shows a graph voltage vs. time, Fig. 9b shows a graph current vs. time and Fig. 9c shows a simplified current waveform. The electric driving power depicted in Figs. 9a and 9b can be simplified as depicted in Fig. 9c. The current waveform is directly proportional to a vibration generated on the piezoelectric element. In reaction, on the ceramic piezoelectric vibrating element, a fast acceleration a1 of the friction portion occurs in the time frame ta1 and a slow deceleration a2 of the friction portion occurs in the time frame ta2 (Fig. 9c), which cause a stick/slip motion of the movable unit, as described with reference to the Figs. 8a to 8c.

As can be seen in Figs, 10a, 10b and 13, the piezoelectric actuator 3 can be of a multilayer type. Fig. 10a is a cross-sectional view of the actuator 3, wherein conductive electrodes 34 are embedded in said piezoelectric actuator 3 in several parallel layers. Said embedded conductive electrodes 34 can be referred to as inner electrodes. Here, the embedded or inner electrodes 34 are stacked in height direction of the piezoelectric actuator. However, it is also possible to arrange the embedded electrodes such as to be stacked in the depth direction or in the width direction, i.e. the two possible perpendicular directions to the orientation direction of the embedded electrodes as shown in Fig. 10a, 10b and 13. It is obvious to the skilled person that the supply of electric voltage to the embedded inner electrodes is normally performed by so-called outer conductive electrodes, which are electrically connected to the embedded inner electrodes, and which are arranged at a corresponding outer surface or at corresponding outer surfaces of the piezoelectric element. As indicated by the arrows, the 'piezoelectric vibrating element has opposite polarization directions in two sections 31a, 31b between the friction element 32 and the fixing portions 33.

However, according to Fig 10b, as indicated by the arrows, the piezoelectric vibrating element has the same polarization directions in two sections between the friction element 32 and the fixing portions. The friction element 32 contacts the friction plate 5 attached to the slider 6 of the movable unit 4. The movable unit 4 is preloaded in the direction of the ferrous stator 2 by means of magnets. The actuator 3 is fixed to the stator 2 at fixing portions, which are located at opposite ends of the actuator 3.

Fig. 11 is a perspective view of an actuator (without friction element 32) according to another preferred aspect of the invention, wherein the piezoelectric vibrating element 31 is substantially embodied as a square plate and has a center through-hole 306. The actuator 3 is provided with an arrangement of conductive electrodes 34, such that, upon application of an electric driving power to the arrangement of electrodes 34, the friction portion (not shown) is displaced along an axis x, which is parallel to the main surface 302 of the actuator. In greater detail, two substantially triangular electrodes 34 are mounted on each of the surfaces of the piezoelectric vibrating element 31. An area, extending diagonally between opposite corners of the square plate and crossing the center through- hole, separates the triangular electrodes 34. The bottom surface of the ceramic piezoelectric vibrating element 31 has the same electrode configuration as the top surface. The axis x is perpendicular to the diagonal line separating the triangular electrodes 34. The fixing portions 33, at which the actuator/vibrating element 31 is fixable to a stator 2, are located on opposite corners of the square plate. The center through-hole 306 is in a centered position between the fixing portions 33.

The actuator shape according to Fig. 11 results in very small fixing portions 33, which have several advantages. Due to the small fixing portions 33, the amount of a fixing means like glue can be very small, and the risk of having areas where no glue is available after having fixed the fixing portions to a stator is considerably reduced in comparison to bigger fixing portions. Thus, application of fixing means is facilitated and can be done faster, while at the same time saving adhesive material. Furthermore, the bonding is more secure. In addition, the small fixing area causes less disturbance of vibration of the piezoelectric element.

Fig. 12 is a perspective view of a piezoelectric actuator 3 according to another preferred aspect of the invention. In this so called "diamond shaped" actuator 3, the piezoelectric vibrating element 31 is substantially embodied as a square plate and the friction element 32 is fixed in the centered through-hole 306 by epoxy resin. The diamond shaped actuator is provided with first and second arrangements of conductive electrodes 34, such that, upon application said electric driving power to said first and second arrangements, the friction portion 32 is displaced along first and second axes x, y, respectively, wherein said first axis x is perpendicular to said second axis y. Said displacement of the friction portion 32 is along a plane which is parallel to the main surface 302 of the actuator. The first and said second axes x, y extend in parallel with the largest diameters of the actuator 3, which are basically imaginary lines connecting the corners of the substantially square plate and cross the center through-hole. In greater detail, four triangular electrodes 34 are arranged on each of the top or bottom surfaces of the piezoelectric vibrating element 31. Areas extending diagonally between opposite corners of the square plate and crossing the center hole separate the triangular electrodes 34. The bottom surface of the ceramic piezoelectric vibrating element 31 has the same electrode configuration as the top surface. Preferably, the diamond shaped actuator 3 is excited at its second longitudinal mode in one diagonal axis. The fixing portions 33, at which the actuator is to be fixed to a stator 2, are located on the four corners of the square plate. The center through-hole 306 is in a centered position between the fixing portions 33. Fig. 13 is a schematic sectional view of a piezoelectric motor 101 having an actuator 103 according to the invention. The actuator 103 of multilayer type according to Fig. 10a is mounted between the stator 102 and the movable unit 104. Unlike the motor according to Fig. 10b, a separate structure 110 supports the movable unit 104 by means of bearing balls 108.

Figs. 14a to 14c show elements of an autofocus module 100 comprising a piezoelectric motor 101 which contains an inventive actuator. The autofocus module 100 further comprises a housing 110, a substantially cylindrical lens barrel 111 , a movable unit/slider 104, which can slide on the lens barrel 111 , and a top cover/shutter 120, which is shown in Figs. 14b and 14c. The dimensions of the housing are about 10,5 mm in width and length. The lens barrel 111 has a diameter of about 8,0 mm. On the backside of the housing 110, which is shown in Fig. 14c, a printed circuit board 140 carrying a focus sensor is fixed to the housing 110. In this embodiment, the actuator 103 of the motor 101 is preferably the actuator according to Fig. 11. The actuator 103 is mounted on a flexible printed circuit board 102 together with a drive IC, position sensor or the like, wherein the flexible printed circuit board 102 forms the stator 102 in the sense of the invention. The actuator 103 is preloaded in the direction of the slider 104 by means of a spring 130, so as to enhance the frictional coupling between the actuator 103 and the slider 104. The slider 104 forms the movable unit in the sense of the invention. Even though not shown, the flexible printed circuit board 102 may comprise a hole, through and from which the spring 130 may project, so as to preload the actuator in the direction of the slider 104. The slider 104 is supported by the bearing balls 108 so as to be movable relative to the housing 110.

Fig. 15 is an exploded perspective view showing elements of another piezoelectric motor containing the inventive actuator. The motor is embodied as a rotary motor, wherein a movable unit is rotatable about a rotation axis z with regard to the stator 202. The actuator 203 is frictionally coupled with the movable unit in an eccentric position with regard to the rotation axis z. The axis x, along which the friction portion of the actuator can be displaced, is tangentially to a circle around said rotation axis z. Preferably, three identical piezoelectric actuators 203, as disclosed in context with Figs. 2 and 3, are used in this rotary motor. The actuators 203 are located on a circular ring type elastic base plate 202 made from steel or other ferrous material, which forms the stator 202 of this embodiment. The opposite ends of the actuators 203 are fixed on base plate 202 using epoxy or cement material. At the center of each actuator 203 a friction element is attached, which is inserted into a centered through-hole and fixed therein by the aid of an adhesive. Fig. 16 shows exploded perspective views of a rotary piezoelectric motor 201 , wherein Fig. 16a is a view from the bottom and Fig. 16b is a view from the top. An annular ring type rotor 204 forms the movable unit. A circular groove 241 is formed on the bottom of the rotor 204 facing the stator 202. The circular groove 241 is used for guiding the friction elements 233 that are located at the centers of piezoelectric actuators 203 and that are inserted into and fixed in a corresponding through-hole. The vibrating piezoelectric elements of the actuators 203 operate at their second longitudinal mode mechanical resonance frequency. When all three piezoelectric elements vibrate at their second mode resonance frequency, each actuator 203 applies a torque on the rotor 204. A torque due to frictional coupling between rotor 204 and the actuators 203 causes the rotor 204 to rotate. In an assembled state, the groove 241 on the rotor 204 is in contact with the friction portions 233 of the actuators 203 keeping the rotor 204 always concentric to the rotation axis z. An annular ring type magnet 207 attached to the rotor 204 applies necessary pre-stressing force in the direction of the stator 202.