WO2012087193A1

WO2012087193A1 - Electromechanical motor

Info

Publication number: WO2012087193A1
Application number: PCT/SE2010/051434
Authority: WO
Inventors: Stefan Johansson; Niklas SNIS
Original assignee: Piezomotor Uppsala Ab
Priority date: 2010-12-20
Filing date: 2010-12-20
Publication date: 2012-06-28
Also published as: DE112010006073T5

Abstract

An electromechanical motor (10) comprises an object (20) to be moved, at least one electromechanical actuator (30) and a control electronics unit (90). The electromechanical actuator has actuator element(s) (40) with electromechanically active material and electrodes (44A-D, 45A-D) for exciting it. The actuator element is attached by a first end (46) to a support structure (50) and by a second end to a drive pad (60). The control electronics unit (90) is electrically connected to the electrodes of the electromechanical actuator. The control electronics provides first electrical signals, causing the drive pad to move in a non-resonant two-dimensional path. The control electronics is arranged for providing second electrical signals, causing the electromechanical actuator to move in a bending mode mechanical resonance. The contacting surface (62) of the drive pad is provided at a distance d from the centre line, larger than 0.1 micrometer times a ratio between the distance between the first and second end and the stroke of the bending mode.

Description

ELECTROMECHANICAL MOTOR

TECHNICAL FIELD

The present invention relates in general to electromechanical motor and driving methods therefore and in particular to such electromechanical motors being operable both with high velocities and high positioning precision.

BACKGROUND

Typically, piezoelectric motors, or more generally electromechanical motors, are either designed to operate in resonance to reach high velocities, quiet operation and good efficiency or designed to manage high precision movement by e.g. quasi static operation. It is theoretically possible to get high precision movements also with resonant motors but in practice the existing solutions will have no or unstable operation when the step size is reduced to the nano-meter level. The problems with modification of a resonant motor for achieving high-precision positioning are to a large extent related to the need for a flexible support close to nodal positions of resonant vibrations. With these flexible backings it will be extremely complicated to control nanometer movements since the flexibility in these backings will typically be several orders of magnitude larger than the desired movement precision considering the commonly encountered force variations and the low stiffness of the supports. Further, a use of a vibrating element with only one contact element portion, also denoted as drive pad, makes the situation even more difficult. To make a movement, the drive pad either has to make a controlled step or a controlled slip relative to a surface of the object to be moved. The typical roughness of the drive pad and moved object in an ultrasonic motor is often in the micrometer range since the powerful resonant driving rapidly causes a substantial wear of the surfaces in contact. In practice, it is impossible to make repeated and controlled nanometer steps on a surface with a roughness that is more than three orders of magnitude larger than the requested step size.

On the other hand, increasing the driving frequency for a quasi-static positioning motor that is designed for nano-positioning into the ultra-sonic and/ or resonant frequency regimes also present severe difficulties. Examples of such motors can e.g. be found in US 6,066,911 , US 6,337,532, US 6,798, 1 17 and US 7,420,321. If ultra-sonic operation without use of mechanical resonances is employed, the increased heat dissipation may easily be so high that it might destroy the electromechanically active volumes. Furthermore, if the frequency of the stepping voltages is coming close to resonances, such resonances are often extremely complex, typically involving actuator elements as well as support members. The result may easily turn out to be a motion pattern that is either unknown or useless for efficient movements. Even if controlled resonant or non-resonant high- frequency operation of quasi-static positioning motors is achieved, the contact surfaces will rapidly be heavily worn and make high-precision positioning impossible.

SUMMARY

A general object of the present invention is to provide driving methods and electromechanical actuators that allows for both high-velocity operation and high precision positioning.

The above object is achieved by methods and devices according to the enclosed independent patent claims. Preferred embodiments are defined by the dependent claims. In general words, in a first aspect, an electromechanical motor comprises an object to be moved, at least one electromechanical actuator, a stator backbone, a spring arrangement and a control electronics unit. The object to be moved is to be moved in a main motion direction. The object to be moved has an interaction surface parallel to the main motion direction. The electromechanical actuator has at least one actuator element and a support structure. The actuator element has volumes of electromechanically active material and electrodes for exciting the volumes of electromechanically active material. The actuator element is attached by only a first end to the support structure. The actuator element has a drive pad attached to a second end opposite of the first end. An actuator element length is defined to be the distance between the first end and the second end along a centre line that is perpendicular to the interaction surface. The support structure of the electromechanical actuator is attached to the stator backbone. The spring arrangement is arranged for providing a normal force between the object to be moved and the electromechanical actuator. The control electronics unit electrically is connected to the electromechanical actuator for enabling provision of electrical signals to the electrodes. The drive pad is arranged with a contacting surface for mechanically interacting with the interaction surface of the object to be moved for causing a motion of the object to be moved. The control electronics is arranged for providing first electrical signals to the electrodes. The first electrical signals to the electrodes cause the actuator element to move the drive pad in a non-resonant two-dimensional path relative to the support structure. The control electronics is further arranged for providing second electrical signals to the electrodes, the second electrical signals to the electrodes causes the electromechanical actuator to move in a bending mode mechanical resonance. The bending mode mechanical resonance has a stroke in the main motion direction. At least a part of the contacting surface of the drive pad is provided at at least a distance d from the centre line in the main motion direction. The distance d is 0. 1 micrometer times a ratio between the actuator element length and the stroke.

In a second aspect, a method for driving an electromechanical motor comprises providing of first electrical signals to electrodes in an actuator element of an electromechanical actuator for exciting volumes of electromechanically active material in the actuator element. The actuator element is of such a kind that is attached by only a first end to a support structure. The actuator element has a drive pad attached to a second end opposite of the first end. The drive pad is arranged for interaction with an interaction surface of an object to be moved in a main motion direction, parallel to the interaction surface. An actuator element length is defined to be the distance between the first end and the second end along a centre line that is perpendicular to the interaction surface. At least a part of the contacting surface of the drive pad is provided at at least a distance d from the centre line in the main motion direction. The distance d is 0.1 micrometer times a ratio between the actuator element length and the stroke. The first electrical signals are configured for causing the actuator element to move the drive pad in a non-resonant two-dimensional path relative to the support structure. The method also comprises providing of second electrical signals to the electrodes. These second electrical signals are configured for causing the electromechanical actuator to move in a bending mode mechanical resonance. The bending mode mechanical resonance has a stroke in the main motion direction.

One advantage with the present invention is that it allows one and the same electromechanical motor to be operated for nano-positioning as well as operation the ultrasonic frequency range. Other advantages and object of particular embodiment of the present invention are further discussed together with the different embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further objects and advantages thereof, may best be understood by making reference to the following description taken together with the accompanying drawings, in which:

FIG. 1 is a schematic illustration of an embodiment of an electromechanical motor;

FIG. 2 is a schematic illustration of the movements of an embodiment of a drive pad during high-precision positioning operation; FIG. 3 is a schematic illustration of the movements of an embodiment of an electromechanical actuator during high-speed operation;

FIGS 4A-E are schematic illustrations of embodiments of electromechanical actuators capable of producing motion trajectories in two dimensions;

FIGS. 5A-F are schematic illustrations of embodiments of drive pads;

FIG. 6A-B are schematic illustrations other embodiments of an electromechanical motor;

FIGS. 7A-C are schematic illustrations of a further embodiment of an electromechanical motor; and

FIG. 8 is a flow diagram of steps of an embodiment of a driving method;

DETAILED DESCRIPTION

Throughout the drawings, the same reference numbers are used for similar or corresponding elements.

The disclosed solutions address the above mentioned problems and propose to modify a quasi-static positioning motor that is designed for nano- positioning to also manage operation in the ultrasonic range and preferably utilizing resonance to reduce input power and heating.

Fig. 1 is a schematic illustration of an embodiment of an electromechanical motor 10. The electromechanical motor 10 comprises an object 20 to be moved, at least one electromechanical actuator 30 and a spring arrangement 80. The object 20 to be moved is to be moved in a main motion direction 9. The object 20 to be moved has an interaction surface 22 that is parallel to the main motion direction 9. A mechanical interaction between the electromechanical actuator 30 and the object 20 to be moved is used for creating the motion. The electromechanical actuator 30 has in this embodiment two actuator elements 40 and a support structure 50. Each actuator element 40 is attached by only a first end 46 to the support structure 50. The actuator elements 40 thus form "legs" sticking out from the support structure. Each actuator element 40 has a drive pad 60 attached to a second end 47. The second end 47 is situated opposite to the first end 46. An actuator element length L can be defined as the distance between the first end 46 and the second end 47 along a centre line 49 that is perpendicular to the interaction surface 22. The drive pad 60 is arranged with a contacting surface 62 for mechanically interacting with the interaction surface 22 of the object 20 to be moved for causing a motion of the object 20 to be moved. Except for occasional contacts with the object 20 to be moved via the drive pads, the actuator elements 40 are not mechanically attached to any other part of the electromechanical motor 10 than the support structure 50 and are therefore free to deform and e.g. create relative motions between different parts of the actuator elements 40 and the respective first ends 46. Each actuator element 40 has volumes 42, 43 of electromechanically active material and electrodes 44, 45 for exciting the volumes 42, 43 of electromechanically active material. When suitable electric signals are provided to the electrodes, the volumes 42, 43 of electromechanically active material change their shape, which causes movements of different parts of the actuator elements, e.g. the second end 47 and the drive pad 60 relative to the respective first ends 46.

The spring arrangement 80 is in this embodiment schematically illustrated as a spring keeping the object to be moved 20 and the electromechanical actuator 30 together. The actual spring arrangement 80 can be any type of arrangement that is arranged for providing a normal force N between the object 20 to be moved and the electromechanical actuator 30. This normal force N is necessary to create the moving action.

The electromechanical motor 10 also comprises a stator backbone 70, to which the support structure 50 of the electromechanical actuator 30 is attached. The electromechanical motor 10 also comprises a control electronics unit 90 that is electrically connected to the electromechanical actuator 30 for enabling provision of electrical signals to the electrodes 44A-D, 45A-D, as illustrated by the connections 91. These electrodes 44A-D, 45A-D are thereby possible, at least in groups, to connect to individual voltages. However, if requested, electrodes could be interconnected to follow a same voltage. Typically, but not necessarily, half of the electrodes are connected to a ground potential. As will be discussed further below, in a preferred embodiment, at least some of the electrodes are connected to enable a measurement of an electrode voltage when not being actively used. In this way, the actuators can also be used as sensors.

In the present embodiment, the electromechanically active material is supposed to be a piezoelectric material and the electrodes are arranged to provide an electrical potential difference between electrodes on either sides of a volume of the piezoelectric material. The electrical field so applied across the material will give rise to geometrical shape changes. Depending on the design, the shape changes may be of different kinds. Other electromechanically active materials, such as electrostrictive and antiferroelectric materials can also be utilized for this purpose, then with suitable electrical signals applied to the electrodes. These materials could be single crystalline as well as polycrystalline or amorphous. The electrode arrangements may be provided on the outer side of the material volume to be excited. However, in particular embodiments, to reduce the necessary voltages that are applied, multilayer techniques may be used. The use of multilayer techniques is as such well known in prior art.

The electromechanical motor of the present embodiment resembles in many part with stepping electromechanical motors of prior art. However, there are very distinct differences. In the present embodiment, the drive pad 60 has a contacting surface 62 that covers more or less half of the second end 47 of the actuator element 40 and the center of this contact surface 62 is not centered on the actuator element 40. As will be discussed further below, a contacting surface 62 that extends to the center, or close to the center, of the element makes it is possible to obtain nanometer positioning in the same way as can be made with similar prior art motors. In other words, at least a part of the contacting surface 62 is provided in the vicinity of the centre line 49 of the actuator element 40. Such nano-positioning can be obtained by actuator elements that can be excited to move the respective drive pad in two different directions, e.g. longitudinally and sideways, relative a common backing by proper electrical signals. In other words, the drive pad 60 is moved in a non-resonant two-dimensional path relative to the support structure 50. This will be further discussed in detail here below.

Fig. 2 is a schematic illustration of the movements of an embodiment of a drive pad during high-precision positioning operation. The electromechanically active volumes of the actuator element are excited by the electrodes. For nano-positioning purposes, the control electronics is arranged for providing electrical signals of a first type to the electrodes. These electrical signals to the electrodes are configured in such a way that they cause the actuator element to move the drive pad in a non-resonant two-dimensional path relative to the support structure. Examples of such voltages and suitable electrode arrangements will be discussed further below. For e.g. bimorph structure, where two independently excitable sides extending and attached to each along the length of the actuator element, a typical motion pattern of the center part of the drive pad is achieved within a rhombic motion area 100 with fixed voltage range. Within this rhombic motion area 100, any type of motion path can be obtained by providing suitable voltage signals. The rhombic motion area 100 has an extension in the main motion direction 9 as well as in the length direction 8 of the actuator element 40.

In the present embodiment, the drive pad has an asymmetric shape, where the contacting surface 62 has end 64 provided essentially at the edge of the actuator element 40 and another end 66 provided essentially in the middle, along the centre line 49. In other words, at least a part of the contacting surface 62 is provided in the vicinity of the centre line 49. With the bending of the bimorph structure, the rhombic area movements will also be accompanied by a tilting of the drive pad 60. To avoid that the part of the contacting surface 62 which is close to the edge of the actuator element 40 interferes with the movement, the actuator element 40 could in such embodiment be driven with an off-set tilting of the actuator element and hence the contacting surface 62. As an example, with an actuator element 40 that has an off-center pad, centered e.g. to the left as in Fig. 2, the drive pad center 66 can move within the left part 101 of the rhombic area 100 without any risk that the left part of the drive pad 60 will get in contact with a flat planar object to be moved. A motion path 102 could be one example that can be used during such conditions. The step length will decrease to half, but one is assured that only the edge between the centre end 66 of the contacting surface 62 will be used for high-precision positioning. Under such circumstances, a high step length is probably not the most important parameter anyway.

When going to high speed operation, it is as mentioned in the background possible to increase the frequency of the stepping used for high-precision positioning. However, such motion patterns are typically not very power efficient. Furthermore, when the frequency comes close to different mechanical resonance frequencies of the actuator assembly, many different kinds of behaviors may occur. However, at high-speed movements, the contact between the actuator elements and the object to be moved doesn't have to be maintained at all instances. It is even preferred in certain configurations to let all actuator elements leave the object for shorter periods of time to enable a low friction motion. What is required from the actuator elements is, however, to provide a motion component in the main motion direction as well as in the direction perpendicular to the interaction surface of the object to be moved. One of the findings leading to the present invention is that a bending motion of an actuator element will lead to somewhat different motion paths for the different parts of the actuator element. Fig. 3 is a schematic illustration of the movements of an embodiment of an electromechanical actuator element 40 during high-speed operation. The broken lines indicate the electromechanical actuator element 40 position (enormously exaggerated) at bent positions. The bending of an actuator element 40 will result in a rotation of the actuator element tip where the drive pad 60 is attached. It can be noticed that a point at the contacting surface 62 of the drive pad situated in the vicinity of the centre line 49 moves essentially back and forth in the main motion direction 9, as illustrated by the arrow 104. However, an edge point 65 has, besides a motion component parallel or antiparallel to the main motion direction 9, a motion component in the length direction 8 of the actuator element 40, as seen from the arrow 105. If such a component is large enough, the edge point 65 can be utilized to exert a force to the interaction surface of the object to be moved, not only in the main motion direction, but also transverse to the main motion direction. By utilizing the inertia of the object to be moved and by adapting the applied normal force, the actuator element 40 can be utilized for providing resonance motions. When the actuator element 40 are driven with sufficiently high frequency the normal force pressing the object against the drive pad 60 will not manage to displace the mechanical components more than fractions of micrometers and the drive pad 60 will release from the object when the drive pad 60 moves downwards. It is sometimes better to press the stator unit against the object to be moved since the mass of the stator is often larger and due to inertia the movement of the mechanical components that are pressed together by normal forces will be reduced. However, the driving frequency, the masses and normal forces should be tuned to achieve the best performance for the intended application. The object can thus be given a velocity with a component in the length direction 8 of the actuator element 40. When the actuator element 40 bends back, the inertia of the object will release the contact between the object and the actuator element 40 and the actuator element 40 can bend back and be ready for a next push. This behavior becomes particularly interesting when a mechanical resonance of the actuator element for the bending mode vibration is reached, since the motion can be achieved with low energy input. In other words, the control electronics is for achieving high-speed movements further arranged for providing second electrical signals to the electrodes. The second electrical signals to the electrodes cause the electromechanical actuator to move in a bending mode mechanical resonance. The bending mode mechanical resonance has a stroke in the main motion direction 9. The interaction surface of the object should in principle not move further down from the top position than half the total stroke of the drive pad edge in contact with the rod, in the direction perpendicular to the main motion direction, to avoid that the drive pad close to the centre line gets in contact with the object. The mechanical wear of this central part can therefore be avoided which makes it possible to maintain precision when the motor is used for nanometer positioning. The drive pad 60 will thus essentially be worn on the edge in contact with the object to be moved.

According to experience, it has been found that a reliable high-speed motion typically can be achieved when the lifting of the object to be moved in the direction of the length of the actuator element becomes larger than 0. 1 micrometer. The stroke of the bending of the actuator element thus has to be sufficient and the distance between the contacting edge 65 and the centre line 49 has to be large enough. To this end, at least a part of the contacting surface of the drive pad is provided at at least a distance d from the centre line 49 in the main motion direction 9. The distance d is at least 0. 1 micrometer times a ratio between the actuator element length L and the stroke. In such a way, an efficient quiet and high speed operation can be achieved by the actuator elements, driven in bending resonance.

Furthermore, in this particular embodiment, the drive pad is asymmetric with respect to a plane that is perpendicular to the main motion direction and passes through the centre line. Such a design is intended for high-speed movements in the direction away from the resonance driving end of the interaction surface.

In the embodiment of Fig. 1, the electromechanical actuator 30 has two actuator elements 40 positioned after each other in the main motion direction 9. This is for accomplishing a two-directional movement. The drive pads 60 are assembled mirror imaged with respect to the center of the stator unit. One actuator element is used for high-speed driving in one direction and the other for the other direction. Together, they can then also accomplish high-precision positioning. It is also beneficial to have the driving points as close to each other as possible. Thus, in this embodiment the drive pad 60 of each actuator element 40 is asymmetric with respect to a plane 48 that is perpendicular to the main motion direction 9 and passes through the centre line 49. Furthermore, an end 64 of one drive pad 60 that is situated at the largest distance from the respective centre line 49 is facing an end 64 of the other drive pad 60 that is situated at the largest distance from that respective centre line 49.

To reduce the risk that the object to be moved will get in contact with the drive pad of the other actuator element which typically is inactive, the actuator elements and drive pad contact edges are in one particular embodiment placed very close to each other. In that way a rather large angular error of the electromechanical actuator 30 relative the object 20 to be moved can be accepted before the other drive pad touches the contacting surface. To further avoid that the central part of the drive pad gets in contact with the object during resonant operation, the inactive actuator element can be retracted longitudinally using a proper bias voltage on the actuator element. The active actuator element can also be tilted by applying proper voltages to the electrodes so that the central part of the drive pad will always be lower than the edge part of the drive pad during the resonant vibration. Typically this will decrease the possible resonant vibration amplitude to half of the maximum value but the driving can be made more smooth and the impact of the drive pad edge against the object to be moved when reestablishing contact will be minimized. The motor has to be designed and built to allow control of the mass of the parts that move due to the motor normal force.

The benefit of having a motor design according to the above embodiments is that the high mechanical wear, that is present during the high-speed operation at the bending mode resonance, becomes concentrated around the end of the drive pad at largest distance the centre line. When then switching into high-precision positioning, the centre edge of the drive pad is still smooth and therefore suitable for positioning tasks down in the nanometer range.

There are different possibilities to achieve movements in two dimensions. Figs. 4A-E illustrates different approaches. Fig. 4A illustrates two actuator elements with a bimorph structure, where each half is excited by electrodes that are provided in planes parallel to the interaction surface of the object to be moved. By using such electrodes, a d33 excitation of the piezoelectric material can be utilized to achieve an extension in the length direction 8. Such electrode configuration is very useful in applications where the electromechanical actuator is manufactured as a monolithic unit. The electrodes can also be positioned along the length of the actuator elements. One such embodiment is illustrated in Fig. 4B, with the electrodes perpendicular to the main motion direction. There, the d3i excitation of the piezoelectric material is utilized. In Fig. 4C, an embodiment with d3i excitation and with the electrodes directed parallel to the main motion direction as well as perpendicular to the interaction surface is depicted.

By using a setup with double bimorphs, such as in the embodiment of Fig. 4D, additional advantages can be achieved. The volumes of electromechanical material 42A-B, 43A-B could have electrodes 44C-F, 45C- F arranged for d3i or d33 activation in analogy with the arrangements in Figs. 4A-C. Two bimorphs are thus created, connected end to end. The bending mode resonance can be operated with the upper and lower bimorphs connected in parallel, e.g. that one side at a time is stretched. During the high-precision operation, the upper and lower bimorphs can be driven opposite to each other, e.g. by connecting electrodes 44C with 45E, 44D with 45F, 45C with 44E and 45D with 44F. Such a connection will result in S- shaped strokes of the actuator element. In such a way, the contacting surface 62 of the drive pad 60 can always be kept parallel to the interaction surface. By having a tiny tilting of the actuator element either by electrically exciting the actuator elements with an offset voltage or by a slightly wedge- shapes drive pad, the entire rhombic motion areas can be utilized and thereby a large step size.

Also a combination of a d3 i or d33 and a dis excitation (a shear excitation), as illustrated in Fig. 4E can be utilized for driving a drive pad in a two- dimensional path. However, such solutions are generally not very manufacturing- friendly.

It is also possible to achieve fine-positioning by using a unimorph structure. By only exciting one side of an electromechanical element, motion having a component in the main motion direction as well as in the direction of the normal of the interaction surface of the object to be moved. The other side can either comprise electrodes without voltage differences or a volume without electrodes. By having two such elements, with transversally directed movement paths, a phase shifted operation can give rise to a controlled motion where the contact with the object to be moved is alternating between the two elements. By a proper design of the transfer properties each element can be in contact during movement in the main motion direction but not during the return motion. A stepwise motion is thus achieved.

Figs. 5A-C are schematic illustrations of further embodiments of different drive pads. In Fig. 5A, at least a part of said drive pad is resilient. This is here achieved by providing a slit 67 between the second end 47 of the actuator element 40 and the outer part of the drive pad 60. This resilience is typically of benefit when operating at high frequencies and at resonances. In Fig. 5B, the drive pad 60 is instead divided into two parts. An edge part 68 built by a material that is inherently somewhat resilient, and a centre part 69, made by a stiffer material. The stiffness of the drive pad is typically of benefit when high-precision positioning is to be performed. In other words, the drive pad is resilient in an end thereof that is situated at the largest distance d from the centre line and stiff in a vicinity of the centre line 49. In order to increase the lifting action of the actuator element during the bending resonance, the distance d could be increased. This is show in Fig. 5C, where the drive pad 60 protrudes outside the side of the actuator element 40 in the main motion direction 9 or opposite thereto.

During high-speed operation, also the interaction surface of the object to be moved will be exposed for wear. Such wear may also influence any subsequent fine-positioning operation. By designing the drive pads properly, such effects can be at least partly mitigated. In Fig 5D a schematic illustration of an embodiment of a drive pad 60 for reduced wear is shown from above and from the side as well as the interaction surface 22 of the object 20 to be moved. The edges 65 between the side 64 and the contacting surface 62 are used when operating at high frequencies and at resonances. An edge 63 in the middle of the drive pad, in the direction perpendicular to the main motion direction 9, is instead provided at a shorter distance from the centre line 49 and this part of the drive pad will then not participate during high-speed operation. The benefit of the drive pad 60 in Fig 5D is that wear on the interaction surface 22 of the moving object is concentrated at two areas 23 corresponding to the contact between the interaction surface 22 of the moving object 20 and the edges 65 of the drive pad 60 hence maintaining a central area 21 on the interaction surface 22 of the moving object 20 completely free of wear. This wear-free area guarantees a smooth surface of the interaction surface 22 of the moving object 20 to be used for high-precision positioning.

The contact edges 65 for high frequency operation can also be configured in other ways. An interchanging of the edges 65 and 63 can e.g. be useful, as illustrated in Fig 5E. In this case, the high-speed operation is performed against the middle part of the interaction surface 22 of the moving object 20.

In Fig. 5F, yet another embodiment of a drive pad 60 according to the same type of ideas is presented. Here, also the edge 66 used for fine-positioning driving is divided into sections, so that only a centre part will be active during fine positioning and then interact with the wear-free centre part 21 of the object 20. These ideas of utilizing different parts on the interaction surface 22 of the object 20 for different types of driving can be implemented by the geometrical shape of the drive pad in various other ways, by interleaving driving edges and non-driving edges, for high-speed operation and for fine-positioning operation, in a direction perpendicular to the main motion direction in a suitable manner. In other words, an edge of an end of the drive pad 60 that is situated at the largest distance d from the centre line 49 extends less than a width, in a direction parallel to the interaction surface and perpendicular to the main motion direction 9, of the drive pad 60.

Fig. 6A is a schematic illustration of an embodiment of another electromechanical motor 10. This embodiment of an electromechanical motor 10 comprises a multitude of actuator elements 30 positioned after each other in the main motion direction 9. Using a multitude of actuator elements makes the risk for accidental contact of the object with any inactive drive pad becomes very small. These actuator elements are preferably placed as far away from each other as possible. In the most simple embodiment with more than one actuator element (not shown), having only two electromechanically actuators, it is preferable to arrange the electromechanically actuators well separated from each other.

The use of actuator elements on both sides of the object to be moved also improves the stability, i.e. in a yoke arrangement, see e.g. Fig. 6B. The actuator elements 40 on the opposite side of the object 20 to be moved will then typically work in the same phase, i.e. actuator elements 40 aligned along the centre line 49 will make contact with the object 20 to be moved more or less at the same time. However, for certain applications in other embodiments, it can sometimes also be convenient to operate the actuator elements 40 out of phase.

In an embodiment, such as in Fig. 6A, having a relatively large number of actuator elements, it may be an advantage to provide the control electronics to be arranged for providing the second electrical signals to the electrodes of the respective actuator elements phase shifted relative each other. The object to be moved will in such an embodiment be kept on a relatively constant height, since there always will be some actuator element in contact or almost in contact with the object. The driving edges will be out of phase from each other, e.g. as illustrated by the dots showing the position of the drive pad edge 65 at a given time and the curve 109 in Fig. 6A. The position in the direction 8 of the actuator element length at this given time differs between the different actuator elements.

Figs. 7A-C are schematic illustrations of a further embodiment of an electromechanical motor 10. The actuator element 40 is here attached to a support structure 50, comprising a beam 51 attached rigidly to the stator backbone 70. The beam 51 is relatively rigid, providing a solid support when relatively slow movements of the actuator element are performed. Likewise, the beam is relatively stiff against rotational movements, such as induced by tilting the actuator element. However, the beam 51 is resilient in a direction 8 of the actuator element length.

If the mechanical support of a single actuator element with a flat drive pad is made more flexible it is possible to utilize also the resonance of the mechanical support to amplify the longitudinal vibrations. As an example is thus the actuator element of Figs. 7A-C. There will in such a case be at least two different driving modes that can be controlled independently. If the actuator element is a bimorph it will be possible to generate tilting movements of the drive pad, i.e. movements in the driving directions, without activating any longitudinal mode, as illustrated in Fig. 7B. On the other hand, if the whole actuator element is contracted or elongated in the longitudinal direction, i.e. the direction of the actuator element length, it is possible to generate a longitudinal vibration where the actuator element 40 and beam 51 vibrate together, see Fig. 7C. With different phase shifts between the both phases of a bimorph element it will be possible to control both longitudinal and bending vibrations at the same time. The drive pad can be moved along an elliptical trajectory or simply move linearly in a direction in between the longitudinal and driving direction. In the latter case the edges of flat drive pads will be used for high velocity resonant driving and the centre portion of the drive pad will have the same surface roughness as it had initially. High precision movement can hence be made in the same way as was described previously.

In other words, the control electronics is arranged for providing the second electrical signals to the electrodes for causing a contraction and expansion of the actuator element in the direction of the actuator element length with a same frequency as the bending mode resonance frequency, superimposed on the bending mode resonance but phase shifted therefrom. The support structure 50 is adapted to give an assembly consisting of the actuator element 40 and the support structure 50 a resonance frequency in the vicinity of the bending mode resonance frequency for a motion along the direction 8 of the actuator element length.

In principle it will be possible to build a positioning motor with only one actuator element. In one embodiment, the actuator element has a drive pad which is thicker at both drive element edges. It is possible to use also a drive pad which has equal thickness over the whole drive element. To drive in one direction the actuator element is first tilted to one side and then vibrated with amplitudes that only allows contact with the drive pad side that is extending furthest in the longitudinal direction, i.e. the edge of the drive pad which is in contact with the object when the actuator element is tilted but not vibrated. To drive in the other direction the actuator element is first tilted to the other direction and then vibrated. As can be understood, this mechanism will only work if both of the drive pad edges are aligned more or less parallel with the object contacting surface. Typically, the actuator element has to be mounted in the motor housing with some means that allow for self-alignment since in many cases the external forces or torques may rotate the object in relation to the initial alignment of the drive pad contact edges. The simplest solution is provision of a plastic film, presented further below.

To achieve long-range nanometer positioning with a drive pad that is thicker at element edges it is necessary to utilize either stick-slip positioning or high speed retraction of the drive pad from the object. Fine short-range nanometer positioning will typically only be made with small angle tilting to avoid using the worn edges of the drive pad. The wear of the drive pad will be confined to the edges and as long as there is not too extensive wear the central part of the drive pad will keep the initial roughness. Stick- slip positioning is made by slow speed movement of the actuator element in the driving direction and when the maximum tilting range has been reached the actuator element will move fast in the opposite direction. This movement should be fast enough to make the drive pad slip from the object contacting surface and a new short-range high resolution movement can be made relative this new position.

The high speed retraction method is similar. Short-range positioning is made by slow speed movement and when the maximum tilting range has been reached the actuator element is retracted in the longitudinal direction at the same time as it is retracted in the driving direction. This will result in a small step and a new short-range high resolution movement can be made relative this new position. There are of course intermediate methods between stick-slip and high speed retraction.

Piezoelectric motors with positioning capacity in the nanometer range will typically need a planarity of the drive pads in the sub-micrometer range. This calls for advanced lapping procedures which both increases cost and adds complex demands on the assembling and design of the motor unit. To reduce the size of motors and simplify the motor production process a self- alignment solution is here proposed. This is of particular interest when the external application controls the alignment of the object relative the motor housing and even further so if only one stator unit is used. The motor will not operate if the two drive pads are not aligned with the object contacting surface and in these examples self-alignment is more or less necessary. The use of plastic intermediate layers between the stiff stator backing and the motor housing has been explored and it has been concluded that this solution is sufficient in most applications where both nanometer precision and high speed resonant driving is needed. This is e.g. illustrated in Fig. 1, where the support structure 50 comprises a plastic intermediate layer 54 provided between a stiff stator backing 52 and the motor housing 70.

The plastic will deform due to the normal force and the drive pad surfaces will align against the object thereby eliminating the need for an exact planarity between stator pairs. Typically, the motor housing can be heated to accelerate deformation or cure the plastic or adhesive for the self-alignment. To achieve faster alignment the drive element can be operated at high voltage and high frequency to generate sufficient heat to make the plastic film deform faster. The performance is further improved if the backing or motor housing has a ridge in the center either integrated or simply as a separate object to avoid complete compression of the plastic film. Most thermosetting adhesives will work as a plastic self-aligning film and the yield stress of the plastic material has to be dimensioned in relation to the torque that will be applied to the object to be moved. Typically, when an external linear bearing is used the self-alignment will be made by assembling the motor unit and attaching the object to the linear bearing and let the plastic film deform until the external torque on the drive rod is minimized.

To maintain resonant operation even when loads, normal forces and temperatures are changing it is convenient to feed back sensor information from the actuator element itself. In most of the designs described there will be at least one part of the actuator element that is not used for driving and this part can be used as a piezoelectric sensor for the vibration amplitude. When an electromechanically active material is exposed for a force and is deformed, an electrical field will appear over the volume. If electrodes are present in the volume, these changes in the electrical field will result in voltage differences between different electrodes. By measuring such voltages on the electrodes, information about the mechanical behavior of the volume can be achieved. Consequently, by arranging the connection of the electrodes in such a way that such voltages can be measured, the actuator element can be utilized as a piezoelectric sensor.

Fig. 8 is a flow diagram of steps of an embodiment of a driving method. The method for driving an electromechanical motor starts in step 200. In step 210, first electrical signals are provided to electrodes in an actuator element of an electromechanical actuator for exciting volumes of electromechanically active material in the actuator element. The actuator element are attached by only a first end to a support structure, and has a drive pad attached to a second end opposite of the first end. The drive pad is arranged for interaction with an interaction surface of an object to be moved in a main motion direction, parallel to the interaction surface. An actuator element length is defined to be the distance between the first end and the second end along a centre line that is perpendicular to the interaction surface. At least a part of the contacting surface of the drive pad is provided at at least a distance d from the centre line in the main motion direction. The distance d is 0.1 micrometer times a ratio between the actuator element length and the stroke. The first electrical signals are configured for causing the actuator element to move the drive pad in a non-resonant two-dimensional path relative to the support structure. In step 220, second electrical signals are provided to the electrodes. The second electrical signals are configured for causing the electromechanical actuator to move in a bending mode mechanical resonance. The bending mode mechanical resonance has a stroke in the main motion direction. The embodiments described above are to be understood as a few illustrative examples of the present invention. It will be understood by those skilled in the art that various modifications, combinations and changes may be made to the embodiments without departing from the scope of the present invention. In particular, different part solutions in the different embodiments can be combined in other configurations, where technically possible. The scope of the present invention is, however, defined by the appended claims.

REFERENCES

US 6,066,911

US 6,337,532

US 6,798, 1 17

US 7,420,321

Claims

1. An electromechanical motor (10), comprising:

an object (20) to be moved in a main motion direction (9), having an interaction surface (22) parallel to said main motion direction (9);

at least one electromechanical actuator (30), having at least one actuator element (40) and a support structure (50);

said actuator element (40) having volumes of electromechanically active material (42, 43) and electrodes (44A-F, 45A-F) for exciting said volumes of electromechanically active material (42, 43);

said actuator element (40) being attached by only a first end (46) to said support structure (50);

said actuator element (40) having a drive pad (60) attached to a second end (77) opposite of said first end (46);

an actuator element length (L) being the distance between said first end (46) and said second end (47) along a centre line (49) that is perpendicular to said interaction surface (22);

a stator backbone (70), to which said support structure (50) of said electromechanical actuator (30) being attached;

a spring arrangement (80), arranged for providing a normal force (N) between said object (20) to be moved and said electromechanical actuator (30);

a control electronics unit (90) electrically connected to said electromechanical actuator (30) for enabling provision of electrical signals to said electrodes (44A-F, 45A-F);

said drive pad (60) being arranged with a contacting surface (62) for mechanically interacting with said interaction surface (22) of said object (20) to be moved for causing a motion of said object (20) to be moved;

said control electronics (90) being arranged for providing first electrical signals to said electrodes (44A-F, 45A-F), said first electrical signals to said electrodes (44A-F, 45A-F) causing said actuator element (40) to move said drive pad (60) in a non-resonant two-dimensional path relative to said support structure (50); said control electronics (90) being further arranged for providing second electrical signals to said electrodes (44A-F, 45A-F), said second electrical signals to said electrodes (44A-F, 45A-F) causing said electromechanical actuator (30) to move in a bending mode mechanical resonance, said bending mode mechanical resonance having a stroke in said main motion direction (9);

at least a part of said contacting surface (62) of said drive pad (60) being provided at at least a distance (d) from said centre line (49) in said main motion direction (9), said distance (d) being 0.1 micrometer times a ratio between said actuator element length (L) and said stroke.

2. The electromechanical motor according to claim 1, characterized in that at least a part of said contacting surface (62) is provided in the vicinity of the centre line (49) .

3. The electromechanical motor according to claim 1 or 2, characterized in that said drive pad (60) is asymmetric with respect to a plane (48) that is perpendicular to said main motion direction (9) and passes through said centre line (49) .

4. The electromechanical motor according to any of the claims 1 to 3, characterized in that said drive pad (60) protrudes outside said actuator element (40) in said main motion direction (9).

5. The electromechanical motor according to any of the claims 1 to 4, characterized in that at least a part of said drive pad (60) is resilient.

6. The electromechanical motor according to claim 5, characterized in that said drive pad (60) is resilient in an end (64) thereof being situated at the largest distance from said centre line (49) and stiff in a vicinity of said centre line (49).

7. The electromechanical motor according to any of the claims 1 to 6, characterized in that at least a part (51) of said support structure (50) is resilient in a direction (8) of said actuator element length (L) .

8. The electromechanical motor according to claim 7, characterized in that said control electronics (90) is arranged for providing said second electrical signals to said electrodes (44A-F, 45A-F) for causing a contraction and expansion of said actuator element (40) in said direction of said actuator element length (L) with a same frequency as said bending mode resonance frequency, superimposed on said bending mode resonance but phase shifted therefrom.

9. The electromechanical motor according to claim 8, characterized in that said support structure (51) is adapted to give an assembly consisting of the actuator element (40) and the support structure (51) a resonance frequency in the vicinity of said bending mode resonance frequency for a motion along said direction of said actuator element length (L) .

10. The electromechanical motor according to any of the claims 1 to 9, characterized in that an edge of an end of said drive pad (60) being situated at the largest distance from said centre line (49) extends less than a width, in a direction perpendicular to said main motion direction (9) and parallel to said interaction surface (22), of said drive pad (60).

11. The electromechanical motor according to any of the claims 1 to 10, characterized in that said electromechanical actuator (30) has at least two actuator elements (40) positioned after each other in said main motion direction (9).

12. The electromechanical motor according to claim 1 1, characterized in that said drive pad (60) of each of said actuator elements (40) is asymmetric with respect to a plane (48) that is perpendicular to said main motion direction (9) and passes through said centre line (49) and in that an end of one drive pad being situated at the largest distance from the respective centre line (49) is facing an end of the other drive pad being situated at the largest distance from the respective centre line (49) .

13. The electromechanical motor according to any of the claims 1 to 12, characterized in that said electromechanical motor (10) comprises a multitude of actuator elements (40) positioned after each other in said main motion direction (9), and in that said control electronics (90) being arranged for providing said second electrical signals to said electrodes of the respective actuator elements (40) phase shifted relative each other.

14. A method for driving an electromechanical motor, comprising the steps of:

providing (210) first electrical signals to electrodes in an actuator element of an electromechanical actuator for exciting volumes of electromechanically active material in said actuator element;

said actuator element being attached by only a first end to a support structure, and having a drive pad attached to a second end opposite of said first end, said drive pad being arranged for interaction with an interaction surface of an object to be moved in a main motion direction, parallel to said interaction surface;

an actuator element length being the distance between said first end and said second end along a centre line that is perpendicular to said interaction surface;

at least a part of said contacting surface of said drive pad being provided at at least a distance d from said centre line in said main motion direction, said distance d being 0. 1 micrometer times a ratio between said actuator element length and said stroke;

said first electrical signals being configured for causing said actuator element to move said drive pad in a non-resonant two-dimensional path relative to said support structure; and

providing (220) second electrical signals to said electrodes, said second electrical signals configured for causing said electromechanical actuator to move in a bending mode mechanical resonance, said bending mode mechanical resonance having a stroke in said main motion direction.