WO2012041370A1

WO2012041370A1 - Electromechanical actuator

Info

Publication number: WO2012041370A1
Application number: PCT/EP2010/064452
Authority: WO
Inventors: Mats Bexell; Per Oskar Lithell
Original assignee: Piezomotor Uppsala Ab
Priority date: 2010-09-29
Filing date: 2010-09-29
Publication date: 2012-04-05
Also published as: DE112010005916T5; DE112010005916B4

Abstract

An electromechanical actuator device (10) comprises a drive actuator (20), a holding actuator (30), an object (50) to be moved, a support piece (40) and biasing means (60). The object (50) to be moved has an interaction surface (52) parallel to a main motion direction (2). The support piece (40) is mechanically attached the drive actuator (20) and the holding actuator (30). The biasing means (60) is arranged for providing a normal force (68). Both actuators (20, 30) have respective volumes of electromechanically active material (24, 34) and respective electrode arrangements (26, 36) configured for enabling activation. The drive actuator (20) is excitable to move a driving interaction portion (25) in two-dimensional paths parallel to the main motion direction (2) and transverse (4) to the interaction surface (52). The holding actuator (30) is excitable to move a holding interaction portion (35) in a direction perpendicular to the main motion direction (2) and transverse (4) to the interaction surface (52). The holding actuator (30) is wider than the drive actuator (20) in the main motion direction (2).

Description

ELECTROMECHANICAL ACTUATOR

TECHNICAL FIELD

The present invention relates in general to electromechanical actuators, and in particular to 5 electromechanical actuators generating a motion by repetition of small steps.

BACKGROUND

Electromechanical actuators, and particularly electromechanical motors, have been widely applied for many different tasks during recent years. High force, small size, high speed, high-precision positioning0 and inexpensive manufacturing are attractive characteristics of many of the prior-art motors. However, the attractive characteristics are often contradictory, and optimizing regarding one aspect often reduces other qualities.

Electromechanical actuator arrangements using a set of drive elements presenting a two-dimensional5 motion have been discussed for a while. In US 6,066,911 , stacks of piezoelectric layers are formed side by side on a common piezoelectric base. This drive element was intended to be driven in the ultrasonic frequency range, and thereby benefit from high speed and high power efficiency. In US 6,337,532, a similar basic approach is used, but the operation is intended for a non-resonant walking operation. The excitation of the piezo-legs is performed in a very controlled manner, giving a smooth o motion and a very accurate positioning. However, the operation frequencies are far below resonance.

A similar approach is disclosed in US 7,067,958. In one of the embodiments presented therein, the idea of using one drive actuator and one holding actuator is presented in order to reduce the heat generation in the volumes of the active electromechanical material. The tip of the drive actuator is 5 movable along a two-dimensional path relative the common support. The tip of the holding actuator(s) is instead only movable linearly perpendicular to the intended motion direction. In short, the drive actuator comes into mechanical interaction with an interaction surface of an object to be moved and applies. By moving the tip of the drive actuator in the intended motion direction, a force is applied on the object, which essentially follows the tip. When the drive actuator is going to return to its original o position, the object is allowed to rest on the holding actuator, while the drive actuator releases its mechanical contact with the object. In this way, a stepwise motion is created by use of only one two- dimensionally movable actuator. The holding actuators are intended to be more or less stationary during the driving, which means that very little heat is produced by these. The main motion is supposed to be used for giving a smooth transfer of the mechanical contact of the object to or from the drive actuator. To this end, the drive actuator and the holding actuator are given essentially the same velocities in the Z direction during the transfer period. The dimensions of the holding actuator(s) were also kept small in order to reduce the volume of the heat generating electromechanical material. This main driving principle has proved to be operating well in most respects. In certain applications, however, minor drawbacks are noted. In applications, where movements of the object are separated by very long periods of inactivity, it has been found that the position of the object could experience very small but indeed displacements also during the inactivity periods. This was particularly noticeable when heave objects or large normal forces were applied.

SUMMARY

An object of the present invention is to improve stability and accuracy of electromechanical actuators having a single set of drive actuators. This object is achieved by electromechanical actuator devices and methods according to the enclosed independent claims. Preferred embodiments are defined in dependent claims. In general words, in a first aspect, an electromechanical actuator device comprises at least one drive actuator, at least one holding actuator, an object to be moved, a support piece and biasing means. The object to be moved has an interaction surface parallel to a main motion direction. The support piece is mechanically attached to a first end of the drive actuator and a first end of the holding actuator. The biasing means is arranged for providing a normal force between the support piece and the object to be moved. Both the drive actuator and the holding actuator have respective volumes of electromechanically active material and respective electrode arrangements configured for enabling activation of respective electromechanically active material. The drive actuator has a drive interaction portion at a second end of the drive actuator. The second end of the drive actuator is opposite to the first end of the drive actuator. The drive interaction portion is intended for providing driving forces on the interaction surface of the object to be moved. The volumes of electromechanically active material and electrode arrangement of the drive actuator are configured to enable a motion of the second end of the drive actuator relative the first end of the drive actuator in two-dimensional paths parallel to the main motion direction and transverse to the interaction surface of the object to be moved. The holding actuator has a holding interaction portion at a second end of the holding actuator. The second end of the holding actuator is opposite to the first end of the holding actuator. The holding interaction portion is intended for providing contact with the interaction surface of the object to be moved. The volumes of electromechanically active material and electrode arrangement of the holding actuator are configured to enable a motion of the second end of the holding actuator relative the first end of the holding actuator in a direction perpendicular to the main motion direction and transverse to the interaction surface of the object to be moved. A largest distance in the main motion direction between any parts of the volumes of electromechanically active material of the holding actuator is larger than a largest distance in the main motion direction between any parts of the volumes of electromechanically active material of the drive actuator.

In a second aspect, a method for driving an electromechanical actuator device is presented. The electromechanical actuator device is of a type having at least one first actuator, in turn having volumes of electromechanically active material, at least one second actuator, in turn having volumes of electromechanically active material, a support piece, and an object to be moved in a main motion direction. A largest distance in the main motion direction between any parts of the volumes of electromechanically active material of the second actuator is larger than a largest distance in the main motion direction between any parts of the volumes of electromechanically active material of the first actuator. The method comprises providing of a normal force between the support piece and the object to be moved perpendicular to an interaction surface of the object to be moved and perpendicular to the main motion direction of said object to be moved. The volumes of electromechanically active material of the first actuator are excited for causing a motion of a first interaction portion of the first actuator relative to a first end of the first actuator, by which the first actuator is attached to the support piece, in two-dimensional paths parallel to the main motion direction and transverse to the interaction surface of the object to be moved. The first interaction portion of the first actuator is provided at a second end of the first actuator opposite to the first end of the first actuator. The volumes of electromechanically active material of the second actuator are excited for causing a motion of a second interaction portion of the second actuator relative to a first end of the second actuator, by which the second actuator is attached to the support piece, in a direction perpendicular to the main motion direction and transverse to the interaction surface of the object to be moved. The second interaction portion of the second actuator is provided at a second end of the second actuator opposite to the first end of the second actuator.

One advantage of the present invention is that it provides an electromechanical actuator that is simple to operate and that presents an excellent stability also when not being operated.

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 actuator device according to the present invention;

FIGS. 2A-G are sketches and diagrams illustrating an embodiment of a motion principle possible to use with the present invention;

FIGS. 3A-B illustrate different embodiments of drive actuators excitable in two-dimensional paths;

FIGS. 4-9 are schematic illustrations of other embodiment of an electromechanical actuator device according to the present invention;

FIG. 10 is a schematic illustration of an embodiment of a drive interaction portion;

FIGS. 11-12 are schematic illustrations of yet other embodiments of an electromechanical actuator device according to the present invention;

FIGS. 13A-C are schematic illustrations of yet other embodiments of an electromechanical actuator device according to the present invention; and

FIG. 14 is a flow diagram of steps of an embodiment of a method according to the present invention.

DETAILED DESCRIPTION

Throughout the drawings, the same reference numbers are used for similar or corresponding elements. Fig. 1 illustrates an embodiment of an electromechanical actuator device 10 according to the present invention. The electromechanical actuator device 10 comprises a drive actuator 20 and a holding actuator 30. A first end 22 of the drive actuator 20 and a first end 32 of the holding actuator 30 are both mechanically attached to a support piece 40. The drive actuator 20 has a volume of electromechanically active material 24 and an electrode arrangement 26. The volume of electromechanically active material 24 and the electrode arrangement 26 of the drive actuator 20 are configured for enabling activation of the volume of electromechanically active material 24 of the drive actuator 20. In the present embodiment, the drive actuator 20 comprises a bimorph 29 structure of the volume of electromechanically active material 24 of the drive actuator 20. The volume of electromechanically active material 24 of the drive actuator 20 has two parts that can be excited independently of each other. This is in the present embodiment realized by having the electrodes of each part activated independently of each other by two terminating conductors 23, 27 being electrically connected to respective electrodes in each part. Ground electrodes and terminating conductors thereto are in this embodiment provided at the back side of the electromechanical actuator device 10. By such an arrangement, a second end 28 of the drive actuator 20 can be moved in two- dimensional paths in a plane that is parallel to an intended main motion direction 2 and extended along the drive actuator 20. The holding actuator 30 has similarly a volume of electromechanically active material 34 and an electrode arrangement 36. The volume of electromechanically active material 34 and the electrode arrangement 36 of the holding actuator 30 are analogously configured for enabling activation of the volume of electromechanically active material 34 of the holding actuator 30. In the present embodiment, the holding actuator 30 is a single stack of electromechanically active material slices and electrodes. By supplying a voltage difference between the electrodes, the electromechanically active material can be made to expand or contract in the direction of the extension of the holding actuator 30. This is in the present embodiment realized by having electrodes activated by a terminating conductor 33. Ground electrodes and terminating conductors thereto are in this embodiment provided at the back side of the electromechanical actuator device 10. By such an arrangement, a second end 38 of the holding actuator 30 can be moved linearly in a direction that is perpendicular to the intended main motion direction 2 and extended along the holding actuator 30,

The electromechanical actuator device 10 of course also comprises an object 50 to be moved in the main motion direction 2. The object 50 has an interaction surface 52 which is parallel to the main motion direction 2. A biasing means 60 is configured to provide a normal force 68 between the support piece 40 and the object 50 to be moved. In the present embodiment, the biasing means 60 comprises a roller 62 and a spring 64, connecting the roller 62 with a point 66 at the support piece 40. This ensures that the object 50 to be moved is held against the actuators of the electromechanical actuator device 10.

The drive actuator 20 has a drive interaction portion 25 provided at the second end 28 of the drive actuator 20. The second end 28 is opposite to the first end 22 of the drive actuator 20. The drive interaction portion 25 thus points away from the support piece 40. The drive interaction portion 25 is intended for providing driving forces on the interaction surface 52 of the object 50 to be moved. The volumes of electromechanically active material 24 of the drive actuator 20 and the electrode arrangement 26 of the drive actuator 20 are configured to enable a motion of the second end 28 of the drive actuator 20 relative the first end 22 of the drive actuator. This means that the drive interaction portion 25 is moved and can be used e.g. to apply a driving force onto the interaction surface 52 of the object 50 to be moved. The motion of the second end 28 of the drive actuator 20 can take place in two- dimensional paths that are situated in a plane that is parallel to the main motion direction 2 and transverse to the interaction surface 52 of the object 50 to be moved. Such transverse direction is indicated by the arrow 4. In other words, the drive actuator 20 is of a kind that can move its tip along the main motion direction 2 as well as transverse to the interaction surface 52, and can thus serve as a driving actuator against the object 50 to be moved. Example of such motion paths will be discussed more in detail further below.

The holding actuator 30 has a holding interaction portion 35. The holding interaction portion 35 is situated at a second end 38 of the holding actuator 30, opposite to the first end 32 of the holding actuator 30. The holding interaction portion 35 thus points away from support piece 40, in analogy with the driving interaction portion 25. The holding interaction portion 35 is intended for providing contact with the interaction surface 52 of the object 50 to be moved. However, unlike the driving interaction portion 25, the holding interaction portion 35 is not intended to apply any driving forces onto the interaction surface 52. The volumes of electromechanically active material 34 of the holding actuator 30 and the electrode arrangement 36 of the holding actuator 20 are configured to enable a motion of the second end 38 of the holding actuator 30 relative the first end 32 of the holding actuator 30. This movement is directed in a direction perpendicular to the main motion direction 2 and transverse 4 to the interaction surface 52 of the object 50 to be moved. The holding interaction portion 35 can thereby be removed from the interaction surface 52 or brought into mechanical contact with the interaction surface 52, but is primarily not intended to impose any forces in the direction of the main motion direction 2.

The voltages that are needed for exciting the volumes of electromechanically active material 24, 34 of the drive actuator 20 and the holding actuator 30, respectively, are typically provided by a power supply 80 connected to the terminating conductors 23, 27, 33. The power supply 80 is configured to control the voltages according to certain voltage curves. The exact shape of such voltage curves depends on the actual geometrical configuration of the drive actuator 20 and the holding actuator 30 and on the requested motion of the object 50 to be moved. In the present embodiment, the electromechanical actuator device 10 comprises only one drive actuator 20 and only one holding actuator 30. In such an embodiment, the motion of the object 50 to be moved may be somewhat unstable, and typically some sort of bearing arrangement 70 is provided to define the path along which the object 50 to be moved is displaced. In applications, where the periods of motion are separated by long periods of inactivity, the object 50 to be moved rests against at least one of the drive actuator 20 and the holding actuator 30. If the position of the drive actuator 20 is different from a straight position, i.e. the drive actuator 20 is displaced sidewards, exposure for the normal force 68 of the object 50 to be moved, in particular if the normal force 68 is high and/or the object 50 to be moved is heavy, the drive actuator 20 might be deformed. Also small changes in voltage during the resting period may change the actual position of the object 50 to be moved. It is therefore beneficial if the object 50 to be moved rests against only the holding actuator 30 during longer periods of inactivity. In other words, when the object 50 to be moved has reached a position in which it will be kept for a longer time, the electromechanical actuator device 10 preferably is put into a parking state, where the normal force and weight of the object 50 to be moved is supported by the holding actuator 30. Since the holding actuator only is controllable along the direction of the normal force, the risk for deformations is lower. However, it has been found that even a holding actuator 30 may be deformed, in particular if the object 50 to be moved is exposed to force components in the main motion direction 2, and further in particular if the width of the holding actuator 30 in the main motion direction 2 is relatively small. Therefore, the holding actuator 30 is made wider than the drive actuator 20 in the main motion direction 2.

A largest distance 31 in the main motion direction 2 between any parts of the volumes of electromechanically active material 34 of the holding actuator 30 can be defined. This largest distance 31 is indicated by arrows in the figure. Similarly, a largest distance 21 in the main motion direction 2 between any parts of the volumes of electromechanically active material 24 of the drive actuator 20 can be defined. According to the present invention, the largest distance 31 of the holding actuator 30 is larger than the largest distance 21 of the drive actuator 20. Preferably, a ratio between the largest distance 31 in the main motion direction 2 between any parts of the volumes of electromechanically active material 34 of the holding actuator 30 and the largest distance 21 in said main motion direction 2 between any parts of the volumes of electromechanically active material 24 of the drive actuator 20 is equal or larger than 1.5 and even more preferable equal or larger than 2.

In embodiments like in Fig. 1 , where the holding actuator 30 comprises one single simple-shaped volume without cavities, the largest distance 31 of the holding actuator 30 becomes typically equal to the width of the holding actuator 30 in the main motion direction 2. Similarly, the he largest distance 21 of the drive actuator 20 typically corresponds to the width of the drive actuator 20 in the main motion direction 2. As will be seen further below, these distances may be more difficult to define for more complex shapes of the actuators. This is in the absolute opposite direction compared to prior art, where the holding actuators were made narrow in order to reduce the volume of electromechanically active material that could generate heat. It has been found that since the holding actuators 30 typically are moved very little, the total heat generation will typically not be very much influenced by the contribution from the holding actuators 30. Furthermore, in applications where only short periods of activity are to be expected, the heat issue is not very important at all.

A broad holding actuator 30 creates a stable support on which the object 50 to be moved can rest safely during inactivity periods. The drive actuator 20 may during parking of the object 50 to be moved be brought to a position without mechanical contact with the interaction surface 52, preferably also without any voltages applied over the electrode arrangement 26.

In Figs. 2A-G, one example of a motion, possible to achieve by e.g. the embodiment of Fig. 1 , is described a little bit more in detail, in order to increase the understanding of the basic motion principle. Note, that the geometrical shape changes of the drive actuator 20 and the holding actuator 30 are extremely exaggerated in the figure in order to visualize the basic principles.

In Fig. 2A, the drive actuator 20 is excited in such a way that the right part of the drive actuator 20 becomes longer. This results in that the entire drive actuator 20 is somewhat expanded and at the same time bends somewhat towards the holding actuator 30. The object 50 to be moved is in this situation only in contact with the drive actuator 20. From this position, the left part of the drive actuator is caused to expand, which cause the drive actuator 20 to bend in a direction away from the holding actuator 30, In Fig. 2B, the expansions of both parts of the drive actuator 20 are equal, and the drive actuator 20 is straight, but still alone supporting the object 50 to be moved. The motion of the drive actuator 20 is continued away from the holding actuator 30 be reducing the expansion of the right part of the drive actuator 20. In Fig. 2C, the drive actuator 20 has almost reached its extreme position of a right bending. At the same time as the bending, the drive actuator 20 also contracts, and it is soon time for the drive actuator 20 to hand over the contact with the object 50 to be moved to the holding actuator 30. In particular embodiments of the operation of the electromechanical actuator device 10, the holding actuator 30 can be actively involved in such handing-over. However, in a simplest operation embodiment, the holding actuator 30 may also be kept at a constant height. In Fig. 2D, the normal force from the object 50 to be moved is now supported by the holding actuator 30. The drive actuator 20 is released from the object 50 to be moved and can freely return to an original position for a next step of driving. This return passes e.g. the situation depicted in Fig. 2E, where the entire drive actuator 20 has its most contracted state. Finally, when the drive actuator 20 comes to the situation as in Fig. 2F, the drive actuator and possibly the holding actuator 30, have to prepare for handing over the contact with the object 50 to be moved to the drive actuator again. The process can then be repeated as required.

In Fig. 2G, a diagram summarizes the motion pattern of the embodiment of Figs. 2A-F. The drive actuator 20 moves along a rhombic path in the X-Z plane, e.g. in a plane parallel to the main motion direction and perpendicular to the interaction surface of the object to be moved. The letters A-F denotes the relations with the Figs. 2A-F. The holding actuator 30 may as indicated above even be stationary, but may also in a typical case be moved up and down in the Z direction, i.e. perpendicular to the interaction surface of the object to be moved. By these motion patterns, a motion of the object to be moved can be achieved with quite simple mechanical and electrical means.

However, it should be noted that the actual detailed motion pattern has a small impact of the basic features of the present invention. Examples of other types of motion patterns of the driving actuator that are useful ion connection with the present invention are motion along other types of polygons, such as e.g. hexagons. Also smooth curved trajectories of the driving actuator are useful in some applications. However, in order to maintain a long stepping length, such solutions typically require somewhat more sophisticated electronics. In the basic version of a motion pattern useful together with the present invention, the holding actuator is passive in respect of creating a motion in the intended drive direction. However, the holding actuators may in more complex configurations also be provided with possibilities to move somewhat along the main motion direction, Such motions may be used e.g. for fine adjustments of a final position. In extreme cases that motion may even contribute to the actual driving, however, typically with a small amount, since a wide actuator typically has a smaller bending stroke than a narrow one. The most important contribution of the holding actuator to the overall operation is however, the motion perpendicular to the main motion direction.

In the embodiment of Fig. 1 , the drive actuator 20 is a bimorph structure, which also is the embodiment that at the moment is believed is the most preferable. However, it is also possible to achieve corresponding functions of the drive actuator 20 by other arrangements. In Fig. 3A, the drive actuator 20 is built from two part actuators. A shear actuator 82 is attached on top of a linear actuator 81 , which in turn is attached to the support piece 40. This structure of a combined actuator is known as such in prior art, see e.g. the U.S. patent 4,928,030. The shear actuator 82 gives a displacement in the main motion direction of the upper surface relative to the lower surface when being excited. In other words, the shear actuator 82 provides a driving force in the man motion direction. The linear actuator 81 is then responsible for the motion perpendicular to the main motion direction. A combination of these to motions can be combined to give a total two-dimension motion in a plane.

In Fig. 3B, another embodiment of a possible drive actuator 20 is illustrated. Here, two linear actuators 81 are arranged on top of each other and on top of the support piece 40. The linear actuators' are arranged in a tilted direction so that an expansion or contraction of the linear actuators 81 will result in motions along respective directions inclined with respect to the main motion direction. Also a combination of such movements can give rise to a two-dimensional motion path useful for a drive actuator 20. In Fig. 3C, yet another embodiment of a possible drive actuator 20 is illustrated. In this embodiment, the drive actuator 20 comprises two bimorph 29A, 29B structures of the volumes of electromechanically active material 24. The bimorphs 29A, 29B are attached one on top of the other. The electrodes are connected in a crossed fashion, which means that the electrodes of the left half of the upper bimorph are provided with the same voltages as the electrodes of the right half of the lower bimorph and vice versa. This means that the two bimorphs 29A, 29B always are bending in opposite direction, which will ensure that the driving interaction portion 25 always will be directed in the same angle with respect to the body to be moved. This is typically reducing wear of the drive actuator since the relative motion between the body to be moved and the drive actuator during the driving part of the motion will be negligible. Also, since the interaction surface of the body to be moved always will be parallel to the driving interaction portion 25, the maximum surface pressure on the drive actuator 20 will also be lower than for a single bimorph actuator.

In the embodiments illustrating actuators in detail, the electrodes have been illustrated as being positioned parallel to the interaction surface of the body to be moved. As is well know as such in prior art, also other configurations of the electrodes are possible, e.g. perpendicular to the interaction surface, either parallel or perpendicular to the main motion direction.

Fig. 4 illustrates another embodiment of an electromechanical actuator device 10 according to the present invention. For increasing the readability of the figure, the object to be moved and the biasing means are omitted. In this embodiment, the electromechanical actuator device 10 comprises more than one holding actuator 30, in this particular embodiment two holding actuators 30. The drive actuator 20 and the holding actuators 30 are in the present embodiment positioned after each other in the main motion direction 2. Such a configuration increase the total stiffness in the main motion direction 2 in situations where the object to be moved is supported by the holding actuators 30. In the present embodiment, the support piece 40 protrudes with protrusions 41 outside on both sides of the outermost holding actuators 30 in the main motion direction. This configuration further increases the stiffness of the attachment of the holding actuators 30, and makes the attachment more symmetric, which typically increases the predictability of the operation. Also in embodiment where drive actuators 20 are positioned at one or both ends, such protrusions 41 are of benefit. In order to achieve an even higher stiffness, the support piece 40 comprises a material that is stiffer than the electromechanically active material 24, 34 of the drive actuator 20 and the holding actuator 30, respectively. This provides for a more controlled and predictable operation of the electromechanical actuator device 10. This can be achieved e.g. by gluing, soldering or by any other method attaching separate actuators on a common piece. However, it is also possible to manufacture the entire assembly of the holding actuator 30, the drive actuator 20 and the support piece 40 in one and the same common production process.

Fig. 5 illustrates yet another embodiment of an electromechanical actuator device 10 according to the present invention. In this embodiment, the electromechanical actuator device 10 comprises one holding actuator 30 and two drive actuators 20. As anyone skilled in the art understands, any number of drive actuators and holding actuators can be configured according to the principles of the present invention. When more than one drive actuator 20 is used, the need for external bearing arrangements is reduced and may even be totally omitted. The stiffness of the holding actuator is of high importance for the present ideas. The stiffness is to a large extent dependent on the amount of material in the main motion direction, typically the maximum dimension of the holding actuator 30. However, inclusions of hollow volumes within the holding actuators 30 will only reduce the stiffness somewhat compared to fully solid holding actuators 30, at least concerning the stiffness in the main motion direction. In Fig. 6, another embodiment of an electromechanical actuator device 10 is illustrated, in which the holding actuators 30 comprises two part volumes 83 of electromechanically active material 34. The upper parts of the part volumes 83 are mechanically connected by a bridge 84, typically a common holding interaction portion 35. A hollow volume 85 is thereby created. In this manner, the holding actuator 30 becomes relatively stiff without use of any large amount of electromechanically active material 34. However, the largest distance 31 in the main motion direction 2 between any parts of the volumes of electromechanically active material 34 of the holding actuator 30 is still larger than the width of the drive actuator 20. In other words, the width of the electromechanically active material 34 that is connected by a common holding interaction portion 35 is larger than the width of the electromechanically active material 24 that is connected by a common actuating interaction portion 25.

These ideas are further utilized in the embodiment of Fig. 7. Here, the holding actuator 30 also presents a hollow volume 85. The drive actuator 20 is in this embodiment positioned within the hollow volume 85 of the holding actuator 30. This particular embodiment provides an extremely compact design with high stiffness in the main motion direction. The holding interaction portion 35 is provided with an opening 86, through which the driving interaction portion 25 can protrude. The set-up is also symmetric which typically simplifies the operation characteristics. The largest distance 31 in the main motion direction 2 between any parts of the volumes of electromechanically active material 34 of the holding actuator 30 is here also larger than the largest distance 21 in the main motion direction 2 between any parts of the volumes of electromechanically active material 24 of the drive actuator 20.

Fig. 8 illustrates an embodiment of an electromechanical actuator device 10 designed for extremely good parking stability. In this embodiment, the electromechanical actuator device 10 comprises two drive actuators 20 and two holding actuators 30A, 30B. During operation, when the object to be moved is moved, only the holding actuator 30A that is positioned between the drive actuators 20 comes into interaction with the object to be moved. The holding actuator 30B is only used during longer inactivity periods, when the object to be moved should be kept at a stationary position. It is an advantage if the holding actuator 30B is situated close to a position for any sensors for measuring the movements. This minimizes unwanted movements, such as e.g. caused by temperature changes. The two holding actuators also present a high stiffness in the direction of the normal force.

During long inactivity periods, it is typically required that the object to be moved should be kept stationary for a long time. Even if the energy consumption within the electromechanically active material (or rather conversion into heat and motion) becomes negligible, the standby power consumption of the power supply is still present. Furthermore, the electromechanical actuator device

10 becomes sensitive to disturbances in voltage or to power failures. It is therefore preferred if one can provide a parking set-up where all applied voltages can be set to zero. Fig. 9 illustrates an embodiment of an electromechanical actuator device 10 when being in a parking state. The dimensions of the drive actuator 20 and the holding actuator 30 are adapted such that the holding interaction portion 35 protrudes further in a direction perpendicular to the interaction surface 52 than the drive interaction portion 25 when respective volumes of electromechanically active material 24, 34 of the drive actuator 20 and the holding actuator 30 are non-activated, i.e. when all applied voltages are zero. In other words, when the drive actuator 20 and the holding actuator 30 are non-activated, a small gap 71 exists between the interaction surface 52 and the drive interaction portion 25 and the object 50 to be moved rests solely on the holding interaction portion 35. A zero voltage is less exposed for voltage drifts and the excitation state becomes insensitive to power failures.

The drive interaction portion 25 can also be designed in different ways depending on the requirements. Fig. 10 illustrates an embodiment of a drive interaction portion 25 having a central ridge 87. This drive interaction portion 25 will give a larger stroke for the motion and it will be stiffer in the direction of the normal force than a drive interaction portion without ridge 87. However, the drive interaction portion 25 will be less stiff in the main motion direction 2. By having an appropriate compromise between the two types of stiffness, the resulting stiffness can be sufficient and still give rise to an increased stepping length.

Also the shape of the holding interaction portion 35 is of importance. Fig. 11 illustrates an embodiment of an electromechanical actuator device 10 where the holding interaction portion 35 comprises two juts 88 separated with a distance. This embodiment ensures a well defined contact plane between the body to be moved and the electromechanical actuator device 10. A larger area of a surface can not be manufactured exactly plane without extreme efforts. This typically means that a body, resting on a relatively flat surface anyway only will have two or a few contact points. When the body is exposed for a load along the surface, the position of these contact points may change and a minor rotation of the body with respect to the surface may be the result. If the contact between the body and the surface instead is restricted from the beginning to two narrow juts 88, the relative position of the body and the holding interaction portion 35 will not change e.g. if the load conditions change. This becomes particular important during long periods of inactivity when the body is supposed to be stationary independent of e.g. shifting load conditions. In all embodiments discussed above, the drive actuator 20 and the holding actuator 30 are positioned after each other in the main motion direction 2. At the contrary, in Fig. 12, the drive actuator 20 and the holding actuators 30 are instead positioned side by side. The choice of configuration depends e.g. on the type of load that the object to be moved is exposed for, or simply on manufacturing considerations, e.g. concerning positions of terminations etc. Anyone skilled in the art realizes that the number of possible configurations is almost infinite.

Also the drive actuator 20 can be configured to obtain a relatively high stiffness while reducing the amount of active material, in an analogue manner to what was illustrated for the holding actuators earlier. In Fig. 13A, the drive actuator 20 is composed by two part actuators, each one configured according to the ideas presented in Fig. 3A, bridged together by a bridge 89, preferably constituted by the driving interaction portion 25. Since the top of each drive actuator part always is directed parallel to the main motion direction, they can be bridged together without influencing the motion pattern if both parts are driven in parallel. Such a set-up will use less total volume of electromechanically active material, but will maintain a relatively high stiffness. In Figs. 13B and 13C analogue embodiments are illustrated based on the drive actuator ideas of Figs. 3B and 3C, respectively.

These bridged drive actuators 20 can of course also be combined with bridged holding actuators e.g. according to the ideas illustrated in Figs. 6, 7 or 11.

In the embodiment of Fig. 1 , one example of a biasing means is illustrated. However, the detailed configuration of the biasing means is not of particular importance for the present invention as long as it provides a normal force between the support piece and the object to be moved. In other words, the biasing means is configured to keep the electromechanical actuator device together, insuring that at least one of the driving interaction portion and the holding interaction portion is pressed towards the interaction surface of the object to be moved. The biasing means could involve spring arrangements, as indicated in Fig. 1 , or spring arrangements of other types. However, other types of force generating principles can also be utilized, exemplified by but not limited to electrostatic forces, magnetic forces, electrodynamic forces etc. Almost any kind of normal force creating devices used in prior art can be used in connection with the present invention.

An electromechanical actuator device according to the present invention could also be configured in a yoke configuration, where a body to be moved in positioned between two sets of actuators. The general setup may e.g. look like the LT20 motor manufactured and commercialized by PiezoMotor

Uppsala AB. The biasing means then typically provides a normal force between the respective support piece, typically by springs or any other well known normal force creating device. The present invention has so far been described as an idea of an improved device. However, the ideas can also be seen as a novel method for driving an electromechanical actuator device. Fig. 14 illustrates steps of an embodiment of such a method according to the present invention. Such a method begins in step 200 and concerns the driving of an electromechanical actuator device having at least one first 5 actuator and at least one second actuator. The first actuator has volumes of electromechanically active material and the second actuator also has volumes of electromechanically active material. The concerned electromechanical actuator device also comprises a support piece and an object to be moved in a main motion direction. The largest distance in the main motion direction between any parts of the volumes of electromechanically active material of the second actuator is larger than a largest o distance in the main motion direction between any parts of the volumes of electromechanically active material of the first actuator. In step 210, a normal force is provided between the support piece and the object to be moved perpendicular to an interaction surface of the object to be moved and perpendicular to the main motion direction of the object to be moved. The volumes of electromechanically active material of the first actuator are excited in step 212 for causing a motion of a first interaction portion of5 the first actuator relative to a first end of the first actuator. The first actuator is attached to the support piece by this first end. The motion is performed in two-dimensional paths parallel to the main motion direction and transverse to the interaction surface of the object to be moved. The first interaction portion of the first actuator is provided at a second end of the first actuator opposite to the first end of the first actuator. In step 214, the volumes of electromechanically active material of the second o actuator are excited for causing a motion of a second interaction portion of the second actuator relative to a first end of the second actuator. The second actuator is attached to the support piece by that first end. The motion is performed in a direction perpendicular to the main motion direction and transverse to the interaction surface of the object to be moved. The second interaction portion of the second actuator is provided at a second end of the second actuator opposite to the first end of the second 5 actuator. The steps 212 and 214 are illustrated as they would occur sequentially. However, depending on the actual application, the steps 212 and 214 may occur at least partly simultaneously. One or both of the steps 212 and 214 may also be repeated. The procedure ends in step 299.

In the embodiments presented above, the electromechanical material is assumed to be a piezoelectric 0 material. However, the present invention can also be used for other types of electromechanical materials. Most actuator materials could be characterised as electromechanical materials, but in the present disclosure we intend electromechanical materials to be materials that change their shape when an electric voltage or current is applied. Typical examples of electromechanical materials according to the meaning in the present disclosure are piezoelectric, electrostrictive and antiferroelectric materials and these materials could be single crystalline as well as polycrystalline or amorphous.

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.

Claims

1. Electromechanical actuator device (10), comprising:

at least one drive actuator (20);

at least one holding actuator (30);

5 an object (50) to be moved, having an interaction surface (52) parallel to a main motion direction (2);

a support piece (40), mechanically attached to a first end (22) of said drive actuator (20) and a first end (32) of said holding actuator (30); and

biasing means (60), providing a normal force (68) between said support piece (40) and said o object (50) to be moved;

both said drive actuator (20) and said holding actuator (30) having respective volumes of electromechanically active material (24, 34) and respective electrode arrangements (26, 36) configured for enabling activation of respective said electromechanically active material (24, 34);

said drive actuator (20) having a drive interaction portion (25) at a second end (28) of said5 drive actuator (20), opposite to said first end (22) of said drive actuator (20), said drive interaction portion (25) being intended for providing driving forces on said interaction surface (52) of said object (50) to be moved;

said volumes of electromechanically active material (24) and electrode arrangement (26) of said drive actuator (20) being configured to enable a motion of said second end (28) of said drive 0 actuator (20) relative said first end (22) of said drive actuator (20) in two-dimensional paths parallel to said main motion direction (2) and transverse (4) to said interaction surface (52) of said object (50) to be moved;

said holding actuator (30) having a holding interaction portion (35) at a second end (38) of said holding actuator (30), opposite to said first end (32) of said holding actuator (30), said holding interaction portion (35) being intended for providing contact with said interaction surface (52) of said object (50) to be moved;

said volumes of electromechanically active material (34) and electrode arrangement (36) of said holding actuator (30) being configured to enable a motion of said second end (38) of said holding actuator (30) relative said first end (32) of said holding actuator (30) in a direction perpendicular to said main motion direction (2) and transverse (4) to said interaction surface (34) of said object (50) to be moved;

a largest distance (31) in said main motion direction (2) between any parts of said volumes of electromechanically active material (34) of said holding actuator (30) being larger than a largest distance (21) in said main motion direction (2) between any parts of said volumes of electromechanically active material (24) of said drive actuator (20).

2. Electromechanical actuator device according to claim 1 , characterized in that a ratio 5 between said largest distance (31) in said main motion direction (2) between any parts of said volumes of electromechanically active material (34) of said holding actuator (30) and said largest distance (21) in said main motion direction (2) between any parts of said volumes of electromechanically active material (24) of said drive actuator (20) is equal or larger than 1.5 and preferably equal or larger than 2. i o

3. Electromechanical actuator device according to claim 1 or 2, characterized in that said electromechanical actuator device (10) comprises more than one drive actuator (20).

4. Electromechanical actuator device according to any of the claims 1 to 3, characterized in that said drive actuator (20) comprises a bimorph structure of said volumes of electromechanically

15 active material (24) of said drive actuator (20).

5. Electromechanical actuator device according to any of the claims 1 to 4, characterized in that said electromechanical actuator device (10) comprises more than one holding actuator (30).

20 6. Electromechanical actuator device according to any of the claims 1 to 5, characterized in that said holding interaction portion (35) protrudes further in a direction perpendicular to said interaction surface (52) than said drive interaction portion (25) when respective volumes of electromechanically active material (24, 34) of said drive actuator (20) and said holding actuator (30) are non-activated.

25

7. Electromechanical actuator device according to any of the claims 1 to 6, characterized in that said support piece (40) comprises material that is stiffer than said electromechanically active material (24, 34) of said drive actuator (20) and said holding actuator (30).

30 8. Electromechanical actuator device according to any of the claims 1 to 7, characterized in that said support piece (40) protrudes outside on both sides of said drive actuator (20) and said holding actuator (30) in said main motion direction (2).

9. Electromechanical actuator device according to any of the claims 1 to 8, characterized in that said holding interaction portion (35) comprises two juts (88) separated with a distance.

10. Electromechanical actuator device according to any of the claims 1 to 9, characterized in 5 that said drive actuator (20) and said holding actuator (30) are positioned after each other in said main motion direction (2).

11. Electromechanical actuator device according to any of the claims 1 to 10, characterized in that said drive actuator (20) is positioned in a hollow volume (85) of said holding actuator (30).

0

12. Method for driving an electromechanical actuator device having at least one first actuator, in turn having volumes of electromechanically active material, at least one second actuator, in turn having volumes of electromechanically active material, a support piece, and an object to be moved in a main motion direction, a largest distance in said main motion direction between any parts of said volumes of5 electromechanically active material of said second actuator being larger than a largest distance in said main motion direction between any parts of said volumes of electromechanically active material of said first actuator, said method comprising the steps of:

providing (210) a normal force between said support piece and said object to be moved perpendicular to an interaction surface of said object to be moved and perpendicular to said main o motion direction of said object to be moved;

exciting (212) said volumes of electromechanically active material of said first actuator for causing a motion of a first interaction portion of said first actuator relative to a first end of said first actuator, by which said first actuator is attached to said support piece, in two-dimensional paths parallel to said main motion direction and transverse to said interaction surface of said object to be 5 moved, said first interaction portion of said first actuator being provided at a second end of said first actuator opposite to said first end of said first actuator; and

exciting (214) said volumes of electromechanically active material of said second actuator for causing a motion of a second interaction portion of said second actuator relative to a first end of said second actuator, by which said second actuator is attached to said support piece, in a direction o perpendicular to said main motion direction and transverse to said interaction surface of said object to be moved, said second interaction portion of said second actuator being provided at a second end of said second actuator opposite to said first end of said second actuator.