WO2010024664A1

WO2010024664A1 - Actuator and method for positioning an object

Info

Publication number: WO2010024664A1
Application number: PCT/NL2009/000165
Authority: WO
Inventors: Jeroen Paul Van Schieveen; Josephus Wilhelmus Spronck
Original assignee: Technische Universiteit Delft; Technologiestichting Stw
Priority date: 2008-08-26
Filing date: 2009-08-26
Publication date: 2010-03-04
Also published as: WO2010024664A8; NL2001921C

Abstract

The invention relates to an actuator for positioning an object in a first direction of movement, comprising a base, at least three positioning members, wherein each of the at least three positioning members has a contact surface which is configured to bear against the object under prestress, and a control device to drive the at least three positioning members in friction drive mode. In addition, the actuator comprises at least one heating device which is connected to the control device, in which the at least one heating device is configured to heat the at least three positioning members, and prestressing means to provide the prestress with which the contact surfaces of the at least three positioning members bear against the object, in which the at least three positioning members are of elongate design and are connected by one end to the base, and in which the contact surface is situated near the other end of the at least three positioning members, and in which the at least three positioning members are made from a material which expands under the effect of heat.

Description

P29491 PC00/MVM

Actuator and method for positioning an object

The invention relates to an actuator for positioning an object in at least a first direction of movement according to the preamble of claim 1.

Such actuators are known from, for example, US 7,301 ,257 B2. In this document, the actuator is provided with piezo elements as positioning members. The actuator can position an object in friction drive mode.

The friction drive mode is characterized by a sequential phase, in which a minority of the positioning members are driven simultaneously, in such a manner that the friction between the contact surface of the respective positioning members and the object is overcome, as a result of which the contact surface slides along the object while the object is held in place by the other positioning members, and a simultaneous phase, in which a majority of, preferably all, the positioning members are driven simultaneously, as a result of which the object is displaced from a first position to a second position. The order of the two phases determines in which direction the object is displaced. By repeating the phases, the object can be moved to a desired position step by step.

It is an advantage of the friction drive mode that drive means are only required to expand the positioning members in the longitudinal direction, as a result of which these slide along the surface of the object. This is contrary to drive modes which use a so-called walking displacement of the positioning members and the contact between contact surface and object is actively interrupted in order to then displace the positioning members along the object. As a result, the friction drive mode is of a more simple design. It is also possible to achieve a more simple design by the positioning members contacting the object at all times, as a result of which three positioning members can both carry and position an object, while the other drive modes require at least four positioning members to both carry and position an object. This is caused by the fact that contact is interrupted, as a result of which it has to be possible for the object to be carried by the other positioning members.

In this application, the expression a direction of movement is understood to mean one of six degrees of freedom, that is to say three translations (x, y, z) and three rotations (Rx, Ry, Rz), with a direction of movement being possible in two directions (positive and negative). With applications where the object has to be positioned accurately and then remain stable in a desired position for a specific time, the above-described actuators do not perform satisfactorily. This is caused inter alia by the existing hysteresis and creep of the piezo elements.

Another drawback is the fact that such actuators cannot easily be scaled to, for example, MEMS applications as a result of the more complex fabrication process. The materials from which piezo elements are normally made cannot usually be readily combined with the lithographic fabrication techniques used for MEMS applications, as a result of which, for example, the integration with the driving electronics is rendered more difficult and the system becomes more complex and more expensive.

It is an object of the invention to provide an actuator which is capable of positioning an object accurately in a direction of movement and can hold the object securely in place in a desired position.

This object is achieved by an actuator according to the preamble of claim 1 , in which the actuator comprises at least one heating device which is connected to the control device, in which the at least one heating device is configured to heat one or more of the at least three positioning members, and the actuator comprises prestressing means to provide the prestress with which the contact surfaces of the at least three positioning members bear against the object, and in which the at least three positioning members are of elongate design and are connected by one end to the base, in which the contact surface is situated near the other end of the at least three positioning members, and in which the at least three positioning members comprise a material which expands under the effect of heat. It is an advantage of an actuator according to the invention that the hysteresis is smaller than with existing actuators, as a result of which the object can be positioned more accurately.

Another advantage is the fact that it is possible to hold the object in place more securely due to the prestressing means, as the friction between the contact surface of a positioning member and the object is determined by the prestress, in other words the degree to which the contact surface is pressed against the object. The prestress is provided by the prestressing means. In addition, the prestressing means can be used to influence the friction, as a result of which the latter can be predicted more accurately. This makes the actuator more reliable. It is another advantage that the actuator becomes quicker, as the step size which can be achieved is increased. The prestressing means can provide the prestress in, for example, a magnetic or electrical manner, but likewise also mechanically. Preferably, the prestressing means comprise a resilient element. Resilient elements have the advantage that they are simple structures which can be used in a wide variety of applications in a simple manner, including MEMS applications.

In an embodiment, the longitudinal direction of the positioning members corresponds substantially to the first direction of movement.

In an embodiment, the at least three positioning members each form a resilient element. This offers the advantage that the prestressing means are integrated in the positioning members, thus simplifying design and fabrication. Preferably, the prestress between the contact surface and the object can be adjusted individually for each positioning member in order to thereby adjust the friction and render the latter more predictable. Being able to influence the friction offers the advantage of being able to make the friction between the individual contact surfaces and the object virtually equal. This improves the operation of the actuator, because the friction drive mode is thus able to perform in an optimum manner. Adjusting the prestress can be effected, for example, by means of changing the deflection of the positioning members by making the position at which the positioning member is attached to the base displaceable. This could be achieved, for example, by means of separate thermal actuators. In a variant thereof, the temperature of some of the at least three positioning members can be adjusted individually without this resulting in a significant expansion of the positioning members, and thus making it possible to individually adjust the stiffness of the three positioning members and therefore the prestress.

In an embodiment, the prestress can only be adjusted during the friction drive mode and not while the object is being securely held in place, as a result of which the thermal actuators do not have to be switched on continuously, which saves energy.

In an embodiment, the at least three positioning members are distributed over substantially opposite sides of the object. This offers the advantage that the prestresses of the various positioning members act substantially in opposition to one another. In an embodiment, the object requires no further guiding. In addition, the object can be clamped between the positioning members, thus increasing the stability. Preferably, the object is mainly carried by the at least three positioning members. This offers the advantage that the positioning and the secure holding of the object depends to a large degree on the positioning members which can easily be controlled by means of the control device. In addition, the object does not require guiding, which makes the design simpler.

With the friction drive mode, it is important that always a minority of the positioning members is driven in the sequential phase. This conforms to a time delay in the expansion of the various positioning members. Such a time delay can be achieved in different ways.

In one variant, it is possible for each positioning member to have its own individual heating device, with the control device being configured to be able to separately control the heating devices sequentially and simultaneously. The positioning members can then be of identical design and the time delay is caused by the control device itself.

In another variant, the positioning members are configured in such a manner that each positioning member has a substantially different thermal time constant compared to the other positioning members, as a result of which the temperature and the associated expansion are not synchronous. However, the cooling of the positioning members can be slightly synchronous. Thus, only one heating device is required in order to heat the positioning members, wherein the positioning members fully expand at different points in time. In an embodiment, the heating device is only required during positioning and can be removed, for example, after positioning has taken place. This would reduce the costs of such a system and makes it possible to serve several systems using one heating device.

In yet another variant, one heating device is provided which is positioned at a different distance to each positioning member, as a result of which the positioning members have to be heated over different distances and therefore do not expand in a synchronous manner. The advantage of this variant is that cooling does take place in a synchronous manner if the positioning members are of identical design.

In an embodiment, the positioning members in groups of at least three positioning members form part of a monolithic structure. This has the advantage that a stable structure is produced with less creep than, for example, piezo elements.

In an embodiment, the base and the positioning members are made of semiconductor material. This increases the stability and makes it possible for the actuator to be produced by means of MEMS fabrication techniques. The MEMS fabrication techniques offer the advantage that the tolerances used during fabrication can be reduced, as a result of which accuracy increases and friction becomes more predictable. In addition, the actuator can be scaled more easily and be made suitable for MEMS applications.

In an embodiment, the control device is configured to be able to position the object in a second direction of movement. In one possible embodiment, a first direction of movement is, for example, a direction of translation in the longitudinal direction of the positioning members and the object can be rotated through a small angle by causing the positioning members to expand differently. In this case, the actuator does not have to be driven in friction drive mode. In a variant thereof, each positioning member is configured to have two heating devices, with the two heating devices being arranged on either side of a plane which runs through the centre of the positioning member and with the plane being defined by the longitudinal direction of the positioning member and the normal to the contact surface of the positioning member. The two heating devices are then able to give the material on either side of the plane a different temperature, as a result of which the positioning members can also move sideways and it is possible to position the object also in a direction of movement at right angles to the plane. The actuator can in this case also be driven in friction drive mode.

If it is desired to position the object over a larger range in a second direction of movement and preferably also in a third direction of movement, it is preferable if the actuator comprises several groups of at least three positioning members, with the positioning members being placed around the object in such a manner that the object can also be positioned in a second direction of movement and preferably also in a third direction of movement, respectively. Each group of at least three positioning members is now capable of displacing the object with respect to the base. Two groups can therefore position the object in at least two directions of movement and three groups can position the object in at least three directions of movement. When the object can be positioned in at least three directions of movement, the directions of movement are preferably one direction of translation and two directions of rotation.

It is possible to position the object in several directions of movement while the actuator is being driven in friction drive mode when at least four positioning members are placed at more or less the same distance around the object.

In an embodiment, at least one heating device comprises a resistive heating element. The resistive heating element can preferably be used as a sensor. A resistive heating element heats up because an electric current is passed through the heating element which has a certain resistance. This resistance depends inter alia on the material properties which, in turn, depend on the temperature of the heating element. This property makes it possible to use the heating element as a sensor. The resistance of the heating element in this case is a measure of the temperature and can be determined by measuring the electric current and the applied voltage across the heating element. The information from the sensor represents information which can be advantageous during positioning, through which, for example, the expansion of the positioning members can be controlled, and while holding the object securely in place, through which undesirable thermal effects can be detected.

In an embodiment, the actuator also comprises guide means which guide the object during positioning. This is advantageous, inter alia, when the object is relatively large and the positioning members are therefore no longer able to guide the object themselves, or when relatively large forces have to be absorbed, for example in order to prevent the positioning members from becoming damaged. The guide means can also provide additional stability in directions of movement other than the direction of movement in which positioning takes place.

An actuator according to the invention is suitable for combination with a control system. In that case, a sensor will measure the position of the object and drive the control device in such a manner that the object moves to the desired position.

The invention also relates to a method for positioning an object in a first direction of movement using an actuator as described above, comprising the following steps:

- the control device driving the at least one heating device in such a manner that a majority of, preferably all, the positioning members experience substantially the same expansion and the object moves from a first position to a second position, and

- the control device driving the at least one heating device in such a manner that with a minority of the positioning members, but in any case with at least one positioning member, the friction force between the contact surface and the object of these positioning members is overcome, as a result of which the contact surface subsequently slides along the object while the other positioning members keep the object substantially in place.

The invention will be explained below in more detail by means of figures and with reference to various non-limiting embodiments. In the figures:

Fig. 1 shows a perspective diagrammatic illustration of an actuator according to the invention, Fig. 2 shows a top view of a second embodiment of an actuator according to the invention, Fig. 3 shows a side view of the actuator from Fig. 2,

Fig. 4 shows a perspective view of a third embodiment of an actuator according to the invention, and

Fig. 5 shows a perspective view of a diagrammatic illustration of a fourth embodiment of an actuator according to the invention.

Fig. 1 shows an embodiment of an actuator 1 according to a first embodiment of the invention. The actuator 1 comprises a base 2 with respect to which an object 3 can be positioned in a first direction of movement, indicated here by arrow A. The actuator 1 furthermore comprises three elongate positioning members 10, 11 , and 12, which are connected to the base 2 at an end 10a, 11a, 12a, respectively. The longitudinal direction of the positioning members 10, 11 , 12 in this case corresponds to the first direction of movement A.

Near the other end 10b, 11 b, 12b, the positioning members 10, 11 , 12 each have a contact surface 10c, 11c, 12c. The contact surfaces 10c, 11c, 12c bear against the object 3 under prestress. This prestress is applied by prestressing means. In this embodiment, the prestressing means comprise resilient elements which are formed by the three positioning members 10, 11 , 12 themselves which are elastic to this end. When the contact surfaces 10c, 11 c, 12c bear against an upper surface 3a of the object 3, the positioning members 10, 11 , 12 are subjected to flexural loads in a direction at right angles to the top upper surface 3a. This results in the contact surfaces 10c, 1 1c, 12c being pressed against the upper surface 3a. As in this embodiment the contact surfaces 10c, 11c, 12c bear against the same side of the object 3, the prestressing means also comprise guide means 4 on the opposite side of the upper surface 3a. As a result thereof, the object 3 is clamped between the positioning members 10, 11 , 12 and the guide means 4, which provides a stable position for keeping the object 3 in place.

Apart from applying the prestress between the contact surfaces 10c, 11c, 12c and the object 3, the guide means 4 also serve to guide the object 3 in the first direction of movement A and to prevent the object 3 from rotating about a horizontal axis.

The positioning members 10, 11 , 12 are made from a material which expands under the effect of heat. In this case, the expansion in the longitudinal direction is the most relevant and is indicated for each positioning member 10, 11 , 12 by arrow 10d, 11d, and 12d, respectively. The actuator 1 comprises three heating devices 15 which are suitable for heating the positioning members 10, 11 , 12. However, embodiments with fewer heating devices are also possible. The heating devices 15 are connected to a control device 16 which is configured in such a manner that the three positioning members 10, 11 , 12 can be driven in friction drive mode.

In friction drive mode, the heating devices 15 are driven in such a manner that in a sequential phase always a minority of the positioning members 10, 11, 12 expands or shrinks in the longitudinal direction, wherein the friction between the contact surface of the respective positioning members and the object is overcome and the contact surface slide along the object 3, while the other positioning members keep the object 3 in its position. Thereafter, the other positioning members are driven in the same manner until all contact surfaces 10c, 11c, 12c have slid along the object 3. The friction drive mode furthermore comprises a simultaneous phase in which all positioning members 10, 11 , 12 expand or shrink simultaneously and the object 3 is moved from a first position to a second position. The sequence of the simultaneous and sequential phases determines in which direction the object 3 is moved. By repeating the two phases, the object 3 can be moved to a desired position step by step.

When this is applied to the embodiment from Fig. 1 , the object 3 can be moved from a first position to a second position in the following manner, with the object 3 being situated further from the base 2 in the second position. First, in the simultaneous phase, the positioning members 10, 11, 12 are heated simultaneously by the heating devices 15. As a result thereof, the positioning members 10, 11 , 12 expand and the object 3 is moved along the guide means 4 from the first position to the second position. This is followed by the sequential phase. In this example, the heating device 15 of the positioning member 10 is first driven in such a manner by the control device 16 that the positioning member 10 cools down. This results in the positioning member 10 shrinking. Initially, this is counteracted by the friction between the contact surface 10c and the upper surface 3a. At a certain moment, the positioning member 10 has cooled down so much that the friction between the contact surface 10c and the upper surface 3a is overcome and the contact surface 10c slides along the upper surface 3a. The force which the contact surface 10c exerts on the object 3 in this phase is distributed over the two other positioning members 11 , 12. This prevents the friction between these contact surfaces 11c, 12c and the upper surface 3a from being overcome, thus keeping the object 3 in the second position by the non-driven positioning members 11 , 12. Subsequently, the positioning member 11 is driven in a way similar to that of the positioning member 10, so that the contact surface 11c slides along the upper surface 3a, while the positioning members 10 and 12 keep the object 3 in place. Finally, the positioning member 12 will be driven in such a manner that the contact surface 12c slides along the upper surface 3a and the positioning members 10 and 11 keep the object 3 in place. The positioning members are now in their original position again and ready to enter a simultaneous phase again in order to move the object another step, if required. It will be clear to those skilled in the art that, in this example, the sequence in which the positioning members 10, 11 , 12 are driven in the sequential phase is arbitrary and does not affect the operation. What is important is that at equal friction, in each case a minority of the positioning members is driven simultaneously. The sequence is of no importance in this case. Preferably, the friction between the individual contact surfaces and the object is virtually equal.

When, on the contrary, the object 3 has to be moved in the opposite direction, the sequential phase will have to be executed first, so that the positioning members 10, 11 , 12 are alternately driven and expand one after the other, with the respective contact surfaces 10c, 11c, 12c sliding along the upper surface 3a. When all contact surfaces 10c, 11c, 12c have slid along the upper surface 3a, the positioning members 10, 11 , 12 can be driven simultaneously and cool down in such a manner that the object 3 is moved in the direction of the base 2.

Furthermore, it is possible for the actuator 1 to be configured in such a manner that the object 3 can also be positioned in a second direction of movement. In this embodiment, this is possible by, for example, rotating the object 3 through a small angle about an axis which is at right angles to the upper surface 3a. The control device causes the positioning members 10, 11 , 12 to expand differently in this case. When the distance between the positioning member 10 and 11 and between 11 and 12 is equal, this can be achieved, for example, by causing the positioning member 12 to expand, but not the positioning member 10 and by causing the positioning member 11 to expand half as much as the positioning member 12. Other variants hereof are also possible.

Fig. 2 shows a top view of an actuator 21 according to a second embodiment of the invention. The actuator 21 comprises a base 22 and four positioning members 30, 31 , 32, 33. The actuator 21 is able to position an object 23 in a first direction of movement, indicated here by arrow B. The four positioning members 30, 31 , 32, 33 are of elongate design and the longitudinal direction substantially corresponds to the first direction of movement B. Near an end 30a, 31a, 32a, 33a, the positioning members 30, 31 , 32, 33 are connected to the base 22. Near another end 30b, 31 b, 32b, 33b, the positioning members 30, 31 , 32, 33 have a contact surface, denoted as 30c, 31c, 32c, 33c. The contact surfaces 30c, 31c, 32c, 33c bear against the object 23 under prestress. The positioning members 30, 31 , 32, 33 are configured as resilient elements and thus form the prestressing means. In this example, the positioning members 30, 31 , 32, 33 are distributed in pairs on opposite sides of the object 23. Thus, the object 23 is clamped between the positioning members and the latter are able to substantially carry the object 23. In order to prevent movements in directions other than the direction of movement B, guide means 24 may be provided. These guide means 24 may also partially carry the object 23, but this is not imperative.

The actuator 21 furthermore comprises heating devices (not shown) which are capable of heating the positioning members 30, 31 , 32, 33. The heating device can be configured as a resistive heating element which is integrated in, on or with positioning members 30, 31 , 32, 33.

In order to drive the heating devices, the actuator 21 furthermore comprises a control device (not shown) which is connected to the heating devices.

This embodiment is particularly suitable for use in a MEMS application. At least the base 22, positioning members 30, 31 , 32, 33 and the guide means 24 may in this case be made from semiconductor material, preferably from a monolithic structure.

Fig. 3 shows a side view of the actuator 21 from Fig. 2. It can clearly be seen here that the object 23 is carried by the positioning members 30, 31 , 32, 33 and, optionally, also by the guide means 24. Similarly to the actuator 1 from Fig. 1 , this embodiment can be driven in friction drive mode in order to move the object 23 in both directions of movement B. After positioning, the positioning members 30, 31 , 32, 33 hold the object 23 securely in the desired position.

When the heating devices are configured as resistive heating elements, the latter can be used as sensors. The information from the sensors can be used during the sequential and simultaneous phase of the friction drive mode in order to measure the temperature and thus the expansion of a positioning member 30, 31 , 32, 33, but can also be used to detect undesirable thermal effects while holding the object securely. When an undesirable thermal effect does occur, the sensors can indicate this, following which the object 23 can be positioned again, if required. In this manner, it can be ensured that the object 23 is in the desired position. Fig. 4 shows an actuator 41 according to the invention in perspective. The actuator 41 comprises a base 42 and various positioning elements 50, 51 , 52, 53 which are arranged around a spherical object 43. At one end, the positioning elements are connected to the base 42. On the invisible side of the object 43, similar positioning elements are provided so that the object 43 is clamped and carried by these positioning elements and no guide means are required to support or guide the object 43 further. An element 45 runs through the object 43. Such an element could, for example, be a fibre from an optical MEMS application.

The positioning elements 50, 51 , 52, 53 may be configured as separate positioning members, with a contact surface bearing against the object 43 at the free end of the positioning members 50, 51 , 52, 53 under prestress. However, it is also possible to arrange several positioning members at the free end of positioning elements 50, 51 , 52, 53 (not shown). Preferably, at least three positioning members are provided. In that case, the positioning members are connected to the positioning elements 50, 51 , 52, 53 near an end and have a contact surface at the other end which bears against the object 43 under prestress. The positioning elements 50, 51 , 52, 53 are able to position the object 43 by means of the friction drive mode in a direction of translation at right angles to an upper surface 42a and two directions of rotation about two axes which are at right angles to one another parallel to the upper surface 42a. As a result thereof, the end of element 45 can be aligned with, for example, a similar element, in three directions of movements which are represented here by the three arrows C, D and E.

It is possible to design the actuator 41 with three positioning members which are evenly distributed over the circumference of the object 43. In that case, at least three positioning members are provided on each positioning element in order to be able to position the object 43 in a direction of movement. In a variant, it is possible for the actuator 41 not to comprise positioning elements, but for the positioning members to be connected to the base 42 directly.

The positioning elements 50, 51 , 52, 53 can also be configured with two parallel heating elements per positioning element in such a manner that the positioning elements can deflect sideways as a result of a temperature difference which is caused by the heating elements in the width direction of the positioning element and the sphere is rotated about an axis along the direction of movement C. In this case, it is also possible to use the friction drive mode.

Fig. 5 is a diagrammatic illustration of a platform 63 which is carried by three groups of three positioning members 70. In this embodiment, the groups are evenly distributed over the circumference of the platform 63. The positioning members 70 comprise a contact surface (not shown) at one end by means of which they bear against the sides of the platform 63. Another end of the positioning members is connected to a base 62.

By driving the groups of three positioning members 70 in the same direction in friction drive mode, with each group of three positioning members being driven in a way similar to the embodiments from Fig. 1 , the platform 63 can be positioned in a first direction of movement. The first direction of movement is indicated by arrow Z.

When at least one of the three groups is not driven or is driven in the opposite direction or if the speed of a group differs from that of the other two, the platform 63 will rotate about a horizontal axis. The platform 63 can than also be positioned in two other directions of movement, with the two other directions of movement being rotations about a horizontal axis. The platform 63 can thus be positioned in a total of three directions of movement.

An actuator according to the invention and in particular the embodiment from Fig. 5 is very suitable for positioning lenses and/or mirrors, but other applications outside of optics are also possible. Thus, sensor components could be aligned with one another in order to thus increase the linearity or the sensitivity of the sensor.

In the embodiments illustrated, when several groups of at least three positioning members have been shown, all groups are arranged parallel to one another. Those skilled in the art will understand that groups of at least three positioning members can also be placed at an angle, for example at right angles to one another. All this depends on the desired directions of movement in which the object has to be positioned.

Claims

C L A I M S

1. Actuator for positioning an object in a first direction of movement, comprising:

- a base,

- at least three positioning members, wherein each of the at least three positioning members has a contact surface which is arranged to bear against the object under prestress, - a control device to drive the at least three positioning members in friction drive mode,

characterized in that

- the actuator comprises at least one heating device which is connected to the control device, in which the at least one heating device is configured to heat one or more of the at least three positioning members,

- the actuator comprises prestressing means to provide the prestress with which the contact surfaces of the at least three positioning members bear against the object,

- the at least three positioning members are of elongate design and are connected by one end to the base,

- the contact surface is situated near the other end of the at least three positioning members,

- the at least three positioning members comprise a material which expands under the effect of heat.

2. Actuator according to claim 1 , in which the longitudinal direction of the positioning members corresponds substantially to the first direction of movement.

3. Actuator according to claim 1 or 2, in which the prestressing means comprise a resilient element.

4. Actuator according to one of the preceding claims, in which the at least three positioning members each form a resilient element.

5. Actuator according to one of the preceding claims, in which the prestress between the contact surface and the object can be adjusted for each positioning member individually.

6. Actuator according to one of the preceding claims, in which the at least three positioning members are distributed over substantially opposite sides of the object.

7. Actuator according to one of the preceding claims, in which the object is mainly carried by the at least three positioning members.

8. Actuator according to one of the preceding claims, in which each positioning member 5 has a heating device and the control device is configured to be able to separately control the heating devices sequentially and simultaneously.

9. Actuator according to one of the preceding claims, in which the at least three positioning members are configured in such a manner that each positioning member has a

10 substantially different thermal time constant compared to the other positioning members.

10. Actuator according to one of the preceding claims, in which the at least three positioning members in groups of at least three positioning members form part of a monolithic structure.

15

11. Actuator according to one of the preceding claims, in which at least the base and the at least three positioning members are made of semiconductor material.

12. Actuator according to one of the preceding claims, in which the control device is 20 configured to be able to position the object in a second direction of movement.

13. Actuator according to one of the preceding claims, in which the actuator comprises several groups of at least three positioning members, wherein the groups are arranged around the object in such a manner that the object can also be positioned in a second

25 direction of movement and preferably also a third direction of movement.

14. Actuator according to one of the preceding claims, in which the at least one heating device comprises a resistive heating element which can also be used as a sensor.

30 15. Actuator according to one of the preceding claims, in which the actuator also comprises guide means which guide the object during positioning.

16. Method for positioning an object in a first direction of movement using an actuator according to one of the claims 1-15, comprising the following steps:

35 - the control device driving the at least one heating device in such a manner that a majority of, preferably all, the positioning members experience substantially the same expansion and the object moves from a first position to a second position, and - the control device driving the at least one heating device in such a manner that with a minority of the positioning members, but in any case with at least one positioning member, the friction force between the contact surface and the object of these positioning members is overcome, as a result of which the contact surface subsequently slides along the object while the other positioning members keep the object substantially in place.