WO2008041171A2

WO2008041171A2 - Actuator assembly and opto-mechanical device comprising an actuator assembly

Info

Publication number: WO2008041171A2
Application number: PCT/IB2007/053977
Authority: WO
Inventors: Johan F. Lurquin; Robby F. Maes; Johannes F. M. Willemse
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2006-10-06
Filing date: 2007-10-01
Publication date: 2008-04-10
Also published as: WO2008041171A3

Abstract

An actuator assembly (100,200,300,400,500) comprises a first part (2), and a second part (3), which comprises a guide (5,6,58,59) for guiding motion of the first part (2) relative to the second part (3). The actuator assembly (100,200,300,400,500) further comprises an electro -mechanical unit (11,17,47,57) for generating a driving force between the first part (2) and the second part (3) along the guide (5,6,58,59) in response to a driving signal, and at least one pair of permanent magnets (20,21,30,31,40,50) attached to one of the first and second parts, and which comprises a first magnetized zone (12,41,51) and a second magnetized zone (13,42,52), which are positioned in a line, for providing a force transverse to the guide (5,6,58,59), acting to hold at least one surface part of the first part (2) against a surface of the guide (5,6,58,59). The respective components of magnetization of the first and the second magnetized zone (12,13,41,42,51,52) of the at least one pair of permanent magnets (20,21,30,31,40,50) parallel to the line are oriented in opposite directions.

Description

Actuator assembly and opto-mechanical device comprising an actuator assembly

FIELD OF THE INVENTION

The invention relates to an actuator assembly comprising a first part, a second part, which comprises a guide for guiding motion of the first part relative to the second part, and an electro -mechanical unit for generating a driving force between the first part and the second part along the guide in response to a driving signal.

The invention also relates to an opto-mechanical device comprising an actuator assembly.

BACKGROUND OF THE INVENTION Examples of such an assembly and device are known. US-Al -2004/0234258 discloses a lens driving device and imaging device. A driven body has an optical lens, a sleeve on one side and a slot on the opposite side, so as to sandwich a light axis together with the sleeve. Further, a guide shaft is a shaft fitting into the sleeve in order to cause the driven body to move along the direction of the optical axis. The guide shaft is a shaft inserted as a brace at the slot and is for preventing the driven body from rotating taking the guide shaft as a center. A flat coil is fixed to the driven body. The position of the driven body is detected by a position detection magnet fitted to the driven body and a magneto -resistive element arranged in a non-contact manner spaced from the magnet. When current flows in the drive coil, thrust parallel with the optical axis direction is generated. Frictional resistance generated between the driven body and the guide shafts becomes fixed because the thrust direction and drive direction are always parallel over the entire region of the drive stroke. Drive characteristics and servo characteristics can be made superior.

A disadvantage of the known assembly is that the servo control to keep the driven body positioned constantly consumes power and makes the lens drive relatively complicated. The known assembly requires the servo control to correct for displacements due to external influences.

SUMMARY OF THE INVENTION It is an object of the invention to provide an assembly and device of the types defined in the opening paragraphs that allow for relatively simple and energy-efficient positioning of the first part relative to the second part of the actuator assembly. The invention is defined by the independent claims. Advantageous embodiments are defined by the dependent claims.

This object is achieved by means of the actuator assembly according to the invention, which is characterized in that the actuator assembly further comprises at least one pair of permanent magnets attached to one of the first and second parts, comprising a first magnetized zone and a second magnetized zone, which are positioned in a line, for providing a force transverse to the guide, acting to hold at least one surface part of the first part against a surface of the guide, wherein respective components of magnetization of the first and second magnetized zone parallel to the line are oriented in opposite directions.

The actuator assembly comprises an electro -mechanical unit for generating a driving force in response to a driving signal, in order to displace the first part with respect to the second part. Because the actuator assembly according to the invention further comprises at least one pair of permanent magnets with a first and a second magnetized zone, wherein respective components of magnetization of the first and second magnetized zone parallel to the line are oriented in opposite directions, for providing a force transverse to the guide, acting to hold at least one surface part of the first part against a surface of the guide, a substantially constant dynamic frictional force, opposing the driving force, is present when the first part is moved along the guide. Since the resistance encountered by the first part is well defined, it is even possible to use open-loop control to position the first part relative to the second part. This is more efficient and simpler than servo control. In the absence of a driving signal, the force transverse to the guide ensures that a static frictional force is maintained, which prevents displacement due to external influences. Due to the presence of the pair of permanent magnets, separate energizing for providing the force transverse to the guide is not required. This enhances the energy-efficiency of the actuator assembly. The first and second magnetized zone, that are oriented in opposing directions, result in an increased efficiency of the at least one pair of permanent magnets and therefore in an increased force transverse to the guide and/or a more energy-efficient actuator assembly. An additional advantage of the assembly is that the area of contact between the first and second part remains substantially constant even in the face of wear, due to the force holding them against each other. The guide on the surface of which friction is generated may be any bar, rod, or other mechanical construct directing the motion of one part of the actuator assembly relative to the other.

In an embodiment of the actuator assembly according to the invention, the actuator assembly further comprises a position measuring sensor attached the other of the first and second parts for measuring a displacement of the first part with respect to the second part as a result of the driving force. This provides for a simple construction in which the relative position of the first part with respect to the second part is controlled.

In an embodiment, the electro-mechanical unit for generating the driving force comprises at least one electrically conductive coil, attached to the other of the first and second parts, for generating a magnetic field acting on the at least one pair of permanent magnets, in response to the driving signal. This variant is simpler in construction, since the permanent magnets provide both for the driving force and the force transverse to the guide. In an embodiment, the other of the first and second parts further comprises a ferromagnetic back plate, wherein the electrically conductive coil is positioned between the ferromagnetic back plate and the at least one pair of permanent magnets. This has the advantage of amplifying the magnetic field created by the electrically conductive coil. The energy efficiency is thus further improved. The permanent magnets may be made smaller, decreasing the mass of the part to which it is attached.

In an embodiment, the at least one pair of permanent magnets is arranged to surround a portion of the first part and the location of the guides is asymmetrical with respect to the surrounding pair of permanent magnets. The surrounding pair of permanent magnets provides for an energy-efficient assembly and the asymmetrical placement of the guides with respect to the surrounding pair of permanent magnets provides for the required asymmetry resulting in a net force, not equal to zero, that acts on the guide. According to another aspect of the invention, there is provided an optomechanical device comprising a lens body and an actuator assembly according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS These and other aspects of the invention will be further elucidated and described with reference to the drawings, in which:

Fig. 1 is an exploded view of a first embodiment of an actuator assembly according to the invention; Fig. 2 is a schematic cross-section of a pair of permanent magnets of an actuator assembly according to the invention;

Fig. 3 is a perspective view of the actuator assembly of Fig. 1;

Fig. 4 is a perspective cross-sectional view of the actuator assembly of Fig. 1; Fig. 5 shows in a cross-sectional view forces F acting on the guidance pins of the embodiment of Fig. 1;

Fig. 6 is a perspective cross-sectional view of a second embodiment of an actuator assembly according to the invention;

Fig. 7 shows a diagram of the output signal Y (voltage or current) of a Hall sensor as a function of the distance X between a magnet and the Hall sensor;

Fig. 8 is an exploded view of a third embodiment of an actuator assembly according to the invention;

Fig. 9 is a perspective cross-sectional view of the actuator assembly of Fig. 8;

Fig. 10 shows in a cross-sectional view forces acting on the guidance pins of the embodiment of Fig. 8;

Fig. 11 is an exploded view of a fourth embodiment of an actuator assembly according to the invention;

Fig. 12 is a perspective view of the actuator assembly of Fig. 11;

Fig. 13 is a perspective cross-sectional view of the actuator assembly of Fig. 11;

Fig. 14 shows in a cross-sectional view forces acting on the guidance pins of the embodiment of Fig. 11;

Fig. 15 is an exploded view of a fifth embodiment of an actuator assembly according to the invention; Fig. 16 is a perspective cross-sectional view of the actuator assembly of

Fig. 15;

Fig. 17 shows in a cross-sectional view forces acting on the barrel of the embodiment of Fig. 15;

Fig. 18 is a diagram containing a graph of the voltage applied to a coil as a function of time in an actuator assembly according to the invention in a first method of driving the actuator assembly; and

Fig. 19 is a diagram containing a graph of the voltage applied to the coil as a function of time in a second method of driving an actuator assembly according to the invention. The figures are not drawn to scale. In general, identical components are denoted by the same reference numerals in the figures.

DETAILED DESCRIPTION OF EMBODIMENTS Implementations of actuator assemblies in opto-mechanical products will be explained below as an example. The description will focus on the field of imaging optics for small applications, such as web cameras, including those integrated in Liquid Crystal Display (LCD) flat panel displays, for mobile phones, Personal Digital Assistants (PDAs), etc. In camera's, for instance, the actuator assembly can be used for auto-focus movement, optical zoom, mechanical shutter actuation, diaphragm opening control, compensation of tilt between optics and imager, x-y movement of an image sensor to compensate for camera shake, etc. More generally the actuator assembly finds application in any field in which an actuator assembly allowing small displacements and requiring low forces to hold the movable parts in position is useful. This includes the field of magnetic and/or optical recording, for example. It also includes applications in toys, such as remotely controlled model cars and planes, game controllers with tactile feedback, etc. Another field of application is that of industrial and/or automotive equipment, e.g. to operate proportional valves for fluids and gasses, mass flow controllers, etc. Yet another field of application is in lighting, to focus small bundles of light, for example by moving a lens in front of a bright Light Emitting Diode (LED). The assembly and method of driving it are particularly suited to mobile applications, where low power consumption is an issue.

In Figs. 1, 3, 4 and 5 a first actuator assembly 100 comprises a first part and a second part, in this example formed as a barrel 2 and a housing 3, respectively. The barrel 2 is movable with respect to the housing 3. In the example (see Fig. 4), the barrel 2 carries a lens body 4. The barrel 2 carries a lens assembly comprising more than one lens in other embodiments.

The barrel 2 is linearly guided to allow focusing of an optical system comprising the lens body 4. To this end, the housing comprises a guide, in this example in the form of two guidance pins 5,6. The barrel 2 comprises holes 8 and 9 through which the guidance pins 5,6 travel. The guidance pins 5,6 are just an example of a mechanical guide. In other embodiments of the actuator assembly, the housing comprises at least one rail and the movable part of the assembly comprises at least one traveler co-operating with one of the rails to guide the motion of one part of the actuator assembly relative to another part. In other embodiments, the part of the actuator assembly carrying a lens assembly is provided with the guide, and the guide co-operates with a frame of a stationary part of the opto-mechanical device.

Two pairs of permanent magnets 20 and 21 are attached to the barrel 2. The pairs of permanent magnets 20 and 21 establish a field of feree acting on the guidance pins 5,6, which are made of a ferromagnetic material. In another embodiment, a mechanical resilient means is used to press a brake shoe against the guidance pins 5,6. However, the use of an electro-mechanical unit for establishing a field of feree has the advantage of requiring fewer components, as well as allowing relatively unconstrained movement of the barrel 2.

A force F provided by the pairs of permanent magnets 20 and 21 and acting on the guidance pins 5,6 is in a direction transverse to the guide, perpendicular to the direction of movement of the barrel 2 relative to the housing 3. Due to this force F, inside surface parts of the holes 8 and 9 are forced against the surface of the guidance pins 5,6. As a result, movement of the barrel 2 along the guidance pins 5,6 is opposed by a substantially constant factional force. The factional force remains constant within a margin defined by fluctuations of the coefficient of friction due to changing environmental conditions, such as humidity, temperature, etc. Because the inside surface parts of the holes 8 and 9 are forced against the surface of the guidance pins 5,6, wear of the barrel 2 will have relatively little effect on the factional force. Another added advantage is that the transverse force removes play in the position of the barrel 2. This is an advantage in opto-mechanical devices, in which the optical axis of the lens body 4 needs to be aligned with respect to other optical components.

It is observed that the holding force may be varied by varying the distance of the pairs of permanent magnets 20,21 to the guidance pins 5,6 by means of varying the position of the holes 8,9. A higher holding force results in a higher static factional force (when the barrel 2 is stationary relative to the housing 3), which may be desirable in applications in which the first actuator assembly is subjected to shocks (e.g. optical pick-up units in optical recording apparatuses). An embodiment such as that of Figs. 1, 3, 4 and 5, in which the pairs of permanent magnets 20,21 exert a force on ferromagnetic guidance pins 5,6 has the advantage of comprising few parts.

To displace the barrel 2 in the direction determined by the guidance pins 5,6, the first actuator assembly 100 comprises an electrically conductive coil 11, with terminals (not shown) for applying a driving signal, such as a driving voltage or a driving current. The electrically conductive coil 11 is attached to the housing 3 in this embodiment. This allows relatively large displacements of the barrel 2, since there are no wires connected to the barrel. Current passing through the electrically conductive coil 11 gives rise to a magnetic field acting on the pairs of permanent magnets 20 and 21.

The pairs of permanent magnets 20 and 21 each comprise a first magnetized zone 12 and a second magnetized zone 13, separated by a transition zone 14, as is shown in Fig. 1. The transition zone 14 is preferably made of a material that has a high magnetic permeability. In another embodiment the first and second magnetized zones 12 and 13 adjoin and no transition zone is present. As is shown in Fig. 2, which is a schematic cross-section of the pair of permanent magnets 20, the first magnetized zone 12 has a magnetization M in a direction parallel to the guidance pins 5,6. The second magnetized zone 13 has a magnetization MM in a direction parallel to the guidance pins 5,6 which is opposite to the direction of the magnetization M of the first magnetized zone 12. The effect is that the pairs of permanent magnets 20 and 21 produce magnetic flux lines perpendicular to its face and perpendicular to the longitudinal axes of the guidance pins 5,6, and also provide opposite north and south poles in a direction parallel to the guidance pins 5,6. The opposite direction of the magnetizations M and MM of the first and second magnetized zones 12 and 13, respectively, wherein the, for example, north pole of the first magnetized zone 12 is adjacent to the north pole of the second magnetized zone 13, provides for a high concentration of magnetic flux lines in the region of both north poles, where the first and second magnetized zones 12 and 13 are adjacent, thereby increasing the energy- efficiency of the actuator assembly. The construction of the pairs of permanent magnets 20 and 21 allows the above- mentioned effect to be achieved without the use of a yoke. The overall configuration is relatively compact, and provides for a reduction of the number of parts of the first actuator assembly 100.

Forces F acting on the guidance pins 5,6 resulting from the magnetic field of the pairs of permanent magnets 20 and 21 are shown in Fig. 5. The resulting forces F are symmetrical around the center of mass of the barrel 2, resulting in very little tilt of the barrel 2 versus the housing 3 and little noise during movement.

A displacement of the first part with respect to the second part can be measured by a position-measuring sensor. Different types of sensors can be used, such as, for example, optical reflector types and magnetic field sensor types. In an embodiment of the actuator a measurement can be made of the displacement between a first part with a magnet and a second part equipped with the position-measuring sensor. Fig. 6 shows a second actuator assembly 200 comprising a hall sensor 28 and the movable pair of permanent magnets 21. The displacement of the pair of permanent magnets 21 is detected by the hall sensor 28 located near the pair of permanent magnets 21. The displacement of the pair of permanent magnets 21 generates a voltage in the hall sensor 28, which is a function of the relative distance between the hall sensor 28 and the pair of permanent magnets 21. Fig. 7 shows an example of such a function, wherein the X-axis represents the distance between the hall sensor 28 and the pair of permanent magnets 21 and the Y-axis represents the voltage generated in the hall sensor 28 as a result of the displacement of the pair of permanent magnets 21. Alternatively, an additional permanent magnet is provided on the barrel 2, which is dedicated for use with the hall sensor 28 and which may be smaller than the pair of permanent magnets 21. Figs. 8, 9 and 10 show a third actuator assembly 300. Like parts are numbered in the same way as in Figs. 1, 3, 4 and 5. In addition to, or instead of, the ferromagnetic guidance pins 5,6, the third actuator assembly 300 comprises a ferromagnetic back plate 16. The third actuator assembly 300 further comprises an electrically conductive coil 17, which is attached to the barrel 2, and two pairs of permanent magnets 30 and 31 that are mounted on the housing 3. The pairs of permanent magnets 30 and 31 are similar in construction to the pairs of permanent magnets 20 and 21 of the first actuator assembly 100. The ferromagnetic back plate 16 is arranged in between the electrically conductive coil 17 and the barrel 2, and the electrically conductive coil 17 is arranged in between the pairs of permanent magnets 30 and 31 and the barrel 2, wherein, for example, the electrically conductive coil 17 is separated from the pairs of permanent magnets 30 and 31 by a small gap. The ferromagnetic back plate 16 interacts with the pairs of permanent magnets 30 and 31 to provide the force holding the inside surface parts of the holes 8 and 9 against the guidance pins 5,6. In addition, the ferromagnetic back plate 16 acts to enhance the magnetic field generated when current is passed through the electrically conductive coil 17. Forces F acting on the guidance pins 5,6 resulting from the magnetic field are shown in Fig. 10.

For a given motor configuration, the force acting the guidance pins 5,6 of the third actuator assembly 300 is fixed, whereas, in the embodiment of the first actuator assembly 100, this force can be determined independently by varying the position of the holes 8 and 9 to vary the distance between the pairs of permanent magnets 20 and 21 and (ferromagnetic) guidance pins 5,6. However, the force F of the third actuator assembly 300 can be changed by, for example, increasing the size of the ferromagnetic back plate 16, in which case the relatively large ferromagnetic back plate 16 allows for use of smaller barrel- mounted pairs of permanent magnets 20,21. Another example of influencing the force F of the third actuator assembly 300 is to choose another material for the ferromagnetic back plate 16 having a different magnetic permeability. Moreover, variation of the dimensions of the electrically conductive coil 17 still allows one to change the distance between the barrel- mounted pairs of permanent magnets 20,21 and the ferromagnetic back plate 16.

Figs. 11, 12, 13 and 14 depict a fourth actuator assembly 400. Like parts are numbered in the same way as in Figs. 1, 3, 4 and 5. In addition to, or instead of, the fourth actuator assembly 400 comprises a square-shaped pair of permanent movable magnets 40 with two opposing magnetized zones 41, 42 and an electrically conductive coil 47. The pair of permanent magnets 40 is arranged around the barrel 2 and fixed to it and the coil 47 is static and attached to the housing 3. A small gap separates the pair of permanent magnets 40 and coil 47.

Resulting forces F that are provided by the pair of permanent magnets 40 and are acting on the guides 5,6 are shown in Fig. 14. Because of the symmetrical construction of the pair of permanent magnets 40, the guides 5,6 of the fourth actuator assembly 400 should not be placed symmetrical in relation to the pair of permanent magnets 40 in order to provide for the net force F acting on the guides 5,6 that is not equal to zero. This is achieved, for example, by a distance from the guides 5,6 to the pair of permanent magnets 40 in the X- direction that is different from a distance from the guides 5,6 to the pair of permanent magnets 40 in the Y-direction. As a result, movement of the barrel 2 along the guides 5,6 is opposed by a frictional force. Figs. 15, 16 and 17 show a fifth actuator assembly 500. Like parts are numbered in the same way as in Figs. 1, 3 and 5. In addition, or instead of, the fifth actuator assembly 500 comprises a ferromagnetic back plate 55, formed as a segment of a cylinder, one cylindrical pair of permanent magnets 50 with two opposing magnetized zones 51,52 and a stationary cylindrical electrically conductive coil 57. The pair of permanent magnets 50 is arranged around the barrel 2 and fixed to it. The coil 57 and the ferromagnetic back plate 55 are static and attached to the housing 3.

The barrel 2 is linearly guided by a first and a second guiding ring 53, 54 to allow focussing of an optical system comprising the lens body 4. To this end, the housing 3 comprises a first and a second bearing cylinder 58, 59 for linearly guiding the barrel 2 via the first and second guiding ring 53, 54.

The ferromagnetic back plate 55 is arranged in between the coil 57 and the barrel 2, and the coil 57 is arranged in between the pair of permanent magnets 50 and the barrel 2, wherein, for example, the coil 57 is separated from the pair of permanent magnets 50 by a small gap. The ferromagnetic back plate 55 interacts with the pair of permanent magnets 50 to provide the force F holding the first and second guiding rings against the first and second bearing cylinders 58, 59 of the housing 3. The force F acting on the barrel 2 resulting from the magnetic field is shown in Fig. 17. In addition, the ferromagnetic back plate 55 acts to enhance the magnetic field generated when current is passed. The first, second, third, fourth and fifth actuator assembly 100, 200, 300, 400 and 500 can be driven in substantially the same way. Each is provided with an electromechanical unit for providing a force transverse to the guidance pins 5,6 or bearing cylinders 58, 59, acting to hold inside surface parts of the barrel 2 against the surface of the guidance pins 5,6 or bearing cylinders 58, 59. Thus, static friction prevents motion of the barrel 2 relative to the housing 3 when no current is passed through the electrically conductive coils 11,17,47,57. This enables the barrel 2 to be displaced relative to the housing 3 using a pulsed current, which reverts to a low level or no current at all between pulses. Friction ensures that the position held by the barrel 2 is maintained. With a substantially constant, and thus predictable, friction force opposing motion of the barrel 2, open- loop control becomes possible. This simplifies the driver construction, compared with stepper motors, piezo drives and voice coil actuators in a servo system.

Figs. 18 and 19 show the voltage applied to terminals of the electrically conductive coils 11,17,47,57 in two different modes of driving. In each, a driver apparatus translates a desired displacement into a series of substantially equal incremental displacements. Each displacement is effected by applying a periodic pulse train for a number of periods corresponding to the number of incremental displacements. The driver apparatus need only be configured to store the distance corresponding to the incremental displacement, together with the waveform of the current required to effect the incremental displacement. It is not necessary to have the driver include memory for storing a parameter field. It will be appreciated that the direction of movement of the first part relative to the second part is changed by changing the polarity of the periodic pulse train that is applied.

In Figs. 18 and 19, ΔTi indicates the duration of one period of the waveform characterizing the periodic pulse train. In Fig. 18, a pulse is applied over a first interval ΔT₂. In the illustrated embodiment, no current is passed through the electrically conductive coils 11,17,47,57 during a second interval ΔT3. This allows the barrel 2 essentially to come to a standstill before the next incremental displacement is effected by applying a further pulse. Instead of applying no current at all during the second interval ΔT₃, a current smaller than that required to generate a sufficiently large driving force to overcome the frictional resistance may be passed through the electrically conductive coils 11,17,47,57. It is observed that both Figs. 18 and 19 relate to embodiments in which voltage pulses of substantially equal amplitude are applied. This is an advantage in simple mobile applications where only one system voltage is available. In more sophisticated embodiments, the pulse height and/or duration are modulated. In the embodiment shown in Fig. 19 the pulse applied during the first interval

ΔTi commences with a first sub-interval ΔT₂ in which a first pulse is applied that has a higher voltage than a second pulse that is applied over a second sub-interval ΔT₄. The higher first pulse provides for a force that compensates the frictional force in a more efficient way. The amount of friction depends partly on the choice of material for the barrel 2 and guidance pins 5,6. In one example, the barrel 2 is made of a liquid crystal polymer, which is wear-resistant and amenable to accurate machining, such as, for example, Polyphenylene Sulfide (PPS). The guidance pins 5,6 can be made of steel with extra polishing being applied, if required. Sapphire bearings in the barrel 2 may be employed in high-end applications. After a step, it is possible to determine the distance of the actual travel using a position-measuring sensor (e.g. a hall sensor). If the distance shows a deviation from the intended step, the width or height of the voltage pulse can be adapted to correct the distance of a next step.

In summary, the invention provides for an actuator assembly comprising a first part, and a second part, which comprises a guide for guiding motion of the first part relative to the second part. The actuator assembly further comprises an electro-mechanical unit for generating a driving force between the first part and the second part along the guide in response to a driving signal, and at least one pair of permanent magnets attached to one of the first and second parts, and which comprises a first magnetized zone and a second magnetized zone, which are positioned in a line, for providing a force transverse to the guide, acting to hold at least one surface part of the first part against a surface of the guide. The respective components of magnetization of the first and the second magnetized zone of the at least one pair of permanent magnets parallel to the line are oriented in opposite directions.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word "comprising" does not exclude the presence of other elements or steps than those listed in a claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements

Claims

CLAIMS:

1. Actuator assembly (100,200,300,400,500), comprising a first part (2), a second part (3), which comprises a guide (5,6,58,59) for guiding motion of the first part (2) relative to the second part (3), - an electro-mechanical unit (11,17,47,57) for generating a driving force between the first part (2) and the second part (3) along the guide (5,6,58,59) in response to a driving signal, and at least one pair of permanent magnets (20,21,30,31,40,50) attached to one of the first and second parts, comprising a first magnetized zone (12,41,51) and a second magnetized zone (13,42,52), which are positioned in a line, for providing a force transverse to the guide (5,6,58,59), acting to hold at least one surface part of the first part (2) against a surface of the guide (5,6,58,59), wherein, respective components of magnetization of the first and second magnetized zone (12,13,41,42,51,52) parallel to the line are oriented in opposite directions.

2. Actuator assembly according to claim 1, wherein the actuator assembly further comprises a position measuring sensor (28) attached the other of the first and second parts for measuring a displacement of the first part (2) with respect to the second part (3) as a result of the driving force.

3. Actuator assembly according to claim 1, wherein the electro-mechanical unit for generating the driving force comprises at least one electrically conductive coil (11,17,47,57), attached to the other of the first and second parts, for generating a magnetic field acting on the at least one pair of permanent magnets (20,21,30,31,40,50), in response to the driving signal.

4. Actuator assembly according to claim 3, wherein the other of the first and second parts further comprises a ferromagnetic back plate (16,55), wherein the electrically conductive coil (11,17,47,57) is positioned between the ferromagnetic back plate (16,55) and the at least one pair of permanent magnets (20,21,30,31,40,50).

5. Actuator assembly according to claim 1, wherein the at least one pair of permanent magnets (40,50) is arranged to surround a portion of the first part (2) and wherein the location of the guides (5,6) is asymmetrical with respect to the surrounding pair of permanent magnets (40,50).

6. Opto -mechanical device comprising a lens body (4) and an actuator assembly (100,200,300,400,500) according to any one of claims 1 to 5.