The invention relates to an actuator for an optical pickup unit in an optical player, comprising a lens holder, a support frame, means for suspending the lens holder in the support frame, which means constrain movement of the lens holder relative to the support frame, allowing an at least limited translation in a focussing direction, substantially parallel to the optical axis of a lens in the lens holder, and actuator means, comprising at least one conductive focussing coil, each coil being positioned relative to a fixed magnetic circuit in such a way that a current flowing through the coil gives rise to a force between the lens holder and the support frame in the focussing direction.

The invention further relates to an optical pickup unit and an optical player comprising such an actuator.

The actuator is one of the key parts in an optical pickup unit for reading and/or writing signals on an optical disk. In a mass production line, one of the reasons for a fall-off reject is so-called TIF (i.e. tilt-in- field) fall-off, which means that the tilt angle of the lens holder is too big. The reason for the fall-off is mainly due to the operators' bad soldering of the suspension means, i.e. a suspension wires, poor productivity of the translator jig, which is to be used for making a translator, and a poor design of the actuator. The hinging properties of the lens holder are also affected by the suspension means; in case of the suspension wires by different length of the wires, bent wires, different phase of wires etc., resulting in different stiffness of the wires. Due to these inaccuracies the TIF angle at extreme positions in focusing direction becomes even worse than at a nominal or central position. This is the reason that actuators with a rather big TIF angle are screened at the end of the assembly line. As such TIF fall-off is checked mainly in the final phase of the assembly lines, it is not easy to eliminate this reject fully in the line.

It is an object of the present invention to provide an actuator in which the TIF reject rate can be reduced by reducing the influence of the suspension means of the lens holder.

To obtain this object, the lens holder in the actuator is providing with magnetizable members effectively opposite the fixed magnetic circuit. The actuator according to the invention is described in claim 1. The magnetizable members will provide a stiffness of the lens holder against movements of the lens holder in focusing direction away from a central or nominal position. In this way, the stiffness of the lens holder is divided into two groups of stiffness, that is the stiffness provided by the (mechanical) suspension means and the stiffness provided by the magnetic attraction of the magnetic circuit. Therefore, in comparison to conventional suspension means, the stiffness of the suspension means according to the present invention can be reduced and consequently, the influence of in accuracies of this suspension means is reduced, especially in extreme positions of the lens holder. When a proper arrangement of the magnetizable members is used, the attraction forces on these iron pieces will counteract a tilting of the lens holder, which further reduces TIF rejects. Preferably, the magnetizable members are arranged on a line where the magnetic flux density from the magnetic circuit is constant. In this arrangements the magnetisible members will be attracted to the neutral position on the magnet where the flux density of the magnet keeps constant, i.e., which is preferably the central or nominal position of the lens holder. Preferably, the magnetizable members comprise iron pieces as these are very simple means to provide stiffness in cooperation with the already present magnetic circuit. They may be relatively small-sized so that they do not add much weight to the mass of the lens holder. For example, such iron piece may have a weight of less than 1 mg. Considering the normal weight of the total moving part of the actuator is supposed to be around 320-350 mg, the portion of the iron pieces in the moving part makes not even a few percent of the total moving mass, so the dynamic characteristics effect of the moving part due to the adding of the iron pieces is negligible.

It is favourable if the magnetizable members are provided on opposite sides of a plane through the central mass of the lens holder since this will enhance the stiffness against undesired tilting.

To obtain a simple arrangement in an embodiment in which the focusing coils are provided in pairs and have a winding axis perpendicular to the focusing direction, each of said magnetizable members is provided substantially central to two focusing coils in each pair. Only two magnetizable members are required in this arrangement.

It is preferred that the means for suspending the lens holder includes wire members, each fixed at one end to the lens holder and at the other end to the support frame, said wire members being relatively flexible, such that the stiffness of the lens holder in focusing direction is caused by the magnetizable members for a substantial percentage. The optimal figure will also depend on the actuator dynamic requirements, i.e. 1 ^st resonant frequency of the actuator, damping ratio, the weight of the moving mass, K- factor (electromagnetic parameter), etc.

The invention will now be explained in further detail with reference to the accompanying drawings in which:

Fig. 1 is a very schematic drawing of an optical player including an optical pick-up unit according to the invention;

Fig. 2 is a perspective view of an embodiment of the actuator according to the invention.

Figs. 3 and 4 are views according to the lines III-III and IV-IV in Fig. 2 respectively, with the support frame removed.

Fig. 5 A and 5B show a schematic diagram of an actuator for an optical pick up unit according to the prior art in a nominal/central position and in the highest position, respectively.

Fig. 6A, and 6B show a schematic diagram of an embodiment of an actuator for an optical pick up unit according to the invention in the nominal/central position and in the highest position, respectively.

Fig. 1 diagrammatically shows an optical player in accordance with the invention, which comprises a turntable 1, which can be rotated about an axis of rotation 3 and driven by an electric motor 5, which is secured on a frame 7. An optically scannable information carrier 9, such as a CD, DVD or Blue ray disk, can be placed on the turntable 1, which information carrier is provided with a disk-shaped substrate 11 on which an information layer 13 having a spiral- shaped information track is present. The information layer 13 is covered with a transparent protective layer 14.

The optical player further comprises an optical pickup unit 15 in accordance with the invention for optically scanning the information track present on the information

layer 13 of the disk 9. The optical pickup unit 15 can be displaced with respect to the axis of rotation 3 mainly in two opposite radial directions Y and Y' by means of a displacement device 17 of the optical player. For this purpose, the optical pickup unit 15 is secured to a slide 19 of the displacement device 17, and the displacement device 17 is further provided with a straight guide 21 provided on the frame 7 and extending parallel to the Y direction, over which guide the slide 19 is displaceably guided, and with an electric motor 23 by means of which the slide 19 can be displaced over the guide 21. In operation, an electrical control unit of the optical player, which is not shown in Fig. 1, controls the motors 5 and 23 so as to cause the disk 9 to rotate about the axis of rotation 3 and, simultaneously, the optical pickup unit 15 to be displaced parallel to the Y-direction, in such a manner that the spiral- shaped information track present on the information layer 13 of the disk 9 is scanned by the optical pickup unit 15. During scanning, the information present on the information track can be read by the optical pickup unit 15, or information can be written on the information track by the optical pickup unit 15. The disk 9 is read by detection of light reflected in the disk 9. For example, a light beam is reflected in the direction of the disk 9 by means of a mirror, which is part of the optical pickup unit 15. If the optical pickup unit 15 is also suited for writing information on the disk, the light beam will have a different power level and/or wavelength during writing, but must also be focussed onto a point in the disk 9, as is the case when the disk 9 is being read. Light reflected by the mirror is focussed onto the disk 9 by means of an objective lens 23, situated in a lens holder 25.

In a typical optical disk system, the information tracks are very closely spaced in the radial direction, in order to fit as much information as possible onto the disk 9. In a Compact Disk (CD) the distance is 1.6 μm, in a Digital Versatile Disk (DVD) the distance is 0,74 μm. There is a tendency towards smaller track distances in newer systems, as sources of (laser) light of smaller wavelengths and objective lenses 23, or lens systems, with a higher numerical aperture become available. In the configuration shown in Figs. 1 and 2, the light beam is aligned in a radial direction relative to the disk 9. The position and orientation of the mirror and objective lens 23, determine the point on the disk 9 at which the light is focused. Smaller distances between successive information tracks are made possible by more accurate actuator arrangements for controlling the position and orientation of the optical pickup unit 15.

As mentioned before, the position of the optical pickup unit 15 as a whole in the radial direction of the disk 9 is controlled by means of the displacement device. However,

fine-tuning of the position of the focusing point in the disk 9 is then carried out by adjusting the position of the lens holder 25 relative to the rest of the optical pickup unit 15. To this end, the optical pickup unit 15 comprises a support frame 27, fixed to or part of the slide 19.

The lens holder 25 is suspended in the support frame in such a way that its movement relative to the support frame is constrained. Referring to Fig. 2, the lens holder 25 is firstly able to carry out translations in a focusing direction z. That is, it can be moved closer or further away from the disk 9. In this way, the exact point in the disk 9 on which the light is focused can be adjusted. Secondly, the lens holder 25 is able to carry out translations in a tracking direction y. By varying the position of the lens holder 25 in the tracking direction, the position on which the light beam 4 is focused can be moved further or closer to the center of the disk 9. Thirdly, the lens holder 25 can be tilted, i.e. it can carry out rotations about a tangent direction x. In this way, the light beam can be focused on the disk 9 in such a way that it is always locally perpendicular to the surface of the disk, despite inclination of the disk. The adjustment of position and orientation of the lens holder 25 is used to adjust for small geometric deviations in the disk 9, or in the information track. In particular, deviations from a perfect plane - an "umbrella- like" shape - can be compensated for, by varying the degree of tilt and the position in the focusing direction. The possibility of translating the lens holder 25 in the tracking direction y, makes it possible to compensate for deviations from a spiral or circular shape of the information track. This becomes more important as a lens 23 with a higher numerical aperture is used. Such a lens can be positioned closer to the disk, and makes it possible to read a disk 9 with narrow and closely spaced information tracks.

To accurately control the position and orientation, the optical pickup unit 6 comprises an actuator and a control circuit (not shown). The control circuit provides the driving signals for the actuator. It is not considered part of the invention and a multitude of possible implementations of a control circuit for this purpose are known, so that no further description is given of the control circuit.

As shown in Figs. 2 - 4, the actuator comprises a first focusing coil 29 and a second focusing coil 31 on each opposite side of a plane through the center of mass of the lens holder 25 and parallel to the focusing direction z and tangent direction x. The winding axis of each coil is perpendicular to the focusing direction z. The focusing coils 29, 31 are fixed to the lens holder 25. A magnetic circuit is provided for each of the focusing coils 29, 31. This magnetic circuit comprises, opposite to each pair of focusing coils 29, 31, a yoke 33

and permanent magnets 35 and 37. These magnets 35, 37, viewed in a direction parallel to the focusing direction z, are arranged next to each other on the closing yoke 33 manufactured from a magnetizable material. The permanent magnets have respectively, a direction of magnetization M directed parallel to the tangent direction x, and a direction of magnetization M' directed parallel to the tangent direction x, but opposite thereto (see Fig. 3). Of course, a yoke and electromagnets could also be used, in principle.

The same magnetic circuit is also used for the actuator arrangement used to control movement in the tracking direction y. The magnetic circuit forms a loop in a plane parallel to the focusing direction z and the tracking direction y. The flux is therefore also parallel to the tangential direction x at a point in the circuit.

The air gaps also provides space for accommodating radial coils 39, which are mounted in the air gap in each magnetic circuit, with their winding axis aligned in the tangential direction x. The radial coils 39 and the magnetic circuit form an actuating arrangement for controlling the position of the lens holder 25 in the tracking direction y. Instead of using only one radial coil 39, it would also be possible to use two.

The preferred means by which the lens holder 25 is suspended in the support frame 27 of the optical pickup unit 15 are formed by six wire members 41. Each is fixed at one end to the lens holder 25, and to a support frame part 43 at the other end. The wire members 41 are made of a resilient material, preferably electrically conductive, e.g. copper, iron, or an alloy. The number of wires could be different, for example, two, four or eight.

The wire members 41 limit the number of degrees of freedom of the lens holder 25. Only translations in the tracking direction y and the focusing direction z are possible. Only tilt about the tangent direction x is allowed. In particular, tilt about the focusing direction z and tracking direction y is not possible. However, in extreme positions of the lens holder 25 in focusing direction (for example -0,65 mm to +0,65 mm from the nominal or central position), especially if already inaccuracies in the wire members are present, a tilt angle TIF (Tilt-In-Field) could arise. This TIF angle is illustrated in Fig. 5B which shows the prior art situation.

Therefore, in order to reduce this tendency, the present invention proposes to reduce the stiffness of the suspension wires 41 and to add magnetic stiffness. The reduction of the wire stiffness is obtained, for example, by making the wires longer or thinner. This reduces the influence on the TIF. This weakened stiffness of the suspension members is compensated by a stiffness caused by magnetic suspension means. These are formed, in this case, by magnetizable members 45 attached to the lens holder 25 effectively opposite the

magnetic circuit formed by the magnets 35, 37. As is also shown in Fig. 6A, 6B, in this embodiment, the magnetizable members 45 are small iron pieces. They may be made for example from 304 or 403 steel (X5CrNil8-l 1 or X7Crl3), but other materials are conceivable. They are arranged symmetrically with respect to the plane through the center of mass of the lens holder 25 and parallel to the focusing direction z and tangent direction x and centrally between the focusing coils 29, 31 in each pair each opposite to the permanent magnets 35, 37.

In the nominal position of the lens holder 25 (Fig. 6A), the center of the magnetizable member 45 in focusing direction z is aligned with a line L where the magnetic flux density of the permanent magnets 35, 37 keeps constant. As a result the magnetizable members 45 are attracted by the magnets 35, 37 in the direction of this nominal position of the lens holder 25. There is thus a magnetic pretension to this nominal position and therefore the magnetizable members 45 add magnetic stiffness to the lens holder 25. The magnetizable members 45 also counteract a tilting movement of the lens holder 25 around the y direction (called alpha direction) and therefore the TIF angle will be further reduced in the extreme positions of the lens holder (Fig. 6B). The magnetizable members will also counteract a tilting movement around the x direction (called beta direction), as appears from Fig. showing the co-operation.

Instead of one magnetizable member 45 per side of the lens holder, it would also be conceivable, for example, to provide two magnetizable members. In this case it would be preferred to arrange these magnetizable members 45 in the center of the focusing coils 29, 31 as indicated by dash lines in Fig. 4. Such arrangement would even better counteract a tilting movement in beta direction.

The ratio between magnetic stiffness delivered by the magnetizable members 45 and the mechanical stiffness by the suspension wires 25 can be chosen according to the specific design, as long as the overall stiffness of the actuator system remains constant. Otherwise the natural frequency will be changed affecting the basic actuator dynamics. In general the smallest mechanical stiffness will result in the best TIF performance, so maximization of the magnetic stiffness is at present preferred. By means of the above-described optical player in accordance with the invention, it is possible, during scanning the information layer 13 of the disk 9, to read information present on the information layer 13 or to write information on the information layer 13. It is to be noted that the invention also relates to optical players and optical pickup

units by means of which only information present on an information layer of an information carrier can be read.

The invention is not limited to the above-described embodiments, which may be varied within the scope of the claims. For example, it is not strictly necessary that the lens holder be suspended by rod-shaped wire members. Further, although a single objective lens 23 is used in the described embodiment, the lens holder may comprise a more elaborate optical system for focusing and/or splitting the beam, depending on the complexity of the optical drive. The arrangement of coil and magnets may be altered according to requirements.

The magnetizable members may also be magnets or other members that cause an attractive force.

In the presently preferred embodiments, the disk is an optical disk. However, it should be understood that the invention can also be used for all kinds of other disks e.g. ferro-electric, magnetic, magneto-optic, near- field, active charge storage disks or other disks using combinations of these techniques or other reading and/or writing techniques. In these cases the lens and laser will be replaced by another reading/writing member which may require cooling.

In general it is noted that, in this application, the expression "comprising" does not exclude other elements, and "a" or "an" does not exclude a plurality. A single processor or unit may fulfill the functions of several elements in the appended claims. Reference signs in the claims shall not be construed as limiting the scope thereof.