Device for accessing optical recording carriers

The invention concerns a device for accessing optical recording carriers, e.g. CD, DVD, blue laser light based formats of optical recording carriers like HD-DVD or BluRay Disk BD, or future formats. In the following, "accessing" is meant to comprise reading access for reading of informations from an optical recording carrier where they are stored, writing access for writing or recording of informations into the optical recording carrier, as well as combinations of reading and writing access. Such a device has an optical scanner often also called pickup, which generates a scanning beam, often a laser beam, to scan the recording carrier via a focusing lens, and detects a detection beam that has been influenced by the optical recording carrier. The optical scanner has a lens holder bearing actuator coils, and it has a magnet configuration for generating a magnetic field permeating the actuator coils. The lens holder serves to hold the focus lens and is movable relative to the pickup and to the recording carrier, in order to make possible to shift the focus lens both in a direction orthogonal to the surface of the optical recording carrier, i.e. in the so- called focus direction, as well as in a direction parallel to the surface of the optical recording carrier, i.e. in a tracking direction orthogonal to information tracks on the carrier. It also makes possible to rotate the focus lens around an axis substantially parallel to the information tracks being accessed, which movement is called tilt. A focus, tracking, and tilt servo, respectively, impose electrical currents on the actuator coils, for moving, i.e. translating and rotating, the lens holder in the desired direction, and into the desired position and orientation.

Publication US 20040022168 A shows such a device, where the magnet configuration consists of main and side magnets of opposite magnetisation. This known arrangement can be seen to have the drawback that the magnetic fields of opposing polarity which point towards the actuator coils cause a decrease of the resulting total magnetic field and a decrease in sensitivity, in particular of the focus servo control .

The invention aims to solve the problem to improve such a device with respect to actuator sensitivity.

According to the invention, the magnet configuration consists of a main magnet with a main magnetisation direction orthogonal to the surface of the magnet configuration, and one or more side magnets, attached to sides of the main magnet, the side magnets having a side magnetisation direction oriented at right angle to the main magnetisation direction. The advantage is that, effective branches of a tracking coil situated in front of the side magnets meet magnetic fields of different strength or even of opposite polarity. This increases the tracking sensitivity, while at the same time the magnetic field permeating a focus coil situated in front of the entire magnet configuration is not being diminished by the side magnets, so that focus servo sensitivity remains high and undiminished. By varying the size, the magnetic field strength and the position of the main and side magnets, the sensitivity of the actuator with respect to focus, tracking and tilt servo can be varied within wide limits.

It is advantageous to attach side magnets as described above onto a main magnet in tracking direction as well as in focus direction. This has the advantage of causing a strengthened magnetic field for the focus and tilt coils.

Advantageously, the side magnets are magnetised in such a way that their pole pointing towards the main magnet is of the same type as the pole of the main magnet pointing towards the actuator coils, with other words, these two poles are either both "North" type poles or both "South" type poles. This has the advantage that, in the area where the two magnets touch, the magnetic field permeating the actuator coils is strengthened.

According to the invention, a further increase in focus sensitivity can be achieved if the width of the focus coil in tracking direction is chosen smaller than the width of the magnet configuration, so that the active branches of the focus coil primarily lie in the region where the magnetic field of the main magnet is strengthened by the side magnets .

Further advantages of the invention and variants thereof are given in the following description of example embodiments. In these,

Fig. 1 shows parts of a device according to the invention in 3D view;

Fig. 2 shows parts of a device according to the invention with a view to the lens holder; and

Fig. 3 shows a variant of a magnet configuration according to the invention.

Fig. 1 shows a part of an optical scanner 1 of a device according to the invention, with a lens holder 2, a focus lens 3 arranged therein, as well as holding wires 4, which connect the lens holder 2 to the holder 5 and constitute mechanically a resilient parallelogram suspension and electrically a conductive connection. Contact points 6 serve for the electrical connection; for reasons of clarity Fig. 1 does not show that they actually are connected to the holder wires 4.

The holder 5 is connected to a base 8 made of ferromagnetic material, the base having straps 9, 10 forming a magnetic shunt together with the magnet configuration 11. The magnetic field as it exists in the clearance or airgap between the magnet configuration 11 and the strap 10 acts upon the coils, not or only partially visible here; and the coils serve as actuators for setting the focus in focus direction 12, the tracking in tracking direction 13, or the tilt in tilt direction 14.

The magnet configuration 11 consists of a main magnet 21 and two side magnets 22 attached to sides of the main magnet 21 in tracking direction 13. The main magnetisation direction 31 of the main magnet 21 is at right angles to the surface of the magnet configuration 11 and also at right angles to the side magnetisation direction 32 of the side magnets 22, shown here only for the front side magnet 22.

Fig. 2 shows a part of a device according to the invention, which is shown here with a view to the lens holder 2, but without the magnets. Tracking coils 16 having vertically oriented effective branches 17, 17' are wound onto first supports 15. Any current through the tracking coils 16

causes a tracking alignment by moving the lens holder 2 to a desired position along the tracking direction 13. In a magnet configuration 11 according to the invention having a main magnet 21 and side magnets 22, the move and hence the sensitivity of tracking alignment is increased because, carrying currents of opposite direction, the two effective branches 17, 17' of the tracking coils 16 are situated, respectively, in the vicinity of one of the opposing pole ends of the side magnets 22. The additional side magnets 22 generate an additional Lorentz force at the outer effective branches 17' of the tracking coils 16, which increases tracking sensitivity. In a device without the side magnets 22, maximum tracking sensitivity could only be reached if only the inner effective branches 17 of the tracking coils 16 lie within the magnetic field of the main magnet 21.

A focus coil 26 is wound into a clearance 25 which runs all around the lens holder 2. Among the branches of the focus coil 26, those oriented in tracking direction 13 constitute the active branches 27, because there the coil wires are completely immersed in the magnetic field of the magnet configuration 11, not shown here, and are oriented perpendicular to the direction of the magnetic field as well as perpendicular to the direction of focus movement 12. Any electrical current through the focus coil 26 causes a focus alignment by moving the lens holder 2 upwards or downwards in focus direction 12. Focus sensitivity is increased because not only are the effective branches 27 of the focus coil 26 exposed, over their entire length, to the undiminished magnetic field of the main magnet 21, but also this magnetic field, in its border areas near the side magnets 22, is actually strengthened by the superposed field

of the side magnets oriented perpendicular to the main magnet field.

Tilt coils 36, 36' , only schematically shown in the Figure, are wound onto second holders 35. Their effective branch 37 lies parallel to the effective branch 27 of the focus coil 26. The two tilt coils 36, 36' are electrically connected in such a way that, interacting with the magnetic field of the magnet configurations not shown here, a tilt alignment by tilting the lens holder 2 in tilt direction 14, corresponding to a rotation around the axis of the track direction 18, is caused. This is achieved by chosing the direction of the currents flowing through the tilt coils 36, 36', depending on magnet polarity, in such a way that, when the two tilt coils 36, 36' are connected in series, the current running through them causes, in the two coils, moving forces in focus direction 12 but of opposite orientation .

Fig. 3 schematically shows a variant of a magnet configuration according to the invention with a main magnet 21, with first side magnets 22 arranged to the left and to the right of it, as well as with a second side magnet 23 arranged underneath the main magnet 21. The second side manget 23 also has a side magnetisation direction 33 perpendicular to the main magnetisation direction 31. Above the magnet configuration 11, tracking coils 16 are shown for clarity displaced vertically upwards, in reality they are arranged behind, and hidden by, the magnet configuration 11, each near the respective first border line 24 between main magnet 21 and first side magnet 22. The Figure also shows, spatially displaced to the back, hence displaced to the top and right in the Figure for clarity reasons, tilt coils 36,

36', which in reality are arranged above a second border line 34 between main magnet 21 and the second side magnet 23. The second side magnet 23 increases the magnetic field of the main magnet 21 in the region of the tilt coils 36, 36', achieving an increased sensitivity.