Electro-optical scanning device and optical player comprising the scanning device

Abstract: An electro-optical scanning device comprises a lens (2) having an optical axis (2a), which lens is movable in a focussing direction (F) to form at least one radiation spot on a surface to be scanned and in a tracking direction (T) oriented transversely of the focussing direction, in order to follow a recording track. The scanning device comprises a stationary section (16) with a magnetic circuit and one air gap (20), and a section (6) which is movable relative to the stationary section and comprising said lens, which movable section comprises at least one focussing coil (12) and two tracking coils (14a, 14b) for driving the lens, the focussing coil having a coil axis (12a) extending parallel to the optical axis. The air gap is situated between the optical axis of the lens and the coil axis of the focussing coil, the mass centre of the movable section, which section is secured to the stationary section by a compliant mechanical suspension (22a, 22b), being situated in or at least near the air gap.

Description: The invention relates to an electro-optical scanning device comprising a movable lens having an optical axis, which lens is movable in a focusing direction to form at least one radiation spot on a surface to be scanned and in a tracking direction oriented transversely of said focusing direction, in order to follow a recording track in said surface, and comprising a stationary section with a magnetic circuit and with a single air gap extending in a plane defined by said focusing direction and said tracking direction, and a movable section which extends in a direction oriented transversely of a plane defined by said focusing direction and said tracking direction and is movably secured to the stationary section by a compliant mechanical suspension and the movable section carrying said lens and comprising a focusing coil having a coil axis extending parallel to said optical axis at least one tracking coil for driving the lens, each of said coils extending into said single air gap. Such a scanning device is known from EP-A-0 359 670. The known device has a stationary body and an actuator body supported so as to be movable in a direction parallel to a rotational axis of a disk and in a radial direction. Holding members, each comprising a thin plate, are secured to an end portion of the actuator body, while a focusing coil is affixed to another end portion of the actuator body, tracking coils being affixed to an outer side of the focusing coil. An objective lens carried by the actuator body is situated between the holding members and the focusing coil. Hinge portions are formed at the ends of the holding members to connect the members to the actuator body and the stationary body, respectively. In order to simplify vibrational modes and to achieve a more stable focusing and tracking servo operation in an embodiment of the known scanning device the center of gravity of the movable actuator body is arranged to coincide with the location of a hinge portion connecting a holding member to the stationary body. A balancer is provided on the side opposite the objective lens with said hinge portion on the fixed side being adopted as a point of symmetry, the balancer being supported by hinge portions. Another electro-optical scanning device is known from US-A-4,321,701. This known electro-optical scanning device comprises a stationary section with a single air gap and a vertically disposed movable section. The movable section, which is guided relative to the stationary section via an intermediate body, comprises a plate-shaped body which extends in the air gap and, secured to this body, a holder for a lens and any other optical elements. The holder is positioned in such a way relative to the plate-shaped body that the optical axis of the lens is situated in the plane of the plate-shaped body. Vertical current conductors serving as tracking coils are arranged at two opposite edges of the plate-shaped body. A focusing coil mounted on the plate-shaped body and having a coil axis oriented transversely of the optical axis of the lens is arranged between said conductors. A drawback of this known scanning device is that, as a result of the presence of the vertically disposed plate-shaped body and as a result of the unfavourable arrangement of the plate-shaped body and the lens holder relative to each other, the movable section has a large dimension viewed along the optical axis of the lens. Therefore, the scanning device has a substantial overall height, which renders the device unsuitable for use in modern optical disc players of small height. Another drawback is the comparatively intricate method of manufacturing the composite plate-shaped body. Moreover, it has been found that in practice the mutual arrangement of the lens and the coils is inconvenient and imposes undesirable constructional limitations. It is an object of the invention to improve the scanning device of the type defined in the opening paragraph in such a manner that an easy-to-manufacture scanning device of suitable shape and having a small overall height can be obtained. To this end the scanning device in accordance with the invention is characterized in that the single air gap is situated between the optical axis of the lens and the coil axis of the focusing coil, the movable section having a mass centre situated in or near said single air gap. In the scanning device in accordance with the invention the lens is disposed in a cantilevered relationship to the coils. This has the advantage that when the scanning device is incorporated in an optical player, for example a CD player or a DATA player, a suitable space is obtained to accommodate optical components, such as a semiconductor laser, which cooperate with the scanning device. The scanning device in accordance with the invention also enables a compact construction to be obtained, specifically the small dimension in a direction viewed along the optical axis being very advantageous. As a result of the special arrangement of the focusing coil this coil provides a counterpoise for the lens and lens-suspension part of the movable section of the scanning device, thereby enabling a well-balanced construction to be obtained in a simple manner. The point of application of the driving forces is then situated in or near the mass centre of the movable section, so as to preclude the occurrence of parasitic resonances. Another advantage of the scanning device in accordance with the invention is that the scanning device can be manufactured from a comparatively small number of easy to assemble parts. It is to be noted that US-A-4,878,214 discloses an objective lens supporting device for an optical pickup including an objective lens. A lens holder is movably supported by supporting rods, in such a manner that the objective lens can be moved in both a direction of the optical axis of the objective lens and a direction along an information recorded surface of an optical disc. The device is provided with a driving mechanism for driving the lens holder. This mechanism comprises a focusing coil wound around a rectangular outer periphery of the lens holder and two pairs of tracking coils fixed to the focusing coil. The mechanism further comprises a pair of magnets retained by a stationary yoke, the lens holder being between the magnets and the magnets being opposed to the focusing coil and tracking coils. An embodiment of the electro-optical scanning device in accordance with the invention, is characterized in that the air gap is bounded by a magnet having a magnetization direction which is oriented transversely of a plane defined by the focusing direction and the tracking direction, the focusing coil surrounding the magnet. In this way a small distance can be obtained between the optical axis of the lens and the coil axis of the focusing coil, which is important for the stiffness of the movable section. A high stiffness of the movable section is desirable in order to achieve that possible parasitic resonances of the lens are situated in a high frequency range. An embodiment of the scanning device in accordance with the invention is characterized in that the suspension comprises suspension rods which extend in a direction oriented transversely of the plane defined by the focusing direction and the tracking direction, which suspension rods are each secured to the stationary section and the movable section at a first end and a second end respectively, said ends being provided with an integral hinge allowing movements about an axis which extends in the focusing direction and about an axis which extends in the tracking direction. An advantage of this embodiment is that it allows such a stiffness to be selected for the suspension that any resonant frequencies resulting from a possible unbalance are low in relation to the desired bandwidth of the control system. The hinges used in the suspension may be constructed as double integral hinges. The suspension can be manufactured by means of an injection-moulding process and can be mounted in a simple manner. In order to obtain a translational movement of the lens in the tracking direction, an embodiment of the scanning device in accordance with the invention is characterized in that the suspension rods extend parallel to one another. Another practical embodiment is characterized in that the number of suspension rods is four, the rods at opposite sides of the plane defined by the optical axis of the lens and the coil axis of the focusing coil being arranged in pairs which form part of a single injection-moulded product. An embodiment which is attractive from the point of view of production-engineering is characterized in that the movable section comprises a separately formed lens holder and coil holder. During manufacture the lens holder and the coil holder can be provided independently of one another with the lens and the coils respectively. Subsequently, the lens holder and the coil holder can be secured to one another, for example by means of snapped connections, ultrasonic welding or cementing. For reasons of symmetry the electro-optical scanning device in accordance with the invention will generally comprise two tracking coils. In order to avoid that during production of such a scanning device separate coils have to be handled and mounted, an embodiment of the scanning device in accordance with the invention is characterized in that the coil holder is assembled from a focusing-coil holder, provided with the focusing coil, and two tracking-coil holders, each provided with a tracking coil. It is to be noted that EP-A-0,212,941 discloses an electro-optical scanning unit having an objective lens secured to a frame via a plastics suspension. The suspension allows translational movements of the lens in the focusing direction and rotary movements of the lens to provide tracking movements. The known unit further comprises a double drive system which a teach of two opposite sides of the suspension is provided with a magnet, a focusing coil and two tracking coils arranged, the coils being disposed in comparatively wide magnetic air gaps. An electro-optical scanning unit comprising a double drive system is also known from EP-A-0,348,845. In this unit an objective lens is accommodated in a lens holder which is secured to a frame by means of two pairs of blade springs. The drive system comprises two sets of coils which are arranged in line viewed in the longitudinal direction of the blade springs, each set comprising one focusing coil and two tracking coils. EP-A-0,343,979 describes an object lens driving device comprising a lens holder with an object lens. In order to enable focusing and tracking movements to be performed by the lens the device has a hinge construction comprising one resilient rotary hinge and two resilient connecting members. The prior-art device further comprises a focus drive device and a tracking drive device. The focus drive device comprises two drive units arranged diametrally relative to the lens and each comprising a focusing coil and a magnet cooperating therewith. The invention also relates to an optical player comprising a scanning device in accordance with the invention, a turntable and a slide, the slide being movable along a radial path relative to the turntable, and the plane defined by the optical axis of the objective and the coil axis of the focusing coil being perpendicular to said radial path. The invention will now be described in more detail, by way of example, with reference to the drawings, in which Figure 1 is an exploded view of an embodiment of the scanning unit in accordance with the invention, Figure 2 is a sectional view of the embodiment shown in Figure 1, and Fig. 3 is a perspective view of a deck comprising a device in accordance with the invention. The electro-optical scanning device in accordance with the invention shown in Figures 1 and 2 is referred to as a "2D-actuator" and is intended for use in CD decks constructed for two-stage radial tracking of optical discs, for example CDs of 8 cm or 12 cm. In such a deck an objective can be translated along a radial path relative to a disc which is rotatable about an axis of rotation. For this purpose the deck comprises, for example, a translatable slide carrying the scanning device. An information track of a rotating disc can be inscribed and/or read without mechanical contact by means of a radiation beam produced by a laser. The scanning device in accordance with the invention is constructed in such a manner that the production process can be almost fully mechanized for large-quantity production at minimal costs. The scanning device is particularly suitable for use in so-called "slim-line" equipment. The scanning device comprises an objective lens 2 mounted in a lens holder 4 and having an optical axis 2a. The lens holder 4 forms part of a movable section 6 of the scanning device, the movable section further comprising a focussing-coil holder 8 and two tracking-coil holders 10a and 10b. The coil holders 8, 10a and 10b are interconnected and connected to the lens holder 4, in the present example by means of snap-in connections. The focussing-coil holder 8 comprises a focussing coil 12 having a coil axis 12a which extends parallel to the optical axis 2a. The tracking-coil holders 10a and 10b each comprise a tracking coil 14a and 14b respectively. The scanning device in accordance with the invention further comprises a stationary section 16, which comprises a permanent magnet 18 and ferromagnetic portions. The magnet 18, which has been magnetized in a direction indicated by an arrow A, forms a magnetic circuit with the ferromagnetic portions of the stationary section 16, a magnetic air gap 20 extending between the magnet 18 and one of the ferromagnetic portions 16a. In the present example the magnet 18 and the yoke portion 16a have facing plane-parallel surfaces bounding the gap 20. As a result of this the magnetic field in the gap may be slightly inhomogeneous, which may give rise to parasitic forces. The likelihood that undesirable parasitic forces occur can be reduced by adapting the shape of the magnet 18 and/or the yoke portion 16a. A highly homogeneous magnetic field in the gap 20 is obtained in that at least one of the surfaces of the stationary section 16 which bound the gap is made concave. The movable section 6 is movably secured to the stationary section 16 by means of a two-part suspension 22a, 22b, the focussing coil 12 and the tracking coils 14a and 14b extending partly in the air gap 20. The movable section, in particular the lens 2, is movable relative to the stationary section in a focussing direction, indicated by a double arrow F, and in a tracking direction, indicated by a double arrow T. When the scanning device in accordance with the invention is employed in a device for translating an objective along a radial path relative to a rotating disc, the laser, generally a semiconductor laser, can be arranged underneath or beside the scanning device, as required. In the last-mentioned case a mirror 24 placed underneath the lens 2 is arranged in the light path. The suspension 22a, 22b comprises four suspension rods 26a, 28a, 26b and 28b, which are preferably made of a plastics , which are arranged in pairs and which each have a double integral hinge 28 at their respective ends. The lens 2 has a neutral position, in which the suspension rods 26a, 28a, 26b and 28b extend in a direction oriented in a plane defined by the focussing direction F and the tracking direction T. The suspension rods 26a and 28a together with two connecting portions 30 and 31 form one integral injection-moulded product. An identical product is formed by the suspension rods 26b and 28b and two connecting portions 32 and 33. The parallel suspension rods are secured to the movable section 6 and the stationary section 16 by means of the connecting portions 30 and 32 and the connecting portions 31 and 33 respectively, projections 34 of the sections 6 and 16 engaging in recesses 36 of the portions 30, 32 and 31, 33 respectively. The double integral hinges 28 allow both movements about an axis which extends in a focussing direction F and in a tracking direction T. As a result of the suspension which is used the focussing movements and the tracking movements are translational movements. An advantage of this is that a satisfactory compromise is obtained between the susceptibility to shocks produced outside the slide carrying the scanning device and the susceptibility to disturbances originating from the slide itself. This is of particular importance if a slide-drive system is employed. The suspension itself is particularly advantageous on account of its inherent damping characteristics and its simple manufacturing and assembly possibilities. In the assembled condition the scanning device in accordance with the invention is arranged on the slide in such a manner that the magnetic air gap 20 extends tangentially of the disc to be scanned. It has been found by experiment that both in the tracking direction and in the focussing direction the scanning device in accordance with the invention enables a high efficiency to be attained as a result of a comparatively small movable mass and a satisfactory ratio between driving forces and the electric currents applied to the coils. Consequently, the scanning device in accordance with the invention is very suitable for low-power uses. Fig. 3 shows a part of a CD player, in particular the player deck, employing an embodiment of the device in accordance with the invention. In the description of the present embodiment parts corresponding to those in the embodiment already described bear the same reference numerals. The deck shown in Fig. 3 comprises a chassis 100 in which an electrically drivable turntable 102 for supporting and centring an optical disc having an information track, for example a CD, is supported so as to be rotatable about an axis of rotation 102a. The deck further comprises mechanical guide means for translating a slide 104 radially relative to the turntable 102 (as indicated by the arrow T). The guide means comprise a guide rod 106, which is secured to the chassis 100, and a guide wall 108 for cooperation with sliding bushes 110 and sliding surfaces 112 respectively of the slide 104. An electric motor, not shown, which is supported in the chassis, serves for driving the slide 104, which may be effected by means of a transmission mechanism. The slide 104 carries an embodiment 114 of the scanning device in accordance with the invention and, consequently, of a type as shown in Figs. 1 and 2, and a semiconductor laser arranged beside the scanning device 114. The scanning device 114 comprises a movable section 6, which comprises an objective 2 having an optical axis 2a parallel to the axis of rotation 102a, a focussing coil 12 having a coil axis 12a, and two tracking coils 14a, 14b. A suspension 22a, 22b connects the movable section 6 to a stationary section 16 of the scanning device, which stationary section is connected to the slide 104 and comprises a magnetic circuit with a permanent magnet 18. The scanning device is positioned in such a way relative to the slide 107 that its longitudinal axis 1 is oriented transversely of the plane defined by the axis of rotation 102a and the optical axis 2a, so that the objective 2 is movable both in the focussing direction F and in the tracking direction T. In operation the slide 104 is moved along a radial path, indicated by the arrow T, relative to the axis of rotation 102a of the turntable 102 for the purpose of coarsely following the information track of the rotating disc, which track is situated in an information plane. Since the track may be slightly off-centred on the optical disc which has been clamped onto the turntable and the turntable may also exhibit slight radial deviations, allowance is made for small radial excursions of the information track during rotation. Any deviations which in operation may arise between the position of the scanning spot formed in the disc by the objective and the track portion to be scanned are reduced by means of a radial tracking system comprising the scanning device 114, in that the objective 2 performs small high-frequency tracking movements in the direction indicated by the arrow T.

Claims: An electro-optical scanning device comprising a movable lens (2) having an optical axis (2a), which lens is movable in a focusing direction (F) to form at least one radiation spot on a surface to be scanned and in a tracking direction (T) oriented transversely of said focusing direction, in order to follow a recording track in said surface, and comprising a stationary section (16) with a magnetic circuit (18,16a) and with a single air gap (20) extending in a plane defined by said focusing direction and said tracking direction, and a movable section (6) which extends in a direction oriented transversely of a plane defined by said focusing direction and said tracking direction and is movably secured to the stationary section by a compliant mechanical suspension (22a, 22b) the movable section carrying said lens and comprising a focusing coil having a coil axis (12a) extending parallel to said optical axis and at least one tracking coil (14a, 14b) for driving the lens, each of said coils extending into said single air gap, characterized in that the single air gap (20) is situated between the optical axis (2a) of the lens (2) and the coil axis (12a) of the focusing coil (12), the movable section (6) having a mass centre situated in or near said single air gap. An electro-optical scanning device as claimed in Claim 1, characterized in that the air gap (20) is bounded by a magnet (18) having a magnetization direction (A) which is oriented transversely of a plane defined by the focusing direction (F) and the tracking direction (T), the focusing coil (12) surrounding the magnet (18). An electro-optical scanning device as claimed in Claim 1 or 2, characterized in that the suspension (22a, 22b) comprises suspension rods (26a, 28a, 26b, 28b) which extend in a direction oriented transversely of the plane defined by the focusing direction (F) and the tracking direction (T), which suspension rods are each secured to the stationary section (16) and the movable section (6) at a first end and a second end respectively, said ends being provided with an integral hinge (28) allowing movements about an axis which extends in the focusing direction and about an axis which extends in the tracking direction. An electro-optical scanning device as claimed in Claim 3, characterized in that the suspension rods (26a, 28a, 26b, 28b) extend parallel to one another. An electro-optical scanning device as claimed in Claim 3 or 4, characterized in that the number of suspension rods (26a, 28a, 26b, 28b) is four, the rods at opposite sides of the plane defined by the optical axis (2a) of the lens (2) and the coil axis (12a) of the focusing coil (12) being arranged in pairs which form part of a single injection-moulded product. An electro-optical scanning device as claimed in Claim 1, 2, 3 or 4, characterized in that the movable section (6) comprises a separately formed lens holder (4) and coil holder (8, 10a, 10b). An electro-optical scanning device as claimed in Claim 6, characterized in that the coil holder is assembled from a focusing-coil holder (8), provided with the focusing coil (12), and two tracking-coil holders (10a, 10b), each provided with a tracking coil (14a, 14b). An electro-optical scanning device as claimed in any one of the preceding Claims, characterized in that the stationary section (16) has a gap-bounding yoke portion (16a) having a concave surface turned away from the air gap (20). An optical player comprising a scanning device as claimed in any one of the preceding Claims, a turntable (102) and a slide (104), the slide being movable along a radial path (T) relative to the turntable and the plane defined by the optical axis (2a) of the lens (2) and the coil axis (12a) of the focusing coil (12) being perpendicular to said radial path.

Inventors: COMPTER JOHAN CORNELIS, HEYMANS PETER MICHEL SILVESTER, HOMBURG FREDERICUS GERARDUS AN, STINESEN BERNARDUS JOHANNES, COMPTER, JOHAN CORNELIS, HEYMANS, PETER MICHEL SILVESTER MARIA, HOMBURG, FREDERICUS GERARDUS ANTONIUS, STINESEN, BERNARDUS JOHANNES
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1991
Main classification: G11B7
IPC codes: G11B7