Apparatus and method for an electromagnetic actuator with two orthogonal axes of motion
FIELD OF THE INVENTION 
     The present invention is related to the field of data storage and retrieval and, more particularly, to an apparatus, generally referred to as an actuator, for both focusing a laser beam on a storage surface of an optical data storage medium and for controlling the radial position on a signal track. The signal track is defined by optically identifiable regions which can be written on the storage surface and/or which can be read through the interaction of a laser beam with the storage surface. 
     BACKGROUND OF THE INVENTION 
     A multiplicity of actuators have been described in the prior art in the read/write heads of optical storage recorder/player units which control both the focusing and the tracking of the laser beam on optically distinct regions. Typically, the objective lens of the actuator is moved by linear moving coil motors and is suspended in such a fashion that the lens can be displaced along two orthogonal axes. One of the orthogonal axes is the vertical axis (i.e., an axis perpendicular to the storage surface) which is used for focusing. The second orthogonal axis is horizontal and is used for positioning the laser beam relative to signal tracks on the storage surface. By way of specific implementation of the suspension of the objective lens in an optical read/write system, the actuator described in the U.S. Pat. No. 5,199,014 incorporates two injection molded plastic structures attached at opposite sides of the lens housing. Other suspension systems found in the related art use wires, i.e., U.S. Pat. No. 4,782,475, rubber structures, or metal blades, i.e., U.S. Pat. No. 5,136,558, etc. In all the foregoing implementations, suspension design requires dynamic performance without resonances in the desired range of operating frequencies (excluding the first natural frequency). The motors of the actuator must provide a predetermined amount of driving force to accelerate the lens and the associated housing. The actuator package should be easily assembled and should be compact enough to fit in the limited volume available in the contemporary optical disc drive mechanisms. The actuator should be capable of being fabricated at low cost despite the fact that most actuators described in the related art are complicated and consist of many different parts requiring a high degree of accuracy combined with an elaborate and time consuming assembly. 
     A need has been felt for a two-axis actuator characterized by a high degree of simplicity, a low number of parts, relaxed requirements with respect to accuracy, and, as a result, a low cost in manufacturing and assembly of the actuator. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the present invention, an actuator includes a stationary member, a lens housing, and four flexure bars coupling the stationary member and the lens housing. The flexure bars are fabricated so that when the lens housing moves relative to the stationary member, the orientation of the lens housing remains fixed. A coil assembly is coupled in the lens housing and, when current is applied to one of the plurality of coils of the coil assembly, the interaction of the current with a magnetic field originating from a magnetic structure causes the lens assembly to move relative to the stationary member. The movement of the lens assembly provides the focusing and the tracking of a radiation field on the optical storage medium. The stationary member, the lens assembly, and the flexure bars are comprised of two identical top and bottom sections, the two sections including structure to determine the relative positions of the top and bottom sections when they are assembled. The top and bottom sections include structure to engage the coil assembly securely, the forces acting on the coil assembly determining the position of the lens housing relative to the stationary member. 
     The use of the single injection molded structure to provide both the top and bottom of the stationary member and the lens housing and to provide the flexure arms reduces the number of elements and assembly. In addition, the molded structure includes projections and apertures which position the two portions during assembly. The coil assembly is rectangular in form and lends itself to easy fabrication. The X cross-section flexure arms, as part of the molded structure, are capable of bending only at the thinned portions and provide the constant spatial orientation when being repositioned. Thus both the number of parts has been reduced and the complexity of and the time for assembly of the actuator is reduced. 
     These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an actuator for an optical storage and retrieval system according to the present invention. 
     FIG. 2 is an exploded view of an optical storage and retrieval unit actuator according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 1, an actuator 10 for an optical storage and retrieval system includes an objective lens 100 mounted in a lens housing 20. The lens housing 20 is coupled to a stationary member 30 by means of four flexure bars 40. Near each end of the flexure bars 40 are thin sections 41,42 and 43,44 oriented in the horizontal and vertical directions, respectively. When a force is applied to the lens housing, these thin sections 41,42 and 43,44 allow the lens housing to be displaced in the vertical and horizontal direction for the focusing and the tracking operations of the actuator 40 relative to the stationary portion 30. A top portion of lens housing 20 and a top portion of stationary member 30 coupled by a first pair of the flexure arms 40 comprises a top section 80 of the actuator, while a bottom portion of the lens housing 20 and a bottom portion of stationary member 30 coupled by a second pair of flexure arms comprises a bottom section 90 of the actuator 10. Both halves of the actuator are identical and fabricated as injection-molded plastic parts. The assembly includes gluing the bottom section 90 and the top section 80 together. The motion of the lens housing 20 in either the vertical or horizontal direction is a result of forces applied along the Z and/or the X axis, cf. FIG. 1. The forces exerted on the lens housing 20 are created by two linear moving coil motors. Each motor is comprised of magnetic structure 50 and magnetic structure 60 combined with the moving coil assembly 70. The coil assembly 70 is located in the lens housing 20. 
     Referring next to FIG. 2, a more detailed understanding of the operation of the invention can be determined from the exploded view of actuator 20. As discussed previously, the top section 80 and the bottom section 90 of the actuator 10 are identical and include portions of the lens housing 20 and the stationary member 30 along with two flexure bars 40. The lens housing, as seen in the bottom portion 90, incorporates a housing frame 25. The housing frame 25 includes two pairs of locating features. A first pair of locating features is the focusing elements 21 to align the focusing coil 71. The second pair of alignment elements are the tracking elements 22 to align the tracking coils 72 and 73. A hollow seating surface 26 in the housing frame 25 (shown in the top section 80) is used to position objective lens 100. The two portions of the lens housing 25 are fitted with a hole 23 and a pin 24. Similarly, the two portions of the stationary member have a pin 31 and a hole 32. The holes 23 and 32 and the pins 24 and 31 align the top section 80 with the bottom section 90 during assembly of the actuator 10. 
     Supporting bars 40 are constructed with an X cross-section that provides rigidity and stiffness of the actuator suspension. To allow the bars to bend in either the vertical or the horizontal direction, two pairs of thin sections 41 and 42 and thin sections 43 and 44 are provided at each end of the bar 40. The thin sections 41 and 44 are oriented vertically and permit motion of the head assembly 20 in the horizontal direction relative to stationary member 30. Thin sections 42 and 43 are oriented horizontally and permit motion of the lens housing in the vertical direction relative to the stationary member 30. 
     Moving coil assembly 70 includes three separate coils having a generally rectangular shape. Focus coil 71 is wound around the vertical (Z) axis in the horizontal plane (i.e., the X-Y plane in FIG. 1). Two tracking coils 72 and 73 are wound around the horizontal (X) axis and are coupled (glued) to the inside of focus coil 71. Tracking coils 72 and 73 are positioned in such a way that the narrow 53 and 63 portions of the U-shaped steel brackets 51 and 61 can be located therebetween. The brackets 51 and 61 belong to each magnetic structure (linear motor) 50 and magnetic structure (linear motor) 60. The magnetic field is created by permanent magnets 52 and 62. The focusing and tracking coils are located in this field. When electric current is applied to each of the coils, a resulting force will be applied to the lens housing 20 because the coil assembly 70 is coupled thereto. The resulting forces are directed along the Z axis or along the X axis and provide for the focusing or the tracking motion of lens 100. 
     It will now be appreciated that there has been presented an actuator which requires a fewer number of parts than the actuators described in the related art. The two identical molded structures which include sections of the lens housing and the stationary member coupled by two flexure arms comprise the major components of the actuator. Similarly, the coil assembly which provides the force to move the lens housing relative to the stationary member, is comprised of a rectangular coil with two smaller rectangular coils coupled thereto and is consequently easy to fabricate and easy to position in the lens housing. The two identical molded structures each have at least one aperture and one projection which position the two identical structures when assembled. Once again, the physical configuration of the identical molded structures results in simplified fabrication. 
     Operation of the present invention is believed to be apparent from the foregoing description and drawings, but a few words will be added for emphasis. The flexure arms 40, which couple the lens housing and the stationary member, have orthogonal thin sections which permit the lens housing to move in two orthogonal directions to permit the focusing and the tracking movements. The flexure arms are part of the identical molded structures so that the degrees of freedom of movement are determined simply by assembling the two molded structures. 
     While the invention has been described with reference to actuators for interaction with optical storage media, it is apparent that the invention is easily adapted to other devices that have parts that require two degrees of freedom of movement while maintaining the spatial orientation of the moving part. While the invention has been described with particular reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements of the preferred embodiment without departing from invention. In addition, many modifications may be made to adapt a particular situation and material to a teaching of the invention without departing from the essential teachings of the present invention. 
     As is evident from the foregoing description, certain aspects of the invention are not limited to the particular details of the examples illustrated, and it is therefore contemplated that other modifications and applications will occur to those skilled in the art. It is accordingly intended that the claims shall cover all such modifications and applications as do not depart from the true spirit and scope of the invention. 
     PARTS LIST 
     10 Optical storage and retrieval actuator 
     20 Lens housing 
     21 Focusing elements 
     22 Tracking elements 
     23 Lens housing portion hole 
     24 Lens housing portion pin 
     25 Housing frame 
     26 Hollow seating surface 
     30 Stationary member 
     31 Stationary member pin 
     32 Stationary member hole 
     40 Flexure bars 
     41, 42, 43, and 44 Thin sections 
     50 Magnetic structure 
     51 Bracket 
     52 Permanent magnet 
     53 Leg 
     60 Magnetic structure 
     61 Bracket 
     62 Permanent magnet 
     63 Leg 
     70 Coil assembly 
     71 Focusing coil 
     72 Tracking coil 
     73 Tracking coil 
     80 Top section 
     90 Bottom section 
     100 Objective lens