Rotary head actuator for optical disk
BACKGROUND OF THE INVENTION 
     The invention relates to rotary actuator systems for optical heads for optical disk drive systems, for example, compact disk read only memories (CD ROMS) and magneto-optical disks. 
     Emerging optical read and write disk drive technologies promise greatly improved storage densities. However, existing magnetic disk drive technologies have established access speed standards which will be difficult to meet. Existing write once and read only disk drives have seek times much longer than their magnetic counterparts. The higher density of optical disks results primarily in more closely spaced data tracks with tracking tolerances tighter by an order of magnitude. Thus, in order to realize their potential competitive advantage, optical drives must meet already challenging seek time specifications while being even more precise. 
     An optical disk can have either many tracks in the form of concentric rings or one long spiral track. With concentric tracks, the track pitch is the distance along a radius from one track to the next. For a spiral, the pitch is the radial distance the read head moves when following the spiral for one revolution of the disk. Track pitch for optical drives is about 1 micron, as determined by the diffraction-limited spot size of an 820 nanometer diode laser. 
     Optical disk drives require a movable optical head positioned by an actuator. The purpose of the head positioning actuator is to seek from one track to another. The total radial excursion of the optical head necessary to cover the data area of the disk is called the stroke. 
     There are two distinct types of head actuators for optical disk drives: rotary and linear. In a rotary head actuator, the body of the actuator pivots on rotary bearings and the head sweeps out an arcuate path. A linear actuator translates along a single axis radially relative to the disk axis. 
     SUMMARY OF THE INVENTION 
     The invention addresses two design goals for rotary actuators for optical heads: reducing moving inertia and reducing image rotation or head skew. 
     In a first aspect of the invention, a focusing element is mounted on the end of an arm connected to a rotary actuator along with a pair of reflectors mounted respectively on the end of the arm and on the pivot axis of the arm. The focusing element is typically an objective lens defining a focal point on the disk. These optical elements together define an optical path via the arm to the disk for an external beam. In the preferred embodiment, the beam is produced by a stationary source aligned with the pivot axis, and the reflectors are 45° mirrors. 
     In a multiple disk system, a plurality of actuator arms of the foregoing design are mounted in spaced parallel relationship on a rotary hub. The reflector on the end of at least one arm is paired with a respective optically switchable reflector on the pivot axis of the hub. In one embodiment, the optically switchable reflectors use removable index matching fluid in a beam splitter. 
     In the second aspect of the invention, the optical head actuator geometry is defined in a particular way. The rotary actuator is positioned such that the pivot axis is located at a distance y. from the disk axis. The focusing element and first reflector are mounted on the ends of the arm at a distance 1 from the pivot axis. The annular data zone of the disk is positioned an average distance r 2  from the disk axis. For a particular disk, y 0  and 1 are chosen so that y 0   2  is approximately equal to the sum of r 2   2  and l 2  so as to minimize image rotation. 
     Incorporating a minimum number of optical components in the moving head minimizes the inertia of the actuator thus increasing the mobility of the head and accordingly decreasing the seek time. The reduction in skew angle accomplished by the prescribed geometrical relationship reduces the requirements for electronic compenstion due to head skew in a rotary optical head. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The drawing is first briefly described as follows. 
     FIG. 1 is a schematic side view of an optical disk drive system with a rotary actuator according to the invention. 
     FIG. 2 is a plan schematic view showing a disk drive with a rotary actuator. 
     FIG. 3A is a schematic representation of the optical head positions in relation to the disk, illustrating the origin of head skew. 
     FIG. 3B is a schematic representation of a rotary actuator arm which reduces head skew according to the invention. 
     FIG. 3C is a schematic representation of two superimposed pit images corresponding to positions 1 and 2 of FIG. 3B. 
     FIG. 4 is a schematic side view of a multiple disk embodiment of the rotary actuator system according to the invention. 
     FIG. 5 is a schematic side view of a cube beam splitter arrangement with index matching fluid to implement the optical switches of FIG. 4. 
     FIG. 6 is a schematic side view of a preferred embodiment of the optical switch system of FIG. 5 in which the spaced beam splitters are united. 
     FIG. 7 is a schematic side view of an alternate embodiment of the optical switch system of FIG. 5. 
    
    
     STRUCTURE 
     The embodiments of the invention described below are envisioned primarily for use in conjunction with magneto-optical disk technology. However, the rotary head actuator is generally applicable to other optical disk technologies such as write once and read only, including audio CD. 
     As shown in FIGS. 1 and 2, a rotary actuator arm 10 is mounted for rotation on pivot axis c adjacent to a storage disk 12 continuously spinning on a rotary axis O coinciding with the center of the disk. The pivot axis of the arm and disk rotation axis are parallel. The arm 10 terminates in an optical head 10a located generally over the annular data portion 12a of the disk 12. 
     As shown in FIG. 1, the rotary actuator arm 10 includes a pair of reflectors 14 and 16 which can be either front surface mirrors or prisms, and a miniature objective lens 18 mounted in the optical head 10a. Optical head 10a can include an objective lens on an air bearing slider with an aperture slit as disclosed in U.S. patent application Ser. No. 07/373,991 filed Jun. 29, 1989 by Neville K.S. Lee entitled &#34;Method for Increasing Linear Bit Density in Magneto-Optical Storage Media&#34;. The reflectors 14 and 16 are mounted respectively on the pivot axis c and the optical axis d of the lens 18. Laser and optics module 20 includes a stationary, nonrotating diode laser, collimators, and detection circuitry, including focus and tracking error detectors. The beam from laser and optics module 20 is directed precisely along the pivot axis c through aperture 10b into the arm and is reflected from pivot axis reflector 14 at right angles to the other reflector 16 in the optical head where it is again reflected at right angles through the lens 18 along its optical axis to the focal point of the lens on the disk 12. 
     As shown in FIG. 2, the other components of the head actuator may follow in general the design of conventional CD players such as the Phillips Model CDB 473 Magnavox Compact Disc Player. In this system, an arm 24 rides on a pair of rotary bearings 26. A half-inch slug of brass or tungsten 28 counterbalances the optical head 10a so that the center of mass of the arm coincides with the pivot axis c. A matched pair of voice coils 30 and 32 are electrically connected in series, one on each side of the pivot to provide balanced torque to avoid side loads on the bearings. The voice coils are made of copper or aluminum. Aluminum is more efficient; however, copper&#39;s ease of soldering may reduce manufacturing costs. 
     The optical head sweeps out an arcuate path p as shown in FIGS. 2 and 3A covering the annular data area 12a. The angle between the actuator and a radius drawn from the center of the disk to the head is defined as head skew. As shown in FIG. 3A, rotary actuators have continuously changing head skew unless the head is counter-rotated as the actuator moves. Changing skew causes rotation of the track image on the optical detector. The optical detector provides three signals: focus error, track error (usually via a so-called quad cell) and a data signal. If the image rotates, the track error signal will change slightly requiring compensation. The effect of variable image skew is not eliminated by removing the optical detector to a stationary platform as shown in FIG. 1. However, the change in skew angle and consequent image rotation can be minimized by choosing the arm length and pivot axis location in a particular way. 
     As shown in FIG. 3B, the length 1 is the arm length from the pivot axis to the optical axis d of the objective lens 18. The length y o  is the distance between the arm pivot axis c and disk rotational axis O and r 2  is the distance from the disk axis to the midpoint in the annular data area of the disk. Thus, the length r 2  is the average track radius. The angle θ represents the skew angle as the angle between the reference radius intersecting the pivot axis and the variable radius intersecting the optical head. 
     It can be shown that the angle θ bears the following relationship to the other parameters: 
     
         COS.sup.-1 [(r.sub.2.sup.2 +y.sub.o.sup.2 -l.sup.2)/2y.sub.o r.sub.2 ] 
    
     It can be further shown that the skew angle is minimized when y o   2  =r 2   2  +1 2 . 
     Examples are given by the following table: 
     
         ______________________________________                                    
Model   y.sub.o    r.sub.2 1       Θ                                
                                        ΔΘ                    
______________________________________                                    
1       6.4         3.375  5        50.7°                          
                                        0.7°                       
        6.4        4.0     5       51.4                                   
        6.4        4.5     5       51.0                                   
2       5.7         3.375  4        43.6°                          
                                        1.0°                       
        5.7        4.0     4       44.6                                   
        5.7        4.5     4       44.3                                   
3       5.0         3.375  3        35.7°                          
                                        1.2°                       
        5.0        4.0     3       36.9                                   
        5.0        4.5     3       36.3                                   
4        3.79      2.0     3        51.9°                          
                                        0.4°                       
         3.79       2.313  3       52.3                                   
         3.79       2.625  3       52.0                                   
______________________________________                                    
 
    
     The foregoing table represents four different models, each one provided with three rows of data. In each group, the second row corresponds to the geometric mean radius of the data area. Thus, the first three groups all have the same size disks with a geometric mean track radius of 4.0 units, e.g., inches. The first and third rows of each group represent other angular positions of the optical head in the data area nearer or farther from the disk axis. The third group represents the familiar 3-4-5 right triangle, in which the effective pivoted arm length is three units and the pivot axis distance from the disk axis is five units. In groups 1 and 2, the arm length 1 is chosen as 5 and 4, respectively, with corresponding alterations in the radial distance of the pivot axis. 
     The fourth model is based on a smaller disk with a geometric mean track radius of 2.313. 
     The difference in the head skew angle given by the values of θ is seen to vary only slightly for any of the four models. As expected, Δθ, namely, the change in skew angle which is desirably minimized, is lowest for the longest arm in model 1. However, for the smaller disk in model 4, Δθ is even smaller, less than a half a degree. 
     The optimal pivot axis location for the split head design with stationary optics 20 is different than for an integrated head with laser, optics and detectors all on the end of the arm. For both split and integrated heads, it turns out that minimum image rotation occurs when the actuator pivot point, the center of the disk and the point where the head is over the geometric mean track form a right triangle. However, for the split head, the track radius is the hypotenuse of the triangle, whereas for the integrated head the actuator arm should be the hypotenuse. Thus, the arm can generally be made shorter for the split head design, which makes it stiffer. 
     As shown in FIG. 4, a multiple disk arrangement taking advantage of the rotary split head design of FIG. 1 includes a plurality of spaced coaxially arranged spinning disks 50, 52, 54 and 56. The disks can be either single.or double-sided in this embodiment, as well as in the single disk embodiment of FIG. 1. The head actuator employs a central elongated hub 58 mounted on the pivot axis c and supporting a plurality of spaced actuator arms 60, 62, 64 and 66 sized to fit between the disks as shown in FIG. 4. Optically switchable pivot axis reflectors 70, 72, 74 and 76 are paired with the optical heads on the arms 60, 62, 64 and 66. The reflectors 70, 72 and 74 either reflect or transmit light depending on their state. 
     Preferably, cube beam splitters 90 made of spaced prisms 92 and 94 with displaceable refractive index matching liquid 96 at the interface are used as the switching medium for reflectors 70-74, as shown in FIG. 4. FIG. 6 shows a preferred embodiment in which adjacent cube beam splitters as in FIG. 5 are joined at 98, for example, eliminating extra air/glass interfaces and facilitating optical alignment. 
     FIG. 7 is an alternate configuration which accomplishes switching with one or more globules of index matching fluid The optical switch arrangement of FIG. 7, although diagonally disposed, is thus effectively,aligned on the pivot axis c. 
     Details of an electrically displaceable index matching liquid system are disclosed in U.S. Pat. Nos. 4,789,228 and 4,818,052, issued Dec. 6, 1988 and Apr. 4, 1989, both to LePesant et al., entitled &#34;Electrically Controlled Optical Switching Device&#34; and &#34;Device for Optical Switching by Fluid Displacement . . . &#34;, respectively, incorporated herein in their entirety. 
     For example, to activate head 64 (FIG. 4), the index matching fluid is removed from the prisms interface of reflector 74, while the index matching fluid remains present at the interfaces of the reflectors 70 and 72. Thus, the light beam from optics module 20 reflected via mirror or prism 80 passes through reflectors 70 and 72 which have been rendered transmissive by the presence of the index matching fluid, but is fully reflected by the reflector 74 to the optical head on arm 64 which directs it to disk 54. To switch to a different arm, the index matching fluid from another one of the reflectors is removed and restored to the interface with reflector 74. 0f course, reflector 76, being the last in line, does not have to be optically switchable but can remain fully reflective at all times. 
     The index matching fluid system can be extended to any number of interfaces. For example, to activate device number n, the index matching fluid is removed from the number n interface while leaving the rest of the interfaces filled. To switch to interface m, the n interface is filled and the fluid is removed from interface m. 
     The geometry discussed in connection with FIG. 3B for the split head design would be equally applicable to the multiple disk embodiment of FIG. 4, i.e., with each arm bearing the same geometrical relationship. 
     The foregoing description of specific embodiments is offered as an illustration rather than a limitation on the scope of the invention. Various modifications, refinements, additions or subtractions can be made to the disclosed embodiments without departing from the principle and spirit of the invention. For example, other means of optical switching can be used to implement a multiple disk design. In any event, the intended scope of the invention is indicated by the appended claims and their respective equivalents.