Abstract:
A holographic spinner is mounted to a drive motor via a center drive hub containing a self-aligning bearing. Rotational drive is coupled through to the spinner by one or more drive pins. This arrangement permits centrifugal forces of the spinner to perpendicularly align the spinner to the rotor axis while simultaneously maintaining disc centration and positive drive about the axis.

Description:
This is a continuation of application Ser. No. 921,411, filed July 3, 1978 (now abandoned). 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to light spot scanning, and more particularly to scanning with the use of holographic optical elements. 
     The primary function of a scanning system is the controlled sampling, or restoration, of information. In an optical scanning system, the information is processed either in parallel by a light beam which can simultaneously illuminate many data sites, or sequentially by a beam which, due to its size, illuminates only a single data site at a time. Interest in sequential optical scanning has expanded in recent years primarily because of new capabilities provided by laser light. Laser scanners are capable of generating high resolution images at high scan rates. Most of the scanning systems devised to manipulate a laser beam include a galvanometer, rotating mirror, acousto-optic elements, or electro-optic elements as the light deflector. It was first demonstrated in 1967 that a rotating hologram can also serve as a deflector element in an image scanning system. 
     Laser line scanners used for imaging applications are generally required to generate a repetitive single scan line. A problem which has been encountered with multi-faceted rotating mirror line scanners is that due to the facet-to-facet nonuniformities and spinner wobble, non-collinear multiple scan lines are formed. An obvious solution to this problem is to fabricate the spinner assembly to such precise mechanical and optical tolerances that the residual error does not detract from the desired level of image quality. The expense of this approach, however, is a decided disadvantage. Holographic scanning provides an alternative by which this problem can be minimized. 
     In a typical arrangement for making a flat holographic spinner, a point light source serves as the object and a normally incident plane light wave as the reference beam. When a hologram thus constructed is illuminated with a plane light wave which is the conjugate of the original reference beam, the hologram functions to reconstruct a wavefront which converges with or without the use of additional optical elements, to form an image of the original point object light source. When the holographic spinner is then rotated about the axis of the reference beam, the reconstructed image spot scans a circle in space. 
     A problem exists in that, if the spinner is rigidly mounted to the rotating shaft but is not perpendicular to the shaft, mechanical wobble of the spinner creates a distorted reconstruction image and/or mechanical breakdown of the spinner. The elastomeric mounting technique disclosed in U.S. Pat. No. 4,067,639 provides significant improvement but causes the spinner to asymptotically approach the perpendicular position without fully reaching it and, hence, a small degree of wobble still exists. 
     It is an object of this invention to provide a holographic spinner which is free from mechanical wobble during rotation. 
     Another object is to provide a holographic spinner which freely rotates into a fully perpendicular position with respect to the axis of rotation. 
     Other objects, advantages and features of this invention may become apparent from the following more detailed description given in connection with the accompanying drawings. 
    
    
     DRAWINGS 
     FIG. 1 is a schematic illustration of a typical geometry for a holographic scanner. 
     FIG. 2 is a schematic illustration of a prior art rigidly mounted holographic spinner experiencing wobble. 
     FIG. 3 is a diagram of the centrifugal forces acting upon a spinning wheel such as the spinner of FIG. 2. 
     FIG. 4 is a cross-sectional view of a spinner mounting arrangement according to this invention. 
     FIG. 5 is a cross-sectional view of a second spinner mounting arrangement. 
     FIG. 6 is a cross-sectional view of a third spinner mounting arrangement. 
    
    
     DESCRIPTION 
     Referring now to FIG. 1, a holographic spinner is shown at 2 mounted for rotation on shaft 4. Holographic spinner 2 as shown here is a reflection hologram and is disposed in the path of a reconstruction light beam 6 which reflects from the face of the hologram as a reconstruction of the original interfering wavefront to which the hologram was originally exposed. Thus, at any instant, an image of the original point source is reconstructed, and the locus of this reconstructed point source as the hologram is spun on the axis of shaft 4 is a circle in space. Two reconstruction points 8,8&#39; are shown. As shown in FIG. 1, the spinner 2 can be sectioned into a plurality of &#34;holographic facets&#34; 2a, 2b which are analogous to the several facets of a polygon mirror scanner. Two such sections or &#34;facets&#34; are shown in FIG. 1, but this is only illustrative. There can be any number of such sections, subject to other limitations, such as the total scan angle required from each facet. Since these several facets are generated photographically on a single flate surface their imaging characteristics can be controlled to a high degree of precision with the result that facet-to-facet differences can be virtually eliminated. With facet-to-facet uniformity achieved, the major remaining element in the system which contributes to the multiple line image problem is mechanical wobble of the spinner element. The following description provides techniques for eliminating wobble in rotating spinners. 
     FIG. 2 shows an exaggerated view of what occurs when a holographic spinner element is rigidly mounted so that the plane containing the holographic facets periodically wobbles. The two views in FIG. 2 illustrate how a beam of light 6 is reflected from the hologram at two different positions of the spinner 2, 180° from each other in rotation. A prior art spinning element rigidly mounted in this fashion will maintain the same amplitude of wobble at all rotation rates until a sufficiently high rotation rate is reached where the spinner shatters or breaks loose from its mounting as a result of centrifugal force. Obviously, the reconstructed point source from such a hologram will define a wobbly circular locus in space, and a multi-faceted hologram will define a plurality of wobbly circular loci in space. 
     The forces acting on a rotating wheel are analyzed in FIG. 3 in which a rigid wheel 12 is shown rotating on the axis of shaft 4 with respect to which wheel 12 is somewhat tilted. As a consequence of the wheel 12 being tilted, its angular momentum vector, L, does not lie along the shaft axis 4 but precesses about it. The vector F in this Figure represents the total centrifugal force acting on a unit element of the wheel. Because of the wheel tilt, the vector F has a radial and a normal component, F r  and F n , respectively, with respect to wheel 12. The magnitude of the centrifugal force vectors are given by way of the following equations: 
     
         F=M W.sup.2 r 
    
     
         F.sub.r =F Cos θ 
    
     
         F.sub.n =F Sin θ 
    
     Where M is the mass of a unit element, r is its radial distance from the center of rotation, W is the angular velocity of the wheel and θ is the angle between the normal to the wheel surface and the rotor axis. 
     As a result of the wheel symmetry, the F r  forces are in a state of equilibrium, as long as they do not exceed the strength of the material, while the F n  forces generate a torque which is normal to the sides of the wheel. If the wheel is rigidly mounted to the rotor this torque tries to bend the wheel since its magnitude increases with radial distance. If, on the other hand, the wheel is coupled to the rotor in a manner which permits some degree of movement, the torque will act to make the sides of the wheel perpendicular to the rotor axis. When this occurs, the F n  vector reduces toward zero and the direction of the angular momentum vector L approaches the rotor axis. 
     In the present technique, the wobble in the holographic spinner 2 is completely eliminated by mounting the spinner on the rotor in the embodiments as illustrated in FIGS. 4, 5 and 6. 
     As shown in the cut-away side view of FIG. 4, motor 14 drives rotor 4 in the indicated rotational direction. Holographic spinner assembly 13, comprising spinner 2 and hub member 15 is linked to rotor 4 via a mounting assembly indicated generally as 16. The dimensions of this mounting assembly have been exaggerated in order to more clearly indicate its function. Assembly 16 comprises a plurality of self-aligning ball bearings 18 which freely rotate within the confines of spherical raceways 20 and seating element 21 which is fixedly attached to the shaft. Drive pin 22 is connected to member 15 by pin slots 24 and is also connected to rotor 4. 
     During operation, as the spinner rotates, drive pin 22 couples the spinner assembly to the rotor thereby synchronizing the angular velocity of spinner to rotor. The self-aligning ball bearing is essentially insensitive to the angular velocity at which the spinner assembly is driven and, therefore, does not rotate through 360°. It does, however, rotate to the extent that a partial transverse rotation is accomplished across the spherical surface of raceway 20. The bearing, therefore, permits the spinner assembly to function as a free swinging gimbal which allows the spinner assembly to rotate in the plane of the rotor as shown by arrows 28. In other words, the centrifugal forces (F n  of FIG. 2) operate to align the spinner 2 precisely normal to the rotor axis 4. Further, if the outer and inner parts of assembly 16 are indexed in the playback mode to the same relative orientation they had during holographic spinner fabrication, the scanner system will be invariant with regard to any concentricity errors which may occur within the raceways of the bearing. 
     There are commercially available several self-aligning bearings which can be used in the fabrication of the system of FIG. 4. Examples are the DSP series manufactured by the Fafnir Ball Bearing Company. 
     FIG. 5 shows a cross-sectional view of a second mounting embodiment in which a spherical bearing is used in place of the self-aligning bearing of FIG. 4. In the figure, rotor 4 is again driven in the indicated rotational direction by a motor (not shown). Mounting assembly 32 comprises a spherical bearing 34 which is connected to guide slots 36 in hub member 37 by drive pin 38. In this embodiment, the drive pin is centralized to minimize drive torque vectors not parallel to the rotor shaft. Bearing 34 again provides a partial transverse motion the extent of which can be limited by appropriate selection of the transverse length of the guide slot 36. Spherical bearings of the LH BVV Series, manufactured by the Boston Gear Company, have been found suitable, after altering, to have one or more guide slots. 
     FIG. 6 is an alternate mounting configuration which allows the spinner asembly some degree of freedom to pivot in the plane of the rotor. In this embodiment, rotor 40 has a tapered end surface 42. Spinner assembly 44 consisting of spinner 46 and hub member 48 is linked to rotor 40 via a drive pin 50 located on tapered surface 42. Side 48A of member 48 is tapered inwardly so as to provide a curved surface 49 which abuts surface 42. 
     A flat spacing washer 52 is mounted to hub member 48 by rubber washer 54 and has an inner aperture diameter wider than the rotor shaft diameter. One or more spherical washer 56 is held in sliding contact against washer 52 by spherical retaining ring 58. Washer 56 and ring 58 thus form in effect, a ball joint assembly. In operation, as the spinner rotates about the axis, it is free to rock or pivot a short distance on the tapered portion of the rotor. This traverse motion is imparted via washers 52 and 54 to the spherical washer 56 which either slides against ring 58 or compresses washer 54 to accomodate the particular motion. As in the other embodiments, perpendicular alignment is achieved with the rotor. If desired, washers 52 and 54 may be eliminated in which case washer 56 would be in sliding contact with the surface of hub member 48. In either case, it would be desirable to apply a lubricant such as graphite or teflon at the points of sliding contact. 
     It has been demonstrated that a holographic spinner mounted to the rotor axis by the arrangements shown in FIGS. 4 through 6 is able to freely rotate to a position of perpendicular alignment with the rotor axis. The techniques disclosed completely eliminate the undesirable wobble of the spinner and greatly improve the scanning performance. The mounting techniques disclosed here could be used to eliminate the rotor wobble in a polygonal or multi-faceted mirror scanners. 
     In the embodiments shown in FIGS. 4, 5 and 6, it may be desirable to lock the spinners into their optimum state in order to avoid perturbations caused, for example, by externally applied forces (movement of machine housing, etc.). This is accomplished by rotating the spinner at its normal operating range until the desired position of perpendicular alignment is achieved. At this point a locking compound (silicone or epoxy) could be applied into the hub area and the constant velocity rotation continued until the compound has solidified locking the bearing into place. Depending on the type of compound, the application could be applied prior to rotating. It is evident that if this procedure is implemented, the drive pin would not be necessary. The drive pin can be eliminated completely provided the bearings are mounted in their respective raceways so that sufficient frictional forces are generated to achieve perpendicular alignment until the solidification technique is implemented. It is desirable that the shaft locking compound setup speed and the operating shaft speed should be the same.