Abstract:
There is provided a scanning system, including a main housing, and at least one optical wedge ( 12 ) rotatable about a shaft means ( 40 ) located within the main housing, wherein a laser beam having an optical axis and incident on the optical wedge ( 12 ), is refracted at least once by the wedge; characterized in that the shaft means ( 40 ) passes through the center of the at least one wedge ( 12 ) and that the optical axis of the incident laser beam is substantially parallel to, but laterally offset from, the shaft means ( 40 ).

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a rotary wedge-type scanner, more particularly, to a wedge-type scanner having an optical axis that is laterally offset relative to its mechanical axis.  
       BACKGROUND OF THE INVENTION  
       [0002]     Scanners are used in electro-optical devices for target acquisition, obstacle warning systems, range measurement, 3-dimensional profiling, and the like, with the object, e.g., in target acquisition, being “painted” by the controlled laser scanning beam. The laser light reflected from the object is received by the detector section of the device and, after processing, produces a real-time image of the object.  
         [0003]     Known solutions for scanning include electro-optical or acousto-optical crystals, the index of refraction of which, and therefore the deflection of the laser beam, can be altered by changing the voltage, respectively the mechanical pressure applied. This solution, however, is useful only for very small apertures and requires complex electronics.  
         [0004]     Also known is the use of mirrors. By rotating two mirrors simultaneously, it is possible to produce a scanning effect. Yet with increasing optical aperture, attainable speeds and accuracy are reduced. Also, when large apertures are required, mirror-type scanners need a large space for two mirrors that rotate in planes perpendicular or parallel to one another. Furthermore, mirrors are very sensitive to environmental conditions in terms of vibrations and temperature fluctuations.  
         [0005]     Rotating wedges are the best solution for fast wide angle scanning and medium-to-large apertures. Scanning by a pair of optical wedges facilitates a design that is not only compact, but, in principle, is also nearly insensitive to environmental conditions. However, the classic wedge-type scanners suffer from serious drawbacks. The optical wedges are mounted in, or bonded to, annular metal mounts which are then press-fitted into the inside diameter of the internal race of large, peripheral bearings. The outer races of the bearings are then pressed into the inside diameter of the main housing of the device.  
         [0006]     With increasing optical aperture, the above-described design becomes very heavy and loses its compactness. Large mounts and bearings have large moments of inertia, slowing down scanner responses. A more serious problem, however, resides in the fact that with increasing bearing diameter, the friction moment of the bearing increases exponentially. The relation between the friction moment and bearing diameter is approximately as follows: 
 
 M=f ( D   2 ) 
 
 wherein: 
 
 M=friction moment, and 
 
 D=bearing diameter. 
 
         [0007]     With larger bearing diameters, the friction moment becomes the principal moment in the system, requiring larger motors that consume more power and produce more heat to be dissipated. A larger torque also demands heavier gear transmissions. Altogether, larger torque demands not only reduce the service life of the system, but also make control of the wedges more difficult, especially when oscillating movement is required, as well as impairing the reading accuracy of the wedge position.  
       DISCLOSURE OF THE INVENTION  
       [0008]     It is thus one of the objects of the present invention to overcome the drawbacks and disadvantages of the known rotary wedge scanners and to provide a scanner that, for a given transmission aperture and angle of deflection, has the lowest possible weight and power consumption and is as compact as possible so as to reduce space requirements.  
         [0009]     It is a further object of the invention to provide a scanner that is capable of operating at high speeds and high accelerations to ensure full flexibility in scanning at a reduced sensitivity to environmental conditions, that does not demand excessive manufacturing tolerances, and in which the friction moment of the bearings, as well as the moment of inertia of the set of rotating components, are both of a minimal value.  
         [0010]     According to the present invention, the above objects are achieved by a scanning system, comprising a main housing, and at least one optical wedge rotatable about a shaft means located within said main housing; wherein a laser beam having an optical axis and incident on said optical wedge, is refracted at least once by said wedge; characterized in that said shaft means passes through the center of said at least one wedge and that the optical axis of said incident laser beam is substantially parallel to, but laterally offset from, said shaft means. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The invention will now be described in connection with certain preferred embodiments and with reference to the following illustrative figures, so that it may be more fully understood.  
         [0012]     With specific reference now to the figures in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice.  
         [0013]     In the drawings:  
         [0014]      FIG. 1  is a schematic illustration of the scanner according to the present invention, in a first extreme position of beam deflection;  
         [0015]      FIG. 2  schematically illustrates a second extreme position of beam deflection;  
         [0016]      FIG. 3  is a perspective view of one of the optical wedges, including the annular gear;  
         [0017]      FIG. 4  is a front view of the scanner assembly;  
         [0018]      FIG. 5  is a perspective rear view of the assembly;  
         [0019]      FIG. 6  is a cross-sectional view of the scanner of  FIG. 4 ;  
         [0020]      FIG. 7  represents a cross-sectional view of wedges, including the annular gear and the bearing assembly, and  
         [0021]      FIG. 8  is an enlarged cross-sectional view of the bearing assembly. 
     
    
     DETAILED DESCRIPTION  
       [0022]     Referring now to the drawings, there are seen in  FIG. 1  two pairs of optical wedges, a front pair of optical wedges  2  comprising two identical wedges and a rear pair of optical wedges  4 , also comprising two identical wedges. Wedges  6 ,  8 ,  10  and  12  of the two pairs are advantageously made of silicon, because of its relatively high index of refraction. In the illustrated embodiment of the invention, the apex angle of wedges  6  and  8  is 5.9° and the apex angle of wedges  10  and  12  is 2.6°. Other apex angles are possible, as well as other high index of refraction optical materials besides silicon.  
         [0023]     One of the principal differences between the prior-art wedge scanners and that of the present invention resides in the fact that in the former, the bearings that permit the wedges to rotate are large, peripheral ones, the disadvantages of which have already been discussed above, while the wedges of the scanner of the present invention rotate about small, central bearings. Accordingly, while in prior-art wedge scanners the optical axis and the axis of rotation are co-linear, in the present invention the optical axis OA is laterally offset from the axis of rotation AR, which, obviously, passes through the center of the wedges. In the present embodiment of the invention, the lateral offset is 55 mm, with the outside diameter D of the wedges being 150 mm. The two wedge pairs  2 ,  4  are provided with partly tapering bores  14 , seen to better advantage in  FIG. 8 , which bores are concentric with the axis of rotation AR and accommodate bearing housings mounting ball bearings.  
         [0024]     The off-center entrance beam has a diameter d of about 25 mm, and the maximal downward deflection represented in  FIG. 1  is α=50°.  FIG. 2  represents the wedge positions producing the maximal upward deflection, which is α′=50°. Accordingly the total angle of scanning is. α+α′=100°. The laser beam is transmitted through the off-axis aperture without obscuration losses.  
         [0025]     Clearly, the central area of the scanner, being occupied by the bearings and their housings, cannot be used for the reception of the reflected beams, and reduces the clear aperture by about 18%. However, this reduction is not significant and can be compensated for by increasing the effective diameter of the wedges by about 10%. Another embodiment envisaged employs a Cassegrain optical system located in front of the scanning system, instead of using a full aperture optical system. The loss of the central receiving area is less meaningful, because in a Cassegrain optical system light does not pass through the center area.  
         [0026]     The four wedges  6 ,  8 ,  10  and  12  can be driven and controlled separately and independently with respect to all magnitudes such as velocity, angular position, acceleration, sense of rotation, scanning pattern and others, by electronic means, using angular sensors and feedback processing. Electronic control is very versatile and allows the change of interrelations of the above magnitudes and of the scan pattern without modifying the mechanical design of the device, thus permitting the same device to serve many applications and furthermore facilitating a change of scan patterns in real time: Also possible is non-patterned 2D and 3D scanning. The preferred scan pattern is the so-called rosette, having different vertical and horizontal dimensions. Of particular usefulness is a scan pattern wider in the horizontal direction, with the rotation axis being in the horizontal plane. Other combinations allow linear scans, including scans in the vertical plane only. Wedge  12  rotates fast, providing a high linear scan speed. Wedge  10  rotates slowly, with wedge  12  malting several tens of turns for one turn of wedge  10 . Both the angular phase difference and the ratio of angular speeds are pre-definable. The rate of rotation of wedge  10  is designated as frame rate. Wedges  8  and  6  rotate in opposite directions, providing deflection in the horizontal direction, versus no. deflection in the vertical direction. The rate of rotation of wedges  8  and  6  equals the frame rate. The initial angular phases of wedges  10 ,  8  and  6  are identical, which means that the scanner provides maximum deflection in the horizontal direction when wedge  10  deflects to the left or right. Wedges  8  and  6  could also provide full turns for maximum width of the scan pattern, while scan patterns of any desired width can be provided by performing partial turns of these wedges, by oscillating them about their central (vertical) positions. For example, the turn rates of wedges  12 ,  10 ,  8  and  6  could be 30, 1, 1 and −1 turns per second, respectively. Different patterns can be produced by different turn-rate combinations, while pattern orientation can be changed by changing the phase differences of wedges  10 ,  8  and  6 .  
         [0027]     The scanner transmits light in both directions, with most of the aperture being used to collect the reflected light into the receiver channel. For the same optics, the transmitting and receiving channels have the same deflection characteristic for the same wavelength, which, in the present embodiment, is 1.5 μm. To obtain identical deflection characteristics also for several different wavelengths, achromatic wedges must be used.  
         [0028]      FIG. 3  shows a subassembly including a wedge, e.g., wedge  6 , annular spur gear  16 , bearing housing  18 , double ball bearing  20 , balancing weight  22  and an index.  
         [0029]     Annular gear  16 , of which there is one for each wedge, is made of aluminum and is bonded to the periphery of its wedge, which therefore defines the position of the gear with respect to the main housing. As the difference of thermal expansion between the silicon of the wedge and the aluminum of gear  16  is rather large, allowance is made for this difference by using a special adhesive of a calculated layer thickness that is compressed with rising ambient temperature and re-expands with falling temperature. Each wedge has a bearing housing  18 , seen to better advantage in  FIG. 8 , which confronts a similar problem, discussed further below in conjunction with  FIG. 8 .  
         [0030]     Each wedge also has two ball bearings mounted in bearing housing  18 , seen to better advantage in  FIG. 8 .  
         [0031]     As the center of gravity of the optical wedges is obviously not located along the mechanical axis AR ( FIG. 1 ), a balancing weight  22  must be provided to move the center of gravity exactly into that axis, to prevent destructive vibrations. By carefully shifting weight  22  along annular gear  16 , a position can be found in which the subassembly is dynamically balanced.  
         [0032]     Index  24  is used in conjunction with the rotary encoders discussed further below.  
         [0033]      FIGS. 4 and 5  are views of the front and rear faces, respectively, of the assembled scanner. Seen is a main housing  26  that, because of the above-mentioned low friction moment, can be made of an aluminum casting instead of a steel one, thereby reducing the total weight of the device by 650 gr. The assembled wedges  6 ,  8 ,  10  and  12  are attached to main housing  26  by a first three-armed spider  28  seated on and supporting bearing housing  18  of wedge  6  and a second three-armed spider  30  seated on and supporting bearing housing  18  of wedge  12 .  
         [0034]     Further seen are high-precision drive motors  32 , one for each wedge, and shaft encoders  34 , also one for each wedge. Both drive motors  32  and shaft encoders  34  are separately mounted on main housing  26 .  
         [0035]     Shaft encoders  34  must measure wedge position with a high degree of accuracy, therefore their gears  36 , which engage with annular gears  16 , must be equipped with anti-backlash mechanisms. Drive-motor gears  38 , on the other hand, may have some backlash in their engagement with annular gears  16 , without interfering with the proper operation of the scanner.  
         [0036]     In the cross-sectional view of  FIG. 6 , there is seen a central shaft  40  which passes through all of the wedges and about which all of the ball bearings  20  rotate, each wedge having two bearings mounted in its bearing housing  18  (also see  FIG. 8 ). Central shaft  40  has two threaded ends of reduced diameter that fit into recesses in the hubs  42 ,  44  of spiders  28 ,  30 .  
         [0037]      FIG. 7  shows a wedge including its annular gear  16  and bearing assembly comprising bearing housing  18  and ball bearings  20 .  
         [0038]      FIG. 8  represents the central portion of  FIG. 7 , at an enlarged scale. Here, at the bearing housing/wedge interface, another differential-expansion problem must be dealt with, as the coefficient of expansion of the silicon wedge is about 2×10 −6 1/C°, while the coefficient of expansion of the bearing housing is about 7×10 −6 1/C°. To prevent potentially destructive stresses from developing, bore  14  in the wedges is partly tapering, as is clearly shown in  FIG. 8 . A special adhesive, of a calculated layer thickness, is used for joining the wedges and their bearing housings  18 . The tapering section of bore  14  facilitates a slight axial movement of the wedges relative to the bearing housings  18 , thereby rendering the radial forces created by expansion differences innocuous. The coaxiality between annular gear  16  and bearing housing  18  has to be as perfect as possible.  
         [0039]     While power transmission in the present embodiment was designed as a gear transmission, power from motors  32  to the wedges could also take the form of belt drives. It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrated embodiments and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes, which come within the meaning and range of equivalency of the claims, are therefore intended to be embraced therein.