Abstract:
An image projection system ( 100 ) has a laser ( 102, 104, 106 ) providing at least one beam ( 103, 105, 107 ) to a scan mirror apparatus ( 130 ) for scanning the at least one beam ( 103, 105, 107 ) in two orthogonal directions ( 404, 406 ). The scan mirror ( 130 ) includes an oscillating portion ( 204, 904 ) disposed contiguous to a frame ( 202 ) and includes a reflective portion ( 218, 918 ) capable of reflecting the beam ( 103, 105, 107 ). Circuitry ( 500 ) is provided for measuring the capacitance between interdigitated teeth ( 212, 214, 912, 914 ) on the frame ( 202 ) and the oscillating portion ( 204, 904 ).

Description:
FIELD 
     The present invention generally relates to laser beam image projection devices, and more particularly to an apparatus for providing feedback describing the position of a scan mirror. 
     BACKGROUND 
     It is known that two-dimensional images may be projected onto a screen by reflecting a laser beam or beams off of an oscillating scan mirror to project a raster pattern including scan lines alternating in direction, for example, horizontally across the screen, with each scan line being progressively displaced vertically on the screen. The laser beam or beams are selectively energized to illuminate pixels on the screen, thereby providing the image. 
     A first scan mirror typically oscillates at a high speed back and forth horizontally while a second scan mirror oscillates at a lower speed vertically. The first scan mirror oscillates at a resonance frequency with the highest velocity in the center while approaching zero as it nears either extreme of its oscillation. The second mirror moves at a constant speed in the orthogonal direction (vertically) from the top of the screen to the bottom, for example, then returns to the top for the next generation of the image. 
     The repetitive oscillation or movement of the mirrors is caused by a drive apparatus for each mirror. Conventional mirror systems include a permanent magnet or a piezoelectric device mounted on each mirror with a drive signal applied to a coil or directly to the piezoelectric device, thereby providing motion to the mirror. A processor providing the drive signal determines the timing at which the lasers must be pulsed to match the speed at which the mirrors are driven, in a synchronous fashion, to illuminate the appropriate pixel. 
     In order for the processor to make an accurate determination of the position of the mirror or mirrors for coordinating the laser beam pulses to improve image convergence between the alternating scans, feedback of the mirror&#39;s position is provided to the processor so the laser pulse may be appropriately timed. One known method of providing this feedback is to mount a magnet on the mirror, which creates a changing magnetic field as the mirror is scanning. The changing electric current generated in an external coil provides the feedback indicating the velocity of the scan mirror. The position can in turn be deduced from this signal. However, mounting a magnet on the mirror increases the inertia, and in turn, the size of the entire mirror structure. 
     Accordingly, it is desirable to provide an apparatus for providing feedback of the mirrors position to improve image convergence without increasing the mass of the mirror. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and 
         FIG. 1  is a top view of a known image projection system; 
         FIG. 2  is a side view of a mirror in accordance with a first exemplary embodiment; 
         FIG. 3  is a perspective front view of an inertial drive for use with the first exemplary embodiment; 
         FIG. 4  is a projection of an image showing scan lines provided from the system of  FIG. 1 ; 
         FIG. 5  is a graph showing angular position over time of a scan mirror of the exemplary embodiments; 
         FIG. 6  is a graph showing capacitance measured with the exemplary embodiment; 
         FIG. 7  is a side view of a mirror in accordance with a second exemplary embodiment; and 
         FIG. 8  is a side view of a mirror in accordance with a third exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background or the following detailed description. 
     An image projection system includes a pulsed light source, for example, red, green, and blue lasers, and a mirror system including an oscillating reflective surface for generating an image comprised of scanned lines. In order to synchronize the pulsed light and the positioning of the mirror, capacitive feedback is obtained that indicates the position of the mirror. The mirror includes a moveable frame (on the order of a few microns) and an oscillating reflective surface disposed contiguous thereto. The moveable frame and oscillating reflective surface have a plurality of first and second teeth, respectively, interdigitized and spaced apart. A circuit determines the capacitance between the first and second teeth as the reflective surface oscillates that correlates to a position of the reflective surface. This capacitance, or position, is then synced with the pulsed light source by a processor to provide an improved convergence of the scanned lines of the image. 
     Referring to  FIG. 1 , a projection system  100  includes three lasers  102 ,  104 ,  106  for emitting a beam of different frequencies. Laser  102  preferably is a semiconductor laser emitting a red beam  103  at about 635-655 nanometers. Lens  110  is a biaspheric convex lens having a positive focal length and is operative for collecting virtually all the energy in the read beam  103  and for producing a diffraction-limited beam with a focus at a specified distance from the lens. 
     The laser  104  preferably is a semiconductor laser emitting a blue beam  105  at about 475-505 nanometers. Another biaspheric convex lens  112  shapes the blue beam  105  in a manner analogous to lenses  110  shaping the red beam  103 . 
     Laser  106  is preferably a laser system including an infrared semiconductor laser having an output beam of 1060 nanometers, and a non-linear frequency doubling crystal. An output mirror (not shown) of the laser  106  is reflective to the 1060 nanometer infrared radiation, and transmissive to the doubled 530 nanometer green laser beam  107 . One or more lenses, for example a biaspheric convex lens  114 , may be used to create the desired beam  107  shape. While lasers  102  and  104  are described as semiconductor lasers and laser  106  is described as a laser system, it should be understood that any type of laser may be used for any of the three beams  103 ,  105 ,  107 . 
     The laser beams  103 ,  105 ,  107  are pulsed at frequencies on the order of 100 MHz. The green beam  107  may require an acousto-optical modulator (not shown) within the laser system  106  to achieve that frequency, if a non-modulated solid-state laser system is used. The green beam  107  is reflected off of mirror  122  towards the scanning assembly  130 . Dichroic filters  124  and  126  are positioned to make the green, blue, and red beams  103 ,  105 ,  107  as co-linear as possible (substantially co-linear) before reaching the scanning assembly  130 . Most importantly, the dichroic mirrors direct all three beams towards the small high-speed scan mirror. Filter  124  allows the green beam  107  to pass therethrough, while reflecting the blue beam  105 . Filter  126  allows the green beam  107  and blue beam  105  to pass therethrough, while reflecting the red beam  103 . The operation of the system described above is described in detail in U.S. Pat. No. 7,059,523 which is incorporated herein by reference. 
     The nearly co-linear beams  103 ,  105 ,  107  are reflected off a first scan mirror  132  and a second scan mirror  134 . One or more additional mirrors (not shown), which may be stationary, may be utilized to direct the beams  103 ,  105 ,  107  in the desired direction and/or for image orientation. 
     Referring to  FIG. 2  and in accordance with a first exemplary embodiment, the scan mirror  132 ,  134  comprises a moveable frame  202  and an oscillating portion  204 . The moveable frame  202  and oscillating portion  204  are fabricated of a one-piece, generally planar, silicon substrate which is approximately 150 microns thick. The frame  202  supports the oscillating portion  204  by means of hinges that includes a pair of co-linear hinge portions  206 ,  208  extending along a hinge axis  210  and connecting between opposite regions of the oscillating portion  204  and opposite regions of the frame  202 . The frame  202  need not surround the oscillating portion  204  as shown. The silicon is etched to form a plurality of teeth  212  defining slots  213  in the frame  202 , and a plurality of teeth  214  defining slots  215  in the oscillating portion  204 . The teeth  212  and  214  are shown on opposed sides of the oscillating portion  204 , but may be disposed on only one side or on adjacent sides as well. The teeth  212  and  214  are electrically conductive, but are electrically isolated from the teeth  212  on the frame  202 , and are interdigiated to form opposed comb structures. The electrical connections  222 ,  224  to the teeth  212  and  214 , respectively, and the isolation therebetween, may be accomplished, for example, by doping the frame  202  in order to make the silicon frame electrically conductive, as is well known in the semiconductor industry, except for a portion  203 . As an alternative to doping the silicon, a thin conductive material, such as silver or gold, for example, may be formed on the frame  202  and oscillating portion  204 . When a conductive material is formed on the frame  202 , the portion  203  would not be coated in order to electrically isolate the scanning mirror portion from the frame. Oscillating portion  204  includes a reflective portion  218  for reflecting the beams  103 ,  105 ,  107 . 
     An inertial drive  302  shown in  FIG. 3  is a high-speed, low electrical power-consuming device that typically is mounted on a printed circuit board  304 . A scan mirror, for example, scan mirror  132  or  134 , is mounted on the inertial drive  302  by piezoelectric transducers  306 ,  308  extending perpendicularly between the frame  202  and the inertial drive  302 , and on opposed sides of the axis  210 . Although only two piezoelectric transducers  306 ,  308  are shown, additional piezoelectric transducers, such as four, may be used. An adhesive may be used to insure a permanent contact between the one end of each transducer  306 ,  308  and the frame  202 . Each transducer  306 ,  308  is coupled by connectors (not shown) to the printed circuit board  304  to receive a periodic alternating voltage. The piezoelectric transducers  306 ,  308  could be mounted on printed circuit boards, ceramic substrates, or any rigid substrate, as long as electrical connections can be made to thereto. 
     One of the scan mirrors, for example scan mirror  132 , oscillates to provide a horizontal scan (direction  404 ) as illustrated on the display  402  in  FIG. 4 . The other of the scan mirrors, for example scan mirror  134 , oscillates to provide a vertical scan (direction  406 ). 
     In operation, the periodic alternating voltage causes the respective transducer  306 ,  308  to alternatively extend and contract in length. When transducer  306  extends, transducer  308  contracts, and vice versa, thereby simultaneously pushing and pulling the frame  202  to twist, or move, about the axis  210 . As the frame moves, the oscillating portion  204  reaches a resonant oscillation about the axis  210 . As the oscillating portion  204  oscillates, the teeth  214  move back and forth with regards to the teeth  212  of the frame  202  creating a change in capacitance. 
     The capacitance may be measured in a manner known by those in the industry by coupling an oscillator circuit (not shown) to the connectors  222 ,  224 . By measuring the capacitance of the oscillator, the capacitance between the teeth  212 ,  214  may be determined. A Colpitts oscillator is one preferred example of the oscillator circuit; however, many types of other oscillator circuits may be used. 
       FIGS. 5 and 6  are graphs, respectively, of the angular position  502  of the oscillating portion  204  measured over time, and the capacitance  602  detected between the teeth  212  of the frame  202  and the teeth  214  of the oscillating portion  204  measured over time. 
     The capacitance between the teeth  212  and  214  is sensed, as the reflective surface  218  oscillates, that correlates to a position of the reflective surface  218 . This capacitance, or position, is then synced with the pulsed light source  102 ,  104 ,  106  by a processor to provide an improved convergence of the scanned lines of the image. 
     Referring to  FIG. 7  and in accordance with a second exemplary embodiment, a single scan mirror  700  may be used instead of the scan mirrors  132 ,  134 . An outer portion of the scan mirror  700  comprises the scan mirror as illustrated in  FIG. 2  and bears a similar numbering scheme. 
     However, the scan mirror  700  differs from that of  FIG. 2  in that an oscillating portion  704  is disposed within the oscillating portion  204 . Oscillating portion  704  includes a reflective portion  718  for reflecting the beams  103 ,  105 ,  107 . Each of the oscillating portions  204 ,  704  are electrically isolated from each other and from the frame  202 . 
     The oscillating portion  704  is fabricated of a one-piece, generally planar, silicon substrate which is approximately 150 microns thick. The oscillating portion  204  supports the oscillating portion  704  by means of hinges that includes a pair of co-linear hinge portions  706 ,  708  extending along a hinge axis  710  and connecting between opposite regions of the oscillating portion  704  and opposite regions of the oscillating portion  204 . The oscillating portion  204  need not surround the oscillating portion  704  as shown. The silicon is etched to form a plurality of teeth  712  defining slots  713  in the oscillating portion  704 , and a plurality of teeth  714  defining slots  715  in the oscillating portion  704 . The teeth  712  and  714  are shown on opposed sides of the oscillating portion  704 , but may be disposed on only one side or on adjacent sides as well. The teeth  712  and  714  are electrically conductive, but are electrically isolated. This isolation may be accomplished, for example, by doping the frame  202  and the oscillating portion  704  in order to make them electrically conductive, as is well known in the semiconductor industry, except for portions  203  and  703 . As an alternative to doping the silicon, a thin conductive material, such as silver or gold, for example, may be formed on the frame  202  and oscillating portion  704 . Oscillating portion  704  includes a reflective portion  718  for reflecting the beams  103 ,  105 ,  107 . Movement of the oscillating portion  704  is accomplished by moving the frame  202  at a resonant frequency. 
     As explained above, an oscillator circuit (not shown) is coupled to conductors  222  and  224  to measure the capacitance between the teeth  212  and  214 . Likewise, another oscillator circuit (not shown) is coupled to conductors  222  and  724  to measure the capacitance between the teeth  214  and  914 . The oscillator circuit determines the capacitance between the teeth  712  and  714 , as the reflective surface  718  oscillates, that correlates to a position of the reflective surface  718 . This capacitance, or position, is then synced with the pulsed light source  102 ,  104 ,  106  by a processor to provide an improved convergence of the scanned lines of the image. 
     While the exemplary embodiments described above include teeth  212 ,  214 ,  712 ,  714 , another exemplary embodiment my exclude the teeth.  FIG. 8  shows such a device  800  wherein the frame  202  and the oscillating portion  204  are adjacent one another without the teeth. While the measurable capacitance changes would be more pronounced with the exemplary embodiment of  FIG. 2 , the capacitive changes would be measurable for device  800 . An exemplary embodiment without teeth is also envisioned for the device  700  of  FIG. 7 . 
     In other exemplary embodiments, the teeth  212 ,  214 , and or  712 ,  714  may be positioned on the frame  202 ,  702  and the oscillating portion  204 ,  704  otherwise than shown above; for example, the reflective portion  218  may overlie the teeth  214 . 
     While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.