Abstract:
A method and apparatus that projects a two-dimensional image is disclosed. The method may include emitting a laser beam, sweeping the laser beam with a first scan mirror along a first scan direction to form a scan line on the projection surface, and sweeping the scan line with a second scan mirror along a second scan direction generally orthogonal to the first scan direction to form a raster pattern of scan lines on the projection surface. The scanner for the first scan mirror is driven with a stimulus waveform, wherein the stimulus waveform has a fundamental frequency that is substantially equal to the resonance frequency of the scanner but also contains harmonics to achieve a nearly constant velocity of the laser beam spot during its scan across the projection surface. The stimulus thereby reduces peak laser output power and corresponding noise generation while maintaining image brightness and image quality.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to image projection systems, and in particular, to a method and apparatus for reducing RF noise generation or required laser output in an image projection system. 
     2. Introduction 
     A portable projector or micro-projector can provide an image larger than the hosting projecting device dimension. Such displays have the potential to overcome the display size limitation on mobile devices and can be used to display documents and images, for example. The displays can also help to overcome the input interface size limitation on mobile devices by displaying virtual input devices, such as keyboards. One technology used for micro-projection is laser scanning, in which red, green and blue laser beams are scanned across a surface to form a full color image. To provide the user of the device with optimum mobility and ease of use, these devices must be small in size, lightweight, and battery powered. However, the requirements necessary to achieve portability result in severe constraints on space, weight, and power dissipation, and increase the amount of heat energy generated by the components of the portable device. 
     In order to accommodate the brightness and image quality requirements, the conventional solutions for minimizing power consumption in laser-scanned projection require an increase in peak laser output power and peak laser drive current. This increase in laser drive current increases the generation of radio frequency (RF) noise. The effects of this additional RF noise could be particularly significant if the projector is integrated within a portable phone. The noise could exacerbate the problem of desense, or loss of receiver sensitivity, leading to a significant degradation of radio performance. 
     For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for reducing peak laser output power and associated RF noise generation in laser-scanned projection. 
     The scanning of the projector laser beam is achieved by one or two mirrors that reflect the beam and move in such a way as to create a raster pattern on a screen. The raster is created by a horizontal fast movement and a slower vertical movement. The vertical movement is linear, creating on the screen a spot that moves at constant velocity. The mirror creating the horizontal movement oscillates in a resonance mode which enables a much faster movement of the spot on the screen. The penalty of resonance movement is that the mirror moves as a harmonic oscillator and, consequently, the spot velocity is sinusoidal. This requires a higher light output from the lasers during the time when the mirror moves faster. The higher current required for driving the lasers during this time can generate higher RF noise. Therefore, there is a need to make the speed of the horizontal movement of the mirror essentially constant, to minimize the generation of excessive RF noise. 
     The patent to Barnea (U.S. Pat. No. 7,042,613) describes a method of linearizing the movement of the mirror consisting in a bouncing mechanism that reverses the movement at the end of its range. This mechanical solution complicates considerably the construction of the mirror. 
     The patent to Bridgelall (U.S. Pat. No. 5,889,269) describes an optical scanner for reading two-dimensional indicia such as bar code symbols and the like and includes a scan element arranged to create a two-dimensional raster pattern which may be scanned across the indicia to be read. In order to improve linearity of response in the vertical direction, perpendicular to the direction of the scan lines, the scan element is driven in that direction by a modified triangular waveform in which the amplitude and/or the phase of at least one of the harmonics has been altered. However, this prior art only applies linearization orthogonal to the fast scanning direction and not parallel to the fast scanning direction. And this prior art is only applied to improve the linearity of response within a two-dimensional raster patter across indicia to be read, and not to minimize RF noise generation associated with image projection near high RF sensitivity communications equipment. 
     It is the object of this invention to provide a means of reducing the laser peak power, peak laser drive current, and generated RF noise by improved linearity of the mirror movement. 
     SUMMARY OF THE INVENTION 
     A method and apparatus that projects a two-dimensional image is disclosed. The method may include emitting a laser beam, sweeping the laser beam with a first scan mirror along a first scan direction to form a scan line on the projection surface, and sweeping the scan line with a second scan mirror along a second scan direction generally orthogonal to the first scan direction to form a raster pattern of scan lines on the projection surface. The scanner for the first scan mirror is driven with a stimulus waveform, wherein the stimulus waveform has a fundamental frequency that is substantially equal to the resonance frequency of the scanner but also contains harmonics to achieve a nearly constant velocity of the laser beam spot during its scan across the projection surface. The stimulus thereby reduces peak laser output power and corresponding noise generation while maintaining image brightness and image quality. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to describe the manner in which the above-recited and other advantages and features of the invention can be obtained, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
         FIG. 1  is a perspective view of a electronic device projecting an image at a working distance therefrom; 
         FIG. 2  is a block diagram of an image projection arrangement for installation in accordance with a possible embodiment of the invention; 
         FIG. 3  is a graph of the moving velocity of spot generated by the laser beam on the screen; and 
         FIG. 4  shows a stimulus waveform adapted to provide a linear scanning motion in accordance to an embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The features and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth herein. 
     Various embodiments of the invention are discussed in detail below. While specific implementations are discussed, it should be understood that this is done for illustration purposes only. A person skilled in the relevant art will recognize that other components and configurations may be used without parting from the spirit and scope of the invention. 
     The invention comprises a variety of embodiments, such as a method and apparatus and other embodiments that relate to the basic concepts of the invention. 
       FIG. 1  illustrates an exemplary diagram of an electronic device  100  for projecting a two-dimensional image in accordance with a possible embodiment of the invention.  FIG. 1  generally identifies a hand-held instrument, for example, a personal digital assistant, in which a lightweight, compact, image projection arrangement  200  ( FIG. 2 ) is mounted and operative for projecting a two-dimensional image  160  at a predetermined distance from instrument  110  and within a particular area  140 . By way of example, an image  160  is situated within a working range of distances relative to the instrument  110 . As shown in  FIG. 1 , the image  160  extends over an optical horizontal scan angle extending along the horizontal direction, and over an optical vertical scan angle extending along the vertical direction, of the image  160 . As described below, the image  160  is comprised of illuminated and non-illuminated pixels on a raster pattern of scan lines swept by a scanner in the arrangement  200 . The shown shape of the instrument  110  represents just one form factor of a housing in which the arrangement  110  may be implemented. The instrument can be shaped as a pen, a cellular telephone, a clamshell, a wristwatch, or any other existing or yet to be discovered shape. This compact, miniature size allows the arrangement  200  to be mounted in housings of many diverse shapes, large or small, portable or stationary, including some having an on-board display  120 , a keypad, and a window through which the image is projected. While described as an image projection device it should be understood that the instrument  110  is capable of projecting a video stream, projecting text, or projecting graphics and for performing cellular phone functions such as voice and text communication. A video stream  160  is a sequence of static images which when displayed in sequence produce motion picture video. The electronic device  100  includes image projection arrangement  200  ( FIG. 2 ) and communication devices for sending and receiving voice, text, and data. 
     The electronic device  100  illustrated in  FIG. 1  and the image projection arrangement  200  and the related discussion are intended to provide a brief, general description of a suitable environment in which the invention may be implemented. Although not required, the invention will be described, at least in part, in the general context of computer-executable instructions, such as program modules, being executed by a processor/controller. Generally, program modules include routine programs, objects, components, data structures, etc. that perform particular tasks or implement particular abstract data types. Moreover, those skilled in the art will appreciate that other embodiments of the invention may be practiced in network computing environments with many types of computer system configurations, including personal computers, hand-held devices, multi-processor systems, microprocessor-based or programmable consumer electronics, network PCs, minicomputers, mainframe computers, and the like. 
     Embodiments may also be practiced in distributed computing environments where tasks are performed by local and remote processing devices that are linked (either by hardwired links, wireless links, or by a combination thereof through a communications network. In a distributed computing environment, program modules may be located in both local and remote memory storage devices. 
       FIG. 2  shows a more detailed exemplary block diagram of an image projection arrangement  200  which may implement one or more modules or functions of the laser power monitoring process. Three lasers  225 ,  230 ,  235  produce laser beams of different colors—red, blue and green—respectively. The laser beams are reflected by mirrors  250  and  270 , the reflected beam is direct to screen  280  where a spot is formed. The light spots formed by each laser are superimposed on one another at screen  280 . If more than one laser is powered simultaneously, then the spot on the screen  280  will have the mixed color of all the actuated lasers. The use of red, blue, and green lasers enables a wide range of colors to be formed. The spot of the desired color is swept in mutually orthogonal directions by the X-mirror  250  and Y-mirror  270 . The mirrors are moved by the X-scanner  245  and the Y-scanner  265 , respectively. The scanners are controlled by their respective drives  240  and  260 . The laser drive  220  is connected to each laser and pulses the lasers ( 225 ,  230 , 235 ) on and off to form a bit-mapped image on screen  280 . The color of the bit-mapped image is a function of the lasers that were actuated for each pixel of the image. Hence, the entire image can be colored all red, blue, or green, or any mixture of these colors. In addition, each pixel can be separately provided with a desired color so that the entire image comprises multiple colors. 
     A light source such as lasers  225 ,  230 , and  235  energized by laser drive  220  emit a laser beam to a first X-mirror  250  oscillatable by an X-scanner  245  for horizontally moving the laser beam back and forth at a first sweep frequency along a first direction, commonly termed “X-axis” sweeping, and, in turn, to a second Y-mirror  270  oscillatable by a Y-scanner  265  for vertically moving the laser beam up and down at a second sweep frequency along a second direction, commonly termed “Y-axis” sweeping. The first and second directions are mutually orthogonal. The first or horizontal sweep frequency is generally much faster than the second or vertical sweep frequency. A control processor  205  is operative for controlling the X-scanner drive  240  and the Y-scanner drive  260  to insure that the X-mirror  250  and the Y-mirror  270  oscillate at the correct phases. An X-scanner feedback signal  290  and a Y-scanner feedback signal  295  transmit information about the mirror positions to processor  205  that monitors the sweep frequencies of the mirrors  250 ,  270  and controls the drives  240 ,  260  so as to maintain the rated phases. A crystal oscillator (not shown) serves as the master clock. In operation, a spot of focused light from the laser ( 225 ,  230 ,  235 ) is swept by the X-scanner  245  at the horizontal frequency along the X-direction to the end of the projection to form a first scan line. Thereupon, the Y-scanner  265  sweeps the spot at the vertical frequency along the Y-direction to form a second scan line. The formation of successive scan lines proceeds in the same manner. In a preferred embodiment, for a display area or a projection screen  140  the resolution of the displayed image  160  in  FIG. 1  is a function of the raster pixels along the height Y-direction) and pixels along the width X-direction). 
     The image  160  is created in the raster pattern by pulsing the lasers  225 ,  230 ,  235  on and off at selected times under the control of processor  205 , which, in turn, controls the laser drive  220 . The lasers  225 - 235  produce visible light and are turned on only when a pixel in the desired image is desired to be seen. Input device  210  provides content to be displayed as an image  160 . An X-mirror feedback circuit and a Y-mirror feedback circuit (not shown) monitor the sweeps of the mirrors  250 ,  270  and assist the drives  240 ,  250  in maintaining the rated phases. The output signal  290  from the X-mirror feedback circuit is used by the control processor  205  to synchronize the signals to the X-scanner drive  240  and laser drive  220  with the position of the X-mirror  250 . 
     In common practice the electromechanical combination of X-scanner  245  and X-mirror  250  commonly exhibits a strong electromechanical resonance versus the frequency of the applied electrical stimulus from X-scanner drive  240 . For a fixed amplitude of the applied electrical stimulus from X-scanner drive  240 , the amplitude of the displacement response of X-mirror  250  is a strong function of the frequency of this applied electrical stimulus, where the maximum amplitude of the displacement response occurs at a resonance frequency ω ◯ . In order to minimize electrical drive power to X-scanner drive  340 , X-scanner drive  240  is commonly driven by a sinusoidal waveform at the resonance frequency ω ◯ . While the application of this sinusoidal frequency stimulus at ω ◯  achieves the target amplitude of displacement of the X-scanner  245  and X-mirror  250  with the least applied electrical power, the resultant displacement of X-mirror  250  becomes substantially sinusoidal in time. Consequently, the velocity of the laser beam that is swept across the projection surface also becomes substantially sinusoidal, exhibiting its highest velocity in the center of the screen, and its lowest velocities near the left and right sides of the screen. This is illustrated graphically by graph  300  at  FIG. 3 , where the spot velocities of both linear  305  and sinusoidal  310  sweeping are plotted versus the relative position on the screen, where the sinusoidal  310  sweep is defined to exhibit that same area over the span of the sweep as the linear sweep. It can be seen in  FIG. 3  that the peak sinusoidal velocity is approximately 50% higher than the constant velocity of the linear sweep. In order to maintain a constant spot brightness across each horizontal line despite the sinusoidal velocity variations, the optical power to the lasers also needs to be modulated in a sinusoidal manner, outputting a peak laser power in the center of the screen, and reduced laser power near the left and right sides of the screen. Analyses have shown that the peak laser output power under this sinusoidal variation is approximately 50% higher than the constant laser output power that would be required to produce the same screen brightness if the velocity of the spot across each horizontal scan line were constant. Consequently, the peak drive current to each laser is also 50% higher than the constant drive current that would be required to produce the same screen brightness if the velocity of the spot across each horizontal scan line were constant. Since the radio frequency (RF) noise associated with the laser drive current is proportional to the magnitude of this current, the RF noise is also 50% higher than the RF noise that would be generated for the same screen brightness if the velocity of the spot across each horizontal scan line were constant. This RF signal can interfere with the operation of electronic device  100 . This noise, additionally, could exacerbate the problem of desense leading to a significant degradation of radio performance functions in electronic device  100 . In general, desense is a loss of receiver sensitvity most often caused by a strong unintended signal that swamps or overloads portable device  110 . Such a strong signal will cause the portable device phone)  110  to become practically non-functional. In such a condition portable device  110  can hardly respond to any weak signals. RF noise from the increase drive current will increase the potential for desense in portable device  110 . 
     There is a need to linearize the scanning pattern of X-mirror  250  because doing so leads to reduced peak laser power consumption, reduced peak laser drive current, and reduced RF noise. In the preferred embodiment, the linearity in the horizontal direction (x) is improved by driving the X-scanner  245  with a modified triangular waveform stimulus as shown in  FIG. 4 . This stimulus waveform will produce a near-linear response from X-scanner  245  and a near-linear sweep of the laser beam along the horizontal direction of the projection area  140 . 
     The drive waveform is a modified triangular waveform in which the harmonics are defined in magnitude and phase to compensate for the natural sinusoidal response of the resonant X-mirror  250 . 
       FIG. 4  is an exemplary stimulus waveform  400  in accordance with a possible embodiment of the invention. In particular, the first view  410  of the waveforms shows the stimulus signal  420  and response  430  from the X-mirror  240  in  FIG. 2 . A closer view  440  with smaller amplitude scale shows the stimulus signal  440  and response  460 . The first view  410  shows the stimulus signal  420  as a triangular pulse waveform and the response  430  as a triangular waveform. The second view  440  clearly shows that the stimulus signal is a modified triangular waveform  440 . The modified triangular waveform is  440  consists of a triangular waveform with a triangular pulse or spikes superimposed at certain intervals. 
     This stimulus waveform S(t) and the response R(t) can be mathematically described by the following equations: 
     
       
         
           
             
               
                 
                   
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     Where A m  is an amplitude, φ m  is a phase, Q is the “Q factor”, ω ◯  is the fundamental frequency of the X-scanner  245  plus X-mirror  240 , t is time, and m is a summing index. As can be seen in equation 1, this stimulus pattern S(t) has a fundamental frequency of ω ◯ , and has additional harmonics. The application of the stimulus pattern S(t) to X-scanner  244  will produce a near-linear response R(t) of the X-mirror  250 . Stimulus waveform S(t) may be generated, for example, by a lookup table contained within ROM  215 . 
     Embodiments within the scope of the present invention may also include computer-readable media for carrying or having computer-executable instructions or data structures stored thereon. Such computer-readable media can be any available media that can be accessed by a general purpose or special purpose computer. By way of example, and not limitation, such computer-readable media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to carry or store desired program code means in the form of computer-executable instructions or data structures. When information is transferred or provided over a network or another communications connection (either hardwired, wireless, or combination thereof to a computer, the computer properly views the connection as a computer-readable medium. Thus, any such connection is properly termed a computer-readable medium. Combinations of the above should also be included within the scope of the computer-readable media. 
     Computer-executable instructions include, for example, instructions and data which cause a general purpose computer, special purpose computer, or special purpose processing device to perform a certain function or group of functions. Computer-executable instructions also include program modules that are executed by computers in stand-alone or network environments. Generally, program modules include routines, programs, objects, components, and data structures, etc. that perform particular tasks or implement particular abstract data types. Computer-executable instructions, associated data structures, and program modules represent examples of the program code means for executing steps of the methods disclosed herein. The particular sequence of such executable instructions or associated data structures represents examples of corresponding acts for implementing the functions described in such steps. 
     Although the above description may contain specific details, they should not be construed as limiting the claims in any way. Other configurations of the described embodiments of the invention are part of the scope of this invention. For example, the principles of the invention may be applied to each individual user where each user may individually deploy such a system. This enables each user to utilize the benefits of the invention even if any one of the large number of possible applications do not need the functionality described herein. In other words, there may be multiple instances of the embodiments in  FIGS. 1-4 , each processing the content in various possible ways. It does not necessarily need to be one system used by all end users. Accordingly, the appended claims and their legal equivalents should only define the invention, rather than any specific examples given.