Abstract:
For use in an imaging system having a plurality of adjacent display units, the improvement comprising a thermal actuator in at least one of the display units for moving a screen thereof away from a chassis thereof when the said display unit exceeds a predetermined temperature, so as to prevent mechanical interference between the adjacent display units.

Description:
FIELD 
     The present invention relates to configurable imaging systems having a plurality of display units for generating respective portions of a composite image, and more particularly to a thermal actuator for use with a display unit of a configurable imaging system to minimize mechanical stress resulting from thermal expansion. 
     BACKGROUND 
     A large number of applications and potential applications exist for imaging systems such as projection displays that are used to display information. Such applications include, but are not limited to, general indoor signage (e.g. shopping malls, arcades, etc.), transportation signage (e.g. arrival/departure times, etc.), in-lobby signage for office buildings, control rooms, restaurants signage, etc. 
     It is known to provide large displays for signage and the like by assembling a multiplicity of individual display units in an array (see, for example, United States Patent Publication No. 2008/0284675, the contents of which are incorporated herein by reference). The construction of each individual display unit may include a chassis for housing projection lamps, electronic circuits, etc., and a rear projection screen. Typically, the chassis is metallic whereas the rear projection screen is plastic; resulting in a mismatch of thermal expansion coefficients (i.e. the screen expands at a faster rate than the chassis). Therefore, in order to build an array of display units capable of operation over a wide range of environmental temperatures, the thermal expansion coefficient mismatch must be accounted for. 
     One method of accounting for thermal expansion is to undersize the screen so that at elevated temperatures the screen does not exceed the chassis size and cause interference with a neighbouring display unit. However, this solution is not desirable since it results in large gaps between adjacent display unit screens at nominal temperatures, in order to accommodate thermal expansion at elevated temperatures. Large gaps between adjacent screens have the potential to interfere with the optical transition from one display to the next, thereby reducing overall image quality. Another, less optimal alternative is to allow minor interference collisions within predetermine tolerance limits that do not cause damage to either the screen or chassis. However, it is difficult to manufacture display units with sufficiently high tolerance limits as to avoid damage caused by interference between adjacent units at elevated temperatures and gaps between units at nominal temperatures. 
     Yet another solution is to fabricate the chassis and display screen from the same material so that the chassis and screen exhibit similar thermal expansion characteristics. To achieve this, the chassis may be constructed of plastic having a comparable coefficient of thermal expansion (CTE) to the screen assembly. As the screen expands, so too does the chassis, thereby maintaining the expansion differential to a minimum. Unfortunately, a plastic chassis has potentially poor performance with respect to dimensional stability, particularly as it relates to component positioning. For example, a plastic chassis may twist and distort as it expands and contracts, resulting in misalignment of the optical components. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments will now be described, by way of example only, with reference to the attached Figures, wherein: 
         FIG. 1  shows a configurable imaging system having a plurality of display units for generating respective portions of a composite image; 
         FIG. 2  shows the configurable imaging system of  FIG. 1  wherein the display screens of certain ones of the display units are pivoted outward, according to an embodiment of the invention, so as to minimize interference between adjacent display screens at elevated operating temperatures; 
         FIG. 3  shows the pivoting mechanism of a display unit shown in  FIGS. 1 and 2 , according to an exemplary embodiment; 
         FIG. 4  is a cross-sectional view of the display unit in  FIG. 3 , showing the screen in a closed position for operation at nominal temperatures; and 
         FIG. 5  cross-sectional view of the display unit in  FIG. 3 , showing the screen pivoted to an open position for operation at elevated temperatures. 
     
    
    
     A skilled person in the art will understand that the drawings are for illustrative purposes only. The drawings are not intended to limit the scope of the applicant&#39;s teachings in any way. 
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     In  FIG. 1 , an exemplary imaging system  10  is shown comprising a plurality of imaging units  20  assembled to form an array. Exemplary units are described in Applicant&#39;s co-pending United States Patent Publication No. 2008/0284675. The front surface of each display unit  20  comprises a rear projection screen  22  that is pivotally connected to a chassis  24 , as described in greater detail below. 
     In order to provide dimensional stability, the chassis  24  of each display unit  20  is configured to generally comprise a rigid frame, whereas the screen  22  is preferably made from a plastic material, such as Polymethyl methacrylate (PMMA), Styrene Methyl Methacrylate Acrylic copolymer (SMMA), glass, acrylic, polycarbonate, Polyethylene terephthalate (PET), or any suitable clear or mostly clear plastic. The rigid frame may also be configured to permit mounting of the display unit  20  to a supporting structure, such as a wall. Non-limiting examples of suitable materials for the chassis include aluminum, magnesium, and glass-filled nylon. 
     Within the chassis  24  of each display unit  20  are a plurality of electronic and optical components (not shown) for displaying images on the screen  22 . According to an exemplary embodiment, the electronic and optical components may include a small rear projector, including a light source, light valve, optics and associated electronics. The light source may, for example, be implemented using LEDs, although it is contemplated that lasers or other light sources may be utilized, the selection and implementation of which would be known to a person of ordinary skill in the art. The chassis  24  may also contain a light engine and associated circuitry (including, for example, a microprocessor, RAM frame buffer, and video processing to provide image capture, resizing, color matching, edge blending, etc). It will be appreciated that the various electronic and optical components generate heat within the unit  20 . 
     As discussed above, each unit  20  projects a portion of a composite image (preferably at SVGA resolution to enable small pixel pitch (under 1 mm)). For example, United States Patent Publication No. 2008/0284675 discloses fully configurable display units (i.e. they are not required to be arranged in rectangular configurations), resulting in significant flexibility in terms of display design. 
     Regardless of the arrangement, coupling mechanisms permit physical registration or alignment of each display unit  20  with each vertically and/or horizontally adjacent display unit  20 , for example via matching protrusions and indentations on respective surfaces of each display unit chassis  24 . 
     As discussed above, where the screen materials (generally comprising the screen, lenticular, diffusion layers, Fresnel, etc.) exhibit thermal expansion characteristics that differ (e.g. exceed) from that of the chassis, an expansion differential can result. Changes in temperature can arise from a number of sources, including, but not limited to operation of the display unit, and changes in the ambient temperature in which the display unit is located. To account for this thermal expansion, it is known to provide a nominal gap between adjacent screens  22  in order to avoid potentially damaging screen compression or collision. While such a gap may be sized large enough to permit for thermal changes in screen size, it will be appreciated that a large gap between adjacent screens may interfere with the optical transition from one display unit  20  to the next, thereby reducing overall image quality. 
     Table 1 provides an exemplary set of thermal expansion characteristics of a rigid chassis compared to a screen. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Thermal Expansion of Chassis and Screen (no interface pad) 
               
             
          
           
               
                   
                   
                 CTE 
                   
                   
               
               
                   
                 Nominal 
                 (mm/m- 
                 Width increase 
                 Width at 
               
               
                 Component 
                 Width (mm) 
                 10 C.) 
                 over 40 C. (mm) 
                 Temp (mm) 
               
               
                   
               
             
          
           
               
                 Chassis 
                 408 
                 0.259 
                 0.42 
                 408.42 
               
               
                 Screen 
                 408 
                 0.67 
                 1.09 
                 409.09 
               
             
          
           
               
                 Gap Required 
                 0.67 
               
               
                   
               
             
          
         
       
     
     While both the chassis and screen are dimensioned with a nominal width of 408 mm, the actual width of each component at operating temperature (e.g. 40° C. higher) differs as the CTE of the chassis is lower than the CTE of the screen. As shown, the screen expands to a total width of 409.09 mm, while the chassis expands to a total width of 408.42, representing a 0.67 mm difference. In this scenario, because the screen expands to a greater extent than the chassis, significant gaps between adjacent screens would be required to avoid potentially damaging compression/collision. 
     Therefore, in accordance with the embodiment of  FIG. 2 , once the temperature of a display unit  20  exceeds a predetermined threshold (e.g. 40° C.), the screen  22  is caused to pivot to a new position, as shown in  FIG. 2 , where interferences (if any) between adjacent units  20  are minimal. 
     According to the embodiment shown in  FIG. 3 , a thermal actuator  26  is mounted on the rigid chassis  24 . The actuator preferably comprises a stationary component mounted to the chassis and a linear translation element in contact with (e.g. connected to) the screen  22  and adapted to be moved by the stationary component so as to push the screen  22  outward and away from the chassis  24  above a predetermined temperature (e.g. 40° C.) or above a percentage of the predetermined temperature (e.g., 75% of the 40° C. screen collision temperature in Table A). For example, the nominal gap between display units  20  may be 0.5 mm at 20° C., such that when the temperature rises from 20° C. to 40° C., the screen may expand by approximately 0.5 mm, which means there is no longer a gap. Any further increase in temperature may then result in activation of the thermal actuator  26  so as to pivot the screen(s)  22  outwardly, such that collision between adjacent screens is averted. 
     A person of skill in the art will appreciate that any of a plurality of thermal actuators may be used. In one embodiment, a mechanical actuator  26  is provided wherein the linear translation element is a piston  28  adapted to be pushed by the thermal expansion of a fluid, such as wax, from an expansion conduit of a reservoir  30  (i.e. the stationary component connected to the chassis  24 ), as shown in  FIGS. 4 and 5 . As the wax expands and the piston  28  extends, the screen  22  is caused to pivot about a hinge  32 . One benefit of the actuator shown in  FIGS. 4 and 5  is that it requires no external power input. Alternatively, the actuator  26  may comprise any of a temperature sensor with a solenoid, a MEMs thermal actuator, electrostatic, magnetic, or piezoelectric device. When the temperature drops below the activation temperature, wax within reservoir  30  contracts thereby allowing the piston  28  to retract (e.g. under spring biasing) so that the screen returns to the closed configuration of  FIG. 4   
     While generally described within the framework of ‘multi-tiled’ displays, the thermal actuator set forth herein can be suitably applied to other imaging units, such as multiple displays in a control room. 
     It will be appreciated that, although embodiments have been described and illustrated in detail, various modifications and changes may be made. While several embodiments are described above, some of the features described above can be modified, replaced or even omitted. All such alternatives and modifications are believed to be within the scope of the invention and are covered by the claims appended hereto.