Abstract:
A magneto-optical liquid crystal display responds to the direction of a provided magnetic field. Display alternatives include a compass-like dial indication of direction, numeric values, and complex pixilated images. By providing a variable magnetic field, the display may be used as a non-contact remote analog gauge or digital indicator.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a division of U.S. patent application Ser. No. 09/927,736 filed Aug. 10, 2001, now U.S. Pat. No. 6,647,771 B2 issued Nov. 18, 2003 entitled ‘EXTERNAL PRESSURE DISPLAY FOR VEHICLE TIRES’, and a continuation of the original parent U.S. Provisional Patent Application Ser. No. 60/228,941 filed Aug. 30, 2000.” Accordingly, all benefits, including, but not limited to priority and filing dates that may accrue to this application pursuant 35 U.S.C. §§ 119, 120, &amp; 121 are claimed. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND 
     Field of Invention 
     Use of an indicator dial responsive to the direction of a magnetic field to report the value of a quantity is well known. U.S. Pat. No. 3,807,232 to Wetterhorn describes a magnetically-coupled pressure readout based on Bourdon-tube and conventional magnetic compass principles. U.S. Pat. No. 6,499,353 to Douglas, et al, discloses a virtually identical apparatus to that of Wetterhorn. 
     U.S. Pat. No. 6,520,006 to Burns describes means alternative to Bourdon tubes for rotating a “sender” magnet in response to pressure stimulus. Burns also discloses readout means alternative to magnetic compasses including arrays of magnetoresistive and Hall-effect sensors as well as induction coils in situations where the “sender” magnet as a whole moves past a pickup sensor, such as in automotive tire pressure sensing application. 
     U.S. Pat. No. 3,876,286 to Deutscher et al. describes orienting nematic liquid crystals in response to electric and magnetic fields and hence affecting light transmission in electrically controlled liquid crystal cells. U.S. Pat. No. 4,128,496 to Cole et al. describes various dichroic liquid crystal compositions for use in displays. 
     U.S. Pat. No. 5,412,500 to Fergason describes a method for continuously rotating the direction of linearly polarized light by a liquid crystal cell in response to an electrical input across the cell. This type of device employs quarter wave phase retardation means and a polarization analyzer to effect polarization rotation and requires monochromatic light. The degree of polarization rotation is controlled by the strength of the electrical field, which has a fixed direction. 
     U.S. Pat. No. 5,798,807 to Prehn proposes a multi-element device for projecting temporally modulated images. In this type of device the degree of polarization rotation is controlled by the strength of the electrical, magnetic, or mechanical impetus, not the orientation of the impetus. 
     U.S. Pat. No. 4,114,990 to Mash et al. describes a cell filled with a cholesteric (i.e., chiral nematic) liquid crystal for rotating the polarization of incident plane light in response to an electrical field applied transversely across the cell. A simulated pointer meter is to be produced by progressively varying the orientation of a polarizing layer over the cell. This type of device depends on controlling (by means of the electric field strength) the balance between homeotropic alignment of the liquid crystal at one inner cell surface and parallel homogeneous alignment at the other inner cell surface, which affects the degree of helical twist of the cholesteric liquid crystal. 
     U.S. Pat. No. 4,768,862 to McAdams describes a liquid crystal cell employing a uniform transverse magnetic field to switch the linear polarization orientation of an incident light beam in a wavelength selective fashion. In this type of devic, the orientation of the liquid crystal director is switched from twisted nematic to homeotropic by increasing the strength of the transverse magnetic field. 
     U.S. Pat. No. 4,214,819 to Pohl et al. proposes using a combination of electric and magnetic fields to increase the switching speed of an electro-optical liquid crystal cell and controlling the color of a display. This type of device uses predetermined electric and magnetic field directions and is controlled by varying the electric filed strength. 
     SUMMARY 
     The invented magnetically magneto-optical liquid crystal display exploits the magneto-optical effects of liquid crystals to indicate direction of the magnetic field at the display. This magnetic field may be provided by a remote sender unit responding to mechanical stimulus such as pressure or to other means for changing or controlling the direction of a magnetic field at the display. Thus the value of a physical or other quantity may be displayed. The provided magnetic field may also be natural, such as the geomagnetic field. 
     The magneto-optical liquid crystal display unit preferably is a thin, flexible patch comprising a liquid crystal cell, and is positioned such that the provided magnetic field has a substantial component in the plane of the cell. Individual pixels comprising the display have predetermined light transmission properties different from one another. The display unit provides a continuous response indicating direction of a vector sum of the magnetic fields across the display as a whole at any particular location and time. 
     Objects and Advantages 
     An important advantage of invented magneto-optical liquid crystal display is that it has no moving parts. 
     Another important advantage is that the invented magneto-optical liquid crystal display is totally passive, requiring no power to display a quantity or to form an image. 
     Yet another important advantage is that the invented magneto-optical liquid crystal display responds globally, to the provided magnetic field applied across the display, which permits production of successive complex images in response to simple stimuli. 
     Thus a further advantage is that the invented magneto-optical liquid crystal display may act as its own, passive, analog-to-digital converter. 
     Another advantage relates both to the ease and accuracy of precisely controlling and then sensing magnetic field direction with magneto-optical liquid crystals as opposed to sensing and parameterizing flux or strength of a magnetic field. 
     An important advantage is that the invented magneto-optical liquid crystal display can be produced at very low cost. 
     Other significant advantages of the invented magneto-optical liquid crystal display are simplicity and robustness. 
     Yet other significant advantages of the invented magneto-optical liquid crystal display relate to its thin profile, small size, and light weight. 
     Another advantage of the invented magneto-optical liquid crystal display relates to mechanical flexibility and ability to bend with and conform to complex surfaces. 
     A related advantage of the invented magneto-optical liquid crystal display relates to gluing or bonding the display to surfaces. 
     Still another advantage of the invented magneto-optical liquid crystal display relates to the capability for employing ambient light. 
     A particular utility of the invented magneto-optical liquid crystal display is in magnetically-coupled pressure gauges, where dangerously low pressure of tires, particularly of the wheels of large trucks and aircraft, can be easily determined by visual inspection, rather than measurement, substantially enhancing safety. 
     Other aspects of the invented magneto-optical liquid crystal display relate to economical, direct, non-contaminating pressure and internal temperature indicators in hazardous environments. 
     Still other aspects of the invented magneto-optical liquid crystal display are associated with a passive display accomplished by prior orientation of pixels of magneto-optical liquid crystal display for placement within an external magnetic field that changes direction responsive to pressure or other transduced property within a closed vessel. 
     Other features of the invented magneto-optical liquid crystal display is that the display responding to the magnetic field can be essentially independent of the wavelength of the light. On the other hand, the display can respond to magnetic field direction with changes in color. Color changes may provide warning or hazard indications. 
     Still other features of the invented magneto-optical liquid crystal display relate to mechanisms for precise control of liquid crystal display systems with a magnetic field. 
     The invented magneto-optical liquid crystal display may be used as an inexpensive flexible magnetic compass permanently bonded to maps and charts to indicate North orientation and as part of novelty items. 
     Still further objects and advantages will become apparent from a consideration of the ensuing description and accompanying drawings. 
    
    
     
       DRAWING FIGURES 
         FIG. 1  is a perspective view of a thin magneto-optical display. 
         FIG. 2  is a perspective cross section through a magneto-optical LCD employing global rotation of a dichroic liquid crystal and a single polarizer. 
         FIG. 3  is a perspective cross section through a magneto-optical LCD employing variably twisting a singly anchored birefringent liquid crystal and dual polarizers. 
       
         
           
                 
               
                 
                 
                 
                 
               
             
                 
                     
                 
                 
                   Reference Numerals In The Drawings 
                 
                 
                     
                 
               
               
                 
                     
                 
               
            
             
                 
                    4 
                   display unit 
                   22 
                   thin optical structure 
                 
                 
                   24 
                   virtual pointer 
                   26 
                   magnetic field 
                 
                 
                   28 
                   dial scale 
                   30 
                   display symbol 
                 
                 
                   40 
                   reflecting layer 
                   42 
                   polarizing layer 
                 
                 
                   44 
                   center layer with liquid crystal 
                   46 
                   polarizing regions or 
                 
                 
                     
                     
                     
                   pixels 
                 
                 
                   48 
                   incident and reflected light 
                   50 
                   polarizer orientation 
                 
                 
                   52 
                   liquid crystal 
                   54 
                   magnetic field vector 
                 
                 
                   56 
                   first polarizing layer 
                   58 
                   center layer with liquid 
                 
                 
                     
                     
                     
                   crystal 
                 
                 
                   60 
                   second polarizing layer 
                   62 
                   reflecting layer 
                 
                 
                   64 
                   liquid crystal 
                   66A 
                   liquid crystal director 
                 
                 
                   66B 
                   liquid crystal director 
                   68 
                   polarizing pixels 
                 
                 
                   70 
                   incident and reflected light 
                   72A–B 
                   linear polarizer 
                 
                 
                     
                     
                     
                   orientation 
                 
                 
                   76 
                   magnetic field vector 
                 
                 
                     
                 
               
            
           
         
       
     
    
    
     DESCRIPTION 
     Preferred Embodiment 
       FIG. 1  shows a thin-film, conformal, magnetically coupled pressure display unit  4 . Display unit  4  is comprised of a thin optical structure  22  whose light transmission and/or reflection properties respond to the direction of an external magnetic field  26 . One display possibility is a virtual pointer  24  that rotates with the direction of the magnetic field  26  in the plane of display unit  4  to indicate a quantity related to magnetic field direction as marked on dial scale  28 . Virtual pointer  24  is a segment of optical structure  22  with greater or lesser light transmission than the average over the structure. An alternative display approach is to make the dial scale markings themselves more or less transmissive as the component of the magnetic field  26  in the plane of display unit  4  rotates, so that the marking corresponding to a quantity related to the direction of magnetic field  26  is the only one visible. A further display alternative is to cause all or part of the display to change color. A color-changing display can indicate dangerously low or high pressure in a pressure display, for example. A visible pressure display symbol  30  can be made to change as the magnetic field rotates in response to pressure changes. This may be accomplished by segmenting symbol  30  into pixels or regions with varying polarization direction properties. While  FIG. 1  illustrates several types of displays, a combination of displays, including all those shown or suggested in  FIG. 1 , and the above description may be used simultaneously. 
     Optical structure  22  is similar in appearance and construction to a typical conventional LCD cell that exploits the electro-optical effect. Various embodiments are possible.  FIG. 2  illustrates the preferred embodiment, which comprises a reflecting layer  40 , a polarizing layer  42 , and a center layer  44  filled with a liquid crystal  52  having dichroic (i.e., polarization dependent) absorption properties. Liquid crystal  52  is preferably a calamitic (nematic or smectic) mixture within which a “guest” dichroic dye is dissolved. Discotic liquid crystal molecules may be added to liquid crystal  52  to improve magnetic responsivity. Ferronematic compounds may be employed to enhance magnetic response as well. 
     Liquid crystal  52  is also weakly anchored to the surfaces of layers  40  and  42  with no preferred orientation. Weak anchoring and a relatively wide spacing (tens of micrometers) between layers  40  and  42  are desirable in order to promote alignment of the director of liquid crystal  52  with an external magnetic field vector  54 . Magnetic field vector  54  should have a substantial component aligned in the plane of the liquid crystal cell comprised of layers  40 ,  42 , and  44 . Magnetic field vector  54  must be strong enough to overcome the wall anchoring tendencies of liquid crystals (i.e., the Freedericksz effect). While all liquid crystals exhibit the magneto-optical effect as well as the more commonly exploited electro-optical effect, the chosen mixture should have relatively strong magneto-optical properties. Upper layer  42  comprises regions, which may be pixels,  46 , each of which possess selectively predetermined light polarizing properties. Regions or pixels  46  linearly polarize natural incident light or block polarized incident linearly polarized light in accordance with polarizer orientation  50 . Regions or pixels  46  may be individual polarizers or be a polarizing film applied selectively to a transparent substrate. 
     Optionally, the ordering of the various layers can be varied. For example, polarizing layer  42  can be replaced by a transparent cover and moved to be in between the liquid crystal layer  44  and reflective layer  40 . 
     An alternative embodiment of the liquid crystal display of the invented magneto-optical liquid crystal pressure gauge cell illustrated in  FIG. 3  comprises a first linearly polarizing layer  56 , a center layer  58  preferably filled with a substantially transparent birefringent calamitic (nematic or smectic) liquid crystal  64 , a second linearly polarizing layer  60 , and a reflective layer  62 . Layer  60  linearly polarizes incident linearly polarized light in accordance with polarizer orientation  72 A. Liquid crystal  64  is preferably a mixture capable of rotating the direction of polarization of incident linearly polarized light. Liquid crystal  64  may also incorporate discotic and/or, possibly, chiral liquid crystal components to improve responsivity to magnetic fields. Ferronematic compounds may be employed to enhance magnetic response as well. Liquid crystal  64  is relatively strongly anchored to the inner surface of first polarizing layer  56  so as to have a director  66 A substantially aligned at the inner surface of first layer  56  with the orientation of preferred polarized light transmission of first layer  56 . Polarizing second layer  60  comprises regions or pixels  68  with predetermined light polarizing properties. Regions or pixels  46  block polarized incident linearly polarized light in accordance with polarizer orientation  72 B. Liquid crystal  64  is relatively weakly anchored to the inner surface of second polarizing layer  60  so that a director  66 B near the inner surface of layer  60  readily aligns with an applied magnetic field vector  76 . Magnetic field vector  76  should have a substantial component aligned in the plane of the liquid crystal cell comprised of layers  56 ,  58 , and  60 . Magnetic field vector  76  should be strong enough to overcome liquid crystal director wall anchoring tendencies at the inner surface of second polarizing layer  60  and weak enough to avoid breaking the wall anchor at the inner surface of first polarizing layer  56 . 
     The embodiments described in reference to  FIGS. 2 and 3  employ reflecting layers  40  or  62  to permit passive operation using incident light. Reflected incident light is selectively blocked or passed to produce a display. Alternatively, transmissive modes of operation may be employed using artificial light sources whereby reflecting layers  40  or  62  are not included in the layered structures. The display is then seen as changes in light transmission through the layered structure. 
     Operation 
     Referring to  FIG. 1 , the magnetic field  26  in the plane of display unit  4  has a variable orientation with respect to the display unit. Magnetic field  26  may be natural or be produced by a combination one or more permanent or electromagnets (not shown). The position of virtual pointer  24 , the markings on dial scale  28 , or the value of visible display symbol  30  to change to show the reading as the relative direction of the magnetic field rotates. 
     Referring to  FIG. 2 , incident light  48  is polarized selectively in regions or pixels  46  and passes into dichroic liquid crystal layer  44 . The polarized light is attenuated in variable degrees according to the difference in angle between the polarization direction and the orientation of the director of dichroic liquid crystal  52 , which aligns substantially with applied magnetic field  54 . A substantial part of the unattenuated component of light passing through liquid crystal layer  44  is reflected back through liquid crystal layer  44  by reflecting layer  40 . Further attenuation of the reflected light will occur within liquid crystal layer  44  before the attenuated and reflected light reemerges from polarizing layer  42 . Thus pixel  46  may appear bright or dark under control of the orientation of magnetic field  54 . 
     In the display system shown in  FIG. 3 , incident light  70  is linearly polarized by first layer  56  and passes into liquid crystal layer  58 . The orientation of the linear polarization of the light entering liquid crystal layer  58  is rotated by the helical twist of the liquid crystal director  64 . The helical twist is produced by torque resulting from anchoring director  66 A to the inner surface of layer  56  and the forces produced by external magnetic field  76  on the liquid crystal layer  58 . Director  66 B at the inner surface of layer  60  is weakly anchored to layer  60  so that most of the span of liquid crystal layer  58  readily aligns with magnetic field  76 . Note that the twist of the liquid crystal director  64  need not be uniform across the span of layer  58 . The pixels  68  comprising second polarizing layer  60  selectively attenuate the polarized light passed by liquid crystal layer  58  in accordance with the degree of helical twist and the particular linear polarization orientation of the individual pixels  68 . Light transmitted by the second polarizing layer  60  is then reflected back through that polarizing layer by reflecting layer  62  and is further attenuated. Reflected light emanating from second polarization layer  60  is transmitted back through liquid crystal layer  58  and first polarizing layer  56  to an observer. Thus pixel  68  may appear bright or dark based upon the orientation of magnetic field  76 . 
     CONCLUSIONS, RAMIFICATIONS, AND SCOPE 
     It can be seen that I have invented a liquid crystal display scheme that responds globally a vector sum of externally provided magnetic fields where the relative orientation of the sum magnetic filed selectively alters light polarization properties of pixels to form images. External magnetic fields may be provided by combinations of permanent magnets, electromagnets, and natural magnetic fields. The strengths and directions of these external provided magnetic fields may further be controlled by varying the orientation or positions of permanent magnets and electromagnets and by controlling the current(s) through any electromagnet(s). 
     Accordingly, it can also be seen that I have invented a convenient, easily visible, passive, robust, compact, flexible, accurate, and economical means for displaying magnetic field direction and the values of quantities that may be used to control or may be related to magnetic field direction, including vehicular tire pressure. Vehicular safety can be enhanced because tire pressure can be visually quickly and cleanly checked without need of attaching/detaching any external devices. 
     It can also be seen that I have invented a flexible and passive magnetic compass, which may be attached or bonded to charts and maps to provide permanent means for geographical orientation. Biasing the magneto-optic liquid crystal display with one or more permanent magnets can provided enhanced compass performance. 
     It can further be seen that I have invented an analog gauge with no moving parts. For example, an analog ammeter may be constructed by passing current through a wire near the invented magneto-optic liquid crystal display. Additionally, this ammeter may also passively digitize and display current digitally. 
     Although the description above contains some specificity, it should not be construed as limiting the scope of the invention but as merely providing illustrations of the scope of this invention: namely utilizing of the magneto-optic properties of liquid crystals in combination with a magnetic field that changes direction responsive to a changing analog signal or parameter, to provide a passive visual indication of the change in the signal or parameter. 
     Various other embodiments and ramifications are possible within scope the basic invention as described above. For example, materials other than liquid crystals such as ferrofluids demonstrate a type of response to magnetic field direction as liquid crystals and have the potential to provide comparable types of passive displays. 
     Also there are many different mechanisms capable of varying direction or rotating a magnetic field transducing mechanical stimulus, including pressure. Furthermore, flux-guiding means, including additives to the liquid crystal material, may be employed to increase the coupling of the tangential magnetic field within the display. More complex layered structures may be enhance the quality as well as color of the visual effects magnetic field direction has on pixels making up the display including multiple polarizing and liquid crystal layers. 
     The references listed below present useful teachings to those wishing to become familiar with and more skilled in fields of magneto-optic effects of liquid crystal compositions and analogous materials:
       Liquid Crystals,  2 nd Ed ., S. Chandrasekhar, Cambridge University Press, 1992.     Introduction to Liquid Crystals—Chemistry and Physics , P. J. Collings and M. Hird, Taylor and Francis, 1998.     Liquid Crystal Devices: Physics and Applications , V. G. Chigrinov, Artech House, 1999.   “ Ferronematics: Enhanced Magneto - Optical Response of a Liquid Crystalline Systems ,” Burylov, S. V. and Y. L. Raikher,  Mat. Sci. Eng. C 2 (1995) 235–241.   “ Static And Dynamic Behaviour of Ferronematics Under Magnetic Fields ,” Motoc, C., and E. Petrescu,  Modern Phys Lett. B.,  12, No 13 (1998) 529–540.   “ Magnetic Field Controlled Optical Phase Retardation in a Hybrid Nematic Cell ,” P. Ziherl, D. Subacius, A. Strigazzi, V. M. Pergamenshchik, A. L. Alexe-lonescu, O. D. Lavrentovich, and S. Zumer,  Liquid Crystals,  24, no. 4 (1998) 607–612.   

     Definitions of cholesteric helix and pitch length and methods of its measurement, are known to those skilled in the art such as can be found in  Electro - optical and Magneto - Optical Properties of Liquid Crystals , Blinov, L. M., John Wiley &amp; Sons Ltd. 1983. The pitch length is modified by adjusting the concentration of the chiral material in the liquid crystal composition. For most concentrations of chiral dopants, the pitch length induced by the dopant is inversely proportional to the concentration of the dopant. 
     Thus the scope of the invention should be determined with respect to the appended claims and equivalents, rather than by the examples described above.