Abstract:
An orientation system adapted to be mounted within a vehicle, comprising a display including a rotational heading indicating compass card fabricated from an image retaining electronic display panel responsive to bearing data signals.

Description:
THE INVENTION 
     This invention relates to a means to provide bearing indications from a vehicle to a source of original or reflected radiation. 
     BACKGROUND OF THE INVENTION 
     Modern technology has produced a wide variety of electronic devices adapted to indicate the bearing and distance of an object from a vehicle. Examples of relatively broad categories of such devices are radio direction finders, target tracking radars, storm cloud tracking radars and atmospheric disturbance detectors. 
     Radio direction finders generally incorporate a simple means to indicate bearing only except in the case of target identification transpondors. These latter systems function in cooperation with a rotatable antenna synchronized to a plan position indicator or PPI oscilloscope adapted to function similar to a tracking radar system except a transpondor is located in the target and adapted to provide a reply which is utilized by the system instead of a radar echo. 
     Radar systems using PPI displays have been used in vehicles such as aircraft for a significant number of years to indicate a large variety of items such as land masses, cities, other aircraft and areas of heavy precipitation. This latter application of radar systems commonly known as weather radar has found wide spread use in aircraft due to its ability to aid a flight crew in avoiding severe turbulence associated with thunderstorms. 
     A relatively newer approach to detecting severe turbulence associated with thunderstorms has recently been provided by systems combining radio receivers and plan position indicators wherein the radio receivers are responsive to the electromagnetic energy generated by lightning. 
     All of the foregoing systems utilize plan position indicators based upon cathode ray tubes which incorporate an electronically produced radial deflection in combination with an electromechanically produced rotational deflection of an electron beam. The rotational deflection or sweep of the beam is generally produced by rotating a deflection coil about the neck of a cathode ray tube in synchronization with a rotating or oscillating antenna. More recent technology has produced systems in which the target data is stored in computer means and displayed on the face of the cathode ray tube as a result of stationary electronic deflection means responsive to bearing related address data for the target data in storage. 
     All of the above plan position indicating devices have a serious drawback when used in a vehicle because the target data is the result of the instantaneous relative bearing at the time the signal is received. This vehicle heading oriented display remains fixed so that when the vehicle turns, the target image which has been retained fails to reflect the change in bearing from the vehicle. This results in enlarged and distorted targets and erroneous targets when high rates of turn are encountered. 
     In the atmospheric disturbance detection devices utilized to detect the presence of thunderstorms, the display means is usually a computer processed relative bearing indication on a cathode ray tube utilizing electronic deflection only. In these systems the display is a function of an automatic direction finding signal processed by a phase responsive antenna system. The received signals are retained in a storage means for a relatively long period of time when contrasted to normal radar return echos and used to generate a picture of severe weather cells. If the vehicle turns, the bearing data presented will be in error as a function of the amount of heading change of the vehicles since the last display update. This could be as much as 180 degrees in some instances and render the display completely useless. 
     OBJECTIVES OF THE INVENTION 
     In view of the obvious inability of the prior art display systems to indicate proper target bearing without complete updating of the display, it is a primary objective of the present invention to provide a plan position indicator which automatically maintains a true target relative bearing presentation even when the vehicle carrying the indicator is turning at a relatively high rate. 
     A further objective of the present invention is to provide a plan position indicator using a liquid crystal display element in the form of a compass heading indicating card. 
     A still further objective of the present invention is to provide a plan position indicator incorporating a dipolar electro-optic indicator in the form of a compass heading card. 
     A still further objective of the present invention is to provide a plan position indicator in the form of a compass card which has a capability of retaining target data in a display mode a relatively long duration of time. 
     It is a further objective of the present invention to provide an atmospheric disturbance indicator capable of storing and displaying disturbance incidents for a relatively long duration while maintaining proper relative bearing with respect to the individual incidents. 
     The foregoing and other objectives of the invention will become apparent in the light of the drawings, specification and claims contained herein. 
     SUMMARY OF THE INVENTION 
     Presented hereby is a plan position indicator incorporating a liquid crystal display means or a dipolar electro-optic display means on a rotatable, compass heading indicator card. The image generating light scattering materials contained on the rotatable card are activated by a grid of electrodes positioned behind the card and fixed with respect to the rotatable card housing or vehicle in which the device is mounted. Target signals activate individual electrodes essentially instantaneously as they are received to cause a target spot to appear on the card. As the vehicle turns that target spot will change in azimuth with respect to the vehicle as long as it is retained on the display. 
     The display materials utilized on the compass card are selected from liquid crystal compounds and dipolar electro-optic compounds adapted to have a relatively long image retention time. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front three-fourths view of a preferred embodiment of the present invention. 
     FIG. 2 is a side cutaway view illustrating a preferred embodiment of the present invention. 
     FIG. 3 is an exploded view of an indicator compass card and pin electrode assembly. 
     FIG. 4 is a sectional view of the electrical connection between individual electrode pins and the cable connector. 
     FIG. 5 is a sectional view of an electrode assembly. 
     FIG. 6 is a plan view of an alternate electrode assembly and functional control potential sources. 
     FIG. 7 is a front three-fourths exploded view illustrating the front and rear electrodes and indicator compass card structure of a preferred embodiment. 
     FIG. 8 is a schematic representation of a preferred embodiment of the invention in combination with an exploded, front three-fourths view of the principle elements comprising the invention. 
     FIG. 9 is a side cutaway view of the front electrode of the embodiment illustrated in FIG. 8. 
     FIGS. 10 and 11 are sectional views of an indicator compass card using dipolar elements. 
    
    
     DESCRIPTION OF THE INVENTION 
     FIG. 1 depicts a typical indicator incorporating the novel concepts of the present invention. The display 10 is a combined target indicator and compass card which incorporates indicia representing the cardinal compass points. Target indication is achieved by the card through its ability to alter its light scattering properties in response to an electric field stimulous. In the preferred embodiment, the card rotates to present the relative magnetic bearing under the indicating arrow 11 at the top of the front panel bezel 12 and due to this rotation, relative target azimuth is maintained. The card may be rotated by any means but preferably it is driven by a magnetic compass, gyrosyn compass or a gyro compass. For instance, the card 10 can be driven by the selysn receiver motor of a gyroscopically stabilized flux gate compass or remote indicating compass in which a magnetic compass rotates a selysn transmitter. A preferred embodiment, which will be discussed in the specification by way of explanation, utilizes a gyro compass mechanism to drive the indicator compass card 10. In this embodiment, knob 13 is provided to set the gyro compass heading indicator card 10 to the proper magnetic heading. 
     In the embodiment illustrated in FIG. 2 the indicator compass card 10 is rigidly affixed to shaft 14 which couples the card to the gyroscopic rotating mechanism via gear train 15. The illustrated gyroscopic mechanism is presented as exemplary only and may be any one of the numerous types available. The gyroscope 104 of the gyrocompass device of FIG. 2 is supported by a base 105 which is essentially parallel to a plane tangent to the surface of the earth. The base 105 is supported on a shaft 106 which permits the gyroscope to rotate freely with respect to the case 16 and 360° about an axis perpendicular to the plane tangent to the earths surface. When the gyroscope 104 is spun up it will remain fixed with respect to the perpendicular axis so that relative rotation between the case 16 and base 105 will occur when the case is rotated, as it would be if fixed in the instrument panel of a turning aircraft. A gear 107 is attached to the base 105. It is adapted to drive gear train 15 which rotates the indicator 10. The worm gear 108 cooperates with gear 109 on the support base 110 which supports shaft 106 so that when knob 13 is turned, it will alter the relationship between the gyroscope heading and indicator position. 
     Positioned immediately behind the rotatable card 10 is an electrode assembly 20 which is rigidly affixed to the indicator case 16. A transparent glass or plastic lens 17 is positioned in the bezel 12 and over the indicator compass card 10 to protect the card and keeps foreign matter out of the assembly. A light 18 is located inside the indicator case 16 to provide target illumination via light transmission through selected portions of the indicator compass card 10. 
     One form of electrode assembly 20 and indicator compass card 10 is illustrated in an exploded view in FIG. 3. This electrode assembly is comprised of a plurality of electrode pins 21 in a circular arrangement adapted to permit a relatively uniform display presentation by generating an electric field between selected pins 21, through the indicating material of indicator compass card 10, and a transparent electrode 22 covering the front face of indicator compass card 10. Ground potential or other required operating voltage is maintained on transparent electrode 22 via a shoe contact 23. The spacing between electrode pins 21 is relatively constant but may be slightly greater around the outer periphery of the electrode assembly as required to permit electrical connection of the pins to target data input cable connectors 24 and 25. 
     The electrode pins 21 are in the form of short pins as illustrated in FIGS. 4 and 5 which may be tied directly to individual conductors 26 that are then combined to form cables 27 coupling the electrode assembly to a target signal processing means. The length of the pin electrodes 21 is selected so that the wires connecting the pins to the signal processing source are spaced apart from the indicator compass card a distance great enough so that any electric fileds they may create will have no affect thereon. The pins may be placed in a relatively thick material forming the electrode support or they may be supported between two thin sheets of dielectric material. In a preferred embodiment a single thick sheet 28 of dielectric material is utilized having a thickness equal to the length of the pins 21 and the pins are headed at the display side to create a larger image. 
     A preferred form of the invention utilized printed wiring 29 to connect the individual pins 21 to the cable receptacles 24 and 25 at the sides of the electrode assembly 20, see FIGS. 3 and 4. This technique is preferred because extremely fine printed wires or conductors may be utilized and because of their thin dimensions they may be placed relatively close to the indicator compass card 10 without creating display interfering electric fields and erroneous targets. 
     The indicator compass card 10 of FIG. 3 is comprised of a dielectric support member 31 and a transparent conductive electrode adapted to form a hollow disc which is filled with a liquid crystal display compound 32. In operation, an electrostatic field is set up between energized electrodes and the transparent conductive electrode causing the liquid crystal display element to alter its light scattering properties so that ambient light or light from a lamp directed on the front of the indicator compass card will create a visible image. In an alternate embodiment, light source such as 18 of FIG. 2 may be placed within the housing 16 and the support dielectric base 31 of FIG. 3 can be fabricated from a translucent material so that target images are formed by the process of selectively permitting light to pass through the indicator compass card. 
     The liquid crystal display compound 32 may be a cholesteric or nematic liquid crystal similar to those listed in Table 1 or a mixture of a cholesteric liquid crystal compound with a nematic liquid crystal compound of the type that exhibits a non-destructive turbulent motion when an electrical current of sufficient magnitude is passed therethrough. An example of such a material is presented in U.S. Pat. No. 3,703,331 on &#34;Liquid Crystal Display Element Having Storage&#34; issued to J. E. Goldmacher et al, on Nov. 21, 1972. As suggested by that patent, the ratios and types of cholesteric and nematic liquid crystals are controlled to provide a display element having the required image retentivity. For atmospheric disturbance presentations this time may be in the order of two minutes. 
     Table 1 
     Nematic Liquid Crystals 
     p-azoxyanisole 
     p-azoxyphenetole 
     p-butoxybenzoic acid 
     p-methoxycinnamic acid 
     butyl-p-anisylidene-p-aminocinnamate 
     anisylidene 
     para-aminophenylacetate 
     p-ethoxybenzylamino-a-methylcinnamic acid 
     1,4-bis(p-ethoxybenzylidene)cyclohexanone 
     4,4&#39;-dihexyl-oxybensene 
     4,4&#39;-diheptyloxybenzene 
     anisal-p-amino-azo-benzene 
     anisaldazine 
     a-benzeneazo-(anisal-α&#39;-naphthylamine) 
     n,n&#39;-nonoxybenzetoluidine 
     p-n-anisylidene-p&#39;-aminophenylacetate 
     p-n-butoxyoenzoic acid dimer 
     p-n butoxybenzylidene-p&#39;-aminophenylacetate 
     p-n-octoxybenzylidene-p&#39;-aminophenylacetate 
     p-n-benzylideneacetate-p&#39;-aminophenylethoxide 
     p-n-anisylidene-p&#39;-aminophenylbutyrate 
     p-n-butoxybenzylidene-p&#39;-aminophenylpentanoate 
     p-n-hexoxybenzylidene-p&#39;-aminophenylacetate 
     p-iso-pentoxybenzylidene-p&#39;-aminophenylacetate 
     p-n-benzylidenebutyrate-p&#39;-aminophenylmethoxide 
     p-n-benzylidenebutyrate-p&#39;-aminophenylhexoxide 
     p-n-nonoxybenzylidene-p&#39;-aminophenylacetate 
     p-n-anisylidene-p&#39;-aminophenylpentanoate 
     p-n-propoxybenzylidene-p&#39;-aminophenylacetate 
     p-n-propoxybenzylidene-p&#39;-aminophenylbutyrate 
     p-n-benzylidenebutyrate-p&#39;-aminophenylpropoxide 
     p-n-benzylideneacetate-p&#39;-aminophenylmethoxide anils of the generic group (p-n-alkoxybenzylidene-p-n-alkylanilines), 
     such as p-methoxybenzylidene-p&#39;-n-butylaniline; nematic compounds of the alkoxybenzylidene-aminoalkylphenone group, such as methoxybenzyl-idene-amino-butyrophenone and methoxybenzylidene aminovalerophenone; mixtures of the above and others. 
     Cholesterol Derivatives and Cholesteric Liquid Crystals 
     Cholesteryl Chloride 
     Cholesteryl Bromide 
     Cholesteryl Iodide 
     Cholesteryl Nitrate 
     Esters derived from reactions of cholesterol and carboxylic acids 
     For example Cholesteryl Crotonate 
     Cholesteryl Nonanoate 
     Cholesteryl Hexanoate 
     Cholesteryl Formate 
     Cholesteryl Chloroformate 
     Cholesteryl Propionate 
     Cholesteryl Acetate 
     Cholesteryl Valerate 
     Cholesteryl Linolate 
     Cholesteryl Linolenate 
     Cholesteryl Oleate 
     Cholesteryl Erucate 
     Cholesteryl Butyrate 
     Cholesteryl Caprate 
     Cholesteryl Laurate 
     Cholesteryl Myristate 
     Ethers of cholesterol such as 
     Cholesteryl Decyl Ether 
     Cholesteryl Oleyl Ether 
     Cholesteryl Dodecycl Ether 
     Carbamates and carbonates of cholesterol such as 
     Cholesteryl Decyl Carbonate 
     Cholesteryl Oleyl Carbonate 
     Cholesteryl Methyl Carbonate 
     Cholesteryl Ethyl Carbonate 
     Cholesteryl Butyl Carbonate 
     Cholesteryl Docosonyl Carbonate 
     Cholesteryl Heptyl Carbamate 
     and alkyl amides and aliphatic secondary amines derived from 3β-amino-Δ 5  -cholestene and mixtures thereof peptides such as poly-γ-benzyl-1-glutamate derivatives of beta sitosterol such as sitosterol chloride and active amyl ester of cyano benzylidene amino cinnamate 
     Stigmasterol 
     Cholesteryl Palmitate 
     Cholesteryl Decanoate 
     Cholesteryl Laurate 
     Cholesteryl Propionate 
     Cholesteryl Heptafluorobutyrate 
     Cholesteryl 2-Furoate 
     Cholesteryl Cinnamate 
     Cholesteryl Cyclohexanecarboxylate 
     Cholesteryl Anisoate 
     Dicholesteryl Phxhalate 
     Cholesteryl p-Nitrobenzoate 
     Cholesteryl p-Phenylazobenzoate 
     Cholesteryl 3.5-Dinitrobenzoate 
     Cholesteryl 2-(Ethozyethoxy) Ethyl Carbonate 
     Cholesteryl 2-(2-Methoxyethoxy) Ethyl Carbonate 
     Cholesteryl Geranyl Carbonate 
     Cholesteryl Octadecyl Carbonate 
     Cholesteryl 2-Propyn-1-yl Carbonate 
     Cholesteryl 2-Methyl-2-propene-1-yl Carbonate 
     Cholesterol Derivatives and Cholesteric Liquid Crystals (Continued) 
     Allyl Cholesteryl Carbonate 
     Cholesteryl 2.2.2-Trifluoroethyl Carbonate 
     Cholesteryl Methyl Carbonate 
     Cholesteryl Cinnamyi Carbonate 
     Cholesteryl p-Menth-1-en-8-yl Carbonate 
     Cholesteryl Nitrate 
     Cholesteryl Propynyl Carbonate 
     3β-Chlorocholest-5-ene 
     Cholesteryl Methanesulfonate 
     5α-Choiestan-3β-yl Chloroformate 
     Cholesteryl Chloroformate 
     5α-Cholestan-3β-ol 
     The alkyl groups in the above compounds are typically saturated or unsaturated fatty acids, or alcohols, having less than about 25 carbon atoms and unsaturated chains of less than about 5 double-bonded olefinic groups. Aryl groups in the above compounds typically comprise simply substituted benzene ring compounds. Any of the above compounds and mixtures thereof may be suitable for cholesteric liquid crystalline films in the advantageous system of the present invention. 
     An alternate embodiment of the invention is illustrated in FIGS. 6 and 7 where in the indicator compass card 40 includes display material 42 contained between two members 41 and 43 and the rear electrode 44 is comprised of a parallel grid of conductors 45. A second electrode 46 is located in front of the rotatable display element 40 and it is comprised of a plurality of parallel, transparent conductors 47 on a transparent substrate 48 oriented so the conductors 47 are perpendicular to the conductors 45 of the rear assembly 44. The electrodes are connected via cable connectors 51 and 52 and cables 53 and 54 to signal processing means 56 and 57 which are adapted to address individual vertical and horizontal electrodes to create an image at a predetermined point on the rotating imaging assembly. This embodiment has certain advantages over the previously discussed embodiment for it requires fewer electrical connections to the display generating means and simplifies the addressing electronics. 
     An erase assembly 58 comprised of a power supply and a switching means is attached to electrodes of the assemblies to permit automatic and selective erasure of the display. Alternately, erase electrodes may be included in any embodiment. For instance in the embodiment illustrated in FIG. 3 the erase electrodes are transparent electrode 22 and a second transparent electrode 122 on the opposite side of the compass indicator card 10 electrically connected to the power source via a shoe contact similar to 23. If desired, the erase electrodes can be mounted on the fixed electrode assemblies to eliminate the need for shoe contacts. This is the preferred form since the imaging conductors can be placed between the erase electrodes and the compass indicator card 10 to prevent the erase electrodes from interfering with the image generating process. 
     The signal processing means 56 and 57 of FIG. 7, or of any embodiment, couple electrical impulses from a signal generating source to the display creating electrodes. The signal generating source may be a radar receiver of the type adapted to provide target data display signals having display coordinates, or it may be a television video signal generator with the sweep scan synchronized to the horizontal conductor pattern of electrode assembly 47 and digitizing signals synchronized with segments of the horizontal sweep impressed on the vertical conductor pattern of electrode 46, or it may be a radio direction finder and signal processing unit such as the Ryan Stormscope WX-7 manufactured by ((Ryan))))) Stormscope, 4800 Evanswood Drive, Columbus, Ohio, 43229 which is capable of receiving electromagnetic radiation disturbances generated by lightning and processing the received signals using radio direction finding techniques so that the disturbance targets will have an azimuth and distance quality. 
     In a preferred form of the instant invention illustrated in FIG. 8, a rear electrode 60 incorporates a plurality of radiating conductors 61 which are electrically connected by leads 68 to cable connector 51. The number of radiating conductors is a function of the dimensions of the conductor and the electrode assembly but preferably the number of conductors is a multiple of 36. The front electrode assembly 62 is comprised of a transparent, dielectric substrate 63 upon which closely spaced circular, transparent conductors 64 are positioned on the side adjacent to the rotatable indicator 40. The transparent, circular conductors 64 are connected to cable connector 52 by means of transparent connectors 65 which pass through the substrate 63 and traverse the side of the substrate opposite the side upon which the circular conductors are positioned. The dielectric substrate 63 is dimensioned so that connectors 65 are maintained a sufficient distance from the rotatable indicating means 40 so that they will not generate electric fields that will influence the rotatable display element 40, A thin, transparent dielectric substrate 66 may be positioned between substrate 63 and conductors 64 to insulate the circular electrodes or conductors 64 from a conductive, transparent sheet 67 positioned over dielectric substrate 63 to form an erasing electrode. This forms a front electrode assembly which is comprised of a sandwich having a primary supporting dielectric body 63 with connection conductors 65 on the front side and a conductive layer 67 on the back side covered by a dielectric layer 66 which in turn is covered by circular electrode conductors 64, see FIG. 9. 
     The individual ray like conductors 61 on electrode assembly 60 are connected via conductive connectors 68 and cable connector 51 to an automatic direction finder receiver 70. The analog data normally used to drive a direction indicating meter is digitized, by any one of the acceptable standard methods well known in the art, to provide a number of inputs adapted to match the number of ray conductors 61 on electrode assembly 60. 
     Signals detected by visual omnirange (VOR) receivers 71 and 72 are digitized by one of the standard conversion techniques such as that utilized in the DVOR/100 digital VOR radial display manufactured by HTI, Redwood Avenue, Los Angeles, Calif., 90066 or the Bendix PX2000 Navigation System manufactured by the Bendix Corporation, Avionics Division, P.O. Box 9414, Ft. Lauderdale, Florida, 33310 to provide a number of azimuth outputs corresponding to the number of ray electrode conductors 61 of electrode assembly 60. 
     The signals detected by the ADF are also applied to a discriminator 76 which includes a band pass filter adapted to pass signals of a predetermined frequency range corresponding to the electromagnetic radiation generated by lightning. The output of the discriminator 76 is applied to an analog to digital converter 74 which converts the signals received from the discriminator to a plurality of digital outputs corresponding to the number of electrode conductor rings 64 on electrode assembly 62. This interconnection is arranged so that the strongest signal will energize the smallest ring electrode at the center of the electrode assembly 62 and the weakest signal will energize the largest circular electrode 64 to provide a range indication. A variable attenuation pad 75 is positioned between discriminator 76 and analog-to-digital converter 74 so that the amplitude of the signal applied to the analog-to-digital converter may be adjusted to cause the concentric ring electrodes 64 to represent signals at predetermined ranges. An amplification network 73 is positioned between the ADF receiver and the analog-to-digital converter so that the effective range of the device may be set to predetermined ranges such as 0 to 10 miles, 0 to 20 miles, 0 to 100 miles, 0 to 200 miles, etc. 
     Signal strength controlling means such as amplifiers or attenuation means 81, 82 and 83 are provided between the VOR receivers 71 and 72 and ADF receiver 70 and the electrode assembly 60 so that an operator may cause signals from a specific receiver to be of a greater amplitude and thus create a brighter display or conversely have a lesser amplitude and create a dimmer display. 
     The conductive sheet electrode 67 of electrode assembly 62 is energized by function selection control 77 which is adapted to apply a constant operating potential to the electrode and thus mask out any effects which might be created by circular electrode conductors 64 or in an alternate mode of operation energization means 77 will produce no operating potential for conductor 67 and thus only targets representing atmospheric electrical discharge will be created on the display means 40. In a third mode of operation, control means 77 provides regular, intermittent activation pulses to electrode 67 so that VOR and ADF data will be displayed as ray lines corresponding to electrodes 61 and atmospheric disturbance data will be alternately displayed as target points where energized circular electrodes 64 correspond with energized ray electrodes 61. In this mode of operation the pilot of an aircraft will have a single display which will indicate relative bearing to two different VOR stations and an ADF station in combination with point targets representing atmospheric disturbances. The operator may, through the signal strength controlling means 81, 82 and 83 vary the intensity of the VOR and ADF azimuth indicating rays so that he may easily distinguish therebetween. 
     In the preceeding embodiments of the present invention, the liquid crystal display element may be replaced with a dipolar electro-optic structure such as those structures disclosed in U.S. Pat. No. 3,512,876 on &#34;Dipolar Electro-Optic Structures&#34; issued to A. M. Marks, May 19, 1970. This display element is comprised of a plurality of small dipolar elements 101 suspended in a liquid 102 as indicated in FIGS. 10 and 11. The dipolar elements 101 are normally randomly orientated as indicated in FIG. 10, but an electric field is generated between two electrodes as in the previous embodiments and it causes the dipolar elements to align themselves as illustrated in FIG. 11, thus altering the light scattering properties of the material. When this display medium is utilized, the image retentivity is controlled by selecting the viscosity of the liquid in which the dipolar elements are dispersed. The heavier the liquid the greater the duration an image will be retained. This is controlled in a preferred embodiment by the controlled application of heat through heater 103 of FIG. 2 to vary the liquid viscosity. 
     The dipolar elements 101 are, in a preferred embodiment, minute particles having one dimension in the order of λ/2η and a perpendicular dimension equal to or less than λ/10η where λ is the wavelength of light and η is the index of refraction of the suspending medium. 
     The displayed targets are erased in the dipolar embodiments by using an alternating current erase signal similar to that used in the liquid crystal embodiments. 
     While preferred embodiments of this invention have been illustrated and described, variations and modifications may be apparent to those skilled in the art. Therefore, I do not wish to be limited thereto and ask that the scope and breadth of this invention be determined from the claims which follow rather than the above description.