US20080074959A1

US20080074959A1 - Electromagnetic write heads and backplanes for use in bi-stable, reflective magneto optical displays

Info

Publication number: US20080074959A1
Application number: US11/863,071
Authority: US
Inventors: Charles O. Edwards; James Caruso
Original assignee: TRED Displays Corp
Current assignee: TRED Displays Corp
Priority date: 2006-09-27
Filing date: 2007-09-27
Publication date: 2008-03-27
Also published as: KR20090060363A; EP2074620A2; KR20090060364A; US8020326B2; JP2010505151A; WO2008039954A2; WO2008039955A2; WO2008039957A2; TW200819820A; WO2008039955A3; CN102016969A; CN102016969B; JP2010505152A; WO2008039954A3; US20080074365A1; US20080072467A1; EP2084700A4; US8077142B2; US20080074368A1; WO2008039957A3

Abstract

A write head for use in a bistable, magnetic optical display comprising one or more conducting coils configured to generate a magnetic field that is substantially perpendicular to the axis of rotation of one or more magneto optical elements of the optical display and parallel to a display viewing plane of the one or more magnetic display elements of the magnetic optical display such that an electrical pulse of less then 150 milliseconds is sufficient to create a magnetic field that alters the bistable state of the magneto optical display such that the display will stay in this new state without the need for ongoing electrical current being applied to the write head.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. Provisional Application No. 60/847,601, filed Sep. 27, 2006, U.S. Provisional Application No. 60/847,603, filed Sep. 27, 2006 and U.S. Provisional Application No. 60/875,514, filed Dec. 18, 2006, all of which are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. The Field of the Invention
Embodiments of the present invention relate to novel electromagnetic write heads that are useful as a backplane for reflective, bi-stable, magneto-optical displays. These electromagnetic write heads are a uniquely designed printed circuit board or similar circuitry which has a unique blend of electronic and magnetic attributes resulting in a low-cost backplane useful to drive a magneto-optical display.
2. The Relevant Technology
The principles of the present invention relate to a new type of write head circuitry that is designed for use with a magneto-optical display. While magneto-optical displays have been envisioned in the past, there has never been disclosed a write head backplane suitable to drive such as display.
By way of introduction, magneto-optical displays utilize a multi-composite material that has both magnetic and optical properties. This multi-composite material, or magneto-optical elements, herein referred to as MOEs, have numerous applications in display-related applications. These MOEs can be actuated with a low power external magnetic field producing a rotation and change in color which can be taken advantage of in unique ways to produce a low-cost, low-power, reflective, large-format, flat panel displays that are bistable and can be manufactured using low-cost, industrial manufacturing processes.
While displays have been developed in the past, these were an array of individual electromagnetic actuators acting like small motors to flip disks for flaps in the work performed by Tijanic (U.S. Pat. No. 5,809,675), Turney (U.S. Pat. No. 5,005,305) and others. Recently, magneto-optical displays have been disclosed by Masatoshi (Jap Patent Publ. 08-197891) and Tadao et al. (Japan Patent Publ. 2002-006346). Masatoshi disclosed a display comprising an array of magnetic optical material wherein a permanent magnetic “pen” is moved across the surface, the magnetic/optical materials move in accordance with the pens magnetic field thus generating an image.
The work by Tadao et al. discloses a multi-layer display with an array of fine magnetic/optical materials combined with a multitude of finely produced layers and an X-Y arrangement of electric wirings in the form of patterned metal. In Tadao, the X-Y array of wires can generate a magnetic field perpendicular to the X-Y wire and display viewing plane. Here again, the important distinction is that the magnetic field in the Tadao case is perpendicular to the plane of the display.
The inventions disclosed herein are significantly different in that they rely on novel electromagnetic circuit architecture such that the magnetic field is primarily parallel to the display plane. This new architecture is needed for the unique MOE type of magneto-optical display that has the magnetic elements arranged in a bi-stable magnetic state parallel with the viewing plane. This results in a vastly superior performing display.

BRIEF SUMMARY

A first embodiment disclosed herein relates to a write head for use in a bistable, magnetic optical display. The write head includes one or more conducting coils configured to generate a magnetic field that is substantially perpendicular to the axis of rotation of one or more magneto optical elements of the optical display and parallel to a display viewing plane of the one or more magnetic display elements of the magnetic optical display such that an electrical pulse of less then 150 milliseconds is sufficient to create a magnetic field that alters the bistable state of the magneto optical display such that the display will stay in this new state without the need for ongoing electrical current being applied to the write head.
A second embodiment disclosed herein relates to a write head for use in a bistable, magnetic optical display. The write head includes a first conducting coil wound in a first direction, a second conducting coil wound in a second direction, wherein the first and second conducting coils are configured to generate a magnetic field that is substantially perpendicular to the axis of rotation and parallel to a display viewing plane of one or more magnetic display elements of the magnetic optical display such that an electrical pulse of less then 150 milliseconds is sufficient to create a magnetic field that alters the bistable state of the magneto optical display such that the display will stay in this new state without the need for ongoing electrical current being applied to the write head
A third embodiment disclosed herein relates to a write head for use in a bistable, magnetic optical display. The write head includes a soft magnetic material and one or more conducting coils that are at least partially disposed proximate the magnetic material, wherein the one or more conducting coils are configured to generate a magnetic field that is substantially perpendicular to the axis of rotation of two or more magneto optical display elements of the magnetic display and parallel to a display viewing plane of the two or more magneto optical display elements of the magnetic optical display; and wherein the soft magnetic material includes a protruding wall that divides the two or more magnetic display elements into two sections, with each section creating a separate, stable magnetic domain.
A fourth embodiment disclosed herein relates to a write head for use in a bistable, magnetic optical display. The write head includes a coil of one or more layers that are wound in one direction with the windings parallel to the viewing plane of one or more magneto optical display elements and a soft magnetic component that is placed proximate the coil and creates a wall that divides the coil winding into two sides by protruding through the coils and dividing the magneto optical elements into two separate opposing magnetic sections, wherein the magneto optical elements of the two separate magnetic sections may be actuated by the coil simultaneously.
This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.
Additional features and advantages will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the teaching herein. The features and advantages of the teaching herein may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features 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 hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

To further clarify the above and other advantages and features of the present invention, a more particular description of the invention will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered 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 illustrates a cross-sectional representation and side-view representation of simple two-MOE, magnetic write head design.

FIG. 2 represents simple single coil magnetic write head with MOEs showing two different bi-stable states.

FIG. 3 illustrates a single pole, single coil magnetic write head design.

FIG. 4 represents end-on view and top-down view of a dual-coil, single pole magnetic write head design.

FIG. 5 illustrates an end-on view of a multi-pole magnetic write plane design.

DETAILED DESCRIPTION

The process of “writing” to a display pixel is done by presenting an external magnetic field (commonly referred to as H) to the pixel. Presenting this external magnetic field rotates one or more “magneto-optical elements” (a component that has both magnetic and optical properties (herein referred to as MOEs) resulting in rotation and thus a color change.
An electromagnetic write head disclosed as an embodiment herein utilizes the principle of an inductive coil. By applying these principles it is possible to create a magnetic write head for a bi-stable, magneto optical display.
Referring to FIG. 1A, an embodiment of write heads for magneto optical displays in accordance with the present invention is illustrated. As shown in FIG. 1, there are typically three principle components used to create write heads for magneto optical displays.
A first component is a coil of conductors, designated at 1, that is designed to create a magnetic field that is perpendicular to the axis of rotation and parallel to the display viewing plane of the MOE in the pixel, designated at 3. The coil 1's magnetic field is also approximately parallel to the direction of the magnetic field created by the MOEs (3) alignment in their two, 180 degree opposed bi-stable states, north-south, north-couth or south-north, south-north.
A second component is a soft magnetic material, designated at 2, which may be, but is not limited to, Iron, an Iron-containing material or a ferromagnetic material. The soft magnetic material 2 is used to amplify the magnetic field created by the coil 1 and may also be used to create a flux circuit within the pixel 3. This soft magnetic material 2 is not required for all designs to make a functioning write head, but it can significantly increase (by 3× or more) the efficiency and magnetic field strength of the write head.
A third component is control electronics (not illustrated) capable of providing a DC current pulse to the write head and also capable of controlling the direction of the DC current such that the current can be reversed and thus the electromagnetic field created by the write head coil 1 can be controlled in either the North-South or South-North direction by reversing the direction of DC current flow. Any suitable control circuitry known in the art may be used.
The two bi-stable states are written by creating a magnetic field (H) that flows in one of the two directions perpendicular to the pixel's axis of rotation (either North-South or South-North). When the write head 1 presents a new magnetic field to the pixel that opposes and is greater then the magnetic field created by the permanent magnetic materials inside the MOEs of the pixel 3, a rotational mechanical force is generated that will realign the MOEs so that their permanent magnetic fields are aligned with the stronger magnetic field created by the write head. The result is the MOEs in this pixel 3 will rotate.
As the MOEs pass 90 degree rotation and start approaching 180 degrees rotation, their permanent magnetic fields will work with the write head 1 magnetic field to rotate the MOEs to the opposing bi-stable state at 180 degrees rotation. Each side of the MOEs is typically coated with a different optical coating (usually a color pigment) and in this way the new optical state is displayed. Because these magnetic display pixels (or group of MOEs, 3) are truly bi-stable once rotation has past 90 degrees, if the MOEs have a strong enough permanent magnetic charge, they will continue rotation on their own without a sustained external magnetic field from the write head 1 and they will self-align into the new bi-stable state.
If the write head 1 magnetic field is oriented in the same direction as the MOEs permanent magnetic field then no change of state and no rotation of MOEs occur. So there is no negative side effect of writing a pixel 3 in the same direction multiple times except that it is wasted energy.
In some embodiments, a backplane is an array of write heads 1 used to control the state of multiple pixels 3 or segments in the display. An efficient electromagnetic write head 1 can control a bi-stable magneto optical pixel 3 using an electromagnetic field generated by a current pulse of less then 150 milliseconds. Longer pulses may be used, but they generally add no benefit and waste power. So typically a useful pulse width will be less then 150 milliseconds even if the actual pulse width is longer. Many configurations of MOE pixels 3 are capable of much faster speeds (less than 50 milliseconds on some current designs with less than 10 milliseconds anticipated).
Because this type of magneto optical display is bi-stable, only pixels that are to be changed need to be “written” with an electromagnetic pulse. No “refresh” of pixels that are already in their target state is needed. Energy is saved if only pixels that need to be changed to the other bi-stable state or are in an unknown state (like at start up) are written. Therefore it is desirable to have a control system that stores the current state of the display pixels, and can intelligently compare that to the next image and then only “writes” those pixels or segments that are different and therefore need to be updated to create the new image.
Most pixel architectures of bi-stable, magneto optical displays have only one section of MOEs per pixel with all MOEs in the section aligned in the same direction of rotation and aligned in the same magnetic direction (North-South, North-South or South-North, South-North). The MOEs will align to create a single prevailing magnetic field in each section. In some pixel or segment designs, it may be desirable to have more then one orientation of the MOEs which will create separate sections inside a single pixel. All sections of a pixel are controlled at the same time and share one set of coils and control electronics. When the MOEs within a pixel for example are separated by a wall and the magnetic field on each side of the wall is rotated 180 degrees by rotating the MOEs, there will be two low energy, bi-stable states (North-South, North-South or South-North, South-North) on the two sides of the pixel.
In some display designs, a single pixel or segment may be divided into two or more sections of MOEs. Each of these sections will follow the same principles of a shared axis of rotation and shared magnetic orientation of all MOEs within the section. In their bi-stable state, the poles of the MOEs in each section have sufficient attraction to overcome friction and self-rotate to align in the same direction creating a prevailing magnetic field H within the pixel. This self-alignment will occur when no external magnetic field is presented. In order to “write” the pixel, the stable state is disrupted by an external magnetic field that must be greater then the permanent magnetic field created by the MOEs in their bi-stable state. If the external magnetic field is not strong enough, then the permanent magnetic field created by MOEs themselves will over power the external field and prevent sufficient rotary force from being created and the MOEs within the section will not rotate. Generally, the external magnetic field created by the write head 1 pulse will need to be 1.5× or greater then the average permanent magnetic field generated by the MOEs in their bi-stable states in order to be sufficient to overcome friction and reliably write the pixel to the new bi-stable state.
Increasing the strength of the write head 1 magnetic field beyond 1.5× of the MOEs magnetic field can be used to improve response time because greater rotational force is created in the MOEs and thus they spin faster and more reliably to the new state. However, increasing the magnetic field strength of the write heads 1 within a backplane generally requires the use of more electrical current which will make the display less power efficient. However, as the magnetic field strength of the write head 1 pulse increases it may be possible to further decrease the pulse width and save some of the energy. If the absolute strength of the write head 1 magnetic field it too weak to generate reliable rotation of the MOEs (less then 1.5×), increasing pulse width alone will not resolve this problem.
If the permanent magnetic field strength of the individual MOEs is too low, actuation by an external field becomes increasingly difficult and the MOEs will not reliably self align when the external field is removed. This is because the rotary force created by the write head 1 pulse on the MOEs is a product of the write head 1 external magnetic field strength times the internal MOE permanent magnetic field strength. If the MOEs have a weak internal magnetic field strength, the rotary force is similarly weakened. If the MOEs have too strong of a permanent magnetic field they can create a strong bond that tightly aligns the MOEs to each other in their bi-stable state. This MOE to MOE magnetic attraction must be overcome by the external magnetic field of the write head in order to actuate the MOEs. Overcoming MOE to MOE magnetic forces that are stronger then needed due to higher then necessary magnetic loading within the MOEs will significantly decrease write head efficiency.
For display applications, the magnetic field strength of the MOEs will generally be less then 40 milliTeslas. The amount of magnetic loading needed in the MOEs varies due to several factors. These include the mass of the MOEs, the diameter of the MOEs, the frictional and mechanical force inhibiting rotation, and display design. In general, it has been observed that decreasing the diameter of the MOEs decreases the magnetic loading required and increases display efficiency and response speed. This is because the mass of the MOEs will decrease linearly while the magnetic field strengths increase exponentially as the dimensions of the MOEs and the write head are decreased. Therefore, smaller geometries have significantly higher effective magnetic field strengths with less mass in the MOEs to rotate.
There are several practical limitations that prevent and/or make decreasing size of the MOEs and the write heads impractical and more costly. As geometries get smaller, tighter tolerancing is needed in the display components. Above +/−25 micron tolerances it is possible to use a wide variety of low cost industrial plastics and PCB processes. Manufacturing costs of the display can increase dramatically if the tolerances required by the components becomes less then +/−10 microns. Also as the pixel size decreases, more individual components are needed to make a display of the same unit area. This increases the number of components, device complexity and assembly costs.
Another limiting factor is the magnetic stability of small particle sizes. If magnetic powders are used in the MOEs, then individual particle sizes of less than 50 microns are less stable and more difficult to manufacture especially when they are added into plastics that are extruded or cast. For the write heads themselves, as the conductive traces decrease in size they increase in resistance. This increases the amount of heat generated by the coils of the write heads in relation to the strength of the electromagnetic fields they generate. Therefore as the write heads decrease in size they become less efficient. It is anticipated that due to these combined factors these electromagnetic write heads and bi-stable magneto optical displays will be most efficient in displays that have pixels sizes of 1 mm per pixel and larger. Of course, the principles of the present invention may also be applied to displays with pixel sizes of less than 1 mm per pixel.
Low Power and Wireless
An efficient electromagnetic backplane consisting of an array of write heads combined with a bi-stable magneto optical frontplane can create a very high efficiency display. For example, in a clock application, a clock display changes the minute digit every 60 seconds, the 10 minute digit changes once every 10 minutes, the hour digital changes once every hour and the ten hour digit changes twice every 12 hours. Therefore, only one write pulse of less than 150 milliseconds is needed every minute to change the selected pixels of a clock display to the new time. In this clock example, no power is used by the clock display for the remaining 59.85 seconds per minute. In an application like signs for gas prices, these displays may only be changed once a day—using no power except for this single write cycle.
Bi-stable, magneto optical displays are also reflective so they use no power to display their optical state when external light is available unlike light emitting signs like light emitting diode (LED) and organic light emitting diode (OLED) that require constant power even in daylight. This ultra-low power usage can make it possible for the backplane and control electronics to be powered by a small self contained power source such as batteries and/or solar cells. For some applications this can eliminate the need for costly infrastructure and wiring especially where the sign may not be near utilities.
Light weight and thin digital signs are desirable because they reduce the cost of installation and they reduce the size and strength of the support structures required to hold the signs. Combining wireless technology for data communications, such as cellular, Bluetooth, IR or other similar systems, can make it possible for these displays to be installed without any hard wiring for power or for data transfer. They can be truly stand alone devices.
Major Components of an Electromagnetic Write Head
The write head design will be impacted by the layout of the pixels or segments they are designed to control. Several designs for write head coils have been developed. Because the write heads that make up the backplane can be the most costly components of a display, a low cost method of fabrication have also been utilized.
The Use of PCBs and Soft Magnetic Plates
Wire winding of the write head coils may be used, but the preferred method of manufacturing is printed circuit boards (PCB) or printed wiring boards (PWBs). The use of PCB (or PWB) manufacturing to create the write heads is desirable because (1) PCB is a proven, high production, low cost manufacturing method, (2) PCB backplanes can be produced in large area arrays of write heads on large PCB substrates, (3) the PCB backplanes can also contain the interconnect traces needed for the control of each write head within the array and (4) the PCB process allows for multi-layer PCBs so the number of windings used in each coil can be increased further by increasing the number of PCB layers used. It is even possible to add components and circuitry to the PCB write head array so the same PCB can contain both the write heads and also the control electronics.
Conventional printed circuit board technology using Copper and Aluminum over fiber-reinforced epoxy (or FR4), polyimide (or Kapton) and other similar substrates is a standard, low cost manufacturing process for creating patterned conductors. Generally, PCBs are used in conjunction with discrete components where the discrete components create the functionality and the traces on the PCB are used primarily for interconnecting these discrete components. Embodiments of the present invention create inductive coil arrays with the patterned conductors of the PCB acting as the coils themselves. The PCB patterned traces are capable of creating magnetic fields sufficient to actuate the display without the need for additional discrete coil components.
The benefit of using an array of coils patterned on a PCB is that the cost of manufacturing is dramatically reduced because addition discrete coil components are eliminated. An array of hundreds and even thousands of coils can be patterned on a single PCB. Also, the PCB process is planar so the write heads and coils are flat and parallel to the MOE display frontplane, creating a smooth surface for rotation of the MOEs against the PCB backplane. Because magnetic field strength decreases geometrically with distance, maintaining the closest possible contact between the MOEs and the PCB backplane is desirable. In many implementations, the MOEs may rest directly on the PCB backplane with no additional separation layer. Because PCB technology is well established, many suppliers commonly provide patterned Cu conductors with widths of 100 microns with 100 micron spacing separating them. Higher resolution processes are also available and improved resolutions are under constant development. In the present invention, the conductors are patterned into windings on each layer of the PCB. Multiple PCB layers can be used to create more windings and increase magnetic field strength. The magnetic field strength created by the coil to actuate a pixel is usually in excess of 1.5× the average field strength across the MOEs in the pixel.
By reversing the current direction through the coils using control electronics the magnetic field is also reversed. This permits the controlling of the two bi-stable states of the pixel. The flux pattern created by the conductor pattern of the coil is most efficient when it approaches perpendicular to the axis of rotation and approximately parallel with the poles of the MOEs. Flux patterns from coils are generally not linear. This is not a problem as long as the majority of the MOEs during the write pulse rotate sufficiently past 90 degrees so that they can continue rotation on their own to the new bi-stable state at the end of the pulse actuation of the coil array.
A flux arc going from a 45 degree angle at the one pole of the write head to parallel at the center and then −45 angle at the other side of the pixel can be a very effective flux pattern. When the magnetic field of the coil is turned off the MOEs continue to rotate because of their internal permanent magnetic charge. They will rotate until they self align to the closest low energy state. For efficient use of energy, the write head coil only needs to be pulsed long enough to create the needed rotation of greater than 90 degrees. This will generally be below 150 milliseconds with many designs achieving less then 50 milliseconds (video speeds possible).
The magnetic field strength of the coil array can be amplified with the use of a “soft” magnetic plate usually made of Iron or an Iron-containing material. The term “soft” in magnetic materials refers to a material that has good permeability and can be magnetically charged by an external field, but does not maintain a permanent magnetic charge after the external field is removed. Generally this is because the “soft” material has a low curie temperature so under normal conditions it is operating at higher then its' curie temperature. This prevents “soft” magnetic material from maintaining a permanent magnetic field. There are many low cost iron alloys with permeability approaching 2,000 or more that are magnetically “soft.”
To reduce cost and increase ease of manufacturing, a single patterned plate of “soft” magnetic material may be shared across an array of coils. It may be desirable for some coil designs to have parts of the soft magnetic plate protrude into the PCB (through holes or slots) to act as center cores for the coils or as shielding between pixels. It also may be desirable to have sections of the plate cut out to provide some magnetic isolation between separate pixels or display sections. Stamped sheet metal, casting, powder forming and other conventional manufacturing technologies may be used to produce these soft ferrous plates. An amplification of the magnetic field strength by 3× or more is possible through the use of these soft plates which results in higher efficiency and reduced EMI (electro magnetic interference). Also in some pixel designs a flux circuit is created within the pixel by using the soft magnetic plates.
Different Write Head and Pixel Designs
The architecture and geometries of the electromagnetic write head and the MOE pixel design should be coordinated together. In this section a pixel is used to refer to a single display element such as a pixel in a raster image. The term pixel may also mean any distinct display segment that is optically controlled as a unit, such as a single segment of a 7-segment numeral display.
Magnetic flux fields, unlike an electrical charge, are always dipoles (two opposing poles North and South). Therefore, a MOE with a magnetic field will always have a North and South pole, unlike electrically charged particles that can be monopoles (just positive or negative charges on a single particle). The write head will also have at least two dominant poles (North and South). When only one set of dipoles is used per pixel, this a called “single pole” design. It is also possible to design pixels and write heads with multiple dipoles (for example North-South, South-North . . . ). These are called “multi-pole” designs.
Single Pole Write Head Designs
Referring now to FIGS. 2A and 2B, a single pole design according to an embodiment of the present invention is illustrated. As shown in FIGS. 2A and 2B, in single pole designs, all MOEs within each individual pixel are oriented in the same direction and have parallel axis of rotation. Across the entire pixel the MOEs align to create one dipole that can be rotated between the two bi-stable states; North-South or South-North. The backplane coil is oriented so that it can apply a magnetic field in either direction of the dipole to create rotation during the write pulse to the target bi-stable state.
In FIG. 2A is shown a schematic cross section of an example of a single pole write head design with MOEs designated at 4 oriented in response to the write head-generated magnetic field designated at 1 in a north/south (left/right) orientation. The magnetic field 1 is generated by the direction of the coils 2 conductive tracks designated at 2. In FIG. 2B, the direction of the current in the conductive coils 2 has been reversed, thus reversing the magnetic field 1 and thus the MOEs 4, which have a permanent magnetic field, rotate to align with the external magnetic field 1. The ferromagnetic material designated at 3 aids in concentrating the magnetic flux lines. The designs of Single Pole write heads can be single coil or dual coil.
Single Pole, Single Coil Design
Single coil designs shown in FIG. 3 are coils 1 that are wound in a single direction around a soft magnetic core 4. The coil 1 runs parallel to the MOEs 2 frontplane and aligns to the net magnetic field dipole of the MOEs in their bi-stable states (North-South or South-North). This also means the coil 1 magnetic field is oriented perpendicular to the MOE axis of rotation. This type of coil can resemble a flattened solenoid. Note that when illustrated a coil in FIG. 3 and in the other figures disclosed herein, a circle with an x in the middle indicates that the coil winding is going into the paper and a circle with a dot in the middle indicates that the coil winding is coming out of the paper.
When actuated by a DC current, a magnetic field is created across the MOEs of the pixel that causes rotation to the target bi-stable state. By reversing the DC current direction through the coil, the other bi-stable state can be achieved. This simple design is effective, but may be more difficult to manufacture then other designs because the soft core material needs to be in the center of the coil layers. If PCB technology is used, there can also be many vias (interconnections between layers) which may require drilling and additional trace spacing. FIG. 3 indicates the PCB substrate materials are typically FR4 or Polyimide but other materials are also used.
Dual Coil-Single Pole Designs
Referring to FIG. 4, in a dual coil-single pole design, two opposing coils represented at 3 are created per PCB layer that are connected in series by means of vias (holes that drilled in a PCB and are platted with conductive metal to connect traces between layers) from one layer to another. A single trace can be used to create both coils using an “S” pattern. One coil is wound clockwise and the other is wound counter clockwise. This design makes it possible to have efficient layouts with minimal vias per layer. Multiple layers can be used to increase the number of winding resulting in increased magnetic field strength. The center poles of a soft magnetic material represented at 2 aid in the focusing of the magnetic field represented at 5.
FIG. 4A shows a cross-sectional view and 4B shows a top view showing the top of the poles 2 thru the MOEs represented at 4. The two coils are positioned perpendicular to the axis of rotation and parallel to the prevailing magnetic poles of the MOEs in the single pole pixel. In a preferred embodiment, a soft magnetic core connects the two coils and creates a flux circuit that resembles a “horseshoe” magnet configuration which is represented in this figure. Here, with the “horseshoe” soft magnetic material 2 and the permanent magnetic MOEs 4, a closed flux circuit is formed. Note that in FIGS. 4A and 4B, reference 1 represent the PCB substrate material.
Although it is not required, this design works well when the pixel design has stronger magnetically charged MOEs (up to 4× stronger or more) in the center of the pixel that act as “Drivers” and lower magnetically charged MOEs at the edges that act as “Passengers”. This is because the center of the dual coils is where the center of the magnetic poles will be. These poles on the write head will usually be inset from the edges of the pixel due to the trace thickness of the windings of the coils. In a driver/passenger pixel configuration, the primary driving force that changes the pixel state is rotation of the driver MOE(s) in the center of the pixel. The passenger MOEs at the edge of the pixel need not be rotated in the desired direction during the write pulse itself. As long as the driver MOE(s) in the center of the pixel are rotated in the proper direction, the passenger MOE(s) at the edges will align themselves to the stronger driver MOE(s) after the write pulse is completed.
Multi-Pole Designs
In a multi-pole pixel design, a single pixel is divided into two or more magnetic sections. Each section may have a different orientation for the magnetic poles. The MOEs of each section may also be oriented differently and even the axis of rotation may be varied. All sections of the pixel, however, are controlled by a common write head element and all sections of the pixel are switched together. The benefit of a multi-pole design is that coil windings by their nature create 360 degrees of rotation per wind. Therefore, magnetic fields are created in all of these directions. In a multi-pole design more efficient use may be made of the magnetic field generated by the full windings. This type of design can be very useful where large individual segments are needed such as 50 mm and larger.
Single Coil-3 Pole Design
Referring to FIG. 5, an additional embodiment of a magnetic write head is illustrated, in this case the single coil-three pole design. This embodiment of the write head has two stable states of the MOEs represented at 3, North-South and South-North, with North represented as N and South represented as S in the figures within the same pixel. It may be more efficient when the aspect ratio of the pixel is greater then 1:1, with the longer dimension corresponding to the axis of rotation of the MOEs and the shorter dimension corresponding to the axis of the write head magnetic field poles. A single coil represented at 1 is created per layer of the PCB with all windings rotating in the same direction (clockwise or counterclockwise). In the longer axis of the pixel, the traces of the coil windings can be straightened to run lengthwise along the pixel. These traces create a magnetic field represented at 2, perpendicular to the current flow across the surface of the pixel and perpendicular to the axis of rotation of the MOEs. The longer the straight section of the traces is, the more efficient this write head design can be. It should also be noted here for clarity that the generated magnetic field 2 is parallel to the viewing plane of the display yet perpendicular to the MOEs axis of rotation.
A soft magnetic backplate, represented at 4, is placed beneath the winding of the pixel. It includes a protruding wall that divides the pixel into the two sections and separates the MOEs into two sides. This wall separates the pixel into two magnetic sections, with each side creating a separate, stable magnetic domain. The center protruding wall of soft magnetic material divides the traces so that the current flow for all traces on each side of the wall runs in the same direction. This creates two sides each with a single flux pattern from the edge of the pixel to the center wall. The axis of rotation for the MOEs in both of the two side sections is the same, but the magnetic orientation of the two optical states is 180 degrees opposed. When one section is North-South the other section will always be South-North and vice versa.
The soft magnetic backplate with a center protruding wall also creates efficient flux circuits on each side such that very little magnetic flux is wasted. This configuration can amplify and direct the magnetic flux to the frontplane of the display with minimal wasted flux projecting backwards, away from the MOE frontplane. When the write pulse is completed, the backplate and center wall also create two closed flux circuits with the MOEs of the frontplane on each of the two sides. These flux circuits on the two sides reinforce the bi-stable states of the MOEs and assists in alignment of the MOEs. Because the single coil-three pole design uses only one coil per layer, it is very efficient for PCB manufacturing with minimal vias.
Optical Edge on Wall
Because the protruding wall of a Single coil-3 pole design (FIG. 5) is visible from the front of the display, to improve the optical quality of this pixel it is also possible to angle the top of the protruding wall at an angle approximately 45 degrees to the two sides of the wall. Using a reflective finish on the angled edge of the wall will cause the viewer to see the reflection of the MOEs optical surface rather then viewing a fixed wall that does not change color state when the display is viewed from the front. This increases the contrast ratio of the two optical states.
Configurations of the Backplane Write Heads
A unique feature of bi-stable reflective magneto optical displays is that the write head is only required to be behind the pixel or segment during the write pulse. In this way it is more like a recordable magnetic media. Because of this, there are three alternatives for write head configurations and backplanes. They are: (1). Full backplane, (2) Moving write head, and (3) Moving frontplane, all three of which are included in the principles of the present invention.
Full Backplane
In this configuration there is a single write head for every pixel or segment. The write head backplane and pixel arrays are attached to each other and aligned. The preferred embodiment is the use of an array of write heads on a PCB that is mounted to a frontplane containing the MOE pixels. These pixels may all be in the same orientation or in some cases they may use a “checkerboard” or other pattern to decrease crosstalk between pixels or create a different layout of display. The key concept is that a single write head is fixed behind each pixel or display segment. In some cases, the size of the overall display may make “tiling” the display into subsections desirable. This is when a display is divided into subassemblies that are then “tiled” together to create the full size display. This approach enables replacement and service of a portion of the display without replacing the entire display. It also allows for custom sizes of displays to be created using the same tile subassemblies simply by varying the number of tiles in X and Y that are assembled to make the final display. For large format applications tiling may be the preferred method.
Moving Write Head
In this embodiment, a single write head or an array or bar of write heads is created. This write head array is then moved across the back of the display or the front. As the write head aligns with each pixel, write pulses are applied and the effected pixels change state. Mechanical motion and encoder systems may be used to move the write head and pulse the write heads at the correct timing and location. In many configurations, it may be desirable to have a write head bar that has enough write heads to span either the entire X or Y direction of the display. In this way, the motion of the write head is only needed in a single direction. If the bar covers the X axis, motion only needs to occur in the Y direction for example. If the write head is smaller then both axis of the display, motion control in both X and Y directions will be needed to cover the entire display.
Moving Frontplane
In some embodiments it may be desirable to fix the write heads and move the frontplane of the display (structure which includes an array of MOEs). In these embodiments, a write head bar that spans the entire axis perpendicular to the axis of the display motion is desirable. Two implementations of this approach are discussed below.
Rotating Cylinder Display:
In this display the frontplane of MOE pixels is configured as a rotating cylinder. The write head is a bar mounted either inside or outside of the cylinder. Inside is generally the preferred implementation because the write head is concealed by the display cylinder. As the display cylinder rotates, an encoder detects the position of the display frontplane and pulses the write head bar with the appropriate image data. This type of display could be very effective for kiosks and large signs that are viewed from all sides
Moving Belt:
It is also possible to make the MOE frontplane using flexible plastics or hinged flat sections (like the treads of a tank). In this way it could be rotated as a belt. Similar to the rotating cylinder, a write head bar is used and an encoder senses the position of the display and controls the timing of the write pulses of the image data to match the moving pixels of the frontplane. This type of display is useful for message boards, stock price “tickers”, news wires and other displays with continually or frequently updating information. The cost of a write head bar can be significantly less then the cost of a full backplane because far less PCB area and electronics are used so this is also a low cost way to make a larger display.
The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope

Claims

1. A write head for use in a bistable, magnetic optical display comprising:

one or more conducting coils configured to generate a magnetic field that is substantially perpendicular to the axis of rotation of one or more magneto optical elements of the optical display and parallel to a display viewing plane of the one or more magnetic display elements of the magnetic optical display such that an electrical pulse of less then 150 milliseconds is sufficient to create a magnetic field that alters the bistable state of the magneto optical display such that the display will stay in this new state without the need for ongoing electrical current being applied to the write head.

2. The write head in accordance with claim 1 further comprising:

control electronics configured to provide an electric pulse signal to the one or more conducting coils such that the one or more conducting coils generate the magnetic field.

3. The write head in accordance with claim 2, wherein the control electronics are further configured to control the direction of the electric pulse provided to the one or more conducting coils such that by reversing the direction of the electric pulse, the direction of the magnetic field generated by the one or more conducting coils may be controlled.

4. The write head in accordance with claim 1, wherein a soft magnetic material is added to the display in close proximity to the one or more conducting coils and wherein the soft magnetic material is one of Iron, an iron-containing material or a ferromagnetic material that can be magnetically charged by an external field, but does not maintain a permanent magnetic charge after the magnetic field is removed.

5. The write head in accordance with claim 4, wherein the soft magnetic material is configured to magnify and/or direct the magnetic field generated by the one or more conducting coils.

6. The write head in accordance with claim 1, wherein the magnetic field generated by the one or more conducting coils is approximately parallel to the direction of a magnetic field created by alignment of the magnetic display elements in a bistable state and wherein causing the generated magnetic field to be greater than the magnetic field of the magnetic display elements will cause the magnetic display elements to rotate to a second bistable state.

7. The write head in accordance with claim 1, wherein the one or more conducting coils are implemented as patterned conductors of a printed circuit board.

8. The write head in accordance with claim 1, wherein the write head is configured to be moveable through the use of mechanical motion and encoder systems such that the write head may generate a magnetic field for an array of pixels of a display comprising one or more of the magnetic display elements.

9. The write head in accordance with claim 1, wherein multiple write heads are configured in an array as part of a backplane of a display and wherein each write head generates the magnetic field for one or more pixels of a display comprising one or more of the magnetic display elements.

10. The write head in accordance with claim 1, wherein the write head is configured to be stationary and wherein a front plane comprising one or more of the magnetic display elements is configured to be moveable such that by moving the front plane, the one or more magnetic display elements are subjected to the magnetic field generated by the write head.

11. A write head for use in a bistable, magnetic optical display comprising:

a first conducting coil wound in a first direction;

a second conducting coil wound in a second direction;

wherein the first and second conducting coils are configured to generate a magnetic field that is substantially perpendicular to the axis of rotation and parallel to a display viewing plane of one or more magnetic display elements of the magnetic optical display such that an electrical pulse of less then 150 milliseconds is sufficient to create a magnetic field that alters the bistable state of the magneto optical display such that the display will stay in this new state without the need for ongoing electrical current being applied to the write head.

12. The write head in accordance with claim 11 further comprising:

control electronics configured to provide an electric pulse signal to the first and second conducting coils such that the first and second conducting coils generate the magnetic field.

13. The write head in accordance with claim 11, wherein a soft magnetic material is added to the display in close proximity to the first and second conducting coils and wherein the soft magnetic material is one of Iron, an iron-containing material or a ferromagnetic material that can be magnetically charged by an external field, but does not maintain a permanent magnetic charge after the magnetic field is removed and wherein the soft magnetic material is configured to magnify and/or direct the magnetic field generated by the first and second conducting coils.

14. The write head in accordance with claim 13, wherein the soft magnetic material generates a flux circuit that resembles an electromagnetic horseshoe pattern.

15. The write head in accordance with claim 11, wherein the one or more conducting coils are implemented as patterned conductors of a printed circuit board.

16. The write head in accordance with claim 15, wherein the printed circuit board is comprised of two or more layers and wherein the two or more layers can be used to increase the number of windings of the first and second coils to thereby increase the strength of the generated magnetic field.

17. The write head in accordance with claim 11, wherein the first and second conducting coils are positioned perpendicular to the axis of rotation of magneto optical elements and parallel to magnetic poles of the magnetic display elements and wherein causing the generated magnetic field to be greater than the magnetic field of the magnetic display elements will cause the magnetic display elements to rotate to a bistable state.

18. A write head for use in a magnetic optical display comprising:

a soft magnetic material;

one or more conducting coils that are at least partially disposed proximate the magnetic material;

wherein the one or more conducting coils are configured to generate a magnetic field that is substantially perpendicular to the axis of rotation of two or more magneto optical display elements of the magnetic display and parallel to a display viewing plane of the two or more magneto optical display elements of the magnetic optical display; and

wherein the soft magnetic material includes a protruding wall that divides the two or more magnetic display elements into two sections, with each section creating a separate, stable magnetic domain.

19. The write head in accordance with claim 18, wherein the protruding wall allows for one or more magnetic display elements in a first section to rotate to a first bistable state and for one or more magnetic display elements in a second section to rotate to a second bistable state when subjected to the generated magnetic field.

20. The write head in accordance with claim 18, wherein a top portion of the protruding wall is configured at an angle approximately 45 degrees to the two sides of the wall to improve optical display.

21. The write head in accordance with claim 18, wherein the soft magnetic material is one of Iron, an iron-containing material or a soft ferromagnetic material that can be magnetically charged by an external field, but does not maintain a permanent magnetic charge after the magnetic field is removed and wherein the magnetic material is configured to magnify and/or direct the magnetic field generated by the one or more conducting coils.

22. The write head in accordance with claim 18, wherein the one or more conducting coils are implemented as patterned conductors of a printed circuit board.

23. A write head for use in a magnetic optical display consisting of:

a coil of one or more layers that are wound in one direction with the windings parallel to the viewing plane of one or more magneto optical display elements; and

a soft magnetic component that is placed proximate the coil and creates a wall that divides the coil winding into two sides by protruding through the coils and dividing the magneto optical elements into two separate opposing magnetic sections;

wherein the magneto optical elements of the two separate magnetic sections may be actuated by the coil simultaneously.

24. The write head in accordance with claim 23, wherein the write is a first write head, the write head further comprising:

a second write head configured the same as the first write head;

wherein the first and second write heads are combined to create additional opposing magnetic sections such that the magneto optical elements of the additional magnetic sections may be actuated simultaneously.