Improved wave switching for electro-optical displays

Abstract

A method for driving “separated contact” electro-optical displays and “isolated electrode” electro-optical displays, such as an electrophoretic display comprising charged pigment particles placed in a solvent that move in response to an applied electric field. The improved method provides a “wave switching” waveform having less visual “dead time” than a conventional wave switching method. The improved method provides a DC-balanced waveform that enables a banner-type display to wave switch from a first color to a second color in a first direction and then return from the second color to the first color in the opposite direction. Such switching was not feasible in conventional devices due to concerns about runaway residual voltage accumulation that could destroy the display.

Description

Technology Field

[0001] Cross-reference regarding related applications

[0002] This application claims priority to U.S. Provisional Application No. 63 / 559,515 filed February 29, 2024. All patents and publications described herein are incorporated by reference in their entirety.

[0003] Technology field

[0004] The present invention relates to a method for driving an electro-optic display, in particular a bistable electro-optic display. The invention is intended, in particular, for use with, but is not limited to, a particle-based electrophoretic display in which one or more types of electrically charged particles exist within a fluid and move through the fluid under the influence of an electric field to change the appearance of the display. The method described herein makes the electrophoretic display appear as if the optical state is changing from one side of the display to the other, and proceeds as a "wave" of color change across the display. Such a method is particularly valuable in digital signage, such as segmented digital signage, where the wave update draws the viewer's attention to the signage. Background Technology

[0005] The term “electro-optic,” as applied to materials or displays, is used herein in the customary sense in the field of imaging technology to refer to a material having first and second display states with at least one optical property that changes from its first display state to its second display state by the application of an electric field to the material. The optical property is typically color that can be perceived by the human eye, but may also be other optical properties such as optical transmittance, reflectance and luminescence, or pseudo-color in the sense of changes in reflectance of electromagnetic wavelengths outside the visible light range in the case of machine-readable displays.

[0006] The term “gray state” is used herein in the conventional sense in the field of imaging technology to refer to an intermediate state between the two extreme optical states of a pixel, and does not necessarily imply a black-and-white transition between these two extreme states. For example, some of the E Ink patents and published applications referenced below describe electrophoretic displays in which the extreme states are white and dark blue, so that the intermediate “gray state” would actually be light blue. In fact, as previously mentioned, a change in optical state may not be a change in color at all. The terms “black” and “white” may be used below to refer to the two extreme optical states of a display and should generally be understood to include extreme optical states that are not strictly black-and-white, such as the aforementioned white and dark blue states. The term “monochrome” may be used below to indicate a driving method that drives a pixel only in its two extreme optical states without any intermediate gray state. The method described herein is not limited to black and white displays and can be used with displays capable of displaying many different colors, such as 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16 or more colors.

[0007] The terms “bistable” and “bistable” are used herein in the customary sense in the art to refer to a display comprising a display element having first and second display states having at least one optical characteristic that is different, so that after any given element is driven, its first or second display state is assumed by an addressing pulse of a finite duration, and after the addressing pulse is terminated, said state will persist at least several times, e.g., at least four times, which is the minimum duration of the addressing pulse required to change the state of the display element. U.S. Patent No. 7,170,670 describes that some particle-based electrophoretic displays capable of grayscale are stable not only in their extreme black and white states but also in intermediate gray states, and the same is true for some other types of electro-optical displays. While it is appropriate to call this type of display “multi-stable” rather than “bistable,” for convenience, the term “bistable” may be used herein to encompass both bistable and multi-stable displays.

[0008] The term "impulse" is used herein by convention in the sense of the integral of voltage with respect to time. However, some bistable electro-optical media act as charge converters, and in such media, an alternative definition of impulse may be used, namely the integral of current with respect to time (equivalent to the total applied charge). Depending on whether the medium acts as a voltage-time impulse converter or as a charge impulse converter, the appropriate definition of impulse must be used.

[0009] Most of the description below will focus on how to drive an electro-optical display through a transition from an initial gray level to a final gray level (which may or may not differ from the initial gray level). The term "waveform" will be used to denote a curve of total voltage versus time used to influence the transition from a specific initial gray level to a specific final gray level. Such a waveform may contain multiple waveform elements; where these elements are essentially rectangular (i.e., a given element involves the application of a constant voltage during a time interval); the elements may be referred to as "pulses" or "drive pulses." The term "drive scheme" refers to a set of waveforms sufficient to influence all possible transitions between gray levels for a particular display. A display may use more than one drive scheme; For example, the aforementioned U.S. Patent No. 7,012,600 teaches that a driving method may need to be modified according to parameters such as the temperature of the display or the time operated during its lifetime, and thus a plurality of different driving methods may be provided for use at different temperatures, etc. A set of driving methods used in this manner may be referred to as a “related driving method set.” It is also possible to use more than one driving method simultaneously in different regions of the same display, and a set of driving methods used in this manner may be referred to as a “simultaneous driving method set.”

[0010] One type of electro-optical display that has been the subject of intensive research and development is the particle-based electrophoretic display, in which multiple charged particles move through a fluid under the influence of an electric field. Compared to liquid crystal displays, electrophoretic displays can possess properties such as good brightness and contrast, a wide viewing angle, state bistable, and low power consumption. Nevertheless, they have not been widely used due to issues regarding long-term image quality. For example, the particles constituting the electrophoretic display tend to settle, resulting in an insufficient service life for the display.

[0011] As mentioned above, electrophoretic media require the presence of a fluid. In most conventional electrophoretic media, this fluid is a liquid, but electrophoretic media can be produced using a gaseous fluid; see, for example, Kitamura, T., et al., “Electrical toner movement for electronic paper-like display”, IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y., et al., “Toner display using insulative particles charged triboelectrically”, IDW Japan, 2001, Paper AMD4-4). See also U.S. Patent Nos. 7,321,459 and 7,236,291. These gas-based electrophoretic media appear to be susceptible to the same type of problem as liquid-based electrophoretic media due to particle precipitation when the medium is used in an orientation that allows such precipitation, for example, in a display where the medium is placed in a vertical plane. In fact, particle sedimentation appears to be a more serious problem in gas-based electrophoretic media than in liquid-based electrophoretic media, because the lower viscosity of gaseous suspensions compared to liquids enables faster sedimentation of electrophoretic particles.

[0012] Numerous patents and applications assigned to or in the names of MIT (Massachusetts Institute of Technology) and E Ink Corporation describe various technologies used for encapsulated electrophoresis and other electro-optical media. These encapsulated media comprise numerous small capsules, each capsule comprising an inner phase containing electrophoretic transport particles in a fluid medium and a capsule wall surrounding the inner phase. Typically, the capsules themselves are immobilized within a polymerizable binder to form a coherent layer positioned between two electrodes. The technologies described in these patents and applications include the following:

[0013] (a) Electrophoretic particles, fluids and fluid additives; see, exe., U.S. Patent Nos. 7,002,728 and 7,679,814;

[0014] (b) capsules, binders, and encapsulation processes; see, exe., U.S. Patent Nos. 6,922,276 and 7,411,719;

[0015] (c) Films and subassemblies comprising electro-optical materials; see, for example, U.S. Patent Nos. 6,982,178 and 7,839,564;

[0016] (d) a backplane, an adhesive layer and other auxiliary layers and a method used in a display; see, for example, U.S. Patent Nos. 7,116,318 and 7,535,624;

[0017] (e) Color formation and color adjustment; see, exe., U.S. Patent Nos. 7,075,502 and 7,839,564;

[0018] (f) Method for driving a display; refer to the above MEDEOD application;

[0019] (g) Applications of displays; see, for example, U.S. Patent Nos. 7,312,784 and 7,312,784; and

[0020] (h) A non-electrophoretic display as described in U.S. Patent Nos. 6,241,921; 6,950,220; 7,420,549; 8,319,759; 8,994,705 and 10,372,008.

[0021] Many of the aforementioned patents and applications recognize that the walls surrounding discrete microcapsules in an encapsulated electrophoretic medium may be replaced by a continuous phase, and thus the electrophoretic medium may produce a so-called polymer-dispersed electrophoretic display comprising multiple discrete droplets of an electrophoretic fluid and a continuous phase of a polymerizable material, wherein the discrete droplets of the electrophoretic fluid within such a polymer-dispersed electrophoretic display may be considered as capsules or microcapsules even if the discrete capsule membrane is not associated with each individual droplet; refer, for example, to the aforementioned U.S. Patent No. 6,866,760. Accordingly, for the purposes of this application, such a polymer-dispersed electrophoretic medium is considered a subspecies of an encapsulated electrophoretic medium.

[0022] A related type of electrophoretic display is the so-called "microcell electrophoretic display." In a microcell electrophoretic display, charged particles and fluids are not encapsulated within microcapsules but are instead retained within a carrier medium, typically within a plurality of cavities formed within a polymerizable film. For example, see U.S. Patent Nos. 6,672,921 and 6,788,449, both assigned to SiPix Imaging, Inc.

[0023] Electrophoretic media are often opaque (for example, because in many electrophoretic media, particles substantially block the transmission of visible light through the display) and operate in a reflective mode, but many electrophoretic displays can be configured to operate in a so-called "shutter mode," where one display state is substantially opaque and the other is light-transmitting. See, for example, U.S. Patent Nos. 5,872,552; 6,130,774; 6,144,361; 6,172,798; 6,271,823; 6,225,971; and 6,184,856. Dielectrophoretic displays, which are similar to electrophoretic displays but depend on variations in electric field intensity, can also operate in a similar mode; see U.S. Patent No. 4,418,346. Other types of electro-optical displays can also operate in a shutter mode. An electro-optical medium operating in shutter mode can be useful for a multilayer structure for a full-color display, in which at least one layer adjacent to the viewing surface of the display operates in shutter mode to expose or conceal a second layer located further away from the viewing surface.

[0024] Encapsulated electrophoretic displays typically do not suffer from the problems caused by clustering and deposition failure modes of traditional electrophoretic devices and offer additional advantages, such as the ability to print or coat displays on various flexible and rigid substrates. (The use of the word "printing" is intended to include all forms of printing and coating, including but not limited to: pre-measuring coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, and curtain coating; roll coatings such as knife over roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; silk screen printing processes; electrostatic printing processes; thermal printing processes; inkjet printing processes; electrophoretic deposition (see U.S. Patent No. 7,339,715); and other similar techniques.) Thus, the resulting display can be flexible. Additionally, because the display medium can be printed (using various methods), the display itself can be manufactured at a low cost.

[0025] Other types of electro-optical media may also be used in the display of the present invention.

[0026] The bistable or multiplestable behavior of particle-based electrophoretic displays and other electro-optical displays exhibiting similar behavior (such displays may be referred to hereinafter as "impulse-driven displays" for convenience) is in stark contrast to the behavior of conventional liquid crystal ("LC") displays. Twisted nematic liquid crystals are not bistable or multiplestable but act as voltage converters, so that when an electric field is applied to a pixel of such a display, it generates a specific gray level in the pixel regardless of the gray level previously present in that pixel. Furthermore, LC displays are driven in only one direction (from a non-transparent or "dark" state to a transparent or "light" state), and the reversal from the light state to the dark state is affected by reducing or removing the electric field. Finally, the gray level of a pixel in an LC display is not sensitive to the polarity of the electric field but only to its magnitude; in practice, for technical reasons, commercial LC displays usually reverse the polarity of the driving electric field at frequent intervals. In contrast, bistable electro-optical displays act as impulse converters in a first-order approximation, so that the final state of the pixel depends not only on the applied electric field and the time during which the electric field is applied, but also on the state of the pixel before the application of the electric field.

[0027] Regardless of whether the electro-optical medium used is bistable, to obtain a high-resolution display, individual pixels of the display must be addressable without interference from adjacent pixels. One method to achieve this objective is to create an "active matrix" display by providing an array of nonlinear elements, such as transistors or diodes, such that at least one nonlinear element is associated with each pixel. The addressing or pixel electrode addressing a single pixel is connected to an appropriate voltage source through the associated nonlinear element. Typically, when the nonlinear element is a transistor, the pixel electrode is connected to the drain of the transistor, and while this arrangement will be assumed in the following description, it is essentially arbitrary, and the pixel electrode may be connected to the source of the transistor. By convention, in a high-resolution array, pixels are arranged in a two-dimensional array of rows and columns, so that any particular pixel is uniquely defined by the intersection of a designated row and a designated column. The sources of all transistors in each column are connected to a single column electrode, and the gates of all transistors in each row are connected to a single row electrode; In other words, it is customary to assign sources to rows and gates to columns, but this is essentially arbitrary and can be reversed if desired. Row electrodes are connected to row drivers, which essentially ensure that only one row is selected at any given moment; that is, to the selected row electrodes, a voltage is applied such that all transistors in the selected row are conductive, and to all other rows, a voltage is applied such that all transistors in the unselected rows remain non-conductive. Column electrodes are connected to column drivers, and the column drivers apply selected voltages to various column electrodes to drive the pixels in the selected rows to their desired optical state.(The aforementioned voltage is conventionally provided to the electro-optical medium side opposite the non-linear array and is for a common front electrode extending across the entire display.) After a preset interval known as the "line address time," the selected row is deselected, the next row is selected, and the voltage on the column driver is changed so that the next line of the display is written. This process is repeated so that the entire display is written row by row.

[0028] Alternatively, for electro-optical media with a significant threshold voltage (most electrophoretic media do not), passive matrix driving may be used. In this type of driving, two sets of parallel elongated electrodes are provided on opposite sides of the electro-optical layer, and the two sets of electrodes are arranged orthogonally to each other so that each pixel is defined by the intersection of one electrode from each of the two sets. Finally, electro-optical displays may use so-called "direct driving," in which a separate conductor is provided for each of the multiple pixels to connect the pixel electrode to the display controller, thereby allowing the display controller to directly control the potential of each pixel electrode.

[0029] Active and passive matrix displays are complex and expensive, particularly for large-area displays, because the cost of the required electrodes tends to be a function of the display area rather than the number of pixels. However, active and passive matrix displays have the flexibility to display any image and thus can display both text and pictures of various point sizes. Direct-drive displays tend to be less expensive but lack flexibility, and even when they can display text, they are typically limited to a single point size and require a very large number of connections between the pixel electrodes and the controller; for example, see U.S. Design Patent No. D485,294, which requires 63 pixels to represent a single character of various versions of the Latin alphabet at a single point size.

[0030] To date, most commercial applications of electrophoretic and similar bistable electro-optical displays have been in small products such as electronic document readers, watches, and solid-state memory devices. However, there is growing interest in applying these displays to furniture and architectural applications. In many furniture and architectural applications, electro-optical displays are intended to provide simple and typically moving geometric patterns. The present invention aims to provide a display and driving method useful for furniture and architectural applications. Furthermore, the described waveform enables a type of "back and forth" waveform switching that was not previously available in prior art waveform switching, namely, waveform switching as described in U.S. Patent Nos. 10,197,883 and 10,551,713, the entirety of which is incorporated by reference.

[0031] As described in the various applications mentioned above, if the waveform applied to the electro-optical display is not DC-balanced, damage to the electrode may occur, particularly in the case of very thin light-transmitting electrodes, typically less than 1 μm. (In this application, the term “light-transmitting” is used in the customary sense in the field of display technology, as described, for example, in U.S. Patent No. 6,982,178, and means transmitting sufficient visible light so that an observer viewing the electro-optical material through the light-transmitting electrode can observe changes in the optical state of the electro-optical material.) To reduce or eliminate such damage to the electrode, at least a portion of one of the first and second electrodes may be provided with a passivation layer disposed between the electrode and the layer of the electro-optical material. Suitable passivation layers are described, for example, in U.S. Patent No. 6,724,519. However, to further prevent the possibility of damage to the electrode, it is necessary to develop a DC-balanced waveform used for the operation of the electro-optical display. Such damage may cause the display to show inaccurate colors or cause the display to stop functioning completely.

[0032] The present invention provides an improved method for driving a spaced contact display, for example, a spaced contact electrophoretic display, wherein the spaced contact display comprises a layer of an electro-optical material, first and second electrodes on opposite sides of the layer of the electro-optical material—at least one of the first and second electrodes is light-transmitting, and at least one of the first and second electrodes has at least two spaced contacts—and a voltage controller arranged to provide a driving voltage signal between two spaced contacts attached to the same electrode.

[0033] In a first aspect, the present invention relates to a method for driving a spaced-contact electro-optical display, comprising the step of providing a spaced-contact electro-optical display, the step of providing the spaced-contact electro-optical display comprising: an electro-optical material layer; a first electrode layer and a second electrode layer on opposite sides of the electro-optical material layer—the first electrode layer or the second electrode layer being light-transmittable—; a first contact and a second contact spaced apart on the first electrode layer and electrically coupled to the first electrode layer; and a voltage controller coupled to the first and second contacts; and the step of providing a first time-varying driving signal to the first contact using the voltage controller—the first time-varying driving signal starts at a duty cycle of 0, ramps to a duty cycle of 1, and then returns to a duty cycle of 0. A method for driving a separated contact electro-optical display, comprising the step of providing a second time-varying driving signal to the second contact using the voltage controller, wherein the second time-varying driving signal starts at a duty cycle of 0, ramps to a duty cycle of -1, ramps to a duty cycle of 1, and then returns to a duty cycle of 0. In some embodiments, the ramp from a duty cycle of -1 to a duty cycle of 1 for the second time-varying driving signal comprises two different slopes of the duty cycle per unit time.In some embodiments, the method further comprises the step of providing a third time-varying drive signal to the first contact using the voltage controller, wherein the first time-varying drive signal starts at a duty cycle of 0, ramps to a duty cycle of -1, and then returns to a duty cycle of 0; and the step of providing a fourth time-varying drive signal to the second contact using the voltage controller, wherein the second time-varying drive signal starts at a duty cycle of 0, ramps to a duty cycle of 1, ramps to a duty cycle of -1, and then returns to a duty cycle of 0. In some embodiments, the first time-varying drive signal and the second time-varying drive signal reach a duty cycle of 1 simultaneously. In some embodiments, the frequency of the first and second drive signals is 30 Hz or higher. In some embodiments, the magnitude of the voltage of the first and second drive signals is 15 V to 30 V. In some embodiments, both the first and second electrodes have at least two spaced contacts, and the voltage controller is arranged to vary the potential difference between the two spaced contacts attached to each electrode. In some embodiments, the electro-optical material comprises an electrophoretic material comprising a plurality of electrically charged particles disposed in a fluid and capable of moving through the fluid under the influence of an electric field. In some embodiments, the electrically charged particles and the fluid are confined within a plurality of capsules, microcells, or individual droplets surrounded by a continuous phase comprising a polymer material. In some embodiments, at least one of the first and second electrodes is interrupted by at least one non-conductive region so that the electric current must follow a non-linear path between the two contacts on the electrode. In some embodiments, at least one of the first and second electrodes is divided into a plurality of sections having different electrical resistances per unit length.In some embodiments, at least one of the first and second electrodes is divided into a plurality of sections having different electric capacitances per unit area. In some embodiments, at least a portion of one of the first and second electrodes is provided with a passivation layer disposed between the electrode and the electro-optical material layer. In some embodiments, the first or second time-varying driving signal includes a sine wave, a triangular wave, a sawtooth wave, or a square wave.

[0034] In another aspect, the present invention relates to a method for driving a spaced-contact electro-optical display, comprising the step of providing a spaced-contact electro-optical display, the step of providing the spaced-contact electro-optical display comprising: an electro-optical material layer; a first electrode layer and a second electrode layer on opposite sides of the electro-optical material layer—the first electrode layer or the second electrode layer being light-transmittable—a first contact and a second contact spaced apart on the first electrode layer and electrically coupled to the first electrode layer, and a voltage controller coupled to the first and second contacts; and the step of providing a first time-varying driving signal to the first contact using the voltage controller—the first time-varying driving signal starts at a duty cycle of 0, ramps to a duty cycle of -1, maintains a duty cycle of -1 for a time sufficient to provide DC balance to the first time-varying driving signal, then ramps to a duty cycle of 1, and then returns to a duty cycle of 0. The present invention provides a method for driving a separated contact electro-optical display, comprising the step of providing a second time-varying driving signal to the second contact using the voltage controller, wherein the second time-varying driving signal starts at a duty cycle of 0, transitions to a duty cycle of 1 by passing through a negative duty cycle impulse until the first time-varying driving signal maintains a duty cycle of -1 and then proceeds to a duty cycle of 1, and then returns to a duty cycle of 0. In some embodiments, the method further comprises the step of providing a third time-varying driving signal identical to the second time-varying driving signal to the first contact; and the step of providing a fourth time-varying driving signal identical to the first time-varying driving signal to the second contact. In some embodiments, the first time-varying driving signal and the second time-varying driving signal reach a duty cycle of 1 simultaneously. In some embodiments, the frequency of the first and second driving signals is 30 Hz or higher.In some embodiments, the magnitude of the voltage of the first and second driving signals is 15V to 30V. In some embodiments, both the first and second electrodes have at least two spaced contacts, and the voltage controller is arranged to vary the potential difference between the two spaced contacts attached to each electrode. In some embodiments, the electro-optical material comprises an electrophoretic material comprising a plurality of electrically charged particles disposed in a fluid and capable of moving through the fluid under the influence of an electric field. In some embodiments, the electrically charged particles and the fluid are confined within a plurality of capsules, microcells, or individual droplets surrounded by a continuous phase comprising a polymer material. In some embodiments, at least one of the first and second electrodes is interrupted by at least one non-conductive region so that the electric current must follow a non-linear path between the two contacts on the electrode. In some embodiments, at least one of the first and second electrodes is divided into a plurality of sections having different electrical resistances per unit length. In some embodiments, at least one of the first and second electrodes is divided into a plurality of sections having different electric capacitances per unit area. In some embodiments, at least a portion of one of the first and second electrodes is provided with a passivation layer disposed between the electrode and the electro-optical material layer. In some embodiments, the first or second time-varying driving signal includes a sine wave, a triangular wave, a sawtooth wave, or a square wave.

[0035] In a preferred embodiment of the spaced contact display of the present invention, both the first and second electrodes have at least two spaced contacts, and a voltage controller is arranged to change the potential difference between the two spaced contacts attached to each electrode. Each electrode may, of course, have more than two spaced contacts, and in such case, it is not essential that the voltage controller be arranged to change the potential of all but one of these contacts independently of each other; for example, the contacts may be divided into two or more groups, and a potential difference may be applied between different groups while the contacts within each group are maintained at the same potential.

[0036] The spaced-contact display of the present invention may have more than one electrode on each side of the electro-optical medium layer. In practice, for a very large display (perhaps covering a very large wall surface), it may be necessary or desirable for the display to be divided into a series of separate modules, each of which has an electro-optical layer interposed between a first electrode and a second electrode. Additionally, the spaced-contact display of the present invention may have a different number of electrodes on each side of the electro-optical medium layer.

[0037] The method disclosed herein is also suitable for use with an "isolated electrode" display, said display comprising a layer of an electro-optical material, a sequence of at least three electrodes disposed adjacent to the layer of the electro-optical material and configured to apply an electric field across the layer of the electro-optical material—wherein at least one surface of the electrode on the layer of the electro-optical material is light-transmitting—and a voltage controller arranged to vary the potential difference between the first electrode and the last electrode of said sequence, wherein (a) each electrode of said sequence is positioned opposite to the layer of the electro-optical material from both the electrode preceding it in said sequence and the electrode following it in said sequence; and (b) each electrode of said sequence has a first edge that overlaps or is adjacent to the electrode preceding it in said sequence and a second edge that overlaps or is adjacent to the electrode following it in said sequence; (c) Each electrode other than the first and last electrodes of the above sequence is electrically isolated so that its potential is controlled by the passage of current through the layer of the electro-optical material. Brief explanation of the drawing

[0038] Figure 1 of the attached drawings is a highly schematic plan view of the spacing contact banner display of the present invention illustrating the positions of contacts for the upper (T) and lower (B) electrode layers. FIG. 2 is a very schematic cross-sectional view along line II-II of FIG. 1 viewed in the direction of the arrow. In particular, the contact for the upper electrode layer is located at the bottom of the display, and the contact for the lower electrode layer is located at the top of the display. FIG. 3 is a schematic plan view of an alternative configuration of a spaced-contact display in which a gap is provided in one electrode layer so that current follows a non-linear path between two spaced-contacts of the electrode layer. FIG. 4 is a schematic plan view of an alternative configuration of a spaced-contact display in which a single electrode is divided into sections having different electrical resistances per unit length. FIG. 5 is a side view of an alternative configuration of a spaced-contact display having regions of variable capacitance per unit area in a single electrode layer. FIG. 6 is a side view of an isolation electrode display of the present invention in which a series of electrodes alternate above and below a layer of electro-optical material. Each electrode of the sequence is positioned on the opposite side of the layer of electro-optical material from both the electrode preceding it and the electrode following it in the sequence. FIGS. 7a to 7d illustrate schematic plan views of various layers of an electrode layer suitable for use with the present invention. FIG. 8 illustrates an exemplary driving method used in the prior art, in which two different time-varying driving waveforms are provided by a voltage controller to electrodes spaced apart at both ends of a banner display. For both waveforms, the voltage and frequency are kept constant, but the duty cycle changes as a function of time (as determined by the frame number). Figure 9 illustrates the change in duty cycle for a constant voltage and frequency. FIG. 10 illustrates an embodiment of an improved driving method of the present invention that provides a DC-balanced waveform allowing less "dead time" during the driving cycle. FIG. 11 illustrates an embodiment of an improved driving method of the present invention that provides a DC-balanced waveform allowing less "dead time" during the driving cycle. FIG. 12 illustrates an exemplary wave-switching banner sign indicating the location of a train station. The sign switches between black text on a white background and white text on a black background as a wave moving from left to right for about 3 seconds. Specific details for implementing the invention

[0039] The present invention relates to an improved method for driving a "separated contact" electro-optical display and an "isolated electrode" electro-optical display. The improved method provides a DC-balanced waveform that reduces the amount of residual voltage remaining between electrodes when the driving cycle is completed. Additionally, the improved method enables a banner-type display to "wave" switch in a first direction and then return in the opposite direction.

[0040] In a spaced-contact display, two electrode layers are disposed on both sides of a layer of an electro-optical medium. Each electrode layer may include two contacts, and each electrode layer may simply have a uniform strip of a conductive film (light-transmitting or not light-transmitting) extending between the two contacts. A voltage controller is coupled to the contacts and configured to apply a potential difference that varies over time between a pair of contacts attached to at least one of the electrode layers. For example, the voltage controller may vary the potential difference (voltage) applied to the first and second contacts at different frequencies. The voltage controller may vary the potential difference as, for example, a sine wave, a triangular wave, a sawtooth wave, or a square wave of a fixed or variable frequency or a variable duty cycle.

[0041] Additionally, in a spaced-contact display, interesting visual effects can be created by using non-uniform electrode layers. For example, at least one of the first and second electrode layers may be interrupted by at least one non-conductive region, thereby causing the current to follow a non-linear path between the two contacts on the electrode. Examples of possible geometric arrangements of such non-linear paths include stripes, spirals, and interdigitated electrodes, which are described below with reference to the drawings. Alternatively, at least one of the first and second electrode layers may be divided into multiple sections having different electrical resistances per unit length and / or multiple sections having different electrical capacitances per unit area. The electrode layer having two contacts may also be provided in the form of multiple conductive traces or regions on the backplane and configured to operate as a bus bar. In some embodiments, the electrode layer may comprise a first plurality of conductive lines, a layer of insulating material applied over the first plurality of conductive lines, a second plurality of conductive lines applied over the layer of insulating material, and a layer of resistive material electrically in contact with the second plurality of conductive lines. The layer of insulating material may be configured to electrically connect each conductive trace in the first plurality of conductive lines to a single conductive line in the second plurality of conductive lines, and each conductive line in the second plurality of conductive lines to a single conductive line in the first plurality of conductive lines. In another embodiment, the electrode layer may comprise a plurality of conductive lines, a layer of insulating material applied over the first plurality of conductive trace lines, a plurality of conductive regions applied over the layer of insulating material, and a layer of resistive material electrically in contact with the plurality of conductive regions.A layer of insulating material may be configured to electrically connect each conductive line in a plurality of conductive lines to a single conductive region, and each conductive region to a single conductive line. To reduce or eliminate such damage to the electrode layer, a passivation layer disposed between the electrode layer and the layer of electro-optical material may be provided on at least a portion of one of the first and second electrode layers. A suitable passivation layer is described, for example, in U.S. Patent No. 6,724,519.

[0042] The method disclosed herein is also suitable for use with a “isolating electrode” display, the isolating electrode display comprising a layer of an electro-optical material, a sequence of at least three electrodes disposed adjacent to the layer of the electro-optical material and configured to apply an electric field across the layer of the electro-optical material—wherein at least one electrode on a surface of the layer of the electro-optical material is light-transmitting—and a voltage controller arranged to vary the potential difference between the first electrode and the last electrode of the sequence,

[0043] (a) Each electrode of the sequence is placed on the opposite side of a layer of electro-optical material from both the electrode preceding it and the electrode following it in the sequence;

[0044] (b) Each electrode of the sequence has a first edge that overlaps with or is adjacent to the electrode preceding it in the sequence and a second edge that overlaps with or is adjacent to the electrode following it in the sequence;

[0045] (c) Each electrode other than the first and last electrodes of the sequence is electrically isolated so that its potential is controlled by the passage of current through a layer of electro-optical material.

[0046] In most cases, an isolated contact display (as defined above) is driven by a voltage controller configured to apply a potential difference between the first electrode and the last electrode of the display. The voltage controller can be arranged to apply a potential difference that varies over time between the first electrode and the last electrode.

[0047] The display and driving method of the present invention may use any type of electro-optical medium described above. In a preferred embodiment, the electro-optical display may comprise an electrophoretic material comprising a plurality of electrically charged particles disposed within a fluid and capable of moving through the fluid under the influence of an electric field. The electrically charged particles and the fluid may be confined within a plurality of capsules or microcells. Alternatively, the electrically charged particles and the fluid may exist as a plurality of individual droplets surrounded by a continuous phase comprising a polymer material. The fluid may be a liquid or a gas. The spaced-contact display comprises a layer of electro-optical material, first and second electrodes on opposite sides of the layer of electro-optical material—at least one of the first and second electrodes having at least two spaced-contacts—and a voltage controller arranged to vary the potential difference between two spaced-contacts attached to the same electrode.

[0048] As described above, most conventional electro-optical displays, whether active matrix type or direct drive type, use a single light-transmitting "common" electrode on the front (top; viewing) side of the electro-optical layer and an array of electrodes (pixel electrodes or direct drive electrodes) on the opposite side (backplane) of the electro-optical layer. The potential difference between each array of electrodes and the common electrode is controlled by a display driver or voltage controller, so that each array electrode (in principle) controls the optical state of the region of the electro-optical medium located between the corresponding array electrode and the common electrode. That is, the optical state depends on the polarity and magnitude of the potential difference and the time it is applied. Typically, the potential on the "common" front electrode is controlled by a so-called "top-plane connection," which may be connected to the back substrate or penetrate the back substrate to provide a stable connection. Refer to FIG. 2. Although it is common practice to provide multiple connections on the front electrode to reduce the risk of bad contacts, such multiple connections are not independently controllable.

[0049] In contrast to displays having multiple upper planar connections, a spaced-contact display using the method of the present invention relies on the potential difference between two or more spaced-contacts on a single electrode layer (typically an elongated electrode layer) to create a potential gradient within that electrode layer. This time-dependent potential difference between different regions of the single electrode layer and the electrode on the opposite side of the electro-optical layer will result in "wave switching" as illustrated in FIG. 12. Furthermore, if multiple contacts are provided on both electrode layers of the display as in the conventional case, a potential gradient will exist within both electrodes, and the potential difference applied at any point in the electro-optical layer will be the difference between the instantaneous potentials at selected parts of the electro-optical material located between the two electrode layers. That is, the potential difference applied to the electro-optical material will vary continuously across the electro-optical layer, resulting in a corresponding continuous variation in the optical state of the electro-optical medium. As illustrated in FIG. 12, the effect is noticeable and considerably fast.

[0050] Since the display of the present invention is intended to operate by developing a potential gradient within the electrode (by providing a potential gradient between two or more contacts attached to the electrode within a single electrode), the resistance provided by the electrode is very important. If the electrode resistance is too low, excessive current may be generated within the electrode, which can cause electronic devices in the voltage controller to short circuit and other problems, such as excessive local heating that can damage the electro-optical layer, for example. On the other hand, if the electrode resistance is excessive, voltage propagation from the separated contacts is limited to a short range, so only a very small area adjacent to the contacts is switched, and a large number of contacts may be required if the entire area of ​​the display needs to be switched. This situation is similar to an isolated electrode embodiment where the effect from individual contacts is limited in terms of range.

[0051] In addition, since various embodiments of the display manufactured according to the present invention may rely on reflected ambient light to view images generated by the electro-optical material, light loss from the light-transmitting electrode must be minimized. For example, in a display according to various embodiments of the present invention, ambient light passes through the light-transmitting electrode twice: the first time when the ambient light travels from the light source to the surface of the electro-optical material, and the second time when the light is reflected from the electro-optical material to the viewer. As mentioned above, the electrode material must form a front electrode of sufficiently high conductivity to ensure sufficient current for uniform driving of the display. While a thicker layer of the electrode material will have greater conductivity, light loss will also increase with thickness because the material is not colorless. Although indium tin oxide (ITO) has high colorability, this color effect can be minimized by applying an extremely thin layer, for example, of about 1,000 to 2,000 Å.

[0052] The optimal electrode resistance will vary depending on the size of the display, the number of contacts, and the characteristics of the specific electro-optical medium used, but the sheet resistance of the light-transmitting electrode material is preferably about 500 to about 50,000 Ohm / sq, more preferably about 1,000 to about 15,000 Ohm / sq, and most preferably about 300 to about 5,000 Ohms / square. If desired, light-transmitting conductors such as PEDOT, carbon nanotubes, graphene, and nanowires can, of course, be used.

[0053] The electro-optical material used in the display of the present invention is typically a bistable display material, such as an electrochromic material, a rotational dichroic material, or an electrophoretic material. Such bistable materials change their electro-optical state only after being exposed to an electric field for a significant period, typically about 0.1 to 1 second. Accordingly, the appearance of the display of the present invention is controlled not only by the potential present in various regions of each electrode as the potential at the separated contact changes, but also by the speed at which the electro-optical material used responds to the electric field to which it is exposed. Additionally, as described in some of the aforementioned applications, some electro-optical materials undergo a phenomenon known as "blooming," in which a change in potential at an electrode affects the electro-optical state of the material over an area larger than the area of ​​the electrode itself. Blooming is often treated as a problem in electro-optical displays because it tends to distort the displayed image, but in at least some of the displays of the present invention, blooming can actually be advantageous for hiding otherwise inactive areas of the display. For example, as previously mentioned, in some displays of the present invention, the first and / or second electrodes may be interrupted by at least one non-conductive region so that the current must follow a non-linear path between two contacts on the electrodes. Blooming can be used to conceal the optical effect of this non-conductive region. In practice, in some cases, it may be desirable to design the electro-optical material to have increased blooming to help with this concealment.

[0054] A conventional display of the present invention may include the following layers in order:

[0055] (a) A transparent conductive layer forming the viewing surface of the display ("front electrode");

[0056] (b) a layer of encapsulated electrophoretic medium;

[0057] (c) a layer of lamination adhesive; and

[0058] (d) A “backplane” comprising a substrate (typically a polymer film) and a conductor that does not need to be transparent.

[0059] At least two regions of each electrode in layers (a) and (d) are cleaned to expose a conductor for an electrical contact that can be addressed independently. Finally, the display includes a voltage controller for driving the front electrode and the backplane to positive and negative potentials relative to each other and for creating a potential gradient within each electrode.

[0060] These displays were manufactured using the following materials. The front electrode was formed from 5 mil (127 μm) polyethylene terephthalate coated with Grade OC300 or 450 ITO on one surface. Alternatively, the front electrode could be coated on the remaining layers of the display without any supporting substrate. The encapsulated electrophoretic medium was substantially the same as described in U.S. Patent No. 8,270,064, and the lamination adhesive was a 25 μm layer, substantially the same as described in U.S. Patent No. 7,012,735, containing 5,000 ppm of tetrabutylammonium hexafluorophosphate dopant to control electrical properties. The backplane was a PET / ITO film similar to that used for the front electrode, but could be replaced with printed carbon conductors or other low-cost transparent or opaque conductors.

[0061] An elongated banner "spaced contact" display is schematically illustrated in FIGS. 1 and FIG. 2 of the drawing. As shown in FIG. 1, the elongated spacing contact display (generally designated as 100) comprises an elongated rectangular light-transmitting electrode (102), which is typically commercially available PET-ITO (Saint-Gobain). The electro-optical display material, generally designated as 106, is supported from below by a substrate (118).

[0062] As illustrated in FIG. 2, the elongated spaced contact display (100) includes an elongated rectangular light-transmitting electrode (102), which includes a PET film (108) that supports an ITO front electrode (110) extending over the entire area of ​​the display (106). An electro-optical display material (106) is in contact with the front electrode (110), which may be an encapsulated electrophoretic medium, and the lower surface of the electro-optical display material (106) is in contact with a lamination adhesive layer (114), and the lamination adhesive layer (114) secures the encapsulated electrophoretic medium (112) to a backplane comprising an ITO electrode layer (116) on a glass substrate (118). As illustrated in FIG. 1, the front electrode (102) is provided with four contacts (T1 to T4) arranged close to the corners of the elongated spaced contact display (100), and the rear electrode (116) is similarly provided with four contacts (B1 to B4) arranged in a similar manner.

[0063] FIG. 2 illustrates the configuration of contacts (T1 to T4) and contacts (B1 to B4). Contacts (B1 to B4) are typically manufactured by kiss-cutting an opening through a PET film (108) using a laser cutter and cleaning the portion beneath the electrophoretic medium (106) and lamination adhesive (114). Similarly, contacts (T1 to T4) are manufactured by cutting an opening through a glass substrate (118) and cleaning the portion above the electrophoretic medium (106) and lamination adhesive (114) by using a solvent or by rubbing with a mechanical means such as manual or electric toothbrush. The resulting opening is filled with a conductive material, such as carbon-filled adhesive, conductive ink, or silver paste, to manufacture contacts that can be individually addressed. Optionally, an insulating material (119), such as a transparent non-conductive polymer, may be applied around the through-hole of the opposite electrode layer to prevent short circuits. A voltage controller capable of independently driving contacts (T1 to T4) and contacts (B1 to B4) to positive and negative potentials in a time-dependent manner is provided by a display controller (not shown) having 12 outputs, each capable of independently programmable to supply any voltage and waveform between ±30 V on each channel, and having high impedance or float states. The controller has one driving line for each output, i.e., direct driving.

[0064] The display illustrated in FIGS. 1 and 2 can be constructed substantially as described in the aforementioned U.S. Patent No. 6,982,178. A PET / ITO film (i.e., PET film (108) and ITO electrode (110)) is coated with or laminated to an electrophoretic medium (106) to form a PET / ITO / electro-optical subassembly. An ITO-glass substrate (i.e., bottom substrate (118) and bottom ITO electrode (116)) is laminated to the subassembly by a lamination adhesive (114). In an alternative embodiment, since the electrophoretic medium (112) does not transmit light, the bottom ITO electrode (116) can be replaced with a non-light-transmitting electrode without any effect. The resulting structure is a fully electro-optical display capable of switching when a proper electrical connection is provided. Using this technique, it is possible to produce individual displays of 16 × 60 inches (406 × 1523 mm) or larger. Other configuration methods may also be used, for example, as described in the aforementioned U.S. Patent No. 6,982,178, after the formation of the front plane laminate (FPL), the FPL can be cut to size before laminating it onto the backplane. The substrate of the kiss-cut area is removed, and then the electrophoretic medium is cleaned. The greater the number of contacts and the spatial distribution of these contacts around the periphery of the display, the more complex the switching pattern that may be affected becomes.

[0065] The display (100) illustrated in FIGS. 1 and 2 can be driven by a voltage controller (not illustrated) by setting the upper contacts (T2 and T3) (located at the diagonal corners of the display (100)) to -20V and +20V, respectively, setting the backplane contacts (B2 and B3) to ground, and allowing all other remaining contacts (T1, T4, B1, B4) to be floating. If this driving pattern is maintained for longer than about 1 second, the optical state of the electrophoretic layer will be half dark and half white, with a diffusion gradient region in the center (see 1.5 seconds in FIG. 12). If a variable voltage pattern is supplied to the driven contacts (T2 and T3) instead of a fixed voltage, a moving pattern is created within the electrophoretic layer. For example, if one contact receives a 20V amplitude sine wave with a frequency of 0.1 Hz and another driving contact on the same electrode receives a 20V amplitude sine wave with a frequency of 0.09 Hz, the switching wave from black to white will move slowly across the display at different speeds and directions that vary over time due to the different frequencies of the two supplied sine waves, namely from left to right or from right to left. The speed and direction of the wave moving from white to black or from black to white can be made constant and repeatable by making the frequencies of the two sine waves the same and giving them a constant phase difference. More complex patterns can be formed by driving two contacts at opposite ends of the diagonal of the display, particularly when the contacts on opposite diagonals are used on the upper and lower electrodes. Much more complex patterns can be generated by providing a larger number of contacts around the periphery of the display.

[0066] The display illustrated in FIGS. 1 and 2 has electrodes in a simple rectangular shape, so that each electrode layer is essentially uniform between the spaced contacts at both ends; however, it should be understood that the method for improved wave switching (described below) is not limited to a rectangular or any specific shape of display, and interesting effects can be produced using a polygonal (e.g., hexagonal or octagonal) display, or a circular or elliptical display. In such cases, one or more contacts may be provided around the periphery of the display and another contact may be provided at the center of the display, so that changes in the electro-optical material can be propagated radially rather than linearly. Furthermore, the present invention is not limited to planar two-dimensional displays but can be applied to three-dimensional objects. Both the electrodes and the electro-optical medium can be deposited on the three-dimensional object; for example, electrodes formed of organic conductors can be deposited from a solution, and the electrophoretic medium can be deposited by a spray technique.

[0067] Additionally, interesting optical effects can be obtained by providing a gap in one or both electrodes, for example by removing or chemically altering the electrode material, so that the current follows a non-linear path between the two contacts on the electrode. FIG. 3 is a schematic plan view of this type of display (referred to as 300 overall). Similar to the elongated spaced-contact display (100) shown in FIG. 1 and 2, the display (300) has an elongated rectangular shape, and strip contacts (302 and 304) are provided at its opposite ends. (The contacts do not need to be pads, nor do they need to have more than one contact at each end.) The electrode layer (306) extending between the contacts (302 and 304) is interrupted by a number of non-conductive regions (308), so that the current (and thus the electro-optical effect) follows a substantially sinusoidal path between the contacts (302 and 304). The non-conductive region (308) does not require a special material and can be achieved by simply scraping off the ITO from the PET over a width sufficient to block a short circuit across the removed ITO, i.e., a width of 5 mm.

[0068] Non-conductive regions, such as the non-conductive region (308) of FIG. 3, can be used to "channel" electro-optical effects in various interesting patterns. For example, a circular, elliptical, or polygonal display may have a single contact at the periphery of the display, a second contact at the center of the display, and a spiral non-conductive region for channeling electro-optical effects along a spiral electrode extending between the two contacts. In the case of a three-dimensional display, a much greater degree of design freedom is available; for example, a display formed on a cylindrical substrate may use a spiral non-conductive region to channel electro-optical effects along a spiral path between contacts provided at opposite ends of the cylindrical substrate.

[0069] As previously mentioned, in the spaced-contact display of the present invention, at least one of the first and second electrodes may be divided into a plurality of sections having different electrical resistances per unit length, and a schematic plan view of such a display (generally referred to as 400) is shown in FIG. 4. The display (400) is generally similar to the display (300) shown in FIG. 3 in that the display (400) has a long rectangular shape with contacts (402 and 404) provided at its opposing ends. Also, like the display (300), the display (400) is provided with a non-conductive region (408). However, the arrangement of the region (408) is different from the arrangement of the region (308) in FIG. 3; The region (408) is formed in the form of two adjacent pairs of regions extending from the opposing long edge of the display (400), leaving a narrow "neck" or "isthmus" of conductive material (410 or 412) between each adjacent pair. Thus, current passing between the contacts (402 and 404) passes sequentially through the low-resistance region (414), the high-resistance neck (410), the low-resistance region (416), the high-resistance neck (412), and the low-resistance region (418).

[0070] FIG. 4 illustrates the formation of regions of varying resistance by changing the width of the electrodes, but of course, other techniques for varying resistance may be employed. For example, the display illustrated in FIG. 4 can be modified by replacing each neck region (410 and 412) with contacts provided on adjacent regions and interconnected via appropriate resistors. To avoid the unsightly presence of visible electrical components, the resistors and associated conductors may be housed within a frame surrounding the display, such as those commonly found in frames for signage. The ability to interconnect the electrode regions "invisibly" by electrical components placed within such a frame provides additional design freedom, namely the ability to electrically arrange electrode segments in an order different from their physical locations. For example, consider a modified version of the display (400) in which the electrodes are divided into five segments (designated as A, B, C, D, E from left to right in FIG. 4) rather than the three segments (414, 416, 418) shown in FIG. 4, and the segments (A through E) are interconnected through conductors and resistors hidden within the frame. The electrical interconnection can be arranged so that the electrode segments are electrically interconnected in the order of (e.g.) A, D, B, C, E, which will produce an electro-optical effect that appears to jump around the display rather than proceeding linearly as in the display (100) shown in FIG. 1 and 2.

[0071] Instead of providing regions of various resistance within the electrode, regions of various capacitance may be used, and a schematic cross-sectional view through such a display (generally indicated as 500) is shown in FIG. 5. The display (500) is generally similar to the displays (300 and 400) shown in FIG. 3 and FIG. 4, respectively, in that it has the shape of a long rectangle having an electrode (506) provided with contacts (502 and 504) at its opposing ends. However, unlike the exemplified electrodes of the displays (300 and 400), the electrode (506) of the display (500) is not interrupted. Instead, by providing a series of spaced electrodes (512) that are entirely grounded on the electro-optical medium side opposite to the electrode (506), regions of various capacitance per unit area are provided in the electrode (506). It will be readily apparent that regions of the electrode (506) positioned opposite the electrodes (512) will have substantially greater capacitance per unit area than regions of the electrode (506) not positioned opposite the electrodes (512), and thus provide variation in the electro-optical performance of the display (500) generally similar to that provided by regions of varying resistance of the display (400). (Typically, an adhesive layer similar to the adhesive layer (114) shown in FIG. 2 will be present between the electro-optical layer (510) and the electrode (506) or between the electro-optical layer (510) and the electrode (512). The adhesive layer has been omitted in FIG. 5 for ease of illustration, but its presence or absence makes no difference to the basic operation of the display (500).)

[0072] Now, one embodiment of the isolation electrode display of the present invention will be described with reference to FIG. 6. Conceptually, the isolation electrode display can be considered as a variation of the variable resistance electrode display of the type shown in FIG. 4, which includes using the electro-optical layer itself as a high-resistance region between low-resistance electrodes. Since this variation places a continuous high-resistance region (electrode) on opposite sides of the electro-optical layer, only a single set of electrodes is required.

[0073] More specifically, as illustrated in FIG. 6, an isolation electrode display (generally designated as 600) has the shape of a long rectangle comprising an electro-optical material layer (610) and a sequence of seven electrodes (612 to 624), each electrode having the shape of a long strip extending across the entire width of the display. The first electrode (612) and the last electrode (624) are each connected to a voltage control unit (schematically indicated as 626), which enables a potential difference that varies over time to be applied between the electrodes (612 and 624). The remaining electrodes (614 to 622) are electrically isolated so that their potential is controlled by the passage of current through the electro-optical material layer (610). Electrodes (612 to 624) alternate between the lower surface and the upper surface (as illustrated) of the layer (610), and the electrodes (614, 618, 622) on the upper surface (which is the viewing surface of the display) are light-transmitting; the electrodes (612, 616, 620, 624) may or may not be light-transmitting. As can be seen in FIG. 6, each of the electrodes (614 to 622) has a first edge (its left edge as illustrated in FIG. 6) that overlaps with the preceding electrode and a second edge (its right edge as illustrated in FIG. 6) that overlaps with the succeeding electrode. As long as they are placed adjacent to each other to leave a conductive path of reasonable length through the layer (610), the adjacent edges do not necessarily have to overlap. It will be understood that the first and second edges of the electrodes do not need to be on opposite sides of the electrodes. For example, the electrodes (612 to 624) may be in the shape of an isosceles triangle, so that the first and second edges are not parallel, or the electrodes may be arranged in a checkerboard shape, in which case some electrodes may have first and second edges that are right angles to each other.

[0074] When a time-varying potential difference is applied between the electrodes (612 and 624) by the voltage controller (626), it will cause complex fluctuations in the potential of the electrodes (614 to 622) depending on factors such as the resistivity of the layer (610), the capacitance between the electrodes, and the polarization within the layer (610), and will cause even more complex fluctuations in the optical state of various parts of the layer (610). Most commonly, as the voltage applied by the voltage controller (626) changes, various parts of the layer (610) will be perceived as "flickering."

[0075] In another embodiment, the display may be manufactured to include a backplane configured to achieve low-power wave switching. Referring to FIGS. 7a through 7d, the backplane may include a rectangular substrate (700) on which a first plurality of conductive lines or traces (710) of various lengths can be printed. One end of each conductive line (710) may be connected to a driving circuit or a voltage controller (not shown). Then, an insulating material layer (720) may be applied over the first plurality of conductive lines (710), except for a plurality of voids (730) that leave the unconnected ends of the first plurality of conductive lines (710) exposed. A second plurality of conductive lines or traces (740) may be applied over the insulating material layer (720), each of which is electrically in contact with the corresponding first conductive line (710) across it, but only one of the first conductive lines (710) is electrically in contact with it. This can be achieved by printing a second plurality of conductive lines (740) on a void (730) in the insulating material (720). The location of the void in the insulating material layer and the second plurality of conductive lines are precisely positioned to prevent electrical short circuits between unrelated conductive lines. Thus, each of the first plurality of conductive lines is electrically connected to only one line within the second plurality of conductive lines, and vice versa. One or more contact pads (751, 752) may also be applied to the exposed end of the first conductive line (710). One or more contact pads (751, 752) may provide a location for electrically connecting the backplane to a light-transmitting front electrode (not shown) of the display. Finally, a resistive material layer (760) may be applied over the second plurality of conductive lines (740), so that the resistive material layer (760) electrically contacts the second plurality of conductive lines (740).The resistive material layer is preferably the top layer of the backplane and will be in direct contact with the front panel laminate (FPL) of the display.

[0076] Each component within the backplane can be easily fabricated using techniques known to those skilled in the art, such as methods for manufacturing multilayer printed circuit boards. Various materials may be used for the various layers of the backplane. For example, materials for the insulating layer include, but are not limited to, dielectric materials, preferably dielectric materials including photocurable solvent-free organic or silicon-based oligomers. Examples of materials that may be used for the resistive layer include, but are not limited to, resistive carbon, ITO filler polymers, PEDOT filler polymers, and metal fillers. Similarly, any conductive material, such as carbon or conductive metals such as silver, nickel, and copper, may be used to print the first and second plurality of conductive lines.

[0077] The material for forming the second plurality of conductive lines preferably has higher conductivity than the material used to form the resistive material layer. The combination of the second plurality of conductive lines and the resistive material layer essentially provides a series of highly conductive bus bars, and since the resistive material layer ensures a uniform voltage around each line, the conductive lines function as individual bus bars. The resistance of the resistive material layer can be selected for the length and spacing between the bus bar lines. Preferably, the total resistance between the bus bar lines is 1 kOhm or more, and more preferably 10 kOhm or more for reduced power consumption. For example, in a configuration where the length-to-spacing ratio of the bus bar lines is 10, providing a resistive layer having a resistivity of 10 kOhms / square results in a total resistance of 1 kOhm between the bus bar lines.

[0078] In another embodiment of the present invention, a method for driving a display having the backplane described above is provided. To drive the display, a driver having two or three levels of output capability may be used, which also preferably has floating (high impedance) output capability. Referring again to FIGS. 7a through 7d, the driver may be connected to a first plurality of conductive lines (710) along the left side of the substrate (700). In the first step, the driver may apply voltage to the leftmost bus bar line ("first bus bar line") of the second plurality of conductive lines (740) and short-circuit or float the remaining bus bar lines. As used herein, "short-circuit" or "short-circuiting" means grounding a conductive line or area, and "float" or "floating" means electrically isolating a conductive line or area. The electro-optical medium within the FPL located near the leftmost bus bar line will be switched immediately, and a color gradient from the switched electro-optical medium to the unswitched electro-optical medium will appear between the first bus bar line and the subsequent bus bar line ("second bus bar line"). If the voltage controller is capable of pulse width modulation (PWM) or voltage modulation (VM) output, the driver can generate a gradient that unfolds more slowly by gradually increasing the duty cycle or voltage. After a period of time, the length of which may be determined by the switching speed required by the application, the driver can apply voltage to the second bus bar line while terminating the voltage application to the first bus bar line, and proceed in the same manner as follows. In this way, the driver can gradually switch the electro-optical medium across the entire display in a controlled manner, while limiting the voltage application only to the region where the gradient exists between the switched electro-optical medium and the unswitched electro-optical medium.Fortunately, advanced function generators are not essential for PWM and VM outputs and can be achieved with, for example, a LabVIEW programming kit, an Arduino board coupled to a suitable voltage source, or a specific ready-made EPD driver available from ULTRACHIP.

[0079] For example, if a wider gradient is desired, the driver can apply voltage to multiple adjacent bus bar lines in a sequentially timed manner to gradually spread the gradient across the entire display. Finally, by applying a pattern of opposite voltages on multiple bus bar lines, a gradient can be generated at any location on the display, and multiple gradients can exist simultaneously. The gradient can start and stop at any point on the display or at any time, and when multiple gradients are generated simultaneously, the gradient can propagate in multiple directions at multiple speeds. Thus, wave complexity depends on the bus bar line spacing and software control for the driver.

[0080] As described above, the insulating material layer between the first and second plurality of conductive lines connects the driving circuit to a remote area of ​​the backplane. Various conductive lines can cross each other without electrically short-circuiting one another. This configuration enables various backplane designs. For example, a backplane having a printed layer similar to the rectangular backplane described above can instead be provided on a circular substrate.

[0081] A (prior art) PWM driving method for the displays of FIGS. 7a through 7d is illustrated in FIG. 8. In FIG. 8, a driving signal having a frequency of 30 Hz and ±15 V has a variable duty cycle in the range of -1 to 1, which can be visualized with reference to FIG. 9. A duty cycle of 0 to -1 (i.e., a negative duty cycle) corresponds to a duty cycle of 0 to 1 illustrated in FIG. 9, except that the voltage is -V, i.e., -15 V. When a blue driving signal is applied to one corner contact (e.g., pad (751)) of an elongated spaced-contact display (700) while an orange driving signal is applied to another corner contact (e.g., pad (752)), a switching effect is produced along the bottom of FIG. 8. It should be understood that the method illustrated in FIG. 8 is generally applicable to driving an elongated spaced-contact display or an isolated electrode display (e.g., FIG. 6). In addition, solid black lines were added to the blue driving signal for easier visualization.

[0082] Returning to Fig. 8, the transition will appear to start on the banner side where the blue driving signal is applied (aka "left"), and end on the side where the orange trace is applied (aka "right"). A dashed black line has been added to the orange driving signal for easier visualization. This plot shows a single transition from one rail state to another, for example, from black to white, as illustrated along the bottom of Fig. 8. Once the white state is reached, it is possible to drive the display from white to black by inverting the duty cycle signal provided to both contacts. However, the wave switching will again occur from left to right.

[0083] Importantly, as illustrated in FIG. 8, the visible transition will appear to be completed around frame 40, when the trailing (orange) waveform rises significantly above 0. [As used herein, "frame" is a unit of time representing a step from the first duty cycle to the next adjacent duty cycle. The size of the frame is somewhat arbitrary, but for optimal performance and to prevent damage to the display, each frame in FIG. 8 is about 100 ms, e.g. 80 ms. Thus, the number of voltage pulses in a given duty cycle is about 2 to 5 in ±V; not as long as illustrated in FIG. 9, which is merely for illustrative purposes.] However, from frame 40 to frame 66, there will be a negligible change in the visual appearance of the display, which means there is a natural rest period before the banner can be switched back to the opposite color. If a wave in the same direction (from left to right) going from white to black follows immediately after the transition shown in Fig. 8, the visual effect will not start until about frame 67, or more than 2 seconds after the first switching appears to be completed.

[0084] Fortunately, two combined transitions (black to white and white to black) moving in the same direction (left to right) result in a DC-balanced pair, which reduces wear and damage to the display, although there is no discernible visual effect for approximately 2 / 5 of the time. However, DC balance is achieved only when moving in the same direction (left to right) while progressing (black to white and white to black). Note that in Fig. 8, the orange trace is DC-balanced, whereas the blue trace is entirely positive and therefore not DC-balanced. Consequently, if the display does not switch back to its original color, excess charge can damage the display. Even worse, if the direction is reversed (right to left) for the return transition (white to black), that is, if a blue trace of opposite polarity is applied to the right and an orange trace of opposite polarity is applied to the left, the DC imbalance doubles (!), which will rapidly degrade the display. Therefore, using the driving method of Fig. 8, it is impossible to achieve back-and-forth wave switching, or "ping-pong switching," without substantially degrading the display. Finally, it should be noted that the orange trace essentially terminates at the rail voltage at 100% duty cycle (see Fig. 9). If a delay or "pause" follows the unidirectional switching, a kickback phenomenon occurs where the electrophoretic medium bounces off the destination rail state, resulting in a final state that is less saturated than desired.

[0085] Considering the limitations of conventional driving technology, there is a need for an improved waveform switching method for the type described above, namely, a separated contact and isolated electrode display. FIG. 10 illustrates an improved waveform that can be used for a long-extended separated contact display using PWM or VM driving. In particular, the waveform of FIG. 10 significantly reduces the relative amount of time during which the optical state of the display does not visibly change; for the driving method related to FIG. 8, the time during which it is not visibly switched is about 40%, while for the driving method related to FIG. 10, the time during which it is not visibly switched is reduced to about 10%. The waveform of FIG. 10 also reduces kickback when a pause is inserted at the end of driving. Two significant changes from the conventional method (Fig. 8) can be seen in the blue and orange lines at the start and end of the switching. At the start, the duty cycle of the orange line (right) increases smoothly (but rapidly) from 0 to -1, and the sudden state jump experienced in the orange line of FIG. 8 is eliminated. At the end of the transition, the duty cycle of the orange line (right) smoothly decreases from 1 to 0, eliminating kickback. Additionally, the visual transition occurs approximately between frames 0 and 55, providing a significant reduction in idle time. Although each end of this waveform remains in a DC unbalanced state, it provides a smooth transition while minimizing the time between successive updates in each rail state. Simultaneously, the blue line (left) is extended so that the visual transition aligns with the orange line; the blue line still returns from a duty cycle of 1 to a duty cycle of 0 as in Fig. 8, but this occurs after the visual transition is complete.

[0086] An alternative driving method for a long-extended spaced-contact display using PWM or VM driving is illustrated in Fig. 11. The driving method of Fig. 11 additionally provides DC balance at the end of each wave switching, but the driving is more complex than Fig. 10 and may require a more sophisticated voltage controller. Both the blue (left) and orange (right) driving methods start with a duty cycle of 0 as in Fig. 10, but the first 28 frames of the transition are used to DC balance both the blue and orange waveforms. Then, starting around frame 30, the wave will start on the left side of the display. Around frame 55, the right side of the display will appear to have completed the transition. To achieve DC balance, both driving methods must run until completion, resulting in approximately 20% of visibly unswitched time, which is worse than Fig. 10. However, DC balancing in the driving method of FIG. 11 allows the display to reverse direction for the next transition, that is, to achieve the desired "ping-pong switching." The reversed direction driving can be achieved essentially by providing a blue driving method to the right contact and an orange driving method to the left contact, or by reversing the direction and polarity of both the blue and orange driving methods while providing modified blue and orange driving methods to the left and right contacts, as done in the previous driving method.

[0087] From the foregoing, it can be seen that the present invention provides a display and driving method that enables the generation of visually interesting patterns and the shifting changes in the optical state of an electro-optical medium (particularly a bistable medium such as an electrophoretic medium) using very simple and inexpensive electrodes. Visually interesting patterns can be used to guide the viewer's gaze or to draw attention to content that would otherwise be static, such as provided by a segmented electrode display, which is illustrated, for example, in FIG. 12.

[0088] It will be apparent to those skilled in the art that numerous changes and modifications can be made to the specific embodiments of the invention described above without departing from the scope of the invention. For example, variable voltage is not limited to simple sine waves; triangular, sawtooth, and square waves of fixed or variable frequency may all be used. Accordingly, the entire description above should be interpreted as exemplary rather than limiting.

Claims

Claim 1 A method for driving a spaced contact electro-optical display, comprising the step of providing a spaced contact electro-optical display, Electro-optical material layer, A first electrode layer and a second electrode layer on opposite sides of the electro-optical material layer - said first electrode layer or said second electrode layer is light-transmitting - , A first contact and a second contact spaced apart on the first electrode layer and electrically coupled to the first electrode layer, and A method for driving a separated contact electro-optical display comprising: providing a separated contact electro-optical display including a voltage controller coupled to the first and second contacts; providing a first time-varying driving signal to the first contact using the voltage controller, wherein the first time-varying driving signal starts at a duty cycle of 0, ramps to a duty cycle of 1, and then returns to a duty cycle of 0; and providing a second time-varying driving signal to the second contact using the voltage controller, wherein the second time-varying driving signal starts at a duty cycle of 0, ramps to a duty cycle of -1, ramps to a duty cycle of 1, and then returns to a duty cycle of 0. Claim 2 A method for driving a spaced-contact electro-optical display according to claim 1, wherein the ramp from a duty cycle of -1 to a duty cycle of 1 for the second time-varying driving signal comprises two different slopes of the duty cycle per unit time. Claim 3 A method for driving a separated contact electro-optical display, further comprising: the step of providing a third time-varying driving signal to the first contact using the voltage controller, wherein the first time-varying driving signal starts at a duty cycle of 0, ramps to a duty cycle of -1, and then returns to a duty cycle of 0; and the step of providing a fourth time-varying driving signal to the second contact using the voltage controller, wherein the second time-varying driving signal starts at a duty cycle of 0, ramps to a duty cycle of 1, ramps to a duty cycle of -1, and then returns to a duty cycle of 0. Claim 4 A method for driving a spaced contact electro-optical display, comprising the step of providing a spaced contact electro-optical display, Electro-optical material layer, A first electrode layer and a second electrode layer on opposite sides of the electro-optical material layer - said first electrode layer or said second electrode layer is light-transmitting - , A first contact and a second contact spaced apart on the first electrode layer and electrically coupled to the first electrode layer, and A method for driving a separated contact electro-optical display comprising: providing a separated contact electro-optical display including a voltage controller coupled to the first and second contacts; providing a first time-varying driving signal to the first contact using the voltage controller, wherein the first time-varying driving signal starts at a duty cycle of 0, ramps to a duty cycle of -1, maintains a duty cycle of -1 for a time sufficient to provide DC balance to the first time-varying driving signal, then ramps to a duty cycle of 1, and then returns to a duty cycle of 0; and providing a second time-varying driving signal to the second contact using the voltage controller, wherein the second time-varying driving signal starts at a duty cycle of 0, transitions to a duty cycle of 1 by passing through a negative duty cycle impulse until the first time-varying driving signal maintains a duty cycle of -1 and then proceeds to a duty cycle of 1, and then returns to a duty cycle of 0. Claim 5 A method for driving a spaced-contact electro-optical display, further comprising: a step of providing a third time-varying driving signal identical to the second time-varying driving signal to the first contact; and a step of providing a fourth time-varying driving signal identical to the first time-varying driving signal to the second contact. Claim 6 A method for driving a spaced-contact electro-optical display according to any one of claims 1 to 5, wherein the first time-varying driving signal and the second time-varying driving signal simultaneously reach a duty cycle of 1. Claim 7 A method for driving a spaced-contact electro-optical display according to any one of claims 1 to 6, wherein the frequency of the first and second driving signals is 30 Hz or higher. Claim 8 A method for driving a spaced-contact electro-optical display according to any one of claims 1 to 7, wherein the magnitude of the voltage of the first and second driving signals is 15V to 30V. Claim 9 A method for driving a spaced-contact electro-optical display according to any one of claims 1 to 8, wherein both the first and second electrodes have at least two spaced-contacts, and the voltage controller is arranged to change the potential difference between the two spaced-contacts attached to each electrode. Claim 10 A method for driving a spaced-contact electro-optical display according to any one of claims 1 to 9, wherein the electro-optical material comprises an electrophoretic material comprising a plurality of electrically charged particles disposed in a fluid and capable of moving through the fluid under the influence of an electric field. Claim 11 A method for driving a spaced-contact electro-optical display according to claim 10, wherein the electrically charged particles and the fluid are confined within a plurality of capsules or microcells or individual droplets surrounded by a continuous phase comprising a polymer material. Claim 12 A method for driving a spaced-contact electro-optical display according to any one of claims 1 to 11, wherein at least one of the first and second electrodes is interrupted by at least one non-conductive region so that the electric current follows a non-linear path between two contacts on said electrode. Claim 13 A method for driving a spaced-contact electro-optical display according to any one of claims 1 to 12, wherein at least one of the first and second electrodes is divided into a plurality of sections having different electrical resistances per unit length. Claim 14 A method for driving a spaced-contact electro-optical display according to any one of claims 1 to 13, wherein at least one of the first and second electrodes is divided into a plurality of sections having different electric capacitances per unit area. Claim 15 A method for driving a spaced-contact electro-optical display according to any one of claims 1 to 14, wherein at least a portion of one of the first and second electrodes is provided with a passivation layer disposed between the electrode and the electro-optical material layer. Claim 16 A method for driving a spaced-contact electro-optical display according to any one of claims 1 to 15, wherein the first or second time-varying driving signal comprises a sine wave, a triangular wave, a sawtooth wave, or a square wave.

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