HK40107107B - A method for driving an electro-optic display - Google Patents

Description

Method for driving electro-optic displays

Cross-references to related applications

This application claims priority to U.S. Provisional Patent Application No. 63/293,947, filed December 27, 2021, and U.S. Provisional Patent Application No. 63/301,747, filed January 21, 2022. The entire contents of the aforementioned provisional patent applications are incorporated herein by reference. Furthermore, the entire contents of any patent, published application, or other publication cited herein are incorporated herein by reference in their entirety.

Technical Field

This invention relates to a method for measuring the electrical characteristics of an electro-optic display. More specifically, this invention relates to a method for measuring the electrical characteristics of an active matrix electrophoretic display module.

Background Technology

Electrophoretic display media are typically characterized by the movement of particles through an applied electric field. They are highly reflective, can be fabricated as bistable, can be scaled up to large areas, and consume very little power. Encapsulated electrophoretic displays also allow for printing. These properties enable encapsulated electrophoretic display media to be used in many applications where conventional electronic displays are not suitable, such as flexible displays.

One specific application of displays is as input devices, such as touchscreens, keypads, or writing tablets. In many cases, it is necessary to sense the state of the display in order to digitize the input. For example, measuring and analyzing certain characteristics of the display can enable the detection of input position. A response event or action may then be generated.

Furthermore, the electrical properties of the encapsulated electrophoretic display medium may vary in response to environmental factors such as temperature and humidity. In some cases, to achieve repeatable optical states in a display, it may be necessary to compensate for changes in the electrical properties of the polymer material containing the encapsulated electrophoretic display medium. Therefore, it is necessary to measure the display parameters that affect the waveform compensation scheme. However, using external display sensors may increase the cost of the display and complicate the manufacturing process. Moreover, external sensors may not be able to accurately measure the parameters inside the display.

Summary of the Invention

Therefore, it is necessary to efficiently and accurately measure the electrical characteristics of the display and apply the waveform to the display pixels based on the measured electrical characteristics.

Accordingly, in one aspect, the subject matter of this document provides a method for driving an electro-optic display comprising an electro-optic material layer disposed between a common electrode and a backplane. The backplane includes a pixel electrode array, and each pixel electrode is coupled to a pixel transistor. A display controller circuit applies a waveform to the pixel electrode array by applying one or more time-dependent voltages between the common electrode and the pixel electrode array via the pixel transistors. The driving method includes applying a first measurement waveform comprising one or more frames to a first portion of the pixel electrodes of the pixel electrode array. During each frame of the first measurement waveform, the same time-dependent voltage is applied to each pixel electrode of the first portion of the pixel electrode. The method further includes measuring a first current flowing through a current measuring circuit coupled between the common electrode and the output of the display controller circuit applying the time-dependent voltage to the common electrode, and determining a first impedance of the electro-optic material adjacent to the first portion of the pixel electrode based on the first current flowing through the current measuring circuit and the time-dependent voltage applied to each pixel electrode of the first portion of the pixel electrode during the first measurement waveform. The method further includes selecting a first driving waveform to be applied to each pixel electrode of the first portion of the pixel electrode based on the first impedance of the electro-optic material adjacent to the first portion of the pixel electrode, and applying the first driving waveform to the first portion of the pixel electrode. The first driving waveform includes a time-dependent voltage sufficient to change the optical state of the electro-optic display near the first portion of the pixel electrode.

In some embodiments, the current measurement circuit includes a resistive element and a differential voltage amplifier, wherein a first input terminal of the differential voltage amplifier is connected to a first terminal of the resistive element, and a second input terminal of the differential voltage amplifier is connected to a second terminal of the resistive element.

In some embodiments, the time-dependent voltage applied to each pixel electrode of the first portion of the pixel electrode includes a uniform voltage pulse. In some embodiments, the time-dependent voltage applied to each pixel electrode of the first portion of the pixel electrode includes a first voltage pulse having a first polarity and a second voltage pulse having a second polarity opposite to the first polarity.

In some embodiments, the first portion of the pixel electrode includes all the pixel electrodes of the pixel electrode array. In some embodiments, the first portion of the pixel electrode includes pixel electrodes located near the outer periphery of the pixel electrode array.

In some embodiments, the method further includes: applying a second measurement waveform comprising one or more frames to a second portion of pixel electrodes of a pixel electrode array, wherein during each frame of the second measurement waveform, the same time-dependent voltage is applied to each pixel electrode of the second portion of pixel electrodes; measuring a second current flowing through a current measurement circuit; determining a second impedance of an electro-optic material near the second portion of pixel electrodes based on the second current flowing through the current measurement circuit and the time-dependent voltage applied to each pixel electrode of the first portion of pixel electrodes during the second measurement waveform; selecting a second driving waveform to be applied to each pixel electrode of the second portion of pixel electrodes based on the second impedance of the electro-optic material near the second portion of pixel electrodes; and applying the second driving waveform to the second portion of pixel electrodes, wherein the second driving waveform includes a time-dependent voltage sufficient to change the optical state of the electro-optic display near the second portion of pixel electrodes.

In some embodiments, the first portion of the pixel electrode includes a pixel electrode from a first region of the pixel electrode array, and the second portion of the pixel electrode includes a pixel electrode from a second region of the pixel electrode array, wherein the pixel electrodes in the first region and the second region do not overlap. In some embodiments, the method further includes: applying a zero-volt waveform to the second portion of the pixel electrode while applying a first measurement waveform to the first portion of the pixel electrode; and applying a zero-volt waveform to the first portion of the pixel electrode while applying a second measurement waveform to the second portion of the pixel electrode.

On the other hand, the subject matter presented herein provides a method for driving an electro-optic display comprising an electro-optic material layer disposed between a common electrode and a backplane. The backplane includes a pixel electrode array, and each pixel electrode is coupled to a pixel transistor. A display controller circuit applies a waveform to the pixel electrode array by applying one or more time-dependent voltages between the common electrode and the pixel electrode array via the pixel transistors. The driving method includes simultaneously activating pixel transistors associated with a first portion of pixel electrodes in the pixel electrode array, and applying a first voltage to the first portion of pixel electrodes. The driving method further includes injecting a measurement waveform from a signal generation circuit via a current measurement circuit coupled between a signal generation circuit and the common electrode, and measuring a first current flowing through the current measurement circuit based on the measurement waveform. The driving method further includes determining a first impedance of the electro-optic material adjacent to the first portion of pixel electrodes based on the first current flowing through the current measurement circuit and the time-dependent voltage applied to each pixel electrode of the first portion of pixel electrodes during the first measurement waveform. The driving method further includes: selecting a first driving waveform to be applied to each pixel electrode of the first portion of the pixel electrode based on a first impedance of the electro-optic material near the first portion of the pixel electrode; and applying the first driving waveform to the first portion of the pixel electrode, wherein the first driving waveform includes a time-dependent voltage sufficient to change the optical state of the electro-optic display near the first portion of the pixel electrode.

In some embodiments, the current measurement circuit includes a resistive element and a differential voltage amplifier, wherein a first input terminal of the differential voltage amplifier is connected to a first terminal of the resistive element, and a second input terminal of the differential voltage amplifier is connected to a second terminal of the resistive element.

In some embodiments, the measured waveform includes a periodic square wave or a sinusoidal voltage waveform. In some embodiments, the measured waveform includes a voltage with an amplitude insufficient to change the optical state of the electro-optical display. In some embodiments, the measured waveform includes an oscillating voltage waveform having multiple frequencies.

On the other hand, the subject matter presented herein provides a method for driving an electro-optic display comprising an electrophoretic display medium disposed between a first common electrode and an array of pixel electrodes, wherein each pixel electrode is coupled to a first terminal of a storage capacitor, and a second terminal of each storage capacitor is coupled to a second common electrode. A display controller circuit is configured to apply a time-dependent voltage to the first and second common electrodes, which are independent of each other. The driving method includes simultaneously activating pixel transistors associated with the first portion of the pixel electrodes. The driving method further includes switching a first switch to disconnect the first common electrode from the display controller circuit and connect the first common electrode to a first terminal of an impedance measurement circuit, and switching a second switch to disconnect the second common electrode from the display controller circuit and connect the second common electrode to a second terminal of the impedance measurement circuit. The driving method further includes injecting a measured waveform voltage from the impedance measurement circuit into the first common electrode, wherein the measured waveform includes a time-dependent voltage. The driving method further includes measuring a first current flowing through the impedance measurement circuit based on the measured waveform, and determining a first impedance of the electro-optic material adjacent to the first portion of the pixel electrodes based on the first current flowing through the impedance measurement circuit and the time-dependent voltage applied to the first common electrode during the measured waveform. The driving method further includes selecting a first driving waveform to be applied to each pixel electrode of the first portion of the pixel electrodes based on a first impedance of the electro-optic material near the first portion of the pixel electrodes. The driving method also includes switching a first switch to disconnect a first common electrode from the impedance measurement circuit and connect the first common electrode to the display controller circuit, and switching a second switch to disconnect a second common electrode from the impedance measurement circuit and connect the second common electrode to the display controller circuit. The driving method further includes applying a first driving waveform to the first portion of the pixel electrodes, wherein the first driving waveform includes a time-dependent voltage sufficient to change the optical state of the electro-optic display near the first portion of the pixel electrodes.

In some embodiments, the measured waveform includes a periodic square wave or sinusoidal voltage waveform. In some embodiments, the measured waveform includes a voltage with an amplitude insufficient to change the optical state of an electro-optical display near the first portion of the pixel electrode. In some embodiments, the measured waveform includes a voltage with an amplitude less than 1 volt. In some embodiments, the measured waveform includes an oscillating voltage waveform having multiple frequencies.

In some embodiments, the first portion of the pixel electrode includes a pixel electrode located near the outer periphery of the pixel electrode array.

Attached Figure Description

Figure 1 is a circuit diagram showing an electrophoretic display according to the subject matter described herein.

Figure 2 shows a circuit model of the electro-optic imaging layer according to the subject matter described herein.

Figure 3 is a graph showing the impedance versus temperature of an exemplary front-plane laminate or “FPL” according to the subject matter described herein.

Figure 4A shows two graphs of exemplary impedance measurements performed on three exemplary display modules in accordance with the topics described herein.

Figure 4B is a graph showing the ghosting performance during different grayscale transitions according to the subject matter described herein.

Figure 5 shows a schematic diagram of one embodiment of an electrophoretic display according to the subject matter described herein.

Figure 6 shows a schematic diagram of one embodiment of an electrophoretic display according to the subject matter described herein.

Figure 7 shows a schematic diagram of one embodiment of an electrophoretic display according to the subject matter described herein.

Figure 8 shows a set of impedance measurements from an exemplary active matrix display module using the method described herein.

Figure 9 shows a schematic diagram of one embodiment of an electrophoretic display according to the subject matter described herein.

Figure 10 shows an exemplary signal sequence for implementing the impedance measurement described herein.

Figure 11 shows a schematic diagram of one embodiment of an electrophoretic display according to the subject matter described herein.

Detailed Implementation

This invention relates to methods for driving electro-optic displays, particularly bistable electro-optic displays, and apparatus used in such methods. More specifically, this invention relates to driving methods that can reduce "ghosting" and edge effects, as well as flicker, in such displays. The invention is particularly, but not exclusively, intended for use with particle-based electrophoretic displays, in which one or more types of charged particles are present in a fluid and move through the fluid under the influence of an electric field to alter the appearance of the display.

The term "electro-optic," when applied here to materials or displays, is used in its conventional meaning in the field of imaging, referring to a material having first and second display states that differ in at least one optical property, which is changed from its first display state to its second display state by applying an electric field to the material. While the optical property is typically color perceptible to the human eye, it can also be another optical property, such as light transmission, reflection, emission, or, in the case of displays intended for machine reading, a false color in the sense of a change in reflectivity at electromagnetic wavelengths outside the visible light range.

The term "gray state" is used herein in its conventional sense in imaging technology to refer to a state between two extreme optical states of a pixel, and does not necessarily imply a black-and-white transition between these two extreme states. For example, several patents and published applications from IENK, mentioned below, describe electrophoretic displays where the extreme states are white and dark blue, making the intermediate "gray state" actually a pale blue. In fact, as already mentioned, a change in optical state may not be a color change at all. In the following text, the terms "black" and "white" can be used to refer to the two extreme optical states of a display and should be understood to generally include extreme optical states that are neither strictly black nor strictly white, such as the aforementioned white and dark blue states. In the following text, the term "monochrome" can be used to refer to a driving scheme that drives pixels only to their two extreme optical states, without any intermediate gray state.

While some electro-optic materials are solid in the sense that they have a solid outer surface, they may, and often do, have internal spaces filled with liquid or gas. For convenience, such displays using solid electro-optic materials will be referred to as "solid-state electro-optic displays" below. Therefore, the term "solid-state electro-optic display" includes rotating dual-color component displays, encapsulated electrophoretic displays, microcell electrophoretic displays, and encapsulated liquid crystal displays.

The terms “bistable” and “bistable” are used herein in their conventional sense in the art, referring to a display including a display element having first and second display states that are different in at least one optical characteristic, such that after any given element is driven by an addressing pulse of finite duration to present its first or second display state, the state will persist for at least several times, for example, at least four times, the minimum duration of the addressing pulse required to change the state of the display element. U.S. Patent No. 7,170,670 indicates that some particle-based electrophoretic displays supporting grayscale are stable not only in their extreme black-and-white states but also in intermediate gray states, as are some other types of electro-optical displays. This type of display is properly referred to as “multistable” rather than bistable, but for convenience, the term “bistable” may be used herein to encompass both bistable and multistable displays.

The term "impulse" is used in this document in its conventional sense, namely the integral of voltage with respect to time. However, some bistable electro-optic media act as charge sensors, and for such media, an alternative definition of impulse can be used, namely the integral of current over time (equal to the total applied charge). The appropriate definition of impulse should be used depending on whether the medium acts as a voltage-time impulse sensor or a charge impulse sensor.

The majority of the following discussion will focus on methods for driving one or more pixels of an electro-optic display through a transition from an initial gray level to a final gray level (which may be different from or the same as the initial gray level). The term "waveform" will be used to describe the overall voltage versus time curve used to achieve the transition from a particular initial gray level to a particular final gray level. Typically, such a waveform will consist of multiple waveform elements; wherein these elements are substantially rectangular (i.e., wherein a given element comprises a fixed voltage applied over a period of time); these elements may be referred to as "pulses" or "drive pulses." The term "drive scheme" refers to a set of waveforms sufficient to achieve all possible transitions between gray levels of a particular display. A display may use more than one drive scheme; for example, as taught in the aforementioned U.S. Patent No. 7,012,600, it may be necessary to modify the drive scheme based on parameters such as the temperature of the display or the time the display has been operating during its lifetime, and thus multiple different drive schemes may be provided for the display to be used at different temperatures, etc. A set of drive schemes used in this way may be referred to as a "set of related drive schemes." As described in the aforementioned MEDEOD applications, more than one driving scheme can be used simultaneously in different areas of the same display, and a set of driving schemes used in this way can be referred to as a "set of synchronized driving schemes".

Several types of electro-optic displays are known. One type of electro-optic display is the rotating bicolor component type, as described, for example, in U.S. Patents 5,808,783, 5,777,782, 5,760,761, 6,054,071, 6,055,091, 6,097,531, 6,128,124, 6,137,467, and 6,147,791 (although this type of display is often referred to as a "rotating bicolor sphere" display, the term "rotating bicolor component" is preferred as it is less precise in some of the aforementioned patents). This display uses a large number of small bodies (typically spherical or cylindrical) and internal dipoles, each body having two or more portions with different optical characteristics. These bodies are suspended within liquid-filled bubble chambers located within a matrix, allowing the bodies to rotate freely. The appearance of a display can be altered by applying an electric field to the display, thereby rotating the subject to various positions and changing which part of the subject is seen through the viewing surface. This type of electro-optic medium is typically bistable.

Another type of electro-optic display uses electrochromic media, such as nanochromic films, which comprise electrodes at least partially formed of semiconductor metal oxides and a plurality of dye molecules reversibly capable of color change attached to the electrodes; see, for example, Nature 1991, 353, 737 by O’Regan, B. et al.; and Wood, D., Information Display, 18(3), 24 (March 2002). See also Bach, U. et al., Adv. Mater., 2002, 14(11), 845. This type of nanochromic film has also been described, for example, in U.S. Patents 6,301,038, 6,870,657, and 6,950,220. This type of medium is also typically bistable.

Another type of electro-optic display is the electrowetting display, which was developed by Philips and described in Hayes, R.A. et al., “Video-Speed Electronic Paper Based on Electrowetting”, Nature, 425, 383-385 (2003). U.S. Patent No. 7,420,549 indicates that this electrowetting display can be fabricated to be bistable.

One type of electro-optic display that has been intensively researched and developed for many years 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 (LCDs), electrophoretic displays can possess the following properties: good brightness and contrast, wide viewing angle, state bistability, and low power consumption. However, long-term image quality issues have hindered their widespread use. For example, the particles constituting an electrophoretic display are prone to settling, leading to a short lifespan for these displays.

As mentioned above, the electrophoretic medium requires the presence of a fluid. In most prior art electrophoretic media, this fluid is a liquid, but electrophoretic media can be manufactured using a gaseous fluid; see, for example, Kitamura, T. et al., “Electricaltoner 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. Patents 7,321,459 and 7,236,291. When such gas-based electrophoretic media are used in a direction that allows particle sedimentation, such as in a sign where the medium is positioned in a vertical plane, they are susceptible to the same type of problems caused by particle sedimentation as liquid-based electrophoretic media. In fact, particle sedimentation is more severe in gas-based electrophoresis media than in liquid-based electrophoresis media because the lower viscosity of gaseous suspensions allows electrophoretic particles to settle more quickly compared to liquid suspensions.

Numerous patents and applications transferred to or registered with MIT and Einkel describe various techniques for encapsulating electrophoretic and other electro-optic media. Such encapsulated media comprise a plurality of small capsules, each capsule comprising an inner phase and a capsule wall surrounding the inner phase, which contains particles capable of electrophoretic movement in a fluid medium. Typically, the capsules themselves are held within a polymer binder to form a coherent layer located between two electrodes. Techniques described in these patents and applications include:

(a) Electrophoretic particles, fluids, and fluid additives; see, for example, U.S. Patent Nos. 7,002,728 and 7,679,814;

(b) Encapsulation, adhesives, and encapsulation processes; see, for example, U.S. Patent Nos. 6,922,276 and 7,411,719;

(c) Microunit structures, wall materials, and methods of forming microunits; see, for example, U.S. Patent Nos. 7,072,095 and 9,279,906;

(d) Methods for filling and sealing microcells; see, for example, U.S. Patent Nos. 7,144,942 and 7,715,088;

(e) Thin films and subassemblies containing electro-optic materials; see, for example, U.S. Patent Nos. 6,982,178 and 7,839,564;

(f) Backplanes, adhesive layers and other auxiliary layers for use in displays, and methods thereof; see, for example, U.S. Patent Nos. 7,116,318 and 7,535,624;

(g) Color formation and color adjustment; see, for example, U.S. Patent Nos. 7,075,502 and 7,839,564;

(h) Applications of displays; see, for example, U.S. Patent Nos. 7,312,784 and 8,009,348;

(i) Non-electrophoretic displays, as described in U.S. Patent No. 6,241,921 and U.S. Patent Application Publication No. 2015/0277160; and applications of packaging and microcell technologies other than displays; see, for example, U.S. Patent Application Publications Nos. 2015/0005720 and 2016/0012710; and

(j) A method for driving a display; see, for example, U.S. Patent Nos. 5,930,026; 6,445,489; 6,504,524; 6,512,354; 6,531,997; 6,753,999; 6,825,970; 6,900,851; 6,995,550; 7,012,600; 7,023,420; 7,034,783; 7,061,166; 7,061,662; 7,116,466; 7,119,772; 7,177,066; 7,193,625; 7,202,847; 7,242. 514；7,259,744；7,304,787；7,312,794；7,327,511；7,408,699；7,453,445；7,492,339；7,528,822；7,545,358；7,583,251；7,602,374；7,612,760；7,679,599；7,679,813；7,683,606；7,688,297；7,729,039；7,733,311；7,733,335；7,787,169；7,859,742；7,952, 557；7,956,841；7,982,479；7,999,787；8,077,141；8,125,501；8,139,050；8,174,490；8,243,013；8,274,472；8,289,250；8,300,006；8,305,341；8,314,784；8,373,649；8,384,658；8,456,414；8,462,102；8,537,105；8,558,783；8,558,785；8,558,786；8,558 855；8,576,164；8,576,259；8,593,396；8,605,032；8,643,595；8,665,206；8,681,191；8,730,153；8,810,525；8,928,562；8,928,641；8,976,444；9,013,394；9,019,197；9,019,198；9,019,318；9,082,352；9,171,508；9,218,773；9,224,338；9,224,342；9,224 ,344; 9,230,492; 9,251,736; 9,262,973; 9,269,311; 9,299,294; 9,373,289; 9,390,066; 9,390,661; and 9,412,314; and U.S. Patent Application Publication Nos. 2003/0102858; 2004/0246562; 2005/0253777; 2007/0070032; 2007/0076289; 2007/0091418; 2007/0103427; 2007/0176912; 2007/0 296452; 2008/0024429; 2008/0024482; 2008/0136774; 2008/0169821; 2008/0218471; 2008/0291129; 2008/0303780; 2009/0174651; 2009/0195568; 2009/0322721; 2010/0194733; 2010/0194789; 2010/0220121; 2010/0265561; 2010/0283804; 2011/0063314; 2011/ 0175875; 2011/0193840; 2011/0193841; 2011/0199671; 2011/0221740; 2012/0001957; 2012/0098740; 2013/0063333; 2013/0194250; 2013/0249782; 2013/0321278; 2014/0009817; 2014/0085355; 2014/0204012; 2014/0218277; 2014/0240210; 2014/0240373; 2014 /0253425; 2014/0292830; 2014/0293398; 2014/0333685; 2014/0340734; 2015/0070744; 2015/0097877; 2015/0109283; 2015/0213749; 2015/0213765; 2015/0221257; 2015/0262255; 2016/0071465; 2016/0078820; 2016/0093253; 2016/0140910; and 2016/0180777.

Many of the aforementioned patents and applications recognize that the walls surrounding discrete microcapsules in an encapsulated electrophoretic medium can be replaced by a continuous phase, thereby producing so-called polymer-dispersed electrophoretic displays, wherein the electrophoretic medium comprises a plurality of discrete droplets of electrophoretic fluid and a continuous phase of polymeric material, and the discrete droplets of electrophoretic fluid within such a polymer-dispersed electrophoretic display can be considered as capsules or microcapsules, even without a discrete capsule membrane associated with each individual droplet; see, for example, the aforementioned No. 2002/0131147. Therefore, for the purposes of this application, such polymer-dispersed electrophoretic media are considered a subclass of encapsulated electrophoretic media.

One related type of electrophoretic display is the so-called "microcell electrophoretic display." In a microcell electrophoretic display, charged particles and suspended fluid are not encapsulated within microcapsules, but are held within multiple chambers formed in a carrier medium (e.g., a polymer membrane). See, for example, International Application Publication No. WO 02/01281 and published U.S. Patent Application No. 2002/0075556, both assigned to Sipix Imaging, Inc.

Many of the aforementioned patents and applications from ENKOOL and MIT also consider microcell electrophoretic displays and polymer dispersion electrophoretic displays. The term "encapsulated electrophoretic display" can refer to all such display types, and they can also be collectively referred to as "microcavity electrophoretic displays" to encompass the morphology of the walls.

Another type of electro-optic display is the electrowetting display, developed by Philips and described in Hayes, R.A. et al., “Video-Speed Electronic Paper Based on Electrowetting,” Nature, 425, 383-385 (2003). Co-pending application No. 10/711,802, filed October 6, 2004, shows that this electrowetting display can be fabricated to be bistable.

Other types of electro-optic materials can also be used. Of particular interest are bistable ferroelectric liquid crystal displays (FLCs), which are known in the art and have exhibited residual voltage behavior.

Although the electrophoretic medium can be opaque (since, for example, in many electrophoretic media, particles essentially block visible light from passing through the display) and operate in reflective mode, some electrophoretic displays can be manufactured to operate in a so-called "shutter mode," in which one display state is substantially opaque and the other is translucent. See, for example, U.S. Patents 6,130,774 and 6,172,798 and U.S. Patents 5,872,552, 6,144,361, 6,271,823, 6,225,971, and 6,184,856. Dielectrophoretic displays, similar to electrophoretic displays but relying on changes in electric field intensity, can operate in a similar mode; see U.S. Patent 4,418,346. Other types of electro-optic displays are also capable of operating in shutter mode.

High-resolution displays can include individual pixels that are addressable and unaffected by interference from neighboring pixels. One method to obtain such pixels is to provide an array of nonlinear elements, such as transistors or diodes, wherein at least one nonlinear element is associated with each pixel to produce an "active matrix" display. The addressing electrode, or pixel electrode, that addresses a pixel is connected to a suitable voltage source via the associated nonlinear element. When the nonlinear element is a transistor, the pixel electrode may be connected to the drain of the transistor, and this arrangement will be assumed in the following detailed description, although it is essentially arbitrary, and the pixel electrode may be connected to the source of the transistor. In a high-resolution array, pixels can be arranged in a two-dimensional array of rows and columns, such 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 may be connected to a single column electrode, and the gates of all transistors in each row may be connected to a single row electrode; similarly, the source-to-row and gate-to-column assignments can be reversed if desired.

The display can be written line by line. Row electrodes are connected to row drivers, which apply voltage to selected row electrodes to ensure all transistors in the selected row are turned on, while applying voltage to all other rows to ensure all transistors in these unselected rows remain off. Column electrodes are connected to column drivers, which apply selected voltages to individual column electrodes to drive the pixels in the selected row to their desired optical state. (The aforementioned voltages are relative to a common front electrode, which may be positioned on the side of the electro-optic medium opposite to the nonlinear array and extend across the entire display. As is known in the art, voltage is relative and is a measure of the charge difference between two points. One voltage value is relative to another. For example, zero voltage (“0V”) means there is no voltage difference relative to another voltage.) After a pre-selection interval known as the “line addressing time,” the selected row is deselected, another row is selected, and the voltage on the column drivers is changed to write the next line of the display.

Exemplary EPD

Figure 1 illustrates a schematic diagram of a pixel 100 of an electrophoretic display or EPD according to the subject matter presented herein. Pixel 100 may include an imaging film 110. In some embodiments, the imaging film 110 may be bistable. In some embodiments, the imaging film 110 may include, but is not limited to, an encapsulated electrophoretic imaging film that may include, for example, charged pigment particles.

An imaging film 110 may be disposed between the front electrode 102 and the rear electrode 104. The front electrode 102 may be formed between the imaging film and the front of the display. In some embodiments, the front electrode 102 may be transparent. In some embodiments, the front electrode 102 may be formed of any suitable transparent material, including but not limited to indium tin oxide (ITO). The rear electrode 104 may be formed opposite to the front electrode 102. In some embodiments, a parasitic capacitance (not shown) may be formed between the front electrode 102 and the rear electrode 104.

Pixel 100 can be one of a plurality of pixels. The plurality of pixels can be arranged in a two-dimensional array of rows and columns to form a matrix, such that any particular pixel is uniquely defined by the intersection of a specified row and a specified column. In some embodiments, the pixel matrix can be an “active matrix” in which each pixel is associated with at least one nonlinear circuit element 120. The nonlinear circuit element 120 can be coupled between a backplane electrode 104 and an address electrode 108. In some embodiments, the nonlinear element 120 can include diodes and/or transistors, including but not limited to MOSFETs. The drain (or source) of the MOSFET can be coupled to the backplane electrode 104, the source (or drain) of the MOSFET can be coupled to the address electrode 108, and the gate of the MOSFET can be coupled to a driver electrode 106 configured to control the activation and deactivation of the MOSFET. (For simplicity, the terminal in the MOSFET coupled to the backplane electrode 104 will be referred to as the drain of the MOSFET, and the terminal in the MOSFET coupled to the address electrode 108 will be referred to as the source of the MOSFET. However, those skilled in the art will recognize that in some embodiments, the source and drain of the MOSFET can be interchanged.)

In some embodiments of the active matrix, the addressing electrodes 108 of all pixels in each column may be connected to the same column electrode, and the driver electrodes 106 of all pixels in each row may be connected to the same row electrode. The row electrode may be connected to a row driver, which can select one or more rows of pixels by applying a voltage sufficient to activate a nonlinear element 120 of all pixels 100 in the selected row to the selected row electrode. The column electrode may be connected to a column driver, which can apply a voltage suitable for driving the pixel to a desired optical state to the addressing electrode 106 of the selected (activated) pixel. The voltage applied to the addressing electrode 108 may be related to the voltage applied to the front plane electrode 102 of the pixel (e.g., approximately 0 volts). In some embodiments, the front plane electrode 102 of all pixels in the active matrix may be coupled to a common electrode.

In some embodiments, the pixels 100 of the active matrix can be written row by row. For example, a row of pixels can be selected by a row driver, and a voltage corresponding to the desired optical state of that row of pixels can be applied to the pixels by a column driver. After a pre-selection interval known as “line addressing time”, the selected row can be deselected, another row can be selected, and the voltage on the column driver can be changed to write another line of the display.

Figure 2 illustrates a circuit model of an electro-optic imaging layer 110 disposed between a front electrode 102 and a rear electrode 104 according to the subject matter presented herein. Resistors 202 and 204 may represent the resistance and capacitance of the electro-optic imaging layer 110, the front electrode 102, and the rear electrode 104 (including any adhesive layers). Resistors 212 and 214 may represent the resistance and capacitance of the laminated adhesive layer. Capacitor 216 may represent the capacitance that may be formed between the front electrode 102 and the rear electrode 104, for example, at the interface contact region between layers, such as between the imaging layer and the laminated adhesive layer and/or between the laminated adhesive layer and the backplane electrode. The voltage Vi across the imaging film 110 of a pixel may include the residual voltage of the pixel.

It has been observed that the performance of electrophoretic displays can vary depending on environmental conditions. For example, the impedance variation of the front planar laminate (“FPL”) may be related to temperature fluctuations. Therefore, impedance measurements, such as the impedance of the front planar laminate, can be used to understand the operating characteristics of the electrophoretic display and the display ink system.

Figure 3 is a graph 300 illustrating the impedance versus temperature curve 305 of an exemplary front planar laminate. Specifically, graph 300 shows the FPL impedance _Zreal on the Y-axis at 10 Hz versus the temperature on the x-axis in ^degrees Celsius. As shown in graph 300, the FPL impedance decreases with increasing temperature. It should be understood that the FPL referred to herein may include, but is not limited to, the light-transmitting conductive layer, electro-optic dielectric layer, and adhesive layer of an electrophoretic display. In some embodiments, this impedance measurement information, instead of temperature measurements, can be used for waveform selection to optimize display performance.

Figure 4A shows two graphs of exemplary impedance measurements performed on three exemplary display modules at different waveform frequencies at 28°C, wherein two of the modules, modules 1 and 3, have very similar impedances, while module 2 shows a different impedance than the other two modules.

Figure 4B is a graph showing the ghosting performance during different grayscale transitions, for example, a 4-bit, 16-grayscale display supporting 1 (black) to 16 (white) grayscale levels, using the same drive waveform used in Figure 4A at 28°C for the transition (e.g., {GT1,GT2,…,GT15}→{GT1,GT2,…,GT16}). The x-axis of the graph represents different grayscale transitions. As shown, the ghosting performance of module 1 is very similar to that of module 3, while module 2 has very different ghosting performance. The ghosting results are related to the impedance measurements shown in Figure 4A. Therefore, it can be inferred that impedance measurements can be used as a measure of optical performance, and modules with similar impedances behave similarly optically and can therefore be driven similarly.

Figure 5 shows a schematic diagram of the electrophoretic display 500. In this embodiment, the controller 502 outputs synchronized line voltages for the source 506 and gate 508 to sequentially scan the display pixels. During each update, a DC voltage can be supplied to the top plane 510 via the Vcom 504 line.

In practice, it may be necessary to use external instruments to access the pixel electrodes of an active matrix display in order to directly measure electrical characteristics by applying a signal. Depending on the signal used as input, the measured electrical characteristics may include, but are not limited to, current, resistance, charge, capacitance, time constant, phase shift, amplitude, and peak frequency.

Referring now to Figure 6, one method for measuring electrical characteristics is to update the active matrix display 600 using a known waveform while simultaneously measuring the electrical response of the ink layer 602 via a common electrode. In this method, the voltage applied to all pixel electrodes is the same during each frame. Therefore, at the end of each frame, a uniform electric field can be formed on the ink within the updated display area. Thus, the electrical response measured via the common electrode can reflect the average electrical characteristics of the updated area. For example, the pixels of the active matrix backplane can be placed at a non-zero voltage while measuring the current through the Vcom 604 line. The current transients during and after the voltage pulse on the backplane can reveal the regional average electrical characteristics of the display 600, such as characteristics related to sheet resistance and sheet capacitance. In this configuration, since the waveform is applied to the pixels by the source and gate drivers in the controller, this method can be used when the active matrix display 400 is operating in normal scanning mode.

As shown in Figure 6, a current measurement circuit 606 can be inserted between the Vcom line 604 and the top plane electrode 608. In some embodiments, this current measurement circuit may include a small resistor (e.g., approximately 500 ohms) and a differential voltage amplifier, wherein the input line of the differential voltage amplifier is connected across the resistor. In this configuration, the resistor's resistance can be small enough not to significantly affect the operation of the display 600, but large enough to provide a voltage signal proportional to the current flowing through the Vcom line 604. In operation, as the display 600 updates, waveforms and images are selected to apply the same time-dependent voltage to all pixels. This time-dependent voltage can be a simple voltage pulse, for example, a uniform voltage applied over a finite length of time. In another embodiment, the time-dependent voltage may include two consecutive pulses with opposite amplitudes. The current as a function of time can be measured over the entire display addressing event. The current can be averaged over a time period covering a portion of the portion following each pulse. This value is approximately proportional to the effective resistance or impedance of the display film. This effective impedance may be related to the electro-optic behavior of the display film and can therefore be used to select an appropriate waveform for updates during standard display operation. This measurement method provides the flexibility to perform selective or localized measurements without affecting the normal scanning operation of the display.

In another embodiment, a subset of display pixels can be driven while measuring the current through the Vcom 604 line. In this configuration, although current transients provide an average electrical characteristic of the display area, averaging is performed only over the area of driven pixels. In this case, "driven" is defined as: driving pixels receiving a non-zero voltage, while the remaining pixels are not driven, i.e., the non-driven pixels receive a zero voltage bias. In this way, the electrical characteristics of only the left half of the display 600 can be measured by applying a non-zero voltage to only the pixels in the left half of the display and measuring the current through the Vcom 604 line. Of course, this example can be extended to measure the electrical characteristics of that area of the display by selecting only pixels in any sub-area of the display to receive a non-zero voltage. This configuration can also be extended to driving a set of discontinuous display pixels.

In operation, various waveforms can be used to perform electrical measurements on specific areas. This document defines a “waveform” as a list of voltages used to achieve a transition from one grayscale level to the same or different grayscale levels. In some embodiments, the appropriate waveform to be applied to that area of the display can be selected using electrical characteristics measured in a certain area of the display as input. This can be done on multiple areas of the display, and various waveforms can be selected to achieve the desired transition in different areas of the display. For example, a complete waveform file can contain many temperature-appropriate waveforms designed to achieve the desired transition over specific temperature bands, one waveform per temperature band. In some embodiments, the appropriate waveform for temperature measurement can be selected using input from a temperature sensor. For example, the controller can use both temperature and area-specific electrical information as input. The area-specific electrical information can be used to provide information about the temperature difference between different parts of the display, and the waveform applied to different parts of the display can be changed based on the electrical measurements. When electrical measurements indicate a temperature difference between the left and right halves of the display, this information, based on a temperature difference estimate inferred from the region-specific electrical measurements, can be used to select two different waveforms, one for the left half and one for the right half. This concept can also be extended to select waveforms solely based on electrical measurements, meaning a temperature sensor is not required.

In one embodiment, the source driver 610 can be used to drive the "driven pixel" at a sufficiently low voltage so that electrical measurements do not significantly interfere with the displayed image. In another embodiment, electrical measurements can be performed using very short voltage pulses so that the displayed image is not significantly disturbed during the electrical measurements. In yet another embodiment, the seam between two adjacent regions to which two different waveforms are applied may be "blurred" by a dithering mask between the two regions, where the dithering pattern determines which pixels receive which of the two waveforms. Dithering can be configured to provide gradients from a region where all pixels receive one waveform and adjacent regions where all pixels receive the second waveform, where local pixel portions receiving each of the two waveforms transition smoothly within the dithered region.

Therefore, when a display has a significant temperature difference on its surface, and when applying a single waveform across the entire display would produce poor performance, the driving method or scheme described herein can selectively apply various waveforms to different parts of the display surface to compensate for temperature variations on the display surface.

In another approach, now referring to Figure 7, the active matrix display 700 can be updated using an external source via a common electrode, and the electrical response of the ink 702 can be measured on the same Vcom line 704. A known voltage signal can be injected via the common electrode, while all pixel electrodes are connected to a known voltage, such as ground. To close the circuit loop, all TFTs of the active matrix are simultaneously turned on. In this configuration, all display electrodes can be considered as a single electrode covering the entire display area. This method offers greater flexibility because any signal can be applied to the common electrode without being limited by the requirements of the scanning operation. The electrical characteristics of the electrophoretic layer can be inferred from the output of the signal generator 706 and the measured electrical response.

In use, for example, a low-amplitude voltage can be applied to the top plane 708 via the signal generator 706 shown in Figure 7. The amplitude of this voltage can be low enough that its interference with the optical state of the display is not noticeable to an unsuspecting observer, yet large enough to allow the current measurement to have a sufficient signal-to-noise ratio to provide reliable information. In some embodiments, a voltage in the range of 10-100 mV can be used. In another embodiment, an oscillating voltage, such as a 50 mV, 1 Hz square wave, or sine wave, can be applied for a finite time. The in-phase portion of the current can be measured to give a value related to the effective resistance of the display. This effective resistance may be related to the electro-optical behavior of the display film, and by doing so, an appropriate waveform can be selected for updates during standard display operation. For example, if sine waves of different frequencies are applied to the top plane 708, the impedance of the display 700 can be calculated from the electrical response at each frequency.

Figure 8 shows a set of impedance measurements from an exemplary active matrix display module using the method described herein. The setup described herein provides flexibility in what type of input signal can be used and what electrical characteristics can be measured, and allows for easy measurement of the entire display module.

Referring now to Figure 9, in one embodiment, impedance measurements can be performed across the entire active matrix display area. In this configuration, the VCOM line of the display, which typically provides a common bias voltage to the front plane or top electrode of the display and the transistor array of the display, can be split into two lines. One line is VCOM_TFT 916, which provides a bias voltage to the active matrix of the display's transistor network or thin-film transistor (TFT), typically by providing a bias voltage to the terminals of the storage capacitors of each pixel opposite to the terminals connected to the pixel electrodes. The other line is VCOM_FPL 912, which provides a bias voltage to the front plane or top electrode of the display.

As shown in Figure 9, this setup or configuration may include: an active matrix display module 901, which includes a display controller circuit that may consist of an active matrix display and source/gate drivers; an EPD controller 902; a power management integrated circuit (PMIC) 903 that controls the supply of high (i.e., VGH) gate voltage and low (i.e., VGL) gate voltage, source voltages (VPOS and VNEG), two VCOM voltages, and signals (e.g., XON signals) that enable all transistors of the active matrix; a host processing unit 904; a temperature sensor 905; and circuitry 906 for performing impedance measurements. This circuitry can be configured to perform measurements at a single frequency or scan different frequencies, and includes one or more switches 907, 908 for normal operation when off and for impedance measurement when on.

In some embodiments, to initiate impedance measurement, the gate high voltage or VGH is first set to a voltage between 2 volts and 10 volts. A signal can then be sent to turn on all transistors in the active matrix. This signal can be a signal known in the art for performing such tasks, such as the XON signal. The XON signal can be enabled from the GPIO of the PMIC 903 or EPD controller 902. The impedance measurement circuit 906 can then be enabled by setting a signal (e.g., IMP-EN 910) to an appropriate voltage level. Once the impedance measurement circuit 906 is enabled, switches 907 and 908 can be set to the ON state, thereby isolating the VCOM_FPL 912 line from the active matrix module 901. A sinusoidal voltage signal of a given frequency V can be sent to the VCOM_FPL line 912, thereby allowing access to the top electrode of the display 901. The amplitude of this signal can be set small (e.g., less than 1 volt) so that it does not excite nonlinearity in the ink system. Furthermore, because the voltage amplitude of the signal is small, the optical effects are minimal when impedance measurement is active and are generally not noticeable to an observer.

In some embodiments, when both the XON signal and the VGH voltage are on, access to all pixelated electrodes can be achieved via ground line GND 914. Furthermore, the current drawn on the VCOM_FPL line 912 can be measured in impedance circuit 906. Voltage and current can be analyzed, and electrical characteristics, such as phase shift, can be obtained. In practice, impedance measurements may be frequency-dependent (f [Hz]). For example, it may be necessary to collect 5 to 10 cycles of measurement data before data analysis can be performed. Therefore, the minimum required measurement time is 5/f [s], at least 5 s at 1 Hz, and only 0.5 s at 10 Hz.

Referring now to the signal diagram shown in Figure 10, after the active update 1002, the IMP-EN switch and the gate high voltage VGH can be turned on. Furthermore, impedance measurements can be performed during the VGH conduction duration 1004. It should be understood that impedance measurements can be performed using the method described herein when the display is idle, in which case a full frequency scan can be performed.

In yet another embodiment, impedance measurements can be obtained from unused areas of the module, such as the boundary region surrounding the outer periphery of the pixel array. In this case, referring now to FIG11, direct access to the bottom electrode of the boundary region can be achieved via the boundary electrode line (e.g., BORDER line 1130). In some embodiments, activating a high gate voltage and the XON signal may not be necessary, and measurements can be performed at any time. As shown in FIG11, XON signal accessibility is no longer required, and measurements can be performed across the BORDER 1130 line and the VCOM_FPL 1112 line.

In another embodiment, a dedicated region can be designed into the active matrix module for impedance measurement purposes.

Based on the topic presented in this paper, using impedance measurements instead of temperature sensor measurements for waveform selection has the advantage of achieving better performance on the display. This is at least in part because impedance is a direct measurement of the ink system, while a separate temperature sensor in the device can only simulate the temperatures experienced by the ink system. Impedance measurements can be used to quantify the aging of the display module. This information can be used to assist in selecting an appropriate waveform to load in order to compensate for module aging. Through a waveform database bound to different impedance data, for a given time and module, the best waveform that best matches the device-level impedance information can be selected.

It will be apparent to those skilled in the art that many changes and modifications can be made to the specific embodiments of the present invention described above without departing from the scope of the invention. Therefore, the entire description above should be interpreted in an illustrative rather than restrictive sense.

Claims (6)

1. A method for driving an electro-optic display, the electro-optic display comprising an electrophoretic display medium disposed between a first common electrode and a pixel electrode array, wherein each pixel electrode is coupled to a first terminal of a storage capacitor, wherein a second terminal of each storage capacitor is coupled to a second common electrode, and wherein a display controller circuit is configured to apply time-dependent voltages to the first common electrode and the second common electrode, which are independent of each other, the driving method comprising: Simultaneously activate the pixel transistors associated with the first part of the pixel electrode; Switch the first switch to disconnect the first common electrode from the display controller circuit and connect the first common electrode to the first terminal of the impedance measurement circuit; Switch the second switch to disconnect the second common electrode from the display controller circuit and connect the second common electrode to the second terminal of the impedance measurement circuit; A measured waveform voltage from the impedance measurement circuit is injected into the first common electrode. The measured waveform includes a time-dependent voltage and the amplitude of the measured waveform is set to be small so as not to excite nonlinearity in the electro-optic display. The first current flowing through the impedance measurement circuit is measured based on the measured waveform; The first impedance of the electrophoretic display medium near the first portion of the pixel electrode is determined based on the first current flowing through the impedance measurement circuit and the time-related voltage applied to the first common electrode during the measurement waveform. Based on the first impedance of the electrophoretic display medium near the first portion of pixel electrodes, a first driving waveform is selected for each pixel electrode to be applied to the first portion of pixel electrodes; Switch the first switch to disconnect the first common electrode from the impedance measurement circuit and connect the first common electrode to the display controller circuit; Switching the second switch disconnects the second common electrode from the impedance measurement circuit and connects the second common electrode to the display controller circuit; and The first driving waveform is applied to the first portion of the pixel electrode, wherein the first driving waveform includes a time-dependent voltage sufficient to change the optical state of the electro-optic display near the first portion of the pixel electrode. 2. The method according to claim 1, wherein the measured waveform includes a periodic square wave or a sinusoidal voltage waveform. 3. The method of claim 1, wherein the measured waveform includes a voltage with an amplitude insufficient to change the optical state of the electro-optic display near the first portion of the pixel electrode. 4. The method of claim 3, wherein the measured waveform comprises a voltage with an amplitude of less than 1 volt. 5. The method of claim 1, wherein the measured waveform comprises an oscillating voltage waveform having multiple frequencies. 6. The method of claim 1, wherein the first portion of the pixel electrode comprises a pixel electrode located near the outer periphery of the pixel electrode array.