Method for driving an electro-optical display

Abstract

A method for driving an electro-optic display including an electro-optic material disposed between a common electrode and a backplane. The backplane includes an array of pixel electrodes, each pixel electrode coupled to one transistor. The display controller applies a waveform to the pixel electrodes. The method includes applying a first measurement waveform to a first portion of pixel electrodes. During each frame of the first measurement waveform, a same time dependent voltage is applied to each of the first portion of pixel electrodes. The method includes determining an impedance of the electro-optical material proximate the first portion of the pixel electrodes based on a measurement of the current flowing through the current measurement circuit and a time dependent voltage applied to each pixel electrode during the first measurement waveform, and selecting a drive waveform based on the impedance of the electro-optical material proximate the first portion of the pixel electrodes.

Description

[0001] This application is a divisional application of an application with a filing date of December 27, 2022, an application number of 202280084612.6, and an invention name of “Method for driving an electro-optical display”.

[0002] CROSS-REFERENCE TO RELATED APPLICATIONS

[0003] This application claims priority to U.S. Provisional Patent Application No. 63 / 293,947 filed on December 27, 2021 and U.S. Provisional Patent Application No. 63 / 301,747 filed on January 21, 2022. The entire contents of the above-mentioned provisional patent applications are incorporated herein by reference. In addition, the entire contents of any patent, published application or other publication cited herein are incorporated by reference in their entirety. Technical Field

[0004] The present invention relates to a method for measuring electrical characteristics of an electro-optical display. More particularly, the present invention relates to a method for measuring electrical characteristics of an active matrix electrophoretic display module. Background Art

[0005] Electrophoretic display media are generally characterized by the movement of particles by an applied electric field, are highly reflective, can be made bistable, can be expanded to large areas, and consume very little power. Encapsulated electrophoretic displays also enable the displays to be printed. These properties allow encapsulated electrophoretic display media to be used in many applications where conventional electronic displays are not suitable, such as flexible displays.

[0006] One particular application of display screens is as input devices, such as touch screens or keypads or 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 location. A response event or action may then be generated.

[0007] In addition, the electrical properties of the encapsulated electrophoretic display medium may change in response to environmental factors such as temperature and humidity. In some cases, in order to achieve a repeatable optical state in the display, it may be necessary to compensate the drive waveform in response to changes in the electrical properties of the polymer material comprising the encapsulated electrophoretic display medium. Therefore, it is necessary to measure the display parameters that affect the waveform compensation scheme. However, the use of external display sensors may increase the cost of the display and complicate the manufacturing process. In addition, the external sensors may not accurately measure the parameters inside the display. Summary of the invention

[0008] Therefore, it is necessary to efficiently and accurately measure the electrical characteristics of the display and apply a waveform to the display pixels according to the measured electrical characteristics.

[0009] Accordingly, in one aspect, the subject matter presented herein provides a method for driving an electro-optical display, the electro-optical display comprising an electro-optical material layer disposed between a common electrode and a backplane. The backplane comprises an array of pixel electrodes, and each pixel electrode is coupled to a pixel transistor. The 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 transistor. The driving method comprises applying a first measurement waveform comprising one or more frames to a first portion of 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 pixel electrodes. The method also comprises measuring a first current flowing through a current measurement circuit coupled between the common electrode and an output of a display controller circuit that applies a time-dependent voltage to the common electrode, and determining a first impedance of an electro-optical material proximate 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 method also comprises selecting a first drive waveform to be applied to each pixel electrode of the first portion of pixel electrodes based on the first impedance of the electro-optical material proximate to the first portion of pixel electrodes, and applying the first drive waveform to the first portion of pixel electrodes. The first drive waveform includes a time-dependent voltage sufficient to change an optical state of the electro-optic display proximate a first portion of pixel electrodes.

[0010] In some embodiments, the current measurement circuit includes a resistive element and a differential voltage amplifier, wherein a first input of the differential voltage amplifier is connected to a first terminal of the resistive element and a second input of the differential voltage amplifier is connected to a second terminal of the resistive element.

[0011] In some embodiments, the time-dependent voltage applied to each pixel electrode of the first portion of pixel electrodes includes a uniform voltage pulse. In some embodiments, the time-dependent voltage applied to each pixel electrode of the first portion of pixel electrodes includes a first voltage pulse having a first polarity and a second voltage pulse having a second polarity opposite to the first polarity.

[0012] In some embodiments, the first portion of pixel electrodes includes all pixel electrodes of the pixel electrode array. In some embodiments, the first portion of pixel electrodes includes pixel electrodes located near a periphery of the pixel electrode array.

[0013] In some embodiments, the method also includes: applying a second measurement waveform including one or more frames to a second portion of pixel electrodes of the 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 the current measurement circuit; determining a second impedance of an electro-optical 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 drive waveform to be applied to each pixel electrode of the second portion of pixel electrodes based on the second impedance of the electro-optical material near the second portion of pixel electrodes; and applying a second drive waveform to the second portion of pixel electrodes, wherein the second drive waveform includes a time-dependent voltage sufficient to change the optical state of the electro-optical display near the second portion of pixel electrodes.

[0014] In some embodiments, the first portion of pixel electrodes includes pixel electrodes from a first region of the pixel electrode array, and the second portion of pixel electrodes includes pixel electrodes from a second region of the pixel electrode array, and the pixel electrodes of 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 pixel electrodes while applying a first measurement waveform to the first portion of pixel electrodes; and applying a zero volt waveform to the first portion of pixel electrodes while applying a second measurement waveform to the second portion of pixel electrodes.

[0015] On the other hand, the subject matter presented herein provides a method for driving an electro-optical display, the electro-optical display including an electro-optical material layer disposed between a common electrode and a backplane. The backplane includes an array of pixel electrodes, and each pixel electrode is coupled to a pixel transistor. The 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 transistor. The driving method includes simultaneously activating pixel transistors associated with a first portion of pixel electrodes of the pixel electrode array, and applying a first voltage to the first portion of pixel electrodes. The driving method also includes injecting a measurement waveform from a signal generating circuit through a current measuring circuit coupled between the signal generating circuit and the common electrode, and measuring a first current flowing through the current measuring circuit based on the measurement waveform. The driving method also includes determining a first impedance of the electro-optical material near the first portion of pixel electrodes 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 pixel electrodes during the first measurement waveform. The driving method also includes: selecting a first driving waveform to be applied to each pixel electrode of the first portion of pixel electrodes based on a first impedance of the electro-optical material near the first portion of pixel electrodes, and applying the first driving waveform to the first portion of pixel electrodes, wherein the first driving waveform includes a time-dependent voltage sufficient to change the optical state of the electro-optical display near the first portion of pixel electrodes.

[0016] In some embodiments, the current measurement circuit includes a resistive element and a differential voltage amplifier, wherein a first input of the differential voltage amplifier is connected to a first terminal of the resistive element and a second input of the differential voltage amplifier is connected to a second terminal of the resistive element.

[0017] In some embodiments, the measurement waveform comprises a periodic square wave or sinusoidal voltage waveform. In some embodiments, the measurement waveform comprises a voltage having an amplitude insufficient to change the optical state of the electro-optic display. In some embodiments, the measurement waveform comprises an oscillating voltage waveform having multiple frequencies.

[0018] On the other hand, the subject matter presented herein provides a method for driving an electro-optical display, the electro-optical 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. The display controller circuit is configured to apply a time-dependent voltage to the first common electrode and the second common electrode independently of each other. The driving method includes simultaneously activating pixel transistors associated with a first portion of pixel electrodes. The driving method also 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 also includes injecting a measurement waveform voltage from the impedance measurement circuit into the first common electrode, wherein the measurement waveform includes a time-dependent voltage. The driving method also includes measuring a first current flowing through the impedance measurement circuit based on the measurement waveform, and determining a first impedance of an electro-optical material near the first portion of 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 measurement waveform. The driving method also includes selecting a first drive waveform to be applied to each pixel electrode of the first portion of pixel electrodes based on a first impedance of the electro-optical material proximate to the first portion of pixel electrodes. The driving method also includes switching a first switch to disconnect the 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 the second common electrode from the impedance measurement circuit and connect the second common electrode to the display controller circuit. The driving method also includes applying a first drive waveform to the first portion of pixel electrodes, wherein the first drive waveform includes a time-dependent voltage sufficient to change an optical state of the electro-optical display proximate to the first portion of pixel electrodes.

[0019] In some embodiments, the measurement waveform comprises a periodic square wave or a sinusoidal voltage waveform. In some embodiments, the measurement waveform comprises a voltage having an amplitude insufficient to change the optical state of the electro-optical display near the first portion of pixel electrodes. In some embodiments, the measurement waveform comprises a voltage having an amplitude less than 1 volt. In some embodiments, the measurement waveform comprises an oscillating voltage waveform having multiple frequencies.

[0020] In some embodiments, the first portion of pixel electrodes includes pixel electrodes located near a periphery of the pixel electrode array. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 is a circuit diagram representing an electrophoretic display according to the subject matter described herein.

[0022] Figure 2A circuit model of an electro-optical imaging layer according to the subject matter described herein is shown.

[0023] Figure 3 is a graph illustrating a graph of impedance versus temperature for an exemplary front plane laminate or "FPL" according to the subject matter described herein.

[0024] Figure 4A Two graphs are shown of example impedance measurements performed on three example display modules according to the subject matter described herein.

[0025] Figure 4B is a graph illustrating ghosting performance during different grayscale transitions according to the subject matter described herein.

[0026] Figure 5 A schematic diagram is shown of one embodiment of an electrophoretic display according to the subject matter described herein.

[0027] Figure 6 A schematic diagram is shown of one embodiment of an electrophoretic display according to the subject matter described herein.

[0028] Figure 7 A schematic diagram is shown of one embodiment of an electrophoretic display according to the subject matter described herein.

[0029] Figure 8 A set of impedance measurements from an exemplary active-matrix display module using the methods described herein is shown.

[0030] Fig. 9 A schematic diagram is shown of one embodiment of an electrophoretic display according to the subject matter described herein.

[0031] Fig.10 An exemplary signal sequence for implementing the impedance measurements described herein is shown.

[0032] Fig.11 A schematic diagram is shown of one embodiment of an electrophoretic display according to the subject matter described herein. DETAILED DESCRIPTION

[0033] The present invention relates to methods for driving electro-optical displays, particularly bistable electro-optical displays, and apparatus for use in such methods. More particularly, the invention relates to driving methods which can reduce "ghosting" and edge effects and reduce 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 change the appearance of the display.

[0034] The term "electro-optical" as applied to a material or display is used herein in its conventional sense in the field of imaging to refer to a material having first and second display states that differ in at least one optical property, the material being changed from its first display state to its second display state by application of an electric field to the material. While the optical property is typically color perceptible to the human eye, it may also be another optical property such as light transmission, reflection, luminescence, or, in the case of displays intended for machine reading, false color in the sense of a change in reflectivity of electromagnetic wavelengths outside the visible range.

[0035] The term "gray state" is used herein in its conventional meaning in imaging technology to refer to a state between two extreme optical states of a pixel, and does not necessarily mean a black-white transition between the two extreme states. For example, several patents and published applications of Iink Corporation mentioned below describe electrophoretic displays in which the extreme states are white and dark blue, so that the intermediate "gray state" is actually light blue. In fact, as already mentioned, the change in optical state may not be a color change at all. In the following, the terms "black" and "white" may be used to refer to the two extreme optical states of the display, and should be understood to generally include extreme optical states that are not strictly black and not strictly white, for example, the aforementioned white and dark blue states. In the following, the term "monochrome" may be used to represent a drive scheme that drives the pixel only to its two extreme optical states, without a gray state in between.

[0036] Some electro-optic materials are solid in the sense that the material has a solid outer surface, but these materials may and usually do have internal spaces filled with liquid or gas. For convenience, such displays using solid electro-optic materials are referred to below as "solid-state electro-optic displays." Thus, the term "solid-state electro-optic display" includes rotating two-color component displays, packaged electrophoretic displays, microcell electrophoretic displays, and packaged liquid crystal displays.

[0037] The terms "bistable" and "bistability" are used herein in their conventional meaning in the art to refer to a display comprising a display element having first and second display states that differ in at least one optical property and such that after any given element is driven to assume its first or second display state by an addressing pulse of finite duration, that state will persist for a time after termination of the addressing pulse that is at least several times, e.g., 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 shows that some particle-based electrophoretic displays that support 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. This type of display is properly referred to as "multistable" rather than bistable, but for convenience the term "bistable" may be used herein to cover both bistable and multistable displays.

[0038] The term "impulse" is used herein in its conventional sense, i.e., the integral of voltage with respect to time. However, some bistable electro-optical media act as charge sensors, and for such media, an alternative definition of impulse may be used, i.e., the integral of current with respect to time (equal to the total charge applied). The appropriate definition of impulse should be used, depending on whether the media acts as a voltage-time impulse sensor or a charge impulse sensor.

[0039] Much of the following discussion will focus on methods of driving one or more pixels of an electro-optical 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 refer to the overall voltage versus time curve used to achieve a transition from a particular initial gray level to a particular final gray level. Typically, such a waveform will include a plurality of waveform elements; wherein the elements are substantially rectangular (i.e., wherein a given element includes the application of a fixed voltage 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 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 the drive scheme may need to be modified based on parameters such as the temperature of the display or the amount of time the display has been operating during its lifetime, and therefore a plurality of different drive schemes may be provided for the display for use at different temperatures, etc. A set of drive schemes used in this manner may be referred to as a "set of related drive schemes". As described in several of the aforementioned MEDEOD applications, more than one drive scheme may also be used simultaneously in different regions of the same display, and a set of drive schemes used in this manner may be referred to as a "set of synchronized drive schemes."

[0040] Several types of electro-optical displays are known. One type of electro-optical display is a rotating two-color component type, as described in, for example, 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 generally referred to as a "rotating two-color ball" display, the term "rotating two-color component" is preferably more accurate because in some of the above-mentioned patents, the rotating component is not spherical). This display uses a large number of small bodies (usually spherical or cylindrical) and internal dipoles, and the small body has two or more parts with different optical characteristics. These bodies are suspended in a liquid-filled vacuole located in the matrix, and the vacuole is filled with liquid so that the body can rotate freely. The appearance of the display is changed by applying an electric field to the display, thereby rotating the body to various positions and changing which part of the body is seen through the viewing surface. This type of electro-optic medium is usually bistable.

[0041] Another type of electro-optical display uses an electrochromic medium, such as an electrochromic medium in the form of a nanochromic film, which includes an electrode formed at least in part by a semiconductor metal oxide and a plurality of dye molecules attached to the electrode that can reversibly change color; see, for example, O'Regan, B. et al. Nature 1991, 353, 737; 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 is also described in, for example, U.S. Patents Nos. 6,301,038, 6,870,657, and 6,950,220. This type of medium is also generally bistable.

[0042] Another type of electro-optical display is the electrowetting display, developed by Philips and described in Hayes, RA et al., "Video-Speed ​​Electronic Paper Based on Electrowetting", Nature, 425, 383-385 (2003). U.S. Patent No. 7,420,549 shows that such an electrowetting display can be made bistable.

[0043] One type of electro-optical display that has been the subject of intensive research and development for many years is the particle-based electrophoretic display, in which a plurality of charged particles are moved through a fluid under the influence of an electric field. Compared to liquid crystal displays, electrophoretic displays can have the following attributes: good brightness and contrast, wide viewing angle, state bistability, and low power consumption. However, long-term image quality issues with these displays have hindered their widespread use. For example, the particles that make up electrophoretic displays tend to settle, resulting in insufficient useful life for these displays.

[0044] As mentioned above, electrophoretic media require the presence of a fluid. In most prior art electrophoretic media, the fluid is a liquid, but electrophoretic media can be made using gaseous fluids; 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. 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 when used in a signboard where the medium is set in a vertical plane, such gas-based electrophoretic media are susceptible to the same type of problems caused by particle sedimentation as liquid-based electrophoretic media. In fact, the particle sedimentation problem is more severe in gas-based electrophoretic media than in liquid-based electrophoretic media because the lower viscosity of the gaseous suspending fluid allows for more rapid sedimentation of the electrophoretic particles compared to the liquid suspending fluid.

[0045] Numerous patents and applications assigned to or in the names of MIT and Ink Corp. describe various techniques for encapsulated electrophoretic and other electro-optical media. Such encapsulated media include a plurality of small capsules, each of which itself includes an inner phase containing particles that are electrophoretically mobile in a fluid medium and a capsule wall surrounding the inner phase. Typically, the capsules themselves are held in a polymer binder to form a coherent layer between two electrodes. The techniques described in these patents and applications include:

[0046] (a) electrophoretic particles, fluids and fluid additives; see, for example, U.S. Patent Nos. 7,002,728; and 7,679,814;

[0047] (b) capsules, adhesives and encapsulation processes; see, e.g., U.S. Pat. Nos. 6,922,276; and 7,411,719;

[0048] (c) microcell structures, wall materials, and methods of forming microcells; see, for example, U.S. Patent Nos. 7,072,095 and 9,279,906;

[0049] (d) Methods for filling and sealing microlocations; see, e.g., U.S. Pat. Nos. 7,144,942 and 7,715,088;

[0050] (e) Films and subassemblies containing electro-optical materials; see, e.g., U.S. Patent Nos. 6,982,178 and 7,839,564;

[0051] (f) Backplanes, adhesive layers and other auxiliary layers and methods for use in displays; see, e.g., U.S. Patents 7,116,318; and 7,535,624;

[0052] (g) color formation and color adjustment; see, e.g., U.S. Patent Nos. 7,075,502 and 7,839,564;

[0053] (h) Display applications; see, e.g., U.S. Patent Nos. 7,312,784 and 8,009,348;

[0054] (i) non-electrophoretic displays, such 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 technology other than displays; see, for example, U.S. Patent Application Publication Nos. 2015 / 0005720 and 2016 / 0012710; and

[0055] (j) Methods for driving displays; see, e.g., 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; 7,312,7 94; 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; 8,139,0 50; 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; 8,810,5 25; 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 No. 2003 / 0102858;2004 / 0246562; 2005 / 0253777; 2007 / 0070032; 2007 / 0076289; 2007 / 0091418; 2007 / 0103427; 2007 / 0176912; 2007 / 0296452; 2008 / 0024429; 2008 / 0024482; 2008 / 0136774; 2008 / 0169821; 2008 / 0218471; 2008 / 0291129; 2008 / 03 03780; 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; 2 012 / 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 / 029 3398; 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. ;

[0056] Many of the aforementioned patents and applications recognize that the walls surrounding discrete microcapsules in encapsulated electrophoretic media can be replaced by a continuous phase, thereby producing a so-called polymer dispersed electrophoretic display, wherein the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymer material, and the discrete droplets of electrophoretic fluid within such a polymer dispersed electrophoretic display can be considered to be capsules or microcapsules, even though there is no discrete capsule membrane associated with each individual droplet; see, e.g., aforementioned No. 2002 / 0131147. Therefore, for the purposes of this application, such polymer dispersed electrophoretic media are considered to be a subclass of encapsulated electrophoretic media.

[0057] A related type of electrophoretic display is the so-called "microcell electrophoretic display." In a microcell electrophoretic display, the charged particles and the suspending fluid are not encapsulated in microcapsules, but are held in a plurality of chambers formed in a carrier medium (e.g., a polymer film). See, e.g., International Application Publication No. WO 02 / 01281 and Published U.S. Patent Application No. 2002 / 0075556, both assigned to Sipix Imaging.

[0058] Many of the aforementioned patents and applications from Ink and MIT also consider microcell electrophoretic displays and polymer dispersed electrophoretic displays. The term "encapsulated electrophoretic display" may refer to all such display types, which may also be collectively referred to as "microcavity electrophoretic displays" to summarize the morphology of the walls.

[0059] Another type of electro-optical display is the electrowetting display, developed by Philips and described in Hayes, RA et al., "Video-Speed ​​Electronic Paper Based on Electrowetting," Nature, 425, 383-385 (2003). In co-pending application No. 10 / 711,802, filed October 6, 2004, it was shown that such an electrowetting display can be made bistable.

[0060] Other types of electro-optical materials may also be used. Of particular interest, bistable ferroelectric liquid crystal displays (FLC) are known in the art and have demonstrated residual voltage behavior.

[0061] Although electrophoretic media can be opaque (because, for example, the particles in many electrophoretic media substantially block visible light from being transmitted through the display) and operated in a 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 one display state is light-transmissive. 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, which are similar to electrophoretic displays but rely on variations in electric field strength, can operate in a similar mode; see U.S. Patent 4,418,346. Other types of electro-optical displays can also operate in a shutter mode.

[0062] A high resolution display may include individual pixels that are addressable without interference from adjacent pixels. One method of obtaining such pixels is to provide an array of non-linear elements such as transistors or diodes, wherein at least one non-linear element is associated with each pixel to produce an "active matrix" display. An addressing electrode or pixel electrode that addresses a pixel is connected to an appropriate voltage source through an associated non-linear element. When the non-linear 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, the pixels may be arranged in a two-dimensional array of rows and columns, such that any particular pixel is uniquely defined by the intersection of a specified row and a specified 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; again, the assignment of sources to rows and gates to columns may be reversed if desired.

[0063] The display can be written to in a row-by-row manner. The row electrodes are connected to a row driver, which can apply a voltage to the selected row electrode to ensure that all transistors in the selected row are turned on, while applying a voltage to all other rows to ensure that all transistors in these unselected rows remain non-conductive. The column electrodes are connected to a column driver, which applies a selected voltage to each column electrode to drive the pixels in the selected row to their desired optical state. (The foregoing voltages are relative to a common front electrode, which can be disposed on the side of the electro-optical medium opposite the non-linear array and extending 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 voltage value. For example, zero voltage ("0V") refers to 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 driver is changed to write the next line of the display.

[0064] Exemplary EPD

[0065] Figure 1 A schematic diagram of a pixel 100 of an electrophoretic display or EPD according to the subject matter presented herein is shown. Pixel 100 may include an imaging film 110. In some embodiments, imaging film 110 may be bistable. In some embodiments, imaging film 110 may include, but is not limited to, an encapsulated electrophoretic imaging film that may include, for example, charged pigment particles.

[0066] The 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 face 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 the front electrode 102. In some embodiments, a parasitic capacitor (not shown) may be formed between the front electrode 102 and the rear electrode 104.

[0067] Pixel 100 may be one pixel among a plurality of pixels. A plurality of pixels may 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 may be an "active matrix" in which each pixel is associated with at least one nonlinear circuit element 120. Nonlinear circuit element 120 may be coupled between backplate electrode 104 and addressing electrode 108. In some embodiments, nonlinear element 120 may include a diode and / or a transistor, including but not limited to a MOSFET. The drain (or source) of the MOSFET may be coupled to backplate electrode 104, the source (or drain) of the MOSFET may be coupled to addressing electrode 108, and the gate of the MOSFET may be coupled to driver electrode 106, which is configured to control activation and deactivation of the MOSFET. (For simplicity, the terminal of the MOSFET coupled to the backplate electrode 104 will be referred to as the drain of the MOSFET, and the terminal of the MOSFET coupled to the addressing electrode 108 will be referred to as the source of the MOSFET. However, one of ordinary skill in the art will recognize that in some embodiments, the source and drain of the MOSFET may be interchanged.)

[0068] In some embodiments of the active matrix, the addressing electrodes 108 of all pixels in each column can be connected to the same column electrode, and the driver electrodes 106 of all pixels in each row can be connected to the same row electrode. The row electrodes can be connected to a row driver, which can select one or more rows of pixels by applying a voltage sufficient to activate the non-linear elements 120 of all pixels 100 in the selected row to the selected row electrode. The column electrodes can be connected to a column driver, which can apply a voltage suitable for driving the pixel to a desired optical state on the addressing electrodes 106 of the selected (activated) pixels. The voltage applied to the addressing electrodes 108 can be related to the voltage applied to the front plane electrodes 102 of the pixels (e.g., a voltage of approximately 0 volts). In some embodiments, the front plane electrodes 102 of all pixels in the active matrix can be coupled to a common electrode.

[0069] In some embodiments, the pixels 100 of the active matrix can be written in a row-by-row manner. For example, a row of pixels can be selected by a row driver, and a voltage corresponding to the desired optical state of the row of pixels can be applied to the pixels by a column driver. After a pre-selection interval known as the "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.

[0070] Figure 2 A circuit model of an electro-optic imaging layer 110 disposed between a front electrode 102 and a rear electrode 104 in accordance with the subject matter presented herein is shown. Resistor 202 and capacitor 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 layer). Resistor 212 and capacitor 214 may represent the resistance and capacitance of a laminating adhesive layer. Capacitor 216 may represent a capacitance that may be formed between the front electrode 102 and the rear electrode 104, for example, at an interfacial contact area between the layers, for example, between the imaging layer and the laminating adhesive layer and / or between the laminating adhesive layer and the backplane electrode. The voltage Vi across the imaging film 110 of a pixel may include a residual voltage of the pixel.

[0071] It has been observed that the performance of an electrophoretic display can vary depending on environmental conditions. For example, changes in the impedance of a front plane laminate ("FPL") can be correlated to temperature fluctuations. Therefore, impedance measurements, such as those of the front plane laminate, can be used to understand the operating characteristics of an electrophoretic display and a display ink system.

[0072] Figure 3 3 is a graph 300 showing a graph 305 of impedance versus temperature for an exemplary front plane laminate. Specifically, the graph 300 shows the impedance at 10 Hz in MΩ cm on the Y-axis. 2 The FPL impedance Z is in units of real 305 is a graph of temperature in degrees Celsius on the x-axis. As shown in graph 300, the FPL impedance decreases as the temperature increases. It should be understood that the FPL referred to herein may include, but is not limited to, a light-transmitting conductive layer, an electro-optical dielectric layer, and an adhesive layer of an electrophoretic display. In some embodiments, the impedance measurement information may be used instead of the temperature measurement value for waveform selection to optimize display performance.

[0073] Figure 4A Two graphs showing exemplary impedance measurements performed at different waveform frequencies on three exemplary display modules at 28°C, wherein two of the modules, modules 1 and 3, have very similar impedances, while module 2 exhibits a different impedance than the other two modules.

[0074] Figure 4Bis a graph showing ghosting performance during different grayscale transitions, for example, a display supporting 4-bit, 16 grayscale levels represented from 1 (black) to 16 (white) using the same Figure 4A The same driving waveform used in the figure at 28°C is used for the transition (e.g., {GT1, GT2, …, GT15}  {GT1, GT2, …, GT16}). The x-axis of the graph represents the different grayscale transitions. As shown in the figure, the ghosting performance of module 1 is very similar to that of module 3, while module 2 has a very different ghosting performance. The ghosting results are similar to Figure 4A Therefore, it can be inferred that the impedance measurement can be used as a measure of optical performance, and that modules with similar impedance behave similarly optically, and can therefore be driven similarly.

[0075] Figure 5 A schematic diagram of an electrophoretic display 500 is shown. In this embodiment, a controller 502 outputs synchronized source 506 and gate 508 line voltages to sequentially scan display pixels. During each update, a DC voltage may be supplied to the top plane 510 via the Vcom 504 line.

[0076] In practice, it may be necessary to use external instrumentation to access the pixel electrodes of an active matrix display to directly measure electrical characteristics by applying a signal. Depending on the signal used as input, the electrical characteristics measured may include, but are not limited to, current, resistance, charge, capacitance, time constant, phase shift, amplitude, and frequency peaks.

[0077] Reference now Figure 6 , one method of measuring electrical characteristics is to update the active matrix display 600 using a known waveform while measuring the electrical response of the ink layer 602 through a common electrode. In this method, the voltage applied to all pixel electrodes during each frame is the same. Therefore, at the end of each frame, a uniform electric field can be formed on the ink within the updated display area. Therefore, the electrical response measured by 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 those related to sheet resistance and sheet capacitance. In this configuration, since the waveforms are 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 operated in normal scanning mode.

[0078] like Figure 6As shown, a current measurement circuit 606 may be inserted between the Vcom line 604 and the top plane electrode 608. In some embodiments, the 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 resistance of the resistor may be small enough so as 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, when the display 600 is updated, the waveform and image are selected so that the same time-dependent voltage is applied to all pixels. The time-dependent voltage may 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 of opposite amplitude. The current as a function of time may be measured throughout the display addressing event. The current over a period of time covering a portion of the rear portion of each pulse may be averaged. This value is approximately proportional to the effective resistance or impedance of the display film. This effective impedance can be related to the electro-optical behavior of the display film and can therefore be used to select an appropriate waveform for updating during standard display operation. This measurement method provides the flexibility to perform selective area or local measurements without affecting the normal scanning operation of the display.

[0079] In another embodiment, a portion of the display pixels can be driven while measuring the current through the Vcom 604 line. In this configuration, although the current transient provides a regional average electrical characteristic of the display, it is averaged only over the area of ​​the driven pixels. In this case, "driving" is defined as: the driven pixels receive a non-zero voltage, while the remaining pixels are not driven, that is, 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 this area of ​​the display by selecting pixels in any sub-area of ​​the display to receive a non-zero voltage. This configuration can also be extended to drive a group of non-contiguous display pixels.

[0080] In operation, various waveforms can be used to perform electrical measurements of specific areas. "Waveform" is defined herein as a set of voltage lists to achieve a transition from a grayscale of one grayscale level to a grayscale of the same or different grayscale levels. In some embodiments, the electrical characteristics measured in a certain area of ​​the display can be used as input to select the appropriate waveform to be applied to the area of ​​the display. This can be done for multiple areas of the display, and various waveforms can be selected to achieve the desired transition in various areas of the display. For example, a complete waveform file can contain many temperature-appropriate waveforms that are designed to achieve the desired transition in a specific temperature band, one waveform for each temperature band. In some embodiments, an input from a temperature sensor can be used to select an appropriate waveform for temperature measurement. For example, the controller can use both temperature and region-specific electrical information as input. Region-specific electrical information can be used to provide information about the temperature difference between various parts of the display, and the waveform applied to various parts of the display can be changed based on electrical measurement values. When the electrical measurements indicate that there is a temperature difference between the left and right halves of the display, this information can be used to select two different waveforms, one for the left half of the display and one for the right half, based on an estimate of the temperature difference inferred from the region-specific electrical measurements. This concept can also be extended to select waveforms based solely on electrical measurements, that is, without the need for a temperature sensor.

[0081] In one embodiment, the source driver 610 can be used to drive the "driven pixels" at a sufficiently low voltage that the electrical measurement does not significantly interfere with the displayed image. In another embodiment, the electrical measurement can be made using very short voltage pulses so that the displayed image is not significantly disturbed during the electrical measurement. In yet another embodiment, the seam between two adjacent areas where two different waveforms are applied may be "blurred" by a dither mask between the two areas, where the dither pattern determines which pixels receive which of the two waveforms. The dithering can be set to provide a gradient from an area where all pixels receive one waveform and an adjacent area where all pixels receive a second waveform, where the local pixel portion receiving each of the two waveforms smoothly transitions within the dithered area.

[0082] Thus, when a display has significant temperature differences across its surface, and applying a single waveform across the entire display would produce poor performance, the drive methods or schemes described herein enable the selection of various waveforms to be applied to various portions of the display surface to compensate for the temperature variations across the display surface.

[0083] In another approach, now refer to Figure 7, the active matrix display 700 can be updated using an external source through 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 through 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 turned on at the same time. In this configuration, all display electrodes can be treated as a single electrode covering the entire display area. This method has greater flexibility because an arbitrary signal can be applied to the common electrode without being restricted 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.

[0084] In use, for example, Figure 7 The signal generator 706 shown applies a low amplitude voltage to the top plane 708. The amplitude of the voltage can be low enough so that its interference with the optical state of the display will not reach a level that is obvious to the casual observer, and it is large enough to allow the current measurement to have a sufficient signal-to-noise ratio, thereby providing reliable information. In some embodiments, a voltage in the range of 10-100 mV can be used here. In another embodiment, an oscillating voltage, such as 50 mV, 1 Hz square wave or sine wave, can be applied for a limited time. The in-phase portion of the current can be measured to give a value related to the effective resistance of the display. Wherein, the effective resistance may be related to the electro-optical behavior of the display film, and by doing so, it can be used to select an appropriate waveform to be updated in 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.

[0085] Figure 8 A set of impedance measurement results from an exemplary active matrix display module using the method described herein is shown.The arrangement described herein provides flexibility in what type of input signals can be used and which electrical characteristics can be measured, and can easily measure an entire display module.

[0086] Reference now Fig. 9 , in one embodiment, impedance measurements can be made for 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 transistor network or active matrix of thin film transistors (TFTs) of the display, typically by providing a bias voltage to the terminal of the storage capacitor of each pixel opposite to the terminal connected to the pixel electrode. The other line is VCOM_FPL 912, which provides a bias voltage to the front plane or top electrode of the display.

[0087] like Fig. 9 As shown, the arrangement or configuration may include: an active matrix display module 901, which includes a display controller circuit that may be composed of an active matrix display and a source / gate driver; an EPD controller 902; a power management integrated circuit (PMIC) 903, which controls the supply of a high (i.e., VGH) gate voltage and a low (i.e., VGL) gate voltage, source voltages (VPOS and VNEG), two VCOM voltages, and a signal (e.g., XON signal) that can turn on all transistors of the active matrix; a host processing unit 904; a temperature sensor 905; and a circuit for performing impedance measurement 906. The circuit may be configured to be able to perform measurements at a single frequency or to scan different frequencies, and includes one or more switches 907, 908 for achieving normal drive when closed and impedance measurement when opened.

[0088] In some embodiments, to start the impedance measurement, the gate high voltage or VGH is first set to a voltage between 2 volts and 10 volts. Subsequently, a signal can be sent to turn on all transistors in the active matrix. The signal can be a signal known in the art for performing such tasks, such as an XON signal. The XON signal can be enabled from the PMIC 903 or the GPIO of the EPD controller 902. Subsequently, the impedance measurement circuit 906 can 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 an open 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 the signal can be set small (e.g., less than 1 volt) so that it does not excite the nonlinearity of the ink system. In addition, because the voltage amplitude of the signal is small, the optical impact is small when the impedance measurement is active and is generally not noticed by the observer.

[0089] In some embodiments, when both the XON signal and the VGH voltage are turned on, access to all pixelated electrodes can be achieved through the ground line GND 914. Moreover, the current drawn on the VCOM_FPL line 912 can be measured in the impedance circuit 906. Therein, the voltage and current can be analyzed and electrical characteristics, such as phase shift, can be obtained. In practice, the impedance measurement may be related to the frequency f [Hz]. For example, it may be necessary to collect 5 to 10 cycles of measurement data before the data can be analyzed. Therefore, the shortest measurement time required is 5 / f [s], which is at least 5s at 1Hz and only 0.5s at 10Hz.

[0090] Reference now Fig.10As shown in the signal diagram, after active update 1002, the IMP-EN switch and the gate high voltage VGH can be turned on. Moreover, during the duration 1004 of VGH being on, an impedance measurement can be performed. It should be understood that the impedance measurement can be performed using the method described herein when the display is idle, in which case a full frequency sweep can be performed.

[0091] In yet another embodiment, impedance measurements may be obtained from unused areas of the module, such as in a border area around the periphery of the pixel array. Fig.11 , direct access to the bottom electrode of the border region can be achieved through the border electrode line (e.g., BORDER line 1130). In some embodiments, it may not be necessary to activate the high gate voltage and XON signal, and measurements can be made at any time. Fig.11 As shown, XON signal accessibility is no longer required and measurements can be made across the BORDER 1130 line and the VCOM_FPL 1112 line.

[0092] In another embodiment, a dedicated area may be designed into the active matrix module for the purpose of impedance measurement.

[0093] According to the subject matter presented herein, waveform selection using impedance measurements rather than using temperature sensor measurements has the advantage of achieving better performance on displays. 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 temperature 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 to compensate for the aging of the module. With a waveform database tied to different impedance data, the best waveform that matches the closest device-level impedance information can be selected for a given time and module.

[0094] It is obvious 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 present invention. Therefore, all contents of the above description should be interpreted in an illustrative rather than a limiting sense.

Claims

1. A method for driving an electro-optic display, the electro-optic display comprising a layer of electro-optic material disposed between a common electrode and a backplane, the backplane comprising an array of pixel electrodes, wherein each pixel electrode is coupled to a pixel transistor, wherein a display controller circuit applies a waveform to the array of pixel electrodes by applying one or more time-dependent voltages between the common electrode and the array of pixel electrodes via the pixel transistors, the driving method comprising: applying a first measurement waveform comprising one or more frames to a first portion of pixel electrodes of the pixel electrode array, wherein during each frame of the first measurement waveform, the same time-dependent voltage is applied to each pixel electrode of the first portion of pixel electrodes; measuring a first current flowing through a current measurement circuit coupled between the common electrode and an output of the display controller circuit, the display controller circuit applying a time-dependent voltage to the common electrode; determining a first impedance of an electro-optic material proximate 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; selecting a first drive waveform to be applied to each pixel electrode of the first portion of pixel electrodes based on the first impedance of the electro-optical material proximate to the first portion of pixel electrodes; as well as The first drive waveform is applied to the first portion of pixel electrodes, wherein the first drive waveform comprises a time-dependent voltage sufficient to change an optical state of the electro-optic display proximate the first portion of pixel electrodes.

2. The method of claim 1 , wherein the current measurement circuit comprises a resistance element and a differential voltage amplifier, wherein a first input of the differential voltage amplifier is connected to a first terminal of the resistance element, and a second input of the differential voltage amplifier is connected to a second terminal of the resistance element. 3 . The method of claim 1 , wherein the time-dependent voltage applied to each pixel electrode of the first portion of pixel electrodes comprises a uniform voltage pulse.

4. The method of claim 1, wherein the time-dependent voltage applied to each pixel electrode of the first portion of pixel electrodes comprises a first voltage pulse having a first polarity and a second voltage pulse having a second polarity opposite to the first polarity. 5 . The method according to claim 1 , wherein the first portion of pixel electrodes includes all pixel electrodes of the pixel electrode array. 6 . The method according to claim 1 , wherein the first portion of pixel electrodes includes pixel electrodes located near a periphery of the pixel electrode array.

7. The method according to claim 1, further comprising: applying a second measurement waveform comprising one or more frames to a second portion of pixel electrodes of the 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 the current measurement circuit; determining a second impedance of electro-optic material proximate to 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 drive waveform to be applied to each pixel electrode of the second portion of pixel electrodes based on the second impedance of the electro-optical material proximate to the second portion of pixel electrodes; as well as The second drive waveform is applied to the second portion of pixel electrodes, wherein the second drive waveform comprises a time-dependent voltage sufficient to change the optical state of the electro-optic display proximate the second portion of pixel electrodes.

8. A method according to claim 7, wherein the first portion of pixel electrodes includes pixel electrodes from a first area of ​​the pixel electrode array, and the second portion of pixel electrodes includes pixel electrodes from a second area of ​​the pixel electrode array, and wherein the pixel electrodes of the first area and the pixel electrodes of the second area do not overlap.

9. The method according to claim 7, further comprising: applying a zero volt waveform to the second portion of pixel electrodes while applying the first measurement waveform to the first portion of pixel electrodes; as well as A zero volt waveform is applied to the first portion of pixel electrodes while the second measurement waveform is applied to the second portion of pixel electrodes.

10. A method for driving an electro-optic display, the electro-optic display comprising a layer of electro-optic material disposed between a common electrode and a backplane, the backplane comprising an array of pixel electrodes, wherein each pixel electrode is coupled to a pixel transistor, wherein a display controller circuit applies a waveform to the array of pixel electrodes by applying one or more time-dependent voltages between the common electrode and the array of pixel electrodes via the pixel transistors, the driving method comprising: Simultaneously activating pixel transistors associated with a first portion of pixel electrodes of the pixel electrode array; applying a first voltage to the first portion of pixel electrodes; injecting a measurement waveform from a signal generating circuit through a current measuring circuit coupled between the signal generating circuit and the common electrode; measuring a first current flowing through the current measurement circuit based on the measurement waveform; determining a first impedance of an electro-optic material proximate 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 a first measurement waveform; selecting a first drive waveform to be applied to each pixel electrode of the first portion of pixel electrodes based on the first impedance of the electro-optical material proximate to the first portion of pixel electrodes; as well as The first drive waveform is applied to the first portion of pixel electrodes, wherein the first drive waveform comprises a time-dependent voltage sufficient to change an optical state of the electro-optic display proximate the first portion of pixel electrodes.

11. The method of claim 10, wherein the current measurement circuit comprises a resistance element and a differential voltage amplifier, wherein a first input of the differential voltage amplifier is connected to a first terminal of the resistance element, and a second input of the differential voltage amplifier is connected to a second terminal of the resistance element.

12. The method of claim 10, wherein the measurement waveform comprises a periodic square wave or a sinusoidal voltage waveform.

13. The method of claim 10, wherein the measurement waveform comprises a voltage having an amplitude insufficient to change an optical state of the electro-optic display.

14. The method of claim 10, wherein the measurement waveform comprises an oscillating voltage waveform having a plurality of frequencies.

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