Patent Application
20120086691
The invention relates to the field of displays, in particular, displays comprising electrofluidic cells.
Up to now, in certain areas of display technology, an electrophoretic electro-optical medium is commonly used, in particular for flexible displays. However, the electrophoretic electro-optical medium is subject to a number of restrictions. The medium has a relatively slow pixel response that makes video display challenging and has a relatively low brightness compared to paper.
Displays based on the electrowetting electro-optical medium may remedy at least some of the restrictions mentioned above. Particular variants using this principle are e.g. described in publications WO2004068208 and U.S. Pat. No. 4,583,824. These variants have a height dimension that is relatively large compared to liquid crystal or electrophoretic displays which hinders the use in flexible displays.
The recently developed Electrofluidic Chromatophore (EFC) variant of a display based on electrowetting has a smaller height dimension and may therefore be more suitable to use in flexible displays.
Because pixels in displays based on the EFC technology have a high reflectivity, these displays can be used in situations ranging from dim ambient lighting to full sun-light.
In the remainder, we will refer to an EFC cell as a pixel cell comprising a fluid holder for holding a polar fluid and a non-polar fluid having differing display properties, the fluid holder comprising a reservoir with a geometry having a small visible area projected in the direction of a viewer, and a channel with a geometry having a large visible area projected in the direction of a viewer, the channel being connected to the reservoir so as to enable free movement of the polar fluid and non-polar fluid between the channel and the reservoir, at least part of a surface of the channel comprising a wetting property responsive to a supply voltage over the pixel cell and defining (i) a stable region wherein the supply voltage stabilizes the amount of polar fluid in the channel; (ii) a fill region that controls the flow of polar fluid into the channel and (iii) a retract region that controls the flow of polar fluid into the reservoir; and at least two pixel cell terminals being configured to supply the supply voltage to at least part of the surface of the fluid holder comprising the wetting property for supply voltage controlled movement of polar fluid.
The EFC pixel cell will respond in dependency of the supplied voltages, in particular, the fill or retract level of the polar fluid will be controlled. The various conditions that the EFC cell can exhibit as a result of these controlled voltages may in the remainder be also referred to as cell display properties or cell states, more particular, a fill state, retract state or stable state, to correspond to the visual appearance a black state, white state or more generally a color state that can be stable or change dependent on the supply voltage.
The state of an EFC cell is not directly related to the voltages at the terminals. Instead, these voltages and their timing control the rate and direction of change of the state. Therefore, to drive a cell to a certain state, differential driving is needed, i.e. driving that takes into account the current state of the cell and applies certain voltages at the cell terminals for a certain time to reach the new, wanted state. Actual voltage levels can be influenced by cell geometry and material properties. In previous applications, an active matrix design has been proposed including pixel-based active elements to provide the required driving signals to the EFC pixel. Due to the complex composition and needed materials for such an active matrix design a desire exists to provide a driving system that has lowered manufacturing costs and still provides acceptable display functionality. It is also an object of this invention to propose an EFC display drive scheme to display content in an energy efficient manner.
According to an aspect of the invention, there is provided a display apparatus comprising a plurality of electrofluidic chromatophore (EFC) pixel cells. Each pixel cell comprises a fluid holder for holding a polar fluid and a non-polar fluid having differing display properties, the fluid holder comprising a reservoir with a geometry having a small visible area projected in the direction of a viewer, and a channel with a geometry having a large visible area projected in the direction of a viewer. The channel is connected to the reservoir so as to enable free movement of the polar fluid and non-polar fluid between the channel and the reservoir. At least part of a surface of the channel comprises a wetting property responsive to a supply voltage over the pixel cell and defining (i) a stable region wherein the supply voltage stabilizes the amount of polar fluid in the channel; (ii) a fill region wherein the amount of polar fluid in the channel increases and (iii) a retract region wherein the amount of polar fluid in the channel decreases. At least two pixel cell terminals are configured to supply the supply voltage to at least part of the surface of the channel comprising the wetting property for supply voltage controlled channel movement of the polar—non-polar fluid front. The display further comprises a driver operative to provide controlled column voltages and any of a predefined row select or non-select voltage via respective column and row electrodes. A circuit board comprises a row electrode and a column electrode each directly connecting the driver to the pixel cell terminals for supplying the supply voltage to the pixel cell terminals as a voltage difference between the row and column electrodes in a passive matrix configuration. Passive-matrix displays typically have no pixel wise active elements such as transistors and are thus simpler in construction than active-matrix displays. Because of this, passive-matrix displays can be fabricated at lower cost and higher yield than active-matrix displays.
While the appended claims set forth the features of the present invention with particularity, the invention, together with its objects and advantages, may be best understood from the following detailed description taken in conjunction with the accompanying drawings of which:
A display controller 10 is arranged to control the driver 8 as a result of pixel image information 101 inputted in the display controller 10. Typically, the display 1 is refreshed a number of times per second. The frame time is defined as the time wherein all the pixels of a display are refreshed once. The frame time comprises a line selection time, wherein the pixel cells 2 connected to a row 7 are activated, followed by a hold time, wherein the other rows are sequentially addressed. Alternatively other update schemes may be provided, e.g. with multiple row addressing, where more than one row is selected and refreshed at a time.
In the following, the operation of the present EFC pixel cell is further explained. Amongst others, it will be shown that there is a stable supply voltage that stops the polar—non-polar fluid front movement in the channel of a pixel cell.
As shown in the picture, circuit board 3 comprises a row electrode and a column electrode each directly connecting the driver 8 to the pixel cell terminals 5, 5′, and 9 for supplying the supply voltage to the pixel cell terminals as a voltage difference between the row and column electrodes in a passive matrix configuration.
While a supply voltage can be typically supplied to a pixel cell 2 via a single row and column electrode 6, 7, pixel cell 2 can be additionally connected with another row electrode 5′, typically, having a further pixel cell terminal that is electrically connected to a further electrode 5′ directly connected to the driver 8 to provide a bias supply voltage or basic supply voltage to the pixel cell as will be explained hereafter. The additional bias electrode can be provided as a patterned electrode parallel to the row electrodes. In such an arrangement a pixel cell intermediate condition can be provided. This condition can be defined as a state of the pixel cell wherein the possible cell display property changes are limited due to the supply of a basic supply voltage to the at least one further pixel cell terminal with the aim to reduce the column voltage required to induce a change in the cell display property. The bias voltage may be dependent on the display property change. The effects of bias voltage are further discussed with reference to
The switching state of an electrofluidic pixel cell 2 is not directly related to the voltages at the terminals 5, 9. Instead, these voltages and their timing control the rate and direction of change of the switching state. Therefore, to drive a cell to a certain switching state, differential driving is needed, i.e. driving that takes into account the current switching state of the cell 2 and applies certain voltages at the cell terminals 5, 9 for a certain time to reach the new desired switching state. One possibility is that a copy of the currently displayed image is kept and used in the calculation of the driving signals for the new image. Another option is to reset the entire display to a known switching state, e.g. the state in which all cells 2 are fully retracted, (in the absence of supply voltage) and apply certain driving signals by driver 8 to display a new image.
Advantageously, this passive matrix configuration can be used for applications with a relatively low information change rate, like e-reading, map navigation, camouflage skinning, (outdoor) signage, shelf-edge labeling etc.
A color display variant may be implemented by using water of different colors for different pixel cells, for example red, green and blue or cyan, magenta and yellow, or by providing a color filter on top of a black and white display or by integrating the color filter in the display on or near the top surface 38 of the channel 33.
The channel 33 is typically 3 to 5 um in height; the thickness of the mesa defining the lower channel wall 37 is typically 40 um. The theoretical switching speed is in the order of milliseconds for both transitions.
Typically, besides a polar fluid 34, the fluid holder 31 also comprises a non-polar fluid (not shown). To generate a cell display property, the polar fluid 34 and the non-polar fluid have differing display properties. A display property may e.g. be a color, also encompassing monochromatic variants or a certain transmission and/or reflection characteristic of the fluid. In one embodiment, the polar fluid 34 has a transmission differing from the non-polar fluid. Typically, the polar fluid 34 comprises water and the non-polar fluid comprises oil. Preferably the water is blackened and the oil is left clear or is diffuse scattering, because blackening water with pigments may yield a more saturated black than blackening oil with dyes. Pigmented blackened water may result in a sufficiently black pixel color with a layer of water with a thickness of only 3 micrometer. This allows a display with a total thickness less than 100 micrometer, which typically is within a suitable thickness range for flexible displays. Typically the water contains ionic content as the conductive element. The non-polar fluid may occupy the space not occupied by the polar fluid 34. The non-polar fluid is preferably immiscible with the polar fluid 34.
In an embodiment, the geometry of the channel 33 and the fluid reservoir 32 are carefully constructed to impart a mutually differing principle radius of curvature. In such embodiments, the fluid reservoir 32 imparts a large principle radius 35 of curvature onto the polar fluid and the channel imparts a small principle radius 36 of curvature onto the polar fluid 34 when the surfaces of the channel 33 and the fluid reservoir 32 are sufficiently hydrophobic. This configuration results in a Young-Laplace force that aims to bring the polar fluid in its energetically most favorable shape, i.e. the droplet shape and urges the polar fluid 34 into the fluid reservoir 32.
On the other hand, however, the polar fluid 34 may be urged into the channel 33 by generating an electromechanical force larger than and opposite of the Young-Laplace force. To control this force, at least part of a surface 38 of the channel 33 and the lower channel wall 37 comprises a wetting property responsive to an applied supply voltage to one or more of the walls of the fluid holder 31. The polar fluid 34 may comprise a conductive element or component. Typically a hydrophobic fluoropolymer is provided on at least part of the surface 38 of the channel 33 and the lower channel wall 37, although other materials having a wetting property responsive to an electric field may be applied.
The electromechanical force is directed opposite to the counteracting force that urges the polar fluid 34 into the fluid reservoir 32 and may be controlled by varying the supply voltage. This counteracting force may be the Young-Laplace force or another, oppositely directed, electromechanical force or a combination of those.
A supply voltage providing a balance of counteracting force and electromechanical force, i.e. a voltage whereby movement of the polar fluid 34 is absent is called the stable voltage. Although the stable voltage may show variation depending on the cell display property, it is in principle unrelated to the cell display property. That is, substantially independent of the fluid front position, the stable voltage will stabilize the fluid front of the polar fluid 34. It is noted that this characteristic may not be found in other display types like electrophoretic or liquid crystal displays. In other words, providing the stable supply voltage to a pixel cell stabilizes the polar fluid 34 in the pixel cell 20.
By applying a supply voltage to at least a part of the channel surface 37, 38 of the channel 33, the induced electric field typically reduces the hydrophobic character of the fluoropolymer and results in an electromechanical force, aiming to bring the polar fluid 34 from the reservoir 32 into the channel 33 that is proportional to the supply voltage over the at least part of the channel surface 37, 38 squared. The supply voltage changes the wetting property of at least part of the surface 37, 38 of the channel 33.
Varying the electromechanical force may be used to control the movement of the polar fluid 34 in the pixel cell 20. Therefore, the pixel cell 20 comprises at least two pixel cell terminals (not shown). The pixel cell terminals are arranged to apply a supply voltage via electrodes (not shown) to the at least part of the surface of the channel 33 comprising the wetting property responsive to an applied supply voltage. The supply voltage may be provided by a combination of voltage differences, from any of a number of electrodes attached to the pixel cell.
a-d shows various schematic connections for connecting the electrofluidic pixel cell to a matrix electrode structure in a 3-terminal configuration. At the top and bottom of the channel, there are planar electrodes 380, 370 covered by dielectric layer 371, 381. The polar water droplet forms the third electrode 390. Alternatively, the top electrode can be left out. Care has to be taken that the wetting properties of the top wall of the channel 31 are optimized in this case. For reading convenience the reference numerals are only indicated in
Electromechanical force on the water-oil front is caused by a voltage across a dielectric stack including the fluoropolymer layer. In passive-matrix configuration, there are row and column electrodes, with a pixel at each crossing.
In more detail, the configuration of
The electromechanical force that pulls the water into the channel is proportional to the applied voltage squared. This results in a symmetrical response around 0V (see
The width of the stable region on the x-axis is non-zero due to the effects of wetting hysteresis or a wetting barrier that is inherent to the materials used in the pixel cell, or that is purposely added to define the stable range by modifying the channel wetting property. The effect of these barriers is to locally increase the width of the stable region to lower voltages and to higher voltages, respectively, yielding preferential states of the oil-water distribution in the pixels. These preferential states can be used as discrete gray levels or as an intermediate starting level from which gray levels can be reached faster and more accurate.
These barriers may be provided by physical structures locally influencing an applied electric field to the channel surface having a wetting property, by physical structures locally influencing the wetting property or by physical structures locally influencing the radius of curvature and thus the Young-Laplace pressure of the polar liquid in the channel. These barriers may also include a change in the chemical composition at the surface which has strong influence on the wetting properties.
The speed of the water front typically is in the order of centimeters per second and preferably between 0 and 50 centimeters per second, as 28 centimeters per second yields a switching speed between the black and the white state of about 1 millisecond for a pixel cell size of 0.2 millimeters (having a 0.28 millimeters diagonal size) when the reservoir is positioned in the corner of the pixel cell, which is compatible with displaying video content on the display apparatus. In this simple calculation only the influence of the electromechanical force and the counteracting force have been taken into account; other forces, such as the drag force, that reduce the speed of the water front with the distance of the water front from the reservoir, have not been taken into account.
Accordingly, via row and column electrodes, driver 8 is directly connected to the pixel cell terminals 2i, 2j on a circuit board comprising row and column electrodes each directly connecting the driver 8 to the pixel cell terminals 2i, 2j for supplying the supply voltage Vpx to the pixel cell terminals as a voltage difference Vcol−Vrow between the row and column electrodes in a passive matrix configuration.
To prevent degenerative effects of unipolar electric fields, typically on a row, column or frame basis the row and column polarity may be periodically inverted relative to each other to invert the polarity of the supply voltage, so as to obtain an average supply voltage being essentially zero with no directional build-up of charges in the pixel cells.
A typical preferred value of the row select voltage Vrsel is about Vsl which typically ranges between 3.5 and 4.5 Volts. A typical preferred value of the row non-select voltage is about 0V, typically ranging between −0.5 and +0.5 V. With a sufficient margin for error it is important that the stable region is sufficiently wide and is centered preferably halfway 0V and the voltage at which fill speed saturation occurs (typically above the 8-9 V). The width of the stable region is a compromise between the allowable signal perturbations, caused e.g. by crosstalk, and needed voltage swings. Advantageously, the stable voltage range has a lower stable voltage and a higher stable voltage differing in a range of 0.5-1.5 V. In practice, the stable voltage region has a width of 1V, centered around 4.5V. Typical voltage swings for such system are in the order of 10V.
Preferably the supply voltage is provided having the width of the stable region at least larger than or equal to the half width of the retract region. This may result in a drive scheme where the pixels of a selected row can either be driven to white or to black or can be kept in their current switching state, while the pixels in the not selected rows are kept in their current switching state. A shift of the voltages Vrow and Vcol is possible when the voltage difference remains the same resulting in unchanged supply voltage Vpx.
In contrast to the
The corresponding alternatives with inverted row polarity are shown in
In contrast,
However, during the row select the fill and retract voltage regions do no longer include a stable bordering condition wherein the amount of polar fluid is kept stable in the channel. Therefore to obtain a stable condition, wherein the pixel cell is kept in a stable state during a row select period, the pixel may be subsequently switched to a fill state and to a retract state with as a net result no change in the switching state. This can for example be provided by pair wise subsequent row select pulses or alternatively with two consecutive row selection periods that are spaced sufficiently close together in time to achieve the same net result.
As indicated in
The cell display property may be expressed as the transmission and/or reflection of the pixel cell at a predefined wavelength or in a range of predefined wavelengths; corresponding to a polar fluid front position in the channel.
Typically, the cell display property is expressed as the transmission and/or reflection of the pixel cell at a predefined wavelength or in a range of predefined wavelengths. The number of cell display properties is generally limited to a number of discrete levels within the complete range of possible transmission and/or reflection values. The pre-defined, discrete transmission and/or reflection values are measurable, physical values that can be represented by a (binary) number and as such can be processed by the controller.
Inversion may in principle be applied to individual pixels or a group of pixels.
Driving can be distributed over more than one frame, thereby lowering the needed voltage swing and also rendering a low-contrast image fast, after which its contrast is improved in one or more subsequent frames.
Pixels can be made multi-stable, enabling removal of all electrode voltages after writing an image without having the water in all pixels retract into the reservoirs.
While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. In particular, unless clear from context, aspects of various embodiments that are treated in various embodiments separately discussed are deemed disclosed in any combination variation of relevance and physically possible and the scope of the invention extends to such combinations. In the embodiments the column voltage range is restricted to a small band defined by the stable region at Vrow=Vrnsel so that rows that are not selected are only driven within the stable voltage range. In the embodiments, row electrodes are substantially similar in structure in respect of the column electrodes and can be interchanged with the column electrodes—that is a supply voltage to the pixel can be provided with a ‘column’ electrode having a column select voltage, whereas the row electrode is then provided with a corresponding row voltage for providing the supply voltage. Such and other variations to the disclosed embodiments can be understood and by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.
