Patent Application
20120050347
This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2010-194777, filed on Aug. 31, 2010 the entire contents of which are incorporated herein by reference.
The present invention relates to a display device including a display element having memorability (i.e, memory).
Display devices that use a material having memorability such as a cholesteric liquid crystal have been developed and applied to electronic paper etc. Because of a difficult manufacturing process that uses a film substrate, the electronic paper tends to suffer from lot-to-lot variability in contrast, brightness, and gamma characteristics of a display element. After manufacture, a long period of use of the display element may cause variations in such characteristics. Such variability and variations over time may hinder desirable display even if the display element is driven under the same driving conditions.
Thus, it is proposed to provide the display element with a brightness sensor that detects the actual display state, for example, in order to perform adjustment such that a desired display state may be obtained. However, providing the display element with the brightness sensor may be difficult in terms of cost and appearance, and may not be preferable especially for a reflective display element that boasts of its easy portability such as electronic paper.
It is also proposed to measure the accumulated energization time for a display element that keeps energized during display in order to predict and correct variations over time. Because the electronic paper is energized during rewriting which occurs irregularly, however, correction that utilizes the accumulated energization time may not be applied to the electronic paper.
Thus, it is difficult to make the display devices free of lot-to-lot variability and variations over time in contrast, brightness, and gamma characteristics of the display element.
Related art is disclosed in Japanese Laid-open Patent Publication No. 2008-065058, and Japanese Laid-open Patent Publication No. 52-140295.
According to one aspect of the invention, a display device includes a display element that maintains its display state even when no longer driven; a capacitance detecting circuit that detects a capacitance of the display element; and a driving condition adjusting circuit that drives the display element under predetermined driving conditions to set the display element to the display state, and that adjusts driving conditions for the display element on the basis of the capacitance of the display element exhibiting the display state detected by the capacitance detecting circuit.
The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed.
Embodiments of the present invention will be specifically described below with reference to the drawings.
As illustrated in
The host control unit 21 includes a main CPU etc., and performs various processes on image data stored in an external storage device or image data acquired via a communication circuit or the like to obtain an image that is suitable for display on the display device. For example, in order to display halftone image data, tone conversion is performed by applying a known tone conversion technique such as an error diffusion method, an ordered dither method, or a blue noise mask method for adaptation to the number of tones that are displayable on the display device. A part of this process may be performed by the control unit 23. The host control unit 21 stores the generated image data in the frame memory 22.
The control unit 23 includes a sub CPU, a microcontroller, a PLD, or the like, and controls various units excluding the host control unit 21. The control unit 23 generates drive data in accordance with the image data read from the frame memory 22 to supply the generated drive data to the segment driver 11 and the common driver 12. In order to facilitate adjustment of the timing to supply the drive data to the segment driver 11 and the common driver 12, it is desirable that the control unit 23 include a buffer 25 that temporarily stores the generated drive data.
The display element 10 is a display element that uses a cholesteric liquid crystal and in which R, G, and B panels are stacked in three layers to enable color display. The display element 10 will be discussed in detail later. The segment driver 11 and the common driver 12 drive the display element 10 in accordance with a simple matrix scheme, and are implemented by a general-purpose driver IC. The segment driver 11 includes three drivers to independently drive the panels in the respective layers. The common driver 12 may include one driver that commonly drives the panels in the three layers.
The power source unit 13 includes a voltage boosting regulator, such as a DC-DC converter, that boosts a voltage of 3 to 5 V supplied from a common power source (not illustrated) of the display device to +50 V in case of a unipolar driver IC, and to about −25 V to +25 V in conjunction with a negative DC-DC converter in case of a bipolar driver IC. As a matter of course, it is desirable that the voltage boosting regulator have a high conversion efficiency for the characteristics of the display element. Preferably, switching of a reset voltage and a writing voltage is performed using an analog switch or a digital potentiometer. A booster circuit including an operational amplifier and a transistor and a smoothing capacitor are disposed in a stage subsequent to the switching circuit to stabilize the drive voltage of the display element 10.
The configuration described above is the same as that of a common display element that uses a cholesteric liquid crystal, and various configurations known in the art may be used. The display element 10 is not limited to a display device that uses a cholesteric liquid crystal, and may be any display element that has memorability.
In the display device according to the first embodiment, the power source unit 13 produces a capacitance detection signal, such as a saw-tooth wave signal or a triangular wave signal, in accordance with a control signal from the control unit 23 to supply the produced capacitance detection signal to a power source terminal of the segment driver 11. Preferably, the power source terminal is not used for writing or the like. The power source unit 13 may adjust the voltage to be supplied to the segment driver 11 and the common driver 12 in accordance with the control signal from the control unit 23.
In the display device according to the first embodiment, further, the current sense amplifier 14 is disposed to detect a current through a signal line for supplying the capacitance detection signal from the power source unit 13 to the segment driver 11. The current detected when the capacitance detection signal is applied to the display element 10 is related to the capacitance of the display element 10. The current sense amplifier 14 outputs a detection signal to the computation unit 24.
The control unit 23 executes a driving condition adjusting mode when the display device is started and in response to a command from a user. The driving condition adjusting mode may be automatically executed immediately when the display device is used for the first time, such as after product shipment, and thereafter may be automatically executed regularly, for example at a frequency of about once a month. The control unit 23 sets the display element 10 to a predetermined display state, causes the power source unit 13 to apply the capacitance detection signal to the display element 10, and causes the computation unit 24 to digitize the detection signal from the current sense amplifier 14 and take the digitized detection signal as detection data. The computation unit 24 acquires the detection data while changing the display state of the display element 10 in accordance with a driving condition adjusting sequence to be discussed later to determine driving conditions for desired display. After the driving condition adjusting mode is finished, the control unit 23 controls the various units under the determined driving conditions.
Next, a display device that uses a cholesteric liquid crystal, which is used as the display device 10 in the display device according to the first embodiment, will be described.
The panels 10B, 10G, and 10R have the same configuration as each other except that they reflect light around wavelengths that are different from each other. The configuration of a typical example of the panels 10B, 10G, and 10R, which is referred to as a panel 10A, will be described below.
As illustrated in
The upper substrate 51 and the lower substrate 53 may both be translucent. However, the lower substrate 53 of the panel 10R may be non-translucent. Examples of the translucent substrate include a glass substrate. Besides the glass substrate, a film substrate made of PET (polyethylene terephthalate), PC (polycarbonate), or the like may also be used.
Typical examples of the material of the electrodes of the upper electrode layer 54 and the lower electrode layer 55 include Indium Tin Oxide (ITO). Besides, a transparent conductive film made of Indium Zinc Oxide (IZO) or the like may also be used.
The transparent electrode of the upper electrode layer 54 is formed on the upper substrate 51 as a plurality of upper belt-like transparent electrodes that are parallel to each other. The transparent electrode of the lower electrode layer 55 is formed on the lower substrate 53 as a plurality of lower belt-like transparent electrodes that are parallel to each other. The upper substrate 51 and the lower substrate 53 are disposed such that the upper electrodes and the lower electrodes cross each other as seen from a direction that is perpendicular to the substrates. Pixels are formed at the crossing points. An insulating thin film is formed on the electrodes. If the thin film is thick, it is necessary to increase the drive voltage. Conversely, absence of the thin film causes a flow of a leak current, which may reduce the accuracy of automatic adjustment according to the present invention. In the example, the relative dielectric constant of the thin film is about 5, which is considerably lower than that of the liquid crystal. Therefore, the thickness of the thin film is preferably about 0.3 μm or less.
The insulating thin film may be formed by a thin film of SiO2, or an organic film made of a polyimide resin, an acrylic resin, or the like known as an orientation stabilizing film.
As described above, a spacer is disposed in the liquid crystal layer 52 to make the spacing between the upper substrate 51 and the lower substrate 53, that is, the thickness of the liquid crystal layer 52, constant. In general, the spacer is formed by spherical bodies made of a resin or an inorganic oxide. However, a fixed spacer formed by coating a surface of a substrate with a thermoplastic resin may also be used. The cell gap formed by the spacer is preferably in the range of 4 μm to 6 μm. If the cell gap is smaller than this range, the reflectivity may be reduced to result in a dark display, and a high threshold abruptness may not be expected. Conversely, if the cell gap is larger than this range, the drive voltage may be increased to make driving by general-purpose parts difficult, although a high threshold abruptness may be retained.
The liquid crystal composition forming the liquid crystal layer 52 is a cholesteric liquid crystal obtained by adding 10 to 40% by weight (wt. %) of a chiral material to a nematic liquid crystal mixture. The amount of the chiral material to be added is defined with the total amount of the nematic liquid crystal component and the chiral material defined as 100 wt. %.
Various nematic liquid crystals known in the art may be used. However, a liquid crystal material with a dielectric constant anisotropy (Δ∈) in the range of 15 to 35 is desirable. A dielectric constant anisotropy of 15 or less generally increases the drive voltage, which makes it difficult to use general-purpose parts in a drive circuit.
On the other hand, a dielectric constant anisotropy of 25 or more may reduce the threshold abruptness, and further may reduce the reliability of the liquid crystal material itself.
Meanwhile, a refractive index anisotropy (Δn) of 0.18 to 0.24 is desirable. A refractive index anisotropy less than this range may reduce the reflectivity in the planar state. A refractive index anisotropy more than this range may significantly increase scatter reflection in the focal conic state, and may result in a high viscosity to reduce the response speed.
Next, bright and dark (white and black) display on the display device which uses a cholesteric liquid crystal material will be described. Display on the display device which uses a cholesteric liquid crystal is controlled in accordance with the state of orientation of the liquid crystal molecules.
In the planar state, light at a wavelength corresponding to the helical pitch of the liquid crystal molecules is reflected. A wavelength λ with maximum reflection is represented by the following formula using an average refractive index n and a helical pitch p of the liquid crystal.
λ=n·p
Meanwhile, a reflection band Δλ increases along with a refractive index anisotropy Δn of the liquid crystal.
In the planar state, incident light is reflected, and thus a “bright” state, that is, white, is displayed. In the focal conic state, on the other hand, light having passed through the liquid crystal layer is absorbed by the light absorbing layer provided under the lower substrate 53, and thus a “dark” state, that is, black, is displayed. A halftone state that falls between the “bright” state (white display) and the “dark” state (black display) is established with the planar state and the focal conic state coexisting with each other. The halftone level is determined by the ratio of the planar state and the focal conic state coexisting with each other.
Next, a method of driving the display element which uses a cholesteric liquid crystal will be described.
When a strong electric field (at VP100 or higher) is produced in the cholesteric liquid crystal, the helical structure of the liquid crystal molecules is completely disrupted during the application of the electric field to establish the homeotropic state in which all the molecules are orientated in accordance with the direction of the electric field. Next, if the application voltage is abruptly reduced from VP100 to substantially zero when the liquid crystal molecules are in the homeotropic state, the helical axis of the liquid crystal becomes vertical with respect to the electrodes to establish the planar state in which light corresponding to the helical pitch is selectively reflected.
On the other hand, in the case where a weak electric field (in the range of VF100a to VF100b) that does not disrupt the helical structure of the cholesteric liquid crystal particles is applied and thereafter removed, or in the case where a strong electric field is applied and thereafter removed slowly, the helical axis of the cholesteric liquid crystal molecules becomes parallel to the electrodes to establish the focal conic state in which incident light is reflected.
If an electric field with an intermediate strength (VF0 to VF100a or VF100b to VP0) is applied and thereafter moved abruptly, a halftone image may be displayed with the planar state and the focal conic state coexisting with each other.
Display is made utilizing the above phenomena.
A simple matrix type display device that uses a cholesteric liquid crystal uses a Dynamic Driving Scheme (DDS) for high-speed rewriting. The display device according to the first embodiment also uses the DDS to display a halftone image. Before rewriting an image, a reset operation may be performed to bring all the pixels into the planar state at the same time. The reset operation is performed by forcibly setting all the outputs of the segment driver 11 and the common driver 12 to respective predetermined values. The reset operation does not require transfer of data for setting the output values, and thus may be executed in a short time. Because power is consumed, the reset operation may not be performed for low power consumption devices.
In order to facilitate description, display of a black-and-white binary image will first be described.
As discussed earlier, the DDS roughly includes three stages, namely a “Preparation” period, a “Selection” period, and an “Evolution” period, which are arranged in this order from the beginning. A Non-Select period is provided before and after these periods. In the Preparation period, the liquid crystal is initialized into the homeotropic state, and a Preparation pulse at a high voltage and with a large pulse width is applied. In the Selection period, a trigger for a branch into the planar state or the focal conic state is provided. In the Selection period, a Selection pulse at a low voltage and with a small pulse width is applied to switch into the planar state, and no pulse is applied to switch into the focal conic state. In the Evolution period, a determination is made on the planar state or the focal conic state depending on the transient state in the preceding Selection period, and an Evolution pulse at an intermediate voltage and with a large pulse width is applied. Each of the Preparation pulse, the Selection pulse, and the Evolution pulse includes a set of positive and negative pulses.
In the Preparation period and the Evolution period, in practice, a plurality of sets of positive and negative Preparation pulses, or Evolution pulses, are applied, rather than a single set of positive and negative pulses with a large pulse width such as those illustrated in
In the case where the DDS is executed in the first embodiment, the common driver 12 outputs six values including GND, and the segment driver 11 outputs four values including GND. Currently, general-purpose driver ICs for the simple matrix scheme have been put into practical use, and may be used as the segment driver 11 or the common driver 12 through mode setting. Thus, the general-purpose driver IC utilized as the segment driver 11 has redundant outputs. In the first embodiment, the redundant outputs of the segment driver 11 are utilized to apply a capacitance detection signal to the display device 10.
The common driver 12 and the segment driver 11 vary their outputs in each quarter of the Selection period. The segment driver 11 outputs a voltage waveform that varies in the order of 42 V, 30 V, 0 V, and 12 V for white display and a voltage waveform that varies in the order of 30 V, 42 V, 12 V, and 0 V for black display. The common driver 12 outputs a voltage waveform that varies in the order of 36 V, 36 V, 6 V, and 6 V in the Non-Select period, a voltage waveform that varies in the order of 30 V, 42 V, 12 V, and 0 V in the Selection period, a voltage waveform that varies in the order of 12 V, 12 V, 30 V, and 30 V in the Evolution period, and a voltage waveform that varies in the order of 0 V, 0 V, 42 V, and 42 V in the Preparation period.
Accordingly, in the Preparation period, a voltage waveform that varies in the order of 42 V, 30 V, −42 V, and −30 V is applied to the liquid crystal of the data electrode for white display, and a voltage waveform that varies in the order of 30 V, 42 V, −30 V, and −42 V is applied to the liquid crystal of the data electrode for black display. In the Evolution period, a voltage waveform that varies in the order of 30 V, 18 V, −30 V, and −18 V is applied to the liquid crystal of the data electrode for white display, and a voltage waveform that varies in the order of 18 V, 30 V, −18 V, and −30 V is applied to the liquid crystal of the data electrode for black display. In the Selection period, a voltage waveform that varies in the order of 12 V, −12 V, −12 V, and 12 V is applied to the liquid crystal of the data electrode for white display, and a voltage waveform at 0 V is applied to the liquid crystal of the data electrode for black display. In the Non-Select period, a voltage waveform that varies in the order of 6 V, −6 V, −6 V, and 6 V is applied to the liquid crystal of the data electrode for white display, and a voltage waveform that varies in the order of −6 V, 6 V, 6 V, and −6 V is applied to the liquid crystal of the data electrode for black display.
As illustrated in
In the Preparation period and the Evolution period, which extend over several to a dozen or so times the duration of the Selection period, a plurality of the Preparation pulses, or the Evolution pulses, of
The set of the Preparation pulse, the Selection pulse, and the Evolution pulse of
In order to display a halftone image, the Selection period is further divided into a plurality of sub periods so that the driving waveform illustrated in
As discussed earlier, a display device that uses a liquid crystal having memorability tends to suffer from lot-to-lot variability in contrast, brightness, and gamma characteristics of a display element, and a long period of the display element may cause variations in such characteristics. Such variability and variations over time in the display element may hinder desirable display even if the display element is driven under the same driving conditions. In particular, the DDS used by the display device according to the first embodiment is narrow in optimum range of the driving conditions, and thus is significantly affected by the variability and variations over time in the display element. Therefore, good display may not be obtained under fixed driving conditions.
In order to adjust the driving conditions, the characteristics of the display element related to display (brightness) are detected to perform adjustment on the basis of the relationship of the detected characteristics with the display (brightness). As discussed earlier, it has been proposed to determine the driving conditions in accordance with the capacitance value. Also in the display device according to the first embodiment, the capacitance of the display element 10 is detected to adjust the driving conditions so as to achieve desirable driving conditions. In the display device according to the first embodiment, however, the capacitance of the display element 10 is directly detected without using a dummy cell, and detection of the capacitance and adjustment of the driving conditions are performed with the display element 10 set to a predetermined display state (white, black, or a halftone level).
As is clear from
The current sense amplifier 14 may be an amplifier that outputs the detected current value as an analog voltage value. The voltage of the voltage signal output from the current sense amplifier 14 is digitized by an A/D converter (ADC) in the computation unit 24 to be used in computation of a capacitance value. Providing a low-pass filter having an appropriate cut-off frequency between the output of the current sense amplifier 14 and the A/D converter further improves the detection accuracy.
The power source unit 13 generates respective voltages to be supplied to the segment driver 11 and the common driver 12 using a voltage dividing circuit. Because the DDS momentarily consumes a large current, it is desirable that the respective voltages formed by the voltage dividing circuit of the power source unit 13 be output via the booster circuit including the operational amplifier Amp and the transistors Tr1 and Tr2 illustrated in
Further, at terminals of the power source unit 13 that output voltages to be supplied to the segment driver 11 and the common driver 12, a smoothing capacitor with a capacitance of about several μF is often provided in a stage subsequent to the damping resistor. At a terminal that outputs a capacitance detection signal illustrated in
The capacitance is detected by the current sense amplifier 14 detecting the current value during charge/discharge that occurs along with application of the capacitance detection signal to the display element 10.
It is found that use of the saw-tooth wave capacitance detection signal allows stable detection of the current during charge/discharge even for the cholesteric liquid crystal, which is poorer in capacitance characteristics than a TFT liquid crystal.
A CR oscillation circuit was prototyped by replacing the test cell with a capacitor. The oscillation frequency of the CR oscillation circuit was measured. As a result, the oscillation frequency in the planar state was about 1.4 times that in the focal conic state, and the oscillation frequency fluctuated significantly to frequently become unstable. Consequently, for the cholesteric liquid crystal, the capacitance was detected more stably in accordance with the current during charge/discharge that occurs along with application of the saw-tooth wave capacitance detection signal than in accordance with detection of the oscillation frequency.
In the detection of the capacitance described above, the capacitance of the display element 10 during white/black display is detected. With the display element 10 brought into a halftone display state, however, the capacitance in the halftone display state may be detected. In the detection of the capacitance described above, in addition, a saw-tooth wave capacitance detection signal is used. However, a triangular wave capacitance detection signal may also be used to perform a similar measurement.
Next, a method of adjusting the driving conditions in the display device according to the first embodiment will be described.
Conditions that may be adjusted in adjusting the driving conditions for the DDS include the respective voltages of the Preparation pulse and the Evolution pulse, the voltage of the Selection pulse for white display, and the pulse width (duty ratio) of the Selection pulse. In the first embodiment, the voltage of the Evolution pulse (Evolution voltage) and the duty ratio of the Selection pulse are adjusted. The Evolution voltage is adjusted because it is a factor that strongly governs the display contrast. The duty ratio of the Selection pulse is adjusted because it is relatively easily adjustable, among factors that produce tone variations, and enables accurate adjustment.
In
In
P indicates variations in capacitance, with respect to the Evolution voltage, of the display element, for which the driving conditions are to be adjusted. When the variations in capacitance P are compared with R of the reference example, C100 and C0 are increased to C100′ and C0′, respectively, the gradient at the intermediate portion is increased, and the capacitance values at 25%, 50%, 90%, etc. in the range between C100 and C0 and the corresponding Evolution voltages are increased.
In the method of adjusting the driving conditions according to the first embodiment, in the first stage, C100′ and C0′ are detected.
In the second stage, the Evolution voltage is determined such that a predetermined capacitance value (for example, 25%, 50%, 90%, etc.) in the range between C100′ and C0′ is obtained by varying the duty ratio of the Selection pulse. In other words, the Evolution voltage is determined such that generally maximum contrast and brightness may be obtained.
In the first embodiment, as described above, the Evolution voltage is varied. However, C100′ and C0′ may not be varied just by varying the Evolution voltage. For example, as illustrated in
Thus, in the first embodiment, the Evolution voltage is set such that C100′ and C0′ correspond to display brightnesses of 100 and 0 (relative values), respectively, and such that variations in capacitance at a halftone portion correspond to variations in duty ratio of the Selection pulse.
In the third stage, variations in duty ratio of the Selection pulse are determined such that variations in capacitance at the halftone portion are linear.
In step S11 of the first step S1, all the pixels of the display element 10 are rendered into the white display state (planar state) in accordance with the DDS. In step S11, in order to ensure that all the pixels are brought into the white display state, the duty ratio of the Selection pulse is set to 100% and, further, the Evolution voltage is set to be higher than normally as illustrated in
In step S12, the capacitance of the display element 10 in the white display state set in step S11 is measured, and the obtained value is set as a 0% point. Thus, C0′ is set as the 0% point.
In step S13, all the pixels of the display element 10 are rendered into the black display state (focal conic state) in accordance with the DDS. In step S13, in order to ensure that all the pixels are brought into the black display state, the duty ratio of the Selection pulse is set to 0% (no Selection pulse) and, further, the Evolution voltage is set to be lower than normal as illustrated in
In step S14, the capacitance of the display element 10 in the black display state set in step S13 is measured, and the obtained value is set as a 100% point. Thus, C100′ is set as the 100% point.
The second step S2 includes steps S21 to S23, which are repeated three to five times as illustrated in step S2R.
In step S21, all the pixels of the display element 10 are rendered into a halftone display state (planar state+focal conic state). The set halftone may be any tone such as 90%, 50%, and 25%. For example, in the case where the set halftone is 25%, the duty ratio of the Selection pulse is set to 25% under the driving conditions stored in advance to bring all the pixels of the display element 10 into a halftone display state in accordance with the DDS. In the case where the set halftone is 90%, the Evolution voltage is set such that generally maximum display contrast may be obtained, which is preferable in terms of display contrast.
In step S22, the capacitance of the display element 10 in the halftone display state set in step S21 is measured.
In step S23, a target capacitance value corresponding to the set halftone is calculated from the capacitances C0′ and C100′ corresponding to the 0% point and the 100% point determined in steps S12 and S14, respectively, to be compared with the measured capacitance value obtained in step S22. Then, the Evolution voltage is adjusted on the basis of the comparison results such that the measured capacitance value reaches the target capacitance value.
Steps S21 to S23 are repeated. When the measured capacitance value obtained in step S22 is approximated to the target capacitance value, step S2 is terminated to proceed to step S3.
The Evolution voltage may be adjusted by any method that adjusts the measured capacitance value to the target capacitance value. Such a method is known as a root-finding algorithm. Typical examples of the method include a Newton's method and a bisection method. Examples in which these methods are applied will be described.
In Newton's method, standard characteristics of variation in capacitance with respect to the Evolution voltage as illustrated in
As illustrated in
As illustrated in
It is known that Newton's method may result in a divergence, rather than a convergence, in the case where an object to which a solution is to be calculated has characteristics that vary very abruptly or characteristics that vary convexly and concavely. In the case where the Evolution voltage is to be adjusted, however, the capacitance varies very monotonically with respect to the Evolution voltage, and thus application of Newton's method generally reliably results in a convergence.
In the bisection method, it is not necessary to store standard characteristics of variation in capacitance with respect to the Evolution voltage.
As illustrated in
As illustrated in
As illustrated in
In general, the bisection method is less likely to result in a divergence, but takes more time to achieve a convergence, than Newton's method. As described above, however, the capacitance varies very monotonically with respect to the Evolution voltage, and therefore repeating the steps five times resulted in a convergence to a generally constant value.
Returning to
In step S31, all the pixels of the display element 10 are brought into a target halftone display state in which any of the halftones for display is displayed. This process is the same as that in step S21.
In step S32, the capacitance of the display element 10 in the target halftone display state set in step S31 is measured.
In step S33, a target capacitance value corresponding to the target halftone display state is calculated to be compared with the measured capacitance value obtained in step S32. Then, the duty ratio of the Selection pulse is determined on the basis of the comparison results such that the measured capacitance value reaches the target capacitance value.
Steps S31 to S33 are repeated. When the measured capacitance value obtained in step S32 is approximated to the target capacitance value, step S3 is terminated.
In case of the DDS, the liquid crystal responds considerably abruptly, and therefore it is inherently difficult to form halftone display. Therefore, about three to seven halftones may be displayed. When the third step is repeated for each of the halftones to determine the duty ratio of the Selection pulse for all the halftones for display, the process proceeds to step S4.
For a capacitance at the halftone portion, a duty ratio of the Selection pulse at which such a capacitance (halftone) is obtained is determined, and the driving conditions are updated using the thus determined duty ratio of the Selection pulse. The duty ratio of the Selection pulse is determined by applying Newton's method or the bisection method to each capacitance at the halftone portion. In case of the DDS, the liquid crystal responds considerably abruptly, and therefore it is inherently difficult to form a halftone display. Therefore, while Newton's method may be used to determine the duty ratio of the Selection pulse, the bisection method is better in finding an optimum value because of the lower risk of resulting in a divergence.
According to the display device of the first embodiment configured described above, it is possible to automatically optimize the driving conditions so as to constantly perform good display even in the case where the characteristics of the display element 10 fluctuate because of lot-to-lot variability and variations over time.
In the display device according to the first embodiment, in detecting the capacitance of the display element 10, the display element 10 is driven in accordance with the DDS such that all the pixels are brought into the same display state. In driving the display element 10 in accordance with the DDS, it is necessary to apply a driving waveform such as that illustrated in
Thus, as illustrated in
In
In the display element according to the first embodiment, in the first step S1, the respective capacitances corresponding to the brightnesses of 0 and 100 (relative values) are determined. In the second step S2, the Evolution voltage is set such that a predetermined capacitance value at a predetermined halftone portion may be obtained from the capacitances determined in the first step S1. In the third step S3, the relationship between the capacitance value at the halftone portion and the duty ratio of the Selection pulse is set using the Evolution voltage determined in the second step S2. In the case where fluctuations between the brightnesses of 0 and 100 (relative values) and the corresponding capacitances are small because of the characteristics of the display element, the first step S1 may be omitted. Also in this case, it is necessary to perform steps S2 and S3 in the case where the characteristics of variation in capacitance with respect to the Evolution voltage of
Conversely, in the case where variations in capacitance value (halftone) with respect to the duty ratio of the Selection pulse illustrated in
In the display device according to the first embodiment, the Evolution voltage and the duty ratio of the Selection pulse are adjusted so as to obtain desired display characteristics. However, there are also other factors of the driving conditions that may vary the display characteristics as discussed earlier. In the case where such factors are to be adjusted, the technique described above in which the capacitance of the display element is detected in different display states and the driving conditions are adjusted on the basis of the detected capacitance may also be applied.
In the display device according to the first embodiment, further, a unipolar driver IC is used. However, a bipolar driver IC may also be used.
Voltages are defined as VP3, VP2, VP1, 0, VN1, VN2, and VN3 in descending order of voltage from the positive side to the negative side. During a positive phase, a voltage difference between SEG-VP3 and COM-VP1 is applied in the Selection period for rendering of white display, and a voltage difference between SEG-VP1 and COM-VP1 is applied in the Selection period for rendering of black display. In the Preparation period and the Evolution period, an average voltage is applied in accordance with the relationship of
Formulas for deriving VP3, VP2, VP1, 0, VN1, VN2, and VN3 for each of SEG and COM from the Evolution voltage will be provided below. A Non-Select voltage is a voltage applied to all the pixels, rendered or unrendered, in none of the Preparation period, the Selection period, and the Evolution period.
SEG
—
VP3=((Evolution voltage)+3*Non-Select voltage)/2
SEG
—
VP2=(((Evolution voltage)+3*Non-Select voltage)−Non-Select voltage)−SEG—VP3
SEG
—
VP1=SEG—VP3−Non-Select voltage*2
SEG
—
VN3=−(SEGVP3)
SEG
—
VN2=−(SEG—VP2)
SEG
—
VN1=−(SEG—VP1)
COM
—
VP3=SEG—VP3
COM
—
VP2=SEG—VP2
COM
—
VP1=SEG—VP1
COM
—
VN3=−(COM—VP3)
COM
—
VN2=−(COM—VP2)
COM
—
VN1=−(COM—VP1)
In the display device according to the first embodiment, the DDS is used. In the case where the conventional driving scheme discussed earlier is used, however, the technique described above in which the capacitance of the display element is detected in different display states and the driving conditions are adjusted on the basis of the detected capacitance may also be applied. A display device according to a second embodiment that uses the conventional driving scheme will be described below.
The cholesteric liquid crystal is brought into a homeotropic state, in which all the liquid crystal molecules are oriented in accordance with the direction of an electric field, when a strong electric field (reset voltage) is applied. The cholesteric liquid crystal is brought from the homeotropic state into the planar state when the application of the electric field is abruptly canceled. When an intermediate electric field (writing voltage) is applied, the cholesteric liquid crystal is brought from the planar state into the focal conic state. The proportion of liquid crystal molecules that are brought into the focal conic state differs in accordance with the application time. Specifically, a short application time results in a small proportion of liquid crystal molecules brought into the focal conic state, and a long application time results in a large proportion of liquid crystal molecules brought into the focal conic state.
The conventional driving scheme enables halftone display with high uniformity, which is difficult to achieve with the DDS, and is advantageous in a generally full-color display.
The display device according to the second embodiment has the same configuration as that illustrated in
The conventional driving scheme includes a reset process in which all the pixels to be rewritten are brought into the homeotropic state by applying a reset voltage, and thereafter brought into the planar state by canceling the application of the reset voltage, and a writing process in which a writing pulse is applied to each of the pixels to display an image by adjusting the application time of the writing pulse.
As described above, the ratio of the coexisting focal conic state varies in accordance with the application time of the writing voltage. There are roughly two methods of varying the application time of the writing voltage. A first method is to vary the application time by varying the width of a pulse. A second method is to vary the application time by varying the number of consecutive short pulses.
In the display device according to the second embodiment, examples of adjustable parameters of the driving conditions include the voltage of the writing pulse in the writing process, the maximum accumulated time of the writing pulses, and the pulse width. These parameters are optimized by applying Newton's method, the bisection method, or the like while measuring the capacitance of the display element set to a display state.
All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiment(s) of the present inventions have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2010-194777 | Aug 2010 | JP | national |
