Method of displaying gray scales on a ferroelectric liquid crystal cell with variable thickness

Abstract: A ferroelectric liquid crystal cell is controlled by applying a strobe pulse (Vs) and a data pulse (Vd1, Vd2, Vd3) to the cell, the magnitude of the data pulse being modulated in order to control the resultant pulse applied to the cell. The resultant pulse comprises a pre-pulse on one polarity and a main pulse of the opposite polarity. When applied to a liquid crystal cell of stepped thickness, the application of data pulses of different magnitudes switches regions of different thicknesses of the cell so as to provide grey level capability.

Description: This invention relates to a ferroelectric liquid crystal cell, a method of controlling such a cell, and to a liquid crystal display (LCD) comprising a plurality of such cells. Two drive schemes commonly used with ferroelectric LCDs are the JOERS/Alvey scheme and the Malvern schemes. As described by PWH Surguy et al in Ferroelectrics 122, 63, 1991, the JOERS/Alvey drive scheme is for use with an LCD having a plurality of rows and columns of electrodes. A two time slot strobe pulse is applied to the rows and a data pulse is applied to the columns. One of the time slots of the strobe pulse is at zero, the other time slot having an amplitude Vs. The strobe pulse is scanned down the plurality of row electrodes. The data pulse has an amplitude Vd and the polarity thereof may be changed between each slot. At each pixel of the LCD, the effective applied electric field is the combination of the strobe pulse and the data pulse. In the time slot wherein the strobe pulse is zero, the magnitude of the effective electric field will be equal to Vd. However, in the other slot, the strobe and data pulses combine and depending upon their polarity, the resultant may have a magnitude greater or less than either of the strobe and data pulses. If the magnitude falls within a predetermined range, switching of the pixel occurs. The Malvern schemes are similar to the JOERS/Alvey scheme, but instead of the strobe pulse being at zero for one time slot and at Vs for the other slot, the strobe pulse is at zero for one time slot and at Vs for several time slots. In order to distinguish between the different Malvern schemes, the schemes are identified by the number of slots over which the strobe pulse is at Vs, for example Malvern-2 denotes the scheme in which the strobe pulse is zero for one slot and at Vs for two slots. The Malvern schemes are described in Liquid Crystals 13, 597, 1993. When used to control a ferroelectric LCD capable of displaying a plurality of grey scales, it is desirable to be able to apply a range of electric fields to the LCD. However, the above described drive schemes are intended for black and white operation. GB-A-2 178 582 relates to a liquid crystal apparatus and driving method for addressing continuous or analogue grey levels. EP 0 510 606 discloses a ferroelectric liquid crystal display with row electrodes for consecutive strobe signals and column electrodes for simultaneous data signals. This display has liquid crystal cells defined where the row electrodes overlap the column electrodes. Each cell is of the multi-threshold type with a continuously varying liquid crystal layer thickness. The data signals comprise pulses whose magnitudes vary in order to control the resultant pulse applied to the cells. The addressing scheme disclosed in EP 0 510 606 is of a type in which each resultant pulse has two polarity inversions. Each data pulse comprises a first pulse of a first polarity and a duration of ΔT/2, a second pulse of the same magnitude but of a second polarity opposite the first polarity and a duration ΔT, and a third pulse of the first polarity and a duration ΔT/2. The strobe signal is a larger magnitude single pulse and is simultaneous with the second data pulse. Thus, the resultant pulse across each cell comprises first and third pulses each of duration ΔT/2 and a second pulse of larger magnitude, opposite polarity and duration ΔT. US 4 712 877 discloses a multi-threshold ferroelectric liquid crystal display in which the liquid crystal layer of each cell is of stepped thickness. According to a first aspect of the present invention there is provided a method as defined in the appended Claim 1. According to a second aspect of the invention, there is provided a ferroelectric liquid crystal crystal cell as defined in the appended Claim 3. Preferred embodiments of the invention are defined in the other appended claims. The use of this method has the advantage that a plurality of different magnitudes of electric field may be applied to each pixel of an LCD, the magnitude of the applied field controlling the effective grey level of each pixel. Since only one phase of data pulse is used, the use of this method has the further advantage of permitting good grey level discrimination whilst reducing pixel pattern dependence. Pixel pattern dependence occurs if data pulses of differing phases are used. The invention will further be described, by way of example, with reference to the accompanying drawings, in which:- Figure 1 is a diagram showing the operation of the drive scheme of an embodiment of the invention; Figure 2 is a graph of slot width vs. data voltage for two liquid crystal cells of different thicknesses; and Figure 3 is a graph of slot width vs. strobe voltage for two liquid crystal cells of different thicknesses and different data voltages. The drive scheme described with reference to the accompanying drawings is intended to be used with a ferroelectric LCD capable of displaying a plurality of grey levels. One such device comprises a liquid crystal layer, each pixel of which comprises a plurality of regions of different thickness of liquid crystal material. The voltage which must be applied to the liquid crystal material in order to change the state of the material is dependent upon the thickness of the liquid crystal material. If each pixel contains two regions of differing thickness of the liquid crystal material, i.e. a single step in thickness, the application of a relatively low voltage data pulse to the pixel will switch both regions of the pixel, whereas a relatively high voltage pulse data will result in none of switched. The application of a relatively intermediate voltage data pulse will switch the thicker of the two regions of the pixel. Of course, the drive scheme may be used with a pixel comprising four steps. It will be recognised that on applying a relatively high voltage data pulse to the pixel, only one of the four steps may be switched, the application of a lower voltage data pulse resulting in the switching of two, three or perhaps all four of the steps of a pixel. Depending upon the number of elements of each pixel which are switched on, the pixel may appear white, black or in one of several intermediate grey levels. The ferroelectric LCD includes a plurality of such pixels arranged in rows and columns. A plurality of first electrodes is arranged so that the pixels forming each row are electrically connected to one another. In addition, a plurality of second electrodes is arranged to electrically connect each of the pixels forming each column. In order to control the state of each pixel of the LCD, a voltage pulse is applied to the electrodes. Since each pixel of the LCD is influenced by the voltage pulse applied to the corresponding first and second electrodes, it will be recognised that each pixel of the LCD may be individually addressed. In order to control a particular pixel, a voltage pulse is applied to the first electrode which is connected to that row of pixels, and that voltage pulse is known as the strobe pulse. A second voltage pulse known as the data pulse is applied to the second electrode interconnecting the appropriate column of pixels. As shown in Figure 1, where the LCD is driven using the drive scheme according to the present invention, the strobe pulse comprises a first time slot for a strobe pre-pulse 1 and a second time slot for a strobe main pulse 2, one of which is at zero potential and the other of which is at a voltage the magnitude of which is referred to as Vs. In the illustrated example, the strobe pre-pulse I is at zero potential and the strobe main pulse 2 is at a potential of magnitude Vs. The data pulse also comprises two slots, the magnitude of the pulse being equal for both of the slots, one slot being positive, and the other negative in order to DC balance the data pulse. At the addressed pixel, the resultant pulse 3 applied to the pixel is the combination of the strobe pulse and the data pulse and, as shown in Figure 1, depending upon the shape and magnitude of the data pulse, the magnitude of the resultant pulse 3 applied to the pixel is variable. Consequently, in this example, the resultant pulse 3 has a first slot for a resultant pre-pulse 4 and a second slot for a resultant main pulse 5. In order to be able to control the different steps of the pixel, the magnitude of the data pulse applied to the appropriate second electrode is adjustable. The application of a first voltage Vd 1 is arranged to switch both the first and second regions of the pixel when the strobe pulse is applied to the appropriate first electrode. The application of a larger voltage Vd 2 to the appropriate second electrode is arranged to switch only a first one of the steps when the strobe pulse is applied to the appropriate first electrode. Figures 2 and 3 are graphs of slot width against Vs and Vd. In Figure 2, the curves show that, for a fixed level of Vs (in the Figure 2 case, Vs = 30V), the magnitude of the data pulse required to switch the liquid crystal is dependent upon the thickness of the liquid crystal layer. The polarity of the values of the abscissa correspond to the polarity of the pre-pulse of the data pulse. Negative values correspond to data pulses which, when applied with the strobe pulses to the addressed pixels, yield resultant pulses having pre-pulses of opposite polarity to their associated main pulses. Positive values correspond to data pulses which, when applied with the strobe pulses to the addressed pixels, yield resultant pulses having pre-pulses of the same polarity as their associated main pulses. Consequently, negative values correspond to the pre-pulse of the resultant pulse having the opposite polarity to the main pulse of the resultant pulse. Similarly, positive values correspond to the pre-pulse of the resultant pulse having the same polarity as the polarity of the main pulse. Therefore, if the slot width is approximately 150 µs, the application of a data pulse of magnitude less than approximately -4V results in switching of both a thick (1.7 µm) region and a thin (1.36 µm) region of liquid crystal material. However, on applying a data pulse of -6V, only the thick region switches, the thin region remaining unchanged. The application of a data pulse of -10V would result in neither region switching. Figure 3 also indicates that there are regions in which, for a given magnitude of strobe pulse and for a given slot width, the application of a data pulse of one magnitude will result in regions of one thickness being switched while others are unchanged, variations in the magnitude of the data pulse determining which thicknesses of liquid crystal material will be switched. In producing the graphs of Figures 2 and 3, the regions comprise regions of cells having parallel rubbed alignment layers having a surface pretilt of approximately 5°. Where the ferroelectric liquid crystal material is of the type which displays a minimum in its response time-voltage characterises, as shown in Figures 2 and 3, it is clear that, for a particular size of time slot width, the application of a data pulse voltage of relatively low magnitude results in both the thick and thin cells being switched whereas the application of a larger magnitude data pulse for the same slot width results in only the thick cell switching, the thin cell remaining in its unswitched state. As shown in Figure 2, there is a band of finite width in which switching of some of the regions may occur, other regions of equal thickness not being switched. In order to control the pixels accurately, it is desirable not to apply electric fields falling within these bands. In Figure 2, these bands are indicated by the shaded areas. It will be recognised that it is desirable to control the LCD using a negative data pulse, the separation of the lines of the graph of Figure 2 being greater for negative data pulses than it is for positive values of the data pulse. Similarly it is desirable to use a strobe pulse of magnitude equal to or less than the minimum switching voltage of the material. In order to control a display comprising a plurality of such pixels, a strobe pulse is applied to one of the first electrodes and an appropriate data pulse is applied to each of the second electrodes. Switching of the desired one(s) of a first row of pixels is thus achieved. A strobe pulse is then applied to another of the first electrodes and appropriate data pulses applied to the second electrodes to achieve switching of the desired one(s) of the pixels forming a second row. This routine is repeated until each row has been switched, the routine then continuing by switching the first row and each successive row. By applying the strobe and data pulses at high speed, a substantially flicker-free display can be achieved. The strobe pulses may be extended in a similar manner to the Malvern schemes. The application of varying magnitude data pulses to the electrodes may reduce the contrast of the display and may in some cases cause flickering. These effects are caused by the variations in the RMS voltage applied to the second electrodes. It is advantageous to reduce these differences in order to reduce the contrast and flickering problems. One method of doing this is to apply a signal to the non-selected rows of pixels using their first electrodes which results in those rows of pixels being subject to an average value of the electric field rather than a varying value. For example, if two data voltages Vd 1 , Vd 2 are used (Vd 1 and Vd 2 having the same phase) and pulses of magnitude (Vd 1 +Vd 2 )/2 (in phase with the data pulses) are applied to the non-selected row electrodes, the same resultant magnitude is applied to all the pixels. However, since the magnitude of the data pulse is not constant, it is not possible to compensate for this effect accurately when more than two data pulse magnitudes are in use. An alternative method is to apply a compensating signal to all of the second electrodes between every ten or so strobe pulses in order to allow the RMS voltage applied to the second electrodes to be the same. Hence, the compensation signal has to be calculated for each set of second electrodes between the ten or so strobe pulses, since the voltage applied to the second electrodes depends upon the data pulses applied to the respective first electrodes. Depending upon how often the compensating signal is applied, the display will be slowed down. Although the preceding description relates to the control of a ferroelectric LCD which is capable of displaying a plurality of grey scale levels due to the liquid crystal layer being of varying thickness, it will be recognised that the described drive scheme could be used in LCDs in which grey scale is achieved by other means and is controlled by the application of electric fields of varying magnitude. If the method of controlling an LCD described above is used in combination with temporal and spatial dither, a large number of grey levels can be achieved. For example, using pixels which are capable of displaying four grey levels in combination with two bits of spatial and two bits of temporal dither, a total of 256 grey levels can be achieved.

Claims: A method of controlling a multi-threshold ferroelectric liquid crystal cell of the type comprising a ferroelectric liquid crystal layer displaying a minimum in its response time-voltage characteristics and having first and second regions of different switching properties, the method comprising applying a strobe pulse (Vs) to a first electrode of the cell and applying a data signal (Vd1, Vd2, Vd3) to a second electrode of the cell, the magnitude of the data signal (Vd1, Vd2, Vd3) being modulated in order to control the resultant signal (3) applied to the cell by the first and second electrodes and being selected from at least three different magnitudes, such that: the resultant signal (3) comprises a pre-pulse (4) of a first polarity in a first time slot and a main pulse (5) of a second polarity opposite the first polarity in the second consecutive time slot; the data signal (Vd1, Vd2, Vd3) comprises a first pulse in the first time slot of a third polarity and a second pulse in the second time slot of a fourth polarity opposite the third polarity; each of the pre-pulse (4) and the main pulse (5) is of rectangular shape; the magnitude of the pre-pulse (4) is less than the magnitude of the main pulse (5); and each time slot has a width such that the first and second regions are switched when the data signal has the smallest of the at least three magnitudes (Vd1), neither of the first and second regions is switched when the data signal has the largest of the at least three magnitudes (Vd3) and only the first region is switched when the data signal has a magnitude (Vd2) between the smallest and the largest magnitudes. A method as claimed in Claim 1, wherein the strobe pulse is extended into at least one further time slot which is consecutive with the second time slot. A multi-threshold ferroelectric liquid crystal cell comprising a ferroelectric liquid crystal layer, first and second electrodes, first means for applying a strobe pulse (Vs) to the first electrode, second means for applying a data signal (Vd1, Vd2, Vd3) to the second electrode, and means for modulating the magnitude of the data signal (Vd1, Vd2, Vd3) in order to control the resultant signal (3) applied to the cell, the data signal (Vd1, Vd2, Vd3) having a magnitude selected from at least three different magnitudes, characterised in that : the ferroelectric liquid crystal layer comprises a layer of liquid crystal material of a type displaying a minimum in its response time-voltage characteristics; the resultant signal (3) comprises a pre-pulse (4) of a first polarity in a first time slot and a main pulse (5) of a second polarity opposite the first polarity in a second consecutive time slot; the data signal (Vd1, Vd2, Vd3) comprises a first pulse in the first time slot of a third polarity and a second pulse in the second time slot of a fourth polarity opposite the third polarity; each of the pre-pulse (4) and the main pulse (5) is of rectangular shape; the magnitude of the pre-pulse (4) is less than the magnitude of the main pulse (5); the cell has first and second regions of different switching properties; and each time slot has a width such that the first and second regions are switched when the data signal has the smallest of the at least three magnitudes (Vd1), neither of the first and second regions is switched when the data signal has the largest of the at least three magnitudes (Vd3), and only the first region is switched when the data signal has a magnitude (Vd2) between the smallest and the largest magnitudes. A ferroelectric liquid crystal cell as claimed in Claim 3, characterised in that the first and second regions have different liquid crystal layer thicknesses. A liquid crystal display comprising a plurality of liquid crystal cells as claimed in Claims 3 or 4, arranged as a plurality of rows and columns, the first electrodes of the cells of each row being electrically connected together, the second electrodes of the cells of each column being electrically connected together, and the liquid crystal layer being a continuous layer extending through each of the cells. A display as claimed in claim 5, characterised in that the first means is arranged to supply the strobe pulse in turn to the rows of the first electrodes and characterised by third means adapted to simultaneously supply to the other rows of the first electrodes a signal being in phase with the data signal and having a magnitude equal to the arithmetic mean of the at least three different magnitudes. A display as claimed in claim 5, characterised by fourth means adapted to periodically supply to the second electrodes of each column between strobe pulses a compensating signal such that the same RMS voltage is applied to each of the second electrodes.

Inventors: BONNETT PAUL, RAYNES EDWARD PETER, TAGAWA AKIRA, BONNETT, PAUL, RAYNES, EDWARD PETER, TAGAWA, AKIRA
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1995
Main classification: G09G3
IPC codes: G02F1, G09G3