GB2208739A

GB2208739A - Liquid crystal devices

Info

Publication number: GB2208739A
Application number: GB8719079A
Authority: GB
Inventors: Carolyn Bowry; Alan Mosley; Beatrice Mary Nicholas
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1987-08-12
Filing date: 1987-08-12
Publication date: 1989-04-12
Anticipated expiration: 2007-08-12
Also published as: GB8719079D0; GB2208739B

Abstract

In a method of driving a matrix of ferroelectric liquid crystal devices in a TDM mode, a strobing signal comprises a first and a second pulse 24, 25 of mutually opposite polarity and either or both of the data ON and data OFF signals 27, 28 comprises n pulses of alternately opposite polarities (where n>1) spanning the same time period as the first pulse 24 or the second pulse 25, or 2n such pulses spanning both those time periods. By use of the relatively short n or 2n data pulses of alternately opposite polarities, the effect of crosstalk due to the multiplexing is reduced, because the liquid crystal director of an unaddressed device moves only a small distance in one direction, followed by a small distance in the opposite direction, in response to the successive pulses. <IMAGE>

Description

Ferroel ectri c Liquid Crystal Devices This invention relates to ferroelectric liquid crystal (FLC) devices, and particularly to a method and apparatus for driving the liquid crystal elements of such displays.

A ferroelectric liquid crystal has a permanent electric dipole which interacts with the applied electric field. Hence, ferroelectric liquid crystals exhibit fast response times, which make them suitable for use in display, switching and information processing applications. An example of an FLC device is described in a paper by N. A. Clarke et al, entitled USubmicrosecond bistable electro-optic switching in liquid crystals" in Appl. Phys. Lett. Volume 36, 1980, pp 899-901.

The stimulus to which an FLC device responds is a dc field, and its response is a function of the applied voltage (V) and the length of time (t) for which it is applied. The response is not a linear function of V x t, and there may be a voltage level at which, irrespective of the length of time for which the voltage is applied, switching of the device will not occur. There may also be a length of time of application of the voltage which will be too short for switching to occur, irrespective Of the magnitude of the voltage.

An FLC device which can be multiplexed needs to have at least two different states (called latched states) which the liquid crystal can adopt in the absence of an applied field. These can be the same states as the states (called switched states) obtained when a field of either polarity is applied, or they can be different states.

The liquid crystal can change from one switched state to another switched state when a field is applied thereto, without necessarily going to a latched state when the field Is removed.

For a given time interval the voltage at which the liquid crystal switches from one state to the other by 10S is called the switching threshold at 10 % switching (T510). The voltage at which the liquid crystal switches fully from one state to the other state is called the switching threshold at 100% switching (T5100). The voltage at which the liquid crystal will go fully into one of the latched states when the field is removed is called the latching threshold at 100% latching (TL1oo). The voltage at which the liquid crystal no longer goes into either of two different states when the field is removed is called the latching threshold at 0% latching (TLo).

A ferroelectric liquid crystal element is switched to one state by the application of a voltage of a given polarity across its electrodes, and is switched to the other state by the application thereto of a voltage of the opposite polarity. It is essential that an overall dc voltage shall not be applied across such an element for an appreciable period, so that the elements remain charge-balanced, thereby avoiding decomposition of the liquid crystal material.

Pulsed operation of such elements has therefore been effected, with a pulse of one polarity being immediately followed by a pulse of the other polarity, so that there is no resultant dc polarisation.

The liquid crystal elements are commonly arranged in matrix formation and are operated selectively by energising relevant row and column lines. Time-division multiplexing is effected by applying pulses cyclically to the row (strobe) lines in sequence and by applying pulses, in synchronism therewith, to selected column (data) lines.

An example of an FLC display driving system is disclosed in an article by T. Harada, M. Taguchi, K. Iwasa and M. Kai in SID 85 Digest, p 131 et seq. This system uses four pulses per refresh cycle, and can therefore be classified as a 4-slot drive system.

A problem which results from the multiplexing operation is the production of crosstalk. Since each data line is coupled to a number of liquid crystal elements which, at any instant, are not being addressed by the strobe pulses, those elements will be subjected to the pulses applied to the data lines. Ideally the data pulses should not, in the absence of corresponding strobe pulses, cause switching of the unaddressed crystal elements. However, it is generally accepted that the data pulses will cause movement of the director of the liquid crystal material in the unaddressed elements, which results in the occurrence of some switching. This switching will degrade the contrast between the ON and OFF elements.

It is an object of the present invention to provide a method and apparatus for driving the elements of an FLC device, in which the effects of crosstalk are reduced.

According to one aspect of the invention there is provided a method of driving a ferroelectric liquid crystal device matrix in a time-division multiplexing mode, comprising applying strobing signals cyclically to strobe lines coupled to two-state liquid crystal elements of the device and applying data signals selectively to data lines coupled to the elements, wherein each strobing signal comprises a first strobe pulse of one polarity followed by a second strobe pulse of the opposite polarity, wherein the data signals comprise a first data signal operative in combination with the strobing signal to set a selected liquid crystal element in a first one of its states, and a second data signal operative in combination with the strobing signal to set the selected liquid crystal element in the other of its states, and wherein one or each of the first and second data signals comprises n pulses of alternately opposite polarities, where n is a positive integer greater than unity, the n pulses together spanning the same time period as said first strobe pulse or said second strobe pulse or both of said first and second strobe pulses.

Preferably n is an odd integer, and preferably the last of the n pulses is of such polarity as will tend to set the selected liquid crystal element in a desired one of its stable states.

According to another aspect of the invention there is provided apparatus for driving a ferroelectric liquid crystal display matrix in a time-divlsion multiplexing mode, comprising means to apply strobing signals cyclically to strobe lines coupled to two-state liquid crystal elements of the device; and means to apply data signals selectively to data lines coupled to the elements, wherein each strobing signal comprises a first strobe pulse of one polarity followed by a second strobe pulse of the opposite polarity, wherein the data signals comprise a first data signal operative in combination with the strobing signal to set a selected liquid crystal element in a first one of its stable states, and a second data signal operative in combination with the strobing signal to set the selected liquid crystal element in the other of its states, and wherein one or each of the first and second data signals comprises n pulses of alternately opposite polarities, where n is a positive integer greater than unity, the n pulses together spanning the same time period as said first strobe pulse or said second strobe pulse or both of said first and second strobe pulses.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, wherein Figure 1 is a block schematic diagram of an FLC drive system; Figure 2 illustrates strobing and data pulses occurring in a previously-known 4-slot drive system; Figure 3 shows the waveforms giving rise to crosstalk on unaddressed liquid crystal elements of the FLC device; Figure 4 illustrates strobing and data pulses occurring in a first embodiment of the invention; Figure 5 is a graph showing schematically the variation in switching threshold voltage of a crystal element with applied pulse width; and Figures 6-12 illustrate strobing and data pulses occurring in respective further embodiments of the invention.

Referring to Figure 1, a display comprises a matrix of ferroelectric liquid crystal elements 1 coupled to row (strobe) and column (data) lines 2 and 3, respectively. For the sake of example, nine of such elements coupled to three strobe lines and three data lines are shown, but there may be any desired number of elements and corresponding lines. A strobe pulse generator 4 is coupled to the strobe lines, and a data pulse generator 5 is coupled to the data lines. The strobe pulse generator continuously applies strobing signals to the strobe lines 2 in sequence, and the data pulse generator applies data signals to the data lines 3, in synchronism with the pulsing of the strobe lines, to set the corresponding elements 1 in the required states.

Figure 2 shows waveforms which would be applied to the lines 2 and 3 in a known 4-slot drive system as disclosed in the above-mentioned article by Harada et al. In Figure 2(a), a strobing signal comprises a positive pulse 6 followed by a negative pulse 7 and, later during the same frame period, a negative pulse 8 followed by a positive pulse 9. All of these pulses are of the same amplitude Vs, and there is therefore no residual dc level. The data signal may comprise a pulse train 10 (Figure 2(b)) for setting the addressed element in the ON state or a pulse train 11 (Figure 2(c)) for setting it in the OFF state. The pulse train 10 comprises positive and negative pulses 12 and 13, respectively, coincident with the pulses 6 and 7, and positive and negative pulses 14 and 15, respectively, coincident with the pulses 8 and 9. The pulses 12, 13, 14 and 15 are all of amplitude Vd.The pulse train 11 comprises pulses 16, 17, 18 and 19 of the same amplitude as the pulses 12, 13, 14, and 15 but of opposite polarity thereto.

It will be apparent that the data pulses are also applied via the lines 3 to those liquid crystal elements 1 which are not being addressed by the strobing signal. This leads to crosstalk, which is inherent in any multiplexing scheme. In order to reduce visible crosstalk effects there are certain conditions which a multiplexing scheme must satisfy, as follows: 1. The data voltage Vd must not be large enough to switch the liquid crystal. Switching the liquid crystal will reduce the contrast of the device.

i.e. Vdz T10 2. The strobe voltage plus the data voltage (Vs + Vd) must be large enough to switch and latch the liquid crystal so that the correct state (On or OFF) of the element is achieved.

i.e. VS + VdB > TL100 3. The strobe voltage minus the data voltage (Vs - Vd) can switch the liquid crystal since it occurs only once in every frame scan. However, it must not latch the liquid crystal, since this will reverse the data required, not must it unlatch the liquid crystal.

i.e. Vs - VdTLo Whilst strobe pulses and data pulses are being applied to the addressed liquid crystal elements, those data pulses are also being applied to those other liquid crystal elements which are coupled to the same data line. Considering the case of a particular crystal element, such as the element E in Figure 1, that element will receive desired strobe pulses and data pulses when its strobe line 2' is addressed by the generator 4, but it will also receive undesired data pulses over its data line 3' when other strobe lines are being addressed, those data pulses being intended for other elements coupled to the line 3'. This is illustrated in Figure 3.Figure 3(a) shows the strobe pulses for a particular strobe line, for example the line 2', and Figure 3(b) shows the data pulses applied to the data line 3', assuming, for the sake of example, that the data signals are all data ON signals. In practice they will be a mixture of data ON and data OFF signals depending upon the data applied to the device.

Figure 3(b) shows desired data signals 20 and 21 applied to the liquid crystal element E and undesired data signals 22 and 23 applied to the element E but intended for the other elements coupled to the data line 3'. The data signals 22 and 23 will cause the liquid crystal element E to tend to switch back and forth, and the resultant movement of the liquid crystal director will cause degradation of the contrast of the device. It should be remembered that every liquid crystal element in the device Kill be affected in this manner during every period when it is not being addressed.

Figure 4 shows strobe and data pulses provided in a first embodiment of the invention. In Figure 4(a) strobe pulses 24 and 25 are of opposite polarities and of unequal amplitudes. The pair of pulses occurs only once during a frame period, taking up two time slots. The system is therefore a 2-slot system. In order to ensure that no net dc voltage is applied to the liquid crystal element, a constant dc voltage 26 is applied to the strobe line. The dc voltage compensates for the difference between the amplitudes of the pulses 24 and 25.

Figure 4(b) shows a data ON signal in one form in accordance with the inventive concept of the present invention. The data signal comprises a string 27 of alternately negative and positive pulses, together filling the time slots corresponding to the strobe pulses 24 and 25. In this example there are six pulses in the string three corresponding to each strobe pulse, but there may be any desired number. It is, in fact, preferable if there is an odd number of pulses corresponding to each strobe pulse, with the last pulse being of such polarity as to urge the addressed element into the desired state. Such arrangement will also ensure that there are more pulses tending to latch the element than those tending to unlatch it.

As there are equal numbers of positive and negative pulses in each cycle,there will be no net dc level. In Figure 4(c) the data OFF signal 28 is the inverse of the data ON signal of Figure 4(b).

The voltage appearing across the liquid crystal element E as a result of the combined strobe and data ON signals is shown in Figure 4(d). The voltage waveform comprises not only desired pulses 29 and 30, but also undesired pulses 31 and 32 which occur while the element E is not being addressed. The pulses 31 and 32 have a much smaller crosstalk effect than the pulses 22 and 23 of Figure 3, as will now be explained.

Referring to Figure 5, it will be seen that the switching threshold voltage of a ferroelectric liquid crystal element varies non-linearly with the width of the applied pulse. Hence, for a long pulse of width t1, such as the pulses 20 of Figure 3, the voltage required to switch the element is much smaller than that required for a short pulse of width t2, such as the pulses of the string 29 in Figure 4(d).

If we now consider the effect of the pulses 29 applied across the liquid crystal element, the first pulse may not be sufficient, in combination with the strobe pulse, to cause switching of the element, because it is too narrow. The voltage then decreases at the end of the first positive-going pulse and then rises again, the effect of the two positive-going pulses being cumulative. It must be arranged that this cumulative effect is sufficient, with the strobe pulse, to latch the element. The amplitude of the negative pulses 30 is selected such that there is no residual dc level as a result of the pulses 29 and 30 taken together. The pulses 30 must be large enough to cause latching of the element, but the combination of the pulses 25 and 28 must not cause the element to latch.

Considering, now, the effect of the unwanted pulse strings 31 and 32, it will be seen that the first positive pulse in each string will tend to cause movement of the director of the crystal liquid material, but due to the narrowness of the pulse the effect will be small. The next pulse is negative, so there will be a small movement in the opposite direction, and so on. It will be apparent that the visual effect of these short pulses, causing small movements of the liquid crystal director in successively opposite directions, can be less than the effect of a long pulse such as the pulses in the undesired data signals 22 and 23 in Figure 3 (b).

Figure 6 shows an alternative to the configuration of Figure 4. In this case the data ON signal 33 and the data OFF signal 34 each comprise an even number (four) of pulses corresponding to each strobe pulse.

An alternative arrangement of short data pulses is shown in Figure 7, in which the strobe signal (Figure 7(a)) is the same as in Figures 4 and 6, but the data ON signal (Figure 7(b)) comprises a train 35 of four pulses of successively opposite polarity, starting with a negative pulse and taking up a time slot corresponding to that of the strobe pulse 24. The data OFF signal in Figure 7(c) comprises a train 36 of four pulses of successively opposite polarities starting with a positive pulse and taking up a time slot corresponding to that of the strobe pulse 25.

Figure 8 shows another embodiment, in which the data ON signal (Figure 8(b)) comprises a zero dc level 37, whilst the data OFF signal comprises a train 38 of alternately negative and positive pulses spanning the two time slots of the strobe pulses 24 and 25.

Alternatively, the data ON signal may comprise a train of pulses and the data OFF signal may comprise a zero dc level.

Figure 9 shows an embodiment in which the data ON signal comprises a train 39 of pulses spanning only the time slot of the strobe pulse 24. The data OFF signal comprises a zero dc level 40.

Alternatively, the data ON signal may comprise a zero dc level and the data OFF signal may comprise a train of pulses spanning only the time slot of the strobe pulse 25.

In Figure 10 the data ON signal is similar to that of Figure 9(b) and the data OFF signal comprises negative and positive pulses 41, 42 of the same width as the strobe pulses 24 and 25.

Figure 11 shows an embodiment in which the data ON signal (Figure 11(b)) comprises a train 43 of an odd number of alternately positive and negative pulses, spanning the time slot of the strobe pulse 24, followed by a negative pulse 44 spanning the time slot of the strobe pulse 25. The figure shows, merely as an example, three pulses in the train 43. As there are not equal numbers of positive and negative pulses in the train to cancel each other, there would be a resultant positive dc voltage across the element. However, the negative pulse 44 cancels that dc voltage. It will be apparent that if the magnitude and the period of the last pulse in the train 43 are 3Vd and t/3,respectively, and the period of the pulse 44 is t, the amplitude of the pulse 44 must be Vd, in order to cancel the dc voltage due to that last pulse.The data OFF signal (Figure 11(c)) is the inverse of the data ON signal.

All of the above configurations are 2-slot schemes.

However, the invention is equally applicable to 4-slot schemes, such as that described above with reference to Figure 3. Figure 12 shows such a scheme in which the data ON signal comprises trains 43, 44 of pulses spanning the time slots of the strobe pulses 6, 7 and the strobe pulses 8, 9 respectively. The data OFF signal is the inverse of the data ON signal.

In every embodiment, the pulse voltages and pulse lengths will be adjusted to suit the particular type of liquid crystal elements and the particular combination of strobing and data signals.

It may be advantageous to provide positive and negative pulses which are of mutually different amplitudes and widths. In any of the configurations described above, the polarity of both the strobe pulses and the data pulses may be reversed. For some liquid crystal elements, keeping the strobe pulses the same as in the above embodiments but reversing the polarities of the data pulses may produce a cumulative switching effect which is advantageous.

Any of the various data ON signals shown in Figures 4 or 6-12 may be used in conjunction with any of the data OFF signals shown in those figures.

In each of the drive arrangements of the present invention a further improvement may be effected by superimposing an ac voltage at, say, 100kHz on the pulses. This helps to sharpen the switching thresholds and may also further improve the contrast ratio of the data ON and OFF states during multiplexing. Furthermore, it may be advantageous to provide periods of zero voltage between successive strobe and/or data pulses.

Claims

1. A method of driving a ferroelectric liquid crystal device matrix in a time-division multiplexing mode, comprising applying strobing signals cyclically to strobe lines coupled to two-state liquid crystal elements of the device and applying data signals selectively to data lines coupled to the elements, wherein each strobing signal comprises a first strobe pulse of one polarity followed by a second strobe pulse of the opposite polarity, wherein the data signals comprise a first data signal operative in combination with the strobing signal to set a selected liquid crystal element in a first one of its states, and a second data signal operative in combination with the strobing signal to set the selected liquid crystal element in the other of its states, and wherein one or each of the first and second data signals comprises n pulses of alternately opposite polarities, where n is a positive integer greater than unity, the n pulses together spanning the same time period as said first strobe pulse or said second strobe pulse or both of said first and second strobe pulses.

2. A method as claimed in Claim 1, wherein the n pulses span the same time period as said first strobe pulse or said second strobe pulse and n is an odd integer.

3. A method as claimed in Claim 1 or Claim 2, wherein the last pulse of said n pulses is of such polarity as will tend to set the selected crystal element in a desired one of its stable states.

4. A method as claimed in any preceding claim, wherein each of said first and second data signals comprises n data pulses, wherein the first of the n pulses of said second signal is of opposite polarity to the first of the n pulses of said first signal, and wherein the n data pulses of said first signal together span the same time period as said first strobe pulse and the n data pulses of said second signal together span the same time period as said second strobe pulse.

5. A method as claimed in any one of Claims 1-3, wherein each of said first and second signals comprises two continuous sets of n data pulses together spanning the same time period as both said first and second strobe pulses, and wherein the first data pulse of said first signal is of opposite polarity to the first data pulse of said second signal.

6. A method as claimed in Claim 1, wherein the first data signal comprises n pulses of alternately opposite polarities, and wherein the second data signal comprises m pulses of alternately opposite polarities where m is a positive integer different from n.

7. A method as claimed in Claim 1, wherein one of said first and second data signals comprises a zero voltage level.

8. A method as claimed in any preceding claims, wherein a period of zero voltage is provided between successive strobe and/or data pulses.

9. A method as defined in any preceding claim, wherein a high frequency ac voltage is superimposed on each strobing and/or data signal.

10. A method of driving a ferroelectric liquid crystal display matrix, substantially as hereinbefore described with reference to Figures 1 and 4-11 of the accompanying drawings.

11. Apparatus for driving a ferroelectric liquid crystal device matrix in a time-division multiplexing mode, comprising means to apply strobing signals cyclically to strobe lines coupled to two-state liquid crystal elements of the device; and means to apply data signals selectively to data lines coupled to the elements, wherein each strobing signal comprises a first strobe pulse of one polarity followed by a second strobe pulse of the opposite polarity, wherein the data signals comprise a first data signal operative in combination with the strobing signal to set a selected liquid crystal element in a first one of its states, and a second data signal operative in combination with the strobing signal to set the selected liquid crystal element in the other of its states, and wherein one or each of the first and second data signals comprises n pulses of alternately opposite polarities, where n is a positive integer greater than unity, the n pulses together spanning the same time period as said first strobe pulse or said second strobe pulse or both of said first and second strobe pulses.