Ferro-electric liquid crystal electro-optical device

Abstract: A ferro-electric liquid crystal electro-optical device uses switching between bi-stable states of ferro-electric liquid crystal molecules (3) and has a driving circuit (Figures 7 and 8) for changing between stable states (7,9) by applying a selected voltage signal consisting of a combination of a chopping pulse to which the liquid crystal molecules are not responsive and a DC pulse to which the liquid crystal molecules are responsive.

Description: This invention relates to ferro-electric liquid crystal electro-optical devices eg. display devices , electro-optical shutters for printers or the like, for effecting electro-optical conversion by utilizing spontaneous polarization of a ferro-electric liquid crystal material and its negative dielectric anisotropy. Ferro-electric liquid crystal electro-optical devices which utilize the spontaneous polarization of ferro-electric liquid crystal material and its negative dielectric anisotropy are known and are, for example, disclosed in Japanese Laid-Open Patent Application No. 176097/1985. Figure 2 of the accompanying drawings is a perspective view of a conventional ferro-electric liquid crystal cell. A pair of transparent glass substrates 1,1 are arranged to face each other. An alignment membrane 2,2 is oriented aniaxially and horizontally, and is disposed on an inner flat surface of the substrate 1,1. A rubbed film of polyimide, for example, is used for each of the alignment membranes. The rubbing direction of the pair of alignment membranes is substantially parallel. Reference numeral 3 represents a ferro-electric liquid crystal material such as a chiral smectic liquid crystal material (which will hereinafter be referred to as "SmC* material"). The liquid crystal material has spontaneous polarization in a direction orthogonal to the major axis of the liquid crystal molecule (hereinafter referred to as the "molecular axis"). Here, those liquid crystal materials which have negative dielectric anisotropy Δε above at least a predetermined frequency are particularly useful. Δε below 0 (Δε < 0) means that dielectric polarization occurs in a direction orthogonal to the molecular axis due to an external electric field having a predetermined frequency range. The molecules of SmC* material 3 are sandwiched between the substrates 1,1 exhibit horizontal alignment due to the influence of the alignment membranes 2,2 as shown in Figure 2 and form a layer. A pair of electrodes 4,5 are arranged to face each other in order to clamp the SmC* material 3 between them and to apply a driving voltage thereto. Figure 3 is a driving waveform diagram of a conventional liquid crystal cell. A first DC pulse having positive polarity is applied between the electrodes 4,5. However, the electrode 4 is kept at ground potential. The liquid crystal molecules are aligned in such a fashion that spontaneous polarization 6 of each liquid crystal molecule is arranged perpendicular to the electrode 4 (see Figure 2). This is a first stable state 7, in which the molecular axis is inclined by +ϑ with respect to a normal 8 to the layer of SmC* material. Next, when AC pulses are applied, dielectric polarization occurs in a direction perpendicular to the molecular long axis because the liquid crystal molecule has negative dielectric anisotropy, and the first stable state is maintained and fixed by dielectric torque. When a second DC pulse having a negative polarity is further applied between the electrodes 4,5, the liquid crystal molecule is responsive to this pulse and the spontaneous polarization 6 of each liquid crystal molecule is aligned in a state where it faces perpendicular to the electrode 5. This is a second stable state 9, where the molecular axis is inclined by -ϑ relative to the normal 8 to the layer of SmC* material (see Figure 2). Thereafter, when AC pulses are applied, this second stable state is maintained. Namely, the first stable state is written by the positive DC pulse, the second stable state is written by the negative DC pulse and the stable state is maintained by the AC pulses. Reverting back to Figure 2, reference numeral 10,10 represents a pair of polarizers whose polarization axes cross each other at right angles. They clamp the SmC* material 3 and optically discriminate between the liquid crystal domains in the second stable state by utilizing birefringence. For example, the first stable state is discriminated as a light cut-off state (hereinafter referred to as "black") and the second stable state, as a light transmission state (hereinafter referred to as "white"). The prior art reference referred to above discloses that the electrode arrangement of the liquid crystal cell is of a matrix structure type such as shown in Figure 4 and the scanning or segment electrodes 4 and the signal or common electrodes 5 are opposed. However, this reference does not disclose a driving waveform and a driving circuit for actually effecting line sequential driving. It is not possible to effect matrix driving using the waveform shown in Figure 3. The present invention seeks to provide a ferro-electric liquid crystal electro-optical device which has a drive circuit for matrix-driving, which uses spontaneous polarization of a ferro-electric liquid crystal material and its negative dielectric anisotropy, and which can write both white and black by one line sequential scanning. In EP-A-0149899 and in an article on pages 128 to 130 of 1985 International Symposium, digest of technical papers, by J.M. Geary, there is disclosed the provision of a steady Ac signal combined with biopolar pulses by which switching is achieved. According to the invention, there is provided a ferro-electric liquid crystal electro-optical device which uses switching between bi-stable of ferro-electric liquid crystal molecules, including driving means for changing the states of the molecules between stable states by applying a selected signal having first and second portions, entirely of respective opposite polarities, one of the first and second portions consisting of a DC pulse of a polarity to change the state of the molecules from one to another state, and the other of the first and second portions consisting of a chopped pulse of opposite polarity ineffective to change the stable state of the molecules, the chopped pulse having a high frequency under which the ferro-electric liquid crystal molecules exhibit negative dielectric anisotropy. The invention also provides a method of operating a ferro-electric liquid crystal electro-optical device, including the steps of changing the state of the liquid crystal molecules by applying a selected signal having first and second portions, entirely of respective opposite polarities, one of the first and second portions consisting of a DC pulse of a polarity to change the state of the molecules from one to another stable state, and the other of the first and second portions consisting of a chopped pulse of opposite polarity ineffective to change the stable state of the molecules, the chopped pulse having a high frequency under which the ferro-electric liquid crystal molecules exhibit negative dielectric anisotropy. The invention also extends to a method of operating a ferro-electric liquid crystal electro-optical device of dot-matrix form, comprising a plurality of segment electrodes and a plurality of common electrodes defining a plurality of display pixels characterised in that in operation, a non-selection alternating voltage of high frequency under which the liquid crystal molecules exhibit negative dielectric anisotropy is applied to the common electrodes other than a selected common electrode a selection signal having first and second portions is applied to the selected common electrode the portions comprising DC pulses of opposite polarity and of an amplitude to change the state of the liquid crystal molecules between stable states, and signal pulses are applied to the segment electrodes each having first and second portions, corresponding in time to the first and second portions of the selection signal, one portion being a chopped pulse of the same high frequency and the other portion having zero level potential, the switching DC pulse being concurrent with the zero level potential portion, whereby a selected voltage effecting either one or the other of the stable states is applied to the display pixels on the selected common electrode. Other aspects of the invention are defined in the dependent claims. The scope of the present invention is defined by the appended claims; and how it can be carried into effect is hereinafter described by way of example and illustrated in the accompanying drawings, in which:- Figure 1 (A) is a waveform diagram of signals applied to matrix dots; Figure 1 (B) of a waveform diagram of signals applied to common electrodes and segments electrodes; Figure 1(C) shows a matrix electrode structure; Figure 2 is a perspective view of a conventional liquid crystal cell; Figure 3 is an operating waveform diagram for the conventional liquid crystal cell; Figure 4 shows the arrangement of electrodes of a liquid crystal cell; Figure 5 is a test waveform diagram useful for explaining the operation of a ferro-electric liquid crystal electro-optical device according to the present invention; Figure 6 is a contrast ratio-v-impressed voltage characteristic diagram useful for explaining the operation of a ferro-electric liquid crystal electro-optical device according to the present invention; Figure 7 is a common electrode drive circuit of a ferro-electric liquid crystal electro-optical device according to the present invention; Figure 8 is a segment electrode drive circuit of a ferro-electric liquid crystal electro-optical device according to the present invention; Figure 9 is a time chart for a common and segment electrode drive circuit of a ferro-electric liquid crystal electro-optical device according to the present invention; and Figure 10 shows an embodiment of a common electrode drive circuit of a ferro-electric liquid crystal electro-optical device according to the present invention generating non-selecting common pulses with a desired amplitude as shown in Figure 1(B). In a ferro-electric liquid crystal electro-optical device of the type which selectively aligns liquid crystal molecules in a first stable state or a second stable state by utilizing the spontaneous polarization of ferro-electric liquid crystal molecules and keeps each of these stable states by utilizing the negative dielectric anisotropy of ferro-electric liquid crystal material, the present invention produces an impressed voltage for producing each stable state by the combination of a chopped pulse to which the liquid crystal molecules are not responsive and a DC pulse to which they are responsive, and arranges these DC pulses so that their phases do not overlap with each other between the impressed voltage for producing the first stable state and the impressed voltage for producing the second stable state. Therefore, when line sequential driving is carried out in a ferro-electric liquid crystal electro-optical device having a matrix electrode arrangement, the first stable state and the second stable state can be written simultaneously into each matrix pixel by one line sequential scanning operation. The present invention will be described with reference to Figure 1. Figure 1(C) shows a matrix electrode construction of a liquid crystal cell. Two scanning or segment electrodes S₁, S₂ and two signal or common electrodes C₁, C₂ are arranged in such a manner as to form four matrix pixels (hereinafter referred to as "dots") D₁ to D₄. The rest of the construction of the liquid crystal cell is the same as shown in Figures 2 and 4. Figure 1(A) shows the waveform applied to each dot. This example shows the waveform for selecting the common electrode C₁ by line sequential scanning and for writing simultaneously white and black to the dots D₁ and D₂ on the common electrode C₁. A waveform which keeps the previous state is applied to the dots D₃ and D₄ on the non-selected common electrode C₂ A chopped positive pulse is applied to the dot D₁ in a first half period of the selected period and a negative DC pulse in a second half period. The molecules of SmC* material do not respond to the chopped pulses but do to the negative DC pulses so that white (second stable state) is written into the dot D₁. A positive DC pulse is applied to the dot D₂ in the first half period of the selection period and a negative chopped pulse in the second half period. The molecules of SmC* material respond to the positive DC pulse in the first half period and black (first stable state) is written into the dot D₂. They do not respond to the chopped pulse in the second half period. As described above the selection period is divided into two half periods so that the first and second half periods are utilized for writing black and white on a time division basis, respectively, and white and black are written simultaneously by one scanning operation. In this case, the invention utilizes the phenomenon that molecules of the SmC* material do not respond to the chopped pulse, and the explanation of this phenomenon will be given below. The AC pulses applied to the unselected dots D₃ and D₄ and the state already written into the dots D₃ and D₄ is maintained by the dielectric torque based upon Δε < 0. When the scanning operation is made line sequentially for a large number of common and segment electrodes (or in other words, when the common electrodes are scanned), re-write of a picture surface of the electro-optical device can be made in one frame. Figure 1(B) shows the waveforms applied to the segment and common electrodes in order to generate the driving waveforms to be applied to the dots D₁ to D₄ shown in Figure 1(A). Waveform (a) represents a common selection signal applied to the common electrode C₁, waveform (b) is a common non-selection signal applied to the common electrode C₂, waveform (c) is a white write signal applied to the segment electrode S₁ and waveform (d) is a black write signal applied to the segment electrode S₂. A circuit for generating these common and segment signals will be described below. The phenomenon that the molecules of SmC* material do not respond to the chopped pulse but do to the DC pulse will be explained. Figure 5 shows test pulses applied to a certain dot in the liquid crystal cell shown in Figures 2 and 4. Waveform (a) consists of DC pulses having a positive polarity and a peak value +V and DC pulses having a negative polarity and a peak value -V with a selection period (3 msec). The display state changes from black to white. Waveform (b) consists of chopped pulses having a peak value of +2V in the first half of the selection period and chopped pulses having a peak value -2V in the second half. Figure 6 is a diagram obtained by examining the contrast ratio when black changes to white during the selection period at each voltage level while the waveforms (a) and (b) are applied with a varying voltage V. In the case of the DC pulse (waveform (a)), a large contrast ratio can be obtained at about 30 V or more. In other words, the molecules of SmC* material shift completely from the first stable state to the second stable state at a threshold value of at least 30 V. In the case of the chopped pulse (waveform (b)), however, the change of the contrast is small even when a pulse having an amplitude of 60 V is applied, and it will be appreciated that molecules of the SmC* material do not completely shift from the first stable state to th second stable state. This can be explained as follows. The properties contributing to the reversion mechanism of the molecules of the SmC* material are believed to be spontaneous polarization and dielectric torque. The spontaneous polarization torque always acts in such a fashion that the spontaneous polarization is in parallel with the direction of electric field, irrespective of the polarity of Δε. The dielectric torque, however, act in such a fashion that the long axis of molecules are perpendicular to the electric field in the case of SmC* material having Δε < 0. In other words, where Δε < 0, the spontaneous polarization torque (which acts in such a fashion that at the initial state where the molecules are about to shift from the first stable state to the second stable state, the long axis of molecules is in parallel with the electric field) and the dielectric torque acts in opposite directions to each other. Therefore, where Δε < 0, response is believed to be slower than where Δε > 0. This dielectric torque is proportional to an effective voltage (rms value of voltage). The effective voltage of the chopped pulse is 2 V₁ while that of the DC pulse is V₁ and the former is greater by a factor of 2 than the latter and acts more strongly by a factor of 2 than the latter. Therefore, response of the chopped pulse is slower than that of the DC pulse and when measurement is made with a predetermined pulse width such as shown in Figure 6, the molecules cannot completely shift from the first stable state to the second stable state and hence, the contrast ratio remains small. Incidentally, the SmC* material used for measurement was Type 3234 of Merck Co having Δε of -2.4. Figure 7 shows a common (or strobe) electrode driving circuit for generating the common selection signal (waveform (a)) and the common non-selection signal (waveform (b)) shown in Fig 1(B). As will be appreciated from Figure 1(B), the necessary voltage levels are +V₁ and -V₁ and the necessary signals for generating the AC pulses at a signal DF₁ for halving the selection period into the first half and the second half and a signal DF₂ for generating the necessary high frequency for holding the stable state (see the time chart of Figure 9). Incidentally, the signal DF₂ is also used for chopping. The drive circuit of Figure 7 comprises a shift register 11, which receives a signal FLM for designating the selection period and a common shift clock CL₁ for distributing line-sequentially the signal FLM to each common electrode. The output of the shift register 11 is connected to a gate group 12. The gate group 12 receives the signals DF₁ and DF₂ and its output controls transmission gates 13 and 14. The input of each transmission gate 13 is at +V₁ potential and its output is applied to a respective common electrode C₁, C₂. The input of each transmission gate 14 is at -V₁ potential, and its output is applied to respective common electrode C₁, C₂. When the output of the shift register 12 is HIGH, the gate group 12 receives the signal DF₁ and renders the transmission gates 13 conductive in the first half period and the transmission gate 14 conductive in the second half period. As a result, the common selection signal represented by waveform (a) in Figure 1(B) appears at the output of the common electrode C₁. When the output of the shift register 12 is LOW, on the other hand, the gate group 12 receives the signal DF₂ and outputs the AC pulse oscillating between +V₁ and -V₁ in synchronism with the signal DF₂ to the common electrode C₂. This is the common non-selection signal represented by waveform (b) in Figure 1(B). Figure 8 shows a signal drive circuit for generating the white write pulses (waveform (c)) and the black write pulses (waveform (d)) to be applied to the segment electrodes S₁, S₂. As can be seen from Figure 1(B), there are three necessary voltage levels, that is, +V₁, 0 and -V₁, which are supplied to the respective segment electrodes through transmission gate 15, 16, 17, 18. The signals for ON-OFF control of each transmission gate are the signals DF₁ and DF₂. A shift register 19 receives a serial video data DATA which is read and stored by a high speed clock CL₂. A latch circuit 20 latches the video data paralleled by the shift register 19, in synchronism with the clock CL₁, and outputs white or black information in accordance with the line sequential timing (clock CL₁). A gate 21 controlled by the output of the latch circuit 20, receives the signals DF₁ and DF₂ as the input signal and produces the output which makes ON-OFF control to each transmission gate. As described already, the output of each transmission gate is applied to each segment electrode. When data appearing at output terminal 0₁ of the latch circuit 20 is white (or HIGH), the gate 21 turns ON the transmission gate 17 and outputs a high frequency signal, which is obtained by alternately turning ON and OFF the transmission gate 15 and 16 by the signal DF₂ and which oscillates between +V₁ and -V₁, to the segment electrode S₁ in the first half of the selection period and turns ON the transmission gate 18 and outputs the 0 level potential in the second half of the selection period. Thus, the white write signal represented by waveform (c) in Figure 1(B) can be obtained at the segment electrode S₁. When the data appearing at output terminal 0₂ of the latch circuit 20 is black (or LOW), the gate 21 similarly outputs the 0 level potential to the segment electrode S₂ in the first half of the selection period and a high frequency oscillating signal between +V₁ and -V₁ in the second half. Thus, the black write signal represented by waveform (d) in Figure 1(B) can be obtained. Figure 10 shows an embodiment of a common (strobe) electrode drive circuit generating non-selecting pulses (waveform (b)) as shown in Figure 1(B) having a desired amplitude. The dielectric torque given to ferro-electric liquid crystal molecules depends on amplitude of applied voltage, applied time and dielectric anisotropy value of the liquid crystal material. Larger amplitude of applied voltage, longer applied time or larger absolute value of dielectric anisotropy generates stronger dielectric torque. The Δε varies according to the kind of SmC* material, ambient temperature etc. Therefore, in order to give necessary torque to the ferro-electric liquid crystal molecules for obtaining high contrast, it is necessary to control the amplitude of the non-selecting signal (waveform (b)). In Figure 10, by setting Vx to a proper value, it is possible to obtain non-selecting signal (waveform (b)) with a desired amplitude. A matrix electro-optical device for writing two black and white optical states by utilizing spontaneous polarization of molecules of SmC* material and their negative dielectric anisotropy divides the selection period into two halves on the time division basis for line sequential scanning and uses the first half for a first stable state and the second half for a second stable state. Therefore, according to the present invention, it is possible to rewrite the picture in one frame and to operate at a high speed. Therefore, the present invention is suitable for displaying moving pictures.

Claims: A ferro-electric liquid crystal electro-optical device which uses switching between bi-stable states of ferro-electric liquid crystal molecules (3), including driving means for changing the states of the molecules between stable states (7,9) by applying a selected signal having first and second portions, entirely of respective opposite polarities, one of the first and second portions consisting of a DC pulse of a polarity to change the state of the molecules from one to another stable state, and the other of the first and second portions consisting of a chopped pulse of opposite polarity ineffective to change the stable state of the molecules, the chopped pulse having a high frequency under which the ferro-electric liquid crystal molecules exhibit negative dielectric anisotropy. A device as claimed in claim 1, characterised in that the driving means is arranged so that a first selected voltage consisting of the chopped pulse followed by the DC pulse is applied to obtain one stable state, and a second selected voltage consisting of the DC pulse followed by the chopped pulse is applied to obtain the other stable state. A device as claimed in claim 2, characterised in that the chopped pulse and the DC pulse of the first selected voltage have a polarities opposite those of the chopped pulse and the DC pulse of the second selected voltage. A device as claimed in claim 1, 2 or 3, characterised in that the driving means is arranged so that the chopped pulse has twice the amplitude of the DC pulse. A device as claimed in any preceding claim , characterised by a dot-matrix construction comprising a plurality of scanning electrodes (S1,S2) and a plurality of signal electrodes (C1,C2) defining a plurality of display pixels (D1, D2, D3, D4). A device as claimed in claim 5, characterised in that, in operation, a selected voltage effecting either one or the other of the stable states of the liquid crystal molecules is applied to each of the display pixels (D1, D2) on a selected scanning line (C1). A device as claimed in claim 5 or 6, characterised in that the driving means is arranged to apply a non-select voltage having a high frequency without DC component to each of the display pixels (D3, D4) on a non-selected scanning line (C2). A device as claimed in claim 7, characterised in that the driving means has a common source (DF2) for the high frequency non-select voltage and the chopped pulse. A device as claimed in any of claims 5 to 8, characterised in that the driving means is arranged to change the amplitude of a non-select voltage applied to each of the display pixels (D3, D4) on a non-selected scanning line (C2). A device as claimed in claim 9, characterised in that the driving means is such that, in operation, the amplitude of the non-select voltage is set so that the liquid crystal molecules (3) are substantially parallel to substrates (1) between which they are sandwiched. A method of operating a ferro-electric liquid crystal electro-optical device, including the steps of changing the states of the liquid crystal molecules by applying a selected signal having first and second portions, entirely of respective opposite polarities, one of the first and second portions consisting of a DC pulse of a polarity to change the state of the molecules from one to another stable state, and the other of the first and second portions consisting of a chopped pulse of opposite polarity ineffective to change the stable state of the molecules, the chopped pulse having a high frequency under which the ferro-electric liquid crystal molecules exhibit negative dielectric anisotropy. A method of operating a ferro-electric liquid crystal electro-optical device of dot-matrix form, comprising a plurality of segment electrodes (S1, S2) and a plurality of common electrodes (C1, C2) defining a plurality of display pixels (D1, D2, D3, D4) characterised in that in operation , a non-selection alternating voltage of high frequency under which the liquid crystal molecules exhibit negative dielectric anisotropy is applied to the common electrodes (C2) other than a selected common electrode (C1), a selection signal having first and second portions is applied to the selected common electrode (C1), the portions comprising DC pulses of opposite polarity and of an amplitude to change the state of the liquid crystal molecules between stable states, and signal pulses are applied to the segment electrodes (S1,S2) each having first and second portions, corresponding in time to the first and second portions of the selection signal, one portion being a chopped pulse of the same high frequency and the other portion having zero level potential, the switching DC pulse being concurrent with the zero level potential portion, whereby a selected voltage effecting either one or the other of the stable states is applied to the display pixels (D1, D2) on the selected common electrode (C1).

Inventors: HARADA TAKAMASA, ITO KOKICHI, SHIMODA SADASHI, TAGUCHI MASAAKI, HARADA, TAKAMASA, ITO, KOKICHI, SHIMODA, SADASHI, TAGUCHI, MASAAKI
Applicants: SEIKO INSTR INC, SEIKO INSTRUMENTS INC.
Application year: 1987
Main classification: G02F1
IPC codes: G09G3, G02F1