CHIRAL SMECTIC LIQUID CRYSTAL ELECTRO-OPTICAL DEVICE AND METHOD OF DRIVING SAME

Abstract: A chiral smectic liquid crystal electro-optical devices uses switching between two states of chiral smectic liquid crystal molecules, the device being driven by utilising the difference (Δτ) between the response time from one of said two states to the other state and the response time from the other state to the one state.

Description: This invention relates to chiral smectic liquid crystal electro-optical devices and methods of driving same. In recent years, liquid crystal devices using a chiral smectic C-phase liquid crystal material have attracted attention. Such devices are used as optical shutters in display devices, cameras and printers that operate at high speeds and have memory retentivity. A widely known example of a ferroelectric liquid crystal compound having the chiral smectic C-phase is a 2-methylbutyl P-(P-n-decyloxybenzylidene)amino). The liquid crystal molecules are arranged in a spiral structure being twisted in each of layers L₁, L₂, L₃, L₄ (Figure 8) by a given azimuthal angle . When a liquid crystal compound having a chiral smectic C-phase is poured into a space between the two substrates B, B' having a gap (of, for example, about 1 µm) which is smaller than the spiral period (usually several µm) thereof to constitute a liquid crystal cell (Figure 9(a)), the liquid crystal molecules lose their spiral structure and establish a state where there is a domain that is tilted in the clockwise direction by an angle ϑ from a direction normal to the layer with the molecular axes thereof being parallel to the substrates B, B', and a domain that is tilted by ϑ in the counter-clockwise direction, i.e., by -ϑ (Figure 9(b)), and further establish electric dipole moments in a direction perpendicular to the molecular axes. Therefore, if one of those domains has an upwardly directed electric dipole moment relative to the substrates B, B', the other domain has a downwardly directed electric dipole moment. If an electric field is applied between the substrates B, B', all of the liquid crystal molecules are orientated at positions tilted by either +ϑ or -ϑ from the direction normal to the layer. Further, if the electric field is applied in the opposite direct, the liquid crystal molecules are inverted and are orientated at positions tilted by either -ϑ or +ϑ. If polarising plates are disposed on both surfaces of the liquid crystal cell and an electric field applied thereto, bright and dark conditions are established by the tilt of the liquid crystal molecules, whereby the cell exhibits the function of a display panel or optical shutter (Figure 10). The liquid crystal panel thus constituted exhibits excellent characteristics, i.e. has a remarkably high response speed of the order of micro-seconds and retains displayed information even after the electric field has been removed. Such a liquid crystal panel, in which the bright and dark conditions are established by means of AC driving signals applied to the liquid crystal material, is disclosed in U.S. Patent No. 4,508,429 and in DE-A-34 14704. In a further liquid crystal panel described in European Patent Application No. 0,149,899 which constitutes prior art in the sense of Article 54 (3) EPC, a change of condition is effected by applying to the liquid crystal material a voltage pulse having a predetermined amplitude and pulse width, and is thereafter maintained by the application of an AC pulse voltage having a reduced amplitude or pulse width. The above memory retentivity is very advantageous from the standpoint of reducing electric power consumption. When display information is to be changed, however, writing of the dark condition and that of the bright condition must be effected by separate frame scannings. Namely, an extended period of time is required for changing display information. The present invention seeks to provide a chiral smectic liquid crystal electro-optical device which is capable of effecting Writing of bright and dark conditions in a single scanning frame at relatively high speed. According to one aspect of the present invention there is provided a chiral smectic liquid crystal electro-optical device in which chiral smectic liquid crystal molecules are switched between a first stable state and a second stable state according to AC driving signals applied thereto, characterised in that said AC driving signals comprise: a first AC pulse signal having a first preceding pulse of one polarity and a first subsequent pulse of the other polarity, each of the first pulses having an equal absolute value of amplitude and an equal pulse width such that the first preceding pulse of one polarity can change the liquid crystal molecules to the first stable state and the first subsequent pulse of the other polarity cannot change the liquid crystal molecules from the first stable state to the second stable state; and a second AC pulse signal having a second preceding pulse of one polarity and a second subsequent pulse of the other polarity, each of the second pulses having an equal absolute value of amplitude and an equal pulse width such that the second preceding pulse of one polarity can change the liquid crystal molecules to the first stable state and the second subsequent pulse of the other polarity can change the liquid crystal molecules from the first stable state to the second stable state. The amplitude values of the first and second pulses may be the same. Alternatively, the pulse widths of the first and second pulses may be the same. According to another aspect of the present invention there is provided a method of driving a chiral smectic liquid crystal electro-optical device in which chiral smectic liquid crystal molecules are switched between a first stable state and a second stable state in dependence upon AC driving signals applied thereto, characterised by the application of a first AC pulse signal to switch the chiral smectic liquid crystal molecules into the first stable state, the first AC pulse signal having a first preceding pulse of one polarity and a first subsequent pulse of the other polarity, each of the first pulses having an equal absolute value of amplitude and an equal pulse width such that a first preceding pulse of one polarity can change the liquid crystal molecules to the first stable state and the first subsequent pulse of the other polarity cannot change the liquid crystal molecules from the first stable state to the second stable state, and by the application of a second AC pulse signal to switch the liquid crystal molecules into the second stable state, the second AC pulse signal having a second preceding pulse of one polarity and a second subsequent pulse of the other polarity, each of the second pulses having an equal absolute value of amplitude and an equal pulse width such that the second preceding pulse of one polarity can change the liquid crystal molecules to the first stable state and the second subsequent pulse of the other polarity can change the liquid crystal molecules from the first stable state to the second stable state. The invention is illustrated, merely by way of example, in the accompanying drawings, in which:- Figure 1 is a perspective sectional view showing an embodiment of a chiral smectic liquid crystal electro-optical device according to the present invention; Figures 2 (a) and 2 (b) are diagrams illustrating the direction of rubbing of substrates in the device of Figure 1; Figure 3 is a diagram showing the applied voltage and the response speed of the device of Figure 1; Figure 4 is a block circuit diagram showing the device of Figure 1; Figures 5 (a) to 5 (e) are diagrams of waveforms illustrating the operation of the circuit of Figure 4; Figures 6 (a) to 6 (e) are diagrams of waveforms illustrating the operation of another embodiment of a chiral smectic liquid crystal electro-optical device according to the present invention; Figure 7 is a diagram showing the relationship between the time of applying the electric field and the concentration of picture elements; Figure 8 is a diagram which schematically illustrates the molecular arrangement of chiral smectic liquid crystal material; Figures 9(a) and 9(b) are schematic diagrams showing the arrangement of molecules when the gap of a display cell is selected to be smaller than the spiral pitch of the liquid crystal molecules; and Figure 10 is a diagram showing the relationship between the domains and the polarisation of smectic liquid crystal material. Referring first to Figure 1 there is shown an embodiment of a chiral smectic liquid crystal electro-optical device according to the present invention. A substrate 1 which constitutes a liquid crystal display panel is composed of a random horizontally orientated layer 1c that is uniformally orientated in all directions. The layer 1c is formed, e.g. by printing or dipping, of a thin polyimide film on the surface of an electrically insulating transparent plate 1b made of, e.g. glass on which are formed segment electrodes 1a, composed of transparent conductive material (Figure 2(a)). A second substrate 2 which together with the substrate 1 constitutes a liquid crystal display panel has a mono-axially orientated layer 2c. The layer 2c is obtained by forming a thin polyimide film on the surface of an electrically insulating transparent plate 2b on which are formed common electrodes 2a which are at right angles to the electrodes 1a. The surface of this thin polyimide film is rubbed in one direction so as to be orientated only in one direction (Figure 2(b)). These two substrates 1 and 2 are disposed so as to be parallel to one another with a gap therebetween. The gap is smaller than the spiral pitch of a ferro-electric chiral smectic liquid crystal compound 3. The random horizontally orientated layer 1c and the monoaxially orientated layer 2c are opposed to each other. The space between the two substrates 1, 2 is filled with a ferro-electric chiral smectic liquid crystal compound 3 obtained by mixing an S-4-O (2-methyl)butyl-resorcylidene-4-alkyl n-octylaniline in equal amounts. The peripheries of the substrates are sealed with a sealing agent to form a cellular structure, thereby to constitute a display panel 6. In Figure 1, reference numerals 4,5 denote polarising plates that are placed on the outer surfaces of the substrates 1,2 respectively. A voltage is applied to the electrodes of the thus-constructed liquid crystal panel to examine the inversion speed of the domains. When a voltage of the same level is applied as shown in Figure 3, the speed τ₁ at which the bright condition changes to the dark condition is different from the speed τ₂ at which the dark condition changes into the bright condition. Namely, there is a difference Δτ in the response time between the two. The present invention seeks to effect writing of the bright condition and that of the dark condition simultaneously by fully utilising the time difference Δτ when the display conditions are switched. Figure 4 illustrates an embodiment of a drive circuit for the display panel 6 of Figure 1. The drive circuit has a common electrode drive circuit 7 connected to the common electrodes 2a of the display panel 6, and a segment electrode drive circuit 8 connected to the segment electrode 1a. When scanning lines successively, the electric field of a first mode or a second mode is applied to the picture elements depending upon the write condition. When not selecting, the electric field of a third mode is applied to the picture elements. That is, when selecting the picture elements, the drive circuit selectively produces: (1) a dark condition write signal which consists of a pulse P₁ (Figure 5(a)) in a negative direction having a voltage |V₁|, and an interval T₁ ranging from time τ₁ required for changing the bright condition into the dark condition to time τ₂ required for changing the dark condition into the bright condition and a pulse P₂ in a positive direction having the voltage |V₁| as well as a first mode signal (Figure 5(b)) consisting of an alternating maintenance signal which has a voltage (V₁/N); and (2) a bright condition write signal which consists of a pulse P₃ (Figure 5(c)) in a negative direction having a voltage |V₁| of the same level and an interval T₂ longer than the time τ₂ that is required for changing the dark condition into the bright condition and a pulse P₄ in a positive direction having the voltage |V₁| and the interval T₂, as well as a second mode signal (Figure 5(d)) consisting of an alternating maintenance signal which has a voltage |V₁/N|. When not selecting the picture elements, the drive circuit produces: (3) a third mode signal (Figure 5(e)) consisting of alternating signals having a voltage |V₁/N| and an interval that does not cause inversion of the liquid crystal molecules. Now operation of the display panel thus constructed will be described. Under the condition where no electric field is applied to the electrodes 1a, 2a, the liquid crystal molecules contained in the narrow gap between the substrates 1,2 are captured at one end by the random horizontally orientated layer 1c. The liquid crystal molecules, however, have freedom in the direction of the plane. Therefore, the other ends of the liquid crystal molecules are biased in one direction by the monoaxially orientated layer 2c. Namely, all of the liquid crystal molecules held between the substrates 1,2 are arranged in one direction and maintain a uniform optical density over the whole surface to form a uniform background without developing any pattern. Under this condition, if the first mode signal is applied, the liquid crystal molecules of selected picture elements are affected by the electric field for a period of time longer than the inversion time τ₁ at the voltage -V₁ of the pulse P₁ but shorter than the inversion time τ₂, and are sufficiently inverted into the dark condition. If the pulse P₂ is applied after the inversion, the liquid crystal molecules of the selected picture elements are not inverted into the bright condition since the interval T₁ of the pulse P₂ is shorter than the time τ₂ for changing the dark condition into the bright condition. Therefore, the dark condition is maintained, i.e. write condition of the pulse P₁ is maintained. Due to the maintenance signal applied after the write signal, the liquid crystal molecules of the selected picture elements are dynamically maintained in the dark condition. If the second mode signal is applied, the liquid crystal molecules of the selected picture elements are affected by the electric field for the period of time T₂ which is longer than the inversion time at the voltage -V₁ of the pulse P₃ and temporarily change into the dark condition. However, since the pulse P₄ that is applied subsequently has the interval T₂ which is longer than the inversion time τ₂ for inverting the dark condition into the bright condition at the voltage V₁, the liquid crystal molecules are inverted from the dark condition written by the pulse P₃, and write the bright condition. Due to the maintenance signal applied after the write signal, the liquid crystal molecules of the selected picture elements are dynamically maintained in the bright condition. Therefore, the writing of the dark condition and that of the bright condition can be effected at a high speed of the order of several hundreds of microseconds. It will be appreciated that, in the above mentioned step, the liquid crystal molecules alternately are affected by voltages having the same voltage level and the same interval. Therefore, the picture elements are free from any residual electric charge. Thus, either the first mode signal or the second mode signal is selected depending upon the condition that is to be written, and the dark condition and the bright condition can be written by one-frame scanning. In this embodiment, the displayed information is maintained by applying an AC voltage having a peak value which is 1/N of the drive voltage after the pattern is displayed. However, since the liquid crystal molecules are captured in a new axis of orientation in the random horizontally orientated layer 1c, directions of the liquid crystal molecules are maintained even after the application of the AC voltage is discontinued, so that the displayed information can be stored. In this embodiment, furthermore, the first mode signal is used for writing the dark condition and the second mode signal is used for writing the bright condition. However, they can be suitably interchanged depending upon the operation characteristics of the liquid crystal panel. Figure 6 shows a second embodiment of a display device according to the present invention which produces a graduation display by utilising changes in the concentration of picture elements relative to the time for applying the electric field as shown in Figure 7. When selecting the picture elements, the device applies: (1) a dark condition write signal which consists of a pulse P₅ (Figure 6(a)) in a negative direction which has a peak voltage |V₁| followed by pulses of voltage V₁/2 modulated with an interval Tx ranging from time τ₁ required for changing the bright condition into the dark condition to the time τ₂ required for changing the dark condition into the bright condition and a pulse P₆ in the positive direction which has a peak voltage |V₁| and followed by pulses of voltage |V₁|/2 modulated with the interval Tx, as well as a fourth mode signal (Figure 6(b)) consisting of an alternating maintenance signal which has a voltage |V₁/N|; and (2) a bright condition write signal which consists of a pulse P₇ (Figure 6(c)) in the negative direction which has a peak voltage |V₁| followed by pulses of voltage |V₁/2| is modulated with an interval Ty longer than the time τ₂ that is required for changing the dark condition into the bright condition, and a pulse P₈ in a positive direction which has a peak voltage |V₁| and which is modulated with the interval Ty, as well as a fifth alternating mode signal (Figure 6(d)) which has a voltage |V₁/N|. When not selecting the picture elements, the device applies: (3) the above mentioned third mode signal (Figure 6(e)). In this embodiment, if the fourth mode signal is applied, the liquid crystal molecules of the selected picture elements change to the dark condition (Figure 7) at a concentration proportional to the volgate -V₁ of the pulse P₅ and the interval Tx. If the pulse P₆ is applied under this condition, the liquid crystal molecules cannot migrate to positions corresponding to the dark condition and maintain the concentration position written by the pulse P₅, since the interval Ty of the peak voltage V₁ is shorter than the inversion time τ₂ for inverting the dark condition into the bright condition. Due to the maintenance signal applied after the write signal, the liquid crystal molecules dynamically maintain the displayed concentration around the selected positions. If the fifth mode signal is applied, the liquid crystal molecules temporarily change into the dark condition, since the interval Ty of the pulse P₇ is longer than the inversion time τ₂. However, since the pulse P₈ which is subsequently applied has the interval Ty which is sufficient to tilt the liquid crystal molecules from the dark condition to the bright condition, the liquid crystal molecules migrate from the dark condition written by the pulse P₇ to the bright condition, and write brightness which is proportional to the interval Ty. Therefore, it is possible to write information having graduation at a very high speed of the order of several hundreds of microseconds in one step. In this embodiment, graduation is provided for both in the dark condition and the bright condition. It will, however, be appreciated that the same effect can be produced even when graduation is provided for in either one of these conditions, for example, for writing the dark condition. In the above mentioned embodiments, the selecting pulses P₁ to P₈ have a constant amplitude and have different pulse widths T₁, T₂, T x , T y in order to utilise the difference between the response time from one of the two states to the other state and the response time from the other state to the one state for driving. In order to utilise the difference of the response time, however, it is also possible to use selecting pulses which have a constant pulse width and have different amplitude between the case of ON to OFF and the case of OFF to ON. In addition to the above mentioned ferro-electric smectic liquid crystal compound, it is also possible to use a chiral smectic liquid crystal compound such as a pyrimide-type liquid crystal compound represented by the general formula: or 2-methylbutyl p-(P-n-decyloxybenzilidene)amino), etc. In the above described embodiments of the present invention, the monoaxially orientated layer and the random horizontally orientated layer composed of polyimide were formed on the surfaces of the substrates. However, it has also been found that the monoaxially orientated film may be composed of an organic material such as polyvinyl alcohol, fluororesin or silane, or an obliquely deposited SiO₂ film in addition to the polyimide. Similarly, the random horizontally orientated film on the other substrate may be composed of organic material such as epoxy, polyvinyl alcohol, fluororesin, polyurethane, silane, phenol or urea, or an inorganic film formed by depositing vapours of SiO₂ or MgF₂ in addition to the polyimide. In the above mentioned embodiments of the present invention, on the inner surface of one of the two substrates a monoaxially alignment treatment is given and on the other inner surface of the other a random homogeneous alignment treatment is given. These treatments are proper in the case of using the liquid crystal compounds mentioned above. However, in the case of using other smectic liquid crystal compounds it may be proper that both of the inner surfaces of the two substrates are treated to produce monoaxial alignment. According to the present invention described above, the liquid crystal panel comprising a smectic liquid crystal compound is affected by a write signal consisting of an AC signal having an interval ranging from the first inversion time to the second inversion time of the liquid crystal compound and an AC signal having an interval greater than one of said two inversion times whichever is the longer. Therefore, the bright and dark conditions can be written within a period of one frame of scanning maintaining memory retentivity and uniform background. Thus, it is possible to realise a display device such as an optical shutter which effects the scanning at high speed.

Claims: A chiral smectic liquid crystal electro-optical device in which chiral smectic liquid crystal molecules are switched between a first stable state and a second stable state according to AC driving signals applied thereto, characterised in that said AC driving signals comprise: a first AC pulse signal having a first preceding pulse (P1) of one polarity and a first subsequent pulse (P2) of the other polarity, each of the pulses of the first AC pulse signal having an equal absolute value of amplitude (V1) and an equal pulse width (T1) such that the first preceding pulse of one polarity can change the liquid crystal molecules to the first stable state and the first subsequent pulse of the other polarity cannot change the liquid crystal molecules from the first stable state to the second stable state; and a second AC pulse signal having a second preceding pulse (P3) of one polarity and a second subsequent pulse (P4) of the other polarity, each of the pulses of the second AC pulse signal having an equal absolute value of amplitude (V1) and an equal pulse width (T2) such that the second preceding pulse of one polarity can change the liquid crystal molecules to the first stable state and the second subsequent pulse of the other polarity can change the liquid crystal molecules from the first stable state to the second stable state. A device as claimed in claim 1 characterised in that the absolute value of amplitude of the pulses of the first AC pulse signal and the absolute value of amplitude of the pulses of the second AC pulse signal are the same. A device as claimed in claim 1 characterised in that the pulse width of the pulses of the first AC pulse signal and the pulse width of the pulses of the second AC pulse signal are the same. A device as claimed in any of claims 1 to 3 characterised in that said AC driving signals further comprise a third AC pulse signal having alternating third pulses of one polarity and the other polarity, each of the pulses of the third AC pulse signal having an equal absolute value of amplitude and an equal pulse width such that the pulses of the third AC pulse signal cannot change the liquid crystal molecules from one to the other of the first and second stable states. A method of driving a chiral smectic liquid crystal electro-optical device in which chiral smectic liquid crystal molecules are switched between a first stable state and a second stable state in dependence upon AC driving signals applied thereto, characterised by the application of a first AC pulse signal to switch the chiral smectic liquid crystal molecules into the first stable state, the first AC pulse signal having a first preceding pulse (P1) of one polarity and a first subsequent pulse (P2) of the other polarity, each of the pulses of the first AC pulse signal having an equal absolute value of amplitude (V1) and an equal pulse width (T1) such that the first preceding pulse of one polarity can change the liquid crystal molecules to the first stable state and the first subsequent pulse of the other polarity cannot change the liquid crystal molecules from the first stable state to the second stable state, and by the application of a second AC pulse signal to switch the liquid crystal molecules into the second stable state, the second AC pulse signal having a second preceding pulse (P3) of one polarity and a second subsequent pulse (P4) of the other polarity, each of the pulses of the second AC pulse signal having an equal absolute value of amplitude (V1) and an equal pulse width (T2) such that the second preceding pulse of one polarity can change the liquid crystal molecules to the first stable state and the second subsequent pulse of the other polarity can change the liquid crystal molecules from the first stable state to the second stable state. A method as claimed in claim 4, characterised in that the absolute value of amplitude of the pulses of the first AC pulse signal and the absolute value of amplitude of the pulses of the second AC pulse signal are the same. A method as claimed in claim 5, characterised in that the pulse width of the pulses the first AC pulse signal and the pulse width of the pulses of the second AC pulse signal are the same. A method as claimed in any of claims 5 to 7 characterised by the application of a third pulse signal having alternating third pulses of one polarity and the other polarity, each of the pulses of the third AC pulse signal having an equal absolute value of amplitude and an equal pulse width such that the pulses of the third AC pulse signal cannot change the liquid crystal molecules from one to the other of the first and second stable states.

Inventors: HARADA, TAKAMASA, TAGUCHI, MASAAKI
Applicants: SEIKO INSTRUMENTS INC.
Application year: 1986
Main classification: G02F1
IPC codes: G09F9, G02F1, G09G3