GB2169431A

GB2169431A - Method of driving a liquid crystal apparatus

Info

Publication number: GB2169431A
Application number: GB08522290A
Authority: GB
Inventors: Morio Ohta; Hideaki Inoue
Original assignee: Casio Computer Co Ltd; Casio Electronics Manufacturing Co Ltd
Current assignee: Casio Computer Co Ltd; Casio Electronics Manufacturing Co Ltd
Priority date: 1984-12-29
Filing date: 1985-09-09
Publication date: 1986-07-09
Also published as: KR900001585B1; GB2169431B; JPH0410050B2; GB8522290D0; FR2575576A1; FR2575576B1; JPS61158315A

Abstract

The present invention relates to a method of driving a liquid crystal light shutter for using a liquid crystal material whose dielectric anisotrophy becomes zero when a specified frequency is inputted is sealed between the common and signal electrodes (13, 14). The signal for driving the liquid crystal light shutters (15-18) comprises a signal the frequency of which is lower than the specified frequency and a signal obtained by superposing a signal of a frequency higher than the specified frequency on a signal of a frequency lower than the specified frequency. The signal for reversely driving the liquid crystal light shutters comprises a signal of a frequency higher than the specified frequency and a silence signal. A frequency fL3 lower than FL may be applied for a short period to further enhance on tendency. A high frequency hysteresis is avoided by the silent signal in the case of off- drive signal. Temperature characteristics are also improved. <IMAGE>

Description

SPECIFICATION Method of driving a liquid crystal apparatus Background of the Invention The present invention relates to a method of driving a liquid crystal apparatus and, more particularly, it relates to a driving method at the time of driving a liquid crystal in a time division manner.

The recording apparatus in which a liquid crystal light shutter is employed is intended to carry out light writing on a photosensitive body by driving opened or closed a plurality of micro shutters in the liquid crystal light shutter by means of the control circuit, to shield or transmit the light of a light source arranged above the liquid crystal light shutter. In the case of this type of recording apparatus, the liquid crystal is required to have high speed response. Therefore, a liquid crystal whose dielectric anisotropy is inverted by the frequency of a signal applied to the micro shutters is employed and driven by two frequencies, one (fH) of which is higher than a frequency (fc) which makes the dielectric anisotropy of the liquid crystal zero, and the other (f,) of which is lower than the frequency (fc).In the case of such liquid crystal light shutter, recording is attained at a density of about 10 dots per 1mm and in the case of size A4, for example, the micro shutters of about 3,000 units are needed per one line.

For the purpose of preventing the number of wires and mounting area from being increased, therefore, the liquid crystal light shutter is usually driven in a time division manner.

This time division drive is of such type that common and signal electrodes are crossed perpendicular to one another in the liquid crystal light shutter to form micro shutters at the crossed portions of both electrodes and recording signals are inputted to the signal electrodes while time-divided selection signals are inputted to the common electrodes.

According to this time division drive or 2time division drive, for example, the light can be transmitted only for half the writing period Tw, and the time period during which the light can be transmitted becomes shorter in the case of n-time division drive, so that the photosensitive body can be short of the amount of exposure light.Therefore, the drive is performed in such a way that the selection signals are used to open or close the micro shutters for a selected time period Tw/n of the one writing period Tw and keep them set for the remaining time period (1 - 1/n) Tfiv (which will be hereinafter referred to as non-selectec time period) of the one writing period T,. In the case of 2-time division drive, for example, a drive pattern signal in which signals of fH, f" fH and *f, shown in Fig. 1A, said signals of fH and *ft being shifted by 180" in phase from said signals of fH and f,, mixed en one writing cycle is formed and applied to the common electrode. Further, one of four drive pattern signals 1-4 shown in Fig. 1B is selected and applied to the signal electrodes.

Four kinds of superposed signals 5-8 shown in Fig. 1C are thus formed and one of these superposed signals is applied to the micro shutters to thereby drive them opened or closed. Since a signal which is shifted by TW/2 in phase from the signal which has been applied to the above-mentioned common electrode is applied to the other common electrode, a superposed signal corresponding to the one shown in Fig. 1C but shifted by TW/2 in phase is applied to the micro shutters to drive them opened or closed.

Although the liquid crystal light shutter which is used to explain the present invention have been formed by a liquid crystal of the GH type, for example, which allows light to be transmitted through it upon its on-operation, a liquid crystal of the TN type which is provided between polarizing plates positioned at an orthogonal Nicol prism and which shields light upon its on-operation may be employed but it should be understood that the liquid crystal light shutter in this case is opened or closed reversely to those of the GH type liquid crystal.

Light transmission characteristics of the micro shutters obtained when the superposed signals 5-8 are applied to the micro shutters are shown in Fig. 1D. A light transmission characteristic 9 is due to the superposed signal 5 which is obtained when the pattern signal 1 is applied to the signal electrodes while the pattern signal shown in Fig. 1A is applied to the common electrodes, with the result that the paired micro shutters are opened. A light transmission characteristic 10 is due-to the superposed signal 6 which is obtained when the pattern signal 2 is applied to the signal electrodes while the pattern signal shown in Fig. 1A is applied to the common electrodes, with the result that ones of the paired micro shutters are open and the others are closed.

Similarly, light transmission characteristics 11 and 12 are due to the superposed signals 7 and 8 which are obtained when the pattern signals 3 and 4 are applied to close ones of the micro shutters and open the others in such a manner as to be reverse to the abovementioned case, or close both of these micro shutters. If.-fH) shown in Fig. 1C represents a superposed signal formed by the signals of f, and fK, (f,-*fH), a superposed signal formed by the signals of f, and *fH and (0) a silent signal.

When the liquid crystal light shutter are driven like this, selected micro shutters can be kept open even for the non-selected time period (1-1/n)Tv, in the case of n-time division drive and the signal f, is applied to the micro shutters for the last time period of one writing /cle Tw to open them and eliminate the hys eresis effect peculiar to the liquid crystal, thereby enabling the liquid crystal light shutters to be opened or closed as if they were driven according ta the static drive.

In the case of the above-described time division drive for the liquid crystal light shutter, however, the liquid crystal light shutters can be kept under their previously-set state for a non-selected time period, using the hysteresis effect of the liquid crystal, but their optimum temperature ranges only from 2 to 3 degrees.

In addition, their optimum temperature changes, depending upon the quality of liquid crystal used, and it was therefore necessary to set a different temperature range in every recording apparatus. Further, the amount of light transmitted at the time of shutter opening is less sufficient compared to the case where the shutters are opened according to the static drive.

Summary of the Invention The present invention is therefore intended to eliminate the above-mentioned drawbacks and the object of the present invention is to provide a method of driving liquid crystal light shutter, by which the temperature range within which the liquid crystal light shutter is operated with reliability is enlarged, the amount of light transmitted is increased.

Brief Description of the Drawings Figures 1A through 1C are waveform diagrams showing signals employed by the conventional driving method, and Figure 1D a view showing light transmission characteristics obtained according to the conventional driving method; Figure 2 shows a plan view of part of the liquid crystal light shutter; Figures 3A through 3C are vaveform diagrams showing signals employed by a driving method of an embodiment of the present invention, and Figure 3D a view showing light transmission characteristics obtained according to the driving method of the present invention; Figures 4A and 4B show characteristics of the amount of transmitted light obtained according to the driving method of the prior art and the present invention when the temperature is changed;; Figures 5A through 5C are waveform diagrams showing signals employed by a driving method of another embodiment of the persent invention, and Figure 5D a view showing light transmission characteristics obtained according to the second embodiment driving mehtod of the present invention; Figures 6A through 6C are waveform diagrams intended to explain a signal of fHX; Detailed Description of the Preferred Embodiment A first embodiment of the present invention will be described with reference to the accompanying drawings.

The embodiment uses 2-time division drive and Fig. 2 shows part of the liquid crystal light shutter. In Fig. 2 common electrodes 13, 14 and signal electrodes 15-16 are crossed perpendicular to one another, and micro shutters 15a-18b and 15b-18b are formed by transparent electrodes each occupying a part of each of the crossed portions of the common and signal electrodes. Recording signals are applied to the signal electrodes 15-18 while selected signals are applied to the common electrodes 13, 14.

Fig. 3A shows recording signals applied to the signal electrodes 15-18 and Fig. 3B shows a selected signal applied to the common electrode 13. Applied to the common electrode 14 is a signal which is shifted by TW/2 in phase from the signal applied to the common electrode 14. Of the recording signals applied to the signal electrodes 15-18, Fig. 3A shows an on-on recording signal 19 for making both of the paired shutters open and an off-off recording signal 20 for rendering both of the paired shutters closed. On-off and off-on recording signals are combinations of the first and second halves of the abovementioned recording signals, similar to waveforms 2 and 3 shown in Fig. 1B.Similariy, as in the case of signal fL, signals f,2 and fL3 in Fig. 3A represent frequencies lower than crossed frequency, but this embodiment of the present invention uses signal having a frequency different from that of the signal fL so as to contain waveforms of integral units within predetermined periods TL and TLS.

Therefore, applied to the micro shutters 15a-18a and 15b-18b to which these recording signals 19, 20 and selection signal have been applied are an on-on drive signal 21 and off-off drive signal 22 which are formed by the recording and selection signals, as shown in Fig. 3C. Different from the conventional onon drive signal 5 and off-off drive signal 8 shown in Fig. 1C, the on-on drive signal 21 contains the low frequency signal f,3 for the period T,3 of the selected one To/2, while the off-off drive signal 22 contains a silent signal (O). The on-off drive signal (not shown) contains the low frequency signal fL3 for a period TL2, while the off-on drive signal contains the period of the silent signal.

The fact that the low frequency signal f,3 is inserted like this during the selected period TW/2 for on-drive forces the orientation of liquid crystal molecuie toward the electric field more strongly and further enhances on-tendency. As shown by the light transmission characteristics in Fig. 3D, therefore, on-on response characteristics 23 and on-off response characteristic 24 of the mocro shutters 15a-18a and 15b-18b enhance the light transmission rate of the shutters for the period TL3. More specifically, the light transmission rate which has been reduced to some levels can be restored to their original one by a signal (f,+fH) of the on-off drive signal 21 to thereby increase the amount of tranmitted light.

By inserting the silent signal (0) for the selected off-drive period instead of the signal fH, it seems that the dielectric relaxation by which the liquid crystal is shifted to on-operation is reduced to approach natural relaxation, thereby worsening the off-response of the liquid crystal, but any large change is not actually caused, as apparent from off-on and off-off response characteristics 25 and 26 in Fig. 3D, and this insertion of the silent signal (0) serves to give good balance to both of the characteristics 25 and 26.

When a waveform into which the low frequency of fL3 is inserted for a part TL3 of the selected time period TW/2 is used, as described above, a more remarkable on-effect can be attained by the signal fL3 in the case of on-drive signal and so-called high requency hysteresis effect can be prevented by the silent signal (0) in the case of off-drive signal.

In the case of the liquid crystal light shutter which repeats its operation, therefore, it can achieve good operation efficiency in its opening and closing operations. Because high frequency components are reduced while low frequency components are increased, the temperature characteristic of the shutter can be improved remarkably and it can perform optimum operation over a wide temperature range. Figs. 4A and 4B show the results, in which 27 and 31 represent on-on drive characteristics, 28 and 32 on-off drive characteristics, 29 and 33 off-on drive characteristics, and, 30 and 34 off-off drive characteristics.

Amounts of light transmitted through the micro shutters by the conventional drive signals (Fig. 4A) are compared with amounts of light transmitted through the micro-shutters by the drive signals of the present invention (Fig. 4B) in relation to temperatures. As compared with the narrow temperature rang of 45-48"C shown in Fig. 4A, the optimum temperature range over which the above-described micro shutters can perform their opening and closing operations with good efficiency is wide in the case of the present invention, ranging from 40"C to 50"C, as shown in Fig. 4B. It should be understood, therefore, from Fig. 4B that the optimum temperature range can be enlarged by the present invention.

A second embodiment of the present invention will be described referring to Figs. 5A to 5D.

This second embodiment is the same as the first one in that the time period during which the signal f,3 is inserted is provided in the selected time period T,/2, but different in that a signal fHy is inserted into the selection signal for a non-selected time period for a time period THY shown in Fig. 5B. Fig. 6B shows the signal fHY enlarged. The signal fH of an off-off recording signal 35 shown in Fig.

5A is shown partially enlarged in Fig. 6A.

When the signal fH is combined with the signal fHy, it forms the waveform shown in Fig. 6C.

More specifically, when the drive signal electrodes and a drive signal shown in Fig. 5B and applied to the common electrodes are applied to the micro shutters, they form the waveforms shown in Fig. 5C, and the waveform shown in Fig. 6C is obtained by enlarging a signal (fH-fHy) in Fig. 5C. Compared with the conventional signal (fL-fH), the signal (fH-fHy) becomes a superposed signal, smaller in duty and accordingly lower in effective value.A signal (fL-fHy) of on-on drive signal 37 and offon drive signal (not shown) which are obtained when the signal ft of an on-on drive signal 36 shown in Fig. 5A and a selection signal shown in Fig. 5B are applied to the micro shutters is different in phase from the signal (fHfHy) but substantially the same in waveform. Therefore, it can be substantially the same signal as the signal (fH-fHy).

When the signal fHY is inserted for the nonselected time period, as described above, it serves to weaken the maintaining of on-operation a little in the case of on-drive but prevents the micro shutters from tending to be open to some extent under a high temperature in the case of off-drive. This fact serves to strengthen the opening operation of micro shutters to some extent for the time period T,3, as shown in Fig. 5D.

As described above, a time division drive of a liquid crystal shutter which achieves a good repetition operation over a longer range of temperatures can be provided by applying a signal f,3 for a time period T,3 of a selection period and further applying a selection signal including a signal fHY for a non-selection period, so as to be best suitable for dielectric characteristics and temperature characteristics of a liquid crystal. As the effect of a signal f,3 applied to the period T,3, the on-characteristics of a liquid crystal light shutter can be improved remarkabiy without deteriorating the off-characteristics thereof.

An operational temperature is not sensitive to a change in the liquid crystal material and its flexibility can be improved.

As described above, the optimium operational temperature range in which a liquid crystal shutter can be controlled accurately is widened to more than 6 degrees as compared with a conventional range of 2 to 3 degrees and an amount of a light radiation on a photosensitive body can be increased.

As a range of an operation temperature is widened, a flexibility is increased with regard to a change in a liquid crystal material.

Claims

1. In a method of driving a recording device having (n) units of common electrodes and plural signal electrodes crossed and op posed to the common electrodes, wherein a liquid crystal material whose dielectric anisotropy becomes zero when a specified frequency is inputted is sealed between the common and signal electrodes to make their crossed portions serve as micro-shutters and these micro-shutters are opened or closed according to signals to be recorded, a method of driving the recording device comprising applying a driving waveform, which sets the micro-shutters opened or closed, to the microshutters for a selected part of time-divided period and applying a driving waveform, which keeps the micro-shutters opened or closed, as set for the selected period just before a nonselected period, to the micro-shutters for the non-selected period, characterized in that said driving waveform which sets the micro-shutters opened or closed for the selected period is a signal including a signal whose frequency is lower than the specified frequency and a superposed waveform obtained when the signal whose frequency, and that a driving waveform which reversely sets the micro-shutters opened or closed for the selected period is a signal including the frequency higher than the specified frequency and a silence signal.

those for the period selected just before is a signal including the frequency higher than the specified frequency and a silence signal.

2. A method of driving the recording device according to claim 1 wherein a first waveform part of the driving waveform which sets the micro-shutters opened or closed during the selected period is a superposed one obtained when the signal whose frequency is lower than the specified frequency is superposed upon a signal whose frequency is higher than the specified frequency, and wherein a first waveform part of the driving waveform which reversely sets the micro-shutters opened or closed for the selected period is the signal whose frequency is higher than the specified frequency.

3. A method of driving the recording device according to claim 1 wherein the signal whose frequency is higher or lower than the specified frequency is applied to the signal lelctrodes for a part of the non-selected period, while a signal which make the superposed waveform to achieve the substatially same driving effect between the common and signal electrodes regardless of the frequency of the signal applied to the signal electrodes is applied to the common electrodes.

4. A method of driving a recording device substantially as hereinbefore described with reference to Figs. 2 to 6 of the accompanying drawings.