Display apparatus

Abstract: A technique for driving a liquid crystal apparatus having an effective display area and a non-display unit area is provided. When a bistable liquid crystal element is used, a voltage must be kept applied to the non-display unit area to maintain a uniform white or black display state. Durability of the non-display unit is particularly demanded. In this invention, however, the non-display unit includes first and second stripe electrodes perpendicular to each other. First and second drive bipolar pulses are respectively applied to the first and second stripe electrodes with a phase difference between these bipolar pulses.

Description: The present invention relates to display apparatus including a ferroelectric liquid crystal (FLC) display panel, a control unit for use therein, and a method of operating the same. Related Background Art Display apparatus using ferroelectric chiral smectic liquid crystals are disclosed in, e.g. U.S.P. Nos. 4,639,089, 4,681,404, 4,682,858, 4,712,873, 4,712,874, 4,712,875, 4,712,877, 4,714,323, 4,728,176, 4,738,515, 3,740,060, 4,765,720, 4,778,259, 4,796,979, 4,796,980, 4,859,036, 4,932,757, 4,932,758, 5,000,545 and 5,007,716. In each display apparatus of these prior arts, transparent electrodes are formed on the inner surfaces of opposite glass substrates aligned by rubbing and spaced apart from each other by a cell gap of 1 to 3 µm to constitute a liquid crystal cell, and a ferroelectric chiral smectic liquid crystal (to be referred to as an FLC hereinafter), is sealed in the liquid crystal cell. This liquid crystal device (panel) is inserted and fixed in a housing and is utilised as a display apparatus. When the liquid crystal panel is mounted using the housing, the upper surface of the housing is higher than the upper surface of the liquid crystal panel. The whole of the panel cannot be observed when viewed from an oblique direction. For this reason, a non-display unit having a width of about 5 to 10 mm is formed around the display unit to improve readability of information displayed on the panel. When a bistable liquid crystal element is used, any signal is preferably applied to this non-display unit to define a non-display unit (i.e. a display frame) for displaying a peripheral portion of an effective display area formed by matrix electrodes in white or black. Since a drive voltage is kept applied to maintain the display frame in a uniform "white" or "black" display state, durability of the liquid crystal apparatus must be improved. It is an object of the present invention to provide a display apparatus having improved durability. It has now been found that the durability of the display apparatus can be improved by modifying the signals applied to the non-display border area, as set forth in the claims appended. In the accompanying drawings: Fig. 1 is a sectional view of a liquid crystal element such as used in the present invention; Figs. 2(a) to 2(c) are plan views of a liquid crystal panel such as used in the present invention; Fig. 3 is a plan view showing a liquid crystal panel; Fig. 4 is a block diagram of a liquid crystal apparatus according to an embodiment of the present invention; Fig. 5 is a waveform diagram for drive signals for displaying a desired pattern on a display unit in the apparatus shown in Fig. 4; Figs. 6A and 6B are a waveform diagram of signals applied to scan electrodes of a common non-display unit and information electrodes of a segment non-display unit, and a waveform diagram of a signal finally applied to non-display electrode intersections when the phases of the above signals are shifted by 1/4 from each other; Fig. 7 is a view illustrating a liquid crystal air gap portion formed between the display and non-display units of the apparatus; Fig. 8 is a waveform diagram of other signals applied to the scan electrodes of the common non-display unit and the information electrodes of the segment non-display unit in the apparatus of Fig. 4; and Fig. 9 is a waveform diagram of still other signals applied to the scan electrodes of the common non-display unit and the information electrodes of the segment non-display unit in the apparatus of Fig. 4. To further the understanding of this invention, preferred embodiments will be described below making reference to the drawings. The following description is given by way of example only. A picture element obtained by aligning an FLC upon formation of a chevron structure has an excellent bright (light) state under the crossed nicols, and a sufficiently high contrast can be obtained. Fig. 1 is a sectional view showing an alignment state of the FLC sealed between substrates 11 and 12. An FLC 13 has layers 15 constituted by a plurality of liquid crystal molecules 14. The plurality of layers 15 are aligned in the same direction and constitute a chevron formation. In this case, the major axis of each of the liquid crystal molecules 14 is preferably inclined at a pretilt angle a of 5° or more from the substrates 11 and 12. The above alignment state is preferably obtained by treating the substrates 11 and 12 by rubbing in directions 16 and 17. Figs. 2(a) to 2(c) are plan views of a panel in which an FLC 13 having a chevron formation is sealed. A seal member 21 seals a gap between the substrates 11 and 12. Although not shown, a first group of scan electrodes for receiving a voltage are arranged in the panels, and a second group of information electrodes perpendicular to the first group of scan electrodes are arranged in the panel to constitute matrix electrodes. The area of the matrix electrodes corresponds to an effective display area 24. A frame display unit 23 defines a non-display area. A normal 22 to each layer 15 (plane) of the FLC 13 is substantially parallel to the rubbing directions 16 and 17. In the element shown in Figs. 2(a) to 2(c), the liquid crystal molecules 14 are uniformly inclined at a tilt angle of + to the left on the plane (spontaneous polarisation is directed from the upper surface of the drawing sheet to the lower surface thereof). According to findings of the experiments of the present inventors, when a voltage (e.g. an 10-Hz AC voltage of ±8V) is applied to the effective display area 24 and the frame display unit 23 in the above state, the liquid crystal molecules 15 start to flow in the right direction. When the voltage is kept applied for a long period of time (e.g. 20 to 50 hours), areas 31 in which the liquid crystal molecules 14 are reduced or depleted are formed in the left portion, as shown in Fig. 3. An area 32 in which the liquid crystal molecules 14 are increased is formed in the right portion. As a result, coloured interference fringes appear over the entire surface of the panel, thereby degrading the display quality. When the liquid crystal molecules 15 in Fig. 2(c) are inclined at a tilt angle of - to the right on the plane (spontaneous polarisation is directed from the lower surface of the drawing sheet to the upper surface), it is also found that the liquid crystal molecules start to flow to the left. Fig. 4 is a block diagram of a liquid crystal display apparatus according to an embodiment of the present invention. This apparatus comprises an FLC panel 401 having 640 (information lines) x 480 (scan lines) pixels. A non-display area adjacent to a display unit 403 is constituted by scan and signal electrodes respectively present in a common non-display unit 405 and a segment non-display unit 407. The apparatus also comprises an FLC panel controller 409 which includes a display unit drive voltage source 411, a non-display unit drive voltage source 426, a non-display unit drive waveshape control unit 413, a logic control unit 415, and a logic control power source 417. These components of the FLC panel controller 409 perform all control operations of the FLC panel 401 such as setup of drive conditions (e.g., a drive waveshape and a drive voltage), control of the segment and common driver groups 419 and 421, control of communication between a data source 423 and the driver groups 419 and 421, and control of drive waveshapes of the non-display unit. Voltages VH and VL (to be described in detail later) are used as drive waveshape voltages of the non-display unit. Fig. 5 shows drive waveshapes for driving a desired pattern on the display unit 403. As shown in Fig. 5, when an information signal m is superposed on scan signals n to n+2, the (n,m) pixel is set in a dark state, and the (n+1,m) and (n+2,m) pixels are set in a bright (light) state. Drive conditions for properly drawing the pattern are V1 = 15 V, V2 = -15 V, V3 = 6 V, V4 = -6 V, and 1H period (one horizontal scan period) = 80 µsec at room temperature. Note that voltage values are potential differences from Vc. Fig. 6A is a waveshape chart of drive waveshapes Wc and Ws respectively applied to the scan electrodes of the common non-display unit 405 and information electrodes of the segment non-display unit 407. Voltage levels are obtained by simple rectangular waves using the voltages VH and VL. The rectangular waves must have periods Tc and Ts (Tc = Ts) for sufficiently switching between the bright and dark states and must have frequencies higher than those visually noticed by an observer, thereby suppressing flickering. According to the present inventors, it is found that flickering can be sufficiently suppressed in a practical level when a frequency of a drive waveshape for the non-display unit is 30 Hz or more (i.e., the period is 33.3 msec or less). In this embodiment, the frequency of the non-display unit drive waveshape is set to be 50 Hz (one period = 20 msec). The period of the signal on the common side is equal to that on the segment side, but the phase of the signal on the common side is shifted relative to that on the segment side by 1/4 because a sufficient voltage is applied to non-display electrode intersections at four corners. More specifically, when in-phase voltages are applied to the electrodes, an electric field is not applied to the non-display electrode intersections 425 to fail to control the intersections 425. As a result of extensive studies of the present inventors, uniform control free from a difference in hue can be performed by shifting the phases of the voltages by 1/8 to 7/8. Fig. 6B shows a waveshape (Wc - Ws) finally applied to the non-display electrode intersection 425 when the phases are shifted from each other by 1/4. When the liquid crystal panel is driven under the above conditions, both the display unit and the non-display unit can be controlled to be set in the uniform display state. With the above arrangement, peak values (i.e., potential differences from Vc) of the respective drive waveshapes shown in Fig. 5 were set to be values defined by the above excellent drive conditions, i.e., V1 = 15 V, V2 = -15 V, V3 = 6 V, and V4 = -6 V, and a drive waveshape voltage for the non-display unit was set to be VH = -VL = 2 to 10 V. The FLC panel was driven under the above conditions. When a predetermined drive period had elapsed, an air gap 701 as a defect was formed between the display unit 403 and the non-display unit 407, as shown in Fig. 7. A relationship between the drive periods until the defect is caused and the drive waveshape voltages VH and VL is shown in Table 1 in which voltages of 2, 4, 5, and 6 V within the range of the present invention, i.e., VH = -VL ≤ 6 V (peak value of information electrode signal) are applied in Examples 1 to 4, and voltages of 8 and 10 V falling outside the range of the present invention are applied in Comparative Examples 1 and 2. VH = -VL [V] Drive Time [hours] Example 1 2 No defect in 1,000 hours Example 2 4 No defect in 1,000 hours Example 3 5 No defect in 1,000 hours Example 4 6 No defect in 1,000 hours Comparative Example 1 8 Defect in 400 hours Comparative Example 2 10 Defect in 350 hours As is apparent from Table 1, in Examples 1 to 4 wherein the peak values (VH and VL) of the non-display drive unit voltages are smaller than or equal to those (V3 and V4) of the nformation signal voltages, durability and reliability can be greatly improved as compared with Comparative Examples 1 and 2. It is assumed that the air gap is formed due to the following mechanism. A ferroelectric liquid crystal layer having a thickness of several µm or less receives a strong electric field of a voltage falling between 20 V and 30 V, and a difference in stress caused by deformation of the layer structure unique to the FLC element is caused to shift liquid crystal molecules within the panel due to some reason. When an electric field stronger than that applied to the display unit is applied to the non-display unit as in the comparative examples, and the period of the drive waveform for the non-display unit is longer than the period of the drive waveform of the display unit by about 100 times, a defect tends to be caused. However, a torque generation mechanism and the like in movement of the liquid crystal are not yet clarified. Another preferred embodiment of the present invention is shown in Figs. 8 and 9. For example, as shown in Fig. 8, when 50-Hz period drive waveshapes Wc and Ws having a 1/2 phase difference are applied to the FLC panel as in Fig. 6A, excellent image quality can be obtained. Even if 50-Hz period drive waveshapes Wc and Ws having a 3/4 phase difference are applied to the FLC panel, as shown in Fig. 9, the non-display unit can be uniformly controlled, thereby obtaining excellent image quality. According to the present invention, as has been described above, since electric fields exceeding the first and second threshold values enough to set the first and second stable states are alternatively applied to the ferroelectric liquid crystal of the non-display unit in a predetermined period, the non-display unit can be substantially maintained at a predetermined brightness level as an intermediate level between the bright (light) and dark states. Since the voltage peak value of the non-display unit drive signal is smaller than the voltage peak value of the information electrode signal, a period for undesirably forming an air gap between the display unit and the non-display unit parallel to the information electrode group can be greatly prolonged. As a result, reliability and durability of the apparatus can be greatly improved.

Claims: A display apparatus comprising a liquid crystal panel (401) having first and second plates (11,12) on which are formed, respectively, scan electrodes and information electrodes which respective scan and information electrodes are arranged perpendicular and facing each other to define a matrix display area (403), first stripe electrodes (405) on said first plate arranged parallel to said scan electrodes, second stripe electrodes (407) on said second plate arranged parallel to said information electrodes, which first and second stripe electrodes (405,407) define a non-display border area (405,407) at the edges of said display area (403), and a ferroelectric liquid crystal material (13) sealed between said first and second plates (11,12); first drive means (421,419) to apply scan signals (n,..) to said scan electrodes and information signals (m) to said information electrodes to display an image in said matrix display area (403); second drive means (413) to apply a first drive signal (W c ) to said first stripe electrodes (405); and third drive means (413) to apply a second drive signal (W s ) to said second stripe electrodes (407); which apparatus is characterised in that: said second drive means (413) and said third drive means (413) are adapted to apply first and second drive signals W c ,W s ), respectively, each having a bipolar pulse waveform and a repetition frequency of not less than 30 Hz, the peak values (V H ,V L ) of the pulse waveform not exceeding those (V 3 ,V 4 ) of said information signals (m), said first and second drive signals (W c ,W s ) having a relative phase shift, whereby respective thresholds for setting stable states of said liquid crystal panel shall be exceeded in each respective half-cycle of said pulse waveform at the intersections of said first stripe electrodes (405) and said information electrodes, said second stripe electrodes (407) and said scan electrodes, and said first stripe electrodes (405) and said second stripe electrodes (407). A display apparatus as claimed in claim 1 wherein the liquid crystal molecules (14) of the liquid crystal material (13) are aligned and arranged in layers (15) in chevron formation. A display apparatus as claimed in either preceding claim wherein the relative phase shift between said first and second drive signals (W c ,W s ) is in the range 1/8 to 7/8 of a cycle, inclusive. A display apparatus as claimed in any preceding claim wherein said first and second drive signals (W c ,W s ) have a pulse width longer than that of said information signals (m). A control unit for use in the display apparatus of claim 1 comprising first drive means (421,419) to apply scan signals (n,..) to said scan electrodes and information signals (m) to said information electrodes to display an image in said matrix display area (403); second drive means (413) to apply a first drive signal (W c ) to said first stripe electrodes (405); and third drive means (413) to apply a second drive signal (W s ) to said second stripe electrodes (407); which control unit is characterised in that: said second drive means (413) and said third drive means (413) are adapted to apply first and second drive signals (W c ,W s ), respectively, each having a bipolar pulse waveform and a repetition frequency of not less than 30 Hz, the peak values (V H ,V L ) of the pulse waveform not exceeding those (V 3 ,V 4 ) of said information signals (m), said first and second drive signals (W c ,W s ) having a relative phase shift, whereby respective thresholds for setting stable states of said liquid crystal panel shall be exceeded in each respective half-cycle of said pulse waveform at the intersections of said first stripe electrodes (405) and said information electrodes, said second stripe electrodes (407) and said scan electrodes, and said first stripe electrodes (405) and said second stripe electrodes (407). A control unit as claimed in claim 5 wherein the relative phase shift between said first and second drive signals (W c ,W s ) is in the range 1/8 to 7/8 of a cycle, inclusive. A control unit as claimed in either preceding claim where said first and second drive signals (W c ,W s ) have a pulse width longer than that of said information signals (m). A method of operating the display apparatus of claim 1 including steps of: applying scan signals (n,...) to the scan electrodes and information signals (m) to the information electrodes to display an image in the matrix display area (403) thereof; applying a first drive signal (W c ) to the first stripe electrodes (405); and applying a second drive signal (W s ) to the second stripe electrodes (407); which method is characterised in that: said first and second drive signals (W c ,W s ) are of bipolar pulse waveform, have a repetition frequency of not less than 30 Hz, peak voltage values (V H ,V L ) not exceeding those (V 3 ,V 4 ) of the information signals (m), and have a relative phase shift, whereby respective threshold levels of the liquid crystal panel for setting stable states are exceeded in each respective half-cycle of said pulse waveform at the intersections of said first stripe electrodes (405) and said information electrodes, said second stripe electrodes (407) and said scan electrodes, and said first stripe electrodes (405) and said second stripe electrodes (407). A method as claimed in claim 8 wherein the relative phase shift between said first and second drive signals (W c ,W s ) is in the range 1/8 to 7/8 of a cycle, inclusive. A method as claimed in either of claims 8 or 9 wherein said first and second drive signals (W c ,W s ) have a pulse width longer than that of said information signals (m).

Inventors: INOUE HIROSHI, KATAKURA KAZUNORI, MIYAMOTO KATSUHIRO, TSUBOYAMA AKIRA, YUKI OSAMU, INOUE, HIROSHI, KATAKURA, KAZUNORI, MIYAMOTO, KATSUHIRO, TSUBOYAMA, AKIRA, YUKI, OSAMU, INOUE, HIROSHI, C/O CANON KABUSHIKI KAISHA, KATAKURA, KAZUNORI, C/O CANON KABUSHIKI KAISHA, MIYAMOTO, KATSUHIRO, C/O CANON KABUSHIKI KAISHA, TSUBOYAMA, AKIRA, C/O CANON KABUSHIKI KAISHA, Yuki, Osamu, c/o Canon Kabushiki Kaisha
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1992
Main classification: G09G3
IPC codes: G09G3