GB2100017A

GB2100017A - Liquid crystal device

Info

Publication number: GB2100017A
Application number: GB8117824A
Authority: GB
Original assignee: Ferranti PLC
Current assignee: Ferranti International PLC
Priority date: 1981-06-10
Filing date: 1981-06-10
Publication date: 1982-12-15
Also published as: GB2100017B

Abstract

A liquid crystal display device includes a first electrode (12) extending over the active area of the device and having a low resistance, and a second electrode parallel to and spaced apart from the first electrode. The second electrode comprises a number of strips (14) connected in series by interconnections (15) and made of a material having a resistance characteristic such that a potential gradient may be developed along the length of the second electrode. A liquid crystal material is contained in the space between the two electrodes, one of which must be transparent. Circuit means (not shown) apply a potential difference between the ends of the second electrode and are operable to vary the distribution of the potential difference relative to the first electrode (12) so as to produce a small transparent area in any desired position on the active area of the device. <IMAGE>

Description

SPECIFICATION Liquid Crystal Device This invention relates to a liquid crystal device for use, amongst other things, as a display device.

Liquid crystal display devices are well-known, usually comprising a first electrode extending over the display area, and one or more shaped electrodes, with a liquid crystal material in the space between them.

The application of an electric field between the first electrode and any of the other electrodes causes the material between them to change its optical characteristics. Frequently the medium is one which becomes opaque when an electric field is applied. It has been necessary to form the electrodes on one side of the material so as to define accurately the shape to be displayed.

British Patent No. 1,131,688 discloses a liquid crystal display device in which two sets of low resistance electrodes are provided, enabling a particular small area to be activated by applying a potential difference across one electrode of each set.

British Patent No. 1,337,637 discloses a similar sort of device in which one electrode covers the entire display area and the other electrode is formed as a set of strips. The strips are formed of resistive material, and one strip is selected to be energised as required. In both of these prior art devices it is necessary to provide circuitry to select the appropriate electrode or electrodes for energisation, and this adds to the complexity of the device.

It is an object of the invention to provide a liquid crystal device in which the activation of any point on the device is simplified.

According to the present invention there is provided a liquid crystal device which includes a first electrode extending over the active area of the device and having a low resistance, a second electrode parallel to and spaced apart from the first electrode and comprising one or more seriesconnected strips each having a resistance characteristic such that a potential gradient may be developed along the length of the electrode, at least one of the electrodes being transparent, a liquid crystal material contained in the space between the first and second electrodes and of a type which is transparent in the absence of an electric field, and circuit means operable to apply a potential difference between the ends of the second electrode and to vary the distribution of said potential difference with respect to the first electrode, so as to produce a small transparent area in a desired position.

Preferably the strips forming the second electrode are formed parallel to one another.

The invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a side view of a liquid crystal display device according to a first embodiment of the invention; Figure 2 is a plan view of part of the device of Figure 1; Figure 3 illustrates the operation of the device of Figures 1 and 2; Figure 4 is a schematic diagram of a control circuit; Figure 5 illustrates the operation of the circuit of Figure 4; Figure 6 illustrates an alternative form of control circuit; Figure 7 is a schematic diagram of a position sensing device; Figure 8 is a diagram of a circuit for use with the embodiment of Figure 7; and; Figure 9 is a schematic diagram of a further embodiment suitable for reading punched card or tape.

Referring now to Figures 1 and 2, a liquid crystal display is built up on two plates 10 and 11 of electrically insulating material. At least one of these plates must be transparent. On the inwardly-facing surface of each of the plates is formed an electrode in the form of a thin deposited or evaporated layer of a suitable material. At least that electrode on the transparent plate must also be transparent. As shown in Figures 1 and 2, electrode 12 on plate 10 is in the form of a continuous layer of low-resistance material so as to form an equipotential surface. The other electrode 13, formed on plate 11, is made up from a plurality of parallel strips 14 of resistive material. The strips are connected in series with one another by low-resistance interconnections 15.Suitable terminations are applied to the electrodes, there being a single termination 16 on the electrode 12, and terminations 17 and 18 on the two ends of electrode 13. The material of electrode 13 is such that a potential gradient may be developed across the electrode. Many more strips will be provided in practice than are shown in Figure 2.

The space between the electrodes 12 and 13 is enclosed by a barrier 19 of suitable material, and the volume so enclosed is filled with a suitable liquid crystal material 20, the layer of material being, say, 101lm thick.

Figure 3 illustrates the operation of the device described above. In Figure 3 (a) the resistive electrode 13 is represented as a resistance, whilst the other electrode 12 is shown as a conductor. A voltage source 21 is connected across the electrode 13 and the electrode 12 is earthed. It will be clear that an electric field will be developed across the material between the two electrodes when the voltage from source 21 is fixed relative to earth potential.

Figure 3(b) shows the voltage distribution across the electrode 13 necessary to place the centre of the electrode at earth potential. The voltage source 21 produces a voltage of 2V varying from +V at one end of the electrode to at the other end. Hence the centre of the electrode is at earth potential. In the case of the simplified 9-strip electrode shown in Figure 2, the centre of the electrode 13 is the centre point of the centre strip. Hence at this point of the display area, and nowhere else, the liquid crystal material will be transparent, and the remainder of the display area will be opaque.

If the voltage applied across the electrode 13 is maintained at a value 2V, but the terminal 17 is earthed, the voltage distribution will be shown in Figure 3(c). In this case the zero potential transparent area will be at the end of the electrode 13 nearest to the terminal 17. It will be seen, therefore, that the transparent area may be moved simply by varying the voltage applied by source 21 with respect to earth, whilst maintaining the voltage itself at a constant value. In practice it is advisable to use a.c.

energising voltages, to avoid any problems due to polarisation of the liquid crystal material. Any suitable periodic waveform may be used, such as sinusoidal or square wave.

Figure 4 illustrates a simple circuit which will enable the transparent area to be produced in any desired position. This circuit uses a multiplier MT as an amplitude modulation to modulate a d.c. control voltage Vc applied to one input with a say, sinusoidal modulating signal Vm. The output of the modulation MT is applied to one end of the electrode 13, say to terminal 17. An inverting amplifierAl sumsthe output of the modulator MT and the sinusoidal signal and has its output connected to the other terminal 18 of electrode 13, to provide an arc voltage 1800 out of phase with that applied to electrode 12.

Figure 5 shows the full range of voltages applied to terminals 17 and 18 as the control voltage Vc is varied. Any particular value of Vc will thus produce the appropriate voltages on terminals 17 and 18 which, with the electrode 12 earthed, will produce the transparent area in the desired position.

An alternative arrangement of the device, leading to a simple control circuit, is shown in Figure 6. in this arrangement, a constant voltage is applied across the electrode 18 which has one end earthed, whilst the electrode 12 has its potential variable over the full range of the voltage across the electrode 13.

Thus if the electrode 12 is held at earth potential, there will be a zero electric field across the liquid crystal material only at the earthed end of electrode 12. Similarly, if the electrode 13 is held at the same potential as the other end of electrode 13, then the zero field, and hence the transparent area, will be at that end of electrode 13. As will be seen from Figure 6, the control circuit now only requires the multiplier MT with its control input and its a.c. modulating input Vm. Any value of the control voltage Vc will produce the transparent area in the appropriate position on the screen.

The application described above is for a display device. However, it is possible to use the liquid crystal device of Figures 1 and 2 for other applications. One of these is a position sensing device. Our copending application No. 8113135 describes a liquid crystal position sensor which scans separately in two coordinates. The device according to the present invention may be used for the same purpose, and will make it necessary to scan in one sense only.

Figure 7 is a schematic diagram of one form of the device, for sensing position about two perpendicular axes. The Figure shows part of a movable body 70 whose position is to be determined relative to a datum position. The body is capable of movement in two perpendicular directions, one of which is shown by the arrow and the other of which is perpendicular to the plane of the drawing. Arranged parallel to the plane of movement of the body 70 is a liquid crystal device 71 of the form described earlier. In this case both electrodes and their supporting plates are transparent. The dimensions of the device 71 are at least equal to the extent of movement of the body 70 in the plane. Carried on the body 70 is a light source 72 which projects a collimated beam of light towards the device so to produce a small spot of light on it.A lens 73 located behind the device focusses any light passing through the device onto a photocell 74. The electrical output from the photocell is applied to a control circuit 75 which controls the voltages applied to the liquid crystal device. The circuit operates to move the transparent area of the liquid-crystal device overthe area until it coincides with the position of the light beam. The resulting output from the photocell 74 is then used to stop the scan, and the coordinates of the position may be determined.

The circuits of Figures 4 and 6 will produce a raster-type scan if the control voltage Vc takes the form of a ramp voltage, varying over the full range in the time required to move the transparent area over the full length of the electrode 13. In its simplest form, therefore, the control circuit 75 may include the circuit of either Figure 4 or Figure 6, together with means for stopping the raster scan when light is received by the photocell 74. If the electrode 13 is of sufficiently linear characteristics, the position of the transparent area may be determined from the time which has elapsed between the start of the scan and an output from the photocell. Figure 8 shows a simple circuit which will perform this function. A counter CT is supplied with clock pulses by way of a gate G.This gate is arranged to be opened when the scan starts and is closed when a photocell output is detected. The counter output is applied to a programmable read-only memory PROM, which converts the counter output directly into the coordinates of the position of the transparent area. The circuit will also have to perform other functions, such as resetting the counter at the beginning of the next scan. In orderto determine the position of the light spot more accurately, it is possible to include in the circuit the phase-sensitive detector and associated circuitry of our copending application No. 8113135 referred to above.

The position-sensing arrangement just described may be used for other purposes. For example, the position of holes in a punched card may be determined by scanning across the surface of the card.

Figure 9 illustrates such an application. The Figure shows a liquid crystal device 91 as already described, together with a light source 92 arranged to illuminate the surface of a punched card 93. Any light passing holes in the card will produce small spots of light on the active area of the device 91. As before, the small transparent area if caused to scan over the active area under the control of the control circuit 94, and any light passing through the device 91 is focussed by lens 95 on to the light-sensitive detector 96. In such an application, where the card 93 will contain a number of holes, the scan will have to cover the entire active area, with the position of each separate spot being recorded.

A somewhat simpler arrangement may be used to read punched tapes, in which the holes appear in rows. The electrode 13 of the device may, in such a case, comprise only one length of resistive material, with the scan taking place along its length. Each row of holes may be read as the tape is indexed past the reading point.

Claims

1. A liquid crystal device which includes a first electrode extending over the active area of the device and having a low-resistance, a second electrode parallel to and spaced apart from the first electrode and comprising one or more seriesconnected strips each having a resistance characteristic such that a potential gradient may be developed along the length of the electrode, at least one of the electrodes being transparent, a liquid crystal material contained in the space between the first and second electrodes and of a type which is transparent in the absence of an electric field, and circuit means operable to apply a potential difference between the ends of the second electrode and to control the distribution of said potential difference with respect to the first electrode so as to produce a small transparent area with a desired position.

2. A device as claimed in Claim 1 in which the strips forming the second electrode are formed parallel to one another.

3. A device as claimed in either of Claims 1 or 2 in which the circuit means is operable to apply a constant potential difference between the ends of the second electrode and to hold the first electrode at a fixed potential with respect to earth.

4. A device as claimed in either of Claims 1 or 2 in which the circuit means is operable to hold the ends of the second electrode at fixed potentials with respect to earth and to vary the potential applied to the first electrode.

5. A device as claimed in any one of the preceding claims in which the circuit means produce alternating potential differences for connection to the second electrode.

6. A device as claimed in Claim 5 in which the circuit means includes means for modulating a d.c.

control voltage with an alternating current signal to provide said alternating potential difference.

7. A liquid crystal device substantially as herein described with reference to Figures 1 to 6 of the accompanying drawings.

8. Apparatus for sensing the position of a spot of light in a plane, which includes a device as claimed in any one of the preceding claims in which the dimensions of the active area are at least equal to the extent of movement of the spot of light and in which the circuit means are arranged to scan the said transparent area over the active area in a predetermined manner, a light sensitive detector operable to detect the passage of light from said spot through the said transparent area, and output means operable to provide an indication of the transparent area when the passage of light is detected.

9. Apparatus as claimed in Claim 8 in which the output means includes timing means for measuring the time between the start of a scan and the presence of an output from the light sensitive detector.

10. Apparatus for determining the position of an object in a plane which includes apparatus as claimed in either of Claims 8 or 9, and a light source carried by the object and operable to provide said spot of light.

11. Apparatus for determining the position of an object in a plane substantially as herein described with reference to Figures 7 and 8 of the accompanying drawings.

12. Apparatus for determining the position of an aperture in an area of material which includes apparatus as claimed in either of Claims 8 or 9, means for supporting the area of material adjacent to the liquid crystal device, and a light source operable to illuminate that surface of the material remote from the said device.

13. Apparatus as claimed in Claim 12 in which the area of material is a punched card.

14. Apparatus for determining the position of an aperture in an area of material substantially as herein described with reference to Figure 9 of the accompanying drawings.