GB2170029A - Display device - Google Patents

Abstract

A device for converting a visual display, such as the digits of a gas meter reading, into electrical form without mechanical interference with the meter uses a liquid crystal cell. Each digit is illuminated and the light reflected from it is projected through the cell which is in its scattering, i.e. non-transmissive mode. A coordinate control array (6,10) for the cell sets it successively to a number of states in each of which it is in its transmissive mode for one of the possible digit values. Thus each such state is compared with the level of light reflected from the digit to be converted. The level which most closely corresponds to the digit is assumed to be that for the digit, and this is indicated as its value. The display may include numerals or text. The digits may be read singly or in parallel. The liquid crystal device 1-12 has a light entry point at 2,3 and exit point facing detectors 13. <IMAGE>

Description

SPECIFICATION Display device This invention relates to an electrical arrangement for converting from a digital, i.e. letter and/or numerals, display to an electrical form suitable for transmission and/or electronic data manipulation.

According to the invention there is provided an arrangement for converting a visually-displayed character digit into an electrical representation, which includes a liquid crystal cell via which the digit to be converted is monitored, the liquid crystal material being normally maintained in its nontransmissive mode, co-ordinate control means for the liquid crystal cell to which electrical signals are applied to set the liquid crystal material successively to a number of patterns of transmissivity each corresponding to one of a number of possible digit or cypher values, a detector adapted to monitorthe light transmitted via the cell, the level of output of light which reaches the detector after traversing the cell indicating whether the pattern to which the cell is set matches the digit to be converted, and output means responsive to such an indication from the detector to indicate the digit's identity electrically.

An embodiment of the invention will now be described with reference to the accompanying drawings, in which Figure 1 is an exploded view of a conversion arrangement embodying the invention, Figure 2 is a simplified block diagram of electronics for use with the arrangement of Figure 1, and Figure 3 shows how an arrangement as in Figure 1 can be used with a domestic electricity or gas meter.

In the arrangement to be described a set of numerals, with or without letters or other symbols intended for direct viewing is made available for transmission as telemetry signals and other electronic handling without any mechanical interference with the display. Where the information displayed relates to quantities for public sale, as for gas and electricity meters, there has to be an effective reconciliation betwen what is transmitted and what is to be displayed on the instrument. Such a device could also be useful to enable the visually handicapped to have some access to ordinary text by providing an input to a computer/voice synthesiser.

The arrangement uses the characteristics of liquid crystals, and consists essentially of two sheets of glass separated by an appropriate spacer, sealed and containing a suitable liquid crystal material. The size of this structure is such as to cover adequately the area of the displayed ciphers/symbols on the existing device to be telemetered, or text to be electronically treated. Such a structure, called 'cell', will now be described. It is assumed herein that the required telemetered value contains nominally six sequential numerical digits presenting theirvaluefor viewing in decimal form. Each number for human observation has an aspect ratio assumed herein to be 3-5, and with the centre lines of succeeding digits 1 1/2 digit widths apart.

The liquid crystal cell is regarded as being broken up into six identical rectangles sequentially disposed upon the long axis of the cell and occupying the active surface of the cell. See Figure 1 attached, where the display is shown at the bottom. it comprises six discs on a common shaft, in normal manner. Each of the rectangles referred to is large enough to cover a digit and half the inter-digit spaces on each side. Thus it will be seen that the nominal cell when related to each individual digit of the display is much larger than that digit. The difference in effective areas is distributed round the outside of the active area, rather like the frame of a picture.

As stated above, the cell consists fundamentally of two matrixed glass plates, separated by a seal and filled with liquid crystal material. The device is however, somewhat more complex, the description starts at the lower glass plate and continues upwards through the structure, see Figure 1.

The lower glass plate 1 has a variety of functions and is suitably prepared along one short edge 2 to serve as an entry point for an external source of illumination. The other three edges are coated with non-reflective material. An alternative light entry is also envisaged, and may use the form of the light conducting bars 3, occupying the 'frame' space and reflecting on every surface except those at entry and facing the required data.

On the upper surface of the plate all save the six areas corresponding exactly to the digit size are covered by layer 4 of a prismatic/holographic material with characteristics suitable for the reflection of a light beam received at low incidence and its reflection broadly at right angles to the incident plane.

This layer consists of a number of contiguous picture frames', and is responsibleforthe illumination of the required display. The layer is placed immediately above the bottom glass plate and within the internal dimension of the seal. This element 4 may be omitted if the 'alternative' method of light entry referred to above is adopted.

A layer of polarising material indicated at 5 may be placed in this area of the structure if required by the particular liquid crystal material chosen.

The bottom glass plate carries within each active area a number of "columns" one set of which is indicated at 6, the connections for these columns are brought out within the space occupied by the reflecting layer as shown at 7. These columns are transparent metallic conductors of indium tin oxide, and the sets are individual to each discrete digit area.

They are on the upper surface of the lower glass plate and brought out at its edges. The dimensions of these conductors will be discussed below.

The next element in the structure is a frame-like spacer/seal 8, above which is a second glass plate 9 on the lower surface of which are a number of continuous "rows" as indicated at 10, again, typically of indium tin oxide. These are common in each discrete area, and their connections are brought out to a short edge.

Immediately above the glass plate 9 is a prismatic layer 11 which effectively covers the digit area, i.e. A position normally occupied by a picture within a frame. This is so arranged that any of the light from the lower plate 1, which is reflected from the digit to be determined, and has thus been transmitted through the liquid crystal material, is reflected to a polished portion of the long edge of the upper glass plate 9 appropriate to the digit in question. A second layer 12 of polarising material may be placed in this area if necessary. Between the two glass plates 1 and 9 separated by the aforementioned spacer 8, a liquid crystal material is placed.

Along the prepared, longer, edge of the upper plate detectors such as 13, one per discrete digit, are arranged, each of which indicate the total amount of light present in the digit spacce, as a result of transmission through the corresponding liquid crystal matrix. Unlike the lower glass plate 1, all edges, other than the long prepared edge, are coated with a reflecting material.

Finally, above the assembly as described, is placed a spring opaque cover 14 pivoted at one of the long edges of the assembly and normally he[d flat against the upper surface of the upper plate, so as to exclude ambient light.

The operation will now be described, using for the purpose of explanation, only one digit, in this case a numerical "4". The liquid crystal material is normally maintained in a scattering mode. The figure "4" on the viewing display is illuminated, generally normal to its surface, by light reflected from within the lower glass plate 1 or bars 3 by the lower prismatic material or directly, depending upon the actual construction adopted. The figure "4" has a high contrast ratio to its background since it is optimised for maximum visibiiity for the human eye.The figure "4" is therefore highly reflective over the body of the figure with a poorly reflective background, By using the "rows" 10 and "columns" 6 driven by electronic circuits to be described below, the liquid crystal material is caused to be in a transmission mode in shapes corresponding to the total range of ciphers to be expected, i.e. in this case 0,1,2...9, sequentially, thus forming a series of masks. Each corresponds to a single numerical digit and as a consequence interferes with the light reflected from the body of the "4" to varying extents.

The light reaching the upper surface of the cell will be at an individual maximum with the mask and the number corresponding. The light which penetrates the cell under any of the conditions mentioned, will be reflected by the upper prismatic material to the edge of the cell where its integrated intensity will be measured by its corresponding detector. By selectively generating a series of digit forms within the liquid crystal material, the required number can be established since only when there is a maximum correspondence between number and form will the detector sense a maximum output. Thus it is effectively a direct observation of the number which by definition eliminates the reconciliation problem.

The drive electronics for the "rows" and "columns" use conventional practice already in common use for similar purposes. There is however, only one set of drive electronics, however many digits there may be in the value to be identified, the process being multiplexed for each individual digit of the value. Multiplexing is facilitated since the sets of columns for each digit are discrete.

The above method of operation may be modified for parallel interrogation of multiple symbols if text, as distinct from numbers, is involved.

In the case under discussion such a matching process takes place for each component digit in sequence, and may be stored in a format best suited to the requirements of the subsequent transmission processes, e.g. binary. When one digit is being matched, the liquid crystal material for all other digits is maintained in a scattering mode.

The liquid crystal material is driven to the transmission mode for all digits if the cover 14 is lifted for visual inspection of the indicated value, and the light source is also then activated to facilitate the inspection.

Two practical difficulties will now be described, with the way to deal with them.

In many numerical values presented for human observation a "cyclometer" principle is used - the succeeding more significant digit being operated by a cam driven by the preceding 9. Since many devices operate relatively slowly there are periods in which digits are half present within the viewing aperture.

Secondly, such devices are usually mass produced and have some "play" such that a digit when fully present within the viewing aperture may occupy a variable, if restricted, set of vertical positions.

The former is dealt with automatically by the above described process, since if a digit is not properly within the aperture no appropriate maximum is produced, as the digit present is compared with complete digit masks. The interrogation process is thus repeated at intervals, until a complete set of digits is available within the aperture. The delays involved are generally of no consequence since readings are normally required only at extended intervals.

To coverthe second point, the dimension, i.e. area generated by the mask is smaller than the effective area of the digit under interrogation to the extent that minor errors in its overall physical placement within the aperture do not give rise to significant variations in the output maxima obtained by the valid comparison process above described. For this reason the dimensions of the "rows" and "columns", their numbers, widths and inter-conductor spacings in effective pixel sizes, is determined by consideration of the physical digits to be identified.

An 8 x 12 matrix with an aspect ratio of 3-5 suffices to give good matching with a 2-1 space-toconductor width ratio for the row and column conductors. Pixel sizes 0.2 x. 0.33mm are used in one example. These figures however, are only given as an example, and are determined by practical engineering and drive cost considerations.

The whole assembly as above described, complete with its cover and electronics is intended to be directly attached to the main viewing aperture already provided for visual interrogation by conventional adhesive or other means, in the case of the interrogation of numerical values (sequential mode), or hand held and move over printed text (parallel mode) when used as part of a reader.

The drive to the crystal display is, in the arrangement described herein, multiplexed, as is the input from the optical sensors concerned with each indi vidual digit. In another arrangement several digits are interrogated - parallel-wise.

The arrangement is intended to address a variety of uses and it is desirable that the font of the symbols to be inspected is capable of being preset to suit the particular operation required. Similarly it has to be recognised that the maximum amount of light for a perfect match for each of a variety of symbols is not the same in each instance, since the amount of light is largely a function of the actual area that constitutes the visible symbol. Hence it is preferable for the system to have a memory indicating the broad level of maxima for each of the ciphers to be interrogated, so as to better distinguish a match between the mask and the required symbol.

It is also preferable for the observed data to be available for telemetering or other treatment in compatible form. In a telemetering system such data would be made available upon a suitable indication from the telemetering system. Further, it is useful to have an internal repetitive start mechanism (clock) to institute a new cycle of interrogation at some convenient interval.

In view of the above requirements, plus the need for a low cost realisation, a microprocessor is used, see Figure 2., in which some facilities are shown come from discrete elements; the actual arrangemenu would however seek to put as many of them as possible directly under the control of the microprocessor. In view of the description already given it is felt that a detailed description of Figure 2 is not needed.

Figure 3 shows in perspective and in plan how an arrangement such as shown in Figures 1 and 2 is installed at a domestic gas or electricity meter. The arrangement is shown at 30, with the power pack at the left.

Where the information is to be telemetered, a terminal device is assumed to exist, which has its own power supplies. A possible embodiment of the above equipment assumes that a low voltage supply is drawn from such an external source to obviate the need for a separate mains driven power supply for the interrogation device. Though Figure 2 shows a mains driven power pack, we also envisage the supply of low voltage power from an external source. In either case the interrogation device is to be fixed directly to the existing glass front of the display to be telemetered or directly presented to the page or other source of text or symbols to be electrically translated, e.g. a reading device for the visually handicapped. In this last case a number of symbols are simultaneously interrogated in place of the sequential mode described above.

The microprocessor is so chosen as to have sufficient on-board memory to accommodate the programming required, and to be able to store the observed digits ready for interrogation by the telemetery or other device. Provision is made for the programme memory to be pre-selected and permanently installed; the data memory may be non voltaile, so that if power is lost, the data memory can be re-established by re-interrogation of the data source.

When the source is used as a transducer for telemetering, we envisage the use of RF transmission methods.from the transducer to a locally situated receiver, e.g. one installed in the van of an employee of the appropriate authority. In this case, the van equipment consists of a short range transmitter/receiver and code generator, so that a common RF signal containing the code of the meter to be read is transmitted with adequate signal level over say 100meter radius.

The household radio receiver/transmitter has associated with it a decoding device which can respond to a discrete address for each household.

All receivers within the area receive the signals and decode, but only the receiver addressed responds, and in so doing generates a 'Flag' signal to the circuit shown in Figure 2. This causes the stored digit trains, for the latest meter reading to be passed to the RF transmitter and thus to the receiver in the vehicle.

The household receiver transmitter equipment for this purpose is conveniently contained in a volume 1" x 1/2" x 2", and may be mains powered from a small additional power pack designed, as indicated earlier, to provide a sufficient low voltage supply for the dynamic mask matching element itself.

The information available at the dynamic mask matching equipment memory may be telemetered by other means, e.g. over the external electricity distribution wiring itself back to the local transformer station, over additional facilities attached to an existing telephone line, or other transmission methods.

Claims (10)

1. An arrangementforconverting avisually displayed character digit into an electrical representation, which includes a liquid crystal cell via which the digit to be converted is monitored, the liquid crystal material being normally maintained in its non-transmissive mode, co-ordinate control means for the liquid crystal cell to which electrical signals are applied to set the liquid crystal material successively to a number of patterns of transmissivity each corresponding to one of a number of possible digit or cypher values, a detector adapted to monitor the light transmitted via the cell, the level of output of light which reaches the detector after traversing the cell indicating whether the pattern to which the cell is set matches the digit to be converted, and output means responsive to such an indication from the detector to indicate the digit's identity electrically.

2. An arrangement as claimed in claim 1, in which the co-ordinate control means includes a set of column conductors on one side of the cell and a set of row conductors on the other side of the cell, which conductors are electrically controlled to set up said pattern.

3. An arrangement as claimed in claim 2, in which said conductors are strips of an indium-tin alloy.

4. An arrangement as claimed in claim 1,2 or 3, in which said digit to be converted is electrically illuminated by alight source such that light is reflected from the digit to be converted into the cell.

5. An arrangement as claimed in claim 1,2,3 or 4, in which the cell includes polarisation material.

6. An arrangement as claimed in claim 1,2,34 or 5, in which the cell includes, between the side sheets ofthe cell, a layer of material which is reflective but has an aperture which corresponds with the area within which a digit to be converted is to be found.

7. An arrangementforconverting a plural digit number or text into electrical form, which includes a number of arrangements each as claimed in any one of claims 1 to 6, one such arrangement being provided for each digit of the number to be converted.

8. An arrangement as claimed in claim 7, and in which when a plural-digit number or text is to be converted, the digits are dealt with successively.

9. An arrangement as claimed in any one of claims 1 to 8, in which the conversion of a single or plural digit number or text is effected automatically in response to the reception of a remote signal by radio or other means.

10. An arrangement for converting a visually displayed single or plural digit character or text into a stored electrical form for onward sequential transmission. substantially as described with reference to the accompanying drawings.