WRITING PAD FOR CHARACTER RECOGNITION APPARATUS

Abstract: Character recognition apparatus includes a writing pad having a pair of sheets (5a, 11a) normally electrically insulated from one another but are adapted to be brought into contact with one another by writing pressure by a writing implement upon the writing pad. The apparatus also has an electrical switching circuit (14) to cause current to flow through the pad along two alternate paths and has a recognition circuit (17) to recognise written characters by virtue of the changes in elec­ trical voltage in the alternate paths. One of the sheets of the writing pad has a central part of high resistivity (5a) and has a border of low resistivity material (4) and being of such a resistivity that the voltage gradient along the length of the pad is linear and the other of the sheets (11a) comprises a layer of conductive material.

Description: This invention has reference to character recognition apparatus, more particularly to such character recognition apparatus which is utilised to recognise characters as the characters are written by hand. Thus, there is described in British Patent Specification No 863431 apparatus for automatically recording hand written characters comprising a supporting surface and an electrical control circuit activated by changes in direction of a writing instrument as it moves over the supporting surface. Furthermore, in British Specifications Numbers 588043 and 130683 there is described a writing pad comprising electrical supply and sensing circuits and a pair of sheets one of which has a resistive circuit, which sheets are moved into contact with one another by a writing implement whereby electric potentials are applied to the surfaces and output potentials related to the position at which the two sheets contact one another are generated. It is an object of the present invention to provide an improved character recognition apparatus and in particular to provide an improved writing pad for use in such character recognition apparatus. According to the present invention a character recognition apparatus comprises a writing pad comprising a pair of sheets normally electrically insulated from one another but adapted to be brought into contact with one another by application of writing pressure by a writing implement upon the writing pad, an electric switching circuit to cause current to flow through the pad along two alternate paths and a recognition circuit to recognise written characters by virtue of the changes in electrical voltages in the alternate paths, characterised in that one of the sheets of the writing pad comprises a central part of high resistivity and has a border of low resistivity material of such resistivity along the length of the pad is linear and the other one of the sheets comprises a layer of conductive material. Character recognition apparatus including a writing pad, in accordance with the preent invention will now be described by way of example with reference to the following drawings wherein: Fig 1. is a diagrammatic cross-sectional view of the writing pad taken on the line 1 - 1 Fig 2; Fig 2. is a plan view of the-lower most sheet of the writing pad; Fig 3. is a view similar to Fig 2 not including a resistive coating; Fig 4. is a circuit diagram; Fig 5. shows a modification of Fig 4; Fig 6. is a further circuit diagram for the writing pad and Fig 7. is a diagrammatic view of part of an alternative arrangement of the writing pad. Referring to the drawings, there is shown in Figs 1 and 2 a writing pad including a rigid base board 1 of non-conducting material (such as that which is usually used as a base for printed circuit boards, for example the material known as fibreglass) which serves as the mechanical supporting part of the writing pad. A layer of conductive connector material 3 is printed on the base board 1. The material of the conductive connector layer is applied in the form of an etched copper layer similar to that used on printed circuit boards or may be applied as an ink which embodies silver particles to provide a coating having a resistance of about 0.01 ohms per square area (and hereinafter referred to as a low resistance). The conductive connector coatings serve to connect the medium resistance borders (to be hereinafter described) to printed connectors 6 at the top edge as shown in Figs 2 and 3) of the sheet 2 to an external electric circuit. Printed on top of the low resistance conductive connector layer 3 is a medium resistance border layer 4 which extends completely around the periphery of the higher resistance coating layer 5 (to be hereinafter described) and overlies the innerpart of the conductive layer 3. The medium resistance border layer 4 overlaps the low resitance layer 3 at the corners thereof. The medium resistance border layer has a width of between three hundreths and five hundreths of an inch (0.03" to 0.05") and maybe printed from ink embodying a mixture of silver and carbon particles to give a resistance of the order of one ohm per square area. This medium resistance area is shown in black on Figs 2 and 3 of the drawings. A higher resistance coating 5 is printed on the board shown in Fig 2 but not in Fig 3 mainly within the border formed by the border layer 4 but partially overlies the border layer 4 as best shown in Fig 2 where it is shown in cross hatching. The main part of the high resistance coating layer 5 comprises a main rectangular part 5a lying within the border layer 4. The other part of the high resistance coating layer also shown in Fig 7 of the drawings in cross hatching comprises a plurality of tuning rectangular parts 5b arranged outside the border layer 4 and which serve to give to the coating layer 5 the property of linear drop in voltage when appropriately driven. The coating which has a resistance of about 1000 ohms per square area may be printed from an in'x having conducting materiai(preferabiy carbon /or silver) and a resin, a plasticizer and a solvent. This coating has a thickness of about 0.001." In order to protect the coating 5 from being scratched during transit or when being loaded into or unloaded from the character recognition apparatus the layer 5 may comprise a first layer having a resistance of about 100,000 ohms per square. The coating is similar in constitution to the main coating 5 but contains a smaller proportion of conducting material and has a thickness of less than 0.001". The low resitance conductive connector layer 3 comprises a series of connector areas arranged at the top edge of the writing pad as shown in Fig 2, to an external circuit. As shown the connector areas 6 are positioned at the end of a conductive lead to a rectangular area 3c adjacent each corner of the high resistance area. These connections are made from the connectors, 6a, 6b, 6c, 6d to the rectangular corner areas 3c adjacent the respective four corners of the rectangular resistive part 5a as best shown in Fig 2. Additional areas of the conductive connector coating 3 are provided in the form of a series of extension lines 10 (Fig 3) extending outwardly from one of the respective corner areas 3c and from beneath the medium resistance border layer 4. These extension lines 10 like the remainder of the low resistance layer 3 are printed to form part of the low resistance layer 3 and are of the same material as that layer. The extension lines 10 each consist of a rectangular part 10a with one line 10b extending from the rectangular part 10a in one direction and a second line 10c extending from the rectangular part 10c but in the opposite direction. Each outer line 10c extends parallel to and outwardly of the line 10b extending from the next adjacent part 10a. The extension lines iota, 10c are overlaid by the tuning rectangular parts 5b which form part of the high resistance coating 5. Extension lines 10b, 10c are intended to be used to ensure that with the selective adjustments to the resistivity of the border areas the effective resistance of the border areas 4 and the extension lines 10b, 10c serves to modify the resistivity of the resistive part 5a and thus the voltage gradient along the length of the pad is linear when connected in an electrical circuit. This is achieved by removing material from the parts 5b which falls between the adjacent lines 10b, 10c parallel with one another or if appropriate removing part of a line 10b or 10c to place the tuning rectangular part 5b on open circuit. The material of the layer 5 of which the rectangular parts 5b are formed is of a high resistance so the removal of a part of the material layer varies the resistance between the adjacent lines 10b, 10c and hence adjusts the effective resistance of the border line 4 to render the voltage gradient across the pad to be linear. Each one of the resistive layer 5 and the border layer 4 are formed by screen printing on the sheet 2. The layers have a thickness of about 0.001 " or even less. Positioned above the resistive layer 5 and separate therefrom is an easily replaceable cushion layer positioned in the area where writing pressure is applied. This cushion layer 11 is intended to prevent an electrical circuit being made when the operators hand rests on the writing surface and consists of two leaf layers 11a, 11b of normally electrically insulating material and between which leaf layers there is provided a layer 12 of a dielectric non-conductive elastomeric gel material. A suitable gel material is that supplied by Dow Corning under the Trade Mark SYLGARD Q3-6527. This material is supplied in two parts and we find that a mixture of the two parts of Part A/Part B in the range of 1.65/1 to 1/1 is suitable. The gel layer has a thickness of between 0.005" to 0.0050" and preferably between 0.010" and 0.012". gel Alternatively the ######## ######### ########## elastomer # material layer may be the material sold under the Trade Mark DYNACON and is described in the specification of U.S. Patent Nos 4054540 and 4120828. The DYNACON layer is of normally high resistance material between the outer surface of the coating and the backing sheet but when pressure is applied at a point on the surface of the pressure sensitive layer by a writing implement such as a pencil or a ball-point pen, then a resistance of that layer in the area which pressure is applied is substancially reduced. The upper leaf layer 11a is a polymeric film coated or treated on the side adjacent the elastomeric gel material with conductive material of low electrical resistance typically less thon 1,000 ohms per square. This may comprise metallised film or a conductive ink coating on the film and preferably comprises a conductive ink which is screen printed onto the lower face of the flexible sheet 12. The conductive material in this layer is connected by an integral electric connection through the conductive material in on electrical circuit to be hereinafter described, to voltage measuring means. The said lower leaf 11b is a volume conductive film having a low electrical resistance through the film and a high electrical resistance across the film. T.1e ratio of through resistance to across resistance should be less than 1/100. It is also required that the resistance through the film should be less than 10,000 ohms per square and the resistance across the film should be greater than 50,000 ohms per square. Such a material may be a filled polymeric film containing conductive material such as carbon such as sold under the Trade Mark Velostat by the 3M's Company or moy consist of an elastomeric film such as silicone film containing finely dispersed nickel particles. The upper leaf layer lia and the lower leaf layer 11b are welded together as by heat sealing 12c at their margins to contain the elastomeric material between the sheets. As shown a further protective flexible sheet 13 is provided over the conductive leaf layer 11 and may comprise a polyester film sheet having a thickness of the order one thousandth of an inch (0.001"). An electrical connection is made between the layer 1 lea and an electrical circuit external of the writing pad. This connection is made through the area 6e adjacent the areas 6b, 6c, an electrical connection is made from these respective areas 6a, 6b, 6c, 6d, 6e to electric circuits to be hereinafter described. An electrical switching circuit as shown in Fig 4 is provided whereby an electrical voltage is connected alternately between the connectors 6a, 6b and the voltage measuring means on the one hand and the connectors 6c, 6d and the voltage measuring means on the other hand. By means of these connections current is caused to flow through the resistive layer first one direction and then through the resistive layer in another direction, preferably at right angles to the first direction. The writing pad so described constitutes a two-dimensional potentiometer by virtue of the alternate electrical connections between the connectors 6b, 6c on the one hand and 6a, 6d on the other hand to make one electrical connection and also between the connectors 6b, 6a on the one hand and 6c, 6d on the other hand to provide alternately generated voltages across the writing pad so that when an electric connection is made between the layer ila and the main coating layer 5a, a voltage is applied to the layer 5a representative of the co-ordinates of the point of contact between the sheets 3 and 11. Referring to the circuit diagrams shown on Fig 4 there is shown a writing pad similar to the pad shown in Fig 1. An electric DC voltage supply is connected to positive and negative terminals 13a and the terminals are connected to switches 14 and thence to the connector leads to 6b, 6c, 6a and 6d. The switches are ganged together so the switches operate together and the switches are arranged so that the connector 6c is always connected to the positive supply and the connector 6a is always connected to the negative supply. Connectors 6b, 6d are alternately connected to the minus and the plus supply respectively in the switch down condition where in the x co-ordinate is measured thence to the plus and minus respectively in the switch down condition wherein the x co-ordinate is measured thence to the plus and minus respectively in the switch up condition, wherein the y co-ordinate is measured. A connection is made from the conducting layer on the leaf layer 1 lea to a voltage measuring means 15. This voltage measuring means is adapted to read the voltage applied across the resistive layer of the writing pad which is representative of a co-ordinate of a point x, y which is representative of the writing position. Thus, if a writing implement is recording a message on the writing pad at any time the position of the writing implement maybe represented by two voltages. The voltage measuring means 15 is connected to an A/D converter 16 and character processor 17 arranged to recognise characters in accordance with the changes in the voltages corresponding to the distances x and y hereinbefore referred to. Fig 5 shows a view similar to Fig 4 except that there is additionally included the connecting leads 8a, 8b positioned between the connector leads 6b, 6c and 6a, 6d respectively. The purpose of the electrodes 8a is to reduce the error caused by the small current shown in the leg of the border between the corners 3c of the connectors 6b and 6c during the condition wherein 6b and 6c are connected to the same supply polarity. (Likewise for electrode 8b with respect to the border leg between corners 6a and 6d). These electrodes 8a, 8b are in the up/down condition (y co-ordinate measured) connected to the positive and negative terminals respectively and in the left/right (x co-ordinate measured) condition are connected to open circuit. Like the circuit shown in Fig 4 the positive and negative terminals are connected through ganged switches to the connectors 6a, 6b, 6c, 6d, 8a, 8b to the writing pad and through the writing pad to the voltage measuring means 15 and to the converter 16, and character processor 17. If desired intermediate electrodes are provided on the shorter side edges of the pad and more than one intermediate electrode may be provided along the longer side edges of the pad. Although we have shown the ganged switches as being of the mechanical kind these switches can be electronic switches. Referring to the circuit shown in Fig 6 there is shown a writing pad having the layer of resistive material 3 and the border layer 4 with connectors 6. An electric supply 18 comprises two leads, one a positive supply lead at a potential of 5 volts and the other lead at apotential of 2 volts are connected through a switch 19 to an operational amplifier 20. The switch 19 is arranged to supply a selected one of the voltages from the supply 18 to one of the inputs 20a to the operational amplifier 20. The output 20c of the operational amplifier 20 is connected to a transistor amplifier circuit 21 comprising a pair of alternately operating transistors 22a, 22b. One of the transistors 22a is a pnp transistor and the transistor 22b is a npn transistor. The output from the operational amplifier 20 is connected to the respective base connections of each one of the transistors 22a, 22b. The collector of the transistor 22a is connected to a zero volts supply and the collector of transistor 22b is connected to a positive 10 volts supply. The emitters of the transistors 22c, 22b are connected together and are connected to one of the corners of one of the connecting electrodes (as shown as area 3c on Figs 2 and 3). A connection is made from a point contact on the inner side of the electrode 3c to the alternative imput 20b connection to the operational amplifier 20. A smoothing circuit 23 is included in the connection between the electrodes 5b and the operational amplifier 20, and this connection serves as a feed back circuit to ensure that the voltage supplied to the electrode 3c remains constant and exactly equal to the reference voltage (shown as +5 or +2 volts). Referring to Fig 7 there is shown a modified form of pad. This pad has a base sheet, with a layer of resistive material 3 printed on it and having a border layer 4 and connecting electrodes 6. In this pad the lines constituting the border layer 4 between adjacent connecting electrodes 5 are each of parabolic shape. In the case of electrodes spaced eight inches apart the depth of the parabola is 0.2 of an inch. By the provision of this low resistance border layer around the resistive layer we are able to reduce or eliminate position measurement errors in the vicinity of the borders due to small current flow in the border layer 4 whose ends are connected to the same power supply. Also a border configuration can be implemented by having the features and advantages of those shown separately in Fig 2. It is desirable that the thickness of the resistive layer 5 shall be constant throughout the length and width of the layer, in order that the resistance of the layer per unit shall remain constant.

Claims: CLAIMS 1. Character recognition apparatus comprising a writing pad comprising a pair of sheets normally electrically insulated from one another but adapted to be brought into contact with one another by writing pressure by a writing implement upon the writing pad, an electrical switching circuit to cause current to flow through the pad along two alternate paths and a recognition circuit to recoganise written characters by virtue of the changes in electrical voltage in the alternate paths characterised in that one of the sheets of the writing pad comprises a central part of high resistivity and has a border of low resistivity -' material of such resistivity that the voltage gradient along the length of the pad is linear and the other one of the sheets comprises a layer of conductive material. 2. A character recognition apparatus according to claim 1 characterised in that the one sheet of the writing pad has a plurality of connector areas in conducting relationship with the part of high resistivity which connector areas are connected through conductive connector layer to a connector at one edge of the writing pad for connecting to the electrical switching circuit. 3. A character recognition apparatus according to claim 1 or 2 choracterised in that a plurality of areas of high resistivity material are produced around the central part of high resistivity selectively connected to the central part of high resistivity by connectors. 4. A character recognition apparatus according to claiml,2 or 3 wherein the sheets of the writing pad are retained apart by an intermediate spacing layer. 5. A character recognition apparatus according to claim 4 wherein the intermediate spacing layer comprises two leaf layers between which is provided a layer of dielectric non-conductive elastomeric gel material. 6. A character recognition apparatus according to claim 2 wherein the connector area comprises rectangular connector areas arranged adjacent each respective corner of the central high resistivity part. 7. A character recognition apparatus according to claim 6 wherein additional connector areas are provided at opposite of the central high resistivity part midway between the corner connector areas. 8. A writing pad comprising a pair of sheets normally electrically insulated from one another but adapted to be brought into contact with one another by writing pressure by a writing implement upon the writing pad, wherein one of the sheets comprises a central part of high resistivity, a plurality of connector areas of low electrical conductivity to connect its part of high resistivity to an external switching circuit characterised in that the part o' high resistivity has a border of low resistivity with a plurality of areas of high resistivity around the border, the plurality of areas of high resistivity are selectively connected to the central part and being of such resistivity that the voltage gradients along the length of the pad is linear. 9. A character recognition apparatus constructed arranged and adapted to operate substancially as herein described with reference to the accompanying drawings.

Inventors: EZEKIEL, MAURICE, LUKIS, LAWRENCE JOSEPH
Applicants: MOORE BUSINESS FORMS, INC.
Application year: 1980
Main classification: G06F3
IPC codes: G06F3, G06K9