WO1993012508A1

WO1993012508A1 - Optical digitising apparatus

Info

Publication number: WO1993012508A1
Application number: PCT/GB1992/002284
Authority: WO
Inventors: Stephen Randall
Original assignee: Eden Group Limited
Priority date: 1991-12-09
Filing date: 1992-12-09
Publication date: 1993-06-24
Also published as: GB9126128D0

Abstract

Optical digitisers comprise a light emitting stylus reacting with a tablet with some means for determining the position of the stylus by locating the spot at which light from the stylus strikes the tablet. In this invention, the digitising tablet comprises a light sensitive region continuously extending over an area with respect to which the position of the stylus is determinable, the region being made of material having an electrical conductivity that varies in dependence on light incident upon it. Electrical measurement means are arranged to provide a signal indicative of the location of the spot of incident light in dependence on this variation.

Description

Optical Digitising Apparatus

Field of The Invention

This invention relates to optical digitising apparatus for determining the co-ordinate position of a stylus with respect to a digitising pad, to allow the user to input data or control signals using the stylus. In. particular it relates to such an apparatus utilising an optical detection approach.

Description of the Prior Art

Computers that solely have keyboards for data entry are insufficient for software applications that use a graphical user interface (GUI) or allow the user to draw or paint. It is necessary to provide some sort of pointing device or cursor control device to take advantage of such applications and a variety of such pointing devices have been developed. For instance, one can commonly use a mouse or trackball. Further, a stylus co-operating with a co-ordinate position detecting device such as a digitising tablet may also be used. This is a particularly convenient arrangement since the stylus can be used not only as a pointing device but also as a writing device too.

It is also well known to provide computers with no keyboard at all. relying instead on all control inputs being made by touching such a stylus to a digitising tablet, commonly overlying the display of the computer. Such computers, known as keyboardless or pen computers, allow the input of graphical data, such as handwriting; such handwriting can be stored as, for example, a bit mapped image or processed by handwriting recognition software and displayed as text.

Digitisers in current commercial production rely generally on electromagnetic inductive coupling between the stylus and the digitising pad to allow the measurement of the position of the stylus with respect to the pad. These devices are somewhat costly and intricate. Major manufacturers include Wacom Inc. and Numonics Inc.

An alternative design uses a pressure sensitive arrangement. For example, in GB 2088063 (Branton), there is shown a rectangular conductive layer overlying a similarly shaped resistive layer, there being an insulating layer between the two. The resistive layer has electrical contacts at each corner such that when one opposed pair of contacts is energised a predetermined electric field is set up; when pressure is applied at any given point by a stylus so that the conductive layer touches the resistive layer under that point, then the potential of the conductive layer rises to that of the resistive layer under that point, giving an indication of the equipotential line above which the point lies. When the alternate contacts are energised, a different field is set up and the corresponding equipotential line can be determined; the intersection of the two lines thus gives the position of the stylus. One of the disadvantages of the pressure sensitive approach is that if the digitising tablet overlies a display, as it does in a pen computer, then an imperfect compromise generally has to be made between the need for structural rigidity, to prevent applied pressure distorting the underlying display, and minimal tablet thinness, to minimise the distance between the stylus contact point and the associated dark pixel.

Optical digitisers are also known, however, but are not in widespread use. For example, EP-407743-A (IBM Corp) discloses a digitising pad which has an upper surface marked with a binary coded pattern readable by a light sensor in the stylus wherein the pattern at any given location uniquely identifies that location. This approach requires a sophisticated stylus which can both illuminate the patterns and focus and detect the reflected light from the patterns and supply data to the computer. One disadvantage with this approach is that the stylus is therefore complex and somewhat costly and has to be tethered to the digitising pad. It would be possible for the stylus to transmit data using a wireless approach such as infra-red band transmissions but this further adds to the cost of the stylus.

Another approach is exemplified by US 3539995 (Brandt) which discloses a digitiser pad comprising a matrix of discrete light activated silicon rectifiers co-operating with a light source in a stylus. Whilst this approach has the advantage over the IBM approach described above in that the stylus itself need only comprise a suitably collimated light source and can therefore be relatively inexpensive and untethered, it suffers from the disadvantages that the resolution is limited to the size of the discrete silicon rectifiers, and that the driving and detection circuitry is complex. Summary of the Invention

In accordance with the present invention, optical digitising apparatus for determining the position of a stylus comprises: a digitising tablet comprising a light sensitive region continuously extending over an area with respect to which the position of the stylus is determinable, the region being made of material having an electrical conductivity that varies in dependence on light incident upon it; a stylus incorporating a light source operable to illuminate a portion of the said region to vary the electrical conductivity of the portion; and electrical measurement means arranged to provide a signal indicative of the location of the portion in dependence on the variation of the electrical conductivity of the said region.

By turning away from the prior art approach of a matrix of discrete light sensitive units, and instead using a continuous region of light sensitive material that extends over the entire area with respect to which the position of the stylus is determinable, the circuitry of the present invention is made considerably simpler. Further, because the stylus incorporates only a simple light source it is both cheap and need not be tethered to the apparatus.

Preferably, the material of the light sensitive region is photo-conductive, i.e becomes substantially electrically conductive when illuminated by the light source in the stylus.

The variation in electrical conductivity may arise with variations in either the intensity or frequency of the incident light, for example.

Conveniently, an increase in the electrical conductivity of the photoconductive region is operable to provide an electrical conduction path for an electrical circuit such that a parameter of the electrical circuit is varied in a manner that allows the position of the stylus with respect to the tablet to be determined.

Preferably, the parameter is one of potential, resistance, impedance or current flow.

In a preferred embodiment, the light sensitive region is a planar sheet and the electrical measurement means comprises: a planar electrically resistive sheet underlying the light sensitive region; a planar electrically conductive sheet translucent to light from the stylus and overlying the light sensitive region; a source of electric potential for connection to the resistive sheet in a first and a second mode whereby, in the first mode, when light from the stylus illuminates the said portion the light sensitive region at that portion becomes electrically conductive and takes the potential of the conducting sheet to a first potential, determined by the potential of the resistive sheet at that portion, and in the second mode whereby, when light from the stylus illuminates the said portion the light sensitive region at that portion becomes electrically conductive and takes the potential of the conducting sheet to a second potential, determined by the potential of the resistive sheet at that portion; measuring means to measure either [1] the value of the potential of the conductive sheet in the first and second modes and thereby provide measured potential signals or [2] the currents flowing between the resistive and conductive sheets and thereby provide measured current signals; and signal means to provide a signal indicative of the position of the stylus from the measured potential signals or the measured current signals.

Additionally, the planar light sensitive region may be rectangular and define the entire area that is useable as a digitising surface. The planar electrically resistive sheet may also be translucent, as will be necessary when the entire digitiser fits over a display.

In another preferred embodiment, the electrical measurement means comprises; a first light sensitive region formed as a network of translucent material of known resistivity laid in a parallel and closely spaced arrangement defining a first coordinatδ direction, the region being formed in a conductive plane at a predetermined potential whereby when light illuminates the light sensitive region at a portion then the first network is varied at that portion to a given potential via the light sensitive region; a second light sensitive region formed as a second network of material of known resistivity laid in a parallel and closely spaced arrangement defining the orthogonal coordinate direction, the region being formed on a second conductive plane at a second predetermined potential, whereby when light illuminates the second light sensitive region at a portion then the second network is varied at a portion to a further potential via the light sensitive region; and meter means for determining the electrical resistance of each network, or an associated parameter, to determine thereby the position of the stylus. Preferably, each network is formed as a single sinuous line and the meter means determines the electrical resistance of the length of each sinuous line from a given position on that line to the position on the line at which the potential is varied, or an associated parameter, to determine thereby the position of the stylus.

In a still further preferred embodiment, the electrical measurement means comprises; a first sinuous line of translucent electrically resistive material laid in a parallel and closely spaced arrangement defining a first coordinate direction, with the said light sensitive region being adjacent and parallel to the first sinuous line; a conductive region at a predetermined potential adjacent the light sensitive region-^hereby when light illuminates the light sensitive region at a portion then the first sinuous line is varied at that portion to the predetermined potential via the light sensitive region; a second sinuous line of translucent electrically resistive material laid in a parallel and closely spaced arrangement defining the orthogonal coordinate direction, with a further light sensitive region being adjacent and parallel to the second sinuous line; the said or a further electrically conductive region at the or a further predetermined potential adjacent the further light sensitive region whereby when light illuminates the light sensitive region at a portion the first sinuous line is varied at that portion to a given potential via the light sensitive region; and meter means for determining the electrical resistance or impedance of the length of each sinuous line from a given position on that line to the position on the line at which the potential is varied, or an associated parameter, to determine thereby the position of the stylus.

In a final preferred embodiment, the electrical measurement means comprises; a first array of parallel and closely spaced narrow linear electrically conductive tracks defining a first axis, the tracks each terminating at a one end in a high impedance strip and being formed on an electrically insulating substrate; a first photoconductive region underlying the first array; a first electrically conductive region underlying the first photoconductive region and being held at a reference potential, whereby when light from the stylus is incident on a given track in the first array then that track is brought into electrical contact with the first conductive region via the first photoconductive region; a second photoconductive region; a second array of parallel and closely spaced narrow linear electrically conductive tracks defining the orthogonal axis, the tracks each terminating at a one end in a high impedance strip and being formed on an electrically insulating substrate; the said first or a second electrically conductive region adjacent to the second photoconductive region and being held at a reference potential, whereby when light from the stylus is incident on a given track in the second array then that track is brought into electrical contact with the said first or the second conductive region via the second photoconductive region; and means for determining the electrical resistance or impedance of each array and associated strip, or an associated parameter, to determine thereby the position of the stylus. \

Preferably, the first sinuous line is formed on the underside of a plastics film having an upper surface against which the stylus contacts in use. Typically, a polycarbonate film such as Lexan Martec is used.

In a convenient embodiment, the digitising apparatus in accordance with the present invention is configured to overlie or underlie or be integral with a display screen, typically an LCD display. This embodiment is appropriate for a keyboardless computer such as is described in PCT/GB88/00666 (Randall). In such a case, the components such as the conductive and resistive sheets and the photo-conductive material are translucent. This embodiment is also appropriate for other display devices that should preferably have user friendly screen interfaces. Using the present invention, for example, a video recorder could have a direct manipulation control panel displayed on the screen of the television set to which it was connected-e.g. an image of a set of buttons corresponding to those found on the recorder itself. The user would direct the light source onto those buttons to emulate pressing them. In this embodiment, as well as the others described herein, it may be desirable to provide the light source with a switch that operates to send a particular signal that the measurement means can detect and respond to appropriately, for instance by issuing a control signal. This embodiment is also useful for display devices that display electronic games. It will be appreciated that the present invention will allow the user to fire a light spot from the light source at a region of the screen to effect target destruction, for example, or control the position of a screen icori such as a spacecraft. The light sensitive material may be a photoconductive material such as zinc oxide or cadmium sulphide. Desirable properties of appropriate materials include low inertia, no dark current and no hysteresis. They may be deposited by a variety of known techniques, such as evaporation or powder sintering.

The emission spectrum of the light source in the stylus will be appropriate to induce photoconductivity. Conveniently, this may be in the infra-red wavelength region. Generally, the light source will emit pulses of light so that the resultant signal is readily differentiable from the relatively constant ambient illumination. Additionally, the light source may emit pulses of light at a variable pulse rate, with the electrical measurement means arranged to determine the value of a user-selectable variable in dependence on the pulse rate. For example, the stylus may have a pressure sensitive tip that can control the pulse rate in dependence on the pressure of the stylus on the tablet. This is particularly convenient for painting or drawing images where the pressure of the stylus can control the density of the displayed colour.

Brief Description of the Drawings

An optical digitising apparatus in accordance with the present invention will now be described with reference to the accompanying drawings in which:

Figure 1 is a schematic perspective view of an optical digitising apparatus in accordance with the present invention in which a planar photoconductive sheet is positioned below an electrically conductive sheet and above an electrically resistive sheet;

Figure 2 is a schematic perspective view of an optical digitising apparatus in accordance with the present invention in which there are a pair of orthogonal and spaced apart sinuous photoconductive tracks, each track being formed in a conductive plane.

Figure 3 is a schematic perspective view of an optical digitising apparatus in accordance with the present invention in which there are a pair of orthogonal and spaced apart sinuous electrically resistive tracks, each track overlying a photoconductive plane, with the photoconductive plane overlying a ground plane; Figure 4 is a schematic perspective view of an optical digitising apparatus in accordance with the present invention in which an array of parallel electrically conductive tracks, with each track terminating in a high impedance strip, overlies a photoconductive plane which itself overlies a plane at a reference potential; there being a further conductive plane below the reference plane and an orthogonal array of parallel electrically conductive tracks below the further conductive plane.

Detailed Description

Referring now to Figure 1, there is shown a stylus 1 operable to shine a narrow collimated beam of light onto a a planar electrically conductive sheet 2 translucent to light from the stylus 1. The conductive sheet 2 is formed of a thin layer of electrically conductive ink or other material in a manner familiar to to those skilled in the relevant arts. The sheet 2 overlies a planar photoconductive sheet 3. The photoconductive sheet 3 itself is formed of a photoconductive material such as cadmium sulphide and overlies a planar electrically resistive sheet 4, formed of resistive ink. In the illustrations of Figures 1 to 4, the various sheets are shown separated for clarity. In practice, the layers may be formed physically abutting one another by, for example, sequential deposition of the material of the respective materials on top of one another. It will be appreciated that the thinness of the present embodiment when such an approach is used may be particularly advantageous.

There is provided a source 5 of electric potential connected to the resistive sheet 4. The source 5 is shown connected in the first of a first and a second mode such that an electric potential is applied to opposite corners of the resistive sheet 4. In the second mode, the electric potential would be applied across the other two corners. There is also provided ^*a measuring means 6 to measure either the value of the potential of the conductive sheet 2 in both the first and second modes and provide measured potential signals, or to measure the currents flowing between the resistive sheet 4 and conductive sheet 2 and provide measured current signals. An A/D converter 7 provides a signal indicative of the position of the stylus 1 from the measured potential signals or the measured current signals. In use, when the resistive sheet 4 is driven in the first mode, then when light from the stylus 1 illuminates a particular portion of the conductive sheet 2, the part of the photoconductive sheet 3 at, i.e immediately underlying that portion becomes electrically conductive and takes the potential of the entire conducting sheet 2 to a first potential, determined by the potential of the resistive sheet at that conduction point. The potential, which is measured by the measuring means 6, determines which equipotential line of the resistive sheet 4 the stylus 1 overlies. When the resistive sheet is driven in the second mode, a further equipotential line is determined in the same manner, the intersection of the two lines being the stylus position. The potential source 5 is capable of driving the resistive sheet 4 between the two modes in rapid sequence so that a moving stylus can be accurately mapped. The detailed implementation of this embodiment will be apparent to to those skilled in the art. The approach bears many similarities to the pressure sensitive approach advocated in GB 2088063, to which further reference may be made.

Referring now to Figure 2, a sinuous photoconductive track 21 is shown having long sides defining the X-axis. The track 21 is formed in an electrically conductive substrate 22 which is held at a reference potential by a potential source (not shown). The substrate 22 is separated from a similar second conductive substrate 23 by an insulating layer 24, typically of Mylar. On the second conductive substrate 23 there is formed a second sinuous photoconductive track 25, here shown having long sides defining the Y-axis. The substrate 23 is also held at a reference potential. In use, when the stylus 1 illuminates a portion of the track 21 or 25 then the track at that portion becomes electrically conductive and takes each track at that portion to a potential related to the reference potential. The position along the length of each track 21 25 at which this occurs can be determined in a number of ways. For example, the very high resistance of track each side of the illuminated portion could be measured using a potentiometer, this resistance being proportional to the length of track. Because of the difficulties in accurately measuring such a high resistance, the alternative approach of Figure 3 may be adopted in some cases.

Referring_^ now to Figure 3, there is shown a sinuous resistive track 31, having long sides defining an X-axis and formed on an insulating Mylar substrate 32. A photoconductive layer 33 underlies the substrate 32 and a ground plane 34 underlies the photoconductive layer 33. A further sinuous resistive track 35, having long sides defining a Y-axis and formed on an insulating Mylar substrate 36, overlies a further photoconductive sheet 37, which itself overlies a ground plane 38. In use, when light from the stylus 1 illuminates a particular portion of the track 31 then the track is grounded at that portion through the photoconductive layer 33. Likewise, track 35 is grounded through layer 37. The resistance or impedance of each limb of each track on either side of the grounded portion can be measured by conventional approaches to yield the position of the stylus 1.

Figure 4 illustrates a refinement of the Figure 3 approach in that instead of a single sinuous resistive track, there is provided a high impedance strip 41, from which an array of conductive tracks 42 emanate, defining an X-axis. The strip 41 and tracks 42 are formed on an insulating Mylar substrate 43, below which is a photoconductive sheet 44 overlying a reference potential plane 45 (which may be a ground plane). Below the reference potential plane 45 there is a further photoconductive sheet 46. Below the photoconductive sheet 46 there is provided a second high impedance strip 47, from which an array of conductive tracks 48 emanate, defining a Y-axis. The strip 47 and tracks 48 are formed on an insulating Mylar substrate 49. In use, when the stylus 1 illuminates a given portion of the conductive tracks 42 48, then the impedance of each strip and array combination varies, by virtue of the electrical contact made between each track at the illumination point and the reference plane 45, in a manner that identifies the position of the illumination. The approach has much in common with the pressure sensitive approach described in GB 2136966, to which further reference may be made.

Claims

1. Optical digitising apparatus for determining the position of a stylus comprising: a digitising tablet comprising a light sensitive region continuously extending over an area with respect to which the position of the stylus is determinable, the region being made of material having an electrical conductivity that varies in dependence on light incident upon it; a stylus incorporating a light source operable to illuminate a portion of the said region to vary the electrical conductivity of the portion; and electrical measurement means arranged to provide a signal indicative of the location of the portion in dependence on the variation of the electrical conductivity of the said region.

2. Optical digitising apparatus as claimed in Claim 1 wherein the material of the light sensitive region becomes substantially electrically conductive when illuminated by the light source in the stylus.

3 Optical digitising apparatus as claimed in Claim 2 wherein an increase in the electrical conductivity of the photoconductive region is operable to provide an electrical conduction path for an electrical circuit such that a parameter of the electrical circuit is varied in a manner that allows the position of the stylus with respect to the tablet to be determined.

4. Optical digitising apparatus as claimed in Claim 3 wherein the parameter is one of potential, resistance, impedance or current flow.

5. Optical digitising apparatus as claimed in Claim 3 wherein the light sensitive region is a planar sheet and the electrical measurement means comprises: a planar electrically resistive sheet underlying the light sensitive region; a planar electrically conductive sheet translucent to light from the stylus and overlying the light sensitive region; a source of electric potential for connection to the resistive sheet in a first and a second mode whereby, in the first mode, when light from the stylus illuminates the said portion the light sensitive region at that portion becomes electrically conductive and takes the-potential of the conducting sheet to a first potential, determined by the potential of the resistive sheet at that portion, and in the second mode whereby, when light from the stylus illuminates the said portion the light sensitive region at that portion becomes electrically conductive and takes the potential of the conducting sheet to a second potential, determined by the potential of the resistive sheet at that portion; measuring means to measure either [1] the value of the potential of the conductive sheet in the first and second modes and provide thereby measured potential signals or [2] the currents flowing between the resistive and conductive sheets and provide thereby measured current signals; and signal means to provide a signal indicative of the position of the stylus from the measured potential signals or the measured current signals.

6. Optical digitising apparatus as claimed in Claim 5 wherein, the planar light sensitive region is rectangular and defines the entire area that is useable as a digitising surface.

7. Optical digitising apparatus as claimed in Claim 3 wherein the electrical measurement means comprises; a first light sensitive region formed as a first network of translucent material of known resistivity laid in a parallel and closely spaced arrangement defining a first coordinate direction, the region being formed in a conductive plane at a predetermined potential whereby when light illuminates the light sensitive region at a portion then the first network is varied at that portion to a given potential via the light sensitive region; a second light sensitive region formed as a second network of material of known resistivity laid in a parallel and closely spaced arrangement defining an orthogonal coordinate direction, the region being formed on a second conductive plane at a second predetermined potential, whereby when light illuminates the second light sensitive region at a portion then the second network is varied at a portion to a further potential via the light sensitive region; and meter means for determining the electrical resistance of the length of each sinuous line from a given position on that line to the position on the line at which the potential is varied, or an associated parameter, to determine thereby the position of the stylus.

8. Optical digitising apparatus as claimed in Claim 7 wherein each network is formed as a single sinuous line and the meter means determines the electrical resistance of the length of each sinuous line from a given position on that line to the position on the line at which the potential is varied, or an associated parameter, to determine thereby the position of the stylus.

9. Optical digitising apparatus as claimed in Claim 3 wherein the electrical measurement means comprises; a first sinuous line of translucent electrically resistive material laid in a parallel and closely spaced arrangement defining a first coordinate direction, with the light sensitive region being adjacent and parallel to the first sinuous line; a conductive region at a predetermined potential adjacent the light sensitive region whereby when light illuminates the light sensitive region at a portion then the first sinuous line is varied at that portion to the predetermined potential via the light sensitive region; a second sinuous line of translucent electrically resistive material laid in a parallel and closely spaced arrangement defining the orthogonal coordinate direction, with a further light sensitive region being adjacent and parallel to the second sinuous line; the said or a further electrically conductive region at the or a further predetermined potential adjacent the further light sensitive region whereby when light illuminates the light sensitive region at a portion the first sinuous line is varied at that portion to a given potential via the light sensitive region; and meter means for determining the electrical resistance of the length of each sinuous line from a given position on that line to the position on the line at which the potential is varied, or an associated parameter, to determine thereby the position of the stylus.

10. Optical digitising apparatus as claimed in Claim 3 wherein the electrical measurement means comprises; a first array of parallel and closely spaced narrow linear electrically conductive tracks defining a first axis, the tracks each terminating at a one end in a high impedance strip and being formed on an electrically insulating substrate; a first photoconductive region underlying the first array; a first electrically conductive region underlying the first photoconductive region and being held at a reference potential, whereby when light from the stylus is incident on a given track in the first array then that track is brought into electrical contact with the first conductive region via the first photoconductive region; a second photoconductive region; a second array of parallel and closely spaced narrow linear electrically conductive tracks defining the orthogonal axis, the tracks each terminating at a one end in a high impedance strip and being formed on an electrically insulating substrate; the said first or a second electrically conductive region adjacent to the second photoconductive region and being held at a reference potential, whereby when light from the stylus is incident on a given track in the second array then that track is brought into electrical contact with the said first or the second conductive region via the second photoconductive region and means for determining the electrical impedance of each array and associated strip, or an associated parameter, to determine thereby the position of the stylus

11. Optical digitising apparatus as claimed in any preceding claim wherein the upper most element is formed on the underside of a plastics film having an upper surface against which the stylus contacts in use.

12. Optical digitising apparatus as claimed in any preceding claim wherein the light sensitive material is zinc oxide or cadmium sulphide.

13. Optical digitising apparatus as claimed in any preceding Claim wherein the light source emits pulses of light whereby the electrical measurement means can differentiate between ambient light and light from the light source.

14. Optical digitising apparatus as claimed in any preceding Claim wherein the light source emits pulses of light at a variable pulse rate, with the electrical measurement means being arranged to determine the value of a user-selectable variable in dependence on the pulse rate .

15. Optical digitising apparatus as claimed in Claim 13 wherein the stylus has a pressure sensitive tip that can control the pulse rate in dependence on the pressure of the stylus on the tablet.

16. Optical digitising apparatus. as claimed in any preceding Claim further adapted to overlie, underlie or be integral with a display device.