GB2088063A - Pad giving electrical positional signals of a contact point - Google Patents

Abstract

An electrically sensitive pad provides at least one output signal responsive to position of an area of contact, e.g. with a stylus (1), with said pad, and comprises: an electrically resistive layer (4) to which is applied at least one electrical field, this layer being substantially uniformly resistive; an electrically insulating mesh (3) and an electrically conductive elastomeric layer (2) for being locally depressed by stylus (1) so that a portion thereof can pass through a mesh aperture and contact said electrically resistive layer (4), thereby enabling the elastomeric layer to sense the potential of said electric field at the area of contact of the elastomeric layer (2) with the resistive layer (4). <IMAGE>

Description

SPECIFICATION Positional determination This invention relates to the determination of the position of an index relative to a field of use within which the index is movable.

By way of illustration and without any restriction to use, it is useful, for cartographic work, computer aided design or computer interaction of any sort which involves graphical, pictorial or in general non-alpha-numeric information of any sort, to be able to identify accurately the position of a "pen" on a "writing pad" in such a way that the positionl coordinates may be input to a computer in digital form to be processed. In the prior art this has been achieved in various ways, such as a "light pen" and T.V.

screen or inductive coupling between a pen and writing pad.

It is known that pressure sensitive devices can be used for positional identification but the object of the present invention is to provide improved apparatus which simplifies the procedure.

A first object of the invention is to provide a method for the production of digital information from which the instantaneous position of an index relative to a field of use may be calculated and apparatus for carrying out that method.

A second object is to provide a "writing pad" structure in which the said method and apparatus are embodied.

According to the present invention a method of providing digital information from which to calculate the instantaneous position of an index relative to a field of use comprises steps of: a. Applying separately at least two electric fields in a uniform resistive surface.

b. Detecting the value of each of the fields at the current situation of the index.

c. Determining the location of the index by comparing the detected values of the fields with information previousiy known about the fields.

The fields generated in the resistive surface will in general be non-linear but for a suitably chosen shape of the resistive surface, the monotonicity of the field enables the position of the index to be uniquely determined.

Where the resistive surface is rectangular and the electric field is applied across the diagonals, the equipotentials are approximately as in Fig. 1 a and Fig. 1 b.

Apparatus for carrying out the method of the invention comprises: a. an electrically sensitive "writing pad" made in three parts sandwiched together, Fig. 2a., the three parts being: 1. a uniform resistive surface.

2. an insulation in grid form.

3. a conductive elastomer.

b. means for applying an electric field across the diagonals of a resistive surface.

c. means for detecting the field at the current position of the index.

d. means for stabilizing the field in the resistive surface.

e. means of applying an electric field to emulate a point source.

The resistive surface advantageously in the form of a rectangle (Fig. 1 a.), has dimensions which may range from a few centimetres square to several square metres. Suitably, the material for the resistive surface can be in the form of a metal film coated on plastic material or a conductive plastic material e.g. Velostat (Registered Trade Mark).

A uniform electric field is applied via analogue semiconductor switches across diagonals of the resistive surface AC then separately across BD.

(Fig. 1 a. and Fig. 1 b).

Negative feedback from two reference points to the inputs of a differential amplifier is used to stabilise the field Fig. 3. Since the functional form of the equipotential is P=a(T)F(x,y) +b(T)G(x,y) where a, b may be functions of both temperature and time, it is essential to use at least two reference points for feedback or mathematical correction.

Ideally the electrical contact with the resistive surface should only be made at a point, however to ensure good electrical contact, the contact area should be large. To emulate an electrical point source, the contact is made such that the contact perimeter lies along the equipotential lines which would be generated by a point source. In the particular case of a rectangle, the contacts are in the form of elliptical quadrants of major and minor axes in the same ratio as the length and breadth of the rectangle. (Fig. 1).

Because of the micro-structural inhomogeneities associated with the surface of conductive films and plastics, electrical contact must be made more uniform by using Electrodag (Registerd Trade Mark), conductive paint or conductive elastomer.

The second part of the sandwich of the "writing pad" consists of a fine mesh, such as lace, or other non-conductive fabric e.g. Kevlar (Registered Trade Mark) nylon, with dimensions of about 0.1 mm gauge and with holes approximately 0.2 to 2 mm diameter.

The third part of the sandwich is a conductive elastomer in sheet form which can be a carbon filled or other conductive particle filled elastomer e.g. natural rubber, Neoprene, Vitron or EPDM (Registered Trade Marks), and is preferably about 0.3 mm to 0.7 mm thick. It should conform to a "fully conductive" rubber British Standard specification.

This top surface is connected to the input of a high impedance amplifier (Fig. 4). Pressure on the top surface with a ball-point pen or other stylus causes contact to be made between the top surface (probe) and the resistive sheet (Fig. 2b).

Hence the position at the point of contact can be determined by measuring the value of the electric fields applied to the resistive sheet at the point of contact and interpolating the co-ordinates.

Using the apparatus described yields areas of contact between the probe and resistive surface which can be as little as 0.1 mm in diameter and thus potentially gives very good resolution. The mesh, unlike previous "writing pads" with centre part formed of powder or liquid, discriminates between stylus pressure and pressure resulting from a hand resting on the upper surface, thus preventing spurious signais reaching the differential amplifier.

To minimise drifting values due to temperature, it is necessary to prevent thermal gradients in the resistive surface. This can be achieved by mounting the surface on a good heat conductive surface (e.g. aluminium sheet). The resistive sheet must be insulated electrically from this heat sink.

One or more heat sinks may be mounted separated by insulating foam.

In order that the nature of the invention may be readily ascertained, an embodiment of the "writing pad" operable in accordance therewith is hereinafter particularly described with reference to the accompanying drawings.

Fig. 1 a, b is a plan view of a rectangular resistive surface with elliptical corner contacts.

Fig. 2 is a side elevation of the "writing pad".

Fig. 3 is a circuit diagram of the "writing pad" and means for applying the electric field with a feedback correction.

Fig. 4 is a circuit diagram of a high impedance amplifier and A/D converter used to amplify the voltage detected by the upper conductive sheet and convert it to a digital value.

Referring to Fig. 1, the resistive surface 1 is a rectangular sheet of uniformly resistive material with electrical contacts made at the corners using contacts 2 in the shape of elliptical quadrants with major and minor axes in the same ratio as the length and breadth of the rectangular sheet. When an electric field is applied to the resistive sheet by applying a potential across AC the equipotential lines are approximately as shown by the dotted lines in la. When a potential is applied across BD the field is approximately as shown in 1 b. 3 and 4 are two reference points used to stabilise the field using the feedback circuit Fig. 3.

Referring to Fig. 2a, which is a side elevation of the "sandwich" part of the "writing pad", 1 is the upper sheet of conductive rubber, 2 is a nylon mesh, 3 is a uniformly resistive sheet, 4 is an electric insulator to insulate the resistive sheet from the beat sink 5, which reduces temperature gradients in the resistive material.

Referring to Fig. 2b, the stylus 1 which may be a ball-point pen pushes the conductive rubber surface 2 through the grid 3 to make contact with the resistive surface 4. During the time that contact is made, the various fields are applied to the resistive surface. The value of these fields are detected using the conductive rubber surface as a probe 7. Using the properties of the shape of the fields the position of the probe may be determined.

Referring to Fig. 3, the circuit diagram shows the resistive surface 1 of the "writing pad" connected within the feedback loop of the differential bridge amplifier 5. The voltages at the reference points 3 and 4 are adjusted using the potentiometers 6 and 7 fed by reference voltage Vref 8. To reduce errors due to fluctuations in voltage, this reference voltage is supplied by the same source as the reference voltage used by the A/D converter in Fig. 4. A potential is applied in turn across each pair of diagonally opposite corners of the resistive surface, the corners being selected by semi-conductor switches 9, 10, 11 and 12, which in turn are controlled by lines 13 and 14.

Referring to Fig. 4. The circuit diagram is of a high impedance amplifier and A/D converter. The value detected by the upper conductive surface when being used as a probe 1 is amplified by the high impedance amplifier 2, whose output is passed to the A/D converter to yield a digital value. The reference voltage used for the A"D converter is supplied by the same source as the reference voltage used by the feedback circuit in Fig. 3 to reduce the errors caused by voltage fluctuations.

The appended claims and accompanying drawings are also part of the disclosure of the present invention. It will be appreciated that the present invention includes alternatives and equivalents within the scope of the above general and detailed descriptions and/or within the scope of the accompanying drawings.

Claims (30)

1. An electrically sensitive pad providing at least one output signal responsive to position of an area of contact with said pad, said pad comprising: an electrically resistive layer for being provided with at least one electrical field, this layer being substantially uniformly resistive; electrically insulating mesh; an electrically conductive elastomeric layer for being locally depressed so that a portion thereof can pass through a mesh aperture and contact said electrically resistive layer, thereby enabling the elastomeric layer to sense the potential of said electric field at the area of contact of the elastomeric layer with the resistive layer.

2. A pad as claimed in claim 1, wherein said resistive layer is substantially in the form of a rectangle.

3. A pad as claimed in claim 1 or 2 wherein said resistive layer comprises a metal coating on plastics material.

4. A pad as claimed in any one of claims 1 to 3, wherein said resistive layer comprises a metal coating on plastics material.

5. A pad as claimed in any one of claims 1 to 3 wherein said resistive layer comprises electrically conductive plastics material.

6. A pad as claimed in any one of claims 1 to 5, wherein mesh apertures each have maximum widths in the range substantially 0.5 to substantially 2.0 mm.

7. A pad as claimed in any one of claims 1 to 6, wherein mesh apertures each have a depth of substantially 0.1 mm.

8. A pad as claimed in any of claims 1 to 7, wherein said mesh is a fabric.

9. A pad as claimed in claim 8, wherein said fabric comprises synthetic polymeric material.

10. A pad as claimed in any one of claims 1 to 9, wherein said elastomeric layer has a thickness in the range substantially 0.3 mm to substantially 0.7 mm.

11. A pad as claimed in any one of claims 1 to 10, wherein said elastomeric layer comprises natural rubber and electrically conductive particles.

12. A pad as claimed in any of claims 1 to 11, comprising at least one electrical field establishment means for establishing said at least one electrical field.

1 3. A pad as claimed in claim 12, wherein said at least one electrical field establishment means is adapted to provide a first said electrical field and a second said electrical field at different times so as to enable position coordinates of said contact area to be determined.

14. A pad as claimed in claim 13, wherein at least one field establishment means is adapted so that said first and second field directions will be substantially 90 degrees to each other.

1 5. A pad as claimed in claim 13, wherein said resistive layer is substantially in the form of a rectangle; and the potentials of the said first and second electrical fields are applied across respective diagonal directions of that rectangular form.

1 6. A pad as claimed in any one of claims 12 to 15, wherein said at least one electrical field establishment means comprises at least one switch means, said switch means being adapted to enable and disable said first electrical field and said switch means being adapted to enable and disable said second electrical field.

1 7. A pad as claimed in claim 16, wherein at least one said switch means is an analogue semiconductor switch.

18. A pad as claimed in any one of claims 12 to 17, wherein said at least one electrical field establishment means has at least one connection area to said resistive layer, the operative edge of this connection area being adapted to lie along a line of equipotential that would be constituted by a notional point source of potential, the point source being notionally at any predetermined location on the resistive layer.

1 9. A pad as claimed in claim 18, wherein the form of said resistive layer is substantially oblong; and said electrical field establishment means is connected at the four corner areas of the resistive layer, each of these connection areas being a quadrant of an ellipse of which the ratio of the major axes to the minor axes is substantially the ratio of the length to the breadth of said oblong.

20. A pad as claimed in any one of claims 12 to 1 9 comprising at least one stabilisation means for stabilising at least one said electrical field, said stabilisation means being adapted to be connected to at least one source of reference potential.

21. A pad as claimed in claim 20, wherein said at least one stabilisation means comprises at least first and second reference locations on said resistive layer, said stabilisation means being able to control said at least one electrical field establishment means by negative feedback thereto in response to the electrical potentials at those reference locations.

22. A pad as claimed in claim 21, wherein said reference locations are respectively at opposite edge regions of said resistive layer and define a notional line passing through a predetermined intermediate region of said resistive layer.

23. A pad as claimed in claim 22, wherein said resistive layer is substantially rectangular and said notional line is substantially parallel to an edge of said rectangular form.

24. A pad as claimed in any one of claims 21 to 23, wherein at least one said stabilising means comprises a differential amplifier having two inputs respectively connected to the resistive layer at the two said reference locations.

25. A pad as claimed in any one of claims 1 to 24, comprising at least one detector means for responding to said sensing of electrical field potential.

26. A pad as claimed in claim 25, wherein at least one said detector means comprises an amplifier for amplifying analogue signals.

27. A pad as claimed in claim 25 or 26, wherein at least one said detector means comprises at least one analogue to digital signai converter.

28. A pad as claimed in claim 27, wherein said analogue to digital converter is adapted to be connected to said source of reference potential.

29. A pad as claimed in any of claims 1 to 28, comprising thermally conductive means for resisting formation of undesirable thermal gradient(s) in said resistive layer.

30. A pad as claimed in claim 1, substantially as hereinbefore described with reference to and as shown in the accompanying drawings.