GB2227319A - Digitising device - Google Patents

Abstract

A digitising pad particularly for use with a navigational aid includes a first set of electrodes crossing a second set of electrodes to form a grid, the electrodes being formed on a flexible substrate by printing conductive material onto the substrate, the pad being flexible as a whole.

Description

DIGITISING DEVICE A digitising device is known which comprises a substrate bearing a pattern of electrodes including first and second sets of electrodes which cross each other. The device can be used with a cursor including a coil which is driven with an AC signal to induce signals in the electrodes or vice versa. These signals can then be processed to determine the position of the cursor on the substrate. Such a digitising device can be used, for example, for digitising drawings, or in a chart plotting and reading system as described in International patent application No W087/07013.

A known digitising device of the above type uses a double-sided printed circuit board. During manufacture, the circuit board initially has copper cladding on both sides. Etch resist is then applied onto the two sides of the board in the pattern of the required electrodes; the exposed copper is then etched away; and then the resist is removed. Appropriate coverings are then applied to the board, and it is mounted on a base to give it the necessary strength.

One aspect of the present invention is concerned with simplifying and reducing the cost of the manufacture of a digitising device. Another aspect of the invention is concerned with producing a digitising device which is more rugged. A further aspect of the invention is concerned with providing a digitising device which is less bulky than the known device.

In accordance with a first aspect of the invention, the electrodes of the digitising device are formed by printing conductive material, such as conductive ink, onto the substrate. In this manner the electrodes can be simply and cheaply produced.

In accordance with a second aspect of the invention, the substrate of the digitising device is flexible and the electrodes are such that they can flex with the substrate. Thus, there is no need to provide a strengthening base for the substrate, and the device can be used by laying the substrate on any conveniently available support surface.

A specific embodiment of the invention will now be described by way of example with reference to the drawings, in which: Figs. la to 1D illustrate steps in the manufacture of a digitising pad according to the invention, showing partial cross-sections of the layers of the pads; Fig. 2 is a perspective view of the digitising pad; Figs. 3 and 4 show arrangements of coarse and fine X electrodes printed on the digitising pad; and Fig. 5 is a schematic diagram of a signal processing unit forming part of the pad.

Referring to Figures 1A to 1D in the manufacture of a digitising pad 10, two sheets 12, 14 of polyester each 0.175mm thick are taken, and a first set of electrodes 16 and a second set of electrodes 18 are printed on the sheets 12, 14 respectively by conventional printing techniques using an electrically conductive ink containing silver or silver and nickel. The ink and printing technique may be similar to that used conventionally for manufacturing membrane keyboards.

A layer 20, 22 of adhesive, such as type "EP54" made by the 3M company, are then spread on the polyester sheets 12, 14 and sets of electrodes 16, 18 to a thickness of 0.025mm. A sheet 24 of polycarbonate of thickness 0.375mm is then taken, and the adhesive sides of the polyester sheets 12, 14 are adhered to the opposite sides of the polycarbonate sheet 24. The digitising pad 10 thus consists, in the following order, of, the outer polyester sheet 12, the first set of electrodes 16, the adhesive layer 20, the polycarbonate spacer sheet 24, the adhesive layer 22, the second set of electrodes 18, and the other outer polyester sheet 14. If desired further protective layers of polycarbonate for example 0.250mm thick may be applied to the outer faces of the polyester sheets 12, 14.

The electrodes may be patterned and arranged in accordance with known techniques. However, for completeness one possible arrangement of electrodes will be briefly described. Referring to figure 2, the sensing portions of the electrodes 16 of the first set for determining an X coordinate are parallel to each other and extend in the direction 26, and the sensing portions of the electrodes 18 of the second set for determining a Y coordinate are also parallel to each other, extending in the direction 28 perpendicular to the direction 26.

The electrodes 16 of the first set include coarse electrodes arranged as shown in Figure 3 and fine electrodes a portion of which are arranged as shown in Figure 4.

Referring to Figure 3, the coarse electrodes of the X set are arranged as four generally rectangular overlapping loops. The ends of each loop are connected to a common line 0 and to respective signal lines A, B, C, D. The coarse X electrodes are used to determine in which of four ranges the X coordinate of the cursor lies.

Referring to Figures 3 and 4, interspersed with the coarse X electrodes are a recurring series of fine X electrodes, the series being designated a' b' c' d' a" b" c" d" Each fine X electrode is also in the form of a loop, narrow in the X direction and extending across nearly the whole width of the pad 10.

The electrodes designated by the same letter, for example, the a' electrodes and the a" electrodes, are connected in series, with electrodes designated "being connected in the opposite sense to the electrodes designated", that is to say when progressing along the series of electrodes if the a' electrodes are traversed in a clockwise dirction, then the a" electrodes are traversed in the opposite anti-clockwise direction. At the ends of the series of electrodes, one of the two conductors is connected to the common line 0 and the other conductor provides a signal line. Thus the fine X electrodes have five input lines 0, a, b, c, d.

The coarse Y electrodes and fine Y electrodes are arranged in a similar manner on the second polyester sheet 14, and when the sheets are assembled, the elongate loops of the fine Y electrodes cross the eleongate loops of the fine X electrodes at right angles.

Referring to Figure 2, a signal processing unit 38 is mounted at one corner of the pad 10 and includes electronic components mounted on a printed circuit board which is connected to the lines of the pad Ax, Bx, Cx, Dx, ax, bx, cx, dx, Ay, By, Cy, Dy, ay, by, cy, dy, 0. The processing unit is also connected to a cable 40 for power supply and data output.

The general operation of the processing unit is similar to that for conventional digitising device and therefore does not need to be described in detail. However, a brief description will be made with reference Figure 5. The signal processing unit 38 comprises an MPU 40 which controls an analogue selector switch 42 cyclically to pass the signal from a selected one of the input signal lines 44 to a buffer amplifier 46. As each line is selected, an integrator 48 is reset and a counter 50 is reset and started. Once reset, the integrator 48 integrates the selected signal, and one of the integrated signal reaches a predetermined value detected by a comparator 52, the counter is stopped and the count is transferred to a shift register 54. The unit therefore acts as a digital volt meter which sequentially detects and transfers to the shift register 54 values indicative of the currents induced in each of the X and Y coarse and fine electrode loops by the cursor. From these values the X, Y coordinates of the cursor can be computed.

In an example of the device described above, the fine X and Y electrodes were pitched at 25mm and 20mm respectively, the cursor comprised a coil of 200 turns and 50mm diameter driven at a frequency of 6kHz. With this example, the position of the cursor could be determined to a resolution of O.lmm.

It will be noted from Figures 3 and 4 that some of the electrodes printed on the same surface cross each other, and it will be realised that steps must be taken to ensure that the crossing electrodes do not touch each other. Since printing is used to form the electrodes, this can be easily accomplished by printing the electrodes in different stages and in between these stages printed small area of a electrically insulating plastics material at the appropriate places. This provides a neater simpler solution compared with the techniques of through-holing and solder links which would be necessary in a printed circuit board form of digitising pad.

In a modification of the pad described above, the first and second sets of electrodes are printed on the same layer, either on opposite surfaces of the same layer, or on the same surface but with insulating material printed between the electrode sets.

In a further modification of the pad described above, a chart, map or plan is printed onto one of the layers so as to be externally visible, or alternatively a pre-printed chart, map or plan is laminated with the digitising pad.

Claims (14)

1. A digitising device comprising a substrate bearing a pattern of electrodes including a first set of electrodes and a second set of electrodes crossing the first set of electrodes, characterised in that the electrodes are formed by printing conductive material onto the substrate in the required pattern.

2. A device as claimed in claim 1, wherein the conductive material is a conductive ink.

3. A device as claimed in claim 1 or 2, wherein the substrate is in the form of a sheet.

4. A device as claimed in claim 3, wherein the sheet is flexible.

5. A device as claimed in any preceding claim, wherein the substrate is formed of first and second layers, the first set of electrodes being printed on the first layer, the second set of electrodes being printed on the second layer, and the first and second layers being bonded together.

6. A device as claimed in claim 5, wherein the first and second layers are bonded together through the intermediary of a third layer, with the first and second sets of electrodes facing towards the third layer.

7. A device as claimed in any of claims 1 to 4, wherein both sets of electrodes are printed onto the same layer of the substrate.

8. A device as claimed in claim 6, wherein the sets of electrodes are printed onto opposite surfaces of said layer.

9. A device as claimed in claim 6, wherein the sets of electrodes are printed onto the same surface of said layer, and insulating material is disposed between the sets of electrodes.

10. A device as claimed in any preceding claim, wherein the electrodes are connected to a signal processing circuit mounted on the substrate.

11. A device as claimed in any preceding claim, wherein the electrodes of the first set are generally parallel to each other, and the electrodes of the second set are generally parallel to each other and orthogonal to the electrodes of the first set.

12. A device as claimed in any preceding claim, wherein the substrate is formed with a chart, map or plan.

13. A device as claimed in any preceding claim, in combination with a cursor movable over the substrate, means for producing a magnetic field between the cursor and the electrodes, and means for processing electrical signals induced by the magnetic field to determine the position of the cursor with respect to the substrate.

14. A digitising device substantially as described with reference to the drawings.