WO1990006563A1

WO1990006563A1 - Digitising device

Info

Publication number: WO1990006563A1
Application number: PCT/GB1989/001478
Authority: WO
Inventors: Hugh John Agnew; Roger Kenneth Woolley; Gwyn David Walter Parfitt
Original assignee: Qudos S.A.
Priority date: 1988-12-09
Filing date: 1989-12-11
Publication date: 1990-06-14
Also published as: AU4747390A; GB2227319A; GB8828864D0

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

The present invention relates to a digitising device and one which is particularly suitable for use with the navigational aid system disclosed in our copending International Patent Application No O87/07013.

It is known to have a digitising device 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 by an AC signal to induce signals in the electrodes or vice versa. These signals can be then processed to determine the position of the cursor on the substrate. Such a digitising device can then be used, for example_/ for digitising drawings or in the case of the aforementioned navigational aid system chart plotting and reading.

A known manner for manufacturing a digitising device of the kind described above 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 awa . The resist is then removed. Appropriate coverings are then applied to the board and it is mounted on a base to give the necessary strength.

The present invention is concerned with providing a digitising device which is cheaper to manufacture than the aforementioned known type of digitising device, is less bulky but is sufficiently rugged particularly for the use referred to earlier in connection with the navigational aid disclosed in our copending International Patent Application No WO87/07013.

According to the present invention a digitising device of the kind described is characterised in that the electrodes are formed by printing conductive material onto the substrate, the device as a whole being flexible.

How the invention may be., carried out will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a perspective view showing the various items and equipment which go to make up a navigational aid of the kind disclosed in our International Patent Application No WO87/07013;

Figures 2A to 2D illustrate steps in the manufacture of a digitising pad according to the invention, showing partial cross-sections of the layers of the pad;

Figure 3 is a perspective view of the digitising device which forms part of the navigation aid as shown in Figure 1;

Figures 4 and 5 show arrangements of coarse and fine electrodes printed on the digitising pad;

Figure 6 is a schematic diagram of a signal processing unit forming part of the digitising pad shown in Figure 2; and

Figure 7 is an enlarged fragmentary view of a small portion of the matrix of electrodes shown in figures 4 and 5.

Referring to Figure 1 the navigation aid comprises a digitising pad 101 and a puck or cursor 200 electrically connected to the digitising pad 101 via a cord 103 and a signal processor or digitiser 104. The signal processor or digitiser 104 is connectable to a computer (not shown) .

By means of a sensor coil in the puck or cursor 200 and a grid of electrodes beneath the surface and built into the digitiser pad 101 it is possible to determine the position of the puck or cursor 200 in terms of x and y coordinates of the puck or cursor 200 on the digitising pad 101. A chart 100 is spread across the digitising pad 101 and held in position by clips 102. The puck or cursor 200 is freely slidable over the surface of the chart 100.

The digitising pad 101 is constructed so that it is suitable for easy stowage within relatively small pleasure and sporting water craft such as yachts and motor cruisers. In such an environment it would be usual for the digitising pad 101 to be placed on a chart table which very often would have hinges which protrude to some .extent above the surface of the table. It is therefore important in this environment that the digitising pad 101 should nevertheless be capable of resting substantially evenly on the chart table. The digitiser pad 101 is therefore manufactured to be flexible as a whole so that it can not only adapt to a degree of unevenness on the chart table but could also possibly be rolled up for stowage.

The way in which the flexible digitiser pad 101 is manufactured will now be described in more detail with reference to the remaining drawings.

Referring to Figures 2A to 2D, 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 manuf cturing 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 ma ^' be patterned and arranged in accordance with known techniques. However, for completeness one possible arrangement of electrodes will be briefly described. Referring to Figure 3, which is a very diagrammatic representation of the digitser pad of Figure 1, 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 4 and fine electrodes a portion of which are arranged as shown in Figure 5.

Referring to Figure 4, 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 4 and 5, 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 a' b' c' d' a" b" c" d" being connected in the opposite sense to the electrodes designated a' b' c' d' a" b" c" d" that is to say when progressing along the series of electrodes if the a' electrodes are traversed in a clockwise direction, 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 3, the signal processing unit 104 mounted at one corner of the pad 10 includes electronic components mounted on a printed circuit board which is connected to the lines of the pad Ax, Bx, Cx, Dx, ax, bx, ex, dx. Ay, By, Cy, Dy, ay, by, ^cYι ^~~Y_* 0_' ^Tne processing unit 104 is also connected to a cable 105 for power supply and data output.

The general operation of the processing unit. 104 is similar to that for a conventional digitising device and therefore does not need to be described in detail. However, a brief description will be made with reference to Figure 6. The signal processing unit 104 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 0.1mm. It will be noted from Figures 4 and 5 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 printing small area of an 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.

Although it has been stated above that the ink and printing technique used to manufacture the digitiser pad may be similar to that used for manufacturing known membrane keyboards there are certain problems which arise as a result of scaling up the process to such a relatively large area as that occupied by the digitiser pad compared with the relatively very small area found in membrane keyboards. The digitiser pad 101 would typically be the size of a half sized British Admiralty Chart i.e. approximately 72 by 53 cm. Furthermore, because of the specific use to which the digitiser pad is to be put it is important that it should be as light as possible consistent with being durable as well as being electrically reliable.

In order to make the digitiser pad 101 as flexible as possible it would clearly be best to make the printed electrodes as thin as possible. However, in order for them to. fulfill their required electrical function they need to have a certain cross-sectional area.

Consequently if these electrodes are made as wide as possible they will tend, when crossing other electrodes, to give rise to a relatively large capacitance effect which in turn will have a deleterious effect on the efficient functioning of the digitiser pad and associated equipment. On the other hand if the electrodes are made relatively narrow but relatively thick in order to overcome this capacitance problem then they will be less flexible.

It can thus be seen that it is not just a simple matter to adopt the known technology for making membrane keyboards to a digitising pad suitable for use with the navigation aid disclosed in our copending International Patent Application WO87/07013. A solution to this problem is illustrated in Figure 7.

This shows, on an extremely large scale, the cross over point between two electrodes 16 and 18.

At the cross over point each electrode is formed with a relatively narrow neck portion 16a and 18a respectively. It has been found with this arrangement that the problem outlined above is minimised and enables the desired electrical characteristics of the digitiser pad to be maintained whilst at the same time allowing it to have the desired physical characteristics of light weight and flexibility.

Other arrangements of the various layers making up the digitiser pad may be employed i.e. the invention is not limited to the order or number of layers shown in Figure 2D.

Claims

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 the device as a whole being flexible. '

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 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.

5. A device as claimed in claim 4, 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.

6. A device as claimed in any of claims 1 to 3, wherein both sets of electrodes are printed onto the same layer of the substrate.

7. A device as claimed in claim 5, wherein the sets of electrodes are printed onto opposite surfaces of said layer.

8. A device as claimed in claim 5, wherein the sets of electrodes are printed onto the same surface of said layer, and insulating material is disposed between the sets of electrodes.

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

10. "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.

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

12. A device as claimed in any preceding claim in which in the area where one electrode crosses over another electrode each electrode has a portion of narrower width than the lengths of the electrode between such cross-over points.

13. A device as claimed in any preceding claim which is sufficiently flexible to be rolled up.

14. A device as claimed in any preceding claim which has means to hold a chart on its upper surface.

15. A device as claimed in which the electrodes . are formed into inductive loops.

16. A device as claimed in claim 15 in which there are large and small loops for coarse and fine adjustment of a cursor movable over the device.

17. 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.