GB2183031A

GB2183031A - Touch sensor

Info

Publication number: GB2183031A
Application number: GB08627126A
Authority: GB
Inventors: Paul Richard Barnard
Original assignee: Individual
Current assignee: Individual
Priority date: 1985-11-14
Filing date: 1986-11-13
Publication date: 1987-05-28
Also published as: GB8627126D0; GB2183031B; GB8528152D0

Abstract

To determine the position of an object P within a designated area 16, a plurality of radiation generators D1,D2 in an array 12 of generators bounding the area 16 are sequentially activated, and radiation detectors PT1,PT2 in an array 14 of detectors on the opposite side of the area 16 detect the resultant shadows for each generator D1 ,D2 activated. The number of times each detector PT1,PT2 detects a shadow, during the sequential activation of the generators D1,D2, is summed, and the detector for which that sum is maximum corresponds to the position of the object P in the direction parallel to the array 14 of detectors. This is repeated for a second array 13 of generators, and a second array 15 of detectors, thereby to determine the position of the object P in the direction parallel to the second array 15 of detectors. From these two position detections, the position of the object P is unambiguously determined. <IMAGE>

Description

SPECIFICATION Touch sensor The present invention relates to a method and apparatus for determining the position of an object on a two-dimensional surface. Since the object detected is usually a finger or other pointer, such apparatus is generally known as a touch sensor.

There are now many situations where it is required to determine the position of an object on a surface such as a television screen (which may be displaying a computer output).

Various methods are known to achieve this, such as conductive meshes on the screen.

However, one of the more popular methods involves the generation of electromagnetic radiation (usually infra-red) and the detection of shadows cast by the object to be detected on an array of radiation detectors.

One known sensor of this type has an array of detectors, usually phototransistors, arranged on three sides of a sguare or rectangle, and two photogenerators at the two free corners of that array. One photogenerator is turned on, and the array of phototransistors scanned until a shadow point is detected at the phototransistor or phototransistors for which the radiation from the radiation generator is blocked by the object to be detected.

Analysis then permits an imaginary line to be drawn from the radiation generator to the photodetectors in the shadow. The first photogenerator is then turned off and the second turned on, and the process repeated, thereby creating a second imaginary line. The point of intersection of these two lines is the location of the object. Such touch sensors enable the position of the object to be detected ex- tremely rapidly, but have the grave disadvantage of low resolution. It is often the case that several adjacent phototransistors detect shadow, and this creates uncertainty in the actual position of the object.

In another form of touch sensor, radiation generators are arranged in an array along two adjacent sides of a rectangle or square, and a corresponding number of detectors, again in the form of phototransistors, arranged along the other two sides. Each radiation generator is switched on in turn, amd when one radiation generator is on, the corresponding phototransistor on the other side of the rectangle or square detects whether a shadow is cast.

Thus, the array may be considered as a grid of lines joining a radiation generator to its corresponding phototransistor and the lines on which the object lies are detected. Again, however, this system is not very accurate.

The present invention seeks to modify this arrangement to permit more accurate results to be obtained. The idea underlying the invention is to provide corresponding arrays of radiation generators and radiation detectors. This was done in UK 2133537, which calculated the position of an object from an analysis of the locations of the emitters and the area of shadow cast by the object on the detectors.

However, the system proposed in that specification reguires complicated analysis, and the present system seeks a more simple way of determining the position of the object. According to the present invention the radiation generators are activated in sequence and, for each generator activated, the radiation signals detected by each of the detectors in one or more of the detector arrays is monitored. In this way a shadow distribution is built up using the sums of the shadow patterns detected by the detector array(s) for each generator. It has been found that this shadow distribution has peaks corresponding to the position of the object being detected.

Preferably, there is a matrix of generators and detectors defining an area within which the object to be detected will be placed.

Thus, for example, two arrays of generators may be arranged on two adjacent sides of a rectangle or square, and two arrays of detectors arranged on the other two sides, leaving a hollow centre. The generators in one array are activated sequentially, and each time a generator is activated, all the detectors in the array on the other side of the matrix are monitored, to detect which of them are in the shadow of the object from the activated generator. By summing the outputs of the detector array for each generator, a shadow distribution is built up which has a peak corresponding to the position of the object parallel to the detector array. This is then repeated for the other generator array and corresponding detector array to obtain the position of the object in the perpendicular direction.

The present invention is not limited to the case where the arrays of the matrix are perpendicular, but this is the easiest arrangement for processing purposes. The number of detectors or generators in each array is not critical, nor does there have to be one-to-one correspondence of generators and detectors in opposite arrays. However, it has been found that one-to-one correspondence gives the most accurate results.

The radiation generators are preferably photodiodes generating infra-red radiation, and the detectors are preferably photosensitive transistors (phototransistors), but other forms of generators and detectors may be used.

In use a matrix of generators and detectors are placed over a surface, which normally will carry information, and an object placed on the surface within the hollow centre of the matrix, e.g. to identify a specific information element.

The position of the object may then be detected as discussed above. The information derived using a sensor according to the present invention may be fed to a computer for suitable analysis. The present invention may be used whenever the two-dimensional position of an object within the matrix has been identified.

An embodiment of the invention will now be described in detail, by way of example, with reference to the accompanying drawings in which: Fig. 1 shows a rectangular matrix of radiation generators and radiation detectors; Fig. 2 shows an arrangement for activating photodiodes of a diode array of the matrix of Fig. 1; and Fig. 3 is a schematic diagram of the processing circuitry for the matrix of Fig. 1.

Referring first to Fig. 1, a matrix 11 consists of a first array 12 of radiation generators, such as photodiodes DI, D2, etc., generating infra-red radiation, and a second array 13 of similar radiation generators arranged perpendicularly to it. The number of radiation generators within the arrays 12 and 13 is not critical; in the present embodiment there are 48 radiation generators in array 12 and 32 in array 13. The other two sides of the matrix 11 are formed by two arrays 14, 15 of radiation detectors such as phototransistors PT1, PT2, etc. Normally there will be the same number of phototransistors in the array 14 as there are photodiodes in the opposite array 12, and similarly the same number of phototransistors in the array 1 5 as there are photodiodes in the array 13. The centre 16 of the matrix 11 is hollow.

In use, the matrix 11 is placed over a surface, to detect the position of an object placed on the surface within the hollow centre 16 of the matrix 11. The surface over which the matrix is placed may be any surface, but normally will be an information carrying surface. For example, it could be the screen of a television displaying information from a computer, or a video game, or could be a surface carrying written information such as a book page. A further alternative is for the surface to show a drawing for which key-points are to be identified and fed into a computer for subsequent retrieval. indeed, in this latter situation, the surface may not initially carry any information whatever, and the position of a pointer detected as something is drawn on the surface.The above discussion of potential uses of the matrix 1 1 is merely given by way of example; many other uses will be immediately apparent to the informed reader.

Suppose now the matrix 11 is placed over a surface and an object such as a finger or other pointer, is placed within the hollow centre 16. The first diode Dl in the array 12 of photodiodes is turned on and this causes infra-red radiation to be emitted across the centre 16 of the matrix 11. The phototransistors PT1, PT2, etc., in the array 14 are scanned sequentially, and circuitry (to be discussed later) notes which phototransistors of the array 14 detect a shadow (i.e. do not receive radiation) because the radiation is being blocked by the object within the centre 16 of the matrix. This is then repeated for the second diode D2 in the array 12, and again the shadow cast by the object on the array 14 is detected. This process is repeated for all the diodes in the array 12, and thereby a shadow distribution is built up by recording how many times each phototransistor detects shadow.Thus, consider an object positioned at point P within the centre 16 of the matrix 11. This will cast a shadow only within the dotted lines 17 and 18, so only those phototransistors within the part 19 of the array 14, will detect shadow. Within that part 19, shadow will be detected more often as the photodiodes are activated in sequence by the phototransistors at the position corresponding to the position of the point P in a direction parallel to the array 14 (as the projection of the width of the phototransistor on the photodiodes through the point P will be wider), and thus a distribution will be built up with the central phototransistors in region 19 detecting a large number of shadows, the peripheral phototransistors in the region 19 detecting an increasingly smaller number of shadows as they are further from the centre, and the phototransistors outside the region 19, not detecting any shadow at all. Thus the peak of the distribution obtained corresponds to the position of the object P in a direction parallel to the array 14. In practice, this position can be determined to within half the spacing of the phototransistors, because if the peak has a maximum corresponding to the position of one phototransistor, then that phototransistor corresponds to the position of the object, whereas if two phototransistors have equal, and maximum, shadow detection, then the object is half-way between those two phototransistors.

This process is then repeated for each photodiode in the array 13, and shadows cast by the object on the array 15 are detected.

Again, a shadow region 20 will be formed, bounded by a line 21. Outside that region 20, the phototransistors of the array 1 5 will not detect shadow, whereas within the region 20, shadow will be detected to varying degrees by the phototransistors, with a peak corresponding to the position P of the object, in a direction parallel to the array 15.

Thus, the position of the object may be determined in two perpendicular directions, which is sufficient to identify its position within the centre 16 of the matrix 11.

The circuitry controlling the operation and detection of the arrays 12 to 15 will now be described with reference to Figs. 2 and 3.

First, Fig. 2 shows how the photodiodes of the arrays 12 and 13 are activated. Two sets of lines 1 to 10 and A to H are formed, with each photodiode D1, D2, etc., joining one of the lines of one set with one of the lines of the other set. As illustrated, the lines 1 to 10 are perpendicular to the lines A to H, but in practice this need not be the case. Furthermore, although the diodes D1, D2, etc., are shown adjacent the lines, so that they form a rectangular array, this is, in fact, not the case as the diodes D1, D2, etc., are in a linear array as shown in Fig. 1. A power line L1 is connected via a resistor R1, and a switching transistor S1 to a power supply (not shown).

When the switching transistor S1 is turned on, a voltage is applied to the power line L1 and hence to the emitters of transistors TA, TB, etc., connected to the lines A, B, etc. An output line L2 is connected to earth, and also to the collectors of transistors T1 to T10, whose emitters are connected to the lines 1 to 10 respectively.

Suppose now that a signal is applied to the base of transistor Ti, to turn it on. Then, one end of each of the diodes D1 to D8 is earthed. Then, suppose a voltage is applied to the base of transistor TA. That transistor becomes conducting and the end of the diode Di, connected to line A, becomes connected, via the resistor R1 and the switching transistor S1, to the power supply. Thus a current flows through the diode D1, causing it to generate light. By removing the voltage from the base of the transistor TA, and applying it to the base of the transistor TB, photodiode D2 can be turned on. By repeating this, all the diodes D1 to D8 can be turned on in sequence.Then, removing the voltage from the base of transistor T1, and applying it to transistor T2, the diodes D9, etc., connected to line 2 may be activated by applying voltages to the basesof transistors TA, TB, etc., in sequence. By repeating this process across the array, each of the diodes of the array 12 (corresponding to diodes connected to lines 1 to 6) and array 13 (corresponding to diodes connected to lines 7 to 10), can be activated.

A similar arrangement is used for the phototransistors PT1, PT2, etc., of the arrays 14 and 15 with phototransistors replacing the photodiodes. The only other change made is that the resistor for this arrangement will be between the line L2 and earth.

Referring now to Fig. 3, the array shown in Fig. 2 corresponds to the block 30, whilst the equivalent circuit to Fig. 2 for the phototransistors is indicated by block 31. The controls for each of the blocks 30 and 31 are similar.

A processor 32 is connected to each of the transistors TA, TB, etc., of the block 30, to activate them in sequence. A similar processor 33 is connected to the coresponding transistors of block 31. A further processor 34 is connected to the transistors T1 to T6 of Fig.

2 (corresponding to the array 12), and a further processor 35 is connected to the transistors T2 to T10 of Fig. 2 (corresponding to the array 13). Similar processors 36, 37 are connected to the block 31 to correspond to the processors 34, 35 respectively. The processors 32, 34, and 35 are connected via a latch 38 to a microprocessor 39, which controls the switching of the transistors TA to TH and T1 to T10. Similarly, the processors 33, 36 and 37 are connected via a latch 40 to the microprocessor 39 to control the switching of the transistors controlling the phototransistors of the arrays 14 and 15. The microprocessor 39 is controlled by a program stored in a memory 41, which controls the activation of the photodiodes D1, D2, etc., and the phototransistors PT1, PT2, etc.Memory within the microprocessor 39 records the information from the phototransistors of the arrays 14 and 15, and so can build up the shadow distribution required, by summing the shadows detected by the phototransistors of each array for all the photodiodes.

Detection is achieved by monitoring the currents flowing through resistor R1, connected to the photodiodes of arrays 12 to 13, and resistor R2 connected to the phototransistors of arrays 14 and 15. The detection circuits are shown schematically by blocks 42 and 43, which feed information concerning the current flowing through the photodiodes, and through the phototransistors (which latter indicates whether or not shadow is being detected) to the microprocessor 39.

The information concerning the position of the object, which information is made up of shadow information from the microprocessor 39 and information concerning the photodiodes and phototransistors activated from blocks 30 and 31, may be fed via lines 44, 45 to an interface, and hence to e.g. a computer, which processes the positional information. Thus, for example, if the system is used to record various positions on a drawing, the computer connected to the system of Fig. 3 may record successively positions on the drawing, hence to build up a complete picture.

Of course, the processing of the information from blocks 30 and 31 is controlled within the microprocessor 39 by a program held within memory 41. It would be possible, by suitable change of this program, to adapt the system to a non-rectangular matrix, although this would make the processing more complicated as the radiation emitted by one radiation generator may cast shadows on both arrays of radiation detectors. Indeed, many other geometries can be envisaged, but the processing becomes more difficult.

As described above there is a one-to-one correspondence between the radiation generators of one array (12 or 13) and the radiation detectors of the opposite array (14 or 15).

This is not essential to the working of the invention, but is believed to give the more accurate result as a relative decrease in the number of radiation generators causes larger shadows to be cast, and a relative decrease in the number of radiation detectors reduces the resolution achievable. The fundamental point, irrespective of the number of photogenerators and photodetectors, is the idea of scanning a plurality of detectors for each radiation generator, and then processing the information to generate a shadow distribution.

Claims

1. A method for determining the position of an object within a designated area, with a plurality of radiation generators and a plurality of radiation detectors positioned at locations around the designated area; the method comprising the steps: (a) activating sequentially at least some of the radiation generators; (b) for each generator activated, detecting which of the radiation detectors do not receive radiation; and (c) for each detector, summing the number of times that detector does not receive radiation during the sequential activation of the generators; thereby to determine the position of the object from the position of the detector(s) for which the sum has the largest value.

2. A method according to claim 1 wherein the designated area is rectangular or square, there is a first set of the emitters located along a first side of the area, a first set of the detectors are located along the opposite side of the area, a second set of the emitters are located along a third side of the area, and a second set of the detectors along the fourth side of the area; and the method involves: carrying out the steps (a) to (c) for the first sets of emitters and detectors, thereby to determine the position of the object in a direction parallel to the first side of the area; and carrying out the steps (a) to (c) for the second sets of emitters and detectors, thereby to determine the position of the object in a direction parallel to the third side of the area.

3. A method according to claim 2, wherein the number of emitters in the first set of emitters equals the number of detectors in the first set of detectors, and the number of emitters in the second set of emitters equals the number of detectors in the second set of detectors.

4. A method of determining the position of an object substantially as any one herein described with reference to the accompanying drawings.

5. An apparatus for determining the position of an object within a designated area, having: a plurality of radiation generators and a plurality of radiation detectors positioned at locations around the designated area; means for activating sequentially at least some of the radiation generators; means for detecting when each of the radiation detectors do not receive radiation; means for summing the number of times the detecting means detects that the corresponding detector does not receive radiation during the operation of the sequential activation means; and means for determining, the detector(s) for which the sum has the largest value during the operation of the sequential activation means.

6. An apparatus according to claim 5 wherein the designated area is rectangular or square, and there is a first set of the emitters located along a first side of the area, a first set of the detectors are located along the opposite side of the area, a second set of the emitters are located along a third side of the area, and a second set of the detectors along the fourth side of there.

7. An apparatus according to claim 6, wherein the number of emitters in the first set of emitters equals the number of detectors in the first set of detectors, and the number of emitters in the second set of emitters equals the number of detectors in the second set of detectors.

8. An apparatus for determining the position of an object substantially as herein described, with reference to and as illustrated in the accompanying drawings.