GB2334573A

GB2334573A - An optical joystick

Info

Publication number: GB2334573A
Application number: GB9802226A
Authority: GB
Inventors: Anthony Mark Sawyer
Original assignee: Penny & Giles Computer Product
Current assignee: Penny & Giles Computer Product
Priority date: 1998-01-30
Filing date: 1998-01-30
Publication date: 1999-08-25
Also published as: GB9802226D0

Abstract

An optical joystick assembly (1) comprises a light source (3) attached to the end of a shaft (2) which is mounted pivotally through a point (6) allowing pivotal movement in any direction about the point. A plurality of light detectors (9 - 12) are arranged relative to the light source, so that the output from the detectors vary as the shaft is moved. The outputs from the sensors are then resolved by circuitry, into signals representative of the position of the joystick, resolved into X and Y components. In an embodiment, a further photodetector in the handle senses the output of the source in order to compensate for temperature variations.

Description

AN OPTTCAL JOYSTICK FIELD OF TIE INVENTION This invention relates to an optical joystick for use as a computer input for example.

BACKGROUND OF THE INVENTION Various types ofjoystick are known which are adapted to input control information to data processing systems. Prior art joysticks employing electromechanical sensors, such as mechanical switches or potentiometers, are easy to manipulate but tend to suffer from wear in their transducer systems (see European Published Patent Application No. 0 083 421 A, for example).

Different designs of optical joystick are also known, but, whilst they unavoidably require multiple light sources and/or detectors, the physical arrangement of prior designs is complicated and so is their electronic circuitry which leads to cost disadvantages (see European Published Patent Application No. 0 295 368 A and US Patent No. 4,533,827A, for example). On the other hand, known optical joystick designs do have the definite advantage of not suffering from mechanical wear in their transducer systems.

OBJECTS AND SUMMARY OF THE INVENTION: It is the principal object of the present invention to provide an optical joystick which is cheap to produce and reliable for application as a computer input device.

In broad terms, the present invention resides in the concept of generating signals which depend on the relative dispositions of a light source and a plurality of detectors, and processing the changing signals from the plural detectors in a simple, logical fashion so as to derive output signals representative of spatial position relative to a predetermined coordinate axis.

More particularly, according to the present invention there is provided an optical joystick comprising: a light source; a plurality of light detectors; said source and detectors being relatively movable by operation of the joystick so that the outputs of the detectors vary as their illumination by the source varies in dependence upon their changing relative positions; and circuitry responsive to increasing outputs from some of said detectors and reducing outputs on others for deriving signals representative of the movement of the joystick resolved into X and Y components.

In accordance with an exemplary embodiment of the invention which will be described hereafter in detail, there are four detectors arrayed in a square and the light source locates at the centre of the square in the home position of the joystick, the joystick being of a kind which is biassed towards a zero, null or home position and returns thereto when there is no manual operation. As the joystick is moved, so light from the light source impinges variably upon the four sensors which provide correspondingly varying outputs. The joystick circuitry resolves these outputs into corresponding joystick positions relative to a predetermined coordinate system. For example, with X and Y axes defined as bisecting respective sides of the abovementioned square, the joystick circuitry in one embodiment is adapted to derive Xaxis and Y.,,s signals as a function of the expressions Taxis = (OP2+0P4) - (OP1+OP3) = = (OPl+OP2) - (Op3+0P4) where OP1, OP2, OP3 and OP4 are the outputs of the four detectors, the X axis bisecting the side of the square having corners at the detectors whose outputs are OP2 and OP4 and the opposite side having corners at the detectors whose outputs are OP1 and OP3, and the Y axis bisecting the side of the square having corners at the detectors whose outputs are OP 1 and OP2 and the opposite side having corners at the detectors whose outputs are OP3 and OP4.

The circuitry in the abovementioned embodiment conveniently comprises an X-axis portion and a separate Y-axis portion, each portion comprising a differential amplifier arranged to receive respective outputs of two of the four detectors on one of its two inputs and respective outputs of the other two detectors on the other of its two inputs, the differential amplifier providing an output according the difference between its two inputs.

As described more filly hereinafter, the circuitry of the joystick of the invention provides XWs and Yaxis signals which are representative of joystick movements in an X, Y coordinate system. The circuitry is simplified and can be implemented at reasonable cost and whilst it trades linearity and precision for these attributes, nonetheless it has utility for many applications where linearity and precision are not top priorities.

The above and further features of the invention are set forth with particularity in the appended claims and will be described hereinafter with reference to the accompanying drawings.

BRIEF DESCRTPTTON OF THE DRAWINGS Figure 1 is a schematic side-elevation view of an optical joystick assembly embodying the present invention, Figure 1 showing the assembly in an undisplaced reference position (a) and in angularly-displaced positions (b) and (c) on either side ofthe reference position (a); Figure 2 is a schematic top plan view of the optical joystick assembly of Figure 1; and Figures 3(a) to (d) are diagrams of the electronic circuitry proposed for use with the optical joystick assembly of Figure 1, Figure 3(a) showing a first group of circuits for converting detected radiation into electrical signals representative thereof, Figure 3(b) showing a second circuit for arithmetically combining the electrical signals produced by the circuits of Figure 3(a), Figure 3(c) showing a third circuit for generating a reference voltage output, and Figure 3(d) showing a fourth circuit diagram for compensating for temperature induced fluctuations of the light source output.

DETAILED DESCRIPTION OF AN EXEMPLARY EMBODIMENT Referring first to Figure 1, there is schematically shown therein a preferred optical joystick assembly 1 embodying the present invention The optical joystick assembly 1, generally indicated in solid line in Figure 1 at a vertical undisplaced or home position, comprises an elongate single-piece carrier in the form of a shaft 2 having a radiation source comprised by an infrared light emitting diode (LED) 3 at its lower end 4 and a hand grip or knob 5 for manipulating the shaft 2 at its upper end. The shaft 2 is mounted at a point 6 along its axial length by means of a gimbal, universal joint or other arrangement (not shown) such as to permit pivotal movement of the shaft in all directions perpendicular to its axis, as shown for example by the arrows 7,7' in the figure, the pivotal mechanism preferably being resiliently-biassed, for example by provision of appropriate springs (not shown) which act on the shaft 2 so as to bias it towards its home position.

As shown in Figures 1 and 2, the shaft 2 is mounted above a printed circuit board 8 carrying an array of sensors 9, 10, 11 and 12 comprised by infrared detector transistors. The four sensors are mounted so that each sensor is at the corner of a notional square having the home position of the joystick mounted LED 3 at its centre as represented in Figure 2, and the mechanical arrangement of the joystick is such that the LED 3 points directly at one of the sensors 9, 10, 11 and 12 when the joystick is at the extreme of its movement in the respective direction in the respective quadrant of the square.

In operation of the thus described joystick arrangement, it will be understood that the sensors 9, 10, 11, 12 provide different outputs in dependence upon the degree of their illumination by the LED 3, this being dependent on the direction in which the LED 3 is pointing as determined by the position of the joystick. The circuitry shown in Figures 3(a), 3(b), 3(c) and 3(d) derives outputs from the four sensors and resolves them into signals representative of the joystick position in X and Y directional components in the plane perpendicular to the axis of shaft 2 in its home position, the X and Y axes being defined as shown in Figure 2 so as to bisect respective sides of the square.

Figure 3(a) shows circuitry associated with each of the sensors 9, 10, 11 and 12 for providing analogue output signals of an amplitude proportional to the intensity of the light received by the respective sensor from the joystick mounted LED 3. As shown, each phototransistor 9, 10, 11, 12 has connected thereto a respective operational amplifier 13, 14, 15, 16 which receives the output of the respective phototransistor on its inverting input and a reference voltage Vref on its non-inverting input. The outputs of the respective op-amps go negative relative to the reference voltage Vref when the respective photo-transistors are illuminated by the LED 3. These outputs are designated OP1, OP2, OP3 and OP4 in Figure 3(a). Figure 3(c) illustrates the generation of Vref from the system supply voltage VCC by means of a resistive voltage divider 17 providing an input to the non-inverting input of an op-amp 18 connected in a negative feedback mode so as to provide a stable Vref output.

Referring now to Figure 3(b), this shows how signals representative of the X-axis and Y-axis displacements of the joystick are derived from the outputs OP1, OP2, OP3 and OP4 of the sensor circuits shown in Figure 3(a).

It is a feature of the present invention that this is effected in accordance with the following relationships: X5 = (OP2+OP4) - (OP 1+OP3), and Yms = (OP 1+OP2) - (OP3+OP4).

In Figure 3(b), there are shown first and second differential amplifiers constructed with operational amplifiers 19 and 20 which respectively determine the Taxis and Y5 signals from the outputs OP1, OP2, OP3 and OP4 of the photosensors. As shown, op-amp 19 has the OP2 and OP4 inputs coupled additively to its non-inverting input and has the OP 1 and OP3 inputs coupled additively to its inverting input, and op-amp 20 has the OP 1 and OP2 inputs coupled additively to its non-inverting input and has the OP3 and OP4 inputs coupled additively to its inverting input. The result is to provide analogue outputs from the op-amps 19 and 20 which are representative of the X-axis and Y-axis displacements of the joystick and the corresponding variations in the illumination of the sensors 9, 10, 11 and 12 by the LED 3.

The circuitry of Figure 3(b) relies upon the proposition that movements of the joystick can be resolved into X and Y components and that the X component can be expressed as a movement towards sensors 11 and 9, corresponding to increases in OP2 and OP4, and away from sensors 12 and 10, corresponding to reductions in OP 1 and OP3, or vice versa, and the Y component can be expressed as a movement towards sensors 12 and 11, corresponding to increases in OP 1 and OP2, and away from sensors 10 and 9, corresponding to reductions in OP3 and OP4. Out of this proposition comes the abovementioned expressions for the Xa s and Y,"is movements. By appropriate selection of the circuit components this method of resolving the differing outputs of sensors 9, 10, 11 and 12 into corresponding X direction and Y direction signals can take account of any lack of uniformity between the response characteristics of the sensors and any directional variations in the their light sensitivity or in the light output of LED 3. In the results, a useable correlation between the movements of the joystick and the variations of the X s and Ya s signals is obtained. Compensation for such variables could be provided in the circuitry and/or by selection of the optical components, but would have a cost penalty. The illustrated circuitry is considered to be suitable for the vast majority of joystick applications where linearity and precision are not top priorities.

Temperature compensation can, however, readily be provided and Figure 3(d) shows one way of accomplishing this. As shown in Figure 3(d), the LED 3 that is mounted at the bottom end of the joystick is driven by the output of an operational amplifier 21 which has a phototransistor 22 connected in a negative feedback circuit such that any variations in the LED output are sensed by the phototransistor 22 and compensated automatically. The phototransistor 22 can, for example, be mounted within the joystick shaft 2 and optically coupled with the light output of the LED by means of a suitably formed optical waveguide in the form of a clear plastics mounting piece for the LED.

Having thus described the present invention by reference to a preferred embodiment, it is to be appreciated that the embodiment is in all respects exemplary and that modifications and variations are possible without departure from the spirit and scope of the invention. For example, the linearity and precision of the embodiment could possibly be improved, if desired, by provision of additional sensors on the circuit board 8 and by corresponding modification of the expressions for Xa s and Yaxis as given hereinbefore.

Alternatively, a simplified arrangement could be obtained by defining the X and Y axes as being aligned with sensors 10 and 11 and sensors 9 and 12 respectively, rather than as shown in Figure 2, in which case the Xaxis and Ya signals could be defined by the expressions: X"", = OP2 - OP3, and Y = OP1 -OP4.

Furthermore, whilst in the described embodiment the LED 3 is moved by the joystick relative to the array of sensors 9, 10, 11 and 12, the arrangement could alternatively be such that the sensor array was moved by the joystick and the LED 3 was stationary. The above-discussed circuitry could also be appropriately biassed to provide a "dead band" region about the home position of the joystick so that small deviations of the joystick from its precise home position did not provide Xaxis and Yaxis outputs; this would take account of the possibility that when the joystick is displaced and then allowed to return towards the home position it might not always return precisely to the home position.

Claims

CLAIMS: 1 An optical joystick comprising: a light source; a plurality of light detectors; said source and detectors being relatively movable by operation of the joystick so that the outputs of the detectors vary as their illumination by the source varies in dependence upon their changing relative positions; and circuitry responsive to increasing outputs from some of said detectors and reducing outputs on others for deriving signals representative of the movement of the joystick resolved into X and Y components.
2. An optical joystick as claimed in claim 1 wherein there are four detectors arrayed in a square and the light source locates at the centre of the square in the home position of the joystick.
3. An optical joystick as claimed in claim 1 wherein X and Y axes are defined as bisecting respective sides of said square and said circuitry is adapted to derive Xaxis and Xaxis signals as a function of the expressions = = (OP2+OP4) - (OP1+OP3) Yaxis = (OP1+OP2) - (OP3+OP4) where OP1, OP2, OP3 and OP4 are the outputs of the four detectors, the X axis bisecting the side of the square having corners at the detectors whose outputs are OP2 and OP4 and the opposite side having corners at the detectors whose outputs are OP1 and OP3, and the Y axis bisecting the side of the square having corners at the detectors whose outputs are OP 1 and OP2 and the opposite side having corners at the detectors whose outputs are OP3 and OP4.
4. An optical joystick as claimed in claim 3 wherein said circuitry comprises separate portions for effecting said X,= and Yw s derivations, said portions each including a differential amplifier arranged to receive respective outputs of two of said detectors on one of its two inputs and respective outputs of the other two of said detectors on the other of its two inputs.
5. An optical joystick as claimed in any of the preceding claims including temperature compensation circuitry.
6. An optical joystick as claimed in claim 5 wherein said temperature compensation circuitry includes a detector responsive to variations in the light output of said light source and means responsive to the output of said detector for adjusting the power supply to the light source.
7. An optical joystick as claimed in any of the preceding claims wherein said light source is mounted at the end of a pivotally-mounted joystick shaft for movement relative to a base plane carrying said detectors.
8. An optical joystick as claimed in any of the preceding claims wherein the light source comprises a light emitting diode and said light detectors comprise phototransistors.
9. An optical joystick as claimed in claim 8 wherein said light emitting diode is adapted to emit infrared light.
10. An optical joystick substantially as herein described with reference to the accompanying drawings.