GB2357835A

GB2357835A - Detecting the incident angle of an optical light beam

Info

Publication number: GB2357835A
Application number: GB9921510A
Authority: GB
Inventors: Michael P Geissler
Original assignee: Individual
Current assignee: Individual
Priority date: 1999-09-13
Filing date: 1999-09-13
Publication date: 2001-07-04
Also published as: GB9921510D0; EP1218769A1; WO2001020361A1; GB2357835A8

Abstract

An angular sensor 2 comprises an array 5 of optical receiving elements 6, where each element is provided with detection means 7. Each element 6 generates radiation in response to incident light, for example by fluorescence, which is reflected internally within the element and directed towards detection means 7. The sensor is disposed so that the outputs from the detecting means 7 vary according to with the orientation of the sensor relative to the incident light source, e.g. 1a-1c. The elements are preferably laminar, where a broad face receives the incident light and the narrow faces form internal mirrors and may have arctuate receiving faces and/ or be disposed so that the light receiving surfaces face in different directions. The sensor may also include a mask with an aperture 4. A system for calibrating the sensor (figure 7) is also disclosed, whereby sets of reference values corresponding to the detector outputs for a number of positions and/ or orientations of the sensor may be compiled and stored.

Description

2357835 OPTICAL ANGULAR SENSOR

Field of the Invention

This invention relates to optical-type angular sensors that can provide an indication of the angular relationship of the sensor to at least one light source and, in preferred forms of the invention, signals which enable the determination of both the angular orientation and the positional location of the sensor relative to an array of liaht sources.

Backaround to the Invention

Optical angular sensors in a variety of forms are known in the art. For example, United Z.

States patent number 5510893) describes, among other things, a position and posture detecting device which is based on a four division PIN photodiode of which each division can independently receive and detect a quantity of incident light. The patent describes how differences in the incident light on the four divisions may be used to compute the angular relationship between the detector and the source of the light. The patent also describes the provision of a fence alone, the common boundaries of each adjacent pair of divisions in order to promote the variation of light incident on the four divisions with changes in the 0 Z> angular relationship of the source and the sensor.

0 Another device, described in United States patent 5393970, comprises a plurality of light receiving elements each comprising two or more closely spaced planar light detectors 1 having their light receiving surfaces facing in different spatial directions. Again, three dimensional location may be determined from signals representing the relative light intensities detected by the light detectors.

Summary of the Invention

The main object of the invention is to provide an improved optical angular sensor, and preferably an optical position and posture sensor.

2 The invention is based on the use of at least two and preferably at least three light collecting elements which respond to incident light to produce electromagnetic radiation (such as light) which is internally reflected by the element to detecting means preferably disposed adjacent a narrow face of the element. The elements may be configured in a variety of ways to cause the outputs of the detecting means to vary in accordance with the ancrular relationship between the elements of the device and the light source. The device may include a shadow mask or other means above or adjacent the elements.

The elements may comprise a fluorescent material, so that light is absorbed by the Z.

material and in response to the incident light is generated, normally at a wave length 1 c greater than the incident light. However, materials employin some other physical W 9 mechanism for the generation of electromagnetic radiation in response to the incident light might be employed.

In one form of the invention, the elements are in the form of flat laminas which are arranged in substantially a common plane and which have their narrow faces formed as internal mirrors. In such an embodiment the device may include a mask defining an aperture, such as a circular aperture, over the array of detectors. However, a different embodiment includes a plurality of such planar elements disposed in different planes, which may be orthogonal planes but need not be. In a yet further embodiment, the lamina elements may comprise part of a spheroidal or ellipsoidal shell.

In all the above forms of the invention, it is generally possible to ensure that the outputs from the detecting means are substantially free of cross-talk and may be sampled for each of a multiplicity of positions of a light source relative to the device so that a look up table can be compiled relating such angular relationship to a respective one of a large multiplicity of sets of values of the outputs of the detectors. Moreover, such a device may be used in conjunction with an array of light sources which may be (operated at different frequencies) whereby the angles between the device and various light sources may be obtained. From those angles position and orientation of the device may be calculated.

Calibration of the angle sensing by means of the compilation of a look up table may readily be accomplished.

3 In general, the invention is intended to provide at least one and preferably more than one of the following advantages, namely: a better signal to noise ratio, an extended dynamic range, increased discrimination against ambient light sources, the avoidance of any need Z> C) C1 for a lens, since the elements may be made substantially larger than, for example, segmented photodiodes. the use of aenerally inexpensive material., increased sensitivit t) 1 Z y and the absence of cross-talk between light receiving elements.

1 Examples of the invention will be described in more detail in the following, and also with reference to the accompanying drawings.

Brief Description of the Drawing

Figure 1 is a general drawing illustrating the use of optical angular sensors with an array Z.

of light sources.

Figure 2 illustrates schematically one embodiment of the invention.

Fl. re 3 illustrates in part the operation of one embodiment of the invention.

U J 1 Figure 4 illustrates part of one embodiment of the invention- Fiaure 5 is one view of a sin-le licht receiving element used for preference in the present invention., and 25 Figure 6 is a plan view of a single light receiving element in accordance with the invention.

Figure 7 illustrates a calibration rig. 30 Figure 8 Is an explanatory diagram.

4 F1y-iire 9 Illustrates a control and processill. Systelli.

Detailed Description of Preferred Embodiment

Ficure 1 of the drawinas is a ceneral schematic view illustrating one mode of use of a c Z sensor accordino, to the invention. The drawing illustrates an array of light sources 1, W which in this particular example are disposed in some selected or arbitrary pattern over a c fi 1 ei ino or top cover of an enclosure. The 1-ht sources could be disposed on a suspended It> C1 gid outdoors. An optical angular sensor 2 has a multiplicity of light receiving elements, ri not specifically referenced in Figure 1, of which the outputs collectively vary as the position or orientation of the device 2 varies relative to the array of light sources. The outputs obtained from the light receiving elements are coupled to and processed in control and processing circuits 3) which may also control, if desired, the light sources 1. One manner of control is to vary the intensities of the light sources at different frequencies so is that the outputs from the various light receiving, elements can simultaneously, by means of respective frequency components, indicate the angular position of the sensor 2 In relation to a multiplicity of the light sources. The circuits 3 may include discriminators to separate the signals components relevant to the respective light sources 1, and processing circuits which relate positional and angular values for the sensor (obtained by a suitably controlled mechanism) to sets of values of the sensor's outputs. Figure 1 includes two other angular Z_ sensors 2a and 2b in different positions and orientations.

In eneral, the light receiving elements, with or without the aid of a mask as will be 9 W described, is disposed so that light from any particular incident light source is distributed to the elements in different portions depending on the angular orientation of the sensor relative to the given light source. Although it would be possible to calculate this angular relationship based on the various proportions of light received by the light receiving Z W elements and the particular geometric arrangement, it is generally preferable to adopt a calibration scheme wherein the sensor 2 is moved to a variety of positions, and set in a variety of different orientations, and the various sets of outputs from the light receiving 0 elements for each positional combination of position and orientation are memorized in the form of a look up table. Then, in use of the system, the set of outputs from the light receivinu elements can be matched against an entry in the look up table to provide an immediate read out of the position and/or angular orientation of the device 2. The latter scheme has the advantaee of avoiding any need for calculation of values and also permits W. =I the automatic correction of imperfections.

The sensor accordina to the invention may In general be employed with a wide variety of light sources, though it is preferable to employ light emitting diodes which may be operated to modulate their light outputs at a selected frequency. The circuitry 3 may of course include, depending on the coding, analog or digital filtering enabling the demodulation of the emitted light and the identification of each light source accordingly.

As indicated herein before, it is known to employ segmented photodiodes as light receiving elements in a context such as that shown in Figure 1. The electronic noise of known light receiving elements increases in proportion to its surface area and accordingly a sensor sensitivity can in general not be increased by increasing the surface area.

Furthermore, a lens is often used to collect greater quantities of light. If light emitting diodes are used at substantial distances from the sensor 2, power has to be increased in accordance with the square of the distance between source and sensor. Brightly light emitting diodes can illuminate the surroundings in undesirable ways.

Known photo sensors generally have a limited range of linear response properties. In such a range the optical power input is directly proportional to the electrical power output. Thus if too much light is incident on a photo sensor, the region of linear response may be exceeded. As a result, known optical angular sensors can only be used within comparatively narrow ranges of optical intensity.

Broadly, the intention of the invention is to enable a substantial increase of the light receiving area without increasing the surface area of a photo sensor. Thereby the signal to 1 1 noise ratio can be increased, allowing higher sensitivity and accuracy, without the need for a lens or more powerful light sources. Thus size and production costs of sensing systems can be reduced.

6 Fl-ure 2 illustrates in general form one embodiment of the invention. The sensor 2 1 IS positioned to receive light from an array of three light sources I a, I b and I c. The sensor _n has an enclosure of which the top wall defines an aperture 4. Although this might be a simple aperture, it would be preferable to dispose a filter such as an ultra-violet filter, as a window in the aperture.

The sensor 2 includes an array 5 of light receiving elements of which one is shown at 6.

As soon to be described, each lleht receiving element generates, in response to the incident light, electromagnetic radiation, preferably in the optical range (by means for example of fluorescence) and each element 6 is associated with a respective detecting means 7 which may be a photodiode.

Flaures, 3 and 4 illustrate one way in which the angular orientation of the sensor relative to the light source may be used to obtain different outputs fi---omthe light receiving elements.

Figure 3 Is a side view whereas Figure 4 is a plan view of an array of four light receiving elements 6a, 6b, 6c and 6d, each of which is in the form of a lamina and has a respective one of a multiplicity of photodiodes (7a-7d) disposed adjacent part of the narrow side face of the lamina. Each photodiode may be bonded to the respective element at a corner thereof Flaure 3) illustrates a light beam 8 from a source (not shown) entering an enclosure 13 obliquely by way of an aper-ture 4 in a mask 10. It therefore illuminates the four light receiving elements in a region 11, The light receiving elements are illuminated differently.

Z_ It may be remarked that it is possible to construct a sensor of this kind employing only two Z.

elements, such as the elements 6a and 6b, thouah a sensor of that kind can in general only sense variation of angle in one plane. In general, at least three light receiving elements are desirable.

Figures 5 and 6 illustrate the manner of operation of a single element for an improved 1 1 sensor according to the invention. Each element 6 is in the form of a lamina or plate of transparent material. The plate has broad fces 12 and narrow faces 13). The narrow faces, 7 except in the region which is adj 've photo sensor 7, are treated to form -71 acent a respect] internal mirrors. Light 8 incident on the plate 6 Is absorbed. The material of the element 6 may, for example, contain fluorescent dye which in response to the incident light and emits light 15 that travels towards the photo sensor 7 either directly or after reflection by the mirrored edge faces or by reflection at the broad faces 12. If the material Is fluorescent, then the llcht emitted by the dye will have a longer wave lenerth than the "dent radiation. This has the advantage that photo sensors are more sensitive as ncl 1 1 wavelength increases.

The element 6 also acts as an optical band pass filter. It Is only sensitive at a particular wavelength. This eliminates the needs for a costly separate band pass filter.

It will be apparent that the lamina or panel 6 can be made of substantial size, since the area is no longer limited by that of the photodiode 7Moreover, the use of a fluorescent element enables the light received and emitted by the fluorescent panel to be distributed equally over the surface of the photo sensor 7. There are no hot spots because light is distributed over a larger area. Furthermore, the sensor may be used at higher levels of light than may be used when the operating range is constrained by the linear range of response of the photodiode.

The collector 6 may comprise an acrylic matrix containing any suitable known dye which exhibits fluorescence.

In the foregoing, the array of light receiving and generating elements is described in Z conjunction with some mask that allows a defined beam of light from a light source to fall, in general, differently on the various elements. The mask may define a variety of apertures of different shape. However, a mask is not essential in all embodiments of the invention.

In particular, a panel comprising an element 6 and its photo sensor 7. may for example be Z disposed on each of a plurality of the faces of a cube. In general, an array of panels such as shown in Figure 6 may be disposed in different planes (either orthogonal or not) and althou-h various forms of mask may be employed, they need not be.

W 8 Furthermore, each ll(ylit receivins element, though larninar, need not be planar. 111 particular, the elements may be disposed as different regions of a spherical or ellipsoidal shell or as different regions of a cylindrical surface. Provided that the outputs obtained from the various light receiving, elements vary as the orientation of the array of elements varies relative to the incident light, any selected configuration of the light elements may employed.

Figure 7 illustrates a calibration rig, comprising a source 1, a sensor 2, a motorised arm 16 which carries the sensor 2 and can move and measure angular position about a first axis and a motorised arm 17 which can move and measure angular position about a second axis. The rig includes spacing bars 18 and 19. The rig includes optical encoders (not shown) for the two axes. The sensor is mounted at the nodal point of the gimbal. The sensor output and the relevant angular position are referenced in a table. Known calculations may be employed to obtain position and orientation information 'In 3D space.

Figure 8 illustrates the circumstances wherein a dished light receiving panel may be desirable to provide precise and angular readings even if a light source is very close. The lines 22 represent parallel rays of a distant light source whereas the lines 23) are divercrent ZD rays from a nearby light source. Line 24 is a centre line of the light sources at the same angle relative to the element 6a. It may be noted that the distance 20 Is not equal to the distance 21 because the angles 2i and 26 are different. Compensation for this inequality may be provided by using dished or arcuate panels.

=1 Figure 9 illustrates the main schematic features of one example of the control and processing system. The system may include four LEDs each on a different frequency. This may be a multitude of LEDs, fitted with microprocessors so that each of them can be controlled to flash on one of the frequencies in use, dimmed (so that a viewing person would not be distracted by LEDs switching on and off) or be turned off completely. The system processor would be in charge of choosing the LEDs in 'view' by the sensor and allocating the right frequency to them.

9

Claims

Claillis

1. An optical angular sensor comprising a plurality of optical receiving elements (6), each of which in response to incident light generates electromagnetic radiation which is reflected internally of the element, and for each element a respective detecting means (7) which responds to the said radiation, the sensor being disposed so that the outputs from the detecting means vary in accordance with the angular orientation of the sensor relative to the 'Incident light.

2. A sensor according to claim 1 wherein each element (6) comprises a lamina of which a 1.7 broad face (12) receives the incident light and of which the narrow faces constitute internal mirrors (14), and wherein the respective detecting means is disposed to receive the radiation from part of a narrow face of the element.

3 3. A sensor according to any foregoing claim wherein the detecting means (7) comprise a 1 g 1 C> photo sensor bonded to said part of the narrow face.

4. A sensor according to any foregoing claim wherein the elements (6) comprises fluorescent material and said radiation is optical.

5. A sensor according to claim 4 wherein each element (6) comprises a transparent synthetic plastic containing a fluorescent dye.

6. A sensor according to any foregoing claim wherein the elements (6) are disposed to 1 1 Z) face in different directions.

7. A sensor according to any foregoing claim wherein each element (6) has a arcuate receiving face.

1

8. A sensor according to any of claims 1 to 6 wherein each element (6) is planar.

9 A sensor accordin. to claim 8 wherein the elements (6) are disposed in substantially a common plane.

10. A sensor according to any foregoing clairn wherein the sensor includes an apertured mask ( 10) disposed in the path of the incident light.

11. An optical angular sensor comprising a plurality of substantially coplanar optical receiving. elements (6), each of which in response to incident light generate electromagnetic radiation which is reflected internally of the element, and for each element a respective detecting means (7) which responds to the said radiation, the sensor being disposed so that the outputs from the detecting means vary in accordance with the angular orientation of the sensor relative to the incident light, wherein each element comprises a lamina of which a broad face (12) receives the incident light and of which the narrow faces constitute internal mirrors (14), and wherein the respective detecting means is disposed to receive the radiation from part of the narrow face of the element and the sensor includes an apertured mask ( 10) disposed in the path of the incident

12. A position sensing system comprising an array of light sources (1) in fixed positions W and a sensor according to any foregoing claim.

13. A position sensing. system according to claim 12 and including means (3) for storing and retrieving a multiplicity of sets of values of said outputs wherein each set corresponds to a respective position and/or orientation of the sensor relative to the said sources.

14. A position sensing system according to claims 12 or 13 including means for W modulatinc, the intensity of said light resources at respective distinct frequencies, and the means for storing includes discriminators which separate signals relating to said light sources according. to the frequency thereof