WO2001020361A1

WO2001020361A1 - Optical angle sensor for use in a position and/or attitude determination system

Info

Publication number: WO2001020361A1
Application number: PCT/GB2000/003509
Authority: WO
Inventors: Michael Paul Alexander Geissler
Original assignee: Geissler Michael Paul Alexande
Priority date: 1999-09-13
Filing date: 2000-09-13
Publication date: 2001-03-22
Also published as: GB9921510D0; GB2357835A8; GB2357835A; EP1218769A1

Abstract

An optical angular sensor comprises a plurality of optical receiving elements (6), each of which is preferably fluorescent. In response to incident light each element generates light which is reflected internally of the element. For each element a respective detecting means (7) responds to the said generated light. The sensor is disposed so that the outputs from the detecting means vary in accordance with the angular orientation of the sensor relative to the incident light. In one embodiment, 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 the respective detecting means is disposed to receive the radiation from part of the narrow face of the element. The sensor may include an apertured mask (10) disposed in the path of the incident light.

Description

_OPT_ICAL _ANGLE SENSOR FOR USE IN A POSITION AND/OR ATTITUDE DETERMINATION

SYSTEM

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 light sources

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

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 along 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 angular relationship of the source and the sensor

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 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

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

The invention is based on the use of one or more 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 narrow faces of the element or elements, which may be configured in a variety of ways to cause the outputs of the detecting means to vary in accordance with the angular relationship between the device and the light source The device may include a shadow mask or other means above or adjacent the element or elements

Each element may comprise a fluorescent material, so that light is absorbed by the material and in response to the incident light is generated, normally at a wave length greater than the incident light However, materials employing some other physical mechanism for the geneiation 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 fiat laminas which are arranged in substantially a common plane and which have their narrow faces formed as internal mirrors The device may include a mask defining an aperture, such as a circular aperture, over the array of detectors However, a different embodiment may include 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

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 for a lens, since the elements may be made substantially larger than, for example. segmented photodiodes, the use of generally inexpensive material, increased sensitivity and the absence of cross-talk between light receiving elements

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

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

Figure 2 illustrates schematically one embodiment of the invention.

Figure 3 illustrates in part the operation of one embodiment of the invention

Figure 4 illustrates part of one embodiment of the invention

Figure 5 is one view of a single light receiving element used for preference in the present invention; and

Figure 6 is a plan view of a single light receiving element in accordance with the invention.

Figure 7 illustrates a calibration rig

Figure 8 is an explanatory diagram

Figure 9 illustrates a control and processing system.

Fi Ogu"- res 10 and 11 illustrate another embodiment of the invention

Figure 1 of the drawings is a general schematic view illustrating one mode of use of a sensor according to the invention The drawing illustrates an array of light sources 1, which in this particular example are disposed in some selected or arbitrary pattern over a ceiling or top cover of an enclosure The light sources could be disposed on a suspended grid outdoors. An optical angular sensor 2 has a multiplicity of light receiving elements, 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 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 sensors 2a and 2b in different positions and orientations.

In general, the light receiving elements, with or without the aid of a mask as will be described, is disposed so that light from any particular incident light source is distributed to the elements in different poπions 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 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 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 receiving 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 advantage of avoiding any need for calculation of values and also permits the automatic correction of imperfections.

The sensor according 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 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 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.

Figure 2 illustrates in general form one embodiment of the invention. The sensor 2 is positioned to receive light from an array of three light sources la, lb and lc. The sensor has an enclosure of which the top wall defines an apeπure 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 light 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

Figures 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 from the 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.

Figure 3 illustrates a light beam 8 from a source (not shown) entering an enclosure 9 obliquely by way of an apeπure 4 in a mask 10 It therefore illuminates the four light receiving elements in a region 1 1, The light receiving elements are illuminated differently.

It may be remarked that it is possible to construct a sensor of this kind employing only two elements, such as the elements 6a and 6b, though 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 sensor according to the invention. Each element 6 is in the form of a lamina or plate of transparent material. The plate has broad faces 12 and narrow faces 13. The narrow faces, except in the region which is adjacent a respective photo sensor 7, are treated to form 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 light emitted by the dye will have a longer wave length than the incident radiation. This has the advantage that photo sensors are more sensitive as wavelength increases The element 6 also acts as an optical band pass filter It is sensitive only at a paπicular 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 7 Moreover, 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 Fuπhermore, 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 element 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 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 paπicular, a panel comprising an element 6 and its photo sensor 7 may for example be 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 although various forms of mask may be employed, they need not be

Furthermore, each light receiving element, though laminar, need not be planar In 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 l, 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 divergent 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 25 and 26 are different Compensation for this inequality may be provided by using dished or arcuate panels

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

In the system shown in Figure 9, each LED in an array 90 of LED's is coupled to a respective channel in a plurality of processing channels 91 Each channel includes, for example, a photosensor 92, a preamplifier 93, a high pass filter 94 and an analogue-to- digital (AD) conversion stage 95 Outputs from the stages are subject to further processing including a compensation for lack of proportionality, in a processing unit 96 The processed digital outputs from the channels are employed to compute angles in an 'angle calculations' block 97 (which may be constituted by software) subject to calibration data 98 Positional and orientation calculations are performed by block 99 An LED coordinator 100 (the system processor) may receive feedback from block 99 and serves to control a respective modulator 101 for each LED Another embodiment is shown in Figures 10 and 1 1 of which Figure 10 is a sectional side view and Figure 1 1 is a plan view. In this embodiment the sensor is composed of a single element 6 and a plurality of detectors 7a to 7d. The detectors are disposed in a manner similar to those in Figure 4 but the single thin planer element 6 is a single panel filled with fluorescent pigment and has the detectors in the form of photodiodes each adjacent part of a narrow face (in this case a corner facet) of the element 6. The light within the element 6 will be alternated as it travels to the photosensors. The alternation may be controlled by roughening the surface of the element and/or altering the concentration of the pigment or other optical alternating material within the element.

In other respects the embodiment in Figures 10 and 11 resembles the one shown in Figures 3 and 4. The incident optical beam 8 enters enclosure 9 by the aperture 4 in the mask 10 to illuminate region 1 1 of element 6. The outputs from detectors will vary according to the position of region 11.

Claims

1. An optical angular sensor comprising at least one optical receiving element (6), which in response to incident light generates electromagnetic radiation which is reflected internally of the element, and a plurality of detecting means (7) which respond 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 the or each element (6) 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 each detecting means is disposed to receive the radiation from a respective part of a narrow face of the element.

3. A sensor according to any foregoing claim wherein the detecting means (7) comprise a photo sensor adjacent said respective part of the narrow face.

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

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

6. A sensor according to any foregoing claim wherein there is a plurality of elements (6) disposed to face in different directions.

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

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

9. A sensor according to claim 8 wherein a plurality of elements (6) are disposed in substantially a common plane.

10. A sensor according to any foregoing claim 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 co-planar 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 light.

12. A position sensing system comprising an array of light sources (1) in fixed positions 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 modulating 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.