GB2231951A

GB2231951A - Detection apparatus and methods

Info

Publication number: GB2231951A
Application number: GB8904718A
Authority: GB
Inventors: Christopher Girling
Original assignee: I E I
Current assignee: I E I
Priority date: 1989-03-02
Filing date: 1989-03-02
Publication date: 1990-11-28
Also published as: GB8904718D0

Abstract

The detection of, for example smoke particles in air is effected in a sampling chamber (25) which is elliptical in transverse cross-section by directing a light beam (32) along the longitudinal axis passing through one focal point of the chamber. Light deflected (scattered) by particles in the path of the beam is multiply reflected from the chamber wall and is directed to a collector or collectors (31, 31') positioned in a plane or planes which includes the major axis of the ellipse. The light source (28) is preferably a semiconductor laser whose intensity is modulated, for example sinusoidally. The apparatus may be used for the detection of low levels of air pollution. <IMAGE>

Description

DETENTION APPARATUS As METHODS SPECIFICATION This invention relates te apparatus for the detection of matter as a disperse phase in a translucent, gaseous, continuous phase, for example the detection of smoke in air. The invention also relates to methods of detection of such matter. The invention is applicable for example to the detection of low levels of air pollution where the pollutant forms a colloidal solution with the air or is a suspension in the air. it is particularly suitable for use where the particle sizes vary from between 0.1 to 10 microns, such as occur in smoke.

The invention is based upon continuous sampling, or repeated samplIng, of the phases present within an optical chamber in which the degree of scatter of a light beam is measured, that degree of scatter being taken as an indication of the concentration of the disperse phase in the continuous phase.

The present invention is concerned primarily with three aspects of the apparatus and method used for carrylng out such procedures. These are: 1. The manner in which the light from the source is introduced into the chamber, and the devices which are used; 2. The manner in which scattered light is collected within the chamber and directed towards a sensor; 3. The nature of the light itself and the electronic circuitry used to produce the light.

In the accompanying drawings, Figs. 1 and 2 are longitudinal sectional views through two alternative prior art constructions of apparatus used for particle detection. In Figs. 1 and 2 the main body 10 of the chamber is of tubular form of no critical crosssectional shape. The disperse and continuous phases are introduced into the chamber through an inlet 16 and leave the chamber through an outlet 12. A light source 13 in a light-prcof housing 14 are provided. Also provided are a lIght sensor 15, together with a lens 16 which focuses light scattered from the sensing zone 17 on to the sensor.

In Fig. 1, the phases are obliged to move through an aperture 18 in a transverse barrier 19 in order to move from the inlet 11 to the outlet 12. A lens 20 focuses light from the light source 13 on to the phases as they pass through the aperture 18. None of this erect light is towards the lens 15 and therefore none of this light is directed to the sensor 15. Thus, in the absence of a disperse phase within the sensing zone 17 to divert the course of the light, no light is detected by the sensor 15. When a disperse phase is present, light will be scattered by it while the disperse phase is present within the sensing zone 17.

Some of tis scattered light will be in the direction towards the lens 16 and will be focused on to the sensor 15. The transverse barrier 19 and the lens 16 are required in order to restrict the field of view of the sensor 15 and thus obscure it from light resulting from spurious reflections off the internal wall surface of the main body of the chamber. The light sensor 15 is focused at a point F within the sensing zone 17 for optimum effect. Due to multiple reflections within the main body of the chamber it is difficult to increase the angle of the axis of the detecting assembly 15, 16 such that the angle of direction of the beam is greater than 1500 without spuriously reflected light entering the detecting assembly. The axis of the detecting assembly and the direction of the beam exist in one plane only.The light source 13 is an incandescent lamp in order to facilitate focusing by the lens 20.

The sensor 15 is a photomultiplier photo-electric element in order to facilitate detection of light of very lo intensit.

In Fig. 2 the light source 13 is a xenon tube which gives a short flash of very high intensity. Due to the nature of a xenon tube it is impossible to focus its light emission accurately. The xenon tube 13 is therefore housed on the side of the main housing 10 and its emitted light is restricted to that part of the main chamber which is to the right-hand side of the transverse barrier 19, as viewed in Fig. 2. The light sensor 15 is focused at the focal point F within the sensing zone 17 by the lens 16. The sensing zone F is tat part of the field of view of the sensor 15 which is directly crossed by the light from the light source 13. A light trap 21 is situated at the end of the field of view in order to avoid spurious reflections from the light source 13 from reaching the sensor 15.

The angle between the main axis of the light source 13 and the direction of sensing by the sensor 15 is 900, and both the main axis of the light source and the direction of sensing exist in one plane only. The disperse and continuous phases pass through the zone 17 and in the absence of a disperse phase no light will be scattered towards the lens 16 or to the sensor 15.

When a disperse phase is present, then light will be scattered and a portion of the scattered light will be in the direction towards the lens 16. Due to the very high intensity of the light source, namely the xenon tube 13, the sensor 15 does not need to be particularly sensitive in this embodiment and may be a semiconductor diode for example. However, since the duration of the light pulse is very short, the diode must be a fast acting diode, such as a PIN diode.

In order to obtain a satisfactory signal-to-noise ratio, these prior art constructions require the use of either a very high intensity light source, such as a xenon flash tube for example, and/or a sensor with a high sensitivity, for example a photo-multiplier tube.

These components have their disadvantages. Both require high voltage supplies in order to operate them, and this is a disadvantage in some hazardous areas.

Both are made in glass envelopes which require some mechanIcal protection, and they are difficult to place accurately within an optical system. High intensity light sources tend to have a limited life when used in certain otherwise advantageous modes, and also suffer from deterioration with age. A xenon tube will only work in very short flashes, and its emitted light is difficult to focus accurately. Therefore, the signal to-noise ratio resulting from its use cannot be enhanced by the use of narrow-band filters, and it is difficult to direct its light so as not to affect the sensor adversely, either directly or from unwanted reflections. Due to these difficulties, existing constructions allocate a volume within a housing on which to focus the light source, and the scattered light from this volume, i.e. sensing zone, is focused by means of spherical lenses on to a sensor. The sensing zone or volume has to be small in order to be well defined, and the angle to the path of the light beam at which the sensing takes place cannot approach 1800. 1800 is the preferred angle because that is the angle at which the scattering is the strongest and most consistent with particle size. Moreover, the angle at which detection of the scattered light takes place and the path of the light source beam exist in one plane only. Within this plane the field of view of the sensor is severely limited. The limitations are the result of the need to prevent the sensor from detecting spurious reflectIons.

Fig. 3 of the accompanying drawings is a polar diagram, which is purely illustrative, but which serves to show the form or configuration of the intensity of light scatter from particles of one size which are illuminated with light at a single wavelength which is of the same order as the particle size and in the single direction of 1800. The continuous curve 22 and the dotted curve 23 illustrate the effect of a variation In particle size. These curves should be rotated continuously round the 1800 axis in order to describe a three dimensional volume in order to represent a more exact illustration of the scatter.In a more practical case, where the particle size and the wavelength of the light may cover a range of dimensions, a different polar diagram would result, which would be the integration of many polar diagrams similar to Fig. 4, in which the relative proportions of light at a specific wavelength to the total light on the one hand, and the relative proportions of the number of particles of a specific size to the total number of particles on the other hand, are taken into account. However, certain generalisations can be made from this illustration.

1. Light is scattered in all planes which include the 1800 axis.

2. Light is scattered consistently in the 1800 direction, regardless of particle size and regardless of the wavelength of the light source.

3. Light scatter outside the main lobe in the 1800 direction is dependent on particle size and on the wavelength of the light. This is particularly so for angles less than 1200.

It is an object of the present invention to provide an improved apparatus and method, with a view to avoIding the disadvantages of the prior art systems discussed above, and taking into account the properties and features of light scatter discussed above.

It is an object of the invention to provide an apparatus and method which provides for increased efficiency and consistency of performance as compared with prior art apparatus and methods. In order to make the most efficient use of the power available from the illuminating source, it is desirable that light scatter in all planes which include the main axis of the illuminatIng beam should be redirected towards a sensor.

It is a further object of the present invention to provide a detector which is based upon the geometrical properties of an ellipsoidal reflector, namely that all rays emanating from a source at one focus are reflected and come together in a plane which includes the major axis of the ellipse.

Broadly in accordance with one aspect of the present invention there is provided a detector comprising a sampling chamber which has an internal light-reflecting surface of elliptical or substantially elliptical cross-section over at least part of the length of the chamber, a light source arranged to direct a light beam along one axis of the elliptical cross-section portion of the chamber and light collecting means positioned in a plane which includes the major axis of the ellipse to collect light which has been reflected from said chamber surface after having been diverted from the light beam.

Broadly in accordance with another aspect of the present invention there is provided a method of detecting particles within a sampling chamber, which comprises directing a light bean along one axis of a sampling chamber which is at least partially elliptical or substantIally elliptical, and reflecting light diverted from particles in the path of the beam towards light collecting means positioned in a plane which includes the major axis of the ellipse.

The apparatus and method of the present invention far more nearly achieve the requirements for an efficient detector than prevIous apparatus and methods.

Preferably, a light source is used which can be modulated at a frequency which will allow the resulting signal to be separate from spurious signals, such as those generated by the sensor (regardless of anv illumination), the light source and the associated circuitry, especially those signals generated at very low frequencies.

Preferably, in order to reduce the dependence of the signal on particle size, those angles of scatter which are between about 1650 and 1800 should be redirected towards the sensor, i.e. those angles of scatter which are included in the main lobes shown in rig. 3 around the 1800 direction.

In order that the invention may be more fully understood, one presently preferred embodiment of apparatus and method in accordance with the invention will now be described by way of example, and with reference to Figs. 4 to 7 of the drawings. In the drawings: Fig. 4 is a longitudinal sectional view through the preferred embodiment of detector; Fig. 5 is a transverse cross-sectional view through the detector taken along the line V-V of Fig. 4; Fig. 6 illustrates one practical embodiment of the apparatus shown in Fig. 4; and, Fig. 7 is a block schematic diagram illustrating the circuitry associated with the light source.

As shown in rig. 4, the detector is broadly similar to that shown in FIgs. 1 and 2, in that it comprIses a housing 25 which defines a chamber 25. The chamber 25 has an inlet 26 and an outlet 27 for the sample which is being monitored. The detector also includes a light source 28 with a focusing lens 29 and a light trap 30 at the far end of the light source axis. A light sensor 31 is provided, with a field of view along an axis which is parallel to the light source axis, but the sensor here has no spherIcal lens to restrict its field of view.The position of this sensor 31 is here shown at the same end of the housing 24 as the light source 28, although in alternative embodiments one could either provide a second sensor 31' at the far end of the housing, or alternatively use just the one sensor 31 instead of the sensor 31.

The light source 28 is focused at a point inside or outside the chamber 25, but in such a way that none of the light falls on the inside surface of the housing 24 and is all directed into the light trap 30. Ideally, the light is focused at infinity, as illustrated by the beam 32 in Fig. 4. In order to achieve such a fine degree of focusing, a monochromatic, coherent, point light source is required, such as may be approximated by a semiconductor laser for example. The light trap 30 should be sufficiently efficient to ensure that the power of light re-emitted will be at least 70dB less than the light which enters the trap. A light sensor placed at any position within the chamber 25 except in the direct path of the light source beam 32, and with any field of view, will only detect light re-emitted from the light trap or light scattered from the light source beam. This removes the restrictions on the light-gathering method, such as those in the prior art constructions discussed above, which allow light in only one plane, which includes the axis of the light source beam and is centered around only one main angler to be redirected towards the sensor.

An important feature of the apparatus of the present invention is shown most clearly in Fig. 5. The housing 24 which defines the sampling chamber 25 is onstructed with an elliptical or approximately elliptical cross-sectional shape for its interior surface :3. This interior surface 33 is highly reflective at the wavelengths of the light source. The following description assumes that the internal wall surface 33 is a perfect ellipse, but it should be understood that the invention is also applicable to a construction where the internal wall surface approximates to an ellipse, such as may be described by eIght or more straight lines tangential to a true ellipse.The ellipse has two focal points at Fl and 2. In such a construction, any path of a light ray which passes through or nearly through, or which originates from or nearly from, one longitudinal axis of a focal point, at any angle in either the longitudinal or a cross-sectional plane, will, if it then strikes the elliptical surface, be reflected in such a way that its new path will be through, or nearly through, the longitudinal axis of the other focal point. Also, if such a light ray strikes a surface other than the elliptical surface, and if-the normal at the point of incidence is in the same plane as the axis of a focal point through which it has passed, then the resulting reflection will be as though it had passed through that axis of a focal point, but at a different point.Thus, if the end walls of the chamber 25 are constructed so that the normals at any point are in a plane that includes the axis of one focal point and also in a plane that includes the axis of the other focal point, then the light will be reflected from these end walls, from a ray whose path has crossed or originated from the a,is of a focal point, such that the new direction of the path of the ray will be through the axis of the other focal point. Such a light path is indicated at P1 in Fig. 4.Moreover, if the wall 34 which defines the entrance to the light trap 30 at the end of the light source beam 32 has a surface whose normals at any point are in a plane which includes the axis of the focal point 1, then any light on a path which originates from, or near, or which passes through or close to the axis of the focal point 1, will be reflected from it on a new path as though its direction had originated from or near or had passed through or close to that axis. The simplest such shape for the wall 34, other than a disc, is a cone. Such a light path is indicated at P2 in Fig.4.The purpose of this sloping wall 34 is to transform the angle of a light path which is very nearly that of the light source beam to one which will strike the elliptical wall of the chamber and still be as though it had passed through or near the axis of the focal point F1.

As can also be seen from Fig. 5, light paths which are constrained by the configuration of the chamber to have passed through or near, or originated at or near, one focus, and which must therefore be reflected to pass through or near the other focus, will during a succession of such reflections approach nearer and nearer to being reflected in a plane which includes the major axis of the ellipse. This can be shown mathematically, but is also evident from simple inspection of the illustration shown in Fig. 5, where a succession of reflections are shown in order, numbered from 1 to 5 and originating from the focal point F1.

Both mathematically and from inspection, it can be determined tat the number of reflections required before a given path from the focal point 1 aligns itself in a plane whIch is within a given angle of the plane including the mayor axis, is a function of the angle of the original longitudinal plane in which the path exists relative to the plane which includes the mayor axis, and the ratio of the lengths of the major and minor axes of the ellipse.

On the basis of these facts, a light source beam 32 whose axis in the direction of travel coincides with the axis of a focal point of an ellipse whose crosssectional shape is that of the chamber, such as that shown in Fig. 4, and whose light is scattered, such as by a disperse phase in a translucent continuous phase, will have its scattered light reflected in such a way that within a certain number of reflections from the Internal wall surface 33 of the chamber, no matter what the original angles of the planes were, the scattered light will be moving in a plane which is nearly coincident with a plane which includes the major axis of the ellipse. It therefore follows that such paths will be incident on the elliptically shaped end walls of the chamber close to a line which is the major axis of the ellipse, and can be said to illuminate it.If it is assumed that the angles in a cross-sectional plane at which scattering occurs are purely random, then the path along the major axis which is illuminated will be evenly illuminated in that direction, and a sensor placed on that path will detect the illumination in direct proportion to that part of the total length of the major axis which the sensor covers.

Referring now to Fig. 6, this shows one practical embodiment of a detector constructed according to the principles shown in Figs. 4 and 5. Here, the chamber 25 in the main body is elliptical in cross-section, and the main body 24 is constructed from 180 linear parts each tangential to a true ellipse. The light source 28 ts a collimated light assembly which is held in a rousing 35 and which IS fixed in position by a wetais g cover 36 so that the beam which it emits is directed through an iris 37.The size of the aperture in the iris 37 is slightly greater than the diameter of the diameter of the collimated beam and removes most of those elements of the beam which are not truly collimatec. The main body 24 is accurately machined so that the light beam passes through a hole in a spoiler 38 and into the light trap which here comprises two coaxial tubes 39 and 40. The path of the light beam rom the source 28 is directed along the axis of the upper focal point of the ellipse. The interior wall surface of the main body 24 and the surface of the spoiler 38 are polished and plated in order to improve the efficiency of reflection. The sample to be monitored is introduced into the housing 24 through holes 41 in the housing wall and which are positioned on the minor axis of the elliptical cross-section of the main body. In this way these entrance holes 41 cause minimum interference to the required reflections.

The sample is moved by an air suction devIce attached to the end of a pipe 42 which draws the sample through the inlet holes 41, the rate of flow being sufficiently high to cause enough turbulence at this point to maximise the speed of dispersion of the sample throughout the chamber 25. In this embodiment the light sensor 31 is positioned at the same end of the housing 24 as the light source 28, and is such that it covers part of the major axis of the elliptical end wall, which is hereinafter called the focal line. A second light sensor in the same wall as the light trap 39, 40 is not used in this embodiment. The spoiler 38 reflects light directly scattered at angles of 178.50 to 150 from 50% of the length of the illuminating beam in the main body.Due to the angle of the spoiler 38, this light is directed back along the main body at an ankle of 40 +/- 7,50 to the elliptical walls of the chamber in a longitudinal plane, and these rays are then reflected a number of times, depending upon their exact angles, from the internal wall surface of the main chamber before reaching the end wall 43 where the light sensor 31 is mounted. These scattered rays are therefore focused, as described above, on the focal line. night which is scattered at an angle which takes it directly to the plane parts of the end wall 44 where the light trap is situated, i.e. not the spoiler 38 or the sample outlet 42, is reflected similarly from that end wall to be focused eventually on the focal line.

tight which is scattered at an angle which takes it directly to the elliptical walls of the main body and on later reflection to the plane parts of either of the end walls 43,44, is also eventually focused on the focal line. Light which does not eventually become focused on the focal line is that light which is scattered at right-angles to the source beam, that light which is scattered such that on subsequent reflections it strikes the spoiler from a path which has just passed through the lower focal axis of the ellipse, that light which strikes the aperture in the iris or the aperture for the sensor, and that light which is lost in the process of reflection. These losses account for approximately 62% of the scattered light.In prior art constructions, more than 93% of the scattered light does not reach the sensor, and this includes much of the light scattered between 150 and 1800 of the main beam.

Reference is now made to Fig. 7. The light source of the detector of the present invention is not pulsed but consists of a source whose intensity can be modulated. This modulation should be such that a signal from an electronic light sensor, when processed through a narrow band bandpass filter, will not give an unreasonable attenuation of the signal but will give high attenuation of any frequencies outside the pass band. Ideally, such modulation is sinusoidal. With the present invention, this leads to similar modulation cf the perceived signal by the sensor 31.This feature allows the electrical noise which inherently exists in the light source and the light sensor and the circuitry associated with those components, which is at a different frequency from that of the modulation, largely to be separated from the required signal by the use of a band pass filter at the frequency of modulation. For this reason, the modulation is at a frequency above the low frequencies at which comparatively very large amplitude spurious signals generated by the light sensor, light source and circuitry occur, and thus enables them to be filtered out. This frequency is dependent upon the components used, but is usually in excess of 100Hz. This arrangement also differentiates between the required signal and signals from external spurious light sources which may enter the light chamber if it is not perfectly light proof.

As shown in Fig.7, the laser light source is represented by a diode L which has an inbuilt monitoring diode D. Power to energise the laser is taken from a power source and regulator P by way of modulating circuits M. The modulation waveform is derived from a waveform generator S and the resulting modulation is accurately maintained by means of a feedback path from a monitoring diode F. The light sensor D is a semiconductor PIN diode and provides a good signal Into a buffer amplifier A, that signal then bering converted to 2 digital signal by an analogue-to dital converter A-D. The digital signal is processed by a digital signal processor DSP in two ways.

Firstly, the processor simulates a narrow band pass filter with a pass band centered at the frequency of modulation and which is about 2Hz between its 3dB attenuation points. Secondly, the processor calculates the amplitude of those resultant, sinusoidal varying numbers which agree in phase with the modulating waveform. Its output OP is a digital representation of this quantity. In this way, a large degree of discrimination against any signal which does not agree closely with the frequency and phase of the required signal is achieved. Synchronisation between the dIgital signal processor DSP and the modulating waveform is maintained perfectly by driving both the waveform generator S and the digital signal processor DSP from a common clock C. This clock is also used to trigger the analogue-to-digital converter A-D.

With the construction of detector as described above in accordance with the invention, and with a sample of air which contains a disperse phase in a concentration which will cause attenuation of 0.005% in the intensity of a beam 300mm long, the resulting output achieved is centered at a level which is twice the output achieved when no disperse phase is present.

Also, this resultant output remains within +/- 10% of this level over the passage of time. This means that the detector of the present invention will indicate accurately the concentration of a disperse phase on a linear scale 0% to 0.05%, and on any higher scale up to 0% to 20%, from where on it ceases to be linear.

Claims

CLAIMS:

1. A detector comprising a sampling chamber which has an internal light-reflecting surface of elliptical or substantially elliptical cross-section over at least part of the length of the chamber, a light source arranged to direct a light beam along the longitudinal axis passing through one focal point of the elliptical cross-section portion of the chamber and light collecting means positioned in a plane which includes the major axis of the ellipse to collect light which has been reflected from said chamber surface after having been diverted from the light beam.

2. A detector according to claim 1, in which the light collecting means is an elongate sensor extending over a portion of the length of the major axis of the ellipse.

3. A detector according to claim 2, in which the elongate sensor is centered on the longitudinal axis passing through the other focal point of the elliptical cross-section portion of the chamber.

4. A detector according to claim 1, in which the light collecting means is positioned on the longitudinal axis passing through the other focal point of the elliptical cross-section portion of the chamber.

5. A detector according to any preceding claim, which includes light collecting means at the same end of the detector as the light source.

6. A detector according to any preceding claim, in which the light from the light source is directed into a light trap at the end of the chamber remote from the source, and in which a wall which surrounds and defines the entrance to the light trap has a surface facing towards the chamber whose normals at any point are in a plane which includes the longitudinal axis through said one focal point.

7. A detector according to claim 6, in which said wall is frusto-conical.

8. A detector according to any preceding claim, in which the sampling chamber is defined by a housing composed of a plurality of sections stacked axially in contact with each other.

9. A detector according to any preceding claim, in which a sample to be monitored for the presence of matter as a disperse phase in a gaseous continuous phase is introduced into the sampling chamber along the minor axis of the elliptical cross-section chamber.

10. A detector according to any preceding claim, in which the light source is a monochromatic, coherent point light source.

11. A detector according to any preceding claim, in which the light source produces a collimated beam.

12. A detector according to any preceding claim, in which the light source is such that its intensity canbe modulated.

13. A detector according to claim 12, in which said modulation is sinusoidal.

14. A detector according to claim 12 or 13, in which the modulation is at a frequency higher than 100 Hz.

15. A detector substantially as hereinbefore described with reference to Figs. 4 to 7 of the accompanying drawings.

16. A method of detecting particles within a sampling chamber, which comprises directing a light beam along the longitudinal axis passing through one focal point of a sampling chamber which is elliptical or substantially elliptical at least in part, and reflecting light diverted from particles in the path of the beam towards light collecting means positioned in a plane which includes the major axis of the ellipse.

17. A method according to claim 16, which includes collecting reflected light along a line extending over a portion of the length of the major axis of the ellipse.

18. A method of detecting particles substantially as hereinbefore described with reference to Figs. 4 to 7 of the accompanying drawings.