U.S. patent application number 11/984511 was filed with the patent office on 2010-11-11 for particle detection.
This patent application is currently assigned to SMITHS DETECTION - WATFORD LIMITED. Invention is credited to Roger Francis Golder, Roger Fane Sewell, Danielle Emma Toutoungi.
Application Number | 20100283450 11/984511 |
Document ID | / |
Family ID | 29226544 |
Filed Date | 2010-11-11 |
United States Patent
Application |
20100283450 |
Kind Code |
A1 |
Golder; Roger Francis ; et
al. |
November 11, 2010 |
Particle detection
Abstract
Particle detection and characterising apparatus has an
insulating tube (50) along which the particles flow. Five
electrodes (51, 52, 55, 56) and (57) spaced along the outside of
the tube are connected via amplifiers (61) and (62) to a processor
(68) arranged to measure the charge on the particles. The apparatus
also includes a laser (11) and photomultiplier tube (23) arranged
to measure the size of the particles so that the nature of the
particles can be characterised from their electrical charge and
size.
Inventors: |
Golder; Roger Francis; (Ely,
GB) ; Sewell; Roger Fane; (Newnham, GB) ;
Toutoungi; Danielle Emma; (Cambridge, GB) |
Correspondence
Address: |
FOLEY AND LARDNER LLP;SUITE 500
3000 K STREET NW
WASHINGTON
DC
20007
US
|
Assignee: |
SMITHS DETECTION - WATFORD
LIMITED
|
Family ID: |
29226544 |
Appl. No.: |
11/984511 |
Filed: |
November 19, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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|
10595082 |
Mar 6, 2006 |
7298127 |
|
|
PCT/GB04/03755 |
Sep 3, 2004 |
|
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11984511 |
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Current U.S.
Class: |
324/96 |
Current CPC
Class: |
G01N 15/1459 20130101;
G01N 15/1031 20130101; G01N 15/0205 20130101 |
Class at
Publication: |
324/96 |
International
Class: |
G01R 31/00 20060101
G01R031/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 5, 2003 |
GB |
0320809.7 |
Claims
1. An apparatus for characterizing a particle comprising: (a) an
electrical charge sensor adapted to determine an electrical charge
on the particle; and (b) an optical device adapted to determine a
second characteristic of the particle, wherein the apparatus is
adapted to provide an indication of the nature of the particle
according to the charge and the second characteristic.
2. The apparatus according to claim 1, wherein the second
characteristic is size.
3. The apparatus according to claim 1, wherein the electrical
charge sensor comprises a pathway for the particle and a plurality
of electrodes spaced along the pathway arranged to provide an
electrical output as the particle passes along the pathway.
4. The apparatus according to claim 3, wherein the pathway is
provided by an electrically insulative tube, and wherein the
plurality of electrodes are provided on an external surface of the
tube.
5. The apparatus according to claim 3, wherein there are five
electrodes spaced along the pathway.
6. The apparatus according to claim 3, wherein the outermost
electrodes are grounded, wherein two electrodes adjacent to
outermost two electrodes are interconnected, and wherein a signal
is derived from the difference between the central electrode and
the two interconnected electrodes.
7. The apparatus according to claim 4, wherein the tube has an
internal diameter of substantially 0.5 mm.
8. The apparatus according to claim 4, further comprising a filter
adapted to prevent particles greater than substantially 10 .mu.m
from entering the tube.
9. A method of characterizing a particle comprising: (a) measuring
charge on the particle; (b) measuring an optical characteristic of
the particle; and (c) providing an output indicative of the nature
of the particle from the combination of both the charge and the
optical characteristic.
10. A method according to claim 9, wherein the optical
characteristic is indicative of the size of the particle.
11. An apparatus for measuring a charge on a particle comprising:
(a) a tube comprising a first end and a second end along which the
particle is arranged to flow, (b) a first and a second outer
electrode, wherein the first outer electrode is located adjacent to
the first end and the second outer electrode is located adjacent to
the second end, (b) a third electrode adjacent to the first outer
electrode and a fourth electrode adjacent to the second outer
electrode, (c) a fifth electrode located between the third and
fourth electrodes, (d) a connection connecting the first and second
outer electrodes to ground, (e) a connection connecting the third
electrode to the fourth electrode and connecting the connected
third and the fourth electrodes to a measuring circuit, and (f) a
connection connecting the fifth electrode to the measuring circuit,
wherein the measuring circuit is adapted to subtract the signals on
the third and fourth electrodes from the signal on the fifth
electrode to derive a signal indicative of the charge on the
particle.
12. An apparatus for characterizing a particle comprising an
electrical charge sensor adapted to determine an electrical charge
on the particle, wherein the electrical charge sensor comprises a
pathway having at least three electrodes spaced along the length of
the pathway comprising a central electrode and two outer
electrodes, wherein the two outer electrodes are connected
together, wherein a charge signal is derived from the difference
between a charge on the central electrode and a charge on the
connected two outer electrodes, and wherein the apparatus is
adapted to derive an indication of the nature of the particles from
the charge signal.
13. The apparatus according to claim 12, further comprising a
second device adapted to determine a second characteristic of the
particle, wherein the apparatus is adapted to characterize a
particle using the combination of both the charge signal and the
second characteristic.
14. The apparatus according to claim 13, wherein the second
characteristic is size.
15. The apparatus according to claim 13, wherein the second device
is an optical device.
16. An apparatus for characterizing a particle in a tube comprising
(a) a plurality of electrodes space along the tube and adapted to
determine an electrical charge on the particle, and (b) a second
device adapted to determine a second characteristic of the particle
unrelated to the electrical charge, wherein the apparatus is
adapted to provide an indication of the nature of the particle
using a combination of both the charge and the second
characteristic.
Description
[0001] This invention relates to particle detection and
characterisation.
[0002] Various techniques are used to detect and identify small
airborne particles, such as ion mobility spectrometry, cyclone
collection or the like. These suffer form various disadvantages
especially when detecting small particles, such as biological
particles.
[0003] It is an object of the present invention to provide an
alternative apparatus and method of characterising particles.
[0004] According to one aspect of the present invention there is
provided apparatus for characterising particles including first
means for determining the electrical charge on the particles and
second means for determining a second characteristic of the
particles, the apparatus being arranged to provide an indication of
the nature of the particles according to the charge and the second
characteristic.
[0005] The second characteristic is preferably size and the means
for determining size may be optical means. The apparatus may
include a plurality of the first means for one of the second means.
The first means may include means defining a pathway for the
particles and a plurality of electrodes spaced along the pathway
arranged to provide an electrical output as the particles pass
along the pathway. The means defining the pathway preferably
includes a tube. There are preferably five electrodes spaced along
the pathway. The outermost electrodes may be grounded, the two
electrodes adjacent the outermost electrodes may be connected
together and a signal may be derived from the difference between
the central electrode and the two interconnected electrodes. The
tube may have an internal diameter of about 0.5 mm. The apparatus
preferably includes means preventing particles greater than about
10 .mu.m from entering the tube.
[0006] According to another aspect of the present invention there
is provided a method of characterising particles including the
steps of measuring the electrical charge on the particles,
measuring a second characteristic of the particles and providing an
output indicative of the nature of the particles from the charge
and the second characteristic.
[0007] The second characteristic is preferably size.
[0008] According to a third aspect of the present invention there
is provided apparatus for measuring the charge on a particle
including a tube along which the particle is arranged to flow,
first and second outer electrodes towards opposite ends of the
tube, third and fourth electrodes adjacent the first and second
electrodes respectively, a fifth electrode between the third and
fourth electrodes, means connecting the first and second electrodes
to ground, means connecting the third and fourth electrodes with
one another and to measuring means, and means connecting the fifth
electrode to the measuring means, the measuring means being
arranged to subtract the signals on the third and fourth electrodes
from the fifth electrode to derive a signal indicative of the
charge on the particle.
[0009] Particle characterisation apparatus according to the present
invention, will now be described, by way of example, with reference
to the accompanying drawings, in which:
[0010] FIG. 1 is a perspective view of the apparatus;
[0011] FIG. 2 is a perspective view of the front side of the
charge-measuring assembly;
[0012] FIG. 3 is a perspective view of the rear side of the
charge-measuring assembly;
[0013] FIG. 4 is a perspective view of a single charge-measuring
unit within the assembly of FIG. 3;
[0014] FIG. 5 is a perspective view of a tube from the unit of FIG.
4;
[0015] FIG. 6 is a graph showing the voltage output of the charge
measuring unit; and
[0016] FIG. 7 is a diagram illustrating the electrical circuit of
the apparatus.
[0017] With reference first to FIG. 1, the apparatus includes a
metal base board 1 on which is mounted a support assembly 2 of
generally rectangular shape and having a longitudinally-extending
recess 3 in which is received a charge-measuring assembly 4 with
cooling fins on its upper surface. The support assembly 2 has a
box-like inlet structure 5 on one side providing a plenum chamber
for air drawn to the inlet side of the charge-measuring assembly. A
box-like outlet structure 6 extending along the opposite side of
the charge measuring assembly 4 includes a plenum chamber and an
outlet opening on the lower surface of the board through which air
can be pumped away from the apparatus. A third structure 7 provides
electrical connection to the assembly 4. Further details of the
charge-measuring assembly 4 are given below.
[0018] The board 1 also supports particle size measuring means in
the form of an optical laser arrangement indicated generally by the
numeral 10. The arrangement includes a laser tube 11 mounted
horizontally on the board 1 to extend parallel to the support
assembly 2. Two inclined mirror assemblies 12 and 13 are oriented
to direct the beam of radiation emerging from one end of the tube
11 axially of the charge measuring assembly 4 and, in particular,
the beam of radiation is incident on a window 14 in one end of the
charge measuring assembly and passes along its length to emerge
from its other end and be absorbed in an optical beam dump 15.
[0019] The charge measuring assembly 4 is shown in greater detail
in FIGS. 2 and 3. It comprises a rectangular metal block 20 with a
forward surface 21 in which ten recesses 22 extend laterally of the
block side-by-side towards its right-hand end. A photo multiplier
tube (PMT) 23 is mounted on the forward surface towards the
left-hand end of the block 20 for measuring the size of the
particles. The rear surface 24 of the block, as shown in FIG. 3,
has a shallow recess 25 extending along most of its length and has
ten circular apertures 26 opening into the recess aligned with
respective ones of the recesses 22 in the forward surface 21. A
groove 27 providing a laser pathway extends diametrically across
the apertures 26. The lower end of the groove 27, as represented in
FIG. 3, opens into the window 14. At its other end, the groove 27
opens into a tubular guide 28 extending to the end of the block 20
and terminated by a coupling 29 to which the optical beam dump 15
is connected.
[0020] Each of the recesses 22 in the block 20 contains a charge
sensor 40 of the kind shown in more detail in FIGS. 4 and 5. Each
sensor 40 includes a rectangular upper plate 41 and a smaller lower
plate 42 separated from one another by two support pillars 43 and
44. An O-ring seal 45 extends around the upper plate 41 to form an
hermetic seal with a respective recess 22, the sensor 40 being held
in place by screws extending into tapped holes (not shown) in the
block 20 through two screw holes 46 at opposite ends of the plate.
The upper plate 41 has an air pipe 47 (FIG. 4). On the underside of
the plate 41 the pipe 47 supports and connects with the upper end
of a charge sensor tube 50, which is shown in more detail in FIG.
5. The lower end of the charge sensor tube 50 is supported in a
recess 70 in the lower plate 42. The recess 70 is circular and
tapers to a reduced diameter at its lower end to reduce stray
capacitance between the metal structure and electrodes on the
charge sensor tube 50. The upper plate 41 also supports an
electrical connector 49 connected to electrodes on the tube 50.
[0021] The tube 50 is shown in more detail in FIG. 5 and is of an
electrically-insulative material such as glass or ceramic, it has a
circular section with an outside diameter of about 1 mm, an inside
diameter of about 0.5 mm and a length of about 30 mm. The tube 50
has five ring-shape electrodes on its outer surface spaced from one
another along the length of the tube. The electrodes are on the
outside of the tube because the air flowing through it may be humid
and conductive. Two electrodes 51 and 52 at opposite ends of the
tube 50 are each 8 mm long and are separated by insulating gaps 53
and 54, each 0.5 mm long, from two intermediate electrodes 55 and
56, which are each 0.5 mm long. A central electrode 57, which is 1
mm long, is separated from the intermediate electrodes by
insulating gaps 58 and 59, each 0.5 mm long. The two outer
electrodes 51 and 52 are connected to ground. The two intermediate
electrodes 55 and 56 are connected together and the signal on these
is subtracted from the signal on the central electrode 57 using the
circuit shown in FIG. 7. The lower end of the tube 50 extends
through the recess 70 and through a circular boss 60 formed on the
lower surface of the lower plate 42. The electrodes 51, 52, 55, 56
57 are located towards the lower end of the tube 50 so that the
distance between the charge measurement electrodes 55, 56 and 57
and the particles' size measurement at the end of the tube is kept
to a minimum.
[0022] The circuit for each sensor 40 comprises two identical
amplifying circuits 61 and 62 connected to the central electrode 57
and the intermediate electrode pair 55 and 56 respectively. Each
amplifying circuit 61 and 62 includes a low-noise charge amplifier
63, a low pass filter 64, a second amplifier 65 and an
analogue-to-digital converter 66. The digital output from each
circuit 61 and 62 is supplied to a subtracting unit 67, which
provides an output signal to a digital processor 68 with suitable
processing to reduce noise. The processor 68 receives nine other
inputs from the other nine sensor units 40. The output signal from
each sensor unit 40 takes the form shown in FIG. 6.
[0023] In operation, atmospheric air is drawn into and through the
apparatus by a pump (not shown) located on the outlet side. The air
is preconditioned with an elutriation filter of known kind to
remove particles outside the range 1 .mu.m to 10 .mu.m. Particles
that are too small would give rise to noise; particles that are too
large would risk clogging the tubes 50. It is important that the
technique used to remove particles outside the desired size range
has only a minimal effect on the charge of the remaining
particles.
[0024] The preconditioned air containing suspect particles flows to
the charge sensing tubes 50 in each of the ten sensors 40. With a
flow rate of about 10 L/min through the apparatus, the flow rate
through each tube 50 is about 100 m/sec. When a particle is present
in the air flowing through a sensor 40 an output response of the
kind shown in FIG. 6 is produced. It can be seen that the width of
the pulse P produced is highly constrained. The width of the pulse
P depends on the diameter of the electrode rings 51, 52, 55, 56 and
57 with larger diameters producing a more spread out signal. Short
duration pulses are important if the apparatus is to be able to
provide reliable identification when there is a high throughput of
particles. The magnitude of the pulse P depends on the charge on
the particle. The output from the sensors 40 is supplied to the
processor 68, which adds a timestamp to the data and is then stored
to hard disc. The laser size measuring arrangement 10 continuously
transmits a beam of laser radiation from the laser tube 11, which
is directed along the groove 27 in the block 20 where it intersects
particles emerging from the lower end of the tubes 50. The output
of the PMT 23 is dependent on the size of the particle and this
output is also supplied to the processor 68. The processor 68,
therefore, receives information about the charge and size of each
particle detected. This is compared with stored information to
provide an output indicative of the nature of the particles.
[0025] It is believed that particles, in particular biological
particles, carrying a relatively high electrical charge are more
likely to have been generated artificially, and therefore present a
possible hazard, because the usual mechanisms for producing small
particles, such as aerosol nozzles and the like involve impact at
high velocity. This causes disruption of the molecules, typically
resulting in a charge on the particles.
[0026] The dimensions of the tube can be varied but its diameter
should be as small as possible compatible with producing flow
through it sufficient to avoid clogging. It is believed that the
minimum practical internal diameter is about 500 .mu.m, with a wall
thickness of about 250 .mu.m. The number of charge sensors can be
varied in order to achieve the desired throughput.
[0027] One possible problem with the charge sensor tubes is that
the particles may interact with the inside of the tubes to produce
tribocharging. Glass and ceramic are known to have a fairly high
propensity to tribocharging, although this is dependent on the
nature of the particles. One possible alternative material is Mylar
(a Registered Trade Mark of E. I. Du Pont de Nemours and Company),
which is a polyester film material close to neutral in the
triboelectric series, although this is only presently available in
film form.
[0028] Instead of using an optical size measuring technique to
identify a second characteristic of the particles it may be
possible to identify the nature of particles from their charge and
some other characteristic.
* * * * *