U.S. patent application number 17/119472 was filed with the patent office on 2021-06-03 for gas sensors.
The applicant listed for this patent is GasSecure AS. Invention is credited to Britta GRENNBERG FISMEN, Kari Anne HESTNES BAKKE, Ib-Rune JOHANSEN, Hakon SAGBERG, Knut Baeroe SANDVEN, Jon TSCHUDI.
Application Number | 20210164895 17/119472 |
Document ID | / |
Family ID | 1000005387013 |
Filed Date | 2021-06-03 |
United States Patent
Application |
20210164895 |
Kind Code |
A1 |
SAGBERG; Hakon ; et
al. |
June 3, 2021 |
GAS SENSORS
Abstract
A gas sensor for measuring concentration of a predetermined gas
includes a light source (2) arranged to emit pulses of light, a
measurement volume (10), a detector (4) arranged to receive light
that has passed through the measurement volume (10), and an
adaptable filter (6) disposed between the light source (2) and the
detector (4). The gas sensor has a measurement state in which it
passes at least one wavelength band which is absorbed by the gas
and a reference state in which said wavelength band is attenuated
relative to the measurement state. A controller is connected to
each of the light source, the detector and the adaptable filter to
change the adaptable filter between one of said measurement state
and said reference state to the other at least once during a gas
sensor operation period.
Inventors: |
SAGBERG; Hakon; (Oslo,
NO) ; GRENNBERG FISMEN; Britta; (Oslo, NO) ;
HESTNES BAKKE; Kari Anne; (Hvittingfoss, NO) ;
TSCHUDI; Jon; (Oslo, NO) ; JOHANSEN; Ib-Rune;
(Oslo, NO) ; SANDVEN; Knut Baeroe; (Oslo,
NO) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
GasSecure AS |
Oslo |
|
NO |
|
|
Family ID: |
1000005387013 |
Appl. No.: |
17/119472 |
Filed: |
December 11, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14362944 |
Jun 5, 2014 |
|
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PCT/GB2012/053021 |
Dec 5, 2012 |
|
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17119472 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01N 21/314 20130101;
G01N 2021/3513 20130101; G01N 2201/128 20130101; G01N 21/3504
20130101; G01N 2201/061 20130101; G01N 2021/317 20130101; G01R
27/2605 20130101; G01N 2201/0696 20130101; G01N 2201/0693
20130101 |
International
Class: |
G01N 21/3504 20060101
G01N021/3504; G01N 21/31 20060101 G01N021/31; G01R 27/26 20060101
G01R027/26 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 5, 2011 |
GB |
1120871.7 |
Claims
1. A gas sensor for measuring a concentration of a gas, the gas
sensor comprising: a light source configured to be switched on to
emit light and to be switched off; a measurement volume; a detector
configured to receive light that has passed through the measurement
volume to output a photo detector signal; an adaptable filter
disposed between the light source and the detector and having a
measurement state in which the adaptable filter passes at least one
wavelength band which is absorbed by the gas and a reference state
in which said wavelength band is attenuated relative to the
measurement state; and a controller connected to each of the light
source, the detector and the adaptable filter, the controller being
configured: to switch the adaptable filter between the measurement
state and the reference state; to switch on the light source at a
start of a gas sensor operation period and to control power to the
light source during said gas sensor operation period; to switch off
the light source at an end of the gas sensor operation period so as
to enter a low power or shutdown mode, wherein the light source is
switched off during the low power or shutdown mode, and wherein:
the control of power to the light source during said gas sensor
operation period includes initiating an initial powering of the
light source during a pre-heat stage of the gas sensor operation
period to preheat the light source to a temperature below a desired
source temperature; the control of power to the light source during
said gas sensor operation period includes controlling power to the
light source during a measurement stage of the gas sensor operation
period to reach a desired source temperature; the measurement stage
of the gas sensor operation period is subsequent to the pre-heat
stage of the gas sensor operation period; and the switching of the
adaptable filter comprises changing the adaptable filter between
one said measurement state and one said reference state during the
measurement stage of said gas sensor operation period and
subsequent to the light source reaching the desired source
temperature; to sample the photo detector signal subsequent to
initiating the initial powering of the light source and prior to
reaching the desired source temperature; to sample the photo
detector signal with the adaptable filter in the measurement state
during said gas sensor operation period and subsequent to the light
source reaching the desired source temperature; and to sample the
photo detector signal with the adaptable filter in the reference
state during said gas sensor operation period and subsequent to the
light source reaching the desired source temperature.
2. The gas sensor of claim 1, wherein: the controller is configured
to sample the photo detector signal subsequent to initiating the
initial powering of the light source and prior to reaching the
desired source temperature a plurality of times to provide a
plurality of samples; and the controller is configured to measure a
rate at which an output from the detector, represented by the
plurality of samples, changes with time.
3. The gas sensor of claim 1, wherein the adaptable filter
comprises a micro-electromechanical system (MEMS).
4. The gas sensor of claim 3, wherein said MEMS filter comprises an
arrangement for measuring a change of capacitance therein for
diagnostic purposes.
5. The gas sensor of claim 1, wherein the adaptable filter
comprises a diffractive optical element having a plurality of
grating bands arranged to be moved by an electrostatic
potential.
6. The gas sensor of claim 1, wherein said light source is the only
light source and wherein said detector is the only detector.
7. The gas sensor of claim 1, wherein the adaptable filter
comprises a plurality of measurement states in each of which the
adaptable filter passes at least one wavelength band which is
absorbed by the gas and for each measurement at least one reference
state in which the wavelength band corresponding to the measurement
state is attenuated relative to said measurement state.
8. The gas sensor of claim 1, wherein the controller is adapted to
change the adaptable filter between said measurement state and one
said reference state a plurality of times during said gas sensor
operation period and subsequent to the light source reaching the
desired source temperature and the sampling of the photo detector
signal is synchronized by the controller to occur each time the
adaptable filter is in the measurement state during said gas sensor
operation period and subsequent to the light source reaching the
desired source temperature and to occur each time the adaptable
filter is in the reference state during said gas sensor operation
period and subsequent to the light source reaching the desired
source temperature.
9. The gas sensor of claim 1, wherein: the control of power to the
light source during said gas sensor operation period heats the
source to a temperature that is low enough not to be measured by
the detector from a time of initiating the initial powering of the
light source during the pre-heat stage up to a time of the
controlling of the power to the light source to reach a desired
source temperature; and the sampling of the photo detector signal
subsequent to initiating the initial powering of the light source
and prior to reaching the desired source temperature occurs with
the light source heated to a temperature that is low enough not to
be measured by the detector.
10. The gas sensor of claim 9, wherein: the controller is
configured to power the light source for a period to provide the
pre-heat stage and to subsequently not power the light source for a
period subsequent to the pre-heat stage and up to a time of the
controlling of the power to the light source to reach a desired
source temperature; and the sampling of the photo detector signal
subsequent to initiating the initial powering of the light source
and prior to reaching the desired source temperature occurs with
the controller not powering the light source.
11. A wireless, battery-operated gas detector unit comprising the
gas sensor of claim 1.
12. A method of measuring a concentration of a gas, the method
comprising the steps of: providing a gas sensor for measuring a
concentration of a gas, the gas sensor comprising a light source
configured to be switched on to emit light and to be switched off,
a measurement volume, a detector configured to receive light that
has passed through the measurement volume to output a photo
detector signal, an adaptable filter disposed between the light
source and the detector and having a measurement state in which the
adaptable filter passes at least one wavelength band which is
absorbed by the gas and a reference state in which said wavelength
band is attenuated relative to the measurement state; and a
controller connected to each of the light source, the detector and
the adaptable filter; switching, with the controller, on the light
source at a start of a gas sensor operation period; switching, with
the controller, off the light source at an end of the gas sensor
operation period so as to enter a low power or shutdown mode,
wherein the light source is switched off during the low power or
shutdown mode; controlling, with the controller, power to the light
source during said gas sensor operation period, including
initiating an initial powering of the light source during a
pre-heat stage of the gas sensor operation period to preheat the
light source to a temperature below a desired source temperature,
and subsequent to said initiating an initial powering, controlling
power to the light source during a measurement stage of the gas
sensor operation period to reach a desired source temperature,
wherein the measurement stage of the gas sensor operation period is
subsequent to the pre-heat stage of the gas sensor operation
period; switching, with the controller, the adaptable filter
between the measurement state and the reference state comprising
changing the adaptable filter between one said measurement state
and one said reference state during said gas sensor operation
period and subsequent to the light source reaching the desired
source temperature; sampling, with the controller, the photo
detector signal subsequent to initiating the initial powering of
the light source and prior to reaching the desired source
temperature; sampling, with the controller, the photo detector
signal with the adaptable filter in the measurement state during
said gas sensor operation period and subsequent to the light source
reaching the desired source temperature; and sampling, with the
controller, the photo detector signal with the adaptable filter in
the reference state during said gas sensor operation period and
subsequent to the light source reaching the desired source
temperature.
13. The method of claim 12, wherein the sampling during said gas
sensor operation period and subsequent to the light source reaching
the desired source temperature provides a gas concentration
measurement using only the light source as a single light source
and using only the detector as a single light detector.
14. The method of claim 12, wherein: the switching of the adaptable
filter between the measurement state and the reference state
comprises changing the adaptable filter between said measurement
state and said reference state a plurality of times during said gas
sensor operation period and subsequent to the light source reaching
the desired source temperature; the sampling of the photo detector
signal occurs each time the adaptable filter is in the measurement
state during said gas sensor operation period and subsequent to the
light source reaching the desired source temperature; and the
sampling of the photo detector signal occurs each time the
adaptable filter is in the reference state during said gas sensor
operation period and subsequent to the light source reaching the
desired source temperature.
15. The method of claim 12, wherein the sampling during said gas
sensor operation period and subsequent to the light source reaching
the desired source temperature provides a gas concentration
measurement using a modulation amplitude based on the photo
detector signal.
16. The method of claim 12, wherein the sampling of the photo
detector signal subsequent to initiating the initial powering of
the light source and prior to reaching the desired source
temperature comprises a plurality of samples and further comprising
measuring a rate at which an output from the detector, represented
by the plurality of samples, changes with time.
17. The method of claim 12, wherein the adaptable filter comprises
a plurality of measurement states in each of which it passes at
least one wavelength band which is absorbed by the gas and for each
measurement at least one reference state in which the wavelength
band corresponding to the measurement state is attenuated relative
to said measurement state, and the method further comprises
switching to each of said measurement states at least once during
said gas sensor operation period and subsequent to the light source
reaching the desired source temperature.
18. A gas sensor for measuring a concentration of a gas, the gas
sensor comprising: a light source configured to be switched on and
to be switched off; a measurement volume; a detector configured to
receive light that has passed through the measurement volume to
output a photo detector signal; an adaptable filter disposed
between the light source and the detector and having a measurement
state in which the adaptable filter passes at least one wavelength
band which is absorbed by the gas and a reference state in which
said wavelength band is attenuated relative to the measurement
state; and a controller connected to each of the light source, the
detector and the adaptable filter, the controller being configured:
to switch on the light source at a start of a gas sensor operation
period to control power to the light source during said gas sensor
operation period and to switch off the light source at an end of
the gas sensor operation period, wherein the control of power to
the light source during said gas sensor operation period includes
initiating an initial powering of the light source to preheat the
light source to a temperature below a desired source temperature
during a pre-heat stage of the gas sensor operation period and
subsequent to the pre-heat stage controlling power to the light
source to reach a desired source temperature; to sample the photo
detector signal subsequent to the pre-heat stage and prior to
reaching the desired source temperature; to switch the adaptable
filter between the measurement state and the reference state,
including changing the adaptable filter between one said
measurement state and one said reference state during said gas
sensor operation period and subsequent to the light source reaching
the desired source temperature; to sample the photo detector signal
with the adaptable filter in the measurement state during said gas
sensor operation period and subsequent to the light source reaching
the desired source temperature; and to sample the photo detector
signal with the adaptable filter in the reference state during said
gas sensor operation period and subsequent to the light source
reaching the desired source temperature.
19. The gas sensor of claim 18, wherein: the controller is
configured to sample the photo detector signal subsequent to the
pre-heat stage and prior to reaching the desired source temperature
a plurality of times to provide a plurality of samples; and the
controller is configured to measure a rate at which an output from
the detector, represented by the plurality of samples, changes with
time.
20. The gas sensor of claim 18, wherein the controller is adapted
to change the adaptable filter between said measurement state and
one said reference state a plurality of times during said gas
sensor operation period and subsequent to the light source reaching
the desired source temperature and the sampling of the photo
detector signal is synchronized by the controller to occur each
time the adaptable filter is in the measurement state during said
gas sensor operation period and subsequent to the light source
reaching the desired source temperature and to occur each time the
adaptable filter is in the reference state during said gas sensor
operation period and subsequent to the light source reaching the
desired source temperature.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of pending U.S.
application Ser. No. 14/362,944, having a 35 U.S.C. 371 (c) (1),
(2) date of Jun. 5, 2014, which is a United States National Phase
Application of International Application PCT/GB2012/053021, filed
Dec. 5, 2012, and claims the benefit of priority under 35 U.S.C.
.sctn. 119 of United Kingdom Application 1120871.7, filed Dec. 5,
2011, the entire contents of which are incorporated herein by
reference.
TECHNICAL FIELD
[0002] This invention relates to gas sensors, particularly sensors
for measuring the concentration of a gas by measuring the
absorption of infra-red light thereby.
TECHNICAL BACKGROUND
[0003] In order to operate gas sensors on battery power for long
periods of time, typically more than one year, the energy
consumption must be low. One way of reducing the energy consumption
is to keep the sensor in sleep or shutdown mode most of the time,
and to turn it on at regular or irregular intervals. A typical
power requirement for a continuously powered infrared sensor is on
the order of 0.1-1 W. If one measurement takes one second to
complete for a non-continuously operated sensor, as an example, and
the required response time is 10 s, the duty cycle becomes 10%,
with a corresponding reduction in energy consumption to 10-100 mW.
In the low end of this range, battery operation becomes a
possibility. The response time requirements will be different for
different applications. There are two modes of operation which may
be required of a gas sensor that can be operated at low duty cycle.
The first is intermittent or sporadic use. Here the gas sensor
would be started with irregular intervals, on demand. The
measurements could be triggered manually or by a second sensor that
monitors for changes in the ambient and estimates a probability
that gas may be present. In this mode, the response time for the
intermittent sensor could be almost as short as for a continuous
sensor, as long as that the wake up time is short enough.
[0004] The second mode is cyclic (or stand-alone) use. For cyclic
measurement the maximum response time will be limited by the cycle
period. As long as the required period/response time is longer than
the time needed for a single measurement, the cyclic mode will
require less power. Again, a sufficiently short wake-up time is
necessary.
[0005] For both these modes to be efficient, it is necessary that
the sensor can be `cold started` in a time interval much less than
the typical time between measurements, and that reliable, accurate
measurements will be available after such a short start-up time.
The present invention aims to provide a sensor and method that
makes this possible. Simple NDIR (non-dispersive infra red) gas
sensors measure concentration using a single light source and a
single detector. These are generally not suitable for safety
applications or applications that require good long-term stability
without recalibration.
[0006] Existing reliable gas sensors use different methods and
configurations to compensate for errors, for example two light
sources and one detector, or two detectors and one light source, or
two of each (doubly compensated). In a state-of-the-art doubly
compensated sensor, one source is provided with a filter for the
`active` wavelength band where the gas absorbs, and the other
source is filtered so that it emits a `reference` wavelength band.
The sources are usually modulated with frequencies in the range of
1-100 Hz. A reference detector monitors the source intensities,
while the main detector measures the light transmitted from the two
sources through the measurement volume and detects if light has
been absorbed by the gas. This set-up compensates for several
errors, such as light loss in the measurement volume, and source
intensity changes. A good compensation, however, depends on a
sufficiently (thermally) stable system. This is of special
importance when the source modulation frequency is low, or if the
two detectors are mounted so that they see different areas of the
source surface. (The temperature on a thermal infra-red source
surface is highly non-uniform). In some cases, a warm-up time of
several minutes is required before the measurement error is
sufficiently low.
SUMMARY
[0007] When viewed from a first aspect the invention provides a gas
sensor for measuring concentration of a predetermined gas
comprising a light source arranged to emit pulses of light, a
measurement volume, a detector arranged to receive light that has
passed through the measurement volume, and an adaptable filter
disposed between the light source and the detector and having a
measurement state in which it passes at least one wavelength band
which is absorbed by the gas and a reference state in which said
wavelength band is attenuated relative to the measurement state
wherein the adaptable filter is arranged to change between one of
said measurement state and said reference state to the other at
least once during each pulse.
[0008] The invention extends to a wireless, battery-operated gas
detector unit comprising a gas sensor as set out above.
[0009] When viewed from a second aspect the invention provides a
method of measuring a concentration of a predetermined gas
comprising passing a pulse of light through a measurement volume to
a detector via an adaptable filter disposed between the light
source and the detector, switching said filter at least once in
each pulse to/from a measurement state in which it passes at least
one wavelength band which is absorbed by the gas and a reference
state in which the wavelength band is attenuated compared to the
measurement state; the method comprising determining said
concentration of gas from the difference in light received by the
detector in said measurement and reference states respectively.
[0010] Thus it will be appreciated that in accordance with the
invention a fully referenced gas concentration measurement can be
taken using a single pulse of light from a single light source and
using a single detector. This enables a low power consumption fast
start-up from cold state and reliable, accurate measurement in a
short measurement period. Thus it opens up the possibility of a
remote, battery-powered wireless sensor unit with a long battery
life but which in the preferred embodiments can have the
reliability and stability of a doubly compensated system.
[0011] In accordance with the invention the adaptable filter
directs the light from the source onto the detector. By changing
its state, the wavelengths of light it passes are changed.
Preferably it comprises a micro-electromechanical system (MEMS).
These can be fabricated so as to be able to change the wavelengths
of light passed. The change can be performed on a timescale less
than one millisecond which means that a short pulse of light can be
used whilst still giving both a measurement and reference period,
thereby limiting the power consumption associated with the
measurement. The MEMS could comprise a diffractive optical element
having a plurality of grating bands arranged to be moved by an
electrostatic potential.
[0012] The MEMS solution is particularly convenient for `cold
starting` the sensor system and performing a complete measurement
using a single pulse of light. This can be done because the
wavelength modulation can be so fast that drift or low-frequency
noise can be filtered, and because the `active` and `reference`
wavelength bands are measured using exactly the same light path.
Drift, non-uniformity, and other error sources will affect the two
measurements equally.
[0013] The invention is not limited to the adaptable filter having
only two states; it may have three or more states. This could
provide a plurality of measurement/reference states--e.g. to allow
the concentrations of different predetermined gases to be measured
or to compensate for the presence of a particular interfering gas
or another known type of disturbance of the spectrum.
[0014] Thus in a set of embodiments the adaptable filter comprises
a plurality of measurement states in each of which it passes at
least one wavelength band which is absorbed by the gas and for each
measurement at least one reference state in which the wavelength
band corresponding to the measurement state is attenuated relative
to said measurement state. The sensor could be arranged such that
each measurement state is used in each pulse or different
measurement states may be used in different pulses--e.g. different
gasses could be measured in alternating light pulses.
[0015] The adaptable filter could, for example, comprise a unitary
structure having a plurality of positions, or it could comprise a
plurality of filter elements each having two or more states and
arranged to give the desired overall states. In either case a MEMS
is preferred.
[0016] As used herein the term `pulse` as applied to light is
intended to mean a temporary emission or increase in light output.
No particular pulse shape is to be inferred and it is not
necessarily the case that outside of pulses there is no light
emission. The length of a pulse may be defined as the length of
time for which the light is above a predetermined threshold. The
pulse width may in some embodiments be between 5 milliseconds and 5
seconds--e.g. between 10 and 1000 milliseconds.
[0017] As discussed previously the pulse frequency may be irregular
where measurement is sporadic or on-demand. Alternatively it may be
regular--e.g. less than once every 10 seconds, or less than once
every 30 seconds, or less than once a minute, or less than once an
hour, or less than once a day.
[0018] The light source could be a thermal source, such as a
filament lamp or heated membrane, or a solid-state source such as a
diode. What is important is that the source emits light in both the
measurement and reference wavelength bands. The adaptable filter
could be switched between its reference and measurement state or
vice versa just once per pulse. Preferably it is switched regularly
between said measurement and reference states a plurality of times
during each pulse. In some embodiments it may be switched more than
10 times per pulse, e.g. more than 25 times or more than 50 times
per pulse. The number of times it switches may be controlled to
give a required accuracy level.
[0019] In a set of embodiments the sensor measures the rate at
which the output from the detector for no input, known as the "dark
level" of the detector, changes with time. This allows a more
accurate gas concentration measurement to be taken since such
changes can then be compensated for.
[0020] A preferred embodiment of the invention will now be
described, by way of example only, with reference to the
accompanying drawings. The various features of novelty which
characterize the invention are pointed out with particularity in
the claims annexed to and forming a part of this disclosure. For a
better understanding of the invention, its operating advantages and
specific objects attained by its uses, reference is made to the
accompanying drawings and descriptive matter in which preferred
embodiments of the invention are illustrated.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] In the drawings:
[0022] FIGS. 1a and 1b are schematic diagrams showing a prior art
doubly-compensated sensor during measurements of clean air and of a
significant amount of the predetermined gas respectively;
[0023] FIGS. 2a and 2b are schematic diagrams showing a sensor in
accordance with the invention during measurements of clean air and
of a significant amount of the predetermined gas respectively;
[0024] FIG. 3 is a graph showing the two states of the filter
element and their relationship to the absorption spectrum of the
gas being measured;
[0025] FIG. 4 is a diagram showing the outputs registered by the
detector in differing circumstances;
[0026] FIG. 5 is a block diagram showing the components of a sensor
system in accordance with the invention;
[0027] FIG. 6 is a drawing of a portion of the MEMS adaptive
filter;
[0028] FIG. 7 is a more detailed sectional view of the filter;
and
[0029] FIG. 8 is a series of graphs showing the variation of
certain parameters during operation.
DESCRIPTION
[0030] Turning first to FIGS. 1 and 2 there may be seen a
comparison between a prior art doubly-compensated sensor in FIGS.
1a and 1 b and an embodiment of the invention in FIGS. 2a and 2b.
The doubly compensated system shown in FIG. 1a is typically
implemented in commercially available detectors for safety
applications. In this doubly compensated system, two light sources
A1, A2 and two detectors B1, B2 ensure that the measurements are
minimally influenced by e.g. dirty optics, light source drift,
temperature. Two different filters C1. C2 are used. One filter, C1
transmits a wavelength band which the gas being measured absorbs.
The other filter C2 is a reference filter that transmits a
neighboring wavelength band.
[0031] As may be seen in FIG. 1b, the light from one infra-red
source A2 passes through the measurement volume D and then to a
beam-splitter E so that it impinges on both filters C1 and C2. If
the gas of interest is present it will absorb light of certain
wavelengths. The light from the other infra-red source A1 does not
pass through the measurement volume D but is directly incident on
the beam-splitter E and so on both filters C1 and C2.
[0032] Absorption by the gas will result in a reduction in the
signal detected by the first detector B1 but will not affect the
signal at the reference detector B2. The difference between the
signals at the respective detectors can be used to calculate the
concentration of gas. Such detectors are in general effective and
reliable in safety-critical applications. However the provision of
two sources and two detectors makes them relatively expensive to
manufacture and they need a relatively large amount of power in
operation. Also, they need a certain warm-up time in order to reach
steady-state with uniform source temperature modulation which is
necessary for reliable measurements.
[0033] An embodiment of the present invention is shown in FIGS. 2a
and 2b. Here there is only a single infra-red source 2 and a single
detector 4. The light passes from the source 2, via a mirror 8 and
an adaptive MEMS filter 6 to the detector 4. As FIG. 2b shows it
passes twice through the measurement volume 10, although this is
not essential. In use the filter element 6 is switched repeatedly
between two different states so that the emergent light has one of
two possible wavelengths associated with the respective states. One
of these wavelengths is in the absorption band of the gas of
interest and the other is not. Thus, as before, the concentration
of gas can be calculated from the output of the detector 4
corresponding to the two respective states. Unlike the prior art
arrangement however the light path is the same for both the
reference and active wavelengths, and there are no beam-splitters.
If the source has a non-uniform intensity, there is dirt on the
optical surfaces, or the detector response changes, both
measurements are affected in the same way. The filter element 6 is
holographic so all light paths contribute to both the active and
reference measurement. The switching between the two states is so
fast that a varying/drifting source can be tolerated.
[0034] FIG. 3 shows the reflection spectra of the filter element 6
in its two states. The solid line 12 shows the reflection spectrum
of the filter during the measurement state. Here it will be seen
that in this state there is a single central peak of wavelengths
passed which coincides with the peak of the absorption spectrum 14
of a hydrocarbon gas (shown superimposed at the top of FIG. 3). In
the measurement state the filter therefore passes a band of
wavelengths which are absorbed by the gas. The light in this
wavelength ban will therefore be affected by the concentration of
gas since this will affect how much of it is absorbed.
[0035] When the filter element is switched to its reference state
however the filter characteristics are changed as shown by the
dashed line 16 so that light is passed in two bands on either side
of the peak in the absorption spectrum 14 and the wavelength band
previously passed in the measurement state (with the central peak)
is significantly attenuated compared to that state. Because the
pass band from the measurement state is attenuated in the reference
state, here the light passed will not be significantly affected by
the concentration of gas since the light which is passed will not
be significantly absorbed by the gas.
[0036] The absorption spectrum 14 shown here is merely illustrative
and may differ for different gasses--e.g. it may have more than one
absorption peak.
[0037] FIG. 4 shows a simplified illustration of the wavelength
band intensities (on the left) and the signal output by the
photodetector 4 (on the right) for different situations. The R
bands are reference bands while the A band is the active band. Thus
when there is no hydrocarbon gas present in the air, the active and
reference bands are the same and the photodetector signal is
unmodulated by the switching of the filter element 6.
[0038] When a hydrocarbon gas is present, light in the active band
is reduced compared to the reference band due to absorption by the
gas. This shows up as a modulation in the photodetector signal
corresponding to the switching between the two states. The
amplitude of the modulation can be used, together with the
difference in the detector output when the source is switched on,
to calculate the concentration of gas.
[0039] If the source or optics are dirty, transmission of light
across both bands will be reduced equally and there will be
constant reduction in the photodetector signal with no
modulation.
[0040] If the source temperature changes between two measurements
this will give different absolute detected levels but there will
again be no modulation and thus a false reading is avoided.
[0041] Finally if there is no signal due to a failed source or
blocked beam, again the reference and active bands will be affected
equally.
[0042] The system is shown in FIG. 5 in the form of a block diagram
representation. The Optical sensor" block represents the optical
sensor hardware that is controlled by a microcontroller. The light
emitted from the source 2 exits through the window to the
measurement cell 10. After returning from the measurement cell 10,
it is filtered by the MEMS filter 6 (Filter module) and is focused
onto the photodetector 4. The ports on the left side are connected
to the microcontroller.
[0043] The light goes through the following stages. The first stage
is generation. The source 2 emits broadband radiation with an
intensity and spectral distribution given by the filament
temperature. A lens (not shown) collects the light for output to
the measurement cell 10.
[0044] The second stage is absorption. The radiation passes twice
through the measurement volume 10, returning to the window and
entrance aperture after reflection in the outer mirror 8. Any
hydrocarbons present will attenuate radiation in a wavelength band
around 3.3 .mu.m, while other gases, contaminants and dirty optics
will attenuate over a broader wavelength range.
[0045] The third stage is filtering. The voltage-controlled MEMS
optical filter alternately selects the 3.3 .mu.m wavelength
measurement band, and a double reference band with peaks on either
side of the 3.3 .mu.m measurement band.
[0046] The fourth stage is detection. A photodetector 4 measures
the filtered light in sync with the filter modulation. The signal
is amplified and sampled by the microcontroller.
[0047] FIGS. 6 and 7 show more details of the MEMS adaptive filter.
The optical surface of the filter element 4 is a diffractive
optical element (DOE) that initially focuses light within a single
wavelength band. In order to change from one filter state to
another, the optical surface is segmented into bands of movable 303
and static 301 surfaces (this is described in greater detail with
reference to FIG. 7). The height difference between these surfaces
determines the degree of constructive or destructive interference
of the diffracted light. A difference of 830 nm or ?/4 is needed
for destructive interference at the center wavelength of 3.3 .mu.m.
Displacement or height difference is achieved by electrostatic
actuation of the movable surfaces 303, which are connected to
springs 305 and suspended above a substrate 304. The restoring
force from the deflected springs 305 balance the electrostatic
force until a critical displacement is reached and the whole frame
305 pulls in towards the substrate 304. Then the resulting height
difference is determined by the depth of an etched recess in the
substrate.
[0048] FIG. 7 shows a sectional view of the filter. Alternating
static beams 102 and movable beams 103 provide the static and
movable surfaces described above. On top of each beam, there is a
diffraction grating relief 101. The static beams 102 are attached
to the substrate 105 by means of e.g. fusion bonding to the silicon
oxide layer 106 whilst the movable beams 103 are able to move in
etched recesses 107 against stops 108.
[0049] The filter element is electrically equivalent to a voltage
dependent capacitor having a capacitance, typically in the range
100 pF to 300 pF initially and increasing with applied voltage. The
microcontroller generates a digital square wave that controls a
single pole, double throw switch, the output of which alternates
between 0V and 24V. The 24V is generated by a step-up regulator. A
sense resistor is used to measure the current flow in and out of
the capacitor, for self test purposes. This is beneficial as it
allows a determination to be made when the filter element is not
working. This is important from a safety point of view since if the
filter does not function in the embodiments disclosed herein a
false negative signal will be given, even in the presence of
gas.
[0050] FIG. 8 shows operation of the optical sensor. Looking along
the horizontal time axis, at point I, the optical sensor is
switched on. During the period between point I and point II the
light source is pre-heated. During the next phase up to point III
the `dark` level and slope are measured. Thereafter up to point IV
the source is heated. In the final phase from point IV to point V
the modulation is measured.
[0051] Plot A shows the photodetector signal. The plot labeled
alpha is the signal when no gas is present. The plot labeled beta
is the signal received when there is a high concentration of the
gas being sensed. The plot labeled gamma is the extrapolated dark
signal, which is used to calculate corrected values of S_SRC (the
increase in signal received resulting from the transmission of
light through the measurement volume) and S_MOD (the amplitude of
the modulation on the received signal corresponding to absorption
of light by the gas in measurement mode) which are explained
further below.
[0052] Plot B shows the signal generated by the microcontroller to
control the operation of the filter element. When the filter
control signal is high, the filter is in the reference state, when
the control signal goes low, the filter switches to the measurement
state.
[0053] Plot C shows the signal sampling. First, the dark signal is
sampled in order to calculate the level and slope of the gamma
curve shown in plot A. Then the signal is sampled in sync with the
filter switching. There may be more than two samples each cycle,
but for simplicity only one pair of samples is shown per cycle. The
values of S_SRC and S_MOD are calculated from the sampled voltages
and the extrapolated dark signal. S_SRC and S_MOD are constant
during the measurement shown in the figure, but may vary if the
source power is not constant. This variation will have little
influence on the measurement if the average values of S_SRC and
S_MOD are used.
[0054] Finally plot D shows the signal from the microcontroller
which controls the light source. First, as mentioned above, the
source is pre-heated to a temperature that is low enough not to be
measured by the detector. The pre-heat stage reduces the time
between point III and IV, the ramp-up time, which is beneficial for
measurement accuracy and power consumption. After measurement of
the dark signal, the source voltage is changed step-wise or
continuously until the correct source temperature is reached. In
the example shown here a constant voltage is applied during the
modulation measurement. In principle the source power voltage may
be controlled during the modulation measurement however.
[0055] In order to calculate the gas concentration, one needs the
following variables: the intensity of the light pulse (S_SRC); and
the amplitude of the light modulation (S_MOD). In addition one
naturally needs system information such as the optical path-length
in the measuring volume, the characteristics of the modulated
filter, the approximate source spectrum, and the spectral response
of the photodetector. The system information is partially given by
design, and partially found from calibration measurements.
[0056] A preferred method of determining the gas concentration from
the measured signals is through the ratio S_NORM=S_MOD/S_SRC. The
sign of S_MOD depends on whether it is in phase with the filter
control signal in plot B. When no gas is present, S_MOD (and thus
S_NORM) is close to zero. The calibrated signal S_CAL is then
calculated as S_CAL=GAIN_S(T)*(S_NORM-S_0(T)), where S_0(T) and
GAIN_S(T) are used to compensate for temperature drift and
individual variations between filters. The coefficients are
determined from calibration measurements using a known gas mixture,
over a range of temperatures. The gas concentration is a nonlinear
function of S_CAL
[0057] The photodetector dark level S_DET may drift a significant
amount during the measurement, which will lead to measurement error
in both S_SRC and S_MOD. To compensate for this, in this embodiment
the rate of change of S_DET is measured, and an extrapolated value
is used when calculating S_SRC.
[0058] Although in the embodiment described the filter has only one
measurement state, it could have multiple such states allowing the
concentrations of multiple gasses to be measured.
[0059] While specific embodiments of the invention have been shown
and described in detail to illustrate the application of the
principles of the invention, it will be understood that the
invention may be embodied otherwise without departing from such
principles.
* * * * *