WO2007080398A1 - Differentiating gas sensor - Google Patents

Differentiating gas sensor Download PDF

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Publication number
WO2007080398A1
WO2007080398A1 PCT/GB2007/000060 GB2007000060W WO2007080398A1 WO 2007080398 A1 WO2007080398 A1 WO 2007080398A1 GB 2007000060 W GB2007000060 W GB 2007000060W WO 2007080398 A1 WO2007080398 A1 WO 2007080398A1
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WO
WIPO (PCT)
Prior art keywords
gas sensor
emitted
filter
radiation
bands
Prior art date
Application number
PCT/GB2007/000060
Other languages
French (fr)
Inventor
Michael J. Smith
Original Assignee
Gas Sensing Solutions Limited
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from GB0600381A external-priority patent/GB0600381D0/en
Priority claimed from GB0600382A external-priority patent/GB0600382D0/en
Application filed by Gas Sensing Solutions Limited filed Critical Gas Sensing Solutions Limited
Publication of WO2007080398A1 publication Critical patent/WO2007080398A1/en

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
    • G01N21/3504—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light for analysing gases, e.g. multi-gas analysis
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/314—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry with comparison of measurements at specific and non-specific wavelengths
    • G01N21/3151—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry with comparison of measurements at specific and non-specific wavelengths using two sources of radiation of different wavelengths
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2201/00—Features of devices classified in G01N21/00
    • G01N2201/12—Circuits of general importance; Signal processing
    • G01N2201/121—Correction signals
    • G01N2201/1215—Correction signals for interfering gases

Definitions

  • the present invention relates to gas sensing, in particular gas sensors having a radiation source (emitter) and a radiation detector, such as non- dispersive infrared (NDIR) gas sensors.
  • a radiation source emitter
  • NDIR non- dispersive infrared
  • a low cost LED Light Emitting Diode
  • a LED does have relatively narrow emission bandwidth, the bandwidth is still broad enough to span the absorption bands of more than one gas. Thus sensing a change in the concentration of one gas may be confused by changes in another's concentration.
  • a gas sensor for differentiating gases in a gas mixture
  • the gas sensor comprising: a radiation source operable to emit radiation, the radiation having first and second emitted bands overlapping in frequency; a modulator configured to modulate the emitted radiation so as to provide each of the first and second emitted bands with a discrete modulation signal; a cavity for conveying the modulated emitted radiation through the gas mixture; a radiation detector operable to receive the modulated emitted radiation so as to provide a received detection signal; and a demodulator, configured to demodulate the discrete modulation signals from the received detection signal thereby providing a demodulated detection signal for each of the first and second emitted bands .
  • the gas sensor further comprises differentiation means configured to differentiate gases in the gas mixture.
  • differentiation means is configured to differentiate gases in the gas mixture using the demodulated detection signals of each of the first and second emitted bands.
  • the differentiation means is configured to differentiate gases by subtracting the demodulated detection signal of the first emitted band from the demodulated detection signal of the second emitted band.
  • the radiation source comprises at least one emitter.
  • the radiation source is configured such that the relative amount of radiation comprising each of the first and second emitted bands is unequal.
  • the radiation source comprises unequal emission areas for each of the first and second emitted bands .
  • the radiation source is configured to provide unequal emission power for each of the first and second emitted bands.
  • the unequal emission power is provided by varying the modulation duty cycle of at least one emitter.
  • the radiation source comprises at least one light emitting diode.
  • the radiation source further comprises at least one filter configured to trim the bandwidth of the at least one emitted band by filtering a portion of the emitted radiation.
  • the at least one filter comprises a band pass filter.
  • the at least one filter comprises a band stop filter .
  • the at least one filter comprises a low pass filter.
  • the at least one filter comprises a high pass filter.
  • the at least one filter is configured to filter the portion of the emitted radiation such that the bandwidth of at least one of the filtered emitted bands is narrower than the bandwidth of its corresponding emitted band.
  • the at least one filter is configured to filter the portion of the emitted radiation such that the bandwidth of at least one of the filtered emitted bands is no greater than 1.5 ⁇ m wide.
  • the at least one filter is configured to trim only one side of the bandwidth of the at least one emitted band.
  • the at least one filter is configured to trim both sides of the bandwidth of the at least one emitted band.
  • At least one emitter comprises the filter.
  • the at least one filter is formed by molecular beam epitaxial growth on at least one emitter.
  • the at least one filter comprises a coating on at least one emitter.
  • the coating comprises a conformal deposited coating.
  • the coating comprises a passivation coating.
  • the coating comprises epoxy resin.
  • the coating comprises cyanoacrylate .
  • the coating comprises glass.
  • the coating comprises acylic resin.
  • the coating comprises polyerethane resin.
  • the coating comprises silicone resin.
  • the coating comprises latex rubber.
  • the coating comprises a UV curable system.
  • the at least one filter further comprises a further material applied to the coating, wherein the further material is configured to enhance the band pass filter effect of the coating.
  • the at least one filter is configured to match the filtered radiation to the optical frequency cut off of the radiation detector.
  • the gas sensor further comprises: a reference radiation detector operable to receive the modulated emitted radiation so as to provide a received reference signal; and a reference demodulator, configured to demodulate the discrete modulation signals from the received reference signal thereby providing a demodulated reference signal for each of the first and second emitted bands.
  • a reference radiation detector operable to receive the modulated emitted radiation so as to provide a received reference signal
  • a reference demodulator configured to demodulate the discrete modulation signals from the received reference signal thereby providing a demodulated reference signal for each of the first and second emitted bands.
  • Figure 1 illustrates in schematic form a gas sensor in accordance with the preferred embodiment of the present invention
  • Figure 2 illustrates in schematic form the absorbance spectrum of three gases and the band widths of two LEDs and a photodiode configured for differentiating CO 2
  • Figure 3 illustrates in schematic form the absorbance spectrum of three gases and the band widths of two LEDs and a photodiode configured for differentiating CO
  • Figure 4 illustrates the absorption band of a filter shown superimposed on a graph of the unfiltered LED emission power.
  • the radiation source comprises two LED emitters 1,2.
  • the radiation source may have multiple LEDs with adjacent or overlapping emission frequency bands.
  • a single LED with multiple emission frequency bands may be used.
  • the bands 3, 4 are overlapping in frequency.
  • a modulator 5 modulates the emitted radiation so as to provide each emitted band with a discrete modulation signal.
  • the cavity 6 conveys the modulated emitted radiation through the gas mixture being sensed.
  • the gas mixture contains for example CO 2 , N 2 O and CO.
  • the photodiode radiation detector 7 receives the modulated emitted radiation and outputs a received detection signal.
  • two, three or more matched emitter/detector pairs may be used.
  • the demodulator 8 demodulates the discrete modulation signals from the received detection signal output from the photodiode. This provides the demodulated detection signal for each emitted band.
  • the processor 9 subtracts one demodulated detection signal from another in order to differentiate the absorption signals of different gases.
  • the modulation used in this embodiment is achieved by driving each LED with a different square wave.
  • the LED emitters are more efficient if driven with strong pulses and then allowed to rest for a period. Such modulation is used to reduce the overall power to the LED whilst maintaining the efficiency.
  • a LED may be driven with 200 to 250 mA, however this may only be maintained if the duty cycle is less than 25% as the LED would quickly overheat.
  • Another modulation requirement is to increase the signal to noise ratio and to make the gas sensor more compatible with standard devices that operate at frequencies well above 1 KHz.
  • Band pass filters 10, 11 may be applied to the surface of the LEDs.
  • a filter may alternatively be a band stop filter, a low pass filter or a high pass filter.
  • the filters are formed as part of the radiation source by molecular beam epitaxy during growth of the LEDs.
  • the structure of each filter is chosen so as to trim the emitted bandwidth by filtering only one side, or both sides, of the emitted bandwidth.
  • the filter enhances the gas sensor's output signal with respect to a specific gas and causes the overall sensor bandwidth and therefore sensitivity to only one gas to be enhanced.
  • the filter is applied as a coating to the surface of the LED, as a conformal / protection / passivation coating designed to protect the LED from environmental problems such as water ingress.
  • the coating is selected because it has an absorption band at a convenient location in the spectrum.
  • a further material may be added to a conventional coating material that enhances, moves or creates an optical filtering effect.
  • the further material may be applied to the processed and mounted chip sets to enhance their desired properties. In this way, single or multiple materials may be used for a combination of functions within the gas sensor application.
  • some conformal coating materials based on epoxies and similar materials naturally absorb radiation in the infrared spectrum at specific frequencies. Additionally, materials such as glass have precise optical cut offs around 4 ⁇ m and some cyanoacrylates have strong IR absorptions around 3.5 ⁇ m.
  • Other suitable coatings are acylic resins, polyerethane resins, silicone resins, latex rubber or a UV curable system. More than one of these coatings may be applied in combination to provide the filter.
  • the composition or mixture of the conformal / protection / passivation coating may be modified to tune the optical absorption to suit the optical requirements of the gas sensing application, with or without the need for conventional optical filter techniques being applied.
  • the use of a conformal / protection / passivation coating to assist optical bandwidth trimming provides a very low cost solution and alternative to conventional (expensive) optical filters.
  • the demodulated detection signal strength can be balanced or tailored to suit the gases being detected by the subtraction. This is achieved by making the relative amount of emitted radiation corresponding to each emitted band unequal. This may be done by varying the number of LED emission elements at an individual optical frequency or at discretely separated optical frequencies within one gas sensor assembly, thereby providing unequal emission areas for each emitted band.
  • Making the relative amount of emitted radiation corresponding to each emitted band unequal may also be achieved by varying the power supply to each emission element. Directly varying a continuous power supply provides unequal emission power for each emitted band instantaneously. Modulating the power supply, for example by changing the duty cycle of a pulsed power supply to each emitter, provides unequal emission power for each emitted band averaged over time.
  • a graph of absort>ance versus wavelength is shown, with indications of emitter bandwidths also shown.
  • the emission bandwidth of the first LED is shown as a rectangle 12.
  • the emission bandwidth of the second LED is also shown as a rectangle 13. These two emission bandwidths are overlapping but both within the detection bandwidth of the photodiode 14.
  • the absorption peaks are shown for the three gases CO 2 15, N 2 O 16 and CO 17.
  • the first LED emits at a bandwidth that spans the three absorption peaks. Therefore the demodulated detection signal for the first LED contains information about absorption by the three gases.
  • the demodulated detection, signal corresponding to the second LED contains information only relating to the two gases N 2 O and CO that have absorption peaks within the emission bandwidth of the second LED.
  • the processor subtracts the demodulated detection signal of the LED from that of the first LED. This provides a detection signal relating only to CO 2 therefore the gas sensor is able to differentiate between absorption signals of CO 2 relative to the other gases.
  • figure 3 shows the absorbance spectrum of three gases, however the LED emitters and photodiode are tuned to have different bands.
  • the bands are configured such that subtraction of the second LED's demodulated detection signal from that of the first provides the CO detection signal thereby differentiating the CO gas signal from that of the other gases.
  • the absorption band 18 of a band stop filter is shown superimposed on a graph of the unfiltered LED emission power 19.
  • the power curve actually shows a dip 20 due to absorption caused by the unintentional filtering effect of the glue used for mounting the LED.
  • the gas sensor may also include a reference diode as well as a detecting photodiode, to allow more accurate . differentiation between the two emitters and additionally assist in temperature compensation.
  • the reference photodiode receives the modulated emitted radiation so as to provide a received reference signal and a reference demodulator demodulates the discrete modulation signals from the received reference signal thereby providing a demodulated reference signal for each of the first and second emitted bands.
  • the reference demodulator demodulates the discrete modulation signals from the received reference signal output from the reference photodiode. This provides the demodulated reference signal for each emitted band.
  • a comparator or for example the processor, uses the demodulated detection signal and demodulated reference signal to perform temperature compensation.
  • the reference photodiode may be used in the same optical path as the detecting photodiode or may be located at another convenient point either having a shorter pathlength with the same optics or having a separate optical path entirely.

Abstract

In an improved non-dispersive infrared (NDIR) gas sensor for differentiating gases in a gas mixture, a radiation source, for example having two LEDs (1,2), emits radiation having first (3) and second (4) emitted bands overlapping in frequency. A modulator (5) is configured to modulate the emitted radiation so as to provide each of the first and second emitted bands with a discrete modulation signal. After the radiation passes through a gas sample chamber (6) , a radiation detector, for example a photodiode (7) , receives the radiation. Demodulated (8) detection signals for each of the first and second emitted bands are used to differentiate gases in the gas mixture, by subtraction. The emitted bands may have different power from different LED areas or duty cycles. Epitaxially grown or coated filters (10,11) on the LEDs trim the bandwidth of the emitted bands .

Description

Differentiating Gas Sensor
The present invention relates to gas sensing, in particular gas sensors having a radiation source (emitter) and a radiation detector, such as non- dispersive infrared (NDIR) gas sensors.
In the field of gas sensing there is a requirement for the accurate differentiation (i.e. discrimination) of individual gases where several gases are present. A low cost LED (Light Emitting Diode) may be used as a radiation source in an NDIR gas sensor. Although a LED does have relatively narrow emission bandwidth, the bandwidth is still broad enough to span the absorption bands of more than one gas. Thus sensing a change in the concentration of one gas may be confused by changes in another's concentration.
In the automotive industry, when sensing of the presence of automotive exhaust gases and CO2 in vehicle cabins, it is important to discriminate the CO2 absorption band from that of the adjacent N2O and CO absorption bands. The need for detection of CO2 in vehicle cabins comes from the use of CO2 refrigerant based air conditioning systems instead of more environmentally harmful fluorocarbon based refrigerants. By providing CO2 based air conditioning systems, automotive manufacturers will be able to avoid emission penalties applied to the disposal and recycling of hydro-fluorocarbons. However, conventional gas sensors having suitable sensitivity for efficient CO2 gas detection and differentiation from other gases are too large and expensive for use in such automotive applications. It would facilitate the use of low cost LED/photodiode based NDIR detectors in such applications if their ability to differentiate individual gases was improved.
It is an object of the present invention to improve differentiation of gases in gas sensors.
According to the present invention, there is provided a gas sensor for differentiating gases in a gas mixture, the gas sensor comprising: a radiation source operable to emit radiation, the radiation having first and second emitted bands overlapping in frequency; a modulator configured to modulate the emitted radiation so as to provide each of the first and second emitted bands with a discrete modulation signal; a cavity for conveying the modulated emitted radiation through the gas mixture; a radiation detector operable to receive the modulated emitted radiation so as to provide a received detection signal; and a demodulator, configured to demodulate the discrete modulation signals from the received detection signal thereby providing a demodulated detection signal for each of the first and second emitted bands .
Preferably, the gas sensor further comprises differentiation means configured to differentiate gases in the gas mixture.
Preferably, differentiation means is configured to differentiate gases in the gas mixture using the demodulated detection signals of each of the first and second emitted bands.
Preferably, the differentiation means is configured to differentiate gases by subtracting the demodulated detection signal of the first emitted band from the demodulated detection signal of the second emitted band.
Preferably, the radiation source comprises at least one emitter.
Preferably, the radiation source is configured such that the relative amount of radiation comprising each of the first and second emitted bands is unequal.
Preferably, the radiation source comprises unequal emission areas for each of the first and second emitted bands . Preferably, the radiation source is configured to provide unequal emission power for each of the first and second emitted bands.
Preferably, the unequal emission power is provided by varying the modulation duty cycle of at least one emitter.
Preferably, the radiation source comprises at least one light emitting diode.
Preferably, the radiation source further comprises at least one filter configured to trim the bandwidth of the at least one emitted band by filtering a portion of the emitted radiation.
Preferably, the at least one filter comprises a band pass filter.
Preferably, the at least one filter comprises a band stop filter .
Preferably, the at least one filter comprises a low pass filter.
Preferably, the at least one filter comprises a high pass filter.
Preferably, the at least one filter is configured to filter the portion of the emitted radiation such that the bandwidth of at least one of the filtered emitted bands is narrower than the bandwidth of its corresponding emitted band. Preferably, the at least one filter is configured to filter the portion of the emitted radiation such that the bandwidth of at least one of the filtered emitted bands is no greater than 1.5μm wide.
Preferably, the at least one filter is configured to trim only one side of the bandwidth of the at least one emitted band.
Alternatively, the at least one filter is configured to trim both sides of the bandwidth of the at least one emitted band.
Preferably, at least one emitter comprises the filter.
Preferably, the at least one filter is formed by molecular beam epitaxial growth on at least one emitter.
Preferably, the at least one filter comprises a coating on at least one emitter.
Preferably, the coating comprises a conformal deposited coating.
Preferably, the coating comprises a passivation coating.
Optionally, the coating comprises epoxy resin.
Optionally, the coating comprises cyanoacrylate .
Optionally, the coating comprises glass. Optionally, the coating comprises acylic resin.
Optionally, the coating comprises polyerethane resin.
Optionally, the coating comprises silicone resin.
Optionally, the coating comprises latex rubber.
Optionally, the coating comprises a UV curable system.
Preferably, the at least one filter further comprises a further material applied to the coating, wherein the further material is configured to enhance the band pass filter effect of the coating.
Preferably, the at least one filter is configured to match the filtered radiation to the optical frequency cut off of the radiation detector.
Preferably, the gas sensor further comprises: a reference radiation detector operable to receive the modulated emitted radiation so as to provide a received reference signal; and a reference demodulator, configured to demodulate the discrete modulation signals from the received reference signal thereby providing a demodulated reference signal for each of the first and second emitted bands.
The present invention will now be described by way of example only with reference to the figures in which: Figure 1 illustrates in schematic form a gas sensor in accordance with the preferred embodiment of the present invention; Figure 2 illustrates in schematic form the absorbance spectrum of three gases and the band widths of two LEDs and a photodiode configured for differentiating CO2; Figure 3 illustrates in schematic form the absorbance spectrum of three gases and the band widths of two LEDs and a photodiode configured for differentiating CO; and Figure 4 illustrates the absorption band of a filter shown superimposed on a graph of the unfiltered LED emission power.
With reference to figure 1, the radiation source comprises two LED emitters 1,2. The radiation source may have multiple LEDs with adjacent or overlapping emission frequency bands. In another embodiment, a single LED with multiple emission frequency bands may be used. In this embodiment the bands 3, 4 are overlapping in frequency. A modulator 5 modulates the emitted radiation so as to provide each emitted band with a discrete modulation signal. The cavity 6 conveys the modulated emitted radiation through the gas mixture being sensed. The gas mixture contains for example CO2, N2O and CO. The photodiode radiation detector 7 receives the modulated emitted radiation and outputs a received detection signal.
In an alternative embodiment two, three or more matched emitter/detector pairs may be used. The demodulator 8 demodulates the discrete modulation signals from the received detection signal output from the photodiode. This provides the demodulated detection signal for each emitted band. The processor 9 subtracts one demodulated detection signal from another in order to differentiate the absorption signals of different gases.
The modulation used in this embodiment is achieved by driving each LED with a different square wave. The LED emitters are more efficient if driven with strong pulses and then allowed to rest for a period. Such modulation is used to reduce the overall power to the LED whilst maintaining the efficiency. A LED may be driven with 200 to 250 mA, however this may only be maintained if the duty cycle is less than 25% as the LED would quickly overheat. Another modulation requirement is to increase the signal to noise ratio and to make the gas sensor more compatible with standard devices that operate at frequencies well above 1 KHz.
Band pass filters 10, 11 may be applied to the surface of the LEDs. A filter may alternatively be a band stop filter, a low pass filter or a high pass filter. In this embodiment the filters are formed as part of the radiation source by molecular beam epitaxy during growth of the LEDs. The structure of each filter is chosen so as to trim the emitted bandwidth by filtering only one side, or both sides, of the emitted bandwidth. The filter enhances the gas sensor's output signal with respect to a specific gas and causes the overall sensor bandwidth and therefore sensitivity to only one gas to be enhanced. In another embodiment, the filter is applied as a coating to the surface of the LED, as a conformal / protection / passivation coating designed to protect the LED from environmental problems such as water ingress. In that case, the coating is selected because it has an absorption band at a convenient location in the spectrum. Additionally, a further material may be added to a conventional coating material that enhances, moves or creates an optical filtering effect. The further material may be applied to the processed and mounted chip sets to enhance their desired properties. In this way, single or multiple materials may be used for a combination of functions within the gas sensor application. Typically, some conformal coating materials based on epoxies and similar materials naturally absorb radiation in the infrared spectrum at specific frequencies. Additionally, materials such as glass have precise optical cut offs around 4μm and some cyanoacrylates have strong IR absorptions around 3.5μm. Other suitable coatings are acylic resins, polyerethane resins, silicone resins, latex rubber or a UV curable system. More than one of these coatings may be applied in combination to provide the filter.
In this embodiment, the composition or mixture of the conformal / protection / passivation coating may be modified to tune the optical absorption to suit the optical requirements of the gas sensing application, with or without the need for conventional optical filter techniques being applied. The use of a conformal / protection / passivation coating to assist optical bandwidth trimming provides a very low cost solution and alternative to conventional (expensive) optical filters. The demodulated detection signal strength can be balanced or tailored to suit the gases being detected by the subtraction. This is achieved by making the relative amount of emitted radiation corresponding to each emitted band unequal. This may be done by varying the number of LED emission elements at an individual optical frequency or at discretely separated optical frequencies within one gas sensor assembly, thereby providing unequal emission areas for each emitted band.
Making the relative amount of emitted radiation corresponding to each emitted band unequal may also be achieved by varying the power supply to each emission element. Directly varying a continuous power supply provides unequal emission power for each emitted band instantaneously. Modulating the power supply, for example by changing the duty cycle of a pulsed power supply to each emitter, provides unequal emission power for each emitted band averaged over time.
With reference to figure 2 a graph of absort>ance versus wavelength is shown, with indications of emitter bandwidths also shown. The emission bandwidth of the first LED is shown as a rectangle 12. The emission bandwidth of the second LED is also shown as a rectangle 13. These two emission bandwidths are overlapping but both within the detection bandwidth of the photodiode 14. The absorption peaks are shown for the three gases CO215, N2O 16 and CO 17. In operation, the first LED emits at a bandwidth that spans the three absorption peaks. Therefore the demodulated detection signal for the first LED contains information about absorption by the three gases. However, the demodulated detection, signal corresponding to the second LED contains information only relating to the two gases N2O and CO that have absorption peaks within the emission bandwidth of the second LED. The processor subtracts the demodulated detection signal of the LED from that of the first LED. This provides a detection signal relating only to CO2 therefore the gas sensor is able to differentiate between absorption signals of CO2 relative to the other gases.
Similarly, figure 3 shows the absorbance spectrum of three gases, however the LED emitters and photodiode are tuned to have different bands. In this case the bands are configured such that subtraction of the second LED's demodulated detection signal from that of the first provides the CO detection signal thereby differentiating the CO gas signal from that of the other gases.
With reference to Figure 4, the absorption band 18 of a band stop filter is shown superimposed on a graph of the unfiltered LED emission power 19. The power curve actually shows a dip 20 due to absorption caused by the unintentional filtering effect of the glue used for mounting the LED.
The gas sensor may also include a reference diode as well as a detecting photodiode, to allow more accurate . differentiation between the two emitters and additionally assist in temperature compensation.
The reference photodiode receives the modulated emitted radiation so as to provide a received reference signal and a reference demodulator demodulates the discrete modulation signals from the received reference signal thereby providing a demodulated reference signal for each of the first and second emitted bands.
The reference demodulator demodulates the discrete modulation signals from the received reference signal output from the reference photodiode. This provides the demodulated reference signal for each emitted band. For each emitted band, a comparator, or for example the processor, uses the demodulated detection signal and demodulated reference signal to perform temperature compensation.
The reference photodiode may be used in the same optical path as the detecting photodiode or may be located at another convenient point either having a shorter pathlength with the same optics or having a separate optical path entirely.
Further modifications and improvements may be made without departing from the scope of the invention herein described.

Claims

Claims
1. A gas sensor for differentiating gases in a gas mixture, the gas sensor comprising: a radiation source operable to emit radiation, the radiation having first and second emitted bands overlapping in frequency; a modulator configured to modulate the emitted radiation so as to provide each of the first and second emitted bands with a discrete modulation signal; a cavity for conveying the modulated emitted radiation through the gas mixture; a radiation detector operable to receive the modulated emitted radiation so as to provide a received detection signal; and a demodulator, configured to demodulate the discrete modulation signals from the received detection signal thereby providing a demodulated detection signal for each of the first and second emitted bands .
2. The gas sensor of claim 1 further comprising a differentiation means configured to differentiate gases in the gas mixture.
3. The gas sensor of claim 2 wherein the differentiation means is configured to differentiate gases in the gas mixture using the demodulated detection signals of each of the first and second emitted bands.
4. The gas sensor of claim 3 wherein the differentiation means is configured to differentiate gases by subtracting the demodulated detection signal of the first emitted band from the demodulated detection signal of the second emitted band.
5. The gas sensor of any previous claim wherein the radiation source comprises at least one emitter.
6. The gas sensor of any previous claim wherein the radiation source is configured such that the relative amount of radiation comprising each of the first and second emitted bands is unequal.
7. The gas sensor of any previous claim wherein the radiation source comprises unequal emission areas for each of the first and second emitted bands.
8. The gas sensor of any previous claim wherein the radiation source is configured to provide unequal emission power for each of the first and second emitted bands.
9. The gas sensor of claim 8 wherein the unequal emission power is provided by varying the modulation duty cycle of at least one emitter.
10. The gas sensor of any previous claim wherein the radiation source comprises at least one light emitting diode.
11. The gas sensor of any previous claim wherein the radiation source further comprises at least one filter configured to trim the bandwidth of the at least one emitted band by filtering a portion of the emitted radiation.
12. The gas sensor of claim 11 wherein the at least one filter comprises a band pass filter.
13. The gas sensor of any of claims 11 to 12 wherein the at least one filter comprises a band stop filter.
14. The gas sensor of any of claims 11 to 13 wherein the at least one filter comprises a low pass filter.
15. The gas sensor of any of claims 11 to 14 wherein the at least one filter comprises a high pass filter.
16. The gas sensor of any of claims 11 to 15 wherein the at least one filter is configured to filter the portion of the emitted radiation such that the bandwidth of at least one of the filtered emitted bands is narrower than the bandwidth of its corresponding emitted band.
17. The gas sensor of any of claims 11 to 16 wherein the at least one filter is configured to filter the portion of the emitted radiation such that the bandwidth of at least one of the filtered emitted bands is no greater than 1.5μm wide.
18. The gas sensor of any of claims 11 to 17 wherein the at least one filter is configured to trim only one side of the bandwidth of the at least one emitted band.
19. The gas sensor of any of claims 11 to 18 wherein the at least one filter is configured to trim both sides of the bandwidth of the at least one emitted band.
20. The gas sensor of any of claims 11 to 19 wherein at least one emitter comprises the filter.
21. The gas sensor of any of claims 11 to 20 wherein the at least one filter is formed by molecular beam epitaxial growth on at least one emitter.
22. The gas sensor of any of claims 11 to 21 wherein the at least one filter comprises a coating on at least one emitter.
23. The gas sensor of claim 22 wherein the coating comprises a conformal deposited coating.
24. The gas sensor of any of claims 22 to 23 wherein the coating comprises a passivation coating.
25. The gas sensor of any of claims 22 to 24 wherein the coating comprises epoxy resin.
26. The gas sensor of any of claims 22 to 24 wherein the coating comprises cyanoacrylate .
27. The gas sensor of any of claims 22 to 24 wherein the coating comprises glass.
28. The gas sensor of any of claims 22 to 24 wherein the coating comprises acylic resin.
29. The gas sensor of any of claims 22 to 24 wherein the coating comprises polyerethane resin.
30. The gas sensor of any of claims 22 to 24 wherein the coating comprises silicone resin.
31. The gas sensor of any of claims 22 to 24 wherein the coating comprises latex rubber.
32. The gas sensor of any of claims 22 to 24 wherein the coating comprises a UV curable system.
33. The gas sensor of any of claims 22 to 32 wherein the at least one filter further comprises a further material applied to the coating, wherein the further material is configured to enhance the band pass filter effect of the coating.
34. The gas sensor of any of claims 22 to 33 wherein the at least one filter is configured to match the filtered radiation to the optical frequency cut off of the radiation detector.
35. The gas sensor of any of previous claim further comprising: a reference radiation detector operable to receive the modulated emitted radiation so as to provide a received reference signal; and a reference demodulator, configured to demodulate the discrete modulation signals from the received reference signal thereby providing a demodulated reference signal for each of the first and second emitted bands .
PCT/GB2007/000060 2006-01-10 2007-01-10 Differentiating gas sensor WO2007080398A1 (en)

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GB0600382.6 2006-01-10
GB0600381.8 2006-01-10
GB0600381A GB0600381D0 (en) 2006-01-10 2006-01-10 Gas sensor with optical filter
GB0600382A GB0600382D0 (en) 2006-01-10 2006-01-10 Differentiating gas sensor

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