The technical field of the disclosure is the analysis of a gas through use of a black-body or gray-body light source and measurement of absorption of a light wave emitted by the light source. The disclosure more specifically relates to the light source, and notably to the way in which it is driven.
Optical methods are frequently used to analyze gases. Sensors allow the composition of a gas to be determined based on the fact that the species from which the gas is composed have spectral absorption properties that are different from one another. Thus, if an absorption spectral band of a gaseous species is known, the concentration of the latter may be determined via an estimation of the absorption of the light passing through the gas, using the Beer-Lambert law. This principle allows the concentration of a gaseous species present in the gas to be estimated.
In the most common methods, the analyzed gas lies between a light source and a photodetector, referred to as the measuring photodetector, the latter being intended to measure a light wave transmitted by the gas to be analyzed, the light wave being partially absorbed by the latter. The light source is usually a source emitting in the infrared, the method used then usually being designated NDIR detection, NDIR being the acronym of nondispersive infrared. Such a principle has been frequently implemented, and is, for example, described in documents U.S. Pat. No. 5,026,992 and WO2007064370.
The comparison between the light wave in the presence of gas and the light wave without gas allows the absorption of the gas to be characterized. It is, for example, a question of determining an amount of a gaseous species in the gas, in the case of the technology referred to as “absorption-based NDIR.”
Generally, the light source is a pulsed source. The measuring photodetector delivers, on each pulse, a signal that is dependent on the intensity of the light wave transmitted by the gas. The signal generated by the photodetector is therefore formed from pulses, the amplitude of which depends on the absorption, by the gas, of the light wave emitted by the light source. The greater the absorption, the lower the amplitude. A measurement of the amplitude of the pulses allows the absorption to be estimated, the latter being correlated with the amount of the gaseous species to which the absorption is attributed. Thus, the measurement of the amplitude allows the amount of the gaseous species that it is desired to determine to be evaluated. The signal delivered by the photodetector comprises minima, which are located outside the pulses of the light source, and maxima, which result from a pulse of the light source. The amplitude may be measured via a comparison of the maxima and minima, or via a spectral analysis of the signal delivered by the photodetector.
The pulses of the light source are usually generated at a frequency of about one Hz. When a sensor is used for a long period, for example several consecutive months, the light source undergoes aging, this resulting in a decrease in the amplitude of the pulses of the emitted light wave. This is why certain devices comprise a reference photodetector, to measure an amplitude of each pulse in a spectral band considered not to be absorbed by the gas. The reference photodetector may also be such that no gas lies between the light source and the reference photodetector. Use of a reference photodetector allows the light wave that would reach the photodetector if there was no absorption to be estimated, so as to take into account a gradual decrease in the amplitude of the pulses of the light source. This is, for example, described in WO2018149799 or in WO2018162848.
It is proposed to improve current devices, so as to delay the aging of the light source and/or to facilitate a possible spectral analysis of the signals generated by the photodetector.
A first subject of the disclosure is a method for measuring an amount of a gaseous species present in a gas, the gaseous species being able to absorb light in an absorption spectral band, the method comprising the following steps:
the method being characterized in that step b) comprises supplying the light source with a pulsed activation signal, the activation signal comprising electrical pulses, each electrical pulse extending between an initial time and a final time, and each electrical pulse comprising:
The initial duration may be within a range from 2 ms to 20 ms, and preferably within a range from 5 ms to 15 ms. The nominal duration may be within a range from 20 ms to 150 ms, and preferably within a range from 50 ms to 100 ms.
The nominal period is consecutive to the initial period: the end of the initial period corresponds to the start of the nominal period.
According to one embodiment, during the initial period, the activation signal is maintained at the initial level during the initial duration.
According to one embodiment, the nominal period extends to the final time.
According to one embodiment, the nominal period is followed by a final period, in the course of which the activation signal decreases, below the nominal level, during a final duration, until the final time. The final duration may be longer than the initial duration. The final duration may be shorter than the nominal duration. According to this embodiment, the end of the nominal period corresponds to the start of the final period.
In the course of the final period, the activation signal may gradually decrease until the final time, for example according to a continuous function, a linear or sinusoidal function for example.
The final duration may be within a range from 10 ms to 100 ms, and preferably within a range from 20 ms to 50 ms.
A second subject of the disclosure is a sensor for determining an amount of a gaseous species in a gas, the sensor comprising:
According to one embodiment, the sensor comprises:
According to one embodiment,
According to one embodiment, the pulse generator is configured such that each pulse comprises a final period, in the course of which the activation signal decreases, below the nominal level, during a final duration, until the final time. The final duration may be within a range from 10 ms to 100 ms, and preferably within a range from 20 ms to 50 ms.
Other advantages and features will become more clearly apparent from the following description of particular embodiments of the disclosure, which are provided by way of nonlimiting examples, and which are shown in the figures listed below.
The gas G comprises a gaseous species Gx an amount cx(k), a concentration, for example, of which it is desired to determine at a measurement time k. This gaseous species absorbs a measurable portion of the light in an absorption spectral band Δx.
The light source 11 is able to emit the incident light wave 12, in the illumination spectral band Δ12, the latter possibly extending between the near ultraviolet and the mid infrared, for example between 200 nm and 10 μm, and most often between 1 μm and 10 μm. The absorption spectral band Δx of the analyzed gaseous species Gx is comprised in the illumination spectral band Δ12. The light source 11 is a pulsed source, the incident light wave 12 being a pulse of duration generally within a range from 100 ms to 1 s. The light source 11 may notably be a filament light source the filament of which is suspended and heated to a temperature within a range from 400° C. to 800° C. Its emission spectrum, in the emission spectral band Δ12, corresponds to the emission spectrum of a black body.
The measuring photodetector 20 is preferably associated with an optical filter 18, defining the measurement spectral band Δ20 encompassing all or part of the absorption spectral band Δx of the gaseous species.
In the example in question, the measuring photodetector 20 is a thermopile, able to deliver a signal depending on the intensity of the detected light wave. Alternatively, the measuring photodetector may be a photodiode or another type of photodetector.
The reference photodetector 20ref is placed beside the measuring photodetector 20 and is of the same type as the latter. It is associated with an optical filter, referred to as the reference optical filter 18ref. The reference optical filter 18ref defines the reference spectral band Δref, which corresponds to a range of wavelengths that are not absorbed by the gaseous species in question. The reference bandwidth Δref is, for example, centered on the wavelength 3.91 μm.
The intensity I20(k) of the light wave 14 detected by the measuring photodetector 20, which is referred to as the measurement intensity, at a measurement time k, depends on the amount cx(k) at the measurement time, according to the Beer-Lambert law:
where:
The comparison between I20(k) and I0(k), taking the form of a ratio
allows the absorption abs(k) generated by the gaseous species in question at the time k to be defined.
The device comprises a processing unit 30, a microprocessor for example, connected to a memory 32. To determine the amount of analyte cx(k) at each measurement time k, the processing unit receives the signals detected by the measuring photodetector 20 and the reference photodetector 20ref, respectively.
During each pulse from the light source 11, it is thus possible to determine μ(cx(k)), this allowing cx(k) to be estimated given that the relationship between cx(k) and μ(cx(k)) is known.
Expression (1) assumes the intensity I0(k) of the incident light wave 12 is known at the measurement time k. This intensity is determined from the light intensity detected by the reference photodetector.
As indicated above, the light source is pulsed. The measurement time k is a time corresponding to a light pulse. Thus, various measurement times correspond, respectively, to various light pulses.
where
The illumination spectrum S of the light source 11 corresponds to the variation in the luminance L(λ, Temp) as a function of λ, when the light source is at a temperature Temp. Generally, the temperature is within a range from 400° C. to 800° C.
The light source 11 is controlled by an electrical pulse generator 10, which delivers an activation signal in order to activate the light source. The activation signal V is an electrical signal with which the light source 11 is supplied. It is formed from electrical pulses impV, each electrical pulse generating an emission of a light pulse imps by the light source. The level of the activation signal sets the amplitude of each light pulse delivered by the light source. By “level,” what is meant is a current or voltage level of the activation signal. In the remainder of the text, the activation signal is considered to set a voltage across the terminals of the light source. Alternatively, it could be a question of a current of the activation signal with which the light source is supplied.
The inventors have supplied a light source 11, such as described above, with an activation signal V formed from regular rectangular pulses, such as shown in
The duration Δt of the electrical pulses of the activation signal V of the light source 11 was 260 ms. The nominal level VN was 1100 mV. The variation V as a function of time in the activation signal normalized by its maximum value has been schematically shown by black dotted lines.
It may be seen that the intensity I of the light wave emitted by the light source takes the form of a light pulse impI, as was expected. However, it may also be seen that the light pulse impI has a latency with respect to the electrical pulse impV. This results in a time lag between the electrical pulse of the activation signal, and the light pulse of the light source 11. The time lag manifests itself:
Due to the existence of the rise time and fall time, each light pulse impI has a shape such as those schematically shown in
As may be seen in
The objective of the disclosure is to modify the shape of each electrical pulse of the activation signal, in order to adjust the shape of each light pulse emitted by the light source so as to avoid the need for an analog filter.
According to a first embodiment, it is sought to reduce the rise time tr of each light pulse, so that the duration of each light pulse may be reduced. According to a second embodiment, it is sought to adapt the shape of the light wave during its fall, so as to facilitate processing of the signal generated by the measuring photodetector 20, and by the optional reference photodetector 20ref.
As mentioned above, each measurement time k is preferably chosen so as to correspond to a maximum intensity Imax of each light pulse. The longer the rise time tr, the more the measurement time must be offset with respect to the start of an electrical pulse of the activation signal V. The offset makes it possible for the measurement time to correspond to a time at which the light pulse is at its highest.
In order to reduce the rise time tr of the light pulse, it is proposed to modulate the amplitude of each electrical pulse of the activation signal V used to control the light source 11. This is the first embodiment of the disclosure, which embodiment will now be described with reference to
At the end of the initial period Ti, the activation signal is brought to a nominal level VN where it remains for a nominal period TN lasting a nominal duration ΔtN. The nominal level corresponds to a conventional supply level of the light source. The nominal period TN extends between the first time t1 and a second time t2. In the embodiment shown in
Generally, the initial duration Δti is shorter than the nominal duration ΔtN.
The initial duration Δti is preferably within a range from 2 ms to 20 ms, and preferably within a range from 5 ms to 15 ms. The nominal duration ΔtN is preferably within a range from 20 ms to 150 ms, and preferably within a range from 50 ms to 100 ms.
In the example shown in
In
It will be recalled that, because of the negligible response time of a photodiode, the pulses shown in
The value of 1000 mV corresponds to the maximum intensity Imax of the light wave emitted in the prior-art configuration.
It is within the ability of those skilled in the art to test various values of Vi, Δti, or even VN, to obtain, at the photodetector, a pulse shape considered to be optimal. The fact that excessively high values of Vi may significantly increase power consumption will possibly be taken into account. Considering such an initial duration Δti, the maximum level Imax is reached more rapidly than in the prior art. This allows the duration of each light pulse to be reduced, because each measurement time k may be brought closer to the initial time of each pulse. Specifically, the measurement time is a time at which the light pulse has reached the maximum intensity Imax. By reducing the rise time tr of the light pulse, the maximum intensity Imax is reached more rapidly. The final time tf of the pulse may be chosen to lie a few ms or tens of ms after the maximum intensity Imax. This results in a reduction in the total duration of the light pulse. By reducing the duration of the light pulse, aging of the light source 11 is delayed. In the example shown in
According to another embodiment, which may be implemented simultaneously with the first embodiment or independently of the latter, the main objective is to optimize the shape of the light pulse during its fall. According to this second embodiment, it is also possible to seek to reduce the rise time tr. According to this embodiment, as shown in
Preferably, the duration Δtf of the final period Tf is shorter than the duration ΔtN of the nominal period TN, it, for example, being at least 1.5 times shorter or at least 2 times shorter than the duration ΔtN of the nominal period TN. Preferably, the duration of the final period Δtf is longer than the duration of the initial period Δti. The duration Δtf of the final period Tf may be within a range from 10 ms to 100 ms, and preferably within a range from 20 ms to 50 ms. During the final period Tf, the activation signal is lower than the nominal level VN, and a fortiori lower than the initial level Vi.
In the example shown in
Preferably, in the course of the final period Tf, the fall of the activation signal follows a monotonic decreasing function, for example, and advantageously, a sinusoidal function.
According to this embodiment, the light pulse impI decreases in a substantially sinusoidal fashion, as shown in
The electrical pulse impV shown in
According to one variant, the pulse does not comprise an initial period Ti. The initial time ti is then the same as the first time t1 of the nominal period TN.
Whatever the embodiment, each electrical pulse of the activation signal V may have the following parameters:
These parameters may be determined on a case-by-case basis, depending on the sought-after technical effect: decrease in rise time tr of the light pulse and/or optimization of the fall of the light wave. The parameters may, for example, be determined by actuating the light source and observing the variation in the signal detected by a photodetector. The latter may be the measuring photodetector 20 used by the sensor, or a specific photodetector, for example a photodiode, used for the purpose of adjusting the parameters of the shape of the electrical pulses from which the activation signal is formed. The power consumption of the light source may also be taken into account.
The signal generated by the measuring photodetector 20 contains three pulses, corresponding to three light pulses generated by the light source.
The signal generated by the photodetector underwent a frequency analysis, after application of a fast Fourier transform, so as to obtain a spectral power corresponding to various frequencies.
This embodiment allows aging of the source to be limited, because it allows the duration ΔtN of the nominal period TN to be reduced. It is estimated that this embodiment allows the duration of the nominal period to be reduced from 260 ms to 45 ms.
Tests were carried out over one week, using a sensor implementing the disclosure (second embodiment) and a prior-art sensor in which the light source was powered at regular intervals such as shown in
Thus, parameterization of the activation signal V according to the disclosure may:
Embodiments of the disclosure will possibly be employed to control light sources with which gas sensors are equipped, for uses such as monitoring air pollution, food processing, monitoring industrial processes, monitoring combustion gases, etc.
Number | Date | Country | Kind |
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1871955 | Nov 2018 | FR | national |
This application is a national phase entry under 35 U.S.C. § 371 of International Patent Application PCT/FR2019/052784, filed Nov. 22, 2019, designating the United States of America and published as International Patent Publication WO 2020/109708 A1 on Jun. 4, 2020, which claims the benefit under Article 8 of the Patent Cooperation Treaty to French Patent Application Serial No. 1871955, filed Nov. 27, 2018.
Filing Document | Filing Date | Country | Kind |
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PCT/FR2019/052784 | 11/22/2019 | WO | 00 |