Method for active photoacoustic noise cancellation

Abstract

A method for calibrating a photoacoustic system (100) for detecting the concentration of fluids, said method comprising a step of prearranging a photoacoustic system for fluids (100) comprising a support (105) arranged to receive a fluid to be examined, a primary optical source (110), a secondary optical source (120), an electronic system comprising a control unit arranged to drive the primary optical source (110) and the secondary optical source (120) by means of two respective periodic signals s1(t) and s2(t), an acoustic transducer (130) arranged to acquire an acoustic response signal p M . The method also comprises the steps of selecting a fluid to be examined having a plurality of absorption peaks p i in the electromagnetic spectrum, associated with corresponding wavelengths λi, with i = 1,2,..., n; in the plurality of absorption peaks p i , selecting an absorption peak p* associated with a corresponding wavelength λ*; defining a first wavelength interval between two end wavelengths λ*1A and λ*1B, where λ*1A <λ*<λ*1B; defining a second wavelength interval between two end wavelengths λ*2A and λ*2B, where λ*2A <λ*<λ*2B, and where λ*2A < λ*1A and λ*2B > λ*1B; by the primary optical source (110), transmitting toward the support (105), without fluid, an electromagnetic radiation having a wavelength λ1, such that λ*1A < λ1 < λ*1B; by the secondary optical source (120), transmitting toward the support (105), without fluid, an electromagnetic radiation having a wavelength λ2, such that λ2 < λ*2A or λ2 > λ*2B; by the acoustic transducer (130), acquiring an acoustic response signal p M = p w1 +p w2 ; iterating the steps of variation of the values of I2 and φ2, emission of the electromagnetic radiations having wavelengths λ1 and λ2, acquisition of the acoustic response signal p M = p w1 +p w2 ; suspending the iteration when the condition p M = p* M is reached, where p* M is a predetermined value.

Description

TITLEMethod for active photoacoustic noise cancellationDESCRIPTIONField of the invention

[0001] The present invention relates to the technical field of photoacoustic detection of fluid concentration.

[0002] In particular, the present invention relates to a method for calibrating a photoacoustic system aimed at reducing background noise.Description of the prior art

[0003] As is known, photoacoustic gas detection systems exploit the selective absorption of light energy by gas molecules to measure their concentration.

[0004] In particular, when the molecules of a gas are illuminated with a modulated (or pulsed) optical source having a specific wavelength, they enter an excited state. Due to the thermoelastic effect, they generate a standing acoustic wave in the medium, the intensity of which is proportional to the density of the excited molecules. This phenomenon makes it possible to derive information about the concentration of the gas being monitored.

[0005] A photoacoustic gas system typically comprises a measuring chamber in which the gas is confined(photoacoustic cell), an excitation optical source, and an acoustic transducer to acquire the signal. However, the signal is typically weak due to the low absorption coefficient of the gases of interest and the low concentrations to be measured.

[0006] From the point of view of electronic measurement, this weakness makes it difficult to measure the gas concentration, due to acoustic interference caused by ambient noise and disturbances on the signal. The mitigation of these two noise components is generally addressed through noise compensation techniques involving the signal acquisition chain. Typically, two identical measuring chambers are used, from which the photoacoustic signal is acquired simultaneously and differentiated.

[0007] Another noise component, which is difficult to compensate electronically or through differential acquisition, is the noise generated by the photoacoustic effect in the walls of the measuring chamber used to contain the gas. The inner walls of the photoacoustic cell are also excited by the modulated optical source, generating an acoustic wave (resulting from vibrational motions of the material) at the same frequency as the gas photoacoustic signal.

[0008] For the purpose of measuring the gas concentration, this component constitutes background noise that overlapsthe useful gas photoacoustic signal. This additive noise is highly interfering and limits the ability to detect the gas to be measured when it is present at low concentrations, affecting the sensor's performance in terms of sensitivity and accuracy.

[0009] The document "Active noise reduction for a differential Helmholtz photoacoustic sensor excited by an intensity-modulated light source", Zhengang Li et al., 2023, describes a method for reducing photoacoustic noise that uses two measuring chambers structured in such a way as to exhibit resonant behavior with opposite phase. In this way, it is possible to isolate the noise component, downstream of the photoacoustic signal acquisition, by comparing the signals.

[0010] However, this method requires the measuring chambers and environmental conditions to be perfectly identical during the measurements. This condition is realistically very difficult to achieve and, in practice, reduces the effectiveness of the actual background noise compensation.Summary of the invention

[0011] It is therefore a feature of the present invention to provide a method for calibrating a photoacoustic system that minimizes background noise during the detection of fluid concentrations.

[0012] It is also a feature of the present invention to provide such a method that is more effective in reducing background noise than known systems.

[0013] These and other objects are achieved by a method for calibrating a photoacoustic system for detecting the concentration of fluids, said method comprising the steps of:- prearranging a photoacoustic system for fluids, comprising:— a support arranged to receive a fluid to be examined;— a primary optical source;— a secondary optical source;— an electronic system comprising a control unit, said electronic system arranged to drive said primary optical source and said secondary optical source (120) by means of two respective periodic signals ^(t) and s2(t) to produce respective excitation electromagnetic radiations modulated in intensity by said periodic signals ^(t) and s2(t), said signal ^(t) having frequency fr, amplitude / xand phase q)1and said signal s2(t) having frequency fr, amplitude / 2and phase <p2;— an acoustic transducer arranged to acquire an acoustic response signal pM;- selecting a fluid to be examined, said fluid having a plurality of absorption peaks pt in the electromagnetic spectrum, associated with corresponding wavelengths Aj, with i= 1,2,- within said plurality of absorption peaks pi, selecting an absorption peak p* associated with a corresponding wavelength A*;- defining a first wavelength interval between two end wavelengths A^Aand A^B, where A^A< A*< A^B;- defining a second wavelength interval between two end wavelengths A2*Aand A2B, where A2A< A*< A2B, and where A2A< A*1Aand A2B> X1B;- by means of said primary optical source, transmitting toward said support, in the absence of said fluid, an electromagnetic radiation having a wavelength Ar, such that A*1A< A±< A^B;- by means of said secondary optical source, transmitting toward said support, in the absence of said fluid, an electromagnetic radiation having a wavelength A2, such that A2< A2Aor ^2> ^2fi'- by means of said acoustic transducer, acquiring an acoustic response signal pM= pwl+ pW2, where pwland pw2are the photoacoustic noise signals due to the optical absorption of said support (105) in response to said electromagnetic radiations havingwavelengths, respectively,and A2;- iterating the steps of:— variation of the values of I2and (p2;— emission of said electromagnetic radiations having wavelengthsand A2;— acquisition of said acoustic response signal PM = Pwi+ Pw2 suspending said iteration when the condition pM=is reached, where p^ is a predetermined value.

[0014] In this way, it is possible to reduce the background noise generated by the support by acting directly on the acoustic response signal, having freedom in the positioning of the optical sources and without needing to modify the material of the support according to the fluid to be examined.

[0015] Furthermore, in this way, it is possible to maximize the ratio between the signal component due to the response of the fluid and the one due to the background noise produced by the support, that is, to maximize the dynamic range of the useful signal. In this way, the useful signal of the fluid can be amplified up to the limits imposed by the saturation of the electronics.

[0016] In particular, said absorption peak p* is the one with the highest value within said plurality of absorption peaks pi.

[0017] Advantageously, the values A*1Aand A1Bare defined in such a way that, at such values, said fluid to be examined has an absorption of 50% with respect to the value of said selected absorption peak p*.

[0018] Advantageously, the values A2yland A2Bare defined in such a way that, at such values, said fluid to be examined has an absorption of 20% with respect to the value of said selected absorption peak p*.

[0019] In particular, the value of p^ is the minimum value of pMreached during said iteration step.

[0020] In particular, the support is a measuring chamber for photoacoustic gas detection.

[0021] Advantageously, the periodic signals s^t) and s2(t) are defined, respectively, by the equations ^(t)= / -£sin(2nfrt+ <pi) and s2(t)= / 2sin(2nfrt+ <p2)•

[0022] In particular, the photoacoustic system may comprise additional optical sources having respective periodic signals S / (t), with j = 1,2,...,m . For each of these signals, the steps defined for the signals of the first two optical sources may be repeated, in order to increase the accuracy in background noise reduction.

[0023] According to another aspect of the present invention, a method is claimed for photoacoustic detection of fluid concentration according to claim 8, which makesuse of the photoacoustic system calibrated by the calibration method according to any one of claims 1 to 7.

[0024] It is a further object of the present invention to provide a photoacoustic system for detecting fluid concentration according to claim 9, which allows background noise to be minimized in an automated manner.Brief description of the drawings

[0025] The invention will be now shown with the following description of some exemplary embodiments, exemplifying but not limitative, with reference to the attached drawings in which:- Fig. 1 schematically shows the following steps of an embodiment of the calibration method for a photoacoustic system according to the present invention;- Fig. 2 shows a possible embodiment of the photoacoustic system according to the present invention.Description of some preferred exemplary embodiments

[0026] With reference to Figs. 1 and 2, the method for calibrating a photoacoustic system 100 for detecting fluid concentration, according to the present invention, comprises a first step of prearranging a photoacoustic system for fluids 100

[0301] comprising a support 105arranged to receive a fluid to be examined, a primary optical source 110, a secondary optical source 120, an electronic system arranged to drive the primary optical source 110 and the secondary optical source 120, and an acoustic transducer 130 arranged to acquire an acoustic response signal pM.

[0027] For example, in the case where the fluid is a gas, the support 105 may be a measuring chamber of the type shown in Fig. 2, in which the gas is arranged to circulate internally .

[0028] In particular, the electronic system is arranged to drive the primary optical source 110 and the secondary optical source 120 by means of two respective periodic signals Si(t) and s2(t) to produce respective excitation electromagnetic radiations modulated in intensity by the periodic signals ^(t) and s2(t) themselves. In general terms, the signal ^(t) has frequency fr, amplitude / xand phase (plfwhereas the signal s2(t) has frequency fr, amplitude / 2and phase (p2• For example, the periodic signals ^(t) and s2(t) may be sinusoidal signals expressed in the form ^(t)= / -£sin2nfrt+ <p±and s2(t)= I2sin2nfrt+ <p2.

[0029] The calibration method then includes a step of selecting the fluid to be analyzed

[0302] and the subsequent selection, among the absorption peaks pt in the electromagnetic spectrum of said fluid, of an appropriateabsorption peak p* associated with a corresponding wavelength A*

[0303] . For example, this absorption peak p* may be the one with the highest value within the plurality of peaks pi.

[0030] Subsequently, the method comprises a step of defining a first wavelength interval between two end wavelengths A^Aand AIB, where A*1A< A*< A^B, and a second wavelength interval between two end wavelengths A2Aand A2B, where A2A< A*< A2B, and where A2A< A*1Aand A2B> A1B

[0304] .

[0031] For example, A*1Aand A^Bmay be defined such that, at those values, the fluid to be examined has an absorption of 50% with respect to the value of the selected absorption peak p*, whereas A2Aand A2Bmay be defined such that, at those values, the fluid to be examined has an absorption of 20% with respect to the value of the selected absorption peak p*.

[0032] The calibration method then includes a step of emitting, by means of the primary optical source 110, an electromagnetic radiation having a wavelength Alfwhere A*1A< Ai< A1B, toward the support 105, in the absence of the fluid to be analyzed

[0305] . Therefore, the value of A±is chosen in such a way that, in the presence of the fluid, the corresponding electromagnetic radiation stimulates both the photoacoustic response of the fluid and that of the support105. During the calibration step, since the fluid is absent,such radiation will produce a photoacoustic response only from the support 105.

[0033] Similarly, the calibration method includes a step of emitting, by means of the secondary optical source 120, an electromagnetic radiation having a wavelength A2, where ^2< ^*2A or A2>A2B, toward the support 105, in the absence of the fluid to be analyzed

[0306] . Therefore, the value of A2is chosen in such a way that, even in the presence of the fluid, the corresponding electromagnetic radiation stimulates only the photoacoustic response of the support 105.

[0034] The calibration method then includes a step of acquiring, by means of the acoustic transducer 130, an acoustic response signal pM. Since the two emission steps are carried out in the absence of the target fluid, the response signal will be in the form PM=PWI+PW2, where pwland pw2are the photoacoustic noise signals due to the optical absorption of the support 105 in response to the electromagnetic radiations having wavelengthsand A2, respectively .

[0035] Subsequently, the method includes an iteration of the steps of varying the values of I2and (p2, emitting the electromagnetic radiations having wavelengthsand A2and acquiring the acoustic response signal pM= pwl+ pw2. This iteration ends when the condition PM=PM is reached, wherePM is a predetermined value. In particular, the value p^ corresponds to the lowest value of pMreached during the iteration, that is, the value that minimizes the background noise produced by the support 105.

[0036] Therefore, the above-described iteration aims to progressively vary the intensity I2and the phase <p2of the periodic signal s2(t) until an acoustic response from the secondary optical source 120 is obtained that is substantially equal in magnitude and in counter-phase with respect to the acoustic response produced by the primary optical source 110.

[0037] In this way, when the photoacoustic system 100, after the calibration, is used for detecting the fluid concentration, the acoustic response signal pMwill have a background noise component reduced to a minimum, and a clearly distinguishable component due to the fluid response.

[0038] Indeed, as mentioned, the electromagnetic radiation having wavelengthproduces both a photoacoustic response from the fluid and a response from the support 105, while the electromagnetic radiation having wavelength A2produces only a response from the support 105. By placing the two acoustic response signals from sources 110 and 120 in counter-phase and with equal intensity, through the abovedescribed calibration, the components due to the response of the support 105 will cancel each other out, while thecomponent due to the fluid response, produced by the primary source 110 alone, will not be cancelled and will be clearly present in the response signal.

[0039] In other words, during fluid detection, the acoustic response signal will be in the form pM= p^ + pwl+ pw2, where Pf±is the photoacoustic response of the fluid. After calibration, the component pwl+ pw2will be minimized, highlighting the component Pf±.

[0040] In this way, the calibration method according to the present invention makes it possible to reduce the background noise generated by the support 105 by acting directly on the acoustic response signal and without requiring downstream electronic compensation. Moreover, the present method allows the use of any type of material for the support 105 regardless of the fluid to be analyzed and without being subject to particular environmental conditions.

[0041] The foregoing description exemplary embodiments of the invention will so fully reveal the invention according to the conceptual point of view, so that others, by applying current knowledge, will be able to modify and / or adapt for various applications such embodiment without further research and without parting from the invention, and, accordingly, it is therefore to be understood that such adaptations and modifications will have to be considered asequivalent to the specific embodiments. The means and the materials to realise the different functions described herein could have a different nature without, for this reason, departing from the field of the invention. It is to be understood that the phraseology or terminology that is employed herein is for the purpose of description and not of limitation.

Claims

CLAIMS1. A method for calibrating a photoacoustic system (100) for detecting the concentration of fluids, said method comprising the steps of:- prearranging a photoacoustic system for fluids (100), comprising:— a support (105) arranged to receive a fluid to be examined;— a primary optical source (110);— a secondary optical source (120);— an electronic system comprising a control unit, said electronic system arranged to drive said primary optical source (110) and said secondary optical source (120) by means of two respective periodic signals ^(t) and s2(t) to produce respective excitation electromagnetic radiations modulated in intensity by said periodic signals ^(t) and s2(t), said signal ^(t) having frequency fr, amplitude / xand phase q)1and said signal s2(t) having frequency fr, amplitude / 2and phase <p2;— an acoustic transducer (130) arranged to acquire an acoustic response signal pM;- selecting a fluid to be examined, said fluid having a plurality of absorption peaks pt in the electromagnetic spectrum, associated withcorresponding wavelengths Aj, with i= 1,2,- within said plurality of absorption peaks pi, selecting an absorption peak p* associated with a corresponding wavelength A*;- defining a first wavelength interval between two end wavelengths A^Aand A^B, where A^A< A*< A^B;- defining a second wavelength interval between two end wavelengths A2*Aand A2B, where A2A< A*< A2B, and where A2A< A*1Aand A2B> X1B;- by means of said primary optical source (110), transmitting toward said support (105), in the absence of said fluid, an electromagnetic radiation having a wavelength Ar, such that A^A< A±< A*1B;- by means of said secondary optical source (120), transmitting toward said support (105), in the absence of said fluid, an electromagnetic radiation having a wavelength A2, such that A2<2^ or ^2> ^2fi'- by means of said acoustic transducer (130), acquiring an acoustic response signal pM= pw±+ pw2 / where pwland pw2are the photoacoustic noise signals due to the optical absorption of said support (105) in response to said electromagnetic radiations having wavelengths, respectively, A±and A2;- iterating the steps of:— variation of the values of I2and (p2;— emission of said electromagnetic radiations having wavelengthsand A2;— acquisition of said acoustic response signal PM = Pwi+ Pw2t suspending said iteration when the condition pM= p^ is reached, where p^ is a predetermined value.

2. The method for calibrating a photoacoustic system (100), according to claim 1, wherein said absorption peak p* is the one with the highest value within said plurality of absorption peaks pi.

3. The method for calibrating a photoacoustic system (100), according to claim 1, wherein the values of A*1Aand A*1Bare defined in such a way that, at such values, said fluid to be examined has an absorption of 50% with respect to the value of said selected absorption peak p*.

4. The method for calibrating a photoacoustic system (100), according to claim 1, wherein the values ofand A*2Bare defined in such a way that, at such values, said fluid to be examined has an absorption of 20% with respect to the value of said selected absorption peak p*.

5. The method for calibrating a photoacoustic system (100),according to claim 1, whereinis the minimum value of pMreached during said iteration step.

6. The method for calibrating a photoacoustic system (100), according to claim 1, wherein said support (105) is a measuring chamber.

7. The method for calibrating a photoacoustic system (100), according to claim 1, wherein said periodic signals ^(t) and s2(t) are defined, respectively, by the equations SiCt)= / -£sin(2?T / rt+ (Pi) and s2(t)= / 2sin(2nfrt+ <p2)•8. A method for photoacoustic detection of fluid concentration, said method comprising the steps of: prearranging a photoacoustic system for fluids (100), comprising:— a support (105) arranged to receive a fluid to be examined;— a primary optical source (110);— a secondary optical source (120);— an electronic system comprising a control unit, said electronic system arranged to drive said primary optical source (110) and said secondary optical source (120) by means of two respective periodic signals s^t) and s2(t) to produce respective excitation electromagnetic radiations modulated in intensity by said periodic signals s^t) and s2(t), said signal s^t) having frequency fr, amplitude / xand phase q)1and said signal s2(t) having frequency frramplitude I2and phase <p2;— an acoustic transducer (130) arranged to acquire an acoustic response signal pM; selecting a fluid to be examined, said fluid having a plurality of absorption peaks pt in the electromagnetic spectrum, associated with corresponding wavelengths Aj, with i= 1,2,calibrating said photoacoustic system for fluids (100) in relation to said selected fluid to be examined by means of the calibration method according to any of claims from 1 to 7; arranging said fluid to be examined in said support (105); by means of said primary optical source (110), transmitting toward said support (105) an electromagnetic radiation having said wavelengthby means of said secondary optical source (120), transmitting toward said support (105) an electromagnetic radiation having said wavelength A2; by means of said acoustic transducer (130), acquiring an acoustic response signal pM= pgas+ pwl+ pw2, where pgasis the photoacoustic signal due to the optical absorption of said fluid to be examined.

9. A photoacoustic system (100) for detecting fluidconcentration, said photoacoustic system (100) comprising: a support (105); a primary optical source (110); a secondary optical source (120); an electronic system comprising a control unit, said electronic system arranged to drive said primary optical source (110) and said secondary optical source (120) by means of two respective periodic signals ^(t) and s2(t) to produce respective excitation electromagnetic radiations modulated in intensity by said periodic signals ^(t) and s2(t), said signal ^(t) having frequency fr, amplitude / xand phase q)1and said signal s2(t) having frequency fr, amplitude / 2and phase <p2; an acoustic transducer (130) arranged to acquire an acoustic response signal pM; said electronic system being also arranged to: acquiring information relating to a fluid to be examined, said fluid having a plurality of absorption peaks pt in the electromagnetic spectrum, associated with corresponding wavelengths Aj, with i= 1,2,...,n; within said plurality of absorption peaks pi, selecting an absorption peak p* associated with acorresponding wavelength A*; defining a first wavelength interval between two end wavelengths A^Aand A^B, where A^A< A*< A^B; defining a second wavelength interval between two end wavelengths A2*Aand A2B, where A2A< A*< A2B, and where A2A< A^Aand A2B> A^B; receiving a start calibration command; checking the absence of fluid in the vicinity of said support (105); actuating said primary optical source (110) for emitting toward said support (105) an electromagnetic radiation having a wavelength Ax, such that A^A< At< A'1B; actuating said secondary optical source (120) for emitting toward said support (105) an electromagnetic radiation having a wavelength A2, such thator A2> A2B; acquiring from said acoustic transducer (130) an acoustic response signal PM=PWI+PW2, where pwland pw2are the photoacoustic noise signals due to the optical absorption of said support (105) in response to said electromagnetic radiations having wavelengths, respectively, A±and A2; iterating the steps of:— variation of the values of I2and (p2;— actuating said primary optical source (110) and said secondary optical source (120) for emitting said electromagnetic radiations having wavelengthsand d2; — acquisition of said acoustic response signalPM = Pwi+ Pw2t suspending said iteration when the condition pM=is reached, where p^ is a predetermined value.

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