Photoacoustic gas sensor

Abstract

The invention relates to a photoacoustic gas sensor (1) comprising a substrate (2); a cover (3) which, together with the substrate (1), forms a measuring cell (4); a microphone (5) provided in the measuring cell (4); and a light source (6) provided in the measuring cell (4). The microphone (5) is provided in the measuring cell (4) such that the center of gravity (411) of the air enclosed in the measuring cell (4) and the center of gravity (521) of the microphone rear chamber (52) substantially coincide in a projection onto the substrate (1).

Description

[0001] Photoacoustic Gas Sensor Technical Field The invention relates to a photoacoustic gas sensor. Prior Art Photoacoustic gas sensors utilize the interaction of electromagnetic radiation ("light") with matter to determine the concentration of a target gas in a carrier gas. Molecules of the target gas can absorb light energy if the light energy corresponds to an energy difference in the rotational / vibrational states of the molecules. Molecules excited in this way can distribute the absorbed energy through collisions with neighboring molecules in the carrier gas: This creates a pressure fluctuation that propagates at the speed of sound and can be measured, for example, with a microphone. The greater the number of molecules in the gas that can absorb the light, the greater the pressure fluctuation and thus the microphone signal.If the light intensity is periodically modulated, the microphone signal is also modulated accordingly, and with, for example, lock-in demodulation of the microphone signal, an offset-free determination of the target gas concentration can be achieved. Pressure fluctuations in the carrier gas, caused by accelerations in the form of vibrations that happen to have the same periodicity, can impair the determination. EP3550286A1 describes a photoacoustic gas sensor where a measuring cell on a substrate forms a measuring volume. A microphone has a bottom opening that points towards the substrate and is communicatively connected to the measuring volume. The scientific article is available via the permalink. Figure 7 on page 7 shows the pressure distribution that arises in a MEMS microphone under acceleration by vibration, calculated using the finite element method. EP2320678A1 discloses an integrated microphone in which an accelerometer is integrated to compensate for vibration effects. CN117388195 discloses an antiseismic photoacoustic cell in which the microphone back chamber is designed such that the center of gravity of the microphone back chamber and the center of gravity of the photoacoustic measuring cell are located on the same side of the microphone diaphragm, in particular where the center of gravity of the microphone back chamber is located further away from the microphone diaphragm than the center of gravity of the photoacoustic measuring cell in a ratio corresponding to the ratio of diaphragm density to air density. Description of the Invention Against this background, a photoacoustic gas sensor is presented here according to independent claims 1 and 3.The photoacoustic gas sensor for detecting a target gas in air comprises a substrate with a first side and a cover on the first side, which together form a measuring cell. A microphone is arranged within the measuring cell, the microphone having a diaphragm of a thickness that defines a pre-chamber and a rear chamber within the microphone. A light source is also arranged within the measuring cell. The measuring cell includes an air volume, the measuring cell having an inlet that allows diffusion of the target gas into the air volume. A distance vector is defined from the geometric center of gravity of the rear chamber to the geometric center of gravity of the air volume. This distance vector has a component parallel to the diaphragm and a component perpendicular to the diaphragm.The microphone is positioned in the measuring cell such that the parallel component has a length of less than 3 millimeters, or such that the perpendicular component deviates in length by less than 3 millimeters from the product of the diaphragm thickness and the relative density of the diaphragm to air, and points towards the rear chamber. When an air-filled, closed volume is subjected to acceleration by vibration, a pressure gradient arises in the volume perpendicular to the direction of acceleration, according to the hydrostatic effect: Δ(Δ) = −Δ ⋅ Δ + Δ0.   , where ^^ ( ^^ ) The pressure difference at location ^^ is referred to as the external pressure, ^^ the density of air, and ^^0  a constant to be determined. Under the assumptions that - the vibration frequency is small compared to a resonance frequency of the volume and therefore the wavelength of sound waves in the air is large compared to the dimensions of the volume, - the acceleration of the air is adiabatic, and - the viscosity of the air can be neglected, then the boundary condition 0 = ∫ ^^(^^)^^^^ applies. Together with the hydrostatic equation, the pressure conditions in the volume can be calculated: ⇒^^0 = ^^^^ ⋅1 ^^∫ ^^^^ ^^ =: ^^^^ ⋅ ^^^^(^^) = ^^^^ ⋅ (^^ − ^^), where ^^ denotes the geometric center of mass of the volume ^^. It should be noted that the volume is only "closed" in the sense that only a negligible amount of air is exchanged with the external volume during a vibration period. In the specific case where the volume is the air in the measuring cell of a photoacoustic gas sensor, an inlet for the target gas is provided. The dimensioning of the inlet also takes into account the desired response time of the gas sensor, which depends on the inlet and the measuring cell volume. The inlet is, for example, covered with a rigid, porous membrane. The microphone also includes a closed volume, namely the so-called rear chamber. The above considerations also apply to these, i.e., the pressure conditions under acceleration also satisfy ^^RK(^^) = ^^^^ ⋅ (^^RK − ^^), where ^^ RKHere, denotes the geometric center of gravity of the microphone's rear chamber. According to the invention, the microphone can now be positioned such that the pressure difference between the two sides of the microphone diaphragm disappears, 0 = ^^MV(^^) − ^^RK(^^) = ^^^^ ⋅ (^^MV − ^^ − (^^RK − ^^)) = ^^^^ ⋅ (^^MV − ^^RK), or can be kept as small as possible in practice. Here, ^^ denotes MV the pressure difference to the outside pressure in the measuring volume and ^^ MVthe geometric center of gravity of the measurement volume. From this arises the condition ^^ ⋅ (^^MV − ^^RK) = 0, meaning the microphone must be positioned in the measurement volume such that the component of the vector difference of the geometric centers of gravity parallel to the direction of acceleration vanishes or is as small as possible. If the acceleration occurs in the plane of the microphone diaphragm, these considerations represent a reasonable approximation. In that case, the diaphragm experiences no displacement due to the acceleration. However, if the acceleration occurs perpendicular to the diaphragm plane, then a term is added to the pressure equalization condition, which reflects the acceleration of the diaphragm itself: ^^MV(^^) = ^^RK(^^) + ^^^^^^ ⋅ ^^^^, where ^^ ^^ the density of the membrane and ^^ ^^denotes a vector whose length corresponds to the thickness of the membrane, is perpendicular to the membrane, and points into the rear chamber. It can then be calculated as a condition that ^^^^ ⋅ (^^MV − ^^) = ^^^^ ⋅ (^^RK − ^^) + ^^^^^^ ⋅ ^^^^ Each acceleration vector can be decomposed into a component perpendicular and parallel to the membrane plane: ^^ = ^^|| + ^^⊥ , and the above conditions for microphone placement are (^^MV − ^^RK)|| = 0 The above considerations for accelerations perpendicular to the diaphragm apply in the case where there is no phase shift between the force on the diaphragm due to the acceleration of its inertial mass and the force due to the pressure difference generated at the diaphragm by the acceleration of the air. In certain embodiments, a non-zero phase shift may prevail in certain vibration frequency ranges, for example, in the low-frequency range of microphones that have an acoustic high-pass filter in the form of a vent in the diaphragm or a digital high-pass filter. The phase shift can start at 90 degrees for vanishing frequencies, pass through 45 degrees at the so-called cutoff frequency, and approach zero degrees for frequencies much higher than the cutoff frequency.This cutoff frequency, also called the low-frequency roll-off cutoff, can be, for example, 25 Hz or below, advantageously 20 Hz or below, or even 15 Hz or below. Due to the phase shift, even with optimal microphone positioning, the RMS value of the pressure difference across the diaphragm does not disappear and is greater the higher the low-frequency roll-off cutoff. The above considerations assume a perfectly rigid cover. For the substrate, such as a printed circuit board, this is a very good approximation due to its considerable thickness. However, for a portion of the cover opposite the substrate, bending can occur under acceleration, for example, if it is made of thin steel. This results in an additional change in volume and thus pressure, which can also be advantageously taken into account.Exact equality of the conditions derived above is not necessary for improved vibration robustness. Under standard conditions of 1013 hectopascals, the differential pressure resulting from acceleration is approximately 13 millipascals per gram and per millimeter of displacement from the optimal position, for example, just under 40 millipascals for 1 gram of acceleration and 3 millimeters of displacement. The physical quantity that a photoacoustic sensor is intended to measure is the concentration of a target gas. However, the physical quantity that is actually measured is a sound pressure acting on the microphone diaphragm. The electrical noise level of typical digital MEMS microphones is on the order of 0.1 millipascals in a 1 Hz wide window at 100 Hz.The pressure generated by the acceleration is therefore many times greater than the noise level in this example and would be incorrectly interpreted by the photoacoustic sensor with a modulation frequency of 100 Hertz as an increased gas concentration. Typical pressure differences, such as those that occur during operation in a measuring cell of a photoacoustic CO2 gas sensor from EP3550286A1, are approximately 5 millipascals per 1000 ppm CO2. In the example above, the error caused by the 3-millimeter offset would thus be expressed in CO2 concentration as approximately 8000 ppm CO2, assuming the vibration frequency exactly matches the photoacoustic excitation frequency. The precise magnitude of the acceleration-induced error in the gas concentration chamber also depends, among other things, on the irradiated light power in the relevant spectral range and the target gas.In one embodiment, the measuring cell of the photoacoustic gas sensor has a volume of less than 10 cubic centimeters, advantageously less than 3 cubic centimeters, and advantageously less than 1 cubic centimeter. With constant incident light power, the signal of a photoacoustic gas sensor decreases inversely to the air volume in the measuring cell. It is therefore advantageous to dimension the measuring cell as small as possible, in particular the area on the substrate within the measuring cell. However, according to the invention, a space-saving arrangement of the microphone and light source within the measuring cell is usually not optimal, because then the microphone cannot maintain the placement described above in at least one direction. In one embodiment, the microphone is a MEMS microphone with a bottom aperture. Such microphones have high sensitivity and a good signal-to-noise ratio due to their relatively large rear chamber.Because, as described above, the geometric center of gravity of the rear chamber plays a significant role in the optimal positioning of the microphone, it is important to note that, in the case of a microphone with a bottom opening, the center of gravity is normally not located above the bottom opening, measured perpendicular to the microphone diaphragm. In one embodiment, the substrate comprises a printed circuit board. The photoacoustic gas sensor can be manufactured particularly cost-effectively if both the microphone and the light source are surface-mount components (SMDs) that are precisely placed and soldered directly onto the circuit board. The cover is also advantageously applied using an SMD process. In one embodiment, the inlet is formed in the cover. To allow the target gas to enter the measuring cell, the photoacoustic gas sensor can have an inlet.The design is particularly simple if the inlet is already formed in the cover, for example, as at least one hole in one side of the cover facing the substrate. The connection between the cover and the substrate can then be made airtight, for example, with a continuous adhesive strip. Advantageously, the at least one hole is covered with a porous membrane. This allows for a larger hole size without the gas exchange between the measuring cell and the environment occurring so rapidly that the photoacoustic signal would be affected. In one embodiment, a control unit is arranged in the measuring cell and configured to modulate the intensity of the light source. To make the photoacoustic gas sensor as compact as possible, it is advantageous to also arrange the control unit for modulating the light source intensity in the measuring cell.An acoustically non-resonant modulation frequency of 500 Hertz or less is advantageous because the photoacoustic signal is inversely proportional to the modulation frequency. Furthermore, such a low modulation frequency allows for the use of slow light sources such as micro-heating plates instead of LEDs or lasers. In one embodiment, the control unit is configured to demodulate an acoustic signal from the microphone to determine a target gas concentration. If the control unit is configured to demodulate the microphone signal at the modulation frequency of the light source, then, for example, the determination and subsequent output of the target gas concentration by the control unit can be achieved by pre-recording and storing the relationship between the target gas concentration and the demodulation value. This allows for particularly simple integration of the photoacoustic gas sensor into consumer devices.Brief description of the drawings: Figure 1 shows a photoacoustic gas sensor with a microphone having a bottom opening. Figure 2 shows a photoacoustic gas sensor with a microphone having a top opening. Figures 3A and 3B show the pressure conditions in a photoacoustic gas sensor under acceleration perpendicular to the microphone diaphragm, with an optimally and a non-optimally positioned microphone with a bottom opening. Figure 4 shows the pressure conditions in a photoacoustic gas sensor with a microphone having a top opening under acceleration perpendicular to the microphone diaphragm. Detailed implementation of the invention: Figure 1 schematically shows a photoacoustic gas sensor 1 according to the invention. It comprises a circuit board 2 and a metal cap 3 with an inlet 42. The metal cap 3 and the circuit board 2 define a measuring cell 4. The target gas, for example CO2 or propane, can diffuse into the measuring cell 4 via the inlet 42.A MEMS microphone 5 with a bottom aperture and a light source 6 are arranged on circuit board 2. For miniaturization and simplified construction, both the microphone 5 and the light source 6 are located inside the measuring cell 4. Setups where the light is introduced into the measuring cell from the outside are more complex and larger: On the one hand, a necessary light-emitting window significantly increases the complexity and thus the cost; on the other hand, more space is required on circuit board 2 if the light source is located outside the measuring cell. In addition to the microphone and light source, other components may also be located in the measuring cell, for example, a light source driver or a microprocessor for signal processing.The air volume 41 enclosed in the measuring cell 4 has a geometric center of gravity 411, which deviates from the geometric center of gravity of the measuring cell more and more components are arranged in the measuring cell, thus reducing the volume accessible to the air. The microphone 5 is mounted on the circuit board 2 such that its bottom opening is spaced away from the circuit board and acoustically connected to the air volume 41 via the air. The microphone can, for example, rest on solder balls or a non-continuous adhesive bead. The microphone has a rear chamber 52, which in turn has a geometric center of gravity 521. The two centers of gravity define a difference vector 7. The microphone 5 is positioned such that the projection of the difference vector onto the plane spanned by the microphone diaphragm is as small as possible, for example, less than 3 millimeters in length.Accelerations parallel to the plane then lead to a minimal pressure difference on both sides of the microphone diaphragm and thus to a minimal error in the concentration output of the sensor. Figure 2 shows an identical photoacoustic sensor, except that here a MEMS microphone 5 with a top aperture is installed. This can be soldered or glued directly onto the circuit board 2 without any concerns about sound reaching the microphone aperture. The microphone back chamber 52 is very small here, and the pressure conditions in the back chamber 52 can be assumed to be approximately constant, equal to the outside pressure. The optimal positioning of the microphone with respect to accelerations parallel to the microphone diaphragm is then again achieved by ensuring that the projection of the difference vector from the back chamber 52 to the geometric center of gravity 411 of the measuring cell onto the diaphragm vanishes.Figures 3A and 3B show the arrangement from Figure 1 under acceleration from left to right. The pressure difference to the ambient pressure is schematically represented by arrows 8, where an upward arrow indicates increased pressure. If the geometric centers of gravity 41 and 521 of the air masses in the two chambers 4 and 52 are close together in the direction parallel to the direction of acceleration, as in Figure 3A, then there is no pressure difference at the microphone diaphragm. However, if the two geometric centers of gravity are far apart, as shown in Figure 3B, then there is a pressure difference at the microphone diaphragm, which leads to its deflection and thus to a distorted concentration reading from the gas sensor. Figure 4 shows the pressure and force relationships for the arrangement from Figure 1 under acceleration perpendicular to the plane of the diaphragm.The microphone diaphragm 51 is subjected to two forces: firstly, the force 92, which originates from the diaphragm mass subjected to acceleration ^^. Secondly, a pressure difference exists across the diaphragm 51 because the geometric centers of gravity 41 and 511 of the measuring cell 4 and the microphone rear chamber 521 are not at the same height when height is measured in the direction of acceleration. This is represented by the difference vector 7 and its component perpendicular to the diaphragm 72. Ideally, the dimensions of the measuring cell and the positioning of the microphone 5 are chosen such that the two forces 91 and 92 cancel each other out.

[0002] List of reference symbols 1 Photoacoustic gas sensor 2 Substrate 3 Cover 4 Measuring cell 41 Air volume 411 Geometric center of gravity of the air volume 42 Inlet 5 Microphone 51 Microphone diaphragm 52 Microphone rear chamber 6 Light source 7 Difference vector of the geometric centers of gravity 71 Component of 7 parallel to the diaphragm 51 72 Component of 7 perpendicular to the diaphragm 51 8 Difference to ambient pressure 91 Force vector resulting from the pressure difference 92 Force vector resulting from diaphragm acceleration 10 Control unit

Claims

Claims 1. Photoacoustic gas sensor (1) for detecting a target gas in air, comprising a measuring cell (4) comprising an air volume (41), a light source (6) in the measuring cell (4) for generating a photoacoustic signal, a microphone (5) in the measuring cell (4) for detecting the photoacoustic signal, wherein the microphone (5) has a diaphragm (51) defining a pre-chamber and a rear chamber (52) in the microphone (5), wherein the microphone (5) is arranged in the measuring cell (4) such that the perpendicular projections onto the diaphragm of the geometric centers of gravity of the rear chamber (52) and of the air volume (41) are less than 3 millimeters apart.

2. Photoacoustic gas sensor (1) according to claim 1, wherein the distance is less than 2 millimeters, preferably less than 1.5 millimeters, preferably less than 1 millimeter, preferably less than 300 micrometers, preferably less than 100 micrometers. 3.Photoacoustic gas sensor (1) for detecting a target gas in air, comprising a measuring cell (4) containing an air volume (41), a light source (6) in the measuring cell (4) for generating a photoacoustic signal, a microphone (5) in the measuring cell (4) for detecting the photoacoustic signal, wherein the microphone (5) has a diaphragm (51) defining a pre-chamber and a rear chamber (52) in the microphone (5), wherein the microphone (5) is arranged in the measuring cell (4) such that the difference between the perpendicular distances to the diaphragm of the geometric centers of gravity of the rear chamber (521) and of the air volume (411) deviates by less than 3 millimeters from the product of the thickness of the diaphragm and the relative density of the diaphragm (51) to air. 4.Photoacoustic gas sensor according to claim 3, wherein the difference preferably deviates by less than 1 millimeter from the product of the thickness of the membrane and the relative density of the membrane (51) to air, preferably less than 300 micrometers, preferably less than 100 micrometers.

5. The photoacoustic gas sensor according to claim 3 or 4, wherein the microphone (5) has a cutoff frequency for low-frequency attenuation of less than 25 Hertz, advantageously less than 20 Hertz, advantageously less than 15 Hertz.

6. The photoacoustic gas sensor according to any one of the preceding claims, wherein the microphone (5) is a MEMS microphone.

7. The photoacoustic gas sensor according to claim 6, wherein the microphone (5) has a bottom opening.

8. The photoacoustic gas sensor according to claim 6, wherein the microphone (5) has a top opening.

9. The photoacoustic gas sensor according to any one of the preceding claims, wherein the measuring cell (4) has a volume of less than 10 cubic centimeters, advantageously less than 3 cubic centimeters, advantageously less than 1 cubic centimeter.

10. The photoacoustic gas sensor according to any one of the preceding claims, wherein the measuring cell (4) is formed by a substrate (2) and a cover (3) applied to the substrate (2). 11.The photoacoustic gas sensor according to claim 10, wherein the cover (3) has an inlet (42) which is covered with a porous membrane.

12. The photoacoustic gas sensor according to any one of the preceding claims, further comprising a control unit (10) arranged in the measuring cell (4) and configured to modulate the intensity of the light source (6).

13. The photoacoustic gas sensor according to claim 12, wherein the control unit (10) is configured to modulate the intensity of the light source (6) at a frequency less than 500 Hertz.

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