• 专利附录
  • 专利说明
  • 权利要求
  • 类似技术
  • 同族专利
  • 引用文献
  • 法律状态
  • 优先权
    • 专利摘要:
    In order to improve the achievable measurement accuracy and the detection limit of a photoacoustic measuring cell (1), it is proposed to choose the ratio of the length L of the resonator cell (2) to the length I of the microphone tube (10), where n is the order of the axial vibration mode of the stationary and the order of the axial oscillation mode of the standing acoustic wave in the microphone tube (10), and at the same time the cross section of the microphone tube (10) is selected so that by the acoustic sensor (11) in dependence the excitation frequency (fM) recorded acoustic signal has a pronounced single maximum (fR).
    公开号:AT511934A2
    申请号:T505902012
    申请日:2012-12-14
    公开日:2013-03-15
    发明作者:
    申请人:Avl List Gmbh；
    IPC主号:
    专利说明:

    Printed: 17-12-2012 E014.1
    102012/50590 AV-3509 AT
    Photoacoustic measuring cell
    The subject invention relates to a photoacoustic measuring cell with a resonator cell, which is bounded on both sides by a chamber having a larger cross-section than the cross section of the resonator cell and is flowed through by a measuring gas, with a laser source 5, the intermittently modulated or pulsed laser beam in the Resonator cell directed, and with a microphone tube, which is arranged in the region of a maximum pressure of the photoacoustic excited in the Resonatorzelie standing acoustic wave, the microphone tube branches off from the resonator cell and at the end of the microphone tube, a sound pickup for receiving an acoustic signal is arranged. Photoacoustic detectors are used inter alia for the measurement of the concentration of solid particles in gaseous media, e.g. for the determination of the Rußmassenkonzentration in vehicle exhaust gases, used and are well known. In this case, a laser beam intermittently modulated or pulsed with a suitably selected operating frequency of the measuring system is directed into a measuring cell filled with measuring gas. During the radiation phases, the particles irradiated by the laser light in the measuring gas absorb the laser light, whereby these and indirectly the measuring gas surrounding the particles are heated. This leads to periodic pressure fluctuations in the gas of the measuring cell, which propagate as sound waves and consequently can be detected with a microphone. The recorded microphone signal is proportional to the particle mass concentration in the measuring cell and is advantageously evaluated for the suppression of noise and signal noise synchronously with the operating frequency and with adapted phase shift, for example with a synchronous rectifier.
    The measuring cell is preferably designed as an acoustic resonator. In order to be able to use the resonance peaking of the acoustic signal to increase the measuring sensitivity, the modulation or pulse frequency of the laser is tuned to a resonant frequency (natural frequency) of the resonator, for which the fundamental oscillation in the axial direction of the measuring cell is generally preferred. By positive interference of the acoustic signals generated by the excited solid particles creates a standing acoustic wave in the axial direction of the resonator. At its axial boundaries pressure magnitudes occur when the boundary has the character of a "firm" termination. The resonator cell actually effective in the measuring cell as a resonator can also be limited on the two sides by acoustic barrier filters (notch filters) in the form of a chamber with a larger cross section, whereby the resonator cell becomes an acoustically "open" resonator on both sides, which in the case the fundamental has a pressure maximum in the center 35 of the resonator row. In such an arrangement, therefore, the sound pressure-measuring microphone should be placed in the center of the resonator. 1/10 * 1 '14-12-2012
    Printed: 17-12-2012 E014.1 10 2012/50590
    AV-3509 AT
    In order to avoid contamination of the window of the measuring cell required for the coupling of the laser light, e.g. To reduce glass windows by the particles present in the sample gas, a counter-current photoacoustic measuring cell has also been proposed, in which the sample gas is supplied from both sides on the outside and is discharged centrally between the two feeders. The measuring gas flowing through the measuring cell thus does not hit the windows, which reduces the pollution. Such a photoacoustic measuring cell is e.g. in EP 1 564 543 B1.
    Due to the ever decreasing particle concentrations in the exhaust gas of internal combustion engines in technical progress, as well as the ever higher demands on the measuring accuracy and the detection limit, also by legal requirements, the demands on the measuring technology are constantly increasing. Conventional improvements in the laser system or the use of more sensitive microphones will therefore no longer be sufficient to meet the demands made on photoacoustic measuring cells.
    It is therefore an object of the present invention to improve a photoacoustic measuring cell 15 with regard to the achievable measuring accuracy and the detection limit.
    This object is achieved in that the length L of the resonator cell and the length I
    Lr fl of the microphone tube are chosen approximately in the ratio - = 2-, where n is the order / w of the axial vibration mode in the resonator cell and m in the microphone tube, and the cross section of the microphone tube is chosen so that by the Schallaufneh-mer in response to the Anregeffequence recorded acoustic signal has a pronounced single maximum. As a result, the standing waves occurring in the resonator cell and in the microphone tube are matched approximately to one another with their own resonant frequency. In general, two resonances which are close to each other in frequency occur in the resonance characteristic of the overall system, but advantageously overlap with a sufficiently small cross-section of the microphone tube to a well-differentiated and clearly pronounced individual resonance of the overall system, so that the frequency dependence of the overall system recorded by the microphone acoustic signal has a well-separated and clearly pronounced single maximum, which means a high gain of the acoustic signal 30. The resonance peak in the microphone tube contributes to the overall resonance peak of the measurement signal and increases the sensitivity of the measuring arrangement. In this way, the measuring accuracy, the resolution and the detection limit of the measuring cell compared to conventional photoacoustic measuring cells are significantly increased.
    Particularly advantageous are the length of the resonator cell in a conventional manner to 35 the resonant frequency of the fundamental acoustic oscillation in the axial direction of the resonator 2/10 '2' 14-12-2012 E014.1
    Printed: 17-12-2012 10 2012/50590
    AV-3509 AT cell in the range of 1000Hz to 12000Hz designed, which is a preferred frequency range for many applications. It has also proven to be particularly advantageous if the microphone tube has a cross section of less than 5 mm 2, in particular less than 1.8 mm 2, or if the cross section has a diameter of the microphone tube smaller than 2.5 mm, in particular less than 1, 5mm, is executed. With these dimensions, favorable results can be achieved with respect to the size and filling times of the measuring cell on the one hand, and a high measuring sensitivity by means of a stable, well-separated and clearly pronounced individual resonance on the other hand.
    If a cooling unit is provided for cooling the sound pickup and / or the microphone tube io, the sound pickup can be effectively protected from excessive temperatures. It is particularly advantageous if a temperature gradient is set by the cooling unit in the microphone tube to form an acoustic lens. In this way, the sound pressure at the end of the microphone tube can be amplified, which further improves the measuring accuracy, the resolution and the detection limit of the measuring cell. The subject invention is explained in more detail below with reference to Figures 1 to 3, which show by way of example, schematically and not by way of limitation advantageous embodiments of the invention. It shows
    1 shows a measuring cell according to the invention with microphone tube,
    2 shows the recorded acoustic signal of a measuring cell according to the invention, and FIG. 3 shows an alternative embodiment of a measuring cell according to the invention.
    The inventive photoacoustic measuring cell 1 according to the embodiment of Figure 1 consists of a cylindrical resonator cell 2 with a circular cross-section having a length L and a diameter D, which is open at both axial ends and by a respective chamber 3, with a diameter larger than the diameter D of the resonator cell 2, 25 is limited. The diameter D is preferably selected such that a laminar volume flow is formed in the resonator cell 2 with a constant flow of measuring gas. At the same time, the diameter D is preferably selected to be so small that substantially only acoustic vibration modes in the axial direction of the resonator cell 2 occur in a broad frequency spectrum, at least in the range of the possible operating frequency of the measuring cell. 30 The two chambers 3 form in a known and proven as an acoustic barrier filter (notch filter). The particle-laden sample gas is supplied via a feed line 4 into the resonator cell 2 and discharged via a discharge line 5. The feed line 4 and the discharge line 5 each open in one of the two chambers 3, wherein the position and the type of supply and discharge of the sample gas in a conventional manner Strömungstech-35 are selected and acoustically favorable. 3/10 -3- 14-12-2012
    Printed: 17-12-2012 E014.1 10 2012/50590
    AV-3509 AT
    From a laser source 6, an intermittently modulated or pulsed laser beam 7 is directed through a glass window, which, as here, the chamber 3 in the axial direction to the outside, directed into the resonator cell 2, advantageously along the longitudinal axis of the resonator cell. 2
    The laser beam leaves the measuring cell 1 on the opposite side also by 5 a glass window. The excitation frequency for the intermittent modulation or for the pulsation of the laser beam 7 is tuned to the frequency of the inventively brought about well-separated and clearly pronounced individual resonance of the overall system (as described below). This can e.g. take place before a measurement, manually, or automatically, for example, by a frequency band is traversed and the frequency 10 of the maximum of the recorded microphone signal is searched.
    The length L of the resonator cell 2 determines the frequency of their acoustic resonances (natural oscillations) and upon excitation with a resonant frequency is formed in the resonator cell 2, a resonant elevated standing acoustic wave 8 from. The notch filters arranged at the ends of the resonator cell effect an acoustically "open" termination of the resonator cell 2 and, according to the fundamental physical relationships, the length L is set at a resonator open at both ends with L ~ n * A / 2, where λ is the wavelength of the standing acoustic wave at the Anrebefirequenz. For the fundamental oscillation n = 1, the length is therefore given as L-A / 2. The diameter D of the resonator cell 2 which remains constant over the length is not relevant for this, as long as essentially only vibrations occur in the axial direction.
    In the region of the maximum pressure of the standing wave 8 in the resonator cell 2, ie here approximately in the middle of the resonator cell 2, a microphone tube 10 is arranged, which opens into the resonator cell 2 with one axial end and at its other axial end a sound pickup 11, such as eg a microphone is arranged. It is irrelevant whether 25 the microphone tube 10 is arranged at right angles to the resonator cell 2, or at any other angle. The microphone tube 10 also represents a resonator in which a standing acoustic wave is formed. In order to tune the resonance of the microphone tube 10 to that of the resonator cell 2 and to combine the two resonance peaks, the length I of the microphone tube 1030 closed on one side is selected according to the fundamental physical relationships with Im * A / 4. For the fundamental oscillation m = 1, the length is consequently Ι-λ / 4, where λ is again the wavelength of the stationary acoustic wave at the operating frequency.
    The maximum pressure of the standing wave 8 in the resonator cell 2, and thus also the preferred axial position of the microphone tube 10, but of course in another structural design, in particular in a non-symmetrical measuring cell 1, also at a different axial position of the resonator cell 2 lie. 4/10 -4- 14-12-2012
    Printed: 17-12-2012 E014.1 10 2012/50590
    AV-3509 AT
    If such a photoacoustic measuring cell 1 with a in the modulation frequency or
    Pulse frequency to the natural frequency of the resonator cell 2 tuned, intermittently modulated or pulsed laser beam 7 excited, results in the resonance frequency fR1 of
    Resonator cell 2 according to the fundamental physical relationship to / R1 = n ^ -, with 5 the speed of sound c in the sample gas. For the resonator cell 2 which is open on both sides, the order n of the oscillation mode is given by a positive natural number, ie ne {1, 2, 3, 4, 5, ...}. Equally, the resonant frequency fR2 of the microphone tube 10 is fK1 = m -. For the unilaterally closed resonator, the order m of the vibration is
    4 L gungsmode given by a positive, odd natural number, so m € {1, 3, 5, 7, 10 If the acoustic signal with the transducer 11 at different modulation or. Recorded pulse frequencies of the laser, one therefore expects a spectrum with many resonances or signal maxima of the two resonators, which is not simply formed by the addition of the two individual spectra of the two resonators, but is characterized by frequency-shifted individual resonances and the typical double maxima coupled rice sonators. Such a signal with two pronounced resonances or
    Maxima, is e.g. represented in the form of the curve 20 in Figure 2.
    Thus, if at the same excitation frequency both in the resonator cell 2 and in the microphone tube 10, a resonant standing wave is sought, then the approximate ratio of the length L of the resonator cell 2 to the length I of the microphone tube 10 with L n .... 20 - = 2 - to choose. / m Surprisingly, however, it was found that despite good coordination of the two resonators, their resonance spectra do not simply merge into a common spectrum of simple resonances. The overall spectrum is characterized, inter alia, by double maxima (such as curve 20 in Fig. 2), whose individual maxima can not be simply assigned to one or the other resonator. Even for the fundamental mode (n = 1 and m = 1) and with good tuning of the two resonators, it had to be noted that although the two resonance frequencies fR1, fR2 approach each other, they are generally approximately the same as shown in FIG Maintain course after the curve 20 and represent no well-separated and clearly pronounced Einzelre- 30 sonanz. However, it was also surprisingly found that a well-separated and clearly pronounced individual resonance of the overall system is formed when the diameter d (or in general the cross-section) of the microphone tube 10 is made sufficiently small. The resonance spectrum of the measuring cell 1 is therefore more surprising 5/10 -15-12-2012 E014.1
    Printed: 17-12-2012 10 2012/50590
    AV-3509 AT
    Depending on a parameter that has no influence on the individual resonance frequencies fRi, fR2 per se.
    Of course, there are still infinitely many harmonics in the frequency spectrum in the system, which, however, are disregarded when determining the individual resonance. 5 With approximate tuning of the individual resonators (over their lengths), one obtains the desired well-separated from each other resonances of the overall system and pronounced individual resonance, if one makes the diameter d at a circular cross-section of the microphone tube 10 sufficiently small. This is illustrated by the curves 21, 22 and 23 in FIG. 2 using the example of a circular cross-section, from which it can be seen that the two pronounced resonances change into a single pronounced resonant frequency f R as the diameter d of the microphone tube 10 is reduced becomes.
    The suitable diameter d of the microphone tube 10 may be e.g. can be easily determined by measurements, possibly using the signal recorded by the microphone 11 with e.g. a synchronous rectifier must be smoothed or filtered before a 15 maximum can be accurately determined. For the measurement results shown in Fig. 2, a measuring cell 1 with a resonator cell 2 with L = 42mm, which corresponds to a resonance frequency of --4000Hz in air, and-matched tuned a microphone tube 10 with 1 = 21 mm used and with a particle-charged Sample gas (here air) filled. The laser was modulated at an excitation frequency fM 20 in the frequency range of 2000 Hz to 7000 Hz, e.g. By means of a known chopper, and the resulting sound pressure signal recorded by means of sound pickup 11 and shown as a voltage Um At a diameter of the microphone tube d = 3mm results in the curve 20 in Figure 2, showing two connected maxima. If the diameter is reduced to d = 2.2 mm, one obtains the curve 21 in FIG. 2, in which the 25 merging of the two maxima from curve 20 is recognized and which is already only a maximum (in the sense of the mathematical extreme value of the curve 21 ) and has a larger signal amplitude. In a further reduction of the diameter to d = 2mm, the resonances push even further together (curve 22 in Figure 2). With a diameter of d = 1.5 mm results in a particularly pronounced individual resonance 30 with only one resonant frequency fR and a high signal amplitude (curve 23 in Fig. 2).
    In order to compare the improvement achieved with curve 23 in FIG. 2, FIG. 2 also shows with curve 24 a typical signal of a conventional photoacoustic measuring cell (without microphone tube 10) with the same dimensions of the resonator cell 2 known from the prior art improved signal strength and thus the improved measurement accuracy and resolution immediately recognizable. 6/10 -6- 14-12-2012
    Printed: 17-12-2012 E014.1 10 2012/50590
    AV-3509 AT
    This behavior is surprisingly independent of the designed resonant frequency of the resonator cell 2. Z.B. results in a length L of the resonator cell 2 of L = 84mm (corresponds approximately to a resonance frequency of 2000Hz) and a matched length I of the microphone tube 10 of l = 42mm with decreasing 5 cross section of the microphone tube 10 also a pronounced pressure maximum in the microphone signal , eg for a circular cross section from a diameter d of about 2.5mm and in particular at d = 1.5mm. The same can be observed at higher resonant frequencies of the resonator cell 2.
    The cross section of the resonator cell 2, or of the chamber 3, or of the microphone tube 10, 10, however, could also be embodied differently than circular, e.g. The circular cross-section could also be approximated by a polygonal (for example, a triangular, rectangular or hexagonal) or rounded (approximately elliptical) cross section. The core of the invention remains unchanged, wherein instead of the diameter d generally the cross section of the microphone tube 10 is reduced, until ausbitdet in the same way a well-separated 15 and clearly pronounced individual resonance of the overall system. Again, it has been found that the above-described effect similarly sets generally from a cross-section of the microphone tube 10 of about 5mm 2, which is about a 2.5mm diameter.
    Of course, the cross section of the microphone tube 10 should not be made arbitrarily small, since otherwise problems with deposits of particles on the microphone tube 10 can be obtained. For an application for measuring soot concentration in the exhaust gas of an internal combustion engine, the measuring cell 1 may be e.g. be designed to a resonant frequency fR in the range of typically 4000Hz. At lower frequencies, in particular less than 1000 Hz, which would be advantageous on account of the resulting larger overall lengths, the associated larger number of particles in the gas volume of the measuring cell 1 and therefore greater excitation and measuring sensitivity, disturbing noises are already produced by the internal combustion engine itself Approximately 12000Hz, the lengths are too small, which would have a negative impact on the excitation and thus on the sensitivity. In order to protect the sound pickup 11 from excessive temperatures, it can also be cooled by a cooling unit, not shown. As cooling, e.g. a Peltier element, a water cooling, a heat exchanger, etc, be provided. Furthermore, it is possible by active, controlled cooling to produce a pronounced temperature gradient in the microphone tube 10, the cooler side being located at the sound pickup 11, which results in a gradient in the density of the measurement gas in the microphone tube 10. A Density 7/10 -7- 14-12-2012
    Printed: 17 * 12-2012 E014.1
    10 2012/50590 AV-3509 AT gradient in the sample gas has the effect of an acoustic lens and additionally amplifies the sound pressure at the end of the microphone tube 10 and thus amplifies the measurement signal.
    Of course, the inventive features can also be applied to a measuring cell 1 as described in EP 1 564 543 A1, as shown in FIG. 3 (the same reference numerals as in FIG. 2 being used for the same components). In this way, the improved sensitivity and resolution caused by the microphone tube 10 according to the subject invention can be combined with the advantages of avoiding deposits of the particles on the glass windows as entry or exit points of the laser beam 7. 10 8/10 -8- 14-12-2012
    权利要求:
    Claims (6)
    [1]
    A photoacoustic measuring cell with a resonator cell (2) which is bounded on both sides by a chamber (3) with a larger cross section than the cross section of the resonator cell (2) and through which a measuring gas flows, with a laser source (6 ), which directs an intermittently modulated or pulsed laser beam (7) into the resonator cell (2), and with a microphone tube (10) which is arranged in the region of a maximum pressure of the acoustically excited in the Resonatorzelie (2) standing acoustic wave (8) in which the microphone tube (10) branches off from the resonator cell (2) and at the end of the microphone tube (10) a sound pickup (11) for receiving an acoustic signal is arranged, characterized in that the ratio of the length L of the resonator cell (2) to Length I I. n of the microphone tube (10) to - = 2- is selected, where n is the order of the axial oscillation mode of the standing acoustic wave in the resonator cell (2) and m denotes the order of the axial vibration mode of the standing acoustic wave in the microphone tube (10), and the cross section of the microphone tube (10) is selected to be picked up by the sound pickup (11) as a function of the exciting frequency (fM) acoustic signal has a pronounced single maximum (fR).
    [2]
    2. Photoacoustic measuring cell according to claim 1, characterized in that the length of the resonator cell (2) is designed for the resonant frequency of the fundamental acoustic oscillation in the axial direction of the resonator cell (2) in the range of 1000Hz to 12000Hz.
    [3]
    3. Photoacoustic measuring cell according to claim 1 or 2, characterized in that the cross section of the microphone tube (10) is smaller than 5mm2, in particular less than 1.8mm2 is selected.
    [4]
    4. Photoacoustic measuring cell according to claim 1 or 2, characterized in that the cross section of the microphone tube (10) is circular and the diameter (d) of the microphone tube (10) is less than 2.5 mm, in particular less than 1.5 mm is selected ,
    [5]
    5. A photoacoustic measuring cell according to one of claims 1 to 4, characterized in that a cooling unit for cooling the sound pickup (11) and / or the microphone tube (10) is provided.
    [6]
    6. photo-acoustic measuring cell according to claim 5, characterized in that by the cooling unit in the microphone tube (10) for forming an acoustic lens, a temperature gradient is adjustable. -9-
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    同族专利:
    公开号 | 公开日
    WO2014090518A1|2014-06-19|
    AT511934B1|2014-06-15|
    AT511934A3|2014-01-15|
    引用文献:
    公开号 | 申请日 | 公开日 | 申请人 | 专利标题
    US4372149A|1980-11-17|1983-02-08|Zharov Vladimir P|Laser-excited spectrophone|
    US6662627B2|2001-06-22|2003-12-16|Desert Research Institute|Photoacoustic instrument for measuring particles in a gas|
    AT6894U3|2004-01-28|2005-01-25|Avl List Gmbh|MEASURING CHAMBER FOR PHOTOACOUS SENSORS|
    CA2461328A1|2004-03-24|2005-09-24|Robert Anthony Crane|A multiplexed type of spectrophone|
    DE102007043951B4|2007-09-14|2009-07-30|Protronic Innovative Steuerungselektronik Gmbh|Device for the detection of molecules in gases|
    US20120118042A1|2010-06-10|2012-05-17|Gillis Keith A|Photoacoustic Spectrometer with Calculable Cell Constant for Quantitative Absorption Measurements of Pure Gases, Gaseous Mixtures, and Aerosols|CN109490211A|2018-11-16|2019-03-19|安徽理工大学|A kind of photoacoustic cell with anti-noise function|
    FR3089250B1|2018-11-30|2020-12-18|Faurecia Systemes Dechappement|Exhaust line comprising a photoacoustic sensor for measuring at least one pollutant gas concentration and motor vehicle comprising such an exhaust line|
    法律状态:
    2018-08-15| MM01| Lapse because of not paying annual fees|Effective date: 20171214 |
    优先权:
    申请号 | 申请日 | 专利标题
    AT505902012A|AT511934B1|2012-12-14|2012-12-14|Photoacoustic measuring cell|AT505902012A| AT511934B1|2012-12-14|2012-12-14|Photoacoustic measuring cell|
    PCT/EP2013/074117| WO2014090518A1|2012-12-14|2013-11-19|Photoacoustic measurement cell|
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