WO2022053230A1

WO2022053230A1 - Optical soot particle sensor

Info

Publication number: WO2022053230A1
Application number: PCT/EP2021/071645
Authority: WO
Inventors: Fabian Purkl; Tobias Graf
Original assignee: Robert Bosch Gmbh
Priority date: 2020-09-09
Filing date: 2021-08-03
Publication date: 2022-03-17
Also published as: DE102020211328A1

Abstract

The invention relates to a soot particle sensor (16) having a laser module (18) which emits a laser beam, wherein the soot particle sensor (16) has optical elements (20) which are arranged in the laser beam for focussing the laser beam into one spot (22) with a top-hat beam profile, and having a detector (26) which is designed to detect temperature radiation (14) and is arranged in the soot particle sensor (16) such that it can detect radiation (14) emitted by soot particles crossing the spot. The soot particle sensor (16) can be used to clearly determine the size of detected soot particles.

Description

description

title

Optical soot particle sensor

State of the art

From the prior art, WO18202433 A1 of the applicant, a soot particle sensor with a laser module having a laser and with a detector set up for the detection of thermal radiation is already known, which has an optical element arranged in the beam path of the laser of the laser module, which is set up for this purpose to focus the laser light emitted by the laser module into a spot. As a result, soot particles that pass through the spot are heated to such an extent that they become a source of radiation, which the detector then detects and generates a corresponding signal.

Disclosure of Invention

The present invention is based on the finding of the inventors that the intensity distribution I of the laser light in the spot in the plane perpendicular to the direction of propagation of the laser light, i.e. the beam profile within the meaning of the invention, is of important importance for the question of the extent to which soot particles are safe with the sensor can be clearly detected in terms of size.

Inhomogeneous beam profiles, such as Gaussian profiles with the intensity distribution Icauß ~ exp - (r/wo) ² , which is radially symmetrical to the propagation direction, where where is the beam valley radius, have the disadvantage that the signal evaluated by the detector when a particle passes through the spot is ambiguous. For example, a relatively large particle (e.g. with diameters > 100 nm) traversing the laser spot in an area with submaximal laser intensity, e.g. further out in the Gaussian profile (e.g. r > wo), a detected signal indistinguishable from a detected signal resulting from a smaller particle crossing the spot in a region of maximum laser intensity, e.g. in the center of the Gaussian profile (e.g. r=0 or r<wo). because the small particle heats up to a higher temperature than the large particle and then emits the same amount of radiation as the large particle from the point of view of the detector, despite the difference in size. This is precisely where the ambiguity of the signals generated with the sensors known from the prior art lies.

Inhomogeneities in the intensity distribution of the laser light in the spot, for example the flanks of the Gaussian intensity distribution, mean that it is not possible to clearly determine the particle size based on the detected signals.

According to the invention, it is provided that the laser light is shaped by appropriate optical elements in such a way that it has a tophat beam profile in the spot, i.e. there has an intensity distribution l _tophat that has a constant value, for example, inside a circle with the radius where and outside of which has zero intensity: l _To phat ~ rect(r/wo) with rect(R) = 1 for R < 1 , and rect(R) = 0 otherwise.

This has the advantage that those particles that pass through the spot are without exception exposed to the maximum laser intensity occurring in the spot. They are thus heated to a defined temperature and consequently glow with an intensity that is only dependent on their size. With the present invention, the size of the particle can thus be unequivocally deduced from the signal generated at the detector.

In the context of the invention, a mathematically idealized intensity distribution perpendicular to the direction of propagation of the laser light, which assumes its maximum value everywhere in an inner region and assumes the value zero everywhere outside the region, is interpreted as a tophat beam profile. Within the scope of the invention, however, real intensity distributions which have a high but finite edge steepness at the edge of the area and/or have small but non-zero intensity fluctuations inside and/or outside the area are also used as a top hat beam profile construed according to the relevant understanding of the person skilled in the art.

The optical elements that are provided according to the invention in order to focus the laser beam into a spot and to shape it into a tophat beam profile in the spot can include conventional optical elements, such as spherical lenses or Fraunhofer elements and/or spherical or plane mirrors. These are, in particular, optical elements that image a laser beam, which always has a Gaussian profile, into a laser beam, which always has a Gaussian profile.

In addition, these optical elements include in particular at least one optical element that changes the beam profile, for example a Gaussian beam (laser beam that always has a Gaussian profile) into a tophat beam (laser beam whose spot is perpendicular to the direction of propagation of the laser beam having tophat beam profile). These are in particular the following optical elements: diffractive optical elements (DOE), refractive beam shapers, diffractive beam shapers, beam-shaping diffusers, Fresnel elements. Such optical elements and their use in connection with laser radiation are known in principle to those skilled in the art.

For the compact implementation of the sensor, it can be provided that a dichroic beam splitter is arranged in the beam path between the laser module and the spot. In this case, it can advantageously be provided that the optical element, which changes the beam profile of the laser beam, is arranged between the laser module and the dichroic beam splitter. The particular advantage is that the radiation emitted by the particles can reach the detector through the beam splitter without having to pass through the optical element that changes the beam profile of the laser beam.

Embodiments of the invention are shown in the drawings and are explained in more detail in the following description. The same reference symbols in different figures denote the same elements or elements that are at least comparable in terms of their function. They show, each in schematic form: 1 shows a measurement principle based on laser-induced incandescence, which is used in the invention;

2 shows a basic structure of a soot particle sensor based on laser-induced incandescence;

3 shows a soot particle sensor based on laser-induced incandescence;

4 shows a first exemplary embodiment of a soot particle sensor according to the invention;

5 shows a second exemplary embodiment of a soot particle sensor according to the invention;

6 shows a third exemplary embodiment of a soot particle sensor according to the invention;

7 shows a fourth exemplary embodiment of a soot particle sensor according to the invention;

8 shows a fifth exemplary embodiment of a soot particle sensor according to the invention;

9 Measurement data obtained by simulation of a soot particle sensor according to the invention (a) and a conventional soot particle sensor (b).

FIG. 1 basically illustrates the measuring principle based on laser-induced incandescence (Lil). High-intensity laser light 10 strikes a soot particle 12. The intensity of the laser light 10 is so high that the energy of the laser light 10 absorbed by the soot particle 12 heats the soot particle 12 to several thousand degrees Celsius. As a result of the heating, the soot particle 12 spontaneously and essentially without a preferred direction emits significant radiation 14 in the form of thermal radiation, also referred to below as LI I light. Part of the radiation 14 emitted in the form of temperature radiation is therefore also emitted in the opposite direction to the incident laser light 10 .

FIG. 2 shows a schematic of a basic structure of an exemplary embodiment of a soot particle sensor 16. The soot particle sensor 16 here has a CW laser module 18 (CW: continuous wave; continuous wave), which is preferred collimated (hereinafter also: parallel) laser light 10 is focused onto a very small spot 22 with at least one optical element 20 arranged in the beam path of the CW laser module 18 . The optical element 20 is preferably a first lens 24. Only in the volume of the spot 22 does the intensity of the laser light 10 reach the high values required for LH.

The dimensions of the spot 22 are in the range of a few μm, in particular in the range of at most 200 μm, so that the soot particles 12 traversing the spot 22 are excited to emit evaluable radiation powers, either through laser-induced incandescence or through chemical reactions (particularly oxidation). As a result, it can be assumed that there is always at most one soot particle 12 in spot 22 and that an instantaneous measurement signal from soot particle sensor 16 only originates from at most one soot particle 12 . The measurement signal is generated by a detector 26 which is arranged in the soot particle sensor 16 in such a way that it detects radiation 14 emanating from the soot particle 12 flying through the spot 22 , in particular thermal radiation. For this purpose, the detector 26 preferably has at least one photodiode 26.1.

It is quite possible that the laser of the laser module 18 is modulated or switched on and off (duty cycle <100%). However, it remains preferred that the laser of the laser module 18 is a CW laser. This enables the use of inexpensive semiconductor laser elements (laser diodes), which makes the entire soot particle sensor cheaper and greatly simplifies the control of the laser module 18 and the evaluation of the measurement signal. However, the use of pulsed lasers is not ruled out.

FIG. 3 shows a soot particle sensor 16 which is suitable for use as a soot particle sensor in the exhaust gas of a combustion process.

The soot particle sensor 16 has an arrangement of an outer protective tube 28 and an inner protective tube 30 . The two protective tubes 28, 30 preferably have a generally cylindrical or prismatic shape. The bases of the cylinder shapes are preferably circular, elliptical or polygonal. The cylinders are preferably arranged coaxially with the axes of the cylinders aligned transverse to the flow of exhaust gas 32 . The inner protective tube 30 protrudes in Direction of the axes beyond the outer protective tube 28 also into the flowing exhaust gas 32 inside. At the end of the two protective tubes 28, 30 facing away from the flowing exhaust gas, the outer protective tube 28 protrudes beyond the inner protective tube 30. The clear width of the outer protective tube 28 is preferably so much larger than the outer diameter of the inner protective tube 30 that between the two protective tubes 28, 30 there is a first flow cross section. The clear width of the inner protective tube 30 forms a second flow cross section.

The result of this geometry is that exhaust gas 32 enters the arrangement of the two protective tubes 28, 30 via the first flow cross-section, then changes direction at the end of the protective tubes 28, 30 facing away from the exhaust gas 32, enters the inner protective tube 30 and exits it is sucked out by the exhaust gas 32 flowing past. A laminar flow results in the inner protective tube 30 . This arrangement of protective tubes 28, 30 is attached to the soot particle sensor 16 transversely to the flow of exhaust gas or in an exhaust pipe.

The soot particle sensor 16 also has the laser module 18 which preferably generates parallel laser light 10 . A beam splitter 34 is located in the beam path of the preferably parallel laser light 10 . The light intensity in this spot 22 is high enough to heat the soot particles 12 transported with the exhaust gas 32 to several thousand degrees Celsius, so that the heated soot particles 12 emit significant radiation 14 in the form of thermal radiation. The radiation 14 is, for example, in the near-infrared and visible spectral range, without the invention being restricted to radiation 14 from this spectral range. Part of this radiation 14 emitted in an undirected manner in the form of temperature radiation, or this LII light, is detected by the optical element 20 and directed onto the detector 26 via the beam splitter 34 . This structure has the particularly important advantage that only one optical access to the exhaust gas 32 is required, since the same optics, in particular the same optical element 20, are used for generating the spot 22 and for detecting the radiation 14 emanating from the soot particle 12. The exhaust 32 is an example of one measuring gas. The measuring gas can also be another gas or gas mixture, for example room air.

The laser module 18 has a laser diode 36 and a second lens 38, which preferably aligns the laser light 10 emitted by the laser diode 36 in parallel. The use of the laser diode 36 represents a particularly cost-effective and easy-to-handle option for generating laser light 10. The preferably parallel laser light 10 is focused by the optical element 20 to form the spot 22.

The optical soot particle sensor 16 preferably has a first part 16.1 that is exposed to the exhaust gas and a second part 16.2 that is not exposed to the exhaust gas and contains the optical components of the soot particle sensor 16. Both parts are separated by a partition 16.3, which runs between the protective tubes 28, 30 and the optical elements of the soot particle sensor. The wall 16.3 serves to insulate the sensitive optical elements from the hot, chemically aggressive exhaust gas 32. In the partition wall 32, a protective window 40 is mounted in the beam path of the laser light 10, through which the laser light 10 impinges into the exhaust gas 32 and via the Radiation 14 emanating from the spot 22 can impinge on the optical element 20 and from there on the detector 26 via the beam splitter 34 .

As an alternative to the exemplary embodiment shown here, the spot 22 can also be generated and the radiation 14 emitted by soot particles in the spot can also be detected via separate optical beam paths.

It is also conceivable to generate the spot 22 with lens combinations other than those given here merely as an exemplary embodiment. In addition, the soot particle sensor 16 can also be implemented with laser light sources other than the laser diodes 36 specified here for exemplary embodiments.

In addition, a filter 42, which is arranged in the beam path between the beam splitter 34 and the detector 26, is provided. The filter 42 is characterized in that it is less permeable for the laser light 10 than for the radiation 14 which emanates from the spot 22 when there is a soot particle 12 there. When the soot particle sensor 16 is installed in an exhaust line of a combustion process, the filter 42 that filters out the excitation light (laser light) in conjunction with the almost complete absence of extraneous/ambient light in the exhaust line allows the use of particularly sensitive detectors 26, e.g. inexpensive SiPM (silicon photomultiplier) or SPAD detectors (SPAD: single-photon avalanche diode). As a result, a light signal generated by a particularly small soot particle and therefore extremely small, which is formed by a few 10 photons, for example, can already be detected. This reduces the dimensions of soot particles, which can just about be detected, to a lower detection limit of 10 to 100 nm.

In the following, specific embodiments of the invention are shown in FIGS. 4 to 8, wherein reference is made to the structure shown in FIG.

The embodiments shown in FIGS. 4 to 8 have in common that optical elements 20a, 20b arranged in the laser beam 17 are provided for bundling the laser beam 17 into a spot 22 with a tophat beam profile in the working plane 15. In the examples illustrated, at least one optical element 20b is included, which changes the beam profile of the laser beam 17 accordingly. Nevertheless, the invention does not fundamentally rule out the alternative of using a laser module 18 which itself emits a laser beam 17 with the desired top-hat beam profile. Then, in addition, optical elements 20a, 20b can be used, through the imaging of which this property is merely retained, for example spherical lenses or Fraunhofer elements and/or spherical and/or plane mirrors and the like.

The embodiments shown in FIGS. 4 to 8 also have in common that the beam splitter 34 already shown in FIG. The radiation 14 emitted by the soot particles 12 traversing the spot 22 traverses the dichroic beam splitter 34 in transmission, however, almost completely and arrives - focused through the further lens 25 - on the detector 26.

In FIG. 4, the optical element 20b, which changes the beam profile of the laser beam 17, is arranged between the laser module 18 and the dichroic beam splitter 34. It is a diffractive optical element.

In FIG. 5, the optical element 20b, which changes the beam profile of the laser beam 17, is arranged between the dichroic beam splitter 34 and the spot 22. It is a diffractive optical element. Alternatively, a refractive beam shaper could also be provided instead of the diffractive optical element.

In FIG. 6, the optical element 20b, which changes the beam profile of the laser beam 17, is also arranged between the dichroic beam splitter 34 and the spot 22. In this case, it is a diffractive optical element in the form of a Fresnel element which, in addition to beam shaping, also effects beam bundling to form a spot 22 . A separate first lens, comparable to the lens 24 in FIGS. 3, 4 or FIG. 5, can therefore be dispensed with in this example.

In FIG. 7, the optical element 20b, which changes the beam profile of the laser beam 17, is arranged between the spot 22 and the dichroic beam splitter 34. FIG. It is a diffractive optical element consisting of a hybrid Frensnel-Fraunhofer element.

In FIG. 8, the optical element 20b, which changes the beam profile of the laser beam 17, is arranged between the spot 22 and the dichroic beam splitter 34. FIG. It is a refractive beam shaper.

It is also possible for the embodiments shown to be combined with one another, ie in particular each of the optical elements 20b shown in the examples can be arranged at any position at which an optical element is arranged in one example. The provision of a plurality of optical elements 20b is also possible in this way. Soot particles 12 with a known diameter were detected with a soot particle sensor according to the invention (see Figure 9a) and with a conventional soot particle sensor, comparable to the soot particle sensor shown in Figure 3, whose beam profile in spot 22 has the form of a Gaussian distribution (see Figure 9b). The peak height of the signal obtained with the detector 26 is shown in FIGS. 9a and 9b in arbitrary units as a function of the particle diameter in nm. Each measured value is shown by an unfilled circle. In the case according to the invention (FIG. 9a), the diameter of the signaling soot particle 12 can be deduced precisely and unambiguously from the measured signal, while this is not the case with the previously known sensor (FIG. 9b) or only to a very limited extent.

Claims

Expectations

1. Soot particle sensor (16) with a laser module (18) emitting a laser beam (17), wherein the soot particle sensor (16) has optical elements (20a, 20b) arranged in the laser beam (17) for bundling the laser beam (17) into a spot (22 ) having a tophat beam profile, and having a detector (26) set up to detect radiation (14), which is arranged in the soot particle sensor (16) in such a way that it detects the soot particles emitted by the spot (22) traversing the radiation (14). able to detect.

2. Soot particle sensor (16) according to claim 1, characterized in that the optical elements (20a, 20b) comprise at least one optical element (20b) which changes the beam profile of the laser beam (17).

3. Soot particle sensor (16) according to claim 2, characterized in that the at least one optical element (20b) that changes the beam profile of the laser beam (17) is a diffractive optical element.

4. Soot particle sensor (16) according to claim 2, characterized in that the at least one optical element (20b) that changes the beam profile of the laser beam (17) is a reactive beam shaper.

5. Soot particle sensor (16) according to claim 2, characterized in that the at least one optical element (20b) that changes the beam profile of the laser beam (17) is a beam-shaping diffuser.

6. Soot particle sensor (16) according to claim 2, characterized in that the at least one optical element (20b) that changes the beam profile of the laser beam (17) is a Fresnel element.

7. Soot particle sensor (16) according to one of claims 2 to 6, characterized in that a dichroic beam splitter (34) is arranged in the beam path between the laser module (18) and the spot (22), and that the optical element (20b) changing the beam profile of the laser beam (17) is arranged between the laser module (18) and the dichroic beam splitter (34).

8. Soot particle sensor (16) according to one of claims 2 to 6, characterized in that a dichroic beam splitter (34) is arranged in the beam path between the laser module (18) and the spot (22), and that the optical element (20b), which changes the beam profile of the laser beam (17), is arranged between the spot (22) and the dichroic beam splitter (34).

9. soot particle sensor (16) according to any one of claims 2 to 8, characterized in that the laser module (18) has a CW diode laser which emits the laser beam (17).