EP3619522A1

EP3619522A1 - Optical soot particle sensor for motor vehicles

Info

Publication number: EP3619522A1
Application number: EP18721709.6A
Authority: EP
Inventors: Radoslav Rusanov; Oliver Krayl; Christoph Daniel Kraemmer
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2017-05-03
Filing date: 2018-04-19
Publication date: 2020-03-11
Also published as: JP7009662B2; CN110621982A; DE102017207402A1; JP2021113820A; US20200056998A1; JP6870116B2; JP2020519867A; US11073480B2; WO2018202433A1; KR20200003800A; KR102486061B1

Abstract

The invention relates to a soot particle sensor (16) comprising a laser module (18) which has a laser and comprising a detector (26) designed to detect thermal radiation (14). The soot particle sensor (16) is characterised in that the laser is designed to generate laser light (10) and in that the soot particle sensor (16) has an optical element (20) positioned in the optical path of the laser of the laser module (18), said element being designed to bundle laser light (10) emitted by the laser module (18) into a spot (22), and in that the detector (26) is positioned in the soot particle sensor (16) such that it detects radiation (14) emitted by the spot (22).

Description

description

title

Optical Soot Particle Sensor for Motor Vehicles Prior Art

The present invention relates to a soot particulate sensor according to the preamble of claim 1. Such a soot particulate sensor comprises a laser module having a laser module and a detector configured for the detection of temperature radiation.

With modern diesel engines powered vehicles are with

Equipped with diesel particulate filters. The functionality of these particle filters must be monitored by on-board diagnostic means in accordance with legal requirements. For motor vehicles, for example, an electrical resistance sensors are used, which is of the

Applicant be manufactured and sold. The operation of these known sensors is based on the formation of conductive soot paths between two interdigital electrodes. With these sensors, the rise time of the

Current after applying a voltage is a measure of the soot concentration. The mass concentration (mg / m ³ exhaust gas or mg / km driving distance) is measured. The calculation of the number concentration (number of particles per m ^{3 of} exhaust gas or per km of driving distance) is very difficult or even impossible with this sensor concept for a variety of reasons. The known sensor is periodically regenerated by being heated by an integrated heating element to at least 700 ° C, whereby the soot deposits burn away.

In the scientific scene, which deals with the influence of fine particles on health, there has long been debate as to which of the sizes total particulate mass (in mg / m ³ or in mg / km) or

Number n of particles (n / m ³ or n / km) in terms of impairment of the Health is the more critical size. It should be noted that especially the small soot particles, which have only a small proportion of the total mass due to their very small mass (m ~ r ³ ), are particularly dangerous. This is due to their high "depth of penetration" into the human body, which results from their small size, so it is foreseeable that the legislation will be on board

Diagnostic means will also dictate the number of particles as soon as corresponding (in terms of performance and price acceptable) solutions are available on the market. The principle of laser induced incandescence (LH) is for the detection of

Nanoparticles (in air) has long been known and is e.g. It is also used intensively for characterizing the combustion process in "glassy" engines in the laboratory or for the characterization of exhaust gases in laboratory environments, whereby the soot particles are subjected to a nanosecond pulse

High-power laser heated to several thousand degrees Celsius, so that they emit significant temperature radiation. This thermally induced

Light emission of the soot particles is measured with a light detector. The method allows the detection of very small soot particles with a

Diameter down to a size of a few 10 nm. This laser induced incandescence forms the preamble of claim 1.

Disclosure of the invention

The present invention differs from this

Prior art high-power nanosecond lasers operate by virtue of the fact that the soot particle sensor has an optical element arranged in the beam path of the laser of the laser module, which is set up to bundle laser light emitted from the laser module into a spot, and in such a way that the detector is arranged in the soot particle sensor. that it detects radiation emanating from the spot. The radiation may be temperature radiation or radiation released by chemical reactions such as oxidation of the soot occurring in the spot.

The sensor according to the invention is also suitable for use as an on-board diagnostic sensor in contrast to the known sensor Motor vehicles. In this case, the soot particle sensor according to the invention also uses the principle of laser-induced incandescence.

A preferred embodiment is characterized in that the laser module is adapted to generate parallel laser light, and that the optical

Element is adapted to bundle outgoing laser laser outgoing parallel laser light in the spot.

In a preferred embodiment, the laser is a favorable CW laser, such as e.g. a diode laser. The prior art is high-priced,

Q-switched solid-state lasers used for LII experiments. The im

Aligemeinen lower power of the CW laser is compensated by a strong focus of the laser light. The laser light of the CW laser is focused on a very small spot via the optical element (e.g., a lens). It is quite possible that the laser will be modulated, but it is preferred to use a CW laser. This allows the use of cost-effective semiconductor laser elements (laser diodes), which makes the entire sensor unit cheaper and greatly simplifies the control and evaluation. Due to the very small spot dimensions (for example a few μm), it can be ensured that at most one soot particle is in the spot and that the measured signal is just from this one soot particle. Thus, a single particle measurement is possible, which the extraction of

Information about the soot particles as its size allows. This provides a clear advantage over other measurement methods for the

Soot particle measurement. The invention advantageously also allows a high measuring speed (at least 1 measurement per second compared to several minutes per measurement) and offers the possibility of measuring the

Particle number. Thus, the invention allows both the determination of

Mass concentration (mg / m ³ or mg / km) and the number concentration (soot particles / m ³ or soot particles / km) of the emitted soot particles.

This is also the use in powered by gasoline engines

Motor vehicles for monitoring a gasoline particle filter used there and for detecting the soot particle emissions of the gasoline engine possible. Especially with gasoline-powered gasoline engines, it is important to be able to measure quickly after the start of the vehicle, since a large part of the soot particles is formed there during the cold start. For gasoline engines, the soot particle count capability is also because of the fineness, ie the small size of the soot particles (little

Mass, high number) also very important. Because automotive sensors (on-board) currently available on the market are unable to

Reliably measuring soot particle counts, this soot particle count capability of the particulate carbon black sensor of the present invention is particularly important and advantageous.

Beyond use in gasoline-fueled gasoline engines, the particulate matter sensor of the invention may be used in any combustion process. Preferred fields of application are the detection of soot particle mass and number concentrations in the on-board

Monitoring the diesel particulate filter in passenger cars and

Lorries as well as off-road in construction machinery, and as a sensor for the measurement of fine dust concentrations, for example in a

Monitoring of indoor air quality or monitoring of emissions from private or industrial incinerators, etc .. The

The particulate sensor according to the invention is based on the principle of the laser

Induced incandescence.

A preferred embodiment is characterized in that the laser is a semiconductor laser element, in particular a laser diode.

It is also preferable that the detector has at least one photodiode. The photodiode is preferably a photodiode sensitive to near-infrared and visible light.

It is further preferred for the soot particle sensor to have a beam splitter which is arranged in the beam path of the preferably parallel laser light such that it directs at least part of the laser light incident from the laser module onto the optical element and incident from the spot

Temperature radiation directed at least in part on the detector. A further preferred embodiment is characterized in that the beam splitter is a polarizing beam splitter, and that the beam splitter is aligned so that it is a predetermined for the incident

Polarization direction having laser light is maximum permeability.

It is also preferred that the soot particle sensor has an optical filter which is arranged in the beam path between the beam splitter and the detector and which is less transparent to the laser light than for outgoing from the spot LII light.

It is further preferred that the laser is adapted to carry laser light

Wavelengths below 500 nm, in particular with a wavelength of 405 nm, 450 nm or 465 nm to emit and that the optical filter is such that it attenuates light with wavelengths below 500 nm or even blocked. It is also possible to use a bandpass filter which does not transmit only a region around the laser wavelength.

A further preferred embodiment is characterized in that the soot particulate sensor has a first part which is adapted to be exposed to a measurement gas, and a second part which does not expose the measurement gas and which contains the optical components of the soot particulate sensor, both parts passing through an impermeable to the sample gas

Partition are separated.

It is also preferable that in the partition wall in the beam path of the laser light, a window is mounted, which is permeable to both the laser light and outgoing from the spot LII light.

It is further preferred that the soot particle sensor has an arrangement of an outer protective tube and an inner protective tube, both having a cylindrical shape, that the protective tubes are arranged coaxially, wherein the axes of the cylindrical shapes preferably parallel to the direction of irradiation of the

Laser light are aligned and the spot lies inside the inner protective tube, that the outer protective tube protrudes beyond the inner protective tube at its end facing the laser and that the inner protective tube protrudes at the opposite end over the outer protective tube. A further preferred embodiment is characterized in that the soot sensor has a shaker module which has an oscillatingly movable element which is mechanically rigidly connected to the laser module, so that an oscillation of the movable part of the shaker module is transmitted to the laser module.

It is also preferred that the shaker module has a piezoelectric actuator having the movable element or has an electromagnetic actuator having the movable element or an actuator having the movable element and operating with magnetostriction.

Further, it is preferable that the soot particle sensor has at least one pair of electrodes disposed in the soot particle sensor on different sides of the spot.

It is also preferable that the soot particulate sensor be a pair of

Sound wave exciters, which is arranged in the interior of the inner protective tube.

It is further preferred that the sound wave exciters are transducers. The

Transducers preferably operate on a piezo or magnetostriction basis or are actuated electrically or electromagnetically and generate a standing ultrasonic wave.

Further advantages emerge from the dependent claims, the

Description and the attached figures.

It is understood that the features mentioned above and those yet to be explained below can be used not only in the particular combination given, but also in other combinations or in isolation, without departing from the scope of the present invention.

Embodiments of the invention are illustrated in the drawings and are explained in more detail in the following description. In this case, the same reference numerals in different figures denote the same or at least functionally comparable elements. In each case, in schematic form:

Fig. 1 is a laser-induced incandescence-based measuring principle used in the invention;

Fig. 2 shows a basic structure of an inventive

Soot-particle sensor;

Fig. 3 shows a first embodiment of an inventive

Soot-particle sensor;

Fig. 4 is based on the subject of Figure 3 second

Embodiment of a soot particle sensor according to the invention;

5 shows a sensitivity of a detector of a soot particle filter

usable silicon photodiode as a function of the wavelength λ of the incident light having first wavelength ranges;

6 likewise the sensitivity of a silicon photodiode which can be used as a detector as a function of the wavelength λ of the incident light with second wavelength ranges;

7 shows a qualitative representation of the LII signal of a soot particle at a sufficiently fast modulation of the intensity of the exciting laser light;

Fig. 8 shows an embodiment of the soot particle sensor of Figure 3 with

connected control and evaluation electronics;

Fig. 9 shows a third embodiment of a soot particle sensor

connected control and evaluation electronics;

Fig. 10 shows details of a fourth embodiment of a

carbon black particle sensor according to the invention; 11 shows a spot in a beam waist of the laser light for various

Combinations of the flow direction of the soot particles carrying gas and the propagation direction of the laser light in a soot particle sensor according to the invention;

Fig. 12 is a qualitative representation of a Lll signal, which in a

spatially oscillating spot results;

Fig. 13 shows a fourth embodiment of an inventive

Soot-particle sensor;

FIG. 14 shows the beam spot defining a spot of laser light together with a soot particle; FIG. Fig. 15 is a Lll signal of a soot particle, which is located on a spatially

oscillating trajectory moved by a laser spot;

Fig. 16 shows a fifth embodiment of an inventive

Soot-particle sensor;

17 is an example of a standing ultrasonic wave interposed between

Sound wave exciters of the soot particle sensor of Figure 16 sets; and

18 shows a comparison of a non-polarizing beam splitter with a

polarizing beam splitter.

FIG. 1 illustrates the measuring principle based on laser-induced incandescence (LH). Laser light 10 of high intensity strikes a soot particle 12. The intensity of the laser light 10 is so high that the absorbed by the soot particles 12 energy of the laser light 10, the soot 12 on several

Heated to one thousand degrees Celsius. As a result of the heating, the soot particle 12 spontaneously and substantially without preferential direction emits radiation 14 in the form of temperature radiation, hereinafter also referred to as LII light. A part of the radiation 14 emitted in the form of temperature radiation is therefore also emitted in the opposite direction to the direction of the incident laser light 10. FIG. 2 schematically shows a basic structure of an exemplary embodiment of a soot particle sensor 16 according to the invention. The soot particle sensor 16 has a CW laser module 18 (CW: continuous wave) whose preferably parallel laser light 10 is arranged with at least one in the beam path of the CW laser module 18 optical element 20 is focused on a very small spot 22. 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 necessary for LH. The invention is not limited to the use of a CW laser. It is also conceivable to use pulsed-powered lasers.

The dimensions of the spot 22 are in the range of some μιτι, in particular in the range of at most 200 μηι, so that the spot 22 traversing soot particles 12 are excited to the emission of evaluable radiation powers, either by laser-induced incandescence or by chemical reactions

(especially oxidation). As a consequence, it can be assumed that there is always at most one soot particle 12 in the spot 22 and that a momentary measurement signal of the soot particle sensor 16 originates from this at most one soot particle 12. The measurement signal is generated by a detector 26 which is arranged in the soot particle sensor 16 so that it detects radiation emanating from the soot particles 2 passing through the spot 22, in particular temperature radiation. For this purpose, the detector 26 preferably has at least one photodiode 26.1. Thus, a single particle measurement is possible, which allows the extraction of information about the soot particles 12 such as size and speed.

Thus, the exhaust gas velocity can be determined, and the calculation of a particle size spectrum is possible. The first size is for the

Calculation of the number concentration of soot particles 12 important. In combination with the second size, the mass concentration can also be calculated. This represents a clear advantage over other measuring methods for soot particle measurement.

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

FIG. 3 shows an advantageous exemplary embodiment of a soot particle sensor 16 according to the invention, 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 general cylindrical shape or prism shape. The bases of the cylindrical shapes are preferably circular, elliptical or polygonal. The cylinders are preferably arranged coaxially, the axes of the cylinders being oriented transversely to the flow of exhaust gas 32. The inner protective tube 30 protrudes in the direction of the axes beyond the outer protective tube 28 into the flowing exhaust gas 32. 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 results in a first flow cross section between the two protective tubes 28, 30. The inside width of the inner protective tube 30 forms a second

Flow cross section. This geometry has the consequence that exhaust gas 32 over the first

Flow cross-section in the arrangement of the two protective tubes 28, 30 enters, then at the end of the protective tubes 28, 30 facing away from the exhaust gas 32, changes its direction, enters the inner protective tube 30 and is sucked out of it by the flowing exhaust gas 32. This results in the inner protective tube 30, a laminar flow. This arrangement of protective tubes 28,

30 is attached to the soot particle sensor 16 across the exhaust flow, or in an exhaust pipe.

The soot particle sensor 6 also has the laser module 18, which preferably generates parallel laser light 10. In the beam path of the preferably parallel laser light 10 is a beam splitter 34. A the beam splitter 34th without deflecting continuous part of the laser light 10 is focused by the optical element 20 to a very small spot 22 in the interior of the inner protective tube 30. In this spot 22, the light intensity 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 significantly

Emit radiation 14 in the form of temperature 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. A portion of this non-directionally emitted in the form of temperature radiation radiation 14, or this LII light is detected by the optical element 20 and directed to the detector 26 via the beam splitter 34. This design has the particularly important advantage that only an optical access to the exhaust gas 32 is required, since the same optics, in particular the same optical element 20 for the production of the spot 22 and for detecting the emanating from the soot particles 12 radiation 14 is used. The exhaust gas 32 is an example of

Sample gas. The sample gas can also be another gas or gas mixture, for example room air.

In the subject matter of FIG. 3, the laser module 18 has a laser diode 36 and a second lens 38, which emit the laser light emitted by the laser diode 36

10 preferably aligned in parallel. The use of the laser diode 36 represents a particularly cost-effective and easy to handle possibility of generating laser light 10. The preferably parallel laser light 10 is focused by the optical element 20 to the spot 22.

The optical particulate sensor 16 preferably has an exhaust gas

exposed first part 16. 1 and a second part 16. 2 not exposed to the exhaust gas, which contains the optical components of the soot particle sensor 16. Both parts are separated by a partition 16.3, between the

Protective tubes 28, 30 and the optical elements of the soot particle sensor extends. The wall 16.3 is used to isolate the sensitive optical elements of the hot, chemically aggressive and "dirty" exhaust 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 is incident into the exhaust 32 and can incident on the outgoing from the spot 22 radiation 14 to the optical element 20 and from there via the beam splitter 34 to the detector 26. As an alternative to the embodiment illustrated here, the generation of the spot 22 and the detection of the soot particles in the spot outgoing

Radiation 14 also take place via separate optical beam paths.

It is also conceivable, the spot 22 with other than here only as

Embodiment to produce specified lens combinations.

In addition, the soot particulate sensor 16 can be realized with other laser light sources than the laser diodes 36 shown here for exemplary embodiments.

FIG. 4 shows another based on the subject matter of FIG

Embodiment. The soot particle sensor 16 of FIG. 4 differs from the soot particle sensor 16 of FIG. 3 by an additional filter 42, which is arranged in the beam path between the beam splitter 34 and the detector 26.

The filter 42 is characterized in that it is less permeable to the laser light 10 than for the radiation 14 emanating from the spot 22 when there is a soot particle 12 there.

This embodiment significantly improves the signal-to-noise ratio of the light falling on the detector 26, because it reduces the amount of laser light 10 due to back reflections of the laser light 0 to the optical

Components of the soot particle sensor 16 would fall on the detector 26, greatly reduced. Such laser light would produce spurious background detector signals which would cause detection of e.g. in the form of temperature radiation of soot particles in the spot 22 outgoing radiation 14 would complicate. The filter 42 removes the troublesome background to the soot particles 12 e.g. reduces pulses of radiation 14 emitted in the form of temperature radiation. The embodiment comprising the filter 42 specifically exploits the narrow bandwidth of laser sources (e.g., laser diodes) by filtering out that narrow bandwidth in front of the light detector 26. It is also conceivable to use a simple edge filter. The signal-to-noise ratio improves greatly.

When the soot particle sensor 16 is installed in an exhaust gas line of a combustion process, filtering out of the filter 42 with the filter 42 permits Excitation light (laser light) in conjunction with the almost complete

Absence of extraneous / ambient light in the exhaust line, the use of particularly sensitive detectors 26, e.g. from cost-effective SiPM (silicon photomultiplier) or SPAD diodes (single-photon avalanche diode). As a result, a light signal generated by a particularly small soot particle and therefore extremely small, which is formed for example by a few 10 photons, can already be detected. Thus, the dimensions of soot particles, which are just yet detectable, decrease to a lower detection limit of 10 to 100 nm.

FIG. 5 shows by way of example the sensitivity of a silicon photodiode which can be used as detector 26 as a function of the wavelength λ of the incident light in arbitrary units. The sensitivity is significant in the range between about 300 nm and 1100 nm. This is transferable to other Si-based detectors 26. Figure 5 also shows a schematic representation of a possible

Wavelength range 44 of the laser light 10 of the exciting laser module 18. Light with these wavelengths is through the optical filter 42 of the

Beam path filtered out before it reaches the detector 26. The detection of e.g. In the form of radiation emitted by temperature radiation 14, which starts from the excited in the spot 22 soot particles 12, takes place in the rest

Wavelength ranges 46, 48 in which the detector 26, which is based on silicon, is still sensitive.

FIG. 6 also initially shows the sensitivity of a silicon photodiode which can be used as detector 26 as a function of the wavelength λ of FIG

incident light in arbitrary units. The sensitivity range is here divided into a first range 50 of comparatively shorter wavelengths and a second range 52 of comparatively larger wavelengths. The wavelengths of the exciting laser light 10 of the laser module 18 are preferably in the first region 50, for example at wavelengths below 500 nm (e.g., 405, 450, 465 nm). The optical filter 42 is preferably a filter that emits light having wavelengths below, e.g. 500 nm strongly attenuated or even largely blocked.

A major advantage of this variant is that it covers almost the entire wavelength range in which a silicon-based detector is sensitive is to use for detection. Another advantage is that this variant allows the use of step filters, the light with

Blocking wavelengths below a lower cut-off wavelength and transmitting light with wavelengths above a cut-off wavelength. Such step filters are usually less expensive than bandwidth filters that use light

Block wavelengths that are narrow, the wavelength of the

Laser wavelength range containing laser light are.

As already mentioned, laser diodes are preferably used as laser light sources. In addition to the advantages already mentioned have laser diodes the

Advantage that their emission of laser light with MHz frequencies can be modulated. This is exploited in the embodiment explained below. The basic idea of this exemplary embodiment consists in a temporal modulation of the intensity of the laser light 10 emitted by the laser module 18. As a result, one takes place with the same frequency

Variation in the intensity of the radiation from a soot particle 12, which is currently in the spot 22, due to LH in the form of thermal radiation emanating 14. A soot particle 12 flying through the spot 22 heats up several times and cools between successive

Each heating up again, so that a periodic Lll signal arises. Lock-in amplification techniques can then be applied to such a Lll signal to improve the signal-to-noise ratio SNR.

A great advantage of this embodiment is that the frequency of the Lll signal is shifted to a high carrier frequency, namely the frequency of the modulation of the intensity of the laser light lying in the MHz range, whereby it reacts much less sensitive to external disturbances, such as They can be triggered, for example, by occurring during driving a motor vehicle vibrations. During driving vibrations occur frequencies of only a few Hz.

Laser power modulation at frequencies in the MHz range is typically not possible with high-power pumped ns lasers typically used for LH. If one goes even further in the direction of fs high-power lasers, then these frequencies are again achieved. The particulate matter sensor 16 used in this embodiment corresponds to the embodiments described so far. The temporal modulation of the intensity of the emanating from the laser module 18 laser light is preferably sinusoidal so that the intensity of the emitted laser light 10 corresponds to the maximum power of the laser module 18 and the lowest intensity of the emitted laser light 10 by the (short) shutdown of

Laser module 18 is reached.

In terms of the waveform and span, however, all sorts of other variations are conceivable. By way of example only, a rectangular or a sawtooth-shaped course of the intensity over time may be mentioned. In a modulation in which the intensity of the laser light 10 changes with a frequency lying in the MHz range, the intensity in the spot 22 in the

Time span in which a soot particle 12 flying through the spot 22 with typical exhaust gas velocities is located in the spot 22 repeatedly assume maximum and minimum values, so that the soot particle 12 periodically heats up and cools down. Thus, the Lll signal of the radiation 14 emitted by the soot particle 12 in the form of temperature radiation oscillates at the same frequency and constant phase to oscillate the intensity of the laser light 10. The frequency of such oscillation would have to be in the range of 100 kHz to 10 MHz. so that a soot particle 12 with a typical flight time of 1 ε to 1 ms in the spot 22 can be illuminated several times. On the other hand, this oscillation must not be faster than the typical heating time and cooling time of the soot particle 12 in the spot 22. This time is between 100 ns and 10 s.

Figure 7 shows a schematic and qualitative representation of the Lll signal 54 of a soot particle 12 in arbitrary units over time t in the event that the soot particle 12 flies through the area of the spot 22 and thereby a temporally sufficiently rapid modulation of the intensity of the exciting Laser light 10 takes place. The modulation is preferably done with a

Modulation frequency ranging from 100kHz to 100MHz. The LII signal 54 maps the intensity of the radiation 14 emitted by the soot particle 12 after excitation by the laser light 10 in the form of temperature radiation. The envelope curve 56 corresponds to the LII signal in the event that the intensity of the laser light 10 is not modulated. A modulation of the stimulating

Laser light 10 causes the soot particle 12 from the exciting laser light Is illuminated and heated again and again, so that the result is the rapidly oscillating signal 58 in which the rapid oscillation of the radiation 14 emitted by the soot particles 12 in the form of thermal radiation is reflected. Lock-in amplification techniques may then be applied to such a signal 58 to improve the signal-to-noise ratio (SNR) and, in particular, to eliminate the interfering signal background 60.

FIG. 8 shows an exemplary embodiment of a soot particle sensor 16 with connected control and evaluation electronics 62. The soot particle sensor 16 corresponds, for example, to that explained with reference to FIG

Soot particle sensor 16, so that the description of Figure 3 is also valid for the figure 8. The control and evaluation electronics 62 may be a separate control unit, or it may be integrated into a control unit which serves to control the combustion process. The control and evaluation electronics 62 has a control module 64 which modulates the intensity of the laser light 10 emanating from the laser module 18, as e.g. has been explained with reference to Figure 7. The signal from the detector 26 is fed to a lock-in amplifier 66, which is also supplied with a signal which maps the modulation of the laser light. As shown in FIG. 8, this signal can be taken directly from the control module 64, or it can be removed from the laser module 18. As a result, the signal of the detector 26 in the signal processing and signal amplification in the control and

Evaluation electronics 62 are correlated with the modulation of the exciting laser light 10, which can be done, for example, by the skilled person to improve the signal-to-noise ratio available lock-in or pseudo random sequence method or signal correlation method.

Such modulation of the laser power in the MHz range is usually not possible with pumped high-power ns lasers which are typically used for LH. However, if one goes further into the fs range, such repetition rates are again possible.

An advantageous embodiment of the object of FIG. 8 is characterized by a filter 42 which is arranged as shown in FIG. 4 and has the properties described above. These Embodiment filters out an influence of an oscillation of the intensity of the laser light 10 backscattered by the optical components on the measurement signal generated by the detector 26. FIG. 9 shows a further exemplary embodiment of a soot particle sensor 16 with connected control and evaluation electronics 62. The soot particle sensor 16 has a shaker module 68. In the illustrated embodiment, a movable element of the shaker module 68 is mechanically rigid with the

Laser module 18 connected so that an oscillation of the movable part of the shaker module 68 transmits to the laser module 18. Incidentally, the soot particle sensor 16 of FIG. 9 corresponds, for example, to the soot particle sensor 16 explained with reference to FIG. 3, so that its description is also valid for the soot particle sensor 16 of FIG. The remaining components of the

Soot particle sensors 16 are not rigid with the moving part of the

Shaker modules 68 connected and therefore do not perform its oscillatory motion. The control and evaluation electronics 62 corresponds to the control and evaluation electronics 62 of Figure 8, so that their description is also valid for the control and evaluation electronics 62 of Figure 9, unless explicitly described otherwise. The embodiment of Figure 9 is based on the idea, the position of the spot 22 in relation to the current position of the

Soot particles 12 to vary. In this case, the spot movement has to be so much faster than the movement of the soot particles 12 with the exhaust gas 32 that the soot particles flying through the spot 22 are repeatedly illuminated and heated while they are in the spot 22, resulting in a periodic LII signal , In such a signal are then preferably lock-in or others

Reinforcement method applied, as has been explained with reference to Figure 8. A difference from FIG. 8 results from the fact that the lock-in amplifier 66 is supplied with the drive signal of the shaker module 68, since this is synchronous with the movement of the spot 22 and thus with the variation of the intensity of the LII signal.

The variation of the position of the spot 22 is generated by a movement of the laser module 18 driven by the shaker module 68. The shaker module 68 has, for example, a piezoactuator, which is actuated by the control module 64. As an alternative to a piezoelectric actuator is the use of a

electromagnetic actuator or magnetostrictive actuator conceivable. Depending on the configuration of the actuator, the oscillation movement can take place parallel or transversely to the laser beam direction, which is represented in FIG. 9 by the indication of possible oscillation directions 70. The

Oscillation direction can also be perpendicular to the plane of the drawing.

FIG. 10 shows details of an embodiment of an alternative to FIG

Soot particle sensor 16, which is adapted to cause the spatial position of the spot 22 to oscillate. The soot particle sensor 16 of FIG. 10 has two shaker modules 68a, 68b, the movable part of which is mechanically coupled to the optical element 20, so that an oscillation movement of the shaker module is transmitted to the optical element 20. Preferably, the mechanical coupling is a rigid connection. The two shaker modules 68a, 68b are preferably driven synchronously and with the same phase and amplitude, so that the optical element 20 is moved back and forth in the propagation direction of the laser light 10, which causes a corresponding movement of the laser beam

Radiation waist causes.

For the variation of the position of the spot 22 are also other suitable

Process conceivable. Also, the movement itself is not limited to a particular shape or direction. It is only important that because of the oscillating movement of the spot 22, the soot particles 12 are each illuminated several times more and less intensively, so that a fluctuation / oscillation of their LII luminous intensity results. A major advantage of varying the position of the spot 22 over one

Variation of the intensity of the laser light 10 is that the intensity of the laser light backscattered by the optical components does not vary with the variation of the position of the spot 22. Thus, in embodiments that operate with the variation of the position of the spot 22, the filter 42 of the embodiment shown in FIG. 45 can be dispensed with

Cost savings possible.

FIG. 11 shows the spot 22 in a beam waist 73 of the laser light 10 for two different combinations of the flow direction 72 of the gas carrying the soot particles 12 and the propagation direction 74 of the laser light 10. In part a) of FIG. 11 (left), the two are Directions 72, 74 parallel to each other. This corresponds to the arrangement of the so far presented soot particulate sensors. In part b) of FIG. 11 (right), the two directions 72, 74 are transverse to one another, which corresponds to an alternatively conceivable construction of the soot particle sensor. In both cases, it is possible to vary the position of the spot 22 both parallel and perpendicular to the respective direction of movement of the soot particles. This results in at least four possible combinations of

Oscillation direction 70 of the spot 22 and propagation direction of the laser light 10. The beam waist 73 is the range of the beam path of the laser light 10 in

Soot particle sensor 16, in which the laser light 10 is most focused. The size of the beam waist 73 is limited downwardly due to optical laws and therefore can not be infinitely small. The spot 22 is the spatial area where the light intensity and thus the energy density and the temperature of the soot particles are high enough to induce the lasers

Incantescence or igniting chemical reactions.

FIG. 12 shows a schematic exemplary representation of the LII signal 76 of the detector 26, which is caused by a soot particle 12, which flies through the spatially oscillating spot 22. The signal 76 corresponds qualitatively to the signal 54 shown in FIG. 7. The envelope curve 78 corresponds to the LII signal in the event that the spot 22 is not moved. Due to the actual, however, existing variation of the position of the spot 22, the soot particle 12 is illuminated and heated again and again, so that a periodic signal 80 is formed, in which the rapid spatial oscillation of the soot particle 12 is formed. Lock-in amplification techniques, or generally signal correlation techniques, may then be applied to such a signal 80 to improve the signal-to-noise ratio, and in particular to eliminate the interfering signal background 82.

Fig. 13 shows a further embodiment of an inventive

Soot particle sensor 16. The soot particle sensor 16 according to FIG. 13 is likewise based on that explained with reference to FIGS. 3 and 4

Soot particulate sensors 16 and 16 additionally include a pair of electrodes 84, 86 disposed in the soot particle sensor 16 on different sides of the spot 22. The electrodes 84, 86 are preferably inside the inner protective tube 30 arranged. These electrodes 84, 86 serve to generate an alternating electric field, which passes through the spot 22. The field generating

AC voltage is applied to the electrodes 85, 86 by the control module 64. As a result, this leads to an externally induced spatial oscillation of the soot particle stream, which periodically modulates the measurement signal. This allows the

Use of correlation techniques (such as lock-in or pseudo-random sequence) in detection, which improves the signal-to-noise ratio by several orders of magnitude. In the illustrated embodiment, the AC voltage supplied to the electrodes 84, 86 is supplied in parallel to an input of a lock-in amplifier 66. At least part of the soot particles carries an electrical charge.

FIG. 14 shows the beam waist 73 of the laser light 10 defining the spot 22 together with a soot particle 12, which is flying straight through the spot 22. As with all other figures, here too, the spot 22 is the spatial area in which the intensity is so high that soot particles 12 passing through are heated so strongly that they are excited to emit radiation, in particular to emit temperature radiation , Without an applied electric field, the soot particle 12 passes through the spot 22 in a uniform movement with the flow velocity of the exhaust gas in the flow direction 72 of the exhaust gas. This situation is shown in part a) of FIG. By applying the alternating electric field, which is oriented transversely to the flow direction 72 of the exhaust gas, the trajectory of the (electrically charged) soot particle 12 is impressed on an oscillation, and it leaves the spot 22 and enters the spot 22 after a reversal of the field direction , On the assumption of a sufficiently large frequency of the alternating electric field, the soot particle 12 is heated periodically and the LH signal emanating from it is periodically modulated.

FIG. 15 shows a schematic representation of the III signal 90 of FIG

Soot particles, which move on an oscillating trajectory through the

Laserspot 22 moves, in arbitrary units over time t. The envelope 92 corresponds to the Lll signal in the event that no alternating electric field is applied. By periodically entering and leaving the laser spot due to the applied alternating field, the soot particle is repeatedly illuminated and heated, so that a periodic Lll signal 94 is formed. To a such signals may be applied as described above for other embodiments, correlation techniques to improve the signal-to-noise ratio (SNR) and eliminate the interfering background 96.

An estimate of the frequencies required for this can be made by the following calculation: The relevant lengths for these processes are due to the axial extent 2zo and lateral extent 2w _{0 of} the beam waist 73

(Laser focus). The dimensions of the spot 22 correspond largely to these dimensions. The lateral extent is through the

Intensity drop to 1 / e ² , as usual in Gaussian radiation optics. The distance z. _c is also referred to as the Rayleigh length and is defined by the beam waist ₀ and the wavelength λ of the laser of the laser module 18:

NOT A WORD '

The minimum necessary frequency for the applied alternating electric field is now given by the condition that a soot particle 12 once leaves the spot 22 on its way through the spot 22 and enters the spot 22 again. This allows the maximum period At _{m X} and thus the minimum frequency

1 _ 2z _Q

_Calculating the exhaust gas flow velocity v _xh : For a beam waist of 2w ₀ = 10 pm, a wavelength of 1 pm as well as a

Exhaust gas velocity of about 1 m / s results, for example, a minimum frequency of about 6 kHz. A typical working frequency should be chosen by a factor of at least 10 higher to allow multiple passes of the

Soot particles 22 through the spot 22 to allow. As a result, the Lll signal is periodically modulated at the frequency f and a detection with

Correlation techniques (lock-in, pseudo-random sequence). This allows a strong suppression of background signals 96, as caused for example by backscattered by optical elements in the beam path light become. Similarly, the use of correlation techniques improves the overall signal-to-noise ratio.

The use of these correlation techniques is not possible with the Q-switched, pulsed ns lasers used in the prior art because of their low repetition rates at high frequencies (in the range of kHz-MHz). With the invention proposed here, this is easily possible.

The electrodes 84, 86 serving to apply the alternating field may themselves be provided with a heating element to heat them at regular intervals so that the settling soot is burned off.

FIG. 16 shows an exemplary embodiment of a soot particle sensor 16, which has sound wave exciters 98, 100 and is based on the soot particle sensor 16 shown in FIG. The pair of acoustic wave exciters 98, 100 is disposed inside the protective tube 30. The sound wave exciters 98, 100 oscillate transversely to the flow direction of the soot particles 12. Die

Sound wave exciters 98, 100 are, for example, electrical transducers, e.g. on a piezo or magnetostriction basis or as speakers

be electromagnetically actuated and generate a standing ultrasonic wave.

FIG. 17 shows an example of such a standing ultrasonic wave 102, which adjusts between the sound wave exciters 98, 100. The excitation frequency of the sound wave exciters 98, 100 is thereby preferably set so that the spot 22 in a velocity bump one between the

Sound wave exciters 98, 100 adjusting standing ultrasonic wave 102 is located. The spot 22 is a space area defined by beam waist 2w ₀ and Rayleigh length 2z ₀ , in which the intensity of the laser light 10 is high enough to "ignite" LH (applies to all embodiments). As a result, the soot particles 12 are pushed periodically and transversely to their original flow direction 72 out of the spot 22 when passing through the spot 22 of the velocity bump and again sucked in, so that a modulation of the

Particle excitation arises, which is reflected in a modulation of the Lll signal. The Lll signal typically decays on a time scale in the range of a few 0 to 100 nanoseconds after excitation. An estimation of the modulation frequencies necessary for this is done completely analogously to the estimation detailed above for the soot particle sensor 16 according to FIG. With the speed of sound in air of about 340 m / s and an operating frequency of f = 60 kHz results in a

Wavelength of the ultrasonic wave of about 5 mm, which is well feasible in a protective tube. As it passes through the velocity bump 22, the soot particle is periodically forced out of focus causing modulation of the particle excitation. As a result, the LII signal is periodically modulated at the frequency f and allows detection with correlation techniques (lock-in, pseudo-random sequence). This allows a strong suppression of background signals, as caused by, for example, backscattered light from optical elements in the beam path. In addition, by varying the excitation frequency of the sound wave exciters 98, 100, the position of the velocity bump of the standing ultrasonic wave 102 can be controlledly shifted, thereby achieving the desired modulation of the detection signal. The technical advantage of this special solution lies in a still further reduced sampling rate in the signal evaluation, which leads to a simpler evaluation circuit (cost) and consequently to a lower one

Power consumption leads.

It is also conceivable to modulate the phases of the oscillations of the two sound wave exciters 98, 100 relative to one another (phase modulation). The sound wave exciters 98, 100 themselves serving to generate the ultrasonic wave can themselves be provided with at least one heating element in order to be able to turn them into

To heat up at regular intervals, so that the settling soot is burned off.

The result is a time profile of the LII signal, as shown in FIG. 15, and as is typical of a soot particle 12 moving through the spot 22 on an oscillating trajectory. The envelope 92 corresponds to the LII signal in the event that no alternating acoustic field is applied. By periodically entering and leaving the laser spot due to the applied alternating field, the soot particle is repeatedly illuminated and heated, so that a periodic Lll signal 94 is formed. Correlation methods can then be applied to such a signal Signal-to-noise ratio (SNR) to improve, as it has already been explained with reference to the thus comparable Figure 12.

FIG. 18 shows a comparison of a non-polarizing beam splitter 234 with a polarizing beam splitter 134. The left half of FIG. 22 relates to the non-polarizing beam splitter 234, while the right half relates to a polarizing beam splitter 134.

The polarizing beam splitter 134 is characterized by the fact that it transmits or reflects light differently depending on the polarization. For a predetermined polarization direction of the incident light results in an almost complete transmission, and for the perpendicular polarization of the incident light results in an almost complete reflection.

Since laser light 10 is generally already polarized, it can

polarizing beam splitter 134 occur in the direction of polarization of the polarizing beam splitter 134 suitably chosen in one direction (way to the spot 22) virtually lossless, while the beam splitter surface 234.1 of the non-polarizing beam splitter 234 already up to 50% of the power transported by the laser light 10 from the Nutzstrahlengang

reflected out. This loss is represented in the left part of FIG. 18 by the arrow 10 pointing to the left. In the case of the beam splitter 134 shown in the right-hand part of FIG. 18, this loss component 10 'does not occur. In other words, by properly choosing the laser polarization and orientation of the laser, the transmitted power at that location can be maximized (to nearly 100%), while with conventional nonpolarizing beamsplitters 234, a power loss of about 50% in transmission through the beam splitter got to. The laser light 10 passes through the beam splitter surface 134.1 of the polarizing beam splitter 134 without being attenuated.

The transmitted light, as described with reference to FIGS. 3 and 4, is focused through the second lens 20 through the protective window 40 to the spot 22 in the inner protective tube 30. With the same power of the laser when using the polarizing beam splitter 134 is therefore twice as much light power in the spot 22 for heating particles available. This embodiment has the advantage that the maximum intensity in the spot 22 is increased with otherwise unchanged soot particle sensor 16, which heats up the soot particles 2 to be measured to higher temperatures and thus increases the radiant output of the heated soot particles 12 of the radiation 14 emitted in the form of temperature radiation , The result is an improved signal-to-noise ratio.

Radiation 14 emitted by soot particles 12 which are located in the spot 22 and emitted in the direction of the lens is preferably detected by the same lens 20 and directed onto the detector 26 via the preferably polarizing beam splitter 134. Since the radiation 14 emitted by the heated soot particle has no preferred polarization, approximately half of the radiation 14 detected by the lens 20 is directed onto the detector 26. Since the soot particles 12 emit only unpolarized radiation 14, only a residual loss remains in the

Detection beam left over, which would occur anyway. The polarizing beam splitter 134 can be used as a beam splitter 34 in all exemplary embodiments according to the invention, that is to say in particular in all soot particle sensors described in this application. Similarly, a non-polarizing beam splitter 234 can be used as a beam splitter 34 in all exemplary embodiments according to the invention, that is to say in particular for all soot particle sensors described in this application.

The use of the polarizing beam splitter 134 therefore has the advantage over the embodiments operating with non-polarizing beam splitters 234 that the increased optical pump power results in a substantially higher performance

Power density and temperature T can be achieved in the spot 22 and thus the spontaneously emitted power rises sharply (Kirchhoffsches

Law of radiation P ~! T ⁴ ). Again, it is true that the soot particle sensor 16 preferably has an optical filter 42, which in the beam path between the

(polarizing) beam splitter 134 and the detector 26 is arranged and having the properties described above. With this filter 42, a shielding of the detector 26 is achieved with respect to the laser light 10, which also improves the signal-to-noise ratio.

Claims

Claims 1. Soot particle sensor (16) with a laser module having a laser

(18) and with a device for detecting temperature radiation (14)

equipped detector (26), characterized in that the

Soot particle sensor (16) has an optical element (20) arranged in the beam path of the laser of the laser module (18), which is set up to bundle laser light (10) emanating from the laser module (18) into a spot (22), and in that Detector (26) in the soot particle sensor (16) is arranged so that it detects radiation (14) emanating from the spot (22).

2. A soot particulate sensor (16) according to claim 1, characterized in that the laser module (18) is adapted to generate parallel laser light (10), and that the optical element (20) is adapted to be of the

Laser module (18) outgoing parallel laser light (10) in the spot (22) to focus. 3. Soot particle sensor (16) according to claim 1 or 2, characterized in that the laser of the laser module (18) is a CW laser.

4. soot particle sensor (16) according to any one of the preceding claims,

characterized in that the laser is a semiconductor laser element, in particular a laser diode (36).

5. soot particle sensor (16) according to any one of the preceding claims,

characterized in that the detector (26) comprises at least one photodiode (26.1).

6. A soot particle sensor (16) according to one of the preceding claims,

characterized by a beam splitter (34) arranged in the beam path of the parallel laser light (10) so as to direct at least part of the laser light (10) incident from the laser module (18) to the optical element (20) and from the Spot (22) ago incident

Directed radiation (14) at least in part on the detector (26). A soot particle sensor (16) according to claim 6, characterized in that the beam splitter (34) is a polarizing beam splitter (134), and in that the polarizing beam splitter (134) is oriented to emit incident laser light (0 ) is maximally permeable.

A soot particle sensor (16) according to claim 6 or 7, characterized by an optical filter (42) which is arranged in the beam path between the beam splitter (34) and the detector (26) and which is less transparent to the laser light (10) than to the spot (22) outgoing

Temperature radiation (14).

A soot particle sensor (16) according to claim 8, characterized in that the laser is adapted to emit Laserücht (10) with wavelengths below 500 nm, in particular 405 nm, 450 nm or 465 nm and that the optical filter (42) so is that it is light with

Wavelengths below 500 nm attenuate or even block.

A soot particle sensor (16) according to one of the preceding claims, characterized by a first part (16.1) which is adapted to be exposed to a measurement gas, and a second part (16.2) which can not be exposed to the measurement gas and which comprises the optical components of the soot particle sensor (16 ), both parts being separated by a partition (16.3) impermeable to the sample gas.

A soot particle sensor (16) according to claim 10, characterized in that in the dividing wall in the beam path of the laser light (10) a window (40) is mounted, both for the laser light (10) and for the spot (22) emanating radiation (14 ) is permeable.

A soot particle sensor (16) according to claim 10 or 11, characterized in that it comprises an outer protection tube (28) and an inner protection tube (30), both having a general cylindrical shape or prism shape, such that the protection tubes are coaxially arranged the axes of the cylinder or prism shapes parallel to

Beam direction of the laser light (10) are aligned and the spot (2) in Interior of the inner protective tube (30) is that the outer protective tube (28) at its end facing the laser beyond the inner protective tube (30) protrudes and that the inner protective tube (28) at the opposite end beyond the outer protective tube (30) protrudes ,

13. A soot particle sensor (16) according to one of the preceding claims,

characterized in that the soot particulate sensor (16) comprises a shaker module (68) having an oscillatingly movable element mechanically rigidly connected to the laser module (18) so that oscillation of the movable part of the shaker module (68) onto the

Laser module (18) transmits.

14, soot particle sensor (16) according to claim 13, characterized in that the shaker module (68) has a movable element having the piezoelectric actuator or a movable element having

electromagnetic actuator or a movable element

comprising magnetostrictive actuator.

15. Soot particle sensor (16) according to any one of claims 1-12, characterized

characterized in that the soot particle sensor (16) comprises a pair of electrodes (84, 86) disposed in the soot particle sensor (16) on different sides of the spot (22).

16, soot particle sensor (16) according to any one of claims 1-12, characterized

characterized in that the soot particle sensor (16) is a pair of

Sound wave exciters (98, 100), which is arranged inside the inner protective tube (30).

17, soot particle sensor (16) according to claim 16, characterized in that the sound wave exciters (98, 100) are electrical transducers which operate on a piezo or magnetostriction basis or are actuated electromagnetically and generate a standing ultrasonic wave.