EP1920233A1

EP1920233A1 - Electrostatic particle sensor

Info

Publication number: EP1920233A1
Application number: EP06792509A
Authority: EP
Inventors: Stefan Wilhelm
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2005-08-24
Filing date: 2006-07-17
Publication date: 2008-05-14
Also published as: US20090295400A1; WO2007023035A1; DE102005039915A1

Abstract

The invention relates to an electrostatic particle sensor for sensing particles (P) in aerosols, particularly soot particles in exhaust gases. Said electrostatic particle sensor comprises a covered electrode (M) which has an effective flow volume (V) and is penetrated by a gas flow to be tested, an inner electrode (I) located inside the covered electrode, and a voltage source (U) that is in electrically conducting connection to both electrodes (M, I). The invention is characterized in that a potential which depends on the throughput (D) through the volume (V) per time unit (t) is impressed upon the voltage source (U).

Description

Electrostatic Particle Sensor

The present invention relates to an electrostatic particle sensor according to the preamble of claim 1.

State of the art:

The environmental impact of particulate matter, in particular due to combustion processes of petroleum products resulting soot particles is constantly increasing. Due to improvements in the combustion technique of petroleum products, such as e.g. Engine and heating system technology, the remaining in the oxidation process particulate residues due to greatly reduced particle sizes are considered to be increasingly critical for the environment.

Furthermore, it is possible to evaluate the quality of the oxidation process on the basis of the number and size of the particles in a defined exhaust gas volume. For example, it is known to use optical measuring methods for determining the exhaust gas quality. However, these have the disadvantage that they are very susceptible to interference by particle deposits on the sensor elements. Furthermore, gravimetric or mobility analysis based methods are known in laboratories, in particular to determine the soot particle number, the soot particle mass or soot particle size distribution in combustion processes. These are sometimes very complex and also have the disadvantage that the measurement of the exhaust gas does not take place directly in the exhaust system, whereby a falsification of the measurement result, partly depending on the age of the sample gas due to it in advance chemical processes unavoidable i st.

In contrast, improved measurement methods, by direct measurement in the exhaust system, are known from DE 198 17 402 C1 and DE 195 36 705 Al. In DE 198 17 402 C2, a plate-shaped capacitor is introduced into an exhaust gas stream and heated to very high temperatures in the range of 500 ° C to 800 ° C to avoid soot deposits and associated Meßwertverfälschungen. This is intended to overcome a disadvantage attributed to DE 195 36 705 A1 to a short circuit formation between two measuring electrodes arranged in a measuring gas line.

The measuring methods of both documents are based on the evaluation of an electrostatic field which is formed between two electrodes, caused by a DC voltage source and changed by particles of an exhaust gas stream adhering to electrical charges.

A disadvantage of these two measuring methods, however, is that only particles of certain sizes can be detected, which are in the range which is able to sense the sensor used in each case on the basis of the physical relationships of the respective measuring method in a position. Particles which are outside this range of particle sizes which can be detected by the respective sensor can therefore not be detected. Thus, with these sensors but no complete quality statement on the measured exhaust gas is possible.

Object and advantages of the invention:

The present invention is therefore based on the object to improve a particle sensor of the type set forth.

This problem is solved by the features of claim 1.

In the dependent claims advantageous and expedient developments of the invention are given. Accordingly, the present invention relates to an electrostatic particle sensor according to the preamble of claim 1, which is characterized in that one for generating an electric field, between a sheath electrode and an inner electrode disposed within this sheath electrode voltage source, one of the gas flow rate per unit time through the effective volume of the sheath electrode dependent potential is impressed.

This approach is based on the finding that by varying the electric field particles, in particular soot particles, with different electrical mobility and thus different mass and thus directly related large can be detected without having to make changes in the effective volume flow between the two measuring electrodes.

Of course, other parameters, such as e.g. the cross section and / or the length of the capacitor formed by the two electrodes or also the speed of the gas flow flowing through this arrangement for the detection of particles of different sizes can be varied. By varying the potential of the voltage source causing the electric field, however, a measuring range change which is particularly easy to handle is made available for a corresponding particle measuring sensor.

In particular, it is considered advantageous in this case if the particle sensor is designed as a cylindrical capacitor, so that an exact determination of the effective volume for the particle determination of the measurement gas volume is possible by means of defined geometric parameters. In addition, due to the radial dependence of the electric field contained therein, a cylindrical capacitor with the same external geometrical dimensions and applied potential offers the possibility of detecting particles with smaller mobility, ie larger mass.

For the interpretation of the parameters essential to this method of meter, and thus also for the determination of the particle size measuring range of the particle measuring sensor are in addition to the geometric variables r _a for the radius of the outer and sheath electrode, r _± for the radius of the inner electrode of length 1 and the potential U of the voltage source for the generation of the electric field also the gas velocity v _gas essential.

In a preferred embodiment, therefore, a gas velocity measuring device is provided which is particularly preferred as a non-invasive measuring device, e.g. is designed as a venturi nozzle. This makes it possible to determine the gas velocity without or at least without significant disturbing influences on the gas flow, which in turn has a positive effect on the measuring accuracy of the particle sensor. It can be arranged before or after the electrode assembly in the gas flow direction.

Of course, however, measuring devices for gas velocity determination in the form of a hot wire and / or an impeller and the like are also possible.

Compared to embodiments in which the effective volume flow is assumed only on the basis of an assumed gas velocity, preferably an average gas velocity, a further reduction of system-related measurement errors can be achieved with these embodiments due to the immediately possible consideration of any changes in the gas flow rate.

Of course, it is also possible in principle to carry out the measuring method without direct speed detection of the measuring gas flow through the sensor volume, but this saving is paid for by a comparatively lower measuring accuracy of the relevant measuring sensor.

Due to the electric field between the two electrodes, that is preferably in the interior of the cylinder capacitor, in the exhaust gas contained, electrically charged particles, in particular Depending on their respective polarity, soot particles are accelerated toward either the outer or the inner electrode. If the particles, in particular soot particles, strike an electrode, they discharge their electrical charge to this electrode. The charge delivery of the charged particles to the electrode can be measured as a current by means of a current measuring device, in particular by means of an electrometer. If the mean charge distribution of the particles is known, this is the average charge per particle, then it can be used to determine the number of particles that have given off a charge to the electrode. The size of the particles is predetermined by the geometric conditions of the measuring arrangement already discussed above, in cooperation with their electrical mobility.

The electric current detected by the electrometer thus corresponds to that electric charge which is transported by those particles of the particle flow in the measurement gas to be evaluated, for which the particle size measurement range is set in each case.

By physico-chemical reactions in the sample gas, a large part of the particles contained therein is electrically charged. However, the charge distribution of the particles is not constant over time, since ion exchange or neutralization in particular takes place through ionic ion recombination, and the particles are predominantly electrically neutral with increasing age of the measurement gas. Depending on the exhaust age, it may therefore be necessary to ionize the soot particles by suitable ion sources. Preferably, direct or indirect high-voltage high-frequency discharge, α-, ß- or γ-radiation, electron radiation or similar ionization sources are provided.

Furthermore, in order to avoid measurement errors, deposits in the measuring arrangement could be removed again by means of a heating device, preferably by burning off.

Exemplary embodiment: The invention will be explained in more detail with reference to the drawings and the following description taken in conjunction therewith.

Show it:

Figure 1 is a schematic representation of an electrostatic particle sensor;

FIG. 2 shows a second embodiment modified from the first embodiment;

Figure 3 is a diagram for the parameter representation of the electrical limit mobility of the particles of a sample gas as a function of the radii ratios of a Mantelbzw. Outer electrode to an inner electrode of the measuring arrangements according to the figures 1 and 2; and

FIG. 4 shows the cross-sectional area of the measuring arrangement, likewise as a function of the radii ratios.

Figures 1 and 2 show two exemplary, symbolically represented structures of electrostatic sensors for measuring particles in Aeorsolen, in particular for the measurement of soot particles in exhaust gases, the exhaust gases are preferably exhaust gases from diesel engines.

Such sensors can be provided as robust measuring devices for the analysis of soot particles directly in the exhaust system, so that they are suitable for a workshop operation on the one hand, and on the other hand also for direct installation in a relevant vehicle, to improve the exhaust quality or basically to improve the motor characteristics.

Another possible application of such sensors is in the heating technology sector. Again, both mobile and immobile applications can be provided. In mobile applications, for example, the current emission values of a heating system can be determined. In immobile applications is a direct Influence on the control process of the heating conceivable, so that, if necessary, a large potential savings in fuel consumption can be achieved by appropriate measures to be initiated soot formation.

1 shows a first embodiment of an electrostatic particle sensor 1 for sensing particles P in aerosols, in particular for sensing soot particles in exhaust gases. This constructed as a cylindrical capacitor, a shell or outer electrode M and an inner electrode I comprehensive sensor is equipped with a voltage source U for supplying the electrodes M, I. According to the invention, the potential of this voltage source U can be adjusted depending on the gas flow rate per unit time through the volume V between the two electrodes M, I and a particle size to be detected. As a result, a variable measuring range for particles of different sizes can be made available with one and the same measuring structure.

Due to the geometric conditions of the capacitor, the strength of the electric field and the velocity of the gas in the condenser, only particles of particular electrical mobility will reach the inner or outer electrode for delivery of the electrical charge attached to them and thus for evidence of their presence in the device being measured gas flow.

The geometric relationships r _a and r _{± of} the cylindrical capacitor 2 together with its length 1 determine the effective volume for the measuring method V. In this embodiment, the inner electrode I is applied via an electrometer 3 to the variable potential of the voltage source U. The mass of this voltage source is connected to the outer electrode, which may optionally also be connected to a vehicle mass 4.

The tube-shaped jacket electrode M of the cylindrical capacitor 2 has a temperature-resistant, insulated lead-through 5 for the electrical connection between the electrometer 3 and the inner electrode I. To ensure that the measurement results obtained with this measurement arrangement are not falsified due to deposits on the inner electrode I and / or to the electrical connection between the electrometer and the inner electrode I during the operating period by deposits and concomitant conductivity changes, is still a heating circuit 6 provided on the switch 7, 8 can be closed. By a second, temperature-independent and trained in the outer electrode M passage 9 through, to the inner electrode I out, the heating circuit 6 is closed.

At appropriate intervals parts of this circuit are heated so much that it adhering particles, especially soot particles burn off to avoid interference with the measurement result. Optionally, such heating periods can be performed clocked, preferably during the heating, no measurement is made to hide interference caused thereby. The heating circuit is supplied by a further voltage source 10.

For determining the gas velocity through the measuring arrangement, a gas velocity measuring device is furthermore provided, which in the present case is particularly preferably designed as a non-invasive measuring device in the form of a Venturi nozzle.

This makes it possible to determine the particle size of the particles P as a function of the gas velocity, the geometric relationships of the measuring arrangement and the strength of the electric field based on the measured electric current, which is caused by the transferred by means of the particles P electric charge.

The flow direction of the gas flow through the measuring arrangement is symbolized by the arrow 12, which is shown symbolically at the inlet of the exhaust pipe 13 between two elements 14 representing an ionization source. The ionization source 14 may preferably be formed as a high voltage source and / or as a high frequency source.

The advantage of this embodiment is that the outer electrode is grounded and can be implemented directly into an exhaust line 13 without insulation. The maximum potential of the voltage source is limited by the electronics of the electrometer.

In contrast to the modified embodiment of Figure 2, the outer electrode is applied to the variable potential of the voltage source U. The inner electrode discharges via the electrometer to ground. In this embodiment, there is the advantage that there is no limitation of the maximum possible potential through the electronics of the electrometer. In contrast to the embodiment in FIG. 1, however, insulation of the jacket or outer electrode M must be provided in relation to the exhaust gas line.

The following two measurement modes are possible with this embodiment:

1. Measuring the number of all diesel soot particles: Operation with constant potential U _max , according to the design of the "electrostatic probe for the measurement of diesel soot", all particles with k> k _{limit are} detected.

2. Measuring mobility (mass, size) distribution:

The potential U is gradually increased from U = 0 V to U = U _max . The distance of the stages and the time periods of the measuring stages determine the resolution of the distribution.

Since all charged soot particles with k> k _{grenz are always} detected during the measurement, the number of soot particles per mobility _interval must be determined by differentiation. By reversing the applied potential U, either positively or negatively charged soot particles can be measured.

Given the outer and inner radius, the length of the electrodes and the applied potential difference and the velocity of the gas v _gas , the following _{limit mobility} k _{grenz results} :

k _grmχ

The limit mobility determines the minimum mobility that a charged particle is allowed to have, given parameters (U, 1, r _a , r _1, v _gas ) within the residence time in the field of the "electrostatic sensor for measuring soot" on the inner electrode to be accelerated. Depending on the application, the parameters (U _max , 1, r _a , r _1, v _gas ) can be adjusted to determine the desired sensitivity, resolving power and bandwidth of the "electrostatic sensor for measuring soot".

To detect possible all diesel soot particles (also with large masses), it is necessary to the interpretation of the parameters (U _max, 1, r _a, r as small a critical mobility kg _re n to achieve. This is because in most applications U _max and limited by technical constraints are strongly determined by the ratio d = τ _& / τ _± The detection sensitivity of the probe, however, is strongly determined by the cross-sectional area A.

_FIGS. 3 and 4 show a depiction of the parameter dependence k _grenz and A as a function of d = I ₃ Zr ₁ . FIG. 3 shows a diagram for the parameter representation of the electrical limit mobility of the particles as a function of the radii ratios of a jacket or outer electrode to the inner electrode of the measuring arrangements according to FIGS. 1 and 2.

Thus, during the measuring process under application of the potential difference U between the casing or outer electrode M having the radius r _a and the internal electrode I τ of radius _± may taking into Considering the electrical mobility k of the particles in the gas stream to be measured the measurement can be performed. As a result, the electric field E perpendicular to the direction of movement of the gas is formed between the two electrodes (inhomogeneities of the electrical electric field E at the edges of the electrodes can be neglected). Charged particles are accelerated in the electric field depending on the polarity either to the outer or inner electrode. Depending on the electrical mobility k of the particle, the constant velocity component u = kE (r) perpendicular to the electrode axis is established.

By knowing the charge distribution on the (soot) particles, it is possible to calculate the number of particles whose electrical mobility is greater than k _limit .

By varying the applied potential difference, it is possible to calculate a mobility spectrum.

With the help of the relationship:

k (m) = U, 2-m ^~ ° ^Al5

(From: WD Kilpatric, An experimental mass-mobility relation for ions in air at atmospheric pressure, Proc., 19 ^th Ann Conf. on Mass Spectroscopy, page 320, 1971) can be determined from the measured electrical mobility, the mass of the particles.

Moreover, by assuming an average density and geometric shape of the soot particles, it is possible to determine their size.

Claims

Claims :

1. Electrostatic particle sensor (1) for sensing particles in aerosols, in particular for sensing soot particles in exhaust gases, comprising a flowed through by a gas flow to be tested jacket electrode (M) with an effective volume (V) and a disposed within the jacket electrode inner electrode ( I), as well as with two electrodes (M, I) in electrically conductive connection voltage source (U), characterized in that the voltage source (U) from the gas flow rate per unit time (t) by the volume

(V) dependent potential is imprinted.

2. Sensor according to claim 1, characterized in that the sensor is designed as a cylindrical capacitor (2).

3. Sensor according to claim 1 or 2, characterized in that the sensor is a gas-speed measuring device

(11).

4. Sensor according to one of the preceding claims, characterized in that the gas velocity measuring device (11) is designed as a non-invasive measuring device.

5. Sensor according to one of the preceding claims, characterized in that the gas velocity measuring device (11) is designed as a Venturi nozzle.

6. Sensor according to one of the preceding claims, characterized in that the gas-speed measuring device comprises a heat wire and / or an impeller.

7. Sensor according to one of the preceding claims, characterized characterized in that a current measuring device (3) is provided for an electric current.

8. Sensor according to one of the preceding claims, characterized in that the current measuring device (3) measures the electrical current, which by a of a particle flow between the two electrodes (M, I) moving, electrically charged particles (P) transported, electrical charge is caused.

9. Sensor according to one of the preceding claims, characterized in that the sensor is an ionization source

(14).

10. Sensor according to one of the preceding claims, characterized in that the ionization source (14) is designed as a high voltage source and / or as a high frequency source.