Particle sensor

Abstract

A particle mass concentration in an aerosol volume can be detected by an optical particle sensor. In order to ensure that different degrees of contamination of the optical particle sensor can be detected by the sensor and can be taken into consideration, the optical particle sensor identifies individual particles at low particle concentrations of up to 1000 particle s/cm.sup.3.

Claims

1. A particle sensor for detecting a particle mass concentration in an aerosol volume, the particle sensor comprising: a light source for irradiating a sample of the aerosol with light that is scattered by the sample; an aerosol photometer for identifying individual particles at low particle concentrations up to 1000 particles/cm.sup.3; an optical particle counter for detecting a static signal component; a DC voltage-coupled amplifier for detecting the static signal component and having an input; a photodetector connected to the input and emitting an output; and an A/D converter loaded with software for scanning and processing the output.

2. The particle sensor according to claim 1, wherein the DC voltage-coupled amplifier has a very small temperature drift.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) The invention will be explained hereinafter in greater detail on the basis of embodiments with reference to the drawing, in which.

(2) FIG. 1 shows a basic depiction of a first embodiment of a particle sensor according to the invention for determining a particle mass concentration in an aerosol;

(3) FIG. 2 shows a basic depiction of a second embodiment of the particle sensor according to the invention;

(4) FIG. 3 shows a graph depicting the characteristic curve of an optical particle counter (OPC)/aerosol photometer (APM) with different degrees of contamination;

(5) FIG. 4 shows a graph depicting the profile of the signal at the photodetector of an optical particle counter (OPC) with different degrees of contamination; and

(6) FIG. 5 shows a graph depicting the profile of the signal at the photodetector of an aerosol photometer (APM) with different degrees of contamination.

SPECIFIC DESCRIPTION OF THE INVENTION

(7) A first embodiment of an optical particle sensor 1 according to the invention, shown in a basic depiction in FIG. 1, includes an aerosol photometer (APM) 2, which is used to determine the particle mass concentration in an aerosol.

(8) The aerosol photometer 2 has a monochromatic light source 3, which can be configured as a laser diode or as a light-emitting diode (LED). The light radiation emitted by the monochromatic light source 3 of the aerosol photometer 2 is bundled in an optical lens 4. The light beam leaving the optical lens 4 passes through a gas flow 5, which entrains the aerosol to be measured. Light is reflected in the direction of a further optical lens 6 by particles of the aerosol contained in the gas flow 5. By means of the two optical lenses 4, 6, the measurement volume 8 or the corresponding measurement chamber 8 depicted in principle in FIG. 1 is provided. The light radiation directed in the measurement volume or in the measurement chamber 8 in the direction of the photodetector 7 on account of the particles provided there and bundled by means of the optical lens 6 is detected at the photodetector 7, wherein a photometer measured value corresponding to the detected light radiation is forwarded to an evaluation unit 9 of the optical particle sensor 1.

(9) The photometer measured value forwarded from the photodetector 7 of the aerosol photometer 2 to the evaluation unit 9 corresponds to the particle loading provided or detected in the measurement volume or in the measurement chamber 8.

(10) In the case of the aerosol photometer (APM) 2 of the embodiment shown on the basis of FIG. 1, a great advantage lies in the fact that the measured value detected in the evaluation unit 9 is independent of the flow rate of the gas flow 5 guiding the aerosol to be measured. In the case of the aerosol photometer (APM) 2, the measurement volume is determined by the optical measurement volume.

(11) Furthermore, the optical particle sensor 1 shown in FIG. 1 on the basis of the first embodiment is designed with an optical particle counter 10. The optical particle counter 10 of the optical particle sensor 1 shown in FIG. 1 likewise has a monochromatic light source 11, which can be formed as a laser diode or light-emitting diode (LED). The monochromatic light source 11 emits light radiation, which is bundled in an optical lens 12. The light beam bundled in the optical lens 12 crosses a gas flow 13, which carries the aerosol to be measured. A measurement volume or a measurement chamber 14 of the optical particle counter (10) is significantly smaller than the measurement volume or the measurement chamber 8 of the aerosol photometer 2. This is achieved in the exemplary embodiment, shown in FIG. 1, of the optical particle counter in that the light emitted by the monochromatic light source 11 is focused much more heavily by means of the optical lens 12 than by the optical lens 4 of the aerosol photometer 2. The measurement volume or the measurement chamber 14 of the optical particle counter 10 is dimensioned under consideration of the expectable values of aerosols to be measured, such that merely a single particle of the aerosol is provided therein. The light radiation reflected in the measurement volume or in the measurement chamber 14 of the optical particle counter 10 is directed by an optical lens 15 to a photodetector 16 of the optical particle counter 10 disposed after the optical lens 15 in the beam path. For each individual particle of the aerosol flowing through the measurement volume or the measurement chamber 14 of the optical particle counter 10 together with the gas flow 13, an individual measured value corresponding to the individual particle is thus forwarded at the photodetector 16 of the optical particle counter 10 to the evaluation unit 9 of the optical particle sensor 1. Each individual measured value corresponds to the light reflected by a single particle of the aerosol to be measured and directed by the optical lens 15 to the photodetector 16 of the optical particle counter 10.

(12) In contrast to the aerosol photometer 2 described above, the optical particle counter 10 detects individual particles. Optical particle counters 10 of this kind are used to measure relatively low particle concentrations, for example in interior spaces. Within the scope of their field of application, that is to say at relatively low to average particle number concentrations, which can usually lie between 1,000 and 20,000 particles/cm.sup.3, high-quality information with regard to the particle number and the particle size distribution the aerosol is possible by means of optical particle counters 10.

(13) Within the measurement range provided or valid for the optical particle counter 10, a measurement signal of the optical particle counter (OPC) 10 always corresponds to exactly one measurement signal of the aerosol photometer (APM) 2. These measurement signal values are determined on account of the performed calibration.

(14) Changes in the optical particle sensor 1, for example on account of contamination by means of deposited particles, always lead to a change in the ratio between the measurement signal value of the optical particle counter 10 on the one hand and the aerosol photometer 2 on the other hand. This change in the ratio between the two measurement signal values makes it possible to draw conclusions relating to the degree of contamination of the optical particle sensor 1, as can be seen most clearly from FIG. 3, in which the characteristic curves of the aerosol photometer (APM) 2 for an optical particle sensor 1 having no contamination, for an optical particle sensor 1 having light contamination, and for an optical particle sensor 1 having heavy contamination are depicted.

(15) In the case of the above-described embodiment of the optical particle sensor 1, the properties of the optical path are used.

(16) Scattered light, which originates from contaminations of the optical particle sensor 1, is received by the photodetectors 7, 16 as a constant measurement signal component.

(17) The evaluation unit 9 of the optical particle sensor 1 explained above on the basis of FIG. 1 has a functional unit 17, by means of which a degree of contamination of the optical particle sensor 1 can be detected from the processed measurement signal of the aerosol photometer 2 and the processed measurement signal of the optical particle counter 10 and can be taken into consideration when preparing the output value of the evaluation unit 9.

(18) In the case of the embodiment of the optical particle sensor 1 according to the invention shown in FIG. 2, this particle sensor comprises merely the optical particle counter 10 with the monochromatic light source 11, the optical lens 12, the measurement volume or the measurement chamber 14, the optical lens 15 and the photodetector 16, wherein the gas flow 13 flows through the optical particle sensor 1.

(19) Usually, the constant component of the measurement signal of the optical particle counter (OPC) 10 is filtered out from the measurement signal, since it does not contain any information relevant for the individual particle measurement. In the case of optical particle counters 10, AC voltage-coupled amplifier stages are therefore used, although these are not depicted in FIG. 2. AC voltage-coupled amplifier stages of this kind can be produced relatively economically and work over a wide temperature range, since all input offset voltages and voltage drifts of the electronic components are also filtered out as a result of the AC voltage coupling.

(20) In the case of the embodiment of the optical particle sensor 1 according to the invention shown in FIG. 2, the effect that the scattered light component and therefore the constant component in the measurement signal changes with a different degree of contamination of the optical particle sensor 1 or the optically effective surfaces thereof is utilized.

(21) In the case of the optical particle counter 10 of the optical particle sensor 1 shown in FIG. 2, it is therefore provided that the static signal component of the measurement signal of the optical particle counter 10 is determined. In the depicted embodiment a DC voltage-coupled amplifier 18 is used for this purpose, the input side of which amplifier is connected to the photodetector 16 of the optical particle counter 10 and the output side of which is connected via an analogue/digital converter 19 to the evaluation unit 9 of the optical particle sensor 1. Accordingly, the output side of the DC voltage-coupled amplifier 18 is scanned by means of the A/D converter 19 and is then processed by software.

(22) Since optical particle sensors 1 used in a motor vehicle are to be operated in a wide temperature range, for example of from 40 degrees C. to 85 degrees C., it is particularly important to use a DC voltage-coupled amplifier 18 that has a very small temperature drift, since the changes to the scattered light and thus the constant signal component of the measurement signal of the photodetector 16 induced by the contamination can be very small.

(23) FIG. 4 shows the signal profile at the photodetector 16 of the optical particle counter 10 with different degrees of contamination. The scattered light caused by contamination leads on the whole to an increase of the measured signal value or the signal level. This offset can be relatively easily determined in the signal profile, since it is the signal level between the particle signals.

(24) This signal value, which is characterizing for the scattered light caused by contamination, is stored and subtracted from the signal value in the following measurements.

(25) With very high degrees of contamination, there is also a small damping of the amplitudes of the particle signals due to the reduced light intensity in the measurement volume, in addition to the signal component caused by the scattered light.

(26) Tests have found that, for a design of the optical particle sensor 1 described above in principle, the link between signal increase and amplitude reduction is a typical characteristic curve.

(27) Since this characteristic curve is determined experimentally for the design in question of the optical particle sensor 1 and is stored in the evaluation unit 9 of the optical particle sensor 1, the amplitude damping factor d belonging to the signal increase can be read in a software-controlled manner from an allocation table and can be used for correction of the measurement signal.

(28) Here: s(t)=photodetector signal at the moment in time t h(t)=the offset value currently stored at the moment in time d(t)=the amplitude damping factor d allocated in the allocation table to the offset h

(29) Thus, the corrected measurement signal m is m(t)=(s(t)h(t))*d(t)

(30) In FIG. 5 the signal profile at the photodetector 7 of the aerosol photometer 2 with different degrees of contamination is shown, wherein the unit of time should be noted.

(31) It is not possible to conclude the degree of contamination of the optical particle sensor 1 on the basis of the signal profile, since it is at no point ensured that a particle mass concentration of 0.0 g/m.sup.3 occurs. Thus, it is not possible to simply assume the lowest value measured in a defined time interval as 0 g/m.sup.3.

(32) If, for example, a particle mass concentration of 20 g/m.sup.3 occurs as lowest value in a defined time interval and if this value is assumed to be a 0.0 g/m.sup.3 value and is stored for offset correction for following measurements, the measurement results subsequently are output too low by this lowest value of 20 g/m.sup.3.

(33) If the optical particle sensor 1, besides the aerosol photometer 2, additionally comprises the optical particle counter 10, as is shown in FIG. 1, the approach described above for the optical particle counter 10 can also be used in conjunction with the aerosol photometer 2.

(34) The sole precondition for regular adjustment of the correction values, specifically offset h and damping factor d, is that the optical particle counter 10 must be able to measure the particle number concentration in a manner free from coincidence errors. With a multi-channel optical particle sensor 1, this can also be enforced in that a switch is made to a channel with a low particle number concentration for the regular adjustment of the correction values.

(35) Since the process of contamination of the optical particle sensor 1 as a result of aerosol deposits is a slowly progressive process, which leads over weeks rather than hours to significant changes of the measurement or output signals, the likelihood of a regular possibility of adjustment of the correction values is relatively high.

(36) If the aerosol photometer 2 and the optical particle counter 10 are separated with regard to the measurement device associated therewith, the correction values determined as described above (offset h and amplitude or level damping factor d) are not applied directly to the measurement signal of the aerosol photometer 2.

(37) Here, the following approach is proposed:

(38) In the case of particle mass concentrations in the measurement range of the optical particle counter 10, the value of the particle mass concentration determined by means of the optical particle counter 10 and the value of the particle mass concentration, determined at the same time by means of the aerosol photometer 2, for different particle mass concentrations are stored. Two value pairs are required, by means of which a straight line can be mathematically calculated. The greatest possible distance between the two value pairs is advantageous.

(39) The method thus to be carried out will be explained hereinafter on the basis of FIG. 3.

(40) The characteristic curve with the circles shows the relationship between the measured value pairs when the optical particle sensor 1 is clean.

(41) There is no offset h and no amplitude damping in the measured value of the aerosol photometer 2.

(42) An offset h means that the straight line plotted between the value pairs intersects the y-axis at the point x=0, not at y=0. The value for y resulting at x=0 corresponds to the offset h.

(43) The gradient of the determined straight line in the clean state of the optical particle sensor 1 has the value 1. In FIG. 3 it is shown that the gradient of the straight line reduces with heavier contamination of the optical particle sensor 1. This is due to the reduction of the light intensity in the measurement volume as a result of the absorption of the light at the contaminated optical outlet and inlet surfaces.

(44) The gradient of the straight line corresponds to the level damping factor p to be applied.

(45) This results in the following function for the correction of the measured values of the aerosol photometer (APM) 2. s(t)=APM photodetector signal at the moment in time (t) h(t)=the offset value currently stored at the moment in time t p(t)=the last line gradient determined This results in the corrected measurement signal m m(t)=(s(t)h(t)/p(t)

(46) A status value for the degree of contamination of the optical particle sensor 1 and thus for the signal quality is generated from the determined correction values. This status value can be queried externally, for example via a diagnosis connection.

(47) In accordance with the degree of contamination of the optical particle sensor 1 determined in the manner described above, it can then be easily decided whether it is necessary to clean or service the optical particle sensor 1.

(48) In the case of an optical particle sensor 1 that has merely one optical particle counter 10 and that is depicted in FIG. 2, only the value for the offset h and the value d for the level damping can be output.

(49) In the case of an optical particle sensor 1 that has both the aerosol photometer 2 and the optical particle counter 10 and that is depicted in FIG. 1, four values can be output on account of the separate measurement systems.

(50) In the case of the optical particle sensors 1 according to the invention, the provided offsets and damping values should advantageously be transformed into a single status value, which for example assigns the signal quality of the optical particle sensor 1 a value between 0 and 100.

(51) Here, it is proposed to use the greater of the two determined offset values in order to generate the status value.

(52) The transformation can be performed for example in accordance with the following calculation rule: status (0 . . . 100) with 100=very good and 0=very poor.

(53) The offset is recalculated into a particle mass equivalent in g/m.sup.3. In a new system this is 0.0 g/m.sup.3.

(54) Status=100 when =100

(55) Status=0 when >100

(56) With the above-described embodiments of optical particle sensors 1, an automatic diagnosis can be provided for identifying degrees of contamination and condensation of an optical measurement path.