WO2012022842A1

WO2012022842A1 - Process and apparatus for particle measurement

Info

Publication number: WO2012022842A1
Application number: PCT/FI2011/050729
Authority: WO
Inventors: Kauko Janka
Original assignee: Pegasor Oy
Priority date: 2010-08-20
Filing date: 2011-08-19
Publication date: 2012-02-23
Also published as: FI20110065A0; FIU20100360U0; EP2606344A1; FI20110066A0; WO2012022844A1; FI20110067A0; WO2012022843A1

Abstract

Process and apparatus for particle measurement, comprising modulating or switching a parameter which affects particle accumulation or deposition into the measurement equipment between at least a mode where particle accumulation or deposition occurs (ON) and another mode where particle accumulation and deposition does essentially not occur (OFF) with a modulation or switching duty cycle of less than 50%.

Description

Process and apparatus for particle measurement

Field of invention

The present invention relates to a method for detecting particles according to the preamble of claim 1 and specifically to a method where the measurement duty cycle is less than 50%. The present invention further relates to a particle sensor for detecting particles according to the preamble of claim 9.

Background of the invention

There is a constant increase in the demand for real-time particle control. Especially the real-time exhaust control of combustion engines, such as vehicles, requires reliable and non-expensive particle monitoring. Requirement for particle control exists also e.g. in indoor air quality monitoring or with air traffic safety.

A major problem in any particle measurement device where particles are charged is the deposition of the charged particles on the inner surfaces of the measurement sensor causing particle accumulation or buildup. This is caused by the repulsive Coulombic force of the charged particles on each other. This repulsive force is especially effective in the space where the particles are charged by an electric discharge, because the strength of the electrostatic field affecting the particle accumulation is further increased by free ions and potentially also by the high-voltage electrode required for the electric discharge.

Accumulation of the charged particles on the inner surfaces of the sensor may block the aerosol flow channel (pathway) or they may reduce the electrical insulation capacity of the electrical insulation

Patent application US 2006/0156791, 20.7.2006, Dekati Oy, describes a method and a sensor device for determining particle emissions from exhaust gases of a combustion engine substantially during the use in an exhaust pipe system or a corresponding exhaust gas duct, in which method emitted particles contained in the exhaust gases are charged and the particle emissions are determined by measuring the electric charge carried by the emitted particles in the exhaust gas duct. The emitted particles are charged by varying the way of charging or the charging power with respect to time in such a manner that as a result of the charging, emitted particles brought into at least two different electrical charge states are present, wherein the charge of the emitted particles is further determined as a difference value/values measured from the emitted particles brought into said at least two different electrical charge states. The application states that the relative lengths in time of the charging cycles generated by the charger C (charger on/off cycles) may vary freely as required by each application. Said lengths in time may also be equal, wherein the charger operates at a pulse ratio of 1 : 1. Thus the application does not teach any advantages of certain duty cycles (charger on/off cycles).

US patent publication US 4,837,440 A, Burtscher et al., 6.6.1989, describes a method for the characterization of particles in aerosol, comprising: using an aerosol, which has been brought to at least one predetermined temperature sufficient for evaporation or decomposition or preventing condensation of molecules on particles of said aerosol; exposing said aerosol to an electromagnetic radiation for activating particles contained in the aerosol to cause said aerosol particles to emit electrons and attain an electric charge correspondingly; and measuring the electric charge of said aerosol particles. The application states that the charging means, e,g, an ultraviolet lamp, may be switched on and off but does neither determine the duty cycle nor teach the advantage of certain duty cycles.

Patent application US 2005/0083633, Riebel et al., 21.4.2005, relates to a device for charging or adjusting the charge of gas-borne particles into a defined charge distribution under utilization of corona discharge in the aerosol space. In addition to an appropriate geometry of the charger and the electrodes, the voltage waveform and the voltage regulation are of great significance for the result. The application further relates to a method for operating the device. The application does not teach the advantage of small duty cycle.

Patent application WO 2010/049870, Koninkl Philips Electronics NV, 6.5.2010, presents a device that is capable of recording the evolution over time of the characteristics of a size distribution of electrically-charged airborne particles in an airflow. The device comprises an air inlet, a particle charging unit, a concentration variation section, a particle sensing section and a data evaluation unit. Specifically, the particle sensing section of the device generates at least two serially obtained measurement signals II and 12 from which the data evaluation unit can infer values for both the average particle diameter dp,and the number concentration N of the size distribution of electrically-charged airborne particles. Also this application fails to teach the advantage of small duty cycle.

Thus the particle sensors of the prior art possess the technical problem of particle accumulation. There is need for a sensor which can measure or monitor particle concentration in real-time and with long measurement periods without frequent sensor cleaning or maintenance. Such need exists particularly with car combustion engine exhaust emission monitoring.

Brief description of the invention

The inventor has surprisingly found a method which will solve the prior art electrical impactor problems described above. The problems are solved by implementing a process where at least one of the parameters: (1) particle charging or (2) free ion and small particle collection or (3) volumetric flow through the particle sensor is switched, preferably on and off, or modulated, over time, so that the duty cycle, i.e. the ratio of sensor ON time (ION) to the cycle time (sensor ON time combined with sensor OFF time, toN+toFF),——— is less than tON+OFF

0,5 (50%). All the parameters (1), (2), and (3) are such parameters that their switching affects particle accumulation on the particle sensor and thus their switching with small duty cycle represents preferred embodiments of the present invention. Charged particles tend to accumulate in the sensor due to Coulombic force, small particles have high diffusion coefficients and thus they accumulate in the sensor due to diffusion and it is obvious that particles may accumulate in the sensor only if and when they are flowing through the sensor. Thus, the generalization of the current invention from the embodiments is that switching any parameter which affects particle accumulation in the sensor with a low duty cycle, decreases or even removes the problem of the prior art. The duty cycle is less than 50%, preferably less than 25% and even more preferably less than or equal to 10%. With less than 50% duty cycle, the maintenance or cleaning period of the sensor may be doubled, if the Coulombic force is the main effect, which has an influence on particle accumulation. In real life Coulombic force seldom is the only mechanism affecting particle accumulation, e.g. with fine particles diffusionphoresis is a vital particle accumulation mechanism. Thus the duty cycle is preferably less than 0,25 (25%) or most preferably 0,1 (10%) or less.

The duty cycle does not have to stay constant during particle monitoring, but a beneficial advantage of the present invention is that the duty cycle can be dynamic, i.e. it's value may be optimized during the measurement as long as it stays below 50%. Not only the duty cycle may be varied, but also the length of the t₀N may be varied as well. The variation and optimization may be based e.g. on the sensor measurement result (particle concentration, time-wise derivative of particle concentration) or on external factors (like combustion engine revolution speed or torque). The length of toN is typically dependent on the ratio of the volumetric flow Q through the sensor to the sensor volume V. When the time- wise resolution is important, the ^-ratio should be high, e.g. 10 1/s or higher, and in such occasions the length of toN can be fractions of a second. In situations, where the particle concentration varies slowly, toN can be tens of seconds or higher.

When the switching/modulation frequency is high, sensor's sensing element output typically does not follow the signal-level variation during the separate cycle, yielding average values over the cycle, instead. Thus sensor's sensing element can be realized so that its output gives essentially average values over a separate cycle, rather than follows momentary sensor's input signal.

In one embodiment of the present invention the duty cycle is varied so that the measurement signal of the sensor's sensing element, which provides data from which particle concentration is determined, is kept essentially constant, i.e. within small variation. With such embodiment the sensing element providing the data may be operated at its optimal measurement area and information in the duty cycle is then used to determine particle concentration.. In this case also the accumulation tendency is reduced extremely effectively with increasing particle concentrations to be measured. Such embodiment could obviously be used also with duty cycles higher than or equal to 50%

It is also possible to widen the measurement range of the particle sensor to be wider than the sensor's sensing element by adapting the duty cycle according to the sensor's signal level so that at higher signal level of the sensor lower duty cycles are used. The duty cycles can be adapted continuously with changing sensor's signal level, or there can be discrete duty- cycle steps. The final measurement value is formed using both the sensor's signal level and used duty-cycle value. Such embodiment of the present invention yields, in addition to the wider dynamic measurement range, also contamination-tendency reduction at higher concentrations. .

In the preferred embodiment of the present invention the particle sensor comprises an electrical discharging unit, typically a corona charger, which charges the particles flowing into the sensor, and the charge carried by the charged particles is then detected. Prior to, in the direction of the sample flow, the means for current measurement lays usually an ion trap which removes free ions, ultrafine particles or fine particles from the sample flow, depending on the electrical field strength of the ion trap. The sensor may also comprise a neutralizer which neutralizes the electrical charge of the particles before they enter the sensor, and the sensor is in this case used in neutralizer (NE) mode. The neutralizer may be a separate unit or the neutralization may be carried out by the electrical discharging unit, e.g. by such a way that the corona charger comprises two charging units with opposite electrical potential or by using a single corona charger in AC (alternating current) mode which produces ions with opposite charges.

The electrical discharging unit of the present invention has a unique feature that it can be switched or modulated between different charging modes. In an embodiment of the present invention the electrical discharging unit is a corona charger which is switched periodically between the ON-mode and OFF-mode, i.e. the corona voltage is periodically switched ON and OFF. Essentially continuous comparison of the measurement results between the ON and OFF modes with less than 50% duty cycle reduces particle accumulation in the sensor. The term "periodical switching" means that the switching may have a fixed frequency or the frequency may vary and also the length of the ON and OFF modes may vary. The length of the ON mode may be less than 100 seconds, less than 1 second or even less than 0,1 seconds. An advantageous feature is that in high-concentration conditions, where the soiling rate is high, extremely low duty cycles can be used without practical degrading the noise properties of the signal. The reason for this is the fact that during even extremely short ON times the high signal levels can yield enough noise-free measurement data. In another embodiment of the present invention, where particles with a high amount of natural charging are measured, it is advantageous to use switching between ON and neutralizer (NE) modes to minimize the particle accumulation, as the NE-mode decreases the Coulombic accumulation of the naturally charged particles. Then the duty is obviously the ratio of to_N to ΪΟΝ+ΝΕ, i.e. duty cycle = ^toN , and duty cycle is less than 50%.

tON+NE

In another embodiment of the current invention, the particle accumulation in the sensor is decreased by switching the operation mode of the ion trap with less than 50% duty cycle. As described above prior to the means for current measurement lays usually an ion trap which removes free ions or ultrafine particles or even fine particles from the sample flow. Increasing the ion trap voltage increases the diameter of trapped particles and with high trap voltage the ultrafine and fine particles which would potentially deposit in the sensor are removed from the sample flow. The ion trap may be switched between high and low voltages, high and View , where Vhi_gh removes ultrafine and /or fine particles and Vi_ow removes free ions with less than 50% duty cycle, which in this case is defined as the ratio of the high-voltage time ty_h to the total cycle time _tv_h+vi, duty cycle = ^tyh .

tvh+vi

In another embodiment of the current invention, the modulated/switched parameter affecting the particle accumulation is the volumetric flow Q through the sensor. Volumetric flow switching is beneficial because it reduces both particle deposition due to Coulombic force as well as particle deposition due to diffusionphoresis. Volumetric flow can be switched e.g. by switching a valve shutting the sample flow to the sensor inlet or from the sensor outlet or by switching or modulating the pump which creates the volumetric flow through the sensor. The term "pump" is here understood as any means of creating the volumetric flow through the sensor and thus pump can be e.g. an ejector pump where either the motive fluid flow or the side fluid flow may be switched or modulated.

In still another embodiment of the present invention particle charging is switched or modulated in synchronically with the volumetric flow, so that particle charging is switched OFF or NE slightly before setting the volumetric flow OFF. This ensures that the particles which are in the sensor when the volumetric flow is switched OFF do not accumulate in the sensor due to Coulombic force. Even though accumulation by diffusionphoresis is still present, this synchronous embodiment is preferably especially with applications where sensor soiling due to charged particles has a large effect.

Brief description of the drawings

In the following, the invention will be described in more detail with reference to the appended schematic drawings, where

Fig. 1 shows one embodiment of the present invention; and

Fig. 2 shows another embodiment of the present invention.

For the sake of clarity, the figures only show the details necessary for understanding the invention. The structures and details which are not necessary for understanding the invention and which are obvious for a person skilled in the art have been omitted from the figures in order to emphasize the characteristics of the invention.

Detailed description of preferred embodiments

The present invention describes a process for particle measurement. The process comprises modulating or switching a parameter which affects particle accumulation or deposition into the measurement equipment between at least a mode where particle accumulation or deposition occurs (ON) and another mode where particle accumulation and deposition does essentially not occur (OFF). Essential for the current invention is that the modulation or switching duty cycle is less than 50% and preferably less than 10 %.

In one embodiment of the present invention the invented process comprises electrically charging at least a fraction of the particles under measurement and measuring the electrical current carried by the charged particles. Ions, charged ultrafine particles or charged fine particles (particle diameter smaller than 100 nm) - or even charged particles larger than 100 nm in diameter - can be removed with the aim of an electrical field of the ion trap.

In the preferred embodiments of the process the modulated or switched parameter which affects particle accumulation or deposition is the electrical charge of the particles, the electrical field strength of the ion trap or the volumetric flow through the measurement apparatus. When the volumetric flow is switched or modulated, it is beneficial to use synchronous switching or modulation of the particle charge and the volumetric flow so that the particle charge is switched or modulated to the OFF mode before the volumetric flow is switched or modulated to OFF mode. This essentially reduces particle deposition due to Coulombic force.

The duty cycle can be adjusted dynamically during the measurement as long as the duty cycle stays below 50%. Preferably the duty cycle is adjusted in such manner that the measurement signal from the sensing signal of the particle measurement apparatus, averaged over the period toN+toFF, is kept essentially constant.

The present invention also includes apparatus 1 (numbers refer to figures 1 and 2) for particle measurement, comprising means 11 for modulating or switching a parameter which affects particle accumulation or deposition into the measurement equipment between at least a mode where particle accumulation or deposition occurs (ON) and another mode where particle accumulation and deposition does essentially not occur (OFF) with less than 50% modulation or switching duty cycle and more preferably with less than 10 % duty cycle.

In one embodiment of the present invention apparatus 1 comprises means 6 for electrically charging at least a fraction of the particles under measurement and means 10 for measuring the electrical current carried by the charged particles. Apparatus 1 also comprises means 8 for removing ions, charged ultrafine particles or charged fine particles with the aim of an electrical field.

In the preferred embodiments apparatus 1 comprises means 1 lb for the modulation or switching of the electrical charge of the particles, means 1 lc for the modulation or switching of the electrical field removing ions, charged ultrafine particles or charged fine particles or means 11a, 1 Id for the modulation or switching of the volumetric flow through apparatus 1. Preferably apparatus Icomprises means for synchronous switching or modulation of the particle charge and the volumetric flow so that the particle charge is switched or modulated to the OFF mode before the volumetric flow is switched or modulated to OFF mode. Figure 1 shows one embodiment of a particle sensor 1 according to the present invention. Particle sensor 1 comprises a sample inlet 2 through which a sample from an aerosol, i.e. a mixture of gas and particles (which may comprise solid and liquid phase), is taken into the sensor 1. Sensor 1 further comprises an outlet 3, through which the measured sample exits sensor 1. Sensor 1 may be particle collecting sensor, such as e.g. a sensor based on measuring the resistivity of collected particles, such as described e.g. in WO 2009/043711, Bosch, or a sensor based on measuring optical attenuation or reflection produced by collected particles. Sensor 1 may also be a non-collecting particle sensor, such as e.g. a sensor based on electrically charging particles and measuring the electrical current carried by the particles, such as described in FI 20080182, Navaro 245 Oy. Preferably the particle sensor is based on charging at least a fraction of the particles in the aerosol sample under measurement and measuring the current escaping from the sensor with the charged particles. The embodiment of Figure 1 comprises particle charging by dielectric barrier discharge (DBD), where particle charging takes place between two electrodes, 6a and 6c. Between the electrodes is also a dielectric layer 6b. Electric discharge occurs on the dielectric surface and at least a fraction of particles passing the gap between electrodes 6a and 6c are charged. Electric discharge is initialized by an alternating electric potential between electrodes 6a and 6c. The electrical potential may be in the order of few kilovolts or few tens of kilovolts and the frequency of the alternating voltage may be e.g. 500 Hz - 500 kHz. The required voltage is applied to electrodes 6a and 6c from the high voltage source 7. Generated ions which are not attached to the particles are collected with a direct electrical field between electrodes 8a and 8b, the direct field being generated by power unit 9. The electrical current carried by the charged particles is measured with a sensitive electrometer 10.

Essential for the present invention is that particle sensor lcomprises means 11 for switching or modulating a parameter which affects particle deposition (which is here used as a synonym for particle accumulation) in the sensor with a duty cycle lower than 0,5 (50%). Modulation or switching is done between two parameter modes, the first one (ON) being a mode where particles may deposit in the sensor and the second one (OFF) being a mode where particle deposition is significantly reduced. Means 11 for parameter switching consists of e.g. 1 la and 1 Id, which modulate power supply to valves 4 and 5, where valve 4 is a valve opening and closing volumetric entry flow to sensor 1 and valve 5 is a valve opening and closing exit flow from sensor 1. Thus means 1 la and l i d may provide a tool for modulating the flow through sensor 1.

Means l i b controls power supply to the high voltage generator 7, which is the power supply unit of the electrical discharge unit 6. Thus, by modulating the power supply through means 1 lb, it is possible to switch/modulate the electrical charge of the particles.

Means 11 c controls power supply to the voltage generator 9, which is the power supply unit of the ion trap 8. Thus, by modulating the power supply through means 1 lc, it is possible to switch/modulate the strength of the electrical field collecting ions, ultrafine particles and/or fine particles.

The technical advantage of modulating or switching at least one parameter which affects particle accumulation (deposition) in the particle sensor, so that the duty cycle of switching/modulation is less than 50%, the sensor maintenance period can be significantly prolonged.

Figure 2 shows a schematic drawing of another embodiment of the present invention. A sample flow from the aerosol under measurement enters into sensor 1 through coupling 2 and the sample flow exits sensor 1 through coupling 3. Sensor 1 may be particle collecting sensor, such as e.g. a sensor based on measuring the resistivity of collected particles, such as described e.g. in WO 2009/043711, Bosch, or a sensor based on measuring optical attenuation or reflection produced by collected particles. Sensor 1 may also be a non- collecting particle sensor, such as e.g. a sensor based on electrically charging particles and measuring the electrical current carried by the particles, such as described in FI 20080182, Navaro 245 Oy. Preferably the particle sensor is based on charging at least a fraction of the particles in the aerosol sample under measurement and measuring the current escaping from the sensor with the charged particles. The embodiment of Figure 1 comprises particle charging by corona charger 6. Essentially clean air is fed through coupling 14 and valve 12 into the sensor 1. This essentially clean air is ionized by the corona charger 6 and the ionized air is mixed with the particle-containing sample flow which enters sensor 1 through coupling 2 and valve 4. Such sensor is in detail explained in applicant's patent application WO

2009/0109688, Pegasor Oy. Ions which are not attached to particles, and possibly also ultrafine and fine particles, are collected by an electrical field arranged between electrodes 8a and 8b, into which electrical voltage is applied from power source 9. The electrical current carried by the charged particles is measured with electrometer 10.

Essential for the present invention is that particle sensor 1 comprises means 11 for switching or modulating a parameter which affects particle deposition (which is here used as a synonym for particle accumulation) in the sensor with a duty cycle lower than 0,5 (50%). Modulation or switching is done between two parameter modes, the first one (ON) being a mode where particles may deposit in the sensor and the second one (OFF) being a mode where particle deposition is significantly reduced. Means 11 for parameter switching consists of e.g. 1 la and 1 Id, which modulate power supply to valves 4 and 5, where valve 4 is a valve opening and closing volumetric entry flow to sensor 1 and valve 5 is a valve opening and closing exit flow from sensor 1. Thus means 1 la and l i d may provide a tool for modulating the flow through sensor 1. With means 1 le it is possible to switch or modulate flow of the essentially clean air passing through valve 12 to an ejector-based particle sensor as the one described in applicant's patent application WO 2009/0109688, and by this way

switch/modulate the volumetric flow through the sensor.

Control unit 13 analyses the measurement signal of sensor 1 and thus is able to provide information on particle concentration as well as the time- wise derivative of the particle concentration. This information can be used to optimize the duty cycle s long as it is below 50%.

It is possible to produce various embodiments of the invention in accordance with the spirit of the invention. Therefore, the above-presented examples must not be interpreted as restrictive to the invention, but the embodiments of the invention can be freely varied within the scope of the inventive features presented in the claims herein below.

Claims

1. Process for particle measurement, comprising:

a. modulating or switching a parameter which affects particle accumulation or

deposition into the measurement equipment between at least a mode where particle accumulation or deposition occurs (ON) and another mode where particle accumulation and deposition does essentially not occur (OFF); and

b. where the modulation or switching duty cycle is less than 50%.

2. Process of claim 1, c ompri sing :

a. electrically charging at least a fraction of the particles under measurement; and b. measuring the electrical current carried by the charged particles.

3. Process of claim 2, comprising removing ions, charged ultrafine particles or charged fine particles with the aim of an electrical field.

4. Process of claim 1-3, comprising the modulation or switching duty cycle of maximum 10%.

5. Process as in claim 2-4, comprising modulation or switching of the electrical charge of the particles.

6. Process as in claim 3-5, comprising modulation or switching of the electrical field removing ions, charged ultrafine particles or charged fine particles.

7. Process as in any of the previous claims, comprising modulation or switching of the volumetric flow through the measurement apparatus.

8. Process of claim 7, comprising synchronous switching or modulation of the particle charge and the volumetric flow so that the particle charge is switched or modulated to the OFF mode before the volumetric flow is switched or modulated to OFF mode.

9. Process as in any of the previous claims, comprising dynamic duty cycle adjustment.

10. Process of claim 9, comprising adjusting the duty cycle in such manner that the

measurement signal from the sensing signal of the particle measurement apparatus, averaged over the period toN+toFF, is kept essentially constant.

11. Apparatus ( 1 ) f or particle measurement, comprising means ( 11 ) for modulating or switching a parameter which affects particle accumulation or deposition into the measurement equipment between at least a mode where particle accumulation or deposition occurs (ON) and another mode where particle accumulation and deposition does essentially not occur (OFF) with less than 50% modulation or switching duty cycle.

12. Apparatus (1) of claim 11, comprising means (11) for modulating or switching a parameter which affects particle accumulation or deposition into the measurement equipment between at least a mode where particle accumulation or deposition occurs (ON) and another mode where particle accumulation and deposition does essentially not occur (OFF) with less than 10% modulation or switching duty cycle.

13. Apparatus (1) of claim 11-12, comprising :

a. means (6) for electrically charging at least a fraction of the particles under

measurement; and

b. means (10) for measuring the electrical current carried by the charged particles.

14. Apparatus (1) of claim 13, comprising means (8) for removing ions, charged ultrafine particles or charged fine particles with the aim of an electrical field.

15. Apparatus (1) of claim 11-14, comprising means (lib) for the modulation or

switching of the electrical charge of the particles.

16. Apparatus (1) of claim 11-15, comprising means (11c) for the modulation or

switching of the electrical field removing ions, charged ultrafine particles or charged fine particles.

17. Apparatus (1) of claim 11-16, comprising means (11a, lid) for the modulation or switching of the volumetric flow through apparatus (1).

18. Apparatus (1) of claim 17, comprising means for synchronous switching or

modulation of the particle charge and the volumetric flow so that the particle charge is switched or modulated to the OFF mode before the volumetric flow is switched or modulated to OFF mode.