Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
  • G01N27/62—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating the ionisation of gases, e.g. aerosols; by investigating electric discharges, e.g. emission of cathode
  • G01N27/68—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating the ionisation of gases, e.g. aerosols; by investigating electric discharges, e.g. emission of cathode using electric discharge to ionise a gas
  • G01N27/70—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating the ionisation of gases, e.g. aerosols; by investigating electric discharges, e.g. emission of cathode using electric discharge to ionise a gas and measuring current or voltage
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N15/00—Investigating characteristics of particles; Investigating permeability, pore-volume or surface-area of porous materials
  • G01N15/06—Investigating concentration of particle suspensions
  • G01N15/0656—Investigating concentration of particle suspensions using electric, e.g. electrostatic methods or magnetic methods

Definitions

• the present invention relates to a particle sensor for measuring particles according to the preamble of claim 1 and specifically a particle sensor for the control of particle emissions from combustion engines with integrated volumetric flow measurement or monitoring.
• the present invention further relates to a process for measuring particles according to the preamble of claim 11.
• Particle sensor should be able to provide at least a rough estimation on the particle size distribution of the aerosol passing through the sensor, as the health effects of the particles seem to be dependent on their particle size or the total surface area of the particles, fine particles dominating adverse health effects.
• 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.
• the measurement method does not provide information on the volumetric flow through the sensor, which information would be valuable for e.g. determining the particle concentration or controlling the sensor operation, such as potential sensor blocking.
• the method also does not provide information on the particle size distribution of the aerosol passing through the particle sensor.
• Particle measurements are frequently carried out by cascade impactors, where particulate matter is withdrawn (preferably isokinetically) from a source and segregated by size.
• Cascade impactors use the principle of inertial separation to size segregate particle samples from a particle laden gas stream.
• Conventional cascade impactors cannot be used in real-time particle size distribution (PSD) measurement and especially in measuring changes in PSD in real time.
• PSD particle size distribution
• ELPITM Electrical Low Pressure Impactor
• the particles are first charged into a known charge level in the corona charger. After charging, the particles enter a cascade low pressure impactor with electrically insulated collection stages.
• the particles are collected in the different impactor stages according to their aerodynamic diameter, and the electric charge carried by particles into each impactor stage is measured in real time by sensitive multi-channel electrometers.
• This measured current signal is directly proportional to particle number concentration and size.
• the particle collection into each impactor stage is dependent on the aerodynamic size of the particles.
• Measured current signals are converted to (aerodynamic) size distribution using particle size dependent relations describing the properties of the charger and the impactor stages.
• the result is particle number concentration and size distribution in real-time.
• ELPF M without operating corona charger, it can be used for particle charge distribution measurements.
• the electrical impactor collects the particles, it is possible also to carry out post-sampling measurements, i.e. weight the samples collected on each impactor stage and/or analyze the composition of the particles collected on each stage.
• post-sampling measurements i.e. weight the samples collected on each impactor stage and/or analyze the composition of the particles collected on each stage.
• impactor is not a useful apparatus
• a major problem in any particle measurement device where particles are charged is the accumulation of the charged particles on the inner surfaces of the measurement sensor. 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 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.
• 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.
• 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 airflow.
• the device comprises an air inlet, a particle charging unit, a concentration variation section, a particle sensing section and a data evaluation unit.
• 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 d p , and the number concentration N of the size distribution of electrically-charged airborne particles.
• Patent specification GB 1 235 856 Maurice Sidney Beck., et al., 16.6.1971
• Patent specification GB 1 235 856 Maurice Sidney Beck., et al., 16.6.1971
• Patent specification GB 1 485 750 Maurice Sidney Beck et al., 14.9.1977, also refers to similar flow measurement arrangement.
• United States Patent US 5,214,386, Hermann Singer, et al., 25.5.1993, describes an apparatus and method for measuring particles in polydispersed systems and particle concentrations of monodispersed aerosols.
• the method includes the contactless measurement of the flow velocities in a pipe or the measurement of the volume flow in the pipe by annular sensors that do not completely surround the pipe.
• the average flow velocity can be
• this measurement requires at least two sensing electrodes.
• 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.
• One way of determining the potential sensor blocking by particle accumulation is to monitor the volumetric flow through the particle sensor.
• Advantageously such volumetric flow control should be realized without increasing the price of the particle sensor.
• the volumetric flow measurement should be combined with other features for reducing particle accumulation.
• the inventor of the present invention has found that the determination of the dynamic transfer function of a particle sensor provides an elegant way of determining also the volumetric flow through the sensor.
• Atmospheric Aerosols Summer Workshop, 19.-20.6.2006, describes the idealized transfer function of an impactor, i.e. the deposition efficiency as function of particle size, can be described by means of a step-function at particle size dso (particle size with 50% collection efficiency on the collection stage).
• dso particle size with 50% collection efficiency on the collection stage.
• real impactors exhibit a deposition
• the inventor has surprisingly found a method which will solve the prior art problems described above, especially the problem of low-cost flow measurement or monitoring.
• the invention comprises a process where an essential parameter of the particle sensor is switched, preferably on and off, or modulated, over time and the essential parameters of the sensor transfer function are determined from the switching/modulation response.
• an essential parameter i.e. a parameter which affects the measurement result of the particle sensor
• the sensor time constant can be determined without the use of several sensing electrodes or equivalent, which has a significant effect on the particle sensor cost.
• the switching/modulation provides additional benefits, such as reduced sensor soiling and possibility for more accurate determination of particle size distribution.
• the process is used with a particle measurement apparatus described in applicant's patent application WO/2009/109688, Pegasor Oy, 11.9.2009, describing a process for measuring particle concentrations in a gas using an ejector for producing an essentially constant sample flow and for efficient mixing of the particle-containing sample and an essentially clean, ionized gas.
• the invention also relates to an apparatus implementing such process.
• the process and the apparatus can be utilized for example in measuring particle concentrations in an exhaust system of a combustion engine.
• the main flow of the ejector consists of essentially clean ionized gas flow.
• the phrase 'essentially clean' means that the particle concentration in the ionized gas is so low that it does not adversely affect the monitoring process.
• the main flow causes suction to the side flow channel and thus a sample flow from the particle-containing gas is sucked to the monitoring apparatus.
• the ionized clean gas forms the main flow and the sample flow forms the side flow.
• WO/2009/109688 states that when the particle concentration of the gas is monitored, it is advantageous to produce an essentially constant gas flow through the measurement apparatus.
• the mass flow in e.g. the exhaust duct of a combustion engine is anything but constant, typically depending on the rotation speed of the engine.
• Using an ejector for sucking the sample flow from the exhaust duct results an essentially constant side flow, the flow being typically pulse-free, i.e. constant.
• Such a flow can then be modulated or switched in a controlled way so that the measurement apparatus works in AC mode, which provides more reliable particle concentration results that using the apparatus in DC mode.
• WO/2009/109688 does not mention volumetric flow measurement using
• the particle charging, (2) free ion and small particle collection, or (3) flow through the sensor are essential parameters which affect the sensor measurement result. They can be switched, preferably on and off, or modulated, over time, and the essential parameters of the transfer function are determined from the switching/modulation response.
• the essential parameters of the transfer function are favorable the delay time t ⁇ and time constant rof the sensor or at least part of the sensor.
• the invented sensor comprises an electrical discharging unit which can be switched or modulated between different charging modes.
• 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.
• the term "periodical switching" means that the switching is continuous for at least the time which is required to determine the essential parameters of the transfer function.
• Switching or modulation 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 10 seconds or even less than a second.
• the duty cycle may vary between 1 and 99%, preferably between 5 and 50% and more preferably between 5 and 20%.
• the sensor may also comprise a neutralizer which neutralizes the electrical charge of the particles.
• 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 is switched between two opposite voltages, ON + and ON " or between three different modes: ON-, OFF-, and NE-modes, where the NE mode describes a mode where either a separate neutralizer or the electrical discharging unit is used to neutralize the particles entering the sensor.
• the senor comprises an ion particle trap which removes free ions or extremely small particles (typically having a diameter of few nanometers or less than 10 nanometers) from the sample flow. Free ion or particle removal is dependent on the strength of the electrical field across the ion trap and the modulating the strength of the electrical field a rough estimation of the particle size distribution of the aerosol passing through the sensor can be determined. It is also possible to connect various particle traps working with different electrical field strengths into series and thus receive a better estimation on the particle size distribution.
• the duty cycle or the length of the ON + , ON " , OFF and NE-modes can be varied during the measurement and thus optimize the operation of the measurement apparatus.
• the duty cycle control may be based on an internal signal of the measurement apparatus e.g. the particle concentration or on the time wise derivative of the particle concentration.
• the duty cycle control may also be based on an external signal, e.g. when measuring particle exhaust from a combustion engine; the external signal may include a change in the momentum of the engine or in the revolution speed of the engine.
• the modulated/switched parameter which affects the sensor measurement result is the volumetric flow Q through the sensor.
• 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.
• Such switching or modulating can be advantageously achieved by using a pulsating pump which at itself creates a modulated flow.
• Such pump may be e.g. a diaphragm pump.
• the response from the switched or modulated mode of the electrical discharge unit is determined by synchronic detection.
• Synchronic detection can be realized by using either analog electronics or digitally. The digital realization can obviously be carried out in a separate computing unit or it may be integrated to a common controller or computing unit, where other control functions of the electrical impactor are carried out as well.
• the main parameters can be determined even continuously when required e.g. due to rapidly changing aerosol composition. If the changes in the measurement environment are not remarkable on a short time interval, and when the maximum time response of the measurement is required, the determination of the main parameters may be carried out with longer intervals.
• a surprising benefit of using a modulated signal instead of one sensor signal is that, in spite of simpler and cheaper practical solution, it can yield better performance.
• the reason of this feature is that one of the signals to be compared/correlated has practically no noise or other disturbances, whereas in prior-art solutions two noisy signals are compared correlated.
• Fig. 1 shows a schematic drawing of an embodiment of the invented sensor
• Fig. 2 describes the time behavior of the ion/particle traps
• Fig 3 describes the determination of the parameters used in the time correction of the ion/particle traps.
• Fig 4 describes the time wise correction of the measurement signal.
• the figure only shows 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.
• the present invention relates to a process for measuring particle concentration, comprising measuring a signal, which is a function of the particle concentration, with a sensing element, switching or modulating a parameter which affects the output of the sensing element and determining the volumetric flow on the basis of the time response which switching or modulation creates to the sensing element output.
• the process comprises electrically charging at least a fraction of the particles entering the measurement apparatus, measuring the electrical current carried by the charged particles and switching or modulating the electrical discharge unit at least between OFF-mode where the electrical discharge unit essentially does not charge the particles and ON-mode where the electrical discharge unit essentially does charge the particles.
• switching or modulating the electrical discharge unit can be carried out at least between NE-mode where the electrical discharge unit essentially electrically neutralizes the particles and ON-mode where the electrical discharge unit essentially electrically charges the particles.
• the measurement process described in WO/2009/109688 is used, including measuring the current escaping from the measurement apparatus with the charged particles and removing ions, charged ultrafme particles or charged fine particles from the aerosol passing through the measurement apparatus with the aim of at least one electrical field.
• the ion/particle trap is switched at least between OFF-mode where the ion/particle trap essentially removes free ions and ON- mode where ion/particle trap essentially removes particles having a diameter smaller than d p .
• the electrical field strength of the ion/particle trap in the ON-mode can be adjusted which further adjusts maximum trapped particle diameter d p .
• One embodiment of the present invention comprises switching or modulating the sample flow through the measurement apparatus between at least two essentially different values.
• the measurement process described in WO/2009/109688 is used, including pumping sample flow through the measurement apparatus with an ejector pump, ionizing the motive fluid of ejector and. switching/modulating the motive fluid flow of ejector.
• the essential parameters of the transfer function of at least a part of the measurement apparatus can be determined e.g. by providing a computational reference signal, comparing the sensing element output to the reference signal, adjusting the reference signal for maximum correlation between the sensing element output and the reference signal, determining the transfer function of at least a part of the measurement apparatus from the reference signal with maximum correlation and determining the volumetric flow through the measurement apparatus using at least some parameters of the computed transfer function.
• the transfer function is determined by providing a computational reference signal following a first-order low-pass filter, determining the delay time t ⁇ and time constant rof the first-order low-pass filter and determining the volumetric flow through the measurement apparatus using the inverse of the sum of td+ T. In most cases only the inverse of 3 ⁇ 4or Tor the inverse of some other combination than the sum of f rf + rcan also be used for the volumetric flow determination.
• the switching/modulation frequency of a parameter affecting the sensing element output is preferably adjusted between 0,01 Hz and 10 Hz and the duty cycle between 1% and 50%.
• the present invention also relates to apparatus 1 for measuring particle
• Apparatus 1 is preferably a particle sensor, which comprises a sensing element 12 for particle concentration measurement, means 22 for switching or modulating a parameter which affects the output of the sensing element 12 and means 23 for determining the volumetric flow on the basis of the response which switching or modulation creates to the sensing element 12 output.
• Figure 1 shows one embodiment of the present invention. This embodiment differs slightly from the preferred embodiment which follows the construction of the particle sensor described in WO/2009/109688.
• Apparatus (1) shown in Figure 1 comprises an electrical discharge unit 20 for electrically charging at least a fraction of the particles entering apparatus 1 through connection 14 a.
• Apparatus 1 further comprises means 12 for measuring the electrical current carried by the charged particles and means 22 for switching or modulating the electrical discharge unit 20 at least between OFF-mode where the electrical discharge unit 20 essentially does not ionize the essentially clean air entering apparatus 1 through connection 14 b and ON-mode where the electrical discharge unit 20 essentially does ionize the essentially clean air entering apparatus 1 through connection 14 b, the ionized air then charging, essentially by unipolar diffusion charging, the particles in the mixing part 14 of apparatus 1.
• Apparatus 1 may further comprise means 12 for measuring the electrical current carried by the charged particles and means 22 for switching or modulating the electrical discharge unit 20 at least between NE-mode where the electrical discharge unit 20 essentially ionizes, by bipolar ionization, the essentially clean air entering apparatus 1 through connection 14b, which unipolar ionized air then further electrically neutralizes the particles in the mixing part 14 of apparatus 1 and ON-mode where the electrical discharge unit 20 essentially ionizes the essentially clean air entering apparatus 1 through connection 14 b, the ionized air then charging, essentially by unipolar diffusion charging, the particles in the mixing part 14 of apparatus 1.
• the electrical discharge unit 20 is preferably a corona charger where the high voltage source 20a produces the required high voltage to the corona needle 20b which is electrically isolated from the body of apparatus lby the electrical isolator 8 .
• the high voltage source 20a produces the required high voltage to the corona needle 20b which is electrically isolated from the body of apparatus lby the electrical isolator 8 .
• unipolar ionization a single corona needle 20b with a suitable high electrical potential is required.
• bipolar ionization either two corona needles 20b working with opposite electrical potentials or a single corona needle 20b switched between opposite electrical potentials is required.
• apparatus 1 further comprises a sensing element 12 which measures the current escaping from apparatus 1 with the charged particles.
• a sensing element 12 which measures the current escaping from apparatus 1 with the charged particles.
• Such escaping current sensing element 12 may also be arranged as described in WO/2009/109688.
• Apparatus 1 further comprises an ion or particle trap 4 for removing ions, charged ultrafine particles, charged fine particles, or charged particles with any size from the aerosol passing through the apparatus 1.
• Apparatus 1 may also comprise more than one ion trap Means 22 switches or modulates the ion/particle trap 4 at least between OFF-mode where the ion/particle trap4 essentially removes free ions (but not particles) and ON-mode where ion/particle trap 4 essentially removes particles having a diameter smaller than d p .
• Diameter dp may be adjusted by adjusting the electrical field strength of the ion/particle trap 4. The electrical field strength is adjusted with the power source 4a which creates an electrical potential between electrode 4b and apparatus body 10.
• Electrode 4b is electrically isolated from the apparatus body 10 with an electrical isolator 8.
• Means 22 may also switch or modulate the sample flow through apparatus 1 between at least two essentially different values. Such switching or modulation obviously requires a flow restriction component adjusted by means 22, and the flow restriction component may be a valve, throttle, or equivalent, obvious for a person skilled in the art.
• apparatus (1) comprises an ejector for pumping the sample flow through apparatus 1 and means (20) for ionizing the motive fluid of ejector and the flow through apparatus 1 is switched or modulated by switching/modulating the motive fluid flow of the ejector.
• Apparatus 1 of Figure 1 further comprises means 27 for determining the essential parameters of the transfer function of apparatus 1. This is preferably realized by providing a computational reference signal, connected to the means 22 for switching or modulating a parameter essentially affecting the sensing element output and means for comparing the sensing element output to the reference signal as well as means for adjusting the reference signal for maximum correlation between the sensing element output and the reference signal and means for computing the transfer function of apparatus 1 from the reference signal with maximum correlation and means 23 for determining the volumetric flow through apparatus 1 using at least some parameters of the computed transfer function.
• the computational reference signal follows at least a first-order low-pass filter and means 27 for determines the delay time td and time constant Tof the first-order low-pass filter, in which case the volumetric flow through apparatus 1 is determined by using the inverse of the sum of td+ T.
• Apparatus 1 is preferably used so that means 22 switches or modulates a parameter affecting the sensing element 12 output essentially continuously and the switching or modulating frequency is preferably between 0,01 Hz and 10 Hz, depending e.g. on particle concentration. Switching or modulating frequency does not need to be constant but it may be varied based on internal reasons (such as particle concentration in apparatus 1) or external reasons (such as change in e.g. combustion engine's torque).
• the duty cycle i.e. ratio——— is preferably as small as possible, because soiling
• means 22 adjust the switching duty cycle between 1% and 50%.
• the determination of the transfer function parameters is realized by describing the time-wise behavior of at least the sensor so, that it comprises a delay component t s and a time constant rwhich describes the behavior of a first-order low-pass filter.
• the volumetric flow passing the sensor is inversely proportional to the sum of the time delay t ⁇ and time constant ⁇ .
• the time-wise behavior of the first-order low-pass filter is similar to the one of a full-mixed reactor.
• modulation initiates a measurement signal receives signals at a certain frequency and with a certain duty cycle, the frequency being preferably higher than 0,01 Hz, more preferably higher than 0,1 Hz and even more preferably higher than 1 Hz and the duty cycle being preferably between 5-20%.
• the differential equation (2) can be approximated by difference equation (3): It is also possible to use ion particle traps in series, in which case the response of the whole system can be described as a serial connection of first-order low-pass filters and delay times, as shown in Figure 2.
• Delay time tdi and time constant n of each trap 4 can be determined by setting them to such values that the model describing the time-wise behavior of each trap has a maximum correlation to the actual measured signal.
• the required impulse for the response is generated by modulating the electrical discharge unit 20 between ON and OFF modes. In another embodiment of the present invention, the required impulse for the response is generated by modulating the electrical discharge unit 20 between ON and NE modes. In yet another embodiment of the present invention, the required impulse for the response is generated by modulating the electrical discharge unit 20 between ON + and ON " modes.
• Figure 3 shows a block diagram of an embodiment where the time- wise parameters are determined.
• Correlator X compares the measurement signal Soi from trap / to the calculated signal ,- and the time parameters t ⁇ # and Tj are set to the values where the correlator X provides a maximum signal C selfish i.e. the correlation between the model and the measured signal is set to maximum.
• the time response of the measured signal can be compensated or correlated by modifying the signal with the inverse function.
• the transfer function following the Laplace notation this means multiplying the signal with the inverse of the transfer function F(s).
• Compensating the time delay is merely applying a time shift.
• Compensating the first- order low-pass filter is achieved by the difference equation

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