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
  • G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
  • G01N27/416—Systems
  • G01N27/417—Systems using cells, i.e. more than one cell and probes with solid electrolytes
  • G01N27/419—Measuring voltages or currents with a combination of oxygen pumping cells and oxygen concentration cells

Definitions

• the invention relates generally to
• Exhaust gas aftertreatment in particular by means of a lambda control in internal combustion engines and in particular a method and a circuit for operating a nitrogen oxide sensor which can be used in such a lambda control to determine the nitrogen oxide concentration in an exhaust gas mixture.
• Lambda control in conjunction with a catalytic converter, is the most effective exhaust gas cleaning process for the gasoline engine today. Only in In conjunction with the ignition and injection systems available today, very low exhaust gas values can be achieved.
• the use of a three-way or selective catalyst is particularly effective.
• the operation of the lambda probe is based on the principle of a galvanic oxygen concentration cell with a solid electrolyte. Lambda probes designed as two-point probes are known to work according to the Nernst principle based on a Nernst cell.
• the solid electrolyte consists of two interfaces separated by a ceramic.
• the ceramic material used becomes conductive at around 350 ° C for oxygen ions, so that when the oxygen content differs on both sides of the ceramic, the so-called Nernst voltage is generated. This electrical voltage is a measure of the difference in the oxygen content on both sides of the ceramic. Since the
• Residual oxygen content in the exhaust gas of an internal combustion engine depends to a large extent on the air-fuel ratio of the mixture supplied to the engine, it is possible to use the oxygen content in the exhaust gas as a measure of the actual air-fuel ratio.
• the senor With the so-called broadband probes, the sensor is designed as a broadband sensor. This is formed by solid electrolyte layers and a number of electrodes. Such a structure emerges from DE 19 912 102 AI, in particular pages 8 and 9 there and FIG. 1 there, to which reference is made in its entirety in the present context. These electrodes are shown schematically in FIG. 1, which is described in detail below. Part of the electrodes mentioned forms a so-called pump cell in this sensor, the other part forms one so-called concentration cell. Furthermore, a so-called first measuring gas space is formed by the solid electrolyte layers.
• a pump voltage U_APE, IPE is applied to the electrodes of the pump cell (FIG. 1), by means of which a constant oxygen partial pressure is set in the first measuring gas space by pumping in or pumping out oxygen.
• a further electrode arranged in a second measuring gas space is operated with one of the electrodes mentioned as a second pump cell. Because of the catalytic material, the further electrode mentioned acts as a NOx-sensitive electrode, on which the NOx is reduced in accordance with the reaction NO -> N2 + 02.
• the above-mentioned reference electrode also acts as a second pump electrode at which the oxygen pumped out of the second measurement gas space is released into the air atmosphere.
• a limit current thus arises at the electrochemical cell, which acts as a further pump cell, which, recorded as a measurement signal, indicates the NOx concentration.
• the sensor described can be used both as a nitrogen oxide (NOx) sensor and as
• Hydrocarbon (HC) sensor can be used.
• NOx measurement signal shows a dependency on the oxygen partial pressure present in the measuring cell. This influence is mainly caused by the electrical interactions of the sensor electrodes that arise in the sensor ceramic.
• the main influence comes from the main pump path shown in Fig. 1 between the outer pump electrode (APE) and the inner pump electrode (IPE), the electrical current level (5 mA ... 10 mA) and thus their pumping capacity being adjusted accordingly with changing oxygen content must become.
• This 02 influence is known to be compensated for by means of suitable evaluation circuits by means of electronic or computational addition or subtraction with an IPE current-dependent factor, the gain of this compensation having to be set specifically for each individual sensor.
• Such a circuit for electronic compensation is shown in block form in FIG. 1.
• the present invention is therefore based on the object of specifying a method mentioned at the outset and a circuit which avoid the abovementioned disadvantages and minimize the aforementioned oxygen influence of the nitrogen oxide signal using the simplest possible technical means and as inexpensively as possible.
• the invention is based on the idea of briefly suppressing the cause of the influence of oxygen on the nitrogen oxide signal, namely the main pumping current between the inner pumping electrode (IPE) and the outer pumping electrode (APE), so that an unadulterated NOx signal is detected in this period can be.
• the invention provides for the (main) pumping current I_Pump to be switched off within a measurement time window T_Mess, ie to be set to the value 0.
• a second variant provides an IPE current control, which sets the main pump current to a constant value> 0 during the measurement time window T_Mess, so that the influence of the main pump current is not completely switched off, but is kept constant while the pump power is reduced less and thus the amplitude of the
• the measurement time window T_Mess is dimensioned such that the pump current flowing between IPE and APE has already decayed within T_Mess and the increase in oxygen concentration within T_Mess due to the current cutoff or reduction has not yet reached the NOx electrode.
• the aforementioned intervention in the pump current can either be carried out regularly with a repetition frequency, the repetition frequency for the current cutoff or reduction being dimensioned such that the disturbance of the oxygen concentration has subsided again at the beginning of a subsequent IPE cutoff or reduction.
• the main pumping current I_Pump can be temporarily switched off or reduced during the operation of the nitrogen oxide sensor and a calibration can be carried out in each case.
• the measurement time window T_Mess is in the range of 10-100 ⁇ sec, preferably 60 ⁇ sec, and the repetition frequency mentioned is in the range of 10-100 Hz, preferably 50 Hz.
• Fig. 1 shows the basic structure of a
• FIG. 2 shows a circuit arrangement according to the prior art for operating the main pumping section shown in FIG. 1;
• Fig. 4 is shown in Fig. 3
• 5 shows measured curves of uncompensated and NOx signals compensated by means of an IPE shutdown according to the invention in comparison; and 6 typical signal curves of the pump voltage U_IPE and of the NOx signal U_NOx detected during an IPE shutdown according to the invention.
• the sensor 10 comprises a first measuring gas chamber 12 which is connected to the measuring gas (here: exhaust gas).
• a first inner pump electrode (IPE) 18 and a second inner pump electrode 20 are arranged in the measurement gas chamber 12.
• a reference gas channel 26 is arranged, which is led out of the body of the sensor 10 at one end and is connected to the air atmosphere.
• the sensor 10 also has one or more gas inlet openings 28 which conduct the measurement gas into the first measurement gas space 12.
• an outer electrode (APE) 30 which is directly exposed to the measurement gas.
• APE outer electrode
• a fifth electrode (LR) 32 which is exposed to the air atmosphere is located in the reference gas channel 26 and is referred to below as an air reference electrode. It should be noted that the fifth Alternatively, electrode 32 can be exposed to the measurement gas.
• the outer electrode 30 and the first inner electrode 18 are operated as pump electrodes of a first pump cell.
• the second inner electrode 20 is connected to the fifth electrode 32, which acts as a reference electrode, as a concentration cell.
• a pump voltage U_APE, IPE is applied to the electrodes 18, 30, by means of which a constant oxygen partial pressure is set in the first measuring gas space 12 by pumping in or pumping out oxygen.
• sample gas is lean (LAMDA> 1)
• oxygen is pumped out of the first sample gas space 12 by the first pump cell.
• sample gas is rich (LAMBDA ⁇ 1)
• the sample gas pumps oxygen into the first sample gas space 12.
• Electrode material and / or a corresponding pump voltage U_APE, IPE ensures that no NOx is pumped off at the electrodes 18, 20 when the oxygen is being pumped.
• the measurement atmosphere which is set to a constant oxygen partial pressure, reaches the second measurement gas space 14 via a connection channel 16, which is only indicated schematically.
• the third inner electrode (measurement electrode 'ME') 22 located in the second measurement gas space 14 is operated with the fifth electrode 30 as a second pump cell. Due to the catalytic material, the fourth inner electrode 24 (here designation 'NO') acts as a NOx-sensitive electrode, on which the NOx is reduced in accordance with the reaction NO -> N2 + 02.
• the reference electrode interacting with the electrode 20 simultaneously acts as a second pump electrode, at which the oxygen pumped out of the second measurement gas space 14 is released into the air atmosphere.
• a limit current thus arises at the electrochemical cell, which acts as a further pump cell, which, recorded as a measurement signal, indicates the NOx concentration.
• FIG. 1 also illustrates the function of the main pump path of the NOx sensor 10.
• LAMBDA 1 to adjust.
• Which value of LAMBDA is present in the first measurement gas space 12 can be determined on the between the inner pump electrode 18 and the Air reference electrode 32 occurring Nernst voltage can be assessed.
• the value of LAMBDA occurring at the inner pump electrode 18 is represented by the electrical voltage between the air reference electrode 32 and ground 34.
• This value 35 forms the actual value of a controlled system 36, from which the setpoint U_Lambda_soll 37 is subtracted by means of a summation element 38.
• the difference signal is fed to the negative input of a two-point controller 40 designed as a differential amplifier, in the present exemplary embodiment an operational amplifier with an amplification factor> 10000.
• the reference input of the two-point controller 40 is at ground 42.
• the output 44 of the two-point controller 40 is connected to the outer pump electrode 30. According to the size and the sign of the difference between U_LR 35 and U_Lambda_soll 37, oxygen is now pumped into or out of the first measuring gas space 12.
• FIG. 2 shows a circuit implementation of the arrangement shown in FIG. 1.
• the voltage U_LR 35 is transmitted via a voltage follower (OP3) 50 to the summation point 38 at the negative input of the two-point regulator (OP5) 40.
• the target value 37 is formed by a negative voltage, which is added to the actual value via a resistor network 52.
• the inner pump electrode 18 is connected to ground 56 by means of a guard amplifier 54.
• Oxygen dependence of a NOx sensor 10 Oxygen dependence of a NOx sensor 10. It can be seen from this illustration that the inner pump electrodes (IPE) 18, 20 are short-circuited and, together with the outer pump electrode (APE) 30, are electrically connected via supply lines to voltage supply connections 100, 102 of the circuit shown. Corresponding connections 104, 106 are provided for the measuring electrode (ME) 22 and the NOx-sensitive electrode (NO) 24.
• the evaluation circuit also has a first circuit part 108 provided for the basic adjustment of the sensor 10. Furthermore, as an addition or. Subtraction stage switched second circuit part 110 is arranged, which serves for the above-described oxygen compensation by means of an IPE current-dependent factor, which by a linear Arrangement of operational amplifiers 112, 114 and 116 is provided.
• the electronics shown in FIG. 3 are also electrically connected to the air reference electrode 32 via a connection 118.
• the evaluation circuit shown in FIG. 3 still has some components that are already known in the prior art, in particular the compensation stage formed from operational amplifiers 112-116 for compensating the 02 influence with an IPE current-dependent factor, to which has already been discussed in connection with FIG. 1.
• the broken line representation of components 112-116 is intended to indicate that these components are not required in the present case.
• the evaluation circuit has an IPE shutdown module 404 connected via connections 400, 402 and 102.
• the shutdown module 404 is also connected via a control line 406 to a NOx measured value acquisition module 408.
• the IPE switch-off module 404 serves to switch off the main pump current I_Pump within a measurement time window T_Mess, ie to set it to the value 0.
• the IPE switch-off module 404 sets the main pump current to a constant value> 0 during the measurement time window T_Mess, so that the influence of the main pump current Although not completely switched off, it is kept constant while the pump power is reduced less and thus the amplitude of the
• the measurement time window T_Mess is dimensioned such that the pump current I_Pump flowing between IPE 18, 20 and APE 30 has already decayed within T_Mess and the increase in oxygen concentration within T_Mess due to the current cutoff or reduction has not yet reached the NOx electrode 24.
• the aforementioned change in the pump current I_Pump is carried out in the exemplary embodiment with a repetition frequency, the repetition frequency for the current cutoff or reduction being dimensioned such that the disturbance of the oxygen concentration has subsided again at the beginning of a subsequent IPE shutdown or reduction.
• the measurement time window T_Mess is in the range of 10-100 ⁇ sec, preferably 60 ⁇ sec, and the repetition frequency mentioned is in the range of 10-100 Hz, preferably 50 Hz.
• the main pumping current I_Pump is temporarily switched off or reduced during the operation of the nitrogen oxide sensor 10 and a calibration is carried out in the process becomes.
• This procedure can be carried out in connection with digital signal processing, by means of which a corresponding correction map is calibrated during adjustment or temporarily during operation.
• the latter procedure can be implemented in the form of a self-learning system.
• the NOx measured value acquisition module 408 shown is used to be able to distinguish the NOx values, which are compensated for oxygen during the measurement time window T_Mess, from the uncompensated measured values.
• the NOx measurement value acquisition module 408 is triggered in time by means of the line 406 and outputs the measurement data acquired within the measurement time window T_Mess via an output line 410 provided for this purpose.
• IPE shutdown module 404 shows the above-described IPE shutdown module 404 in the upper half of the figure and the likewise described NOx measured value detection module 408 in the lower half of the picture in greater detail.
• the voltage at the resistance of the guard amplifier 54 which is proportional to the current I_IPE, is tapped as the actual value detection, the output of the guard amplifier 54 4, and the input of the guard amplifier 54 to the shown terminal, 3 '.
• the voltage difference present at the terminals, 2 'and, 3' forms the input signal for an integral controller 500, which is connected in the function of a switch-off controller.
• the output of the integral controller 500 is coupled to the summation point 38 of the pump current controller 40 via a switch 514 and a voltage follower 502 and via the terminal 1, 1 'shown with a resistor 400 shown in FIG. 3.
• the integral controller 500 changes the voltage at the summation point 38 of the pump current controller 40 such that the pump current I_IPE is regulated to I_setpoint after a transient process.
• a switch can be provided in the line between the outer pump electrode 30 and the regulator output of the pump current regulator 40, by means of which the pump current I_IPE immediately, ie without the settling process mentioned can be set to zero.
• the integral controller 500 can be configured such that the time constant for the integration can be switched over and, for example, can be switched over between a relatively short first time constant and a second time constant that is relatively long compared to the first time constant.
• the first time constant can be set until shortly before the pump current I_IPE passes through zero and only then can it be switched to the second time constant, which then causes an aperiodic transient process.
• the switch 514 is open before the start of T_Mess.
• the resistor 510 ensures that no voltage is fed in at the summation point 38 during the regular regulator operation and thus that there is no influence on the current regulation.
• the switch 503 is opened and the switch 514 is closed, so that the intervention of the integral controller 500 can act on the pump current controller 40.
• the circuit arrangement shown contains a circuitry implementation of the NOx determination method shown in FIG. 2.
• the resulting signal is used in the IPE switch-off method according to the invention without any changes in the circuitry, the influence of the stray current compensation known in the prior art, shown in FIG. 3, being set to zero using specially provided (not shown) potentiometers.
• the functions of the three amplifier stages 112, 114 and 116 shown in FIG. 3 crossed out in FIG. 3) have practically no effect.
• the NOx measured value acquisition is controlled synchronously with the integral controller 500.
• a switch 526 is initially opened.
• a holding element composed of a capacitor 524 and a voltage follower 520 has an output voltage (terminal 13, 13 ') which corresponds to the current state of charge of the capacitor 524.
• the switch 526 is closed, so that the capacitor 524 is charged in accordance with the input voltage present at terminal 12, 12 '.
• the terminal 12, 12 ' is connected to the NOx signal output of the evaluation circuit shown in FIG. 3.
• FIG. 5 shows a comparison of measured courses of an uncompensated NOx signal and NOx signals sampled during a measurement time window T_Mess, compensated according to the invention, in each case with changing 02 concentrations and changing NOx concentrations.
• a recorded NOx signal is plotted over time. If the value of the NOx signal is also recorded and stored shortly before the start of the measurement time, the compensated and the uncompensated curve can be compared.
• flanks of the 02 concentration changes cannot be compensated for using the method according to the invention.
• U (I_IPE) and U (I_NOx) show the waveforms of U (I_IPE) and U (I_NOx) during the measurement time with the sampling times for the compensated and uncompensated signal.
• U (I_IPE) and U (I_N0) denote the output voltages of the above-described guard amplifiers 54, 55, which are present at the electrodes, IPE '18 and, N0' 24.
• U (IPE) denotes the IPE at the positive input of the guard amplifier 54 applied voltage.

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• Chemical & Material Sciences (AREA)
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• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
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• General Health & Medical Sciences (AREA)
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• Immunology (AREA)
• Pathology (AREA)
• Measuring Oxygen Concentration In Cells (AREA)
• Exhaust Gas After Treatment (AREA)

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• 2001
  • 2001-09-17 DE DE10145804A patent/DE10145804B4/de not_active Expired - Lifetime
• 2002
  • 2002-08-02 US US10/489,798 patent/US7455761B2/en not_active Expired - Fee Related
  • 2002-08-02 WO PCT/DE2002/002860 patent/WO2003027655A2/de active Application Filing
  • 2002-08-02 JP JP2003531158A patent/JP2005504292A/ja not_active Abandoned
  • 2002-08-02 EP EP02767071A patent/EP1430296A2/de not_active Withdrawn

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