FR2794863A1 - ELECTRICAL ASSEMBLY OF A NOx MEASUREMENT SENSOR AND ITS METHOD OF USE - Google Patents

Abstract

The pumping currents (IP0, IP1, IP2) in a NOx measuring sensor are produced by current sources (U10, U11, U12) controlled by a voltage. As a replacement for the pumping current (IP0, IP1 IP2), the control voltage (VS0 VS1 VS2) of the current source (U10, U11, U12) is used as the feedback variable in the regulation circuit considered. controlled by a voltage, so that the current measurement can be dispensed with. The first pumping intensity (IP0) is set as a function of the regulation deviation of a regulator (C1) provided for the second pumping intensity.

Description

The invention relates to an assembly for a NOx sensor which

  captures the NOx concentration in a gas, and which comprises an external electrode exposed to the gas to be measured, a first measurement cell, a second measurement cell, which is connected to the first measurement cell, and a reference electrode which is exposed to the surrounding air, which assembly comprises: a first control circuit which regulates in the first measurement cell, by means of a first pumping intensity, an oxygen concentration different from that existing in the gas to be measured, and a second control circuit which regulates in the second measurement cell, by means of a second pumping intensity, an oxygen concentration different from that existing in the first measurement cell. The invention also relates to a method of using a NOx measurement sensor comprising a first measurement cell exposed to the gas to be measured via a first diffusion barrier and a second measurement cell connected to the first

  via a second diffusion barrier.

  For measuring the concentration of NOx in a gas, for example, in the exhaust gases of an internal combustion engine, it is known to use a thick film measurement sensor. Such a measurement sensor is described, for example, in the publication by N. Kato et al; Thick film ZrO2 NOx Sensor for the Measurement of Low NOx Concentration, Society of Automotive Engineers, Publication 980170, 1989, or in that of N. Kato et al, Performance of Thick Film NOx Sensor on Diesel and Gasoline Engines ", Society of Automotive Engineers , Publication 970858, 1997. This measurement sensor has two measurement cells and is composed of a zirconium oxide that conducts oxygen ions. It performs the following measurement principle: in a first measurement cell, to which the gas to be measured is conveyed via a diffusion barrier, a first oxygen concentration is regulated by means of a first pumping intensity of oxygen ions, however that NOx decomposition should not occur. a second measurement cell, which is connected to the first via a diffusion barrier, the oxygen content is further lowered by means of a second oxygen ion pumping intensity The decomposition of NOx on a measurement electrode contained in the second measurement cell leads to a third intensity of pumping of oxygen ions which is a measurement of the concentration of NOx. The entire NOx sensor is brought to a high temperature, for example 750 C, by means of

  an electric heating element.

  To adjust the pumping intensities of oxygen ions, take a Nernst voltage in the measuring cells. The Nernst voltage taken from the first cell is used as a reference quantity for a regulator which regulates the first pumping intensity of oxygen ions by means of a control voltage. A similar regulator is provided for the second measurement cell, for adjusting the second oxygen concentration by means of a second pumping intensity of oxygen ions. In the case of analog arrangements, each

  regulator must be built or calculated separately.

  In the case of regulators produced in digital form by means of a microcontroller, one needs, for each voltage of Nernst, two A / D ports on the A / D converter of the microcontroller because the voltage can only be measured compared to the potential of

microcontroller reference.

  The object of the present invention therefore consists in providing an assembly for a measurement sensor which detects a concentration of NOx in a gas, and a method of using such a measurement sensor, in which fewer regulators are required. , or in which

  the regulation can be carried out more simply.

  This problem is solved by an assembly of the type defined in the introduction and which is characterized by a differential component which detects a regulation difference between the real value of the second pumping intensity in the second circuit and a setpoint for this second intensity of pumping, and a regulating member which regulates the first intensity of

  pumping according to the regulation deviation.

  This problem is also solved by a method of the type defined in the introduction and which is characterized in that a signal is formed which depends on the diffusion of oxygen from the first measurement cell into the second measurement cell, and that a pumping intensity with which a different oxygen concentration than that existing in the gas to be measured is established in the first measuring cell, is adjusted so that a

  predetermined oxygen diffusion.

  According to the invention, the Nernst voltage in the first measurement cell is dispensed with, and the first pumping intensity of oxygen ions is regulated by taking as a reference quantity an oxygen diffusion which is determined from the control difference of the second oxygen ion pumping intensity in the second measurement cell. This makes it possible to give the regulator a simpler construction, or to save an A / D port on the microcontroller since the signal indicative of the diffusion of oxygen which serves as a reference quantity is obtained directly during the regulation of the second

  pumping intensity of oxygen ions.

  This procedure is based on the observation that the regulator for the intensity of pumping of oxygen ions in the first measurement cell, establishes the concentration of oxygen in the first measurement cell at a predetermined value. This first predetermined value influences the diffusion of oxygen towards the second measurement cell. In this way, a signal indicative of this diffusion of oxygen can also be used as a reference quantity for the predetermination or the regulation of the first intensity of pumping of oxygen ions out of the first measurement cell. We can dispense with measuring the

  Nernst voltage in the first cell.

  According to other advantageous features of the assembly according to the invention: the adjustment member is a first current source controlled by a voltage placed in the first control circuit, which circulates the pumping intensity of a first electrode to an external electrode and whose regulation voltage is produced by a first regulator forming the differential component, which regulates the second pumping intensity; - in the second control circuit, a second intensity source controlled by a voltage circulates an oxygen ion pumping intensity from a second electrode to the external electrode, and a regulator produces the adjustment voltage of the second voltage-controlled current source, is connected to the second electrode and to the reference electrode, takes a Nernst voltage between these electrodes and adjusts the control voltage of the second voltage-controlled current source so giving a predetermined Nernst voltage which represents a measurement of the oxygen concentration in the second measurement cell; a third intensity source controlled by a voltage which circulates in the second measurement cell a third pumping intensity from a measurement electrode to the external electrode and a second regulator produces the adjustment voltage of the third source d the current controlled by a voltage, is connected to the measurement electrode and to the reference electrode, takes a Nernst voltage between these two electrodes and adjusts the adjustment voltage so that a predetermined Nernst voltage is obtained, the value of the adjustment voltage being a measure of the intensity, and therefore of the concentration of NOx to be measured; - all the regulators are formed by a microcontroller which produces an output signal, modulated in pulse width from which a reference voltage source and a transistor assembly, downstream of which is connected a low-pass, produce the voltage adjustment in the form of a DC voltage proportional to the duty cycle of the pulses of the output signal; - the assembly includes an analog regulator

  for regulating the first pumping intensity.

  The invention is described in more detail below with reference to the drawings. In these drawings: FIG. 1 shows a schematic representation of a NOx measurement sensor comprising the assembly according to the invention, FIG. 2 shows a schematic representation of a NOx measurement sensor comprising an assembly according to the state of the art, FIG. 3 shows a block diagram of an internal combustion engine in which a sensor for measuring NOx can find use, FIG. 4 shows a block diagram for the illustration of the production of an adjustment voltage for a current source controlled by a voltage which is used in the mounting of the measurement sensor according to FIG. 2, and, FIG. 5 shows a diagram of a source of intensity

voltage controlled.

  A sensor for measuring NOx 1 is shown diagrammatically in a sectional view in each of FIGS. 1 and 2. FIG. 2 shows a measurement sensor comprising an assembly according to the state of the art. This measurement sensor 1 is used in the device shown in FIG. 3 as a measurement sensor 24 for determining the concentration of NOx in the exhaust path 27 of an internal combustion engine 20. For this, the measurement results of the NOx measurement sensor 24 are read by a control unit 23 which is connected to the NOx measurement sensor 24, and they are sent to the apparatus 25 for controlling the operation of the internal combustion engine 20, which controls a device 21 for supplying fuel to the combustion engine internal 20 such that a NOx-reducing catalyst 28, which is in this case upstream of the NOx measurement sensor 24 in the exhaust gas path 27 of the combustion engine

  internal 20 exhibits optimal working behavior.

  The measurement sensor 1 which is composed of a solid electrolyte 2, ZrO2 in this case (see Fig. 2) receives via a diffusion barrier 3 the exhaust gas to be measured in which it s is to determine the NOx concentration. The exhaust gases diffuse in a first cell 4 through the diffusion barrier 3. The oxygen content in this measurement cell is measured by taking a Nernst V0 voltage between a first electrode 5 and a reference electrode 11 which is exposed to the surrounding air. The reference electrode 11 is here arranged in an air duct 12 into which the surrounding air enters through an opening 14. The entire sensor

  measure 1 is heated by a heating element 13.

  The voltage of Nernst V0 is taken by a regulator which fixes a control voltage VP0, which circulates a first intensity of pumping of oxygen ions IP0 through the solid electrolyte 2 of the measurement sensor 1 between the first electrode 5 and a external electrode 6. A predetermined oxygen concentration is then established in the first measurement cell 4 which is measured by means of the Nernst Vl voltage between the electrode 5 and the reference electrode 11. The measurement of the first pumping intensity of oxygen ions which is necessary for regulation, is carried out by means of a ROM measurement resistor and a VOM voltmeter. These devices are produced in most cases in the form of an A / D converter which has a certain

internal resistance.

  This first control circuit thus establishes a predetermined oxygen concentration in the first measurement cell 4. The second measurement cell 8 is connected to the first measurement cell 4 via another diffusion barrier 7. The gas which is in the first measuring cell 4 diffuses towards the second measuring cell 8 through this diffusion barrier 7. A second concentration of oxygen is established in the second measuring cell by means of a second control circuit . For this, a second Nernst Vl voltage is taken between a second electrode 9 and the reference electrode 11 and transmitted to a regulator which produces a second control voltage VP1 which circulates an oxygen ion pumping intensity IP1 out of the second measurement cell 8. Here too, the second oxygen ion pumping intensity IP1 is picked up, for regulation, by means of a measurement resistor RlM and a

V1M voltmeter.

  This second control circuit circulates the oxygen ion pumping intensity IP1 so that an oxygen concentration is established

  predetermined in the second measurement cell 8.

  This oxygen concentration is chosen so that the NOx are not affected by the operations which take place, and in particular so that no decomposition takes place. The NOx are now pumped in a third pumping intensity of oxygen ions IP2, from a measurement electrode 10 to the external electrode 6, passing over this measurement electrode 10 which is contained in the second measurement cell 8 that -ci can be catalytic constitution. Since the residual oxygen content in the measuring cell 8 is sufficiently lowered, this pumping intensity of oxygen ions IP2 is carried almost exclusively by oxygen ions which result from the

  NOx decomposition on the measuring electrode 10.

  The pumping intensity IP2 thus constitutes a measure of the concentration of NOx in the measurement cell 8, and therefore in the exhaust gases to be measured. This value is measured by means of a measurement resistor R2M and a voltmeter V2M and, like the previous pumping intensities, it circulates thanks to a control voltage VP2 which is predetermined by a regulator which takes a voltage of Nernst V2 between the electrode

  measurement 10 and the reference electrode 11.

  The mounting of the NOx measurement sensor 1 is already known in the form described so far. On the contrary, in one embodiment of the invention, one dispenses with sensing the first voltage of Nernst V0 and one uses the assembly shown in FIG. 1. Here, all the pumping intensities IP0, IP1 and IP2 are applied by intensity sources UI0, UI1 and UI2 controlled by a voltage. Naturally other arrangements are also

known to those skilled in the art.

  For each current source controlled by a voltage, the circuit shown in FIG. 5. Fig. 5 shows by way of example a first source of intensity UI0. The regulating voltage VS0 is applied to an operational amplifier 35, at the inverting input through a resistor 36. The inverting input is moreover back-coupled to the output of

  the operational amplifier through a resistor 37.

  The non-inverting input of the operational amplifier 35 is connected to ground through a resistor 38 and it is back-coupled to the output of the operational amplifier 35 through a resistor 39. A resistor 40 is connected between the combination nodes of the back-coupling branch no

  inverting and reverse inverting branch.

  The IPO intensity can be taken at the output 33 of the UPO intensity source controlled by a voltage. This arrangement has the advantage that the intensity IPO is defined at a fixed level by the regulating voltage VSO independently of the impedance of the resistance

  load which is in this case that of the pumping circuit.

  The IPO intensity measurement is not necessary since VSO and IPO are linked by the following equation:

IPO = VSO / R,

  when R is the ohmic value of resistance 40, and resistors 36 to 39 all have the same ohmic value, which is much greater than the value

resistance 40 resistance.

  If the inputs of the operational amplifier are switched, the ohmic value of resistance 40 must be much smaller than that of resistors 36 to 39. Resistors 37 to 39 then have the same ohmic value and the ohmic value resistance should be chosen as large as that of the circuit

  series composed of resistors 36 and 39.

  Thanks to the use of intensity sources controlled by a voltage, the measurement of pumping intensities IPO, IP1 and IP2 can be omitted since their control voltage VSO, VS1 and VS2 is directly linked to the pumping intensity, as described above. The ROM, RlM and R2M measurement resistors, as well as the corresponding VOM, VlM and V2M voltmeters used in

  the state of the art are therefore unnecessary.

  Furthermore, the measurement of the Nernst VO voltage is also deleted. To replace this regulation, the regulating voltage VSO is drawn from the voltage source UIO controlled by a voltage for the first pumping intensity of oxygen ions IPO from a signal from the regulator C1 which regulates the second pumping intensity of oxygen ions IP1 in the second measuring cell 8. The regulation deviation IP_D from the regulator C1 is then used as a signal in a manner which remains

still to be described.

  The regulators C1, C2 used in the assembly of FIG. 1 are preferably microcontrollers or a single microcontroller. The setting voltages VS0, VS1, VS2 can be sent to the digital / analog port of the microcontroller (s). However, the following arrangement is preferably used, shown in FIG. 4, to produce one of the adjustment voltages VS0, VS1, VS2

with the microcontroller.

  In Fig. 4, there is shown a microcontroller 30 which transmits at its output port a signal PW0 modulated in pulse width. This signal PW0 is sent to a transistor assembly 31 to which is further connected a reference voltage source UR. The transistor assembly transmits the reference voltage source UR as a function of the signal PW0 modulated in pulse width

  to a low-pass 32 which, thanks to its pass-through properties

  low, produces from this signal the analog setting voltage VS0, the level of which is proportional to the duty cycle of the pulses. This adjustment voltage VS0 is used for controlling the intensity source UI0 controlled by a voltage, which produces the pumping intensity IPO at its output 33. This diagram for producing the pumping intensity IP0 is also used for the production of the pumping intensities IP1 and IP2, and a single microcontroller 30 can produce the signals PW0 or, respectively PW1 and PW2 at its three outputs. Of course, you can also use three

independent microcontrollers.

  The adjustment voltage VS0 is determined by the regulator C1 of the first oxygen ion pumping intensity IP1 in the following manner. An IP1_S setpoint is set in advance for IP1. This oxygen ion pumping intensity must eliminate from the second measurement cell 8 the molecules of 02 which have reached this second measurement cell from the first measurement cell 4, via the diffusion barrier. 5. The prior setting of the set value IP1 _S thus corresponds to a predetermined oxygen diffusion from the first measurement cell 4 to the second measurement cell 8. The regulation difference IP1_D is calculated from the difference between the set value IPi_S and the actual value of the first oxygen ion pumping intensity IP1, which is known on the basis of the regulating voltage VSl, by the relation cited above existing between the regulating voltage and the intensity pumping oxygen ions. The adjustment voltage VS0 for the first current source UI0 controlled by a voltage is then chosen so that IP1_D disappears. The already known regulation, on a value determined for the voltage of Nernst V0 in the first measurement cell 4, is then replaced by a regulation on a reference value IP1_S of the second pumping intensity of oxygen ions IP1. In this way, on the one hand, it is possible to dispense with detecting the voltage of Nernst V0, which eliminates the ports of A / D converters used in the case of microcontrollers 30. On the other hand, the regulation can be presented by simpler way since it is not necessary to build a separate regulation circuit with special regulator for the first intensity

pumping of oxygen ions IP0.

Claims (8)

  1.Mounting for a NOx measurement sensor (1) which detects the concentration of NOx in a gas, and which comprises an external electrode (6) exposed to the gas to be measured, a first measuring cell (4), a second cell measurement (8), which is connected to the first measurement cell (4), and a reference electrode (11) which is exposed to the surrounding air, which assembly comprises: - a first control circuit which establishes in the first measurement cell (4) having a first pumping intensity (IP0) an oxygen concentration different from that existing in the gas to be measured, and - a second control circuit which adjusts in the second measurement cell (8) having a second pumping intensity (IP1), a different oxygen concentration than that existing in the first measuring cell (4), characterized by - a differential component (Ci) which determines a regulation difference (IP1_D) between the actual value from the d second pumping intensity (IP1) in the second circuit and a setpoint (IP1_S) for this second pumping intensity (IP1), and - an adjusting member (UI0) which regulates the first pumping intensity (IP0) in function away from regulation (IP1_D).   2. Installation according to claim 1, characterized in that the regulating member is a first intensity source (UI0) controlled by a voltage placed in the first control circuit, which circulates the pumping intensity (IP0) from a first electrode (5) to an external electrode (6) and whose adjustment voltage (VS0) is produced by a first regulator (C1) forming the differential component, which regulates the second pumping intensity (IP1).   3. Assembly according to any one of the claims   above, characterized in that, in the second control circuit, a second intensity source (UI1) controlled by a voltage circulates a pumping intensity (IP1) of oxygen ions from a second electrode (9) to the external electrode (6), in that a regulator (Cl) produces the adjustment voltage (VS1) of the second intensity source (UI1) controlled by a voltage, is connected to the second electrode (9) and to the reference electrode (11), takes a Nernst voltage (V1) between these electrodes and adjusts the adjustment voltage (VSl) of the second intensity source (UI1) controlled by a voltage so as to give a voltage of Predetermined nernst which represents a measure of the oxygen concentration in the second cell of measure (8).   4. Assembly according to any one of the claims   previous, characterized by a third intensity source (IP2) controlled by a voltage which circulates in the second measurement cell (8) a third pumping intensity (IP2) from a measurement electrode (10) to the electrode external (6) and a second regulator (C2) which produces the regulation voltage of the third intensity source (IP2) controlled by a voltage, is connected to the measurement electrode (10) and to the reference electrode (11), takes a Nernst voltage (V2) between these two electrodes and adjusts the adjustment voltage (UI2) so that a predetermined Nernst voltage (V2) is obtained, the value of the adjustment voltage (UI2) being a measure of intensity (IP2), and   therefore the concentration of NOx to be measured.   5. Assembly according to any one of the claims   previous, characterized in that all the regulators (C1, C2) are formed by a microcontroller (30) which produces an output signal (PWl, PW2), modulated in pulse width from which a reference voltage source ( UR) and a transistor assembly (31), downstream of which is connected a low-pass (32), produce the adjustment voltage (VSl, VS2) in the form of a DC voltage proportional to the ratio   cyclic pulses of the output signal (PW1, PS2).   6. Assembly according to any one of claims 1   to 4, characterized by an analog regulator for   regulation of the first pumping intensity (IP0).   7. Method for using a sensor for measuring the concentration of NOx in a gas, comprising a first measurement cell exposed to the gas to be measured via a first diffusion barrier and a second measurement cell communicating with the first by means of a second diffusion barrier, characterized in that a signal is produced which depends on the diffusion of oxygen from the first measurement cell into the second measurement cell, and that an intensity of pumping with which an oxygen concentration different from that existing in the gas to be measured is established in the first measurement cell so that diffusion occurs of predetermined oxygen.   8. Method according to claim 7, characterized in that one adjusts in the second measurement cell having a second pumping intensity, an oxygen concentration different from that prevailing in the first measurement cell and that from l regulation difference between the actual value of the second pumping intensity and its set value, the signal is drawn which   is a function of oxygen diffusion.