Method and apparatus for measuring NOx gas concentration

Abstract: A nitrogen oxide concentration detector has a first measurement chamber (2) into which is introduced a measurement gas via a first diffusion resistance (1); an oxygen concentration detection electrode (7a) for measuring the oxygen concentration in the measurement gas in said first measurement chamber (1); a first oxygen ion pump cell (6) for pumping out oxygen in the measurement gas from said first measurement chamber (2) based on the potential of said oxygen concentration detection electrode (7a); a second measurement chamber (4) into which the gas is introduced from said first measurement chamber (2) via a second diffusion resistance (3); and a second oxygen ion pump cell (8) having a pair of electrodes (8a,8b) across which a voltage is applied to decompose NOx in the second measurement chamber (4) to pump out dissociated oxygen to cause a current Ip2 corresponding to the NOx concentration to flow in the second oxygen ion pump cell (8). Variation of NOx concentration is a function of variation of Ip2. The concentration obtained based on the Ip2 is corrected responsive to the oxygen concentration in the measurement gas. Particularly, a coefficient of the variation of the Ip2, gain, in said function is corrected responsive to the oxygen concentration in the measurement gas.

Description: This invention relates to methods and apparatus used for detecting the NOx gas concentration in exhaust gases of an internal combustion engine intended for industrial use and for driving an automobile, vessel or aircraft, or in combustion gases of a boiler or the like. This invention also relates to a method and apparatus for measuring the composition of exhaust gases of a combuston device or an internal combustion engine, in particular the NOx gas concentration. This invention also relates to a method and apparatus for controlling a NOx gas sensor used for measuring the NOx gas concentration in an ambient atmosphere in which the oxygen concentration in a measurement gas is changed acutely. This invention also relates to a method and apparatus for detecting the exhaust gas concentration for measuring the concentration of harmful gas components contained in the exhaust gas from the internal combustion engine. Recently, in dealing with an intensification of exhaust gas regulations, research has been constructed into controlling the operation of an engine or the catalyst based on direct measurement of the NOx in e.g., engine exhaust gases. The resulting NOx gas sensor, which uses an oxygen ion conductor, such as ZrO 2 , causes the measurement gas (detection gas) to be introduced into a first measurement chamber and a first oxygen ion pumping cell in the first measurement chamber pumps oxygen out, to an extent such as not to decompose NOx at all. Then, the gas lowered in oxygen concentration is introduced into a second measurement chamber where oxygen is further pumped out by a second oxygen ion pumping cell to cause NOx from the NOx-gas contained in the remaining gas to be decomposed. This decomposition is sensed as an electric current. In such NOx gas sensor, the NOx gas concentration is detected based on the current flowing across a pair of electrodes provided in the second oxygen ion pumping cell. In general, a pair of electrodes are arranged on both sides of the oxygen ion conductor in such sensor. One of the electrodes is exposed to the atmosphere in the first measurement chamber, while the other electrode is exposed to a reference atmosphere containing oxygen. The electrical voltage applied across the first oxygen ion pumping cell is varied, based on an output of an oxygen partial pressure detection cell adapted for measuring the oxygen concentration in the first measurement chamber, in order to control the oxygen concentration in the first measurement chamber to a constant value. An NOx sensing device operating as described above is known from European patent application EP 0 678 740 A. In this device, the NOx concentration is obtained from the current flowing through the second oxygen ion pumping cell by multiplying the current with a constant sensitivity coefficient, K, and subtracting a constant A indicative of a small amount of oxygen. An NOx sensor having two differently electrochemically active electrodes is known from US patent US 5,397,442. In this device, oxygen and/or NOx in the exhaust gas are either concurrently for sequentially dissociated at the different electrodes. An electrochemical NOx sensor comprising a sensitivity corrector to correct the output signal on the basis of humidity information is known from Japanese patent application JP 3-277961 A. However, with the known devices, various problems have been encountered in the course of investigations toward the present invention. First, the output gain of the NOx gas sensor varies depending on the oxygen concentration upon measuring the NOx gas concentration, which results in an error in the NOx gas concentration measurement. The present inventors have further found that, if the oxygen concentration in the measurement gas is varied significantly, delay is caused in voltage control for the first oxygen ion pumping cell, thereby making it difficult to measure the NOx gas concentration correctly. In particular, in lean burn engines using gasoline or a diesel fuel, which recently are used in increasing numbers, the oxygen concentration in the exhaust gas is varied significantly under the driving conditions. Thus, if the PID control is performed based on the potential of an oxygen partial pressure detection cell, it is difficult to correctly measure the NOx concentration in the exhaust gas if only the method of varying and controlling the pump capability of the first oxygen ion pumping cell is used. In the above-described current limiting type sensor, one problem is that the detection current flowing in the oxygen ion pumping cell is as small as several µA. Moreover, since decomposition of harmful gas components (such as NOx) is controlled or suppressed by the catalytic action on the electrode surface, another problem is that, on prolonged use, catalytic activity is changed causing a shift in the zero point (i.e., a detection output of the gas sensor specifying that the concentration of the pre-set component is substantially zero). On the other hand, in an oxide semiconductor type gas sensor, which is based on the principle that changes in electrical resistance of an oxide semiconductor is proportional to the amount of a pre-set gas component adsorbed on the oxide semiconductor, suffers from poor reproducibility. Further, if moisture in the measurement gas is electrolyzed, causing it to be decomposed or dissociated, then in the second oxygen ion pumping cell, a current will flow due to oxygen yielded on electrolysis not only of NOx but also of moisture, resulting in an incorrect measurement of the NOx concentration. Moreover, in a lean burn engine using gasoline or a diesel fuel, the moisture in the exhaust gas is varied significantly under driving conditions. Consequently, it is difficult to make a correct measurement of the NOx concentration in the exhaust gases. Thus, none of these types of gas sensor is reliably used as a sensor for a prolonged time and under significantly varying environmental conditions, as in the case of a sensor mounted on an internal combustion engine, in particular the sensor mounted on an exhaust gas system of a vehicle. It is therefore the object of the present invention to overcome the drawbacks and disadvantages of the prior art. This object is solved by the method according to independent claims 1 and 7 and the apparatus for measuring the NO x concentration according to independent claims 9 and 12. Further advantageous features, aspects and details of the invention are evident from the dependent claims, description, examples and figures. According to one advantage, the present invention provides a method and apparatus for accurately measuring NOx gas concentration in engine exhaust. According to a second advantage the present invention provides a method and apparatus for controlling the NOx gas sensor which enables the NOx gas concentration to be measured accurately even if the oxygen partial pressure in the measurement gas is changed. According to a third advantage the present invention provides a method and apparatus for measuring an exhaust gas concentration for a prolonged time. Still further advantages of the present invention will become apparent in the entire disclosure. According to the present invention, there is provided a measurement method and apparatus for measuring the NOx gas concentration in which the NOx gas concentration obtained as a NOx gas sensor output is corrected in response to the oxygen concentration in the detection gas (measurement gas). This is based on a correspondence between the gas sensor output and a current caused to flow by pumping out oxygen dissociated on decomposition of NOx. An NOx gas sensor according to the invention includes a first measurement chamber into which a detection gas is introduced via a first diffusion resistance, an oxygen concentration detection electrode for measuring the oxygen concentration in the detection gas in the first measurement chamber, and a first oxygen ion pump cell for pumping oxygen contained in the detection gas out from the first measurement chamber to the outside and/or inside of the first measurement chamber based on the potential of an oxygen concentration detection electrode. The pumping is generally to an extent such that NOx is not entirely decomposed, or in some case to an extent such that nitrogen oxide does not decompose into oxygen in the measurement gas. There also is a second measurement chamber into which the gas is introduced from the first measurement chamber via a second diffusion resistance, and a second oxygen ion pump cell having a pair of electrodes across which a voltage is applied to decompose the oxygen nitride in the second measurement chamber. The dissociated oxygen is pumped out, causing a current corresponding to the nitrogen oxide concentration (second oxygen pump current) to flow in the second oxygen ion pump cell. According to an aspect of the first feature of the present invention, the NOx gas concentration can be found more correctly by a simple method even if the oxygen concentration in the detection gas does vary. Since the output of the oxygen concentration detection electrode has a specific relation with the oxygen concentration in the detection gas, both the oxygen concentration and the NOx gas concentration can be measured by a single NOx gas sensor, so that the NOx gas concentration, based on the oxygen concentration in the detection gas can be corrected using the sole sensor. Thus, the NOx gas concentration measurement apparatus according to the present invention can be applied to the exhaust system of the internal combustion engine in order to find the air-to-fuel ratio along with the NOx gas concentration, thus allowing a sole instrument to display multiple functions. The coefficient of the second oxygen pump current exhibiting oxygen concentration dependency, used for finding the NOx gas concentration, can easily be calculated preferably using the least square method. Thus, by pre-formulating a table correlating a coefficient with the oxygen concentration, the NOx gas concentration can be obtained correctly in real-time. If this method is applied to the NOx detection system for an internal combustion engine, it becomes possible to construct a highly flexible combustion control system that can adapt itself to changes in the oxygen concentration and in the NOx gas concentration. In a second aspect of the first feature of the present invention, the variation (varying amount) of the NOx gas concentration is a function of the variation (varying amount) of the second oxygen pump current. The coefficient of the variation of the second oxygen pump current in this function ('gain') is varied in response to the oxygen concentration in the detection gas in order to find the NOx gas concentration. In a third aspect, the gain as a coefficient of the variation (varying amount) of the second oxygen pump current is a function of the oxygen concentration in the detection gas. A detection gas having a known oxygen concentration and NOx gas concentration is pre-charged into a sensor to measure the second oxygen pump current. Using values of the NOx gas concentration and the second oxygen pump current, the variation of the second oxygen pump current to the variation of the NOx gas concentration at a predetermined oxygen concentration is found, e.g., by the least square method. Using the gain value at the predetermined oxygen concentration, the coefficient of the oxygen concentration as a function of the oxygen concentration in the detection gas and the gain is found, e.g., by the least square method. In a fourth aspect, the gain is represented as a function of the oxygen concentration in the detection gas. A detection gas having a known oxygen concentration and a known NOx gas concentration is pre-charged to measure the second oxygen pump current. Using the nitrogen oxide concentration and the second oxygen pump current, the variation of the second oxygen pump current to the variation of the NOx gas concentration ('gain at a pre-set oxygen concentration') is found, e.g., by the least square method. The gain at the pre-set oxygen concentration is selected and used depending on the oxygen concentration in the detection gas. *Note variation of NOx gas concentration divided by variation of the second oxygen current is defined as "gain" in this disclosure. In a fifth aspect, the gain is represented in terms of a logarithm function of the oxygen partial pressure in the detection gas. Preferably, a value of the oxygen partial pressure is used as the oxygen concentration in third aspect of the first feature of the invention. In a sixth aspect, values of the second oxygen pump current for substantially zero NOx gas concentration and for a pre-set NOx concentration, obtained while varying the oxygen concentration in the detection gas, are measured. The gain at the pre-set oxygen concentration and the second oxygen pump current for the substantially zero NOx gas concentration for the pre-set oxygen concentration (hereinafter referred to as 'offset at the pre-set oxygen concentration') are measured. From the gain and the offset corresponding to the oxygen concentration in the detection gas and the second oxygen pump current, the NOx gas concentration is found. In a seventh aspect, the NOx gas concentration obtained based on the second oxygen pump current is corrected based on an output of the oxygen concentration detection electrode that is changed with the oxygen concentration in the detection gas. In an eighth aspect, the variation (varying amount) of the NOx gas concentration is a function of the variation (varying amount) of the second oxygen pump current. There is provided gain selection means selecting the coefficient of the variation of the second oxygen pump current (hereinafter referred to as 'gain') in this function responsive to the output of the oxygen concentration detection electrode. There is also provided processing means calculating the NOx gas concentration based on the gain selected by the gain selection means based on the second oxygen pump current. The gain selection means can selectively use the coefficient of the gain as determined by ,e.g., the least square method and the oxygen concentration in the detection gas depending on an output of the oxygen concentration detection electrode. Alternatively, a previously found gain at a pre-set oxygen concentration can be selected depending on the output of the oxygen concentration detection electrode. Preferably, the gain selection means and the processing means may be constructed in a micro-computer connected to a nitrogen oxide concentration sensor. Detection of the oxygen concentration can be based on the output of the oxygen concentration detection electrode means directly or indirectly detecting the oxygen concentration or the current or the voltage representing the oxygen concentration. Preferably, the oxygen concentration is detected based on the potential of the oxygen concentration detection electrode. Alternatively, the oxygen concentration is detected based on a voltage applied across the first oxygen ion pump cell controlled based on the output (potential) of the oxygen concentration detection electrode, and the first oxygen pump current flowing in the first oxygen ion pump cell. Thus, the gain of the nitrogen oxide concentration can be corrected based preferably on the voltage applied across the first oxygen ion pump cell and the first oxygen pump current. Next, the aspects of the second feature of the present invention is hereinafter briefly explained. The aspects of the second feature is applicable with advantage to the NOx gas sensor to which the aspects of the first feature is applied with advantage. In the aspects of the second feature, the method for measuring the NOx gas concentration is comprised of several elements. That is, in a first aspect, if the oxygen partial pressure (concentration) is changed, the detection output of the NOx gas concentration based on the second oxygen ion pump current is corrected in accordance with the change. In this first aspect of the second feature, the NOx gas concentration can be measured accurately even if the oxygen concentration in the measurement gas would vary acutely. That is, even in an atmosphere in which the oxygen concentration varies significantly, the NOx gas concentration can be found correctly in real-time without control delay. By applying the control method of the aspects of the second feature to the nitrogen oxide detection system for an internal combustion engine, it becomes possible to construct a combustion control system which can adapt itself flexibly to changes in the oxygen concentration and in NOx gas concentration. The control device can be constructed both by software and by hardware. In a second aspect, there is provided an oxygen partial pressure detection cell having an oxygen partial pressure detection electrode for detecting the oxygen partial pressure in the first measurement chamber. A detection output of the NOx gas concentration is corrected based on an output of the oxygen partial pressure detection cell. In a third aspect, the voltage applied to the first oxygen ion pumping cell is controlled based on an output of the oxygen partial pressure detection cell. The detection output of the NOx gas concentration is calibrated based on the variation (varying amount) of the current flowing in the first oxygen ion pumping cell (referred to hereinafter as 'first oxygen ion pump current'). In a fourth embodiment, the voltage applied across the second oxygen ion pumping cell is controlled responsive to variations in the oxygen partial pressure in the measurement gas. In a fifth embodiment, if the oxygen partial pressure is low, the voltage applied across the second oxygen ion pumping cell is lowered. If the oxygen partial pressure is high, the voltage applied across the second oxygen ion pumping cell is raised. In a sixth embodiment, there is provided an oxygen partial pressure detection cell for detecting the oxygen partial pressure in the first measurement chamber. If, with the output of the oxygen partial pressure detection cell and the output of the second oxygen ion pumping cell as inputs, the output of the oxygen partial pressure detection cell is changed, a NOx gas concentration detection output based on the second oxygen ion pump current is corrected. In a seventh embodiment, there is provided an oxygen partial pressure controller for controlling the first oxygen ion pump current so that the output of the oxygen partial pressure detection cell will be constant. There is also provided a memory having the pre-stored relation between the variation of the output of the oxygen partial pressure detection cell and the offset of the second oxygen ion pump current. Responsive to the variation in the output of the oxygen partial pressure detection cell, pre-set data is read out from the memory. Based on the read-out data, the value of the offset of the second oxygen ion pump current is varied to correct the NOx gas concentration detection output. In a second aspect, the gas sensor is a NOx sensor. In a third aspect, the gas sensor is a sensor exploiting the oxygen ion pumping operation of the solid electrolyte. In a fourth aspect, it is assumed that the ratio of the air to the fuel supplied to the engine (air-to-fuel ratio, A/F ratio) corresponds to the amount of the moisture in the exhaust gases. Based on the air-to-fuel ratio, gas concentration detection signal and the true gas concentration value, a map containing correction data of the gas concentration detection signal corresponding to the air-to-fuel ratio is pre-formulated. From this pre-formulated map, pre-set correction data corresponding to the air-to-fuel ratio as determined by the engine operating conditions are read out. The gas concentration detection signal is corrected by the read-out correction data. In a fifth aspect, a specific or certain voltage is applied across the first oxygen ion pump cell to sufficiently pump oxygen in the measurement gas generally to an extent of not entirely decomposing NOx, or in some case to an extent of not decomposing NOx. A current having a parameter corresponding to the oxygen concentration in the measurement gas flows in the first oxygen ion pump current. The second oxygen ion pump cell decomposes NOx in the gas left over in pumping to pump out dissociated oxygen. A current corresponding to the NOx gas concentration flows in the second oxygen ion pump cell. A NOx gas concentration detection signal corresponding to the electric current value obtained from the second oxygen ion pump cell is corrected responsive to a signal corresponding to the electric current value obtained from the first oxygen ion pump cell. In a still further advantage, a residual gas is formed under the condition that allows and compensates for substantially decomposition of NO in the first flow channel or generally an upstream region in the flow channel. This condition is established such that the in-flowing NOx from the ambient gas compensates the decomposed amount of NO to bring an equilibrium state in the residual gas. The NOx is understood to be substantially the sum of NO and an amount of NO2 in an equilibrium determined generally by the temperature in which the rate of NO2 decreases as the temperature rises. For example, at room temperature it is almost NO2, at 300 to 400°C 50/50 and at 700°C or above NO2 is 5% or less. The sensor is preferably operated at about 700°C or above and up to about 900°C more preferably at 750 to 850°C. Thus the role of NO2 in NOx is relatively small or negligible under the carefully operated conditions, or NO2 may be regarded as NO under certain condition which will be explained later. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a cross-section along the short side direction of a NOx gas sensor according to an embodiment of the invention. Fig. 2 is a flowchart for illustrating the principle of detecting the nitrogen oxide concentration by a sensor like shown in Fig. 1. Figs. 3A and 3B are conceptual views for illustrating the reason the gain is changed by the oxygen concentration, where Figs. 3A and 3B illustrate a ratio of the gas components in each chamber with the oxygen concentration in the detection gas of 0% and more than 0%, respectively. Fig. 4 is a flowchart for illustrating a method for measuring the NOx gas concentration according to the operation of an embodiment of the present invention and a method for measuring the oxygen concentration and the air-to-fuel ratio. Fig. 5 is a graph that illustrates the dependency of the gain of the NOx gas concentration on the oxygen concentration in the detection gas. Fig. 6 illustrates a NOx gas sensor control device according to a comparative example. Fig. 7 is a block diagram for illustrating a NOx gas concentration measurement device according to a comparative example. . Fig. 8 is a block diagram for illustrating a NOx gas concentration measurement apparatus according to an embodiment of the present invention. Fig. 9 illustrates the results of measurement by the NOx gas concentration measurement apparatus in depence of a detection cell potential. Fig. 10 illustrates the results of measurement by the NOx gas concentration measurement apparatus in dependence of a 2 nd pumping voltage. Fig. 11 illustrates the results of measurement by the NOx gas concentration measurement apparatus according to an embodiment of the present invention. Fig. 12 shows the results of measurement of the concentration of exhaust gases of a 1.5L lean burn gasoline engine using the NOx gas measurement device according to a comparative example. Fig. 13 shows the results of measurement of exhaust gases of a 1.5 L lean burn gasoline engine using a NOx gas measurement apparatus according to an embodiment of the present invention. Fig. 14 illustrates a NOx gas sensor control apparatus according to another embodiment of the present invention. Fig. 15 is a schematic view for illustrating the structure of a NOx gas sensor employed in an embodiment of the present invention. Fig. 16 illustrates the cross-section indicated by arrow A in Fig. 15. Fig. 17 illustrates the layout of a NOx gas sensor shown in Fig. 15. Fig. 23 is a perspective view of the sensor for illustrating the position of the cross-section shown in Fig. 1. Fig. 24 is a diagram for illustrating the principle of measuring the NOx gas concentration. Fig. 25 illustrates a cross-section in a short side direction of a NOx gas sensor used in an embodiment of the present invention and schematics of a control system. Fig. 26 is a perspective view showing a cross-section in a longitudinal direction of the NOx gas sensor shown in Fig. 1. Fig. 27 shows a layout of a NOx gas sensor shown in Fig. 25. Fig. 28 illustrates a manufacturing example and the detailed layout of a NOx gas sensor used for measurement. Fig. 34 illustrates an example in which a NOx gas sensor according to an embodiment of the various aspects of the present invention is fitted to a metal body. Figs. 35A to 35D illustrate a NOx gas concentration sensor having two chamber according to further aspects of the present invention, wherein Fig. 35A is a cross-sectional view for illustrating such NOx gas concentration sensor, Fig. 35B is a longitudinal cross-sectional view thereof; Fig.35C is a schematic enlarged cross-sectional view of the first measurement chamber and Fig.35D is a plan view of the second measurement chamber. Fig. 36 illustrates a sensor of a reference example for the sensor shown in Figs. 35A-35D. Referring to the drawings, the principle of the first feature of the present invention is explained. The present invention involves a method for calibrating changes in the gain based on the oxygen concentration (or the amount of change of the nitrogen oxide concentration/amount of change of the second oxygen pump current) to measure the NOx gas concentration correctly. The present inventors have already proposed a sensor and a method for detecting the gas concentration, in particular the nitrogen oxide concentration. Fig. 1 and 2 show the schematic structure of the NOx gas sensor to which the method for measuring the NOx gas concentration according to the present invention is applied, and the process of detecting the NOx gas concentration by this sensor, respectively. The sensor of Fig. 1 has a first oxygen ion pumping cell 6 including two sets each of diffusion resistance portions, oxygen ion pumping cell and a measurement chamber and a pair of electrodes provided on both sides of a first solid electrolyte layer, an oxygen concentration measurement cell 7 including a pair of oxygen concentration detection electrodes provided on both sides of the second solid electrolyte layer, and a second oxygen ion pumping cell 8 including a pair of electrodes arranged on both sides of the third solid electrolyte layer. An insulating layer is formed between the solid electrolyte layers. Between the first oxygen ion pumping cell 6 and the oxygen concentration measurement cell 7 is formed a first measurement chamber 2 defined by an insulating layer and a solid electrolyte layer. Similarly, by an insulating layer and a solid electrolyte layer, a second measurement chamber 4 is defined above the second oxygen ion pumping cell 8. In a wall surface surrounding the first measurement chamber 2 are formed plural first diffusion holes (portion) 1 having a diffusion resistance. In a midportion of the first measurement chamber 2 is formed a second diffusion hole (portion) 3 in a spaced-apart relation from the first diffusion holes 1. The second diffusion hole 3 is passed through the oxygen concentration measurement cell 7 and the solid electrolyte layer to provide communication between the first and second measurement chambers 2 and 4 with a diffusion resistance. In the sensor shown in Fig. 1, the process of detecting the NOx gas concentration in the exhaust gas is as shown in Fig. 2, steps 201 to 205. Thus, the NOx gas concentration can be found by exploiting the fact that the second oxygen pump current IP2 is proportionate to the amount of oxygen yielded on decomposition of NOx. Meanwhile, the first diffusion resistance and the second diffusion resistance correspond in Fig. 1 to a gas diffusion resistance possessed by the first diffusion holes 1 and the second diffusion hole 3, respectively. As a practical matter, in a low oxygen concentration atmosphere lower than a specific value, oxygen cannot be pumped out completely from the first measurement chamber due to constraint such as NOx decomposition. Therefore, the oxygen pumped out from the second measurement chamber is both oxygen yielded on NOx decomposition in the second measurement chamber and oxygen not pumped in the first measurement chamber and diffused into the second measurement chamber. That is, the current flowing in the second oxygen ion pumping cell is affected by both the residual oxygen concentration in the second measurement chamber and the NOx gas concentration. Therefore, the effect of residual oxygen needs to be precluded for enabling correct measurement of the NOx gas concentration. Thus, it may be contemplated to use different values of 'offset' depending on the oxygen concentration. Specifically , (i) for various values of oxygen concentration, the nitrogen oxide concentration is set to zero, and the amount of the current flowing at this time through the second oxygen ion pumping cell (this amount of current being hereinafter referred to as 'offset') is measured, and (ii) the current flowing at this time through the second oxygen ion pumping cell is measured using a detection gas having a known standard NOx gas concentration. The 'gain' of the variation of the second oxygen ion pumping cell current is thus determined throughout the various values of oxygen concentration. This gain is given by the following equation: 'Gain' = (standard NOx gas concentration)/(generated current amount - offset) The values of the offset, thus found, varied with the oxygen concentration, and a perpetually constant gain value, are pre-stored in storage means, such as memory. During measurement, the offset values and the gain as well as the current flowing in the second oxygen ion pumping cell are entered to a micro-computer to calculate the NOx gas concentration. Since the offset is varied with the oxygen concentration, as described above, pre-set values that correlate with the oxygen concentration values are pre-stored as a map. Depending on the output of the oxygen concentration measurement cell, pre-set data (offset values) are read out from this map to correct or calculate the NOx gas concentration. However, the present inventors have found, as a result of this investigations, that gain calibration according to oxygen concentration is required for correct measurement of the NOx gas concentrations occurring on the order of several ppm. The reason the gain is varied by the oxygen concentration is possibly as follows. Figs. 3(A) and 3(B), conceptually show one reason why the gain is changed by the oxygen concentration, and also show the change in the proportions of the gas concentration between the first and second measurement chambers. In the sensor shown in Fig.A1, oxygen is pumped by the first oxygen ion pumping cell from the first measurement chamber so that the concentration of oxygen flowing from the first measurement chamber into the second measurement chamber will be equal to the concentration prescribed in the oxygen concentration measurement cell (constant electromotive force). The following proportions prevail as the proportions of the oxygen concentration of respective components (NO, CO and the like) flowing into the second measurement chamber. Referring to Fig. 3A, if the oxygen concentration in the measurement gas is zero, there is no oxygen pumped out by the first oxygen ion pumping cell, so that the proportions of the gas concentration of various components flowing into the second measurement chamber are not changed. However, referring to Fig. 3B, the amount of oxygen pumped out by the first oxygen ion pumping cell is increased with increased oxygen concentration in the gas being detected. As a result, the proportions of the various gas components flowing into the second measurement chamber are increased corresponding to the decreased amount of oxygen as compared to that of the detection gas. That is, the higher the oxygen concentration in the detection gas, the higher becomes the NOx gas concentration in the second measurement chamber. Thus, the amount of the current in the second oxygen ion pumping cell proportionate to the amount of oxygen yielded on NOx decomposition is increased to increase the sensor sensitivity to the NOx gas concentration (the gain is lowered). For more correct calculation of the NOx gas concentration, the gain lowering caused by the increased sensor sensitivity cannot be disregarded to render it necessary to make corrections. The present inventors provide means for correcting the gain in response to the oxygen concentration in the detection gas based on the foregoing information. Referring to Figs. 2, 6 and 26, the principle pertaining to the second feature of the present invention and the invention underlying this principle are explained. Fig. 6 is a transverse cross-sectional view for illustrating the schematic structure of a NOx gas sensor to which the control method according to the second feature of the present invention is advantageously applied. The transverse cross-section of Fig. 6 corresponds to a cross-section of the sensor shown as a longitudinal cross-section in Fig. 26. The sensor of Fig. 6 has a first oxygen ion pumping cell 6, including two sets each of diffusion resistance portions, an oxygen ion pumping cell and a measurement chamber and a pair of electrodes 6a, 6b provided on both sides of a first solid electrolyte layer. An oxygen concentration (partial pressure) measurement cell 7 including a pair of oxygen concentration detection electrodes 7a, 7b provided on both sides of the second solid electrolyte layer, and a second oxygen ion pumping cell 8 including a pair of electrodes 8a, 8b arranged on both sides of the third solid electrolyte layer, also are used. These layers are laminated in order and an insulating layer is formed between the solid electrolyte layers. Between the first oxygen ion pumping cell 6 and the oxygen concentration measurement cell 7 is formed a first measurement chamber 2 by an insulating layer and the solid electrolyte layers. Similarly, by an insulating layer and the solid electrolyte layers, a second measurement chamber 4 is defined above the second oxygen ion pumping cell 8. In a wall surface surrounding the first measurement chamber 2 are formed plural first diffusion holes 1 having a diffusion resistance. In a mid portion of the first measurement chamber 2 is formed a second diffusion hole 3 in a spaced-apart relation from the first diffusion holes 1. The second diffusion hole 3 is passed through the oxygen concentration measurement cell 7 and the solid electrolyte layers to provide communication between the first and second measurement chambers 2 and 4 with a diffusion resistance. Actually, in a low oxygen concentration atmosphere lower than certain specific value, oxygen cannot be pumped out completely in the first measurement chamber due to a constraint of NOx decomposition. Therefore, the oxygen pumped out from the second measurement chamber is made up of both an oxygen portion yielded on NOx decomposition in the second measurement chamber and an oxygen portion that was not pumped out of the first measurement chamber and was diffused into the second measurement chamber. That is, the current flowing in the second oxygen ion pumping cell is affected by both the residual oxygen concentration in the second measurement chamber and the NOx gas concentration. Therefore, the effect of residual oxygen needs to be precluded for enabling correct measurement of the NOx gas concentration. Thus, according to the invention which forms the basis with respect to the second feature of the present invention, the effect of oxygen is precluded in the following manner. That is, (i) measurement gases in which the nitrogen oxide concentration is previously set to 0 and the oxygen concentration has various different values (gases with various value of the oxygen concentration) are charged into a measurement unit and measurement is made on the current flowing in the second oxygen ion pumping cell (this current is hereinafter referred to as 'offset'); (ii) the measurement gas with the standard NOx gas concentration is charged into a measurement device and measurement is made on the current flowing in the second oxygen ion pumping cell; and (iii) from these measured values, the 'gain' of the variation of the second oxygen ion pump current is set. This gain is given by the following equation: 'Gain' = (standard NOx gas concentration)/(generated current amount - offset) The value of the offset, thus found, varied with the oxygen concentration, and the gain values, are pre-stored in storage means, such as memory. During measurement, the offset and the gain as well as the current flowing in the second oxygen ion pumping cell are entered to a micro-computer to calculate the NOx gas concentration . However, it has turned out there is a problem as follows. Namely, if the oxygen concentration in the measurement gas varies, the oxygen concentration in the first measurement chamber is not constant due to, for example, control delay, even if it is attempted to pump out oxygen by the first oxygen ion pumping cell to provide a constant oxygen concentration in the first measurement chamber using a control method such as PID. Thus, there may be an occurrence that the oxygen concentration in the first measurement chamber is lowered excessively, such that NOx is decomposed in the first measurement chamber and an output is decreased when measuring NOx in the second measurement chamber, or that the oxygen concentration is raised. In such case, excess oxygen which should inherently be pumped out in the first measurement chamber is also measured, with a result that an output is increased to render it impossible to measure the NOx quantity accurately. The control method and apparatus according to the second feature of the present invention is explained by contrasting it to the control method and apparatus according to the comparative example. First, the control method and apparatus according to a comparative example is explained with reference to Figs. 6 and 7. The structure of component parts shown in Fig. 6 is similar to that shown in Fig. 1. In Fig. 6, a reference electrode 7b of the oxygen concentration measurement cell 7 is electrically connected to an input terminal of a differential amplifier 30 to enter the output voltage of the oxygen concentration measurement cell 7. To an output terminal of the differential amplifier 30 is connected an input terminal of a controller 31. The controller 31 performs PID control based on the offset of the voltage of the reference power source from the electromotive force of the oxygen concentration measurement cell 7 to control the voltage applied across a pair of electrodes 6a, 6b of the first oxygen ion pumping cell 6 so that the electromotive force of the oxygen concentration measurement cell 7 will be equal to the voltage of the reference power source. The control apparatus according to the comparative example shown in Fig. 7 has a constant- Vs-controller 41 equivalent to the differential amplifier 30 and the controller 31 shown in Fig. 6, and a controller 40 having first and second oxygen ion pump currents as inputs and the oxygen concentration and NOx concentration values as outputs. The constant-Vs-controller 41 has the output of the oxygen concentration measurement cell 7 as input and controls the voltage applied across the first oxygen ion pumping cell 6 to render the electromotive force of the oxygen concentration measurement cell 7 constant. The controller 40 has, as inputs, the first oxygen ion pump current flowing in the first oxygen ion pumping cell 6 and the second oxygen ion pump current flowing in the second oxygen ion pumping cell 8. An oxygen concentration calculation unit 40a of the controller 40 finds the oxygen concentration based on the first oxygen ion pump current and outputs the resulting oxygen concentration while outputting an oxygen concentration correction signal. A NOx concentration calculation unit 40b of the controller 40 has, as input, the oxygen concentration calibration signal and the second oxygen ion pump current, and outputs a nitrogen oxide concentration value corrected for oxygen concentration based on the input signals. That is, the system shown in Fig. 7 presupposes the constant oxygen partial pressure (concentration) steady state and executes the oxygen concentration correction for detecting the NOx gas concentration based on the first oxygen ion pump current. On the other hand, in the control device for the NOx gas sensor according to the second feature of the present invention, shown in Fig. 8, an oxygen concentration detection output (oxygen concentration detection voltage) of the constant-Vs-controller 41 is directly entered to the NOx concentration calculation unit 40b, which then corrects the nitrogen oxide concentration for oxygen concentration based further on the rate of change (variation) of the detection output of the oxygen concentration (detected voltage of the oxygen concentration) in addition to the above-mentioned oxygen concentration correction signal and the second oxygen ion pump current. It is possible with this control device to correct the detection output of the NOx gas concentration responsive to the rate of change (variation) of the oxygen concentration even if the oxygen concentration in the measurement gas is fluctuated significantly (in a non-steady state of the oxygen partial pressure), to preclude the effect of fluctuations of the oxygen concentration for accurately detecting the NOx gas concentration. Referring to Fig. 4, the method for measuring the NOx gas concentration, oxygen concentration and air-to-fuel ratio according to an embodiment of the first feature of the present invention is explained. First, using a detection gas having known oxygen concentration and NOx concentration values, the gain and the offset of the NOx gas concentration corresponding to the oxygen concentration, gain of the oxygen concentration (electric current value per a standard oxygen concentration), and offset (first oxygen pump current for 0% oxygen concentration) are found. Next, a detection gas having unknown NOx gas concentration etc. are measured. Referring to Fig. 4, the first oxygen pump current IP1 is measured (step 401). Then, based on IP1, the oxygen concentration is calculated and found (411 to 412). An air-to-fuel ratio is then calculated and outputted (421 to 422). Also, responsive to the oxygen concentration, calculated from IP1, the offset and the gain of the NOx gas concentration, previously found, are read out and, using these read-out values and the measured value of the second oxygen pump current IP2, a NOx gas concentration is calculated and outputted (steps 402 to 407). The sequence of calculations shown in Fig. 4 can be executed by a micro-computer connected to a sensor. Alternatively, a measurement device, such as ammeter, may be connected to the sensor and calculations may be executed based on the displayed results, too. Alternatively, gain correction of the NOx gas concentration, exhibiting certain specific oxygen concentration dependency, may be executed by utilizing the output of the first oxygen ion pumping cell and by varying the amplification factor of a NOx gas concentration output circuit using an analog circuit. Next, a preferred embodiment for the second feature of the present invention is explained. That is, PID control is performed by the circuit configuration shown in Fig. 6, so that the electromotive force generated in the oxygen partial pressure detection electrodes in the first measurement chamber will be constant. If at this time a control target voltage of the oxygen partial pressure detection cell (setting of the electromotive force generated in the oxygen partial pressure detection cell) is changed, the second oxygen ion pump current is changed. Therefore, offsets of the second oxygen ion pump current relative to the voltage setting of the oxygen partial pressure detection cell are previously measured and entered as a map in a memory of a controller or the like. If, during rapid changes in the oxygen concentration in the measurement gas, an electromotive force generated across the oxygen partial pressure detection cell is changed, an offset value corresponding to this electromotive force is read out to increase or decrease this amount of offset with respect to the second oxygen ion pump current for enabling more accurate detection of the NOx gas concentration. The oxygen partial pressure detection cell for controlling the oxygen concentration in the first measurement chamber may be used for performing the correction. Also preferably, a reference electrode for the oxygen partial pressure detection cell may be provided in a reference oxygen chamber to cause a minor current to flow to assure a constant oxygen concentration in the atmosphere around the reference electrode to enable correct detection of the oxygen concentration in the measurement chamber. FIRST EMBODIMENT A first embodiment according to the first feature of the present invention is explained in detail. In the present embodiment, the NOx gas concentration was found by the following correcting method, using the NOx gas sensor shown in Figs. 25 to 28. The common measurement conditions were as follows: temperature of the measurement gas: 300°C gas composition: NO (0 to 1500 ppm); 02 (0 to 16%); CO2 10%; balance being N2 heater power: 18 to 25 W (20 W corresponds to 800°C in terms of the sensor temperature) Preliminarily, the values of the second oxygen ion pump current for the NOx gas concentration at substantially zero and for a pre-set concentration (1500 ppm) were measured at various oxygen concentration values. Using measured values of the second oxygen ion pump current as obtained for various values of the concentration of the injected NO at the pre-set oxygen concentration, gain values for various oxygen concentration values (=variation of the NOx gas concentration/variation of the second oxygen ion pump current) was found by the least square method. Table 1 and Fig. 5 show these results. Referring to Fig. 5, it is seen that the larger the oxygen concentration, the lower becomes the gain, so that, for finding the correct NOx gas concentration, it is necessary to correct the gain by oxygen concentration. The method for finding the coefficients of the equation for calculation in order to calculate an adequate gain by substitution of an arbitrary oxygen concentration using the gain values at each oxygen concentration shown in Table 1 is hereinafter explained. RELATION OF OXGEN CONCENTRATION AND GAIN O 2 [%] Gain [ppm/µA] 0 134.520 1 134.040 4 129.300 7 124.872 10 122.150 16 116.144 20 110.400 The methods for correcting the NOx gas concentration 1 to 6 (Examples 1 to 6) obtained on the basis of the second oxygen ion pump current are hereinafter explained. Example A1: Correcting method 1 The oxygen concentration in the detection gas was set to 0, 1, 7 and 16%, the injected NO concentration was set to 0, approximately 500, approximately 1000 and approximately 1500 ppm and, for each combination of these conditions, the second oxygen ion pump current was measured. Then, assuming that the relation between the gain and the oxygen concentration is represented by the first-order equation of the least square method (case of n=1 in equation (4) as later explained) Gain=Gain0+Gainc x O2concent Gain0 : VALUE OF GAIN FOR OXYGEN CONCENTRATION =0% Gainc : GRADIENT The gain at the pre-set oxygen concentration shown in Table 1 was substituted in the equation (1) and the above coefficients (GAINo, GAINc) were determined using the least square method. The value of the gain as found by substituting an optional oxygen concentration in the equation (1) and the value of ΔIp2 shown in the following Table B2 were substituted into the equation (2), respectively, in order to find the NOx gas concentration. NOx=Gain x ΔIp2 where ΔIp2=Ip2measured-Ip2offset Ip2 measured: VALUE OF 2ND OXYGEN PUMP CURRENT (MEASUREMENT) Ip2 offset :VALUE OF 2ND OXYGEN PUMP CURRENT FOR NOx=0% and O2=0% For comparison, the gain values for the oxygen concentration values of 0, 1, 7 and 16% were averaged and, using the resulting averaged value as a coefficient of the equation (2), the NOx gas concentration was found. Table 2 shows the above measured value, calculated value, difference between the measured and calculated values, difference between the true value of the NOx gas concentration (injected NO concentration (A)) and the calculated value (B), and the results of the Comparative Example. CALUCULATION VALUE OF NOx CONCENTRATION AND ERROR BETWEEN CALUCUATION VALUE AND TRUE VALUE IN EXAMPLE A1 AND COMPARATIVE EXAMPLE O 2 [%] SET NO [ppm] (A) ΔIp2[µA] EXAMPLE A1 NOx[ppm](B) (A)-(B) COMP. EX. NOx[ppm](C) (A)-(C) 1 0 0.00 0 0 0 0 1 494 3.72 494 0 462 32 1 979 7.25 963 16 900 79 1 1469 10.95 1455 14 1359 110 7 0 0.00 0 0 0 0 7 485 4.15 513 -28 515 -30 7 967 7.95 982 -15 987 -20 7 1444 11.92 1472 -28 1480 -36 16 0 0.00 0 0 0 0 16 482 4.27 468 14 530 -48 16 958 8.50 930 28 1055 -97 16 1425 12.80 1399 26 1589 -164 EXAMPLE A1: Gain o =134.444 COMP. EX.: Gain=124.124 Gain c = -1.633 It is seen from Table 2 that the concentration can be measured more accurately with the present correction method 1 (in particular, a lean oxygen area). Example A2: Correction method 2 It is assumed that the relation between the gain and the oxygen concentration is represented by the following polynominal of the least square method of the following equation (4). where i SHOWS DEGREE, IN PARTICULARLY Gaini FOR i=0 SHOWS VALUE OF GAIN FOR O 2 =0% In the above equation (4), the gain value for the pre-set oxygen concentration shown in Table 1 for i=2 was substituted and, using the least square method, the above coefficients (GAINi: i = 0 to 2) were determined. Subsequently, the NOx gas concentration was found in the same way as in the correction method 1. Table 3 shows the above measured values, calculated values, difference between the true values of the NOx gas concentration (injected NO concentration (A)) and the calculated values (B) and the results of the Comparative Example (same as that shown in Table 2). It is seen from Table 3 that the NOx gas concentration can be found more accurately by the present correction method 2. CALUCULATION VALUE OF NOx CONCENTRATION AND ERROR BETWEEN CALUCUATION VALUE AND TRUE VALUE IN EXAMPLE A2 AND COMPARATIVE EXAMPLE O 2 [%] SET NO [ppm] (A) ΔIp2[µA] EXAMPLE 2 NOx[ppm](B) (A)-(B) COMP.EX. NOx[ppm](c) (A)-(C) 1 0 0.00 0 0 0 0 1 494 3.72 487 7 462 32 1 979 7.25 949 30 900 79 1 1469 10.95 1434 35 1359 110 7 0 0.00 0 0 0 0 7 485 4.15 511 -26 515 -30 7 967 7.95 978 -11 987 -20 7 1444 11.92 1467 -23 1480 -36 16 0 0.00 0 0 0 0 16 482 4.27 475 7 530 -48 16 958 8.50 945 13 1055 -97 16 1425 12.80 1423 2 1589 -164 EXAMPLE A2: Gain(i=0)=132.252 COMP.EX.: Gain=124.124 Gain(i=1)= -1.523 Gain(i=2)= 0.152 Example A3: Correction Method 3 The oxygen partial pressure was set to 10 -3 , 0.01, 0.07 and to 0.16 and the injected NO concentration was set to 0, approximately 500, approximately 1000 and approximately 1500. For each combination of these conditions, the second oxygen ion pump current was measured, respectively. Meanwhile, the oxygen partial pressure corresponds to the concentration of the above correction methods 1 and 2, expressed in terms of the partial pressure, while measured values of the injected NO concentration and the second oxygen ion pump current and the comparative example are the same as those shown in the above correction methods 1 and 2. It was assumed that the relation between the gain and the logarithm of the oxygen partial pressure was represented by the following equation (5) : Gain=Gain0+Gainc x log (p02) Gain0:VALUE OF GAIN FOR OXYGEN PARTICIAL PRESSURE =1 Gainc:GRADIENT The NOx gas concentration was found by the equation (2) by a method similar to the correction method 1 except that representation by partial pressure is used in place of that by percentage (%). Table 4 shows the above measured values, calculated values, difference between true values of the NOx gas concentration (injected NO concentration (A)) and the calculated value (B) and the results of comparative example. It is seen that the concentration can be measured more accurately by the present correction method 3. CALUCULATION VALUE OF NOx CONCENTRATION AND ERROR BETWEEN CALUCUATION VALUE AND TRUE VALUE IN EXAMPLE A3 AND COMPARATIVE EXAMPLE O 2 [%] SET NO [ppm] (A) ΔIp2[µA] EXAMPLE A3 NOx[ppm](B) (A)-(B) COMP.EX. NOx[ppm](c) (A)-(C) 1 0 0.00 0 0 0 0 1 494 3.72 422 72 462 32 1 979 7.25 822 157 900 79 1 1469 10.95 1448 21 1359 110 7 0 0.00 0 0 0 0 7 485 4.15 441 44 515 -30 7 967 7.95 844 123 987 -20 7 1444 11.92 1575 -131 1480 -36 16 0 0.00 0 0 0 0 16 482 4.27 440 42 530 -48 16 958 8.50 876 82 1055 -97 16 1425 12.80 1690 -265 1589 -164 EXAMPLE A3: Gain o = 96.1 COMP.EX.: Gain=124.124 Gain c = -8.56 Example A4: Correction method 4 Next, gain values for a number of oxygen concentration values were found by a method similar to that for finding the gain for each value of oxygen concentration shown in Table 1. The results are shown in Table 5: RELATION OF OXGEN CONCENTRATION AND Gain O 2 [%] Gain [ppm/µA] O 2 [%] Gain [ppm/µA] 0 134.520 11 121.252 1 134.040 12 120.059 2 131.987 13 118.866 3 130.794 14 117.674 4 129.300 15 116.481 5 128.408 16 116.144 6 127.216 17 114.095 7 124.872 18 112.903 8 124.830 19 111.710 9 123.637 20 110.400 10 122.150 21 109.324 In distinction from the correction method 1, the values of the gain for respective oxygen concentration values shown in Table 5 were substituted into the above equation (2) to find the NOx gas concentration. That is, the values of the gain (Table 5) for optional oxygen concentration values were stored and the stored gain values were read out depending on the oxygen concentration in the detection gas. The NOx gas concentration was calculated from the product of the read-out gain value and ΔIp2. The comparative Example is the same as that shown in Table 2. Table 6 shows the above measured values, calculated values, difference between the true values of the NOx gas concentration (injected NO concentration (A)) and the calculated values (B) and the results of the Comparative Example. It is seen from Table 6 that the NOx gas concentration can be measured more accurately by the present correction method 4. CALUCULATION VALUE OF NOx CONCENTRATION AND ERROR BETWEEN CALUCUATION VALUE AND TRUE VALUE IN EXAMPLE A4 AND COMPARATIVE EXAMPLE O 2 [%] SET NO [ppm] (A) ΔIp2[µA] EXAMPLE A4 NOx[ppm)(B) (A)-(B) COMP.EX. NOx[ppm](c) (A)-(C) 1 0 0.00 0 0 0 0 1 494 3.72 499 -5 462 32 1 979 7.25 972 7 900 79 1 1469 10.95 1468 1 1359 110 7 0 0.00 0 0 0 0 7 485 4.15 523 -38 515 -30 7 967 7.95 1002 -35 987 -20 7 1444 11.92 1502 -58 1480 -36 16 0 0.00 0 0 0 0 16 482 4.27 496 -14 530 -48 16 958 8.50 987 -29 1055 -97 16 1425 12.80 1487 -62 1589 -164 EXAMPLE A4: Gain= the value of COMP.EX.: Gain=124.124 Gain in TABLE A5 Example 5 The oxygen concentration in the detection gas can be found by measuring the current flowing in the first oxygen ion pumping cell. Thus, oxygen and NO of the pre-set concentrations were injected and measurement was made of the first oxygen ion pump current Ip1 flowing at a zero oxygen concentration. The relation of proportionality according to the equation (6) exists between the oxygen concentration in the detection gas and the first oxygen ion pump current: O2 [%] = (Ip1-BO2) /GO2 B02:VALUE OF 1ST OXYGEN PUMP CURRENT FOR O2=0% G02:AMOUNT OF CURRENT/UNIT OF OXYGEN CONCENTRATION Table 7 shows the results of the oxygen concentration as found by the above equation (6). From Table 7, the oxygen concentration can be found correctly to an error value of the order of 0.2% by the present method. That is, even if the oxygen concentration in the detection gas is unknown, the oxygen concentration can be found correctly from the first oxygen pump current so that the gain and the offset for measuring the NOx gas concentration in meeting with the oxygen concentration in the detection gas can be determined correctly to find the NOx gas concentration correctly. This also indicates that the present sensor can measure the NOx gas concentration and the oxygen concentration simultaneously. Moreover, these measured values can be used to find the air-to-fuel ratio. CALUCULATION VALUE OF OXYGEN CONCENTRATION USING 1ST OXYGEN PUMPING CURRENT SET O 2 [%] SET NO [ppm] (A) Ip1[mA] O 2 (CAL.) [%] 0.96 0 0.350 1.09 6.75 0 2.413 6.55 6.68 485 2.392 6.49 15.34 958 5.666 15.15 SECOND EMBODIMENT A second embodiment incorporating the second feature of the present invention is explained. The NOx gas sensor of the present embodiment represents an application of the NOx gas sensor explained with reference to Figs. 15 and 19 to 21A and 21B to the control configuration shown in Fig. 8. (Measurement Example B1) First, measurements were carried out of the gain and the offset of the second oxygen ion pump current when a measurement gas containing 1500 ppm of NO and 7% of oxygen was injected into the control system, with the second oxygen ion pumping cell potential remaining constant, and the setting voltage of an oxygen partial pressure detection cell was varied. The results are sown in Fig. 9. Also, measurements were made of the gain and the offset of the second oxygen ion pump current when the measurement gas containing 1500 ppm of NO and 7% of oxygen was injected, with the setting voltage of the oxygen partial pressure detection cell remaining constant, and the second oxygen ion pumping cell potential was varied. The results are shown in Fig. 10. It is seen from Fig. 9 that, if the setting voltage of the oxygen partial pressure detection cell is lowered, the oxygen left in the measurement gas diffused into a second measurement chamber is increased to increase the second oxygen ion pump current, whereas, if the setting voltage is increased, the second oxygen ion pump current is decreased. On the other hand, it is seen from Fig. 10 that if the setting voltage of the oxygen partial pressure detection cell is constant, with the oxygen concentration in the first measurement chamber being kept constant, the second oxygen ion pump current is decreased or increased as the second oxygen ion pump current is lowered or raised, respectively. Therefore, if characteristics shown in Figs. 9 and 10 are exploited and the second oxygen ion pumping cell voltage is (a)lowered or (b)raised when the electromotive force generated in the oxygen partial pressure detection cell is (a)high or (b)low, respectively, the effects of oxygen in the measurement gas and changes in the oxygen concentration cancel each other to permit accurate measurement of the concentration of the NOx gas in the measurement gas. Fig. 11 shows the relation of the second oxygen ion pump current versus the difference (variation) of the potential between the reference potential (450 mV) and a measured potential of the electrode 7b of the oxygen partial pressure detection cell (Fig. 6) and the second oxygen ion pump current. It is seen from Fig. 11 that a substantially linear relation exists between the two. Thus, by holding the relation in a memory of a controller 40 (map formulation) and by reading out a pre-set offset corresponding to changes in the electromotive force in the oxygen partial pressure detection cell caused by rapid changes in the oxygen concentration in the measurement gas, and further by increasing or decreasing the measured second oxygen ion pump electric current value depending on the read-out offset value, the correct value of the NOx gas concentration can be found based on the corrected second oxygen ion pumping cell output. (Measurement Example B2) The previously-described control device (see Fig. 8) was applied to a lean burn gasoline engine vehicle of 1.5 L to measure the NOx gas concentration in the exhaust gases. As a comparative example, similar measurements were conducted using a controller without correction by an output of the oxygen partial pressure detection cell (see Fig. 7). The setting voltage of the oxygen partial pressure detection cell was set to 450 mV. Simultaneously, true values were measured using an analyzer based on the FTIR method (the analyzer output is indicated at (3) FTIR). Figs. 12 and 13 show the measured results of the Comparative Example and the Example, respectively. It is seen from Figs. 12 and 13 that, in actual vehicle running, the oxygen concentration is varied with time such that control delay occurs in the control operation of controlling the oxygen partial pressure detection cell aiming at a constant potential. Fig. 12 shows that, if the oxygen partial pressure detection cell current is varied suddenly, the NOx sensor outputs an abnormal value (based on the second oxygen ion pump current) thus indicating that the NOx sensor output (2) differs significantly from the analyzer output (3) such that measured values of the NOx gas concentration were not correct. On the other hand, Fig. 13 shows that, by reading out a pre-set offset corresponding to the difference in potential (changes in the electromotive force) between the measured potential of the electrode 7b of the oxygen partial pressure detection cell (see Fig. 6) and the reference potential (450 mV) and by performing the correction of increasing or decreasing the measured second oxygen ion pump electric current value based on the read-out offset value, the NOx sensor output peaks on occurrence of abrupt changes in the oxygen partial pressure detection cell potential assume heights less than one-half as compared to Fig. 12, with the NOx sensor output (2) being substantially equivalent to the analyzer output (3), which demonstrates that measured values of the NOx gas concentration were correct. In the above-described embodiment, the correction is carried out in the inside of the controller 40 shown in Fig. 8. Alternatively, a pre-set circuit may be added to the system shown in Fig. 6 (a pre-set circuit may be added to the sensor used in the above embodiment). Fig. 14 illustrates a control system according to a further embodiment of the third feature of the present invention. The system shown in Fig. 14 differs from the system shown in Fig. 6 in that an amplifier 32 is arranged between the reference electrode 7b of the oxygen detection cell 7 and the outer electrode 8b of the second oxygen ion pumping cell. The system of Fig. 14 varies the voltage applied across the second oxygen ion pumping cell responsive to the difference (changes in potential) between the potential produced across the electrode 7b (see Fig. 6) of the oxygen partial pressure detection cell and the reference potential to effect the above controller by the hardware in place of the software type correction. That is, the pump voltage applied across the second oxygen ion pumping cell is controlled so that the pump voltage will be varied as the potential of the oxygen partial pressure detection cell varies. It was found by a measurement test similar to that of the above embodiment that, with the system shown in Fig. 14, correct NOx gas concentration measurement is similarly possible as in the above embodiment as compared to the case in which the voltage applied across the second oxygen ion pumping cell is controlled to a constant value. THIRD EMBODIMENT An embodiment according to the third feature of the present invention is explained. In the present embodiment, the NOx sensor having a structure shown in Fig. 15 is used. The NOx sensor of Fig. 15 includes a first oxygen ion pumping cell 6, having a pair of electrodes 6a, 6b provided on both sides of a solid electrolyte layer 5-1, an oxygen concentration measurement cell 7 having a pair of oxygen partial pressure detection electrodes 7a, 7b, provided on both sides of a solid electrolyte layer 5-2, and a second oxygen ion pumping cell 8 having a pair of electrodes 8a, 8b provided on the surfaces of a solid electrolyte layer 5-3 and a solid electrolyte layer 5-4, stacked in this order. Between the solid electrolyte layers 5-1, 5-2, 5-3 and 5-4 are formed insulating layers 11-1, 11-2 and 11-3, respectively. Between layers of the first oxygen ion pumping cell 6 and the oxygen concentration measurement cell 7 is defined the first measurement chamber (gap portion) 2 by the left and right side insulating layer 11-1 and the upper and lower side solid electrolyte layers 5-1 and 5-2. Similarly, the second measurement chamber 4 (gap portion) is defined above the second oxygen ion pumping cell 8 by the insulating layer 11-3 and the solid electrolyte layers 5-3 and 5-4. Moreover, first diffusion hole 1 having a diffusion resistance (diffusion resistance portion) is provided on each side in the short side direction of the sensor (front and back sides in Fig. 15) on one side in the first measurement chamber 2. On the other side in the first measurement chamber 2 is formed with an opening of the second diffusion hole 3 (diffusion resistance portion) separated from the first diffusion holes 1. The second diffusion hole 3 is passed through the oxygen concentration measurement cell 7 and the solid electrolyte layer 5-3 to establish communication between the first and second measurement chambers 2 and 4 with a diffusion resistance. In the present sensor, electrodes 8a, 8b of porous metal (such as Pt or Rh alloys) are formed on the same surface of the solid electrolyte layer 5-4 making up the second oxygen ion pumping cell 8. Although the electrodes 8a, 8b are isolated from each other via the insulating layer 11-3, oxygen ions are conducted via the solid electrolyte layer 5-4 to cause the second oxygen pump current Ip2 to flow by this oxygen ion conduction. The electrode 8b is prohibited by the solid electrolyte layer 5-4, insulating layer 11-3 and a lead (line) 8d from direct contact with the sensor atmosphere. Moreover, oxygen pumped out by the second oxygen ion pumping cell 8 can be led to outside via porous lead 8d having a diffusion resistance. Moreover, leads 8c (see Fig. 16) and 8d (see Fig. 16) are electrically connected to the electrodes 8a, 8b, while the lead 8d electrically connected to the outer electrode 8b of the second measurement chamber 4 is porous to permit oxygen ion diffusion. Thus, oxygen decomposed by the NOx gas and pumped from the electrode 8a to the electrode 8b is discharged via lead 8d. Fig. 16 shows the planar cross-section taken along arrow line A in Fig. 16. It is seen by referring to Fig. 16 that the lead 8d is contacted with outside air (atmospheric air or ambient atmosphere of the measurement gas) for communication between outside air and the electrode 8b via a diffusion resistance. Manufacturing Example C Next, a manufacturing method for a NOx sensor shown in Fig. 15 is explained. Fig. 17 is a layout view of the NOx sensor shown in Fig. 15. Although the sheet and the paste shown in Fig. 17 are in the green state, the same reference numerals as those used for the NOx sensor shown in Fig. 15 are used. The ZrO green sheet and the paste for the electrode are layered from upper left to lower left and then from upper right to lower right in Fig. 17, dried and fired to form an integral sensor. The paste materials, such as insulating coat or electrodes, are layered by screen printing on a pre-set ZrO2 green sheet. The manufacturing example of the component parts such as ZrO2 green sheet is as described before. As shown in the layout drawing of Fig. 27, this NOx gas sensor is fabricated by stacking and firing green sheets formed of an oxygen ion conducting solid electrolyte material. On the other hand, porous electrodes are formed by screen printing in the green sheet. Using this NOx gas sensor, a test for measuring the NOx gas concentration in the measurement gas was carried out. Referring to Fig. 28, a manufacturing example and the layout of the NOx gas sensor used for measurement are explained in detail. MANUFACTURING EXAMPLE D Referring to Fig. 28, ZrO2 green sheets and pastes for electrodes and so forth are laminated from upper left to lower left, then to upper right and finally to lower right to fabricate a unitary detector. Insulating coatings, electrodes or the like paste materials are laminated by screen printing on a pre-set ZrO2 green sheet. A manufacturing example for manufacturing various components such as ZrO2 green sheet is now explained. Molding ZrO2 Sheet ZrO2 powders were calcined in an atmospheric oven at 600° C for two hours. 30 kg of the calcined ZrO2 powders, 150 g of a dispersant and 10 kg of an organic binder were charged into a trommel along with 60 kg of balls. The resulting mass was mixed for about 50 hours for dispersion and added to with 4 kg of an organic binder dissolved in 10 kg of an organic solvent. The resulting mass was mixed for 20 hours to produce a slurry having a viscosity of 10 Pa·s. From this slurry, a ZrO2 green sheet about 0.4 mm thick was fabricated and dried at 100°C for one hour. Paste for Printing (1) For first oxygen ion pump electrode 6a, an oxygen partial pressure detection electrode (oxygen reference electrode b) 7b and second oxygen ion pump electrodes 8a, 8b: 20 g of platinum powders, 2.8 g of ZrO2 powders and a suitable quantity of the organic solvent were charged into a crusher (or a pot mill), mixed for four hours for dispersion and added to with 2 g of an organic binder dissolved in 20 g of the organic solvent. The resulting mass was added to with 5 g of a viscosity adjustment agent and mixed for four hours to produce a paste with a viscosity of the order of 150 Pa·s. (2) For first oxygen ion pump electrode 6b, oxygen partial pressure detection electrodes (oxygen reference electrode a) 7a: 19.8 g of platinum powders, 2.8 g of ZrO2 powders, 0.2 g of gold powders and a suitable quantity of the organic solvent were charged into a pulverizer (or a pot mill), mixed for four hours for dispersion and added to with 2 g of an organic binder dissolved in 20 g of the organic solvent. The resulting mass was added to with 5 g of a viscosity adjustment agent and mixed for four hours to produce a paste with a viscosity of the order of 150 Pa·s. (3) For insulating coats and protective coats: 50 g of alumina powders and a suitable amount of the organic solvent were charged into a pulverizer (or a pot mill) and mixed for 12 hours for dissolution. The resulting mass was added to with 20 g of a viscosity adjustment agent and mixed for three hours to fabricate a paste with a viscosity of the order of 100 Pa·s. (4) For Pt-containing Porous Materials (Lead Wires): 10 g of alumina, 1.5 g of platinum powders, 2.5 g of an organic binder and 20 g of an organic solvent were charged into a pulverizer 9 or a pot mill) and mixed for four hours. The resulting mass was added to with 10 g of a viscosity adjustment agent and mixed for four hours to fabricate a paste with a viscosity of the order of 100 Pa·s. (5) For first diffusion hole 1: 10 g of alumina powders having an average grain size of about 2µm, 2 g of an organic binder and 20 g of an organic solvent were charged into a pulverizer (or a pot mill) and mixed for four hours. The resulting mass was added to with 10 g of a viscosity adjustment agent and mixed for four hours to fabricate a paste with a viscosity of the order of 400 Pa·s. (6) For carbon Coat: 4 g of carbon powders, 2 g of an organic binder and 40 g of an organic solvent were charged into a pulverizer (or a pot mill) and mixed for dispersion. The resulting mass was added to with 5 g of a viscosity adjustment agent and mixed for four hours to fabricate a paste. By forming the carbon coat by printing, electrical contact between electrodes, for example, can be eliminated. The carbon coat is used for forming first and second measurement chambers. Since carbon is burned off during firing, there is no carbon coat layer in the sintered body. For Second Diffusion Hole 3: 20 g of alumina powders, with a mean particle size of about 2 µm, 8g of an organic binder and 20 g of an organic solvent were charged into a pulverizer (or a pot mill) and mixed for one hour. The resulting mass was granulated and pressed by a metal mold press under a pressure of approximately 2 t/cm 2 to fabricate a press-molded product (in the green state) in the from of a column 0.8 mm thick with a diameter of 1.3mm. This press-molded product in the green state was inserted into a pre-set point of green sheets of the second and third ZrO2 green sheets and press-bonded together. The resulting product was then fired to form the second diffusion hole 3 in the gas sensor. Laminating ZrO2 After pressure bonding the second and third layers, a portion to be passed through by the second diffusion hole 3 is punched. After this punching, a green columnar-shaped molded product, which serves as the second diffusion hole 3, is embedded, and the third to fourth layers of ZrO2 green sheets are pressure-bonded together under a force of pressure of 5 kg/cm 2 for a pressing time duration of one minute. Binder Removal and Firing The pressure-bonded molded product was fired at 1500°C for one hour after removal of the binder at 400°C for two hours. Example of Measurement A NOx gas sensor having the following size was fabricated in accordance with the above-described manufacturing example. A NOx gas concentration measurement test was conducted by controlling the gas sensor as shown in Fig. 25. The NOx gas sensor used for measurement is of a longitudinal length of 50 mm, a width (short-side direction) of 4 mm and a thickness (laminating direction) of 1.3 mm. The first oxygen ion pump cell is of a thickness of 0.3 mm, while the electrodes 6a, 6b is of a longitudinal length and a short-side length of 7 mm and 2 mm, respectively. The first measurement chamber is of a longitudinal length and a short-side length of 7 mm and 2 mm, respectively, with a height being 50 µm. The second measurement chamber is of the longitudinal length and a short-side length of 7 mm and 2 mm, respectively, with a height being 50 µm. The first diffusion hole is of a longitudinal length and a short-side length of 2 mm and 1 mm, respectively, with a thickness being 50 µm. The second diffusion hole is of a diameter of 1 mm. FIFTH EMBODIMENT Fig. 34 shows an embodiment of a sensor assembly in which a NOx gas sensor device according to the present invention is assembled in a metal shell (main metal fixture). This sensor assembly is so secured that a lower portion of the sensor device formed with an inlet for the measurement gas is located in a protector having holes. A heater is annexed to the sensor device, extending along its length. An outer portion of the heater annexed to the sensor device is coated with a sealing material. The sealing material (for a sensing portion of the sensor device) is of a porous material to permit passage of a gas. The sealing material for being secured in the metal fixture is of an air-proofing material. A holder is provided on an outer peripheral side of the sealing material, and a stainless steel material and a talc material are enclosed between the holder and the main metal shell for a film securing. A caulking force is applied in the axial direction for stably holding the sensor device in the assembly. Extending from the metal shell, a first outer tube and a second outer tube are assembled coaxially and retained relative to each other. The first outer tube is extended into and retained by the metal shell. Within the second outer tube, water-proofing rubber is sealed. An electrode formed on the sensor is electrically connected, via an electrode lead, to an end of a sheathed lead wire with the opposite end connected to a electronic control circuit. SIXTH EMBODIMENT Figs. 35A to 35D illustrate a NOx gas concentration sensor for use in an embodiment of the present invention. Fig. 35A is a cross-sectional view taken along the longitudinal direction, Fig. 35B is a plan view of the first measurement chamber, Fig. 35C is a schematic enlarged cross-sectional view of the first measurement chamber and Fig. 35D is a plan view of the second measurement chamber. The sensor shown in Figs. 35A to 35D includes a layer of a first oxygen ion pumping cell 66 having a solid electrolyte layer and electrodes 66a (positive electrode), 66b (negative electrode) provided on both sides of the solid electrolyte layer, a layer of an oxygen concentration measurement cell 67 having a solid electrolyte layer and oxygen partial pressure detecting electrodes provided on both sides of the solid electrolyte layer, a solid electrolyte layer, and a layer of the second oxygen ion pumping cell 68 having a solid electrolyte layer and oxygen ion pump electrodes 68a, 68b provided within and outside the second measurement chamber 64 formed on one sides of the solid electrolyte layer, layered in this order. The first measurement chamber 62 is defined by left and right insulating layers and upper and lower solid electrolyte layers, while the second measurement chamber 64 is defined above the layer of the second oxygen ion pumping cell 68. Two first diffusion holes 61, 61 are opened, apart from each other, to the first measurement chamber 62 for introducing the measurement gas via a diffusion resistance. The second diffusion hole 63 is passed through the layers of the oxygen concentration measurement cell 67 and the solid electrolyte layer for establishing communication between the first and second measurement chambers 62, 64 to supply a gas containing at least NOx and O2 from the first measurement chamber 62 via a diffusion resistance of a sensor diffusion hole 63 into the second measurement chamber 64. Between the layers of the solid electrolyte are formed alumina insulating layers. Although not shown, heating layers for heating the entire sensor are bonded with a cement layer for sandwiching the entire sensor in the stacking direction. The electrodes are connected via leads formed between the layers to outside of the sensor, such as to a power source. Referring to Fig. 35D, the electrodes 78a, 78b of the second oxygen ion pumping cell 68 are electrically connected to leads 78c, 78d. One of the features of the present invention is that the first measurement chamber 62 and the second measurement chamber 64 are arranged in substantially superposed state and that the first diffusion holes 61, 61 are formed on both sides of the sensor, instead of on its distal end, with a porous material being charged into the second diffusion hole 63, an insulating film being arranged between the solid electrolyte layers and with the electrodes of the cells being insulated from one another. The second measurement chamber 64, defining a void, may be charged with a porous material. As compared to the sensor of Fig. 36, which is a comparative example with respect to the sensor of Figs. 35A to 35D, the sensor of Figs. 35A to 35D is characterized by that the lengths of the electrodes 66b in the first oxygen ion pump current 66 as measured in a direction along the flowing direction of the measurement gas flowing from the first diffusion hole 61 towards the second diffusion hole 63 is shorter than the longitudinal length of the first measurement chamber 62 (A>B). The inner electrode 66b is not formed to the position directly above the second diffusion hole 63. By the electrode 66b of the first oxygen pumping cell 66, being shorter in length, the oxygen concentration gradient in the first measurement chamber 62 in particular in the measurement gas flow direction proceeding from a first diffusion aperture (portion) 61 towards a second diffusion aperture (portion) 63 is lowered. That is the oxygen concentration difference from one to the opposite end of the electrode 66b is reduced, while the electromotive force generated at the distal end portions of the electrode 66a, 66b is suppressed, as shown in Fig. 35C. Moreover, with suppression of the electro-motive force generated in the electrodes 66a, 66b, the first oxygen pumping cell voltage Vpl is reduced for reducing the temperature dependency and oxygen concentration dependency in the NOx gas concentration measurement. This discloses an important design rule for making a most preferred sensor. This effect is brought about by the fact that the first oxygen pumping cell voltage Vpl required for pumping out excess oxygen is lowered so that there occurs no dissociation or decomposition of the NO gas other than that by oxygen pumping-out in the first measurement chamber. Specifically, the NO gas flowing into the second measurement chamber is not too decreased to prevent the Ip2 from being too lowered. The NOx gas concentration was measured using the embodiment shown in Figs. 35A to 35D and a sensor of the comparative example as shown in Fig. 36. The sensor of the embodiment was of a longitudinal length of 50 mm, a width (short side length of 4 mm and a thickness (in the stacking direction) was 1.3 mm. The first oxygen ion pump cell 66 was 0.3 mm in thickness, while the electrodes 66a, 66b were of a longitudinal length A of 8 mm and a longitudinal length B of 4 mm and a short-side length of 2 mm. The first measurement chamber 62 was of a longitudinal length A of 7 mm, a short-side length of 2 mm and a height of 50 mm. The first diffusion hole 61 was of a longitudinal length of 50 m, while the second diffusion hole 63 was of a size of 1 mm and a distance from the end (right-hand end) of the first diffusion hole 61 equal to 5.5 mm. The sensor of the comparative example (Fig. 36) is of the same size as the sensor of the embodiment except that the longitudinal size of the electrodes 46a, 46b is 7 mm (A=B). Generally, the NOx concentration in the gas is determined based on the following phenomenon: Namely, almost NO2 will be deemed to be dissociated into NO and 1/2O2 in a gas atmosphere of high temperature, e.g., at 800°C under normal pressure in which a very small amount of NO2 in an amount of e.g., 0.1% (1000 ppm) is present together with other gases, thus which is deemed to be equivalent to the presence of 0.1% of NO. The measurement is carried out basically under the following principle. (1) At a high temperature (700°C or above), the amount of NO2 contained as nNOx is dissociated into nNO + n/2O1-x , and subjected to removal of the generated n/2 O 1-x ; (2) nNO of step (1) is dissociated into n/2 N2 + n/2 02. This n/2 O2 is transmitted as oxygen ion through the oxygen ion conductor to measure a current caused by the oxygen ion transmission, through which a value proportionate to the current is obtained. Namely, by determining n of NO, the amount of NOx can be determined. Ideally, the sensor should forward the NO generated at step (1) without subjecting to decomposition. However, a certain amount of dissociation of the generated NO at step (1) may occur even under the best suited conditions dissociation rate between NO and (N+O) is governed by influences of various parameters, such as the applied voltage at step (1) and materials and design/configuration of the electrodes. Therefore, it is almost preferred to compensate for substantial decomposition of NO in the step (1). Typically, the composition can be performed by the inverse of the dissociation rate of NO into (N + O) (e.g., 60 to 95%). Generally, the method for determining NOx concentration in a measurement gas is carried out as follows. The determination of the NOx concentration is carried out under a varying condition of the NOx gas concentration in the measurement gas in the course that the measurement gas is allowed to travel through a flow channel facing a ceramic body having an electrically controllable conducting state of oxygen ions. in step (1), the flow of the gas into the flow channel is restricted. Between steps (2) and (3), the flow of the residual gas to the electrode of the step (3) is restricted, too. The flow channel may comprise a first flow channel and a second flow channel communicating with the first flow channel. The formation of the residual gas of step (2) is performed in the first flow channel, and wherein NOx in step (3) is dissociated in the second flow channel. In step (2) the residual gas may be formed under the condition that allows and compensates for substantial decomposition of NOx in the flow channel. Further modifications are possible generally as follows. If a pre-set minor current is allowed to flow in the oxygen concentration measurement cell, its electrode may be used as an auto-generating reference pole. The merit of this auto-generating reference pole is that the reference oxygen concentration is less susceptible to changes in oxygen concentration in air. In the electrode structure, Au components may be carried by particles relatively coarser than the Au particles (that is zirconia particles as main electrode components) so that the Au components are dispersed finely. By adding a component carried by a catalyst adjustment method on the powders of an oxygen ion conductive solid electrolyte, in addition to the porous powders, such as Pt powders, the interface resistance generated across particles is lowered to improve the oxygen expelling capability. For example, one or more of Au, Ag, Ni, Mn, Co, Cu, Ba, Mg, Ca, Na, K and Li may be selected and used as NOx dissociation suppressing capability, other than fine Au particles, as film, if so desired.

Claims: A method for measuring the NOx concentration in an exhaust gas using a NOx concentration detector, said NOx concentration detector having: a first diffusion resistance (1) ; a first measurement chamber (2) ; an oxygen concentration detection cell (7) comprising an oxygen concentration detection electrode (7a, 7b) for measuring the oxygen concentration in the measurement gas present in said first measurement chamber (2) ; a first oxygen ion pump cell (6) for pumping oxygen contained in said exhaust gas from said first measurement chamber (2) to outside and/or from outside to inside of said first measurement chamber (2) based on the potential of said oxygen concentration detection electrode (7a, 7b); a second diffusion resistance (3); a second measurement chamber (4); and a second oxygen ion pump cell (8) having a pair of electrodes (8a, 8b); said method comprising the steps of: introducing said exhaust gas into said first measurement chamber (2) via said first diffusion resistance (1); operating said first oxygen ion pump cell (6) such that oxygen is pumped to an extent not to allow all NOx in said exhaust gas to decompose into oxygen, whereby a first oxygen pump current (Ip1) is caused to flow in the first oxygen ion pump cell (6) , and thus forming a residual gas; measuring the oxygen concentration in said exhaust gas, based on said first oxygen pump current (Ipl); introducing said residual gas from said first measurement chamber (2) into said second measurement chamber (4) via said second diffusion resistance (3) ; applying a voltage across said pair of electrodes (8a, 8b) to decompose nitrogen oxide in said residual gas in said second measurement chamber (4) and to pump out dissociated oxygen, whereby a second oxygen pump current (Ip2) is caused to flow in the second oxygen ion pump cell (8); determining a basic NOx concentration from said second oxygen pump current (Ip2); and determining said Nox concentration by applying a correction to said basic NOx concentration, characterized in that said correction is determined based on said first oxygen pump current (Ip1) responsive to said oxygen concentration in said exhaust gas introduced into said first measurement chamber (2), and said correction comprises a gain correction, wherein said gain is the coefficient of variation of said determined basic NOx concentration with variation of said second oxygen pump current (Ip2), and wherein said gain correction is the variation of said coefficient responsive to said measured oxygen concentration in said exhaust gas. The method according to claim 1, wherein said correction further comprises an offset correction according to said measured oxygen concentration in said exhaust gas. The method according to claim 1 or 2, wherein: a detection gas having a known oxygen concentration and a known NOx concentration is injected into said detector so as to measure said second oxygen pump current (Ip2) ; a variation of said second oxygen pump current (Ip2) versus a variation of said known NOx concentration at a pre-set oxygen concentration, referred to as gain at pre-set oxygen concentration, is found, using values of said known NOx concentration and values of said second oxygen pump current (Ip2) ; said pre-set oxygen concentration in said detection gas is varied; said second oxygen pump current (Ip2) values for substantially zero concentration and a non-zero pre-set concentration of Nox are measured at each pre-set oxygen concentration; said gain at pre-set oxygen concentration, and said second oxygen pump current (Ip2) for substantially zero concentration of NOx at said pre-set oxygen concentration, referred to as offset at pre-set oxygen concentration, are measured; and the NOx concentration is found from said gain and offset at pre-set oxygen concentration, corresponding to said oxygen concentration in said exhaust gas, and from said second oxygen pump current (Ip2). The method according to claim 3, wherein said gain used for gain-correcting is selected from said gain at pre-set oxygen concentration, responsive to said oxygen concentration in said exhaust gas. The method according to claim 3 or 4, wherein said gain is represented in terms of a logarithm of an oxygen partial pressure. The method according to claim 1, wherein a voltage applied across said first oxygen pump cell (6) is controlled based on an oxygen partial pressure in said first measurement chamber. A method for controlling an NOx concentration detector, said detector having: a first diffusion resistance (1) ; a first measurement chamber (2) ; oxygen partial pressure detection cell (7) for detecting the oxygen partial pressure in the measurement gas present in said first measurement chamber (2); a first oxygen ion pump cell (6) for pumping oxygen contained in the exhaust gas from said first measurement chamber (2) to outside and/or from outside to inside of said first measurement chamber (2) based on the potential of said oxygen partial pressure detection cell (7) ; a second diffusion resistance (3); a second measurement chamber (4); and a second oxygen ion pump cell (8) having a pair of electrodes (8a, 8b) provided on an inside and an outside of said second measurement chamber (4); said method comprising the steps of: introducing said exhaust gas into said first measurement chamber (2) via said first diffusion resistance (1) ; operating said first oxygen ion pump cell (6) such that oxygen is pumped to an extent not to allow all NOx in said exhaust gas to decompose into oxygen, whereby a residual gas is formed; introducing the residual gas from said first measurement chamber (2) into said second measurement chamber (4) via said second diffusion resistance (3) ; applying a voltage across said pair of electrodes (8a, 8b) to decompose nitrogen oxide in the residual gas in said second measurement chamber (4) and to pump out dissociated oxygen, whereby a flow of current corresponding to the NOx concentration is caused; characterized in that said method further comprises the step of controlling said voltage applied across said second oxygen ion pump cell (8) depending on the oxygen partial pressure in the measurement gas. The method according to claim 7, wherein, if the oxygen partial pressure is low or high, said voltage applied across said second oxygen ion pump cell (8) is lowered or raised, respectively. An apparatus for measuring the NOx concentration in an exhaust gas, said apparatus comprising an NOx concentration detector having: a first measurement chamber (2) into which is introducible said exhaust gas via a first diffusion resistance (1); an oxygen concentration detection cell (7) comprising an oxygen concentration detection electrode (7a, 7b) for detecting the oxygen concentration in the measurement gas present in said first measurement chamber (2) ; a first oxygen ion pump cell (6) for pumping oxygen contained in said introduced exhaust gas from said first measurement chamber (2) to outside and/or from outside to inside of said first measurement chamber (2) based on a potential of said oxygen concentration detection electrode, thus forming a residual gas, and for determining the oxygen concentration in said exhaust gas by measuring a first oxygen pump current (Ip1) flowing across said first oxygen ion pump cell (6); a second measurement chamber (4) into which the residual gas is introducible from said first measurement chamber (2) via a second diffusion resistance (3); and a second oxygen ion pump cell (8) having a pair of electrodes (8a, 8b) across which a voltage is applicable to decompose NOx in said residual gas in said second measurement chamber (4) to pump out dissociated oxygen, whereby a second oxygen pump current (Ip2) is caused to flow in the second oxygen ion pump cell (8); said apparatus further comprising a NOx concentration processor having: a first input for receiving an output from said second oxygen ion pump cell (8) representing said second oxygen pump current (tip2), said NOx concentration processor determining a basic NOx concentration from said second oxygen pump current (Ip2) ; characterized by a second input for receiving an output of said first oxygen ion pump cell (6) representing said detected oxygen concentration in said exhaust gas introduced into said first measurement chamber (2) ; and processing circuit means adapted for obtaining said NOx concentration by applying a correction to said basic NOx concentration, said correction being determined from said second input, wherein said correction comprises a gain correction, wherein said gain is the coefficient of variation of said determined basic NOx concentration with variation of said second oxygen pump current (Ip2), and wherein said gain correction is the variation of said coefficient responsive to said measured oxygen concentration in said exhaust gas. The apparatus according to claim 9, wherein said processing circuit means are further adapted to apply an offset correction to said basic NOx concentration. The apparatus according to claim 10, further comprising: a constant oxygen partial pressure controller adapted to control said first oxygen ion pump cell (6) to provide a constant output of said oxygen concentration detection cell (7); and a storage unit having pre-stored therein a relation between a variation of an output across said oxygen concentration detection electrodes (7a, 7b) and an offset of said second oxygen ion pump current; wherein pre-set data is readable from said storage unit responsive to a variation of an output of said oxygen concentration detection cell (7), and an offset value of the second oxygen ion pump current (Ip2) is variable based on said data to correct said detection output of said nitrogen oxide NOx concentration detector. An apparatus for measuring the NOx concentration in an exhaust gas, said apparatus comprising an NOx concentration detector having: a first diffusion resistance (1) ; a first measurement chamber (2); an oxygen partial pressure detection cell (7) for detecting the oxygen partial pressure in the measurement gas present in said first measurement chamber (2); a first oxygen ion pump cell (6) for pumping oxygen contained in the exhaust gas from said first measurement chamber (2) to outside and/or from outside to inside of said first measurement chamber (2) based on a potential of said oxygen partial pressure detection cell (7); a second diffusion resistance (3); a second measurement chamber (4); and a second oxygen ion pump cell (8) having a pair of electrodes (8a, 8b) provided on an inside and an outside of said second measurement chamber (4); characterized by a voltage controller (32) adapted for controlling the voltage applied across said second oxygen ion pump cell (8) depending on the oxygen partial pressure in the measurement gas.

Inventors: ANDO MASASHI, ISHIDA NOBORU, KUZUYA YASUHISA, NADANAMI NORIHIKO, OKUMURA TATSUO, OSHIMA TAKAFUMI, OTSUKA TAKAKI, SATO YOSHIKUNI, SUGAYA SATOSHI, ANDO, MASASHI, ISHIDA, NOBORU, KUZUYA, YASUHISA, NADANAMI, NORIHIKO, OKUMURA, TATSUO, OSHIMA, TAKAFUMI, OTSUKA, TAKAKI, SATO, YOSHIKUNI, SUGAYA, SATOSHI, Ando, Masashi, c/o NGK Spark Plug Co., Ltd., ISHIDA, NOBORU, C/O NGK SPARK PLUG CO., LTD., Kuzuya, Yasuhisa, c/o NGK Spark Plug Co., Ltd., Nadanami, Norihiko, c/o NGK Spark Plug Co., Ltd., Okumura, Tatsuo, c/o NGK Spark Plug Co., Ltd., OSHIMA, TAKAFUMI, C/O NGK SPARK PLUG CO., LTD., Otsuka, Takaki, c/o NGK Spark Plug Co., Ltd., Sato, Yoshikuni, c/o NGK Spark Plug Co., Ltd., Sugaya, Satoshi, c/o NGK Spark Plug Co., Ltd.
Applicants: NGK SPARK PLUG CO, NGK SPARK PLUG CO. LTD.
Application year: 1998
Main classification: G01N27
IPC codes: F01N11, F02D41, F01N3, G01N33, G01N27