Abstract:
A connector  2   b  for an NOx sensor is connected via a cable C to an end of a sensor body  2   a  through which a first pump current and a second pump current flow according to the concentration of oxygen and that of NOx, respectively, contained in a gas to be measured. The connector  2   b  is provided with not only terminals for inputting a signal to and outputting a signal from the sensor body  2   a  but also a label resistor RL having resistance corresponding to characteristics (relationship between oxygen concentration and first pump current and that between NOx concentration and second pump current) of the sensor body  2   a  and label signal output terminals T 01  and T 02  connected to opposite ends of the label resistor RL. On the basis of characteristics of the sensor body  2   a , which are identified from the resistance of the label resistor RL which, in turn, is identified via the terminals T 01  and T 02 , the oxygen concentration and the NOx concentration are accurately obtained from detected values of the first and second pump currents. This enables an NOx sensor to always perform accurate measurement irrespective of variations in characteristics among NOx sensors.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a connector for an NOx sensor equipped with input and output terminals for inputting a signal to and outputting a signal from an NOx sensor that measures an NOx concentration. 
     NOx sensors for measuring the concentration of nitrogen oxides (NOx) contained in gas to be measured (hereinafter referred to as a measurement gas) are disclosed in, for example, EP-A1-0678740 and SAE Paper No. 960334, pp. 137-142, 1996. Such a conventional NOx sensor is composed of oxygen-ion conductive solid electrolyte layers that form a first measurement chamber and a second measurement chamber. The first measurement chamber communicates with a measurement gas via a first diffusion-controlling layer, and the second measurement chamber communicates with the first measurement chamber via a second diffusion-controlling layer. A first oxygen-pumping cell and an oxygen-concentration-measuring cell are formed adjacent to the first measurement chamber and are each composed of a solid electrolyte layer sandwiched between porous electrodes. A second oxygen-pumping cell is formed adjacent to the second measurement chamber and is composed of a solid electrolyte layer sandwiched between porous electrodes. 
     BACKGROUND OF THE INVENTION 
     In order to obtain the concentration of NOx contained in a measurement gas, such as exhaust from an internal combustion engine, without influence of other gas components (such as oxygen, carbon monoxide, and carbon dioxide) contained in the measurement gas, the above Nox sensor undergoes the following control. 
     Electrical current (hereinafter referred to as first pump current) is caused to flow to the first oxygen-pumping cell such that an output voltage from the oxygen-concentration-measuring cell attains a predetermined value, thereby controlling the concentration of oxygen contained in the first measurement chamber to a very low level (for example, about 1000 ppm). The measurement gas whose oxygen concentration has been thus controlled to the low level enters the second measurement chamber. A constant voltage is applied to the second oxygen-pumping cell in such a direction that oxygen is pumped out from the second measurement chamber. Then, by virtue of a catalytic function of the porous electrodes of the second oxygen-pumping cell, the second oxygen-pumping cell decomposes NOx contained in the measurement gas into nitrogen and oxygen and pumps out the thus-obtained oxygen from the second measurement chamber. At this time, on the basis of current flowing through the second oxygen-pumping cell, the NOx concentration of the measurement gas is obtained. 
     The first pump current is controlled such that the concentration of oxygen contained in the first measurement chamber attains low level, but not zero, for the following reason. If the concentration of oxygen contained in the first measurement chamber is controlled to zero, the porous electrodes that constitute the first oxygen-pumping cell cause decomposition of NOx contained in the measurement gas that has entered the first measurement chamber. This disables measurement of NOx concentration through use of the second oxygen-pumping cell. 
     The above NOx sensor can obtain not only the concentration of NOX contained in the measurement gas from the second pump current but also the concentration of oxygen contained in the measurement gas from the first pump current. 
     However, a characteristic indicative of the relationship between NOx concentration and second pump current and that between oxygen concentration and first pump current somewhat differ among NOx sensors. If, on the assumption that NOx sensors have certain common characteristics indicative of the relationships, the NOx concentration and the oxygen concentration of a measurement gas are obtained from the second and first pump currents, respectively, detected by the NOx sensor, some NOx sensors may fail to provide sufficiently high measurement accuracy. 
     An object of the present invention is to provide sufficiently high accuracy in measurement by an NOx sensor regardless of variations in characteristics among NOx sensors. 
     According to the present invention there is provided a connector for an NOx sensor, which connector comprises input and output terminals for inputting a signal to and outputting a signal from the NOx sensor. The NOx sensor comprises a first measurement chamber and a second measurement chamber. The first measurement chamber comprises a first oxygen-pumping cell and an oxygen-concentration-measuring cell and communicates with a measurement gas via a first diffusion-controlling layer. The first oxygen-pumping cell and the oxygen-concentration-measuring cell are each formed of an oxygen-ion conductive solid electrolyte layer sandwiched between porous electrodes. The second measurement chamber comprises a second oxygen-pumping cell and communicates with the first measurement chamber via a second diffusion-controlling layer. The second oxygen-pumping cell is formed of an oxygen-ion conductive solid electrolyte layer sandwiched between porous electrodes. A first pump current is caused to flow to the first oxygen-pumping cell such that an output voltage from the oxygen-concentration-measuring cell is maintained at a constant value so as to control the concentration of oxygen contained in the first measurement chamber to a constant level. A constant voltage is applied to the second oxygen-pumping cell in such a direction that oxygen is pumped out from the second measurement chamber. There is detected a second pump current that flows through the second oxygen-pumping cell as a result of the application of the constant voltage and according to the concentration of NOx contained in a measurement gas. 
     The connector for an NOx sensor is equipped with a label resistor having resistance corresponding to at least either a relationship between the concentration of oxygen contained in the measurement gas and the first pump current or a relationship between the concentration of NOx contained in the measurement gas and the second pump current, and is equipped with a pair of label signal output terminals connected to opposite ends of the label resistor. 
     The thus-configured connector for an NOx sensor is used in the following manner. The resistance of the label resistor is detected via the label signal output terminals to thereby identify characteristics of the NOx sensor concerned. Subsequently, the first pump current is caused to flow to the first oxygen-pumping cell such that an output voltage from the oxygen-concentration-measuring cell becomes constant, i.e., such that the concentration of oxygen contained in the first measurement chamber becomes constant. Also, a constant voltage is applied to the second oxygen-pumping cell in such a direction that oxygen is pumped out from the second measurement chamber. The NOx concentration and the oxygen concentration of the measurement gas are respectively obtained from the second pump current, which flows through the second oxygen-pumping cell according to the concentration of NOx contained in the measurement gas, and the first pump current, which flows according to the concentration of oxygen contained in the measurement gas, as well as on the basis of a characteristic(s) of the NOx sensor identified by the resistance of the label resistor. 
     SUMMARY OF THE INVENTION 
     Accordingly, through use of the connector of the present invention for an NOx sensor, a characteristic (at least either relationship between NOx concentration and second pump current or relationship between oxygen concentration and first pump current) of an NOx sensor can be identified by the resistance of the label resistor. Through employment of the identified characteristic(s) of the NOx sensor, the NOx concentration and the oxygen concentration of the measurement gas can be obtained at a sufficiently high degree of accuracy. 
     Through identification of the relationship between NOx concentration and second pump current, variations in an obtained value of NOx concentration among NOx sensors can be compensated for. Through identification of the relationship between oxygen concentration and first pump current, the concentration of oxygen contained in the measurement gas can be accurately obtained; thus, through compensation of the second pump current on the basis of the thus accurately obtained oxygen concentration, the NOx concentration of the measurement gas can be accurately obtained against variations in the concentration of oxygen contained in the measurement gas. 
     Further, through identification of both of the above relationships implemented by a minor additional element of a label resistor, the measurement accuracy of the NOx sensor can be significantly improved. 
     The resistance of the label resistor may correspond to at least the sensitivity of the second pump current to variations in the concentration of NOx contained in the measurement gas. 
     Problems Solved by the Invention 
     In the accompanying drawings FIG. 6B schematically shows the relationship between the second pump current and the concentration of NOx contained in the measurement gas. Offset of the second pump current (second pump current at an NOx concentration of 0%) and sensitivity (slope of the graph) differ among NOx sensors. 
     Through use of a relative value, as opposed to an absolute value, for the second pump current in execution of control, offsets are canceled; thus, through mere compensation for variations in sensitivity, measurement can be performed accurately. 
     The resistance of the label resistor may correspond to at least the first pump current as measured when the oxygen concentration of the measurement gas is identical to that of the atmosphere. 
     Again, FIG. 6A schematically shows the relationship between the first pump current and the concentration of oxygen contained in the measurement gas. As shown in FIG. 6A, the first pump current has an offset of 0 and is substantially proportional to the oxygen concentration. Thus, when the first pump current at a certain oxygen concentration is known, the sensitivity of the first pump current to the oxygen concentration can be easily obtained. 
     Accordingly, from a first pump current as measured when the measurement gas assumes an oxygen concentration identical to that of the atmosphere, the relationship between the first pump current and the concentration of oxygen contained in the measurement gas can be identified. On the basis of the thus-identified characteristic relationship, the concentration of oxygen contained in the measurement gas can be accurately obtained. 
     Through use of the thus-obtained oxygen concentration, the oxygen concentration dependency of an NOx sensor can be compensated for. Also, the thus-obtained oxygen concentration can be applied to various kinds of control that are performed on the basis of the oxygen concentration of the measurement gas. 
     The resistance of the label resistor may correspond to not only oxygen concentration but also to air-fuel ratio or excess air factor. 
     The invention will be further described by way of example with reference to the accompanying drawings, in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front view showing an NOx sensor equipped with a connector according to an embodiment of the present invention as well as the state of attachment of a label resistor to the connector; 
     FIG. 2 is a circuit diagram showing the configuration of an NOx concentration detection apparatus which employs an NOx sensor equipped with the connector according to the embodiment; 
     FIG. 3 is an exploded perspective view showing the structure of an NOx sensor body; 
     FIG. 4 is a table of ranks for NOx sensors; 
     FIG. 5 is a flowchart showing processing to be executed by ECU of the NOx concentration detection apparatus; 
     FIGS. 6A and 6B show graphs schematically showing the relationship between oxygen concentration and first pump current and that between NOx concentration and second pump current. 
     Description of Symbols used in the drawings: 
       2 : NOx sensor equipped with connector 
       2   a : sensor body 
       2   b : connector 
       4 : first pump cell 
       6 : Vs cell 
       8 : second pump cell 
       12 : heater 
       18 ,  22 ,  24 : solid electrolyte layer 
       20 : first measurement chamber 
       26 : second measurement chamber 
       28 : spacer 
       30 : NOx concentration detection apparatus 
       38 : label detection circuit 
       40 : drive circuit 
       42 : detection circuit 
       44 : heater electrification circuit 
       46 : temperature sensor 
     RL: label resistor 
     RR: reference resistor 
     T 01 , T 02 : label signal output terminal 
     T 11 , T 12 : first pump cell control terminal 
     T 21 , T 22 : Vs cell control terminal T 31 , T 32 : second pump cell control terminal T 41 , T 42 : heater control terminal 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a front view showing an NOx sensor equipped with a connector according to an embodiment of the present invention. FIG. 2 is a circuit diagram showing the entire configuration of an NOx concentration detection apparatus which employs an NOx sensor equipped with the connector according to the embodiment and which is used for detecting the concentration of NOx contained in exhaust from a vehicle. FIG. 3 is an exploded perspective view of a sensor body. 
     As shown in FIGS. 1 and 2, an NOx sensor  2  equipped with a connector according to the present embodiment includes a sensor body  2   a  and a connector  2   b  connected to an end of the sensor body  2   a  via a cable C. The sensor body  2   a  includes a first oxygen-pumping cell (hereinafter referred to as a first pump cell)  4 , an oxygen-concentration-measuring cell (hereinafter referred to as a Vs cell)  6 , a second oxygen-pumping cell (hereinafter referred to as a second pump cell)  8 , and heaters  12  and  14 . A first measurement chamber  20  is formed between the first pump cell  4  and the Vs cell  6  and communicates with the exterior of the sensor body  2   a  via a diffusion-controlling layer  4   d . A second measurement chamber  26  is formed between the Vs cell  6  and the second pump cell  8  and communicates with the first measurement chamber  20  via diffusion-controlling layers  6   d  and  22   d.    
     As shown in FIG. 3, in the sensor body  2   a , the first pump cell  4  includes a sheet-like solid electrolyte layer  4   a  and rectangular porous electrodes  4   b  and  4   c  formed on opposite sides of the solid electrolyte layer  4   a . Lead portions  4   b   1  and  4   c   1  extend from the porous electrodes  4   b  and  4   c , respectively. Further, a round hole is formed in the solid electrolyte layer  4   a  in such a manner as to penetrate the porous electrodes  4   b  and  4   c  at their central portions. The thus-formed round hole is filled with a porous filler to thereby form a diffusion-controlling layer  4   d.    
     The Vs cell  6  includes a sheet-like solid electrolyte layer  6   a  similar to the solid electrolyte layer  4   a  of the first pump cell  4  and circular porous electrodes  6   b  and  6   c  formed on opposite sides of the solid electrolyte layer  6   a . Lead portions  6   b   1  and  6   c   1  extend from the porous electrodes  6   b  and  6   c , respectively. Further, a round hole is formed in the solid electrolyte layer  6   a  in such a manner as to penetrate the porous electrodes  6   b  and  6   c  at their central portions. The thus-formed round hole is filled with a porous filler to thereby form a diffusion-controlling layer  6   d.    
     The porous electrodes  4   b  and  4   c  of the first pump cell  4  and the porous electrodes  6   b  and  6   c  of the Vs cell  6  are located on the solid electrolyte layers  4   a  and  6   a , respectively, such that their centers are aligned with each other. Accordingly, when the first pump cell  4  and the Vs cell  6  are arranged in layers, the diffusion-controlling layers  4   d  and  6   d  face each other. The circular porous electrodes  6   b  and  6   c  of the Vs cell  6  have a size smaller than that of the rectangular porous electrodes  4   b  and  4   c  of the first pump cell  4 . An insulation film formed of alumina or a like material is formed on opposite surfaces of the Vs cell  6  so as to cover the lead portions  6   b   1  and  6   c   1  from outside in order to prevent current leakage from the lead portions  6   b   1  and  6   c   1 . Further, a leakage resistance portion  6   f  is formed between the lead portions  6   b   1  and  6   c   1  in order to leak part of oxygen from the porous electrode  6   c  to the side of the porous electrode  6   b  through control of current supply, which will be described later. 
     The first pump cell  4  and the Vs cell  6  are arranged in layers with a solid electrolyte layer  18  interposed therebetween. The solid electrolyte layer  18  has the same shape as that of the solid electrolyte layers  4   a  and  6   a . The solid electrolyte layer  18  has a rectangular hole formed therein in a position corresponding to the porous electrodes  4   c  and  6   b  and has a size greater than that of the porous electrode  4   c . The thus-formed rectangular hole serves as a first measurement chamber  20 . 
     Also, a solid electrolyte layer  22 , which has the same shape as that of the solid electrolyte layers  4   a  and  6   a , is placed on the Vs cell  6  on the side of the porous electrode  6   c . The solid electrolyte layer  22  has a round hole formed therein in a position corresponding to the diffusion-controlling layer  6   d  of the Vs cell  6 . The thus-formed round hole is filled with a porous filler to thereby form a diffusion-controlling layer  22   d.    
     As in the case of the first pump cell  4 , the second pump cell  8  includes a sheet-like solid electrolyte layer  8   a  and rectangular porous electrodes  8   b  and  8   c  formed on opposite sides of the solid electrolyte layer  8   a . Lead portions  8   b   1  and  8   c   1  extend from the porous electrodes  8   b  and  8   c , respectively. The second pump cell  8  and the solid electrolyte layer  22  are arranged in layers with a solid electrolyte layer  24  interposed therebetween. The solid electrolyte layer  24  is formed in the same manner as is formed the solid electrolyte layer  18 . As a result, a rectangular hole formed in the solid electrolyte layer  24  serves as a second measurement space  26 . 
     Heaters  12  and  14  are placed on the opposite sides of the above-described laminate of the first pump cell  4 , the Vs cell  6 , and the second pump cell  8 , i.e. outside the first pump cell  4  and the second pump cell  8 , respectively, such that a predetermined gap is formed between each of the heaters  12  and  14  and the laminate through use of spacers  28  and  29 . 
     The heater  12  ( 14 ) includes a heater substrate  12   a  ( 14   a ) having a shape similar to that of the solid electrolyte layers  4   a ,  6   a , and  8   a , a heater wiring  12   b  ( 14   b ), and a lead portion  12   b   1  ( 14   b   1 ) extending from the heater wiring  12   b  ( 14   b ). The heater wiring  12   b  ( 14   b ) and the lead portion  12   b   1  ( 14   b   1 ) are formed on the side of the heater  12  ( 14 ) that faces the first pump cell  4  (second pump cell  8 ). The spacer  28  ( 29 ) is interposed between the heater  12  ( 14 ) and the first pump cell  4  (second pump cell  8 ) and in a position corresponding to the lead portion  12   b   1  ( 14   b   1 ) so that the heater  12  ( 14 ) faces the porous electrode  4   b  ( 8   c ) of the first pump cell  4  (second pump cell  8 ) with a gap formed therebetween. 
     The connector  2   b  has label signal output terminals T 01  and T 02 ; first pump cell control terminals T 11  and T 12  connected to the electrodes  4   b  and  4   c , respectively, of the first pump cell  4 ; Vs cell control terminals T 21  and T 22  connected to the electrodes  6   b  and  6   c , respectively, of the Vs cell; second pump cell control terminals T 31  and T 32  connected to the electrodes  8   b  and  8   c , respectively, of the second pump cell  8 ; and heater control terminals T 41  and T 42  for supplying current to the heaters  12  and  14 . A label resistor RL is connected to the label signal output terminals T 01  and T 02 . The label resistor RL has a resistance corresponding to a rank indicative of a characteristic of the sensor body  2   a.    
     An NOx concentration detection apparatus  30  determines the concentration of Nox contained in exhaust gas through use of the above-described NOx sensor  2  equipped with the connector and attached to an exhaust pipe of an internal combustion engine carried in a vehicle. 
     As shown in FIG. 2, the NOx concentration detection apparatus  30  includes a label detection circuit  38  for supplying electricity to the label resistor RL via the terminals T 01  and T 02  of the connector  2   b  so as to output a voltage level VR according to the resistance of the label resistor RL; a drive circuit  40  for supplying electricity to the first pump cell  4  and Vs cell  6  of the sensor body  2   a  via the terminals T 11 , T 12 , T 21 , and T 22  of the connector  2   b  and for detecting first pump current IP 1  flowing to the first pump cell  4 ; a detection circuit  42  for applying a constant voltage to the second pump cell  8  of the sensor body  2   a  via the terminals T 31  and T 32  of the connector  2   b  and for detecting second pump current IP 2  which flows through the second pump cell  8  to which the constant voltage is applied; a heater electrification circuit  44  for supplying electricity to the heaters  12  and  14  of the sensor body  2   a  via the terminals T 41  and T 42  of the connector  2   b  so as to heat the cells  4 ,  6 , and  8 ; and an electronic control unit (hereinafter referred to as ECU)  50 , which includes a microcomputer, for controlling the drive circuit  40  and the heater electrification circuit  44  and for determining the NOx concentration and the oxygen concentration of exhaust gas on the basis of detection signals VIP 1  and VIP 2  issued from the drive circuit  40  and the detection circuit  42 , respectively. The porous electrodes  4   c  and  6   b  (terminals T 12  and T 21 )—which face the first measurement chamber  20 —of the first cell  4  and the Vs cell  6 , respectively, are grounded via a resistor R 1 . The porous electrodes  4   b  and  6   c  (terminals T 11  and T 22 ) are connected to the drive circuit  40 . 
     The label detection circuit  38  applies a source voltage VDD to an end (terminal T 01 ) of the label resistor RL via a reference resistor RR and grounds the other end (terminal T 02 ) of the label resistor RL. The source voltage VDD is divided by the label resistor RL and the reference resistor RR. A resultant divided voltage is input to the ECU  50  as a voltage level VR. 
     The drive circuit  40  includes a resistor R 2  and a differential amplifier AMP. A constant voltage VCP is applied to an end of the resistor R 2 , and the other end of the resistor R 2  is connected to the porous electrode  6   c  of the Vs cell  6  (terminal T 22 ). A negative input terminal of the differential amplifier AMP is connected to the porous electrode  6   c  of the Vs cell  6  (terminal T 22 ); a reference voltage VC 0  is applied to a positive input terminal of the AMP; and an output terminal of the AMP is connected to the porous electrode  4   b  of the first pump cell  4  (terminal T 11 ) via a resistor R 0 . 
     The drive circuit  40  operates in the following manner. First, a constant small current iCP is fed to the Vs cell  6  via the resistor R 2  to thereby pump out oxygen from the first measurement chamber  20  into the porous electrode  6   c . Since the porous electrode  6   c  is blocked by the solid electrolyte layer  22  and communicates with the porous electrode  6   b  via the leakage resistance portion  6   f , the concentration of oxygen contained in the blocked space of the porous electrode  6   c  is maintained at a constant level through application of the small current iCP to the Vs cell  6 . Thus, the blocked space serves as an internal reference oxygen source. 
     When the porous electrode  6   c  of the Vs cell  6  serves as an internal reference oxygen source, an electromotive force is generated in the Vs cell  6  in accordance with the difference in oxygen concentration between the first measurement chamber  20  and the internal reference oxygen source. As a result, a voltage Vs built on the side of the porous electrode  6   c  (terminal T 22 ) corresponds to the concentration of oxygen contained in the first measurement chamber  20 . Since the voltage Vs is applied to the differential amplifier AMP, the differential amplifier AMP outputs a voltage in accordance with the deviation of the input voltage from the reference voltage VC 0  (VC 0 —input voltage). The output voltage is applied to the porous electrode  4   b  of the first pump cell  4  via the resistor R 0 . 
     As a result, the first pump current IP 1  flows to the first pump cell  4 . Through control of the first pumping current IP 1 , control is performed such that a constant electromotive force is generated in the Vs cell  6  (between the terminals T 21  and T 22 ) (in other words, such that the oxygen concentration of the first measurement chamber  20  becomes constant). 
     That is, when the measurement gas enters the first measurement chamber  20  via the diffusion-controlling layer  4   d , the drive circuit  40  controls the concentration of oxygen contained in the first measurement chamber  20  to a constant level. 
     The thus-controlled concentration of oxygen contained in the first measurement chamber  20  is set such that a small amount of oxygen (e.g., 1000 ppm) exists, thereby preventing decomposition of NOx contained in the measurement gas contained in the first measurement chamber  20  which would otherwise result from supply of the first pump current IP to the first pump cell  4 . The reference voltage VC 0  for determining this concentration of oxygen is set to 100 mV to 200 mV. The resistor R 0  disposed between the output terminal of the differential amplifier AMP and the porous electrode  4   b  (terminal T 11 ) is adapted to detect the first pump current IP 1 . A voltage VIP 1  built across the resistor R 0  is input to the ECU  50  as a detection signal corresponding to the first pump current IP 1 . 
     A constant voltage VP 2  is applied between the porous electrodes  8   b  and  8   c  of the second pump cell  8  of the sensor body  2   a  (terminals T 31  and T 32 ) via a resistor R 3 , which is a component of the detection circuit  42 . The constant voltage VP 2  is applied to the second pump cell  8  in such a direction that the porous electrodes  8   c  (terminal T 32 ) and  8   b  (terminal T 31 ) become a positive electrode and a negative electrode, respectively, so that current flows from the porous electrode  8   c  (terminal T 32 ) to the porous electrode  8   b  (terminal T 31 ) to thereby pump out oxygen from the second measurement chamber  26 . The constant voltage VP 2  is set to such a voltage, for example 450 mV, that there can be decomposed NOx contained in the measurement gas which has flowed from the first measurement chamber  20  into the second measurement chamber  26  via the diffusion-controlling layers  6   d  and  22   d , to thereby pump out oxygen generated through the decomposition. 
     The resistor R 3  is adapted to convert to a voltage VIP 2  the second pump current IP 2  flowing through the second pump cell  8  (between the terminals T 31  and T 32 ) as a result of application of the constant voltage VP 2  and adapted to input the voltage VIP 2  to the ECU  50  as a detection signal corresponding to the second pump current IP 2 . 
     In the thus-configured NOx concentration detection apparatus  30 , the drive circuit  40  controls to a constant level the concentration of oxygen contained in the measurement gas which has flowed into the first measurement chamber  20  via the diffusion-controlling layer (first diffusion-controlling layer)  4   d . The measurement gas controlled to a constant oxygen concentration flows from the first measurement chamber  20  into the second measurement chamber  26  via the diffusion-controlling layers (second diffusion-controlling layers)  6   d  and  22   d . Accordingly, the first pump current IP 1  flowing to the first pump cell  4  varies in accordance with the concentration of oxygen contained in the measurement gas. The second pump current IP 2  flowing through the second pump cell  8  varies in accordance with the concentration of NOx contained in the measurement gas. 
     The detection signals VIP 1  and VIP 2  corresponding to the first and second pumping currents IP 1  and IP 2 , respectively, and the voltage level VR corresponding to the label resistor RL are converted to digital data by an unillustrated A/D converter. Reading and carrying out a predetermined computation on the digital data, the ECU  50  determines the concentration of oxygen and NOx contained in the measurement gas. 
     In order to secure accuracy of measuring the oxygen and NOx concentrations, the temperature of the sensor body  2   a  must be controlled to a constant level. To meet this end, the ECU  50  controls current supplied to the heaters  12  and  14  (between terminals T 41  and T 42 ) from the heater electrification circuit  44  so that the sensor temperature attains a target value. 
     FIG. 4 is a table showing the relationship between variations in characteristics of the sensor body  2   a  and ranks identified by the resistance of the label resistor RL. 
     As shown in FIG. 4, according to the present embodiment, variations in the first pump current IPI as measured when the oxygen concentration of the measurement gas is set identical to that of the atmosphere (20.9%) (hereinafter referred to as atmospheric data) are classified into six categories; and variations in gain of the second pump current IP 2  as measured while the NOx concentration of the measurement gas is varied with the oxygen concentration of the measurement gas held at 0% (hereinafter referred to as 0% O 2  data) are classified into six categories. 
     Specifically, the atmospheric data are classified into the following six categories (unit: mA): 6.500 to 6.749; 6.750 to 6.999; 7.000 to 7.249; 7.250 to 7.499; 7.500 to 7.749; and 7.750 to 7.999. A correction coefficient β is assigned values of −2 to 3 for the respective categories. 
     The 0% O 2  data are classified into the following six categories (unit: ppm/μA): 155 to 159; 160 to 164; 165 to 169; 170 to 174; 175 to 179; and 180 to 184. A correction coefficient α is assigned values of −2 to 3 for the categories. 
     Since the atmospheric data are classified into six categories and the 0% O 2  data are classified into six categories, a total of 36 categories are established. The 36 categories are respectively named ranks 1 to 36. Each rank is assigned a different combination of values for the correction coefficients α and β. 
     In the present embodiment, the reference resistor RR of the label detection circuit  38  has a resistance of 30.1 kΩ; the source voltage VDD is 5 V; and the resistance of the label resistor RL is set as described in the upper field of each cell of the table of FIG. 4 so that the voltage level VR differs by not less than 100 mV between ranks 1 to 36. The lower field of each cell carries the voltage level VR to be input to the ECU  50  when the resistance of the label resistor RL is set as described in the upper field. 
     In each of ranks 1 to 36, the center value of the corresponding atmospheric data and the center value of the corresponding 0% O 2  data are standard values Ast(β) and GP 2 st(α), respectively, representative of the rank. 
     Specifically, in ranks 1 to 6 to which a correction coefficient β of −2 is assigned, Ast(β) is 6.625 mA; in ranks 7 to 12 to which a correction coefficient β of −1 is assigned, Ast(β) is 6.875 mA; in ranks 13 to 18 to which a correction coefficient β of 0 is assigned, Ast(β) is 7.125 mA; in ranks 19 to 24 to which a correction coefficient β of 1 is assigned, Ast(β) is 7.350 mA; in ranks 25 to 30 to which a correction coefficient β of 2 is assigned, Ast(β) is 7.625 mA; and in ranks 31 to 36 to which a correction coefficient β of 3 is assigned, Ast(β) is 7.875 mA. 
     In rank (6n+1) (where n=0, 1, . . . , 5) to which a correction coefficient α of −2 is assigned, GP 2 st(α) is 157 ppm/μA; in rank (6n+2) to which a correction coefficient α of −1 is assigned, GP 2 st(α) is 162 ppm/μA; in rank (6n+3) to which a correction coefficient α of 0 is assigned, GP 2 st(α) is 167 ppm/μA; in rank (6n+4) to which a correction coefficient α of 1 is assigned, GP 2 st(α) is 172 ppm/μA; in rank (6n+5) to which a correction coefficient α of 2 is assigned, GP 2 st(α) is 177 ppm/μA; and in rank (6n +6) to which a correction coefficient α of 3 is assigned, GP 2 st(α) is 182 ppm/μA. 
     Since the categories of the atmospheric data are identical in data span, the standard value Ast(β) of the atmospheric data for a rank whose correction coefficient is β is calculated by Expression (1) below through use of the correction coefficient β and the standard value Ast(0) (=7.125 mA; standard value Ast(β) at a correction coefficient β of 0). 
     
       
         Ast(β)=Ast(0)+0.250×β(mA)  (1) 
       
     
     As shown in FIG. 6, the first pump current IP 1  and the oxygen concentration are directly proportional. Thus, in the case of the correction coefficient β, the standard gain GP 1 st(β) in the first pump current IP 1  with respect to the oxygen concentration can be calculated by Expression (2) below through use of the standard value Ast(β) and the fact that the oxygen concentration of the atmosphere is 20.9%. 
     
       
         GP 1 st(β)=20.9/Ast(β) (%/mA)  (2) 
       
     
     
       
         =20.9/(Ast(0)+0.250×β)  (2a) 
       
     
     Since the categories of the 0% O 2  data are also identical in data span, the standard value (standard gain) GP 2 st(α) of the 0% O 2  data for a rank whose correction coefficient is α is calculated by Expression (3) below through use of the correction coefficient α and the standard value GP 2 st(0) (=167 ppm/μA; standard value GP 2 st(α) at a correction coefficient α of 0). 
     
       
         GP 2 st(α)=GP 2 st(0)+5×(ppm/μA)  (3) 
       
     
     Through use of the thus-calculated standard gains GP 1 st(β) and GP 2 st(α) and Expressions (4) and (5) below, the oxygen concentration and the NOx concentration of the measurement gas can be obtained from a detected value of the first pump current IP 1  and a detected value of the second pump current IP 2 , respectively. 
     
       
         Oxygen concentration=IP 1 ×GP 1 st(β)  (4) 
       
     
     
       
         NOx concentration=IP 2 ×GP 2 st(α)+IP 2 off  (5) 
       
     
     where IP 2 off is the offset of the second pump current that flows at an NOx concentration of 0%. 
     When the resistance of the label resistor RL is set as described above, the ECU  50  executes processing as represented by the flowchart of FIG. 5 in order to measure the oxygen concentration and the NOx concentration of the measurement gas. 
     In step S 110 , the ECU  50  reads the voltage level VR output from the label detection circuit  38 . Subsequently, in step S 120 , on the basis of values of the correction coefficients α and β for the rank 1-36 corresponding to the read voltage level VR, the ECU  50  calculates the standard gain GP 1 st(β) for the first pump current IP 1  with respect to the oxygen concentration and the standard gain GP 2 st(α) for the second pump current IP 2  with respect to the NOx concentration in accordance with the above Expressions (2a) and (3), respectively. 
     In step S 130 , the ECU  50  reads the detection signals VIP 1  and VIP 2 . In step S 140 , the ECU  50  calculates the first pump current IP 1  (=VIP1/R0) and the second pump current IP 2  (=VIP2/R1). 
     In step S 150 , on the basis of the standard gain GP 1 st(β) calculated in step S 120  and the first pump current IP 1  calculated in step S 140 , the ECU  50  calculates the oxygen concentration in accordance with Expression (4). In step S 160 , on the basis of the standard gain GP 2 st(α) calculated in step S 120  and the second pump current IP 2  calculated in step S 140 , the ECU  50  calculates the NOx concentration in accordance with Expression (5). Then, the ECU  50  returns to step S 130  and repeats steps S 130  through S 160 . 
     Herein, the NOx concentration and the oxygen concentration are simply measured. However, since the relationship between the NOx concentration and the second pump current IP 2  depends on the oxygen concentration, the NOx concentration obtained in step S 160  may be corrected for the oxygen concentration obtained in step S 150 . 
     As described above, according to the present embodiment, in the NOx sensor  2  equipped with the connector, the label resistor RL whose resistance corresponds to rank assigned to the sensor body  2   a  is attached to the connector  2   b.    
     Thus, according to the present embodiment, through identification of the resistance of the resistor RL, a characteristic of the sensor body  2   a  can be identified. On the basis of the identified characteristic of the sensor body  2   a , the NOx concentration and the oxygen concentration of the measurement gas can be accurately measured. 
     Also, according to the present embodiment, both relationships between NOx concentration and second pump current and between oxygen concentration and first pump current serve as characteristics of the sensor body  2   a  and are made to correspond to the resistance of a single label resistor RL. 
     Specifically, two different characteristics of the sensor body  2   a  can be identified simply by adding a single resistor and two terminals for identifying the resistance of the resistor to the connector  2   b . Thus, through employment of minimal configuration, detection accuracy can be significantly improved. 
     Further, according to the present embodiment, the ranks have identical spans of atmospheric data and 0% O 2  data, so that the standard values (standard gains) Ast(β) and GP 2 st(α) that represent the atmospheric data and the 0% O 2  data, respectively, in each rank, can be calculated through use of simple expressions that employ the correction coefficients β and α and Ast(0) and GP 2 st(0), respectively. 
     Thus, according to the present embodiment, in the NOx concentration detection apparatus  30 , there is no need for storing the standard values Ast(β) and GP 2 st(α) for every rank; accordingly, the required storage capacity can be reduced. 
     While the embodiment of the present invention, as herein disclosed, constitutes a preferred form, it is to be understood that other forms might be adopted. 
     For example, in the above-described embodiment, the standard values Ast(β) and GP 2 st(α) are calculated through use of α and β. However, if a sufficiently large storage capacity is available, a table that contains all standard values may be prepared, so that relevant standard values may be read through input of the correction coefficients α and β. 
     The above-described embodiment employs 36 ranks because the voltage level VR differs by at least 100 mV between ranks so that the ECU  50  can reliably identify the voltage level VR. However, the number of ranks may be increased so long as the ECU  50  can reliably identify the voltage level VR. 
     While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.