Abstract:
A sensor control device, and a gas sensor and sensor control device for applying a voltage to a sensor element of the gas sensor including an oxygen ion conductor and a pair of electrodes formed on the oxygen ion conductor of the gas sensor. The sensor control device includes a potential output terminal electrically connected to one of the pair of electrodes and a potential reference terminal electrically connected to the other of the pair of electrodes constituting the sensor element. The sensor control device applies a target voltage to the sensor element via the potential output terminal, corrected to take into account variation in the potential of the potential reference terminal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sensor control device provided with a potential reference terminal and a potential output terminal, the sensor control device applying a voltage to a sensor element connected between the potential reference terminal and the potential output terminal. 
     2. Description of the Related Art 
     A sensor control device provided with a potential reference terminal and a potential output terminal has been used to apply a voltage to a sensor element connected between the potential reference terminal and the potential output terminal. 
     An example of such a sensor control device is an NOx sensor control device that applies a voltage to an NOx sensor as disclosed in WO03/083465 ( FIG. 1  and  FIG. 8 ). 
     The NOx sensor control device applies a voltage to the NOx sensor element (Ip 2  cell) connected between the potential reference terminal (Vcent terminal) and the potential output terminal (Ip 2  terminal) by setting the potential of the potential reference terminal (Vcent terminal) to be a reference point target potential, and by setting the potential of the potential output terminal (Ip 2  terminal) to be an output point target potential. 
     Both the reference point target potential and the output point target potential are set in such a manner that the voltage applied to the NOx sensor element (Ip 2  cell) remains at the target-applied voltage. 
     In the sensor control device in the related art, however, even in the case where the potential of the potential output terminal (Ip 2  terminal) is set to be the output point target potential at a high degree of accuracy, there is a possibility that the actual voltage applied to the sensor element assumes a voltage value different from the target applied voltage. This can occur when the potential of the potential reference terminal (Vcent terminal) fluctuates due to influence of noise or the like. 
     More specifically, because the sensor control device in the related art is configured so that the potential of the potential terminal (Ip 2  terminal) and the potential of the potential reference terminal (Vcent terminal) are independently controlled, a problem arises in that even when the potential of the potential output terminal (Ip 2  terminal) is set at a high degree of accuracy, the voltage applied to the sensor element varies when the potential of the potential reference terminal (Vcent terminal) fluctuates. 
     SUMMARY OF THE INVENTION 
     The invention was made in view of the foregoing problems, and an object thereof is to provide a sensor control device capable of suppressing fluctuation of the voltage applied to the sensor element connected between the potential reference terminal and the potential output terminal even when the potential of the potential reference terminal fluctuates. 
     The above objective is achieved in a first aspect of the invention by providing a sensor control device including a potential reference terminal and a potential output terminal, said sensor control device being adapted for applying a voltage to a sensor element having an oxygen ion conductor and a pair of electrodes formed on the oxygen ion conductor and for connecting the potential reference terminal to one of the pair of electrodes and the potential output terminal to the other of the pair of electrodes, said sensor control device comprising: 
     a reference potential setting section for setting the potential of the potential reference terminal to a reference point target potential; 
     an output potential setting section for setting the potential of the potential output terminal to an output point target potential so that a target voltage is applied to the sensor element with reference to the potential of the potential reference terminal set to the reference point target potential; and 
     an output point potential correction section for correcting the output point target potential according to fluctuation in the potential of the potential reference terminal from the reference point target potential, so that the voltage applied to the sensor element remains at the target applied voltage, 
     said output potential setting section adjusting the potential of the potential output terminal so as to assume the corrected output point target potential. 
     More specifically, the sensor control device is configured to correct the output point target potential in response to the extent of fluctuation in potential between the actual potential of the potential reference terminal and the reference point target potential, and to set the potential of the potential output terminal so to assume the corrected output point target potential. In this manner, the voltage applied to the sensor element remains at the target applied voltage. 
     By correcting the output point target potential in response to the extent of fluctuation in potential of the potential reference terminal as described above, the sensor control device is able to markedly suppress deviation in the voltage applied to the sensor element from the target applied voltage. This is the case even when the actual potential of the potential reference terminal fluctuates due to the influence of noise and the like. 
     Hence, according to the invention, even when the potential of the potential reference terminal fluctuates, it is possible to suppress fluctuation in the voltage applied to the sensor element connected between the potential reference terminal and the potential output terminal. 
     In the case where the potential of the potential reference terminal is higher than the reference point target potential, the output point potential correction section is able to correct the potential of the potential output terminal by raising the potential of the potential output terminal above the output point target potential by a quantity corresponding to the increase in potential (the amount of fluctuation in potential) of the reference point target potential. Conversely, in the case where the potential of the potential reference terminal is below the reference point target potential, the output point potential correction section is able to correct the potential of the potential output terminal by lowering the potential of the potential output terminal below the output point target potential by a quantity corresponding to the decrease in potential of the reference point target potential. 
     The above object is also achieved in a second aspect of the invention by providing a gas sensor and a sensor control device for applying a voltage to a sensor element of the gas sensor, said gas sensor comprising: 
     a first measurement chamber into which a gas to be measured is introduced via a first diffusion resistor portion; 
     a first oxygen ion pump cell comprising a first oxygen ion conductor and first and second electrodes formed on the first oxygen ion conductor so that the first electrode is arranged in the first measurement chamber for pumping oxygen into or out of the gas to be measured that has been introduced into the first measurement chamber; 
     a second measurement chamber into which the gas to be measured after having been acted on in the first measurement chamber is introduced via a second diffusion resistor portion; 
     a second oxygen ion pump cell comprising a second oxygen ion conductor and third and fourth electrodes formed on the second oxygen ion conductor so that the third electrode is arranged in the second measurement chamber, wherein a current corresponding to the concentration of a particular gas in the second measurement chamber flows in the second oxygen ion pump cell; 
     a reference oxygen chamber set to have a reference oxygen partial pressure atmosphere; and 
     an oxygen partial pressure detection cell comprising a third oxygen ion conductor and fifth and sixth electrodes formed on the third oxygen ion conductor so that the fifth electrode is arranged in the first measurement chamber and the sixth electrode is arranged in the reference oxygen chamber; 
     said sensor control device comprising a potential output terminal and a reference output terminal electrically connected to said third and fourth electrodes of the second oxygen ion pump cell, respectively, or to said fifth and sixth electrodes of the oxygen partial pressure detection cell, respectively, 
     said sensor control device further comprising: 
     a reference potential setting section for setting the potential of the potential reference terminal to a reference point target potential; 
     an output potential setting section for setting the potential of the potential output terminal to an output point target potential so that a target voltage is applied to the sensor element with reference to the potential of the potential reference terminal set to the reference point target potential; and 
     an output point potential correction section for correcting the output point target potential according to fluctuation in the potential of the potential reference terminal from the reference point target potential, so that the voltage applied to the sensor element remains at the target applied voltage, 
     said output potential setting section adjusting the potential of the potential output terminal so as to assume the corrected output point target potential. 
     In the gas sensor element having two oxygen ion pump cells and two measurement chambers, the current flowing into the second oxygen ion pump cell is extremely small. Hence, even when the potential of the potential reference terminal used as a reference of the applied voltage to the second oxygen ion pump fluctuates to a small extent, current flowing into the second oxygen ion pump cell does fluctuate, which current fluctuation can significantly deteriorate the measurement accuracy of the concentration of a particular gas. 
     By making the second oxygen ion pump cell in the gas sensor element the sensor element to which a voltage is applied in the sensor control device of the invention, which is a preferred embodiment of the invention, it is possible to suppress fluctuation in the voltage applied to the second oxygen ion pump cell, which can in turn enhance the detection accuracy of the concentration of a particular gas. 
     The sensor control device may be configured such that the output point potential correction section comprises a non-inverting amplifier circuit, and the potential of the potential reference terminal is applied to the input of the non-inverting amplifier. 
     By configuring a non-inverting amplifier circuit comprising an analog circuit as the output point potential reference correction section, it is possible to achieve an inexpensive circuit capable of correcting the output point target potential with a proper response to fluctuation in the potential of the potential reference terminal. 
     The sensor control device may be provided with plural potential output terminals, in which case plural output potential setting sections and output point potential correction sections are provided corresponding to respective plural potential output terminals. 
     By providing plural potential output terminals, output potential setting sections, and output point potential correction sections, not only is it possible to apply a voltage to the respective components of a sensor element that require an external voltage, but it is also possible to suppress fluctuation in the value of the voltage applied to the respective components induced by a fluctuation at the potential reference terminal. 
     Hence, according to the invention, even when the potential of the potential reference terminal fluctuates, it is possible to suppress fluctuation in voltage applied to each of the plural components. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view schematically showing the configuration of an NOx detection device provided with a sensor control device according to the invention; 
         FIG. 2  is a cross section showing the internal configuration of a gas sensor element; 
         FIG. 3  is a circuit diagram of a Vp 2  setting circuit connected to an Ip 2  driver and a second pump cell; and 
         FIG. 4  is a view showing measurement results of the change in voltage applied to the second pump cell in response to fluctuation in the potential of a Vcent terminal (potential reference terminal) in the sensor control device according to one embodiment of the invention and a sensor control device of the related art. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, an embodiment of the invention will be described with reference to the drawings. However, the present invention should not be construed as being limited thereto. 
       FIG. 1  is a view schematically showing the configuration of an NOx detection device  1  provided with a sensor control device  50  to which the invention is applied. 
     The NOx detection device  1  is used to detect a particular gas (NOx in this embodiment) in the exhaust gas of an internal combustion engine (engine). The NOx detection device  1  includes a gas sensor element  10  and the sensor control device  50  as well as an unillustrated heater and heater control circuit. 
     The sensor control device  50  not only carries out energization control of the gas sensor element  10 , but also detects a sensor signal. The heater heats the gas sensor element  10  to maintain it at the operating temperature (in other words, the activation temperature), and the heater control circuit carries out energization control for the heater. 
     The gas sensor element  10  is a sensor element for detecting NOx, and includes a first pump cell  111 , an oxygen concentration detection cell  112 , and a second pump cell  113 . 
     The gas sensor element  10  is heated to the activation temperature by the heater provided separately. 
       FIG. 2  is a cross section showing the internal configuration of the gas sensor element  10 . 
     The gas sensor element  10  has a structure in which the first pump cell  111 , the oxygen concentration detection cell  112 , and the second pump cell  113  are laminated via insulation layers  114  and  115  primarily made of alumina, and includes a first measurement chamber  159  into which a gas to be measured is taken via a first diffusion resistor  116  made of porous alumina. 
     The gas sensor element  10  is able to pump oxygen into and out of the first measurement chamber  159  by means of the first pump cell  111 . It is configured such that the oxygen concentration detection cell  112  is able to measure the difference between the oxygen concentration of a reference oxygen chamber  118  in which the oxygen concentration is controlled to be constant and that of the first measurement chamber  159  (that is, the oxygen concentration in the interior of the first measurement chamber  159 ). 
     The first pump cell  111  comprises a first solid electrolyte layer  131  made of zirconia having oxygen ion conductivity, and a first electrode  135  and a second electrode  137  disposed to sandwich the first solid electrolyte layer  131 . The first electrode  135  and the second electrode  137  are made of platinum or platinum alloy, and a protection layer  122  made of a porous material is formed on the surface of each electrode. 
     The second pump cell  113  comprises a second solid electrolyte layer  141  made of zirconia having oxygen ion conductivity, and a third electrode  145  and a fourth electrode  147  disposed on the second solid electrolyte layer  141  at the surface facing the insulation layer  115 . The third electrode  145  and the fourth electrode  147  are made of platinum or platinum alloy. 
     The oxygen concentration detection cell  112  comprises a third solid electrolyte layer  151  for detection made of zirconia having oxygen ion conductivity, and a fifth electrode  155  for detection and a sixth electrode  157  for reference disposed so as to sandwich the solid electrolyte layer  151  for detection. The electrode  155  for detection and the electrode  157  for reference are made of platinum or platinum alloy. 
     When the gas sensor element  10  is heated to the activation temperature (for example, 750° C.) and brought into an activated state, a first pump current Ip 1  flowing into the first pump cell  111  is controlled by the sensor control device  50  so that voltage Vs applied to the electrodes of the oxygen concentration detection cell  112  stays at a pre-set constant voltage (for example, 425 mV). The first pump current Ip 1  is generated as oxygen ions migrate in the first solid electrolyte layer  131  between the electrodes (between the first electrode  135  and the second electrode  137 ) in the first pump cell  111 . 
     The gas sensor element  10  includes a second measurement chamber  161  to the rear (on the right in the drawing) of the first measurement chamber  159 . A second diffusion resistor  117  made of a porous material is formed between the first measurement chamber  159  and the second measurement chamber  161 . The second measurement chamber  161  is formed so that it penetrates through the oxygen concentration detection cell  112  in the lamination direction and the third electrode  145  of the second pump cell  113  is disposed therein. 
     In the gas sensor element  10 , when a voltage is applied to the second pump cell  113  by the sensor control device  50 , NOx present in the second measurement chamber  161  undergoes dissociation (reduction) due to the catalytic action of the third electrode  145  of the second pump cell  113 . Then, a second pump current Ip 2  begins to flow as oxygen ions obtained as the result of the dissociation migrate in the second solid electrolyte layer  141  between the electrodes (the third electrode  145  and the fourth electrode  147 ) in the second pump cell  113 . 
     More specifically, the second pump cell  113  is configured so that second pump current Ip 2  corresponding to the concentration of a particular gas component (NOx (oxides of nitrogen)) present in the second measurement chamber  161  flows between the electrodes (the third electrode  145  and the fourth electrode  147 ). The sensor control device  50  connected to the gas sensor element  10  is thus able to detect the NOx concentration on the basis of the magnitude and the integral value of the second pump current Ip 2 . 
     Referring to  FIG. 1  again, the sensor control device  50  includes an Ip 1  driver  51 , a PID control circuit  52 , an operational amplifier  53 , an Rpvs measurement circuit  54 , and an Ip 2  driver  56 . 
     The sensor control device  50  is connected to the first pump cell  111  of the gas sensor element  10  via a wire  42  and a wire  43 , to the second pump cell  113  of the gas sensor element  10  via the wire  42  and a wire  44 , and to the oxygen concentration detection cell  112  (Vs cell  112 ) of the gas sensor element  10  via a wire  41  and the wire  42 . 
     The sensor control device  50  also includes an unillustrated output terminal, and the output terminal is connected to an analog input terminal of an unillustrated electronic control unit (hereinafter, also abbreviated as ECU) of the internal combustion engine. The electronic control unit (ECU) controls the respective portions of the internal combustion engine systematically, and performs various kinds of control processing, such as air-fuel ratio control and exhaust gas control of the internal combustion engine. 
     The sensor control device  50  outputs a voltage signal proportional to the magnitude of a current flowing into the first pump cell  111 , a voltage signal proportional to the magnitude of a current flowing into the second pump cell  113 , a voltage signal proportional to the voltage across the electrodes (between the electrodes Vs+ and Vs−) of the oxygen concentration detection cell  112 , the voltage at each terminal of the gas sensor element  10 , the internal resistance of the gas sensor element  10 , and so forth to the ECU. 
     The Ip 1  driver  51  is an operational amplifier used to drive Ip 1  current into the first pump cell  111 . The inverting input terminal (−) is connected to a Vcent terminal (potential reference terminal), the non-inverting input terminal (+) is connected to a reference potential output circuit  74 , and the output terminal is connected to the Ip 1 + terminal. The reference potential output circuit  74  outputs reference potential KVcent used as the reference in the sensor control device  50 , and this output potential (several V) corresponds to the reference point target potential. 
     The first pump cell  111  of the gas sensor element  10  is connected onto the path from the Vcent terminal to the Ip 1 + terminal via a Vs-/Ip-terminal, the wire  42 , and the wire  43 . The Ip 1  driver  51  forms a negative feedback circuit due to this configuration, and therefore controls the Ip 1  current in such manner that the potential of the Vcent terminal remains at the reference potential KVcent (the output potential of the reference potential output circuit  74 ). 
     The PID control circuit  52  controls the first pump current Ip 1  in such a manner that the voltage (electromotive force Vs) across the electrodes of the oxygen concentration detection cell  112  (Vs cell  112 ) remains at the Vs control target value, referring to the potential of the Vcent terminal. The Vs control target value is output from a Vs control target value output circuit  73 . 
     The PID control circuit  52  comprises a PID arithmetic circuit together with respective resistors and capacitors connected to a P 1  terminal, to a P 2  terminal, and to a P 3  terminal serving as input and output signal lines of the sensor control device  50 . The PID control circuit  52  performs a PID operation using the deviation, ΔVs, of the electromotive force Vs of the oxygen concentration detection cell  112  from the Vs control target value, and sets the potential of a Pout terminal so that the electromotive force Vs of the oxygen concentration detection cell  112  approaches the Vs control target value. 
     The Ip 1  current is controlled through the control operation by the PID control circuit  52  and the control operation by the Ip 1  driver  51  as described above. 
     More specifically, in a case where the electromotive force Vs of the oxygen concentration detection cell  112  is higher than the Vs control target value, the oxygen partial pressure in the first measurement chamber  159  is lower than the reference oxygen partial pressure. In such case, the PID control circuit  52  sets the potential of the Pout terminal so as to generate the Ip 1  current that pumps in oxygen by means of the first pump cell  111  to make up for the shortfall of oxygen, through the PID calculation based on the deviation ΔVs. 
     On the other hand, in a case where the electromotive force Vs of the oxygen concentration detection cell  112  is lower than the Vs control target value, the oxygen partial pressure in the first measurement chamber  159  is higher than the reference oxygen partial pressure. In such a case, the PID control circuit  52  sets the potential of the Pout terminal so as to generate Ip 1  current that pumps out excess oxygen by means of the first pump cell  111  through the PID calculation based on the deviation, ΔVs. 
     In addition, a constant current source circuit  72  of +several μA is connected to a Vs+ terminal, and the oxygen reference is generated by supplying an Icp current to the oxygen concentration detection cell  112  by this configuration. 
     The operational amplifier  53  connected between the Vs+ terminal and the PID control circuit  52  is part of a voltage follower circuit. Accordingly, because a high impedance is seen from the PID control circuit  52  side of the Vs+ terminal, the operational amplifier  53  restricts the flow of supply current from the constant current source circuit  72  of several +μA into the PID control circuit  52 . 
     The Rpvs measurement circuit  54  measures the temperature of the gas sensor element  10  on the basis of internal resistance Rpvs of the oxygen concentration detection cell  112 , and it comprises an operational amplifier, a resistor, a capacitor, and so forth. The Rpvs measurement circuit  54  gives rise to a change in voltage corresponding to the internal resistance value of the oxygen concentration detection cell  112 , this value correlating with the element temperature when a specific current flows through the oxygen concentration detection cell  112  at a specific cycle. The Rpvs measurement circuit  54  then multiplies the resulting difference in voltage across the terminals of the oxygen concentration detection cell  112  by a constant number using an operational amplifier, and outputs a VRpvs voltage signal that varies within a range of 0 to 4.5 V. The VRpvs voltage signal is output to the unillustrated ECU. 
     When the Rpvs measurement circuit  54  sends the measurement current into the oxygen concentration detection cell  112 , the Rpvs measurement circuit  54  disconnects the connection between the PID control circuit  52  and the operational amplifier  53  by driving a switch SW interposed between the PID control circuit  52  and the operational amplifier  53 . In this manner, a change in voltage caused by the measurement current does not give rise to a change in an output of the PID control circuit  52 . In other words, the Rpvs measurement circuit  54  measures the internal resistance Rpvs of the oxygen concentration detection cell  112 , while the PID control circuit  52  and the operational amplifier  53  are disconnected by the switch SW. 
     The Ip 2  driver  56  is an operational amplifier that is used to send the Ip 2  current into the second pump cell  113 . The non-inverting input terminal (+) is connected to a Vp 2  setting circuit  57 , the inverting input terminal (−) is connected to an Ip 2 /Vp 2  terminal, and the output terminal is connected to the Ip 2 /Vp 2  terminal via a detection resistor element  78  of the Ip 2  current. In short, the Ip 2  driver  56  controls the potential of the Ip 2 /Vp 2  terminal so as to be equal to the output potential of the Vp 2  setting circuit  57 . 
     The Vp 2  setting circuit  57  is provided to set the potential of the potential output terminal (the Ip 2 /Vp 2  terminal) so that the voltage applied to the second pump cell  113  is the target applied voltage TVp 2 , referring to the Vcent terminal (potential reference terminal). 
     The circuit diagram of the Vp 2  setting circuit  57  is shown in  FIG. 3 . 
     In  FIG. 3 , connections with the Ip 2  driver  56  and the second pump cell  113  in addition to the Vp 2  setting circuit  57  are illustrated schematically. 
     As is shown in  FIG. 3 , the Vp 2  setting circuit  57  includes a target potential output circuit  75  and an output point potential correction circuit  76 . 
     The output potential (output point target potential Va) of the target potential output circuit  75  is set to be the potential of the potential output terminal (the Ip 2 /Vp 2  terminal) when voltage applied to the second pump cell  113  is the target applied voltage TVp 2 , referring to the reference potential KVcent set to be the output potential (rated value) of the reference potential output circuit  74 . The voltage value at which the second pump cell  113  changes to the state where NOx undergoes dissociation (reduction) is set to be the target applied voltage TVp 2 . 
     The output point potential correction circuit  76  includes an operational amplifier  81 , first through fourth resistor elements R 1  through R 4 , and a constant potential output circuit  83 . 
     In the operational amplifier  81 , the non-inverting input terminal (+) is connected to the Vcent terminal (potential reference terminal), the inverting input terminal (−) is connected to the constant potential output circuit  83  via the first resistor element R 1 , and the output terminal is connected to the non-inverting input terminal (+) of the Ip 2  driver  56  via the third resistor element R 3 . The constant potential output circuit  83  outputs a constant potential Vb. 
     The first resistor element R 1  is connected between the inverting input terminal (−) of the operational amplifier  81  and the constant potential output circuit  83 , and the second resistor element R 2  is connected between the inverting input terminal (−) and the output terminal of the operational amplifier  81 . 
     The third resistor element R 3  is connected between the output terminal of the operational amplifier  81  and an output terminal  77  of the Vp 2  setting circuit  57 . The fourth resistor element R 4  is connected between the output terminal  77  and the output terminal of the target potential output circuit  75 . The output terminal  77  is connected to the non-inverting input terminal (+) of the Ip 2  driver  56 . 
     In this embodiment, the resistance of the first resistor R 1  is the same as the second resistor R 2 , while resistance of the third resistor R 3  is the same as the fourth resistor R 4 . 
     In the Vp 2  setting circuit  57  provided with the target potential output circuit  75  and the output point potential correction circuit  76  as described above, the potential of the output terminal  77  (the potential Vout after the Vp 2  is corrected) is expressed by Mathematical Formula 1 below using the potential Vcent of the Vcent terminal, the output point target potential Va, and the constant potential Vb. 
     
       
         
           
             
               
                 
                   Vout 
                   = 
                   
                     Vcent 
                     + 
                     
                       Va 
                       2 
                     
                     - 
                     
                       Vb 
                       2 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Mathematical 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ] 
                 
               
             
           
         
       
     
     Because the Ip 2  driver  56  controls the potential of the Ip 2 /Vp 2  terminal so as to be equal to the potential (the potential Vout after the Vp 2  is corrected) of the output terminal  77  of the Vp 2  setting circuit  57 , the potential of the Ip 2 /Vp 2  terminal is set to be the potential Vout after the Vp 2  is corrected. 
     The actual voltage Vp 2  applied to the second pump cell  113  is determined according to the voltage difference between the potential of the Vcent terminal (potential reference terminal) and the potential of the Ip 2 /Vp 2  terminal. If the voltage drop in a resistor element  79  connected between the Vs-/Ip-terminal and the Vcent terminal is a constant voltage value, Vd, then the voltage Vp 2  applied to the second pump cell  113  can be expressed by Mathematical Formula 2:
 
 Vp 2 =V out− V cent− Vd   [Mathematical Formula 2]
 
     Mathematical Formula 3 is obtained by substituting the potential Vout after the Vp 2  is corrected which is expressed by including Mathematical Formula 1 into Mathematical Formula 2. 
     
       
         
           
             
               
                 
                   
                     Vp 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                   
                   = 
                   
                     
                       Va 
                       2 
                     
                     - 
                     
                       Vb 
                       2 
                     
                     - 
                     Vd 
                   
                 
               
               
                 
                   [ 
                   
                     Mathematical 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     3 
                   
                   ] 
                 
               
             
           
         
       
     
     It is seen from Mathematical Formula 3 that because the voltage Vp 2  applied to the second pump cell  113  (the voltage Vp 2  across the terminals of the second pump cell  113 ) can be expressed by a constant value (the output point target potential Va, the constant potential Vb, and the constant voltage value Vd), it does not vary with the potential Vcent of the Vcent terminal (potential reference terminal). 
     As described above, the voltage Vp 2  (the voltage Vp 2  across the terminals of the second pump cell  113 ) applied to the second pump cell  113  does not vary with fluctuation in the potential of the Vcent terminal because the output point potential correction circuit  76  corrects the potential (the potential Vout after the Vp 2  is corrected) of the output terminal  77  of the Vp 2  setting circuit  57  in response to fluctuation in the potential of the Vcent terminal. 
     More specifically, when the potential of the Vcent terminal (potential reference terminal) becomes higher than the reference point target potential, the output point potential correction circuit  76  raises the potential of the output terminal  77  above the output point target potential Va by a quantity corresponding to the increase of the Vcent terminal potential. Conversely, when the potential of the Vcent terminal decreases below the reference point target potential, the output point potential correction circuit  76  lowers the potential of the output terminal  77  below the output point target potential Va by a quantity corresponding to the decrease in the Vcent terminal potential. 
     As described above, even when the potential of the Vcent terminal fluctuates (deviates from the reference potential KVcent) for any reason, the sensor control device  50  in the NOx detection device  1  is able to suppress fluctuation in the voltage Vp 2  applied to the second pump cell  113  by correcting the potential of the Ip 2 /Vp 2  terminal according to the fluctuation in Vcent terminal potential. 
     Hence, because the sensor control device  50  corrects the potential (output point target potential) of the output terminal  77  according to the fluctuation in potential of the Vcent terminal (potential reference terminal), even when the potential of the Vcent terminal (potential reference terminal) fluctuates due to the influence of noise and the like, it is possible to prevent the voltage Vp 2  applied to the second pump cell  113  from deviating from the target applied voltage TVp 2 . 
     The sensor control device  50  is therefore able to suppress deterioration in control accuracy of voltage applied to the second pump cell  113  even where the potential of the Vcent terminal (potential reference terminal) fluctuates due to noise. 
     The change of the voltage Vp 2  applied to the second pump cell  113  in response to a fluctuation in the potential of the Vcent terminal (potential reference terminal) in the sensor control device  50  of this embodiment and that in a sensor control device of the related art were measured, and the measurement results are set forth in  FIG. 4 . 
     The sensor control device in the related art used for the measurement was built by replacing the Vp 2  setting circuit  57  of the sensor control device  50  with the target potential output circuit  75  alone, so that the output terminal of the target potential output circuit  75  is connected to the non-inverting input terminal (+) of the Ip 2  driver  56 . In short, the sensor control device of the related art used for the measurement was built by eliminating the output point potential correction circuit  76  from the sensor control device  50  described above. 
     As is shown in  FIG. 4 , according to the measurement result of the sensor control device in the related art, the applied voltage Vp 2  decreases as the potential Vcent of the Vcent terminal (potential reference terminal) becomes higher, and the voltage Vp 2  applied to the second pump cell  113  therefore varies with a fluctuation in the potential of the Vcent terminal. In other words, in the sensor control device in the related art, because a fluctuation in the potential of the Vcent terminal is not corrected when the potential of the Ip 2 /Vp 2  terminal is set, when the potential of the Vcent terminal fluctuates, the voltage Vp 2  applied to the second pump cell  113  is different from the target applied voltage TVp 2 . 
     On the other hand, the measurement results of the sensor control device  50  of this embodiment confirm that the applied voltage Vp 2  remains substantially constant even when the potential Vcent of the Vcent terminal (potential reference terminal) fluctuates. Thus, the voltage Vp 2  applied to the second pump cell  113  hardly varies with a fluctuation in the potential of the Vcent terminal. 
     It is clear from the above measurement results that even when the potential of the Vcent terminal fluctuates, it is possible to suppress fluctuation in the voltage Vp 2  applied to the second pump cell  113  with the sensor control device  50  of this embodiment. 
     In this embodiment, the sensor control device  50  corresponds to the sensor control device, the Vcent terminal corresponds to the potential reference terminal, the Ip 2 /Vp 2  terminal corresponds to the potential output terminal, and the gas sensor element  10  (to be more specific, the second pump cell  113 ) corresponds to the sensor element. Also, the Ip 1  driver  51  and the reference potential output circuit  74  correspond to the reference potential setting section, the Ip 2  driver  56  and the target potential output circuit  75  correspond to the output potential setting section, and the output point potential correction circuit  76  corresponds to the output point potential correction section. 
     While one embodiment of the invention has been described, it should be apparent to those skilled in the art that the invention is not limited thereto, and that various changes in form and detail of the invention as shown and described above may be made. It is intended that such changes be included within the spirit and scope of the claims appended hereto. 
     For example, based on fluctuation in the potential of the Vcent terminal, not only may the potential of the Ip 2 /Vp 2  terminal be corrected, but also the potential of the Vs+ terminal may be corrected. 
     To be more specific, the sensor control device  50  applies a voltage not only to the second pump cell  113 , but also to the oxygen concentration detection cell  112 , and the voltage Vs applied to the oxygen concentration detection cell  112  is determined according to the difference between the Vcent terminal (potential reference terminal) potential and the Vs+ terminal potential. Hence, by providing a Vs output point potential correction circuit that corrects the potential of the Vs+ terminal according to a fluctuation in the potential of the Vcent terminal (potential reference terminal), it is possible to suppress fluctuation in the voltage applied to the oxygen concentration detection cell  112  even when the potential of the Vcent terminal (potential reference terminal) fluctuates. 
     The sensor control device configured as described above has plural potential output terminals and plural output potential setting sections and output point potential correction sections corresponding to the plural potential output terminals. The sensor control device configured as described above is able to apply a voltage to each cell in a gas sensor element provided with plural cells that requires an external voltage, and is also able to suppress fluctuation in the voltage applied to each cell induced by a fluctuation at the Vcent terminal (potential reference terminal). 
     In the sensor control device configured as described above, the Vcent terminal corresponds to the potential reference terminal, and the Ip 2 /Vp 2  terminals and the Vs+ terminals correspond to the plural potential output terminals. Also, the output point potential correction circuits  76  and the Vs output point potential correction circuits correspond to the plural output point potential correction sections, the Ip 2  drivers  56  and the target potential output circuits  75  correspond to the plural output potential setting sections, and the PID control circuits  52  and the Vs control target value output circuits  73  correspond to the plural output potential setting sections. 
     The sensor element to be controlled is not limited to the NOx sensor element described above, and for example, it may be any sensor element configured to include a single cell that requires voltage control such as a UEGO sensor (Universal Exhaust Gas Oxygen sensor). Further, the sensor element is not limited to a gas sensor element for detecting a gas, and the sensor control device of the invention is able to control any sensor that measures a subject when an external voltage is applied, such as a temperature sensor, a humidity sensor, and a load sensor. 
     This application is based on Japanese Patent Application No. 2005-320916 filed Nov. 4, 2005, the disclosure which is incorporated herein by reference in its entirety.