Patent Publication Number: US-2009229343-A1

Title: Sensor control apparatus and sensor control system

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sensor control apparatus connectable to a gas sensor for detecting a specific component in a gas to be measured (hereinafter also referred to as “to-be-measured gas”), such as exhaust gas exhausted from an internal combustion engine, and which controls and drives the gas sensor. 
     2. Description of the Related Art 
     A gas sensor has conventionally been known for detecting NO X  (nitrogen oxides) within exhaust gas exhausted from the engine of an automobile. Such a gas sensor includes a first pumping cell and a second pumping cell. The first pumping cell pumps oxygen out of a to-be-measured gas (exhaust gas) introduced into a first measurement chamber or pumps oxygen into the first measurement chamber from outside the gas sensor, to thereby adjust the oxygen concentration of the to-be-measured gas to a predetermined level. The second pumping cell decomposes NO X  contained in the to-be-measured gas introduced from the first measurement chamber to a second measurement chamber and having an adjusted oxygen concentration, whereby a current corresponding to the concentration of NO X  flows between electrodes of the second pumping cell. A heater is provided in the gas sensor, and solid electrolyte members which constitute the cells are heated, whereby the cells are maintained in an activated state. 
     Such a gas sensor is connected to a sensor control apparatus. As a result of the sensor control apparatus driving the gas sensor, a current corresponding to the oxygen concentration flows through the first pumping cell (specifically, between the electrodes of the first pumping cell), and a current corresponding to the NO X  concentration flows through the second pumping cell. Current signals output from the cells are converted to voltage signals by a signal processing circuit within the sensor control apparatus, and are output to an external engine control unit (ECU or the like) as an oxygen concentration signal and an NO X  concentration signal. Meanwhile, supply of electricity to the heater, which partially constitutes the gas sensor, is controlled by a heater control circuit within the sensor control apparatus, whereby the heater current undergoes ON/OFF control. 
     The current from which the NO X  concentration is detected (that is, the current flowing through the second pumping cell) is on a nA (nano-ampere) order, whereas the heater current is on an A (ampere) order. Further, the signal processing circuit and the heater control circuit are formed on a common circuit board within the sensor control apparatus. Therefore, the conventional sensor control apparatus has a problem in that noise generated at the time of switching the heater ON/OFF is transmitted to the signal processing circuit, and accuracy in detecting NO X  concentration is lowered. A gas-concentration detection apparatus which has addressed such a problem is known (see, for example, Patent Document 1). In the gas-concentration detection apparatus, a ground pattern which sets a reference potential in a sensing circuit (signal processing circuit) and a ground pattern which sets a reference potential in a heater control circuit are provided such that the ground patterns diverge from a ground terminal portion. In the gas-concentration detection apparatus, since the flow of heater current to the sensing current can be prevented, the reference potential in the sensing circuit can be stabilized. 
     However, in the case where the ground terminal portion is commonly used, a problem arises in that, when the heater is ON, the large heater current load changes the output voltage of the signal processing circuit. This problem can be avoided by converting the output voltage (analog) of the signal processing circuit to a digital value and sampling the digital value during a period in which the heater is OFF. However, in the case where the digital values sampled without consideration of the ON/OFF states of the heater are averaged, such average value can deviate from the actual value. Accordingly, in order to minimize the influence of the heater current load on the signal processing circuit, desirably the ground of the signal processing circuit and the ground of the heater control circuit are separately provided. 
     Further, there is a demand for recent sensor control apparatuses for providing communication control for communicating with an ECU via serial communication such as a CAN (Controller Area Network), as well as electricity supply control for controlling the supply of electricity to a sensor element within a gas sensor, and electricity supply control for controlling the supply of electricity to a heater (see, for example, Patent Document 2). In this case, a sensor control apparatus must be designed such that a ground pattern which sets a reference potential for a communication circuit is additionally provided on a common circuit board within the sensor control apparatus so as to newly provide a communication ground. 
     [Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2004-212284 
     [Patent Document 2] Japanese Patent Application Laid-Open (kokai) No. 2000-171435 
     3. Problems to Be Solved by the Invention 
     However, in the case where the ground pattern for the signal processing circuit, the ground pattern for the heater control circuit, and the ground pattern for the communication circuit are separately provided, the wiring for grounding the respective ground patterns becomes complex, which is undesirable. 
     SUMMARY OF THE INVENTION 
     The present invention has been achieved for solving the above-mentioned problems of the prior art, and an object thereof is to provide a sensor control apparatus which can minimize output fluctuation of a signal processing circuit and a communication circuit even while a large current is being supplied to a heater, and which can simplify the layout of ground wiring. 
     In accordance with a first aspect (1) of the invention, the above object has been achieved by providing a sensor control apparatus connectable to a gas sensor, the gas sensor including a detection element for detecting concentration of a specific gas in a to-be-measured gas, and a heater for heating the detection element to an element activation temperature. The sensor control apparatus comprises a signal processing circuit which controls supply of electricity to the detection element and detects a voltage signal output from the detection element corresponding to the concentration of the specific gas; a heater control circuit which controls supply of electricity to the heater; and a communication circuit which outputs, as a concentration signal, the voltage signal detected by the signal processing circuit to a first external device by means of serial communication, wherein the signal processing circuit, the heater control circuit, and the communication circuit are implemented on a common circuit board; a power-system ground to which the heater control circuit is connected and a signal-system ground to which the signal processing circuit and the communication circuit are connected are independently provided on the circuit board; and the signal-system ground includes a first electrical path which establishes electrical connection between the circuit board and the first external device, and the power-system ground includes a second electrical path which is provided independently of the first electrical path and establishes electrical connection between the circuit board and a second external device different from the first external device. 
     In a preferred embodiment (2), the sensor control apparatus has a configuration according to (1) above, and is further characterized in that the gas sensor is an NO X  sensor for detecting concentration of NO X  as the concentration of the specific gas in the to-be-measured gas. 
     In a preferred embodiment (3), the sensor control apparatus has a configuration according to (1) or (2) above, and is further characterized in that the sensor control apparatus controls a sensor of an internal combustion engine; the first external device is an engine control unit for controlling the internal combustion engine; and the second external device is a battery for supplying electric power to the heater and the sensor control apparatus. 
     In accordance with a second aspect (4) of the invention, the above object has been achieved by providing a sensor control system comprising a gas sensor including a detection element for detecting concentration of a specific gas in a to-be-measured gas, and a heater for heating the detection element to an element activation temperature; and a sensor control apparatus according to (1) above connected to the gas sensor. 
     EFFECT OF THE INVENTION 
     According to the sensor control apparatus ( 1 ) of the invention, the power-system ground and the signal-system ground are independently provided on the circuit board. In addition, the first electrical path of the signal-system ground for establishing electrical connection between the circuit board and the first external device and the second electrical path of the power-system ground for establishing electrical connection between the circuit board and the second external device are provided independently of one another. Therefore, even when electric current is supplied to the heater, the above configuration can prevent the large heater current flowing through the heater control circuit from influencing the respective outputs of the signal processing circuit and the communication circuit. Accordingly, accuracy in detecting the concentration of the specific gas can be improved. Further, since the signal processing circuit and the communication circuit are connected to a single signal-system ground, the layout of the ground wiring can be simplified. 
     According to the sensor control apparatus ( 2 ) of the invention, the following additional effect is achieved. Namely, the gas sensor control is suitably used for controlling a gas sensor which handles a very weak current, such as an NO X  sensor which generates a current corresponding to the NO X  concentration as one type of concentration-representing signal. That is, configuration ( 2 ) can prevent heater current flowing through the heater control circuit from influencing the respective outputs of the signal processing circuit and the communication circuit. Accordingly, accuracy in detecting NO X  concentration can be improved. 
     According to the sensor control apparatus ( 3 ) of the invention, the following additional effect is achieved. Namely, when the sensor control apparatus is used to control a sensor of an internal combustion engine, the engine control unit can receive an accurate concentration signal output from the sensor control apparatus via the communication circuit. In this manner, the engine controller can accurately control the internal combustion engine on the basis of the concentration signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view schematically showing the configuration of an exhaust system and related components of an internal combustion engine system  1 . 
         FIG. 2  is a view schematically showing the configuration of a sensor control apparatus  2  and an NO X  sensor  10  connected to the sensor control apparatus  2 . 
         FIG. 3  is a view schematically showing the arrangement of electronic components, etc., mounted on a circuit board  20  of the sensor control apparatus  2 . 
         FIG. 4  is a graph showing fluctuation of CAN output voltages (CAN communication signals) and a voltage fluctuation of a signal ground in a sensor control apparatus in which a single GND terminal is commonly used. 
         FIG. 5  is a graph showing fluctuation of CAN output voltages (CAN communication signals) and a voltage fluctuation of a signal ground in the sensor control apparatus of the present embodiment in which two GND terminals are separately provided. 
         FIG. 6  is a graph showing a fluctuation of the NO X  output of the sensor control apparatus in which a single GND terminal is commonly used. 
         FIG. 7  is a graph showing a fluctuation of the NO X  output of the sensor control apparatus of the present embodiment in which two GND terminals are separately provided. 
     
    
    
     DESCRIPTION OF REFERENCE NUMERALS 
     Reference numerals used to identify various features shown in the drawings include the following:
       2 : sensor control apparatus     8 : battery (second external device)     9 : engine control unit (first external device)     10 : NO X  sensor (gas sensor)     20 : circuit board     26 : Ip1 cell/Vs cell control circuit     27 : Ip2 cell control circuit     28 : heater control circuit     29 : CAN circuit     30 : sensor terminal section     31 : external circuit terminal section     100 : sensor element     180 : heater   

     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A sensor control apparatus according to an embodiment of the present invention will next be described in detail with reference to the drawings. However, the present invention should not be construed as being limited thereto. 
     First, referring to  FIG. 1 , briefly described is the configuration of an internal combustion engine system  1  to which a sensor control apparatus  2 , which is an example sensor control apparatus according to the present invention, is attached. The sensor control apparatus  2  controls an NO X  sensor  10  capable of detecting the concentration of NO X  (specific gas) in exhaust gas (a to-be-measured gas). The sensor control apparatus  2  is connected with the NO X  sensor  10 , and constitutes the sensor control system together with the NO X  sensor  10 .  FIG. 1  is a view schematically showing the configuration of an exhaust system and related components of the internal combustion engine system  1 . 
     As shown in  FIG. 1 , the internal combustion engine system  1  includes an engine  5  for driving an automobile. An exhaust pipe  6  is connected to the engine  5  so as to discharge exhaust gas exhausted from the engine  5  outside the automobile. An NO X  selective reduction catalyst  7  for cleaning the exhaust gas is provided in the middle of the path of the exhaust pipe  6 . The NO X  selective reduction catalyst  7  causes NO X  to react with an NO X  reducer, whereby NO X  is converted to N 2  and H 2 O, which are harmless, through know chemical reactions. Although not illustrated, an injector for injecting aqueous urea solution into the exhaust gas flowing through the exhaust pipe  6  is provided upstream of the NO X  selective reduction catalyst  7  (on the upstream side of the flow path of the exhaust gas). 
     The NO X  sensor  10  for detecting the concentration of NO X  in the exhaust gas having passed trough the NO X  selective reduction catalyst  7  is disposed in the path of the exhaust pipe  6  located downstream of the NO X  selective reduction catalyst  7 . The NO X  sensor  10  is electrically connected via a harness (a bundle of signal wires)  4  to the sensor control apparatus  2 , which is disposed at a position separate from the NO X  sensor  10 . The NO X  sensor  10  detects NO X  concentration under control of the sensor control apparatus  2 . The sensor control apparatus  2  drives the NO X  sensor  10 , while receiving electrical power from a battery  8 . The sensor control apparatus  2  outputs a detection signal (concentration signal), which represents the NO X  concentration detected by use of the NO X  sensor  10 , to an engine control unit  9  (hereinafter also referred to as the “ECU  9 ”), which is connected to the sensor control apparatus  2  via a CAN (Controller Area Network) for automobiles  91 . 
     Next, the sensor control apparatus  2  and the NO X  sensor  10  will be described with reference to  FIG. 2 .  FIG. 2  shows the schematic configuration of the sensor control apparatus  2  and the NO X  sensor  10  connected to the sensor control apparatus  2 .  FIG. 2  shows, in section, the internal structure of a front end portion of the sensor element  100  of the NO X  sensor  10 . The left side in  FIG. 2  is the front side of the sensor element  100 . 
     The NO X  sensor  10  has a structure such that the sensor element  100  assuming the form of a narrow elongated plate is held in a housing (not shown) used to attach the NO X  sensor  10  to the exhaust pipe  6  (see  FIG. 1 ). The harness  4  with a connector for taking out a signal output from the sensor element  100  extends from the NO X  sensor  10 . Further, the harness  4  is connected to a sensor terminal section  30  of the sensor control apparatus  2 , which is mounted at a position separate from the NO X  sensor  10 , as described above. Thus, the NO X  sensor  10  and the sensor control apparatus  2  are electrically connected. 
     The structure of the sensor element  100  will next be described. As shown in  FIG. 2 , the sensor element  100  is configured such that three plate-like solid electrolyte members  111 ,  121  and  131  are arranged in layers with insulators  140  and  145  of alumina or the like intervening therebetween. A heater  180  is provided on the external side (lower side in  FIG. 2 ) of the solid electrolyte member  131 . The heater  180  includes laminated sheet-like insulating layers  181  and  182 , which contain a predominant amount of alumina, and a heater pattern  183 , which contains a predominant amount of Pt and is embedded between the insulating layers  181  and  182 . 
     The solid electrolyte members  111 ,  121  and  131  are formed from zirconia and have oxygen-ion conductivity when heated to an activation temperature. Porous electrodes  112  and  113  are provided on respective opposite surfaces of the solid electrolyte member  111  with respect to the direction of lamination of the sensor element  100  such that the electrodes  112  and  113  sandwich the solid electrolyte member  111 . The electrodes  112  and  113  are formed from Pt, a Pt alloy, cermet which contains Pt and ceramic, or a like material. A porous protective layer  114  of ceramic is formed on the surface of each of the electrodes  112  and  113  for protecting the electrodes  112  and  113  from deterioration, which could otherwise result from exposure to a poisonous component contained in the exhaust gas. 
     By causing a current to flow between the electrodes  112  and  113 , the solid electrolyte member  111  can pump oxygen, in either direction, between an atmosphere in contact with the electrode  112  (atmosphere external to the sensor element  100 ) and an atmosphere in contact with the electrode  113  (atmosphere in a first measurement chamber  150 , described below). In the present embodiment, the solid electrolyte member  111  and the electrodes  112  and  113  are collectively called a first oxygen pump cell (hereinafter also referred as the “Ip1 cell”)  110 . 
     Next, the solid electrolyte member  121  is disposed so as to face the solid electrolyte member  111  with the insulator  140  intervening therebetween. Also, porous electrodes  122  and  123  are provided on respective opposite surfaces of the solid electrolyte member  121  with respect to the direction of lamination of the sensor element  100 , such that the electrodes  122  and  123  sandwich the solid electrolyte member  121 . Similarly, the electrodes  122  and  123  are formed from Pt, a Pt alloy, cermet which contains Pt and ceramic, or a like material. The electrode  122  is formed on a side toward the solid electrolyte member  111 . 
     A small space serving as the first measurement chamber  150  is formed between the solid electrolyte members  111  and  121 . The electrode  113  on the solid electrolyte member  111  and the electrode  122  on the solid electrolyte member  121  are disposed in the first measurement chamber  150 . When exhaust gas flowing through the exhaust pipe  6  (see  FIG. 1 ) is introduced into the sensor element  100 , the exhaust gas first enters the first measurement chamber  150 . A first diffusion resistance portion  151  formed of porous ceramic is provided in the first measurement chamber  150  at a position located toward the front end of the sensor element  100 . More specifically, the first diffusion resistance portion  151  serves as a partition between the interior and the exterior of the first measurement chamber  150 , and is adapted to limit inflow of the exhaust gas per unit time into the first measurement chamber  150 . Similarly, a second diffusion resistance portion  152  formed of porous ceramic is provided in the first measurement chamber  150  at a position located toward the rear end of the sensor element  100 . The second diffusion resistance portion  152  serves as a partition between the first measurement chamber  150  and an opening portion  141  communicating with a second measurement chamber  160 , described below, and is adapted to limit flow per unit time of the gas. 
     The solid electrolyte member  121  and the two electrodes  122  and  123  can cooperatively generate an electromotive force according to the difference in partial pressure of oxygen between atmospheres (an atmosphere in the first measurement chamber  150  and in contact with the electrode  122  and an atmosphere in a reference-oxygen chamber  170 , described below, and in contact with the electrode  123 ) separated from each other by the solid electrolyte member  121 . In the present embodiment, the solid electrolyte member  121  and the two electrodes  122  and  123  are collectively called an electromotive force cell or oxygen concentration cell (hereinafter also referred as the “Vs cell”)  120 . 
     Next, the solid electrolyte member  131  is disposed so as to face the solid electrolyte member  121  with the insulator  145  intervening therebetween. Porous electrodes  132  and  133  are provided on the solid electrolyte layer  131  on a side opposite an interface with the solid electrolyte layer  121  and are formed from Pt, a Pt alloy, cermet which contains Pt and ceramic, or a like material. 
     The insulator  145  is absent at a position corresponding to the electrode  132  so as to form an independent small space serving as a reference-oxygen chamber  170 . The electrode  123  of the Vs cell  120  is disposed in the reference-oxygen chamber  170 . The reference-oxygen chamber  170  is filled with a porous body of ceramic. Also, the insulator  145  is absent at a position corresponding to the electrode  133  so as to form an independent small space serving as the second measurement chamber  160 , which is separated from the reference-oxygen chamber  170  by the insulator  145 . An opening portion  125  and the opening portion  141  are provided in the solid electrolyte member  121  and the insulator  140 , respectively, so as to communicate with the second measurement chamber  160 . As mentioned previously, the first measurement chamber  150  and the opening portion  141  are in fluid communication by means of the second diffusion resistance portion  152  intervening therebetween. 
     The solid electrolyte member  131  and the two electrodes  132  and  133  can cooperatively pump oxygen between atmospheres (an atmosphere to which the electrode  132  is exposed and an atmosphere in the second measurement chamber  160  and in contact with the electrode  133 ) separated from each other by the insulator  145 . In the present embodiment, the solid electrolyte member  131  and the two electrodes  132  and  133  are collectively called a second pumping cell (hereinafter also referred to as the “Ip2 cell”)  130 . 
     Next, the configuration of the sensor control apparatus  2  will be described. As shown in  FIG. 2 , a power supply circuit  21 , a microcomputer  22 , a CAN circuit  29 , an Ip1 cell/Vs cell control circuit  26 , an Ip2 cell control circuit  27 , a heater control circuit  28 , etc., are implemented on a circuit board  20  of the sensor control apparatus  2 . The power supply circuit  21  receives electric power from the battery  8 , to which the power supply circuit  21  is connected via a BAT terminal of an external circuit terminal section  31 . The power supply circuit  21  is grounded at the ECU  9 , to which the power supply circuit  21  is connected via a GND1 terminal. The microcomputer  22 , the CAN circuit  29 , the Ip1 cell/Vs cell control circuit  26 , and the Ip2 cell control circuit  27  are connected to the power supply circuit  21  so as to receive electric power necessary for driving the respective circuits. 
     The microcomputer  22  includes a CPU  23  having a known structure, ROM  24 , RAM  25 , a signal input/output section  221  connected to the CPU  23 , and an A/D converter  222  connected to the signal input/output section  221 . The Ip1 cell/Vs cell control circuit  26  and the Ip2 cell control circuit  27  are connected to the A/D converter  222 . Further, the heater control circuit  28  is connected to the CPU  23 . The microcomputer  22  is connected to the ground potential of the ECU  9  via the GND1 terminal of the external circuit terminal section  31 . Notably, in the following description, the expression “is connected to the ground potential” may be simply expressed as “is grounded.” With the above-described configuration, the Ip1 cell/Vs cell control circuit  26 , the Ip2 cell control circuit  27 , and the heater control circuit  28  drive the sensor element  100  and the heater  180  under control of the microcomputer  22 . Further, the microcomputer  22  calculates oxygen concentration and NO X  concentration from the current Ip1 (specifically, a voltage signal converted from the current Ip1) and current Ip2 (specifically, a voltage signal converted from the current Ip2), respectively, which are input via the A/D converter  222  and the signal input/output section  221 . 
     Next, the Ip1 cell/Vs cell control circuit  26  will be described. As shown in  FIG. 2 , the Ip1 cell/Vs cell control circuit  26  is composed of a reference-voltage comparison circuit  261 , an Ip1 drive circuit  262 , a Vs detection circuit  263 , and an Icp supply circuit  264 . The reference-voltage comparison circuit  261  is adapted to compare the voltage Vs between the electrodes  122  and  123  of the Vs cell  120  detected by the Vs detection circuit  263  with a reference voltage (e.g., 425 mV), and outputs the result of the comparison to the Ip1 drive circuit  262 . The Ip1 drive circuit  262  is adapted to supply current Ip1 which flows between the electrodes  112  and  113  of the Ip1 cell  110  connected to the Ip1 drive circuit  262  via an IP1 terminal and a COM terminal of the sensor terminal section  30 . Further, the Ip1 drive circuit adjusts the magnitude and direction of the current Ip1 based on the output of the reference-voltage comparison circuit  261 . The Ip1 drive circuit  262  also detects the current Ip1 flowing between the electrodes  112  and  113  of the Ip1 cell  110 . The detected current Ip1 (specifically, a voltage signal converted from the current Ip1) is output to the microcomputer  22 . 
     The Vs detection circuit  263  is adapted to detect voltage Vs developed between the electrodes  122  and  123  connected to the Vs detection circuit  263  via a VS terminal and the COM terminal of the sensor terminal section  30 . The Vs detection circuit  263  outputs the detected voltage to the reference-voltage comparison circuit  261 . The Icp supply circuit  264  supplies a current Icp which flows between the electrodes  122  and  123  of the Vs cell  120  for pumping out oxygen from the first measurement chamber  150  into the reference-oxygen chamber  170 . The electrode  113  of the Ip1 cell  110  exposed to the first measurement chamber  150 , the electrode  122  of the Vs cell  120  exposed to the first measurement chamber  150 , and the electrode  133  of the Ip2 cell  130  (described below) exposed to the second measurement chamber  160  are connected to a reference electric potential of the Ip1 cell/Vs cell control circuit  26  via the COM terminal of the sensor terminal section  30 . Further, the Ip1 cell/Vs cell control circuit  26  is grounded at the ECU  9  via the GND1 terminal of the external circuit terminal section  31 . 
     Notably, based on a comparison of a previously set reference voltage with the voltage Vs developed between the electrodes  122  and  123  of the Vs cell  120  performed by the reference-voltage comparison circuit  261 , the magnitude and direction of the current Ip1 are adjusted such that the voltage between the electrodes  122  and  123  of the Vs cell  120  substantially coincides with the reference voltage. As a result, the Ip1 cell  110  pumps out oxygen from the first measurement chamber  150  to the exterior of the sensor element  100  or pumps oxygen into the first measurement chamber  150  from the exterior of the sensor element  100 . In other words, the Ip1 cell  110  adjusts the oxygen concentration in the first measurement chamber  150  such that the voltage between the electrodes  122  and  123  of the Vs cell  120  is maintained at a constant value (reference voltage). Typically, the Ip1 cell  110  adjusts the oxygen concentration in the first measurement chamber to a low, constant value without substantially decomposing NO X  contained in the to-be-measured gas. The to-be-measured gas in the first measurement chamber  150  having a reduced oxygen concentration is introduced into the second measurement chamber  160  via the diffusion resistance  152 . 
     Next, the Ip2 cell control circuit  27  will be described. As shown in  FIG. 2 , the Ip2 cell control circuit  27  includes an Ip2 detection circuit  271  and a Vp2 application circuit  272 . The Ip2 detection circuit  271  is adapted to detect a current Ip2 flowing from the electrode  132  to the electrode  133  of the Ip2 cell  130 . The Ip2 detection circuit  271  is connected to the electrode  132  via an IP2 terminal of the sensor terminal section  30 , and is connected to the electrode  133  via the COM terminal of the sensor terminal section  30 . Notably, the detected current Ip2 (specifically, a voltage signal converted from the current Ip2) is output to the microcomputer  22 . The Vp2 application circuit  272  is adapted to apply a predetermined voltage Vp2 (e.g., a voltage of 450 mV of sufficient magnitude to decompose NO X  present in the second measurement chamber  160  into oxygen and nitrogen) between the electrodes  132  and  133  of the Ip2 cell  130 , whereby oxygen is pumped out from the second measurement chamber  160  into the reference-oxygen chamber  170 . The Ip2 cell control circuit  27  is grounded at the ECU  9  via the GND1 terminal of the external circuit terminal section  31 . 
     Next, the heater control circuit  28  will be described. As shown in  FIG. 2 , the heater drive circuit  28  is controlled by the CPU  23  and is adapted to supply current to the heater pattern  183  of the heater  180 , to thereby heat the solid electrolyte members  111 ,  121  and  131  (namely, the Ip1 cell  110 , the Vs cell  120  and the Ip2 cell  130 ). The heater control circuit  28  includes known switching elements (e.g., an FET) for turning ON and turning OFF supply of electricity to the heater pattern. 
     The heater pattern  183  is a single electrode pattern extending in the heater  180 . One end of the heater pattern  183  is connected to the BAT terminal of the external circuit terminal section via an HTR(+) terminal of the sensor terminal section  30 , so that electric power from the battery  8  is supplied to the one end of the heater pattern  183 . The other end of the heater pattern  183  is connected to the heater control circuit  28  via an HTR(−) terminal of the sensor terminal section  30 . The heater control circuit  28  is connected to the ground potential of the battery  8  via a GND2 terminal of the external circuit terminal section  31 . That is, unlike the other circuits, only the heater control circuit  28  is grounded via the GND2 terminal at the battery  8 , which is independent from the ECU  9 . In such a configuration, switching operation of the switching elements of the heater control circuit  28  is effected through PWM (pulse width modulation) power-supply control performed by the CPU  23 , whereby well-known control for supplying current to the heater pattern  183  is performed. Notably, in order to perform PWM power-supply control for supplying current to the heater pattern  183 , the heater control circuit  28  may detect the impedance of the sensor element  100  (specifically, the impedance of the Vs cell  120 ) and calculate the duty ratio of electrical power to be supplied to the heater  180  such that the detected impedance coincides with a target value. Alternatively, the heater control circuit  28  may calculate the duty ratio of electric power supplied to the heater  180  based on the operation state of the internal combustion engine. Since the specific method for performing PWM power-supply control for supplying current to the heater pattern  183  is known, its description is omitted. 
     Next, the CAN circuit  29  will be described. As shown in  FIG. 2 , the CAN circuit  29  is adapted to communicate with the ECU  9  through a CAN (Controller Area Network). The CAN circuit  29  is connected to the CPU  23  via the signal input/output section  221 , and is connected to CAN(+) and CAN(−) terminals of the external circuit terminal section  31 . The CAN(+) and CAN(−) terminals are connected to the ECU  9  via a CAN  91 . Thus, CAN communications can be performed between the CPU  23  and the ECU  9 ; and information representing oxygen concentration based on the current Ip1 and information representing NO X  concentration based on the current Ip2, which concentrations are calculated by the microcomputer  22  (the CPU  23 ), are output through the signal input/output section  121 . Further, the CAN circuit  29  is grounded at the ECU  9  via the GND1 terminal of the external circuit terminal section  31 . 
     Next, the layout of the above-described circuits on the circuit board  20  of the sensor control apparatus  2  will be described with reference to  FIG. 3 .  FIG. 3  is a view schematically showing the layout of electronic components, etc., mounted on the circuit board  20  of the sensor control apparatus  2 . Notably, in order to facilitate description and understanding, the circuit board  20  is assumed to have the form of a rectangular plate; of four edges, an edge along which the sensor terminal section  30  and the external circuit terminal section  31  are disposed will be referred to as the “lower end”; and the edge opposite the lower end will be referred to as the “upper end.” Further, of the two remaining edges, an edge on the side toward the sensor terminal section  30  will be referred to as the “left end,” and an edge on the side toward the external circuit terminal section  31  will be referred to as the “right end.” 
     As shown in  FIG. 3 , the power supply circuit  21 , the microcomputer  22 , the Ip1 cell/Vs cell control circuit  26 , the Ip2 cell control circuit  27 , the heater control circuit  28 , the CAN circuit  29 , the sensor terminal section  30 , and the external circuit terminal section (an ECU terminal section)  31  are implemented on the single circuit board  20  of the sensor control apparatus  2 . 
     The Ip2 cell control circuit  27  is disposed between the sensor terminal section  30  and the upper end and along the left end. The power supply circuit  21  and the heater control circuit  28  are disposed between the external circuit terminal section  31  and the upper end and on the right-end side. The power supply circuit  21  is disposed between the heater control circuit  28  and the upper end. The microcomputer  22  is disposed near the upper end and is located between the Ip2 cell control circuit  27  and the power supply circuit  21 . 
     Further, the Ip1 cell/Vs cell control circuit  26  is disposed between the microcomputer  22  and the sensor terminal section  30 . The Ip1 cell/Vs cell control circuit  26  is disposed adjacent to the Ip2 cell control circuit  27  but away from the heater control circuit  28 . The CAN circuit  29  is disposed on the lower-end side of the power supply circuit  21 . 
     The sensor terminal section  30  includes the terminals (the IP1 terminal, the IP2 terminal, the VS terminal, the COM terminal, the HTR(+) terminal and the HTR(−) terminal) to which the wires of the harness  4  for connection with the NO X  sensor  10  (see  FIG. 1 ) are connected, the terminals being disposed in a row within the sensor terminal section  30 . The sensor terminal section  30  is disposed on the plate face of the circuit board  20  along one edge. 
     The external circuit terminal section  31  is disposed along the same edge along which the sensor terminal section  30  is disposed, such that the terminal sections  30  and  31  are located adjacent to each other. The external circuit terminal section  31  includes the above-described terminals arranged in a row; that is, the terminals (CAN(+), CAN(−)) to which the CAN  91  for communicating with the ECU  9  is connected; the terminal (BAT) to which a signal line extending from the battery  8  is connected; the terminal (GND2 terminal) for grounding the Ip1 cell/Vs cell control circuit  26  and the Ip2 cell control circuit  27 , which are drive circuits of a signal system, and the heater control circuit  28 , which is a drive circuit of a power system; and the terminal (GND1 terminal) for grounding the CAN circuit  29 . 
     As described above, in the external circuit terminal section  31 , the GND1 terminal for grounding the signal-system drive circuits (specifically, the power supply circuit  21 , the microcomputer  22 , the Ip1 cell/Vs cell control circuit  26 , the Ip2 cell control circuit  27 , and the CAN circuit  29 ) and the GND2 terminal for grounding the power-system drive circuit (specifically, the heater control circuit  28 ) are provided independently of each other. Further, the GND1 terminal is grounded to the ground potential of the ECU  9 , and the GND2 terminal is grounded to the ground potential of the battery  8 . That is, the ground of the power system and the ground of the signal system are provided independently of each other on the circuit board  20  and in the electrical path for establishing electrical connection between the circuit board  20  and the ECU  9  and the battery  8 , which are external devices. For example, in the case where the drive circuits of the signal system and the drive circuit of the power system share a common ground, during a period of time in which the heater control circuit  28  is ON, the output values of the Ip1 cell/Vs cell control circuit  26  and the Ip2 cell control circuit  27  become greater than their actual values. Such a problem can be avoided by sampling digital values, obtained through A/D conversion of the output values, during a period in which the heater control circuit  28  is OFF. However, in the case where the digital values sampled without consideration of the ON/OFF states of the heater control circuit  28  are averaged, the oxygen concentration calculated on the basis of the current Ip1 and the NO X  concentration calculated on the basis of the current Ip2 tend to fluctuate greatly. In addition, since the current flowing through the CAN circuit  29  is very small as compared with the heater current, during a period in which the heater control circuit  28  is ON, the output voltage of the CAN circuit  29  also fluctuates. In contrast, in the case where, as in the present embodiment, the ground for the drive circuits of the signal system and the ground for the drive circuit of the power system are provided independently of each other, the influence of the heater current on the drive circuits of the signal system can be suppressed. 
     Further, since, independently of the heater control circuit  28 , the power supply circuit  21  and the microcomputer  22  are grounded at the ECU  9  via the GND1 terminal, the influence of the heater current on the outputs of the power supply circuit  21  and the microcomputer  22  can be prevented. 
     Moreover, the GND1 terminal is commonly used for grounding the Ip1 cell/Vs cell control circuit  26  and the Ip2 cell control circuit  27  and for grounding the CAN circuit  29 . Normally, the external circuit terminal section  31  must have three GND terminals, including that for the heater control circuit  28 . However, the configuration of the present embodiment can reduce the number of the GND terminals to two. Thus, the layout of the ground wiring of the sensor control apparatus  2  can be simplified. 
     Next, an operation for detecting the oxygen concentration and the NO X  concentration by use of the NO X  sensor  10  will be described. As the temperature of the heater pattern  183  increases as a result of supply of drive current thereto from the heater control circuit  28 , the solid electrolyte members  111 ,  121  and  131  shown in  FIG. 2  and constituting the sensor element  100  of the NO X  sensor  10  are heated and thus activated. By this procedure, the Ip1 cell  110 , the Vs cell  120  and the Ip2 cell  130  become operable. 
     The exhaust gas flowing through the exhaust pipe  6  (see  FIG. 1 ) is introduced into the first measurement chamber  150  while its flow rate is limited by the first diffusion resistance portion  151 . Meanwhile, the Icp supply circuit  264  supplies the current Icp which flows through the Vs cell  120  from the electrode  123  to the electrode  122 . Thus, oxygen contained in the exhaust gas can receive electrons from the electrode  122  of negative polarity exposed to the first measurement chamber  150 , to thereby become oxygen ions. The oxygen ions flow through the solid electrolyte member  121  and move into the reference-oxygen chamber  170 . That is, as a result of the current Icp flowing between the electrodes  122  and  123 , oxygen contained in the first measurement chamber  150  is transferred to the reference-oxygen chamber  170 . 
     The Vs detection circuit  263  detects the voltage between the electrodes  122  and  123 . The reference-voltage comparison circuit  261  compares the detected voltage with the reference voltage (425 mV). The result of the comparison is output to the Ip1 drive circuit  262 . By means of adjusting the oxygen concentration within the first measurement chamber  150  such that the difference in electric potential between the electrodes  122  and  123  is maintained at a constant value of around 425 mV, the oxygen concentration of the exhaust gas contained in the first measurement chamber  150  approaches a predetermined value (10 −8  atm to 10 −9  atm). 
     In the case where the oxygen concentration of the exhaust gas introduced into the first measurement chamber  150  is lower than the predetermined value, the Ip1 drive circuit  262  supplies current Ip1 to the Ip1 cell  110  such that the electrode  112  assumes a negative polarity. In this manner, oxygen is pumped into the first measurement chamber  150  from the exterior of the sensor element  100 . By contrast, in the case where the oxygen concentration of the exhaust gas introduced into the first measurement chamber  150  is higher than the predetermined value, the Ip1 drive circuit  262  supplies current Ip1 to the Ip1 cell  110  such that the electrode  113  assumes a negative polarity. In this manner, oxygen is pumped out of the first measurement chamber  150  to the exterior of the sensor element  100 . The oxygen concentration can be detected from the magnitude and flow direction of the current Ip1 at this time. Notably, the oxygen concentration is calculated by the microcomputer  22  on the basis of the current Ip1 (specifically, a voltage signal converted from the current Ip1, typically as a voltage drop across a series connected resistor). 
     The exhaust gas whose oxygen concentration has been adjusted in the first measurement chamber  150  as described above is introduced into the second measurement chamber  160  via the second diffusion resistance portion  152 . In the second measurement chamber  160 , NO X  contained in the exhaust gas contacts the electrode  133  and is decomposed (reduced) into N 2  and O 2  by the catalytic effect of the electrode  133 . Oxygen generated through decomposition receives electrons from the electrode  133 , to thereby become oxygen ions. The oxygen ions flow through the solid electrolyte member  131  and move into the reference-oxygen chamber  170 . At this time, residual oxygen not pumped out of the first measurement chamber  150  similarly moves into the reference-oxygen chamber  170  through the Ip2 cell  130 . Thus, the current flowing through the Ip2 cell  130  consists of a current stemming from NO X  and a current stemming from the residual oxygen. 
     Since the residual oxygen not pumped out of the first measurement chamber  150  is adjusted to a predetermined concentration as mentioned previously, the current stemming from the residual oxygen can be considered substantially constant. Thus its effect on variation in the current stemming from NO X  is small. Therefore, a change in the current flowing through the Ip2 cell  130  is proportional to the NO X  concentration. In the sensor control apparatus  2 , the microcomputer  22  detects the current Ip2 flowing through the Ip2 cell  130  (specifically, a voltage signal converted from the current Ip2) by use of the Ip2 detection circuit  271 , and performs known calculation processing for compensating for offset current stemming from the residual oxygen, to thereby detect the NO X  concentration of the exhaust gas. 
     Next, evaluation tests were performed so as to confirm the effect of the present invention achieved by separately providing the ground for the drive circuits of the signal system and the ground for the drive circuit of the power system. In the evaluation tests, the influence of ON/OFF switching of the heater control circuit  28  on the output values of the sensor control apparatus  2  was examined. Specifically, in Example 1, fluctuations of output voltages (V) of the CAN-H and CAN-L lines (of opposite polarities) of the CAN communication bus and fluctuation of the output voltage (V) of the signal ground, which is a reference voltage for signals, were examined. In Example 2, fluctuation of the output voltage (V) of the Ip2 detection circuit  271  was examined. Notably, in both Examples 1 and 2, a sensor control apparatus in which the GND terminal for the drive circuits of the signal system and the GND terminal for the drive circuit of the power system are rendered common was used as a comparative sample. 
     Example 1 
     First, the results of Example 1 will be described with reference to  FIGS. 4 and 5 .  FIG. 4  is a graph showing fluctuation in the CAN output voltages (CAN communication signal) and voltage fluctuation of the signal ground in a sensor control apparatus in which a single GND terminal is commonly used.  FIG. 5  is a graph showing fluctuation in the CAN output voltages (CAN communication signal) and voltage fluctuation of the signal ground in the sensor control apparatus of the present embodiment in which two GND terminals are separately provided. 
     First, the results of examination of the output fluctuations of the sensor control apparatus in which a single GND terminal is commonly used will be described. As shown in  FIG. 4 , when the heater control circuit  28  was turned ON at time t 1 , the voltage of the signal ground, which had been 0 (V) during a previous OFF period, instantaneously increased, and then dropped to 0.3 (V). After that, the voltage of the signal ground was maintained at 0.3 (V) during a period in which the heater control circuit  28  was ON. Next, when the heater control circuit  28  was turned OFF at time t 3 , the voltage instantaneously dropped by a large amount, but immediately returned to 0 (V). After that, the voltage was maintained at 0 (V) during a period in which the heater control circuit  28  was OFF. 
     Meanwhile, the output voltages of the CAN-H and CAN-L lines were found to fluctuate as follows. When the heater was turned ON at time t 1 , the voltages of the CAN-H and CAN-L lines, which had been 2.5 (V) during the previous OFF period, instantaneously increased, and then dropped to 2.8 (V). After that, the voltages of the CAN-H and CAN-L lines were maintained at 2.8 (V) during a period in which the heater control circuit  28  was ON. Next, during communication between the CAN circuit  29  and the ECU  9  at time t 2 , the voltage of the CAN-H line instantaneously increased to 4.3 (V), and the voltage of the CAN-L line instantaneously dropped to 1.2 (V). After that, these voltages returned to 2.8 (V). When the heater control circuit  28  was turned OFF at time t 3 , the voltage instantaneously dropped by a large amount, but immediately returned to 2.5 (V). After that, the voltages were maintained at 2.5 (V) during a period in which the heater control circuit  28  was OFF. That is, during the period in which the heater control circuit  28  was ON, the voltages of the CAN-H and CAN-L lines increased by 0.3 (V), which is equal to the increase in output voltage of the signal ground during that period. 
     Next, the results of examination of the output fluctuation of the sensor control apparatus (the apparatus of the present invention) in which two GND terminals are separately provided will be described. As shown in  FIG. 5 , when the heater control circuit  28  was turned ON at time t 1 , the voltage of the signal ground, which had been 0 (V) during a previous OFF period, instantaneously dropped, but immediately returned to 0 (V). After that, the voltage of the signal ground was about 0 (V) during a period in which the heater control circuit  28  was ON. Next, when the heater control circuit  28  was turned OFF at time t 3 , the voltage instantaneously dropped by a small amount, but immediately returned to 0 (V). After that, the voltage was maintained at 0 (V) even when the heater control circuit  28  was turned ON and OFF. 
     Meanwhile, the output voltages of the CAN-H and CAN-L lines were found to fluctuate as follows. When the heater control circuit  28  was turned ON at time t 1 , the output voltage, which had been 2.5 (V) during the previous OFF period, instantaneously dropped by a small amount, but immediately returned to 2.5 (V). After that, the output voltage was maintained at 2.5 (V) during a period in which the heater control circuit  28  was ON. Next, during communication between the CAN circuit  29  and the ECU  9 , the voltage of the CAN-H line instantaneously increased to 4.0 (V), and the voltage of the CAN-L line instantaneously dropped to 0.9 (V). After that, these voltages returned to 2.5 (V). When the heater control circuit  28  was turned OFF at time t 3 , the voltages instantaneously increased by a small amount, but immediately returned to 2.5 (V). After that, the voltages were maintained at 2.5 (V) even after the heater control circuit  28  was turned OFF. 
     The results of Example 1 show that, in the case of the sensor control apparatus employing a single common GND terminal, during a period in which the heater control circuit  28  is ON, the output voltage of the signal ground increases, and the output voltages of the CAN-H and CAN-L lines increase by an amount corresponding to the increase in the output voltage of the signal ground. The results also show that the output voltages greatly change at the time of turning the heater control circuit  28  ON and OFF. These results show that the sensor control apparatus was in a state in which the output voltages of the CAN-H and CAN-L lines are apt to be influenced by the heater current flowing through the heater control circuit  28 . This is because the current flowing through the CAN circuit  29  is very small compared with the current flowing through the heater control circuit  28 , and the GND terminal for the heater control circuit  28  and the GND terminal for the CAN circuit  29  are rendered common. Presumably, because of the influence of the heater current, the output voltages of the signal ground, the CAN-H line and the CAN-L line increased when the heater control circuit  28  was ON. In such a state, the CAN output fluctuates during a period in which the heater control circuit  28  is ON, and there is a possibility that the ECU  9  will fail to stably receive the CAN communication signal, which contains NO X  concentration information, etc., from the sensor control apparatus via the CAN  91 . 
     In contrast, in case of the sensor control apparatus  2  of the present invention, fluctuation was hardly observed in the outputs of the signal ground and the CAN-H and CAN-L lines irrespective of ON/OFF switching of the heater control circuit  28 . Further, the fluctuations at the time of turning the heater control circuit  28  ON and OFF were small compared with the case where a common GND terminal was used. Thus, the influence of the heater current flowing through the heater control circuit  28  on the CAN circuit  29  can be minimized. Presumably, because the influence of the heater current is minimized, the output voltages of the signal ground, the CAN-H line, and the CAN-L line hardly change even when the heater control circuit  28  is turned ON. That is, in the sensor control apparatus  2  of the present invention, since the output of the CAN circuit  29  is stable irrespective of the ON/OFF state of the heater control circuit  28 , the ECU  9  can stably receive the CAN communication signal, which contains NO X  concentration information, etc., from the sensor control apparatus  2  via the CAN  91 . 
     Example 2 
     Next, the results of Example 2 will be described with reference to  FIGS. 6 and 7 .  FIG. 6  is a graph showing fluctuation of the NO X  output of the sensor control apparatus in which a single GND terminal is commonly used.  FIG. 7  is a graph showing fluctuation of the NO X  output of the sensor control apparatus of the present embodiment in which two GND terminals are separately provided. In Example 2, an NO X  output voltage converted from the Ip2 current detected by the Ip2 cell control circuit  27  was measured as an analog output representing the NO X  concentration. 
     First, the results of examination of the output fluctuation of the sensor control apparatus in which a single GND terminal is commonly used will be described. As shown in  FIG. 6 , when the heater control circuit  28  was turned ON at time t 5 , the NO X  output voltage, which had been 1.50 (V) during a previous OFF period, instantaneously increased and decreased, and then changed to 1.70 (V). After that, without returning to 1.50 (V), the NO X  output voltage remained at 1.70 (V) during a period in which the heater control circuit  28  was ON. Next, when the heater control circuit  28  was turned OFF at time t 6 , the NO X  output voltage instantaneously dropped by a large mount, but returned to 1.50 (V) after that. 
     Next, the results of examination of the output fluctuation of the sensor control apparatus (the apparatus of the present invention) in which two GND terminals are separately provided will be described. As shown in  FIG. 7 , when the heater control circuit  28  was turned ON at time t 5 , the NO X  output voltage, which had been 1.50 (V) during a previous OFF period, instantaneously increased and decreased, but immediately changed to 1.52 (V). After that, the NO X  output voltage remained at 1.52 (V) during a period in which the heater control circuit  28  was ON. Next, when the heater control circuit  28  was turned OFF at time t 6 , the NO X  output voltage instantaneously increased and decreased, but immediately returned to 1.50 (V). After that, the NO X  output voltage remained at 1.50 (V) during a period in which the heater control circuit  28  was OFF. 
     The results of Example 2 show that, in the case of the sensor control apparatus which employs a single common GND terminal, during a period in which the heater control circuit  28  is ON, the NO X  output voltage increases greatly. The results also show that the output voltage greatly changes at the time of turning the heater control circuit  28  ON and OFF. This demonstrates that the sensor control apparatus was in a state in which the NO X  output voltage is apt to be influenced by the heater current flowing through the heater control circuit  28 . This is because the current (on the nA order) flowing through the Ip2 cell control circuit  27  is very small compared with the heater current flowing through the heater control circuit  28 , and the GND terminal for the heater control circuit  28  and the GND terminal for the Ip1 cell/Vs cell control circuit  26  and the Ip2 cell control circuit  27  are rendered common. Presumably, because of the influence of the heater current, the NO X  output voltage increased when the heater control circuit  28  was ON. In such a state, during a period in which the heater control circuit  28  is ON, the NO X  output voltage fluctuates, and it is difficult to accurately detect NO X  concentration. 
     In contrast, in case of the sensor control apparatus  2  of the present invention, although the NO X  output voltage increased when the heater control circuit  28  was turned ON, the amount of increase was slight, and a difference was hardly observed. Further, fluctuations at the time of turning the heater control circuit  28  ON and OFF were quite small compared with the case where a common GND terminal is used. This is because the ground for the drive circuit of the power system and the ground for the drive circuits of the signal system are provided separately. Thus, the influence of the heater current flowing through the heater control circuit  28  on the output of the Ip2 cell control circuit  27  can be minimized. Presumably, because of the minimized influence of the heater current, the NO X  output voltage of the sensor control apparatus  2  hardly changed even when the heater control circuit  28  was turned ON. Accordingly, in the sensor control apparatus  2  of the present invention, since the output representing the NO X  concentration is stable irrespective of the ON/OFF state of the heater control circuit  28 , the NO X  concentration can be detected accurately. 
     As described above, in the sensor control apparatus  2  of the present embodiment, the GND1 terminal for connecting the drive circuits of the signal system to the ground potential and the GND2 terminal for connecting the drive circuit of the power system to the ground potential are provided in the external circuit terminal section  31  independently of each other. Further, the GND1 terminal is connected to the ground potential of the ECU  9  (the first external device), and the GND2 terminal is connected to the ground potential of the battery  8  (the second external device). That is, the ground for the drive circuit of the power system and the ground for the drive circuits of the signal system are provided independently of each other on the circuit board  20  and in the electrical path for establishing electrical connection between the circuit board  20  and the ECU  9  and the battery  8 , which are external devices. By virtue of this configuration, even when the heater control circuit  28  is turned ON, the influence of the heater current on the Ip1 cell/Vs cell control circuit  26  and the Ip2 cell control circuit  27  of the signal system and the CAN circuit  29  for communication can be suppressed. Accordingly, accuracy in detecting the NO X  concentration can be improved. 
     Further, since the GND1 terminal is shared by the ground for connecting the Ip1 cell/Vs cell control circuit  26  and the Ip2 cell control circuit  27  associated with the sensor element  100  to the ground potential and the ground for connecting the CAN circuit  29  to the ground potential, the number of GND terminals to be provided in the external circuit terminal section  31  can be decreased. Thus, the layout of the ground wiring in the sensor control apparatus  2  can be simplified. 
     Notably, the present invention is not limited to the above-described embodiment, and may be modified in various ways. For example, in the above-described embodiment, one end of the heater pattern  183  is connected to the BAT terminal via the HTR(+) terminal of the sensor terminal section  30 , and the other end of the heater pattern  183  is connected to the heater control circuit  28  via the HTR(−) terminal. However, this circuit configuration may be modified such that one end of the heater pattern  183  is connected via the HTR(+) terminal to the heater control circuit  28 , which is then connected to the BAT terminal, and the other end of the heater pattern  183  is connected to the ground potential of the battery  8  via the HTR(−) terminal. 
     INDUSTRIAL APPLICABILITY 
     The present invention is applicable not only to a sensor control apparatus connected to an NO X  sensor, but also to a sensor control apparatus connected to a heated gas sensor for detecting the concentration of other specific gases within a to-be-measured gas, such as a hydrogen sensor, an HC sensor, etc. 
     It should further be apparent to those skilled in the art 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. 
     This application is based on Japanese Patent Application No. 2008-63509 filed Mar. 13, 2008, incorporated herein by reference in its entirety.