Patent Publication Number: US-7909970-B2

Title: Controller for gas concentration sensor

Description:
This is a Divisional of application Ser. No. 10/825,293, filed Apr. 16, 2004, (issued as U.S. Pat. No. 7,393,441), which in turn claims the benefit of Japanese Patent Application Nos. JP 2003-118337 and JP 2003-285183, filed Apr. 23, 2003 and Aug. 1, 2003, respectively. The entire disclosures of the prior applications are hereby incorporated by reference herein in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a controller for a gas concentration sensor, and more particularly to a gas concentration sensor controller that is suitable for the control of a gas concentration sensor installed in an internal-combustion engine&#39;s exhaust path. 
     2. Background Art 
     As disclosed by Japanese Patent JP-A No. 28575/2000, there is a conventionally known device that includes an oxygen sensor installed in an internal-combustion engine&#39;s exhaust path. The oxygen sensor for this device generates an output in accordance with the oxygen concentration in an exhaust gas that flows in the exhaust path. There is a correlation between the oxygen concentration in the exhaust gas and the exhaust air-fuel ratio. With the conventional device, it is therefore possible to obtain the information about the exhaust air-fuel ratio in accordance with the oxygen sensor output. 
     The above device is capable of detecting the device impedance of the oxygen sensor by varying the voltage V 0 , which is applied to the oxygen sensor, from a reference voltage to a sweep voltage. If a ΔV 0  change occurs in the applied voltage V 0 , the associated current I changes by ΔI, which corresponds to the device impedance Rs. Therefore, the above conventional device calculates the device impedance Rs in accordance with the voltage change ΔV 0  and current change ΔI, which arise when the applied voltage V 0  changes from the reference voltage to the sweep voltage. 
     As described above, the above conventional device acquires the information about the exhaust air-fuel ratio in accordance with the oxygen sensor&#39;s output, and detects the device impedance by applying the sweep voltage to the oxygen sensor. While the sweep voltage is applied to the oxygen sensor, the output value of the oxygen sensor is affected by the sweep voltage. Therefore, while the sweep voltage is applied, the oxygen sensor&#39;s output does not correspond to the exhaust air-fuel ratio. 
     The oxygen sensor includes an impedance component and a capacitance component. Therefore, the oxygen sensor&#39;s output does not revert to a value corresponding to the exhaust air-fuel ratio for some time after the application of the sweep voltage is stopped. Consequently, the above conventional device may erroneously detect the exhaust air-fuel ratio during the time interval between the instant at which the sweep voltage is applied to the oxygen sensor for device impedance detection purposes and the instant at which the influence of the sweep voltage disappears. 
     SUMMARY OF THE INVENTION 
     The present invention has been made to solve the above problems and provides a controller for a gas concentration sensor, which is capable of detecting the device impedance of the gas concentration sensor and accurately detecting the information about the exhaust air-fuel ratio. 
     The above object of the present invention is achieved by a controller for a gas concentration sensor that generates an output correlating with the oxygen concentration in a detected gas. The controller includes an impedance detection unit for applying an impedance detection voltage to the gas concentration sensor to detect a device impedance of the gas concentration sensor. The controller also includes a reverse voltage application unit for applying the same voltage as generated by the gas concentration sensor itself or a voltage that shifts from the same voltage toward an opposite direction against a direction of the impedance detection voltage to the gas concentration sensor for a specified period of time after the impedance detection voltage is applied to the gas concentration sensor. 
     The above object of the present invention is also achieved by a controller for a gas concentration sensor that generates an output correlating with the oxygen concentration in a detected gas. The controller includes an impedance detection unit for applying an impedance detection voltage to the gas concentration sensor to detect a device impedance of the gas concentration sensor. The controller also includes a data invalidation unit for invalidating the output of the gas concentration sensor for a specified period of time after the impedance detection voltage is applied to the gas concentration sensor. 
     The above object of the present invention is also achieved by a controller for a gas concentration sensor that generates an output correlating with the oxygen concentration in a detected gas. The controller includes an impedance detection unit for applying an impedance detection voltage to the gas concentration sensor at specified time intervals to detect a device impedance of the gas concentration sensor. The controller further includes an impedance detection time interval setup unit for increasing the specified time intervals with an increase in the device impedance. 
     Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a configuration of a first embodiment of the present invention; 
         FIGS. 2A ,  2 B,  2 C,  2 D and  2 E are timing diagrams illustrating an operation that is performed in the first embodiment of the present invention; 
         FIG. 3  is an equivalent circuit diagram of an oxygen sensor shown in  FIG. 1 ; 
         FIG. 4  shows waveforms of sensor output that is generated before and after a voltage is applied to the oxygen sensor shown in  FIG. 1 ; 
         FIG. 5  is a flowchart of a control routine executed in the first embodiment of the present invention; 
         FIG. 6  illustrates the procedure to be performed in the routine shown in  FIG. 5  to calculate ON time of second port; 
         FIG. 7  shows a relationship between device impedance Rs and a waveform of sensor output that is generated before and after a voltage is applied to the oxygen sensor shown in  FIG. 1 ; 
         FIG. 8  is a flowchart of a control routine executed in a second embodiment of the present invention; 
         FIG. 9  shows a map of data invalidation count AD 1  that is referred to in the control routine shown in  FIG. 8 ; 
         FIGS. 10A and 10B  are timing diagrams illustrating an outline of an operation that is performed in a third embodiment of the present invention; 
         FIG. 11  is a flowchart of a control routine executed in the third embodiment of the present invention; 
         FIG. 12  shows a typical map of the device impedance calculation interval T 1  that can be used in the third embodiment of the present invention; 
         FIG. 13  shows a relationship between device temperature and device impedance Rs of the oxygen sensor shown in  FIG. 1 ; 
         FIG. 14  is a flowchart of a control routine executed in a fourth embodiment of the present invention; and 
         FIG. 15  shows a map of device impedance calculation interval T 1  that is referred to in the control routine shown in  FIG. 14 . 
     
    
    
     BEST MODE OF CARRYING OUT THE INVENTION 
     Embodiments of the present invention will now be described with reference to the accompanying drawings. Like elements in the drawings are designated by like reference numerals and will not be described again. 
     First Embodiment 
     [Circuit Configuration Description] 
       FIG. 1  is a diagram showing a configuration of a first embodiment of the present invention. As shown in  FIG. 1 , the system according to the first embodiment includes an oxygen sensor  10  and an ECU (Electronic Control Unit)  20 . In the present embodiment, the oxygen sensor  10  is installed in the exhaust path of an internal-combustion engine to generate a sensor output in accordance with the oxygen concentration in exhaust gas. More specifically, the oxygen sensor generates a sensor output that indicates whether exhaust air-fuel ratio is rich or lean. 
     In  FIG. 1 , the oxygen sensor  10  is equivalently indicated as an element that includes an impedance component and an electromotive force component. In other words, the oxygen sensor  10  is an electromotive sensor that generates a voltage in accordance with the oxygen concentration in a detected gas. In the present embodiment, the oxygen sensor  10  is connected to the ECU  20  so that the OX1B terminal is on a high-voltage side, and that the E2 terminal is on a low-voltage side. The ECU  20  monitors the voltage generated between the OX1B terminal and E2 terminal to judge whether the exhaust air-fuel ratio is rich or lean. 
     The device impedance Rs of the oxygen sensor  10  has such a temperature characteristic that the higher the temperature of the oxygen sensor  10 , the smaller the value of the device impedance Rs. To permit the oxygen sensor  10  to function normally, it is necessary to maintain the oxygen sensor  10  at an active temperature. Since the temperature of the oxygen sensor  10  correlates with the device impedance Rs, being able to detect the device impedance Rs accurately is useful in order to control the temperature to the above active temperature. Further, if the device impedance Rs can be accurately detected, it is possible to perform a diagnostic check on the oxygen sensor  10  based on the detected value. As described above, it is required that the device impedance Rs of the oxygen sensor  10  be accurately detected. 
     To comply with the above requirements, the ECU  20  is capable of accurately detecting the device impedance Rs of the oxygen sensor  10 . The ECU  20  for use in the present embodiment is a unit that is capable of acquiring the information about the exhaust air-fuel ratio and detecting the device impedance Rs of the oxygen sensor  10  in accordance with the voltage generated by the oxygen sensor  10  (the voltage developed between the OX1B terminal and E2 terminal). The circuit configuration and functionality of the ECU  20  will now be described in detail. 
     The ECU  20  includes a first switching device  22 . A constant voltage (input voltage) of 5 V is supplied to the first switching device  22 . The gate of the first switching device  22  communicates with a first port  24 . The ECU  20  issues an ON command to the first port  24  as needed to turn ON the first switching device  22 . 
     The first switching device  22  is connected to a first sampling point  28  via a second resistor  26 . The first sampling point  28  is electrically connected to OX1B external terminal of the ECU  20  via a first resistor  30  and electrically connected to the E2 external terminal of the ECU  20  via a first capacitor  31 . 
     The first sampling point  28  is connected to a first analog-to-digital converter (ADC 1 )  32  via a filter circuit having a small time constant. The filter circuit comprises two series-connected resistors  34 ,  36  and a capacitor  38 , which is positioned between an input terminal of the first analog-to-digital converter  32  and a ground wire. A diode  40  is connected between the two resistors  34 ,  36  to maintain the potentials for their joints at a voltage below 5 V for protection purposes. 
     The first analog-to-digital converter  32  is capable of converting an analog signal, which is supplied to its input terminal, to a digital signal. The potential of the first sampling point  28  is supplied to the input terminal of the first analog-to-digital converter  32  via the aforementioned filter circuit having a small time constant. Therefore, the first analog-to-digital converter  32  can accurately digitize and output the potential of the first sampling point  28  even when it varies at a high frequency. As detailed later, the ECU  20  recognizes the digital signal generated by the first analog-to-digital converter  32  in a specified situation as the potential of the first sampling point  28 , and uses it in a detection process for the device impedance Rs. 
     Further, a second switching device  42  is connected to the first sampling point  28  via a third resistor  41 . The gate of the second switching device  42  is connected to a second port  44 . The ECU  20  issues an ON command to the second port  44  as needed to turn ON the second switching device  42 . Therefore, when the ECU  20  issues an ON command to the second port  44 , the first sampling point  28  is electrically connected to the E2 external terminal via the third resistor  41 . 
     In the ECU  20 , a second sampling point  50  is formed between the first resistor  30  and the OX1B external terminal. The second sampling point  50  is connected to one end of an output detection resistor  52 , which is positioned parallel to the oxygen sensor  10 . The impedance of the output detection resistor  52  is sufficiently greater than the device impedance Rs of the oxygen sensor  10 . Therefore, if no input voltage is supplied to the second sampling point  50  (the first switching device  22  is OFF), a voltage equivalent to the electromotive force of the oxygen sensor  10  is generated at the second sampling point  50 . If, on the other hand, an input voltage is supplied to the second sampling point  50  (the first switching device  22  is ON), a voltage equivalent to the product of the current I flowing to the oxygen sensor  10  and the device impedance Rs is generated at the second sampling point  50 . 
     A second analog-to-digital converter (ADC 2 )  54  is connected to the second sampling point  50  via a filter circuit having a small time constant. The filter circuit comprises two series-connected resistors  56 ,  58  and a capacitor  60 , which is positioned between an input terminal of the second analog-to-digital converter  54  and a ground wire. A diode  62  is connected between the two resistors  56 ,  58  to maintain the potentials for their joints at a voltage below 5 V for protection purposes. 
     The second analog-to-digital converter  54  is capable of converting an analog signal, which is supplied to its input terminal, to a digital signal. The input terminal of the second analog-to-digital converter  54  is connected to the second sampling point  50  via the aforementioned filter circuit having a small time constant. Therefore, the second analog-to-digital converter  54  can accurately digitize and output the potential of the second sampling point  50  even when it varies at a high frequency. As detailed later, the ECU  20  recognizes the digital signal generated by the second analog-to-digital converter  54  in a specified situation as the potential of the second sampling point  50 , and uses it in a detection process for the device impedance Rs. 
     Further, the second sampling point  50  is connected to a third analog-to-digital converter (ADC 3 )  68  via a filter circuit, which comprises a resistor  64  and a capacitor  66 . The filter circuit, which is positioned before the third analog-to-digital converter  68 , has a sufficiently great time constant so that only the low-frequency components of a voltage at the second sampling point  50  are allowed to pass. Therefore, the third analog-to-digital converter  68  can accurately generate a digital signal equivalent of a steady-state voltage at the second sampling point  50  without being affected by noise or the like. As detailed later, the ECU  20  recognizes the digital signal generated by the third analog-to-digital converter  68  in a specified situation as the output signal of the oxygen sensor  10 , and uses it in a process for detecting the oxygen concentration in a detected gas. 
     [ECU Operation Description] 
     (Process for Detecting the Oxygen Concentration Information) 
     The ECU  20  turns OFF the first port  24  except in an attempt to detect the device impedance Rs of the oxygen sensor  10 . When the first port  24  is OFF, the first switching device  22  turns OFF so that the steady-state potential of the second sampling point  50  is equivalent to the electromotive force of the oxygen sensor  10  (see  FIG. 1 ). In this instance, the output of the third analog-to-digital converter  68  is equal to the sensor output of the oxygen sensor  10 . Under such a situation, the ECU  20  detects the digital signal generated by the third analog-to-digital converter  68  at specified time intervals (e.g., at 4 msec intervals), and acquires the information about the oxygen concentration in the exhaust gas in accordance with the detected signal value. 
     (Process for Calculating the Device Impedance Rs) 
       FIGS. 2A to 2E  are timing diagrams illustrating the operation that the ECU  20  performs in a mode for calculating the device impedance Rs of the oxygen sensor  10 . More particularly,  FIGS. 2A and 2B  show waveforms of the status of the first port  24  and the second port  44 , respectively.  FIGS. 2C to 2E  show waveforms indicating the changes in the potentials that are supplied to the input terminals of the first to third analog-to-digital converters  32 ,  54 ,  68 , respectively. 
     In the mode for calculating the device impedance Rs, the ECU  20  generally turns OFF the second port  44  (see  FIG. 2B ). In this instance, the second switching device  42  turns OFF so that, inside the ECU  20 , only the first capacitor  31  is connected parallel to a series circuit for the first resistor  30  and the sensor device  10 . The parallel circuit formed by such elements is hereinafter referred to as the “R 1 /Rs−C1 parallel circuit”. The ECU  20  includes the output detection resistor  52 , which is connected parallel to the oxygen sensor  10 . However, the resistance value of the output detection resistor  52  (e.g., 1.5 MΩ) is sufficiently greater than the value of the device impedance Rs of the oxygen sensor  10  (not greater than several tens of kilohms). It is therefore assumed that the existence of the output detection resistor  52  is ignorable. 
     When the calculation of the device impedance Rs is demanded, the ECU  20  turns ON the first port  24  with the second port  44  left OFF (see  FIG. 2A ). When the first port  24  turns ON, the first switching device  22  turns ON so that an input voltage of 5 V begins to be applied to the second resistor  26 . This voltage passes through the second resistor  26 , works on the first sampling point  28 , and is applied to the R 1 /Rs−C1 parallel circuit. 
     When the above voltage begins to be applied to the first sampling point  28 , the potential VS 1  at that point subsequently rises in accordance with the time constant τ and finally converges to a value that is determined according to the ratio between the resistance value R 2  of the second resistor  26  and the combined resistance value R 1 −Rs of the first resistor  30  and oxygen sensor  10 . In this instance, the resulting convergence value VS 1  and the time constant τ are respectively expressed by Equations (1) and (2) shown below.
 
 VS 1=5( R 1+ Rs )/( R 2+ R 1+ Rs )  Equation (1)
 
τ= C 1/{1/( Rs+R 1)+1 /R 2}  Equation (2)
 
     In the circuit shown in  FIG. 1 , the potential VS 1  at the first sampling point  28  is supplied to the first analog-to-digital converter  32 . Therefore, the output of the first analog-to-digital converter  32  varies in the same manner as the value VS 1 , which is indicated by Equations (1) and (2). The waveform shown in  FIG. 2C  shows that the output of the first analog-to-digital converter  32  varies in such a manner after the first port  24  is turned ON. 
     In a process in which the potential VS 1  at the first sampling point  28  varies as described above, the current I flows to the oxygen sensor  10  as indicated by the following equation:
 
 I=VS 1/( R 1+ Rs )  Equation (3)
 
     In this instance, the potential VS 2  at the second sampling point  50  can be expressed as follows using the current I and device impedance Rs:
 
 VS 2= Rs·I   Equation (4)
 
     Since the potential VS 1  at the first sampling point  28  varies according to the time constant τ, the current I, which satisfies Equation (3), and the potential VS 2  at the second sampling point  50 , which satisfies Equation (4), both vary according to the time constant τ. In the circuit shown in  FIG. 1 , the potential VS 2  at the second sampling point  50  is supplied to the second analog-to-digital converter  54 . Therefore, the output of the second analog-to-digital converter  54  varies in the same manner as the value VS 2 , which is expressed by Equations (4) and (2). The waveform shown in  FIG. 2D  indicates that the output of the second analog-to-digital converter  54  varies in such a manner after the first port  24  is turned ON. 
     The current I, which flows to the oxygen sensor  10 , can be expressed as follows using the potential VS 1  at the first sampling point  28 , the potential VS 2  at the second sampling point  50 , and the resistance value R 1  of the first resistor  30 :
 
 I =( VS 1− VS 2)/ R 1  Equation (5)
 
     From Equations (4) and (5), the device impedance Rs can be expressed as follows: 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           R 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           s 
                         
                         = 
                           
                         ⁢ 
                         
                           V 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           S 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             2 
                             / 
                             I 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           V 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           S 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             2 
                             · 
                             R 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             1 
                             / 
                             
                               ( 
                               
                                 
                                   V 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   S 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   1 
                                 
                                 - 
                                 
                                   V 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   S 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   2 
                                 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   6 
                 
               
             
           
         
       
     
     As described above, in the circuit according to the present embodiment, the device impedance Rs of the oxygen sensor  10  can be calculated from the potentials VS 1 , VS 2  that are developed at the first sampling point  28  and second sampling point  50  after the first port  24  is turned ON. Influence of a leak current or the like which has been existing from before the turning ON of the first port  24  is superposed over the potential VS 1  at the first sampling point  28  and the potential VS 2  at the second sampling point  50  of after the turning ON of the first port  24 . For accurate calculation of the device impedance Rs, therefore, it is preferred that the influence of the leak current or the like be eliminated. 
     Therefore, the ECU  20  determines the difference ΔVS 1  between the potential VS 1  prevailing immediately before the first port  24  is turned ON (hereinafter referred to as “VS 1 OFF”) and the potential VS 1  prevailing after the first port  24  is turned ON (hereinafter referred to as “VS 1 ON”), determines the difference ΔVS 2  between the potential VS 2  prevailing immediately before the first port  24  is turned ON (hereinafter referred to as “VS 2 OFF”) and the potential VS 2  prevailing after the first port  24  is turned ON (hereinafter referred to as “VS 2 ON”), applies the determined differences to Equation (6), and calculates the device impedance Rs using the following equation: 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           R 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           s 
                         
                         = 
                           
                         ⁢ 
                         
                           Δ 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           V 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           S 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             2 
                             · 
                             R 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             1 
                             / 
                             
                               ( 
                               
                                 
                                   Δ 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   V 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   S 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   1 
                                 
                                 - 
                                 
                                   Δ 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   V 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   S 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   2 
                                 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             
                               ( 
                               
                                 
                                   V 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   S 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   2 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   ON 
                                 
                                 - 
                                 
                                   V 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   S 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   2 
                                   ⁢ 
                                   OFF 
                                 
                               
                               ) 
                             
                             · 
                             R 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             1 
                             / 
                           
                         
                       
                     
                   
                   
                     
                       
                           
                         ⁢ 
                         
                           { 
                           
                             
                               ( 
                               
                                 
                                   V 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   S 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   1 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   ON 
                                 
                                 - 
                                 
                                   V 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   S 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   1 
                                   ⁢ 
                                   OFF 
                                 
                               
                               ) 
                             
                             - 
                             
                               ( 
                               
                                 
                                   V 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   S 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   2 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   ON 
                                 
                                 - 
                                 
                                   V 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   S 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   2 
                                   ⁢ 
                                   OFF 
                                 
                               
                               ) 
                             
                           
                           } 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     7 
                     ) 
                   
                 
               
             
           
         
       
     
     However, if the influence of the leak current or the like is insignificant and the value VS 1 OFF is nearly equal to the value VS 2 OFF, the relationship expressed by Equation (7) need not always be used. In such an instance, the device impedance Rs should be calculated from Equation (6) (while assuming that VS 1 =VS 1 ON and that VS 2 =VS 2 ON). 
     As described earlier, the first analog-to-digital converter  32  enables the ECU  20  to detect the potential of the first sampling point  28 . Further, the second analog-to-digital converter  54  enables the ECU  20  to detect the potential of the second sampling point  50 . Therefore, when the calculation of the device impedance Rs is demanded, the ECU  20  performs the following calculation steps: 
     (i) Immediately before the first port  24  is turned ON, the ECU  20  detects the output of the first analog-to-digital converter  32  as VS 1 OFF and the output of the second analog-to-digital converter  54  as VS 2 OFF. 
     (ii) Upon termination of the above detection sequence, the ECU  20  turns ON the first port  24 . 
     (iii) When the period required for VS 1  to reach its convergence value (e.g., 135 μsec) elapses after the first port is turned ON, the ECU  20  detects the output of the first analog-to-digital converter  32  as VS 1 ON and the output of the second analog-to-digital converter  54  as VS 2 ON. 
     (iv) Upon termination of the above detection sequence, the ECU  20  turns the first port  24  back OFF. 
     (v) The ECU  20  substitutes the values VS 1 OFF, VS 1 ON, VS 2 OFF, and VS 2 ON, which are detected in processing steps (i) through (iii) above, into Equation (7) to calculate the device impedance Rs. 
     [Description of Problems with Calculating the Device Impedance Rs] 
       FIG. 3  is an equivalent circuit diagram that strictly illustrates the characteristics of the oxygen sensor  10 . As shown in this figure, the oxygen sensor  10  has a capacitance component in addition to an electromotive component and an impedance component. When a voltage is applied to a circuit having such components, an electrical charge is stored in the capacitance component. Before the stored electrical charge is released, the voltage developed across the terminals of the oxygen sensor  10  is higher than the voltage generated by the electromotive component even after the voltage application to the oxygen sensor  10  is stopped. In other words, the sensor output (output voltage) of the oxygen sensor  10  represents an excessive value for the oxygen concentration in the exhaust gas until the electrical charge stored in the capacitance component is released. 
       FIG. 4  shows waveforms of sensor output that is generated before and after a voltage is applied to the oxygen sensor  10  for the purpose of calculating the device impedance Rs. The waveform indicated by a broken line is a sensor output waveform that is obtained when the electrical charge stored in the oxygen sensor  10  is spontaneously released after the first port  24  is turned OFF. The waveform indicated by a solid line is a sensor output waveform that is obtained when the electrical charge stored in the oxygen sensor  10  is forcibly released immediately after the status of the first port  24  is changed from ON to OFF. In the circuit configuration shown in  FIG. 1 , the electrical charge stored in the oxygen sensor  10  can be forcibly released by turning ON the second port  44 , that is, by turning ON the second switching device  42 . More specifically, the waveform indicated by the solid line in  FIG. 4  is obtained when the second port  44  is ON for only a specified period following the instant at which the status of the first port  24  is changed from ON to OFF. 
     As indicated by the broken line in  FIG. 4 , when the electrical charge stored in the oxygen sensor  10  is spontaneously released, the sensor output exceeds the output generated by the oxygen sensor  10  itself for some time (for a period of three samplings in the example shown in  FIG. 4 ) after the voltage application to the oxygen sensor  10  is stopped. Since such an excessive sensor output (output marked x) does not correspond to the oxygen concentration in the exhaust gas, it should not be used as a basis for acquiring the information about oxygen concentration. 
     On the other hand, when the electrical charge stored in the oxygen sensor  10  is forcibly released, that is, when the second port  44  is ON for a specified period only, the sensor output downs to the ground potential, then converges to the inherent output immediately after the voltage application to the oxygen sensor  10  is stopped, as indicated by the solid line in  FIG. 4 . In this instance, the sensor output of the oxygen sensor  10  properly corresponds to the oxygen concentration in the exhaust gas immediately after the voltage application to the oxygen sensor  10  is stopped. Therefore, the ECU  20  changes the status of the first port  24  from ON to OFF in order to calculate the device impedance Rs, then forcibly releases the electrical charge stored in the oxygen sensor  10  by keeping the second port  44  ON for a specified period only. 
       FIGS. 2A and 2B  indicate that the second port  44  turns ON at the moment the first port  24  turns OFF, and that the second port  44  turns OFF in a specified period of time. In the present embodiment, the ECU  20  acquires the output of the oxygen sensor  10  that is supplied to the third analog-to-digital converter  68 , at 4 msec sampling intervals.  FIGS. 2C to 2E  indicate that all the input voltages to the first to third analog-to-digital converters  32 ,  54 ,  68  converge to the inherent output generated by the oxygen sensor  10  itself within a 4 msec period when the second port is turned ON as described above after the voltage application to the oxygen sensor  10  is started. When the input voltage for the third analog-to-digital converter  68  varies in this manner, all the sensor outputs sampled by the ECU  20  properly correspond to the oxygen concentration in the exhaust gas. As a result, the system according to the present embodiment is always capable of correctly detecting the information about the oxygen concentration in the exhaust gas without being affected by the voltage application for calculating the device impedance Rs after the calculation process for the device impedance Rs terminates. 
       FIG. 5  is a flowchart illustrating how the ECU  20  executes a control routine in order to implement the above functionality. It is assumed that the routine shown in  FIG. 5  is an interrupt routine that is started at time intervals (for instance, of 4 msec) at which the output of the oxygen sensor  10  should be sampled. 
     The routine shown in  FIG. 5  first increments the TCOUNT counter (step  100 ). The TCOUNT counter is cleared each time the calculation process for the device impedance Rs is performed, and used to count the time elapsed after the clearing. 
     Next, the count reached by the TCOUNT counter is checked to determine whether it coincides with the device impedance calculation interval T 1  (step  101 ). If it is found in step  101  that TCOUNT is less than T 1 , the current processing cycle comes to an end after the third analog-to-digital converter  68  acquires the output of the oxygen sensor  10  (AD acquisition) (step  102 ). According to the above process, the output of the oxygen sensor  10  is acquired at sampling time intervals as the output representing the information about the oxygen concentration in the exhaust gas until TCOUNT reaches to T 1  after the calculation process for the device impedance Rs is terminated. 
     If it is found in step  101  that TCOUNT is greater than or equal to T 1 , the calculation process for the device impedance Rs is performed at this time point and the TCOUNT counter is cleared to zero (step  104 ). More specifically, processing steps (i) through (v), which are described earlier, are performed in the calculation process for the device impedance Rs. That is, a voltage is applied to the oxygen sensor  10  with the first port turned ON, and then a process is performed to calculate the device impedance Rs in accordance with the resulting changes in the VS 1  and VS 2  values. 
     When the first port  24  turns OFF upon termination of the calculation process for the device impedance Rs, the ON time for the second port  44  is calculated (step  106 ). As described earlier, the ECU  20  turns ON the second port  44  to forcibly release the electrical charge stored in the oxygen sensor  10  after the device impedance Rs is calculated. In this instance, it is preferred that the second port  44  be turned OFF immediately after termination of forced release of electrical charge. In step  106 , therefore, the ON time is set as needed for forced release of electrical charge. 
       FIG. 6  illustrates the procedure to be performed in step  106  to set the ON time for the second port  44 . The left-hand drawing in  FIG. 6  is a map that defines the relationship between the device impedance Rs and the ON time for the second port  44 . The right-hand drawing in  FIG. 6  is a map that defines the relationship between the sensor output generated by the electromotive force of the oxygen sensor  10  and the ON time for the second port  44 . In step  106 , the ON time values determined by these two maps are multiplied together, and the resulting time is set as the ON time for the second port  44 . 
     According to the maps shown in  FIG. 6 , the greater the device impedance Rs, the longer the ON time setting for the second port  44 . When the first port  24  turns ON to apply a voltage to the oxygen sensor  10 , the resulting potential increases with an increase in the device impedance Rs. The higher the resulting potential, the larger the amount of electrical charge stored in the oxygen sensor  10 . Therefore, the time required for forcibly releasing the electrical charge after the first port  24  is turned OFF increases with an increase in the device impedance Rs. When processing step  106  is performed, the ON time for the second port can be set to meet such requirements. 
     According to the maps shown in  FIG. 6 , the higher the sensor output generated by the oxygen sensor  10 , the shorter the ON time setting for the second port  44 . After the voltage application to the oxygen sensor  10  is stopped with the first port  24  turned OFF, the smaller the difference between the normal output and the output generated upon voltage application, the more quickly the sensor output of the oxygen sensor  10  reverts to the normal output. More specifically, the present embodiment causes the sensor output to quickly revert to the normal value after the first port  24  is turned OFF if the normal sensor output voltage of the oxygen sensor  10  is high. It is therefore preferred that the ON time setting for the second port  44  decrease with an increase in the normal sensor output of the oxygen sensor  10 . When processing step  106  is performed, the ON time for the second port can be set to meet such preferences. 
     In the routine shown in  FIG. 5 , the ON process for the second port  44  is then performed (step  108 ). More specifically, step  108  is performed to keep the second port  440 N for a period that is set in step  106 . When step  108  is completed to perform the process, the sensor output of the oxygen sensor  10  reverts to the normal output, that is, the output correctly representing the oxygen concentration in the exhaust gas, immediately after the first port  24  is turned OFF. 
     When the sampling timing arrives again after termination of processing step  108 , it is determined that the condition imposed in step  101  is not met at this time; therefore, processing step  102  is performed again. The sensor output of the oxygen sensor  10  has already reverted to the normal output level. Thus, the ECU  20  can correctly detect the sensor output corresponding to the oxygen concentration in the exhaust gas from the sampling timing arriving immediately after the device impedance Rs is calculated. 
     As described above, the routine shown in  FIG. 5  is capable of forcibly releasing the electrical charge stored in the oxygen sensor  10  in the process for calculating the device impedance Rs by turning ON the second port  44 . Further, this routine can correctly detect the information about the oxygen concentration in the exhaust gas on each sampling cycle without being affected by the voltage application for calculating the device impedance Rs. As a result, the apparatus according to the present embodiment is capable of implementing the function for correctly detecting the device impedance Rs of the oxygen sensor  10  as well as the function for accurately detecting the information about the oxygen concentration in the exhaust gas. 
     In the first embodiment described above, a positive voltage is applied to a positive terminal of the oxygen sensor  10  at the time of calculating the device impedance Rs, and then the potential of the positive terminal of the oxygen sensor  10  is lowered (grounded) to quickly restore the sensor output to its normal level. However, there is an alternative method for quickly restoring the sensor output to normal. More specifically, the alternative is to apply a negative voltage to a negative terminal of the oxygen sensor  10  at the time of calculating the device impedance Rs, and then to raise the potential of the oxygen sensor&#39;s negative terminal for quick restoration of the sensor output. 
     In the first embodiment described above, the voltage applied to the oxygen sensor  10  is reduced to zero with a view toward quickly restoring the sensor output to normal after voltage application to the oxygen  10 . Alternatively, however, the voltage to be applied to the oxygen sensor  10  for quick restoration of the sensor output may be the sensor output voltage that is usually generated by the oxygen sensor  10  or may be a voltage that shifts from the usually generated output toward an opposite direction against a direction of the voltage applied to calculate the device impedance Rs. 
     In the first embodiment described above, the ECU  20  controls only the oxygen sensor (which generates an output that varies depending on whether the exhaust air-fuel ratio is rich or lean). However, the present invention is not limited to the control of the oxygen sensor. The present invention may also be applied to an air-fuel ratio sensor, which generates an output representing the oxygen concentration (air-fuel ratio) in a detected gas. 
     Second Embodiment 
     A second embodiment of the present invention will now be described with reference to  FIGS. 7 through 9 . The system according to the second embodiment can be implemented by using the configuration shown in  FIG. 1  and causing the ECU  20  to execute a routine shown in  FIG. 8 , which will be described later, instead of the routine shown in  FIG. 5 . 
     In the first embodiment described earlier, a voltage is applied to the oxygen sensor  10  for the purpose of calculating the device impedance Rs, and then the electrical charge stored in the oxygen sensor  10  is forcibly released so as not to acquire the sensor output that incorrectly corresponds to the oxygen concentration in the exhaust gas. On the other hand, the system according to the second embodiment does not acquire the sensor output as a proper value while the sensor output is affected by voltage application to the oxygen sensor  10 . In this manner, the second embodiment does not acquire the sensor output when it contains an error. 
       FIG. 7  shows sensor output changes that occur before and after the voltage for calculating the device impedance Rs is applied to the oxygen sensor  10 . The broken line in  FIG. 7  represents a waveform that is obtained when the device impedance Rs is large and the sensor output is significantly increased upon voltage application. The solid line in  FIG. 7  represents a waveform that is obtained when the device impedance Rs is small and the sensor output is insignificantly increased upon voltage application. 
     When the oxygen sensor  10  has a large device impedance Rs as shown in  FIG. 7  (as indicated by the broken line), it takes a relatively long time for the sensor output to decrease to its inherent value, that is, a value corresponding to the oxygen concentration in the exhaust gas, after the sensor output is increased upon voltage application. Further, if the device impedance Rs of the oxygen sensor  10  is small, the sensor output reverts to its inherent value within a relatively short period of time after voltage application. Therefore, if the sensor output is sampled at fixed time intervals, the number of times the sensor output containing an error is acquired, that is, the number of times the data to be invalidated is acquired in the present embodiment, is larger when the device impedance Rs is large than when the value Rs is small. In the present embodiment, therefore, the ECU  20  applies a voltage to the oxygen sensor  10  for the purpose of calculating the device impedance Rs, and then invalidates the sampled data (sensor output) as the data containing an error by a number of times that accords to the calculated device impedance Rs. 
       FIG. 8  is a flowchart illustrating a control routine that the ECU  20  executes to implement the above functionality in accordance with the present embodiment. Like steps in  FIGS. 5 and 8  are designated by like reference numerals and will be briefly described or will not be described again. 
     In the routine shown in  FIG. 8 , if it is found in step  101  that the calculation interval T 1  for the device impedance Rs has elapsed (TCOUNT is greater than or equal to T 1 ), following the processing step  104  (which calculates the device impedance Rs and clears the TCOUNT counter), data invalidation number of times AD 1  is calculated (step  110 ).  FIG. 9  shows a typical map that defines the relationship between the data invalidation number of times AD 1  and the device impedance Rs. The ECU  20  stores a map, which looks like the one shown in  FIG. 9 . Step  110  is performed to reference the map and calculate the data invalidation number of times that corresponds to the device impedance Rs calculated in step  104 . According to the map shown in  FIG. 9 , the larger the device impedance Rs is, thus the more likely the effect of the voltage application is remained in the sensor output, the higher the data invalidation number of times AD 1  is set. 
     Next, the routine shown in  FIG. 8  performs a process for clearing the ADCOUNT counter (step  112 ). The ADCOUNT counter counts the number of times the sensor output is sampled after the calculation process for the device impedance Rs. Upon completion of processing step  112 , the ECU  20  terminates the current processing cycle. 
     If it is found in step  101  that TCOUNT is less than T 1 , after sampling the sensor output in step  102 , the routine shown in  FIG. 8  increments the ADCOUNT counter (step  114 ). According to the above increment process, it is possible to sample the sensor output as well as to increment the ADCOUNT counter upon every sampling interval (upon every routine startup) until elapse of the device impedance calculation interval T 1  is determined. 
     Next, the routine shown in  FIG. 8  judges whether the count reached by the ADCOUNT counter is not greater than the data invalidation count AD 1  (step  116 ). If it is judged that ADCOUNT is less than or equal to AD 1 , the sensor output (data) sampled in step  102  is invalidated (step  118 ). When it is found that ADCOUNT is greater than AD 1 , processing step  118  is skipped so that the current processing cycle terminates without invalidating the acquired data. 
     As described above, the routine shown in  FIG. 8  can invalidate the sampled sensor output by a specified data invalidation number of times AD 1  after a voltage is applied to the oxygen sensor  10  for the purpose of calculating the device impedance Rs. The data invalidation number of times AD 1  can be set in accordance with the device impedance Rs so that it corresponds to a period during which an error will be superposed over the sensor output. Therefore, the system according to the present embodiment is capable of implementing the function for calculating the device impedance Rs of the oxygen sensor  10  as well as the function for constantly detecting the information about the oxygen concentration in the exhaust gas with high accuracy. 
     In the second embodiment described above, the data invalidation number of times AD 1  is set in accordance with the device impedance Rs of the oxygen sensor  10 . However, there is an alternative method for setup. For AD 1  setup purposes, the normal sensor output value of the oxygen sensor  10  may alternatively be taken into account as is the case with the first embodiment. More specifically, the smaller the difference between the normal sensor output value of the oxygen sensor  10  and the sensor output value attained upon voltage application, thus the more likely the time required for data convergence is to be short, the lower the data invalidation number of times can be set. 
     In the second embodiment described above, the ECU  20  controls only the oxygen sensor (which generates an output that varies depending on whether the exhaust air-fuel ratio is rich or lean). However, the present invention is not limited to the control of the oxygen sensor. The present invention may also be applied to an air-fuel ratio sensor, which generates an output representing the oxygen concentration (air-fuel ratio) in a detected gas. 
     Third Embodiment 
     A third embodiment of the present invention will now be described with reference to  FIGS. 10A ,  10 B and  11 . The system according to the third embodiment can be implemented by using the configuration shown in  FIG. 1  and causing the ECU  20  to execute a routine shown in  FIG. 11 , which will be described later, instead of the routine shown in  FIG. 5 . 
     In the present embodiment, the ECU  20  performs a process for calculating the device impedance Rs each time the device impedance calculation interval T 1  elapses, as is the case with the first embodiment. When the device impedance Rs is to be calculated, a voltage is applied to the oxygen sensor  10  as described earlier. The output of the oxygen sensor  10  does not accurately correspond to the oxygen concentration in the exhaust gas until the influence of voltage application disappears. Therefore, the information about the oxygen concentration in the exhaust gas can be accurately detected in accordance with the sensor output only during the time interval between the instant at which the effect of the voltage application in the sensor output disappears and the instant at which the device impedance calculation interval T 1  elapses. 
     The time required for the influence of voltage application upon the sensor output of the oxygen sensor  10  to disappear increases with an increase in the device impedance Rs. Therefore, if the device impedance calculation interval T 1  is fixed, although a sufficient period for correctly detecting the information about the oxygen concentration in the exhaust gas can be acquired as far as the device impedance Rs is low, such a period cannot be acquired in a situation where the device impedance Rs is high. 
       FIGS. 10A and 10B  are timing diagrams illustrating the operations that the present embodiment performs to handle the above situation. Waveform shown in  FIG. 10A  represents the sensor output that the oxygen sensor  10  generates when the device impedance Rs is high. Waveform shown in  FIG. 10B  represents the sensor output that the oxygen sensor  10  generates when the device impedance Rs is low. In  FIGS. 10A and 10B , the period shown with T 1  designates the above-mentioned device impedance calculation interval. The period shown with T 2  designates a period during which sensor output sampling is prohibited. 
     As indicated in  FIG. 10A , the system according to the present embodiment sets a long device impedance calculation interval T 1  and a long sampling prohibition period T 2  if the device impedance Rs is high. If, on the other hand, the device impedance Rs is low, the system sets a short device impedance calculation interval T 1  and a short sampling prohibition period T 2  as indicated in  FIG. 10B . When the values T 1  and T 2  are set in this manner, an adequate period can be obtained for normal sensor output acquisition without regard to the length of the period that is required for the influence of voltage application for calculating the device impedance Rs upon the sensor output to disappear. As a result, the system according to the present embodiment can properly detect the information about the oxygen concentration in the exhaust gas at all times without regard to the device impedance Rs of the oxygen sensor  10 . 
       FIG. 11  is a flowchart illustrating a control routine that the ECU  20  executes to implement the above functionality. Like steps in  FIG. 11  and  FIG. 5  or  8  are designated by like reference numerals and will be briefly described or will not be described again. 
     The routine shown in  FIG. 11  first increments the TCOUNT counter and sets the device impedance calculation interval T 1  and the sampling prohibition period T 2  (step  120 ). In the present embodiment, the ECU  20  stores a map for defining the value T 1  in relation to the device impedance Rs and a map for defining the value T 2 . In step  120 , these maps are referenced to set the values T 1  and T 2 . 
     Upon termination of processing step  120 , processing step  100  is performed to judge whether TCOUNT is greater than or equal to T 1 . If it is found that TCOUNT is greater than or equal to T 1 , step  104  is performed to calculate the device impedance Rs and clear the TCOUNT counter, and then the current processing cycle comes to an end. If, on the other hand, it is found that TCOUNT is less than T 1 , step  122  is performed to judge whether TCOUNT is greater than or equal to T 2 . 
     If it is found that TCOUNT is not greater than or equal to T 2 , it can be judged that the sampling prohibition period T 2  has not yet elapsed after calculation of the device impedance Rs. In this instance, the routine shown in  FIG. 11  terminates the current processing cycle thereafter without sampling any sensor output. If, on the other hand, it is found that TCOUNT is greater than or equal to T 2 , it can be judged that the sampling prohibition period T 2  has already elapsed. In this instance, step  102  is performed thereafter to sample the sensor output, and then the current processing cycle comes to an end. According to the process described above, the sensor output can be acquired at every sampling time interval only after the end of the sampling prohibition period T 2  and before the end of the device impedance calculation interval T 1 . 
     The T 1 -related map and T 2 -related map for use in step  120  are both set so that the higher the device impedance Rs, the greater the value T 1  or T 2 . More specifically, the map concerning the device impedance calculation interval T 1  defines the relationship between the values Rs and T 1  so that a time period in which the oxygen sensor  10  can generate normal sensor output is always sufficiently acquired without regard to the value of the device impedance Rs. Further, the map concerning the sampling prohibition period T 2  defines the relationship between the value T 2  and the device impedance Rs so that the sampling prohibition period T 2  is equal to a period during which the sensor output of the oxygen sensor  10  remains affected by voltage application. Therefore, the routine shown in  FIG. 11  can always provide a sufficient period for normal output generation by the oxygen sensor  10  without regard to the value of the device impedance Rs, and sample correct sensor outputs only. Therefore, the system according to the present embodiment is capable of implementing the function for accurately detecting the device impedance Rs as well as the function for accurately detecting the information about the oxygen concentration in the exhaust gas. 
     In the third embodiment described above, the device impedance calculation interval T 1  and the sampling prohibition period T 2  are set in accordance with the device impedance Rs. However, there is an alternative method for such setup. Setup may alternatively be performed while considering the normal sensor output value of the oxygen sensor  10 , as is the case with the first embodiment. More specifically, the smaller the difference between the normal sensor output value of the oxygen sensor  10  and the sensor output value attained upon voltage application, i.e., the more likely the time required for data convergence is to be short, the lower the T 1  and T 2  can be set. 
     In the third embodiment described above, the ECU  20  controls only the oxygen sensor (which generates an output that varies depending on whether the exhaust air-fuel ratio is rich or lean). However, the present invention is not limited to the control of the oxygen sensor. The present invention may also be applied to an air-fuel ratio sensor, which generates an output representing the oxygen concentration (air-fuel ratio) in a detected gas. 
     Fourth Embodiment 
     A fourth embodiment of the present invention will now be described with reference to  FIGS. 12 through 15 . The system according to the fourth embodiment can be implemented by using the apparatus according to the third embodiment and causing the ECU  20  to execute a routine shown in  FIG. 14 , which will be described later, instead of the routine shown in  FIG. 11 . 
     As described earlier, the apparatus according to the third embodiment increases the device impedance calculation interval T 1  with an increase in the device impedance Rs.  FIG. 12  shows a typical map of the device impedance calculation interval T 1  that can be used with the apparatus according to the third embodiment (see step  120  in  FIG. 11 ). According to this map, in a region where the device impedance Rs is between the high convergence value RsH and the low convergence value RsL, the device impedance calculation interval T 1  is determined so that it is linear in relation to the device impedance Rs. In a region where the device impedance Rs is above the high convergence value RsH or below the low convergence value RsL, however, the resulting determined device impedance calculation interval T 1  coincides with a specified maximum value T 1 max or minimum value T 1 min. According to such a map, the intervals at which the voltage for detecting the device impedance Rs is applied to the oxygen sensor  10  increase with an increase in the device impedance Rs and decrease with a decrease in the device impedance Rs. 
       FIG. 13  illustrates the relationship between the device temperature and device impedance Rs of the oxygen sensor  10 . As indicated in this figure, the device impedance Rs decreases with an increase in the device temperature of the oxygen sensor. When the voltage for device impedance detection is applied to the oxygen sensor  10 , the current I flowing to the oxygen sensor  10  increases with a decrease in the device impedance Rs. Therefore, the current I increases with an increase in the device temperature of the oxygen sensor  10 . 
     According to the map shown in  FIG. 12 , the device impedance calculation interval T 1  is set to the minimum value when the device impedance Rs is below the low convergence value RsL, that is, when a large current I flows upon voltage application. In such an instance, since the device impedance Rs is frequently detected, it becomes that a large current I frequently flows through the oxygen sensor  10 . The larger the current I flowing through the oxygen sensor  10  is and the longer the period of time during which the current I flows, the greater the damage caused to the oxygen sensor  10  will be. Therefore, if the device impedance calculation interval T 1  is determined in accordance with the map shown in  FIG. 12 , it is likely that the oxygen sensor  10  will be significantly damaged after the device temperature is adequately raised. Under these circumstances, the present embodiment employs a long device impedance calculation interval T 1  to decrease the frequency of voltage application to the oxygen sensor  10  in a region where the device impedance Rs is sufficiently low. 
       FIG. 14  is a flowchart illustrating a control routine that the ECU  20  according to the present embodiment performs in order to implement the above functionality. The routine shown in  FIG. 14  is the same as the routine shown in  FIG. 11  except that step  120  is replaced by step  130 . Like steps in  FIG. 14  and  FIG. 11  are designated by like reference numerals and will be briefly described or will not be described again. 
     In the routine shown in  FIG. 14 , step  130  is performed to a) increment the TCOUNT counter, b) set the device impedance calculation interval T 1 , and c) set the sampling prohibition period T 2 . Processes a) and c) are performed by the same method as in the third embodiment (see step  120  in  FIG. 11 ). Process b) is performed to set the device impedance calculation interval T 1  by referencing the map shown in  FIG. 15 . 
     The  FIG. 15  shows a typical map that the ECU  20  stores in order to set the device impedance calculation interval T 1 . A specified threshold value RsTH, which is smaller than the low convergence value RsL, is set on the horizontal axis of the map shown in  FIG. 15 . Within a region where the device impedance Rs is above the threshold value RsTH, this map is designed in the same manner as the map shown in  FIG. 12  is. Within a region where the device impedance Rs is below the threshold value RsTH, the map shown in  FIG. 15  is designed so that the device impedance calculation interval T 1  promptly reaches the maximum value T 1  max when the sensor impedance Rs decreases. 
     The threshold value RsTH shown in  FIG. 15  is defined as the smallest device impedance Rs that does not cause any undue damage to the oxygen sensor  10  when the device impedance calculation interval T 1  is set to the minimum value T 1 min. In other words, the map shown in  FIG. 15  sets the device impedance calculation interval T 1  to the minimum value T 1 min when the device impedance Rs of the oxygen sensor  10  is RsTH. In this instance, the voltage for device impedance detection is repeatedly applied to the oxygen sensor  10  at intervals of the minimum value T 1 min. Upon each voltage application, the current I (=V/RsTH) obtained by dividing the applied voltage V by the value RsTH flows to the oxygen sensor  10 . The threshold value RsTH represents the device impedance Rs for generating the largest current I that can repeatedly flow at intervals of T 1 min without causing any undue damage to the oxygen sensor  10 . 
     The routine shown in  FIG. 14  repeatedly performs a process for detecting the device impedance Rs at device impedance calculation intervals T 1  that is set in accordance with the map shown in  FIG. 15 . In this instance, the damage caused to the oxygen sensor  10  upon voltage application is maximized when the device impedance Rs coincides with the threshold value RsTH, due to the relationship between the voltage application intervals (T 1 ) and the current I generated upon the voltage application. As described earlier, the threshold value RsTH is set so that the oxygen sensor  10  does not receive any undue damage in such an instance. In the present embodiment, therefore, a repeated execution of the process for detecting the device impedance Rs will not unduly damage the oxygen sensor  10  without regard to the value of the device impedance Rs. 
     As described above, the apparatus according to the present embodiment can protect the oxygen sensor  10  from undue damage by decreasing the frequency of device impedance detection within a region where the device impedance Rs of the oxygen sensor  10  is sufficiently low. As a result, the apparatus according to the present embodiment adequately prevents the oxygen sensor  10  from deteriorating while providing the same advantages as the apparatus according to the third embodiment. 
     The present invention, which is configured as described above, provides the following advantages. 
     According to a first aspect of the present invention, the gas concentration sensor&#39;s output can converge to the voltage generated by the gas concentration sensor itself, that is, the output corresponding to the exhaust air-fuel ratio immediately after the impedance detection voltage is applied to the gas concentration sensor. Therefore, the present invention can implement the function for detecting the device impedance and the function for accurately detecting the information about the exhaust air-fuel ratio. 
     According to a second aspect of the present invention, the greater the device impedance of the gas concentration sensor is, thus the longer the period of time for the influence of the impedance detection voltage to disappear, the longer the period (specified period) of voltage application for canceling the influence can be. Therefore, the present invention can efficiently negate the influence of the impedance detection voltage within a short period of time. 
     According to a third aspect of the present invention, the smaller the difference between the voltage generated by the gas concentration sensor itself and the impedance detection voltage is, thus the influence of the impedance detection voltage is more likely to disappear in a short time, the shorter the period (specified period) of voltage application for canceling the influence can be. Therefore, the present invention can efficiently negate the influence of the impedance detection voltage within a short period of time. 
     According to a fourth aspect of the present invention, the gas concentration sensor&#39;s output can be invalidated during a specified period of time during which the gas concentration sensor&#39;s output is affected by the impedance detection voltage. Therefore, the present invention can implement the function for detecting the device impedance and the function for accurately detecting the information about the exhaust air-fuel ratio. 
     According to a fifth aspect of the present invention, the greater the device impedance of the gas concentration sensor is, thus the longer the period of time during which the influence of the impedance detection voltage remains, the longer the data invalidation period (specified period) can be. Therefore, the present invention can effectively avoid erroneous detection of the exhaust air-fuel ratio, which is based on the gas concentration sensor&#39;s output. 
     According to a sixth aspect of the present invention, the greater the device impedance of the gas concentration sensor is, the longer the time intervals (specified time intervals) at which the impedance detection voltage is applied can be. Without regard to the magnitude of the device impedance, therefore, the present invention can properly provide a period during which the gas concentration sensor&#39;s output correctly corresponds to the exhaust air-fuel ratio. 
     According to a seventh aspect of the present invention, the output acquisition period can begin at the end of a period (specified period) during which the gas concentration sensor&#39;s output is affected by the impedance detection voltage. Further, the greater the device impedance is, thus the longer the period of time during which the influence of the impedance detection voltage remains, the longer the above-mentioned specified period can be. Without regard to the magnitude of the device impedance, therefore, the present invention can recognize a period during which the gas concentration sensor&#39;s output correctly corresponds to the exhaust air-fuel ratio as an output acquisition period. 
     According to an eighth aspect of the present invention, the time intervals (specified intervals) at which the impedance detection voltage is applied can be longer in a situation where the device impedance is below a predefined threshold value than in a situation where the device impedance coincides with the predefined threshold value. In other words, in a situation where a large current flows upon impedance detection because of low device impedance, the present invention can decrease the frequency of device impedance detection. Therefore, the present invention effectively prevents the gas concentration sensor from being excessively damaged in a situation where the device impedance is low. 
     Further, the present invention is not limited to these embodiments, but variations and modifications may be made without departing from the scope of the present invention. The entire disclosure of Japanese Patent Application No. 2003-285183 filed on Aug. 1, 2003 including specification, claims, drawings and summary are incorporated herein by reference in its entirety.