Patent Publication Number: US-2004045823-A1

Title: Noiseless gas concentration measurement apparatus

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Technical Field of the Invention  
       [0002] The present invention relates generally to a noiseless circuit structure of a gas concentration measuring apparatus equipped with a gas sensor.  
       [0003] 2. Background Art  
       [0004] Typical gas sensors for use in automotive internal combustion engines uses an oxygen ion conductive solid electrolyte material such as zirconia. For instance, gas sensors are known which have formed therein a gas chamber and a cell which is made up of a pair of electrodes affixed to a solid electrolyte body to pump oxygen molecules (O 2 ) into or out of the gas chamber. Such a type of gas sensor works to transfer oxygen ions as carriers through the solid electrolyte body in response to application of voltage to the electrodes to pump the oxygen molecules into or out of the gas chamber. Gas sensors are known which include a plurality of cells of the above type in order to measure the concentration of NOx (nitrogen oxide), CO (carbon monoxide), and HC (hydro carbon).  
       [0005] Gas sensors of the above type usually include a first and a second gas chamber and a first and a second pump cell. The first pump cell works to pump oxygen molecules out of the first gas chamber to decrease the concentration of oxygen within the second gas chamber to a lower level. The second pump cell includes electrodes made up of metal active with NOx and works to reduce or oxidize gasses within the second gas chamber through a surface of one of the electrodes exposed to the second gas chamber to change the concentration of oxygen on the surface of the electrode. This causes an electric current to flow between the electrodes which is used to determine the concentration of NOx. Specifically, increased accuracy of determining the concentration of NOx is ensured by keeping the oxygen molecules remaining within the second gas chamber as small as possible and actuating the second pump cell quickly when the concentration of oxygen within the second gas chamber changes.  
       [0006]FIG. 17 illustrates a map listing relations between voltage applied to the pump cell and resultant current flowing through the pump cell. The map shows that increasing the voltage applied to the pump cell (which will also be referred to as a pump cell-applying voltage below) results in increased ability of the pump cell to pump the oxygen molecules, thereby increasing the current flowing between the electrodes of the pump cell (which will also referred to as a pump cell current below). The pump cell current is saturated at a value (i.e., a limiting current) indicative of the concentration of oxygen outside the gas chamber, that is, the concentration of oxygen contained in the gasses entering the gas chamber. When the concentration of oxygen outside the gas chamber increases, it will require increasing of the pumping ability of the pump cell, so that a lower limit of the pump cell-applying voltage needed to produce the limiting current. To this end, a target value of the pump cell-applying voltage is determined by look-up using the map of FIG. 17 as a function of the pump cell current indicating a pumped amount of oxygen to output a command voltage to adjust the pump cell-applying voltage.  
       [0007] The pump cell current-to-pump cell-applying voltage relation usually varies, as shown in FIG. 18, between pump cells due to an individual difference therebetween arising from the production tolerance. It is, thus, required to optimize the map, as illustrated in FIG. 17, for each gas sensor to absorb the individual difference.  
       [0008] Nowadays, microcomputers are expected to be suitable for optimizing the map. Fine adjustment of the map is achieved only by rewriting data in a ROM of the microcomputer. This is also useful for saving costs.  
       [0009] The use of the microcomputer to adjust the pump cell-applying voltage poses the following problem. The microcomputer works to output a feeding signal specifying the pump cell-applying voltage from an A/D converter. The feeding signal usually has one of discrete values. This may, as shown in FIG. 19, result in stepwise changes in the pump cell-applying voltage, thereby causing the pump cell current to have spiky peaks (i.e., a current change ΔI) as a function of susceptance of the pump cell. The spiky peaks contribute to a reduction in accuracy of determining the concentration of oxygen (O 2 ) and may also result in a difficulty in determining the pump cell-applying voltage correctly. Further, gas sensors designed to measure the concentration of NOx or CO as a function of a deviation of the concentration of oxygen arising from reduction or oxidization of NOx or CO also have a problem of reduction in accuracy of determining the concentration of NOx or CO.  
       SUMMARY OF THE INVENTION  
       [0010] It is therefore a principal object of the present invention to avoid the disadvantages of the prior art.  
       [0011] It is another object of the present invention to provide a noiseless circuit structure of a gas concentration measuring apparatus.  
       [0012] According to one aspect of the invention, there is provided a gas concentration measuring apparatus which may be employed in burning control of an automotive internal combustion engine. The gas concentration measuring apparatus comprises: (a) a gas sensor including a sensor base and a pump cell, the sensor base including a solid electrolyte body which defines within the sensor base a gas chamber into which gases are admitted through a given diffusion resistance, the pump cell being made up of a first and a second electrode affixed to the solid electrolyte body with the first electrode exposed to the gas chamber and responsive to application of electricity to the first and second electrodes to pump a given gas component out of and into the gas chamber selectively to produce a sensor signal in the form of an electrical change as a function of a pumped amount of the oxygen; (b) an electricity control circuit working to produce a feeding signal having one of discrete electrical values to control the electricity applied to the first and second electrodes of the pump cell; (c) a sensor signal detecting circuit working to detect the sensor signal outputted form the pump cell and produce a sensor output as a function of concentration of the given gas component; and (d) a change limiting circuit working to limit a change in the sensor signal to within a given range, thereby removing noises from the sensor signal which arise from susceptance of the pump cell at the time of a switch between the discrete electrical values of the feeding signal.  
       [0013] In the preferred mode of the invention, the change limiting circuit is implemented by an integrating circuit which works to integrate the sensor signal.  
       [0014] The electricity control circuit works to determine a target value of the feeding signal as a function of the sensor signal.  
       [0015] A second pump cell is further provided which works to produce a pump signal as a function of concentration of the given gas component within a second gas chamber formed within the gas base downstream of the gas chamber. The electricity control circuit may alternatively work to determine the target value of the feeding signal as a function of the pump signal.  
       [0016] The electricity control circuit may alternatively be designed to produce a voltage modulated by a PWM signal and convert the modulated voltage into a DC voltage to be applied to the first and second electrodes of the pump cell.  
       [0017] The electricity control circuit works to produce the DC voltage within a range between binary voltage levels.  
       [0018] The electricity control circuit includes a modulating circuit working to switch the voltage between the binary voltage levels using the PWM signal.  
       [0019] According to the second aspect of the invention, there is provided a gas concentration measuring apparatus which comprises: (a) a gas sensor including a sensor base and a pump cell, the sensor base including a solid electrolyte body which defines within the sensor base a gas chamber into which gases are admitted through a given diffusion resistance, the pump cell being made up of a first and a second electrode affixed to the solid electrolyte body with the first electrode exposed to the gas chamber and responsive to application of electricity to the first and second electrodes to pump a given gas component out of and into the gas chamber selectively to produce a sensor signal in the form of an electrical change as a function of a pumped amount of the given gas component; (b) an electricity control circuit working to produce a feeding signal having one of discrete electrical values to control the electricity applied to the first and second electrodes of the pump cell; (c) a sensor signal detecting circuit working to detect the sensor signal outputted form the pump cell and produce a sensor output as a function of concentration of the given gas component; and (d) a blurring circuit working to blur a change in the sensor signal, thereby removing noises from the sensor signal which arise from susceptance of the pump cell at the time of a switch between the discrete electrical values of the feeding signal.  
       [0020] In the preferred mode of the invention, a change limiting circuit is further provided which works to limit the change in the sensor signal to within a given range prior to blur the change in the sensor signal.  
       [0021] The blurring circuit is implemented by an integrating circuit which works to integrate the sensor signal.  
       [0022] The electricity control circuit works to determine a target value of the feeding signal as a function of the sensor signal.  
       [0023] A second pump cell is further provided which works to produce a pump signal as a function of concentration of the given gas component within a second gas chamber formed within the gas base downstream of the gas chamber. The electricity control circuit may alternatively work to determine a target value of the feeding signal as a function of the pump signal.  
       [0024] The electricity control circuit may alternatively be designed to produce a voltage modulated by a PWM signal and convert the modulated voltage into a DC voltage to be applied to the first and second electrodes of the pump cell.  
       [0025] The electricity control circuit works to produce the DC voltage within a range between binary voltage levels.  
       [0026] The electricity control circuit includes a modulating circuit working to switch the voltage between the binary voltage levels using the PWM signal.  
       [0027] According to the third aspect of the invention, there is provided a gas concentration measuring apparatus which comprises: (a) a gas sensor including a sensor base and a pump cell, the sensor base including a solid electrolyte body which defines within the sensor base a gas chamber into which gases are admitted through a given diffusion resistance, the pump cell being made up of a first and a second-electrode affixed to the solid electrolyte body with the first electrode exposed to the gas chamber and responsive to application of electricity to the first and second electrodes to pump a given gas component out of and into the gas chamber selectively to produce a sensor signal in the form of an electrical change as a function of a pumped amount of the given gas component; (b) an electricity control circuit working to produce a feeding signal having one of discrete electrical values to control the electricity applied to the first and second electrodes of the pump cell; (c) a sensor signal detecting circuit working to detect the sensor signal outputted form the pump cell and produce a sensor output as a function of concentration of the given gas component; and (d) a blurring circuit working to blur the feeding signal produced by the electricity control circuit, thereby removing noises from the sensor signal which arise from susceptance of the pump cell at the time of a switch between the discrete electrical values of the feeding signal.  
       [0028] In the preferred mode of the invention, the blurring circuit is implemented by an integrating circuit which works to integrate the feeding signal.  
       [0029] The electricity control circuit works to determine a target value of the feeding signal as a function of the sensor signal.  
       [0030] A second pump cell may be provided which works to produce a pump signal as a function of concentration of the given gas component within a second gas chamber formed within the gas base downstream of the gas chamber. The electricity control circuit may alternatively work to determine a target value of the feeding signal as a function of the pump signal.  
       [0031] The electricity control circuit may alternatively be designed to produce a voltage modulated by a PWM signal and convert the modulated voltage into a DC voltage to be applied to the first and second electrodes of the pump cell.  
       [0032] The electricity control circuit works to produce the DC voltage within a range between binary voltage levels.  
       [0033] The electricity control circuit includes a modulating circuit working to switch the voltage between the binary voltage levels using the PWM signal. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0034] The present invention will be understood more fully from the detailed description given hereinbelow and from the accompanying drawings of the preferred embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments but are for the purpose of explanation and understanding only.  
     [0035] In the drawings:  
     [0036]FIG. 1 is a circuit block diagram which shows a gas concentration measuring apparatus according to the first embodiment of the invention;  
     [0037]FIG. 2 is a longitudinal sectional view which shows a gas sensor employed in the gas concentration measuring device of FIG. 1;  
     [0038]FIG. 3 is a sectional view taken along the line III-III in FIG. 2;  
     [0039]FIG. 4 is a sectional view taken along the line IV-IV in FIG. 2;  
     [0040]FIG. 5 is a flowchart of a program performed to determine voltage to be applied to a pump cell;  
     [0041]FIG. 6 shows changes in pump cell current indicating the concentration of oxygen (O 2 );  
     [0042]FIG. 7 shows a change in voltage applied to a pump cell;  
     [0043]FIG. 8 shows a noise-caused change in pump cell current;  
     [0044]FIG. 9 is a circuit block diagram which shows a gas concentration measuring device according to the second embodiment of the invention;  
     [0045]FIG. 10 is a circuit block diagram which shows a gas concentration measuring device according to the third embodiment of the invention;  
     [0046]FIG. 11 is a circuit block diagram which shows a gas concentration measuring device according to the fourth embodiment of the invention;  
     [0047]FIG. 12 is a circuit block diagram which shows a gas concentration measuring device according to the fifth embodiment of the invention;  
     [0048]FIG. 13 is a circuit block diagram which shows a gas concentration measuring device according to the sixth embodiment of the invention;  
     [0049]FIG. 14 is a longitudinal sectional view which shows a gas sensor employed in the gas concentration measuring device of FIG. 13;  
     [0050]FIG. 15 is a circuit block diagram which shows a gas concentration measuring device according to the seventh embodiment of the invention;  
     [0051]FIG. 16 is a circuit block diagram which shows a gas concentration measuring device according to the eighth embodiment of the invention;  
     [0052]FIG. 17 shows a pump cell current-to-pump cell applying voltage map as employed in conventional gas concentration measuring devices;  
     [0053]FIG. 18 shows a variation in pump cell-applying voltage arising from a production tolerance of gas sensors; and  
     [0054]FIG. 19 shows a relation between a pump cell-applying voltage changing stepwise and a resultant change in pump cell current. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0055] Referring now to the drawings, wherein like numbers refer to like parts in several views, particularly to FIG. 1, there is shown a gas concentration measuring device according to the first embodiment of the invention which consists essentially of a gas sensor  1  and a control circuit implemented by a CPU  20 . The gas sensor  1  is installed, for example, in an exhaust pipe of an automotive internal combustion engine and exposed to exhaust gasses emitted from the engine. The control circuit is installed in a vehicle cabin or on a lower portion of a vehicle body and coupled with the gas sensor  1  through a wire cable. The control circuit is responsive to outputs from the gas sensor  1  to determine the concentration of nitrogen oxide (NOx), HC (hydro carbon), and CO (carbon monoxide) contained in exhaust gasses of the engine. In the following discussion, the gas sensor  1  is assumed to measure the concentration of NOx.  
     [0056] The gas sensor  1  is, as clearly shown in FIGS.  2  to  4 , formed by a lamination of oxygen ion-conductive solid electrolyte layers  111  and  112  made of zirconia, insulating layers  113  and  114  made of alumina, and a layer  115  made of an insulating material such as alumina or a solid electrolyte material such as zirconia which are laid overlap each other in a thickness-wise direction of the gas sensor  1  in the form of a rectangular plate. The insulating layer  114  interposed between the solid electrolyte layers  111  and  112  has formed therein an opening to define two gas chambers  101  and  102 , as will also be referred to as a first and a second chambers below, which communicate with each other through an orifice  103 . The first and second chambers  101  and  102  are arrayed in a lengthwise direction of the gas senor  1 . The second chamber  102  which is located closer to a base portion (i.e., atmospheric side) of the gas sensor  1  is two times wider than the first chamber  101  which is located closer to a head portion (i.e., gas-sensitive side) of the gas sensor  1 .  
     [0057] Air ducts  104  and  105  are formed outside the solid electrolyte layers  111  and  112 , respectively. The air ducts  104  and  105  communicate with the atmosphere at the side of the base portion of the gas sensor  1 . The first air duct  104  extends over the first chamber  104  through the solid electrolyte layer  112 . The second air duct  105  extends over the second chamber  102  through the solid electrolyte layer  111 . The installation of the gas sensor  1  in an exhaust system of an automotive engine is achieved by inserting the gas sensor  1  partially into an exhaust pipe through a holder and communicating the air ducts  104  and  105  with the atmosphere. Specifically, the air ducts  104  and  105  are filled with air showing a reference oxygen concentration.  
     [0058] The solid electrolyte layer  111  has formed therein a pinhole  106  leading to the first chamber  101 . A porous diffusion layer  116  is formed on the solid electrolyte layer  111  to avoid intrusion of exhaust fine particles into the firs chamber  101  and serves to provide limiting current characteristics. The pinhole  106  works to admit the gasses to be measured into the first chamber  101  which are flowing outside the porous diffusion layer  116 .  
     [0059] The solid electrolyte layer  112  has formed on opposed surfaces thereof electrodes  121  and  122  exposed to the first chamber  101  and the air duct  104 , respectively, and defines a pump cell  1   a  together with the electrodes  121  and  122 . The electrode  121  exposed to the first chamber  101  is made of noble metal such as Au—Pt which is inactive with respect to NOx, that is, hardly decomposes NOx. The electrode  121  exposed to the first chamber  101  will also be referred to as a chamber-side pump electrode. The electrode  122  exposed to the air duct  104  will also be referred to as an air-side pump electrode.  
     [0060] The solid electrolyte layer  111  has formed on opposed surfaces thereof electrodes  125 ,  123 , and  124 . The electrode  125  exposed to the air duct  105 , as can be seen in FIG. 4, serves as an electrode common to the electrodes  123  and  124 . The solid electrolyte layer  111  defines a monitor cell together with the electrodes  123  and  125  and a sensor cell  1   c  together with the electrode  124  and  125 . The electrode  123  of the monitor cell  1   b  exposed to the second chamber  102  is made of noble metal such as Au—Pt which is inactive with respect to NOx, that is, hardly decomposes NOx. The electrode  124  of the sensor cell  1   c  exposed to the second chamber  102  is made of noble metal such as Pt which is active with respect to NOx, that is, serves to decompose or ionize NOx. The electrode  123  exposed to the second chamber  102  will also be referred to as a chamber-side monitor electrode. The electrode  124  exposed to the second chamber  102  will also be referred to as a chamber-side sensor electrode. The electrode  125  exposed to the air duct  105  will also be referred to as an air-side sensor/monitor electrode.  
     [0061] The layer  115  defining the air duct  104  together with the solid electrolyte layer  112  has embedded therein a Pt-made patterned conductor which works as a heater  13  for heating the whole of the gas sensor  1  (especially, the solid electrolyte layers  111  and  112 ) up to a desired activation temperature. The heater  13  is of an electrical type generating Joule heat.  
     [0062] The exhaust gasses of the engine flowing outside the gas sensor  1 , as described above, enters the first chamber  101  through the porous diffusion layer  116  and the pinhole  106 . Application of voltage to the pump cell  1   a  through the electrodes  121  and  122  with the electrode  122  connected to a positive terminal of a voltage source causes oxygen molecules contained in the exhaust gasses to undergo dissociation or ionization, so that the oxygen (O 2 ) is pumped out of the first chamber  101  to the air duct  104 . If the concentration of the oxygen (O 2 ) is lower than a desired level in the first chamber  101 , a reverse voltage is applied to the pump cell  1   a  to pump oxygen molecules into the first chamber  101  from the air duct  104  so as to keep the concentration of oxygen (O 2 ) within the first chamber  101  constant.  
     [0063] Increasing the voltage applied across the electrodes  121  and  122  of the pump cell  1   a  causes the majority of flow of oxygen (O 2 ) into the first chamber  101  from the pinhole  106  to depend on a diffusion resistance of the pinhole  106 , so that a limiting current is produced in the pump cell  1   a  which is a function of the concentration of oxygen (O 2 ) contained in the exhaust gasses flowing outside the gas sensor  1 . Since the chamber-side pump electrode  121 , as described above, hardly decomposes NOx, NOx gas stays within the first chamber  101 .  
     [0064] The exhaust gasses having entered the first chamber  101  diffuse into the second chamber  102 . Specifically, the O 2  molecules in the exhaust gasses are usually not dissociated by the pump cell  1   a  completely, so that residual O 2  molecules flow into the second chamber  102  and reach the monitor cell  1   b  and the sensor cell  1   c.  The application of given voltage to the monitor cell  1   b  and the sensor cell  1   c  with the common electrode  125  coupled to the positive terminal of the voltage source causes the gasses within the second chamber  102  to be decomposed so that oxygen ions are discharged to the air duct  105 , thereby producing limiting currents in the monitor cell  1   b  and the sensor cell  1   c.  Only the chamber-side sensor electrode  124  of the electrodes  123  and  124  exposed to the second chamber  102  is, as described above, active with NOx, so that the current flowing through the sensor cell  1   c  will be greater than that flowing through the monitor cell  1   b  by a value equivalent to the amount of oxygen ion arising from the dissociation or decomposition of NOx on the chamber-side sensor electrode  124  of the sensor cell  1   c.  Determination of the concentration of NOx contained in the exhaust gasses is, therefore, achieved by finding a difference between the currents flowing through the monitor cell  1   b  and the sensor cell  1   c.  EPO 987 546 A2, assigned to the same assignee as that of this application, teaches control of an operation of this type of gas sensor, disclosure of which is incorporated herein by reference.  
     [0065] Referring back to FIG. 1, the control circuit consists of the CPU  20 , a pump cell circuit  3   a,  a monitor cell circuit  3   b,  and a sensor cell circuit  3   c.    
     [0066] The pump cell circuit  3   a  consists of operational amplifiers  41  and  52 , a D/A converter  211 , an A/D converter  221 , a resistor  61 , and a reference voltage source  51 . The D/A converter  211  receives a voltage command signal outputted from the CPU  20  and converts it into an analog voltage signal, which is, in turn, inputted as a feeding signal to the operational amplifier  41  serving as a voltage follower. The operational amplifier  41  works to apply the voltage Vp′ to the air-side pump electrode  122  of the pump cell  1   a.  The operational amplifier  52  which serves as a voltage follower receives an output voltage of the reference voltage source  51  and applies a reference voltage Vp″ to the chamber-side pump electrode  121  of the pump cell  1   a.  The resistor  61  is disposed in a line extending between the operational amplifier  52  and the chamber-side pump electrode  121 . The resistor  61  works as a pumped oxygen amount detector. Specifically, the voltage is developed across the resistor  61  as a function of amount of oxygen pumped by the pump cell  1   a  and inputted to the A/D converter  221 . When the voltage (i.e., Vp′-VP″), as will be referred to as a pump cell-applying voltage Vp below, is applied across the electrodes  121  and  122  of the pump cell  1   a , it will cause the current Ip to flow between the electrodes  121  and  122 , which is measured by the CPU  20  as a voltage drop across the resistor  61 .  
     [0067] Each of the monitor cell circuit  3   b  and the sensor cell circuit  3   c  is similar in structure to the pump cell circuit  3   a  and includes operational amplifiers and a resistor. The monitor cell circuit  3   b  works to apply the voltage Vm across the electrodes  123  and  125  of the monitor cell  1   b,  as will be referred to as a monitor cell-applied voltage below, and measure the current flowing between the electrodes  123  and  125 , as will be referred to as a monitor cell current Im below). Similarly, the sensor cell circuit  3   c  works to apply the voltage Vs across the electrodes  124  and  125  of the sensor cell  1   c,  as will be referred to as a sensor cell-applied voltage below, and measure the current flowing between the electrodes  124  and  125 , as will be referred to as a sensor cell current Is below). The monitor cell circuit  3   b  is substantially identical in structure with the pump cell circuit  3   a  and works to control the monitor cell-applied voltage Vm through an output of a D/A converter.  
     [0068] The control circuit also works to determine the impedance of the pump cell  1   a,  the monitor cell  1   b,  or the sensor cell  1   c.  In practice, such a determination is achieved by measuring the impedance between the electrodes  123  and  125  of the monitor cell  1   b  which will be referred to as sensor impedance below. The determination of the sensor impedance is achieved by shifting an output voltage of the D/A converter of the monitor cell circuit  3   b  either to a positive side or a negative side instantaneously (e.g., for several tens or several hundreds of μsec.) to add an ac component to the monitor cell-applied voltage Vm and measuring a resultant change in the monitor cell current Im through the CPU  20 . Specifically, the CPU  20  determines the sensor impedance based on the changes in the monitor cell-applied voltage Vm and the monitor cell current Im.  
     [0069] The heater  13  is supplied with power from a storage battery (not shown). Specifically, the CPU  20  outputs a pulse width modulated (PWM) signal to the heater  13  trough a heater driver (not shown) to control a supply of power to the heater  13 . The CPU  20  determines the duty cycle of the PWM signal as a function of the sensor impedance. The sensor impedance has a value that is a function of the temperature of the solid electrolyte layers  111  and  112 . The CPU  20  adjusts the duty cycle of the PWM signal so as to bring the sensor impedance into agreement with a preselected target one under feedback control, thereby keeping the temperature of the solid electrolyte layers  111  and  112  at a required activation temperature.  
     [0070] The operation of the gas concentration measuring apparatus of this embodiment will be described blow.  
     [0071]FIG. 5 shows a sequence of logical steps or program executed by the CPU  20  to control the pump cell-applying voltage Vp.  
     [0072] After entering the program, the routine proceeds to step  101  whether the time the pump cell-applying voltage Vp should be adjusted has been reached or not. The adjustment of the pump cell-applying voltage Vp is to be achieved at an interval of, for example, 10 ms. If a NO answer is obtained, then the routine repeats step  101 . Alternatively, if a YES answer is obtained, then the routine proceeds to step  102  wherein the A/D converter  211  samples the voltage appearing across the resistor  61  to measure the pump cell current Im (which will also be referred to as an A/D-sampled value below).  
     [0073] Operations in steps  103  to  105  are to limit a change in output of the pump cell  1   a  within a given range. In the following steps, “X” generally indicates the A/D-measure value, “X i ” indicates the A/D-sampled value in a current program cycle, and “X i−1 ” indicates the A/ D-sampled value one program cycle earlier.  
     [0074] In step  103 , it is determined whether a change in A/D-sampled value X, that is, an absolute value of a difference between the values X i  and X i−1  is greater than or equal to a preselected upper change limit ΔX or not.  
     [0075] If a YES answer is obtained (i.e., |X i −X i−1 |≧ΔX), the routine proceeds to step  104  wherein one of values X i−1 ±ΔX is determined as having being derived in this program cycle. Specifically, if X i ≧X i−1 , meaning that the value X i  in this program cycle has become greater than the value X i−1  in the previous program cycle over the upper change limit ΔX, the value X i−1 +ΔX is determined to be the value X i  as derived in this program cycle. Alternatively, if X i ≦X i−1 , meaning that the value X i  in this program cycle has become smaller than the value X i−1  in the previous program cycle over the upper change limit ΔX, the value X i−1 −ΔX is determined to be the value X i  as derived in this program cycle. Specifically, if a change in the pump cell current Ip is greater than the upper change limit ΔX, the value X is corrected to be within a range of ±ΔX.  
     [0076] Alternatively, if a NO answer is obtained (i.e., |X i −X i−1 |&lt;ΔX), then the routine proceeds to step  105  wherein the A/D-sampled value X i  as derived in this program cycle is used as it is.  
     [0077] After step  104  or  105 , the routine proceeds to step  106  wherein a blur operation is performed.  
     [0078] Specifically, the A/D-sampled value X i  is corrected by the following equation.  
       X   i   =X   i−1 +( X   i   −X   i−1 )/ k    
     [0079] where k indicates a preselected blurring coefficient.  
     [0080] After step  106 , the routine proceeds to step  107  wherein using the value X i  derived in step  106  (i.e., a blurred value of the pump cell current Ip), a target value of the pump cell-applying voltage Vp is determined by looking up a pump cell current-to-applied voltage map.  
     [0081] The routine proceeds to step  108  wherein a feed control operation is performed to change the voltage Vp now being applied to the pump cell  1   a  to the target one as determined in step  107 . Specifically, the output voltage Vp′ of the D/A converter  211  is changed to the target one.  
     [0082] In the CPU  20 , as illustrated in FIG. 1, the above operations are represented by blocks. Specifically, the change limiting circuit  201  performs steps  103  to  105 . The blurring circuit  202  performs step  106 . The pump cell-applying voltage controller  203  performs step  107 . The oxygen concentration signal output circuit  204  works to output the A/D-sampled value as blurred in step  106  as an A/F indicative of the concentration of oxygen (O 2 ) contained in the exhaust gasses.  
     [0083] With the above described sequential operations, the D/A converter  211  provides the output voltage Vp′ having one of discrete values, so that the pump cell-applying voltage Vp will have one of discrete values. Thus, if the pump cell current Ip, as sampled by the A/D converter  221 , contains peaks, as shown in FIG. 19, they are eliminated by the change limiting circuit  201  and the blurring circuit  202  to produce the noiseless pump cell current Ip, thereby enabling the pump cell-applying voltage Vp to be determined correctly. This results in improved accuracy of determining the concentration of NOx.  
     [0084] The elimination of noise in the pump cell current Ip serves to prevent the pump cell-applying voltage Vp from being changed undesirably, thus resulting in stability of oxygen (O 2 ) remaining in the first and second chambers  101  and  102  which improves the accuracy of determining the concentration of NOx using the monitor cell  1   b  and the sensor cell  1   c.    
     [0085] The beneficial effects offered by the first embodiment will also be described below with reference to FIGS. 6, 7, and  8 .  
     [0086]FIG. 6 shows a time-sequential change in concentration of O 2  contained in exhaust gasses of a diesel engine during a fuel cut. An upper line represents the value of the pump cell current Ip (i.e., the A/D-sampled value) sampled and outputted by the A/D converter  221 . A lower line represents the value of the pump cell current Ip after being blurred by the blurring circuit  202 . Either value increases up to a normal atmospheric concentration of O 2  due to the fuel cut, but however, the value of the pump cell current Ip after being blurred by the blurring circuit  202  increases smoothly without any spiky noises.  
     [0087]FIG. 7 shows a time-sequential change in pump cell-applying voltage Vp as determined as a function of the pump cell current Ip. FIG. 8 shows a time-sequential change in pump cell current Ip immediately after being sampled at an interval of 10 ms by the A/D converter  221  (i.e., the A/D-sampled value). In the illustrated case, the pump cell current Ip increases at a maximum rate of 0.05 mA/10 ms due to the fuel cut. The concentration of O 2  changes most greatly during the fuel cut, therefore, a maximum value of a response rate of the pump cell current Ip may be determined as 0.05 mA/10 ms. The peak of the rate of change in the pump cell current Ip reaches about 0.2 mA/10 ms. This is because spiky noises are added to the pump cell current Ip which arise from the susceptance made up of the parasitic capacitance between the electrodes  121  and  122  of the pump cell  1   a  and the capacitance of the solid electrolyte layer  112  due to stepwise changes in the pump cell-applying voltage Vp.  
     [0088] If a change in the pump cell-applying voltage Vp is defined as ΔV1, and the impedance of the pump cell  1   a  is defined as ZAC, then the a change ΔI1 in the pump cell current Ip may be expressed by a relation of ΔI1=ΔV1/ZAC. A maximum value of the pump cell-applying voltage change ΔV1 depends upon a resolution of the D/A converter  211 . When the D/A converter  211  is implemented by a 12-bit D/A converter that is now available, an LSB will be 1.22 mV. If the impedance ZAC of the pump cell  1   a  is 20 Ω, the pump cell-applying voltage change ΔV1 due to the fact that the output voltage of the D/A converter  211  has a discrete value is calculated approximately as 60 μA. This will result in a great error equivalent to an A/F (air/fuel) ratio of one (1) corresponding to 1% concentration of O 2 , for example, when the A/F ratio is twenty three (23) which is usually employed in lean burn or direct injection gasoline engines or great EGR control of diesel engines.  
     [0089] A reduction in such error without sacrificing the rate of response of the pump cell current Ip to a change in concentration of O 2  in the engine is, therefore, achieved by setting a limit of the pump cell current change ΔI1 to 60 μA. The lower line, as illustrated in FIG. 6, indicates the data when the upper change limit ΔX, as used in the change limiting operation in steps  103  to  105 , is 60 μA.  
     [0090] A further reduction in noise added to the pump cell current Ip is achieved by blurring the A/D-sampled value in steps  106 . The blurring coefficient k is preferably determined in terms of a required effect of removing the spiky noises from the pump cell current Ip and the response rate of the pump cell current Ip. The inventors of this application have found experimentally that ⅛ to {fraction (1/16)} are suitable for the blurring coefficient k. The lower line indicates the data when the blurring coefficient k is {fraction (1/16)}.  
     [0091] The change limiting operation in steps  103  to  105  and the blurring operation in steps  106  are not necessarily performed together, but a desired reduction in noise added to the pump cell current Ip may be achieved by at least one of them.  
     [0092]FIG. 9 shows a gas concentration measuring device according to the second embodiment of the invention. The same reference numbers as employed in the first embodiment will refer to the same parts, and explanation thereof in detail will be omitted here.  
     [0093] The gas concentration measuring device includes a pump cell circuit  3   a A and a CPU  20 A. The pump cell circuit  3   a A includes an operational amplifier  62  serving as a voltage follower and a low-pass filter  63 . The voltage appearing at a junction of the resistor  61  and the operational amplifier  52  is inputted to the operational amplifier  62 . An output of the operational amplifier  62  is inputted to the A/D converter  212  through the low-pass filter  63 . The low-pass filter  63  is implemented by an integrating circuit made up of a resistor  631  and a capacitor  632 . The pump cell current Ip sampled by the A/D converter  212  is inputted to the CPU  20 A.  
     [0094] The CPU  20 A includes the pump cell-applying voltage controller  203  and the oxygen concentration signal output circuit  204 . The pump cell-applying voltage controller  203  is responsive to input of the pump cell current Ip (i.e., the A/D-sampled value) to control the pump cell-applying voltage Vp.  
     [0095] The low-pass filter  63  works to smooth or blur the output of the operational amplifier  62  (i.e., the pump cell current Ip), thereby eliminating, like the first embodiment, spiky peaks appearing at the pump cell current Ip during a transition period in which the pump cell-applying voltage Vp is changed stepwise. The structure of this embodiment is lower in control load than that of the first embodiment.  
     [0096] The gas concentration measuring device of this embodiment, unlike the first embodiment, does not work to remove the spiky peaks from the pump cell current Ip before being subjected to the blurring operation. Sufficient removal of the spiky peaks from the pump cell current Ip, thus, requires decreasing the cut-off frequency of the low-pass filter  63  (e.g., to 0.5 Hz). The structure of this embodiment is useful for applications in which a certain degree of response delay is allowed.  
     [0097]FIG. 10 shows a gas concentration measuring device according to the third embodiment of the invention. The same reference numbers as employed-in the above embodiments will refer to the same parts, and explanation thereof in detail will be omitted here.  
     [0098] The adjustment of the pump cell-applying voltage Vp is achieved by changing an output of the D/A converter  211  in the first embodiment, but it is accomplished using another means in this embodiment.  
     [0099] The CPU  20 B includes a pump cell-applying voltage controller  203 B which works to determines a duty cycle of a PWM signal as a function of the pump cell-applying voltage Vp as derived using an applying voltage map and output it.  
     [0100] The PWM signal is inputted to a gate of an FET  433  of the pump cell circuit  3   a B. The FET  433  makes up a modulating circuit  43  together with resistors  431  and  432  which is designed to modulate an output of a voltage source  42  in response to the PWM signal. The voltage source  42  works to output a constant voltage. The modulating circuit  43  provides a power supply signal to the air-side pump electrode  122  through a low-pass filter  44 . The resistors  431  and  432  and the FET  433  are connected in series between the voltage source  42  and ground. The voltage source  42  supplies the voltage to the low-pass filter  44  through the resistor  431 . The low-pass filter  44  is implemented by an integrating circuit made up of resistors  441  and  442 , capacitors  443  and  444 , and an operational amplifier  445 .  
     [0101] In operation, when the PWM signal inputted to the gate of the FET  433  has a logical one (1) to turn on the FET  433 , it will cause the resistance at an input side of the low-pass filter  44  to be decreased by an amount equivalent to the resistance of the resistor  432  disposed electrically between the input of the low-pass filter  44  and ground. Specifically, the voltage inputted to the low-pass filter  44  has a binary discrete value which is either a logical one (1) or zero (0) depending upon if the PWM signal has the logical one (1) or zero (0). A ratio of a high-level time for which the discrete value has the logical one (1) to a low-level time for which the discrete value has the logical zero (0) is set by the duty cycle of the PWM signal. In this way, the output voltage of the voltage source  42  is modulated by the PWM signal outputted by the CPU  20 B.  
     [0102] The voltage output of the modulating circuit  43  is smoothed or blurred by the low-pass filter  44  and applied to the air-side pump electrode  122  of the pump cell  1   a.  The applied voltage, thus, has substantially a constant value in the form of a DC signal within a range between the logical one (1) and zero (0) which is determined by the duty cycle of the PWM signal. Specifically, the longer the on time of the duty cycle of the PWM signal, the lower the level of the voltage applied to the air-side pump electrode  122 .  
     [0103] The range of the level of the voltage inputted to the low-pass filter  44  is between a high and a lower level determined by the resistance values of the resistors  431  and  432 . Thus, increase in resolution of the pump cell-applying voltage Vp is achieved by selecting the resistance values of the resistors  431  and  432  appropriately. The inventors of this application have found experimentally that the cut-off frequency of the low-pass filter  44  is preferably 107 Hz.  
     [0104]FIG. 11 shows a gas concentration measuring device according to the fourth embodiment of the invention which is different from the first embodiment in control of the pump cell-applying voltage Vp. The same reference numbers as employed in the above embodiments will refer to the same parts, and explanation thereof in detail will be omitted here.  
     [0105] The CPU  20 C includes a pump cell-applying voltage controller  203 C. The monitor cell circuit  3   b C includes operational amplifiers  72  and  82 , and an A/D converter  222 . An output of a reference voltage supply  71  is inputted to the operational amplifier  72 . The operational amplifier  72  applies a reference voltage Vm′ to the air-side sensor/monitor electrode  125  of the monitor cell  1   b.  Similarly, an output of a reference voltage supply  81  is inputted to the operational amplifier  82 . The operational amplifier  82  applies a reference voltage Vm″ to the chamber-side monitor electrode  123  of the monitor cell  1   b.  Specifically, when a monitor cell-applying voltage Vm is inputted across the electrodes  123  and  125 , it will cause the monitor cell current Im to flow between the electrodes  123  and  125 , which is detected as a voltage drop of the resistor  83  by the A/D converter  222 .  
     [0106] The pump cell-applying voltage controller  203 C of the CPU  20 C works to determine the pump cell-applying voltage Vp so as to bring the monitor cell current Im into agreement with a preselected one under feedback control. For instance, a PID control using the proportional and the integral is performed to determine the pump cell-applying voltage Vp and control the output voltage Vp′ of the D/A converter  211 . The pump cell-applying voltage controller  203 C is implemented logically by the CPU  20 .  
     [0107] The output voltage Vp′ of the D/A converter  211  has, like the above embodiments, a discrete value, thus causing the pump cell current Ip to have spiky peaks, which results in a decrease in accuracy of determining the concentration of oxygen (O 2 ). The elimination of the spiky peaks of the pump cell current Ip is, like the first embodiment, achieved by subjecting samples of the pump cell current Ip collected by the A/D converter  221  to the change limiting operation and the blurring operation in the change limiting circuit  201  and the blurring circuit  202 .  
     [0108]FIG. 12 shows a gas concentration measuring device according to the fifth embodiment of the invention which is different from the fourth embodiment in control of the pump cell-applying voltage Vp. The same reference numbers as employed in the above embodiments will refer to the same parts, and explanation thereof in detail will be omitted here.  
     [0109] The CPU  20 D includes a pump cell-applying voltage controller  203 D. The monitor cell circuit  3   b D includes operational amplifiers  74  and  86 , the A/D converter  222 , a low-pass filer  85 , and a resistor  84 . An output of a reference voltage source  73  is inputted to the operational amplifier  74 . The operational amplifier  74  applies a reference voltage Vo to the air-side sensor/monitor electrode  125  of the monitor cell  1   b.  The resistor  84  having a greater resistance value is joined to the chamber-side monitor electrode  123  of the monitor cell  1   b.  The voltage developed across the resistor  84  is inputted to the low-pass filter  85 . The monitor cell  1   b  is designed to produce an electromotive force em between the electrodes  123  and  125  as a function of a ratio of a partial pressure of oxygen (O 2 ) within the chamber  102  to that within the air duct  105 . A change in concentration of oxygen within the chamber  102  will result in a change in voltage inputted to the low-pass filter  85 . The electromotive force em shows approximately 0.9V when the concentration of oxygen (O 2 ) within the chamber  102  is higher, drops greatly when it reaches a value corresponding to the stoichiometric amount of air, and has approximately 0.1V when it decreases to a rich-side.  
     [0110] The low-pass filter  85  consists of a resistor  851  and a capacitor  852 . An output voltage of the low-pass filter  85  is inputted to the A/D converter  222  through the operational amplifier  86 .  
     [0111] The pump cell-applying voltage controller  203 D of the CPU  20 D works to determine the pump cell-applying voltage Vp as a function of the electromotive force em. For instance, the electromotive force em produced by the monitor cell  1   b  changes, as described above, within a range between 0.9V and 0.1V across a middle voltage equivalent to the stoichiometric amount of air. The pump cell-applying voltage controller  203 D, thus, determines the pump cell-applying voltage Vp so that the electromotive force em may reach 0.45V and controls an output of the D/A converter  211 . The pump cell-applying voltage controller  203 D is implemented logically within the CPU  20 D.  
     [0112] The output voltage Vp′ of the D/A converter  211  has, like the above embodiments, a discrete value, thus causing the pump cell current Ip to have spiky peaks, which results in a decrease in accuracy of determining the concentration of oxygen (O 2 ). The elimination of the spiky peaks of the pump cell current Ip is, like the first embodiment, achieved by subjecting samples of the pump cell current Ip collected by the A/D converter  221  to the change limiting operation and the blurring operation in the change limiting circuit  201  and the blurring circuit  202 .  
     [0113] The low-pass filter  85  serves to smooth a sudden change in electromotive force em to avoid an undesirable change in the pump cell-applying voltage Vp, thus resulting in improved convergence of the concentration of oxygen within the chamber  102 .  
     [0114]FIG. 13 shows a gas concentration measuring device according to the sixth embodiment of the invention which is different from the fifth embodiment in structure of the gas sensor. The control of the pump cell-applying voltage Vp is identical with that in the fifth embodiment. The same reference numbers as employed in the fifth embodiment will refer to the same parts, and explanation thereof in detail will be omitted here.  
     [0115] The gas sensor  1 E, as clearly shown in FIG. 14, is formed by a strip-like lamination of solid electrolyte layers  151 ,  152 , and  153  made of zirconia, a gas-diffusion-rate limiting layer  154  made of insulating material such as porous alumina, and a solid electrolyte layer  155  made of zirconia having a heater  17  embedded therein.  
     [0116] The solid electrolyte layer  152  and the gas-diffusion-rate limiting layer  154  form a common layer interposed between the solid electrolyte layers  151  and  153 . The gas-diffusion-rate limiting layer  154  is located closer to the head portion of the gas sensor, while the solid electrolyte layer  152  is located closer to the base portion of the gas sensor. The solid electrolyte layer  152  and the gas-diffusion-rate limiting layer  154  have formed therein openings to define first and second chambers  141  and  142  arrayed in a lengthwise direction of the gas sensor. The gas-diffusion-rate limiting layer  154  works to admit gasses to be measured into the first chamber  141  and establish gas communication between the first and second chambers  141  and  142 .  
     [0117] The layer  155  defines an air duct  143  between itself and the solid electrolyte layer  153 . The air duct  143  extends over the first and second chambers  141  and  142  and communicates with the atmosphere. In a case where the gas sensor  1 E is installed in an exhaust pipe of an automotive internal combustion engine, the air duct  143  is exposed outside the exhaust pipe.  
     [0118] Electrodes  161  and  162  are affixed to opposed surfaces of the solid electrolyte layer  151  to, form a pump cell  1   d.  The electrode  161  exposed to the chamber  141  is made of a noble metal such as Au—Pt that is inactive with NOx, that is, hardly decomposes NOx.  
     [0119] Electrode  163  and  165  are affixed to opposed surfaces of the solid electrolyte layer  153  to form a monitor cell  1   e.  The electrode  163  is exposed to the first chamber  141 . The electrode  165  is exposed to the air duct  143 . The electrode  163  exposed to the first chamber  141  is made of a noble metal such as Au—Pt that is inactive with NOx. The electrode  165  extends up to the second chamber  142  and works as a common electrode shared with a sensor cell if and a second pump cell  1   g,  as will be described below.  
     [0120] An electrode  164  is affixed to a surface of the solid electrolyte layer  153  exposed to the second chamber  142 . The electrode  164  forms the sensor cell  1   f  together with the common electrode  165 .  
     [0121] An electrode  166  is affixed to a surface of the solid electrolyte layer  151  exposed to the second chamber  142  to form the second pump cell  1   g  together with the solid electrolyte layers  151  to  153  and the electrode  165 .  
     [0122] The electrode  164  of the sensor cell if exposed to the second chamber  142  is made of a noble metal such as Pt that is active with NOx, that is, works to decompose or ionize NOx. The electrode  166  of the second pump cell  1   g  is made of a noble metal such as Au—Pt that is inactive with NOx.  
     [0123] A patterned conductor is embedded in the layer  155  which makes up the heater  17  to heat the whole of the gas sensor  1 E up to a required activation temperature. The heater  17  is of an electrical type generating Joule heat.  
     [0124] The monitor cell  1   e  produces an electromotive force em as a function of the concentration of O 2  within the first chamber  141 . The monitor cell circuit  3   e,  like the fifth embodiment, consists of the reference voltage source  73 , the operational amplifier  74 , the resistor  84 , the low-pass filter  85 , and the operational amplifier  86  and works to measure the concentration of oxygen (O 2 ) remaining within the first chamber  141 .  
     [0125] The pump cell-applying voltage controller  203 E of the CPU  20 E works to determine the pump cell-applying voltage Vp so that the electromotive force em produced by the monitor cell  1   b  may reach a given voltage (e.g., 0.45V) and controls an output of the D/A converter  211 , thereby discharging the oxygen (O 2 ) from the first chamber  141  so that the concentration of O 2  is kept at a constant lower level. This also discharges O 2  from the second chamber  142  to keep the concentration of O 2  within the second chamber  142  at substantially the same lower level as in the first chamber  141 .  
     [0126] The second pump cell circuit  3   g  works to apply the voltage Vp2 across the electrodes  165  and  166  with the electrode  165  connected to a positive terminal of a power supply to discharge O 2  from the second chamber  142 . Upon application of the voltage Vp2, the electrodes  165  and  166  produce the pump cell current Ip2.  
     [0127] The sensor cell circuit  3   c  works to apply the voltage Vs across the electrodes  165  and  164  with the electrode  165  connected to a positive terminal of a power supply to discharge O 2  from the second chamber  142 . Upon application of the voltage Vs, the electrodes  165  and  164  produce the sensor cell current Is as a function of concentration of NOx within the second chamber  142 .  
     [0128] The above operations are known in the art, and explanation thereof in more detail will be omitted here.  
     [0129] The output voltage Vp′ of the D/A converter  211  has, like the above embodiments, a discrete value, thus causing the pump cell current Ip to have spiky peaks, which results in a decrease in accuracy of determining the concentration of oxygen (O 2 ). The elimination of the spiky peaks of the pump cell current Ip is, like the first embodiment, achieved by subjecting samples of the pump cell current Ip collected by the A/D converter  221  to the change limiting operation and the blurring operation in the change limiting circuit  201  and the blurring circuit  202 .  
     [0130] The low-pass filter  85  serves to smooth a sudden change in electromotive force em to avoid an undesirable change in the pump cell-applying voltage Vp, thus resulting in improved convergence of the concentration of oxygen within the chamber  102 .  
     [0131]FIG. 15 shows a gas concentration measuring device according to the seventh embodiment of the invention which is different from the second embodiment of FIG. 9 in that a low-pass filter  45  is used instead of the low-pass filter  63  in FIG. 9. The same reference numbers as employed in the second embodiment will refer to the same parts, and explanation thereof in detail will be omitted here.  
     [0132] The gas concentration measuring device includes a pump cell circuit  3   a F and a CPU  20 F. The pump cell circuit  3   a F has the low-pass filter  45  to which an output voltage of the D/A converter  211  is inputted. The low-pass filter  45  is implemented by an integrating circuit made up of a resistor  451  and a capacitor  452  and works to output the pump cell-applying voltage Vp′ to the air-side pump electrode  122  of the pump cell  1   a.    
     [0133] The CPU  20 F includes the pump cell-applying voltage controller  203  and the oxygen concentration signal output circuit  204 . The pump cell-applying voltage controller  203  is responsive to input of the pump cell current Ip (i.e., the A/D-sampled value) to control the pump cell-applying voltage Vp.  
     [0134] The low-pass filter  45  works to smooth or blur the output of the operational amplifier  41  (i.e., the pump cell-applying voltage Vp′), thereby eliminating, like the first embodiment, spiky peaks appearing at the pump cell current Ip during a transition period in which the pump cell-applying voltage Vp′ is changed stepwise. The structure of this embodiment is lower in control load than that of the first embodiment.  
     [0135] Removal of as much of the spiky peaks of the pump cell current Ip as possible requires decreasing the cut-off frequency of the low-pass filter  45 . The inventors of this application have found experimentally that when the cut-off frequency is 0.5 Hz, and a minimum resolution of the D/A converter  211  is 2 mV, a change in pump cell current Ip arising from a 2 mV change in pump cell-applying voltage Vp′ is reduced from 0.1 mA (corresponding to an A/F ratio of 1.2) to 0.005 mA (corresponding to an A/F ratio of 0,06).  
     [0136]FIG. 16 shows a gas concentration measuring device according to the eighth embodiment of the invention which is a modification of the third embodiment of FIG. 10. The same reference numbers as employed in the third embodiment will refer to the same parts, and explanation thereof in detail will be omitted here.  
     [0137] The gas concentration measuring device includes a low-pass filter  46  working to smooth or blur an output of the modulating circuit  43 . The low-pass filter  46 , like the low-pass filter  44  of FIG. 10, consists of resistors  461  and  462 , capacitors  463  and  464 , and an operational amplifier  465 . The low-pass filter  46  has a cut-off frequency lower than that of the low-pass filter  44 , thereby providing additional effects of limiting a change in the pump cell-applying voltage Vp to within a desired range and smoothing the pump cell-applying voltage Vp in addition to smoothing an output of the voltage source  42  that is modulated by the PWM signal outputted by the CPU  20 G. This eliminates the need for the blurring circuit  202  and the change limiting circuit  201  as employed in the structure of FIG. 10, thus resulting in a decrease in operation load of the CPU  20 G.  
     [0138] The pump cell-applying voltage controller  203 B works to determine the pump cell-applying voltage Vp as a function of the pump cell current Ip sampled by the A/D converter  221 .  
     [0139] Smoothing the pump cell-applying voltage Vp to a degree which eliminates the spiky peaks of the pump cell current Ip requires decreasing the cut-off frequency of the low-pass filter  46  greatly. The inventors of this application have found experimentally that the cut-off frequency of the low-pass filter  46  is preferably 0.5 Hz.  
     [0140] While the present invention has been disclosed in terms of the preferred embodiments in order to facilitate better understanding thereof, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modifications to the shown embodiments witch can be embodied without departing from the principle of the invention as set forth in the appended claims.