JP4821928B2 - Gas concentration detector - Google Patents

Description

  The present invention relates to a gas concentration detection device that uses a limiting current type gas concentration sensor to detect the concentration of a characteristic gas component in a gas to be detected.

  As this type of gas concentration detection device, there is one that uses a limiting current type gas concentration sensor to detect, for example, NOx in the exhaust gas of an automobile. As a basic structure, the gas concentration sensor includes a first cell (pump cell) and a second cell (sensor cell). In the first cell, oxygen in the exhaust gas introduced into the chamber is discharged or pumped, and the second cell Then, the NOx concentration (concentration of the specific gas component) is detected from the gas after passing through the first cell. The first cell and the second cell output current signals in accordance with the oxygen concentration and the NOx concentration with the application of voltage.

  Some gas concentration sensors include a monitor cell that generates an electromotive force according to the oxygen concentration in the chamber in addition to the first cell and the second cell, and controls the voltage applied to the first cell in the sensor. As a conventional method, a method of PID feedback of the applied voltage by an electromotive force of a monitor cell has been proposed (for example, refer to the Japan Society for Automotive Engineers Academic Lecture Preprints 971 1997-5 9731958, SAE970858). That is, in this method, the applied voltage of the first cell is feedback-controlled each time based on the deviation between the target value of the monitor cell electromotive force set to keep the inside of the chamber at a predetermined low oxygen concentration and the actual monitor cell electromotive force. It was like that.

  However, when the gas concentration sensor detects the exhaust gas concentration, the exhaust gas is first introduced in the vicinity of the first cell, and then flows downstream in the chamber and reaches the monitor cell. Therefore, when the exhaust gas concentration changes, the oxygen concentration in the vicinity of the monitor cell differs from the oxygen concentration in the vicinity of the first cell due to the delay until the exhaust gas reaches the monitor cell, and it takes time for the oxygen concentration in the vicinity of the monitor cell to stabilize. . In particular, in the gas concentration sensor in which the chamber in which the first cell is provided and the chamber in which the monitor cell is provided are communicated via a throttle passage (diffusion passage), the above problem becomes significant. As a result, when feedback control is performed on the voltage applied to the first cell by the electromotive force of the monitor cell, an oscillation phenomenon occurs due to a shift in the feedback control phase.

  For example, if the exhaust gas is switched to lean and a large amount of oxygen is introduced into the chamber, the monitor cell cannot be immediately determined to be lean. Therefore, in the feedback by the electromotive force of the monitor cell, oxygen discharge in the first cell becomes insufficient, and residual oxygen remains more than necessary. Thereafter, when the state of the exhaust gas lean is determined with a delay in the monitor cell, an excessive voltage is applied to the first cell so as to rapidly discharge residual oxygen. Thereafter, even if the residual oxygen in the chamber is completely exhausted, it is determined that the monitor cell is lean for a while. Then, after knowing that oxygen in the chamber has been exhausted too much, the applied voltage of the first cell is suddenly changed to the lower side.

  By repeating the above phenomenon, the applied voltage control system of the first cell oscillates, and the amount of residual oxygen in the chamber fluctuates greatly periodically. As a result, there is a problem that the residual oxygen in the second cell increases and the offset current accompanying the decomposition of the residual oxygen increases, or NOx decomposition occurs in the first cell and the NOx decomposition amount in the second cell decreases. As a result, the detection accuracy of the NOx concentration (concentration of the specific gas component) decreases.

Similarly, when the reaction rate of each cell decreases due to changes over time (element degradation), exhaust gas temperature, etc., the residual oxygen amount in the chamber fluctuates, and the detection accuracy of NOx concentration decreases due to the fluctuation. . For example, if a rich component (HC, etc.) adheres to the electrode of the monitor cell during rich gas due to electrode deterioration, the monitor cell misrecognizes that the exhaust gas is switched to lean even if the exhaust gas is switched to lean. The voltage applied to the first cell is controlled so as to further pump oxygen. Thereafter, when the HC adhering to the electrode of the monitor cell reacts with oxygen and leaves, the applied voltage of the first cell is controlled so that the monitor cell recognizes the lean state and discharges the oxygen in the chamber. Since the oxygen concentration is excessively increased, an excessive applied voltage is applied to the first cell by the increased amount.

  Similarly, when the exhaust gas is switched from lean to rich, if the reaction rate of the monitor cell decreases due to changes over time and a lean component (oxygen, etc.) adheres to the electrode of the monitor cell, the monitor cell misrecognizes that it is in a lean state, and the chamber The applied voltage of the first cell is controlled so as to discharge oxygen inside. That is, an excessive applied voltage that decomposes to NOx in addition to oxygen is applied to the first cell.

  By repeating the above phenomenon, the applied voltage control system of the first cell also oscillates, and the amount of residual oxygen in the chamber fluctuates greatly periodically. As a result, the NOx concentration detection accuracy decreases.

  The present invention has been made paying attention to the above problems, and an object of the present invention is to provide a gas concentration detection device capable of accurately detecting the concentration of a specific gas component in a detection gas. It is.

  The gas concentration detection apparatus of the present invention is based on the premise that a gas concentration sensor including at least a first cell, a second cell, and a monitor cell is used, and gas concentration detection is performed based on a detection result by the gas concentration sensor.

In the gas concentration detection device according to claim 1, the resistance value of the solid electrolyte element provided with the first cell is detected (element resistance detection means), and the first resistance is detected based on the detected element resistance of the first cell. An applied voltage characteristic that predetermines the relationship between the current value of the cell and the applied voltage, or an applied voltage that is set as needed by the applied voltage characteristic is corrected (applied voltage correcting means). Further, the applied voltage to the first cell is controlled based on the corrected applied voltage characteristic or applied voltage (applied voltage control means). More specifically, the applied voltage correction means corrects the offset of the applied voltage straight line, which is the applied voltage characteristic, and changes the offset according to the element resistance of the first cell. In this case, the offset of the applied voltage straight line is preferably increased as the element resistance of the first cell is increased.

  That is, when the element resistance of the first cell changes, the oxygen discharge amount in the first cell varies even with the same applied voltage. Therefore, the amount of residual oxygen in the chamber fluctuates, but it is possible to control the applied voltage so as to optimize the amount of residual oxygen in the chamber by correcting the applied voltage based on the element resistance of the first cell. Become. As a result, the concentration of the characteristic gas component in the gas to be detected can be accurately detected.

  Further, in the present invention, unlike the conventional technique in which the monitor cell electromotive force is fed back to the target value by a PID method or the like, the applied voltage of the first cell is controlled using a predetermined applied voltage characteristic. Therefore, there is no problem that the applied voltage control system of the first cell oscillates due to a response delay in the monitor cell, and the residual oxygen amount in the chamber greatly fluctuates periodically. Therefore, a factor that causes a decrease in gas concentration detection accuracy is eliminated.

Further, in the case of the conventional technique in which the applied voltage control is performed by feeding back the monitor cell signal, the transition of various signals is as shown in FIG. 22B when the gas concentration changes to the lean side. That is, when the gas concentration changes, a predetermined dead time is required until the change is detected by the monitor cell, and the monitor cell signal fluctuates immediately after the gas concentration change is detected by the monitor cell. Further, in order to prevent oscillation due to the difference in reaction speed between the first cell and the monitor cell, the feedback gain must be reduced, and therefore, the applied voltage (first cell voltage) of the first cell changes slowly. Therefore, since the residual oxygen is insufficiently discharged, that is, the residual oxygen is excessive, an excessively large current signal is output as the second cell current. Actually, accurate gas concentration detection cannot be performed until the response time until the second cell current or the monitor cell signal becomes stable.

  On the other hand, according to the present invention, as shown in FIG. 22 (a), when the first cell current changes when the gas concentration changes, the first cell voltage is immediately controlled to a desired target value while following the change. The As a result, residual oxygen does not become excessive in the chamber, and the monitor cell signal does not fluctuate. Therefore, in the second cell, a current corresponding to the concentration of the specific gas component can be detected with high response.

  The “monitor cell output” referred to in the present specification includes not only the electromotive force itself of the monitor cell that functions as an oxygen concentration battery but also a current output obtained by applying a predetermined voltage to the monitor cell electrode.

The block diagram which shows the outline | summary of a gas concentration detection apparatus. Sectional drawing which shows the structure of a gas concentration sensor. The VI characteristic figure which shows the characteristic of each cell of a gas concentration sensor. The VI characteristic figure which shows the applied voltage characteristic of a pump cell. The VI characteristic figure which shows the characteristic of each cell of a gas concentration sensor. The flowchart which shows the main routine by a microcomputer. The flowchart which shows the correction | amendment procedure of a pump cell applied voltage characteristic. The flowchart which shows the change speed setting procedure of a pump cell applied voltage. The figure which shows the outline | summary by which the change speed of a pump cell applied voltage is variably set. The VI characteristic figure which shows the applied voltage characteristic of a pump cell. The flowchart which shows the main routine by a microcomputer. The flowchart which shows the correction | amendment procedure of a pump cell applied voltage characteristic. The VI characteristic figure which shows the pump cell characteristic of a gas concentration sensor. The VI characteristic figure which shows the characteristic of each cell of a gas concentration sensor. The flowchart which shows the correction | amendment procedure of a sensor cell electric current. The VI characteristic figure which shows the characteristic of each cell of a gas concentration sensor. The flowchart which shows the correction | amendment procedure of a sensor cell electric current. The VI characteristic figure which shows a pump cell characteristic. The flowchart which shows the correction | amendment procedure of a pump cell applied voltage characteristic. The circuit diagram which shows the structure of LPF in another form. Sectional drawing which shows the structure of the gas concentration sensor in another form. The figure which shows the mode of response, such as various signals at the time of gas concentration change. The equivalent circuit diagram of a gas concentration sensor. The figure which shows the mode of the peak electric current generation | occurrence | production at the time of a voltage change.

(First reference example)
Prior to the description of an embodiment of the present invention, a first reference example will be described with reference to the drawings. The gas concentration detection apparatus in the present reference example is applied to, for example, a gasoline engine for automobiles, and uses a limiting current type gas concentration sensor to detect oxygen concentration from exhaust gas as a detection target gas and to detect a specific gas. The NOx concentration as the component concentration is detected.

  First, the configuration of the gas concentration sensor will be described. The gas concentration sensor has a three-cell structure consisting of a pump cell (first cell), a monitor cell, and a sensor cell (second cell), and is a specific example of a so-called composite gas sensor that can simultaneously detect oxygen concentration and NOx concentration in exhaust gas. The sectional structure will be described in detail with reference to FIG.

  In the gas concentration sensor 100, solid electrolytes (solid electrolyte elements) 141, 142 made of an oxygen ion conductive material have a plate shape, and are spaced apart from each other by a predetermined interval via a spacer 143 made of an insulating material such as alumina. Are stacked. Among these, a pinhole 141a is formed in the solid electrolyte 141 on the upper side of the figure, and exhaust gas around the sensor is introduced into the first chamber 144 through the pinhole 141a. The first chamber 144 communicates with the second chamber 146 via a constriction 145 as a diffusion passage. Reference numeral 147 denotes a porous diffusion layer.

  Further, the solid electrolyte 142 on the lower side of the figure is provided with a pump cell 110 and a monitor cell 120. The pump cell 110 functions to exhaust or pump oxygen in the exhaust gas introduced into the first chamber 144, and At that time, the oxygen concentration in the exhaust gas is detected. The monitor cell 120 generates a current output when an electromotive force or a voltage is applied depending on the oxygen concentration in the second chamber 146. Here, the pump cell 110 has a pair of upper and lower electrodes 111 and 112 with a solid electrolyte 142 interposed therebetween, and in particular, the electrode 111 on the first chamber 144 side is a NOx inert electrode (an electrode that is difficult to decompose NOx gas). . Similarly, the monitor cell 120 has a pair of upper and lower electrodes 121 and 122 with a solid electrolyte 142 interposed therebetween, and in particular, the electrode 121 on the second chamber 146 side is a NOx inert electrode (an electrode that hardly decomposes NOx gas). is there. The pump cell 110 and the monitor cell 120 decompose oxygen present in the chambers 144 and 146 and discharge it from the electrodes 112 and 122 to the atmosphere passage 150 side.

  The sensor cell 130 is provided facing the monitor cell 120 and has a pair of upper and lower electrodes 131 and 132 with a solid electrolyte 141 interposed therebetween. The sensor cell 130 detects the NOx concentration from the gas after passing through the pump cell 110, and discharges oxygen generated when NOx is decomposed in the second chamber 146 from the electrode 132 to the atmosphere passage 148 side.

  An insulating layer 149 is provided on the lower surface of the solid electrolyte 142 in the figure, and an atmospheric passage 150 is formed by the insulating layer 149. In addition, a heater 151 for heating the entire sensor is embedded in the insulating layer 149.

  In the gas concentration sensor 100 having the above configuration, the exhaust gas is introduced into the first chamber 144 through the porous diffusion layer 147 and the pinhole 141a. When the exhaust gas passes through the vicinity of the pump cell 110, a decomposition reaction occurs by applying a voltage between the electrodes 111 and 112 of the pump cell 110, and oxygen flows through the pump cell 110 according to the oxygen concentration in the first chamber 144. Is put in and out. At this time, since the electrode 111 on the first chamber 144 side is a NOx inactive electrode, NOx in the exhaust gas is not decomposed in the pump cell 110, but only oxygen is decomposed and discharged to the atmospheric passage 150.

  Thereafter, the exhaust gas that has passed in the vicinity of the pump cell 110 flows into the second chamber 146, and the monitor cell 120 generates an output corresponding to the residual oxygen concentration in the gas. The output of the monitor cell 120 is detected as a monitor cell current by applying a predetermined voltage between the electrodes 121 and 122 of the monitor cell 120. Further, by applying a predetermined voltage between the electrodes 131 and 132 of the sensor cell 130, NOx in the gas is reduced and decomposed, and oxygen generated at that time is discharged to the atmospheric passage 148. At that time, the current flowing through the sensor cell 130 is detected as the NOx concentration contained in the exhaust gas.

  Next, the characteristics of the pump cell 110, the monitor cell 120, and the sensor cell 130 will be described with reference to FIGS. 3A to 3C show basic characteristics under constant oxygen concentration and NOx concentration. First, the pump cell characteristics will be described with reference to FIG.

  FIG. 3A is a VI characteristic diagram showing the relationship between the pump cell applied voltage Vp and the pump cell current Ip as the pump cell characteristic, and the pump cell 110 has a limit current characteristic with respect to the oxygen concentration. The limit current region is composed of a straight line portion slightly rising to the right with respect to the V axis, and the region shifts in a direction in which the voltage value increases as the oxygen concentration increases (lean). The slope of the resistance dominant region having the same characteristics generally matches the element impedance Rip of the pump cell 110.

  Here, the relationship between the pump cell current Ip and the applied voltage Vp is defined in advance as an applied voltage characteristic, and the pump cell applied voltage Vp is variably controlled according to the pump cell current Ip at that time according to the applied voltage characteristic. It has become. That is, an applied voltage straight line LX1 having a slope = a and an offset = b is defined as an applied voltage characteristic of the pump cell 110, and the pump cell applied voltage Vp is set on the applied voltage straight line LX1. Incidentally, the electrode 111 (electrode on the first chamber 144 side) of the pump cell 110 is a NOx inactive electrode. However, if the pump cell applied voltage Vp is too large, the NOx is decomposed, so the applied voltage straight line LX1 is the pump cell 110. It is set to such an extent that NOx gas is not decomposed. Actually, the applied voltage straight line LX1 is set so as to maintain the inside of the first chamber 144 at a predetermined low oxygen concentration (near the stoichiometry). For example, a slight residual oxygen (surplus oxygen) of about several ppm to several tens ppm is generated. The pump cell applied voltage Vp is controlled so as to remain in the first chamber 144.

  FIG. 3B is a VI characteristic diagram showing the relationship between the monitor cell applied voltage Vm and the monitor cell current Im as the monitor cell characteristic. The monitor cell 120 has a limit current characteristic with respect to the oxygen concentration. That is, the monitor cell 120 generates an output corresponding to the oxygen concentration in the second chamber 146, and outputs a current value Im1 by applying a predetermined voltage Vm1 to the monitor cell 120 at that time. In this case, if the inside of the second chamber 146 together with the first chamber 144 is maintained at a predetermined low oxygen concentration (for example, about several ppm to several tens of ppm) by oxygen in and out of the pump cell 110, for example, about 0.5 to 2 μA. Monitor cell current Im flows.

  FIG. 3C is a VI characteristic diagram showing the relationship between the sensor cell applied voltage Vs and the sensor cell current Is as the sensor cell characteristic. The sensor cell 130 has a limit current characteristic with respect to the NOx concentration in the gas. . In this case, by applying a predetermined voltage Vs1 to the sensor cell 130, a current value Is1 corresponding to the NOx concentration in the second chamber 146 is output.

FIG. 1 shows a schematic configuration of a gas concentration detection apparatus using the gas concentration sensor 100 having the above configuration. The microcomputer 170 is configured by a well-known logical operation circuit including a CPU, a memory, an A / D, a D / A converter, and the like. The pump cell 110, the monitor cell 120, and the sensor cell 130 each have a power supply circuit. In each power supply circuit, the pump cell current Ip is measured by the current detector 171 and the monitor cell current Im is measured by the current detector 172. Is is measured by the current detector 173. The current values measured by these current detectors 171 to 173 are taken into the microcomputer 170, respectively.

  The microcomputer 170 detects the oxygen concentration in the exhaust gas based on the pump cell current Ip measured by the current detector 171, and sets the pump cell applied voltage Vp as needed according to the pump cell current Ip using the applied voltage straight line LX1 shown in FIG. . At this time, the pump cell applied voltage Vp is set to such an extent that NOx is not decomposed by the pump cell 110. Further, the microcomputer 170 corrects the applied voltage characteristic of the pump cell 110 according to the monitor cell current Im measured by the current detector 172. Further, the microcomputer 170 detects the NOx concentration in the exhaust gas based on the sensor cell current Is measured by the current detector 173.

  In this reference example, the element impedance (element resistance) is detected for the pump cell 110 using the sweep method. That is, when the impedance of the pump cell 110 is detected, the microcomputer 170 instantaneously switches the pump cell applied voltage Vp to at least one of the positive side and the negative side, and this applied voltage becomes sinusoidal by the LPF (low pass filter) 180. As it is, it is applied to the pump cell 110. The frequency of the AC voltage is desirably 10 kHz or more, and the time constant of the LPF 180 is set at about 5 μsec. Then, the element impedance of the pump cell 110 is calculated from the voltage change amount and current change amount at that time. Note that the LPF 180 may be realized by a primary filter including a resistor and a capacitor, for example.

  On the other hand, in the microcomputer 170, a detection range (current detection range) of the pump cell current Ip is set in advance, which is defined as an oxygen concentration detection range in terms of the VI characteristic of the pump cell 110 shown in FIG. That is, in the present reference example, as an example, the range of Ip = −1.8 mA to 2 mA is used as the oxygen concentration detection range, and the applied voltage straight line LX1 is set so that the pump cell applied voltage Vp is variable at least within the oxygen concentration detection range. Is set. The applied voltage straight line LX1 is prepared in advance in the microcomputer 170 as map data. Note that, in the detection unnecessary region outside the oxygen concentration detection range, it is not necessary to change the pump cell applied voltage Vp any more, so the applied voltage straight line LX1 is made upright.

  Next, an outline of correction of the pump cell applied voltage characteristic will be specifically described with reference to FIG. 5A to 5C show the characteristics of the pump cell 110, the monitor cell 120, and the sensor cell 130 when the oxygen concentration and the NOx concentration are constant, as in FIGS. 3A to 3C. In the figure, the characteristic (circle numeral 1 in the figure) indicated by a solid line is the basic characteristic explained in FIG.

  When the element impedance Rip of the pump cell 110 increases due to a decrease in exhaust gas temperature, individual differences, etc., the pump cell characteristic changes from the basic characteristic (circle numeral 1 in the figure) shown by the solid line in FIG. It changes to the circled number 2), and the pump cell applied voltage Vp set on the applied voltage straight line LX1 cannot sufficiently decompose oxygen. Incidentally, this phenomenon is caused by the fact that the limiting current region is slightly inclined to the right in the figure, and the amount of oxygen that becomes insufficiently decomposed increases as the pump cell characteristic falls as shown by the dotted line. Therefore, the residual oxygen in the first and second chambers 144 and 146 increases, and the monitor cell characteristic changes from a solid line to a dotted line as shown in FIG. 5B (the current value of the monitor cell changes from Im1 to Im2). . Further, in the sensor cell 130, the amount of oxygen decomposed simultaneously with NOx increases. As shown in FIG. 5C, the sensor cell characteristic similarly changes from a solid line to a dotted line, which becomes a NOx concentration detection error. (Detection error = Is2−Is1).

On the other hand, when the element impedance Rip of the pump cell 110 decreases due to an increase in exhaust gas temperature or individual differences, the pump cell characteristic changes from a basic characteristic (circled numeral 1 in the figure) indicated by a solid line in FIG. In the pump cell applied voltage Vp set on the applied voltage straight line LX1, the residual oxygen is completely decomposed and the pump cell 110 decomposes to NOx. Therefore, as shown in FIG. 5B, the monitor cell characteristic changes from a solid line to a two-dot chain line (the current value of the monitor cell changes from Im1 to Im3). Further, since a part of NOx is decomposed by the pump cell 110, the sensor cell characteristic similarly changes from a solid line to a two-dot chain line as shown in FIG. 5C, and this becomes a detection error of the NOx concentration. Appears (detection error = Is3-Is1).

  As described above, when the element impedance Rip of the pump cell 110 changes carelessly, the amount of residual oxygen in the first and second chambers 144 and 146 changes, which causes a detection error of NOx concentration. Therefore, in the apparatus of this reference example, it is considered that the change in the residual oxygen amount is grasped from the monitor cell characteristic (monitor cell current Im), and thereby the applied voltage characteristic of the pump cell 110 is corrected.

  That is, when the element impedance Rip of the pump cell 110 rises and the residual oxygen amount increases, it is necessary to increase the oxygen decomposition amount in the pump cell 110. Therefore, based on the monitor cell current value Im2 at that time, FIG. The applied voltage straight line shown in a) is changed from LX1 (solid line) to LX2 (dotted line). In this case, the slope of the applied voltage straight line LX1 (the slope a in FIG. 3) is reduced, and the pump cell applied voltage Vp is corrected in the increasing direction. Accordingly, the residual oxygen in the first and second chambers 144 and 146 is reduced to return to the normal amount, and both the monitor cell characteristics in FIG. 5B and the sensor cell characteristics in FIG. 5C become characteristics indicated by solid lines. Therefore, the NOx concentration detection error is eliminated.

  Further, when the element impedance Rip of the pump cell 110 decreases and NOx is decomposed in the pump cell 110, it is necessary to reduce the amount of oxygen decomposition in the pump cell 110. Therefore, based on the monitor cell current value Im3 at that time, The applied voltage straight line shown in FIG. 5A is changed from LX1 (solid line) to LX3 (two-dot chain line). In this case, the slope of the applied voltage straight line LX1 (the slope a in FIG. 3) is increased, and the pump cell applied voltage Vp is corrected in the decreasing direction. Accordingly, the residual oxygen in the first and second chambers 144 and 146 is increased to return to a normal amount, and both the monitor cell characteristics in FIG. 5B and the sensor cell characteristics in FIG. 5C become characteristics indicated by solid lines. Therefore, the NOx concentration detection error is eliminated.

  Next, the control procedure of the pump cell applied voltage realized by the microcomputer 170 will be described in detail. FIG. 6 is a flowchart showing an outline of a main routine executed by the microcomputer 170. The routine is started when the microcomputer 170 is turned on.

  In FIG. 6, first, in step 100, it is determined whether or not a predetermined time Ta has elapsed since the previous detection of the oxygen concentration and the NOx concentration. The predetermined time Ta is a time corresponding to the detection cycle of the oxygen concentration and the NOx concentration, and is set to about Ta = 4 msec, for example. Then, on the condition that step 100 is YES, the process proceeds to step 110, and the subsequent oxygen concentration and NOx concentration detection processing is performed.

  That is, in step 110, the applied voltage characteristic of the pump cell 110 is corrected according to the monitor cell current Im at that time. The correction process of the pump cell applied voltage characteristic is shown in the flowchart of FIG. 7, and details thereof will be described later. At this time, the aforementioned applied voltage straight line LX1 given as map data is corrected as necessary. Thereafter, in step 120, the pump cell current Ip measured by the current detector 171 is read. In the following step 130, the pump cell application is applied in accordance with the read pump cell current Ip using the corrected applied voltage straight line LX1 in step 110. The voltage Vp is set.

Thereafter, in step 140, the change rate of the pump cell applied voltage Vp is set. The change rate setting process of the pump cell applied voltage is shown in the flowchart of FIG. 8, and details thereof will be described later.

  Thereafter, in step 150, the pump cell application voltage Vp set in step 130 is output in accordance with the changing speed set in step 140. In subsequent step 160, the oxygen concentration (A / F) in the exhaust gas is detected based on the pump cell current Ip in a state where the pump cell current Ip is stable after the applied voltage is output. At this time, the NOx concentration in the exhaust gas is detected based on the sensor cell current Is. Incidentally, regarding the sensor cell 130, a predetermined sensor cell applied voltage Vs (fixed value) is always output from the microcomputer 170.

  After the detection of the oxygen concentration and the NOx concentration, in step 170, it is determined whether or not a predetermined time Tb has elapsed since the previous element impedance detection. The predetermined time Tb is a time corresponding to the detection cycle of the element impedance Rip, and for example, a time such as 128 msec or 2 sec is selectively set according to the engine operating state. Then, on the condition that step 170 is YES, the element impedance Rip is detected in step 180, and heater energization control is performed in the subsequent step 190.

  Here, at the time of detecting the element impedance Rip in the step 180, the pump cell applied voltage Vp is operated, and the voltage is about several tens to 100 μsec on the positive side or the negative side with respect to the applied voltage for detecting the residual oxygen concentration so far. It can be switched once in a time. Then, the change amount of the pump cell applied voltage Vp and the change amount of the pump cell current Ip at that time are read, and the element impedance Rip is calculated from the voltage change amount and the current change amount (Rip = voltage change amount / current change amount). .

  At the time of heater energization control in step 190, the heater energization is controlled so that the element impedance Rip matches a desired target value. As an example, when the element temperature of the gas concentration sensor 100 is low and the element impedance Rip is relatively large, the heater 151 is energized by, for example, full energization control with a duty ratio of 100%. Further, when the element temperature rises, a control duty ratio is calculated using a known PID control method or the like, and the heater 151 is energized by the duty ratio.

  6 is the “applied voltage correction means” described in the claims, step 130 is the “applied voltage setting means”, step 140 is the “change speed setting means”, and step 150. Corresponds to “applied voltage control means”.

Next, the procedure for correcting the pump cell applied voltage characteristic (step 110 in FIG. 6) will be described with reference to the flowchart in FIG.
In FIG. 7, first, in step 111, it is determined whether or not it is a predetermined correction timing now. For example, step 111 is positively determined every 10 to several 100 msec. In the next step 112, the monitor cell current Im measured by the current detector 172 is read. Thereafter, in step 113, the deviation ΔIm of the monitor cell current is calculated. That is, the difference between the target monitor cell current Imtg (for example, about 0.5 to 2 μA) for maintaining the second chamber 146 at a predetermined low oxygen concentration (near the stoichiometry) and the measured monitor cell current Im is a deviation ΔIm. (ΔIm = Imtg−Im).

Thereafter, in step 114, the correction amount ΔVp of the pump cell applied voltage is calculated from the deviation ΔIm of the monitor cell current. At this time, the correction amount ΔVp is an increase / decrease width of the pump cell applied voltage necessary to eliminate the deviation ΔIm of the monitor cell current. If the measured value of the monitor cell current is higher than the target value (when ΔIm <0), Correction amount ΔVp is set. Further, when the measured value of the monitor cell current is lower than the target value (when ΔIm> 0), the negative correction amount ΔVp is set.

  Thereafter, in step 115, the inclination correction amount a1 of the applied voltage straight line LX1 is calculated based on the absolute value of the correction amount ΔVp. Here, the absolute value of the correction amount ΔVp and the inclination correction amount a1 are approximately proportional. Thereafter, in step 116, it is determined whether the correction amount ΔVp is positive or negative. If it is positive, the process proceeds to step 117, and the inclination of the applied voltage straight line LX1 is decreased by the correction amount a1. On the other hand, when the correction amount ΔVp is negative, the routine proceeds to step 118, where the inclination of the applied voltage straight line LX1 is increased by the correction amount a1. Finally, in step 119, the slope of the corrected applied voltage straight line is stored in the memory.

  According to the above step 117, the applied voltage straight line is switched from, for example, LX1 to LX2 in FIG. 5A, and the pump cell applied voltage Vp is corrected in the direction of reducing the residual oxygen in the first chamber 144 (in the direction of more decomposition). The Further, according to step 118 described above, the applied voltage straight line is switched from, for example, LX1 to LX3 in FIG. 5A, and the pump cell applied voltage Vp is corrected in a direction to increase the residual oxygen in the first chamber 144. In addition, according to steps 117 and 118, the monitor cell current Im is controlled to the target monitor cell current Imtg (= Im1 in FIG. 5), whereby the oxygen concentrations in the first and second chambers 144 and 146 are constant. Retained.

  When the applied voltage characteristic is corrected as described above, the microcomputer 170 uses the corrected applied voltage characteristic at a period shorter than the period of correction of the applied voltage characteristic (for example, 10 to several 100 msec), for example, at a period of 4 msec. The pump cell applied voltage Vp is controlled. That is, in another process (not shown), the microcomputer 170 uses the applied voltage straight line whose inclination is corrected, and controls the pump cell applied voltage Vp on the applied voltage straight line according to the current pump cell current Ip.

Next, the procedure for setting the change rate of the pump cell applied voltage (step 140 in FIG. 6) will be described with reference to the flowchart in FIG.
In FIG. 8, first, in step 141, the pump cell application voltage Vp set in step 130 of FIG. 6, that is, the target value of the pump cell application voltage, and the voltage value currently applied to the pump cell 110 are calculated. A deviation ΔV of the voltage Vp is calculated.

  Thereafter, in step 142, the absolute value of the deviation ΔV is determined, and in subsequent steps 143 to 147, the change rate of the pump cell applied voltage Vp is appropriately set according to the absolute value of the deviation ΔV. That is, the degree of | ΔV | is divided into a plurality of stages. The larger the | ΔV |, the faster the change rate of the pump cell applied voltage Vp. Conversely, the smaller | ΔV | Reduce the speed. Further, when | ΔV | is relatively small, the Vp change cycle is set to be variable so that the Vp change cycle becomes longer.

More specifically, a description will be given with reference to FIG.
(1) If | ΔV | <0.01V, change speed = 2 mV / 60 msec (step 143).
(2) If 0.01V ≦ | ΔV | <0.02V, change speed = 2 mV / 40 msec (step 144).
(3) If 0.02V ≦ | ΔV | <0.06V, change speed = 2 mV / 20 msec (step 145).
(4) If 0.06 V ≦ | ΔV | <0.1 V, change speed = 4 mV / 20 msec (step 146).
(5) If | ΔV | ≧ 0.1 V, change speed = 8 mV / 20 msec (step 147).

  Note that the numerical values described in the flowchart of FIG. 8 and FIG. 9 are merely examples, and can be arbitrarily changed. However, the rate of change may be increased as the absolute value of the deviation ΔV increases. If it is FIG. 9, what is necessary is just to be the upward trend.

According to the reference example described in detail above, the following effects can be obtained.
In this reference example, unlike the conventional technique in which the monitor cell electromotive force is fed back to the target value by the PID method or the like, the pump cell applied voltage Vp is controlled using a predetermined applied voltage characteristic (applied voltage map). Therefore, there is no problem that the applied voltage control system of the pump cell 110 oscillates due to a response delay or the like in the monitor cell 120 and the residual oxygen amount in the chamber greatly fluctuates periodically. Therefore, the factor that causes a decrease in the detection accuracy of the NOx concentration is eliminated.

  Also, as shown in FIG. 22A, when the first cell current (pump cell current) changes during the gas concentration change (NOx concentration change), the first cell voltage (pump cell applied voltage) immediately follows the change. It is controlled to a desired target value. As a result, residual oxygen does not become excessive in the chamber, and the monitor cell signal does not fluctuate. Therefore, in the second cell (sensor cell 130), the current corresponding to the NOx concentration can be detected with high response.

  In particular, when the first chamber 144 and the second chamber 146 are communicated with each other via the throttle portion (diffusion passage) 145, the problem of response delay in the monitor cell 120 becomes remarkable. Is resolved.

  Further, the applied voltage characteristic of the pump cell 110 is corrected by the monitor cell current Im (monitor cell output), and the pump cell applied voltage Vp is controlled based on the corrected applied voltage straight line, so that the residual in the first and second chambers 144 and 146 remains. The amount of oxygen can be optimized. That is, the monitor cell current Im converges to the target value and is always kept constant, and the residual oxygen amounts in the first and second chambers 144 and 146 can be controlled as desired. Therefore, the problem that the NOx concentration is erroneously detected due to the change in the residual oxygen amount can be solved. Furthermore, even if the activation state of the pump cell 110 changes due to changes in exhaust gas temperature due to rapid acceleration or deceleration, individual differences, element deterioration, etc., the NOx concentration in the exhaust gas can be detected with high accuracy.

  In particular, since the pump cell applied voltage characteristics are corrected with a period (10 to several 100 msec) longer than the period of applied voltage control (for example, 4 msec), responsiveness changes due to deterioration of the gas concentration sensor 100 or individual differences are taken into account. The correction is performed in the cycle. Therefore, the oscillation phenomenon of the control system during the applied voltage control can be more reliably suppressed.

  In addition, since the rate of change of the pump cell applied voltage Vp is variably set, it is possible to quickly discharge or pump residual oxygen by the pump cell 110. Therefore, the NOx concentration detection response is improved and the detection accuracy is further improved. In particular, when the deviation ΔV of the pump cell applied voltage is relatively small, the cycle of Vp change is lengthened, so that the influence of the peak current at the time of Vp change can be eliminated. That is, when Vp is changed, a peak current (tailing) is generated as a pump cell current, but after the peak current is settled, it is changed to the next applied voltage. Therefore, highly reliable applied voltage control can be realized.

23 shows an equivalent circuit of the gas concentration sensor, where Rg is the particle resistance of the solid electrolyte (zirconia element) to oxygen ions, and Rh and Ch are the grain boundaries at the interface of the solid electrolyte particles. Resistance and grain boundary capacitance, and Rf and Cf are electrode interface resistance and electrode interface capacitance, respectively. In this case, as shown in FIG. 24, when the voltage applied to the gas concentration sensor is changed, a peak current is generated immediately after the voltage change as a sensor current due to the influence of charges stored in the Ch and Cf. In contrast to this situation, according to the above configuration, the applied voltage of the pump cell can be controlled without being affected by the capacity characteristics of the gas concentration sensor.

  On the other hand, the characteristics shown in FIG. 10 may be set as the applied voltage characteristics of the pump cell 110. That is, in FIG. 10, the applied voltage straight line LX1 described above is set within the oxygen concentration detection range, and another applied voltage straight line L11 is set on the lean side of the detection range, and another applied on the rich side. An applied voltage straight line L12 is set. Here, the applied voltage straight line L11 is composed of a primary straight line connecting the point A which is the lean limit of the oxygen concentration detection range and the point B on Vp = 0 (lean side electromotive force), and the applied voltage straight line L12 is , Consisting of a linear line connecting point C, which is the rich limit of the oxygen concentration detection range, and point D on Vp = 0.9 V (rich side electromotive force). In other words, the applied voltage straight line LX1 within the oxygen concentration detection range has a slope that is substantially equivalent to the slope of the resistance dominating region, whereas the applied voltage straight lines L11 and L12 outside the same range are positive and negative with respect to the applied voltage straight line LX1. With a slope of

  By using the characteristics shown in FIG. 10, in the region outside the oxygen concentration detection range, the increase in detection current is limited by the applied voltage straight lines L11 and L12. That is, even when element cracking or the like occurs, the pump cell current Ip is limited in the vicinity of the oxygen concentration detection range, and the inconvenience that an excessive current flows can be avoided. For example, in the lean region, the detected current is limited at point E in the figure. Therefore, problems such as heat generation due to excessive increase in the detection current are solved.

  Furthermore, by limiting the detection current as described above, erroneous detection of element impedance can be prevented. In other words, when there is no current limitation when a defect such as an element crack occurs, the current at the time of impedance detection deviates greatly from the oxygen concentration detection range (current detection range), and a correct value cannot be detected. Therefore, there is a risk of erroneous detection of element impedance. On the other hand, when current limiting is performed, the current at the time of impedance detection does not deviate significantly from the oxygen concentration detection range (current detection range), and the element impedance can be detected correctly.

As the applied voltage straight lines L11 and L12, it is also possible to set a primary straight line with a constant detection current, that is, a primary straight line parallel to the Vp axis in the figure (horizontal axis in the figure).
Next, the second and third reference examples and the first and second embodiments of the present invention will be described with a focus on differences from the first reference example.

(Second reference example)
At the start of operation of the gas concentration sensor 100 such as when the engine is started, the first and second chambers 144 and 146 are in an atmospheric state and are filled with a large amount of oxygen (for example, the oxygen concentration in the atmosphere = 20. 9%). Therefore, since the chambers 144 and 146 are in a state of excessive residual oxygen, the detection accuracy of the NOx concentration is deteriorated. In this case, it takes about 5 to 10 minutes to discharge the residual oxygen in the chambers 144 and 146 at a normal pump cell applied voltage (A / F = 10 to 0.1 to 0.7 V in the atmosphere). , NOx concentration cannot be detected at an early stage.

  Therefore, in this reference example, immediately after the engine is started, a voltage (0.8 to 1.2 V) higher than the normal pump cell applied voltage is applied to the pump cell 110 for about 1 minute to forcibly increase the oxygen discharge capacity in the pump cell 110. Configure as follows. As a result, the residual oxygen in the chambers 144 and 146 can be quickly discharged, and the detection of the NOx concentration can be started at an early stage.

Actually, the flowchart of FIG. 11 is executed. The processing of FIG. 11 is performed by the microcomputer 170 in place of FIG. 6, and only the main part is extracted and shown to the extent that the changed part from FIG. 6 can be understood. The same steps as those in FIG. 6 are given the same step numbers.

  In FIG. 11, the process proceeds to step 201 on condition that step 100 is YES, and the time from the engine start is counted. In the following step 202, it is determined whether or not an initial control time for forcibly discharging residual oxygen has elapsed. The initial control time is, for example, about 1 minute. If the initial control time has elapsed (if step 202 is YES), the process proceeds to step 110 and subsequent steps, and normal processing such as correction of pump cell applied voltage characteristics and setting of pump cell applied voltage Vp is performed.

  If the initial control time has not elapsed (if step 202 is NO), the process proceeds to step 203 to determine whether or not the monitor cell current Im at that time is a normal value. This determination is to determine whether or not the monitor cell current Im is within a range that can be originally taken. For example, if Im ≦ 4 μA, it is determined to be a normal value. If the monitor cell current Im is a normal value, it can be determined that the chambers 144 and 146 are not in a state of excessive residual oxygen even immediately after startup. Therefore, the process proceeds directly to step 110 which is a normal process. For example, when the engine is restarted immediately after the engine is stopped, since the residual oxygen in the chambers 144 and 146 is small, step 203 is positively determined.

  If the monitor cell current Im is not a normal value (Im> 4 μA), the process proceeds to step 204 where the residual oxygen in the chambers 144 and 146 is forcibly discharged, for example, as high as about 0.8 to 1.2V. The voltage is set as the pump cell applied voltage Vp. Thereafter, the process proceeds to step 150, and the pump cell applied voltage Vp set to a high voltage as described above is output.

  As described above, according to the second reference example, the oxygen discharge capacity of the pump cell 110 at the time of starting the engine is forcibly increased. Therefore, the residual oxygen in the chambers 144 and 146 is exhausted quickly, and the detection of the NOx concentration can be started early.

  Further, since the high voltage is applied to the pump cell 110 on the condition that the monitor cell current Im deviates from the specified range (4 μA or less), it is possible to prevent the above-described high voltage from being unnecessarily applied when the engine is restarted. . That is, a desired forced oxygen discharge operation can be realized only when necessary.

(First embodiment)
When the element impedance of the pump cell 110 is changed, the oxygen discharge amount in the pump cell 110 is changed even at the same applied voltage, and the residual oxygen amount in the chambers 144 and 146 is changed. Therefore, in the present embodiment, it is proposed that the applied voltage correction is performed based on the element impedance of the pump cell 110.

  Specifically, in the main routine of FIG. 6, the content of applied voltage correction in step 110 is changed as follows. That is, in the first reference example, the applied voltage correction is performed according to the flowchart of FIG. 7, but in the present embodiment, the applied voltage correction is performed according to the flowchart of FIG. 12 instead. The process of FIG. 12 corresponds to the “applied voltage correction means” recited in the claims, and step 180 of FIG. 6 corresponds to the “element resistance detection means”.

In FIG. 12, first, in step 301, it is determined whether or not it is a predetermined correction timing now. For example, step 301 is positively determined every 10 to several 100 msec. In step 302, the element impedance Rip of the pump cell 110 is read, and in the subsequent step 303, the applied voltage straight line LX according to the element impedance Rip.
The inclination of 1 is corrected. Finally, in step 304, the slope of the corrected applied voltage straight line is stored in the memory.

  Step 303 will be specifically described with reference to FIG. When the element impedance Rip is larger than the specified value, the applied voltage line is switched from LX1 to LX2 in the figure, and the inclination is reduced. As a result, the pump cell applied voltage Vp is corrected in the direction in which the residual oxygen in the first chamber 144 is reduced (in the direction in which more oxygen is decomposed). Further, when the element impedance Rip is smaller than the specified value, the applied voltage straight line is switched from LX1 to LX3 in the figure to increase the inclination. As a result, the pump cell applied voltage Vp is corrected in the direction of increasing the residual oxygen in the first chamber 144.

  According to the first embodiment, it is possible to control the pump cell applied voltage Vp so as to optimize the residual oxygen amount in the chambers 144 and 146. As a result, the NOx concentration in the exhaust gas can be accurately detected.

(Third reference example)
Next, a third reference example will be described. In short, in the first reference example, the applied voltage characteristic of the pump cell 110 is corrected according to the monitor cell current. However, in this reference example, the output of the sensor cell 130 is corrected according to the monitor cell current instead. Since the configuration of the gas concentration detection apparatus can be applied to FIG. 1, the description thereof is omitted.

  The outline of the sensor cell current correction will be specifically described with reference to FIG. 14A to 14C show the characteristics of the pump cell 110, the monitor cell 120, and the sensor cell 130 when the oxygen concentration and the NOx concentration are constant, as in FIGS. 3A to 3C. In the figure, the characteristic (circle numeral 1 in the figure) indicated by a solid line is the basic characteristic explained in FIG.

  Here, in FIG. 14A, another applied voltage straight line LXA is provided on the lower voltage side (left side in the figure) than the applied voltage straight line LX1 in FIG. Thus, the pump cell applied voltage Vp is set. Shifting the applied voltage straight line to the low voltage side means that the oxygen concentration in the first and second chambers 144 and 146 slightly increases, and the element impedance of the pump cell 110 is temporarily increased due to an increase in exhaust gas temperature or individual differences. Even if Rip decreases, NOx decomposition in the pump cell 110 does not occur.

  As described above, the inside of the first chamber 144 is maintained at a predetermined low oxygen concentration, and the oxygen flows into the second chamber 146 as residual oxygen. Therefore, the sensor cell 130 offsets the residual oxygen content to the NOx decomposition current. The sensor cell current Is is detected at a current level obtained by adding the currents. In this case, if the residual oxygen amount is always constant and the offset current is constant, it is possible to relatively evaluate the NOx concentration level. However, if the offset current changes due to fluctuations in the residual oxygen amount, the corresponding amount of NOx is increased. Density detection error occurs.

  For example, when the element impedance Rip of the pump cell 110 increases due to a decrease in exhaust gas temperature or individual differences, the pump cell characteristic changes to the characteristic indicated by the dotted line in FIG. 14A (change from the circled number 1 to the circled number 2 in the figure). As a result, the residual oxygen in the first and second chambers 144 and 146 increases. Therefore, as shown in FIG. 14B, the monitor cell characteristic changes to a dotted line (the current value of the monitor cell changes from Im1 to Im2). Further, as shown in FIG. 14C, the sensor cell characteristic similarly changes to a dotted line (the current value of the sensor cell changes from Is1 to Is2). In this case, the current value Is2 includes an offset current corresponding to the current value Im2 on the monitor cell 120 side, and this appears as a detection error of the NOx concentration.

Therefore, in the apparatus of this reference example, the change in the residual oxygen amount is monitored by the monitor cell characteristic (monitor cell current I
m), and consider correcting the sensor cell current Is accordingly.
That is, when the monitor cell current value is Im1 or the monitor cell current is Im2, the monitor cell current values Im1 and Im2 are respectively subtracted from the sensor cell current values Is1 and 1s2 that are actually measured. Then, in any case, the sensor cell current value is corrected to “IsA”, which is a numerical value of only the NOx decomposition current not including the offset current component. Therefore, when the NOx concentration of the exhaust gas is constant, the NOx concentration detection value (sensor cell current) of the same level is always obtained, and the NOx concentration detection error is eliminated.

  When the element impedance Rip of the pump cell 110 decreases due to an increase in exhaust gas temperature or individual differences, the pump cell characteristic changes from the basic characteristic indicated by a solid line to the characteristic indicated by a two-dot chain line in FIG. The number of residual oxygen in the first and second chambers 144 and 146 becomes very small. In this case, the monitor cell current value is very weak Im3, and the influence of the offset current is negligible, so that the sensor cell current value may not be corrected.

  When the offset current is corrected by subtracting the monitor cell current value from the sensor cell current value, if the catalytic capacity of each electrode in the monitor cell 120 and the sensor cell 130 is the same, that is, if the electrode size is the same and the material is the same, The difference between the two may be simply obtained, but if the catalytic ability of each electrode is different, it is desirable to obtain the difference by multiplying by a coefficient corresponding to the difference in ability. Thereby, the reaction current with respect to residual oxygen becomes equivalent in the monitor cell 120 and the sensor cell 130, and the simple comparison becomes possible.

  Next, a sensor cell current correction procedure realized by the microcomputer 170 will be described with reference to the flowchart of FIG. The process of FIG. 15 corresponds to “second cell output correcting means” recited in the claims.

  In FIG. 15, first, in step 401, it is determined whether or not it is the NOx concentration detection timing now. For example, step 401 is positively determined every 8 msec. In step 402, the monitor cell current Im measured by the current detector 172 is read. In the subsequent step 403, the sensor cell current Is measured by the current detector 173 is read.

  Thereafter, in step 404, a coefficient K for correcting the difference in electrode size and material in the monitor cell 120 and the sensor cell 130 is obtained, and the coefficient K is multiplied by the monitor cell current Im to calculate “Im ′”. By this step 404, the difference in the catalytic ability of the electrodes in the monitor cell 120 and the sensor cell 130 is corrected, and the reaction current for residual oxygen becomes equivalent. Incidentally, if the catalytic ability of the electrode in each of the cells 120 and 130 is the same, the coefficient K = 1. Of course, instead of the monitor cell current Im, the sensor cell current Is may be multiplied by the coefficient.

  Finally, in step 405, the monitor cell current Im 'in step 404 is subtracted from the sensor cell current Is in step 403, and the value is set as a corrected sensor cell current IsB. Thus, by correcting the sensor cell current Is and calculating “IsB”, an error corresponding to the offset current in the sensor cell current value is corrected. For example, in FIG. 14, the “IsA value” is obtained as the corrected sensor cell current IsB.

  However, the NOx concentration detection process of FIG. 15 corresponds to the process of step 160 in the main routine of FIG. Therefore, steps 402 to 405 in FIG. 15 may be performed in step 160 in FIG.

As described above, according to the third reference example, the monitor cell current Im (monitor cell output) is taken in, and the sensor cell current Is is corrected by the monitor cell current Im. Therefore, the residual in the second chamber 146 even at the same NOx concentration. The problem that the sensor cell current Is changes due to the fluctuation of the oxygen amount is solved. Therefore, the NOx concentration in the exhaust gas can be detected with high accuracy.

  Also, in the present reference example, similarly to the first reference example, the applied voltage control system of the pump cell (first cell) oscillates unlike the conventional technique in which the monitor cell electromotive force is fed back to the target value by the PID method or the like. The problem that the amount of residual oxygen in the chamber fluctuates greatly periodically does not occur. Therefore, a factor that causes a decrease in gas concentration detection accuracy is eliminated.

  In the gas concentration sensor 100 described above, the monitor cell 120 and the sensor cell 130 are provided at relatively close positions, and each of the cells 120 and 130 uses the same gas in the second chamber 146 as a detection target. Therefore, the monitor cell current Im can be regarded as equivalent to the offset current, and the sensor cell current Is can be corrected by the monitor cell current Im.

(Second Embodiment)
Next, a second embodiment of the present invention will be described focusing on differences from the above respective reference examples and the first embodiment. In short, in the present embodiment, an apparatus having both the function of correcting the applied voltage characteristic of the pump cell 110 according to the monitor cell current and the function of correcting the output of the sensor cell 130 according to the monitor cell current will be described below. Since the configuration of the gas concentration detection apparatus can be applied to FIG. 1, the description thereof is omitted.

  As an outline, the residual oxygen amount in the second chamber 146 is measured by the monitor cell current, and the applied voltage characteristic of the pump cell 110 is corrected according to the residual oxygen amount. Thereby, the residual oxygen amount (oxygen concentration) in the first and second chambers 144 and 146 is kept constant. However, at this time, the current corresponding to the oxygen concentration in the pump cell 110 is mA (milliampere) level, the sensor cell current due to NOx is μA (microampere) level, and the current level is greatly different. Therefore, the pump cell 110 does not decompose NOx and It is difficult to completely discharge residual oxygen in consideration of control resolution. That is, if the residual oxygen is made zero, there is a possibility that the NOx is slightly decomposed, and the influence appears greatly in the sensor cell current value. Therefore, the pump cell applied voltage characteristics are corrected to such an extent that NOx is not decomposed by the pump cell 110. As a result, since some residual oxygen remains in the first and second chambers 144 and 146, the residual oxygen is detected by the monitor cell 120, and the sensor cell current output is corrected according to the result.

  The outline of the correction of the pump cell applied voltage characteristic and the correction of the sensor cell current will be specifically described with reference to FIG. 16A to 16C show the characteristics of the pump cell 110, the monitor cell 120, and the sensor cell 130 under the constant oxygen concentration and NOx concentration, as in FIGS. 3A to 3C. In the figure, the characteristic (circle numeral 1 in the figure) indicated by a solid line is the basic characteristic explained in FIG.

First, a case will be described in which the element impedance Rip of the pump cell 110 is held at the target value, and correction regarding the applied voltage characteristic (applied voltage straight line LX1) of the pump cell 110 is not necessary. That is, when the pump cell characteristic becomes the basic characteristic shown by a solid line in FIG. 16A (circle numeral 1 in the figure), the applied voltage line LX1 controls the pump cell applied voltage Vp to such an extent that the pump cell 110 does not decompose NOx. As a result, a small amount of residual oxygen flows into the second chamber 146. Accordingly, the monitor cell current value Im1 is measured as shown in FIG. 16B, and the sensor cell current value Is1 is measured as shown in FIG. 16C. At this time, since the current value Is1 includes an offset current corresponding to the amount of residual oxygen, an offset current (corresponding to the monitor cell current Im1) is subtracted from the current value Is1. As a result, a current value “IsC” that does not include an offset current corresponding to residual oxygen is obtained.

  On the other hand, when the element impedance Rip of the pump cell 110 increases due to a decrease in exhaust gas temperature or individual differences, the pump cell characteristic changes to the characteristic indicated by the dotted line in FIG. 16A (circled numeral 2 in the figure). Based on the monitor cell current value Im2 at that time, the applied voltage straight line shown in FIG. 16A is changed from LX1 (solid line) to LX2 (dotted line). Thereby, the residual oxygen amounts in the first and second chambers 144 and 146 are controlled to be constant, and both the monitor cell characteristics in FIG. 16B and the sensor cell characteristics in FIG. At this time, since the residual oxygen amount in the second chamber 146 is kept constant, the sensor cell 130 measures the current value Is1, and this current value Is1 includes an offset current corresponding to the residual oxygen amount. Then, the offset current is subtracted from the current value Is1. Thus, a current value “IsC” that does not include an offset current corresponding to residual oxygen is obtained.

  On the other hand, when the element impedance Rip of the pump cell 110 decreases due to an increase in exhaust gas temperature or individual differences, the pump cell characteristics change to the characteristics indicated by the two-dot chain line in FIG. Accordingly, the applied voltage straight line shown in FIG. 16A is changed from LX1 (solid line) to LX3 (two-dot chain line) based on the monitor cell current value Im3 at that time. Thereby, the residual oxygen amounts in the first and second chambers 144 and 146 are controlled to be constant, and both the monitor cell characteristics in FIG. 16B and the sensor cell characteristics in FIG. At this time, since the residual oxygen amount in the second chamber 146 is kept constant, the current value Is1 is measured in the sensor cell 130. By subtracting the offset current from the current value Is1, the amount corresponding to the residual oxygen is measured. A current value “IsC” not including the offset current is obtained.

  In order to realize the operation of the above-described embodiment, the microcomputer 170 performs the above-described processing of FIG. 7 as “first correction means” described in the claims, and corrects the applied voltage characteristics of the pump cell 110. . At this time, the microcomputer 170 controls the pump cell applied voltage Vp using the corrected applied voltage characteristic at a cycle of 4 msec, for example. In other words, an applied voltage straight line whose slope is corrected is used, and the pump cell applied voltage Vp is controlled on the applied voltage straight line according to the pump cell current Ip at that time.

  Further, the microcomputer 170 performs the process of FIG. 17 in which a part of FIG. 15 is changed as the “second correction means” described in the claims. In FIG. 17, the process with the same step number as in FIG. 15 is performed in accordance with FIG. That is, in step 450 immediately after step 402, it is determined whether or not the monitor cell current Im detected at that time (Im value in step 402) has converged to or near the target value. In this case, the pump cell applied voltage is corrected according to the deviation between the monitor cell current Im and the target value (the monitor cell current for maintaining the inside of the second chamber 146 at a predetermined low oxygen concentration) according to FIG. As a result of the correction, if the monitor cell current Im has converged with respect to the target value, the processing after step 403 is performed. That is, when the residual oxygen amount in the second chamber 146 is maintained at a predetermined amount, the sensor cell current Is is captured and the sensor cell current Is is corrected (steps 403 to 405). If the monitor cell current Im does not converge to the target value, the process waits until it converges.

  According to FIG. 17 described above, the sensor cell current Is is captured and corrected while the residual oxygen amount (oxygen concentration) in the second chamber 146 is stabilized as a result of the applied voltage control. Therefore, the sensor cell current Is can be corrected more accurately.

As described above, according to the second embodiment, the pump cell applied voltage characteristic is corrected by the monitor cell current Im (monitor cell output) and the sensor cell current output is corrected. Therefore, the chamber 144, regardless of the change in the limit current characteristic of the pump cell 110. The amount of residual oxygen in 146 is kept constant, and a sensor cell current output that is not affected by the residual oxygen content is obtained. As a result, the NOx concentration in the exhaust gas can be accurately detected.

  As a means for correcting the pump cell applied voltage characteristic, the process of FIG. 12 may be used instead of the process of FIG. That is, the pump cell applied voltage characteristic is corrected according to the element impedance of the pump cell 110. In such a case as well, the NOx concentration in the exhaust gas can be detected with high accuracy.

  Also, in the second embodiment, similarly to each of the reference examples and the first embodiment, the pump cell (first cell) is different from the conventional technique in which the monitor cell electromotive force is fed back to the target value by the PID method or the like. The applied voltage control system does not oscillate, and the amount of residual oxygen in the chamber does not fluctuate greatly periodically. Therefore, a factor that causes a decrease in gas concentration detection accuracy is eliminated.

  In particular, the applied voltage control period is, for example, 4 msec, while the pump cell applied voltage characteristic correction period is set to 10 to several hundreds msec and the sensor cell output correction period is set to 8 msec. The above correction is performed in a cycle that takes into account a change in response due to a difference or the like. Therefore, the oscillation phenomenon of the control system during the applied voltage control can be more reliably suppressed.

In addition to the above, the present invention can be embodied in the following forms.
In the reference example and the embodiment described above, when correcting the applied voltage characteristic of the pump cell 110, as described with reference to FIG. 5 and the like, correction is performed to increase or decrease the slope of the applied voltage straight line LX1, but this configuration is replaced. Thus, a correction for increasing or decreasing the offset (offset b in FIG. 3) of the applied voltage straight line LX1 is given. That is, as shown in FIG. 18, for example, when the element impedance Rip of the pump cell 110 increases due to a decrease in exhaust gas temperature or individual differences, the pump cell characteristic changes to the characteristic shown by the dotted line in FIG. Therefore, the applied voltage line is changed from LX1 (solid line) to LX10 (dotted line) based on the monitor cell current at that time. Further, when the element impedance Rip of the pump cell 110 decreases due to an increase in exhaust gas temperature or individual differences, the pump cell characteristic changes to the characteristic indicated by a two-dot chain line in FIG. 10 (circle numeral 3 in the figure). Based on the monitor cell current, the applied voltage straight line is changed from LX1 (solid line) to LX11 (two-dot chain line). By changing the applied voltage straight line in this way, the amount of residual oxygen in the second chamber 146 is kept substantially constant. Thereby, the NOx concentration in the exhaust gas can be accurately detected.

  In such a case, the microcomputer 170 corrects the pump cell applied voltage characteristics according to the flowchart of FIG. 19 which is the “applied voltage correcting means” described in the claims. This process is obtained by changing a part of the process of FIG. 7. In steps 111 to 114, as described above, the monitor cell current Im is read, the monitor cell current deviation ΔIm is calculated, and the correction amount ΔVp of the pump cell applied voltage is calculated. Each calculation is performed. Then, in step 501, it is determined whether or not ΔVp ≦ 0. If ΔVp> 0, the process proceeds to step 502, and the offset of the applied voltage straight line LX1 is increased by the correction amount ΔVp. If ΔVp ≦ 0, the process proceeds to step 503, and the offset of the applied voltage straight line LX1 is reduced by the correction amount ΔVp. Finally, in step 504, the offset of the corrected applied voltage straight line is stored in the memory.

  According to the above step 502, the applied voltage straight line is switched from LX1 to LX10 in FIG. 18, for example, and the pump cell applied voltage Vp is corrected in the direction of reducing the residual oxygen in the first chamber 144 (in the direction of more decomposition). Further, according to the above step 503, the applied voltage straight line is switched from, for example, LX1 to LX11 in FIG. 18, and the pump cell applied voltage Vp is corrected in the direction of increasing the residual oxygen in the first chamber 144.

  Similarly, when the pump cell applied voltage characteristic is corrected according to the element impedance as shown in FIG. 12, the offset of the applied voltage straight line may be corrected. In this case, the offset of the applied voltage straight line is preferably increased as the element impedance of the pump cell 110 is increased.

  Further, as the applied voltage correction of the pump cell 110, after setting the pump cell applied voltage Vp according to a predetermined applied voltage characteristic (map data), the correction may be performed on the set value of the Vp. That is, in FIG. 6, the pump cell applied voltage characteristic is corrected according to the monitor cell output, and the pump cell applied voltage Vp is set using the corrected applied voltage characteristic (steps 110 to 130). The pump cell application voltage Vp is set using the predetermined application voltage characteristic as it is without correcting the pump cell application voltage characteristic. Then, the pump cell applied voltage Vp after the setting is corrected according to the monitor cell output. Even in this configuration, there is an excellent effect as described above. The same applies to the case where correction is performed in accordance with the element impedance of the pump cell 110 (first embodiment).

  The time constant of the LPF 180 in FIG. 1 is changed between when impedance is detected and when gas concentration is detected. As a simple configuration, the LPF 180 is configured as shown in FIG. That is, in FIG. 20, the switch SW is used to switch the resistance value of the LPF 180 to either R1 or R2 (where R1> R2), and the time constant is changed. More specifically, when the normal oxygen concentration is detected, the time constant is increased by operating in the state shown in the figure, and when the impedance is detected, the switch SW is switched to reduce the time constant. It is also possible to change the time constant by changing the capacitor capacitance value. As described above, the change rate of the pump cell applied voltage Vp is variably set by switching the time constant of the LPF 180.

  As the gas concentration sensor, it can be applied to other than the gas concentration sensor 100 shown in FIG. For example, the gas concentration sensor 300 having the structure shown in FIG. 21 can be applied. In the gas concentration sensor 300 of FIG. 21, one solid electrolyte 301 is provided with a first pump cell 310 and a second pump cell 320, and the other solid electrolyte 302 is provided with a monitor cell 330 and a sensor cell 340 as a second cell. Is provided. Further, a first chamber 303 and a second chamber 304 are provided between the solid electrolytes 301 and 302, and both chambers 303 and 304 communicate with each other via a constricted portion 305 as a diffusion passage. Note that exhaust gas is introduced into the first chamber 303 through the pinhole 306. Reference numerals 307 and 308 are atmospheric passages, and reference numeral 309 is a heater.

  The gas concentration sensor 300 is obtained by dividing a pump cell as a first cell into two pump cells 310 and 320 in order to increase the efficiency and accuracy of oxygen in and out of the first and second chambers 303 and 304. However, even in such a configuration, as in the reference example and the embodiment described above, by implementing at least one of the application voltage correction of the pump cell 110, the sensor cell output correction, and the like, by embodying the gas concentration detection device, The NOx concentration in the exhaust gas can be detected with high accuracy. In addition to the sensor structure having three cells shown in FIG. 2 and the four cells shown in FIG. 21, a sensor structure having five or more cells may be used.

  In addition to the gas concentration sensor capable of detecting the NOx concentration, the present invention can also be applied to a gas concentration sensor capable of detecting the HC concentration and the CO concentration as the concentration of the specific gas component. In this case, the first cell (pump cell) discharges oxygen in the gas to be detected, and the second cell (sensor cell) detects HC concentration and CO concentration by decomposing HC and CO from the discharged gas components. To do. Furthermore, it can be used for gas concentration detection devices other than those for automobiles, and gases other than exhaust gas can be used as gas to be detected. Even in such a configuration, the detection accuracy of the gas concentration is improved by applying the present invention.

  DESCRIPTION OF SYMBOLS 100 ... Gas concentration sensor, 110 ... Pump cell (1st cell), 120 ... Monitor cell, 130 ... Sensor cell (2nd cell), 144 ... 1st chamber, 145 ... Restriction part, 146 ... 2nd chamber, 170 ... Microcomputer, 300 ... gas concentration sensor, 303 ... first chamber, 304 ... second chamber, 305 ... throttle part, 310, 320 ... pump cell (first cell), 330 ... monitor cell, 340 ... sensor cell (second cell).

Claims (1)

A first cell for exhausting or pumping oxygen in the gas to be detected introduced into the chamber; a second cell for detecting the concentration of the specific gas component from the gas after passing through the first cell; A gas concentration detection apparatus that uses a gas concentration sensor including at least a monitor cell for detecting a residual oxygen concentration, and performs gas concentration detection based on a detection result by the gas concentration sensor,
Element resistance detecting means for detecting the resistance value of the solid electrolyte element provided with the first cell;
Based on the detected element resistance of the first cell, an applied voltage characteristic that preliminarily defines the relationship between the current value of the first cell and the applied voltage, or an applied voltage that is set at any time based on the applied voltage characteristic, is applied. Means,
Applied voltage control means for controlling the applied voltage to the first cell based on the corrected applied voltage characteristic or applied voltage;
And the applied voltage correction means corrects an offset of the applied voltage straight line, which is an applied voltage characteristic, and changes the offset according to the element resistance of the first cell .

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