JP6684169B2 - Gas sensor control device - Google Patents

Description

  The present invention relates to a gas sensor control device used in an internal combustion engine of an automobile or the like.

  Conventionally, as sensors used in automobiles, A / F sensors and the like that detect the oxygen concentration in exhaust gas have been known. The A / F sensor detects the oxygen concentration and thus the air-fuel ratio of the exhaust by utilizing the fact that the magnitude of the current flowing through the sensor element changes according to the oxygen concentration in the exhaust. The detection result of the air-fuel ratio is used in an air-fuel ratio control system configured by an engine ECU or the like, and stoichiometric combustion control or the like in which the air-fuel ratio is feedback-controlled in the vicinity of stoichiometry (theoretical air-fuel ratio) is realized.

  The voltage-current (VI) characteristic of the sensor element is used to detect the air-fuel ratio. That is, by utilizing the fact that the value of the element current in the limit current region of the VI characteristic increases / decreases with the increase / decrease in the air-fuel ratio of exhaust gas, the voltage in the limit current region is used when calculating the air-fuel ratio of exhaust gas. Is applied to detect the current flowing through the sensor element.

  Further, the sensor element is heated by the heater so as to be in an active state when detecting the air-fuel ratio. The heating by the heater is controlled by the amount of electricity supplied to the heater. The amount of electricity supplied to the heater is controlled so as to eliminate the deviation between the element resistance value and the target value which is the resistance corresponding to the activation temperature of the sensor element. That is, element resistance feedback control is performed.

  Here, when the sensor element deteriorates, the element resistance increases. As a result, the slope of the resistance-dominated region of the VI characteristic becomes smaller, and the limit current region changes. In such a case, the detection of the air-fuel ratio may be adversely affected. Therefore, in the conventional technology, in the element resistance feedback control, by keeping the target element resistance value constant, the change in the voltage width in the limit current region is prevented, and the detection accuracy of the air-fuel ratio is maintained (for example, Patent Document 1).

JP, 9-292364, A

  However, in a situation where the deterioration of the sensor element has progressed, when the control is performed while keeping the target value in the element resistance feedback control constant, there is a concern that the temperature of the sensor element may rise. Then, due to the rise of the element temperature, the deterioration of the sensor element is further promoted and the life of the sensor may be shortened.

  The present invention has been made to solve the above problems, and a main object of the present invention is to provide a gas sensor control device capable of suppressing the temperature rise of the sensor element while ensuring the detection accuracy of the oxygen concentration. .

  The present invention is applied to a gas sensor including a sensor element of a limiting current type having a solid electrolyte layer and a pair of electrodes, and a heater for heating the sensor element, in a state where a voltage is applied to the pair of electrodes, an internal combustion engine. While outputting a limiting current according to the oxygen concentration in the exhaust gas of the engine, by using the correlation between the element resistance value of the sensor element and the temperature of the sensor element, by feedback control the element resistance value to a target value, A gas sensor control device for performing element temperature control in the sensor element, wherein a deterioration determination unit that determines that the degree of deterioration of the sensor element is below a predetermined level, and the degree of deterioration of the sensor element is below the predetermined level. If it is determined that there is a deterioration of the sensor element while keeping the width of the limiting current region reduced with the deterioration of the sensor element within a predetermined range. A target setting section to change to the side to increase based on the case, characterized in that it comprises a.

  In the limiting current type gas sensor, the element resistance value may increase as the sensor element deteriorates.In a situation where the element resistance value is increasing, the target value of the element resistance value is kept constant and feedback control is performed. When performing, there is a concern that the element temperature will rise. Further, in a situation where the element resistance value is increasing, the slope of the resistance-dominant region in the VI characteristic (limit current characteristic) of the gas sensor becomes small, so it is considered that the limit current region becomes narrow.

  Specifically, in the V-I characteristic of the gas sensor, the boundary voltage on the high voltage side of the limiting current region (that is, the boundary voltage with the water splitting region) is almost unchanged even if the element deterioration occurs, while it is low. The boundary voltage on the voltage side (that is, the boundary voltage with the resistance-dominated region) changes to the high voltage side as the sensor element deteriorates. Therefore, the limit current region becomes narrower under the condition that the element resistance value increases.

  In this respect, in the above configuration, when the degree of deterioration of the sensor element is equal to or lower than a predetermined level, the target value is changed to a side that is increased based on the degree of deterioration of the sensor element while keeping the width of the limiting current region within the predetermined range. I decided to do it. In such a case, since the target value is changed while the width of the limiting current region is secured, it is possible to detect the oxygen concentration using the limiting current region. On the other hand, by changing the target value of the element resistance value to the larger side, it is possible to suppress the temperature rise of the sensor element that occurs when the sensor element deteriorates under the feedback control. Thereby, it is possible to suppress the temperature rise of the sensor element while ensuring the detection accuracy of the oxygen concentration.

The block diagram which shows a sensor element. The block diagram which shows the electric constitution of a sensor control circuit. The figure which shows the VI characteristic of a sensor element. The figure which shows the equivalent circuit of a sensor element. The figure which shows VI characteristic at the time of deterioration of a sensor element. The figure which shows the relationship between the logarithmic value of element resistance value, and element temperature. The figure which shows the relationship between the logarithmic value of element resistance value, and element temperature. The flowchart which shows the process sequence of target value setting. The figure which shows the relationship between a heater duty value and DC resistance Ri. The figure which shows the relationship between (DELTA) Zac and (DELTA) VP. Explanatory drawing for demonstrating the transition of the target value Zactg. The figure which shows the relationship between element temperature and time. The flowchart which shows the process sequence of target value setting in 2nd Embodiment. Explanatory drawing for demonstrating the transition of the target value Zactg. Explanatory drawing for demonstrating the transition of the target value Zactg.

(First embodiment)
Hereinafter, the control system according to the present embodiment will be described with reference to the drawings. The present embodiment embodies an A / F sensor control system that detects exhaust gas emitted from an engine mounted on a vehicle as a gas to be detected and detects an oxygen concentration (air-fuel ratio: A / F) in the exhaust gas. There is.

  First, the configuration of the A / F sensor 10 will be described with reference to FIG. The A / F sensor 10 has a sensor element 20 having a laminated structure. FIG. 1 shows a sectional structure of the sensor element 20. Actually, the sensor element 20 has a long shape extending in the direction orthogonal to the paper surface of FIG. 1, and the entire element is housed in a housing or an element cover. The sensor element 20 has a solid electrolyte layer 21, a diffusion resistance layer 22, a shielding layer 23, and an insulating layer 24, which are stacked one above the other in the figure. A protective layer (not shown) is provided around the element. The rectangular plate-shaped solid electrolyte layer 21 is a sheet made of partially stabilized zirconia, and a pair of upper and lower electrodes 25 and 26 are arranged to face each other with the solid electrolyte layer 21 interposed therebetween. The diffusion resistance layer 22 is made of a porous sheet for introducing exhaust gas to the electrode 25, and the shielding layer 23 is made of a dense layer for suppressing permeation of exhaust gas. An exhaust chamber 27 is provided in the diffusion resistance layer 22 so as to surround the electrode 25.

  The insulating layer 24 is made of high thermal conductive ceramics such as alumina, and an air duct 28 is formed at a portion facing the electrode 26. A heater 29 is embedded in the insulating layer 24. The heater 29 is composed of a linear heating element that generates heat when energized by a battery power source, and heats the entire element by the generated heat. The duty of the amount of electricity supplied to the heater 29 is controlled so that the sensor element 20 remains active.

  In the sensor element 20 having the above structure, the exhaust gas around the sensor element 20 is introduced from the side portion of the diffusion resistance layer 22, then flows into the exhaust chamber 27 through the diffusion resistance layer 22, and reaches the electrode 25. When the exhaust gas is lean, oxygen in the exhaust gas is decomposed by the electrode 25 and discharged from the electrode 26 to the air duct 28. On the contrary, when the exhaust gas is rich, oxygen in the atmosphere duct 28 is decomposed by the electrode 26 and discharged to the electrode 25 side.

  Next, a specific configuration of the sensor control circuit 30 will be described with reference to FIG. The sensor control circuit 30 has a microcomputer 40 and a detection circuit section. The detection circuit unit detects the element current IL flowing in the sensor element 20 and the impedance Zac of the sensor element 20, and outputs the detection result to the microcomputer 40. The microcomputer 40 is composed of a well-known logical operation circuit including a CPU, various memories and the like. Further, the microcomputer 40 grasps the air-fuel ratio of the exhaust gas based on the detection result of the element current IL and appropriately executes the air-fuel ratio feedback control and the like, and also performs the energization control of the heater 29 based on the detection result of the impedance Zac. .

  In the sensor control circuit 30, a reference voltage power supply 43 is connected to the positive terminal of the sensor element 20 via an operational amplifier 41 and a current detection resistor 42, and an applied voltage control circuit 44 is connected to the negative terminal of the sensor element 20. Has been done. In this case, the point A at one end of the current detection resistor 42 is held at the same voltage as the reference voltage Vf (for example, 2.2V). The element current flows through the current detection resistor 42, and the voltage at point B changes according to the element current. For example, when the exhaust gas is lean, a current B flows from the S + terminal to the S− terminal in the sensor element 20, so that the B point voltage rises. When the exhaust gas is rich, a current flows from the S− terminal to the S + terminal, and the B point voltage decreases. .

  As a basic configuration, the applied voltage control circuit 44 monitors the voltage at point B, determines the voltage to be applied to the sensor element 20 according to the voltage value, and controls the voltage on the S− side. An amplifier circuit is connected to the points A and B at both ends of the current detection resistor 42, and the A / F output voltage which is the output of the amplifier circuit is taken into the A / D input terminal of the microcomputer 40.

  When the impedance Zac is detected, an AC signal having a predetermined frequency is combined with the reference voltage power supply 43 and output to the impedance detection circuit 45. In this case, the voltage at the point B changes by alternating current around the reference voltage power supply 43. Then, the impedance detection circuit 45 detects the amplitude of the AC signal and outputs a signal corresponding to the amplitude as an impedance detection signal. In this case, the microcomputer 40 calculates the impedance Zac from the change amount of the AC voltage and the change amount of the AC current.

  Here, the voltage-current characteristic (VI characteristic) shown in FIG. 3 is used to detect the A / F value. In FIG. 3, the horizontal axis represents the applied voltage VP of the sensor element 20, and the vertical axis represents the element current IL. In the characteristic line of FIG. 3, the straight line portion (flat portion) parallel to the voltage axis that is the horizontal axis is the limit current region that specifies the element current IL as the limit current. The increase / decrease in the element current IL in the limit current region corresponds to the increase / decrease in the air-fuel ratio (that is, the degree of lean rich). That is, the element current IL increases as the air-fuel ratio becomes leaner, and the element current IL decreases as the air-fuel ratio becomes richer. A low voltage side of the limiting current region has a resistance control region having a direct current resistance Ri of the sensor element 20 as a proportional coefficient, and a high voltage side of the limiting current region has water decomposition in which water in exhaust gas is decomposed. There are areas. That is, the limiting current region is defined as a voltage range between the boundary voltage with the resistance control region and the boundary voltage with the water decomposition region.

  Here, the element resistance of the sensor element 20 will be described using the equivalent circuit of FIG. The solid electrolyte layer 21 has a bulk resistance R1 which is a resistance inside the particles of the solid electrolyte layer 21 and a grain boundary resistance R2 which is a resistance between the particles. On the other hand, the electrode interface, which is the interface between the solid electrolyte layer 21 and the electrodes 25 and 26, has an interfacial resistance R3 that prevents passage of the substance at the electrode interface. C2 is the electrostatic capacity (grain boundary capacity) between the particles of the solid electrolyte layer 21, and C3 is the electrostatic capacity (interface capacity) between the solid electrolyte layer 21 and the electrodes 25 and 26. That is, the equivalent circuit of the sensor element 20 is represented as a series connection of the bulk resistance R1, the parallel connection body of the grain boundary resistance R2 and the grain boundary capacitance C2, and the parallel connection body of the interface resistance R3 and the interface capacitance C3. be able to. The impedance Zac indicates a value that is close to the resistance of the solid electrolyte layer 21, that is, the sum of the bulk resistance R1 and the grain boundary resistance R2. Further, the direct current resistance Ri represents the total of the bulk resistance R1, the grain boundary resistance R2, and the interface resistance R3. Note that the element resistance value of the sensor element 20 when a low-frequency (for example, 0.1 Hz) AC voltage is applied is a value approximate to the DC resistance Ri.

  The DC resistance Ri is obtained by a known method. For example, after the engine is stopped, a voltage is applied at a low frequency (for example, 0.1 Hz), and the DC resistance Ri can be obtained based on the current value detected according to the applied voltage and the voltage value. . In addition, as another method, it is possible to acquire the DC resistance Ri from the relationship between the voltage value and the current value under the situation where the element current IL is not detected (for example, 0.2 V).

  By the way, when detecting the air-fuel ratio, the sensor element 20 is heated by the heater 29 so that the sensor element 20 is activated. The energization of the heater 29 is controlled so as to eliminate the deviation between the impedance Zac and the target value Zactg which is the impedance corresponding to the activation temperature of the sensor element 20. That is, element resistance feedback control is performed.

  In the conventional element resistance feedback control, the target value Zactg is controlled to be constant. However, in such a case, the element resistance value increases as the sensor element 20 deteriorates, and the element temperature rises. As a result, the sensor element 20 may be further deteriorated. Therefore, in the control of the microcomputer 40, the target value Zactg is changed to a larger value based on the degree of deterioration of the sensor element 20. Specifically, even when the sensor element 20 is deteriorated, the target temperature is kept constant at the target temperature for activating the sensor element 20 or the target temperature is set within a predetermined range from the target temperature. Change the value Zactg.

  FIG. 5 shows the VI characteristic when the sensor element 20 deteriorates. In FIG. 5, the VI characteristic in the initial state is shown by a solid line, and the VI characteristic in a deteriorated state is shown by a broken line. When the sensor element 20 deteriorates, the slope of the linear line in the resistance-dominated region decreases as the DC resistance Ri, that is, the element resistance value of the sensor element 20 increases. In such a case, the boundary point VA on the low voltage side of the limit current region shifts to VA1 in the high voltage direction, whereas the position of the boundary point VB on the high voltage side of the limit current region is generally due to deterioration of the sensor element. It does not change. As a result, as the sensor element 20 deteriorates, the width of the straight line portion parallel to the horizontal axis in the limiting current region becomes narrower.

  Here, when the target value Zactg is changed to be increased by the control according to the present embodiment, the DC resistance Ri increases as the target value Zactg increases. As a result, the voltage width of the limiting current region in the VI characteristic becomes narrow. In this case, there is a concern that the accuracy of air-fuel ratio detection will decrease.

  Therefore, in the present embodiment, the target value Zactg is changed to be larger while keeping the width of the limiting current region within a predetermined range. That is, by setting the width of the limit current region to a predetermined value or more, the target value Zactg is increased and the increase in the element temperature is suppressed while ensuring the width of the limit current region in which the air-fuel ratio can be detected. In setting the target value Zactg, there is a trade-off relationship between the element temperature of the sensor element 20 and the accuracy of air-fuel ratio detection. In the present embodiment, the target value Zactg in element resistance feedback control is taken into consideration while considering the balance between the two. Set.

  Here, the setting of the target value Zactg will be described in detail. 6 and 7 are graphs showing the relationship between the element temperature on the horizontal axis and the logarithmic value of the element resistance value on the vertical axis. In FIG. 6, initial characteristics and deterioration characteristics are shown for the total resistance (DC resistance Ri) including the electrolyte resistance and the electrode reaction resistance and the electrolyte resistance (impedance Zac). In this case, in the deteriorated state, the logarithmic value of each resistance increases as compared with the initial state. In addition, since the activation energy required for activation in the sensor element 20 does not change regardless of the presence or absence of deterioration, the slope of the logarithmic value of each resistance with respect to the element temperature is the same before and after the deterioration. In FIG. 6, the logarithmic value of each resistance is represented as a linear function with respect to the element temperature. When the element resistance feedback control is performed with the target resistance value kept constant, the element temperature shifts from the initial target temperature T0 to T1 due to deterioration.

  Further, if the DC resistance Ri becomes excessively large, the slope of the resistance dominant region in the VI characteristic becomes too small, and the limit current region becomes excessively narrow. Therefore, there is concern that the accuracy of detecting the air-fuel ratio may be adversely affected. Therefore, in FIG. 6, the deterioration allowable value is set as the upper limit of deterioration allowable for all resistances, and further, the deterioration allowable value of the electrolyte resistance is set based on the deterioration allowable value of all resistances.

  Then, in the present embodiment, the element temperature is kept constant even if the target value Zactg is increased in the correlation between the element temperature and the logarithmic value of the element resistance value, and the target value is set according to the degree of deterioration of the sensor element 20. Change to the side that increases Zactg. That is, in the present embodiment, the target value Zactg is set using the logarithmic value of the element resistance value as the degree of deterioration of the sensor element 20. A procedure for setting the target value Zactg will be described with reference to FIG. In FIG. 7, as an initial state, the correlation between the element temperature and the logarithmic value of the element resistance value is represented by the characteristic line L0, and on the characteristic line L0, Zac0 corresponding to the target temperature T0 is set as the initial target value Zactg. Has been. Then, when the sensor element 20 deteriorates, the characteristic line shifts from L0 to L1. In this case, if feedback control is performed with the target value Zactg kept at Zac0, the element temperature rises from the initial target temperature T0 to T1, but in the present embodiment, a new target value Zactg is set on the characteristic line L1. I am going to do it. Specifically, the element temperature is maintained at the target temperature T0, and then the target value Zactg is changed from Zac0 to Zac1.

  In FIG. 7, “ZacH” is a value corresponding to the deterioration allowable value of the electrolyte resistance, and the element resistance value is limited to ZacH or less, so that the limiting current region in the VI characteristic is within a predetermined range, The limit current region is suppressed from becoming too narrow. In this case, since the element resistance value is ZacH or less, it is determined that the deterioration degree of the sensor element 20 is less than or equal to a predetermined level. Then, in a state where the element resistance value is equal to or lower than ZacH, the increase of the element temperature is suppressed and the target value Zactg is changed to the increasing side in multiple stages.

  Specifically, the heater energization amount (for example, the duty value) in the state where the feedback control is performed is acquired, and the target value Zactg is kept constant, and the heater energization amount becomes larger than the predetermined value each time. The value Zactg is changed to be increased based on the degree of deterioration of the sensor element 20. In this case, the fact that the heater energization amount becomes larger than the predetermined value means that the element temperature gradually rises as the element deteriorates while Zactg is kept constant, and the rise in the element temperature is suppressed. Therefore, Zactg is updated to a larger value. This corresponds to the process of changing the target value Zactg to the larger side based on the degree of deterioration of the sensor element 20.

  Further, in the present embodiment, if the element resistance value is equal to or lower than ZacH, that is, if the degree of deterioration of the sensor element 20 is within the predetermined level, the target value Zactg is changed to the larger side, while the element resistance value is higher than ZacH. The target value Zactg is maintained at a predetermined upper limit if the deterioration degree exceeds a predetermined level. That is, the heater control is performed while suppressing the increase in the element temperature until the element resistance value reaches ZacH, and the heater control is performed while allowing the increase in the element temperature when the element resistance value reaches ZacH.

  Next, the setting process of the target value Zactg will be described with reference to the flowchart of FIG. This processing is repeatedly executed by the microcomputer 40 at a predetermined cycle.

  In FIG. 8, first, in step S11, it is determined whether the engine is stopped. If step S11 is YES, it will progress to step S12, and if NO, this process will be complete | finished as it is. In step S12, the impedance Zac which is the element resistance value is calculated. When calculating the impedance Zac, a well-known impedance detection method can be used. Specifically, the applied voltage VP of the sensor element 20 is temporarily changed at a predetermined AC frequency (for example, 5 kHz), and the current value I that changes in response thereto is detected. Then, the impedance Zac is calculated based on the current value I.

  Then, in step S13, for example, the energization amount (duty value) of the heater 29 is acquired as a parameter indicating the temperature rise of the sensor element 20. The duty value of the heater 29 is calculated from the state in which the energization amount of the heater 29 is stable under the element resistance feedback control. FIG. 9 shows the relationship between the duty value of the heater 29 and the DC resistance Ri. In FIG. 9, the duty value increases as the DC resistance Ri increases. This is because the element temperature rises as the sensor element 20 deteriorates, and as a result, the energization amount of the heater 29 increases. That is, the duty value of the heater 29 is a parameter indicating the temperature rise of the sensor element 20.

  In step S14, it is determined whether the duty value is larger than the predetermined threshold value Dth. The threshold value Dth may be any value as long as the temperature rise of the sensor element 20 can be determined, and is set, for example, based on the reference duty value under the condition where the feedback control is performed at the current target value Zactg. Then, if step S14 is NO, it is determined that it is not necessary to change the target value Zactg, and the process proceeds to step S15. In step S15, the target value Zactg is maintained as it is at the present setting, and this processing is ended.

  If step S14 is YES, it will progress to step S16 and it will be determined whether the element temperature has reached the predetermined upper limit temperature. That is, in the case where the target value Zactg is kept constant under the condition that the element resistance feedback control is executed and the element temperature is allowed to rise further, the duty value gradually increases as the element deteriorates, The element temperature rises accordingly. Assuming this, in step S16, it is determined whether the duty value is smaller than a predetermined upper limit threshold Dh corresponding to the element upper limit temperature. If step S16 is NO, the process proceeds to step S22. In step S22, it is determined that the sensor element 20 is abnormal, and a predetermined diagnostic process is executed. For example, the diagnosis data is saved, the diagnosis lamp is turned on, and the alarm device is activated.

  On the other hand, if step S16 is YES, the process proceeds to step S17. In step S17, a new target value Zactg in element resistance feedback control is calculated based on the degree of deterioration of the sensor element 20. As shown in the description of FIG. 7, when calculating the target value Zactg, the correlation between the logarithmic value of the element resistance value and the element temperature is used. Here, according to the degree of deterioration, the target value Zactg is increased with the element temperature kept constant, and Zacx is calculated as a new target value Zactg.

  Succeedingly, in a step S18, it is determined whether or not the deterioration degree of the sensor element 20 is equal to or lower than a predetermined level. Specifically, it is determined whether or not the calculated new target value Zactg is smaller than the electrolyte resistance deterioration allowable value ZacH. If Zacx is smaller than the allowable deterioration value ZacH (step S18: YES), the process proceeds to step S19. In step S19, Zacx is set to a new target value Zactg. In such a case, the element resistance feedback control in the next drive cycle is performed with Zacx as the target value Zactg.

  On the other hand, if step S18 is NO, the process proceeds to step S21, and the target value Zactg is set to the deterioration allowable value ZacH. That is, when the degree of deterioration of the sensor element 20 exceeds a predetermined level, the target value Zactg is maintained at the deterioration allowable value ZacH to ensure the detection accuracy in air-fuel ratio detection. The deterioration allowable value ZacH is determined based on the deterioration allowable value of the total resistance determined by the voltage width of the limit current region.

  Succeedingly, in a step S20, the applied voltage VP in the limit current region is corrected in accordance with the change amount of the target value Zactg. Specifically, the applied voltage VP is corrected to the high voltage side in accordance with the shift of the boundary point on the low voltage side of the limit current region to the high voltage direction. Here, the change amount ΔZac of the target value Zactg and the correction amount ΔVP have a correlation as shown in FIG. 10, for example. From FIG. 10, the larger the change amount ΔZac, the larger the correction amount ΔVP is calculated. It should be noted that the corrected voltage value may be set to be an intermediate point between the boundary point on the low voltage side and the boundary point on the high voltage side after the shift of the limit current region.

  FIG. 11 is an explanatory diagram for explaining setting of the target value Zactg based on the degree of deterioration of the sensor element 20. In FIG. 11, the characteristic line L0 represents the correlation between the element temperature and the logarithmic value of the element resistance value in the initial state of the sensor element 20. The characteristic line L0 shifts in the order of L0 → L1 → L2 → L3 → L4 as the deterioration of the sensor element 20 progresses. FIG. 11 illustrates a configuration in which the target value Zactg is changed while maintaining the target temperature T0.

  First, in the initial state of the sensor element 20, element resistance feedback control is performed with Zac0 corresponding to the target temperature T0 as the target value Zactg. After that, when the state of the characteristic line L1 is reached, the target value Zactg is changed to Zac1 of P1 at which the element temperature remains the target temperature T0 on the characteristic line L1. Further, when the state of the characteristic line L2 is reached, the target value Zactg is changed to Zac2 of P2 at which the element temperature remains the target temperature T0 on the characteristic line L2. The change to Zac1 and the change to Zac2 are performed based on the heater duty value becoming larger than the threshold value Dth. Both Zac1 and Zac2 are smaller than the deterioration allowable value ZacH.

  After that, when the sensor element 20 further deteriorates and enters the state of the characteristic line L3, Zac3A of P3A becomes a value larger than the deterioration allowable value ZacH. In this case, the target value Zactg is changed to ZacH which is the deterioration allowable value. As a result, the element temperature of the sensor element 20 rises from T0 to T3. Then, in the state of the characteristic line L4 that has further deteriorated, the target value Zactg is maintained at ZacH. In such a case, the element temperature of the sensor element 20 rises from T3 to T4.

  FIG. 12 shows the relationship between the element temperature of the sensor element 20 and the time in element resistance feedback control. As shown in FIG. 12, in the conventional control, the element temperature rises remarkably with the passage of time, whereas in the control of the present embodiment, the temperature rise of the element temperature can be suppressed.

  According to this embodiment described in detail above, the following excellent effects can be obtained.

  In the limiting current type A / F sensor 10, it is considered that the element resistance value increases with the deterioration of the sensor element 20, and under the situation where the element resistance value is increasing, the target value Zactg of the element resistance value is increased. When the feedback control is performed while keeping the temperature constant, there is a concern that the element temperature will rise. Further, in a situation where the element resistance value is increasing, the slope of the resistance governing region in the VI characteristic of the A / F sensor 10 becomes small, so that the limiting current region may be narrowed.

  In consideration of this point, when the degree of deterioration of the sensor element 20 is equal to or lower than a predetermined level, the target value Zactg is increased based on the degree of deterioration of the sensor element 20 while keeping the width of the limiting current region within a predetermined range. I changed it to. In such a case, the target value Zactg is changed while ensuring the width of the limiting current region, so that it is possible to detect the oxygen concentration using the limiting current region. On the other hand, by changing the target value Zactg of the element resistance value to the larger side, it is possible to suppress the temperature rise of the sensor element 20 that occurs when the sensor element 20 deteriorates under feedback control. As a result, the temperature rise of the sensor element 20 can be suppressed while ensuring the accuracy of detecting the oxygen concentration.

  Specifically, based on the correlation between the element resistance value and the element temperature, the target value Zactg is changed to be larger so that the element temperature is constant or within a predetermined range. Thereby, the temperature rise of the sensor element 20 can be suppressed as much as possible.

  The target value Zactg of the sensor element 20 is set to the target value Zactg each time the heater energization amount is acquired while the element resistance feedback control is performed, and the heater energization amount becomes larger than the predetermined value under the condition where the target value Zactg is constant. The configuration is changed to a larger side based on the degree of deterioration. In this case, by monitoring the rise of the element temperature by the heater energization amount, the target value Zactg can be appropriately updated while preventing the element temperature from excessively rising.

  Further, if the degree of deterioration of the sensor element 20 is below a predetermined level, the target value Zactg is changed to a larger side, and if it exceeds the predetermined level, the target value Zactg is maintained at the deterioration allowable value ZacH. Even if the degree of deterioration exceeds the predetermined level, the width of the limiting current region is secured by maintaining the predetermined upper limit value as the target value. As a result, the accuracy of detecting the oxygen concentration can be ensured.

  Further, an abnormality determination unit is provided to determine whether the deterioration limit has been reached based on the further deterioration of the sensor element 20. In this case, the further use of the A / F sensor 10 can be prevented under the condition that the deterioration has progressed seriously. As a result, feedback control and the like can be performed using the sensor element 20 in an appropriate state.

  The logarithmic value of the impedance Zac has a predetermined correlation with the element temperature, and can be expressed as a linear function with respect to the element temperature, for example. Considering this point, the logarithmic value of the impedance Zac is obtained, and the target value Zactg is set based on the correlation between the logarithmic value and the element temperature. In this case, the relationship between the element resistance value and the element temperature can be easily obtained according to the degree of deterioration of the sensor element 20 each time. This makes it possible to properly obtain the target value Zactg of the element resistance value for making the element temperature a predetermined temperature.

  By changing the target value Zactg to be larger, the voltage width of the limiting current region in the VI characteristic becomes narrower. In order to accurately detect the oxygen concentration of exhaust gas, it is necessary to stably apply a voltage in the limit current region. In consideration of this point, when the target value Zactg is set to be increased, the value of the applied voltage is increased according to the change amount. In such a case, the voltage value applied to the sensor element 20 can be corrected according to the fluctuation of the voltage width in the limiting current region, and the oxygen concentration can be detected accurately.

(Second embodiment)
Hereinafter, the second embodiment will be described focusing on the differences from the first embodiment. In the present embodiment, it is determined that the degree of deterioration of the sensor element 20 has reached a predetermined intermediate level with a degree of deterioration smaller than a predetermined level, and it is determined that the degree of deterioration of the sensor element 20 has reached the intermediate level. In addition, the gain of the feedback control is increased to raise the assumed temperature of the element temperature.

  The setting process of the target value Zactg in this embodiment will be described with reference to the flowchart of FIG. This process is replaced with the process of FIG. 8 and repeatedly executed by the microcomputer 40 at a predetermined cycle. The same steps as those in FIG. 8 are designated by the same step numbers, and the description will be simplified.

  In FIG. 13, through steps S11 to S16, and in step S17, Zacx is calculated based on the degree of deterioration of the sensor element 20. Then, in a succeeding step S31, it is determined whether or not Zacx indicating the degree of deterioration of the sensor element 20 is smaller than an intermediate value ZacM which is set as a value smaller than the allowable deterioration value ZacH of the electrolyte resistance. At this time, it is determined whether the degree of deterioration of the sensor element 20 has reached a predetermined intermediate level. If Zacx is smaller than the intermediate value ZacM (step S31: YES), the process proceeds to step S19. In step S19, Zacx at that time is set to a new target value Zactg.

  If Zacx is greater than or equal to the intermediate value ZacM (step S31: NO), the process proceeds to step S32. In step S32, it is determined whether or not the process is the first time since Zacx ≧ ZacM. If YES, the process proceeds to step S33, and the feedback gain is changed to be increased. At this time, for example, the proportional term gain is increased. By increasing the feedback gain, even if the element resistance deviation is the same, the heater duty value increases, and the element temperature is expected to rise accordingly. The temperature rise here is preferably about 20 to 50 ° C., for example.

  Then, in step S18, it is determined whether Zacx is smaller than the electrolyte resistance deterioration allowable value ZacH. At this time, it is determined whether the degree of deterioration of the sensor element 20 is below a predetermined level. If Zacx is smaller than ZacH (step S18: YES), the process proceeds to step S19. In step S19, Zacx at that time is set to a new target value Zactg.

  If NO in step S18, the process proceeds to step S21, and the target value Zactg is set to the allowable deterioration value ZacH. After that, in step S20, the applied voltage VP in the limit current region is corrected in accordance with the change amount of the target value Zactg.

  FIG. 14 is an explanatory diagram for explaining the setting of the target value Zactg based on the degree of deterioration of the sensor element 20. In FIG. 14, similarly to FIG. 11 described above, the characteristic line L0 represents the correlation between the element temperature and the logarithmic value of the element resistance value in the initial state of the sensor element 20, and the characteristic line L0 indicates deterioration of the sensor element 20. Accordingly, it is assumed that the shift is L0 → L1 → L2 → L3 → L4.

  In FIG. 14, when the characteristic line shifts from L0 → L1 → L2 along with the deterioration of the sensor element 20 and then exceeds the intermediate value ZacM to reach the state of the characteristic line L3, the assumed element temperature changes from T0 to T5. Be changed. In this case, the element resistance feedback control is continued after the element temperature is set to T5 by changing the feedback gain. Then, the target value Zactg is updated according to further deterioration of the sensor element 20, that is, as the characteristic line shifts from L3 to L4. The target value Zactg is executed in multiple stages until Zacx reaches ZacH.

  As described above, in the present embodiment, the intermediate value ZacM and the allowable deterioration value ZacH are used as the determination threshold values for determining the deterioration degree of the sensor element 20, and the feedback gain is set when the deterioration degree of the sensor element 20 reaches the intermediate value ZacM. The assumed temperature of the element temperature is increased by increasing the temperature. In this case, the element resistance value can be appropriately controlled while limiting the increase in the element temperature. Therefore, similarly to the above, it is possible to ensure the detection accuracy of the oxygen concentration.

  The present invention is not limited to the above embodiment and may be implemented as follows, for example.

  In the above embodiment, when the target value Zactg is increased, the target value Zactg is calculated so that the element temperature becomes constant. In this respect, the target value Zactg may be calculated so that the element temperature rises within a predetermined range. For example, the target value Zactg may be calculated so that the temperature rise of the element temperature and the logarithmic value of the element resistance value have a predetermined correlation. FIG. 15 shows the transition of the target value Zactg in such a case.

  In FIG. 15, similar to FIG. 11, characteristic lines L0 to L4 according to the degree of deterioration of the sensor element 20 are shown. Note that, unlike FIG. 11 described above, in FIG. 13, the target value Zactg is increased while increasing the element temperature within a predetermined value as the deterioration progresses. In the initial state of the sensor element 20, element resistance feedback control is performed with Zac0 as the target value Zactg. After that, when the state of the characteristic line L1 is reached, the target value Zactg is changed to Zac1x which is on the characteristic line L1 and becomes T1x in which the element temperature has risen by a predetermined temperature from the target temperature T0. In such a case, the element temperature is changed along the characteristic line LX formed by, for example, a linear function, and rises from T0 to T1x. Further, when the state of the characteristic line L2 is reached, the target value Zactg is changed to Zac2x which is on the characteristic line L2 and becomes T2x in which the element temperature has risen by a predetermined temperature from T1x. In such a case, the element temperature rises from T1x to T2x. After that, when the state of the characteristic line L3 is reached, the target value Zactg is changed to ZacH which is the deterioration allowable value. Along with this, the element temperature rises from T2x to T3x. Then, in the state of the characteristic line L4, the target value Zactg is maintained at ZacH. According to this configuration, it is possible to allow the temperature of the sensor element 20 to rise, and to reduce the increase range of the temperature rise as compared with the conventional constant control. Thereby, the temperature rise of the sensor element 20 can be suppressed.

  In the above embodiment, the target value Zactg is set when the engine is stopped. However, the setting is not particularly limited as long as the degree of deterioration of the sensor element 20 can be determined. For example, the setting may be such that this process is performed immediately after the engine is started. According to this configuration, when the target value Zactg is newly set, the target value Zactg is immediately used for the element resistance feedback control during the drive cycle.

  In the above embodiment, the target value Zactg is calculated by using the correlation between the logarithmic value of the element resistance value and the element temperature. In this respect, there is no particular limitation as long as the target value Zactg can be calculated based on the degree of deterioration of the sensor element 20. The target value Zactg is acquired using, for example, a map preset according to the duty value of the heater 29.

  In the second embodiment described above, a plurality of intermediate levels for determining the degree of deterioration of the sensor element 20 may be set. In this case, when the degree of deterioration of the sensor element 20 reaches the intermediate level 1, the feedback gain is increased in anticipation of an increase in the element temperature, and thereafter, when the degree of deterioration of the sensor element 20 reaches the intermediate level 2, the feedback gain is increased again. It is advisable to increase the feedback gain in anticipation of an increase in element temperature.

  The gas sensor may be a so-called two-cell structure gas sensor including an electromotive force cell and a pump cell, in addition to the A / F sensor 10 of the above embodiment. Further, it may be a NOx sensor that detects the NOx concentration in the exhaust gas. Further, as the sensor element 20, it is possible to use a cup type structure other than the laminated type structure.

  10 ... A / F sensor, 20 ... Sensor element, 21 ... Solid electrolyte layer, 25, 26 ... Electrode, 29 ... Heater, 40 ... Microcomputer.

Claims (8)

It is applied to a gas sensor (10) provided with a limiting current type sensor element (20) having a solid electrolyte layer (21) and a pair of electrodes (25, 26), and a heater (29) for heating the sensor element,
While applying voltage to the pair of electrodes, while outputting a limiting current signal according to the oxygen concentration in the exhaust gas of the internal combustion engine, the element resistance value of the sensor element and the element temperature which is the temperature of the sensor element, A gas sensor control device (40) for performing element temperature control in the sensor element by feedback controlling the element resistance value to a target value using the correlation of
A deterioration determination unit that determines that the degree of deterioration of the sensor element calculated based on the element resistance value is below a predetermined level,
When it is determined that the degree of deterioration of the sensor element is less than or equal to the predetermined level, the target value, the target setting unit that changes to the side to increase based on the degree of deterioration of the sensor element,
Equipped with
In the sensor element, the boundary voltage on the high voltage side of the limit current area, which is the voltage area in which the limit current signal can be output, is unchanged regardless of the deterioration of the sensor element, and the low voltage side of the limit current area. The boundary voltage of the sensor element changes to a high voltage side as the sensor element deteriorates, the width of the limiting current region decreases as the sensor element deteriorates, and the degree of deterioration of the sensor element is equal to or less than the predetermined level. Due to this, the width of the limiting current region becomes a predetermined value or more,
The target setting unit, when the degree of deterioration of the sensor element is determined to the a predetermined level or less, before Symbol the target value, the gas sensor control unit to change to the side to increase based on the deterioration degree of the sensor element . The gas sensor control according to claim 1, wherein the target setting unit maintains the target value at a predetermined upper limit value corresponding to the predetermined level when it is determined that the degree of deterioration of the sensor element exceeds the predetermined level. apparatus. Under the condition that the target value is maintained at the upper limit value by the target setting unit, an abnormality that determines that the sensor element has reached the deterioration limit based on the further progress of the deterioration degree of the sensor element The gas sensor control device according to claim 2 , further comprising a determination unit. The target setting unit, when changing the target value, in the correlation between the element resistance value and the element temperature, the element temperature is constant even if the target value is increased, or the element temperature rises. The gas sensor control device according to any one of claims 1 to 3, wherein the target value is changed to a side in which the target value is increased so as to be within a predetermined range. An energization amount acquisition unit that acquires the energization amount of the heater in the state of performing the feedback control is provided,
Claim wherein the target setting unit, each time the energization amount in a state that the target value constant is larger than a predetermined value, that the target value is changed to the side to increase based on the deterioration degree of the sensor element 5. The gas sensor control device according to any one of 1 to 4 . A second determination unit that determines that the degree of deterioration of the sensor element has reached a predetermined intermediate level having a smaller degree of deterioration than the predetermined level;
When it is determined that the degree of deterioration of the sensor element has reached the intermediate level, a gain changing unit that increases the gain of the feedback control in order to increase the assumed temperature of the element temperature,
The gas sensor control device according to any one of claims 1 to 5, further comprising: Along with applying an alternating voltage of a predetermined frequency to the pair of electrodes, as the element resistance value, a resistance calculation unit that calculates an electrolyte resistance value that is a resistance value of the solid electrolyte layer,
A logarithmic value of the electrolyte resistance value, a deterioration degree calculation unit that calculates as the deterioration degree,
Equipped with
The gas sensor control device according to claim 1, wherein the target setting unit sets the target value based on a correlation between the logarithmic value and the element temperature.   When the target setting unit sets the target value to a larger side, a voltage correction unit that corrects the voltage applied to the sensor element to a larger side according to the change amount of the target value is provided. The gas sensor control device according to any one of 1 to 7.

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