JP4893652B2 - Gas sensor control device - Google Patents

Description

  The present invention relates to a gas sensor control device that is connected to a gas sensor such as a NOx sensor and detects the concentration of a specific component from the output of the sensor.

  In recent years, exhaust gas regulations and fuel efficiency regulations have been increasingly strengthened, and for example, a technique related to NOx emission reduction of a diesel engine, a technique related to abnormality detection of a NOx purification device, and the like are required. Also, even in gasoline engines, NOx emissions increase with the expansion of combustion in the lean region for the purpose of improving fuel efficiency, etc. Technology for suppressing NOx emissions, technology for detecting abnormalities in NOx purification devices, etc. Is required. Under these circumstances, demand for NOx sensors is increasing. As the NOx sensor, a multi-cell sensor element using a zirconia solid electrolyte is used.

  By the way, in gas sensors including the above-mentioned NOx sensor, it is necessary to detect that the function of the gas sensor is normal, and detection items include disconnection determination of sensor elements. In an exhaust sensor for automobiles, disconnection determination may be defined by laws and regulations. As an abnormality detection technique for a gas sensor, for example, an A / F sensor is detected, and sensor disconnection is detected based on element impedance. Specifically, abnormality detection is performed based on the amount of current change when the applied voltage is swept and changed at the time of impedance detection, or the impedance value calculated at that time. By performing abnormality detection based on the element impedance, it is possible to determine whether the sensor output is normal or abnormal even if the sensor output is “0”. In other words, when air-fuel ratio feedback control with the stoichiometric air-fuel ratio (stoichiometric) as a target is performed, the sensor output is maintained at almost “0”, but even in such a case, an abnormality such as disconnection can be detected. It was.

  However, in a circuit that detects a weak current such as a NOx detection signal, for example, when the weak current detection and the impedance detection are realized by the same circuit, the NOx detection accuracy may be lowered. That is, the current level at the time of impedance detection is in the order of mA, whereas the current level of the NOx detection signal is in the order of nA, and the current level differs by about 4 to 5 digits. Therefore, it is difficult to detect both NOx current detection and impedance detection with high accuracy, resulting in a decrease in detection accuracy of NOx concentration.

Further, a technique has been proposed in which a switch is provided on a current path through which a NOx detection current flows in a detection circuit, and the switch is opened and closed (see, for example, Patent Document 1). If such a technique is used, the NOx detection circuit and the impedance detection circuit can be appropriately switched, and a signal having a current level as required can be taken out.
JP 2005-326388 A

  However, in the configuration in which the switch is provided on the current path through which the NOx detection current flows in the detection circuit as described above, there is a concern that the NOx current detection may be affected due to the switch. That is, for example, when a semiconductor switching element is used for the switch, a leakage current (leakage current) is generated in the switching element, which is a few tens of nA. For this reason, there is a possibility that a measurement error may occur when a weak current is measured as in NOx concentration detection, and there is room for improvement.

  The main object of the present invention is to provide a gas sensor control device capable of appropriately determining an abnormality such as an activation failure or disconnection while suppressing adverse effects on gas concentration detection accuracy.

  Hereinafter, means for solving the above-described problems and the effects thereof will be described.

  A gas sensor control device according to the present invention includes a sensor element having a solid electrolyte body and a pair of electrodes disposed on the solid electrolyte body, and a specific component in a gas to be detected in a voltage application state between the pair of electrodes. It is connected to a gas sensor that generates an element current corresponding to the concentration. In this control apparatus, the element current is measured by the current measurement resistor, and the measurement result of the element current by the current measurement resistor is output as the element current measurement value by the output circuit. Further, the concentration of the specific component (oxygen concentration, NOx concentration, etc.) is calculated from the measured element current value output from the output circuit in the state of voltage application by the applied voltage setting circuit.

In the first invention, the switch means (switch circuit 71 in FIG. 4) is provided on the path where the element current does not flow in the electrical path that electrically connects the sensor side terminal of the current measuring resistor and the applied voltage setting circuit. The potential difference between both ends of the current measuring resistor is made zero by closing the switch means, and the electromotive force of the sensor element is detected in this state. Then, abnormality determination of the sensor element or the sensor circuit is performed based on the detected electromotive force. The switch means may be temporarily closed during the gas concentration detection based on the measured element current value to detect the electromotive force.

  According to the above configuration, by setting the potential difference between both ends of the current measurement resistor to zero, it is possible to create a state where no element current flows, that is, a state where the element current is approximately 0 nA, and it is possible to detect the electromotive force of the sensor element suitably. At this time, if an abnormality such as breakage or activation failure occurs in the sensor element or an abnormality such as disconnection occurs in the sensor circuit, the sensor electromotive force is not an appropriate value. Abnormality judgment becomes possible.

  In the present invention, since the switch means is provided on the path through which the element current does not flow, an error occurs in the measured element current due to the leakage current caused by the switch means, specifically, the semiconductor switching element such as a transistor. Such inconveniences can be avoided. In particular, when measuring a weak element current such as a NOx detection current, if an error occurs in the current measurement value due to the presence of the switch means, the influence on the gas concentration detection becomes large, but such inconvenience is caused. Can be avoided.

  As described above, according to the present invention, it is possible to properly determine abnormality such as sensor disconnection while suppressing adverse effects on gas concentration detection accuracy.

  The state where the potential difference between both ends of the current measuring resistor is “zero” corresponds to a state where the current flowing through the sensor element is 0 nA or approximately 0 nA. In this case, in an actual circuit configuration, a minute current may flow due to the presence of various circuit elements and the like, and it may be considered that the state of “element current = 0 nA” is not strictly met. Even when a current flows, it corresponds to a state in which the potential difference between both ends of the current measurement resistor is “zero”.

As a configuration for reducing the potential difference between both ends of the current measurement resistor, the sensor side terminal voltage of the current measurement resistor is fed back to the applied voltage setting circuit with the switch means closed as in the second aspect of the invention. Thus, the setting voltage of the setting circuit may be the same voltage as the sensor-side terminal voltage.

As a sensor electromotive force detection method, as in the third invention, the electromotive force of the sensor element may be detected by the sensor-side terminal voltage of the current measurement resistor in a state where the switch means is closed. Alternatively, as in the fourth invention, the voltage at both the positive and negative electrodes of the sensor element may be measured with the switch means closed, and the electromotive force of the sensor element may be detected from the difference between the measured voltage values.

In any of the third and fourth inventions , the sensor electromotive force can be suitably detected. However, the electromotive force of the sensor element can be detected more accurately by detecting the electromotive force of the sensor element based on the difference between the voltage measurement values at the positive and negative electrodes of the sensor element as in the fourth invention .

In the fifth invention, the sensor side terminal of the current measurement resistor is connected to the reference potential portion (for example, ground) via the bias resistor. In short, the sensor electromotive force is not generated in an abnormality occurrence state such as disconnection, and the circuit output becomes indefinite. In this regard, according to the above configuration, even when no sensor electromotive force is generated, the sensor-side terminal voltage of the current measurement resistor is held at a predetermined voltage by the bias resistor. Therefore, the circuit output is stable even in the state where no electromotive force is generated, and the sensor electromotive force as an abnormal value can be detected.

In the sixth aspect of the invention, the first feedback path (feedback input path L1 in FIG. 4) for feeding back the output of the output circuit to the applied voltage setting circuit and the voltage at the sensor side terminal of the current measuring resistor are fed back to the applied voltage setting circuit. A second feedback path (feedback input path L2 in FIG. 4) to be input is provided, and switch means is provided in the second feedback path of the two feedback paths. At the time of normal concentration detection, only the first feedback path is made conductive among the two feedback paths, and the applied voltage setting circuit is set according to the output of the output circuit fed back via the first feedback path. Voltage setting. Further, at the time of detecting the electromotive force, only the second feedback path of the two feedback paths is made conductive, and the applied voltage is determined according to the sensor-side terminal voltage of the current measurement resistor that is fed back via the second feedback path. The voltage is set by the setting circuit, and the potential difference between both ends of the current measuring resistor is set to zero.

  According to the above configuration, by appropriately switching the feedback path to the applied voltage setting circuit side, the gas concentration detection can be temporarily interrupted to detect the sensor electromotive force.

In the seventh invention, a voltage follower or non-inverting amplifier circuit is provided in an electrical path that electrically connects the sensor side terminal of the current measuring resistor and the applied voltage setting circuit, and the voltage follower or non-inverting amplifier circuit and the applied voltage are provided. Switch means is provided on the path to the setting circuit. In such a case, the element current does not flow on the output side of the voltage follower or the non-inverting amplifier circuit. Therefore, a path in which the element current does not flow is provided in the electrical path between the sensor side terminal of the current measurement resistor and the applied voltage setting circuit. Can be provided. And by providing the switch means in this path, it is possible to suppress the adverse effect on the device current by the switch means.

In the eighth invention, the applied voltage setting circuit includes an operational amplifier having a negative feedback section, and a current measurement resistor is provided outside the negative feedback section on the output side of the operational amplifier. According to the above configuration, the element current can be measured by measuring the voltage of at least the non-sensor side terminal of the current measurement resistor. In addition, since the output terminal voltage of the operational amplifier, that is, the voltage of the anti-sensor side terminal of the current measurement resistor can be controlled, the anti-sensor side terminal voltage can be increased or decreased with respect to the sensor side terminal voltage. It is possible to control the potential difference between both ends of the current measurement resistor. Therefore, the potential difference between both ends of the current measuring resistor can be made zero.

According to a ninth aspect of the invention, there is provided terminal voltage measuring means for measuring the voltage of the terminal portion connected to each electrode of the sensor element. And in addition to the abnormality determination by a sensor electromotive force, abnormality determination of a sensor element or a sensor circuit is implemented based on each terminal voltage. As a result, in addition to abnormalities such as breakage, activation failure, and disconnection of the sensor element, it is possible to detect abnormalities in the power supply short and ground short on each electrode side of the sensor element.

According to a tenth aspect of the invention, there is provided voltage application stopping means for stopping voltage application to the sensor element when it is determined that an abnormality has occurred. Thereby, the bad influence to the sensor element by continuing voltage application at the time of abnormality can be suppressed, and the sensor element can be aimed at by extension.

As in the eleventh aspect, electromotive force detection may be performed on condition that the sensor element is in an active state. That is, for example, at the time of starting the sensor, the sensor electromotive force is properly detected after the sensor element rises to a predetermined activation temperature and completes the activation. ADVANTAGE OF THE INVENTION According to this invention, the detection failure of the electromotive force resulting from a sensor element being inactive (element temperature is low) can be suppressed. Thereby, the accuracy of abnormality detection can be improved.

The gas sensor control device of the present invention is suitably applied to the following gas sensors (a twelfth aspect of the present invention) . That is, the gas sensor includes a sensor element having a first cell and a second cell each composed of a solid electrolyte body and a pair of electrodes arranged on the solid electrolyte body, and oxygen in the gas to be detected introduced into the gas chamber. The amount is adjusted to a predetermined concentration level in the first cell, and the concentration (NOx concentration, etc.) of the specific component is detected from the gas after the oxygen amount adjustment in the first cell in the second cell. In the gas sensor control device, the element current generated in the second cell is measured by a current measuring resistor. In this case, the specific component measured in the second cell is a concentration of NOx, HC, etc. other than oxygen, and the device current for detecting the concentration is weak. For example, the device current when detecting the NOx concentration is on the order of nA (nanoampere). In this regard, according to each of the above-described characteristic configurations, the gas concentration can be suitably detected even with a weak element current.

In the thirteenth invention, the electromotive force detection may be performed on the condition that the oxygen concentration in the gas chamber becomes a low oxygen level which is a predetermined concentration level. That is, for example, when the sensor is started, the sensor electromotive force is properly detected after surplus oxygen in the gas chamber is sufficiently discharged by the first cell. According to the present invention, it is possible to suppress an electromotive force detection failure caused by the presence of excessive oxygen in the gas chamber (too much excess oxygen). Thereby, the accuracy of abnormality detection can be improved.

It is considered that the residual oxygen concentration in the gas chamber increases and decreases, and the electromotive force in the second cell changes according to the residual oxygen concentration. Therefore, in the fourteenth aspect , the residual oxygen concentration in the gas chamber is detected, and the abnormality determination value is variably set according to the detected residual oxygen concentration in the gas chamber. And abnormality determination of a sensor element or a sensor circuit is implemented based on the abnormality determination value and each detected electromotive force. Thereby, even if the residual oxygen concentration in the gas chamber is increased or decreased, a highly accurate abnormality determination can be realized.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, an embodiment of the invention will be described with reference to the drawings. In the present embodiment, a NOx concentration detection system that uses a NOx sensor provided in an exhaust pipe of an in-vehicle engine and detects the NOx concentration in the exhaust based on the output of the NOx sensor will be described. The on-board engine is, for example, a diesel engine. An NOx purification catalyst (such as a NOx storage reduction catalyst or an ammonia selective reduction catalyst) serving as an exhaust purification device provided in the exhaust pipe of the engine is abnormal based on the output of the NOx sensor. Diagnosis etc. are to be implemented. For example, a NOx sensor is provided downstream of the NOx purification catalyst, and the NOx purification catalyst is abnormal when the NOx concentration (NOx purification rate) calculated from the output of the NOx sensor exceeds a predetermined abnormality determination value. Diagnosed.

  First, the sensor element 10 constituting the NOx sensor will be described with reference to FIG. The sensor element 10 has a so-called laminated structure, and its internal structure is shown in FIG. The horizontal direction in the figure corresponds to the longitudinal direction of the sensor element 10. The right side of the figure is the element base end side (exhaust pipe attachment site side), and the left side of the figure is the element tip side. The sensor element 10 has a so-called three-cell structure including a pump cell, a sensor cell, and a monitor cell, and each cell is laminated and configured. Since the monitor cell has a function of discharging oxygen in the gas like the pump cell, the monitor cell may be referred to as an auxiliary pump cell or a second pump cell.

  In the sensor element 10, the solid electrolyte bodies 11 and 12 made of an oxygen ion conductive material such as zirconia are formed in a sheet shape, and are stacked at a predetermined interval above and below the figure via spacers 13 made of an insulating material such as alumina. ing. Among these, an exhaust inlet 11a is formed in the upper solid electrolyte body 11 in the figure, and exhaust around the sensor element is introduced into the first chamber 14 through the exhaust inlet 11a. The first chamber 14 communicates with the second chamber 16 via the throttle unit 15. On the upper surface of the solid electrolyte body 11 in the figure, a porous diffusion layer 17 for taking in and out the exhaust gas with a predetermined diffusion resistance is provided, and an insulating layer 19 for defining an air passage 18 is provided.

  Further, an insulating layer 21 made of alumina or the like is provided on the lower surface of the solid electrolyte body 12 in the figure, and an atmospheric passage 22 is formed by the insulating layer 21. A heater (heating element) 23 is embedded in the insulating layer 21 for heating the entire sensor. In this case, the pump 23, the monitor cell 34, and the sensor cell 35 are heated by the heater 23, and activation of each of these cells 31, 34, 35 is promoted. The heater 23 generates thermal energy by power supply from a battery power source (not shown).

  The solid electrolyte body 12 on the lower side of the figure is provided with a pump cell 31 so as to face the first chamber 14. The pump cell 31 takes in and out oxygen in the exhaust gas introduced into the first chamber 14. The residual oxygen concentration in the chamber 14 is adjusted to a predetermined concentration. The pump cell 31 has a pair of upper and lower electrodes 32 and 33 provided with the solid electrolyte body 12 sandwiched therebetween, and in particular, the electrode 32 on the first chamber 14 side is a NOx inert electrode (an electrode that is difficult to decompose NOx). Yes. The pump cell 31 decomposes oxygen present in the first chamber 14 in a state where a voltage is applied between the electrodes 32 and 33 and discharges it from the electrode 33 to the atmosphere passage 22 side.

  Further, a monitor cell 34 and a sensor cell 35 are provided in the upper solid electrolyte body 11 in the drawing so as to face the second chamber 16. After the surplus oxygen is discharged by the pump cell 31 described above, the monitor cell 34 generates a current output in accordance with the electromotive force or voltage application according to the residual oxygen concentration in the second chamber 16. The sensor cell 35 detects the NOx concentration from the gas in the second chamber 16.

  The monitor cell 34 and the sensor cell 35 are arranged side by side at positions close to each other, and have a configuration in which electrodes 36 and 37 are provided on the second chamber 16 side and a common electrode 38 is provided on the atmosphere passage 18 side. That is, the monitor cell 34 is constituted by the solid electrolyte body 11, the electrode 36 and the common electrode 38 disposed so as to face each other, and the sensor cell 35 is similarly configured to face the solid electrolyte body 11 and the electrode 37 disposed so as to face it. And the common electrode 38. The electrode 36 (electrode on the second chamber 16 side) of the monitor cell 34 is made of a noble metal such as Au—Pt that is inactive to NOx, whereas the electrode 37 (electrode on the second chamber 16 side) of the sensor cell 35 is active for NOx. It consists of noble metals such as platinum Pt and rhodium Rh. For convenience, in the drawing, the monitor cell 34 and the sensor cell 35 are shown side by side with respect to the flow direction of the exhaust gas. However, in actuality, these cells 34 and 35 are arranged at the same position with respect to the flow direction of the exhaust gas. It is like that.

  Here, the pump cell 31, the monitor cell 34, and the sensor cell 35 are provided side by side in the longitudinal direction of the sensor element 10, and the pump cell 31 is provided on the distal end side of the sensor element 10, and the base end side (exhaust pipe attachment side) ) Are provided with a monitor cell 34 and a sensor cell 35.

  In the sensor element 10 configured as described above, the exhaust is introduced into the first chamber 14 through the porous diffusion layer 17 and the exhaust introduction port 11a. When this exhaust gas passes through the vicinity of the pump cell 31, a decomposition reaction occurs by applying a pump cell applied voltage Vp between the pump cell electrodes 32 and 33, and the exhaust cell passes through the pump cell 31 according to the oxygen concentration in the first chamber 14. Oxygen is taken in and out. At this time, since the electrode 32 on the first chamber 14 side is a NOx inactive electrode, NOx in the exhaust gas is not decomposed in the pump cell 31, but only oxygen is decomposed and discharged from the electrode 33 to the atmospheric passage 22. By the action of the pump cell 31, the inside of the first chamber 14 is maintained in a predetermined low oxygen concentration state.

  The gas that has passed through the vicinity of the pump cell 31 (the gas after the oxygen concentration adjustment) flows into the second chamber 16, and the monitor cell 34 generates an output corresponding to the residual oxygen concentration in the gas. The output of the monitor cell 34 is detected as a monitor cell current Im when a predetermined monitor cell application voltage Vm is applied between the monitor cell electrodes 36 and 38. Further, when a predetermined sensor cell applied voltage Vs is applied between the sensor cell electrodes 37 and 38, NOx in the gas is reduced and decomposed, and oxygen generated at that time is discharged from the electrode 38 to the atmosphere passage 18. At this time, the NOx concentration in the exhaust gas is detected by the current (sensor cell current Is) flowing through the sensor cell 35.

  The NOx sensor circuit 40 has a microcomputer 41 and a control circuit unit (details will be described later with reference to FIG. 2). The microcomputer 41 and the control circuit unit connect the electrodes 32 and 33 of the pump cell 31 to each other. The pump cell voltage Vp applied to the monitor cell 34, the monitor cell voltage Vm applied between the electrodes 36 and 38 of the monitor cell 34, and the sensor cell voltage Vs applied between the electrodes 37 and 38 of the sensor cell 35 are controlled. The microcomputer 41 sequentially receives the measured values of the pump cell current Ip, the monitor cell current Im, and the sensor cell current Is, and the microcomputer 41 calculates the oxygen concentration and NOx concentration in the exhaust based on these measured values.

  FIG. 2 is a block diagram showing an outline of the NOx sensor circuit 40. The NOx sensor circuit 40 includes a heater drive circuit unit in addition to the illustrated circuit units, but is not illustrated in FIG.

  In FIG. 2, the NOx sensor circuit 40 includes a positive terminal PS + and a negative terminal PS− connected to the electrodes 32 and 33 of the pump cell 31, and a common terminal connected to the common electrode 38 of the monitor cell 34 and the sensor cell 35. COM + and negative terminals MS− and SS− connected to the electrodes 36 and 37 of the monitor cell 34 and the sensor cell 35, respectively, are provided.

  A pump cell drive circuit unit 42 that variably sets a pump cell applied voltage to be applied to the pump cell 31 is connected to the positive terminal PS + of the pump cell 31, and an Ip detection circuit that detects the pump cell current Ip is connected to the negative terminal PS−. The unit 43 is connected. In the pump cell drive circuit unit 42, the pump cell applied voltage is controlled according to the pump cell current Ip detected by the Ip detection circuit unit 43. The pump cell current Ip detected by the Ip detection circuit unit 43 is sequentially input to the microcomputer 41.

  A sensor cell / monitor cell driving circuit 44 for applying a common voltage to each of the cells 34 and 35 is connected to the positive common terminal COM + of the sensor cell 35 and the monitor cell 34, and the negative side of each of the cells 35 and 34 is connected. Connected to the terminals SS- and MS- are an Is detection circuit unit 45 for detecting the sensor cell current Is and an Im detection circuit unit 46 for detecting the monitor cell current Im. A microcomputer 41 is connected to the Is detection circuit unit 45 and the Im detection circuit unit 46, and current measurement values VS1 and VM1 measured by the detection circuit units 45 and 46 in accordance with the sensor cell current Is and the monitor cell current Im are obtained. It is sequentially input to the microcomputer 41. In the sensor cell / monitor cell drive circuit unit 44, the Is detection circuit unit 45, and the Im detection circuit unit 46, the terminal voltages at the respective terminals COM +, SS−, and MS− are measured, and the terminal voltages (Vcom, VS2, and VM2). Are sequentially input to the microcomputer 41. Details will be described later.

  In addition, the sensor cell / monitor cell drive circuit unit 44 is connected to a sensor cell / monitor cell protection circuit unit 48 for stopping the application of voltage to each of the cells in order to protect the monitor cell 34 and the sensor cell 35 when an abnormality occurs.

  Hereinafter, the details of each circuit unit constituting the NOx sensor circuit 40 will be described. However, in the present embodiment, since the circuit configuration relating to the pump cell 31 is not different from the existing one, the description of the pump cell drive circuit unit 42 and the Ip detection circuit unit 43 is omitted.

  FIG. 3 is a circuit configuration diagram of the sensor cell / monitor cell drive circuit unit 44. In FIG. 3, a resistance voltage dividing circuit 51 composed of two resistors is connected to the constant voltage source (constant voltage Vcc), and the divided voltage VX1 of the resistance voltage dividing circuit 51 is input to the + input terminal of the operational amplifier 52. . A common terminal COM + is connected to an output terminal of the operational amplifier 52 via a switch circuit 53 and a protective resistor 54. A protective resistor 55 is provided in the negative feedback section of the operational amplifier 52. A capacitor 56 for ESD (electrostatic discharge) is connected to the common terminal COM +.

  In addition, a voltage follower 58 is connected to the point A1 in the figure having the same voltage as the common terminal COM + via a protective resistor 57. In the drive circuit unit 44, the voltage of the common terminal COM + is used as the common terminal voltage Vcom. Is output.

  The switch circuit 53 is configured to be turned ON / OFF (open / close) based on a voltage application stop signal SG1 input from a sensor cell / monitor cell protection circuit unit 48 described later. To the switch circuit 53. In this configuration, when SG1 = low (voltage application is permitted), the switch circuit 53 is closed, and the divided voltage VX1 of the resistance voltage dividing circuit 51 is applied to the common terminal COM +. Further, when SG1 = high (when voltage application is stopped), the switch circuit 53 is opened, and the application of the divided voltage VX1 to the common terminal COM + is cut off.

  Next, the configuration of the Is detection circuit unit 45 will be described with reference to FIG. In FIG. 4, a current measuring resistor 61 and a differential amplifier circuit 62 are connected in series to the negative terminal SS− of the sensor cell 35. In this case, in particular, the current measuring resistor 61 is provided on the output side of the operational amplifier constituting the differential amplifier circuit 62 and outside the negative feedback section (outside the feedback system). A resistance voltage dividing circuit 63 that divides the constant voltage Vcc by two resistors is connected to the + input terminal of the differential amplifier circuit 62, and a feedback input path L1 is connected to the − input terminal.

  A voltage follower 65 is connected to the B1 point (sensor side terminal of the current measurement resistor 61) on the negative terminal SS− side of the both ends (B1 point and B2 point) of the current measurement resistor 61 via the protective resistor 64. The output terminal of the voltage follower 65 is connected to the + input terminal of the differential amplifier circuit 66. Further, the point B2 (the non-sensor side terminal of the current measuring resistor 61) is connected to the negative input terminal of the differential amplifier circuit 66. Therefore, when the sensor cell current Is flows through the current measuring resistor 61, a potential difference is generated at both ends (points B1 and B2) of the current measuring resistor 61 according to the sensor cell current Is, and the potential difference is predetermined by the differential amplifier circuit 66. And then output as a sensor cell current measurement value VS1.

  The sensor cell current measurement value VS1 that is the output of the differential amplifier circuit 66 is input to the negative input terminal of the differential amplifier circuit 62 through the feedback input path L1. Specifically, the output terminal of the differential amplifier circuit 66 as an “output circuit” and the − input terminal of the differential amplifier circuit 62 as an “applied voltage setting circuit” are connected by a feedback input path L1. In the middle of the feedback input path L1, a switch circuit 67 for interrupting (opening and closing) the path L1 and an LPF (low pass filter) 68 for removing noise including a resistor and a capacitor are provided. Normally, the switch circuit 67 is closed, and the sensor cell current measurement value VS1 which is the output of the differential amplifier circuit 66 is fed back to the differential amplifier circuit 62. Note that the switch circuit 67 is configured by, for example, a semiconductor switching element such as a transistor (the same applies to each switch circuit described later).

  The output voltage of the voltage follower 65 is the same as the voltage at point B1 (that is, the voltage at the negative terminal SS− of the sensor cell 35), and the output voltage is output as the sensor cell terminal voltage VS2.

  The output terminal of the voltage follower 65 and the + input terminal of the differential amplifier circuit 62 are connected by a feedback input path L2, and the path L2 is intermittently opened / closed in the middle of the feedback input path L2. A switch circuit 71 is provided. Normally, the switch circuit 71 is open. When the switch circuit 71 is closed, the sensor cell terminal voltage VS2 that is the output of the voltage follower 65 is fed back to the differential amplifier circuit 62. Here, since the voltage follower 65 has a large input impedance and no element current flows on the output side thereof, the feedback input path L2 can be a path through which no element current flows. A switch circuit 71 is provided on this path.

  The switch circuits 67 and 71 provided in the feedback input paths L1 and L2, respectively, are configured to be turned ON / OFF (open / close) based on a high / low binary circuit switching signal SG2 input from the microcomputer 41. The circuit switching signal SG2 is input to one switch circuit 67 as it is and also input to the other switch circuit 71 via the inverting circuit 72. In the present embodiment, when SG2 = high, the switch circuit 67 is closed, the switch circuit 71 is opened, and only the feedback input path L1 of the two feedback input paths L1 and L2 is brought into conduction. Further, when SG2 = low, the switch circuit 67 is opened and the switch circuit 71 is closed, and only the feedback input path L2 of the two feedback input paths L1 and L2 is brought into conduction. In short, the switch circuits 67 and 71 are opened and closed in a manner in which the opening and closing timing is reversed, so that only one of the feedback input paths L1 and L2 is made conductive.

  When detecting the NOx concentration at normal time, that is, when measuring the sensor cell current Is flowing according to the NOx concentration in the exhaust, a high signal is output from the microcomputer 41 as the circuit switching signal SG2, and the output of the differential amplifier circuit 66 is output. VS1 is input to the negative input terminal of the differential amplifier circuit 62 via the feedback input path L1. Then, the output of the differential amplifier circuit 62 increases or decreases according to the output VS1 of the differential amplifier circuit 66. At this time, the output VS1 increases as the sensor cell current Is increases, and the output of the differential amplifier circuit 62 decreases accordingly.

  On the other hand, when the potential difference between both ends of the current measuring resistor 61 is set to zero and the current flowing through the current measuring resistor 61 is set to 0 nA, a low signal is output from the microcomputer 41 as the circuit switching signal SG2, and the voltage follower is output. The output VS2 of 65 is input to the + input terminal of the differential amplifier circuit 62 via the feedback input path L2. At this time, according to the differential amplifier circuit 62, the voltage at the counter-sensor side terminal (point B2) of the current measurement resistor 61 is adjusted to the same voltage as the sensor-side terminal (point B1) of the current measurement resistor 61. . As a result, the potential difference between both ends of the current measuring resistor 61 becomes zero, and the current does not flow through the current measuring resistor 61 (current = 0 nA). In such a case, the state in which no current flows through the current measuring resistor 61 corresponds to a state in which the NOx concentration = 0 ppm, and the output VS1 of the differential amplifier circuit 66 is originally a predetermined value corresponding to the residual oxygen concentration in the chamber, but is temporarily offset. If there is an error, the output value is shifted by the error. Therefore, the offset error can be obtained from the output.

  Further, when a low signal is output as the circuit switching signal SG2 from the microcomputer 41, no current flows through the current measuring resistor 61. Therefore, the negative terminal SS- of the sensor cell 35 corresponds to the sensor cell electromotive force. Voltage is generated and measured as the sensor cell terminal voltage VS2.

  Of both ends (points B1 and B2) of the current measuring resistor 61, a bias current resistor 75 and an ESD compatible capacitor 76 are connected to the point B1. That is, one end of the bias current resistor 75 and the ESD-compatible capacitor 76 is connected to the sensor side terminal of the current measuring resistor 61, and the other end is grounded. The resistance value of the bias current resistor 75 is, for example, 1 MΩ or more.

  Here, since the bias current resistor 75 is connected to the B1 point (the sensor side terminal of the current measuring resistor 61), the sensor cell electromotive force is generated as described above in a state where an abnormality such as disconnection or element cracking occurs. In the measurement, the sensor cell terminal voltage VS2 can be a fixed voltage. In other words, a value corresponding to the electromotive force abnormality can be acquired as the sensor cell terminal voltage VS2. That is, no electromotive force is generated in the sensor cell 35 in an abnormal state such as disconnection or cracking of the element, and the sensor cell terminal voltage VS2 (point B1 voltage in the figure) becomes indefinite. However, according to the above configuration in which the bias current resistor 75 is provided. For example, even when the sensor electromotive force is not generated, the sensor cell terminal voltage VS2 can be held at a predetermined voltage (a voltage corresponding to the resistance value of the bias current resistor 75). Therefore, even in such a state where no electromotive force is generated, the sensor cell terminal voltage VS2 is stable, and the sensor electromotive force as an abnormal value can be detected.

  In the present embodiment, the low potential side of the bias current resistor 75 is connected to the ground. However, the present invention is not limited to this, and a configuration in which the bias current resistor 75 is connected to another reference potential portion having a fixed potential may be employed. For example, a configuration in which one end of the bias current resistor 75 is connected to a power supply circuit, or a configuration in which the bias current resistor 75 is connected to a circuit unit that outputs a predetermined voltage within the range of the ground voltage to the power supply voltage may be employed.

  When the bias current resistor 75 is provided as described above, since a current flows through the bias current resistor 75, the current flowing through the current measuring resistor 61 may be reduced accordingly. Therefore, the current flowing through the bias current resistor 75 may be measured in advance, and the measured current may be corrected.

  The Im detection circuit unit 46 has a circuit configuration similar to that of the Is detection circuit unit 45, and the description and detailed description thereof are omitted because the description is duplicated. That is, the circuit of FIG. 4 is used as it is as the Im detection circuit unit 46. As shown in FIG. 2, a monitor cell circuit switching signal SG3 is output from the microcomputer 41 to the Im detection circuit unit 46, and the state of residual oxygen concentration detection in the normal state is detected by the circuit switching signal SG3. The state where the potential difference between both ends of the current measurement resistor is zero (the state where the current = 0 nA) is switched (similar to the circuit switching signal SG2 described above). Further, the Im detection circuit unit 46 outputs the monitor cell current measurement value VM1 instead of the sensor cell current measurement value VS1 of FIG. 4, and also outputs the monitor cell terminal voltage VM2 instead of the sensor cell terminal voltage VS2. Yes. In a state where the potential difference across the current measuring resistor is zero, the monitor cell electromotive force is measured by the monitor cell terminal voltage VM2.

  In the microcomputer 41 shown in FIG. 2, the sensor cell current measurement value VS1 output from the Is detection circuit unit 45 and the monitor cell current measurement value VM1 output from the Im detection circuit unit 46 are input, and based on these input values ( Is-Im) value is calculated. Based on the (Is-Im) value, the NOx concentration in the exhaust gas is calculated.

  Next, the configuration of the sensor cell / monitor cell protection circuit unit 48 will be described with reference to FIG. In this sensor cell / monitor cell protection circuit unit 48, for example, a power supply short circuit abnormality in circuit parts (circuit parts connected to the positive common terminal COM +, negative terminal SS−, MS−) on both the positive and negative sides of the sensor cell 35 and the monitor cell 34. Or a ground short fault is detected. In the present embodiment, the sensor cell / monitor cell protection circuit unit 48 corresponds to “voltage application stopping means”.

  In FIG. 5, the protection circuit section 48 includes a common terminal voltage Vcom output from the sensor cell / monitor cell drive circuit section 44, a sensor cell terminal voltage VS2 output from the Is detection circuit section 45, and an Im detection circuit section 46. The monitor cell terminal voltage VM2 to be output is input. In addition, an abnormality determination signal SG4 is input from the microcomputer 41 to the protection circuit unit 48. The abnormality determination signal SG4 will be described in detail later. Briefly, the abnormality determination signal SG4 is a binary signal in which SG4 = high when normal and SG4 = low when an abnormality occurs. Then, the sensor cell / monitor cell protection circuit unit 48 generates a voltage application stop signal SG1 based on these input signals, and outputs the signal SG1 to the sensor cell / monitor cell drive circuit unit 44. Details thereof will be described below.

  The sensor cell / monitor cell protection circuit unit 48 includes five comparison circuits 81 to 85. The operation of each of the comparison circuits 81 to 85 is as follows.

  The first comparison circuit 81 compares the common terminal voltage Vcom (normally 4.4 V) with a reference voltage Vref1 (for example, 4.6 V). In this case, Vcom <Vref1 is normal and the output of the first comparison circuit 81 is low, but when Vcom> Vref1 is abnormal, the output of the first comparison circuit 81 becomes high. For example, when a power supply short circuit occurs at a portion connected to the common terminal COM +, the output of the first comparison circuit 81 becomes high.

  The second comparison circuit 82 compares the sensor cell terminal voltage VS2 (normally 4.0V) with a reference voltage Vref2 (eg, 3.8V). In this case, when normal, VS2> Vref2 and the output of the second comparison circuit 82 is low, but when abnormal, VS2 <Vref2, the output of the second comparison circuit 82 becomes high. For example, when a ground short occurs at a portion connected to the negative terminal SS− of the sensor cell 35, the output of the second comparison circuit 82 becomes high.

  The third comparison circuit 83 compares the monitor cell terminal voltage VM2 (normally 4.0V) with a reference voltage Vref3 (for example, 3.8V). In this case, when normal, VM2> Vref3 and the output of the third comparison circuit 83 is low, but when abnormal, VM2 <Vref3, the output of the third comparison circuit 83 becomes high. For example, when a ground short occurs at a portion connected to the negative terminal MS− of the monitor cell 34, the output of the third comparison circuit 83 becomes high.

  The fourth comparison circuit 84 compares the common terminal voltage Vcom with the sensor cell terminal voltage VS2. In this case, Vcom> VS2 is normal and the output of the fourth comparison circuit 84 is low, but when Vcom <VS2 is abnormal, the output of the fourth comparison circuit 84 becomes high. For example, when a ground short occurs at a portion connected to the common terminal COM +, or when a power short occurs at a portion connected to the negative terminal SS− of the sensor cell 35, the output of the fourth comparison circuit 84 becomes high. Become.

  The fifth comparison circuit 85 compares the common terminal voltage Vcom with the monitor cell terminal voltage VM2. In this case, Vcom> VM2 is normal and the output of the fifth comparison circuit 85 is low, but when Vcom <VM2 is abnormal, the output of the fifth comparison circuit 85 becomes high. For example, when a ground short circuit occurs at a part connected to the common terminal COM +, or when a power supply short circuit occurs at a part connected to the negative terminal MS− of the monitor cell 34, the output of the fifth comparison circuit 85 becomes high. Become.

  Although not shown, the reference voltages Vref1 to Vref3 are all generated by a resistance voltage dividing circuit that divides the constant voltage Vcc by two resistors.

  The outputs of the five comparison circuits 81 to 85 and the abnormality determination signal SG4 from the microcomputer 41 are input to the OR circuit 86. In this case, if any of the plurality of inputs of the OR circuit 86 is high, a high signal is output as the voltage application stop signal SG1. If SG1 = high, the switch circuit 53 is opened in the sensor cell / monitor cell drive circuit unit 44 as described above, and voltage application to the common terminal COM + is cut off (see FIG. 3). That is, when an abnormality such as a power supply short circuit or a ground short circuit occurs in the sensor cell 35 and the monitor cell 34, or when a high level abnormality determination signal SG4 is output from the microcomputer 41, the voltage to the sensor cell 35 and the monitor cell 34 The application is stopped to protect each of these cells. More specifically, by preventing overcurrent to the sensor cell 35 and the monitor cell 34, it is possible to suppress breakage of the sensor element and the like.

  Next, sensor output correction value calculation processing executed by the microcomputer 41 and abnormality detection processing based on sensor electromotive force will be described. In the sensor output correction value calculation process, the potential difference between both ends of the current measurement resistor in the Is detection circuit unit 45 and the Im detection circuit unit 46 is temporarily set to zero during the NOx concentration detection, and the output correction value is determined by the circuit output in that state. (In particular, in this embodiment, an offset correction value) is calculated. In addition, the abnormality detection process is based on the electromotive force value of the sensor cell 35 or the monitor cell 34 obtained by temporarily setting the potential difference between both ends of the current measuring resistor to zero as described above, and thus disconnection, element cracking, element activation failure, etc. The presence or absence of abnormalities is detected.

  First, sensor output correction value calculation processing will be described with reference to the flowchart of FIG. The process shown in FIG. 6 is repeatedly executed by the microcomputer 41 at a predetermined time period. Here, a procedure for calculating an offset correction value in the output value (VS1) of the Is detection circuit unit 45 will be described.

  In FIG. 6, in step S <b> 11, it is determined whether it is now the timing for calculating the offset correction value. In this embodiment, the calculation cycle of the offset correction value is 10 seconds, and step S11 is affirmed every time 10 seconds elapse. The calculation cycle of the offset correction value is desirably set according to, for example, the speed at which the circuit temperature changes. If it is the calculation timing of the offset correction value, the process proceeds to step S12 to determine whether or not the sensor cell 35 has been heated to a predetermined activation temperature (for example, 750 ° C.). Specifically, the temperature rise state of the sensor cell 35 is determined based on the elapsed time from the start of the engine, the heater input power, or the detected impedance value in the sensor cell 35.

  If the sensor cell 35 has been heated to a predetermined activation temperature, the process proceeds to step S13, and the circuit switching signal SG2 output to the Is detection circuit unit 45 is switched from high to low. Thereby, in the Is detection circuit unit 45, the conduction switching of the feedback input paths L1 and L2 to the differential amplifier circuit 62 (here, switching from L1 to L2) is performed, and accordingly, the current flowing through the current measurement resistor 61 is changed. Intentionally set to 0 nA. In the subsequent step S14, standby processing is performed to wait for output stabilization after the circuit switching signal SG2 is switched from high to low.

  Then, after waiting for a predetermined time by the standby process, in step S15, the output VS1 of the differential amplifier circuit 66 is read, and the offset correction value Foff is calculated from the VS1 value. In the present embodiment, the VS1 value at that time is converted into a current to obtain an offset correction value Foff, and the offset correction value Foff is stored in a backup device (for example, EEPROM or backup RAM). In other words, the offset correction value Foff is stored in the backup device as a learning value and is appropriately updated.

  Thereafter, in step S16, the circuit switching signal SG2 is switched from low to high. As a result, the feedback input path to the differential amplifier circuit 62 is returned to L1, and accordingly, the Is detection circuit unit 45 is returned to the normal NOx concentration detection state. In the subsequent step S17, standby processing is performed to wait for output stabilization after the circuit switching signal SG2 is switched from low to high. Then, after waiting for a predetermined time by the standby process, normal NOx concentration detection is resumed (step S18).

  The offset correction value Foff calculated as described above is appropriately used for correcting the sensor cell current Is (current converted value of VS1) that is sequentially measured when NOx concentration is detected. That is, the corrected sensor cell current is calculated by subtracting the offset correction value Foff from the sensor cell current Is measured at the time of detecting the NOx concentration (corrected sensor cell current = Is−Foff), and the NOx concentration is calculated based on the corrected sensor cell current. Calculated.

  Actually, the offset correction value is calculated not only for the Is detection circuit unit 45 but also for the Im detection circuit unit 46, and the NOx concentration is calculated using both offset correction values in the two detection circuit units 45 and 46. Calculation is performed. In this case, the sensor cell offset correction value is subtracted from the sensor cell current Is (measured value) to calculate a corrected sensor cell current, and the monitor cell offset correction value is subtracted from the monitor cell current Im (measured value) to be corrected. The post-monitor cell current is calculated, and the NOx concentration is calculated based on the difference between the corrected sensor cell current and the corrected monitor cell current (= corrected sensor cell current−corrected monitor cell current).

  Here, as shown in FIG. 7, in the NOx sensor circuit 40, an offset error occurs for each of the sensor cell current Is, the monitor cell Im, and (Is−Im). In the figure, “sensor output” is a current value actually generated in the sensor element 10, and “circuit detection value” is the NOx sensor circuit 40 (Is detection circuit unit 45, Im detection circuit) with respect to the actual sensor output. Measurement value measured by the unit 46).

  In such a case, the offset error with respect to the sensor output is obtained as an offset correction value, and the sensor cell current Is and the monitor cell current Im are corrected using the offset correction value, thereby calculating the NOx concentration caused by the offset error of the circuit detection value. Decrease in accuracy can be suppressed.

  FIG. 8 is a flowchart showing an abnormality detection process based on the sensor cell electromotive force. This process is repeatedly executed by the microcomputer 41 at a predetermined time period.

  In FIG. 8, in step S <b> 21, it is determined whether or not it is an abnormality detection timing now. In the present embodiment, the abnormality detection cycle is set to 0.5 seconds, and step S21 is affirmed every time 0.5 seconds elapse. If it is an abnormality detection timing, the process proceeds to step S22, and it is determined whether or not the sensor cell 35 has been heated to a predetermined activation temperature (for example, 750 ° C.) (same as step S12). In step S23, it is determined whether or not the oxygen in the chambers 14 and 16 of the sensor element 10 has been sufficiently exhausted and the residual oxygen concentration has reached a predetermined low oxygen level after engine startup. For example, the state of residual oxygen discharge is determined based on the elapsed time from the start of the engine.

  When both steps S22 and S23 are affirmed, the process proceeds to step S24, and the circuit switching signal SG2 output to the Is detection circuit unit 45 is switched from high to low. Thereby, in the Is detection circuit unit 45, the conduction switching of the feedback input paths L1 and L2 to the differential amplifier circuit 62 (here, switching from L1 to L2) is performed, and accordingly, the current flowing through the current measurement resistor 61 is changed. Intentionally set to 0 nA. In the subsequent step S25, standby processing is performed to wait for output stabilization after the circuit switching signal SG2 is switched from high to low.

  Then, after waiting for a predetermined time by the standby process, in step S26, the common terminal voltage Vcom and the sensor cell terminal voltage VS2 are read, and the electromotive force value of the sensor cell 35 is detected based on the Vom value and the VS2 value. Specifically, the electromotive force value of the sensor cell 35 is calculated by subtracting the sensor cell terminal voltage VS2 (the measured electromotive force value of the sensor cell negative terminal) from the common terminal voltage Vcom (the measured electromotive force value of the sensor cell positive terminal). To do. At this time, the electromotive force value of the sensor cell 35 is stored in a backup device (for example, EEPROM or backup RAM).

  Thereafter, in step S27, it is determined whether or not the electromotive force value detected in step S26 is within a predetermined normal range. Specifically, the inside of the chamber of the sensor element 10 is basically in a weak lean state, and the electromotive force of the sensor cell 35 has a voltage value of about 0.2V. Therefore, the range of 0.2V ± 0.1V (range of 0.1-0.3V) is the normal range. However, considering that the normal sensor cell applied voltage is 0.4V (= 4.4V-4.0V), the normal range may be 0.1 to 0.4V.

  If the electromotive force value is within the normal range, the process proceeds to step S28, and normality determination is performed assuming that no abnormality such as disconnection or element cracking has occurred. If the electromotive force value is not within the normal range, the process proceeds to step S29, where it is determined whether or not an electromotive force abnormality has occurred continuously a predetermined number of times. If the electromotive force abnormality has continuously occurred a predetermined number of times, the process proceeds to step S30, and abnormality determination is performed on the assumption that abnormality such as disconnection or element cracking has occurred.

  When it is determined that an abnormality such as disconnection or element cracking has occurred, a high signal is output as the abnormality determination signal SG4 to the sensor cell / monitor cell protection circuit unit 48 in step S31.

  Thereafter, in step S32, the circuit switching signal SG2 is switched from low to high. As a result, the feedback input path to the differential amplifier circuit 62 is returned to L1, and accordingly, the Is detection circuit unit 45 is returned to the normal NOx concentration detection state. In the subsequent step S33, standby processing is performed to wait for output stabilization after the circuit switching signal SG2 is switched from low to high. Then, after waiting for a predetermined time by the standby process, normal NOx concentration detection is resumed (step S34).

  Although not shown, the abnormality detection process based on the monitor cell electromotive force is similarly performed for the monitor cell 34. The procedure follows the procedure of FIG. Briefly, in the Im detection circuit unit 46, the potential difference between both ends of the current measuring resistor is set to zero, and the monitor cell electromotive force is detected by the monitor cell terminal voltage VM2 in this state. And abnormality determination is implemented by whether the monitor cell electromotive force is in the normal range (the range of 0.1-0.3V, or the range of 0.1-0.4V). Thereby, an abnormality such as disconnection or element crack is detected in the monitor cell 34.

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

  In the Is detection circuit unit 45 (or Im detection circuit unit 46), a switch circuit 71 is provided on a path through which no element current (sensor cell current, monitor cell current) flows, and the electromotive force of the sensor cell 35 is generated with the switch circuit 71 closed. Since the abnormality is detected and the abnormality is determined based on the electromotive force, the occurrence of the abnormality can be suitably detected when an abnormality such as element cracking, activation failure, or disconnection occurs.

  In this case, in particular, since the switch circuit 71 is provided on the path through which the element current does not flow (in other words, the switch circuit is not provided on the path through which the element current flows), the switch circuit 71 It is possible to avoid the inconvenience that an error occurs in the measured element current due to the leakage current, specifically, the leakage current due to the semiconductor switching element such as a transistor. That is, even if a leakage current is generated in the switch circuit 71, there is no influence on the element current measurement (even if an influence occurs, it is extremely small). When measuring a weak NOx detection current as in this embodiment, if an error occurs in the current measurement value due to the presence of the switch circuit, the effect on the NOx concentration detection will be large. Can be avoided.

  With the switch circuit 71 closed, both positive and negative terminal voltages (common terminal voltage Vcom and sensor cell terminal voltage VS2) of the sensor cell 35 are measured, and the electromotive force of the sensor cell 35 is detected based on the difference between these voltages (monitor cell) 34 is the same). Thereby, an electromotive force can be detected accurately. However, it is possible to detect the electromotive force only by the sensor cell terminal voltage VS2.

  In the Is detection circuit unit 45 (or the Im detection circuit unit 46), switch circuits 67 and 71 are provided in the two feedback input paths L1 and L2, respectively, and a switch circuit is selected depending on whether normal NOx concentration detection or electromotive force detection is performed. 67 and 71 are opened and closed, and the feedback input path to be in a conductive state is appropriately switched. Accordingly, by appropriately switching the feedback input path to the differential amplifier circuit 62, the NOx concentration detection can be temporarily interrupted to perform the electromotive force detection.

  Since the sensor side terminal of the current measurement resistor 61 is connected to the ground (reference potential portion) via the bias current resistor 75, the bias current resistor 75 causes the sensor side terminal of the current measurement resistor 61 even when no sensor electromotive force is generated. The terminal voltage can be maintained at a predetermined voltage. Therefore, the circuit output is stable even in the state where no electromotive force is generated, and the sensor electromotive force as an abnormal value can be detected.

  In the sensor cell / monitor cell protection circuit unit 48, the abnormality determination is performed based on the common terminal voltage Vcom, the sensor cell terminal voltage VS2, and the monitor cell terminal voltage VM2 that are the terminal voltages of the sensor cell 35 and the monitor cell 34 (actually, The configuration is such that the abnormality determination signal SG4 is output based on each terminal voltage). As a result, it is possible to detect abnormalities such as element cracks, activation failures, and disconnections based on sensor electromotive force, as well as abnormalities in power supply shorts and ground shorts on the electrode sides of sensor cell 35 and monitor cell 34 based on each terminal voltage. It will be possible.

  When it is determined that an abnormality such as disconnection has occurred, the abnormality determination signal SG4 is set to a high signal, and voltage application to the sensor cell 35 and the monitor cell 34 is stopped in the sensor cell / monitor cell driving circuit unit 44. Thereby, the bad influence to the sensor element by continuing the voltage application to each cell at the time of abnormality occurrence can be suppressed, and the protection of the sensor element can be achieved.

  In the NOx sensor circuit 40 that assumes that a weak current flows in the first place, an excessive current flows to the sensor element when various abnormalities (especially, a power supply short circuit or a ground short circuit at the terminal portion) occur, and the sensor element is destroyed. Adverse effects such as changes in output characteristics. In this regard, the sensor element can be protected by stopping the voltage application to each cell when an abnormality occurs as described above.

  In the abnormality detection process (FIG. 8), it is a condition that the sensor cell 35 (or the monitor cell 34) is in a temperature active state and oxygen in the chambers 14 and 16 of the sensor element 10 is sufficiently exhausted after the engine is started. Therefore, the sensor electromotive force can be properly detected. That is, it is possible to suppress electromotive force detection failure caused by the sensor element 10 being inactive (element temperature is low) and electromotive force detection failure caused by excessive excess oxygen in the chamber. Thereby, the accuracy of abnormality detection can be improved.

  Similarly, in the abnormality detection process (FIG. 8), a standby time is provided for waiting for output stabilization when switching the switch circuits 67 and 71, so that the sensor electromotive force can be detected in a stable state, and the abnormality detection accuracy can be improved. Can be increased. In the standby processing, instead of waiting for a predetermined time, the sensor electromotive force may wait until the amount of change (change rate) per time becomes equal to or less than a predetermined time.

  In the Is detection circuit unit 45 (or Im detection circuit unit 46), the offset correction value Foff is calculated from the output VS1 (or VM1) of the differential amplifier circuit 66 with the switch circuit 71 closed. In this configuration, when an offset error occurs in the NOx sensor circuit 40, an offset correction value Foff corresponding to the offset error can be suitably obtained. Further, since the switch circuit 71 is provided on the path through which the element current does not flow, it is possible to avoid the inconvenience that an error occurs in the element current measurement value due to the leakage current caused by the switch circuit 71 as described above.

  Since the offset correction value Foff can be suitably calculated as described above, and the adverse effect caused by the leakage current of the switch circuit can be eliminated, the detection accuracy of the NOx concentration can be improved. Further, even when an output error occurs in the NOx sensor circuit 40 due to temperature characteristics or a change with time, and further, the output error changes, the NOx concentration can be detected appropriately while appropriately eliminating the output characteristics.

  By closing the switch circuit 71, the potential difference between both ends of the current measuring resistor 61 is set to zero, and the offset correction value Foff is calculated from the output VS1 (or VM1) of the differential amplifier circuit 66 in a state where the potential difference is zero. Thereby, the offset correction value Foff can be suitably calculated from the output VS1 (or VM1) in the measurement state at the NOx concentration = 0 ppm.

  In addition, since the current measurement resistor 61 is provided outside the negative feedback section of the differential amplifier circuit 62, it becomes possible to control the output of the differential amplifier circuit 62 (the terminal voltage on the side opposite to the sensor of the current measurement resistor 61). The potential difference between both ends of the current measuring resistor 61 can be variably adjusted. Therefore, the potential difference between both ends of the current measuring resistor 61 can be made zero.

  In the configuration in which the common drive circuit unit 44 is connected to the positive electrodes of the sensor cell 35 and the monitor cell 34, and the Is detection circuit unit 45 and the Im detection circuit unit 46 are connected to the negative electrodes of the cells 35 and 34, respectively. A switch circuit 71 is provided in each of the Is detection circuit unit 45 and the Im detection circuit unit 46, and the offset correction values of the detection circuit units 45 and 46 are respectively determined by the current measurement values VS1 and VM1 acquired by the detection circuit units 45 and 46, respectively. It was set as the structure to calculate. Thereby, the characteristic variation (circuit error) of each detection circuit unit 45 and 46 can be calculated for each cell. Therefore, compared with the case where a switch circuit is provided in the sensor cell / monitor cell driving circuit unit 44 which is a common driving circuit unit for the cells 34 and 35, the accuracy of the calculated offset correction value can be increased.

  In the calculation process of the sensor output correction value (FIG. 6), the offset correction value Foff is calculated on condition that the sensor cell 35 (or the monitor cell 34) is in the temperature active state. The correction value Foff can be obtained with high accuracy.

  Similarly, in the sensor output correction value calculation process (FIG. 6), a waiting time for waiting for output stabilization is provided when switching the switch circuits 67 and 71, so that the sensor cell current measurement value VS1 is acquired in a state where the circuit output is stable. Therefore, the NOx concentration value and the offset correction value Foff can be obtained accurately. In the standby process, instead of waiting for a predetermined time, it may wait until the amount of change (change rate) of VS1 per time becomes equal to or less than a predetermined value.

(Other embodiments)
The present invention is not limited to the description of the above embodiment, and may be implemented as follows, for example.

  In the above embodiment, the “applied voltage setting circuit” is configured by the differential amplifier circuit 62 in the Is detection circuit unit 45 (see FIG. 4), but this is changed to make the “applied voltage setting circuit” a non-inverting amplifier circuit. It is also possible to configure by. The circuit configuration shown in FIG. 9 will be described focusing on the differences from FIG. Common components are given the same reference numerals. In FIG. 9, the configuration related to the voltage input is changed in accordance with the adoption of the non-inverting amplifier circuit as the applied voltage setting circuit.

  In FIG. 9, a non-inverting amplifier circuit 101 is provided as an applied voltage setting circuit. The negative input terminal of the non-inverting amplifier circuit 101 is connected to the sensor side terminal (point B1) of the current measuring resistor 61, and the voltage at the point B1 is held at the voltage of the positive input terminal of the non-inverting amplifier circuit 101. The non-inverting amplifier circuit 101 is connected to the + input terminal of the resistance voltage dividing circuit 63 through the switch circuit 102 and to the output terminal of the voltage follower 65 through the switch circuit 71. .

  The switch circuits 102 and 71 are configured to be turned ON / OFF (open / close) based on the circuit switching signal SG2 input from the microcomputer 41, and the circuit switching signal SG2 is input to one switch circuit 102 as it is. Then, the signal is input to the other switch circuit 71 through the inverting circuit 103. In the present embodiment, when SG2 = high, the switch circuit 102 is closed and the switch circuit 71 is opened, and the divided voltage VX3 of the resistance voltage dividing circuit 63 is input to the + input terminal of the non-inverting amplifier circuit 101. When SG2 = low, the switch circuit 102 is opened, the switch circuit 71 is closed, and the output of the voltage follower 65 is input to the + input terminal of the non-inverting amplifier circuit 101. In short, the switch circuits 102 and 71 are opened and closed in a manner in which the opening and closing timing is reversed, whereby the input voltage of the non-inverting amplifier circuit 101 is changed.

  In this configuration, when the NOx concentration is normally detected, the circuit switching signal SG2 is set to a high signal, and the voltage VX3 is applied to the negative terminal SS− of the sensor cell 35. Thereby, the sensor cell current Is according to the NOx concentration in the exhaust gas is measured. On the other hand, when the offset correction value is calculated, the circuit switching signal SG2 is a low signal, and the output VS2 of the voltage follower 65 is input to the + input terminal of the non-inverting amplifier circuit 101 via the feedback input path L2. The As a result, the potential difference between both ends of the current measuring resistor 61 becomes zero, and no current flows through the current measuring resistor 61 (current = 0 nA). Therefore, the offset correction value can be calculated from the sensor output VS1 at that time. Also, the sensor electromotive force can be detected by the sensor cell terminal voltage VS2.

  In the above embodiment, when various abnormalities such as disconnection occur, the sensor cell / monitor cell drive circuit unit 44 is configured to protect the sensor by stopping voltage application. Also good. Specifically, in the sensor cell / monitor cell drive circuit unit 44, the protective resistor 54 has a large resistance value (several hundred kΩ to 1 MΩ) to limit it with a predetermined upper limit current (for example, aging current), or the operational amplifier 52 The current output is limited. Thereby, for example, even if an abnormality such as a power supply short circuit or a ground short circuit occurs at the negative terminal of the sensor cell 35, the maximum current flowing in the cell is limited, and the sensor element can be protected. In this case, the cell applied voltage may be configured to be suppressed to an electric aging voltage or less for adjusting the sensor characteristics.

  In the above embodiment, the Is detection circuit unit 45 is configured such that no element current flows through the feedback input path L2, and the voltage is applied to the electrical path that electrically connects the sensor side terminal of the current measurement resistor 61 and the differential amplifier circuit 62. Although the follower 65 is provided, a non-inverting amplifier circuit may be provided instead of the voltage follower 65. That is, in this case, the switch circuit 71 is provided in the path (feedback input path L2) between the non-inverting amplifier circuit and the differential amplifier circuit 62.

  In the above embodiment, as described with reference to FIG. 2, the sensor cell current measurement value VS1 and the monitor cell current measurement value VM1 are input to the microcomputer 41, and the microcomputer 41 calculates the (Is-Im) value. It is also possible to change this as follows. That is, for example, an [Is-Im] calculation circuit unit configured by a differential amplifier circuit is provided, and the sensor cell current measurement value VS1 and Im detection output from the Is detection circuit unit 45 are provided in the [Is-Im] calculation circuit unit. The monitor cell current measurement value VM1 output from the circuit unit 46 is input. The calculation circuit unit calculates an (Is-Im) value and outputs the (Is-Im) value to the microcomputer 41.

  In the abnormality detection process, the abnormality determination value for detecting an abnormality based on the sensor electromotive force (normal range in step S27 in FIG. 8) is variably set according to the residual oxygen concentration in the chambers 14 and 16. Also good. Specifically, the residual oxygen concentration is detected by the pump cell current Ip, and the abnormality determination value is reduced as the residual oxygen concentration increases. Further, when the detection gas is a rich gas (when residual oxygen concentration = 0), the abnormality determination value is increased. Thereby, even if the residual oxygen concentration in the chamber is increased or decreased, a highly accurate abnormality determination can be realized. It is also possible to set an abnormality determination value based on the pump cell current Ip itself.

  -You may make it perform abnormality determination based on a sensor electromotive force on condition that a sensor cell current or a monitor cell current is lower than a regulation value. That is, when a disconnection abnormality is detected, if the sensor cell current or the monitor cell current is measured, it can be determined that no disconnection abnormality has occurred. Further, if the sensor cell current or the monitor cell current is not measured, it is conceivable that an activation failure has occurred, the current value is originally 0, in addition to the occurrence of disconnection abnormality. Therefore, when the sensor cell current or the monitor cell current is lower than the specified value, it is possible to identify the disconnection abnormality by executing the abnormality determination.

  In the above embodiment, a sensor element having a so-called three-cell structure composed of a pump cell, a sensor cell, and a monitor cell is applied, but this may be changed. For example, a sensor element having a so-called two-cell structure composed of a pump cell and a sensor cell is applied. When a monitor cell (third cell) is used, the monitor cell may be an electromotive force cell that outputs an electromotive force.

  The specific component to be detected may be other than NOx. For example, it may be a gas sensor that detects HC or CO in the exhaust. In this case, surplus oxygen in the exhaust is discharged by the pump cell, and HC and CO are decomposed from the gas after the surplus oxygen is discharged by the sensor cell to detect the HC concentration and the CO concentration.

  -It can also be embodied as a sensor control device for a gas sensor provided in an intake passage of the engine or a gas sensor used for other types of engines such as a gasoline engine in addition to a diesel engine. The gas sensor may be a gas to be detected other than exhaust gas or used for purposes other than automobiles.

The block diagram which shows the element internal structure of a NOx sensor, and a NOx sensor circuit. The block diagram which shows the outline | summary of a NOx sensor circuit. The circuit block diagram of a sensor cell / monitor cell drive circuit part. The circuit block diagram of an Is detection circuit part. The circuit block diagram of a sensor cell / monitor cell protection circuit part. The flowchart which shows the calculation process of a sensor output correction value. The figure for demonstrating the offset error of sensor cell current Is, monitor cell Im, and (Is-Im). The flowchart which shows the abnormality detection process based on a sensor cell electromotive force. The circuit block diagram of the Is detection circuit part in another embodiment.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Sensor element, 11 ... Solid electrolyte body, 14 ... 1st chamber (gas chamber), 16 ... 2nd chamber (gas chamber), 23 ... Heater, 31 ... Pump cell (1st cell), 32, 33 ... Electrode, 34 ... monitor cell (third cell), 35 ... sensor cell (second cell), 36-38 ... electrode, 40 ... NOx sensor circuit, 41 ... microcomputer (electromotive force detection means, abnormality determination means), 44 ... sensor cell / monitor cell drive Circuit part 45 ... Is detection circuit part 46 ... Im detection circuit part 48 ... Sensor cell / monitor cell protection circuit part (voltage application stop means) 61 ... Current measuring resistor 62 ... Differential amplifier circuit (application voltage setting circuit) , 65 ... voltage follower, 66 ... differential amplifier circuit (output circuit), 71 ... switch circuit (switch means), 75 ... bias current resistor, 101 ... non-inverting amplifier circuit (applied voltage setting circuit) , 102 ... switching circuit, L1 ... feedback input path (first feedback path), L2 ... feedback input path (second feedback path).

Claims (13)

Disposed in the solid electrolyte body and said solid electrolyte body possess the electrode serving as a pair, one of the pair of electrodes is provided in the gas chamber of gas to be detected is introduced, the air chamber and the other being introduced air a sensor element that provided on, which is connected to the gas sensor to produce a device current corresponding to the concentration of a specific component of the detection gas in the gas chamber at the voltage application state to between the pair of electrodes ,
A current measuring resistor connected to one electrode of the sensor element and measuring the element current;
An output circuit for outputting a measurement result of the element current by the current measurement resistor as an element current measurement value;
An applied voltage setting circuit that is connected to the non-sensor side terminal of the current measuring resistor and sets an applied voltage to be applied to the sensor element;
In the gas sensor control device for calculating the concentration of the specific component from the measured element current value output from the output circuit in the state of voltage application by the applied voltage setting circuit,
An electrical path for electrically connecting the sensor-side terminal of the current measurement resistor and the applied voltage setting circuit and inputting the voltage of the sensor-side terminal of the current measurement resistance to the input unit of the applied voltage setting circuit is the electrical path Is provided with a voltage follower or a non-inverting amplifier circuit to prevent the element current from flowing,
Switch means provided on a path between the voltage follower or the non-inverting amplifier circuit and the applied voltage setting circuit in the electrical path, and opens and closes the electrical path;
By closing the switch means, the potential difference between both ends of the current measuring resistor is made zero, and in this state, an electromotive force for detecting the electromotive force of the sensor element generated between the pair of electrodes according to the oxygen concentration in the detected gas is detected. Power detection means;
An abnormality determining means for performing an abnormality determination of the sensor element or sensor circuit based on the electromotive force detected by the electromotive force detecting means;
A gas sensor control device comprising:   With the switch means closed, the sensor side terminal voltage of the current measuring resistor is fed back to the applied voltage setting circuit to set the setting voltage of the setting circuit to the same voltage as the sensor side terminal voltage. The gas sensor control device according to claim 1, wherein a potential difference between both ends of the current measurement resistor is set to zero.   3. The gas sensor control device according to claim 1, wherein the electromotive force detection unit detects an electromotive force of the sensor element based on a sensor-side terminal voltage of the current measurement resistor in a state where the switch unit is closed.   3. The electromotive force detection means measures a voltage at both positive and negative electrodes of the sensor element with the switch means closed, and detects an electromotive force of the sensor element based on a difference between the measured voltage values. The gas sensor control apparatus according to 1.   The gas sensor control device according to any one of claims 1 to 4, wherein a sensor-side terminal of the current measurement resistor is connected to a reference potential portion via a bias resistor. A first feedback path for feedback-inputting the output of the output circuit to the applied voltage setting circuit, and a second feedback path for feedback-inputting the voltage of the sensor-side terminal of the current measurement resistor to the applied voltage setting circuit;
The switch means is provided in a second feedback path of the two feedback paths,
During normal concentration detection, only the first feedback path of the two feedback paths is in a conductive state, and the voltage is applied by the applied voltage setting circuit in accordance with the output of the output circuit that is fed back via the first feedback path. While letting you configure
When the electromotive force is detected by the electromotive force detection means, only the second feedback path of the two feedback paths is made conductive, and the sensor side terminal voltage of the current measuring resistor is fed back via the second feedback path. The gas sensor control device according to any one of claims 1 to 5, wherein voltage setting is performed by the applied voltage setting circuit so that a potential difference between both ends of the current measurement resistor is zero. The applied voltage setting circuit is constituted by an operational amplifier having a negative feedback section, one out of the negative feedback unit an output side of the operational amplifier the current measurement resistor of claims 1 to 6 is provided The gas sensor control device according to claim 1. Terminal voltage measuring means for measuring the voltage of the terminal portion connected to each electrode of the sensor element,
The abnormality determining means, the addition to the abnormality determination by the electromotive force, the terminal voltage any one of claims 1 to 7 based on the terminal voltage measured by the measuring means carrying out the abnormality determination of the sensor element or the sensor circuit The gas sensor control device according to Item. The gas sensor control device according to any one of claims 1 to 8 , further comprising a voltage application stop unit that stops voltage application to the sensor element when it is determined by the abnormality determination unit that an abnormality has occurred. The gas sensor control device according to any one of claims 1 to 9 , wherein the electromotive force detection means performs electromotive force detection on condition that the sensor element is in an active state. The gas sensor, the oxygen of the each have a first cell and the second cell becomes more solid electrolyte body and said solid electrolyte body arranged pair of electrodes as a sensor element, the detection target gas introduced into the gas chamber Adjusting the amount to a predetermined concentration level in the first cell and detecting the concentration of the specific component from the gas after the oxygen amount adjustment in the first cell in the second cell;
The gas sensor control device according to any one of claims 1 to 10 , wherein an element current generated in the second cell is measured by the current measurement resistor. The gas sensor control device according to claim 11 , wherein the electromotive force detection unit performs electromotive force detection on the condition that the oxygen concentration in the gas chamber becomes a low oxygen level that is the predetermined concentration level. Means for detecting residual oxygen concentration in the gas chamber;
The abnormality determination unit variably sets an abnormality determination value according to the detected residual oxygen concentration in the gas chamber, and determines abnormality of the sensor element or sensor circuit based on the abnormality determination value and each detected electromotive force. The gas sensor control device according to claim 11 or 12 , wherein

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