JP2006053126A - Gas concentration detector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enhance computation precision for element resistance, and to favorably execute improvement in exhaust gas emission, diagnosis for a sensor trouble and the like, based thereon. <P>SOLUTION: A voltage switching circuit 35 changes temporarily an application voltage of a sensor element 10 by a prescribed amount, when detecting an impedance, in a sensor control circuit 100. A ΔI detecting circuit 32 measures a response change of a current in response to the prescribed amount of voltage change, and outputs a measured value as a current change amount signal. A microcomputer 200 registers the voltage change amount in the voltage switching circuit 35 preliminarily in its inside, and computes an element resistance value of the sensor element 10, based on the registered voltage change amount and the current change amount signal output from the ΔI detecting circuit 32. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a gas concentration detection device.

  For example, a limit current type air-fuel ratio sensor (so-called A / F sensor) is known that detects an oxygen concentration (air-fuel ratio) in an exhaust gas discharged from a vehicle engine as a detected gas. That is, the A / F sensor has a sensor element composed of a solid electrolyte body and a pair of electrodes provided on the solid electrolyte body, and an element current corresponding to the oxygen concentration each time a voltage is applied to the sensor element. Is configured to flow. The element current flowing in the sensor element is measured, and the oxygen concentration (air-fuel ratio) is detected from the measurement result.

  In the A / F sensor, since the oxygen concentration can be accurately detected on the assumption that the sensor element is in an active state, element resistance detection (impedance detection) of the sensor element is performed in order to know the active state of the sensor element. Specifically, in the sensor circuit connected to the A / F sensor, the sensor applied voltage is changed to the positive side or the negative side, the voltage change amount at that time is measured, and the current change amount responding to the voltage change is measured. . The element resistance is calculated based on the ratio between the voltage change amount and the current change amount (see, for example, Patent Document 1).

Here, the calculated value of the element resistance is used for heater control and abnormality detection for activating the sensor element. In recent years, legal regulations related to exhaust gas and sensor fault diagnosis are being strengthened. Therefore, improvement in the calculation accuracy of element resistance is desired. In addition, at the present time, when calculating the element resistance, it is necessary to measure the change amount of the sensor applied voltage in consideration of individual differences and the like, and the measurement of the current change amount responding to the voltage change is a requirement. Therefore, considering that a measurement error occurs in each measurement, the accuracy of the element resistance is reduced accordingly. Therefore, it is desired to improve the technology regarding the calculation of the element resistance. In addition, simplification of the circuit configuration is required, and specifically, it is desired to reduce the A / D conversion input signal for measurement signals.
Japanese Patent Laid-Open No. 9-292364

  The main object of the present invention is to provide a gas concentration detection device capable of improving the calculation accuracy of element resistance, and thus enabling suitable implementations such as exhaust gas emission improvement and sensor failure diagnosis. .

  The present invention applies a voltage to a sensor element made of a solid electrolyte body, measures the element current flowing through the sensor element in the voltage application state, and detects the gas concentration in the gas to be detected based on the measured value of the element current. This is based on the assumption that the gas concentration detection device is configured to do so. In the gas concentration detection device, a current or voltage that temporarily applies a predetermined amount of voltage change or current change on the current path connected to the sensor element and responds to the predetermined amount of voltage change or current change. Measure the amount of response change. Further, the element resistance value (element impedance) of the sensor element is calculated using only the current or voltage response change amount (measured value of the response change amount measuring means) as a calculation parameter (calculation variable). It is also possible to treat the current or voltage response change amount (measured value of the response change amount measuring means) as it is as the element resistance value.

  In general, when calculating the element resistance, the applied change amount of the voltage or current to be applied positively and the response change amount of the current or voltage in response thereto are respectively measured, and the measured value of the applied change amount and the response change amount of the applied change amount are measured. The element resistance value is calculated from the ratio to the measured value. If the applied change amount is a fixed value, the response change amount substantially corresponds to the element resistance. Therefore, the element resistance value can be calculated using only the measured value of the response change amount as the calculation parameter as described above. In this case, by using only the measured value of the response change amount as a calculation parameter, the measurement of the applied change amount is not a requirement, and the measurement error caused by the measurement of the applied change amount can be excluded from the calculation error of the element resistance. Therefore, the calculation accuracy of the element resistance can be improved. As a result, the sensor element can always be held in a good state, and suitable implementations such as improvement of exhaust gas emission and sensor failure diagnosis are possible.

  Further, the applied change amount of the voltage or current by the change applying means is registered in advance (internal storage), and the applied applied change amount and the response change amount of the current or voltage (measured value of the response change measuring means) The element resistance value of the sensor element may be calculated based on the above. Even in this configuration, as described above, the measurement of the applied change amount is no longer a requirement, and the measurement error caused by the measurement of the applied change amount can be excluded from the calculation error of the element resistance. Therefore, the calculation accuracy of the element resistance can be improved. As a result, the sensor element can always be held in a good state, and suitable implementations such as improvement of exhaust gas emission and sensor failure diagnosis are possible.

  A response change amount measurement circuit amplifies the measured value of the current or voltage response change amount and outputs it as a response change amount signal. Further, the microcomputer A / D converts the response change amount signal and uses the A / D value. In the configuration in which the element resistance value is calculated, only an error during measurement and amplification of the response change amount and an A / D input error are error factors in the element resistance calculation. In this case, since the error factor is reduced as compared with the configuration in which the applied change amount is similarly measured, the calculation accuracy of the element resistance can be improved. In addition, regarding the calculation of the element resistance, A / D conversion in the microcomputer only needs to be a response change amount signal, and the number of A / D converters can be reduced.

  As another application of the present invention, the present invention is also applicable to an electromotive force output type gas sensor that generates an electromotive force between a pair of electrodes in accordance with the oxygen concentration in the gas to be detected or the concentration of a specific component containing at least oxygen. Is possible. In such a case, a change applying means (for example, an AC power source) and a series circuit unit including a resistance element and a capacitance element (for example, a coupling capacitor) are provided on a current path connected to the electrode of the sensor element. Then, a predetermined amount of voltage change or current change is temporarily applied on the current path connected to the electrode of the sensor element, and the terminal voltage of the sensor element is measured at the time of the voltage change or current change. The element resistance value (element impedance) is calculated using only the measured value of as a calculation parameter (calculation variable). Note that the terminal voltage of the sensor element is measured as a divided value by the resistance component of the resistance element and the capacitance element and the resistance component of the sensor element.

  Also in this configuration, as described above, by using only the measured value of the terminal voltage of the sensor element that changes in response to a voltage change or the like as a calculation parameter, the measurement of the applied change amount is not a requirement, and the measurement of the applied change amount is performed. The measurement error caused by can be eliminated from the calculation error of the element resistance. Therefore, the calculation accuracy of the element resistance can be improved. As a result, the sensor element can always be held in a good state, and suitable implementations such as improvement of exhaust gas emission and sensor failure diagnosis are possible.

  In addition, in the configuration in which the capacitive element is provided on the current path connected to the sensor element as described above and the element resistance value is detected based on the terminal voltage of the sensor element, the resistance increases with the passage of time after application of a voltage change or the like. In the process in which the voltage across the element converges (the convergence process after the peak), the correlation between the sensor terminal voltage and the element resistance value (impedance) is maintained, and the element resistance value can be calculated accurately. Therefore, for example, when an LPF is provided for the purpose of reducing heater noise generated when ON / OFF control of a heater with a built-in sensor is performed, the measured value of the sensor terminal voltage is smoothed by the LPF, and it is difficult to measure the peak value. Even if, the element resistance value can be accurately calculated from the measured value of the convergence process of the sensor terminal voltage. Thereby, the improvement of the calculation accuracy of the element resistance value and the sufficient countermeasure against the heater noise can be realized.

  Note that the capacitive element preferably has a capacitance sufficiently smaller than the capacitive component of the sensor element. In this case, the convergence of the sensor terminal voltage after applying the voltage change or current change by the change applying means is dominated by the charge speed to the capacitive element, and the sensor terminal voltage is subject to a change in capacity due to individual differences or deterioration of the sensor element. Not affected. Therefore, the element resistance value can be accurately calculated without being affected by the capacitance change due to individual differences or deterioration of the sensor elements.

  In addition, the applied change amount of the voltage or current by the change applying unit is registered in advance (internal storage), and the registered applied change amount and the terminal voltage of the sensor element at the time of the voltage change or current change by the change applying unit are measured. The structure which calculates the element resistance value of a sensor element based on a value may be sufficient. Even in this configuration, as described above, the measurement of the applied change amount is no longer a requirement, and the measurement error caused by the measurement of the applied change amount can be excluded from the calculation error of the element resistance. Therefore, the calculation accuracy of the element resistance can be improved. As a result, the sensor element can always be held in a good state, and suitable implementations such as improvement of exhaust gas emission and sensor failure diagnosis are possible.

  In the configuration in which the measured value of the terminal voltage of the sensor element is amplified and output to the microcomputer, the input signal is A / D converted by the microcomputer, and the element resistance value is calculated using the A / D value. Only the voltage measurement, amplification error, and A / D input error are error factors in element resistance calculation. In this case, since the error factor is reduced as compared with the configuration in which the applied change amount is similarly measured, the calculation accuracy of the element resistance can be improved. In addition, regarding the calculation of the element resistance, A / D conversion in the microcomputer only needs to be a response change amount signal, and the number of A / D converters can be reduced.

  In a configuration that does not have an applied change amount measurement circuit that measures and outputs an applied change amount of voltage or current by the change applying means, it is possible to reduce the size and simplification of an IC chip or the like for constituting a gas concentration detection device. Become.

  The electric circuit constituting the change applying means is preferably one in which the electric characteristics are finely adjusted so that the applied change amount of the voltage or current has a desired accuracy. Thereby, the accuracy of the applied change amount of the voltage or current applied by the change applying means is improved (that is, the error is reduced). Therefore, the element resistance value is calculated using only the current or voltage response change amount (or only the sensor terminal voltage) as an operation parameter, or the pre-registered applied change amount and current or voltage response change amount (or sensor terminal voltage). In the configuration in which the element resistance value is calculated based on the above, more reliable element resistance calculation can be realized. Specifically, a trimming technique called active trimming or function trimming may be used. By performing active trimming etc., fine adjustment of the electric characteristics is more reliable than when adjusting the electric characteristics at the time of making a thin film or a semiconductor element in the process of manufacturing a sensor control circuit (IC circuit, etc.). Can be. Here, the fine adjustment of the electrical characteristics to achieve the desired accuracy improves the A / D accuracy, the LSB error when quantizing, the circuit error, etc., and as a result, improves the detection accuracy of the element resistance. I need it.

(First embodiment)
Hereinafter, a first embodiment in which a gas concentration detection apparatus of the present invention is embodied will be described with reference to the drawings. In the present embodiment, an air-fuel ratio detection device that detects an oxygen concentration (air-fuel ratio, hereinafter also referred to as A / F) in the same gas using exhaust gas (combustion gas) discharged from an on-vehicle engine as a detection gas is embodied. The air-fuel ratio detection result is used in an air-fuel ratio control system constituted by an engine ECU or the like. In the air-fuel ratio control system, stoichiometric combustion control in which the air-fuel ratio is feedback-controlled in the vicinity of the stoichiometry, lean combustion control in which the air-fuel ratio is feedback-controlled in a predetermined lean region, and the like are appropriately performed.

  First, the configuration of an A / F sensor as a gas concentration sensor will be described with reference to FIG. The A / F sensor has a sensor element 10 having a laminated structure, and FIG. 2 shows a cross-sectional configuration of the sensor element 10. Actually, the sensor element 10 has a long shape extending in the direction orthogonal to the paper surface of FIG. 2, and the entire element is accommodated in a housing or an element cover.

  The sensor element 10 includes a solid electrolyte 11, a diffusion resistance layer 12, a shielding layer 13, and an insulating layer 14, which are stacked on the top and bottom of the drawing. A protective layer (not shown) is provided around the element 10. The rectangular plate-shaped solid electrolyte 11 (solid electrolyte body) is a sheet made of partially stabilized zirconia, and a pair of upper and lower electrodes 15 and 16 are disposed opposite to each other with the solid electrolyte 11 interposed therebetween. The electrodes 15 and 16 are made of platinum Pt or the like. The diffusion resistance layer 12 is made of a porous sheet for introducing exhaust gas into the electrode 15, and the shielding layer 13 is made of a dense layer for suppressing permeation of the exhaust gas. Each of these layers 12 and 13 is formed by molding a ceramic such as alumina or zirconia by a sheet forming method or the like, but the gas permeability varies depending on the difference in the average pore diameter and porosity of the porosity.

  The insulating layer 14 is made of ceramics such as alumina or zirconia, and an air duct 17 is formed at a portion facing the electrode 16. In addition, a heater 18 made of platinum Pt or the like is embedded in the insulating layer 14. The heater 18 is a linear heating element that generates heat when energized by a battery power source, and heats the entire element by the generated heat. In the following description, the electrode 15 is also referred to as a diffusion layer side electrode, and the electrode 16 is also referred to as an atmosphere side electrode. In the present embodiment, a terminal connected to the atmosphere side electrode 16 is a positive side terminal (+ terminal), and a terminal connected to the diffusion layer side electrode 15 is a negative side terminal (−terminal).

  In the sensor element 10, the surrounding exhaust gas is introduced from the side portion of the diffusion resistance layer 12 and reaches the diffusion layer side electrode 15. When the exhaust gas is lean, oxygen in the exhaust gas is decomposed by the diffusion layer side electrode 15 by applying a voltage between the electrodes 15 and 16, is ionized and passes through the solid electrolyte 11, and then is discharged from the atmosphere side electrode 16 to the atmosphere duct 17. Is done. At this time, a current (positive current) flows in the direction from the atmosphere side electrode 16 to the diffusion layer side electrode 15. On the other hand, when the exhaust gas is rich, oxygen in the atmosphere duct 17 is decomposed by the atmosphere side electrode 16, ionized and passes through the solid electrolyte 11, and then discharged from the diffusion layer side electrode 15. And it reacts with unburned components such as HC and CO in the exhaust gas. At this time, a current (negative current) flows in the direction from the diffusion layer side electrode 15 to the atmosphere side electrode 16.

  FIG. 3 is a drawing showing basic voltage-current characteristics (VI characteristics) of the A / F sensor. In FIG. 3, a flat portion parallel to the voltage axis (horizontal axis) is a limit current region for specifying the element current Ip (limit current) of the sensor element 10, and the increase / decrease in the element current Ip is the increase / decrease in the air / fuel ratio (that is, , Lean and rich). That is, the element current Ip increases as the air-fuel ratio becomes leaner, and the element current Ip decreases as the air-fuel ratio becomes richer.

  In this VI characteristic, the lower voltage side than the limit current region is a resistance dominant region, and the slope of the primary straight line portion in the resistance dominant region is specified by the DC internal resistance Ri of the sensor element 10. The DC internal resistance Ri changes according to the element temperature, and the DC internal resistance Ri increases as the element temperature decreases. That is, at this time, the slope of the primary straight line portion of the resistance dominating region becomes small (the straight line portion lies down). Further, when the element temperature rises, the DC internal resistance Ri decreases. That is, at this time, the slope of the primary straight line portion of the resistance dominating region becomes large (the straight line portion stands up). RG in the drawing represents an applied voltage characteristic (applied voltage line) for determining the applied voltage Vp to the sensor element 10.

  FIG. 1 is an electric circuit diagram showing a configuration of the sensor control circuit 100. In FIG. 1, a reference power source 23 is connected to the + terminal T1 connected to the atmosphere side electrode 16 of the sensor element 10 via an operational amplifier 21 and a current detection resistor 22 as shown in the figure. An applied voltage control circuit 30 is connected to the negative terminal T2 connected to the electrode 15 via an operational amplifier 24 and a resistor 25. The point A at one end of the current detection resistor 22 is held at the same voltage as the reference voltage Ref1. The element current Ip flows through the current detection resistor 22, and the voltage at the point B changes according to the element current Ip. When the exhaust gas is lean, the element current Ip flows from the + terminal T1 to the − terminal T2 in the sensor element 10, so that the voltage at the point B rises. Conversely, when the exhaust gas is rich, the sensor element 10 has the + Since the element current Ip flows toward the terminal T1, the B point voltage decreases. This point B voltage is output to the microcomputer 200 via the element current output circuit 31 as an A / F output that is a detection result of the air-fuel ratio. The microcomputer 200 takes in the A / F output from the A / D port AD1 and performs A / D conversion. The element current output circuit 31 is constituted by, for example, an S / H (sample hold) circuit. The B point voltage at the time of air-fuel ratio detection is sampled, and the sample value is sequentially updated and outputted within a predetermined gate-on period. This A / F output is appropriately used for air-fuel ratio feedback control or the like.

  The applied voltage control circuit 30 determines the voltage to be applied to the sensor element 10 according to the A / F output (sample hold value of the B point voltage) while monitoring the A / F output. As in the voltage characteristic RG, the applied voltage control is basically performed so as to increase the applied voltage when the element current Ip increases (that is, when the point B voltage increases). However, the sensor applied voltage may be a fixed value.

  Moreover, in this air-fuel ratio detection apparatus, the element impedance of the sensor element 10 is detected using a so-called sweep method. That is, the voltage switching circuit 35 changes the sensor applied voltage in an alternating manner based on the voltage switching signal from the microcomputer 200. The voltage switching signal is periodically output from the microcomputer 200 to the voltage switching circuit 35. For example, every 128 msec, the sensor applied voltage is detected from the normal applied voltage for air-fuel ratio detection (control voltage by the applied voltage control circuit 30) for impedance detection. Is temporarily switched to the applied voltage.

  In such a case, the point B voltage is measured by the ΔI detection circuit 32 and the measured value is output to the microcomputer 200 as a current change amount signal. The microcomputer 200 takes in the current change amount signal from the A / D port AD2 and performs A / D conversion. The ΔI detection circuit 32 is configured, for example, by connecting an HPF (high-pass filter) and a P / H (peak hold) circuit in series. The HPF and the P / H circuit allow a predetermined detection period corresponding to the impedance detection period. The change amount of the alternating current at point B measured during the gate-on period is output. The peak-held point B voltage is reset every time the gate is turned off.

  The microcomputer 200 calculates the element impedance based on the AC voltage change amount and the current change amount responding to the voltage change at the time of impedance detection. The energization of the heater 18 is controlled so that the element impedance is maintained at a predetermined target value. As a result, the sensor element 10 is held at a constant target temperature (for example, 750 ° C.). In the present embodiment, the voltage switching circuit 35 corresponds to “change applying means”, the ΔI detection circuit 32 corresponds to “response change amount measuring means”, and the microcomputer 200 corresponds to “element resistance calculating means”.

  In particular, in the present embodiment, the voltage change amount by the voltage switching circuit 35 is registered in advance in the memory of the microcomputer 200 as a fixed value (0.2 V in the present embodiment), and the registered value of the voltage change amount is detected when impedance is detected. And the element impedance is calculated based on the measured value of the current change amount. That is, measurement of the voltage change amount that is changed by the voltage switching circuit 35 is not a requirement, and therefore, the sensor control circuit 100 does not include the voltage change amount measurement circuit.

  Further, accompanying the configuration in which the voltage change amount of the voltage switch circuit 35 is not measured, active trimming is performed on the voltage switch circuit 35 so that the voltage change amount by the voltage switch circuit 35 accurately becomes a desired value. Has been given. This active trimming is also called function trimming, and the electrical characteristics are finely adjusted so that the voltage change amount becomes a desired value with the voltage switching circuit 35 in the operating state.

  Next, the calculation procedure of the element impedance by the microcomputer 200 will be described based on the flowchart of FIG. The process of FIG. 4 is executed by the microcomputer 200 every predetermined time (for example, every 128 msec).

  In FIG. 4, in step S <b> 101, a voltage switching signal is output to the voltage switching circuit 35 of the sensor control circuit 100. In response to this voltage switching signal, the sensor control circuit 100 switches the applied voltage of the sensor from the voltage value for detecting the air-fuel ratio so far to the voltage value for detecting impedance in an AC manner. In the present embodiment, the amount of voltage change to the positive side or the negative side is 0.2V. Thereby, an AC change of about 1 kHz to 20 kHz, for example, is applied to the sensor applied voltage, and the element current changes in response to the AC voltage change.

  Thereafter, in step S102, based on the current change amount signal (the output of the ΔI detection circuit 32 in FIG. 1), the current change amount ΔI in response to the applied voltage change is detected. In the subsequent step S103, the element impedance Zac is calculated from the voltage change amount (0.2 V) of the voltage switching circuit 35 previously registered in the memory and the detected current change amount ΔI (Zac = 0.2 / ΔI). ). When the element impedance Zac is calculated in this way, the calculated value is appropriately used for heater control for sensor activation, sensor failure diagnosis, and the like.

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

  When impedance is detected, the element impedance is calculated based on the voltage change (applied change) registered in the memory in advance and the current change ΔI (measured value of response change) measured at the time of temporary voltage change. did. In other words, while the voltage change amount is a fixed value, the element impedance is calculated using only the current change amount ΔI (measured value of the response change amount) as an operation parameter. As a result, the measurement of the voltage change amount that is the applied change amount is no longer a requirement, and the measurement error caused by the measurement of the voltage change amount can be excluded from the calculation error of the element impedance. Therefore, the calculation accuracy of the element impedance can be improved. As a result, the sensor element 10 can always be held in a good state, and suitable implementations such as improvement of exhaust gas emission and sensor failure diagnosis are possible.

  In such a case, since the voltage change amount measuring circuit for measuring and outputting the voltage change amount is not provided, the IC chip or the like constituting the sensor control circuit 100 can be reduced in size or simplified. In addition, with respect to impedance detection, A / D conversion in the microcomputer 200 only requires a current change amount signal (response change amount signal), and the number of A / D converters can be reduced.

  In addition, even if an inexpensive A / D converter or a reference power supply regulator (for example, a 5V regulator) is used, the influence of the accuracy reduction can be halved, and the deterioration of the calculation accuracy of the element impedance can be suppressed.

  Since active trimming is performed on the voltage switching circuit 35, the accuracy of the voltage change amount (applied change amount) at the time of impedance detection is improved (that is, the voltage change error is reduced). Therefore, a more reliable impedance calculation can be realized in the configuration in which the element impedance is calculated using the voltage change amount registered in advance.

  When the control accuracy of the voltage change by the voltage switching circuit 35 is improved, the voltage change amount at the time of impedance detection can be reduced, and accordingly, the current change amount ΔI as the response change amount can be reduced. In this case, the resistance value of the current detection resistor 22 can be increased, and as a result, the resolution of air-fuel ratio detection by A / F output can be increased.

(Second Embodiment)
Next, an application example to the O2 sensor will be described. The O2 sensor has a cup-shaped sensor element constituting an electromotive force cell, and an electromotive force is generated between the electrodes of the sensor element in accordance with the oxygen concentration in the exhaust gas. The O2 sensor includes a heater for heating the sensor element.

  FIG. 5 is an electrical configuration diagram showing the configuration of the sensor control circuit. As shown in FIG. 5, a resistor 61 and an LPF (low-pass filter) 62 are connected to one terminal of the O2 sensor 60, respectively. When an electromotive force is generated by the O2 sensor 60 according to the oxygen concentration in the exhaust gas, an O2 output corresponding to the electromotive force at each time is output via the LPF 62. The O 2 output output from the LPF 62 is taken into the AD port of the microcomputer 300. The microcomputer 300 performs rich / lean determination of the air-fuel ratio based on the O2 output.

  The LPF 62 is a filter for eliminating noise and AC signals superimposed on the O2 output. As will be described later, the terminal voltage (sensor terminal voltage) of the O2 sensor 60 changes in an AC manner for impedance detection. In addition, a decrease in the accuracy of the O2 output due to the influence is suppressed.

  As a configuration for impedance detection, a series circuit including an AC voltage source 63, a voltage dividing resistor 64, and a coupling capacitor 65 is connected to one terminal of the O2 sensor 60, and the O2 sensor 60 and the coupling capacitor 65 are connected. A series circuit including an HPF (high pass filter) 66, a P / H circuit (peak hold circuit) 67, and an amplifier circuit 68 is connected between the two. Here, the output voltage of the AC voltage source 63 is Va, the terminal voltage on the voltage dividing resistor 64 side among both terminals of the coupling capacitor 65 is Vb, the terminal voltage on the O2 sensor 60 side is Vc, and the voltage on the output side of the HPF 66 is. Is Vd, and the output voltage of the amplifier circuit 68 is Ve. The voltage Vc is an electromotive force corresponding to the oxygen concentration in each case during normal oxygen concentration detection. As described above, the voltage Vc is approximately 0.9 V in a rich atmosphere and approximately 0 V in a lean atmosphere. The resistance value of the voltage dividing resistor 64 is R.

  The P / H circuit 67 includes an input comparator that takes in an input signal, a capacitor that has one end connected to the output side of the comparator and the other end grounded, and holds the peak value of the input signal. Further, the P / H circuit 67 is configured to include an LPF therein, and when noise or the like is superimposed on the input signal, the noise is removed. For example, when PWM control is performed as the heater energization control, the heater energization is repeatedly turned on / off, so that the current is always turned on / off. Therefore, it is considered that a change in magnetic flux due to this change in current propagates to the sensor harness bundled together with the heater harness to generate heater noise. This heater noise is removed by the LPF in the P / H circuit 67. . Of course, noise other than heater noise is also removed by the LPF.

  The AC voltage source 63 corresponds to “change imparting means”, and sweeps and changes the voltage Va to a positive side and a negative side at a predetermined frequency in accordance with a command from the microcomputer 300. At this time, a current flows through a current path including the voltage dividing resistor 64, the coupling capacitor 65, and the O2 sensor 60 according to the change of the voltage Va, and the voltage Vc, which is the sensor terminal voltage, is the element impedance of the O2 sensor 60 and the voltage dividing resistor. The voltage value is divided by 64 resistance values. The voltage Vc is taken into the AD port of the microcomputer 300 as the impedance detection voltage Ve through the HPF 66, the P / H circuit 67, and the amplifier circuit 68.

In the microcomputer 300, the element impedance Zac is calculated by the following equation (1).
Zac = Vc / {(Va−Vc) / R} (1)
In the above equation (1), the voltage Va and the resistance value R are fixed values, and the element impedance Zac can be calculated by measuring the voltage Vc. That is, the microcomputer 300 takes in the voltage Vc through the HPF 66, the P / H circuit 67, and the amplifier circuit 68, and calculates the element impedance Zac based on the taken-in value.

  Particularly in the above configuration, the voltage Va, which is the voltage change amount of the AC voltage source 63, is registered in advance in the memory of the microcomputer 300, and when impedance is detected, the registered value of the voltage change amount (voltage Va) and the sensor terminal voltage (voltage Vc). ) And the element impedance is calculated. That is, the measurement of the voltage change amount (voltage Va) is not a requirement, and therefore, the sensor control circuit does not include the voltage change amount measurement circuit.

  Further, accompanying the configuration in which the voltage change amount (voltage Va) is not measured, active trimming is performed on the AC voltage source 63 so that the voltage change amount by the AC voltage source 63 becomes a desired value with high accuracy. ing. This active trimming is also referred to as function trimming, and the electrical characteristics are finely adjusted so that the amount of voltage change becomes a desired value with the AC voltage source 63 operating.

  The sweep frequency of the voltage Va by the AC voltage source 63 is about 10 kHz, for example, and the resistance value R of the voltage dividing resistor 64 is about 1 kΩ. The capacitance of the coupling capacitor 65 is preferably 0.1 to 1 μF considering the influence on the O 2 output, and is preferably 0.2 μF or less because of cost and size constraints. However, it is considered that the larger the capacitance, the more advantageous for impedance detection. In the present embodiment, the capacitor capacity is 0.1 μF. Incidentally, the capacitance component of the O 2 sensor 60 is, for example, about 1000 μF when it is normal and about 100 μF when it is deteriorated, and the capacitance of the coupling capacitor 60 is sufficiently smaller than the capacitance component of the O 2 sensor 60.

  Here, when the sensor terminal voltage (voltage Vc) is measured as described above at the time of AC voltage change, the sensor terminal voltage Vc has a value whose peak value has a correlation with the element impedance, and in the convergence process. It is not affected by variations in sensor capacity. Therefore, the calculation accuracy of the element impedance can be ensured. In particular, since the coupling capacitor 65 has a capacitance sufficiently smaller than the capacitance component of the O2 sensor 60, the convergence of the sensor terminal voltage Vc is dominated by the charge speed to the coupling capacitor 65, and the sensor terminal voltage Vc. Is not affected by capacitance changes due to individual differences or deterioration of the O 2 sensor 60. Therefore, the element impedance can be accurately calculated without being affected by the capacitance change due to individual differences or deterioration of the O 2 sensor 60.

  As described above, according to the second embodiment, when impedance is detected, the voltage change amount (voltage Va) registered in the memory in advance and the sensor terminal voltage (voltage Vc) measured at the time of temporary voltage change are used. Therefore, the measurement of the voltage change amount (voltage Va) that is the applied change amount is not a requirement, and the measurement error caused by the measurement of the voltage change amount can be excluded from the calculation error of the element impedance. Therefore, the calculation accuracy of the element impedance can be improved. As a result, the O2 sensor 60 can always be maintained in a good state, and it is possible to perform suitable implementations such as improvement of exhaust gas emission and sensor failure diagnosis.

  In addition, as in the first embodiment, the IC chip constituting the sensor control circuit can be reduced in size and simplified, the A / D converter in the microcomputer 300 can be reduced, etc. An effect is obtained.

  In addition, this invention is not limited to the content of description of the said embodiment, For example, you may implement as follows.

  In the first embodiment, the voltage change amount (applied change amount) registered in advance in the memory of the microcomputer 200 and the current change amount ΔI (response change amount) measured at the time of a temporary voltage change when impedance is detected. The element impedance is calculated based on the measured value), but it is also possible to change the configuration and handle the current change amount ΔI (measured value of the response change amount) as the element impedance value as it is. That is, when the voltage change amount is a fixed value, the element impedance Zac and the current change amount ΔI are in an inversely proportional relationship (or a proportional relationship if the element admittance), and ΔI value can be obtained without converting the ΔI value into a Zac value. Based on the values, heater control for sensor activation, sensor failure diagnosis, and the like can be performed.

  In each of the above-described embodiments, the element impedance is calculated as the element resistance value. Instead, an element admittance that is the reciprocal of the element impedance may be calculated (element admittance = ΔI / 0. 2V).

  In each of the above embodiments, when detecting the impedance, the sensor applied voltage is changed in an alternating manner to measure the current response at that time, but this is changed to change the sensor element current in an alternating manner to change the voltage response at that time. Configure to measure. The configuration will be described with reference to FIG. In the configuration of FIG. 6, the difference from FIG. 1 is that the switch circuit 53 is connected to the + terminal T1 of the sensor element 10, and the current detection resistor 22 is connected to one switching terminal of the switch circuit 53. The current switching circuit 51 is connected to the other switching terminal. The current switching circuit 51 changes the element current in an alternating manner based on a current switching signal output from a microcomputer or the like when impedance is detected. Further, a ΔV detection circuit 52 is connected to point C in the figure, and the output of the ΔV detection circuit 52 is used as a voltage change amount signal. In the current switching circuit 51, it is preferable that the dynamic electrical characteristics are finely adjusted to achieve a desired accuracy by active trimming or the like.

  In the configuration shown in FIG. 6, the current change amount as the applied change amount is registered in advance in the memory of the microcomputer, and the current change amount (applied change amount) registered in the memory is temporarily detected when the impedance is detected. The element impedance is calculated based on the voltage change amount ΔV (measured value of the response change amount) measured when the current changes. In other words, while the current change amount is a fixed value, the element impedance is calculated using only the voltage change amount ΔV (measured value of the response change amount) as an operation parameter. According to this configuration, the measurement of the current change amount that is the applied change amount is not a requirement, and the measurement error caused by the measurement of the current change amount can be excluded from the element impedance calculation error. Therefore, the calculation accuracy of the element impedance can be improved. In the above configuration, the current switching circuit 51 corresponds to the “change applying unit”, and the ΔV detection circuit 52 corresponds to the “response change amount measuring unit”.

  In the configuration of the sensor control circuit 100 (see FIG. 1), an amplifier circuit may be added to the output stage of the ΔI detection circuit 32 for outputting a current change amount signal at the time of impedance detection. That is, as shown in FIG. 7, for example, an amplifier circuit unit with an amplification factor β (β = 2) is provided. In such a case, the resistance value constituting the amplifier circuit unit may be trimmed to improve the signal amplification accuracy, thereby absorbing the operation-side detection error.

  In the first embodiment, the A / F sensor having the sensor element structure shown in FIG. 2 has been described. However, the present invention can also be applied to an A / F sensor having another sensor element structure. . For example, the present invention is not limited to a configuration having a single-layer solid electrolyte body, but may be applied to an A / F sensor having a configuration having a two-layer solid electrolyte body or a configuration having a three-layer solid electrolyte body, The present invention can be applied not only to the A / F sensor having a structure but also to an A / F sensor having a cup-type structure.

  Another specific example of the sensor will be described. The sensor element SE1 shown in FIG. 8 has two layers of solid electrolytes 81 and 82. One solid electrolyte 81 has a pair of electrodes 83 and 84 facing each other, and the other solid electrolyte 82 has a pair of electrodes. Electrodes 85 and 86 are arranged to face each other. In addition, although the electrodes 83 to 85 are seen at two places on the left and right objects in the figure, they are the same member connected at any part of the front and back of the paper. In the sensor element SE1, a pump cell 91 is configured by the solid electrolyte 81 and the electrodes 83 and 84, and an oxygen detection cell 92 is configured by the solid electrolyte 82 and the electrodes 85 and 86. Each of the electrodes 83 to 86 is connected to the sensor control circuit 100. In FIG. 8, reference numeral 87 is a gas introduction hole, reference numeral 88 is a porous diffusion layer, reference numeral 89 is an atmospheric duct, and reference numeral 90 is a heater. The oxygen detection cell 92 is generally also referred to as an electromotive force cell or an oxygen concentration detection cell.

  In the A / F sensor having the sensor element structure, the oxygen detection cell 92 generates a binary (0 V or 0.9 V) electromotive force output depending on whether the exhaust gas is lean or rich with respect to stoichiometry. For example, in the case of lean, the electromotive force output of the oxygen detection cell 92 is reduced, and conversely, in the case of being rich, the electromotive force output of the oxygen detection cell 92 is increased. In such a case, the applied voltage of the pump cell 91 is controlled so that the electromotive force output of the oxygen detection cell 92 becomes the stoichiometric value (0.45 V).

  The sensor element structure shown in FIG. 9 may be used. 9 includes three layers of solid electrolytes 101, 102, and 103. A pair of electrodes 104 and 105 are disposed opposite to the solid electrolyte 101, and a pair of electrodes 106 and 107 are disposed on the solid electrolyte 102. Opposed. In this sensor element SE2, a pump cell 111 is constituted by the solid electrolyte 101 and the electrodes 104, 105, and an oxygen detection cell 112 is constituted by the solid electrolyte 102 and the electrodes 106, 107. Further, the solid electrolyte 103 constitutes a wall material for securing the oxygen reference chamber 108. In FIG. 9, reference numeral 109 denotes a porous diffusion layer, and reference numeral 110 denotes a gas detection chamber. Note that the oxygen detection cell 112 is also generally called an electromotive force cell or an oxygen concentration detection cell, like the oxygen detection cell 72 of FIG.

  In addition to the A / F sensor whose oxygen concentration is a detection target, the present invention can be applied to a gas concentration sensor whose detection target is another component concentration. For example, a composite type gas concentration sensor has a plurality of cells formed of a solid electrolyte body, and the first cell (pump cell) discharges or draws out oxygen in the gas to be detected and detects the oxygen concentration. In the second cell (sensor cell), the specific component concentration is detected from the gas after oxygen discharge. This gas concentration sensor is embodied as, for example, a NOx sensor for detecting NOx concentration in exhaust gas, and by applying the present invention, element resistance (element impedance or element admittance) can be calculated with high accuracy. . At this time, the element resistance may be detected for any one of the first cell and the second cell. Further, in addition to the first cell and the second cell, a gas concentration sensor having a plurality of cells such as a third cell (monitor cell or second pump cell) for detecting the residual oxygen concentration after the oxygen is discharged. good.

  In addition to the gas concentration sensor capable of detecting the NOx concentration, the present invention can also be applied to a gas concentration sensor capable of detecting the HC concentration and the CO concentration as the specific component concentration. In this case, the gas concentration sensor discharges surplus oxygen in the gas to be detected by the pump cell, and decomposes HC and CO from the gas after the surplus oxygen is discharged by the sensor cell to detect the HC concentration and the CO concentration.

  As an electromotive force output type gas sensor, in addition to a sensor that generates an electromotive force between electrodes according to the oxygen concentration (O2 concentration) in exhaust gas, an electromotive force is generated between electrodes according to the concentration of NOx, CO, etc. containing oxygen components. It may be a sensor that generates That is, when NOx and CO are decomposed at one electrode to generate oxygen ions and a difference in oxygen partial pressure occurs on both sides across the solid electrolyte body, an electromotive force is generated according to the difference in oxygen partial pressure. At this time, an electromotive force based on the Nernst equation is generated. The present invention can also be applied to a gas sensor having such a configuration.

  Furthermore, it can be used for gas concentration detection devices other than those for automobiles, and gas other than exhaust gas can be used as a detection gas.

It is a block diagram which shows the sensor control circuit in embodiment of invention. It is sectional drawing which shows the structure of a sensor element. It is a figure which shows the output characteristic of an A / F sensor. It is a flowchart which shows the calculation process of element impedance. It is a block diagram which shows the sensor control circuit in 2nd Embodiment. It is a block diagram which shows a sensor control circuit in another form. It is a block diagram which shows the sensor control circuit in another form. It is sectional drawing which shows the structure of another sensor element. It is sectional drawing which shows the structure of another sensor element.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Sensor element, 11 ... Solid electrolyte, 22 ... Current detection resistance, 32 ... ΔI detection circuit, 35 ... Voltage switching circuit, 51 ... Current switching circuit, 52 ... ΔV detection circuit, 60 ... O2 sensor, 63 ... AC voltage source 64 ... voltage dividing resistor, 65 ... coupling capacitor, 100 ... sensor control circuit, 200, 300 ... microcomputer.

Claims (8)

Gas concentration detection that applies a voltage to a sensor element made of a solid electrolyte body, measures the element current flowing through the sensor element in the voltage application state, and detects the gas concentration in the gas to be detected based on the measured value of the element current In the device
Change applying means for temporarily applying a predetermined amount of voltage change or current change on a current path connected to the sensor element;
A response change amount measuring means for measuring a response change amount of a current or voltage in response to the predetermined amount of voltage change or current change;
An element resistance calculation means for calculating an element resistance value of the sensor element using only the response change amount of the current or voltage measured by the response change amount measurement means as an operation parameter;
A gas concentration detection device comprising: Gas concentration detection that applies a voltage to a sensor element made of a solid electrolyte body, measures the element current flowing through the sensor element in the voltage application state, and detects the gas concentration in the gas to be detected based on the measured value of the element current In the device
Change applying means for temporarily applying a predetermined amount of voltage change or current change on a current path connected to the sensor element;
A response change amount measuring means for measuring a response change amount of a current or voltage in response to the predetermined amount of voltage change or current change;
The applied change amount of voltage or current by the change applying unit is registered in advance, and the sensor is based on the registered applied change amount and the current or voltage response change amount measured by the response change measuring unit. Element resistance calculating means for calculating the element resistance value of the element;
A gas concentration detection device comprising:   The response variation measuring unit includes a response variation measuring circuit that amplifies a measured value of the current or voltage response variation and outputs the response variation signal as a response variation signal, while the element resistance calculation unit includes the response variation signal. The gas concentration detection apparatus according to claim 1, further comprising a microcomputer that performs A / D conversion of the gas and calculates an element resistance value using the A / D value. A sensor element comprising a solid electrolyte body and a pair of electrodes provided between the solid electrolyte bodies, and is generated between the electrodes according to the oxygen concentration in the gas to be detected or the concentration of a specific component containing at least oxygen. In a gas concentration detection device applied to a gas sensor that generates electric power,
Change applying means for temporarily applying a predetermined amount of voltage change or current change on a current path connected to the sensor element;
A series circuit unit composed of a resistance element and a capacitance element connected between the change applying means and the sensor element;
Terminal voltage measuring means for measuring the terminal voltage of the sensor element;
An element resistance calculation means for calculating an element resistance value of the sensor element, using only the terminal voltage of the sensor element measured by the terminal voltage measurement means at the time of voltage change or current change by the change applying means,
A gas concentration detection device comprising: A sensor element comprising a solid electrolyte body and a pair of electrodes provided between the solid electrolyte bodies, and is generated between the electrodes according to the oxygen concentration in the gas to be detected or the concentration of a specific component containing at least oxygen. In a gas concentration detection device applied to a gas sensor that generates electric power,
Change applying means for temporarily applying a predetermined amount of voltage change or current change on a current path connected to the sensor element;
A series circuit unit composed of a resistance element and a capacitance element connected between the change applying means and the sensor element;
Terminal voltage measuring means for measuring the terminal voltage of the sensor element;
The sensor element measured in advance by the terminal voltage measuring means when the applied change amount of voltage or current by the change applying means is registered in advance and the voltage change or current change by the change applying means is registered. Element resistance calculation means for calculating the element resistance value of the sensor element based on the terminal voltage of
A gas concentration detection device comprising:   The terminal voltage measuring means includes an output circuit that amplifies and outputs a measured value of the terminal voltage of the sensor element, while the element resistance calculating means performs A / D conversion on the output signal of the output circuit, and the A The gas concentration detection apparatus according to claim 4, further comprising a microcomputer that calculates an element resistance value using the / D value.   The gas concentration detection device according to any one of claims 1 to 6, further comprising an applied change amount measuring circuit that measures and outputs an applied change amount of voltage or current by the change applying unit.   The gas concentration detection device according to any one of claims 1 to 7, wherein the electric circuit that constitutes the change applying means has finely adjusted electric characteristics so that an applied change amount of voltage or current has a desired accuracy.

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