JP4569701B2 - Gas concentration detector - Google Patents

Description

  The present invention relates to a gas concentration detection apparatus that detects the gas concentration of a specific component in the gas using exhaust gas or the like of an in-vehicle engine as a detection gas.

  This type of gas concentration detection device is embodied, for example, as an air-fuel ratio detection device that detects an oxygen concentration (air-fuel ratio: A / F) in the same gas using exhaust gas discharged from an in-vehicle engine as a detected gas. . The detection result of the air-fuel ratio is used in an air-fuel ratio control system constituted by an engine ECU or the like, and the stoichiometric combustion control in which the air-fuel ratio is feedback-controlled near the stoichiometric (theoretical air-fuel ratio) or the air-fuel ratio in a predetermined lean region. Lean combustion control and the like for feedback control are realized. In recent years, exhaust gas regulations and abnormality detection regulations (OBD) have been increasingly strengthened, and it is desired to improve controllability such as stoichiometric control. In addition to the lean region corresponding to the air-fuel ratio control range, the air-fuel ratio is not limited to the atmospheric state. There is a need to expand the detection range. For example, as a response to OBD (detection of a failure that deteriorates the exhaust gas of parts related to exhaust gas regulations), deterioration such as clogging of the sensor at the time of combustion cut in the engine's predetermined operating state (exhaust gas is equivalent to the atmosphere) (decrease in sensor gas current) ) Must be detected. It is also important to improve the fuel consumption along with the improvement of exhaust gas emission, and it is also important to feedback control the rich state at the time of high engine load.

  Further, in a lean combustion engine in which a NOx occlusion reduction type catalyst is mounted in the exhaust system, a large amount of NOx is occluded in the catalyst with the lean combustion, and the NOx occlusion capacity gradually decreases. In addition to this, since the fuel contains sulfur, the NOx storage reduction catalyst is poisoned with sulfur. Therefore, rich air-fuel ratio control is performed in the rich region in order to recover the NOx storage capacity of the catalyst and to recover sulfur poisoning. In view of these circumstances, in the air-fuel ratio control system described above, there is a strong demand to extend the air-fuel ratio detection range to enable wide-range air-fuel ratio detection and to further improve the air-fuel ratio detection accuracy within the same range.

  Conventionally known A / F sensors (oxygen concentration sensors) for detecting the air-fuel ratio include A / F sensors having a cup-type structure and A / F sensors having a laminated structure. Briefly describing the sensor structure, in the cup-type A / F sensor, the solid electrolyte is formed into a cup shape having a U-shaped cross section, and a pair of electrodes are installed on the inner and outer surfaces of the solid electrolyte, and the outer side. Is provided with a diffusion layer to constitute a sensor element. Further, a rod-shaped heater is disposed in the hollow portion of the solid electrolyte, and the entire sensor element is heated and maintained in an active state by the heat generated by the heater. The hollow portion of the solid electrolyte constitutes an air duct.

  On the other hand, in the laminated A / F sensor, the solid electrolyte and the diffusion layer are formed in an elongated plate shape, and further, including an insulating layer that forms an air duct, these are laminated to constitute a sensor element. A pair of electrodes is disposed opposite to the solid electrolyte, and a heater is embedded in the insulating layer.

  When the cup type A / F sensor and the laminated type A / F sensor are compared in terms of structure, the cup type A / F sensor has a relatively large volume (mass) of sensor elements heated for activation. Become. For this reason, there are problems that the time required to complete activation at the time of cold start becomes long, and that early activation becomes difficult, and that heater power consumption necessary for activating the element increases. On the other hand, in the stacked A / F sensor, early activation can be realized because the heater can be integrated with the sensor element, the volume of the sensor element can be reduced, and the like. Moreover, the advantage that the power consumption of the heater can be reduced is also obtained. For these reasons, the shift from the cup-type A / F sensor to the stacked A / F sensor is progressing while the sensor structure is being improved.

  In the case of a stacked A / F sensor, the volume of the air duct is reduced as the entire element is reduced in size. In this case, with the downsizing of the air duct, it is necessary to reduce the current flowing through the sensor element (element current). In other words, when exhaust gas is rich, oxygen is pumped out from the air duct side to the exhaust gas side. At this time, if the element current is large, the amount of oxygen transfer increases and the size of the air duct increases. Therefore, it is necessary to reduce the device current in order to realize miniaturization. Specifically, it is conceivable to reduce the device current by reducing the electrode or increasing the rate limiting rate of the diffusion layer (for example, decreasing the porosity of the diffusion layer).

  Next, a circuit configuration for current detection by the A / F sensor will be described. FIG. 16 is an electrical configuration diagram showing a conventional sensor control circuit.

  In FIG. 16, a reference voltage power supply 153 is connected to the positive terminal (+ terminal) of the sensor element 150 via an operational amplifier 151 and a current detection resistor 152, and an operational amplifier is connected to the negative terminal (−terminal) of the sensor element 150. An applied voltage control circuit 155 is connected via the 154. In this case, the point A at one end of the current detection resistor 152 is held at the same voltage as the reference voltage Vf. The element current flows through the current detection resistor 152, and the B point voltage changes according to the element current. For example, in the case of lean, the current flows from the positive terminal of the sensor element 150 to the same negative terminal, so that the point B voltage increases. In the case of rich, the current flows from the negative terminal of the sensor element 150 to the positive terminal so To do. The applied voltage control circuit 155 controls the applied voltage (that is, the D point voltage) to the sensor element 150 according to the B point voltage. The point B voltage is output as an A / F output to a microcomputer (not shown) or the like via the operational amplifier 156.

  On the other hand, in order to keep the sensor element 150 in a predetermined active state, it is necessary to maintain the AC impedance Zac of the element at a target value, and the heater is energized according to the deviation between the detected value of the impedance Zac and the target value. Be controlled. In this case, the applied voltage control circuit 155 changes the D point voltage in an alternating manner, and the current change amount ΔI obtained by dividing the D point voltage change amount ΔV and the B point voltage change amount by the resistance value of the current detection resistor 152 at that time. Thus, the impedance Zac is detected (Zac = ΔV / ΔI).

  Here, the difference between the sensor characteristics and the air-fuel ratio detection resolution of the cup-type A / F sensor and the laminated A / F sensor will be described with reference to the circuit configuration of FIG. The air-fuel ratio detection range of the A / F sensor is from A / F = 11 (hereinafter referred to as A / F11) to the atmospheric state.

First, specific numerical conditions for the cup-type A / F sensor are as follows. That is, “element current in the atmosphere: 25 mA, element current in A / F 11: −13 mA, AC impedance Zac: 22Ω, DC internal resistance Ri: 30Ω, voltage change when detecting Zac: ± 0.3 V, current detection resistance : 63Ω, reference voltage: 2.5V ”. In such a case, in the stoichiometric state, the point B voltage is 2.5 V, which is the same as the point A voltage. Further, the A / F output (point B voltage) output via the operational amplifier 156 shows the following numerical values at the time of the atmosphere and at the time of the A / F 11 respectively.
Output (atmosphere) = 2.5V + 63Ω × 25mA = 4.075V
Output (A / F11) = 2.5V + 63Ω × (−13 mA) = 1.661V
The A / F output is usually taken in via a 10 to 12 bit A / D converter in a microcomputer or the like. For example, when the detection resolution of A / F 11 to the atmosphere when 10 bit A / D is used is calculated as follows: Become.
(4.075-1.681) / 5V × 1024 = 490
If the current change corresponding to 1 A / F in the vicinity of stoichiometry is 2 mA, the detection resolution is as follows.
2mA × 63Ω / 5V × 1024 = 25
Further, the point D voltage changes with a predetermined width on both the positive and negative sides at the time of impedance detection, but the B point voltage shows the following numerical values in accordance with the positive side change of the D point voltage in the atmosphere and at A / F11.
Point B voltage = 4.075V + 63Ω × (0.3V / 22Ω) = 4.934V
Point B voltage = 1.681V + 63Ω x (0.3V / 22Ω) = 2.54V
On the other hand, when the D point voltage is changed to the negative side, the B point voltage changes to the negative side, and the minimum value of the B point voltage is as follows (A / F11 time is the minimum).
Point B voltage = 1.681V + 63Ω x (-0.3V / 22Ω) = 0.822V
According to the above numerical values, even if the D point voltage is changed to both positive and negative sides at the time of impedance detection, the B point voltage at that time is within the operating voltage range (0 to 5 V) of the A / D converter. In short, it can be seen that the design is made so that proper impedance detection can be performed.

On the other hand, in the case of the multilayer A / F sensor, the element current is reduced as described above, and the element current is about 1/10 that of the cup-type A / F sensor. The specific numerical conditions are as follows. That is, “element current in the atmosphere: 2.5 mA, element current at A / F 11: −1.3 mA, AC impedance Zac: 28Ω, DC internal resistance Ri: 60Ω, voltage change when detecting Zac: ± 0.3 V , Current detection resistance: 185Ω, reference voltage: 2.5V ”. In such a case, in the stoichiometric state, the point B voltage is 2.5 V, which is the same as the point A voltage. Further, the A / F output (point B voltage) output via the operational amplifier 156 shows the following numerical values at the time of the atmosphere and at the time of the A / F 11 respectively.
Output (atmosphere) = 2.5V + 185Ω × 2.5mA = 2.9625V
Output (A / F11) = 2.5V + 185Ω × (−1.3 mA) = 2.2595V
This multilayer A / F sensor is also designed so that the B point voltage and the D point voltage are normally measured during impedance detection, and proper impedance detection can be performed. Incidentally, when the voltage values at the points B and D at the time of impedance detection are calculated in the same manner as the cup type A / F sensor, when the voltage at the point D is changed to the positive side with a width of 0.3 V,
B point voltage = 4.9446V in atmospheric condition
In A / F11, B point voltage = 4.2416V
It becomes. Further, when the point D voltage is changed negatively with a width of 0.3 V, the minimum value of the point B voltage = 0.277 V (A / F11 time is minimum).

In the stacked A / F sensor, for example, the detection resolution of A / F11 to the atmosphere when 10-bit A / D is used is calculated as follows.
(2.9625-25.595) / 5V × 1024 = 144
Therefore, the resolution is only about 0.3 times (144/490 = 0.294) compared to the cup-type A / F sensor.

If the current change corresponding to 1 A / F in the vicinity of stoichiometric is 0.2 mA, the detection resolution is as follows.
0.2mA × 185Ω / 5V × 1024 = 7
This also has a resolution of about 0.3 times (7/25 = 0.28) compared to the cup-type A / F sensor.

  Here, the reason why the detection accuracy of the laminated A / F sensor is lower than that of the cup-type A / F sensor will be described below.

  As described above, in the multilayer A / F sensor, the element current is reduced to about 1/10 of the cup-type A / F sensor. Therefore, when the air-fuel ratio detection range is A / F11 to the atmosphere, this air-fuel ratio detection is performed. The element current range corresponding to the range is 38 mA (-13 to 25 mA) for the cup-type A / F sensor and 3.8 mA (-1.3 to 2.5 mA) for the stacked type A / F sensor. Further, the alternating current at the time of impedance detection is 13.6 mA (= 0.3 V / 22Ω) in the cup-type A / F sensor, and 10.7 mA (= 0.3 V / 28Ω) in the laminated A / F sensor. . In this case, the ratio of the impedance detection current to the element current at the time of A / F detection is 35.8% (13.6 mA / 38 mA = 0.358) in the cup-type A / F sensor, whereas the stacked type A For the / F sensor, this is 281.6% (10.7 mA / 3.8 mA = 2.816).

  That is, in the laminated A / F sensor, the ratio of the impedance detection current to the element current at the time of A / F detection is remarkably large. As a result, the resistance value of the current detection resistor for detecting the element current becomes relatively small, resulting in a problem that the A / F detection resolution is lowered.

  Incidentally, if the DC internal resistance of the sensor element (same as the AC impedance) of the stacked A / F sensor is increased, the impedance detection current decreases, and the ratio of the impedance detection current to the element current at the time of A / F detection decreases. However, if the DC internal resistance of the sensor element is increased, the sensor characteristics (see characteristics in FIG. 3) change, and problems such as the inability to perform applied voltage control as desired occur. Therefore, in reality, it is desirable to keep the DC internal resistance of the sensor element substantially constant.

  By the way, as a prior art for improving the detection accuracy on the premise that a wide air-fuel ratio detection range is required, a gas concentration detection device of Patent Document 1 (Japanese Patent Laid-Open No. 11-37971) has been proposed. FIG. 17 shows the configuration of the sensor control circuit according to Patent Document 1.

  Only the difference from FIG. 16 will be described. In FIG. 17, two resistors 161 and 162 are connected in series as current detection resistors, and either point B or point C in the figure is selectively connected via the switch 163. Are connected to an operational amplifier 156. In this case, the switch 163 is switched according to the A / F value in each case. Specifically, for example, in the atmosphere, the switch 163 is switched to the point C side, the element current is measured by the resistor 161, and the measurement result is output via the operational amplifier 156. Further, at the time of stoichiometry, the switch 163 is switched to the point B side, the element current is measured by the resistor 161 and the resistor 162, and the measurement result is output via the operational amplifier 156.

  With the above configuration, detection accuracy is ensured in a wide air-fuel ratio detection range, and detection accuracy particularly in the vicinity of the stoichiometry is improved.

  However, even in the configuration of FIG. 17, when the element current is reduced when the stacked A / F sensor is used, the ratio of the impedance detection current to the element current at the time of A / F detection is also greatly increased. Therefore, the problem that the detection resolution of A / F falls arises. Therefore, the problem of a decrease in detection accuracy when the element current becomes small is still a matter of concern.

JP-A-11-37971

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a gas concentration detection device capable of improving the gas concentration detection accuracy in a desired gas concentration detection range. .

  As a premise in the gas concentration detection device of the present invention, the current flowing through the element when a voltage is applied to the sensor element is measured by a current detection resistor, and the measured signal is A / D converted by an A / D converter. . Further, the gas concentration calculation is performed based on the signal after A / D conversion. In particular, in the first aspect of the present invention, a plurality of detection ranges are set within the entire detection range of the gas concentration, and a signal measured by the current detection resistor has a predetermined amplification factor in at least one detection range. After being amplified by the amplifier having, it is output to the A / D converter. In other detection ranges, the signal is output to the A / D converter without passing through an amplifier.

  In short, even if the gas concentrations are the same, the signal level input to the A / D converter is expanded to the positive side or the negative side by amplifying the measured signal. As a result, the gas concentration detection resolution can be increased. In addition, an amplifier is provided corresponding to any one of the plurality of detection ranges, and optimal amplification processing can be performed for each individual detection range. As a result, the detection accuracy can be improved in a desired gas concentration detection range.

  In the gas concentration detection apparatus according to the first aspect, a device resistance detection means is also provided. That is, the applied voltage or current of the sensor element is changed in an alternating manner, and the change amount of the current or voltage at that time is measured by the current detection resistor to detect the resistance value of the sensor element.

The present invention is particularly useful when a gas concentration sensor having a relatively small element current is used. For example, a desirable effect can be obtained when a gas concentration sensor having a stacked structure is used. Specifically, the sensor element has a large current change at the time of detecting the element resistance with respect to the element current at the time of detecting the gas concentration. In other words, if the element current is small, there is a current imbalance in which the current change at the time of detecting the element resistance is larger than the element current at the time of detecting the gas concentration when the current detection resistor is also used for detecting the element resistance. Arise. According to the present invention, it is possible to detect a gas concentration with high accuracy even when current imbalance occurs.
In the first aspect of the present invention, when a fixed reference voltage is applied to the first terminal of the current detection resistor and the applied voltage or current of the sensor element is changed in an alternating manner, the current detection resistor first The device resistance value is detected by measuring the voltage value of two terminals. Then, switch means for opening or closing between the amplifier and the second terminal of the current detection resistor is provided. In other words, when the sensor resistance value is detected, if the applied voltage or current of the sensor element is changed in an alternating manner, the signal that is the output of the amplifier may change carelessly, which may adversely affect the gas concentration detection. is there. On the other hand, as described above, by providing the switch means between the amplifier and the second terminal of the current detection resistor, adverse effects on the gas concentration detection can be eliminated.

  In a second aspect of the present invention, a relatively wide first detection range and a relatively narrow second detection range are set as the plurality of detection ranges. Then, a wide range output corresponding to the first detection range is output without passing through the amplifier, and a narrow range output corresponding to the second detection range is output through the amplifier. In this case, it is possible to improve the gas concentration detection accuracy (detection resolution) particularly in the second detection range.

  In a third aspect of the present invention, as the plurality of detection ranges, an entire gas concentration detection range and a specific range which is a part thereof are set. A wide-range output corresponding to the entire detection range is output without passing through the amplifier, and a narrow-range output corresponding to the specific range is output through the amplifier. In this case, the detection accuracy (detection resolution) of the gas concentration can be improved particularly in a specific range.

  According to a fourth aspect of the present invention, the sensor element is a laminated element having a plate-shaped solid electrolyte, a diffusion layer and an insulating layer laminated thereon, and an oxygen reference chamber. In this stacked sensor element, there is a large space restriction for installing the oxygen reference chamber (or atmospheric duct), and the element current is inevitably reduced because the volume that can be secured as the oxygen reference chamber is reduced. Even if current imbalance occurs, highly accurate gas concentration detection is possible.

  As described in claim 5, the gas concentration sensor of the present invention preferably has a sensor element capable of detecting the air-fuel ratio of the combustion gas over a wide area. Specifically, for example, the air-fuel ratio can be detected in a wide range of rich to atmospheric conditions. In this case, the air-fuel ratio detection accuracy can be improved within a desired air-fuel ratio detection range.

  The third aspect improves the gas concentration detection accuracy (detection resolution) in a specific range. In this case, as described in the sixth aspect, the specific range includes a stoichiometric air-fuel ratio (stoichiometric). It should be set to a range. Thereby, the detection accuracy of the stoichiometry and the air-fuel ratio in the vicinity thereof is improved, so that highly accurate stoichiometric control can be realized.

  In the invention according to claim 7, the sensor element is formed by stacking a pump cell for taking in and out oxygen in the detection gas and an oxygen detection cell for outputting a signal corresponding to the oxygen concentration of the detection gas. The pump cell is controlled so that the output signal of the oxygen detection cell becomes a predetermined value. Even with such a sensor element, current unbalance may occur due to a reduction in the element current. However, even in such a situation, highly accurate gas concentration detection is possible.

  In the case of claim 7, as described in claim 8, the internal resistance of the oxygen detection cell is converted into a voltage, or as described in claim 9, the gas concentration sensor includes: An air-fuel ratio sensor that can detect the air-fuel ratio of the combustion gas in a wide range may be used. As the air-fuel ratio sensor, for example, the air-fuel ratio can be detected in a wide range of rich to atmospheric conditions, and the detection accuracy of the air-fuel ratio is improved in a desired air-fuel ratio detection range.

  In addition, as described in claim 10, the gas concentration sensor has a plurality of cells formed of a solid electrolyte, of which the first cell discharges or pumps out oxygen in the gas to be detected, and the second cell. Then, it is good to detect the gas concentration of a specific component from the gas after oxygen discharge. This gas concentration sensor is embodied as, for example, a NOx sensor that detects the NOx concentration in the exhaust gas. The application of the present invention improves the detection accuracy of the NOx concentration.

  According to an eleventh aspect of the present invention, the voltage of both terminals of the current detection resistor is respectively input to the positive and negative input terminals of the amplifier, and the feedback current path of the amplifier is for absorbing the feedback current. An operational amplifier is installed. In other words, if a feedback current flows in the feedback current path of the amplifier, the signal measured by the current detection resistor may change carelessly, which may adversely affect gas concentration detection. On the other hand, the adverse effect on the gas concentration detection can be eliminated by providing the operational amplifier for absorbing the feedback current as described above.

More preferably, as described in claim 12 , when a capacitor for holding a voltage level immediately before the switch means is opened (when OFF), the amplifier and the switch means are connected. good. Thereby, even when the element resistance value is detected, an appropriate gas concentration output (actually a signal immediately before opening the switch) can be obtained.

As described above, in the case where the sensor signal is amplified for each detection range using amplifiers having different amplification factors, the output of the amplifier having a large amplification factor exceeds the operating voltage range of the A / D converter, and the A / D converter There is concern about latch-up. In this case, in order to protect the A / D converter, it is desirable to limit the output of the amplifier (that is, the A / D input) near the maximum operating voltage of the A / D converter. Therefore, in the invention described in claim 13 , a clamp circuit for limiting the output of the amplifier near the maximum operating voltage of the A / D converter is provided at the output portion of the amplifier. As a result, the A / D converter can be protected.

Specific as the clamp circuit as described in claim 14, a diode connected in a forward direction with respect to the constant voltage source of maximum operating voltage equivalent of the A / D converter from the output path of the amplifier It is good to have. In this case, the output of the amplifier is limited by a voltage value obtained by adding the voltage drop of the diode to the constant voltage source (corresponding to the maximum operating voltage of the A / D converter).

Or, as described in claim 15, wherein the clamp circuit includes a pnp-type transistor having a base terminal connected to a reference voltage source with an emitter terminal connected to the output path of the amplifier, the base of the transistor The input voltage may be a voltage value obtained by subtracting the voltage drop between the base and the emitter from a constant voltage source corresponding to the maximum operating voltage of the A / D converter. In this case, if the base input voltage of the transistor is a voltage value obtained by subtracting the voltage drop between the base and the emitter from a constant voltage source (corresponding to the maximum operating voltage of the A / D converter), the voltage value of the emitter terminal is A The voltage value corresponds to the maximum operating voltage of the / D converter, and the output of the amplifier is limited in the vicinity of the voltage value corresponding to the maximum operating voltage.

It is a block diagram which shows the air fuel ratio detection apparatus in embodiment of invention. It is sectional drawing which shows the structure of an A / F sensor. It is a figure which shows the output characteristic of an A / F sensor. It is a figure which shows the relationship between element current and A / F value. It is a circuit diagram which shows the structure of another sensor control circuit. It is a circuit diagram which shows the structure of another sensor control circuit. It is a circuit diagram which shows the structure of the sensor control circuit in 2nd Embodiment. It is a circuit diagram which shows the structure of the sensor control circuit in 3rd Embodiment. It is a circuit diagram which shows the structure of the sensor control circuit in 4th Embodiment. It is a circuit diagram which shows the structure of another sensor control circuit. It is a circuit diagram which shows the structure of another sensor control circuit. It is sectional drawing which shows the structure of another A / F sensor. It is a circuit diagram which shows the structure of a sensor control circuit. It is a circuit diagram which shows the structure of a sensor control circuit. It is sectional drawing which shows the structure of another A / F sensor. It is a circuit diagram which shows the structure of the sensor control circuit in a prior art. It is a circuit diagram which shows the structure of the sensor control circuit in a prior art.

(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 the oxygen concentration (air-fuel ratio: A / F) in the gas using the exhaust gas (combustion gas) discharged from the vehicle-mounted engine as the detection gas is embodied. The detection result of the fuel ratio is used in an air-fuel ratio control system configured 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 realized. Further, in the present embodiment, wide-area air-fuel ratio detection corresponding to recent or future exhaust gas regulations and abnormal detection regulations (OBD), NOx occlusion release of NOx occlusion reduction type catalysts installed in the exhaust system, sulfur poisoning regeneration In order to perform such control, the air-fuel ratio can be detected in a wide range from the rich region (for example, A / F11) to the atmospheric state.

  First, the configuration of the A / F 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. The rectangular plate-shaped solid electrolyte 11 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 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, spinel or zirconia by a sheet forming method or the like, but the gas permeability varies depending on the average pore diameter and porosity of the porosity. Yes.

  The insulating layer 14 is made of high thermal conductive ceramic such as alumina, and an air duct 17 is formed at a portion facing the electrode 16. A heater 18 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 sensor element 10, the surrounding exhaust gas is introduced from the side portion of the diffusion resistance layer 12 and reaches the electrode 15. When the exhaust gas is lean, oxygen in the exhaust gas is decomposed by the electrode 15 and discharged from the electrode 16 to the atmospheric duct 17. On the contrary, when the exhaust gas is rich, oxygen in the air duct 17 is decomposed by the electrode 16 and discharged from the electrode 15 to the exhaust side.

  FIG. 3 is a diagram showing the voltage-current characteristics (VI characteristics) of the A / F sensor. In FIG. 3, the straight line portion parallel to the V-axis (horizontal axis) is a limit current region for specifying the element current (limit current) of the sensor element 10, and the increase / decrease in the element current is an increase / decrease in the air-fuel ratio (ie, lean).・ Rich level). That is, the device current increases as the air-fuel ratio becomes leaner, and the device current decreases as the air-fuel ratio becomes richer. In addition, LX1 in the figure represents an applied voltage straight line (applied voltage characteristic) for determining the applied voltage to the sensor element 10, and its slope is generally in the resistance dominant region (lower voltage side than the limit current region). (Slant part).

  In particular, in the present embodiment, the air-fuel ratio detection range is a wide area from A / F11 to the atmosphere, the element current is −1.3 mA in A / F11, and the element current is 2.5 mA in the atmospheric state. These numerical values are approximately 1/10 compared with the A / F sensor having a cup-type structure.

  FIG. 4 is a drawing showing the relationship between A / F on the horizontal axis and device current I on the vertical axis. According to the figure, it can be seen that as the A / F shifts to the lean side, the current change corresponding to the unit A / F (the slope of the characteristic shown in the figure) decreases. In the vicinity of stoichiometric (A / F = 14.5), the current change corresponding to 1 A / F is 0.2 mA, which is also about 1/10 compared with the A / F sensor having a cup structure. .

  Next, the configuration of the sensor control system as the main part of the present invention will be described with reference to FIG. The sensor control system is provided with a microcomputer (hereinafter abbreviated as “microcomputer”) 20 and a sensor control circuit 30, thereby detecting A / F and sensor elements based on the detection results of the A / F sensor (sensor element 10). Ten impedances (element impedance Zac) are detected.

  In FIG. 1, a microcomputer 20 is composed of a well-known logic operation circuit including a CPU, various memories, an A / D converter, an I / O port, and the like. A current signal (detected by a sensor control circuit 30 described later) Analog signal) is taken in via an A / D converter, and an A / F value calculation and an element impedance Zac calculation are appropriately performed. The A / D converter has, for example, a 10-bit resolution, and the operating voltage range is 0 to 5V. The A / F value calculated by the microcomputer 20 is output to an engine ECU (not shown), for example, and used for air-fuel ratio feedback control and the like.

  In the sensor control circuit 30, a reference voltage power supply 33 is connected to the positive terminal (+ terminal) of the sensor element 10 via the operational amplifier 31 and the current detection resistor 32, and the negative terminal (−terminal) of the sensor element 10. ) Is connected to an applied voltage control circuit 35 via an operational amplifier 34. In this case, the point A (first terminal) at one end of the current detection resistor 32 is held at the same voltage as the reference voltage Vf. The element current flows through the current detection resistor 32, and the voltage at the point B (second terminal) changes according to the element current. For example, when the exhaust gas is lean, the current flows from the + terminal of the sensor element 10 to the same − terminal, so that the voltage at the point B rises. The voltage drops. The applied voltage control circuit 35 monitors the point B voltage and determines the voltage to be applied to the sensor element 10 according to the voltage value (for example, determined based on the applied voltage straight line LX1 in FIG. 3). To control the D point voltage. However, when A / F detection is performed only in the vicinity of the stoichiometry, the applied voltage can be fixed.

  Further, operational amplifiers (differential amplifiers) 36 and 37 constituting an amplifier are connected to points A and B at both ends of the current detection resistor 32, and outputs OP1 and OP2 of the operational amplifiers 36 and 37 are connected to the microcomputer 20. A / D0 and A / D1. The operational amplifiers 36 and 37 have a parallel relationship, the operational amplifier 36 has an amplification factor of 5 times, and the operational amplifier 37 has an amplification factor of 15 times. The operational amplifiers 36 and 37 are battery driven. The outputs OP1 and OP2 (current signals of the sensor element 10) of the operational amplifiers 36 and 37 are A / F detection signals for A / F detection, and the microcomputer 20 performs A / F based on the acquired A / D values. Calculate the value. That is, in the present embodiment, two sensor A / F detection signals are output from the sensor control circuit 30 to the microcomputer 20.

  Here, the operational amplifier 36 corresponds to a “first amplifier”, and is configured such that A / F detection is possible for the entire detection range of air-fuel ratio (for example, A / F 11 to the atmosphere) by the output OP 1 of the operational amplifier 36. . The operational amplifier 37 corresponds to a “second amplifier”, and the output OP2 of the operational amplifier 37 performs A / F detection for a specific range (for example, A / F12 to A / F22) including the stoichiometry in the entire air-fuel ratio detection range. It is configured to be possible. In the following description, the output OP1 is also referred to as a wide range detection signal, and the output OP2 is also referred to as a stoichiometric detection signal. The former corresponds to “wide range output” and the latter corresponds to “narrow range output”. However, the verification will be described later.

  On the other hand, the microcomputer 20 is configured to instruct to temporarily change the voltage applied to the sensor element 10 in an alternating manner, and to detect the element impedance Zac based on the current change amount at that time. More specifically, when the impedance is detected, the applied voltage control circuit 35 receives a command from the microcomputer 20, and the applied voltage to the sensor element 10 (D-point voltage in the figure) is positive and negative with a predetermined width (eg, 0.3V). To change. At this time, the microcomputer 20 inputs the voltage change at point D through A / D3. Further, the voltage at the B point changes according to the element impedance each time the voltage at the D point changes, and the microcomputer 20 inputs the voltage change at the B point through A / D2. In such a case, the microcomputer 20 obtains a change amount ΔV of the D point voltage (A / D3 input) and a current change amount ΔI obtained by dividing the change amount of the B point voltage (A / D2 input) by the resistance value of the current detection resistor 32. Is used to calculate the element impedance Zac (Zac = ΔV / ΔI). In the impedance detection, the current flowing through the sensor element 10 may be changed in an alternating manner, and the element impedance Zac may be calculated from the amount of change in current or voltage at that time.

  Impedance detection is performed in a predetermined cycle (that is, every predetermined time), and the execution timing is commanded from the microcomputer 20 to the applied voltage control circuit 35. Further, the microcomputer 20 controls energization to the heater 18 so that the element impedance Zac is maintained at a predetermined target value. As a result, the sensor element 10 is held in a predetermined active state.

  Next, the detection resolution of the A / F is verified for the sensor control circuit 30 configured as described above. The air-fuel ratio detection range of the A / F sensor is A / F11 to the atmosphere, and specific numerical conditions of the sensor element 10 and the sensor control circuit 30 are as follows. That is, “element current in the atmosphere: 2.5 mA, element current at A / F 11: −1.3 mA, element current at A / F 12: −0.79 mA, element current at A / F 22: 0.884 mA , AC impedance Zac: 28Ω, voltage change when detecting Zac: ± 0.3V, current detection resistance: 185Ω, reference voltage: 2.5V ”. The operational amplifiers 36 and 37 operate with a single power supply and are clamped at 5V, and therefore are configured to output no voltage other than 0 to 5V.

In this case, the operational amplifier 36 has a detection range from A / F11 to the atmosphere, and its output OP1 shows the following numerical values at the time of the atmosphere and at the time of A / F11.
OP1 = 2.5V + 185Ω × 2.5mA × 5 times = 4.8125V
OP1 = 2.5V + 185Ω × (−1.3 mA) × 5 times = 1.2975V
On the other hand, the operational amplifier 37 has a detection range of A / F12 to A / F22, and its output OP2 shows the following numerical values at A / F22 and A / F11, respectively.
OP2 = 2.5V + 185Ω × 0.884mA × 15 times = 4.953V
OP2 = 2.5V + 185Ω × (−0.79 mA) × 15 times = 0.30775V
From the above, the output OP1 of the operational amplifier 36 falls within the operating voltage range (0 to 5V) of the A / D converter (A / D0 in FIG. 1) in the air / fuel ratio detection range of A / F11 to the atmosphere. An appropriate signal output can be realized in the fuel ratio detection range. Further, the output OP2 of the operational amplifier 37 falls within the operating voltage range (0 to 5 V) of the A / D converter (A / D1 in FIG. 1) in the air-fuel ratio detection range of A / F11 to A / F22. An appropriate signal output can be realized in a specific air-fuel ratio detection range at the center.

Under the above numerical conditions, A / F11 to atmospheric detection resolution when 10-bit A / D is used is calculated as follows.
(4.8125-1.2975) / 5V × 1024 = 720
In this case, a resolution (720/144 = 5) that is five times that of the prior art (the circuit configuration of FIG. 16) is obtained.

If the current change corresponding to 1 A / F in the vicinity of stoichiometric is 0.2 mA, the detection resolution is as follows.
0.2mA × 185Ω × 15 times / 5V × 1024 = 114
In this case, it becomes 0.009 A / F per 1 LSB, and the requirement of high-precision control near the stoichiometry (for example, resolution of 0.01 A / F or less) can be satisfied.

Incidentally, at the time of impedance detection as described above, the element impedance Zac is calculated from the amount of change in the point B voltage and the amount of change in the point D voltage in FIG. 1, and the voltage at each point is also converted into an A / D converter at that time. It is designed to be within the operating voltage range (0 to 5 V) of (A / D2, A / D3 in FIG. 1). That is, at the time of impedance detection in the atmosphere and at the time of impedance detection at A / F11, with the positive change of the D point voltage, the B point voltage shows the following numerical values respectively, which are below the maximum operating voltage of the A / D converter. It has become.
Point B voltage = 2.9625V + 185Ω × (0.3V / 28Ω) = 4.9446V
Point B voltage = 2.2595V + 185Ω x (0.3V / 28Ω) = 4.2416V
On the other hand, the B point voltage changes to the negative side with the negative change of the D point voltage, and the minimum value of the B point voltage indicates the following numerical value (A / F11 time is the minimum).
Point B voltage = 2.2595V + 185Ω x (-0.3V / 28Ω) = 0.2774V
According to the above numerical values, even if the D point voltage is changed to both positive and negative sides at the time of impedance detection, the B point voltage at that time is within the operating voltage range (0 to 5 V) of the A / D converter, and the appropriate impedance is obtained. It can be seen that detection can be performed.

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

  The current signal measured by the current detection resistor 32 is amplified by operational amplifiers 36 and 37 that are provided for each of a plurality of detection ranges and each have a predetermined amplification factor, and then are sent to the microcomputer 20 (A / D converter). Therefore, even when the same air-fuel ratio is detected, the signal level input to the A / D converter is expanded to the positive side or the negative side, and the air-fuel ratio detection resolution can be increased. . In addition, since the operational amplifiers 36 and 37 are provided for each detection range of the air-fuel ratio, it is possible to perform optimum amplification processing for each individual detection range. As a result, it is possible to improve the detection accuracy in a desired wide-range air-fuel ratio detection range (a range including high lean and high rich). At this time, it is possible to arbitrarily differentiate the accuracy of individual detection ranges.

  The air-fuel ratio detection apparatus of the present embodiment is particularly useful when using a stacked A / F sensor. In other words, the sensor element 10 having a laminated structure causes a current imbalance at the time of impedance detection with respect to the element current at the time of A / F detection, so that current imbalance occurs. In the air-fuel ratio detection range, the detection accuracy can be improved.

  Further, since the amplification factor of the operational amplifier 37 is increased with respect to the specific range (A / F12 to A / F22) which is a part of the entire detection range (A / F11 to the atmosphere) of the air-fuel ratio, the detection of the air-fuel ratio is performed particularly in the specific range. Accuracy (detection resolution) can be improved. That is, since the specific range is set to a range including the stoichiometry, the accuracy of detecting the stoichiometry and the air-fuel ratio in the vicinity thereof is improved, and thus high-accuracy stoichiometric control can be realized.

When the operational amplifier 37 having a gain of 15 times is used as described above, the output OP2 is at atmospheric pressure and at A / F11 time.
OP2 = 2.5V + 185Ω × 2.5mA × 15 times = 9.4375V
OP2 = 2.5V + 185Ω × (−1.3 mA) × 15 times = −1.1075 V, which exceeds the range of 0 to 5 V that is the operating voltage range of the A / D converter. Moreover, a negative voltage cannot be output by circuit operation with a vehicle-mounted battery (single power supply). However, since the A / F detection signal is output in two systems as described above, the air-fuel ratio detection range that cannot be detected by the output OP2 of one operational amplifier 37 is detected by the output OP1 of the other operational amplifier 36. As a result, A / F detection can be realized for the entire desired detection range.

  The configuration shown in FIG. 1 can be modified as shown in FIGS. 5 and 6 show a circuit configuration in which a part of the configuration of the sensor control circuit 30 is changed. For convenience, the microcomputer 20 and the configuration related to the A / D capture of the point B voltage and the point D voltage are duplicated in FIG. Omitted (the same applies hereinafter).

  In short, in the configuration of FIG. 1, the operational amplifiers 36 and 37 amplify the voltage difference between the points A and B at both ends of the current detection resistor 32. However, in the configuration of FIG. Amplifies the voltage difference from the B point voltage. That is, since the point A voltage is the same as the reference voltage Vf, the reference voltage Vf is input to the operational amplifiers 36 and 37 via the operational amplifier 38.

  In the configuration of FIG. 1 above, the feedback of the operational amplifiers 36 and 37 is at point A, so that the feedback current of the operational amplifiers 36 and 37 flows to the current detection resistor 32 and an error may occur in the air-fuel ratio detection. In the configuration, the operational amplifier 38 for absorbing the feedback current is installed in the feedback current path of the operational amplifiers 36 and 37, and adverse effects on the air-fuel ratio detection can be eliminated. That is, the air-fuel ratio detection accuracy can be maintained. In general, the reference voltage Vf is often generated by dividing the two resistors, and the feedback current from the operational amplifiers 36 and 37 flows to change the reference voltage Vf. However, the operational amplifier 38 is not necessary when the reference voltage Vf can be sinked / sourced.

  Further, in FIG. 6, a switch 39 as a switch means is arranged on the path from the point B terminal (second terminal) of the current detection resistor 32 to each operational amplifier 36, 37, and the switch 39 is turned off when impedance is detected. . In other words, when the impedance is detected, if the voltage applied to the sensor element 10 is changed in an alternating manner, the current flowing through the current detection resistor 32 changes, which causes the current signals output from the operational amplifiers 36 and 37 to change carelessly. As a result, the air-fuel ratio detection may be adversely affected. On the other hand, by providing the switch 39 as described above, adverse effects on the air-fuel ratio detection can be eliminated.

  In this case, a capacitor 40 may be provided as shown, and the current signal level immediately before the switch 39 may be held when the switch 39 is turned off, that is, the current signal level immediately before the impedance detection. As a result, even when the element impedance Zac is detected, an appropriate air-fuel ratio output (actually a current signal immediately before the switch is turned off) can be obtained.

(Second Embodiment)
A second embodiment of the present invention will be described below with a focus on differences from the first embodiment.

In the configuration of FIG. 1, the operational amplifiers 36 and 37 are arranged in parallel to output the A / F detection signal in two systems. However, in the configuration of FIG. Although output, an operational amplifier is provided only in one output system. Specifically, in FIG. 7, among the two systems that output the A / F detection signal, the wide range detection signal (corresponding to the A / D0 input in FIG. 1) is output without passing through the operational amplifier, whereas the stoichiometric detection signal (Corresponding to the A / D1 input in FIG. 1) is output via the operational amplifier 41. The amplification factor of the operational amplifier 41 is 15 times.
The detection resolution of A / F is verified for the sensor control circuit 30 in FIG. The air-fuel ratio detection range of the A / F sensor is A / F11 to the atmosphere as in the first embodiment, and other specific numerical conditions of the sensor element 10 and the sensor control circuit 30 are also the same as those in the first embodiment. The same is true (see conditions above).

In such a case, the wide range detection signal is output as a point B voltage, and the point B voltage indicates the following numerical values in the atmosphere and at A / F11, respectively.
Point B voltage = 2.5V + 185Ω × 2.5 mA = 2.9625V
Point B voltage = 2.5V + 185Ω × (−1.3 mA) = 2.2595V
On the other hand, the operational amplifier 41 has a detection range of A / F12 to A / F22, and the stoichiometric detection signal (OP3 in the figure) shows the following numerical values at A / F22 and A / F11, respectively.
OP3 = 2.5V + 185Ω × 0.884mA × 15 times = 4.953V
OP3 = 2.5V + 185Ω × (−0.79 mA) × 15 times = 0.30775V
From the above, the wide range detection signal is within the operating voltage range (0 to 5V) of the A / D converter (A / D0 in FIG. 1) in the air / fuel ratio detection range of A / F11 to the atmosphere, and the wide range air / fuel ratio detection An appropriate signal output in the range can be realized. The stoichiometric detection signal (output of the operational amplifier 41) is within the operating voltage range (0 to 5 V) of the A / D converter (A / D1 in FIG. 1) in the air-fuel ratio detection range of A / F11 to A / F22. Thus, an appropriate signal output can be realized in a specific air-fuel ratio detection range centered on stoichiometry.

Under the above numerical conditions, the detection resolution of A / F11 to the atmosphere when 10 bit A / D is used is as follows.
(2.9625-25.595) / 5V × 1024 = 144
If the current change corresponding to 1 A / F in the vicinity of stoichiometric is 0.2 mA, the detection resolution is as follows.
0.2mA × 185Ω × 15 times / 5V × 1024 = 114
In this case, it becomes 0.009 A / F per 1 LSB, and the requirement of high-precision control near the stoichiometry (for example, resolution of 0.01 A / F or less) can be satisfied.

  In the case of the present embodiment, the wide-range detection signal has the same detection accuracy as that of the conventional technique (circuit configuration in FIG. 16), but can detect a wide range of air-fuel ratio. In addition, sufficient detection accuracy can be secured for the stoichiometric detection signal provided separately from the wide-range detection signal.

  Although the illustration of the impedance detection is omitted for the sake of convenience, the element impedance Zac is calculated from the change amount of the point B voltage and the change amount of the D point voltage in FIG. At that time, the voltage at each point is designed to be within the operating voltage range (0-5V) of the A / D converter (A / D2, A / D3 in FIG. 1), and proper impedance detection is performed. It can't be changed.

  Also in the sensor control circuit 30 of FIG. 7, it is possible to adopt the configurations of FIGS. 5 and 6 in the first embodiment described above. That is, the configuration of FIG. 5 is adopted, and the operational amplifier 41 is configured to amplify the voltage difference between the reference voltage Vf and the point B voltage, or the configuration of FIG. When a switch and a capacitor are arranged in the middle of the path to F detection (wide range detection, stoichiometric detection), and the current flowing through the current detection resistor 32 changes during impedance detection, the current change is inadvertently output. Or may be prevented.

(Third embodiment)
A third embodiment of the present invention will be described below with a focus on differences from the first embodiment. In the configuration of FIG. 8 in the present embodiment, a wide range of A / F detection signals are output in one system via the operational amplifier 42. The amplification factor of the operational amplifier 42 is 5 times.

The detection resolution of A / F is verified for the sensor control circuit 30 in FIG. The air-fuel ratio detection range of the A / F sensor is A / F11 to the atmosphere as in the first embodiment, and other specific numerical conditions of the sensor element 10 and the sensor control circuit 30 are also the same as those in the first embodiment. The same is true (see conditions above). In such a case, the A / F detection signal (OP4 in the figure) shows the following numerical values in the atmosphere and at A / F11, respectively.
OP4 = 2.5V + 185Ω × 2.5mA × 5 times = 4.8125V
OP4 = 2.5V + 185Ω × (−1.3 mA) × 5 times = 1.2975V
Under the above numerical conditions, the detection resolution of A / F11 to the atmosphere when 10 bit A / D is used is as follows.
(4.8125-1.2975) / 5V × 1024 = 720
In this case, a resolution (720/144 = 5) that is five times that of the prior art (the circuit configuration of FIG. 16) is obtained.

If the current change corresponding to 1 A / F in the vicinity of stoichiometric is 0.2 mA, the detection resolution is as follows.
0.2mA × 185Ω × 5 times / 5V × 1024 = 37
In this case, it becomes 0.03 A / F per 1 LSB, and although the control control in the vicinity of the stoichiometry is somewhat lowered, the detection resolution is expected to be improved as compared with the conventional technique (circuit configuration in FIG. 16).

  According to the present embodiment, the air-fuel ratio detection resolution can be increased, and an appropriate signal output can be realized in a wide air-fuel ratio detection range (A / F11 to the atmospheric range).

  Although the illustration of the impedance detection is omitted for convenience, the element impedance Zac is calculated from the change amount of the B point voltage and the change amount of the D point voltage in FIG. At that time, the voltage at each point is designed to be within the operating voltage range (0-5V) of the A / D converter (A / D2, A / D3 in FIG. 1), and proper impedance detection is performed. It can't be changed.

  Also in the sensor control circuit 30 of FIG. 8 described above, the configurations of FIG. 5 and FIG. 6 in the first embodiment described above can be adopted. That is, the configuration of FIG. 5 is adopted, and the operational amplifier 42 is configured to amplify the voltage difference between the reference voltage Vf and the B point voltage, or the configuration of FIG. 6 is adopted, and the operational amplifier 42 is connected from the B point terminal of the current detection resistor 32. A switch and a capacitor may be arranged in the middle of the path leading to, and when the current flowing through the current detection resistor 32 changes during impedance detection, the current change may be prevented from being inadvertently output.

(Fourth embodiment)
In each of the first to third embodiments, the case where the current detection resistor 32 is provided on the positive terminal side of the sensor element 10 and the applied voltage control circuit 35 is provided on the negative terminal side has been described. In this embodiment, this is reversed, and the current detection resistor 32 is provided on the negative terminal side of the sensor element 10 and the applied voltage control circuit 35 is provided on the positive terminal side. FIG. 9 shows a modification of the configuration of FIG. Although illustration and detailed description are omitted, the circuit configuration similar to that of the present embodiment (configuration in which the + and-of the sensor element is reversed) can be applied to the configurations of FIGS. For convenience of explanation, the same reference numerals are given to the respective components in FIG.

  In FIG. 9, the applied voltage at the time of A / F detection and the AC voltage change at the time of impedance detection are given to the + terminal of the sensor element 10 via the operational amplifier 34. When the exhaust gas is lean, a current flows through the current detection resistor 32 in the direction of D → E, and the E point voltage becomes lower than the D point voltage. On the other hand, in the case of rich, a current flows in the direction of E → D, and the point E voltage becomes higher than the point D voltage. As a result, the potential logic with respect to the air-fuel ratio is reversed, and the feedback current of the operational amplifiers 36 and 37 flows to point E. In this case, since the feedback current is absorbed by the operational amplifier 31, the current flowing through the current detection resistor 32 is not affected. Therefore, it is not necessary to separately provide an operational amplifier for absorbing the feedback current (the operational amplifier 38 in the figure) as shown in FIGS. 5 and 6, and the circuit can be simplified.

  In the sensor control circuit 30 of FIG. 9, a switch and a capacitor are disposed in the path from the point E of the current detection resistor 32 to the operational amplifiers 36 and 37, and the current flowing through the current detection resistor 32 changes when impedance is detected. In doing so, the current change may be prevented from being inadvertently output.

  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 configurations of FIG. 1 and FIG. 7 described above, a case where a voltage (5 V or more) exceeding the operating voltage range of the A / D converter is output via the operational amplifier is considered. Specifically, in the operational amplifiers 37 and 41 having an amplification factor of 15 times, the output exceeds the operating voltage range of the A / D converter, and there is a concern about latch-up of the A / D converter. In this case, in order to protect the A / D converter, it is desirable to limit the output of the operational amplifier (that is, the A / D input) near the maximum operating voltage of the A / D converter. Therefore, in order to protect the A / D converter, a configuration in which a 5V clamp circuit is installed at the output portion of the A / F detection signal will be described below.

  In the configuration shown in FIG. 10, clamp circuits 51 and 52 are provided at the output portions of the operational amplifiers 36 and 37, respectively. The clamp circuit 51 includes diodes 51a and 51b and a resistor 51c, and the clamp circuit 52 includes diodes 52a and 52b and a resistor 52c. More specifically, in the clamp circuit 51, a diode 51a is connected to the point P on the output path of the operational amplifier 36 in the forward direction with respect to the constant voltage Vcc (5V). If it is 7V, the point P in the figure is held at a maximum of “5V + 0.7V”. The constant voltage Vcc corresponds to a constant voltage source corresponding to the maximum operating voltage of the A / D converter. The same is true for the other clamp circuit 52. Therefore, the outputs of the operational amplifiers 36 and 37 are limited near the maximum operating voltage of the A / D converter.

  In the configuration shown in FIG. 11, clamp circuits 53 and 54 are provided at the output portions of the operational amplifiers 36 and 37, respectively. The clamp circuit 53 includes a pnp transistor 53a, a diode 53b, and resistors 53c and 53d, and the clamp circuit 54 includes a pnp transistor 54a, a diode 54b, and resistors 54c and 54d. More specifically, in the clamp circuit 53, the emitter terminal of the transistor 53a is connected to the point Q on the output path of the operational amplifier 36, and the base terminal is composed of a constant voltage Vcc (5V), a diode 53b, and a resistor 53c. Connected to a reference voltage source. In this case, the diode 53b is connected in the reverse direction to the constant voltage Vcc (5V), and if the voltage drop of the diode 53b is 0.7V, the base input voltage of the transistor 53a is held at 4.3V. The If the voltage drop between the emitter and base of the transistor 53a is 0.7V, the Q point in the figure is held at about 5V. The same is true for the other clamp circuit 54. Therefore, the outputs of the operational amplifiers 36 and 37 are limited near the maximum operating voltage of the A / D converter.

  In the above embodiment, two detection ranges are set, that is, the entire air-fuel ratio detection range (A / F11 to atmosphere) and the specific range (A / F12 to A / F22), and the air-fuel ratio detection is performed for each of these detection ranges. Although it is configured to increase the accuracy, it is also possible to set three or more detection ranges and increase the air-fuel ratio detection accuracy for each detection range. In a broad sense, the present invention is not limited to the above ranges, but a relatively wide first detection range and a relatively narrow second detection range are set, and the first amplifier with an amplification factor m corresponds to the first detection range. It is also possible to obtain a wide range output and obtain a narrow range output corresponding to the second detection range by a second amplifier having an amplification factor n (n> m).

  In addition, it is also possible to divide the entire detection range of the air-fuel ratio into a plurality of sections and set each as a plurality of detection ranges. For example, in the case where A / F11 to the atmosphere is the entire detection range, the detection range of A / F11 to A / F22 and the detection range of A / F22 to the atmosphere are set. In this case, the plurality of detection ranges may be provided apart from each other or partially overlapped. Even in this configuration, the same effect can be obtained by applying the above-described embodiments.

  In each of the above embodiments, the element impedance is calculated from the current change amount measured by the current detection resistor 32. However, the current change amount (or voltage change amount) is measured regardless of the current detection resistor 32, and the element impedance is calculated. It is also possible to adopt a configuration for calculating. Further, it can be implemented as a gas concentration detection device that does not require impedance detection.

  In the above embodiment, the A / F sensor having the sensor element structure of FIG. 2 has been described. However, the present invention can be applied to an A / F sensor having another sensor element structure. For example, the sensor element 60 shown in FIG. 12 has two layers of solid electrolytes 61, 62. One solid electrolyte 61 has a pair of electrodes 63, 64 facing each other, and the other solid electrolyte 62 has A pair of electrodes 65 and 66 are arranged to face each other. Note that the electrodes 63 to 65 are seen in two places on the left and right objects in the figure, but they are the same member connected at any part of the front and back of the page. In the present sensor element 60, a pump cell 71 is configured by the solid electrolyte 61 and the electrodes 63 and 64, and an oxygen detection cell 72 is configured by the solid electrolyte 62 and the electrodes 65 and 66. The sensor element 60 has the laminated structure as in the sensor element 10 described above. In FIG. 12, reference numeral 67 denotes a gas introduction hole, reference numeral 68 denotes a porous diffusion layer, reference numeral 69 denotes an air duct, and reference numeral 70 denotes a heater.

  The potential of the electrode 66 of the oxygen detection cell 72 is input to the − input terminal of the comparator 82, and the comparison voltage Vref is input to the + input terminal of the comparator 82. A current detection resistor 83 is connected between the electrode 63 of the pump cell 71 and the output of the comparator 82, and points A and B of both terminals of the current detection resistor 83 are taken out as sensor outputs. Yes.

  In the A / F sensor having the sensor element structure, the oxygen detection cell 72 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 the stoichiometric. For example, in the case of lean, the electromotive force output of the oxygen detection cell 72 decreases, and the output of the comparator 82 (point B voltage in the figure) increases. Therefore, a current flows through the current detection resistor 83 in the direction of B → A. On the other hand, when it is rich, the electromotive force output of the oxygen detection cell 72 increases, and the output of the comparator 82 (point B voltage in the figure) decreases. Therefore, a current flows through the current detection resistor 83 in the direction of A → B. The oxygen detection cell 72 is also generally called an electromotive force cell or an oxygen concentration detection cell.

  FIG. 13 is a circuit diagram showing the configuration of the sensor control circuit 80 for the A / F sensor having the sensor element structure of FIG. In FIG. 13, a reference voltage power supply 81 is connected to a common terminal of the pump cell 71 and the oxygen detection cell 72. Further, a closed circuit having an operational amplifier 82 and a current detection resistor 83 is formed through each of the cells 71 and 72, and a comparison voltage Vref (0.45V) is generated at the non-inverting terminal (+ terminal) of the operational amplifier 82. A voltage generation circuit 84 is connected. As described above, the current flows through the current detection resistor 83 in the direction of B → A during lean, and the current flows through the current detection resistor 83 in the direction of A → B when rich. In such a case, the pump cell 71 is feedback-controlled so that the output voltage of the oxygen detection cell 72 becomes a predetermined value (however, various feedback control circuits have already been disclosed, and the illustration and detailed description thereof will be given here. (Omitted).

  Further, an operational amplifier 85 having a gain of 5 and an operational amplifier 86 having a gain of 15 are connected to both terminals A and B of the current detection resistor 83, respectively. In this case, the operational amplifier 85 corresponds to a “first amplifier”, and A / F detection can be performed for the entire detection range of the air-fuel ratio (for example, A / F 11 to the atmosphere) by the output of the operational amplifier 85. Further, the operational amplifier 86 corresponds to a “second amplifier”, and A / F detection is possible for a specific range (for example, A / F12 to A / F22) including the stoichiometry in the entire detection range of the air-fuel ratio by the output of the operational amplifier 86. It becomes. The output of the operational amplifier 85 corresponds to “wide range output”, and the output of the operational amplifier 86 corresponds to “narrow range output”.

  The above-described configuration shown in FIG. 13 also has the excellent effects described above. In other words, since the operational amplifiers 85 and 86 are provided for each detection range of the air-fuel ratio, it is possible to perform an optimum amplification process for each individual detection range. As a result, it is possible to improve the detection accuracy in a desired wide-range air-fuel ratio detection range (a range including high lean and high rich). In particular, the detection accuracy of the stoichiometry and the air-fuel ratio in the vicinity thereof is improved, so that highly accurate stoichiometric control can be realized.

  In the sensor control circuit 80 of FIG. 13, the points A and B of both terminals of the current detection resistor 83 are not fixed and fluctuate. However, in the sensor control circuit 90 shown in FIG. We propose a configuration that can fix the terminal.

  In FIG. 14, a voltage (for example, 3 V) equivalent to the reference voltage Vf <b> 1 is applied to the common terminal of the pump cell 71 and the oxygen detection cell 72 through the operational amplifier 93. That is, the point B voltage in the figure is fixed at 3V. Further, a closed circuit having a feedback circuit 91 and a current detection resistor 92 is configured through the oxygen detection cell 72. The reference voltage Vf2 in the feedback circuit 91 is, for example, 2.55V.

  The operation of the sensor control circuit 90 will be described taking the rich time as an example. When rich, the point C1 in the figure rises to 3.45 V due to the electromotive force of the oxygen detection cell 72, so the potential at the point C2 in the feedback circuit 91 drops. Then, the output of the feedback circuit 91, that is, the point A voltage increases. That is, when rich, a current flows through the current detection resistor 83 in the direction of A → B. On the other hand, current flows through the current detection resistor 83 in the direction of B → A during lean.

  Further, an operational amplifier 94 having a gain of 5 and an operational amplifier 95 having a gain of 15 are connected to both terminals A and B of the current detection resistor 92, respectively. In this case, the operational amplifier 94 corresponds to a “first amplifier”, and A / F detection can be performed for the entire detection range of the air-fuel ratio (for example, A / F 11 to the atmosphere) by the output of the operational amplifier 94. The operational amplifier 95 corresponds to a “second amplifier”, and A / F detection can be performed for a specific range (for example, A / F12 to A / F22) including the stoichiometry in the entire detection range of the air-fuel ratio by the output of the operational amplifier 95. It becomes. The output of the operational amplifier 94 corresponds to “wide range output”, and the output of the operational amplifier 95 corresponds to “narrow range output”.

  The configuration shown in FIG. 14 also has the excellent effects described above. That is, the detection accuracy can be improved in a desired wide-range air-fuel ratio detection range (a range including high lean and high rich). In particular, the detection accuracy of the stoichiometry and the air-fuel ratio in the vicinity thereof is improved, so that highly accurate stoichiometric control can be realized. In the configurations of FIGS. 13 and 14, the second to fourth embodiments described above can be applied as appropriate.

  In addition, the sensor element structure shown in FIG. 15 may be used. The sensor element 100 of FIG. 15 includes three layers of solid electrolytes 101, 102, and 103, and 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 the present sensor element 100, a pump cell 111 is constituted by the solid electrolyte 101 and the electrodes 104 and 105, and an oxygen detection cell 112 is constituted by the solid electrolyte 102 and the electrodes 106 and 107. Further, the solid electrolyte 103 constitutes a wall material for securing the oxygen reference chamber 108. The sensor element 100 has a laminated structure, which is the same as the sensor element 10 described above. In FIG. 15, 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.

  The potential of the electrode 107 of the oxygen detection cell 112 is input to the − input terminal of the comparator 113, and the comparison voltage Vref is input to the + input terminal of the comparator 113. A current detection resistor 114 is connected between the electrode 104 of the pump cell 111 and the output of the comparator 113, and points A and B of both terminals of the current detection resistor 114 are taken out as sensor outputs. Yes. In this case, a current flows through the current detection resistor 114 in the direction of B → A when lean, and conversely, a current flows through the current detection resistor 114 in the direction of A → B when rich. The configuration related to the sensor control circuit is as described above, and the description thereof is omitted.

  The two-cell sensor element described with reference to FIGS. 12 and 15 can be configured to detect the internal resistance of the oxygen detection cell by appropriately operating the voltage applied to the oxygen detection cell. That is, in a periodic internal resistance detection period, a voltage change corresponding to the internal resistance value of the oxygen detection cell is caused by flowing a predetermined measurement current through the oxygen detection cell, and the voltage change amount is measured. Thereby, the internal resistance of the oxygen detection cell is converted into a voltage and can be output. More specifically, a constant current (current for resistance measurement) having a polarity opposite to the internal electromotive force generated in the oxygen detection cell is supplied to the oxygen detection cell for a predetermined time every predetermined time. Measure the voltage change at both ends. In such a case, the internal resistance of not only the oxygen detection cell but also the pump cell can be detected simultaneously by causing a constant current of reverse polarity to flow not only to the oxygen detection cell but also to the pump cell. After a constant current having the opposite polarity is passed, a constant current having the opposite polarity (for example, positive if the resistance measurement current is negative) is allowed to flow for a predetermined time to detect the concentration. You may make it aim at the early return to a possible state.

  Further, an AC pulse having a constant frequency (for example, about several kHz) is applied to the oxygen detection cell, and a change in the terminal voltage of the oxygen detection cell at that time is monitored. In this case, since the change in the terminal voltage of the oxygen detection cell corresponds to the internal resistance value of the oxygen detection cell, the internal resistance of the oxygen detection cell can still be detected.

  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 gas concentration. For example, a composite type gas concentration sensor has a plurality of cells formed of a solid electrolyte, and in the first cell (pump cell), oxygen in the detection gas is discharged or pumped and the oxygen concentration is detected. In the second cell (sensor cell), the gas concentration of the specific component is detected from the gas after oxygen discharge. This gas concentration sensor is embodied as, for example, a NOx sensor that detects the NOx concentration in the exhaust gas. The application of the present invention improves the detection accuracy of the NOx concentration. 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 being able to detect the NOx concentration, the present invention can also be applied to a gas concentration sensor that can detect the HC concentration and the CO concentration as the gas concentration of the specific component. In this case, surplus oxygen in the gas to be detected 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. 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.

10 ... sensor element,
11 ... Solid electrolyte,
12 ... diffusion layer,
14 ... Insulating layer,
17 ... Air duct,
20 ... Microcomputer,
30 ... Sensor control circuit,
32 ... Current detection resistor,
36, 37 ... operational amplifier,
39 ... switch,
40: Capacitor,
41, 42 ... operational amplifier,
51-54 ... clamp circuit,
51a, 52a ... diodes
53a, 54a ... pnp transistors,
60 ... sensor element,
61, 62 ... solid electrolyte,
69 ... Atmospheric duct,
71 ... pump cell,
72. Oxygen detection cell,
80 ... sensor control circuit,
83 ... current detection resistor,
85, 86 ... operational amplifier,
90 ... sensor control circuit,
92 ... Current detection resistor,
94, 95 ... operational amplifier,
100: Sensor element,
101, 102 ... solid electrolyte,
108 ... oxygen reference room,
111 ... pump cell,
112 ... Oxygen detection cell.

Claims (15)

A gas concentration sensor having a sensor element made of a solid electrolyte and capable of detecting a gas concentration of a specific component in a gas to be detected over a wide area, and a current detection resistor for measuring a current flowing through the element when a voltage is applied to the sensor element And an A / D converter for A / D converting a signal measured by the current detection resistor , performing a gas concentration calculation based on the signal after A / D conversion, A fixed reference voltage is applied to one terminal, and when the applied voltage or current of the sensor element is changed in an alternating manner, the voltage value of the second terminal of the current detection resistor is measured to determine the resistance value of the sensor element. In the gas concentration detection device, in which the sensor element has a large current change at the time of element resistance detection with respect to the element current at the time of gas concentration detection,
A plurality of detection ranges are set in the entire detection range of gas concentration, and the signal measured by the current detection resistor is amplified by an amplifier having a predetermined amplification factor in at least one detection range, and then the A / D Output to the converter, and output to the A / D converter without passing through the amplifier in other detection ranges ,
A gas concentration detection apparatus comprising a switch means for opening or closing between the amplifier and the second terminal of the current detection resistor .   A relatively wide first detection range and a relatively narrow second detection range are set as the plurality of detection ranges, and a wide-range output corresponding to the first detection range is output without an amplifier, and the second detection The gas concentration detection apparatus according to claim 1, wherein a narrow range output corresponding to the range is output via an amplifier.   A total detection range of gas concentration and a specific range which is a part of the detection range are set as the plurality of detection ranges, and a wide range output corresponding to the total detection range is output without using an amplifier, and the specific range is supported. The gas concentration detection device according to claim 1, wherein the narrow-range output is output through an amplifier.   The gas concentration detection device according to any one of claims 1 to 3, wherein the sensor element is a laminated element having a plate-shaped solid electrolyte, a diffusion layer and an insulating layer laminated thereon, and an oxygen reference chamber.   The gas concentration sensor according to any one of claims 1 to 4, wherein the gas concentration sensor has a sensor element capable of detecting an air-fuel ratio of combustion gas over a wide area.   The gas concentration detection device according to claim 3, wherein the gas concentration sensor has a sensor element capable of detecting an air-fuel ratio of combustion gas in a wide range, and the specific range is set to a range including a theoretical air-fuel ratio. .   The sensor element is a stacked element formed by stacking a pump cell for taking in and out oxygen in a gas to be detected and an oxygen detection cell for outputting a signal corresponding to the oxygen concentration of the gas to be detected. 4. The gas concentration detection device according to claim 1, wherein the pump cell is controlled so that the output signal becomes a predetermined value.   The gas concentration detection device according to claim 7, wherein the gas concentration detection device has a configuration for converting an internal resistance of the oxygen detection cell into a voltage.   The gas concentration detection device according to claim 8, wherein the gas concentration sensor is an air-fuel ratio sensor capable of detecting an air-fuel ratio of combustion gas over a wide area.   The gas concentration sensor has a plurality of cells formed of a solid electrolyte, of which the first cell discharges or pumps out oxygen in the gas to be detected, and the second cell has a gas of a specific component from the gas after oxygen discharge. The gas concentration detection device according to any one of claims 1 to 4, wherein the gas concentration detection device detects the concentration.   11. The amplifier according to claim 1, further comprising a configuration in which voltages at both terminals of the current detection resistor are respectively input to positive and negative input terminals of the amplifier, and an operational amplifier for absorbing a feedback current is installed in a feedback current path of the amplifier. The gas concentration detection apparatus according to any one of the above. 12. The gas concentration detection apparatus according to claim 1 , wherein a capacitor for holding a voltage level immediately before the switch means is connected between the amplifier and the switch means when the switch means is opened. Detection device. The output of said amplifier, the gas concentration detection apparatus according to an output of the amplifier to one of the A / D converter maximum operating Claim clamp circuit is provided for limiting the vicinity voltage 1 to 12 of. 14. The gas concentration detection device according to claim 13 , wherein the clamp circuit includes a diode connected in a forward direction from an output path of the amplifier to a constant voltage source corresponding to a maximum operating voltage of the A / D converter. Gas concentration detector. 14. The gas concentration detection device according to claim 13 , wherein the clamp circuit includes a pnp transistor having an emitter terminal connected to an output path of the amplifier and a base terminal connected to a reference voltage source. A gas concentration detection apparatus in which a base input voltage is a voltage value obtained by subtracting a voltage drop between a base and an emitter from a constant voltage source corresponding to the maximum operating voltage of the A / D converter.

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