JP2008216233A - Sensor control device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To set a plurality of different gas concentration detection ranges and perform a proper gas concentration detection in these detection ranges. <P>SOLUTION: In a sensor control circuit 30, a current detection resistance 32 is connected to a sensor element 10 for measuring an element current. An inverting amplification circuit 38 is connected to the current detection resistance 32. An A/F output voltage which is the output of the inverting amplification circuit 38 is output to a microcomputer 20. In the inverting amplification circuit 38, an offset set circuit 50 is connected to one signal input terminal of an operational amplifier 39. The offset set circuit 50 is used to apply an offset to an A/F output voltage. The offset set circuit comprises a switch element 51, a resistance connected in series to the switch element 51, and a power supply circuit 55 having two partial pressure resistances 53, 54. The switch element 51 is turned on/off according to the offset selection signal output from the microcomputer 20. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a sensor control device that controls 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.

  This type of sensor control 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 (combustion gas) discharged from an in-vehicle engine as a detected gas. Has been. 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 for feedback control of the air-fuel ratio in the vicinity of the stoichiometric (theoretical air-fuel ratio) or the air-fuel ratio in a predetermined lean region. Lean combustion control with feedback control is 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 combustion control. In addition to the lean region corresponding to the air-fuel ratio control range, the air condition is not limited to the atmospheric state. There is a need to expand the fuel ratio detection range. For example, it is necessary to detect deterioration such as clogging of a sensor at the time of fuel cut in a predetermined operation state of the engine as OBD correspondence (failure diagnosis of parts related to exhaust gas regulation). 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.

  In order to enable air-fuel ratio detection in a wide air-fuel ratio detection range and to improve air-fuel ratio detection accuracy in a specific range, which is a limited air-fuel ratio detection range, the applicant of this application A technique has been proposed in which a plurality of amplifier circuits each having a different amplification factor are provided in each section, and an air-fuel ratio detection signal is obtained through the plurality of amplifier circuits (see, for example, Patent Document 1). According to the above prior art, it is possible to realize highly accurate air-fuel ratio detection within a desired air-fuel ratio detection range.

  However, in the above-described prior art, since a plurality of amplifier circuits (operational amplifiers) are essential, there is a possibility that inconveniences such as an increase in circuit size and an increase in the number of terminals for signal input / output may occur. Therefore, there is room for improvement.

  In addition to switching between a wide range and a narrow range as the air-fuel ratio detection range, a technology that enables the setting of the stoichiometric range near the stoichiometric range and the lean range in the lean range is also proposed. Has been. For example, the sensor control device of Patent Document 2 has a configuration in which an element current signal flowing through a sensor element is input to a differential amplifier circuit, and signal amplification is performed at a predetermined amplification factor by the differential amplifier circuit. In addition, the offset voltage is input to one input terminal of the differential amplifier circuit. By switching the offset voltage, the stoichiometric range near the stoichiometric range or the lean narrow range in the lean range can be used as the air-fuel ratio detection range. Can be set.

  FIG. 14 is a circuit diagram showing a part of the gas sensor control circuit disclosed in Patent Document 2. FIG. 14 is a circuit diagram corresponding to FIG. 2 in Patent Document 2. In the circuit diagram, a configuration in which the signal amplification factor is variable is omitted.

  In FIG. 14, a current detection resistor 101 is a shunt resistor for detecting an element current, and buffers 102 and 103 are connected to both ends thereof. In the operational amplifier 111 constituting the differential amplifier circuit 110, the other end of the resistor 112 having one end connected to the buffer 102 is connected to the inverting input terminal (− terminal), and between the − terminal and the output terminal. Is connected to a resistor 113. Similarly, in the operational amplifier 111, a non-inverting input terminal (+ terminal) is connected to the other end of a resistor 114 having one end connected to the buffer 103, and a two-position switching switch 116 is connected via a resistor 115. ing. The switch 116 is switched between a state connected to the power source V11 determined by the voltage dividing resistors 117 and 118 and a state connected to the power source V12 (≠ V11) determined by the voltage dividing resistors 119 and 120. It has become.

  In the above configuration, the offset voltage of the differential amplifier circuit 110 is switched by switching the switch 116, and accordingly, the stoichiometric narrow range near the stoichiometric range or the lean narrow range in the lean region can be switched.

However, in the sensor control device disclosed in Patent Document 2, the resistance value on the non-inverting input terminal (+ terminal) side of the operational amplifier 111 is changed by switching the switch 116, and the amplification factor and the like fluctuate accordingly. Arise. That is, in the operational amplifier 111, the values of the resistors 112 and 113 on the inverting input terminal (− terminal) side are R11 and R12, the values of the resistors 114 and 115 on the non-inverting input terminal (+ terminal) side are R13 and R14, and Consider a case where the resistance value determined by the voltage resistors 117 and 118 is R15, and the resistance value determined by the voltage dividing resistors 119 and 120 is R16. In this case, in the differential amplifier circuit 110, basically,
(1) R11 = R13 and (2) R12 = R14 + R15 or (3) R12 = R14 + R16
As a result, signal amplification at a desired amplification factor (= R12 / R11) is possible. However, if R15 ≠ R16, either (2) or (3) is not established. Therefore, when the switch 116 is switched, it is considered that there is a problem that the signal amplification factor and the gas concentration detection range fluctuate unexpectedly.
JP 2004-205488 A JP 2006-275628 A

  The main object of the present invention is to provide a sensor control device that enables a plurality of different gas concentration detection ranges to be set, and can perform appropriate gas concentration detection in each of the detection ranges. is there.

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

  In the first aspect of the present invention, a current measuring resistor for measuring a current flowing through the sensor element and an element current signal measured by the current measuring resistor are input to at least one of positive and negative signal input terminals, An amplification circuit that amplifies the input element current signal, an offset setting circuit connected to an intermediate point between the input resistance and the feedback resistance on the feedback path side of the amplification circuit, and an offset setting state by the offset setting circuit, Offset switching means for switching according to the density value detected each time.

  In short, in the present invention, in a sensor control device that amplifies an element current signal input to at least one of positive and negative signal input terminals using an amplifier circuit, the input current between the input resistance and the feedback resistor on the feedback path side of the amplifier circuit is between By connecting an offset setting circuit to the intermediate point, it is possible to set an offset for the output signal of the amplifier circuit. That is, the amplifier circuit and the offset setting circuit described above constitute an adding circuit, and the output signal of the amplifier circuit is set by the offset setting circuit to an element current signal corresponding to the magnitude of the current flowing through the sensor element. The signal is obtained by adding the offset value. Accordingly, it is possible to set different gas concentration detection ranges with a configuration using one amplifier circuit. In this case, since the current addition amount flowing from the offset setting circuit to the amplifier circuit side is known in advance, the gas concentration can be accurately detected in consideration of the current addition amount.

  Here, as a comparison object, when the differential amplifier circuit shown in FIG. 14 is used, it is considered that the signal amplification factor and the gas concentration detection range fluctuate unexpectedly as described above. This is because, in the differential amplifier circuit of FIG. 14, the value of the feedback resistance and the value of the ground resistance become inconsistent with the switching of the offset voltage (in FIG. 14, either R12 = R14 + R15 or R12 = R14 + R16 is not established). ). On the other hand, according to the circuit configuration of the present invention in which the offset setting circuit is connected to the intermediate point between the input resistor and the feedback resistor on the feedback path side of the amplifier circuit, the value of the feedback resistor and the grounding are changed with the switching of the offset voltage. Since the resistance value does not fluctuate, this problem can be avoided.

  As described above, according to the present invention, a plurality of different gas concentration detection ranges can be set, and appropriate gas concentration detection can be performed in each of these detection ranges.

  Here, the amplifier circuit is an inverting or non-inverting amplifier circuit that inputs an element current signal measured by a current measuring resistor to one of positive and negative signal input terminals and amplifies the input element current signal. (See FIGS. 1, 9, and 13). Alternatively, the amplifier circuit may be a differential amplifier circuit that inputs terminal voltage signals at both ends of a current measuring resistor as element current signals to positive and negative signal input terminals and amplifies the voltage difference (see FIG. 12). ). In any case, it is possible to realize a configuration in which a state in which the value of the feedback resistance of the amplifier circuit and the value of the ground resistance match is maintained before and after switching of the offset setting state by the offset switching means.

  More specifically, regarding the offset setting, the offset switching means is not connected to the state where the offset setting circuit is connected to the intermediate point between the input resistance and the feedback resistance on the feedback path side of the amplifier circuit. (Claim 2).

  Alternatively, the offset setting circuit may variably set a plurality of offset values, and the offset switching means may selectively set any one of the plurality of offset values.

  According to each of the above-described configurations, the offset setting circuit can be connected or not connected, or the offset value can be variably set, so that a difference corresponding to the offset value can be made as the gas concentration detection range. Easy to set.

  According to a third aspect of the present invention, a sensor control device according to claim 3 inputs a current measuring resistor for measuring a current flowing through a sensor element and an element current signal measured by the current measuring resistor to one signal input terminal. An inverting or non-inverting amplifier circuit that amplifies an element current signal, an offset setting circuit connected to a signal input terminal for inputting the element current signal or the other signal input terminal in the amplifier circuit, and the offset setting circuit And an offset switching means for switching the offset setting state according to the density value detected each time.

  In short, in the present invention, in a sensor control device that amplifies an element current signal using an inverting or non-inverting amplifier circuit, an offset setting circuit is connected to one of the signal input terminals of the amplifier circuit. An offset can be set for the output signal. That is, the amplifier circuit and the offset setting circuit described above constitute an adding circuit, and the output signal of the amplifier circuit is set by the offset setting circuit to an element current signal corresponding to the magnitude of the current flowing through the sensor element. The signal is obtained by adding the offset value. Alternatively, an offset value set by the offset setting circuit is added to a reference signal (a signal input terminal different from a signal input terminal for inputting an element current signal) input to the amplifier circuit, and the amplifier circuit Output signal is offset. Accordingly, it is possible to set different gas concentration detection ranges with a configuration using one amplifier circuit. In this case, since the current addition amount flowing from the offset setting circuit to the amplifier circuit side is known in advance, the gas concentration can be accurately detected in consideration of the current addition amount.

  Here, when a differential amplifier circuit is used as a comparison object instead of an inverting or non-inverting amplifier circuit (see FIG. 14 described above), the offset setting circuit is connected to one signal of the amplifier circuit as described above. Although the offset is set by connecting to the input terminal, in such a configuration, it is considered that the signal amplification factor and the gas concentration detection range fluctuate unexpectedly as described above. On the other hand, according to the circuit configuration of the present invention using an inverting or non-inverting amplifier circuit, such a problem can be avoided.

  As described above, according to the present invention, a plurality of different gas concentration detection ranges can be set, and appropriate gas concentration detection can be performed in each of these detection ranges.

  More specifically, regarding the offset setting, the offset switching means may switch between a state in which the offset setting circuit is connected to a signal input terminal of the amplifier circuit and a state in which the offset setting circuit is not connected.

  Alternatively, the offset setting circuit may variably set a plurality of offset values, and the offset switching means may selectively set any one of the plurality of offset values.

  According to each of the above-described configurations, the offset setting circuit can be connected or not connected, or the offset value can be variably set, so that a difference corresponding to the offset value can be made as the gas concentration detection range. Easy to set.

  The sensor control device of the present invention can be applied as an air-fuel ratio detection device that detects an air-fuel ratio based on an element current signal, with exhaust gas discharged from an internal combustion engine as a detection target.

  In this case, as described in claim 6, the air-fuel ratio detection range is a part of the entire air-fuel ratio range that can be detected by the sensor element and is used for stoichiometric combustion control (stoichiometric zoom range RG2). ) And a lean detection range (lean zoom range RG3) that is also part of the entire air-fuel ratio range and is used for lean combustion control. Then, the offset switching means determines whether to perform air-fuel ratio detection in either the stoichiometric detection range or the lean detection range according to the status of each air-fuel ratio control, and performs offset switching according to the determination result. It is good to carry out.

  The stoichiometric detection range and the lean detection range are both narrow detection ranges that form part of the total air-fuel ratio range, but are detection ranges in which the magnitude of the air-fuel ratio is different. As a result, an appropriate air-fuel ratio can be detected in each detection range. As a result, stoichiometric combustion control and lean combustion control can be suitably performed.

  In addition, as described in claim 7, a rich detection range (rich zoom range RG4) that is a part of the total air-fuel ratio range and is used for rich combustion control may be further defined as the air-fuel ratio detection range. Is possible. In this case, the offset switching means may switch between the stoichiometric detection range, the lean detection range, and the rich detection range in accordance with the status of each air-fuel ratio control.

  Thereby, in addition to stoichiometric combustion control and lean combustion control, rich combustion control can be suitably performed. In the case where the air-fuel ratio detection range is the rich detection range, an offset on the positive and negative sides may be given as compared to the case where the lean detection range is set. That is, when the air-fuel ratio detection range is the lean detection range, a current corresponding to the offset flows from the offset setting circuit to the amplifier circuit, whereas when the air-fuel ratio detection range is the rich detection range, Conversely, a current corresponding to the offset is caused to flow from the amplifier circuit to the offset setting circuit.

  Here, as described in claim 8, each of the detection ranges may be set as a detection range including a stoichiometric point. As a result, each detection range includes a state where the element current = 0 mA (stoichiometric detection state). In this case, for example, an error in circuit characteristics can be determined based on variations in the output signal of the amplifier circuit when the sensor element is in the stoichiometric detection state.

  Specifically, as described in claim 9, by temporarily stopping the voltage applied to the sensor element, the current flowing through the sensor element is forcibly cut off, and the output of the amplifier circuit at that time An error in circuit characteristics may be detected based on the signal. For example, the voltage application is stopped by opening the voltage application path to the sensor element, and the element current is cut off. In this case, the stoichiometric detection state can be forcibly set, and an error in circuit characteristics can be determined based on variations in output signals of the amplifier circuit at that time.

  In the invention according to claim 10, a plurality of amplification resistors for determining an amplification factor are provided in the amplifier circuit, and any one of the plurality of amplification resistors is configured to variably set the amplification factor of the amplifier circuit. Amplification rate switching means for switching between input resistance and feedback resistance of the amplifier circuit is provided.

  In the above configuration, the gas concentration detection resolution can be adjusted by variably setting the amplification factor (gain) of the amplifier circuit. Therefore, the detection accuracy can be increased in a desired gas concentration detection range. At this time, the variable setting of the amplification factor of the amplifier circuit is performed by distributing the input resistance and feedback resistance for a plurality of amplification resistors. Therefore, unlike the conventional configuration in which a plurality of different amplification factors are realized by using a plurality of amplifier circuits (operational amplifiers), the configuration can be simplified. As described above, the configuration can be simplified and the detection accuracy can be increased in the desired gas concentration detection range.

  Considering the combination with the inventions of claims 1 to 5, in addition to setting the gas concentration detection range having different offset setting states, it is possible to set the gas concentration detection range having different widths. As a result, a wider variety of gas concentration detection ranges can be set, and a practically desirable configuration can be realized.

  In the configuration of the tenth aspect, it is desirable that a switch element is provided on the signal input path of the operational amplifier constituting the amplifier circuit, and that the amplifying resistor is switched in accordance with the operation of the switch element. That is, the signal input path of the operational amplifier is generally high impedance, and according to the configuration in which the amplification factor is changed by the switch element provided on the high impedance signal input path, the switch element has a resistance component. However, the resistance component can be ignored. Therefore, signal amplification in the amplifier circuit can be performed with high accuracy.

  In the invention according to claim 11, when a plurality of detection ranges that are wide and narrow are defined in advance as the gas concentration detection range and the gas concentration detection is performed within a relatively narrow detection range, the amplification factor of the amplifier circuit When the gas concentration is detected within a relatively wide detection range, the amplification resistor is switched so as to reduce the amplification factor of the amplification circuit. In this case, accuracy-priority concentration detection can be performed in a relatively narrow detection range, whereas concentration detection can be performed while expanding the detection range in a relatively wide detection range.

  When applied as an air-fuel ratio detection device, offset switching and amplification factor switching may be performed as follows.

That is, as described in claim 12, as the air-fuel ratio detection range, the entire air-fuel ratio range (global range RG1) that can be detected by the sensor element, and a part of the total air-fuel ratio range and stoichiometric combustion control And a lean detection range (lean zoom range RG3) which is also a part of the entire air-fuel ratio range and is used for lean combustion control. And
(1) When the full air-fuel ratio range is used, no offset is given by the offset switching means, and the gain is reduced by the gain switching means,
(2) When using the stoichiometric detection range, an offset is not given by the offset switching means, and an amplification factor is increased by the amplification factor switching means,
(3) When using the lean detection range, an offset is given by the offset switching means, and an amplification factor is increased by the amplification factor switching means.

  According to the above configuration, the air-fuel ratio detection range can be easily changed to any of the total air-fuel ratio range, the stoichiometric detection range, and the lean detection range. As a result, stoichiometric combustion control and lean combustion control can be suitably performed. In addition to this, it is possible to suitably perform sensor abnormality diagnosis by atmospheric detection.

  Further, as described in claim 13, a rich detection range (rich zoom range RG4) that is a part of the total air-fuel ratio range and is used for rich combustion control may be further defined as the air-fuel ratio detection range. Is possible. When the rich detection range is used, it is preferable that an offset on the positive and negative sides of the lean detection range is given by the offset switching unit, and an amplification factor is increased by the amplification factor switching unit.

  Thereby, in addition to stoichiometric combustion control, lean combustion control, etc., rich combustion control can be suitably implemented.

  In the invention described in claim 14, the amplifier circuit is configured to obtain an output by an operational amplifier having a constant current source or a pull-down resistor in an output stage. In this case, the output voltage range can be expanded to the lower limit side with a relatively simple configuration. That is, the expansion of the output voltage range can be realized by an operational amplifier having a small chip area, and the sensor control circuit can be miniaturized.

(First embodiment)
Hereinafter, a first embodiment of a sensor control device according to the present invention will be described with reference to the drawings. The present embodiment embodies an air-fuel ratio detection device that detects the oxygen concentration (air-fuel ratio: A / F) in the exhaust gas (combustion gas) discharged from the on-board engine as a detected gas. This 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 realized. In the present embodiment, a wide range of air-fuel ratio detection corresponding to recent or future exhaust gas regulations and abnormality detection regulations (OBD), rich combustion control during rich combustion operation, and NOx occlusion reduction type catalyst installed in the exhaust system. The air-fuel ratio can be detected in a wide range from the rich region (for example, A / F11) to the atmospheric state in order to perform control such as occlusion NOx release and sulfur poisoning regeneration.

  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 layer 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 solid electrolyte layer 11 is a partially stabilized zirconia sheet, and a pair of upper and lower electrodes 15 and 16 are disposed opposite to each other with the solid electrolyte layer 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, a straight line portion parallel to the V-axis (horizontal axis) is a limit current region for specifying the element current IL (limit current) of the sensor element 10, and the increase / decrease in the element current IL is an increase / decrease in the air / fuel ratio (that is, , Lean and rich). That is, the element current IL increases as the air-fuel ratio becomes leaner, and the element current IL decreases as the air-fuel ratio becomes richer. Note that LX1 in the figure represents an applied voltage straight line (applied voltage characteristic) for determining the applied voltage Vp to the sensor element 10, and its inclination is generally in the resistance dominant region (lower voltage side than the limit current region). To the sloped part).

  Next, the electrical configuration of the sensor control system as the main part of the present invention will be described with reference to FIG.

  In FIG. 1, a microcomputer (hereinafter abbreviated as “microcomputer”) 20 and a sensor control circuit 30 are provided as the main components of the sensor control system, and the element current flowing through the A / F sensor (sensor element 10) is thereby controlled. Measurement and calculation of an A / F value based on the element current value are performed. The microcomputer 20 is configured by a well-known logical operation circuit including a CPU, various memories, an A / D converter, and the like. The microcomputer 20 inputs an A / F output voltage corresponding to the element current value from the sensor control circuit 30, and The A / F value is calculated from the A / D value of the A / F output voltage. 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 sequentially output to the engine ECU 25.

  The engine ECU 25 has a stoichiometric feedback control function and a lean feedback control function during normal driving of the vehicle, a rich feedback control function when the engine is heavily loaded, a sensor abnormality diagnosis function under an air atmosphere accompanying fuel cut, and the like. These are all performed based on the actual air-fuel ratio (detection A / F by the A / F sensor).

  Specifically, as the stoichiometric feedback control, for example, the target air-fuel ratio is set to stoichiometric (A / F = 14.7), and the fuel injection by the injector is performed so that the actual air-fuel ratio detected by the A / F sensor matches the target air-fuel ratio. Control the amount (precision stoichiometric control). As the lean feedback control, a lean target air-fuel ratio (for example, A / F = 30) is set, and the fuel injection amount by the injector is controlled so that the actual air-fuel ratio detected by the A / F sensor matches the lean target air-fuel ratio ( Precision lean combustion control). As rich feedback control, a rich target air-fuel ratio (for example, A / F = 10) is set when a high load increase is performed such as when the vehicle is accelerating or when climbing a hill, and the actual air-fuel ratio detected by the A / F sensor is The fuel injection amount by the injector is controlled so as to coincide with the rich target air-fuel ratio. Further, as a sensor abnormality diagnosis at the time of fuel cut, when the gas atmosphere in the exhaust pipe becomes an atmospheric state (that is, a known atmosphere) due to the fuel cut, the output value (element current value) of the A / F sensor is atmospheric. Whether or not the sensor element has deteriorated is determined based on whether or not the value is appropriate.

  Here, in the stoichiometric feedback control and the lean feedback control, it is necessary to detect the air-fuel ratio with high accuracy in the stoichiometric vicinity region including the stoichiometry or a predetermined lean control region, whereas in the rich feedback control and sensor abnormality diagnosis, It is necessary to detect the air-fuel ratio in a wide range from the rich region to the ultra-lean region (atmosphere). Therefore, in the present embodiment, as the air-fuel ratio detection range, the entire range RG1 including rich to super-lean (atmosphere), the stoichiometric zoom range RG2 that is the stoichiometric vicinity region, and the lean zoom range RG3 that is a predetermined lean control region. And switching of the air-fuel ratio detection range (switching of the ranges RG1 / RG2 / RG3) is performed according to which of the processes related to the air-fuel ratio is performed.

  The entire range RG1 corresponds to a “total air-fuel ratio range” that can be detected by the sensor element 10. Further, the stoichiometric zoom range RG2 is a part of the entire range RG1 (total air-fuel ratio range) and corresponds to a “stoichiometric detection range” used for stoichiometric combustion control, and the lean zoom range RG3 is also the entire range RG1 (all This is a part of the air-fuel ratio range) and corresponds to a “lean detection range” used for lean combustion control.

  FIG. 4 is a diagram showing the relationship between A / F and sensor output voltage (microcomputer input value) for the three air-fuel ratio detection ranges RG1 to RG3. Note that the range of the A / F output voltage is defined according to the A / D processing range on the microcomputer 20 side, and is in the vicinity of 0V to 5V as shown in the figure.

  As shown in FIG. 4, the entire range RG1 is set in the range of A / F = 10 to the atmosphere, the stoichiometric zoom range RG2 is set in the range of A / F = 13 to 20, and the lean zoom range RG3 is set to A / F. = 14 to 35. Each of these ranges RG1 to RG3 is set as a detection range straddling the stoichiometric point. At this time, according to the air-fuel ratio detection in the entire range RG1, the air-fuel ratio can be detected in the entire region assumed as the use region in this system. Further, according to the air-fuel ratio detection in the stoichiometric zoom range RG2 and the lean zoom range RG3, the air-fuel ratio detection resolution can be increased within a limited voltage range (A / D operating voltage range).

  FIG. 5 is a diagram showing the relationship between the element current IL and A / F. As shown in FIG. 5, the element current IL = 0 mA at stoichiometry (A / F = 14.7). Further, when the lean region and the rich region are compared, the A / F sensitivity with respect to the element current IL is different, and the former has higher A / F sensitivity. This depends on the sensitivity characteristics of the sensor element 10. In this case, even if the device current range (IL width on the horizontal axis) is substantially the same between the stoichiometric zoom range RG2 and the lean zoom range RG3, the substantial air-fuel ratio detection range (A / F width on the vertical axis) is different. It can be confirmed.

  For this reason, in the present embodiment, the device current range (in other words, the A / F output voltage range) is the same for the stoichiometric zoom range RG2 and the lean zoom range RG3, but the substantial air-fuel ratio detection range is different. It is supposed to be.

  The above-described stoichiometric feedback control, lean feedback control, rich feedback control, and sensor abnormality diagnosis are not the processes that are simultaneously performed in duplicate, but are alternatively performed. Therefore, the air-fuel ratio detection by the entire range RG1 and the air-fuel ratio detection by the stoichiometric / lean zoom ranges RG2, RG3 are not required at the same time, and the air-fuel ratio detection ranges of the RG1, RG2, RG3 are adjusted according to the situation. The air / fuel ratio is detected (A / F value is calculated) based on the detection range.

  Returning to the description of FIG. 1, in the sensor control circuit 30, the reference voltage power supply 33 is connected to the negative terminal (− terminal) of the sensor element 10 via the operational amplifier 31 and the current detection resistor 32 (current measurement resistor). An applied voltage control circuit 35 is connected to the positive terminal (+ terminal) of the sensor element 10 via an operational amplifier 34. A switch 36 is provided on the output terminal side of the operational amplifier 34. The switch 36 is a current interrupting means provided to interrupt the current flowing through the sensor element 10, and is normally held in a closed state, and is opened in response to an open command signal from the microcomputer 20. ing.

  In this case, the point A at one end of the current detection resistor 32 is held at the same voltage as the reference voltage Vf. The element current IL flows through the current detection resistor 32, and the voltage at the point B changes according to the element current IL. For example, when the exhaust gas is lean, the current flows from the positive terminal to the negative terminal of the sensor element 10, so that the point B voltage decreases. When the exhaust gas is rich, the current flows from the negative terminal to the positive terminal of the sensor element 10, so that the point B voltage is To rise. 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 voltage at point C. However, when the A / F detection is performed only in the vicinity of the stoichiometry, the applied voltage can be fixed.

  Further, an inverting amplifier circuit 38 is connected to points A and B at both ends of the current detection resistor 32, and an A / F output voltage as an output of the inverting amplifier circuit 38 is applied to an A / D input terminal of the microcomputer 20. It is captured. In the microcomputer 20, the A / F value is calculated based on the A / D value of the A / F output voltage that is sequentially taken. The inverting amplifier circuit 38 includes an operational amplifier 39, three amplification resistors 41, 42, and 43 connected in series, and a switch element 44 formed of, for example, a MOS transistor. The resistance values of the amplifying resistors 41, 42, and 43 are R1, R2, and R3, respectively.

  Specifically, as the configuration of the inverting amplifier circuit 38, the element current signal detected by the current detection resistor 32 is input to the negative side input terminal (inverted input terminal) of the operational amplifier 39. Further, a switch element 44 is provided on a signal input path connected to the negative side input terminal (inverted input terminal) of the operational amplifier 39, and the contact point a and the contact point b which are switching contacts of the switch element 44 are the above three. Of the amplifying resistors 41, 42 and 43, they are connected to both ends of the central amplifying resistor 42. In this case, normally, the switch element 44 is in a state of electrically connecting the negative input terminal of the operational amplifier 39 and the contact a as shown in the figure. When a gain switching signal is output from the microcomputer 20, a switching operation is performed in the switch element 44 so that the negative input terminal of the operational amplifier 39 and the contact b are conductively connected.

Here, in a state where the negative side input terminal of the operational amplifier 39 and the contact point a of the switch element 44 are conductively connected (state shown in the figure), the amplification resistor 41 is the input resistance of the inverting amplification circuit 38, and the amplification resistors 42 and 43. Becomes the feedback resistance of the amplifier circuit 38. Therefore, the amplification factor GA of the inverting amplifier circuit 38 in this state is
GA = (R2 + R3) / R1 (Formula 1)
It becomes.

When the negative input terminal of the operational amplifier 39 and the contact point b of the switch element 44 are conductively connected, the amplifying resistors 41 and 42 are the input resistors of the inverting amplifier circuit 38 and the amplifying resistor 43 is the amplifier circuit 38. It becomes a feedback resistor. Therefore, the gain GB of the inverting amplifier circuit 38 in this state is
GB = R3 / (R1 + R2) (Formula 2)
It becomes.

  When the amplification factors GA and GB are compared, GA> GB. That is, when the conduction contact of the switch element 44 is the contact a, the amplification factor is relatively high, and when the conduction contact of the switch element 44 is the contact b, the amplification factor is relatively low. is there. Therefore, when the conduction contact of the switch element 44 is switched from the contact point a to the contact point b, the amplification factor of the inverting amplification circuit 38 is changed from a high amplification factor to a low amplification factor. In the present embodiment, the gain GA when the conductive contact of the switch element 44 is the contact a is “× 15”, and the gain GB when the contact b is the contact b is “× 5”.

  In combination with the above-described air-fuel ratio detection range, when air-fuel ratio detection is performed in the stoichiometric / lean zoom ranges RG2 and RG3, which are relatively narrow detection ranges, the amplification factor of the amplifier circuit 38 is set to increase the detection resolution. The conduction point of the switch element 44 is a contact a. On the other hand, when air-fuel ratio detection is performed in the entire range RG1, which is a relatively wide detection range, the amplification factor of the amplifier circuit 38 may be reduced by giving priority to the detection range expansion, and the conduction contact of the switch element 44 is reduced. Contact point b. In such a case, the microcomputer 20 receives information on each air-fuel ratio control status from the engine ECU 25, and outputs a gain switching signal to the switch element 44 based on the received information. Thereby, the amplification factor is switched in the amplification circuit 38 in correspondence with the air-fuel ratio detection range.

  An offset setting circuit 50 is connected to the midpoint between the amplification resistors 41 and 42. The offset setting circuit 50 applies an offset to the A / F output voltage that is the output of the inverting amplifier circuit 38, and the air-fuel ratio detection range is changed by the offset application. The inverting amplifier circuit 38 and the offset setting circuit 50 constitute an adder circuit. Therefore, the inverting amplifier circuit 38 outputs the A / F output voltage as a voltage signal obtained by adding the offset signal to the element current signal corresponding to the magnitude of the element current IL.

  The offset setting circuit 50 includes a switch element 51 formed of, for example, a MOS transistor, a resistor 52 connected in series to the switch element 51, and a power supply circuit 55 having two voltage dividing resistors 53 and 54. Has been. The switch element 51 is turned on / off based on an offset switching signal output from the microcomputer 20.

  In this case, if the offset switching signal is a low signal, the switch element 51 is turned off. Accordingly, the connection between the inverting amplifier circuit 38 and the offset setting circuit 50 is substantially cut off, and the inverting amplifier circuit 38 operates regardless of the offset setting circuit 50. That is, offset setting is not performed for the A / F output voltage.

  On the other hand, if the offset switching signal is a high signal, the switch element 51 is turned on. Therefore, the inverting amplifier circuit 38 and the offset setting circuit 50 are connected, and the inverting amplifier circuit 38 operates in a state in which the offset setting by the offset setting circuit 50 is reflected. That is, an offset current determined by the resistor 52 and the power supply circuit 55 flows into the inverting amplifier circuit 38, and an offset is given to the A / F output voltage corresponding to the offset current.

  In combination with the above-described air-fuel ratio detection range, when air-fuel ratio detection is performed in the stoichiometric zoom range RG2, the switch element 51 is turned off, and as shown in FIGS. 4 and 5, the air-fuel ratio detection centered on stoichiometry is performed. A range is realized (the same applies to the entire range RG1). Further, when air-fuel ratio detection is performed in the lean zoom range RG3, the switch element 51 is turned off, and an air-fuel ratio detection range in a predetermined lean region is realized as shown in FIGS.

  Next, a specific procedure for switching the air-fuel ratio detection range in the air-fuel ratio detection apparatus will be described. FIG. 6 is a flowchart showing the range switching process. This process is repeatedly executed by the microcomputer 20 at a predetermined time period.

  In FIG. 6, in step S11, a startup process is executed. This start-up process is a process that is started only immediately after the engine is started, and details thereof will be described later.

  Thereafter, in steps S12 to S14, the engine control state is determined each time. That is, in step S12, it is determined whether or not the ECU signal transmitted from the engine ECU 25 is a signal indicating that lean combustion control is being performed. For example, lean combustion control is performed during light load traveling such as constant traveling at high speed. In such a case, step S12 is affirmed and the process proceeds to step S15.

  In step S13, it is determined whether or not the ECU signal is a signal indicating that rich combustion control is being performed. Further, in step S14, it is determined whether or not the ECU signal is a signal indicating that fuel cut (F / C) is in progress. If the rich combustion control is being performed or the fuel is being cut, either step S13 or S14 is affirmed and the process proceeds to step S16. When all of Steps S12 to S14 are denied, it is considered that the stoichiometric combustion control is currently being performed, and the process proceeds to Step S17.

  In step S15, the air-fuel ratio detection range is set to the lean zoom range RG3. Specifically, the switch element 44 of the inverting amplification circuit 38 is operated to the contact a side and the switch element 51 of the offset setting circuit 50 is turned on (however, if the operation state is already maintained, the same state is maintained. The same applies to steps S16 and S17). As a result, the air-fuel ratio detection range in a state where the amplification factor is relatively high and the offset is applied, that is, the lean zoom range RG3 is set.

  In step S16, the air-fuel ratio detection range is set to the entire range RG1. Specifically, the switch element 44 of the inverting amplifier circuit 38 is operated to the contact b side, and the switch element 51 of the offset setting circuit 50 is turned off. Thereby, the air-fuel ratio detection range in a state where the amplification factor is relatively low and no offset is given, that is, the entire range RG1 is set.

  In step S17, the air-fuel ratio detection range is set to the stoichiometric zoom range RG2. Specifically, the switch element 44 of the inverting amplifier circuit 38 is operated to the contact a side, and the switch element 51 of the offset setting circuit 50 is turned off. As a result, the air-fuel ratio detection range in a state where the amplification factor is relatively high and no offset is applied, that is, the stoichiometric zoom range RG2 is set.

  Note that the switching conditions of the above ranges RG1 to RG3 are not limited to the above-described conditions (steps S12 to S14), and other conditions may be applied.

  FIG. 7 is a flowchart showing details of the start-up process started in step S11. This start-up process is for determining the characteristic error of the sensor control circuit 30. Specifically, the current (element current) flowing through the sensor element 10 is cut off (the switch 36 in FIG. 1 is opened). Thus, switching to each range RG1 to RG3 is performed, and the characteristic error in each range RG1 to RG3 is determined based on the A / F output voltage at that time. At this time, the characteristic error is diagnosed depending on whether or not the A / F output voltage becomes a value corresponding to the element current IL = 0 mA. Details of the processing procedure will be described below.

  In FIG. 7, in step S21, switching to the stoichiometric zoom range RG2 is performed, and in the subsequent step S22, the A / F output voltage in such a state is read. In step S23, switching to the lean zoom range RG3 is performed, and in the subsequent step S24, the A / F output voltage in this state is read. Further, in step S25, switching to the entire range RG1 is performed, and in the subsequent step S26, the A / F output voltage in such a state is read.

  Finally, in step S27, the A / F output voltage read in steps S22, S24, and S26 is compared with the abnormality determination value (value corresponding to element current IL = 0 mA), and from the result, the sensor control circuit 30 is compared. The characteristic error is determined.

  Since neither the stoichiometric zoom range RG2 nor the entire range RG1 is given an offset, there is a correlation between the respective characteristic errors, and the other characteristic error can be obtained from one characteristic error. Therefore, it is possible to omit either the processing of steps S21 and S22 or the processing of steps S25 and S26. For example, when the characteristic error of the entire range RG1 is obtained based on the characteristic error in the stoichiometric zoom range RG2, the characteristic error of the stoichiometric zoom range RG2 is multiplied by the ratio of the amplification factors of both ranges. What is necessary is just to obtain | require the characteristic error of RG1. It is also possible to carry out the abnormality diagnosis process as described above other than at the time of starting. For example, it can be carried out during engine operation or when the engine is stopped.

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

  In the inverting amplifier 38, the connection / non-connection of the offset setting circuit 50 to one signal input terminal of the operational amplifier 39 can be switched, and switching between the stoichiometric zoom range RG2 and the lean zoom range RG3 can be performed by switching the connection / non-connection. It was possible. In this case, in the lean zoom range RG3 in which the offset is set, the amount of current that flows from the offset setting circuit 50 to the operational amplifier 39 is known in advance, and therefore the air-fuel ratio is accurately detected by taking the amount of current added into account. Can do.

  According to the above configuration using the inverting amplifier circuit 38 as the amplifier circuit for element current amplification, unlike the case of using the differential amplifier circuit (see FIG. 14 described above), the signal amplification factor and the air-fuel ratio are unexpectedly different. The problem that the detection range fluctuates can be avoided. Further, in the configuration of FIG. 14 using the differential amplifier circuit, signals are input to both input terminals of the operational amplifier 111 via the buffers 102 and 103. However, in the configuration of the present embodiment, no buffer is required. The configuration can also be simplified.

  In the present embodiment, the element current IL flowing through the current detection resistor 32 is measured at an intermediate point (point B in FIG. 1) between the operational amplifier 31 and the current detection resistor 32. Thus, the feedback current fed back through the amplification resistors 41 to 43 can be prevented from flowing into the current detection resistor 32 (that is, in this case, the feedback current flows from the point B in FIG. 1 to the operational amplifier 31 side). In this case, if a feedback current flows into the current detection resistor 32, it is necessary to install a buffer or the like in the signal input path of the operational amplifier 39 in order to suppress a decrease in detection accuracy of the element current due to the feedback current. In this configuration, it is not necessary to install such a buffer.

  In the inverting amplifier circuit 38, which of the plurality of amplification resistors 41 to 43 becomes an input resistor or a feedback resistor is switched by the switch element 44, and the amplification factor of the inverting amplifier circuit 38 is variably set in accordance with the switching. This makes it possible to adjust the air-fuel ratio detection resolution appropriately. Therefore, the detection accuracy can be increased in the desired air-fuel ratio detection range.

  The variable setting of the amplification factor of the inverting amplifier circuit 38 is performed by allocating the input resistance and feedback resistor associated with the switching of the switch element 44. Therefore, a plurality of different amplification factors can be realized by using a plurality of amplifier circuits (op amps). Unlike the previous configuration, the configuration can be simplified. With the simplification of the circuit configuration, it is possible to reduce the size and cost of the circuit and reduce the number of terminals.

  As described above, according to the present embodiment, it is possible to improve the detection accuracy in the desired air-fuel ratio detection range while simplifying the circuit configuration.

  By providing the switch element 44 on the signal input path (input line connected to the negative side input terminal) of the operational amplifier 39, the A / F output voltage in the inverting amplifier circuit 38 can be amplified with high accuracy. That is, since the signal input path of the operational amplifier 39 is generally high impedance, even if the switch element 44 has a resistance component, the resistance component can be ignored. Therefore, the accuracy of signal amplification in the inverting amplifier circuit 38 is improved.

  High-accuracy stoichiometric combustion control (stoichiometric feedback control) can be realized based on the air-fuel ratio detection result in the stoichiometric zoom range RG2, and high-accuracy lean combustion control (lean) based on the air-fuel ratio detection result in the lean zoom range RG3. Feedback control) can be realized. Further, based on the air-fuel ratio detection result in the entire range RG1, the deterioration diagnosis of the A / F sensor by the atmospheric detection at the time of fuel cut, the rich feedback control at the time of increasing the high load, and the like can be suitably performed. It is also possible to suitably cope with direct injection lean fuel control in which combustion control is performed in an ultra lean region.

  Since each air-fuel ratio detection range is set as a detection range including a stoichiometric point, when diagnosing a characteristic error on the circuit, the diagnosis can be performed easily and suitably.

  In the configuration of FIG. 1, the configuration example in which the current detection resistor 32 and the inverting amplifier circuit 38 are connected to the positive terminal side of the sensor element 10 has been described. However, the current detection resistor 32 and the inverting amplifier circuit 38 are connected to the sensor element 10. It may be configured to be connected to the negative terminal side.

(Second Embodiment)
In the second embodiment, a rich zoom range RG4 that is a predetermined rich control region is defined as the air-fuel ratio detection range. The rich zoom range RG4 is set, for example, within a range of A / F = 10-15. In this case, as shown in FIG. 8, in addition to the above-described three ranges (entire range RG1, stoichiometric zoom range RG2, and lean zoom range RG3), a rich zoom range RG4 may be defined. These ranges are appropriately switched according to which of the processes related to the air-fuel ratio is performed, as described above. FIG. 9 shows a circuit configuration when the four air-fuel ratio detection ranges RG1 to RG4 are switched. In FIG. 9, the configuration of the offset setting circuit 60 is different from that in FIG. 1, and the difference will be described below.

  The offset setting circuit 60 is connected to an intermediate point between the amplifying resistors 41 and 42 as in the above-described offset setting circuit 50 (see FIG. 1). The offset setting circuit 60 includes a switch element 61 configured by, for example, a MOS transistor. The switch element 61 has three switching contacts a, b, and c, and resistors 62 and 63 are connected in series to the contacts a and c, respectively. Each of the resistors 62 and 63 is connected to a power supply circuit 67 having three voltage dividing resistors 64, 65 and 66. Specifically, the resistor 62 is intermediate between the voltage dividing resistors 64 and 65. The resistor 63 is connected to the intermediate point between the voltage dividing resistors 65 and 66. The switch element 61 is turned on / off based on an offset switching signal output from the microcomputer 20.

  In this case, the offset switching signal is a control signal for operating the conduction contact of the switch element 61 to any one of a, b, and c. When the offset switching signal is a signal for operating the conduction contact of the switch element 61 to the contact b, the connection between the inverting amplifier circuit 38 and the offset setting circuit 60 is substantially cut off, and the inverting amplifier circuit 38 is connected to the offset setting circuit. Operates independently of 60. That is, offset setting is not performed for the A / F output voltage.

  On the other hand, when the offset switching signal is a signal for operating the conduction contact of the switch element 61 to the contact a or the contact c, the offset setting is performed by the power supply circuit 67 for the A / F output voltage. At this time, one of the two offset values is selectively set by switching to the contact point a or the contact point c.

  Here, when the conduction contact of the switch element 61 is operated to the contact a, the voltage V1 set by the power supply circuit 67 is applied to the inverting amplifier circuit 38 via the switch element 61 and the like, and the conduction contact is the contact. When operated to b, the voltage V2 set by the power supply circuit 67 is applied to the inverting amplifier circuit 38 via the switch element 61 and the like. The voltages V1 and V2 have a relationship of V1> V2. Further, the voltages V1 and V2 respectively apply positive and negative offsets to the inverting amplifier circuit 38. That is, the voltage V1 has a voltage value that can cause a current corresponding to the offset (positive current toward the operational amplifier 39) to flow from the offset setting circuit 60 to the inverting amplification circuit 38 by applying the voltage. On the other hand, the voltage V2 has a voltage value that allows a current corresponding to the offset (a negative current toward the operational amplifier 39) to flow into the offset setting circuit 60 from the inverting amplifier circuit 38 side by applying the voltage. Specifically, when it is assumed that both ends of the current detection resistor 32 become 2.2 V [volt] at the time of detecting stoichiometry, for example, V1≈4 V [volt] and V2≈1 V [volt].

  In combination with the above-described air-fuel ratio detection range, when air-fuel ratio detection is performed in the stoichiometric zoom range RG2, the switch element 61 is operated to the contact b, and as shown in FIG. Range is realized. Further, when air-fuel ratio detection is performed in the lean zoom range RG3, the switch element 61 is operated by the contact a, and an air-fuel ratio detection range in a predetermined lean region is realized as shown in FIG. Further, when air-fuel ratio detection is performed in the rich zoom range RG4, the switch element 61 contact c is operated, and an air-fuel ratio detection range in a predetermined rich region is realized as shown in FIG.

  According to the second embodiment described in detail above, the air-fuel ratio detection accuracy is improved even in the rich zoom range RG4. Therefore, in addition to stoichiometric combustion control and lean combustion control, rich combustion control can be suitably performed. For example, it is possible to improve the control accuracy when performing rich combustion control at the time of high load increase or rich combustion control for function regeneration of an exhaust purification device (lean NOx catalyst) or the like.

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

  As the operational amplifier 39 constituting the inverting amplifier circuit 38, the configuration of FIG. In this case, as shown in the equivalent circuit of FIG. 10A, the base of the transistor Q2 is connected to the collector of the input transistor Q1, and the base of the output transistor Q3 is connected to the emitter of the transistor Q2. Similarly, the base of the output transistor Q4 is connected to the collector of the input transistor Q1. A resistor 81 is provided between the output transistors Q3 and Q4. A base of the transistor Q5 is connected to one end side (transistor Q3 side), and an output terminal is connected to the other end side (transistor Q4 side). . Further, a constant current circuit 82 is connected between the power supply line and the collector of the input transistor Q1, and a constant current circuit 83 is connected between the emitter of the transistor Q5 and the ground line. According to the above configuration, by connecting the constant current circuit 83 to the output terminal, an inflow of current (current sink) from the output terminal to the constant current circuit 83 occurs, and accordingly, a voltage near the lower limit value of the power supply voltage range. Output is possible. That is, the range of the output voltage can be expanded on the lower limit side.

  10B, a pull-down resistor 85 may be connected to the output terminal of the operational amplifier 39. In this case, an inflow of current from the output terminal to the pull-down resistor 85 occurs, and voltage output in the vicinity of the lower limit value of the power supply voltage range becomes possible as described above. That is, the range of the output voltage can be expanded on the lower limit side.

  10A and 10B, the output voltage range can be extended with an operational amplifier that is inexpensive and has a small chip area. Thereby, size reduction as a sensor control circuit can be achieved. Incidentally, as a device current amplification operational amplifier, it has been proposed to use a rail-to-rail operational amplifier capable of obtaining an input / output amplitude in the vicinity of the power supply voltage (for example, Japanese Patent Application Laid-Open No. 2006-275628). In addition, it is considered that there is a disadvantage that the chip area becomes large.

  In the above embodiment, the inverting amplifier circuit is used as the amplifier circuit for amplifying the element current. However, it is possible to change this and use a non-inverting amplifier circuit.

  In the above embodiment, the lean zoom range RG3 is defined as an air-fuel ratio detection range including a stoichiometric point. However, it is possible to change this and define it as an air-fuel ratio detection range that does not include a stoichiometric point. For example, the lean zoom range RG3 is defined in the range of A / F = 20 to 35. Similarly, the rich zoom range RG4 can be defined as an air-fuel ratio detection range that does not include a stoichiometric point.

  In the above embodiment, the sensor element (A / F sensor) having the element structure shown in FIG. 2 has been described. However, the present invention can be applied to a sensor element having another element structure. For example, instead of the one-cell type sensor element, a two-cell type sensor element having a pump cell and an electromotive force cell is used. In other words, instead of a configuration having one layer of solid electrolyte, a configuration having two layers of solid electrolyte or a configuration having three layers of solid electrolyte is adopted. Further, the present invention can be applied to a sensor element having a cup-type structure instead of a sensor element having a laminated structure.

  Hereinafter, two configuration examples of the sensor element having a two-cell structure will be described with reference to FIGS.

  The sensor element 130 shown in FIG. 11A has two solid electrolyte layers 131 and 132, and a pair of electrodes 133 and 134 are arranged opposite to each other, and the other solid electrolyte layer 131 is disposed. A pair of electrodes 135 and 136 are disposed opposite to the layer 132. The electrodes 133 to 135 appear to be two places on the left and right sides of 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 130, the pump cell 141 is configured by the solid electrolyte layer 131 and the electrodes 133 and 134, and the oxygen detection cell 142 is configured by the solid electrolyte layer 132 and the electrodes 135 and 136. The sensor element 130 has a laminated structure as in the sensor element 10 described above. In FIG. 11A, reference numeral 137 denotes a gas introduction hole, reference numeral 138 denotes a porous diffusion layer, reference numeral 139 denotes an air duct, and reference numeral 140 denotes a heater.

  The potential of the electrode 136 of the oxygen detection cell 142 is input to the negative input terminal of the comparator 145, and the comparison voltage Vref is input to the positive input terminal of the comparator 145. A current detection resistor 146 is connected between the electrode 133 of the pump cell 141 and the output of the comparator 145, and points A and B of both terminals of the current detection resistor 146 are extracted as sensor outputs. Yes.

  In the sensor element 130 having the above structure, the oxygen detection cell 142 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 142 decreases, and the output of the comparator 145 (point B voltage in the figure) increases. Therefore, a current flows through the current detection resistor 146 in the direction of B → A. On the contrary, when it is rich, the electromotive force output of the oxygen detection cell 142 increases, and the output of the comparator 145 (point B voltage in the figure) decreases. Therefore, a current flows through the current detection resistor 146 in the direction of A → B. The oxygen detection cell 142 is also generally called an electromotive force cell or an oxygen concentration detection cell.

  Further, the sensor element 150 shown in FIG. 11B has three solid electrolyte layers 151, 152, and 153, and a pair of electrodes 154 and 155 are disposed opposite to the solid electrolyte layer 151. A pair of electrodes 156 and 157 are arranged opposite to each other. In the present sensor element 150, a pump cell 161 is configured by the solid electrolyte layer 151 and the electrodes 154 and 155, and an oxygen detection cell 162 is configured by the solid electrolyte layer 152 and the electrodes 156 and 157. Further, the solid electrolyte layer 153 constitutes a wall material for securing the oxygen reference chamber 158. The sensor element 150 has a laminated structure, which is the same as the sensor element 10 described above. In FIG. 11B, reference numeral 159 denotes a porous diffusion layer, and reference numeral 160 denotes a gas detection chamber. Note that the oxygen detection cell 162 is also generally referred to as an electromotive force cell or an oxygen concentration detection cell, like the oxygen detection cell 72 of FIG.

  The potential of the electrode 157 of the oxygen detection cell 162 is input to the negative input terminal of the comparator 165, and the comparison voltage Vref is input to the positive input terminal of the comparator 165. A current detection resistor 166 is connected between the electrode 154 of the pump cell 161 and the output of the comparator 165, and points A and B of both terminals of the current detection resistor 166 are extracted as sensor outputs. Yes. In this case, a current flows through the current detection resistor 166 in the direction of B → A when lean, and conversely, a current flows through the current detection resistor 166 in the direction of A → B when rich.

  The configuration of the sensor control circuit of the gas concentration sensor having the two-cell sensor element will be described with reference to FIG. FIG. 12 shows the configuration of the sensor control circuit 170 related to the sensor element 130 of FIG.

  In the sensor control circuit 170 of FIG. 12, a reference voltage power source 171 is connected to a common terminal of the pump cell 141 and the oxygen detection cell 142. In addition, a closed circuit including these cells 141 and 142, an operational amplifier 172, and a current detection resistor 173 is configured, and a comparison voltage Vref (0) is connected to a non-inverting input terminal (positive input terminal) of the operational amplifier 172. .45V) is connected to a comparison voltage generation circuit 174. When lean, a current flows through the current detection resistor 173 in the direction of B → A, and conversely, when rich, a current flows through the current detection resistor 173 in the direction of A → B (the operational amplifier 172 is connected to the operational amplifier 145 in FIG. 11A). The current detection resistor 173 corresponds to the current detection resistor 146). In such a case, the pump cell 141 is feedback-controlled so that the output voltage of the oxygen detection cell 142 becomes a predetermined value (however, various feedback control circuits have already been disclosed, and illustration and detailed description thereof are omitted here).

  A differential amplifier circuit 180 is connected to points A and B at both ends of the current detection resistor 173. The differential amplifier circuit 180 inputs each terminal voltage at both ends of the current detection resistor 173 and amplifies the voltage difference. The A / F output voltage that is the output of the differential amplifier circuit 180 is a microcomputer (illustrated). Is omitted. The differential amplifier circuit 180 includes an operational amplifier 181, a switch element 182 is provided on a signal input path connected to the positive input terminal (non-inverting input terminal), and a contact that is a switching contact of the switch element 182. a and a contact b are respectively connected to both ends of the central amplifying resistor 184 among the three amplifying resistors 183, 184 and 185 connected in series. In this case, normally, the switch element 182 is in a state of electrically connecting the positive input terminal of the operational amplifier 181 and the contact a as shown in the figure.

  In addition, a switch element 186 is provided on a signal input path connected to the negative side input terminal (inverted input terminal) of the operational amplifier 181, and contacts c and d, which are switching contacts of the switch element 186, are connected in series. Of the three amplification resistors 187, 188, and 189 thus formed, they are respectively connected to both ends of the central amplification resistor 188. In this case, normally, the switch element 186 is in a state of electrically connecting the negative input terminal of the operational amplifier 181 and the contact c as shown in the figure.

  When a gain switching signal is output from a microcomputer (not shown), switching operations are simultaneously performed in the switch elements 182 and 186. In this case, the amplification factor of the differential amplifier circuit 180 is changed in accordance with the switching operation of the switch elements 182 and 186. In which case (that is, in which air-fuel ratio detection range) the gain is increased or decreased is determined according to the above description, and the description is omitted here.

  Note that in the differential amplifier circuit 180, “positive input resistance value = negative input resistance value” and “feedback resistance value = input resistance value” before and after switching of the switch elements 182 and 186. Is retained.

  Further, on the feedback path side of the differential amplifier circuit 180, an offset setting circuit 190 is connected to an intermediate point between the amplifier resistors 187 and 188. The offset setting circuit 190 applies an offset to the A / F output voltage that is the output of the differential amplifier circuit 180, and the air-fuel ratio detection range is changed by the application of the offset. The differential amplifier circuit 180 and the offset setting circuit 190 constitute an adder circuit. Therefore, the differential amplifier circuit 180 outputs the A / F output voltage as a voltage signal obtained by adding the offset signal to the element current signal corresponding to the magnitude of the element current IL.

  The offset setting circuit 190 includes a switch element 191 composed of, for example, a MOS transistor, a resistor 192 connected in series to the switch element 191, and a power supply circuit 195 having two voltage dividing resistors 193 and 194. Has been. The switch element 191 is turned on / off based on an offset switching signal output from a microcomputer (not shown).

  In this case, if the offset switching signal is a low signal, the switch element 191 is turned off. Therefore, the connection between the differential amplifier circuit 180 and the offset setting circuit 190 is substantially cut off, and the differential amplifier circuit 180 operates independently of the offset setting circuit 190. That is, offset setting is not performed for the A / F output voltage.

  On the other hand, if the offset switching signal is a high signal, the switch element 191 is turned on. Therefore, the differential amplifier circuit 180 and the offset setting circuit 190 are connected, and the differential amplifier circuit 180 operates in a state in which the offset setting by the offset setting circuit 190 is reflected. That is, an offset current determined by the resistor 192 and the power supply circuit 195 flows into the differential amplifier circuit 180 (the operational amplifier 181), and an offset is given to the A / F output voltage according to the offset current.

  In combination with the above-described air-fuel ratio detection range, when air-fuel ratio detection is performed in the stoichiometric zoom range RG2, the switch element 191 is turned off, and an air-fuel ratio detection range centered on stoichiometry is realized (the entire range RG1 is also The same). When air-fuel ratio detection is performed in the lean zoom range RG3, the switch element 191 is turned off, and an air-fuel ratio detection range in a predetermined lean region is realized.

  In FIG. 12, when the offset setting state is switched, there are a state where the offset setting circuit 190 is connected to an intermediate point between the input resistance and the feedback resistance on the feedback path side of the differential amplifier circuit 180 and a state where it is not connected. Can be switched. In such a case, the state where the value of the feedback resistance of the differential amplifier circuit 180 and the value of the ground resistance match before and after the switching is maintained. Therefore, the disadvantage that the signal amplification factor and the gas concentration detection range fluctuate due to offset switching is avoided, and proper air-fuel ratio detection can be realized.

  In the sensor control circuit 170 of FIG. 12, the points A and B at both ends of the current detection resistor 173 are not fixed and fluctuate. However, in the sensor control circuit 200 shown in FIG. 13 below, one terminal of the current detection resistor Can be fixed.

  In the sensor control circuit 200 shown in FIG. 13, a voltage (for example, 3 V) equivalent to the reference voltage Vf <b> 1 is applied to the common terminal of the pump cell 141 and the oxygen detection cell 142 through the operational amplifier 203. That is, the point B voltage in the figure is fixed at 3V. In addition, a closed circuit including the oxygen detection cell 142, the feedback circuit 201, and the current detection resistor 202 is configured. The reference voltage Vf2 in the feedback circuit 201 is, for example, 2.55V.

  The operation of the sensor control circuit 200 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 142, so the potential at the point C2 in the feedback circuit 201 drops. Then, the output of the feedback circuit 201, that is, the voltage at point A increases. That is, when rich, a current flows through the current detection resistor 202 in the direction of A → B. On the contrary, current flows through the current detection resistor 202 in the direction of B → A during lean.

  A non-inverting amplifier circuit 210 is connected to points A and B on both ends of the current detection resistor 202, and an A / F output voltage, which is an output of the amplifier circuit 210, is supplied to a microcomputer (not shown). Is output. The amplifier circuit 210 includes an operational amplifier 211. A switch element 212 is provided on a signal input path connected to the negative side input terminal (inverted input terminal), and a contact a that is a switching contact of the switch element 212 is provided. The contact b is connected to both ends of the central amplifying resistor 214 among the three amplifying resistors 213, 214, and 215 connected in series. In this case, normally, the switch element 212 is in a state of electrically connecting the negative input terminal of the operational amplifier 211 and the contact a as shown in the figure.

  When a gain switching signal is output from a microcomputer (not shown), a switching operation is performed in the switch element 212. In this case, the amplification factor of the amplifier circuit 210 is changed with the switching operation of the switch element 212. In which case (that is, in which air-fuel ratio detection range) the gain is increased or decreased is determined according to the above description, and the description is omitted here.

  Further, on the feedback path side of the amplifier circuit 210, an offset setting circuit 220 is connected to an intermediate point between the amplification resistors 213 and 214. The offset setting circuit 220 applies an offset to the A / F output voltage that is the output of the amplifier circuit 210, and the air-fuel ratio detection range is changed by the application of the offset. The amplifier circuit 210 and the offset setting circuit 220 constitute an adder circuit. Therefore, the amplifier circuit 210 outputs the A / F output voltage as a voltage signal obtained by adding the offset signal to the element current signal corresponding to the magnitude of the element current IL.

  The offset setting circuit 220 includes a switch element 221 configured by, for example, a MOS transistor, a resistor 222 connected in series to the switch element 221, and a power supply circuit 225 having two voltage dividing resistors 223 and 224. Has been. The switch element 221 is turned on / off based on an offset switching signal output from a microcomputer (not shown).

  In this case, if the offset switching signal is a low signal, the switch element 221 is turned off. Therefore, the connection between the amplifier circuit 210 and the offset setting circuit 220 is substantially cut off, and the amplifier circuit 210 operates independently of the offset setting circuit 220. That is, offset setting is not performed for the A / F output voltage.

  On the other hand, if the offset switching signal is a high signal, the switch element 221 is turned on. Therefore, the amplifier circuit 210 and the offset setting circuit 220 are connected, and the amplifier circuit 210 operates in a state in which the offset setting by the offset setting circuit 220 is reflected. That is, an offset current determined by the resistor 222 and the power supply circuit 225 flows into the amplifier circuit 210 (the operational amplifier 211), and an offset is given to the A / F output voltage according to the offset current.

  In combination with the above-described air-fuel ratio detection range, when air-fuel ratio detection is performed in the stoichiometric zoom range RG2, the switch element 221 is turned off, and an air-fuel ratio detection range centered on stoichiometry is realized (the entire range RG1 is also The same). When air-fuel ratio detection is performed in the lean zoom range RG3, the switch element 221 is turned off, and an air-fuel ratio detection range in a predetermined lean region is realized.

  In the circuit configuration of FIG. 13 as well, when the offset setting state is switched, the offset setting circuit 220 is connected to an intermediate point between the input resistance and the feedback resistance on the feedback path side of the differential amplifier circuit 210 as described above. The state of not being connected is switched. In this case, the disadvantage that the signal amplification factor and the gas concentration detection range fluctuate unexpectedly due to the offset switching is avoided, and appropriate air-fuel ratio detection can be realized.

  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 component. 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.

  The present invention can also be applied to a gas concentration sensor that can detect HC concentration and CO concentration as gas concentration components. 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.

  The sensor control device of the present invention can be applied not only to a gas sensor (sensor element) used for a gasoline engine but also to a gas sensor (sensor element) used for another type of engine such as a diesel engine. It can be used as a sensor control device for applications other than automobiles, or a gas other than exhaust gas can be used as a detected gas.

The block diagram which shows the electrical structure of a sensor control system. Sectional drawing which shows the structure of an A / F sensor. The figure which shows the voltage-current characteristic (VI characteristic) of an A / F sensor. The figure which shows the relationship between A / F and a sensor output voltage. The figure which shows the relationship between A / F and element current. The flowchart which shows a range switching process. The flowchart which shows the detail of the process at the time of starting. The figure which shows the relationship between A / F and a sensor output voltage. The block diagram which shows the electric constitution of a sensor control system in 2nd Embodiment. The figure which shows another structure regarding an operational amplifier. Sectional drawing which shows the structure of the sensor element of 2 cell structure. The block diagram which shows the electric constitution of a sensor control system in another embodiment. The block diagram which shows the electric constitution of a sensor control system in another embodiment. The circuit diagram for demonstrating a prior art.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Sensor element, 11 ... Solid electrolyte layer, 20 ... Microcomputer, 25 ... Engine ECU, 30 ... Sensor control circuit, 32 ... Current detection resistance, 38 ... Inversion amplifier circuit, 39 ... Operational amplifier, 41-43 ... Resistance for amplification, 44 ... switch element, 50 ... offset setting circuit, 51 ... switch element, 60 ... offset setting circuit, 61 ... switch element.

Claims (14)

In a sensor control device that controls 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,
A current measuring resistor for measuring a current flowing through the sensor element;
An element current signal measured by the current measuring resistor is input to at least one of positive and negative signal input terminals, and an amplifier circuit that amplifies the input element current signal;
An offset setting circuit connected to an intermediate point between the input resistor and the feedback resistor on the feedback path side of the amplifier circuit;
An offset switching means for switching the offset setting state by the offset setting circuit according to the density value detected each time;
A sensor control device comprising:   2. The offset switching means switches between a state in which the offset setting circuit is connected and a state in which the offset setting circuit is not connected to an intermediate point between an input resistance and a feedback resistance on the feedback path side of the amplifier circuit. The sensor control device according to 1. In a sensor control device that controls 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,
A current measuring resistor for measuring a current flowing through the sensor element;
An inverting or non-inverting amplifier circuit that inputs an element current signal measured by the current measuring resistor to one signal input terminal and amplifies the input element current signal;
An offset setting circuit connected to a signal input terminal for inputting the element current signal or the other signal input terminal in the amplifier circuit;
An offset switching means for switching the offset setting state by the offset setting circuit according to the density value detected each time;
A sensor control device comprising:   The sensor control device according to claim 3, wherein the offset switching means switches between a state in which an offset setting circuit is connected to a signal input terminal of the amplifier circuit and a state in which the offset setting circuit is not connected. The offset setting circuit is capable of variably setting a plurality of offset values,
The gas concentration detection device according to claim 1, wherein the offset switching unit selectively sets any one of the plurality of offset values. The exhaust gas of the internal combustion engine is to be detected, and the air-fuel ratio detection range is a part of the total air-fuel ratio range that can be detected by the sensor element and is used for stoichiometric combustion control. It is applied as an air-fuel ratio detection device that is defined as a part and a lean detection range used for lean combustion control,
The offset switching means determines whether to perform air-fuel ratio detection in either the stoichiometric detection range or the lean detection range according to the status of air-fuel ratio control each time, and performs offset switching according to the determination result The sensor control apparatus according to claim 1. As the air-fuel ratio detection range, a rich detection range that is a part of the total air-fuel ratio range and is used for rich combustion control is applied as an air-fuel ratio detection device further defined,
The offset switching means determines whether to perform air-fuel ratio detection in one of the stoichiometric detection range, the lean detection range, or the rich detection range according to the status of each air-fuel ratio control, and according to the determination result The sensor control apparatus according to claim 6, wherein offset switching is performed.   The sensor control device according to claim 6 or 7, wherein each of the detection ranges is set as a detection range including a stoichiometric point.   Circuit characteristic detection that temporarily stops the voltage applied to the sensor element to forcibly cut off the current flowing through the sensor element and detects an error in the circuit characteristic based on the output signal of the amplifier circuit at that time 9. The sensor control device according to claim 8, further comprising means. While providing a plurality of amplification resistors for determining the amplification factor in the amplifier circuit,
6. An amplification factor switching means for switching which of the plurality of amplification resistors becomes an input resistance or feedback resistor of the amplification circuit so as to variably set the amplification factor of the amplification circuit. The sensor control device according to 1. As a gas concentration detection range, a sensor control device that predefines a plurality of detection ranges that are wide and narrow,
The amplification factor switching means increases the amplification factor of the amplification circuit when performing gas concentration detection within a relatively narrow detection range, and when performing gas concentration detection within a relatively wide detection range. The sensor control device according to claim 10, wherein the amplification resistor is switched so as to reduce an amplification factor of the amplification circuit. The exhaust gas of the internal combustion engine is to be detected, and as the air-fuel ratio detection range, a total air-fuel ratio range that can be detected by the sensor element, and a stoichiometric detection range that is a part of the total air-fuel ratio range and that is used for stoichiometric combustion control And an air-fuel ratio detection device that is also defined as a lean detection range that is part of the entire air-fuel ratio range and is used for lean combustion control,
When using the full air-fuel ratio range, no offset is given by the offset switching means, and the amplification factor is reduced by the amplification factor switching means,
When using the stoichiometric detection range, no offset is given by the offset switching means, and the gain is increased by the gain switching means,
The sensor control device according to claim 10 or 11, wherein when the lean detection range is used, an offset is given by the offset switching unit and an amplification factor is increased by the amplification factor switching unit. As the air-fuel ratio detection range, a rich detection range that is a part of the total air-fuel ratio range and is used for rich combustion control is applied as an air-fuel ratio detection device further defined,
13. When the rich detection range is used, the offset switching unit applies an offset to the positive / negative opposite side of the lean detection range, and the amplification factor switching unit increases the amplification factor. Sensor control device.   The sensor control device according to claim 1, wherein the amplifier circuit is configured to obtain an output by an operational amplifier including an output stage having a constant current source or a pull-down resistor.

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