JPH10153576A - Air-fuel ratio sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an air-fuel ratio sensor by which air-fuel ratio can be detected with good accuracy irrespective of the change in power-supply voltage or in the ambient temperature. SOLUTION: The air-fuel ratio sensor is provided with a current control circuit 20, by which a pumping current Ip flowing to a pumping element 7 is controlled bidirectionally in such a way that the output voltage Vs of a battery element 11 becomes a prescribed value corresponding to a theoretical air-fuel ratio, a first output circuit 22 which is constituted as a well-known differential amplifier circuit by making use of an operational amplifier OP3 as the center and which corresponds to a voltage across both ends of a detecting resistance Rd installed in the route of the pumping current Ip , and a second output circuit 24 which outputs a second air-fuel ratio signal Do , whose signal level is changed according to the magnitude relationship of the voltage across both ends of the detecting resistance Rd . The signal level of a first air-fuel ratio signal Vo corresponding to the theoretical air-fuel ratio can be measured on the basis of a timing at which the second air-fuel ratio signal Do is inverted. As a result, an air-fuel ratio can be reset correctly even when the relationship is changed due to the influence of a power-supply voltage and an ambient temperature.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an air-fuel ratio sensor for detecting an air-fuel ratio of an air-fuel mixture supplied to various combustion devices such as an internal combustion engine from an oxygen concentration in exhaust gas.

[0002]

2. Description of the Related Art Conventionally, as one type of air-fuel ratio sensor of this type, two detection elements each having a plate-shaped oxygen ion conductive solid electrolyte and porous electrodes on both surfaces thereof are provided. The porous electrode is disposed so as to be in contact with the measurement gas chamber in which the diffusion of exhaust gas is restricted, and the porous electrode of one of the detection elements which is not in contact with the measurement gas chamber is disposed so as to be in contact with a reference gas such as the atmosphere. Air-fuel ratio sensors (for example, Japanese Patent Application Laid-Open Nos. 61-138156 and 61-186649)
Is known.

In this type of air-fuel ratio sensor, a detection element in contact with a reference gas is used as an oxygen concentration cell element, and the other detection element is used as an oxygen pump element, so that the voltage generated across the oxygen concentration cell element becomes constant. By bidirectionally controlling the pump current flowing through the pump element and extracting a signal corresponding to the pump current, an air-fuel ratio signal that continuously changes from a lean region to a rich region of the air-fuel ratio can be obtained.

That is, in the oxygen concentration cell element, a voltage is generated in accordance with the ratio between the oxygen partial pressure of the reference gas and the oxygen partial pressure in the measurement gas chamber. By bidirectionally controlling the pump current flowing through the oxygen pump element so that the oxygen partial pressure of the air becomes constant, and detecting the current value, a detection signal corresponding to the air-fuel ratio can be obtained.

The output of such an air-fuel ratio sensor is taken into a CPU or the like via an A / D converter and is used for controlling an internal combustion engine.
Usually, the converter and the CPU are often operated by a single power supply such as + 5V. For this reason, in the air-fuel ratio sensor, it is necessary to take out a detection signal according to the pump current having both positive and negative polarities as a single-polarity signal. An output circuit that detects and amplifies the voltage between both ends and outputs a signal obtained by level-shifting the amplified voltage signal so as to fluctuate around a predetermined reference voltage (for example, の of the power supply voltage) as a detection signal is provided. Is provided.

That is, when such an air-fuel ratio sensor is used, a predetermined voltage set in advance is set as an output voltage corresponding to a case where the exhaust gas has a stoichiometric air-fuel ratio, and the predetermined voltage is used as a reference value to detect a detection signal. The air-fuel ratio is determined from the output voltage.

[0007]

However, in order to output a predetermined voltage from the air-fuel ratio sensor when the exhaust gas has a stoichiometric air-fuel ratio, a power supply is usually provided by using a Zener diode or a voltage dividing resistor. By lowering or dividing the voltage, a reference voltage (reference voltage) must be created inside the detection circuit. However, when the power supply voltage fluctuates or the characteristics of these circuit elements change due to fluctuations in the ambient temperature, the reference voltage also changes. That is, if the power supply voltage or the ambient temperature changes, the predetermined voltage set in advance does not correspond to the voltage actually output from the air-fuel ratio sensor at the stoichiometric air-fuel ratio, and an error occurs in the detection value. There was a problem.

In particular, in recent years, in order to purify exhaust gas from an internal combustion engine, a so-called precision λ that controls the internal combustion engine so that the air-fuel ratio precisely approaches the stoichiometric air-fuel ratio (ie, excess air ratio λ = 1). Control is known, but in this control, λ needs to be detected in 0.001 units. On the other hand, when the engine is controlled in the lean burn state, this type of air-fuel ratio sensor must detect the air-fuel ratio up to the range of λ = 3.0.
It is necessary to detect with a resolution of 3.0 = 1/3000 or more. This is equivalent to encoding with 12 bits in the A / D converter. If it is necessary to detect the output voltage with such a high resolution, if the above-mentioned reference voltage changes even a little due to a disturbance, the accuracy of the output will be reduced. This makes it impossible to perform good control.

The present invention has been made to solve the above problems.
It is an object of the present invention to provide an air-fuel ratio sensor capable of accurately detecting an air-fuel ratio regardless of fluctuations in a power supply voltage and an ambient temperature.

[0010]

Means for Solving the Problems and Effects of the Invention The present invention has been made to achieve the above object, and comprises two detecting elements each having a porous electrode formed on both surfaces of an oxygen ion conductive solid electrolyte. A detection element portion disposed facing the measurement gas chamber in which inflow of exhaust gas is restricted, and an oxygen concentration cell element that outputs one of the detection elements according to an oxygen concentration in the measurement gas chamber; Operated as an oxygen pump element that moves oxygen ions between porous electrodes formed on both sides of the detection element, and the output signal of the oxygen concentration cell element is
Current control means for bidirectionally controlling a pump current flowing through the oxygen pump element so as to indicate that the oxygen concentration in the measurement gas chamber is an oxygen concentration corresponding to the stoichiometric air-fuel ratio, A detection resistor disposed in the current path and a first air-fuel ratio signal that changes according to the voltage across the detection resistor and that is set to a predetermined voltage when the exhaust gas is at the stoichiometric air-fuel ratio. Air-fuel ratio output means for outputting an air-fuel ratio signal, and comparing means for comparing the magnitude of the voltage across the detection resistor and outputting the comparison result as a second air-fuel ratio signal. It is characterized by.

In the air-fuel ratio sensor according to the present invention, the current control means determines that the output signal of the oxygen concentration cell element has an oxygen concentration in the measurement gas chamber corresponding to the stoichiometric air-fuel ratio. By controlling the pump current flowing through the oxygen pump element as shown, the detection resistance indicates the pump current according to the oxygen concentration in the exhaust gas and, consequently, the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine. Flows.

That is, if the air-fuel ratio is rich (lower than the stoichiometric air-fuel ratio: λ <1), a pump current flows in the direction of supplying oxygen to the measurement gas chamber, while the air-fuel ratio becomes lean (than the stoichiometric air-fuel ratio). High: If λ> 1), on the contrary, a pump current flows in the direction of drawing oxygen from the measurement gas chamber.

The air-fuel ratio signal output means outputs a first air-fuel ratio signal that changes in accordance with the voltage across the detection resistor and is set to a predetermined voltage when the exhaust gas has a stoichiometric air-fuel ratio. Generate. That is, the first air-fuel ratio signal corresponds to the pump current, and thus corresponds to the air-fuel ratio.

On the other hand, the comparing means compares the magnitude of the voltage across the detection resistor and outputs the result of the comparison as a second air-fuel ratio signal. That is, when the direction in which the pump current flows is inverted, the signal level of the second air-fuel ratio signal is inverted, that is, the signal level is different depending on whether the state of the air-fuel ratio is lean or rich.

Therefore, according to the air-fuel ratio sensor of the present invention,
If reference value setting means or the like for detecting the first air-fuel ratio signal is provided at the timing when the second air-fuel ratio signal is inverted, the value of the first air-fuel ratio signal corresponding to the stoichiometric air-fuel ratio is measured, and Since the measured value can be set as a reference value when calculating the air-fuel ratio from the first air-fuel ratio signal, even if the reference voltage created inside the air-fuel ratio sensor changes due to fluctuations in the power supply voltage or ambient temperature, The air-fuel ratio can be obtained with high accuracy.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic configuration diagram showing an air-fuel ratio sensor of an embodiment to which the present invention is applied and peripheral devices thereof, and FIG. 2 is a cross-sectional view of a detection element portion of the air-fuel ratio sensor.

As shown in FIG. 1, an air-fuel ratio sensor 2 according to the present embodiment includes a detection element 2a provided in an exhaust pipe of an internal combustion engine.
And a detection circuit section 2b that controls the detection element section 2a and outputs later-described first and second air-fuel ratio signals corresponding to the oxygen concentration in the exhaust gas.

Here, first, as shown in FIG. 2, a detecting element portion 2a comprises a first sensor element (pump element) 7 having porous electrodes 5 and 6 formed on both sides of a solid electrolyte substrate 4, and a solid electrolyte A second sensor element (battery element) 11 having porous electrodes 9 and 10 formed on both sides of a substrate 8, and a spacer 12 laminated between the two elements 7 and 11 to form a measurement gas chamber a. ing. On the opposite side of the battery element 11 from the measurement gas chamber a, an air chamber b formed by a wall surface 13 made of a heat-resistant and air-tight member such as ceramics is provided for bringing the porous electrode 10 into contact with the atmosphere. Have been.

Also, one porous electrode 5 of the pump element 7
, The pump element control terminal A, the one porous electrode 10 of the battery element 11 has the battery element control terminal B, and the pump element 7 and the remaining porous electrodes 6 and 9 of the battery element 11 have the common terminal C.
Is connected. Further, the pump element 7 and the battery element 11
A support 15 on which a screw portion 14 for attaching to an exhaust pipe is engraved is attached to a lower peripheral portion of the wall surface 13 via a heat-resistant and insulating adhesive member 16. In other words, the detection element section 2a is attached to the exhaust pipe 1 by screwing the screw section 14 of the support base 15 to the detection element section mounting screw section 17 formed on the exhaust pipe 1 and tightening the screw section. Have been.

As a material for the solid electrolyte substrates 4 and 8 constituting the pump element 7 and the battery element 11, a yttria-zirconia solid solution and a calcia-zirconia solid solution are known. In addition, cerium dioxide, thorium dioxide, Each solid solution of hafnium, perovskite type solid solution, trivalent metal oxide solid solution and the like can be used. As the porous electrodes provided on both surfaces of the solid electrolyte substrates 4 and 8, platinum, rhodium, or the like having a catalytic action of an oxidation reaction is used. Further, as a material of the spacer 12, alumina, spinel, forsterite, Steatite, zirconia and the like are used.

Next, as shown in FIG. 1, the detection circuit section 2b provides an output voltage Vs between the terminals B and C, that is, the output voltage Vs of the battery element 11.
Is bidirectionally controlled between the terminals A and C, that is, the pump current Ip flowing through the pump element 7 so that the predetermined value (450 mV in this embodiment) corresponding to the case where the measured gas chamber a has the stoichiometric air-fuel ratio. Current control circuit 20 and operational amplifier OP
3 and resistors R11 to R17, and a first air-fuel ratio signal Vo corresponding to a voltage between both ends of a detection resistor Rd described later.
, A first output circuit 22 that outputs a current, the operational amplifier OP4, the resistors R21 to R25, and the transistor TR, and the magnitude of the voltage across the detection resistor Rd, that is, the current through which the pump current Ip flows through the detection resistor Rd. A second output circuit 24 that outputs a second air-fuel ratio signal Do whose level changes according to the direction.

The output of the current control circuit 20 is connected to the pump element control terminal A, a predetermined voltage Vb (4 V in this embodiment) is applied to the non-inverting input, and the resistance Rx
The operational amplifier OP1 that operates to maintain the potential of the inverting input connected to the common terminal C at the predetermined voltage Vb via the
And a PID circuit 21 for performing PID control based on a voltage signal obtained from the battery element control terminal B so that the output voltage Vs of the battery element 11 becomes a predetermined value corresponding to the stoichiometric air-fuel ratio.
And a detection resistor Rd having one end connected to the inverting input of the operational amplifier OP1 and the other end connected to the output of the PID circuit 21.
And an operational amplifier OP2 provided as a buffer circuit for extracting the potential of the detection resistor Rd on the operational amplifier OP1 side without affecting the pump current Ip.

The operational amplifier OP1 and the detection resistor Rd
The resistance value of the resistor Rx connected between the common terminal C and
It is selected so that the voltage drop due to the pump current Ip is sufficiently small. The PID control performed by the PID circuit 21 is as follows.
Output a value obtained by adding an appropriate weight to each of a signal proportional to the deviation signal of the non-control signal (voltage signal from the control terminal B), a signal obtained by integrating the deviation signal, and a signal obtained by differentiating the deviation signal. This is a well-known control that performs control as follows.

In the current control circuit 20 configured as described above, the output voltage Vs of the battery element 11 indicates that the oxygen concentration in the measurement gas chamber a is the oxygen concentration corresponding to the stoichiometric air-fuel ratio. In addition to the power supply voltage V
Even if the predetermined voltage Vb fluctuates due to fluctuation of DD (8 V in this embodiment) or the like, the output voltage Vs always indicates that the oxygen concentration in the measurement gas chamber a corresponds to the stoichiometric air-fuel ratio. Then, the operation is performed so that the pump current Ip = 0.

When the air-fuel ratio to be detected is lean (λ> 1), the pump current Ip is directed from the pump element control terminal A to the common terminal C in order to extract oxygen from the measurement gas chamber a. That is, the detection resistor Rd flows from the operational amplifier OP1 side to the PID circuit 21 side, and when rich (λ <1), to supply oxygen to the measurement gas chamber a from the common terminal C to the pump element control terminal A , That is, the detection resistor Rd flows from the PID circuit 21 side to the operational amplifier OP1 side.

That is, when the detection resistor Rd is lean, the potential of the detection resistor Rd on the operational amplifier OP1 side increases, and when the detection resistor Rd is rich, the potential of the detection resistor Rd on the PID circuit 21 side increases. The first output circuit 22 is connected to the operational amplifier OP
3 is a known differential amplifier circuit centered on the reference numeral 3 and the divided voltage value of the power supply voltage VDD by the resistors R15 and R16 is set as a reference voltage Vc (4 V in this embodiment).
As shown in the graph, the leaner the air-fuel ratio, the higher the reference voltage Vc.
A first air-fuel ratio signal Vo that is larger and richer than the reference voltage Vc and becomes equal to the reference voltage Vc at the stoichiometric air-fuel ratio (λ = 1) is output.

On the other hand, the second output circuit 24 is composed of an operational amplifier OP4 used as a comparator, resistors R21 to R25 and a transistor configured as inverted output circuits for inverting the output of the operational amplifier OP4 and converting the output to a predetermined signal level. As shown in FIG. 3B, the second air-fuel ratio signal Do is output when the air-fuel ratio is lean, becomes high level when the air-fuel ratio is rich, becomes low level when the air-fuel ratio is rich, and is inverted when the air-fuel ratio is the stoichiometric air-fuel ratio. .

The air-fuel ratio sensor 2 thus configured is
For example, it is used together with the ECU 26 that controls an internal combustion engine for a vehicle. The ECU 26 includes a CPU, a ROM,
The microcomputer mainly includes a microcomputer including a RAM, and operates the air-fuel ratio sensor 2 when the internal combustion engine is started, and outputs a first air-fuel ratio signal Vo input via an A / D converter and a stoichiometric air-fuel ratio. The air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine is obtained from the reference value Vf corresponding to the above, and the operation control of the internal combustion engine is performed based on the obtained air-fuel ratio.

FIG. 5 is a flowchart showing a reference value setting process which is repeatedly executed by the ECU 26 using the idle time of the operation control. Before this processing is started, the reference value Vf is changed to the power supply voltage VDD by the initialization processing executed immediately after the power supply to the ECU 26 is turned on.
And a reference voltage Vc previously determined based on the resistance values of the resistors R15 and R16, the second air-fuel ratio signal Do is read, and the signal level is set as a comparison value Dp described later.

When the present process is started, first, in S110, the second air-fuel ratio signal Do is read, and in S120, the signal level is changed to the second air-fuel ratio at the time of the previous execution of the present process. It is determined whether or not it is equal to the comparison value Dp representing the signal level of the signal Do. If the comparison value Dp is equal to the comparison value Dp, that is, if the air-fuel ratio is kept lean or rich, the actual The process ends.

On the other hand, in S120, the second
If the air-fuel ratio signal Do is not equal to the comparison value Dp, that is, if the air-fuel ratio is changing from lean to rich, or from rich to lean, S130
Move to Then, in S130, the first air-fuel ratio signal Vo is read, and in S140, the read value Vo is set as a new reference value Vf.
Then, the second air-fuel ratio signal D read in S110
After setting the signal level of “o” as a new comparison value Dp, the process ends.

As described above, according to the air-fuel ratio sensor 2 of the present embodiment, the first air-fuel ratio signal V corresponding to the air-fuel ratio is obtained.
Not only o, but also at the stoichiometric air-fuel ratio, the signal level is inverted to output the second air-fuel ratio signal Do. Therefore, at the timing when the signal level of the second air-fuel ratio signal Do is inverted, By detecting the first air-fuel ratio signal Vo, the signal level of the first air-fuel ratio signal Vo corresponding to the stoichiometric air-fuel ratio (λ = 1), that is, the reference voltage Vc can be measured.

According to the ECU 26 which performs control using the output of the air-fuel ratio sensor 2, the second air-fuel ratio signal D
o detected at the timing when the signal level of
The reference value Vf is reset by the air-fuel ratio signal Vo. Therefore, the reference voltage Vc changes when the power supply voltage fluctuates or the resistance values of the voltage dividing resistors R15 and R16 change due to the influence of the ambient temperature, and the reference value Vf set in the ECU 26 corresponds to the stoichiometric air-fuel ratio. Even if it disappears, if the signal level of the second air-fuel ratio signal Do is inverted, the reference value Vf is promptly reset to a value that correctly corresponds to the stoichiometric air-fuel ratio. The air-fuel ratio can be determined with high accuracy from the air-fuel ratio signal Vo, and the operation control of the internal combustion engine can be accurately performed.

Although one embodiment of the present invention has been described above, the present invention is not limited to the above-described embodiment, and can be implemented in various modes without departing from the gist of the present invention. For example, in the above-described embodiment, the detection element unit 2a is configured such that the porous electrode 10 formed on the surface of the battery element 11 opposite to the measurement gas chamber a is in contact with the atmosphere. A device that generates a reference oxygen source by passing a minute pumping current through the battery element may be used.

In the above-described embodiment, the ECU 26 detects the change in the signal level of the second air-fuel ratio signal Do in accordance with the control program, and then reads the first air-fuel ratio signal Vo. A circuit for latching the signal level of the first air-fuel ratio signal Vo with the air-fuel ratio signal Do may be provided.

Further, in the above embodiment, the first and second air-fuel ratio signals Vo and Do are separately input to the ECU 26. However, the addition circuit adds the first and second air-fuel ratio signals Vo and Do. May be provided so that the addition result is input to the A / D converter of the ECU 26.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of an air-fuel ratio sensor of an embodiment.

FIG. 2 is a cross-sectional view illustrating a configuration and a mounting state of a detection element unit.

FIG. 3 is a graph showing characteristics of first and second air-fuel ratio signals.

FIG. 4 is an explanatory diagram showing waveforms of first and second air-fuel ratio signals.

FIG. 5 is a flowchart illustrating a reference value setting process executed by an ECU.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Exhaust pipe 2 ... Air-fuel ratio sensor 2a ... Detection element part 2b ... Detection circuit part 4, 8 ... Solid electrolyte substrate 5, 6, 9, 10 ... Porous electrode 7 ... Pump element
DESCRIPTION OF SYMBOLS 11 ... Battery element 12 ... Spacer 13 ... Wall surface 14 ... Screw part 1
DESCRIPTION OF SYMBOLS 5 ... Support base 16 ... Adhesive member 17 ... Screw part 20 ... Current control circuit 21 ... PID circuit 22 ... First output circuit 24 ... Second output circuit 26 ... ECU OP1-OP4 ... Operational amplifier TR
... Transistors R11-R17, R21-R25 ... Resistance

Claims (2)

[Claims] 1. A detection element comprising two detection elements each having a porous electrode formed on both sides of an oxygen ion-conductive solid electrolyte body facing a measurement gas chamber in which inflow of exhaust gas is restricted. And an oxygen concentration cell element that outputs a signal corresponding to the oxygen concentration in the measurement gas chamber, and oxygen that moves oxygen ions between porous electrodes formed on both sides of the detection element. A pump that operates as a pump element and flows through the oxygen pump element so that the output signal of the oxygen concentration cell element indicates that the oxygen concentration in the measurement gas chamber is the oxygen concentration corresponding to the stoichiometric air-fuel ratio. Current control means for bidirectionally controlling the current; a detection resistor provided in the current path of the pump current; and a predetermined resistor which varies according to the voltage between both ends of the detection resistor and when the exhaust gas has a stoichiometric air-fuel ratio. No electricity And an air-fuel ratio signal output means for generating a first air-fuel ratio signal set so as to be a pressure. The air-fuel ratio sensor comprising: An air-fuel ratio sensor comprising a comparison means for outputting a signal as a second air-fuel ratio signal. 2. A reference value setting means for detecting the first air-fuel ratio signal at a timing when the second air-fuel ratio signal is inverted, and setting the detected value as a reference value corresponding to a stoichiometric air-fuel ratio. The air-fuel ratio sensor according to claim 1, wherein: