JP3736921B2 - Air-fuel ratio sensor - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an air-fuel ratio sensor that detects an air-fuel ratio of an air-fuel mixture supplied to various combustion equipment such as an internal combustion engine from an oxygen concentration in exhaust gas.
[0002]
[Prior art]
Conventionally, as one of the air-fuel ratio sensors of this type, two detection elements having porous electrodes on both sides of a plate-like oxygen ion conductive solid electrolyte are used, and one porous electrode of each detection element is an exhaust gas. And a porous electrode on the side not contacting the measurement gas chamber of one of the detection elements is arranged so as to be in contact with a reference gas such as the atmosphere. Fuel ratio sensors (for example, Japanese Patent Laid-Open Nos. 61-138156 and 61-186849) are known.
[0003]
In this type of air-fuel ratio sensor, the oxygen sensor device is used as a detection element on the side in contact with the reference gas, and the other sensor element is used as an oxygen pump device, so that the voltage generated at both ends of the oxygen concentration cell device is constant. By controlling the flowing pump current bidirectionally and taking out a signal corresponding to the pump current, an air-fuel ratio signal that continuously changes from the lean region to the rich region of the air-fuel ratio can be obtained.
[0004]
In other words, in the oxygen concentration cell element, a voltage corresponding to the ratio of the oxygen partial pressure of the reference gas and the oxygen partial pressure in the measurement gas chamber is generated, so that this voltage becomes constant, that is, the oxygen content in the measurement gas chamber. If the pump current flowing through the oxygen pump element is controlled bidirectionally so that the pressure becomes constant and the current value is detected, a detection signal corresponding to the air-fuel ratio can be obtained.
[0005]
By the way, the output of such an air-fuel ratio sensor is taken into a CPU or the like via an A / D converter and used for control of an internal combustion engine or the like. In many cases, it is operated with a single power source.
For this reason, in an air-fuel ratio sensor, it is necessary to take out a detection signal corresponding to a pump current having both positive and negative polarities as a unipolar signal. Usually, a detection resistor is provided in the current path of the pump current, An output circuit that detects and amplifies the voltage at both ends, and outputs as a detection signal a signal whose level is shifted so that the amplified voltage signal fluctuates around a predetermined reference voltage (for example, 1/2 of the power supply voltage). Is provided.
[0006]
That is, when such an air-fuel ratio sensor is used, a predetermined voltage set in advance is used as an output voltage corresponding to the case where the exhaust gas has the stoichiometric air-fuel ratio, and the predetermined voltage is used as a reference value from the output voltage of the detection signal. The air-fuel ratio will be obtained.
[0007]
[Problems to be solved by the invention]
However, when the exhaust gas has the stoichiometric air-fuel ratio, in order to output a predetermined voltage from the air-fuel ratio sensor, the power supply voltage is usually lowered or divided using a Zener diode or a voltage dividing resistor. Thus, a reference voltage (reference voltage) must be created in the detection circuit. However, when the power supply voltage fluctuates or the characteristics of those circuit elements change due to fluctuations in ambient temperature, the reference voltage also changes. That is, when 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 theoretical air-fuel ratio, and an error occurs in the detected value. There was a problem.
[0008]
In particular, in recent years, so-called precision λ control for controlling an internal combustion engine so that the air-fuel ratio is close to the theoretical air-fuel ratio (that is, the excess air ratio λ = 1) is known in order to purify exhaust gas from the internal combustion engine. However, in this control, it is necessary to detect λ 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 a range of λ = 3.0. It is necessary to detect with a resolution of 0.0 = 1/3000 or more. This is equivalent to encoding with 12 bits in the A / D converter, but when the output voltage needs to be detected with such high resolution, if the reference voltage changes even slightly due to disturbance, the accuracy of This makes it impossible to perform good control.
[0009]
In order to solve the above problems, an object of the present invention is to provide an air-fuel ratio sensor capable of accurately detecting an air-fuel ratio regardless of fluctuations in power supply voltage and ambient temperature.
[0010]
[Means for Solving the Problems and Effects of the Invention]
The present invention made to achieve the above object
A detection element unit 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;
One of the detection elements is an oxygen concentration cell element that outputs a signal corresponding to the oxygen concentration in the measurement gas chamber, and the other is an oxygen pump element that moves oxygen ions between porous electrodes formed on both sides of the detection element. The pumping current flowing through the oxygen pump element is controlled so that the output signal of the oxygen concentration cell element indicates that the oxygen concentration in the measurement gas chamber is an oxygen concentration corresponding to the stoichiometric air-fuel ratio. Current control means for controlling in the direction,
A detection resistor disposed in the current path of the pump current;
An air-fuel ratio signal output means that outputs 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 has a stoichiometric air-fuel ratio;
An air-fuel ratio sensor comprising:
Comparing means is provided for comparing the magnitude of the voltage across the detection resistor and outputting the comparison result as a second air-fuel ratio signal.
[0011]
In the air-fuel ratio sensor of the present invention configured as described above, the current control means indicates that the output signal of the oxygen concentration cell element indicates that the oxygen concentration in the measurement gas chamber is at an oxygen concentration corresponding to the stoichiometric air-fuel ratio. As described above, by controlling the pump current flowing through the oxygen pump element, a pump current corresponding to the oxygen concentration in the exhaust gas and, further, the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine flows through the detection resistor.
[0012]
That is, if the air-fuel ratio is rich (lower than the theoretical 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 is lean (higher than the theoretical air-fuel ratio: λ If> 1), on the contrary, a pump current flows in the direction of drawing out oxygen from the measurement gas chamber.
[0013]
The air-fuel ratio signal output means generates a first air-fuel ratio signal that changes according to the voltage across the detection resistor and is set to a predetermined voltage when the exhaust gas has the stoichiometric air-fuel ratio. In other words, the first air-fuel ratio signal corresponds to the pump current and eventually corresponds to the air-fuel ratio.
[0014]
On the other hand, the comparison means compares the magnitude of the voltage across the detection resistor and outputs the comparison result as a second air-fuel ratio signal. In other words, the signal level of the second air-fuel ratio signal is reversed when the direction in which the pump current flows is reversed, that is, the signal level varies depending on whether the air-fuel ratio is lean or rich.
[0015]
Therefore, according to the air-fuel ratio sensor of the present invention, if the reference value setting means for detecting the first air-fuel ratio signal is provided at the timing when the second air-fuel ratio signal is inverted, the first air fuel ratio sensor corresponding to the theoretical air-fuel ratio is provided. The value of the air-fuel ratio signal can be measured and the measured value can be set as a reference value for determining the air-fuel ratio from the first air-fuel ratio signal. Even if the reference voltage created in step 1 changes, the air-fuel ratio can be obtained with high accuracy.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
An 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 and its peripheral devices according to an embodiment to which the present invention is applied, and FIG. 2 is a cross-sectional view of a detection element portion of the air-fuel ratio sensor.
[0017]
As shown in FIG. 1, an air-fuel ratio sensor 2 according to this embodiment includes a detection element unit 2a provided in an exhaust pipe of an internal combustion engine, and a later-described first element corresponding to the oxygen concentration in exhaust gas by controlling the detection element unit 2a. And a detection circuit unit 2b for outputting the first and second air-fuel ratio signals.
[0018]
First, as shown in FIG. 2, the detection element unit 2 a includes a first sensor element (pump element) 7 in which porous electrodes 5 and 6 are formed on both sides of the solid electrolyte substrate 4, and a solid electrolyte substrate 8. A second sensor element (battery element) 11 in which porous electrodes 9 and 10 are formed on both sides, and a spacer 12 that is stacked between these elements 7 and 11 to form a measurement gas chamber a are provided. 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 airtight member such as ceramics is provided in order to bring the porous electrode 10 into contact with the atmosphere. It has been.
[0019]
Further, one porous electrode 5 of the pump element 7 has a pump element control terminal A, and one porous electrode 10 of the battery element 11 has a battery element control terminal B, the remaining porous parts of the pump element 7 and the battery element 11. A common terminal C is connected to the electrodes 6 and 9.
Further, a support base 15, which is provided with a screw portion 14 for attachment to the exhaust pipe, is attached to the lower peripheral edge portions of the pump element 7, the battery element 11, and the wall surface 13 via a heat-resistant and insulating adhesive member 16. It has been. That is, the detection element portion 2a is attached to the exhaust pipe 1 by screwing the screw portion 14 of the support base 15 into the screw portion 17 for attaching the detection element portion formed on the exhaust pipe 1 and tightening. Has been.
[0020]
In addition, yttria-zirconia solid solution and calcia-zirconia solid solution are known as materials of the solid electrolyte substrates 4 and 8 constituting the pump element 7 and the battery element 11, and each of cerium dioxide, thorium dioxide, and hafnium dioxide. A solid solution, a perovskite solid solution, a trivalent metal oxide solid solution, or the like can be used. In addition, 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 for oxidation reaction is used. Further, as the material of the spacer 12, alumina, spinel, forsterite, Steatite, zirconia, etc. are used.
[0021]
Next, as shown in FIG. 1, the detection circuit unit 2b has a predetermined value (this value) corresponding to a case where the output voltage Vs between the terminals B and C, that is, the battery element 11, is the stoichiometric air-fuel ratio. In the embodiment, it is constituted by a current control circuit 20 for controlling the pump current Ip flowing between the terminals A and C, that is, the pump element 7 in two directions, an operational amplifier OP3, and resistors R11 to R17. A first output circuit 22 that outputs a first air-fuel ratio signal Vo corresponding to a voltage across the detection resistor Rd, an operational amplifier OP4, resistors R21 to R25, and a transistor TR. And a second output circuit 24 that outputs a second air-fuel ratio signal Do whose level changes in accordance with the magnitude relationship, that is, the current direction in which the pump current Ip flows through the detection resistor Rd.
[0022]
Among these, the current control circuit 20 has an output 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 output is connected to the common terminal C via the resistor Rx. Based on the operational amplifier OP1 that operates to hold the potential of the inverted input at the predetermined voltage Vb and the voltage signal obtained from the battery element control terminal B, the output voltage Vs of the battery element 11 corresponds to a predetermined air / fuel ratio. A PID circuit 21 that performs PID control so as to be a value, 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 OP1 of the detection resistor Rd The operational amplifier OP2 is provided as a buffer circuit for taking out the potential of the side end without affecting the pump current Ip.
[0023]
The resistance value of the resistor Rx connected between the operational amplifier OP1 and the detection resistor Rd and the common terminal C 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 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 in which control is performed by using a value added with an appropriate weight as an output.
[0024]
In the current control circuit 20 configured as described above, the output voltage Vs of the battery element 11 operates so as to indicate that the oxygen concentration in the measurement gas chamber a is an oxygen concentration corresponding to the stoichiometric air-fuel ratio. In addition, even if the predetermined voltage Vb fluctuates due to fluctuations in the power supply voltage VDD (8 V in this embodiment), the output voltage Vs is always the oxygen concentration in the measurement gas chamber a corresponding to the stoichiometric air-fuel ratio. The operation is performed so that the pump current Ip = 0 when the concentration is indicated.
[0025]
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 draw oxygen from the measurement gas chamber a, that is, the detection resistance. When Rd flows from the operational amplifier OP1 side to the PID circuit 21 side and is rich (λ <1), in order to supply oxygen to the measurement gas chamber a, the common terminal C is directed to the pump element control terminal A, That is, the detection resistor Rd flows from the PID circuit 21 side toward the operational amplifier OP1 side.
[0026]
That is, if it is lean, the potential at the end of the detection resistor Rd on the side of the operational amplifier OP1 is increased, and if it is rich, the potential on the end of the detection resistor Rd on the side of the PID circuit 21 is increased.
The first output circuit 22 is configured as a known differential amplifier circuit centered on the operational amplifier OP3, and the divided voltage value of the power supply voltage VDD by the resistors R15 and R16 is used as the reference voltage Vc (4V in this embodiment). 3), as shown in FIG. 3A, the leaner the air-fuel ratio, the higher the reference voltage Vc, and the richer the lower the reference voltage Vc. When the stoichiometric air-fuel ratio (λ = 1), the reference voltage Vc. The first air-fuel ratio signal Vo that is equal to is output.
[0027]
On the other hand, the second output circuit 24 includes an operational amplifier OP4 used as a comparator, and resistors R21 to R25 and a transistor TR configured as an inverting output circuit that inverts the output of the operational amplifier OP4 and converts it into a predetermined signal level. As shown in FIG. 3B, a second air-fuel ratio signal Do is output, which is at a high level when the air-fuel ratio is lean and at a low level when the air-fuel ratio is rich, and the signal level is inverted when the air-fuel ratio is the stoichiometric air-fuel ratio.
[0028]
The air-fuel ratio sensor 2 configured in this way is used together with an ECU 26 that controls an internal combustion engine for a vehicle, for example.
The ECU 26 is mainly composed of a microcomputer comprising a CPU, a ROM, and a RAM. The ECU 26 operates the air-fuel ratio sensor 2 when the internal combustion engine is started, and is input via an A / D converter. The air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine is obtained from the air-fuel ratio signal Vo and the reference value Vf corresponding to the stoichiometric air-fuel ratio, and operation control of the internal combustion engine is performed based on the obtained air-fuel ratio. .
[0029]
Here, FIG. 5 is a flowchart showing a reference value setting process that is repeatedly executed in the ECU 26 using the idle time of the operation control.
The reference value Vf is determined in advance based on the power supply voltage VDD and the resistance values of the resistors R15 and R16 by the initialization process executed immediately after the power supply to the ECU 26 is turned on before this process is started. The second air-fuel ratio signal Do is read, and the signal level is set as a comparison value Dp described later.
[0030]
When this process is started, first, the second air-fuel ratio signal Do is read in S110, and in the subsequent S120, the signal level of the second air-fuel ratio signal Do when the previous main process is executed is determined. It is determined whether or not it is equal to the comparison value Dp representing the signal level, and if it is equal to the comparison value Dp, that is, if the air-fuel ratio continues to be lean or rich, this processing is terminated. To do.
[0031]
On the other hand, if the previously read second air-fuel ratio signal Do is not equal to the comparison value Dp in S120, that is, the air-fuel ratio has changed from lean to rich or from rich to lean. If so, the process proceeds to S130.
In S130, the first air-fuel ratio signal Vo is read. In the subsequent S140, the read value Vo is set as a new reference value Vf. In the subsequent S150, the second air read in the previous S110 is set. After the signal level of the air-fuel ratio signal Do is set as a new comparison value Dp, this process is terminated.
[0032]
As described above, according to the air-fuel ratio sensor 2 of the present embodiment, not only the first air-fuel ratio signal Vo corresponding to the air-fuel ratio but also the second air-fuel ratio whose signal level is inverted at the stoichiometric air-fuel ratio. Since the signal Do is output, the theoretical air-fuel ratio (λ = 1) is detected by detecting the first air-fuel ratio signal Vo at the timing when the signal level of the second air-fuel ratio signal Do is inverted. It is possible to actually measure the signal level of the first air-fuel ratio signal Vo corresponding to the reference voltage Vc.
[0033]
Then, according to the ECU 26 that performs control using the output of the air-fuel ratio sensor 2, the reference value Vf is obtained from the first air-fuel ratio signal Vo detected at the timing when the signal level of the second air-fuel ratio signal Do changes. Has been reset. Therefore, the reference voltage Vc changes due to the fluctuation of the power supply voltage or the resistance values of the voltage dividing resistors R15 and R16 due to the influence of the ambient temperature, and the reference value Vf set in the ECU 26 corresponds to the theoretical 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 quickly reset to a value that correctly corresponds to the stoichiometric air-fuel ratio. Thus, the air-fuel ratio can be obtained from the air-fuel ratio signal Vo with high accuracy, and the operation control of the internal combustion engine can be performed with high accuracy.
[0034]
As mentioned above, although one Example of this invention was described, this invention is not limited to the said Example, In the range which does not deviate from the summary of this invention, it can implement in various aspects.
For example, in the above 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 small pumping current through the battery element may be used.
[0035]
In the above embodiment, the ECU 26 detects the change in the signal level of the second air-fuel ratio signal Do according to the control program and then reads the first air-fuel ratio signal Vo. A circuit that latches the signal level of the first air-fuel ratio signal Vo with the signal Do may be provided.
[0036]
Further, in the above embodiment, the first and second air-fuel ratio signals Vo and Do are separately input to the ECU 26, but an addition circuit for adding the first and second air-fuel ratio signals Vo and Do is provided. The addition result may be 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 showing a reference value setting process executed by the 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 11 ... Battery element 12 ... Spacer 13 ... Wall surface 14 ... Screw portion 15 ... Support base 16 ... Adhesive member 17 ... Screw portion 20 ... Current control circuit 21 ... PID circuit 22 ... First output circuit 24 ... Second output circuit 26 ... ECU OP1-OP4 ... Operational amplifier TR ... Transistor R11 to R17, R21 to R25 ... resistance

Claims (1)

A detection element unit 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;
One of the detection elements is an oxygen concentration cell element that outputs a signal corresponding to the oxygen concentration in the measurement gas chamber, and the other is an oxygen pump element that moves oxygen ions between porous electrodes formed on both sides of the detection element. The pumping current flowing through the oxygen pump element is controlled so that the output signal of the oxygen concentration cell element indicates that the oxygen concentration in the measurement gas chamber is an oxygen concentration corresponding to the stoichiometric air-fuel ratio. Current control means for controlling in the direction,
A detection resistor disposed in the current path of the pump current;
Air-fuel ratio signal output means for generating a first air-fuel ratio signal that changes according to the voltage across the detection resistor and is set to a predetermined voltage when the exhaust gas has a stoichiometric air-fuel ratio;
An air-fuel ratio sensor comprising:
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 ;
Reference value setting means for detecting the first air-fuel ratio signal at a timing at which the second air-fuel ratio signal is inverted and setting the detected value as a reference value corresponding to the theoretical air-fuel ratio is provided. An air-fuel ratio sensor characterized by the above.

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