JP3372150B2 - Oxygen concentration detector - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a resistance value detecting type oxygen concentration detecting device for detecting an oxygen concentration using a sensor element whose resistance value changes according to the oxygen concentration in an ambient atmosphere, and more particularly to an internal combustion engine. The present invention relates to an oxygen concentration detection device suitable for detecting the air-fuel ratio of a fuel mixture supplied to a combustion device from the oxygen concentration in the exhaust gas of various combustion devices such as.

[0002]

2. Description of the Related Art Conventionally, in this type of device, a metal oxide semiconductor made of titania or the like has been used as a sensor element. However, this sensor element has a resistance value which changes at the same time as an ambient oxygen concentration. , The resistance value also changes depending on the temperature of itself. That is, the resistance value is low when the temperature has a negative temperature coefficient (hereinafter referred to as element temperature), and conversely, the resistance value increases when the element temperature is low. Therefore,
This type of device has a problem that the oxygen concentration cannot be accurately detected unless the detection signal is compensated.

Therefore, in the air-fuel ratio detecting device for determining whether the air-fuel ratio of the fuel mixture supplied to the internal combustion engine is rich or lean using such a resistance value detecting type oxygen concentration detecting device, conventionally, for example, As disclosed in Japanese Patent Publication No. 61-33127, the air-fuel ratio determination standard is set to an intermediate value between the maximum value and the minimum value of the detection signal so that the air-fuel ratio can be accurately determined. Based on the intermediate value, by changing the resistance value of the reference resistor for detecting the element resistance connected in series to the sensor element, the fluctuation range (maximum value-minimum value) of the detection signal is increased to increase the air-fuel ratio. For example, as disclosed in Japanese Patent Application Laid-Open No. 1-267449, the resistance value of the reference resistor for detecting element resistance is reduced when the exhaust temperature is high, and when the exhaust temperature is low, the detection accuracy is improved. By increasing the resistance value of the reference resistor, the resistance value of the reference resistor is changed according to the change of the element resistance accompanying the change of the element temperature so that the detection signal does not change greatly due to the change of the element temperature. , Have been proposed.

However, in the device disclosed in Japanese Patent Publication No. 61-33127, it is assumed that the intermediate value between the maximum value and the minimum value of the detection signal corresponds to the element temperature. Since the resistance value of the device is changed, some effect can be obtained when the element temperature is stable, but when the element temperature (and by extension its resistance value) changes, the element temperature at that time is changed. There is a problem in that it is not possible to obtain an intermediate value of the detection signal corresponding to, and when the element temperature suddenly changes due to the operating state of the internal combustion engine, it is not possible to obtain an accurate detection signal.

Further, in the device disclosed in Japanese Patent Laid-Open No. 1-267449, it is assumed that the exhaust temperature corresponds to the element temperature, and the resistance value of the reference resistor is changed. Although it is affected by temperature, the element temperature and the exhaust temperature do not match, so the resistance value of the reference resistor cannot be changed accurately according to the element temperature, and an accurate detection signal cannot be obtained. There was a problem. Further, in particular, in this device, a plurality of resistors (two in the embodiment) having different resistance values are provided as reference resistors connected in series to the sensor element, and are connected to the sensor element as reference resistors according to the exhaust temperature. Since the resistors are switched, it is only possible to change the resistance value of the reference resistor in steps, and even if the exhaust temperature and the element temperature match, it is possible to correspond to the oxygen concentration. It is not possible to obtain an accurate detection signal.

On the other hand, as an apparatus capable of solving such a problem, for example, as disclosed in Japanese Patent Laid-Open No. 3-180748, a heater is laminated on the same substrate as the sensor element, and the heater is energized to provide a sensor. A device for actively stabilizing the element temperature by heating the element, or a heater for keeping the element temperature constant as disclosed in, for example, Japanese Patent Laid-Open No. 5-288711. A metal resistor having a positive temperature coefficient is used so that the resistance value of the heater becomes constant (in other words, the element temperature becomes the target temperature), and the energizing current of the heater is controlled. Proposed. Further, according to this type of device, since the element temperature is stabilized by using the heater, a highly accurate detection signal corresponding to the oxygen concentration can be obtained.

[0007]

However, in this type of device, since the element temperature is stabilized by heating by the heater, the element temperature exceeds the target temperature due to the energization control of the heater as the temperature of the surrounding atmosphere rises. If this happens, there is a problem in that the detection signal cannot be temperature-compensated, and the detection signal has a value different from the actual oxygen concentration.

For example, in an oxygen concentration detecting device used for air-fuel ratio control of an internal combustion engine, a sensor element has been placed very close to the internal combustion engine in order to improve its starting characteristics. Depending on the operating conditions of the engine, the temperature of the exhaust gas directly hitting the sensor element and the temperature of the exhaust pipe to which the sensor element is attached may reach a high temperature higher than the target temperature of the sensor element. Then, in such a case, the element temperature cannot be controlled by the heater, so that the oxygen concentration in the exhaust gas (in other words, the air-fuel ratio) cannot be accurately detected from the detection signal.

The present invention has been made in view of these problems, and in an oxygen concentration detecting device in which the temperature of a sensor element is controlled by a heater, even if the element temperature exceeds a temperature controllable by the heater, the element It is an object to obtain a stable detection signal without being affected by temperature.

[0010]

In order to achieve the above object, the invention according to claim 1 has a negative temperature coefficient, and the resistance value changes according to the oxygen concentration in the ambient atmosphere. An element and a heater for heating the sensor element, which has a positive temperature coefficient, are laminated on the same substrate, and the resistance value of the heater is detected, and the resistance value is a predetermined target. Heater control means for controlling the detection element part to a target temperature by controlling the energization current of the heater so as to have a predetermined value corresponding to the temperature, a reference resistor connected in series with the sensor element, and the sensor. A constant voltage source for applying a DC voltage to the series circuit of the element and the reference resistor, the detection means for outputting the connection point voltage between the sensor element and the reference resistor as a detection signal representing the oxygen concentration, In the equipped oxygen concentration detector, A switching element connected in parallel correction resistor to the reference resistor of the serial detection means, said switching element, ON during the pulse width modulation signal having a predetermined duty ratio
Based on the resistance value of the heater detected by the heater control means and the PWM control means for OFF control, the duty of the pulse width modulation signal is set so that the ON time of the switching element becomes longer as the resistance value becomes higher. A duty ratio setting means for setting a ratio, and a low-pass filter having a cutoff frequency lower than the frequency of the pulse width modulation signal and processing the detection signal output from the detection means. And

In the oxygen concentration detecting device according to the present invention thus constructed, the heater control means controls the energizing current of the heater so that the resistance value of the heater becomes a predetermined value corresponding to a predetermined target temperature. By controlling, the temperature of the detection element unit (and thus the sensor element) is controlled to the target temperature.
Therefore, the temperature of the sensor element (element temperature) is usually controlled to the target temperature by the energization control of the heater, and the detection signal output from the detection means has a value corresponding to the oxygen concentration.

On the other hand, when the ambient temperature around which the detection element section is disposed becomes high and the temperature of the detection element section exceeds the target temperature accordingly, the heater control means compensates the temperature of the detection signal by the heater energization control. It becomes impossible, and the detection signal output from the detection means changes according to the element temperature.

However, according to the present invention, in addition to the reference resistor for detecting the element resistance which is connected in series to the sensor element, the correction resistor which can be connected in parallel via the switching element and has a resistance value lower than that of the reference resistor. Is provided, the PWM control means controls ON / OFF of the switching element with a pulse width modulation signal having a predetermined duty ratio, and the duty ratio setting means sends the duty ratio of the pulse width modulation signal to the heater control means. Based on the resistance value of the heater detected by
The higher the resistance value of the heater, the more the switching element turns on.
Set it so that the time is long.

Therefore, the reference resistor and the parallel circuit of the reference resistor and the correction resistor are alternately connected in series to the sensor element according to the duty ratio of the pulse width modulation signal. The average of resistance values (hereinafter referred to as comparative resistance) is a value from the upper limit value of the resistance value of the reference resistor to the resistance value of the parallel circuit of the reference resistor and the correction resistor. The comparison resistance is determined by the duty ratio of the pulse width modulation signal. The duty ratio is set by the duty ratio setting means as the resistance value of the heater (and the temperature of the sensor element) increases. Is set to be long, the comparative resistance becomes smaller as the temperature of the sensor element becomes higher (in other words, as the element resistance becomes smaller). That is, the comparison resistance changes according to the element resistance of the sensor element by the operations of the PWM control means and the duty ratio setting means.

Therefore, according to the present invention, even if the temperature of the detection element portion rises and the element temperature cannot be controlled by the heater energization control of the heater control means, the average of the detection signals output from the detection means is obtained. The value can be controlled to a value corresponding to the actual oxygen concentration.

Further, the average value of the detection signals output from the detecting means in this manner is a value corresponding to the oxygen concentration,
Since this detection signal pulsates according to the frequency of the pulse width modulation signal, the oxygen concentration cannot be detected from the absolute value of the detection signal. Therefore, in the present invention, a low-pass filter having a cutoff frequency lower than the frequency of the pulse width modulated signal is provided, and the low-pass filter processes the detection signal output from the detection means.

Therefore, according to the oxygen concentration detecting device of the present invention, the detection signal corresponding to the oxygen concentration can always be obtained without being affected by the temperature change of the sensor element. Further, in particular, in the present invention, unlike the conventional device, the reference resistor itself connected in series with the sensor element is not changed, or the correction resistor is not simply connected in parallel to the reference resistor, but the correction for the reference resistor is performed. The resistors are connected in parallel by pulse width modulation control (PWM control) using a pulse width modulation signal, and the resistance value of the heater corresponding to the element temperature is used to set the duty ratio of the pulse width modulation signal. So
The resistance value (comparison resistance) of the resistor connected in series with the sensor element can be accurately set to a value corresponding to the element resistance without any response delay, and the oxygen concentration can be detected with extremely high accuracy. Therefore, if the oxygen concentration detection device of the present invention is used in an air-fuel ratio control device such as an internal combustion engine, it becomes possible to improve the control accuracy of the air-fuel ratio.

When the oxygen concentration detecting device of the present invention is used in an air-fuel ratio control device such as an internal combustion engine as described above, the frequency of the pulse width modulation signal is at least a lean-rich air-fuel ratio by the air-fuel ratio control. It is necessary to set it higher than the control frequency to be reversed (in the case of an internal combustion engine for an automobile, it is about 0.5 to 3 Hz), and it is preferable to set the frequency about 1000 times the control frequency. Is desirable.

Further, in this case, the low-pass filter for filtering the detection signal changes its cutoff frequency so as not to cut the signal component of the control frequency.
It is necessary to set the frequency higher than the control frequency and lower than the frequency of the pulse width modulation signal, and specifically, it is desirable to set the cutoff frequency to about 10 to 30 times the control frequency.

By the way, in the oxygen concentration detecting device of the present invention, the control for connecting the correction resistor in parallel with the reference resistor by the PWM control means (hereinafter referred to as comparative resistance control).
Is to supplement the temperature compensation of the detection signal by the heater energization control. Basically, if it is executed in a region where the temperature compensation cannot be performed by the heater energization control, that is, in the region where the element temperature exceeds the target temperature. Good. But in reality,
Even if the heater energizing current is controlled, the temperature of the detection element portion may temporarily drop from the target temperature due to the flow of the gas to be measured or the like. For example, when the detection element unit is attached to the exhaust pipe of an internal combustion engine to detect the air-fuel ratio, the flow of exhaust gas becomes rapid during acceleration of the internal combustion engine, etc. The temperature may temporarily drop from the target temperature.

Therefore, it is desirable to execute such comparative resistance control even in a region where the temperature of the detecting element portion temporarily drops. For that purpose, as described in claim 2, in the duty ratio setting means, the heater is used. When the resistance value of is equal to or higher than the resistance value corresponding to a temperature lower by a predetermined temperature than the target temperature, the duty ratio may be set according to the resistance value. That is, in this way, the detection signal can be temperature-compensated by the comparative resistance control even in a region where the element temperature temporarily drops, and the oxygen concentration detection accuracy can be further improved. .

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION As an embodiment of the present invention, an air-fuel ratio detecting device for detecting the air-fuel ratio of a fuel mixture supplied to an internal combustion engine for an automobile will be described below.

First, FIG. 2 shows the structure of the detection element portion 2 which is attached to an exhaust pipe of an internal combustion engine via a housing or the like (not shown). As shown in FIG. 2, the detection element portion 2 of the present embodiment includes a flat plate-shaped electrically insulating member 10 made of alumina or the like.
Of the electrode patterns 12, 14 on the side surface of the
And a heating resistor pattern 16 that constitutes a heater HT for heating is formed around it, and an opening 18a is provided on the pattern surface so as to expose the tips of the electrode patterns 12 and 14. And an electrically insulating member 1
A flat plate-like electrically insulating member 18 made of the same material as that of 0 and thinner than this is joined, and the opening 18a is filled with a metal oxide semiconductor 20 made of titania or the like that constitutes the sensor element TS. It is produced by firing.

The heating resistor pattern 16 is made of a metal resistor having a positive temperature coefficient, and the electrode pattern 1
2 and 14 are made of a low resistance metal body. Further, electrode terminals (not shown) for connection with an external device are provided at both ends of the heating resistor pattern 16 and the end portions of the electrode patterns 12 and 14 opposite to the metal oxide semiconductor 20, respectively.

Next, FIG. 1 shows the entire control device which is connected to the heater HT and the sensor element TS via the electrode terminals and lead wires provided in the detection element section 2 and which conducts the heater and detects the air-fuel ratio. 3 is a schematic configuration diagram showing the configuration of FIG. As shown in FIG. 1, a battery BT is connected to the heater HT via an ignition switch IG of an internal combustion engine and a heater energizing circuit 4, and a sensor element TS detects oxygen concentration (in other words, air-fuel ratio) for detection. The circuit 6 is connected.

The heater energizing circuit 4 is a central processing unit (hereinafter, referred to as CP) in an electronic control unit (hereinafter, referred to as ECU) for controlling the internal combustion engine when the ignition switch IG is turned on.
O by the control signal PWM1 output from 8)
The heater HT is turned off from the positive side of the battery BT when turned off.
Switch 4a consisting of a bipolar transistor, MOS-FET, etc., for connecting / disconnecting the current-carrying path to
And a detection resistor RSH (resistance value: about 100 mΩ) provided in the energization path from the semiconductor switch 4a to the heater HT and for detecting the value of current flowing in the heater HT,
It consists of

Therefore, in the heater HT, when the semiconductor switch 4a is in the ON state when the ignition switch IG for operating the internal combustion engine is ON, a current flows through the detection resistor RSH, and in response to this current. The heater HT generates heat and the temperature of the entire detection element unit 2 including the sensor element TS rises.

On the other hand, the detection circuit 6 outputs an output voltage V from a constant voltage circuit (not shown) which generates a constant voltage from the battery voltage.
By applying c to one end of the sensor element TS and grounding the other end through the reference resistor Rc, a current is passed through the sensor element TS. At the connection point between the sensor element TS and the reference resistor Rc, the cutoff frequency 50H
Low-pass filter of about z (hereinafter referred to as LPF) 6a
Is provided, and the connection point voltage is filtered by the LPF 6a to generate a detection signal Vs, which is output to the CPU 8.

Further, one end of a correction resistor RA is connected to a connection point between the sensor element TS and the reference resistor Rc, and the other end of the correction resistor RA is an NPN type whose emitter is grounded. The collector of the transistor TR1 (corresponding to the switching element of the present invention) is connected. And
The base of the transistor TR1 is connected to the CPU 8 via the resistor R1 and is also grounded via the resistor R2. In addition, the transistor TR1 is turned on by the control signal PWM2 output from the CPU8.
When turned off, the correction resistor RA is added to the reference resistor Rc.
This is to switch whether or not to connect in parallel.

Next, the ECU operates by being supplied with power from the battery BT through the ignition switch IG, and, for example, based on the detection signal Vs output from the detection circuit 6, the fuel mixture supplied to the internal combustion engine. Various control processes for internal combustion engine control, such as air-fuel ratio control, which determines whether the air-fuel ratio of air is lean or rich, and controls the amount of fuel injected and supplied to the internal combustion engine so that the air-fuel ratio becomes the stoichiometric air-fuel ratio. It is a well-known device for executing the above, and is mainly composed of a microcomputer including a CPU 8 and ROM, RAM and the like (not shown). Further, various circuits for controlling the engine such as the heater energizing circuit 4 and the detecting circuit 6 are provided. ing.

In this embodiment, the CPU 8 detects the resistance value (heater resistance) RH of the heater HT from the voltage across the detection resistor RSH, and the heater resistance RH corresponds to the target temperature (eg 700 ° C.). Control signal PWM1 is turned on so that the semiconductor switch 4a is turned on / off.
The F control is performed to control the energization current of the heater HT, and the control signal PWM2 is generated according to the control processing as the heater control means and the heater resistance RH (and thus the element temperature of the sensor element TS). By performing ON / OFF control of the transistor TR1 with the control signal PWM2, temperature compensation of the detection signal Vs (comparison resistance control) is performed.
The control processing as the PWM control means and the duty ratio setting means is also executed.

The control signals PWM1 and PWM2 output from the CPU 8 are both pulse width modulation signals whose duty ratio is controlled. In this embodiment, the heater control control signal PWM1 is shown in FIG. As shown, 50msec.
Is generated as one cycle (frequency: 20 Hz), and the control signal PWM2 for controlling the comparative resistance is 1 m as shown in FIG.
It is generated with sec. as one cycle (frequency: 1 kHz).

The transistor TR1 is turned on / off by the control signal PWM2, so that the sensor element TS has the reference resistor Rc, the reference resistor Rc, and the reference resistor Rc in accordance with the duty ratio of the control signal PWM2. The parallel circuit of the correction resistor RA is alternately connected in series, and the average (comparison resistance) Rc 'of the resistance values is, as shown in FIG. 5, the reference resistor Rc when the duty ratio is 0%. The upper limit, the lower limit of the resistance value of the parallel circuit of the reference resistor Rc and the correction resistor RA when the duty ratio is 100%,
As the duty ratio of the control signal PWM2 (DUTY
Ratio).

Next, control processing executed by the CPU 8 for heater control and comparative resistance control will be described with reference to the flowchart shown in FIG. Figure 6 (a)
As shown in, the CPU 8 controls the heater control signal PW.
The heater energization control process is executed every 50 msec., Which is the same as the cycle of M1. In this process, first, in S110, the control signal PWM1 is set to a high level to turn on the semiconductor switch 4a and start energizing the heater HT.
Further, in the subsequent S120, the energization time DH calculated in the previous process in the process (the initial value is set immediately after the CPU 8 is started) is set in the timer, and in S130, the control signal PW is set.
A predetermined time (2 in this embodiment) after M1 is set to High level
msec.) It waits until a predetermined time elapses by judging whether or not it has elapsed. Then, when a predetermined time has elapsed, in S140, the battery BT side voltage VH1 and the heater HT side voltage VH2 of the detection resistor RSH are read, respectively, and S1
At 50, the heater resistance RH is calculated using the following equation based on the read voltage values VH1 and VH2 and the resistance value of the detection resistor RSH.

RH = RSH.VH2 / (VH1-VH2) That is, in this embodiment, as shown in FIG.
The heater resistance measurement timing is secured for 2 msec. Immediately after the start of energization, and after 2 msec. Has passed after the start of energization,
When the current flowing through the heater HT becomes stable, the heater resistance RH is detected from the voltage across the detection resistor RSH.

When the heater resistance RH is calculated in S150, the flow advances to S160 to update the energization time DH of the heater HT. As shown in FIG. 3, this energization time DH is for setting the ON time within one cycle (50 msec.) Of the control signal PWM1, in other words, for setting the duty ratio of the control signal PWM1, and in S160, The heater resistance RH calculated above and the heater resistance RHC when the temperature of the detection element section 2 (and by extension the sensor element TS) is the target temperature (700 ° C.) (value set in advance as a judgment reference value)
And RH> RHC, B = 0.5 mse
c., when RH = RHC, B = 0, and when RH <RHC, B = -0.5 msec., the update reference value B of the energization time DH is set, and the set update reference value B and the coefficient are set. Based on A and the previously set energization time DH (n-1), the next energization time DH is calculated using the following equation.

DH = DH (n-1) + AB The energization time DH thus calculated is used to set the timer in S120 when the process is executed next time. Then, in S120, the energization time DH
When is set, a timer built in the CPU 8 measures the energization time DH, and when the energization time DH elapses, the DH timer interrupt process shown in FIG. 6B is started.
Then, in this DH timer interrupt process, the control signal PWM1
Is set to the Low level, the semiconductor switch 4a is turned off to stop energizing the heater HT (S30).
0). Therefore, the semiconductor switch 4a performs the heater resistance RH by the heater energization control process and the DH timer interrupt process.
Is turned on and off repeatedly with 50 msec. As one cycle so that becomes the judgment reference value RHC.
By the OFF operation, the temperature of the detection element unit 2, and further, the temperature of the sensor element TS is controlled to the target temperature (700 ° C.).

Next, when the energization time DH is updated in S160, this time the process proceeds to S170, in which the calculated heater resistance RH is the resistance value (control start resistance) RHO at which the comparative resistance control should be executed. It is determined whether or not the above. In addition,
The control start resistance RHO has a predetermined temperature (6 in this embodiment) at which the temperature of the detection element section 2 is lower than the target temperature (700 ° C.).
The heater resistance RH at 50 ° C.) is set.

The heater resistance RH is the control start resistance R
If it is HO or higher, the process proceeds to S180 and the comparison resistance (R
The duty ratio DT for the control of c ') is calculated using the map shown in FIG. 7, and if the heater resistance RH has not reached the control start resistance RHO, the duty for this comparative resistance control is executed in S190. Set the ratio DT to 0.

As shown in FIG. 7, the map for calculating the duty ratio DT is set such that the larger the heater resistance RH, in other words, the higher the temperature of the sensor element TS, the larger the duty ratio DT. Has been done. This is because the sensor element TS has a negative temperature coefficient, and as shown in FIG. 8, the element resistance decreases as the temperature rises. In the present embodiment, the element accompanying the temperature change of the sensor element TS. This map is set so that the comparison resistance Rc 'can be changed at the same change rate in response to the change in resistance.

In this way, when the duty ratio DT is set in S180 or S190, the process proceeds to S200, and the control value TON of the comparison resistor Rc 'is updated based on the set duty ratio DT. . This control value TON
As shown in FIG. 4, one cycle of the control signal PWM2 (1 m
sec.), and in S200, the detection circuit 6 is operated at a ratio corresponding to the duty ratio DT set above.
The time TON for turning on the transistor TR1 therein is calculated. Then, when the ON time TON for the comparative resistance control is calculated in this way, the process is once ended.

Next, the control value (O
(N time) TON is set to the high level for the ON time TON in the Rc 'control process shown in FIG. 6C executed every 1 msec. For the comparative resistance control (S400). Used for. As a result, the transistor TR1 in the detection circuit 6 is turned on / off at a very high speed at a ratio corresponding to the duty ratio DT, and the average resistance (that is, the average resistance) of the resistors connected in series to the sensor element TS (that is, The comparison resistance Rc ′) changes according to the temperature of the sensor element TS.

As described above, according to the air-fuel ratio detecting apparatus of this embodiment, the temperature of the sensor element TS is the target temperature (7
Not only the energization of the heater HT is controlled so that the temperature becomes 00 ° C.), the duty ratio DT for the comparative resistance control is obtained from the heater resistance RH, and the transistor TR1 is turned on by the control signal PWM2 according to the duty ratio DT.・ OFF
By performing (pulse width modulation control), the sensor element TS
It is adapted to control a comparison resistor Rc 'which is connected in series with the.

Therefore, according to the present embodiment, for example, a resistor connected in series with the sensor element TS is a reference resistor Rc.
As shown in the dotted line in Figure 9, the LPF
The detection signal Vs input to the CPU 8 via 6a raises the temperature from the target temperature (700 ° C.) of the sensor element TS even though the air-fuel ratio is the theoretical air-fuel ratio with the excess air ratio λ = 1. Accordingly, the value on the lean side does not change, and as shown by the solid line in FIG.
The value can always be set to a value corresponding to the oxygen concentration (air-fuel ratio) in the exhaust gas without being affected by the temperature change of the sensor element TS. Therefore, according to this embodiment, the CPU 8 can always execute the air-fuel ratio control of the internal combustion engine with high accuracy without being affected by the temperature of the sensor element TS.

Further, in this embodiment, since the comparative resistance control is performed based on the heater resistance RH corresponding to the temperature of the sensor element TS, the comparative resistance Rc 'can be controlled with respect to the temperature change of the sensor element TS without a response delay. In particular, in the present embodiment, since the comparative resistance control is performed in the temperature range (650 ° C.) lower than the target temperature (700 ° C.) or higher, the sensor element TS becomes the target temperature or higher. The detection signal Vs can be temperature-compensated by the comparative resistance control not only in the region where the temperature of the detection signal Vs cannot be compensated by the energization control of the heater HT but also when the sensor element TS temporarily falls below the target temperature. .

Therefore, according to the present embodiment, as shown by the dotted line in FIG. 10, the detection signal Vs is rich or abrupt due to a rapid temperature change of the sensor element TS that occurs during transient operation such as acceleration or deceleration of the internal combustion engine. There is no large change to the lean side, and as shown by the solid line in FIG. 10, even if there is a transient temperature change of the sensor element TS, the detection signal Vs corresponds to the actual oxygen concentration (air-fuel ratio). Can be made.

Although one embodiment of the present invention has been described above, the present invention is not limited to the above embodiment and can take various modes. For example, in the above embodiment, the air-fuel ratio detection device for detecting the air-fuel ratio from the oxygen concentration in the exhaust gas of the internal combustion engine for automobiles has been described, but the present invention is a sensor element made of a resistor having a positive temperature dependence such as titania. If it is a resistance value detection type oxygen concentration detecting device that detects the oxygen concentration using, the same effect can be obtained by applying it in the same manner as the above-mentioned embodiment.

Further, for example, in the above embodiment, the reference resistor Rc is provided on the ground side of the sensor element TS, and the correction resistor RA is connected in parallel with the reference resistor Rc by a switching element (transistor TR1) which functions as a so-called low side switch. Although the detection circuit 6 that is connected is used, for example, as shown in FIG. 11, the reference resistor Rc is provided on the power source side of the sensor element TS, and the correction resistor RA is connected in parallel to this. Even if the detection circuit 30 described above is used, the same effect as that of the above embodiment can be obtained.

That is, the detection circuit 30 applies the output voltage Vc from a constant voltage circuit (not shown) to one end of the sensor element TS via the reference resistor Rc, and directly grounds the other end of the sensor element TS. , A current is passed through the sensor element TS, and at the connection point between the sensor element TS and the reference resistor Rc, as in the detection circuit 6 of FIG. LPF3 to generate
0a is connected. Further, one end of the correction resistor RA is connected to the connection point between the sensor element TS and the reference resistor Rc, and the other end of the correction resistor RA is connected to the power line with its emitter connected to a PNP type transistor. The collector of TR12 is connected. A resistor R14 is connected between the base and emitter of the transistor TR12, and the base of the transistor TR12 is connected to the collector of an NPN transistor TR11 via a resistor R13. This transistor TR11
Is turned on / off by the control signal PWM2 from the CPU8.
F is for turning on the transistor TR12 by turning on the base current of the transistor TR12 when it is turned on. Its emitter is grounded and its base is a resistor R.
It is connected to the control signal PWM2 output port of the CPU 8 via 11 and is grounded via the resistor R12.

Therefore, the detection circuit 3 thus constructed
Even at 0, the control signal PWM output from the CPU 8
According to 2, the correction resistor RA can be connected in parallel to the reference resistor Rc, and the average of the resistors (comparison resistance Rc ') connected in series to the sensor element TS can be controlled. Since the detection signal Vs is generated by filtering the connection point voltage between the sensor element TS and the reference resistor Rc with the LPF 30a, the transistor TR is generated.
There is little pulsation due to 12 switching. Therefore, even if this detection circuit 30 is used, the control signal PWM2
Is generated in the same manner as in the above embodiment, it is possible to obtain the same effect as in the above embodiment.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing an overall configuration of a control device of an embodiment that performs heater energization and air-fuel ratio detection.

FIG. 2 is an exploded perspective view illustrating a configuration of a detection element unit according to an embodiment.

FIG. 3 is an explanatory diagram showing a control signal PWM1 for controlling a heater.

FIG. 4 is an explanatory diagram showing a control signal PWM2 for comparison resistance control.

FIG. 5 is an explanatory diagram showing a relationship between a control signal PWM2 and a comparison resistance Rc ′.

FIG. 6 is a flowchart showing a control process executed by a CPU.

FIG. 7 is an explanatory diagram showing a map used to set the duty ratio DT of the control signal PWM2 from the heater resistance.

FIG. 8 is an explanatory diagram showing changes in element resistance corresponding to changes in temperature of the sensor element.

FIG. 9 is an explanatory diagram showing changes in the detection signal with respect to changes in the temperature of the sensor element.

FIG. 10 is an explanatory diagram showing changes in a detection signal during transient operation of the internal combustion engine.

FIG. 11 is an electric circuit diagram illustrating another configuration example of the detection circuit.

[Explanation of symbols]

2 ... Detection element HT ... Heater TS ... Sensor element 4 ... Heater energizing circuit 4a ... Semiconductor switch RSH
Detecting resistor 6, 30 Detecting circuit Rc Reference resistor RA Compensating resistor TR1, TR12 Transistor (switching element) 6a, 30a Low pass filter (LPF) 8 Central processing unit (CPU) 10, 18 Electrically insulating member 12 ... Electrode pattern 16 ... Heating resistor pattern 20 ... Metal oxide semiconductor

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-9-49818 (JP, A) JP-A-3-180748 (JP, A) JP-A-1-267449 (JP, A) JP-A-5- 288711 (JP, A) JP 55-2932 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) G01N 27/12

Claims (2)

(57) [Claims] 1. A sensor element which has a negative temperature coefficient and whose resistance value changes in accordance with the oxygen concentration in the ambient atmosphere, and a heater which has a positive temperature coefficient and heats the sensor element are the same. The detection element unit laminated on the substrate and the resistance value of the heater are detected, and the energization current of the heater is controlled so that the resistance value becomes a predetermined value corresponding to a predetermined target temperature, and the detection is performed. A heater control means for controlling the element part to a target temperature, a reference resistor connected in series with the sensor element, and a constant voltage source for applying a DC voltage to a series circuit of the sensor element and the reference resistor, An oxygen concentration detecting device comprising: a detection unit that outputs a connection point voltage between the sensor element and the reference resistor as a detection signal indicating the oxygen concentration, wherein a correction resistor is connected in parallel to the reference resistor of the detection unit. Switching element to Based on a resistance value of the heater detected by the heater control means, a PWM control means for controlling ON / OFF of the switching element with a pulse width modulation signal having a predetermined duty ratio, and the switching element increases as the resistance value increases. Of duty ratio setting means for setting the duty ratio of the pulse width modulation signal so as to increase the ON time of the detection signal, and a detection signal output from the detection means having a cutoff frequency lower than the frequency of the pulse width modulation signal. An oxygen concentration detection device comprising: a low-pass filter that processes a signal. 2. The duty ratio setting means sets the duty ratio according to the resistance value when the resistance value of the heater is equal to or higher than a resistance value corresponding to a temperature lower than the target temperature by a predetermined temperature. The oxygen concentration detection device according to claim 1, wherein the oxygen concentration detection device is set.