JPH11344466A - Heater control device of gas concentration sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To well control a heater while simplifying constitution. SOLUTION: The sensor element part 60 of an A/F(air/fuel ratio) sensor 30 is constituted by laminating a solid electrolyte and a heater 64 to integrate them. The A/F sensor 30 outputs a linear A/F detection signal proportional to the concn. of oxygen in exhaust gas accompanied by the application of voltage. A microcomputer 20 operates the heater 64 through a heater control circuit 25 to hold the sensor element part 60 (solid electrolyte) to predetermined active temp. In this case, the microcomputer 20 detects element resistance on the basis of the voltage applied to the sensor element part 60 and the sensor current flowing accompanied by the applied voltage, and controls the supply of a current to the heater 64 on the basis of the element resistance. That is, the open control of heater working current is performed corresponding to the temp. rising capacity of the heater until effective element resistance becomes detectable, and the control for feeding back element resistance to an objective value is performed when the effective element resistance becomes detectable.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a heater control device for a gas concentration sensor.

[0002]

2. Description of the Related Art For example, in an oxygen concentration sensor for detecting the oxygen concentration (air-fuel ratio) in exhaust gas, a sensor current corresponding to the oxygen concentration flows with the application of a voltage, and the oxygen concentration or the air-fuel ratio is determined by the sensor current. It can be detected linearly. Also,
In this sensor, it is necessary to maintain a sensor element made of a solid electrolyte at a predetermined activation temperature. Usually, a heater is attached to the sensor to control the amount of electricity supplied to the heater. As one method of such heater control, there is a method of detecting a heater resistance and controlling power supplied to the heater so that the heater resistance becomes a predetermined value.

[0003] For example, in Japanese Patent Application Laid-Open No. Hei 8-278279, "a heater control device for an oxygen sensor", a heater temperature (heater temperature) is detected based on a heater resistance, and the heater temperature reaches an initial heating temperature of the heater. Until all power is turned on. After reaching the initial heating temperature, the power supply to the heater is controlled according to the heater temperature at that time. Further, an element temperature is detected based on the resistance value of the sensor element, and when the element temperature reaches a predetermined value, energization to the heater is controlled according to the sensor element temperature. The control device has a detection circuit for detecting the heater voltage and the heater current, and the heater resistance is detected from the heater voltage and the heater current.

[0004]

In order to maintain the sensor element at a relatively high activation temperature, a large current must be supplied to the heater. In this case, in order to accurately and efficiently detect the heater current, a high-precision and low-resistance heater current detection resistor is required in the detection circuit, and such a request causes a complicated configuration. Further, since the resistor for detecting the heater current is a heating element, the heat capacity in the circuit must be taken into consideration, which causes a problem that the degree of freedom in circuit design is hindered.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a heater for a gas concentration sensor capable of performing good heater control while simplifying the configuration. It is to provide a control device.

[0006]

In the heater control apparatus for a gas concentration sensor according to the present invention, a sensor element for detecting the concentration of a specific component in a gas to be detected and a sensor element which generates heat when a power supply voltage is applied to heat the sensor element. It has a heater for heating to a predetermined activation temperature, and it is assumed that the sensor element and the heater are arranged close to each other and applied to a gas concentration sensor in which the temperature difference between them is always equal to or less than a predetermined value. And

According to the first aspect of the present invention, a current detecting means for applying a voltage to the sensor element and detecting a value of a current flowing according to a concentration of a specific component in a gas to be detected at the time, Heater control means for determining the element resistance or the element temperature of the sensor element based on the obtained current value, and controlling the amount of power to the heater based on the obtained element resistance or element temperature.

In short, the present invention is based on the premise that the sensor element and the heater are arranged close to each other and the temperature difference between them is always equal to or less than a predetermined value, and the heater is used by using only one of the element resistance and the element temperature. Is performed. That is, unlike the conventional device, the detection of the heater resistance is not required in the heater control. In such a case, a resistor through which a large amount of heater current flows becomes unnecessary, and accordingly, the circuit configuration can be simplified. Further, since the resistor as the heating element can be eliminated, the influence of heat generated by the resistor can be suppressed, and the degree of freedom in circuit design is improved. As a result, the object of the present invention is to achieve good heater control while simplifying the configuration.

The present invention is particularly effective in a laminated sensor in which a heater is stacked on a sensor element having a solid electrolyte and the solid electrolyte and the heater are integrated (claim 5). That is, in such a stacked sensor, the solid electrolyte and the heater are provided close to each other, so that, for example, when the sensor is started from a cold state, the element temperature changes following the change in the heater temperature. In this case, appropriate heater control can be performed by monitoring the element temperature (or element resistance) without monitoring the heater temperature.

In practice, the heater control means may feedback control the element resistance or the element temperature to a target value. In this case, highly accurate and highly reliable heater control can be realized.

According to the third aspect of the present invention, the amount of current supplied to the heater is controlled to be open in accordance with the heater temperature raising capability until an effective element resistance or element temperature of the sensor element can be detected. In this case, it is possible to suppress element cracking due to excessive temperature rise of the sensor element while improving the temperature rising characteristic of the sensor element. By improving the temperature rise characteristics of the sensor element,
The time from the sensor cold state to the activation can be shortened, and the sensor output can be used early.

According to the present invention, a voltage value of a power supply for supplying electric power to the heater is obtained, and a heater power supply amount is corrected according to the voltage value. In practice, the heater power supply amount may be corrected to decrease as the power supply voltage increases, and the heater power supply amount may be corrected to increase as the power supply voltage decreases. In this case, even when the power supply voltage fluctuates, it is possible to carry out heater energization without excess and deficiency.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to the drawings. The air-fuel ratio detection device according to the present embodiment is applied to a gasoline injection engine mounted on an automobile. In the air-fuel ratio control system of the engine, the air-fuel ratio detection device detects the air-fuel ratio based on the detection result of the air-fuel ratio detection device. The fuel injection amount is controlled at a desired air-fuel ratio. In the following description, a schematic configuration of an air-fuel ratio control system, a procedure for detecting an air-fuel ratio (A / F) using an air-fuel ratio sensor, and a procedure for controlling energization of a heater provided in the sensor will be described in detail.

FIG. 1 is an overall configuration diagram showing an outline of an air-fuel ratio control system according to the present embodiment. In FIG. 1, the air-fuel ratio detection device 15 includes a microcomputer (hereinafter, referred to as a microcomputer 20) which is a center of the internal calculation, and the microcomputer 20 controls an engine control ECU 16 for realizing fuel injection control, ignition control, and the like. And are communicably connected to each other. A limiting current type air-fuel ratio sensor (hereinafter, referred to as an A / F sensor 30) is attached to an exhaust pipe 12 extending from an engine main body 11 of the engine 10. It outputs a linear air-fuel ratio detection signal (sensor current signal) proportional to the oxygen concentration.

The microcomputer 20 includes a well-known CPU, ROM, RAM, and the like for executing various arithmetic processes, and controls a bias control circuit 40 and a heater control circuit 25, which will be described later, according to a predetermined control program. Microcomputer 2
0 operates with power supplied by the battery power supply + B.

The A / F sensor 30 has a stacked sensor element portion (cell) 60, and its structure is shown in FIG.
This will be described with reference to FIG. Here, FIG. 2 is a cross-sectional view showing the overall configuration of the A / F sensor 30, FIG. 3 is a cross-sectional view of a sensor element section 60 constituting the A / F sensor 30, and FIG. FIG. 2 is an exploded perspective view illustrating a configuration.

As shown in FIG. 2, the A / F sensor 30
Is a cylindrical metal housing 31 screwed to the exhaust pipe wall.
The element cover 32 is attached to the lower opening of the housing 31. In the element cover 32,
The distal end (lower end) of the long plate-shaped sensor element section 60 is provided. The element cover 32 has a bottomed double structure, and has a plurality of exhaust ports 32a for taking exhaust gas into the inside of the cover.

The sensor element section 60 extends upward in the drawing so as to penetrate the insulating member 33 provided in the housing 31, and a pair of lead wires 34 is connected to its upper end. A body cover 35 is swaged to the upper end of the housing 31. Further, a dust cover 36 is attached above the main body cover 35, and the upper part of the sensor is protected by the double structure of the main body cover 35 and the dust cover 36. Each cover 35, 36 is provided with a plurality of air ports 35a, 36a for taking air into the covers.

Next, the configuration of the sensor element section 60 will be described with reference to FIGS. The sensor element unit 60 is roughly divided into
It comprises a solid electrolyte 61, a gas diffusion resistance layer 62, an air introduction duct 63, and a heater 64, and is formed by laminating these respective members. A protective layer 65 is provided around each member.

The rectangular plate-shaped solid electrolyte 61 is a sheet made of partially stabilized zirconia, and a porous measurement electrode 66 made of platinum or the like is formed on the upper surface (gas diffusion resistance layer 62 side) by a screen printing method or the like. At the same time, a porous atmosphere-side electrode 67 also made of platinum or the like is formed on the lower surface (at the side of the air introduction duct 63) by a screen printing method or the like. Lead wires 66 a and 67 a are connected to the measurement electrode 66 and the atmosphere-side electrode 67.

The gas diffusion resistance layer 62 has a gas permeable layer 62a made of a porous sheet for introducing exhaust gas to the measurement electrode 66, and a gas shielding layer 62b made of a dense layer for suppressing the transmission of exhaust gas. . Each of the gas permeable layer 62a and the gas shielding layer 62b is formed by molding a ceramic such as alumina, spinel, or zirconia by a sheet molding method or the like, but the gas permeability is different due to a difference in porosity average pore diameter and porosity. It has become something.

The air introduction duct 63 is made of a ceramic having a high thermal conductivity such as alumina, and an air chamber 68 is formed by the duct 63. The air introduction duct 63 plays a role of introducing air into the atmosphere-side electrode 67 in the atmosphere chamber 68. Incidentally, the atmosphere chamber 68 is provided with the cover 3 shown in FIG.
The air ports 5a and 36a communicate with the air ports 35a and 36a.

A heater 64 is provided on the lower surface of the air introduction duct 63.
Is attached. The heater 64 is connected to a battery power source +
A heating element 64a that generates heat when energized from B and an insulating sheet 64b that covers the heating element 64a are provided. Lead wires 64c are connected to both ends of the heating element 64a. However, in addition to the configuration of FIG. 3, the heating element 64a may be embedded in the solid electrolyte 61,
A configuration in which the heating element 64a is embedded in the gas diffusion resistance layer 62 is also possible.

In the sensor element section 60, the exhaust gas reaching the measuring electrode 66 does not enter the gas permeable layer 62a from the vertical direction (up and down direction in the drawing), but the gas permeable layer 62a
Invading from the side of That is, since the surface of the gas permeable layer 62a is covered with the gas shielding layer 62b, the exhaust gas cannot enter from the vertical direction, but enters the inside of the transmission layer 62a from a side surface direction orthogonal to the direction. In such a case, the gas diffusion amount of the gas permeable layer 62a depends on the horizontal dimension of the gas permeable layer 62a (actually, the distance between the side surface of the gas permeable layer 62a and the measurement electrode 66). Therefore, a uniform and stable sensor output can be obtained even if the hole diameter of the gas permeable layer 62a varies.

In the A / F sensor 30 having the above-described configuration, the sensor element section 60 generates a limit current corresponding to the oxygen concentration in a lean region from the stoichiometric air-fuel ratio point. In this case, the sensor element section 60 (solid electrolyte 61) can detect the oxygen concentration with linear characteristics, but a high temperature of about 600 ° C. or more is required to activate the sensor element section 60, In addition, since the activation temperature range of the sensor element section 60 is narrow, the engine 1
The active state cannot be maintained by heating only with the exhaust gas of 0. Therefore, in the present embodiment, the sensor element section 60 is maintained in the active temperature range by controlling the heating of the heater 64 (the heating element 64a). In the region richer than the stoichiometric air-fuel ratio, the concentration of unburned gas such as carbon monoxide (CO) changes almost linearly with respect to the air-fuel ratio. Generates a limiting current.

The voltage-current characteristics of the A / F sensor 30 will be described with reference to FIG. According to FIG. 5, the current flowing into the solid electrolyte 61 of the sensor element unit 60 and the solid electrolyte 6
It can be seen that the applied voltage to 1 has a linear characteristic. In such a case, the straight line parallel to the voltage axis (horizontal axis) specifies the limit current of the sensor element unit 60, and the increase / decrease of this limit current (sensor current) depends on the increase / decrease of the A / F value (that is, lean).・ Degree of richness). In other words, the limit current increases as the A / F value becomes leaner,
The limit current decreases as the A / F value becomes richer.

In this voltage-current characteristic, a voltage range smaller than a linear portion parallel to the voltage axis is a resistance dominant region, and the inclination of the primary linear portion in the resistance dominant region is equal to the solid electrolyte 61 in the sensor element section 60. (Referred to as element resistance). This element resistance changes with the temperature change.
When the temperature of 0 decreases, the slope decreases due to an increase in element resistance.

FIG. 6 is a graph showing the relationship between the limiting current value on the horizontal axis and the A / F value corresponding to the limiting current value on the vertical axis. As an example, the specific numerical values of the heat resistance of the A / F sensor 30 are as follows: element heat resistance = 900 to 950 ° C., heater heat resistance = 1000 to 1100 ° C., maximum element temperature change rate = 150 ２００200 ° C./s, maximum value of heater temperature change rate = 200 ° C./s, maximum value of element-heater temperature difference = 200 ° C.

In FIG. 1, the A / F sensor 3
A bias command signal Vr for applying a voltage to 0 (sensor element unit 60) is input from the microcomputer 20 to the D / A converter 21 and is converted into an analog signal Vb by the D / A converter 21. The signal is input to an LPF (low-pass filter) 22. The output voltage Vc from which the high-frequency component of the analog signal Vb has been removed by the LPF 22 is input to a bias control circuit 40 for applying a voltage for A / F detection or element resistance detection to the A / F sensor 30. You. At the time of A / F detection, the A / F at that time is determined using the characteristic line L1 in FIG.
While the applied voltage Vp corresponding to the value is set, a single-shot voltage having a predetermined time constant consisting of a predetermined frequency signal is applied during element resistance detection.

The current detection circuit 5 in the bias control circuit 40
0 detects the value of the current flowing with the application of the voltage to the A / F sensor 30. The analog signal of the current value detected by the current detection circuit 50 is input to the microcomputer 20 via the A / D converter 23. The heater 64 (heating element 64 a) provided in the A / F sensor 30 includes a heater control circuit 25.
Controls its operation. That is, the heater control circuit 25 controls the amount of power to the heater 64 by the duty ratio signal corresponding to the element resistance of the A / F sensor 30.

Next, the configuration of the bias control circuit 40 is shown in FIG.
This will be described with reference to the electric circuit diagram of FIG. In FIG. 7, the bias control circuit 40 has a reference voltage circuit 44. The reference voltage circuit 44 has a constant voltage V
cc is divided to generate a constant reference voltage Va.

A non-inverting input terminal of an operational amplifier 45a in the amplifier circuit 45 is connected to a voltage dividing point of a reference voltage circuit 44 held at a reference voltage Va, and an output terminal of the operational amplifier 45a is connected to a sensor current detecting circuit. One terminal 42 of the sensor element unit 60 (A / F sensor 30) is connected via 50. The terminal 42 is a terminal connected to the atmosphere-side electrode 67 of the sensor element section 60, and the same voltage Va as the reference voltage Va of the reference voltage circuit 44 is always applied to the terminal 42. The terminal 42 is connected to the inverting input terminal of the operational amplifier 45a, and the voltage Va of the terminal 42 is A / D
The data is taken into the converter 23.

The sensor current detection circuit 50 includes an operational amplifier 4
A current detection resistor 50a connected between the output terminal of the A / F sensor 30 and the terminal 42 of the A / F sensor 30;
The sensor current Ip corresponding to the / F value is detected. The voltage Vd at the connection point between the operational amplifier 45a and the current detection resistor 50a is taken into the A / D converter 23.

On the other hand, the operational amplifier 47a in the amplifier circuit 47
The LPF 22 is connected to the non-inverting input terminal. The output terminal of the operational amplifier 47a is connected to the inverting input terminal of the operational amplifier 47a and the other terminal 41 of the sensor element unit 60. The terminal 41 is a terminal connected to the measurement electrode 66 of the sensor element unit 60. The terminal 41 has a command voltage Vc which is an output after passing through the LPF 22.
The same voltage Vc is applied.

With the above configuration, a constant voltage Va is always supplied to one terminal 42 of the sensor element section 60. When a voltage Vc lower than the constant voltage Va is supplied to the other terminal 41 of the sensor element unit 60 via the LPF 22, the sensor element unit 60 is positively biased. Also,
The other terminal 4 of the sensor element unit 60 via the LPF 22
1 is supplied with a voltage Vc higher than the constant voltage Va,
The sensor element unit 60 is negatively biased.

Next, the operation of the air-fuel ratio detecting device 15 configured as described above will be described. FIG. 8 is a flowchart showing an outline of a main routine performed by the microcomputer 20. The routine is started when the microcomputer 20 is powered on.

In FIG. 8, the microcomputer 20 first determines in step 100 whether or not a predetermined time Ta has elapsed since the previous A / F detection. The predetermined time Ta is a time corresponding to an A / F value detection cycle, and is, for example, Ta = 4 ms.
Set to about. A predetermined time T from the previous A / F detection
If a has elapsed (YES in step 100), the microcomputer 20 proceeds to step 110, and executes an A / F value detection process according to FIG. 9 described later. If step 100 is NO, the microcomputer 20 ends this routine once.

Here, the A / F detection routine shown in FIG. 9 will be described.
The voltage Vp is applied to the sensor element unit 60 of the sensor 30.
The applied voltage Vp depends on the A / F value (limit current value I
For example, it is set as a value on the characteristic line L1 in FIG. 5 according to p).

Thereafter, the microcomputer 20 proceeds to step 112
Then, a current value flowing through the sensor element section 60 when Vp is applied, that is, a limit current value (sensor current) Ip detected by the current detection circuit 50 is read. Further, the microcomputer 20
In step 113, the limit current value Ip at each time is converted into an A / F value using the limit current value-A / F map shown in FIG. Further, the microcomputer 20 outputs the A / F value obtained as described above to the engine control ECU 16 in the subsequent step 114, and then returns to the original routine of FIG.

After detecting the A / F value, the microcomputer 20 returns to FIG.
In step 120, it is determined whether or not a predetermined time Tb has elapsed since the previous detection of the element resistance. The predetermined time Tb is a time corresponding to a detection cycle of the element resistance ZAC, and is selectively set according to, for example, an engine operating state. In the present embodiment, Tb = 2 s (second) in a normal state where the change in the A / F value is relatively small (during a steady operation of the engine 10).
When the A / F value changes suddenly (when the engine 10 is started or when it is in a transient operation), the predetermined time Tb is variably set to Tb = 128 ms (millisecond).

If step 120 is YES, the microcomputer 20 detects the element resistance ZAC in step 130, and controls energization of the heater 64 in step 140. The processes of steps 130 and 140 are respectively performed according to FIGS. 10 and 11 described later. If step 120 is NO, the microcomputer 20 ends this routine once.

Next, the procedure for detecting the element resistance ZAC in step 130 of FIG. 8 will be described with reference to FIG.
In this embodiment, when detecting the element resistance ZAC,
The so-called "AC impedance" is determined using a sweep method.

In FIG. 10, the microcomputer 20 first operates the bias command signal Vr in step 131, and changes the applied voltage Vp (A / F detection voltage) up to that point in a one-time change to the positive side. . The voltage application time for detecting the element resistance is about several tens to 100 μs. Thereafter, the microcomputer 20 reads the voltage change amount ΔV at that time and the sensor current change amount ΔI detected by the current detection circuit 50 in step 132. Further, in the following step 133, the microcomputer 20 calculates the element resistance ZAC from the ΔV value and ΔI value (ZAC = ΔV / ΔI), and thereafter returns to the original routine of FIG.

According to the above processing, the LPF 2 shown in FIG.
A voltage having a predetermined time constant is applied to the A / F sensor 30 one by one via the bias control circuit 40. As a result, as shown in FIG. 12, a peak current ΔI (current change amount) is detected after a lapse of t time from the application of the voltage, and an element resistance ZAC is detected from the voltage change amount ΔV and the peak current ΔI at that time. (ZAC = ΔV / Δ
I). In such a case, by applying a one-shot voltage to the A / F sensor 30 via the LPF 22, the occurrence of an excessive peak current is suppressed, and a highly reliable element resistance ZAC can be detected.

The element resistance ZAC obtained as described above is
FIG. 13 shows the relationship with the element temperature. That is,
As the element temperature is lower, the element resistance ZAC is dramatically increased. Incidentally, the activation temperature of the A / F sensor 30 (about 700 ° C.)
Corresponds to the element resistance ZAC ≒ 90Ω.

Next, the procedure for controlling the energization of the heater in step 140 of FIG. 8 will be described with reference to FIG. The microcomputer 20 first determines in step 141 whether or not the detected element resistance ZAC has reached a predetermined value (500 Ω in the present embodiment). Here, the predetermined value = 500Ω is
This is a value for determining that the effective element resistance ZAC can be detected. At the beginning of the low temperature start of the engine 10, the element resistance ZAC is so large that it cannot be detected, and the microcomputer 20 determines that “ZAC> 500Ω”.

If ZAC> 500Ω, the microcomputer 20
Determines the duty ratio Duty for heater energization in step 142 according to, for example, the relationship of FIG. According to step 142, the heater energization is controlled to be open. In FIG. 14, the duty ratio Duty is set according to the temperature raising capability of the heater 64, and the duty ratio Duty is basically set to a lower value as the heater temperature raising capability is higher. If the heater temperature raising ability is lower than the P1 value, Duty = 100% is set, and if the heater temperature raising ability is higher than the P2 value, Duty = 50% is set.

By the way, the heater heating stress refers to the engine 1
0 corresponds to the temperature increase rate from the low temperature start. for that reason,
The heater temperature raising capability may be estimated based on the temperature raising speed, and the heater temperature raising capability may be obtained from the estimated value. As the heating rate, a method of calculating from the heating data in the heater heating process at the time of the previous running of the vehicle, a method of estimating from the total operating time of the engine 10 (or the total running distance of the vehicle), and the like can be applied.

On the other hand, if ZAC ≦ 500Ω, the microcomputer 20 performs feedback control in steps 143 to 146 so as to maintain the element resistance ZAC at a predetermined value. That is, the current element resistance ZAC and its target value ZACr
The duty ratio Duty for energizing the heater is determined by using a PID control method or the like so that ef matches.

Specifically, the microcomputer 20 executes step 1
At 43, the element resistance ZAC at the previous processing is set to the previous value "ZAC
In step 144, the detected element resistance ZAC (the current detection value in FIG. 10) is read. Further, the microcomputer 20 calculates a proportional term Gp, an integral term Gi, and a differential term Gd by the following formula at step 145.

Gp = Kp · (ZAC−ZACref) Gi = Gi + Ki · (ZAC−ZACref) Gd = Kd · (ZAC−ZAC0) where “Kp” is a constant of proportionality and “Kp”
“i” represents an integration constant, and “Kd” represents a differentiation constant. And
The microcomputer 20 adds the proportional term Gp, the integral term Gi, and the differential term Gd in step 146 to add the duty ratio Duty.
Is calculated (Duty = Gp + Gi + Gd).

After determining the duty ratio Duty as described above, the microcomputer 20 proceeds to step 147, and calculates the final duty ratio Dfn by correcting the duty ratio Duty. Specifically, the final duty ratio Dfn is calculated by using Dfn = Duty + FK, using a correction value FK corresponding to the voltage value of the battery power supply + B. Correction value FK
Is set, for example, based on the relationship in FIG. FIG.
According to the above, based on the battery power supply + B (actually, the voltage generated by the alternator) = 13 V, a correction value FK of less than “1” is set for + B ≧ 13 V, and a correction value of “1” or more for + B <13 V The value FK is set. When correcting the duty ratio, an exhaust gas temperature correction may be performed in addition to the above + B correction. In this exhaust gas temperature correction, a smaller correction value is given as the exhaust gas temperature is estimated to be higher.

The microcomputer 20 proceeds to step 14
At 8, the element temperature is guarded by the maximum allowable value, and then a duty ratio signal for energizing the heater is output to the heater control circuit 25 of FIG. In step 148, the heater is energized by the calculated final duty ratio Dfn.
It is determined whether the element temperature exceeds the maximum allowable value of “900 ° C.” or the maximum allowable value of the element temperature change rate of “150 ° C./s”. If it is assumed that the maximum allowable value is exceeded, the energization duty is regulated to “0” or a value (approximately 0.1 to 1%) at which the element temperature is surely reduced.
However, the restricted duty ratio may be set according to the conversion speed of the A / D converter.

As described above, in the present embodiment, in the heater control, the energization amount open control independent of the heater temperature (or the heater resistance) and the element resistance feedback control similarly independent of the heater temperature are performed. Is effective when the stacked A / F sensor 30 is applied as in the present embodiment. That is, in the case of the stacked A / F sensor 30, since the sheet-shaped element portion and the heater are arranged in a stacked manner, the heat generated by the heater is easily transmitted to the element portion, and the temperature difference between the element temperature and the heater temperature is small. . for that reason,
Even if the heater temperature or the heater resistance is not detected, the heater energization control can be performed.

FIG. 16 is a time chart more specifically showing the characteristic operation of the present embodiment. Here, FIG. 16A shows an A / F sensor constituted by a stacked sensor as in the present embodiment, in which the temperature difference between the element temperature and the heater temperature is relatively small, and FIG. An A / F sensor constituted by a non-type sensor and having a relatively large temperature difference between the element temperature and the heater temperature will be described. Note that, in FIG. 16, the A / F sensor 3
0 indicates a process of increasing the temperature from the cold state, and the element resistance ZAC at the initial stage of the heater energization start (initial engine start).
Is too high to detect.

In FIG. 16A, the heater temperature and the element temperature change with a maximum temperature difference of about 200 ° C.
In this case, before time t1 when ZAC> 500Ω,
The energization duty ratio Duty of the heater 64 is controlled to be open (step 142 in FIG. 11). Also, ZAC ≦ 5
After the time t1 at which 00Ω is reached, feedback control is performed so that the element resistance ZAC matches the target value ZACref (steps 143 to 146 in FIG. 11). As a result, the heater temperature converges to about 780 ° C. and the element temperature becomes about 700
Converges to ° C.

On the other hand, in FIG. 16B, FIG.
In order to obtain the same transition of the element temperature and the element resistance ZAC as in the above, a sharp rise in the heater temperature is required at the beginning of the engine start. Therefore, the heater temperature overshoots, and the temperature difference between the element temperature and the heater temperature becomes very large. Furthermore, the heater temperature is maintained at a relatively high temperature (for example, about 850 ° C.) in order to maintain the active state of the sensor element unit.

When comparing FIGS. 16A and 16B, in FIG. 16A, the element temperature changes following the change in the heater temperature. Therefore, it is understood that proper heater control can be performed by monitoring the element temperature (element resistance) without monitoring the heater temperature.

According to the above-described embodiment, the following effects can be obtained. (A) In the present embodiment, the solid electrolyte 61 (sensor element) and the heater 64 are arranged close to each other, and the temperature difference between them is always equal to or less than a predetermined value.
And the energization control of the heater 64 is performed based on the element resistance ZAC obtained from the sensor current. That is, unlike the conventional device, the detection of the heater resistance is not required in the heater control. In such a case, a resistor through which a large amount of heater current flows becomes unnecessary, and accordingly, the circuit configuration can be simplified. Further, since the resistor as the heating element can be eliminated, the influence of heat generated by the resistor can be suppressed, and the degree of freedom in circuit design is improved. As a result, the object of the present invention is to achieve good heater control while simplifying the configuration.

(B) Until the effective element resistance ZAC can be detected, the heater power supply is open-controlled in accordance with the heater temperature raising capability. In this case, the sensor element unit 6
The element cracking due to excessive temperature rise of the element portion 60 can be suppressed while improving the temperature rise characteristic of 0. Sensor element 6
By improving the temperature rise characteristic of 0, the time from the cold state of the sensor to the activation can be shortened, and the sensor output can be used early.

(C) When the effective element resistance ZAC can be detected, the element resistance ZAC is feedback-controlled to the target value ZACref. Therefore, highly accurate and highly reliable heater control can be realized.

(D) The heater power supply (Duty) is corrected according to the battery power supply + B. In this case, even when the battery power supply + B fluctuates, it is possible to carry out the heater energization without excess and deficiency.

The embodiment of the present invention can be embodied in the following forms other than the above. In the above embodiment, when the effective element resistance ZAC can be detected, the element resistance ZAC
(Steps 143 to 146 in FIG. 11), this is changed. For example, the element resistance ZAC may be converted into an element temperature, and the element temperature may be feedback-controlled to a target value. In such a case, as in the above embodiment, while simplifying the configuration,
Good heater control can be performed.

In the above embodiment, the effective element resistance ZA
In the period until C can be detected, the duty ratio Duty is determined according to the temperature raising capability of the heater 64 (FIG. 14), but this is changed. For example, the degree of deterioration of the sensor element unit 60 or the heater 64 is obtained, and the duty ratio Duty is determined according to the degree of deterioration. In other words, as the deterioration of the heater 64 progresses, the heater resistance increases and the heating rate decreases. Therefore, as the degree of deterioration of the heater 64 increases, the duty ratio Duty is set to a larger value.

In the above-described embodiment, the "AC impedance" is obtained by using the sweep method when detecting the element resistance ZAC. However, this is changed and the "DC impedance" is detected. Specifically, a “negative voltage Vn” in a resistance dominant region that does not affect the limit current generation region is applied to the A / F sensor 30,
The element resistance Ri is detected based on the current value In (negative current value) at that time (Ri = Vn / In). When the DC impedance is detected in this way, a configuration for applying a single alternating voltage in a predetermined frequency range to the sensor element unit 60 becomes unnecessary, and the LPF 22 in FIG. 1 and the like can be omitted.

In the above embodiment, the PID control is performed in the feedback control of the element resistance or the element temperature. However, this may be changed to another control such as PI control or P control. In the above embodiment, the plate-shaped solid electrolyte layer, the diffusion resistance layer, and the heater are respectively laminated, and
Although the / F sensor is configured, this configuration is changed. For example, the sensor may be embodied as a sensor in which a sheet-like heater is attached to a sensor element portion (solid electrolyte and diffusion resistance layer) having a cup-shaped cross section. In short, any configuration may be used as long as the sensor element unit and the heater are arranged at close positions so that the temperature difference between the sensor element unit and the heater is equal to or lower than a predetermined temperature.

The present invention can be applied to devices other than the air-fuel ratio detecting device using the A / F sensor. That is, NOx, H
The present invention is also applicable to a gas concentration detection device using a gas concentration sensor capable of detecting gas concentration components such as C and CO. By using the same method as that of the above-described embodiment also in the other gas concentration detecting device, it is possible to perform excellent heater control while simplifying the configuration.

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram showing an outline of an air-fuel ratio control system according to an embodiment of the present invention.

FIG. 2 is a cross-sectional view showing the overall configuration of the A / F sensor.

FIG. 3 is a sectional view of a sensor element unit.

FIG. 4 is an exploded perspective view of each member constituting the sensor element unit.

FIG. 5 is a VI characteristic diagram of the A / F sensor.

FIG. 6 is a graph showing a relationship between a limit current value of an A / F sensor and an air-fuel ratio.

FIG. 7 is an electric circuit diagram showing a detailed configuration of a bias control circuit.

FIG. 8 is a flowchart showing a main routine by a microcomputer in the air-fuel ratio detection device.

FIG. 9 is a flowchart showing a procedure for detecting an A / F value.

FIG. 10 is a flowchart showing a procedure for detecting element resistance.

FIG. 11 is a flowchart showing a control procedure of heater energization.

FIG. 12 is a waveform chart showing a sensor voltage and a sensor current when detecting an element resistance.

FIG. 13 is a graph showing a relationship between element resistance and element temperature.

FIG. 14 is a diagram showing a relationship between a heater temperature raising capacity and a power supply duty ratio.

FIG. 15 is a diagram showing a relationship between a battery power supply + B and a correction value FK.

FIG. 16 is a time chart more specifically showing a characteristic operation in the embodiment.

[Explanation of symbols]

10: engine, 15: air-fuel ratio detection device, 20: microcomputer (microcomputer) as heater control means,
Reference numeral 30: A / F sensor (limit current type air-fuel ratio sensor) as a gas concentration sensor; 40, bias control circuit; 50, current detection circuit as current detection means; 60, sensor element portion; 61, solid electrolyte; Heater, 64a heating element, + B battery power supply.

 ──────────────────────────────────────────────────の Continuing from the front page (72) Inventor Toshiyuki Suzuki 1-1-1, Showa-cho, Kariya-shi, Aichi Prefecture Inside DENSO Corporation

Claims (5)

[Claims] 1. A sensor element for detecting a concentration of a specific component in a gas to be detected, and a heater for generating heat by energizing a power supply voltage to heat the sensor element to a predetermined activation temperature, The present invention is applied to a gas concentration sensor in which a sensor element and the heater are disposed close to each other and a temperature difference between the two is always equal to or less than a predetermined value. Current detection means for detecting a current value flowing in accordance with the concentration; obtaining an element resistance or an element temperature of the sensor element based on the detected current value; and supplying a current to the heater based on the obtained element resistance or the element temperature. And a heater control means for controlling the temperature of the gas concentration sensor. 2. A heater control device for a gas concentration sensor according to claim 1, wherein said heater control means performs feedback control of an element resistance or an element temperature to a target value. 3. The gas according to claim 1, wherein the amount of current supplied to the heater is controlled to be open in accordance with the heater temperature raising capacity until an effective element resistance or element temperature of the sensor element can be detected. Heater control device for density sensor. 4. The gas concentration sensor according to claim 1, wherein a voltage value of a power supply for supplying electric power to said heater is obtained, and a heater power supply amount is corrected according to the voltage value. Heater control device. 5. The gas concentration sensor according to claim 1, wherein the gas concentration sensor is formed by stacking a heater on a sensor element having a solid electrolyte, and integrating the solid electrolyte and the heater. Heater control device for density sensor.