JPH06102241A - Wide ranging air/fuel ratio sensor - Google Patents

Abstract

PURPOSE:To stabilize the temperature of a sensor even when the temperature of a gas to be measured changes significantly by controlling an applied voltage to a heater so as to keep the applied voltage to the heater constant in a low gas temperature range and the resistance of the heater constant in a high gas temperature range. CONSTITUTION:A heater applied voltage adjusting means 4 adjusts an applied voltage to a heater 3. When a gas temperature range judging means 7 determines whether a detection value of a heater current is above a specified value or not, a heater applied voltage control means 8 determines a control value so that the heater applied voltage is kept constant when the heater current is above the specified value or so that a heater resistance is determined from detection values of a current of the heater and the applied voltage to the heater to keep the resistance of the heater constant when the heater current is below the specified value. The results are outputted to the heater applied voltage adjusting means 4.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wide range air-fuel ratio sensor, and more particularly to a wide range air-fuel ratio sensor for measuring nitrogen oxide concentration.

[0002]

2. Description of the Related Art As a nitrogen oxide concentration measuring device, there has been proposed a device which is light in weight and portable (see Japanese Patent Laid-Open No. 21543/1990).

To explain this, as shown in FIG. 9, a pair of wide-range air-fuel ratio sensors (also abbreviated as "sensors" as appropriate).
32A and 32B are sensor bodies 11A and 11B and sensor bodies 11A and 11B, respectively, which are provided in the exhaust pipe 31.
Circuit that controls the current flowing into the sensor (sensor control circuit) 2
5A and 25B, one sensor 32A is a type that is insensitive to nitric oxide NO, the other sensor 32B
Is a type sensitive to NO. In order to distinguish the two, A and B are added after the code.

Arithmetic unit 3 comprising a microcomputer
Reference numeral 5 denotes sensor outputs _{IP (A)} and _{IP (B)} from the sensor control circuit.
Enter a, the NO concentration _{X NO X NO = {(I} P (B) -α (B)) - (γ O2 (B) / γ O2 (A)) ×
(IP _(A) -α _(A) )} / η _NO · γ _{O2 (B)} where γ _{O2 (A)} ; the sensitivity coefficient of the sensor 32A for O ₂ γ _{O2 (B)} ; for the sensor 32B for O ₂ . Sensitivity coefficient η _NO ; Ratio of sensitivity coefficient of sensor 32B to NO and sensitivity coefficient of sensor 32B to O ₂ α _(A) ; Zero output of sensor 32A α _(B) ; Zero output of sensor 32B.

Although a description of how this formula is obtained is omitted, according to this formula, the sensitivity coefficients γ _{O2 (A)} and γ _{O2 (B)} for O ₂ and the zero output α _(A) , If α _(B) is obtained in advance, it means that the NO concentration can be obtained by calculation using I _{P (A)} and I _{P (B)} actually measured by the sensors 32A and 32B. Even a trace component (several thousands ppm) such as the concentration can be reliably measured.

For this reason, the sensor sensitivity setters 33A, 33
B, the sensitivity coefficient γ _{O2 (A)} , γ of each sensor 32A, 32B
_{O2 (B) and} the zero outputs α _(A) and α _{(B) of the} sensors 32A and 32B are preset by the zero output setters 34A and 34B, and these values are input to the arithmetic unit 35. There is. The calculated NO concentration is output to the analog display (or digital display) by the output device 36.

[0007]

The sensor outputs _{IP (A)} and _{IP (B)} are greatly affected by the sensor temperature (specifically, the temperatures of the sensor bodies 11A and 11B). It is desirable that the sensor temperature be stable even if the temperature of the exhaust gas guided to the main bodies 11A and 11B changes greatly. As shown in FIGS. 10 and 11, the heaters 28A and 29A have constant voltage sources 29A and 29B. By applying a constant voltage from the sensor main body 11A, 11B
Is heated to an appropriate temperature.

However, in an automobile engine in which the exhaust gas temperature greatly changes depending on operating conditions, when the exhaust gas reaches a high temperature of, for example, about 500 ° C. or higher, the sensor body tends to overheat due to the high temperature exhaust gas. ,
The sensor temperature rises sharply. In this case, temperature compensation is not performed, and an error occurs in the sensor output corresponding to the increase in the sensor temperature.

Therefore, according to the present invention, the heater is applied so that the heater applied voltage (voltage applied to the heater) becomes constant in the low temperature region of the gas to be measured and the heater resistance becomes constant in the high temperature region of the gas to be measured. By controlling the voltage, even when the measured gas temperature changes significantly,
The purpose is to stabilize the sensor temperature.

[0010]

In the present invention, as shown in FIG. 1, a heater 3 for heating a sensor, a means 4 for adjusting a voltage applied to the heater 3, an actual heater current and a heater applied voltage. 5, 6 for detecting respectively
And means 7 for determining whether or not the detected value of the heater current is a predetermined value or more. Based on the result of the determination, the heater applied voltage becomes constant when the heater current is a predetermined value or more, and the heater current is predetermined. When the value is lower than the value, the heater resistance is obtained from the detected values of the heater current and the heater applied voltage, the control amount is determined so that the heater resistance becomes constant, and the control amount is output to the heater applied voltage adjusting means 4. And.

[0011]

When the heater applied voltage is controlled to be constant, the sensor temperature is maintained at a substantially constant value in the low temperature region of the gas to be measured (heater current is above a predetermined value).

In this case, since the heater resistance gradually increases as the temperature of the gas to be measured rises, the heater current decreases.

When the temperature of the gas to be measured further rises, the degree of increase in the sensor temperature becomes more severe than the degree of decrease in the heater current due to the increase in the heater resistance, and the sensor temperature rises rapidly. That is, even if the temperature of the gas to be measured becomes high, if the constant voltage control is continued, the sensor temperature cannot be maintained at a constant value.

On the other hand, in the present invention, the fact that the gas to be measured is in the high temperature range is judged from the fact that the actual heater current is below a predetermined value. At this time, the actual heater resistance is determined, and this heater is determined. The resistance is controlled to be constant. In the high temperature region of the gas to be measured, the sensor is sufficiently heated by the high temperature gas to be measured. Therefore, in the high temperature region, the amount of heating by the heater is reduced to suppress an increase in the sensor temperature.

Thus, even in the case of a gas to be measured whose gas temperature greatly changes from a low temperature to a high temperature, such as exhaust gas from an automobile engine, the change in the sensor temperature can be suppressed to a small level, and the accuracy of the sensor output can be improved for all gases. Good retention over temperature range.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 2 and 3 show a wide-range air-fuel ratio sensor applied to a nitrogen oxide concentration measuring device.
0, corresponding to FIG. The same parts as those in FIGS. 10 and 11 are designated by the same reference numerals.

Since the basic operating principle and basic characteristics of the sensor are the same for all the sensors, an outline of the type sensitive to NO will be given below. As shown in FIG. A pair of electrodes 14B, 15 sandwiching a solid electrolyte (for example, zirconia) 13B.
The pumping cell 12B in which B is arranged is formed in layers,
The gas to be measured (exhaust gas) is guided to the vicinity of the inner electrode 14B via the introduction hole 17B penetrating the center of the solid electrolyte 13B.

The pumping cell 12B pumps oxygen O ₂ into and out of the diffusion chamber 16B. When a current is passed in the direction of the solid arrow, O ₂ is pumped out from the inner electrode 14B side to the outside of the cell. On the contrary, when flowing in the direction of the dashed arrow, O _{2 is} pumped in from the exhaust gas outside the cell.

The inner electrode 14B made of platinum serves as a catalyst having a characteristic of decomposing NO in a region where the oxygen partial pressure in the gas to be measured is low, and conversely, not decomposing NO in a region where the oxygen partial pressure is high. So that the vicinity of the electrode 14B maintains a low oxygen partial pressure equilibrium state between both electrodes 14B, 1
When the current flowed in 5B (this current value referred to as sensor output) controls the I _{P (B),} the gas components in the exhaust gas of the electrode 14B vicinity _{(O 2, CO, H 2} , HC, NO) concentration of I _{P (B)} increases as it increases. That is, the sensor output I
_{P (B)} has a characteristic proportional to the concentration of each gas component.

A sensing cell 19B having a pair of electrodes 21B and 22B sandwiching an oxygen ion conductive solid electrolyte 20B is for detecting the oxygen partial pressure in the vicinity of the electrode 14B, and is close to the electrode 14B. If the one electrode 21B provided is the reference electrode and the other electrode 22B provided on the side of the atmosphere introducing chamber 23 is the measurement electrode, the measurement electrode follows the oxygen partial pressure near the electrode 14B according to Nernst's formula. Output voltage Vs is output.

Therefore, a voltage (constant value) corresponding to a low oxygen partial pressure is set as a reference voltage V _EB , and this V _EB and the voltage Vs from the measurement pole are input to a differential amplification amplifier 27B as a comparator, and V _When I _{P (B)} is increased or decreased so that there is no difference between _EB and Vs, the oxygen partial pressure near the electrode 14B is kept low.

The pumping currents I _{P (A)} and I _{P (B)} controlled in this way are input to the arithmetic unit 35 as sensor outputs as shown in FIG.

By the way, since the sensor outputs _{IP (A)} and _{IP (B)} are greatly influenced by the sensor temperature, the sensor temperature is stable even if the exhaust gas temperature as the gas to be measured changes greatly. Is desirable.

In this case, in the low temperature range of the exhaust gas, a constant voltage is applied to the heaters 28A and 28B shown in FIGS. 2 and 3 to heat the sensor bodies 11A and 11B to a proper temperature, thereby keeping the sensors constant. Although it is possible to maintain the temperature, when the temperature of the exhaust gas becomes high, the sensor temperature suddenly rises due to this high temperature exhaust gas, and an error occurs in the sensor output corresponding to the temperature increase.

To deal with this, the heaters 28A, 2
8B and the constant voltage sources 29A, 29B between the heater control unit 4
1A and 41B are provided, and the heater applied voltage is controlled to be constant in the low temperature range of exhaust gas (constant voltage control).
When the exhaust gas reaches a high temperature range, the heater resistance is controlled to a constant value (constant resistance control).

FIG. 4 shows details of the heater control units 41A and 41B. However, since the details are the same, the subscripts A and B are omitted.

In FIG. 4, a differential amplifier 42 is provided for detecting the actual voltage applied to the heater, and a circuit 43 for detecting the heater current includes a reference resistor 44 connected in series with the heater 28 and this resistor 44. And a differential amplification amplifier 45 for detecting a current flowing through.

The heater current and heater applied voltage detected by the current detection circuit 43 and the differential amplification amplifier 42 are input to the arithmetic processing unit 47 including a microcomputer via the A / D converter 46. The heater resistance can be obtained as the quotient of the heater applied voltage and the heater current.

The heater applied voltage adjusting section 48 includes a transistor 49 as a switching element, an operational amplifier 50, D
The A / A converter 51 is used.

Of these, the NPN transistor 49 has a reference resistor 44 and a voltage source 29 connected to its emitter and collector, respectively, and an operational amplifier 50 connected in series to its base.

The transistor 49 is turned on and off depending on whether the base current flows or does not flow. The output of the heater applied voltage adjusting unit 48 (output waveform at point D) is shown in FIG.
This is an ON-OFF pulse with a constant cycle A, and when the ON duty (B / A at the ON time ratio of the constant cycle) is 100%, the power supply voltage becomes the point D potential as it is.

However, when the ON duty becomes smaller than 100%, the effective voltage at point D becomes lower than the power supply voltage as shown in FIG. That is, if the heater applied voltage should be lowered, the ON duty should be reduced. O
The heater applied voltage can be adjusted by changing the N duty.

The ON-duty signal is instructed from the arithmetic processing section 47.

FIG. 6 shows the arithmetic processing in the arithmetic processing unit 47. In the figure, the actual heater current I is compared with a predetermined value Is, and if I> Is, the exhaust gas temperature is low, so
N duty is set to 100% (step 2, FIG. 6).
3, 4).

On the other hand, when I ≦ Is, it is judged that the exhaust gas temperature is high, and feedback control is performed so that the heater resistance becomes constant. For example, from the actual heater current I and the heater applied voltage V1 to the heater resistance R1 (=
V1 / I) is calculated, and when this is compared with the reference resistance Rs, if R1> Rs, the heater applied voltage must be lowered. Therefore, the heater applied voltage V1 detected this time is reduced by a constant value ΔV ( V1−ΔV) is put in V of the memory (steps 2, 5, 7, 8, 9 in FIG. 6), and conversely, if R1 <Rs, the heater applied voltage is increased, so the heater applied voltage V1 detected this time is kept constant. The value (V1 + ΔV) obtained by adding the value ΔV is put into V (steps 2, 5, 7, and 11 in FIG. 6).

The value that enters V is the target value of the heater applied voltage. Therefore, the ON duty is calculated by looking up the table having the contents of FIG. 7 from the value of V so that the target value of the heater applied voltage is obtained, and this is output (FIG. 6).
Steps 10 and 4). An ON-OFF pulse is generated from the ON duty by a pulse generator (not shown), and this is output to the voltage adjusting unit 48.

The reference resistance Rs corresponds to the heater resistance when the heater current I matches the predetermined value Is.

Even if the heater applied voltage is increased from R1 <Rs, the target value of the heater applied voltage (that is, V1 + ΔV) does not exceed the reference applied voltage Vmax (step 13 in FIG. 6). , 14).

In this way, the arithmetic processing section 47 performs constant voltage control when the exhaust gas temperature is low (when I ≧ Is) and constant resistance control when the exhaust gas temperature is high (when I <Is). Is switched over.

The operation of this example will be described below with reference to FIG.

When the temperature of the exhaust gas introduced into the sensor body is low, the ON duty becomes 100%, and the power supply voltage from the constant voltage source 29 is directly applied to point D in FIG. A voltage is applied, which keeps the sensor temperature at a constant value around 800 ° C., as shown in FIG.

Since the heater resistance R1 gradually increases as the temperature of the exhaust gas rises, the heater current I
Comes down. When the exhaust gas temperature further rises, the degree of increase in the sensor temperature becomes more severe than the degree of decrease in the heater current I due to the increase in the heater resistance R1, and the sensor temperature rises sharply. That is, even if the exhaust gas temperature is in the range of about 500 ° C. or higher, the sensor temperature cannot be maintained at a constant value if the constant voltage control is continued.

On the other hand, in this example, the predetermined value I corresponding to the heater current when the exhaust gas temperature is about 500.degree.
When the actual heater current I becomes lower than that at s, the actual heater resistance R1 is obtained, and the heater applied voltage is controlled so that this matches the reference resistance (constant value) Rs. In the high temperature range where the exhaust gas is about 500 ° C. or higher, the sensor body is sufficiently heated by the high temperature exhaust gas. Therefore, the heating amount of the heater is reduced as the temperature becomes higher, so that the rise of the sensor temperature is suppressed. Of.

As a result, even when the exhaust gas changes drastically from a low temperature to a high temperature, the change in the sensor temperature is suppressed to a small level as shown in FIG. 8, and the accuracy of the sensor output is kept good over the entire gas temperature range. .

[0045]

According to the present invention, it is determined whether the detected value of the heater current is a predetermined value or more, and when the heater current is a predetermined value or more, the heater applied voltage becomes constant, and the heater current is set to a predetermined value. When it falls below, the heater resistance is calculated from the detected values of the heater current and the heater applied voltage, and the heater applied voltage is controlled so that this heater resistance is constant, so the gas temperature is low, such as exhaust gas from an automobile engine. Even if the gas to be measured changes drastically from high temperature to high temperature, the change in sensor temperature can be kept small,
The accuracy of the sensor output can be kept good over the entire gas temperature range.

[Brief description of drawings]

FIG. 1 is a diagram corresponding to the claims of the present invention.

FIG. 2 is a circuit diagram of a sensor control circuit and a sensor control circuit of a wide range air-fuel ratio sensor 32A of a type that is insensitive to NO according to an embodiment.

FIG. 3 is a circuit diagram of a sensor control structure and a sensor control circuit of a NO-sensitive wide-range air-fuel ratio sensor 32B according to an embodiment.

FIG. 4 is a block diagram of heater control units 41A and 41B.

FIG. 5 is an output waveform diagram of a heater applied voltage adjustment unit 48.

FIG. 6 is a flowchart for explaining a process executed by an arithmetic processing unit 47.

FIG. 7 is a characteristic diagram of effective output with respect to ON duty.

FIG. 8 is a waveform chart for explaining the operation of the embodiment.

FIG. 9 is a system diagram of a conventional example.

FIG. 10 is a circuit diagram of a sensor control structure and a sensor control circuit of a conventional wide range air-fuel ratio sensor 32A that is insensitive to NO.

FIG. 11 is a circuit diagram of a sensor control structure and a sensor control circuit for a wide range air-fuel ratio sensor 32B of a conventional type that is sensitive to NO.

[Explanation of symbols]

 3 heater 4 heater applied voltage adjusting means 5 heater current detecting means 6 heater applied voltage detecting means 7 gas temperature range determining means 8 heater applied voltage control means 11A, 11B sensor body 25A, 25B sensor control circuit 28 heater 28A, 28B heater 29 fixed Voltage source 29A, 29B Constant voltage source 32A, 32B Wide range air-fuel ratio sensor 41A, 41B Heater control unit 42 Differential amplification amplifier 43 Heater current detection circuit 47 Operation processing unit 48 Heater applied voltage adjustment unit

Claims (1)

[Claims] 1. A heater for heating a sensor, a means for adjusting an applied voltage to the heater, a means for detecting an actual heater current and a heater applied voltage, respectively, and a detected value of the heater current is a predetermined value or more. A means for determining whether the heater applied voltage is constant when the heater current is equal to or higher than a predetermined value based on the determination result, and the heater current and the heater applied voltage are detected when the heater current is lower than the predetermined value. Calculate the heater resistance from the value,
Determine the control amount so that this heater resistance is constant,
A wide range air-fuel ratio sensor, characterized in that it is provided with means for outputting this to the heater applied voltage adjusting means.