JPS62278443A - Air/fuel ratio measuring apparatus - Google Patents

Abstract

PURPOSE:To achieve a higher reliability while eliminating control operation based on erroneous counts and protecting a detector completely, by shifting a timer to a counting standby position or a time counting position with a voltage comparator. CONSTITUTION:A changeover switch 11 is provided between the output side of a driving amplifier 7 and a pumping current supply electrode 5(5a-5d) to connect the electrode 5 selectively to the output side of the amplifier 7 or a constant current circuit 15. A timer 12 turns the switch 11 to the constant current circuit 15 at the counting standby position and at the time counting position and to the output side of a detector 1 after the end of an increment counting. A voltage comparator 14 is provided between the switch 11 and the timer 12. When the voltage between each of the pumping current supply electrodes 5a-5d is outside the range of the upper limit and lower limit assumed to damage electrodes, the voltage comparator 14 sets the timer 12 to the counting standby position while it does the timer 12 to the time counting position when the voltage is within the range of both the limits. The timer 12 is also set to the counting standby position simultaneously with the closing of a power source of an air/fuel ratio measuring apparatus.

Description

Detailed Description of the Invention 3. Detailed Description of the Invention (Field of Industrial Application) The present invention relates to an air-fuel ratio measuring device, and in particular, the sensor (air-fuel ratio detector) constituting the measuring device is The present invention relates to an air-fuel ratio measuring device that can be sufficiently protected from the risk of scratches.

(Prior Art) It has been known that zirconia, which is made into a ceramic, is an electrical insulator at room temperature, but becomes a conductor at high temperatures (approximately 500'C or higher). It is also known that zirconia at high temperatures has the property of selectively permeating only oxygen (hereinafter referred to as "selective oxygen permeation property").

From the selective oxygen permeability characteristic, an electromotive voltage can be obtained that corresponds to the difference in oxygen concentration on both main surfaces of the zirconia plate. That is,
If the electromotive force and the oxygen concentration on one main surface of the zirconia plate are known, the oxygen concentration on the opposite main surface can be calculated from a certain formula (Nernst's formula), so zirconia can be used as an oxygen concentration detection element. be able to.

Furthermore, when a current is passed through the zirconia plate between its two main surfaces, the zirconia plate has the function of moving oxygen in a direction opposite to the direction of the current. That is, it has a pumping effect of pumping oxygen from one main surface side of the zirconia plate to the other main surface side.

Conventionally, there has been known an air-fuel ratio detector that combines the oxygen concentration detection function and pumping function of zirconia.

FIG. 2 is a circuit diagram showing an example of a conventional air-fuel ratio measuring device using the air-fuel ratio detector.

In the figure, 1 is an air-fuel ratio detector (hereinafter simply referred to as a detector). This detector 1 includes a first zirconia 2 that performs an oxygen concentration detection operation, a second zirconia 3 that performs a pumping operation, a hollow constriction 4 provided between the first and second zirconia, and the second zirconia 3 that performs a pumping operation. A thin film electrode 5 made of, for example, platinum is formed on each of the main surfaces of the first and second zirconias 2 and 3.

A small hole 6 is provided in the second zirconia 3. Note that the portion of the small pores 6 may be formed of a porous coating film having characteristics equivalent to those of the small pores 6.

Further, among the electrodes 5, the electrodes 5b and 5C inside the first and second zirconia 2 and 3 are grounded, and the first
The outer electrode 5a of the zirconia 2 is connected to the drive amplifier 7 (-
) input terminal and also the outer electrode 5d of the second zirconia 3
are connected to the output terminals of the drive amplifier 7, respectively.

A reference voltage Er is applied to the (+) input terminal of the drive amplifier 7. Note that the detector 1 is detachable from the circuit shown in FIG.

With the air-fuel ratio measuring device having the above configuration, the air-fuel ratio can be detected from, for example, exhaust gas between internal combustion engines. When detecting the air-fuel ratio, the outer main surface of the first zirconia 2 (
The electrode 71 5 a side) is in contact with the atmosphere, and the outer main surface of the second zirconia 3 (electrode 5 d side) is in contact with the exhaust gas flowing due to concentration diffusion. Therefore, hollow chamber 4
Exhaust gas flows into the interior through small holes 6.

The operation shown in FIG. 2 will be explained below separately for lean combustion with a large air-fuel ratio and rich combustion with a small air-fuel ratio.

(1) Operation during lean burn During lean burn, a relatively large amount of residual oxygen exists in the exhaust gas. When such exhaust gas flows into the hollow chamber 4, the oxygen concentration CV in the hollow seedling 4 and the oxygen concentration Cr in the atmosphere
The difference in concentration between the two is relatively small. For this reason, the electromotive force generated in the first zirconia 2, that is, between the electrodes 5a and 5b (
The voltage F (between the oxygen concentration detection electrodes) becomes smaller than the reference voltage Er.

Therefore, during -8N combustion, the output of the drive amplifier 7 becomes positive, and a pumping current in the direction of the arrow (positive direction) corresponding to the difference between the reference voltage Er and the electromotive force E flows through the electrode 5d to the second zirconia 3. supplied to

As a result, the oxygen in the hollow chamber 4 is pumped out to the outer main surface side of the second zirconia 3 by the pumping action of the zirconia. When the amount of oxygen pumped out by the pumping current in the positive direction and the amount of oxygen flowing into the hollow chamber 4 become balanced, the pumping current stabilizes at a certain constant value.

(2) Operation during rich combustion During rich combustion, relatively large amounts of hydrogen and carbon monoxide are present in the exhaust gas. When such exhaust gas flows into the hollow chamber 4 through the small hole 6, it combines with oxygen in the hollow chamber 4 to become water and carbon dioxide. As a result, the oxygen concentration CV in the hollow chamber 4 decreases significantly, and the oxygen concentration Cr in the atmosphere decreases.
The difference in concentration between the

For this reason, the voltage (electromotive voltage generated in the first zirconia 2) E applied to the (-) input terminal of the drive amplifier 7 becomes larger than the reference voltage Er. Therefore, during rich combustion, the output of the drive amplifier 7 becomes negative, and a pumping current in the direction of arrow EIIB (negative direction) corresponding to the difference between the reference voltage Er and the electromotive voltage E is supplied to the second zirconia 3.

As a result, oxygen (oxygen in the exhaust gas) on the outer main surface side of the second zirconia 3 is taken into the hollow chamber 4 by the pumping action of the zirconia.

When the amount of oxygen taken in by the negative pumping current and the amount of oxygen combined with hydrogen or carbon monoxide are balanced, the pumping current settles to a certain value.

In the air-fuel ratio measuring device configured and operated as described above, the air-fuel ratio is calculated from a well-known formula based on the pumping current during the operations of (1) and (2) above. , - The carbon oxide concentration can be calculated, and the calculated value can be calculated by calculation.

By the way, when using the detector 1, there is a restriction that excessive power must not be supplied to the second zirconia 3. The reason is that if excessive power is supplied, the second
This is because the zirconia 3 will be heated and the electrode 5d, and therefore the detector 1, will be thermally damaged.

The power consumption in the second zirconia 3 is determined by the product of the pumping current and the voltage between the electrodes 5d and 5C (between the pumping current supply electrodes).

As is clear from the operation description in FIG. 2, the pumping current has a positive or negative value determined in proportion to the oxygen concentration or hydrogen or carbon oxide concentration of the exhaust gas (measured gas). Further, the voltage between the electrodes 5d and 5C is mainly determined by the product of the above-mentioned bombing current and the internal resistance between the applied voltages ti5d and 5C.

By the way, the internal resistance between the electrodes 5d and 5C mainly depends on the temperature of the detector 1, and when the temperature of the detector 1 is low (approximately 600
'C or less), the internal resistance is extremely large, especially at air temperature, reaching several tens of MΩ.

Therefore, in order to eliminate excessive power supply to the second zirconia 3 and avoid heat damage to the detector 1, the detector 1 is installed inside the detector itself at the same time as the power is turned on to the air-fuel ratio measuring device. or an external heater to heat it to the operating temperature (usually around 700'C).

However, in the following case, the internal resistance between the electrodes 5d and 5C is large and excessive power is supplied to the second zirconia 3, so the supply of pumping current is blocked,
It is necessary to prevent the detector 1 from being damaged by heat.

(a) When the detector 1 has not reached the above-mentioned operating temperature after the power is turned on to the air-fuel ratio measuring device.

(b) When the air-fuel ratio measuring device is replaced with a new detector 1 while it is in use, and the detector 1 has not yet reached the operating temperature described above.

(C) When the heater that heats the detector 1 is disconnected; (d) When the detector 1 is excessively cooled by the gas to be measured.

In addition, in the above case, the detector 1 does not operate correctly,
Therefore, it is also impossible to calculate an accurate air-fuel ratio.

Furthermore, in the following cases, the pumping current becomes excessive, so it is necessary to limit this to prevent thermal damage to the detector 1.

(e) When the oxygen concentration or hydrogen or carbon oxide concentration in the gas to be measured is excessively high.

FIG. 3 is a circuit diagram showing an example of a conventional air-fuel ratio measuring device devised to prevent thermal damage to the detector 1 as much as possible. In the figure, the same reference numerals as in FIG. 2 indicate the same or equivalent parts.

The feature of FIG. 3 is that a pumping current limiting circuit 8 and a switch circuit 10 which is opened and closed by the output signal of a timer 9 are provided between the output terminal of the drive amplifier 7 and the electrode 5d.

The timer 9 starts counting by a known appropriate means when the air-fuel ratio measuring device is powered on, and outputs a count-up signal to the switch circuit 10 when the scheduled time has elapsed. As a result, the switch circuit 10 detects that the expected time (usually 10 to
After 30 seconds), it becomes closed. As a result, a pumping current is supplied to the second zirconia 3.

Further, the pumping current is limited to the predetermined value by the pumping current limiting circuit 8 when the positive and negative currents thereof become larger than the predetermined value.

Therefore, according to the circuit shown in FIG. 3, in the case (a), the pumping current can be prevented from being supplied, and also in the case (e), where the pumping current becomes excessive, the pumping current can be stopped. It is possible to limit the number of appointments. Therefore,
Thermal damage to the detector 1 caused by these is prevented.

(Problems to be Solved by the Invention) However, in the circuit shown in FIG. 3, in the cases (b), (C), and (d) above, where the pumping current must be blocked, the supply of the pumping current cannot be blocked. Therefore, in these cases, there is a problem that the detector 1 is damaged by heat.

The present invention has been made to solve the above-mentioned problems.

(Means and operations for solving the problems) In order to solve the above problems, the present invention provides a method for solving the above-mentioned problems by disposing the drive amplifier between the output side of the drive amplifier and one side of the pumping current supply voltage. a switching means for selectively connecting one of the supply electrodes to the output side of the drive amplifier or a constant current circuit; a voltage comparator that is provided between the output side of the switching means and the timer and that puts the timer in a counting standby state or a time counting state; When the voltage between the pumping current supply electrodes is outside the expected upper and lower limit voltage ranges that may damage the pumping current supply electrodes, the timer is put into a counting standby state and the upper and lower limit voltages are The timer is in a time counting state when it is within the range, and the timer is in a counting standby state at the same time as the power of the air-fuel ratio measuring device is turned on.

(Example) The present invention will be described in detail below with reference to the drawings.

FIG. 1 is a circuit diagram of an embodiment of the present invention. In the figure, the same reference numerals as in FIG. 3 represent the same or equivalent parts.

In FIG. 1, a changeover switch 11 is provided between an electrode 5d and a drive amplifier 7, and an output signal from a timer 12 switches a movable contact 11a to one of fixed contacts 11b and 11c.

That is, when the timer 12 is in a counting standby state or a time counting state, the movable contact 11a is in contact with the fixed contact 11C, and the constant current circuit 15 consisting of the resistor R1 and the power source 13 is connected to the electrode 5d.

Furthermore, when the timer 12 completes counting (counts up) the scheduled time and outputs a count-up signal, the movable contact 11a contacts the fixed contact 11b, so the output terminal of the drive amplifier 7 and the electrode 5
d is connected.

The current supplied from the power supply 13 to the second zirconia 3 is
The current output from the pumping current limiting circuit 8 (several mA
It is regulated by the resistor R1 so that it has a sufficiently smaller value (within μA) than the positive and negative currents.

The voltage EO of the power supply 13 is set sufficiently higher than the upper limit voltage (positive voltage) p of the voltage comparator 14, which will be described later. Note that the timer 12 enters a counting standby state by a known appropriate means at the same time as the power of the air-fuel ratio measuring device is turned on.

When the voltage applied to the second zirconia 3 exceeds a predetermined upper limit voltage vp or is smaller than a lower limit (negative voltage) Vm, the voltage comparator 14 puts the timer 12 into a counting standby state, and When the voltage Vp is within the upper and lower limits of 7 m, the timer 12 is operated to count time.

The absolute value of the upper and lower limit voltage Vp, 7 m is larger than the absolute value (about 2 V) of the voltage that can occur in the second zirconia 3 in the normal operating state of the detector 1, and is larger than the maximum value of the pumping current limiting circuit 8. The power determined by the product of the output current and the voltage generated across the second zirconia 3 at this time is set to be equal to or lower than the voltage at which the detector 1 is equal to the maximum power that will not damage it. According to the inventor's experiments, it is usually about 3 to 6V.

Further, the (-) input terminal side of the drive amplifier 7 is grounded through a resistor R2. The value of this resistance R2 is smaller than the internal resistance value (several tens of MΩ) between the oxygen concentration detection electrodes (between electrodes 5a and 5b) of the detector 1 at room temperature, and the value (several tens of MΩ) during heating (over 600'C) It is set to a value that is sufficiently larger than the number of Ω).Usually, it can be set to about 1MΩ.

Next, the operation of this embodiment will be explained.

(1) When the power is turned on to the air-fuel ratio measuring device, the timer 12
is in a counting standby state and the changeover switch 11 is in the state shown, so that the electrode 5d is connected to the constant current circuit 15. Immediately after the power is turned on, the detector 1 is at almost air temperature, so the electrode 5d is heated.

The internal resistance between 5C and 5C is very high.

As a result, the voltage generated between the electrodes 5d and 5C due to the minute current supplied from the power supply 13 through the resistor R1 has a value higher than the upper limit voltage Vp of the voltage comparator 14. As a result, the timer 12 maintains the counting clear state and does not start counting.

(2) When current is applied to the air-fuel ratio measuring device, the detector 1
A heater built into or externally attached to the
(not shown) starts heating the detector 1. As a result, the temperature of the detector 1 increases, and the internal resistance between the electrodes 5d and 5C rapidly decreases accordingly.

The internal resistance between the electrodes 5d and 50 is a certain value (for example, 10Ω
), and the voltage between the electrodes 5d and 5C is the upper limit voltage V
When the voltage becomes equal to or less than p, the voltage comparator 14 changes the timer 12 from a counting standby state to a time counting state. This causes timer 12
starts counting operation.

(3) When the scheduled time for the detector 1 to reach the operating temperature has elapsed, the timer 12 outputs a count-up signal. As a result, the movable contact 11a of the changeover switch 11 is switched to the fixed contact 11b side.

Thereby, the electrode 5d is connected to the output terminal of the drive amplifier 7 through the pumping current limiting circuit 8, and the normal operation of the air-fuel ratio measuring device is started. Note that the internal resistance between the electrodes 5d and 5C at the operating temperature is usually about 100Ω.

Next, a description will be given of the operation when replacing the detector 1 while the air-fuel ratio measuring device is in use, that is, while the measuring device is powered on.

When the detector 1 is disconnected from the circuit shown in Figure 1 for replacement, etc., the (-) input terminal side of the drive amplifier 7 becomes ground potential due to the resistor R2, so the reference voltage on the (+) input terminal side It becomes lower than Er.

As a result, the voltage applied to the voltage comparator 14 is the upper limit voltage ■p
, the timer 12 enters the counting clear state, and the changeover switch 11 enters the state shown. Since the voltage EO of the power supply 13 is set sufficiently higher than the upper limit voltage Vp as described above, this state is maintained.

Then, when the replaced new detector 1 is connected, the air-fuel ratio measuring device starts normal operation through the operations (2) and (3) described above.

Note that since the resistance R2 is set to be sufficiently larger than the internal resistance between the electrodes 5a and 5b during normal operation of the air-fuel ratio measuring device, it has no effect on the operation of the measuring device during normal operation. I won't give it.

Next, the operation when the detector 1 is excessively cooled by the gas to be measured will be described.

As described above, when the detector 1 is excessively cooled by the gas to be measured, as the temperature of the detector 1 decreases, the electrodes 5d,
50, the internal resistance increases.

At this time, assuming that the bombing current output from the drive amplifier 7 does not change, the voltage between the electrodes 5d and 5C increases with this increase in internal resistance, and the upper and lower limit voltages of the voltage comparator 14 p, ■When it exceeds m, timer 12
is in the counting clear state, and the changeover switch 11 is in the state shown.

That is, the normal operating state of the air-fuel ratio measuring device is interrupted.

From this state, when the temperature of the second zirconia 3 rises and the voltage between the electrodes 5d and 5C falls within the range of the upper and lower limit voltage p1m of the voltage comparator 14, the timer 12 immediately shifts to the counting operation state. . After that, when the scheduled time elapses, a count-up signal is output from the timer 12, and the movable contact 1
1a switches to the fixed contact 11b side, the air-fuel ratio measuring device enters a normal operating state.

At this time, if the temperature of the detector 1 has not returned to a level at which the air-fuel ratio measuring device can maintain the normal operating state, the timer 12 is in the counting waiting state → the fixed contact 11C of the movable contact 11a
Switching to side → Start of calculation operation of timer 12 → Timer 1
The cyclic operation of outputting a count-up signal from 2→switching the movable contact 11a to the fixed contact 11b→counting standby state of the timer 12 is repeated.

Note that if the detector 1 is supercooled by the gas to be measured and the oxygen concentration or hydrogen or carbon oxide concentration of the gas to be measured is too high, the temperature will be lower than in other cases of supercooling. Therefore, even if the increase in internal resistance between the electrodes 5d and 50 due to the temperature drop of the detector 1 is small, the voltage between the electrodes 115d and 5C will exceed the upper limit voltage vp or the lower limit voltage m, so the timer 12 It is easy to understand that the counting state becomes clear even more quickly.

Next 1. The operation when the heater that heats the detector 1 is disconnected will be explained.

When the heater is disconnected, the temperature of the detector 1 decreases, and the internal resistance between the electrodes 5d and 5C increases accordingly. When the voltage between the electrodes 5d and 50 increases with this increase in internal resistance and exceeds the upper limit voltage vp or lower limit voltage vm of the voltage comparator 14, the timer 12 is brought into a counting clear state and the changeover switch 11 is turned on. The state shown in the figure will be reached.

If the heater is disconnected, the internal resistance does not decrease, so this state continues thereafter. If the heater is disconnected when the air-fuel ratio measuring device is powered on, the timer 12 is put into a counting standby state at the same time as the power is turned on, and continues in this state.

In the above explanation, the (-) input terminal side of the drive amplifier 7 is
Although the resistor R2 is connected to the ground through the resistor R2, the resistor R2 is not necessarily required.

That is, even if the resistor R2 is not present, when the detector 1 is disconnected, the voltage applied from the drive amplifier 7 to the voltage comparator 14 will exceed the upper limit voltage Vp or be smaller than the lower limit voltage Vm. This is because this is normal, and therefore the operation at the time of replacing the detector is performed normally.

However, if the resistor R2 is provided, the drive amplifier 7 (
-) Since the input terminal side is not in a floating state, the operation of the drive amplifier 7 becomes stable when replacing the detector.

Furthermore, when the timer 12 is forced into a counting standby state by the voltage comparator 14, if an indicator is provided to indicate this, it will be easy to determine whether the detector 1 is disconnected or the heater is disconnected. In addition, if a lamp serving as an indicator repeatedly turns on and off within a relatively short period of time, it can be easily detected that the temperature of the gas to be measured is too low. .

(Effects of the Invention) As is clear from the above description, according to the present invention, the following effects are achieved.

(1) If a new detector is replaced or connected while the air-fuel ratio measuring device is in use, or if the heater that heats the detector is disconnected,
Alternatively, even if the detector is excessively cooled by the gas to be measured, excessive power is not supplied to the pumping current supply electrode, so the detector can be completely protected.

(2) Also, when the timer is forcibly reset by the voltage comparator, an indicator to that effect can be provided to indicate that either the detector is disconnected or the heater is disconnected. It can be easily distinguished, and it can also be easily detected that the temperature of the gas to be measured is too low.

(3) Furthermore, since the present invention does not measure the air-fuel ratio when the detector is not in a normal operating state, control operations based on erroneous counts will not occur even when the present invention is installed in an actual vehicle. The reliability of the vehicle can be improved without causing any damage.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of one embodiment of the present invention. FIG. 2 is a circuit diagram showing an example of a conventional air-fuel ratio measuring device. FIG. 3 is a circuit diagram showing another example of a conventional air-fuel ratio measuring device. 1...Detector, 2...First zirconia, 3...
Second zirconia, 4... hollow chamber,

Claims (4)

[Claims] (1) A zirconia plate for oxygen concentration detection, a zirconia plate for pumping, an oxygen concentration detection electrode formed on both main surfaces of the zirconia plate for oxygen concentration detection, and a pumping current formed on both main surfaces of the zirconia plate for pumping. supply electrode,
A hollow chamber is formed between the opposing surfaces of both the zirconia plates, and a cover is formed on the pumping zirconia plate in order to introduce the gas to be measured on the outer main surface side of the pumping zirconia plate into the hollow chamber. An air-fuel ratio detector consisting of a measuring gas introduction means, one input terminal connected to one of the oxygen temperature detection electrodes, and the other input terminal connected to a reference voltage,
An air-fuel ratio measuring device comprising a drive means for outputting positive and negative pumping current to the pumping zirconia plate according to the difference between the voltage between the oxygen concentration detection electrodes and a reference voltage, the output side of the drive means and the pumping current supply. a switching means provided between one of the electrodes and selectively connecting one of the pumping current supply electrodes to the output side of the driving means and one of the constant current circuit; The constant current circuit is provided with a timer that performs switching control to the output side of the driving means after counting up, and a timer that is provided between the output side of the switching means and the timer, and the timer is in a counting standby state or a time counting state. and a voltage comparator that sets the timer to a counting standby state when the voltage between the pumping current supply electrodes is outside the range of the predetermined upper limit voltage and lower limit voltage. An air-fuel ratio measuring device characterized in that the timer is placed in a time counting state when the voltage is within a lower limit voltage range, and the timer is placed in a counting waiting state at the same time as the power of the air-fuel ratio measuring device is turned on. (2) The above-mentioned claim is characterized in that, when the absolute value of the positive and negative pumping currents of the drive means becomes larger than a predetermined value, pumping current limiting means is provided for limiting the absolute value to the predetermined value. The air-fuel ratio measuring device according to item 1. (3) One input terminal side of the driving means is grounded via a resistor, and the value of the resistor is set to be smaller than the internal resistance value between the oxygen concentration detection electrodes at room temperature and the internal resistance during heating. The air-fuel ratio measuring device according to claim 1 or 2, wherein the air-fuel ratio measuring device is set larger than the above value. (4) Claims 1, 2, or 3 further include display means for displaying that the timer is forced into a counting standby state by a voltage comparator.
The air-fuel ratio measuring device described in .