JP4229565B2 - NOx sensor - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a gas concentration detection device, and more specifically, oxygen gas, NOx, SOx, CO, CO, contained in combustion exhaust gas discharged from a boiler, an automobile or the like.₂, H₂The present invention relates to a gas concentration detection device that detects the concentration of an oxygen-bonded gas such as O or a combustible gas such as hydrocarbon (HC).
[0002]
[Prior art]
In recent years, the amount of nitrogen oxides (NOx), carbon monoxide (CO), various hydrocarbons (HC), etc. discharged from various combustion devices such as gasoline and diesel vehicles, boilers, etc. can be made. As much as possible, reduction is a problem, and various gas concentration detection devices for controlling the amount of these gases discharged from a combustion device or the like and monitoring the operation of the exhaust gas purification device have been proposed.
[0003]
Above all, the gas concentration using oxygen pumping action of oxygen ion conductive solid electrolyte represented by stabilized zirconia (hereinafter, simply referred to as “solid electrolyte”) or generation of electromotive force due to formation of oxygen concentration cell. Since the detection device is small and has a relatively simple structure, the NOx sensor for monitoring the amount of NOx gas contained in the exhaust gas of the automobile, and the ratio of air to fuel sent to the engine of the automobile are managed. It is used for oxygen sensors.
[0004]
In general, a high temperature (for example, about 600 ° C. or more in the case of stabilized zirconia) is required for the solid electrolyte to exhibit oxygen ion conductivity. Therefore, a gas concentration detection device using a solid electrolyte usually includes an electrochemical cell in which electrodes are formed on both sides of the solid electrolyte, and a heater for heating the electrochemical cell, and the electrochemical cell and the heater. Has a structure in which an insulating layer is interposed therebetween.
[0005]
However, when the gas concentration detector is heated to a high temperature using a heater, an offset current that does not correspond to the component concentration of a specific gas (detected gas) contained in the gas to be measured may flow. If the offset current or the value of the voltage generated corresponding thereto is larger than the value of the current or voltage output from the electrochemical cell, or the offset current depending on the operating state of the gas concentration detector. When the value fluctuates greatly, it may cause a significant decrease in gas detection accuracy.
[0006]
For example, a gas concentration detection device for detecting the concentration of a small amount of NOx gas contained in exhaust gas detects a small amount of NOx gas contained in the exhaust gas without being affected by oxygen, which is the main component of the exhaust gas. In general, an electrochemical cell for removing oxygen from the gas to be measured (hereinafter referred to as “oxygen pump cell”) and a gas to be detected included in the gas to be measured from which oxygen has been removed. (In this case, an electrochemical cell (hereinafter referred to as “gas detection cell”) for detecting NOx gas is provided.
[0007]
Since the concentration of NOx contained in the exhaust gas is usually several hundred ppm or less, when such a low concentration of NOx is measured by a gas concentration detector, its output is on the order of μA. However, in this type of gas concentration detection device, even if the gas to be measured having a NOx concentration of 0 ppm is supplied and the oxygen concentration in the gas to be measured is controlled to be, for example, 1 ppm or less by the oxygen pump cell, the offset flowing into the gas detection cell The current value may be a value corresponding to 100 ppm, which is much higher than a value corresponding to 1 ppm.
[0008]
This offset current is considered to occur due to various causes. For example, it is said that a small amount of oxygen diffuses and penetrates into the gas detection cell due to insufficient airtightness of the solid electrolyte laminate, thereby causing an offset current to flow. In addition, for example, it is said that when the gas detection cell becomes high temperature, a small amount of electron conduction occurs in the solid electrolyte constituting the cell, and this appears as an offset current. Alternatively, it is said that a leak current from the heater is applied to the gas detection cell and appears as an offset current.
[0009]
In order to reduce the value of the offset current or to maintain the value of the offset current at a constant value, various methods have been conventionally proposed. For example, Japanese Patent Laid-Open No. 10-318979 proposes a gas sensor in which a heater pattern is dense on the front main pump cell (oxygen pump cell) side and rough on the rear measurement pump cell (gas detection cell) side. Yes. Further, the publication proposes a gas sensor provided with a heater control means for controlling the power of the heater so that the impedance of the solid electrolyte constituting the measurement pump cell is constant.
[0010]
Also, for example, in Japanese Patent Application Laid-Open No. 11-23521, a heater power supply unit that supplies power to a heater and a cell power supply unit that applies a voltage to a cell are electrically separated using an insulated DC-DC converter. A method of driving a heating chemical sensor is disclosed.
[0011]
Furthermore, in Japanese Patent Publication No. 6-84950, a second solid electrolyte provided with a protective electrode is disposed between a heating unit provided with a heater and an electrochemical cell (gas detection cell), and electrochemical An electrochemical device in which at least one electrode of a static cell and a protective electrode are electrically connected is disclosed.
[0012]
[Problems to be solved by the invention]
Japanese Patent Application Laid-Open No. 10-318979 discloses that if the temperature of the gas detection cell is lowered to 700 ° C. by changing the heater pattern, the resistivity of the electron conduction of the solid electrolyte can be increased to 1 MΩ or more. It is disclosed that the offset value due to the electron conduction can be reduced. However, since the temperature of the gas detection cell usually rises to near the exhaust gas temperature, the temperature of the gas detection cell also rises to 700 ° C. or higher under the exhaust gas conditions of automobiles generally exceeding 700 ° C. Therefore, this method cannot be expected to greatly reduce the offset value.
[0013]
In addition, in this publication, when the impedance of the solid electrolyte constituting the gas detection cell is controlled to be constant using the heater control means, the temperature of the solid electrolyte can be kept constant. It is disclosed that the offset current can be kept constant. However, since there is a delay in heat conduction between the gas detection cell and the heater, there is an upper limit on the temperature control speed. Therefore, when the engine load of a car changes greatly and the exhaust gas temperature changes suddenly, even if the power control of the heater is performed precisely, the temperature of the gas detection cell cannot be avoided and the offset value is kept constant. It is difficult to do.
[0014]
Moreover, in the gas concentration detection apparatus provided with the heater, it is desirable that the electrical insulation between the heater and the gas detection cell is as high as possible. Therefore, a method of covering the periphery of the heating element with an insulator or providing the heating element on the electric insulator is employed. However, since the insulating property of an insulating material such as alumina decreases exponentially in a high temperature range exceeding 800 ° C., the voltage applied to the heater leaks to the gas detection cell via the insulating layer although it is a small amount. In addition, the leak current or the leak voltage generated thereby greatly fluctuates due to slight temperature fluctuations, so that if the output of the gas detection cell is very small, the gas detection performance is significantly reduced.
[0015]
In order to solve this problem, Japanese Patent Application Laid-Open No. 11-23521 discloses that the heater circuit and the gas detection circuit are electrically separated. However, in a state where both circuits are electrically separated, electrical noise such as electromagnetic waves, electromagnetic induction, and electrostatic induction generated from an automobile ignition system or the like is likely to be applied to the gas detection cell and the current detection unit. Therefore, these components that do not correspond to the gas concentration must be excluded by the filter circuit, and the responsiveness as the gas concentration detection device is lowered. Even if the two circuits are separated, the insulation between the heater circuit and the gas detection circuit decreases in a high humidity environment, etc., so that the leakage current also increases and the detection accuracy as a gas concentration detection device decreases. .
[0016]
On the other hand, as disclosed in Japanese Examined Patent Publication No. 6-84950, it is conceivable to provide a protective electrode in the heater to remove leakage current from the heater. However, in order to enhance the shielding effect, it is necessary to dispose a protective electrode in the vicinity of the joint surface between the gas detection cell and the heater, and there is a problem that the strength of the joint portion is lowered. In order to avoid this, when a protective electrode is provided in the heater portion, the sensor structure becomes complicated and expensive in order to obtain a complete shielding effect.
[0017]
  The problem to be solved by the present invention is that the absolute value of the offset current flowing through the gas detection cell and its temperature dependency are small, the concentration of the gas to be detected can be detected with high accuracy, and the response is excellent.NOx sensorIs to provide.
[0018]
[Means for Solving the Problems]
  In order to solve the above-described problems, the present invention provides an oxygen pump cell that discharges oxygen contained in a gas to be measured, in which electrodes are formed on both surfaces of a solid electrolyte, and a gas detection cell in which electrodes are formed on both surfaces of the solid electrolyte. A NOx sensor comprising: a detection cell power source for applying a voltage to the gas detection cell; a heater for heating the gas detection cell; and a heater power source for supplying power to the heater. A connection circuit that connects one terminal and one terminal of the heater power supply, a leakage voltage that is provided on the connection circuit and is generated between the heater and the gas detection cell, or from the heater A leak canceling means for canceling the leak current flowing through the gas detection cell;The leak canceling means is a voltage dividing means for dividing the heater power supply and applying a voltage to the gas detection cell in a direction opposite to the leak voltage,The gist of the invention is that the terminal voltage of the leak canceling means is configured to be 0.4 to 0.6 times the terminal voltage of the heater power supply.
[0019]
  According to the present invention having the above configurationNOx sensorSince the heater circuit and the gas detection circuit are connected by the connection circuit, the influence of electrical noise caused by electromagnetic waves or the like can be eliminated without using a filter circuit or the like. Further, since the voltage or current is applied to the gas detection cell in the direction opposite to the leak voltage or leak current by the leak canceling means, the absolute value of the offset current and the temperature dependence thereof due to the leak can be suppressed to a small value. for that reason,NOx sensorWithout reducing the responsiveness ofNOxGas detection accuracy can be improved.
[0020]
DETAILED DESCRIPTION OF THE INVENTION
First, by using a gas concentration detection device (hereinafter referred to as a “detection device”) having a gas detection cell and a heater for heating the gas detection cell, the heater operation is affected by the offset current of the gas detection cell. An experiment was conducted to examine whether it had an effect. First, the configuration of the detection device used in the experiment (hereinafter referred to as “experimental detection device”) will be described.
[0021]
FIG. 1 shows a cross-sectional view of the experimental detection apparatus. In FIG. 1, an experimental detection device 10 is a device for detecting an oxygen-bonded gas such as NOx gas contained in a gas to be measured using the oxygen pump action of the solid electrolyte, and includes a solid electrolyte 12a, a partition wall It consists of the laminated body of the layer 14, the solid electrolyte 12b, and the insulating layer 34a.
[0022]
A gas introduction chamber 16 and an air introduction chamber 18 are provided inside the experimental detection apparatus 10. The gas introduction chamber 16 is a closed hole provided with a diffusion rate-determining portion 14 a for introducing the gas to be measured into the inside thereof by diffusion, and its inner wall is constituted by the solid electrolytes 12 a and 12 b and the partition wall layer 14. The air introduction chamber 18 is a closed hole having an opening 18a for introducing air into the inside thereof, and its inner wall is constituted by the solid electrolyte 12b and the insulating layer 34a.
[0023]
The diffusion rate controlling part 14a is not particularly limited as long as it can give a predetermined diffusion resistance to the gas to be measured that diffuses inside the gas introduction chamber 16. In the example shown in FIG. 1, a slit is provided in the partition wall layer 14 to form a small hole, which is used as the diffusion rate controlling portion 14 a. However, a porous material is provided in the opening of the gas introduction chamber 16, and this is used as the diffusion rate controlling portion. It may be used as 14a.
[0024]
The experimental detection apparatus 10 includes an oxygen pump cell 20, a pump cell power supply 24, a gas detection cell 26, a detection cell power supply 30, a heater unit 34, and a heater power supply 38.
[0025]
The oxygen pump cell 20 includes a solid electrolyte 12a and an anode 22a and a cathode 22b provided on both surfaces thereof, and a pump cell power supply 24 for applying a predetermined voltage is connected to the anode 22a and the cathode 22b. The oxygen pump cell 20 is an electrochemical cell for selectively discharging only oxygen contained in the gas to be measured introduced into the gas introduction chamber 16 to the outside of the experimental detection device 10, and is located in the vicinity of the diffusion rate controlling portion 14a. Is provided. The anode 22 a and the cathode 22 b are provided on the outer wall side of the experimental detection device 10 and the inner wall side of the gas introduction chamber 16, respectively.
[0026]
The gas detection cell 26 includes a solid electrolyte 12b, and an anode 28a and a cathode 28b provided on both surfaces thereof. A detection cell power source 30 for applying a predetermined voltage to the anode 28a and the cathode 28b, and an anode 28a. And a gas detection cell output ammeter 32 for measuring the current flowing between the cathode 28b. The gas detection cell 26 is an electrochemical cell for decomposing the oxygen-bonded gas contained in the gas to be measured and measuring the amount of oxygen generated at that time, and is provided at the subsequent stage of the oxygen pump cell 20. The anode 28 a is provided on the inner wall side of the atmosphere introduction chamber 18, and the cathode 28 b is provided on the inner wall side of the gas introduction chamber 16.
[0027]
The heater section 34 is formed by forming a heater 36 made of a metal and a ceramic component inside the insulating layer 34a. Further, both ends of the heater 36 are connected to a heater power source 38 via lead portions 36a and 36b. The heater unit 34 is used to heat the solid electrolytes 12a and 12b to a temperature at which the oxygen pump functions, or to keep the experimental detection device 10 at a predetermined temperature. In the example shown in FIG. Is provided at the lower end of the experimental detection apparatus 10.
[0028]
Further, the experimental detection apparatus 10 includes an experimental circuit 40 for examining the influence of the heater operation on the offset current. The experimental circuit 40 is a circuit for connecting the cathode 28 b of the gas detection cell 26 and either the + terminal or the − terminal of the heater power supply 38 using the open / close switch 42 and the changeover switch 44. Further, high input resistance voltmeters 46 are connected to both ends of the open / close switch 42 so that a potential difference generated between the cathode 28b and the + terminal or the − terminal can be measured.
[0029]
Next, a general method for detecting the oxygen-bonded gas contained in the gas to be measured using the experimental detection apparatus 10 shown in FIG. 1 will be described. First, the entire experimental detection apparatus 10 is placed in the gas to be measured, and the heater unit 34 is used for the experiment so that the solid electrolytes 12a and 12b are maintained at a temperature (for example, 700 ° C.) exhibiting an oxygen pump action. The detection device 10 is heated. At this time, the gas to be measured flows into the gas introduction chamber 16 from the diffusion control part 14a by diffusion.
[0030]
The gas to be measured that has flowed in from the diffusion rate controlling part 14a reaches the oxygen pump cell 20 by diffusion. The cathode 22b of the oxygen pump cell 20 uses an electrode having a high reducibility only with respect to oxygen at a predetermined voltage or lower, so that an appropriate size is provided between the anode 22a and the cathode 22b using the pump cell power source 24. When a voltage is applied, only the oxygen gas contained in the gas to be measured is selectively discharged out of the experimental detection apparatus 10 by the oxygen pump action.
[0031]
Further, the measurement gas from which the oxygen concentration has been removed reaches the gas detection cell 26 by diffusion. At this time, when a predetermined voltage is applied between the anode 28a and the cathode 28b of the gas detection cell 26, first, the oxygen-bonded gas is adsorbed on the cathode 28b, and the oxygen-bonded gas is decomposed at the interface between the cathode 28b and the solid electrolyte 12b. Oxygen generated by the decomposition of the oxygen-bonded gas is discharged to the atmosphere introduction chamber 18 side by the oxygen pump action of the solid electrolyte 12b. At that time, since a current proportional to the oxygen-bonded gas concentration flows between the anode 28a and the cathode 28b, the concentration of the oxygen-bonded gas can be detected by measuring the magnitude with the gas detection cell output ammeter 32. Become.
[0032]
Next, the experimental procedure will be described. The experimental detector 10 is N with a purity of 99.99%.₂When exposed to a gas atmosphere (NOx = 0), the cathode 28b of the gas detection cell 26 has this N₂A gas was introduced as a gas to be measured. On the other hand, air was introduced into the anode 28a.
[0033]
A voltage of 9 to 13 V was applied to the heater 36 by a heater power supply 38, and the temperature of the experimental detection apparatus 10 was changed to 700 to 900 ° C. In addition, a predetermined voltage was applied to the oxygen pump cell 20 by the pump cell power supply 24 so that the oxygen in the gas introduction chamber 16 could be discharged out of the experimental detection device 10. Furthermore, a voltage of 0.5 V was applied to the gas detection cell 26 from the detection cell power supply 30 to set the operation state in which the gas detection cell output ammeter 32 can detect the dissociation current resulting from the decomposition of the NOx gas.
[0034]
Therefore, in this operating state, the atmosphere of the experimental detection apparatus 10 is almost pure N.₂Because of the gas atmosphere, the value of the current flowing through the gas detection cell 26 should be extremely low, but if the oxygen gas flows into the gas introduction chamber 16 or due to electrical factors, the gas detection cell If a current flows through the output ammeter 32, this can be measured as an offset current.
[0035]
In this state, the open / close switch 42 is closed, and the selector switch 44 is set to the A side (shown in FIG. 1) so that the potential of the cathode 28b of the gas detection cell 26 becomes equal to the potential of the positive terminal or the negative terminal of the heater power supply 38. The change in the offset current was examined by connecting to the + terminal) or the B side (− terminal). The results are shown in FIG.
[0036]
As shown in FIG. 2, when the positive terminal of the heater power supply 38 and the cathode 28b of the gas detection cell 26 were connected, the offset current increased with an increase in temperature. On the other hand, when the negative terminal of the heater power supply 38 and the cathode 28b of the gas detection cell 26 were connected, the offset current decreased with increasing temperature. In particular, when the heating temperature of the experimental detection device 10 exceeded 800 ° C., the offset current changed remarkably with respect to the temperature in all cases, and the absolute value reached several μA at 900 ° C.
[0037]
In addition, when the anode 28a, which is the other electrode of the gas detection cell 26, and the positive terminal or the negative terminal of the heater power supply 38 are connected, the change behavior with respect to the magnitude of the offset current and the heating temperature is almost the same as in FIG. It was the same.
[0038]
On the other hand, when the anode 28a or the cathode 28b of the gas detection cell 26 was not connected to either the positive terminal or the negative terminal of the heater power supply 38, such a change in the offset current did not occur. From the above results, it can be inferred that the large change in the offset current shown in FIG. 2 is caused by electrical leakage from the heater 36 itself or its lead portions 36a and 36b.
[0039]
Next, in order to examine the path through which the offset current flows, the changeover switch 44 is connected to the + terminal or the − terminal of the heater power supply 38 with the open / close switch 42 shown in FIG. 1 opened. Using the high input resistance voltmeter 46, the voltage generated between the cathode 28b and the + terminal or the − terminal of the heater power supply 38 was measured.
[0040]
As a result, it was found that a voltage 0.4 to 0.6 times the voltage applied to the heater 36 was generated at the cathode 28b. Further, the ratio of this voltage to the voltage applied to the heater 36 (hereinafter referred to as “leakage voltage ratio”) is obtained by changing the voltage applied to the heater 36 in order to change the temperature of the experimental detection device 10. However, it turned out that it does not change a lot.
[0041]
The reason why the above results are obtained can be considered as follows. FIG. 3A shows a longitudinal sectional view of the experimental detection apparatus 10 in a state where the cathode 28b of the gas detection cell 26 and the negative terminal of the heater power supply 38 are connected. As described above, the experimental detection apparatus 10 has a structure in which the gas detection cell 26 is stacked on the insulating layer 34a including the heater 36, and the oxygen pump cell 20 is stacked on the insulating layer 34a via the partition wall layer 14. have.
[0042]
Since the insulating layer 34a is generally made of highly insulating alumina or the like, current does not leak from the heater 36 to the gas detection cell 26 near room temperature. However, since insulating properties such as alumina decrease exponentially at high temperatures, a part of the current supplied to the heater 36 by the heater power supply 38 leaks when the temperature of the experimental detection device 10 increases. Then, as indicated by the arrow in FIG. 3A, it is considered that the gas flows into the gas detection cell output ammeter 32 through the insulating layer 34a and the gas detection cell 26.
[0043]
If this experimental detection apparatus 10 is represented by an equivalent circuit, it can be represented as shown in FIG. Here, r in FIG.₁Is the resistance of the heater 36, R₁And R₂Is the resistance of the insulating layer 34a, r₂And r₃Represents the resistance of the solid electrolyte 12b. R₄And r₅Represents the resistance between the solid electrolyte 12b and the anode 28a and between the solid electrolyte 12b and the cathode 28b, respectively. Further, the applied voltage of the heater power supply 38 is set to V₁, The applied voltage of the detection cell power supply 30 is V₂And
[0044]
In this case, since the minus terminal of the heater power supply 38 and the cathode 28b of the gas detection cell 26 are connected, if the potential at this point is 0 (V), the potential of the gas detection cell 26 is αV.₁(V) (where α is the resistance R₁, R₂, R₂And r₃(Partial pressure ratio determined). On the other hand, the potential of the positive terminal of the detection cell power supply 30 is V₂Therefore, in the end, the gas detection cell output ammeter 32 has a potential difference V expressed by the following equation (1).₋An offset current corresponding to
[0045]
[Expression 1]
V₋= V₂-ΑV₁
[0046]
On the other hand, an equivalent circuit when the positive terminal of the heater power supply 38 and the cathode 28b of the gas detection cell 26 are connected can be represented as shown in FIG. In this case, if the potential of the positive terminal of the heater power supply 38 is 0 (V), the potential of the gas detection cell 26 is − (1−α) V.₁(V). On the other hand, the potential of the positive terminal of the detection cell power supply 30 is V₂Since (V) remains, eventually, the gas detection cell output ammeter 32 has a potential difference V expressed by the following equation (2).₊An offset current corresponding to
[0047]
[Expression 2]
V₊= V₂+ (1-α) V₁
[0048]
From the equations (1) and (2), the voltage V applied to the heater 36 in order to raise the temperature of the experimental detection device 10.₁When V is increased, V₊Increases and V₋Can be seen to decrease. Further, when the temperature of the experimental detection device 10 rises, the resistance R of the insulating layer 34a₁, R₂It is understood that the absolute value of the offset current increases accordingly. Further, the voltage V applied to the gas detection cell 26₂Is generally a voltage V applied to the heater 36.₁Considerably smaller than r and r₂, R₃, R₄, R₅Is R₁, R₂For example, in the connection state as shown in FIG. 3A, the current flowing when the open / close switch 42 is closed assuming that the leakage voltage ratio is substantially equal to α in the connected state shown in FIG. It can be seen that the offset current is 32.
[0049]
In the above-described experiment, the leak voltage ratio was 0.4 to 0.6 regardless of the temperature of the gas detection cell 26, but this is because the structure of the experimental detection device 10 is substantially symmetrical, and the heater 36. This indicates that the voltage can be regarded as leaking from a substantially central point. Further, when the structure and arrangement of the heater 36 are different, the leakage voltage ratio is expected to be different. In this case, the leakage voltage ratio can be obtained by calculation from the structure and arrangement of the heater. ing.
[0050]
By the way, as shown in FIG. 2, when one electrode of the gas detection cell 26 and one terminal of the heater power supply 38 are not connected, that is, when the gas detection circuit and the heater circuit are completely separated, The absolute value of the current and its temperature dependency can be reduced. However, when the detection device is used in an environment (for example, in an automobile) that is significantly affected by electromagnetic waves, electromagnetic induction, or electrostatic induction, if the gas detection circuit and the heater circuit are electrically separated, electric waves such as electromagnetic waves Noise affects the electrical output of the gas detection cell. In order to remove this electrical noise, it is effective to provide a filter circuit. However, if a filter circuit is provided, the responsiveness of the detection device is reduced, and a sudden change in the detected gas concentration is faithfully monitored. It becomes difficult.
[0051]
On the other hand, when one electrode of the gas detection cell 26 and one terminal of the heater power supply 38 are connected, that is, when the gas detection circuit and the heater circuit are not electrically separated, the potential between them is an electric potential. Therefore, the influence of electrical noise can be eliminated. However, in the detection device equipped with the heater, the voltage applied to the heater 36 has an electrical effect on the gas detection cell 26 and the detection electronic circuit, so that the heater power supply 38 and the electrode lead of the gas detection cell 26 are connected. Various considerations are necessary.
[0052]
In particular, when a very small amount of NOx contained in the gas to be measured is measured, the output becomes very small on the order of several μA. For example, in the case of the experimental detection apparatus 10 shown in FIG. 1, as shown in FIG. 2, the offset current value at 900 ° C. corresponds to a NOx concentration of several hundred ppm. If the voltage varies depending on the voltage applied to the heater 36, the detection performance is significantly deteriorated. Further, in a temperature range exceeding 800 ° C., even if a temperature change of only 10 ° C. occurs in the sensor, a detection error close to 100 ppm occurs, so that it cannot be used for automobiles that require concentration detection of several hundred ppm or less.
[0053]
As a method of excluding this, for example, stabilization of the temperature of the detection device can be considered. However, when the detection device is used in automobile exhaust gas, the exhaust gas temperature also changes suddenly due to an abrupt change in engine load. Therefore, in order to keep the temperature fluctuation of the detection device within 10 ° C., a heat retention mechanism is required. As a result, the gas flowability is hindered, and the responsiveness is remarkably deteriorated.
[0054]
On the other hand, from the above-described experiment, when one electrode of the gas detection cell 26 and one terminal of the heater power supply 38 are connected, the offset current is mainly a current leak that flows from the heater 36 to the gas detection cell 26. It was found that it was caused. Further, it was found that the leak voltage ratio is substantially constant regardless of the temperature of the experimental detection device 10 and the voltage applied to the heater 36. Therefore, the leakage voltage generated between the heater 36 and the gas detection cell 26 (αV shown in FIGS. 3B and 3C).₁Or-(1-α) V₁If a voltage acting in the opposite direction to the gas detection cell 26 is applied to the gas detection cell 26, the generation of offset current due to leakage can be suppressed. It is considered possible.
[0055]
  The present invention is based on such a concept,One terminal of the detection cell power supplyOne terminal of the heater power supply is connected by a connection circuit, and a leak canceling means for canceling a leak voltage generated on the connection circuit or a leak current flowing from the heater to the gas detection cell is provided. To do.
[0056]
Here, the leak canceling means is not particularly limited as long as it can apply a voltage or current acting in the opposite direction to the leak voltage or leak current to the gas detection cell. In addition, the absolute value of the voltage or current applied to the gas detection cell is preferably the same as the absolute value of the leak voltage or leak current.
[0057]
A suitable example of the leak canceling means is a voltage applying means for applying a leak voltage to the gas detection cell. Since the leakage voltage ratio is mainly determined by the structure of the heater, for example, when a constant voltage power source is used as the heater power source, a constant voltage power source that generates a voltage at a constant ratio with respect to the heater power source is used as a voltage applying unit. Can be used. On the other hand, when it is necessary to change the voltage applied to the heater for the temperature control of the heater, a voltage source that outputs a voltage at a predetermined ratio in accordance with the change in the voltage applied to the heater is used as a voltage application unit. It is desirable to use an amplifier or the like.
[0058]
Moreover, when using various voltage sources, amplifiers, etc. as a voltage application means, it is preferable that a voltage application means is a low impedance. If a low impedance voltage applying means is used as the leak canceling means, there is an advantage that the gas detection accuracy is improved because it is less susceptible to electrical noise such as electromagnetic waves.
[0059]
The leak canceling means may be a voltage dividing means that divides the heater power supply and applies a voltage to the gas detection cell in a direction opposite to the leak voltage. When the heater power supply is divided using a voltage dividing means and this voltage is applied to the gas detection cell, there is an advantage that a constant ratio of voltage can always be output to the gas detection cell even if the output of the heater power supply fluctuates. Further, when the heater power supply is divided using a resistor, it is preferable to insert a capacitance in parallel with the resistor. Thereby, the impedance of the voltage dividing circuit can be reduced, and the influence of electrical noise can be reduced.
[0060]
Examples of gas to be detected that can be measured using the gas detection cell include oxygen gas, NOx, SOx, CO, and CO.₂, H₂Examples include oxygen-bonded gases such as O and flammable gases such as hydrocarbons (HC), but the present invention includes a gas detection cell and a heater for heating the gas detection cell, regardless of the type of gas to be detected. Any detection device can be applied and is not particularly limited. In addition, the present invention is not limited to a detection apparatus having one gas detection cell, and is also applied to a detection apparatus that detects two or more types of detected gases at the same time using two or more gas detection cells. it can.
[0061]
Furthermore, as a method of measuring the concentration of the gas to be detected using an electrochemical cell, a method of measuring the concentration of the gas to be detected using the electrochemical oxygen pump action of the solid electrolyte, Although there is a method for measuring the concentration of a gas to be detected by utilizing generation of electromotive force due to formation, the present invention is applicable to any detection device using any method and is particularly limited. is not. Below, the detection apparatus provided with the leak cancellation means is demonstrated concretely based on an Example.
[0062]
【Example】
Example 1
FIG. 4 is a cross-sectional view of the detection device 50 according to the first embodiment. In FIG. 4, the detection device 50 connects the cathode 28 b of the gas detection cell 26 and the negative terminal of the heater power supply 38 with a connection circuit 52, and is generated between the heater 36 and the gas detection cell 26 on the connection circuit 52. A canceling power supply 54 is provided for applying a voltage in the opposite direction to the leaked voltage. The other points are the same as those of the experimental detection apparatus 10 shown in FIG.
[0063]
In addition, about the detection apparatus 50 shown in FIG. 4, when the temperature of the gas detection cell 26 was heated to 800 degreeC or more and the leak voltage ratio was measured previously, it was 0.53. Therefore, in this embodiment, a low-impedance voltage source that generates a voltage 0.53 times the voltage applied to the heater power supply 38 is used as the cancellation power supply 54.
[0064]
Using this detection device 50, the offset current was measured under the same conditions as in the experiment described above. The results are shown in FIG. For comparison, FIG. 5 also shows the result of FIG. 2 obtained in the above-described experiment (offset current when the cathode 28b of the gas detection cell 26 and the negative terminal of the heater power supply 38 are simply connected). It was.
[0065]
As can be seen from FIG. 5, when the canceling power supply 54 is provided on the connection circuit 52, the change in the offset current is remarkably reduced even when the heating temperature of the gas detection cell 26 is significantly changed to 700 to 900.degree. Further, since a low impedance power source was used as the canceling power source 54, the influence of electromagnetic induction or the like on the gas detection cell 26 and the current detection unit was not seen, and the detection accuracy was greatly improved.
[0066]
(Example 2)
FIG. 6 is a cross-sectional view of the detection device 60 according to the second embodiment. In FIG. 6, the detection device 60 divides the heater power supply 38 instead of the cancellation power supply 54 used in the detection device 50 of the first embodiment, and applies a voltage 62 to the gas detection cell 26 in a direction opposite to the leak voltage. Is provided on the connection circuit 52.
[0067]
Further, in this embodiment, in order to avoid an increase in impedance of the connection circuit 52 from the negative terminal of the heater power supply 38 through the resistor 62 to the cathode 28b of the gas detection cell 26 due to the resistor 62, the resistor 62 is connected in parallel with the resistor 62. An electric capacity 64 was inserted and used as a voltage dividing means. In this embodiment, the resistor 62 is 2 kΩ, but a resistance value between 100 Ω and 100 kΩ can be applied. Other points have the same configuration as that of the detection device 50 shown in FIG.
[0068]
Using this detection device 60, the voltage dependency ratio of the resistor 62 was changed from 0.4 to 0.6, and the temperature dependence of the offset current was examined. The other measurement conditions were the same as in the experiment described above. The results are shown in FIG.
[0069]
From FIG. 7, the temperature dependence of the offset current changes according to the voltage dividing ratio of the resistor 62. When the voltage dividing ratio of the resistor 62 is between 0.45 and 0.55, the temperature dependence of the offset current is reduced. I understand that. In particular, it was found that the temperature dependence of the offset current becomes extremely small when the voltage division ratio is around 0.5.
[0070]
When the ambient temperature changes, in order to control the temperature of the gas detection cell 26 to be constant, it is necessary to change the voltage applied to the heater 36 as in feedback control. In this embodiment, since the heater power supply 38 is divided by the resistor 62, even in such a case, there is an advantage that a voltage with a constant ratio corresponding to the change in the voltage applied to the heater 36 can be easily obtained. .
[0071]
In particular, when the voltage applied to the heater 36 changes with time like a rectangular wave in order to control the temperature of the gas detection cell 26 to be constant, the leak current from the heater 36 is an alternating current with a large amplitude. It becomes current. In the conventional detection device, since this alternating current is superimposed on the output current of the gas detection cell 26, current components that do not correspond to the gas concentration must be excluded using a filter circuit or a high-speed switch. . For this reason, there is a problem in that the response as a detection device is lowered, or an output current that is continuous in time cannot be obtained. However, in the case of the present embodiment, such a situation does not occur, and the detection accuracy can be greatly improved by a simple method.
[0072]
(Example 3)
FIG. 8 is a cross-sectional view of the detection device 70 according to the third embodiment. In FIG. 7, the detection device 70 includes an oxygen detection cell 72 for detecting the residual oxygen concentration in the measurement gas introduced into the gas introduction chamber 16 in addition to the gas detection cell 26.
[0073]
The oxygen detection cell 72 includes a solid electrolyte 12 b and electrodes 74 a and 74 b formed on both surfaces thereof, and is disposed adjacent to the gas detection cell 26. The electrode 74a is provided on the atmosphere introduction cell 18 side, and the electrode 74b is provided on the gas introduction chamber 16 side.
[0074]
The oxygen detection cell 72 is an electrochemical cell that detects the residual oxygen concentration in the gas introduction chamber 16 by measuring an electromotive force generated between the electrodes 74 a and 74 b with an oxygen concentration detection output voltmeter 76. . The value measured by the oxygen concentration detection output voltmeter 76 is fed back to the pump cell power source 25 that applies a voltage to the oxygen pump cell 20, and the pump cell power source 25 keeps the oxygen concentration in the gas introduction chamber 16 constant. The applied voltage is controlled.
[0075]
Further, in the detection device 70, in order to eliminate the influence of the leakage voltage from the heater 36 to the oxygen detection cell 72, the negative terminal of the heater power supply 38 and the electrode of the oxygen detection cell 72 provided on the gas introduction chamber 16 side. 74 b is connected by a connection circuit 77, and a second cancellation power supply 78 for the oxygen detection cell 72 is provided on the connection circuit 77.
[0076]
The leak voltage ratio from the heater 36 to the oxygen detection cell 72 was measured in advance and was almost 0.5. Therefore, the second cancellation power source 78 has 0.5 times the voltage applied by the heater power source 38. A voltage source that generates a voltage of 1 is used. The other points are the same as those of the detection device 50 shown in FIG.
[0077]
In the present embodiment, a cancellation power source 54 for excluding the influence of the leakage voltage from the heater 36 to the gas detection cell 26, and a second cancellation power source for excluding the influence of the leakage voltage from the heater 36 to the oxygen detection cell 72. 78, the detected gas such as NOx and the residual oxygen concentration contained in the measured gas can be detected with high accuracy. Therefore, if the offset current corresponding to the residual oxygen concentration is subtracted from the output current of the gas detection cell 26, the detection accuracy of the gas to be detected can be further improved.
[0078]
Further, for example, when measuring the concentration of NOx gas contained in the exhaust gas of an automobile, if the engine load changes suddenly, the oxygen concentration in the exhaust gas changes greatly, and the residual oxygen concentration in the measured gas that reaches the gas detection cell 26 May fluctuate. On the other hand, according to the present embodiment, the applied voltage of the pump cell power supply 25 is feedback controlled based on the oxygen concentration detected by the oxygen detection cell 72, so that the residual oxygen concentration in the gas to be measured can be kept constant. it can. Therefore, even when the composition of the gas to be measured varies greatly, the offset current caused by the residual oxygen concentration is maintained at a constant value, and the detection accuracy of the gas to be detected can be further improved.
[0079]
(Example 4)
FIG. 9 is a cross-sectional view of the detection device 80 according to the fourth embodiment. In FIG. 9, a detection device 80 includes a gas detection cell between the anode 28 a and the cathode 28 b of the gas detection cell 26 instead of the detection cell power supply 30 and the gas detection cell output ammeter 32 used in the detection device 50 of the first embodiment. An output voltmeter 82 is connected. In this case, the detected gas concentration is measured from the magnitude of the electromotive force generated between the anode 28a and the cathode 28b due to the formation of the oxygen concentration cell.
[0080]
In the present embodiment, since the heater 36 having the same structure as that of the first embodiment is used, the canceling power supply 54 is provided with a voltage source that generates a voltage approximately 0.5 times the voltage applied to the heater 36. Using. Other points have the same configuration as that of the detection device 50 of the first embodiment, and a description thereof will be omitted.
[0081]
In FIG. 9, when measuring the concentration of a small amount of gas to be detected with the cancellation power supply 54 removed, the leakage voltage from the heater 36 is significantly larger than the electromotive force output from the gas detection cell output voltmeter 82. Become. Further, when a leak voltage is generated, a leak current flows through the gas detection cell 26. Therefore, the electromotive force output from the gas detection cell output voltmeter 82 includes an electromotive force component that does not correspond to the concentration of the gas to be detected, and the detection accuracy of the gas to be detected is greatly reduced.
[0082]
On the other hand, in the present embodiment, the influence of the leakage voltage is excluded by the cancellation power supply 54, so that the electromotive force output from the gas detection cell output voltmeter 82 is only the electromotive force component corresponding to the concentration of the detected gas. Become. Therefore, even if the detected gas concentration is extremely low, such as NOx gas contained in the exhaust gas, the detected gas can be detected with high accuracy.
[0083]
Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the scope of the present invention.
[0084]
For example, in the above embodiment, the case where oxygen-bonded gas such as NOx gas is used as the detection gas has been mainly described, but the same applies to the case where flammable gas such as hydrocarbon is used as the detection gas. By applying, detection accuracy can be greatly improved.
[0085]
Further, for example, in the second embodiment, the heater power supply is divided, but a voltage source other than the heater power supply provided in the detection device is divided, and the voltage is detected in the reverse direction to the leak voltage via the connection circuit. You may make it apply to.
[0086]
Further, for example, in Example 3, the atmosphere is introduced into the atmosphere introduction chamber 18 side, and the residual oxygen concentration is measured with reference to the atmosphere. However, the electrochemical for forming an electrode serving as a reference for measuring the oxygen concentration is used. A cell may be provided in the detection device, and the residual oxygen concentration may be measured using the electrode as a reference electrode. In this case, the detection accuracy of the residual oxygen concentration can be improved by canceling out the leak voltage or leak current generated at the reference electrode by the leak canceling means, as in the third embodiment.
[0087]
Furthermore, in the third embodiment, when the magnitude of the leak voltage from the heater 36 to the gas detection cell 26 and the magnitude of the leak voltage from the heater 36 to the oxygen detection cell 72 are substantially the same, the cancellation power supply 54 And the second cancellation power source 74 may be shared, and thereby the same effect as in the above embodiment can be obtained.
[0088]
【The invention's effect】
  The present inventionAn oxygen pump cell that discharges oxygen contained in the gas to be measured, with electrodes formed on both sides of the solid electrolyte;A gas detection cell having electrodes formed on both sides of the solid electrolyte;A detection cell power source for applying a voltage to the gas detection cell;A heater for heating the gas detection cell and a heater power supply for supplying power to the heater were provided.NOx sensorInOne terminal of the detection cell power supply,A connection circuit that connects one terminal of the heater power supply, and a leakage voltage that is provided between the heater and the gas detection cell or that leaks from the heater to the gas detection cell. And a leak canceling means.The terminal voltage of the leak canceling means is configured to be 0.4 to 0.6 times the voltage between the terminals of the heater power supply.So, even under high temperature usage environment with sudden temperature fluctuation, the influence of offset current due to leakage voltage or leakage current is excluded,NOxThe gas detection accuracy is improved. In addition, since the heater circuit and the gas detection circuit are not electrically separated, they are not easily affected by electrical noise, and the responsiveness does not deteriorate.
[0089]
Further, as a leak canceling means, a voltage applying means for applying a voltage to the gas detection cell in a direction opposite to the leakage voltage, or a voltage dividing means for dividing the heater power supply and applying a voltage to the gas detection cell in the opposite direction to the leakage voltage. Is used, there is an effect that the absolute value of the offset current and its temperature fluctuation can be reduced with a simple structure even when the leakage voltage fluctuates due to the fluctuation of the voltage applied to the heater.
[0090]
As described above, according to the present invention, offset current due to leakage current can be reduced without using a protective electrode, so that a detection device with a simple configuration can be obtained, and gas detection of minute current output and minute voltage output can be achieved. Even if it is a cell, the concentration of the gas to be detected can be measured with high accuracy. For example, if this is used for combustion control of an automobile engine, detection of the concentration of NOx gas contained in the exhaust gas of an automobile, etc. This is an invention that is extremely effective in terms of industry.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of an experimental detection apparatus.
FIG. 2 is a diagram showing the influence of a heating temperature and a connection state between a heater power supply terminal and a gas detection cell electrode on an offset current.
3 (a) is a longitudinal sectional view of the experimental detection apparatus shown in FIG. 1, and FIGS. 3 (b) and 3 (c) show a heater power source minus terminal and a gas detection cell, respectively. It is a figure which shows the equivalent circuit of the detection apparatus for experiment in the case of connecting the cathode of this, and the case where the + terminal of a heater power supply and the cathode of a gas detection cell are connected.
4 is a cross-sectional view of the detection device according to Embodiment 1. FIG.
FIG. 5 is a diagram showing temperature dependence of offset current of the detection device shown in FIG. 4;
FIG. 6 is a cross-sectional view of a detection device according to a second embodiment.
7 is a diagram showing the influence of the partial pressure ratio and the heating temperature on the offset current of the detection device shown in FIG. 6. FIG.
FIG. 8 is a cross-sectional view of a detection device according to a third embodiment.
FIG. 9 is a cross-sectional view of a detection device according to a fourth embodiment.
[Explanation of symbols]
12a, 12b Solid electrolyte
26 Gas detection cell
28a Anode (electrode)
28b Cathode (electrode)
36 Heater
38 Heater power supply
50, 60, 70, 80 Gas concentration detector (detector)
54, 78 Counter power supply (voltage application means)
62 Resistance (voltage dividing means)

Claims (1)

An oxygen pump cell that discharges oxygen contained in the gas to be measured with electrodes formed on both sides of the solid electrolyte, a gas detection cell with electrodes formed on both sides of the solid electrolyte, and detection that applies a voltage to the gas detection cell In a NOx sensor comprising a cell power source, a heater for heating the gas detection cell, and a heater power source for supplying power to the heater,
A connection circuit for connecting one terminal of the detection cell power source and one terminal of the heater power source;
A leak canceling means for canceling a leak voltage generated between the heater and the gas detection cell, or a leak current flowing from the heater to the gas detection cell, provided on the connection circuit;
The leak canceling means is a voltage dividing means for dividing the heater power supply and applying a voltage to the gas detection cell in a direction opposite to the leak voltage,
A NOx sensor characterized in that the terminal voltage of the leak canceling means is 0.4 to 0.6 times the terminal voltage of the heater power supply.

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