JP2000304732A - Gas concentration sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a gas concentration sensor with which a gas concentration can be measured with high accuracy. SOLUTION: A recessed part 34a is formed so that the bottom face is to be nearly parallel to a reflecting face 34 in an edge part which comes into contact with the sidewall of a measuring chamber in the reflecting face 34. The distance between the edge part of the reflecting face 34 and an ultrasonic element 33 is made longer than the distance between the central part of the reflecting face 34 and the ultrasonic element 33. As a result, different-route waves which are propagated along the sidewall of the measuring chamber are reflected by the bottom face of the recessed part 34a so as to be propagated. That is to say, as compared with a case in which the reflecting face 34 is a plane, the propagation distance of the different-route waves becomes long, and the different-route waves are not synthesized near the modulation point of straight waves. That is to say, since the modulation point of the straight waves can be detected precisely, the time between the modulation point of transmitted waves and the modulation point of receives waves can be measured as the propagation time of ultrasonic waves, and a gas concentration can be measured with high accuracy.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas concentration of a combustible gas such as evaporative fuel in intake air supplied to an intake pipe of an engine for an internal combustion engine, or a fuel gas or exhaust gas of a fuel cell. The present invention relates to a gas concentration sensor that measures a gas concentration of a gas component in the gas.

[0002]

2. Description of the Related Art Conventionally, as a fuel supply system from a fuel tank to an engine, a fuel pumping fuel from a fuel tank by a fuel pump to a fuel pipe via a fuel pipe to an injector has been known. Supply system.

[0003] Separately from this, a second supply system is provided in which a canister temporarily absorbs evaporated fuel generated in a fuel tank, purges fuel accumulated in the canister, and sends the purged gas to an intake pipe. is there. Therefore, in the engine, in addition to the fuel injected from the injector, evaporated fuel such as purge gas (hereinafter simply referred to as purge gas) is burned in the cylinder.

As described above, if the air-fuel ratio deviates from the stoichiometric air-fuel ratio in the combustion control by supplying the purge gas to the engine separately from the injected fuel, the CO, H
The purifying ability of C and NOx is drastically reduced, and as a result, CO, HC, NOx and the like in the exhaust gas are increased.

Therefore, in order to use the purge gas as the main fuel system for combustion, for example, when starting the engine, particularly when the catalyst is inactive, the concentration of the purge gas is measured with high accuracy and the supply amount is controlled optimally. It is extremely important to do so. As a sensor for measuring the purge gas, for example, a sensor using ultrasonic waves (ultrasonic sensor) is considered, and its development is being promoted, but is not always sufficient.

That is, this type of ultrasonic sensor causes an ultrasonic element to transmit and receive an ultrasonic wave having a modulation point, and based on the time between the modulation points of the transmitted and received waves (ie, the propagation time of the ultrasonic wave), the purge gas Although there is something that detects concentration, the ultrasonic wave actually received by the ultrasonic element has the highest sound pressure,
This is a composite wave of a component (straight wave) propagating along the shortest path and a component (different path wave) having a relatively low sound pressure and a longer propagation distance than the direct wave.

That is, a different path wave propagating slightly behind a direct wave is synthesized near the modulation point of the direct wave, making it difficult to detect the modulation point of the direct wave to be measured. There was a problem that measurement was not possible. Therefore, there is a problem that it is extremely difficult to accurately control the concentration of the purge gas based on the measurement result of the concentration of the purge gas.

In recent years, fuel cells have been actively developed as clean power sources for automobiles. There are several types of fuel cells, such as a molten carbonate type and a phosphoric acid type. The polymer electrolyte type (PEFC), which has advantages such as easy start / stop, high output density, and small size and light weight, has attracted special attention. ing.

[0009] The polymer electrolyte fuel cell uses hydrogen as its fuel, and it is generally accepted that this hydrogen is obtained as a reformed gas of methanol through a reformer. On this occasion,
It is extremely important to measure the hydrogen concentration in the reformed gas for more efficient power generation. Since the gas concentration sensor for measuring the hydrogen concentration is assumed to be used in combustible gas, it is desired that the operating temperature be low. As such an example, Japanese Patent Publication No. Hei 7-31153 discloses a sensor using a proton conductor membrane.
Not completely unheated. Also, a current measurement type sensor using Nafion as an electrolyte has been proposed,
The electrode is poisoned by a large amount of CO generated at the start of fuel,
There is also a problem that the humidity dependency is large.

By the way, as a method of detecting the gas concentration without heating and reducing the poisoning and the humidity dependency due to the miscellaneous gas components, a method using an ultrasonic sensor as used in the engine has been proposed. . However, especially when a gas component having a small molecular weight such as hydrogen gas is to be detected, the speed of sound becomes extremely high, and it is difficult to detect a modulation point because a different path wave is likely to overlap a direct wave. A problem has arisen that the difficulty of measuring the propagation time becomes more apparent.

Further, when receiving such a high-speed sound wave, the switching noise of the controller and the reverberation of the ultrasonic wave generated at the time of transmission of the ultrasonic wave also overlap with the received wave returned from the reflection surface, and There was a problem that was hindered. The present invention has been made to solve the above problems, and has an object to accurately measure a propagation time of a straight wave and to be used for a specific gas such as a purge gas or a fuel cell. An object of the present invention is to provide a gas concentration sensor capable of measuring a gas concentration of a specific gas or the like with high accuracy.

[0012]

According to the first aspect of the present invention, there is provided a measuring chamber having an inflow hole and an outflow hole through which a gas to be measured flows in and out, facing each other in the measuring chamber. An ultrasonic element that is provided on one of the two wall surfaces to transmit the ultrasonic wave toward the other wall surface and that can receive a reflected wave of the ultrasonic wave reflected on the wall surface as a reflection surface, The ultrasonic element transmits an ultrasonic wave having at least one modulation point and receives the reflected wave, and uses the modulation point to calculate a propagation time from the transmission of the ultrasonic wave to the reception of the reflected wave. And a gas concentration detecting means for detecting a gas concentration of the specific gas in the gas to be measured based on the propagation time. And the distance between the ultrasonic element and And gist of the gas concentration sensor, characterized in that formed so as to be longer than the distance between the central portion and the ultrasound element of the reflecting surface.

Thus, in the gas concentration sensor according to the first aspect, the distance between the edge of the reflection surface and the ultrasonic element is longer than the distance between the center of the reflection surface and the ultrasonic element. Accordingly, the propagation distance of the different path wave propagating along the side wall of the measurement chamber becomes longer, so that the different path wave can be prevented from being synthesized near the modulation point of the direct wave.

In other words, since the modulation point of the direct wave can be detected accurately, the propagation time can be measured accurately, and the gas concentration can be measured with high accuracy. Then, by precisely adjusting the gas concentration based on the measurement result with high accuracy, for example, air-fuel ratio control or the like can be suitably performed.

According to a second aspect of the present invention, there is provided a measurement chamber provided with an inflow hole and an outflow hole through which a gas to be measured flows in and out, and is provided on one of two opposite wall surfaces in the measurement chamber. An ultrasonic element that transmits an ultrasonic wave toward the other wall surface and that can receive a reflected wave of the ultrasonic wave reflected by the wall surface as a reflecting surface, and at least one modulation point for the ultrasonic element. And transmitting the ultrasonic wave having the received reflected wave, measuring the propagation time from the transmission of the ultrasonic wave to the reception of the reflected wave using the modulation point, based on the propagation time, Gas concentration detection means for detecting the gas concentration of the specific gas in the gas to be measured, in a gas concentration sensor, the area of the reflection surface of the measurement chamber,
The gas concentration sensor is characterized in that the area is larger than the area of the opening surface of the ultrasonic element.

As described above, in the gas concentration sensor according to the second aspect, the area of the reflection surface of the measurement chamber is at least equal to the area of the opening surface of the ultrasonic element (more specifically, the area of a portion for transmitting and receiving ultrasonic waves). Equal, preferably larger. Therefore, among the ultrasonic components transmitted in parallel with the direct wave, there is no component incident on the side wall of the measurement chamber, so that there is no different-path wave component that propagates in a time very close to the propagation time of the direct wave. Therefore, the modulation point of the direct wave can be detected accurately, so that the propagation time can be measured accurately, and the gas concentration can be measured with high accuracy. Then, by precisely adjusting the gas concentration based on the measurement result with high accuracy, for example, air-fuel ratio control or the like can be suitably performed.

However, when the area of the reflection surface is equal to the area of the aperture surface of the ultrasonic element, for example, a different path wave incident on the side wall of the measurement chamber near the reflection surface may be a problem.
In other words, this component, after being incident on the side wall of the measurement chamber, propagates between the reflecting surface and the ultrasonic element along the side wall in parallel with the direct wave, so that the time is relatively close to the propagation time of the direct wave. It propagates.

Therefore, more preferably, the area of the reflection surface should be larger than the area of the aperture surface of the ultrasonic element. In this case, the different path wave incident on the side wall of the measurement chamber propagates along the side wall between the reflection surface and the ultrasonic element after being incident on the side wall of the measurement chamber. Therefore, the propagation time of the direct wave is sufficiently shorter than that of the different path wave, so that the modulation point of the direct wave can be detected more accurately, and the propagation time can be measured correctly.

The distance between the edge of the reflection surface and the ultrasonic element in the gas concentration sensor according to claim 2 is defined as claim 1.
If the distance is longer than the distance between the central portion of the reflection surface and the ultrasonic element as described above, the propagation time of the above-mentioned different path wave is further increased, and the detection accuracy of the modulation point of the direct wave can be further improved.

Further, according to a third aspect of the present invention, the gas concentration detecting means causes the ultrasonic element to transmit and receive an ultrasonic wave with at least one frequency modulation, and utilizes a modulation point of the ultrasonic wave. The gas concentration sensor according to claim 1 or 2, wherein the gas concentration of the specific gas is detected based on the obtained propagation time.

A gas concentration sensor according to a third aspect exemplifies the invention of the first or second aspect. For example, when an ultrasonic wave whose frequency is modulated only once from F1 to F2 is transmitted, the received wave reflects the frequency change. Therefore, for example, by measuring the time between frequency switching points in a direct wave (that is, each modulation point of a transmitted wave and a reflected wave),
The propagation time is known. That is, the propagation time of the ultrasonic wave can be accurately measured regardless of the strength of the signal.

According to a fourth aspect of the present invention, the gas concentration detecting means causes the ultrasonic element to transmit and receive an ultrasonic wave containing at least one antiphase component, and utilizes a modulation point of the ultrasonic wave. The gas concentration sensor according to claim 1 or 2, wherein the gas concentration of the specific gas is detected based on the propagation time obtained in the above.

A gas concentration sensor according to a fourth aspect exemplifies the invention of the first or second aspect. For example, 1
If an ultrasonic wave into which the opposite phase component (180 degrees) of the point is introduced is transmitted, no signal waveform appears at the opposite phase point of the transmission wave. The opposite phase point is also reflected on the reflected wave, which is a received wave, and a portion having no signal waveform appears. Therefore,
For example, the propagation time can be determined by measuring the time between the opposite phase points in the direct wave (that is, the point where there is no signal waveform of each of the transmitted wave and the reflected wave). That is, the propagation time of the ultrasonic wave can be accurately measured regardless of the strength of the signal.

The modulation points to be introduced into the transmission wave are:
It may be a switching point of phase by general phase modulation.
In other words, at least one phase switching point (for example, when the phase is changed from θ degrees to (θ
+)), The phase change point is reflected in the received wave. So, for example,
The propagation time can be determined by measuring the time between the phase switching points of the direct wave (that is, the phase switching points of the transmitted wave and the reflected wave). That is, the propagation time of the ultrasonic wave can be accurately measured regardless of the strength of the signal.

According to a fifth aspect of the present invention, the gas concentration detecting means transmits an ultrasonic wave from the ultrasonic element, reflects the ultrasonic wave on the reflecting surface, and then transmits the ultrasonic wave to the ultrasonic element side. And reflects the reflected gas again on the reflecting surface, measures the propagation time until the reception of the reflected wave after the first reflected wave, and detects the gas concentration of the specific gas based on the propagation time. The gist of the present invention is a gas concentration sensor according to any one of claims 1 to 4.

According to a fifth aspect of the present invention, in the gas concentration sensor according to any one of the first to fourth aspects, the ultrasonic wave propagation time is measured with higher accuracy. For example, when the characteristics of, for example, the molding material of the ultrasonic element change due to aging or the like, the propagation time of the first reflected wave (first reflected wave) in the deteriorated product is equal to the propagation time of the new first reflected wave. If the gas concentration of the specific gas is measured based on the propagation time of the new first reflected wave, the gas concentration cannot be detected accurately. On the other hand, the reflected wave (for example, the second reflected wave) after the first reflected wave is simply reflected on the surface of the ultrasonic element and is not affected by the structure inside the element, and therefore, as shown in FIG. Is deteriorated, the fluctuation of the propagation time is small, and the influence of the deterioration is small.

Therefore, in the gas concentration sensor according to the fifth aspect, not the first reflected wave (the first reflected wave) which is susceptible to the deterioration but the reflected waves after the second reflected wave which are not easily affected by the deterioration. The gas concentration of the specific gas is detected based on the propagation time. Thus, the gas concentration can be measured with high accuracy. Therefore, by precisely adjusting the gas concentration based on the measurement result with high accuracy, for example, air-fuel ratio control can be suitably performed.

According to a sixth aspect of the invention, there is provided a gas concentration sensor according to any one of the first to fifth aspects, wherein the specific gas is fuel vapor of an engine for an internal combustion engine. . The gas concentration sensor according to claim 6,
FIG. 3 illustrates the type of a specific gas to be measured by a gas concentration sensor. Here, as the specific gas, a fuel vapor such as a purge gas is measured. Thereby, the gas concentration of the fuel gas can be accurately measured, so that the air-fuel ratio control or the like can be suitably performed.

As shown in FIG. 21, the received waveform includes not only a direct wave and a different path wave but also switching noise of a controller and reverberation of an ultrasonic wave generated at the time of transmission. appear. When the speed of sound is sufficiently low as shown in FIG. 21A, these waveform components appear at regular intervals, so that the position of each waveform can be clearly grasped. Therefore, it is possible to accurately measure the propagation time based on the straight wave.

However, as shown in FIG. 21 (b), when the speed of sound is particularly high, a direct wave is received before the reverberation component is sufficiently converged. It may overlap with the component. Further, since the difference in propagation time between the direct wave and the different path wave cannot be sufficiently obtained, the different path wave may overlap the latter half of the direct wave. In such a case, it is difficult to detect the waveform position of the direct wave corresponding to the specific waveform position of the transmission wave, so that it is difficult to accurately measure the propagation time.

Therefore, as a specific configuration for solving such a problem, the invention described in claim 7 is based on a measuring chamber provided with an inflow hole and an outflow hole through which a gas to be measured flows in and out; Provided on one of the two wall surfaces facing each other in the room, transmitting ultrasonic waves toward the other wall surface and receiving reflected ultrasonic waves reflected from the other wall surface as a reflection surface The ultrasonic element, the ultrasonic element, to transmit the ultrasonic wave and receive the reflected wave, the propagation time from the transmission of the ultrasonic wave to the reception of the reflected wave, the waveform of the ultrasonic wave In the gas concentration sensor comprising: measurement by detecting a specific position of, and based on the propagation time, gas concentration detection means for detecting the gas concentration of the specific gas in the measured gas, the measurement chamber, Center and front of reflective surface The distance between the ultrasonic element is summarized as gas concentration sensor, characterized in that was formed to have a higher 60 mm.

The invention according to claim 7 particularly assumes a case where the concentration of a gas component having a relatively small molecular weight is measured by an ultrasonic sensor. In other words, the distance between the ultrasonic element and the reflecting surface is specifically defined so that the gas concentration can be effectively detected even when the sound speed becomes extremely large when measuring the gas concentration. It is.

The distance of 60 mm is obtained from the experimental result described later. When the distance is defined as described above, the propagation time of the reflected wave can be sufficiently taken, and the reverberation of the transmitted wave can be obtained. After the components converge, the reflected wave reaches the ultrasonic element. As a result, it is possible to avoid interference with the received wave due to the reverberation component overlapping the reflected wave, and it is possible to accurately detect the arrival of the reflected wave. This is particularly convenient when measuring the propagation time with reference to the head position of the direct wave.

However, by simply defining the distance between the central portion of the reflecting surface and the ultrasonic element as described above, the problem of the overlap between the reflected wave (straight wave) and the reverberation component can be solved, but it is different from the straight wave. The problem of overlap with path waves cannot be solved. This is not a problem when the propagation time is measured with reference to the head position of the direct wave, but when the measurement is performed with reference to the modulation point of the direct wave, the modulation point can be accurately determined due to the overlap of different path waves. This is a problem because it cannot be detected.

That is, when the speed of sound is sufficiently low as shown in FIG. 22A, each waveform component appears at a fixed interval, so that the waveform position can be clearly grasped. Therefore, the propagation time can be accurately measured. In this case, since the distance between the central portion of the reflecting surface and the ultrasonic element is sufficiently secured according to the configuration described in claim 7, the propagation time is longer than that in FIG.

However, in the case where the speed of sound is particularly high as shown in FIG. 22B, even if the problem that the first half of the direct wave overlaps with the reverberation component is solved by the configuration according to the seventh aspect. Since the difference between the propagation times of the direct wave and the different path wave does not change at all, the problem that the different path wave overlaps the latter half of the direct wave is not solved. Therefore, when measuring the propagation time with reference to the modulation point of the direct wave, it is difficult to detect the position of the direct wave corresponding to the specific waveform position of the transmission wave, and it is difficult to measure the propagation time. Becomes

According to an eighth aspect of the present invention, in the measurement chamber, a concave portion having a bottom surface parallel to the reflective surface is provided at an edge of the reflective surface, and the bottom surface of the concave portion and the ultrasonic element are connected to each other. The gas concentration sensor according to any one of claims 1 to 7, wherein the distance is set to be 18 mm or more longer than the distance between the center of the reflection surface and the ultrasonic element. I do.

According to the present invention, the distance between the bottom surface of the concave portion formed on the reflection surface and the ultrasonic element is specifically defined. In this case, even when the speed of sound becomes extremely large, a configuration capable of effectively detecting the gas concentration is specifically shown.

The distance of 18 mm is also obtained from the experimental result described later. When the distance is defined in this manner, a sufficient difference between the propagation distance of the direct wave and the propagation distance of the different path wave can be obtained. Therefore, the different path wave arrives after the arrival of the straight wave. As a result, it is possible to avoid the interference of the received wave due to the overlapping of the different path wave and the straight wave, and it is possible to accurately detect the arrival of the reflected wave.

FIG. 17 shows a reception waveform when the configuration specified by claims 7 and 8 is adopted. In this way, even when the sound speed is high, each waveform component appears at a fixed interval, so that the position of each waveform can be clearly grasped. Therefore, it is possible to accurately measure the propagation time based on the straight wave.

According to a ninth aspect of the present invention, the specific gas is:
A gas concentration sensor according to any one of claims 1 to 8, wherein the gas concentration sensor is hydrogen gas. The gas concentration sensor according to the ninth aspect indicates the type of the specific gas to be measured by the gas concentration sensor. In particular, the gas concentration sensors according to the seventh and eighth aspects can remarkably exhibit their effectiveness. This is shown as a specific gas. Hydrogen gas is one of the gases whose molecular weight is small and the sound speed at the time of measuring the gas concentration becomes extremely large as the mixing ratio increases. A fuel cell or the like may be used as a measurement object of the hydrogen gas.

[0042]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment (embodiment) of a gas concentration sensor according to the present invention will be described below with reference to the drawings. (Embodiment 1) In this embodiment, a gas concentration sensor using an ultrasonic wave is used to measure the gas concentration of evaporated fuel.

First, the system configuration in this embodiment will be described. FIG. 1 is a system configuration diagram including a gas concentration sensor. As shown in FIG. 1, in the present embodiment, a throttle valve 3 for adjusting an intake air amount, a fourth gas concentration sensor 24 for detecting a gas concentration of purge gas, Injector 6 for injecting fuel
Is arranged.

On the other hand, an oxygen sensor (all-area air-fuel ratio sensor) 8 for detecting the oxygen concentration in the exhaust gas and a three-way catalyst 9 for purifying the exhaust gas are arranged in the exhaust pipe 7 of the engine 1 from the upstream side. . Further, as a path for supplying fuel to the engine 1, a first supply system for supplying liquid fuel and a second supply system for supplying gaseous fuel are provided.

As the first supply system, a gasoline tank 11 is provided via a first supply path 12 and a fuel pump 15.
It is connected to the injector 6. Therefore, fuel is supplied from the gasoline tank 11 to the injector 6 via the first supply path 12 by the fuel pump 15, and is injected from the injector 6 into the intake pipe 2.

On the other hand, as a second supply system, the gasoline tank 11 is connected to a canister 14 via a second supply path 13, and the canister 14 is connected to a throttle valve 3 via a third supply path 16 and a purge valve 17. And the fourth gas concentration sensor 24 is connected to the intake pipe 2.

In this embodiment, the second supply path 1
3, a first gas concentration sensor 21, a second gas concentration sensor 22, and a third gas concentration sensor 23 for detecting the gas concentration of the evaporated fuel, respectively, in the canister 14, and in the third supply path 16 between the canister 14 and the purge valve 17. Is arranged.
Any one of the first to third gas concentration sensors 21 to 23 may be arranged. In addition, the fuel purged (discharged by evaporation) from the canister 14 is referred to as purge gas.

Therefore, the fuel evaporated from the gasoline tank 11 is once adsorbed by the canister 14, and the outside air is appropriately introduced into the canister 14 to purge the fuel. Then, the evaporated fuel (purge gas) generated by the purge is supplied to the intake pipe 2 between the throttle valve 3 and the fourth gas concentration sensor 24 after the gas flow rate is adjusted by the purge valve 17.

The system includes an electronic control unit (EC) for controlling the supply of the purge gas and controlling the air-fuel ratio.
U) 26. The ECU 26 includes first to first
Fourth gas concentration sensors 21 to 24 (hereinafter gas concentration sensor 2)
5), signals from various sensors such as an oxygen sensor 8 and an air flow meter 10, and outputs control signals to various actuators such as a purge valve 17, a throttle valve 3, and an injector 6. In addition, EC
U26 also turns on the gas concentration sensor 25.
It also outputs a control signal such as OFF.

Next, the structure and the basic principle of the gas concentration sensor 25 of this embodiment will be described. First, the structure of the gas concentration sensor 25 will be described. The gas concentration sensor 25 of the present embodiment is an ultrasonic gas concentration sensor that generates an ultrasonic wave using a piezoelectric element, and in particular, an ultrasonic transmission / reception element (element ASSY) for both transmitting and receiving ultrasonic waves. Is used.

More specifically, as shown in FIG. 2, the gas concentration sensor 25 includes a driving / calculating circuit 31, a measuring chamber 32 into which intake air containing fuel vapor is introduced, and an opposing inside of the measuring chamber 32. An ultrasonic transmitting / receiving element (hereinafter, also simply referred to as an ultrasonic element) 33 provided at one of the two end portions to be measured;
In order to reflect the ultrasonic wave inside, the reflection surface 34 provided at a predetermined distance L at the other end opposite to the end where the ultrasonic element 33 is provided, and the intake air flows in and out. An inlet 36 and an outlet 37 (also referred to as an inlet 36 and an outlet 37 in the figure). A concave portion 34 a is formed at an edge portion of the reflection surface 34 that is in contact with the side wall of the measurement chamber 32 so that the bottom surface is substantially parallel to the reflection surface 34.
Therefore, the distance between the bottom surface of the concave portion 34 a and the ultrasonic element 33 is longer by L ′ than the distance between the central portion of the reflection surface 34 and the ultrasonic element 33.

The ultrasonic element 33 includes a piezoelectric element 38 and a matching layer 39 adhered to the end face of the piezoelectric element 38 on the measurement chamber 32 side.
An output extraction lead 41 extracted from the piezoelectric element 38 so as to extract a sensor output from the piezoelectric element 38,
Piezoelectric element 38, matching layer 39 and output lead 41
And an element case 43 in which the end on the side of the piezoelectric element 38 is fixed by a molding material 42 inside. The matching layer 39
The end face on the measurement chamber 32 side is the measurement chamber 32 of the element case 41.
It is arranged so as to substantially coincide with the end face on the side. A resin thin film having excellent oil resistance and heat resistance is bonded to the matching layer 39 and the end face of the element case 43 on the measurement chamber 32 side.

Next, the configuration of the driving / calculating circuit 31 of the gas concentration sensor 25 corresponding to the gas concentration detecting means of the present invention will be described. As shown in the block diagram of FIG. 3, a CPU (microcomputer) 51 is used for driving and calculating the gas concentration sensor 25. Transmission and reception of ultrasonic waves are switched using transmission / reception switches (SW) 52a and 52b.

At the time of transmission, a voltage is applied to the ultrasonic element 33 using a driver to transmit the ultrasonic wave. On the other hand, at the time of reception, a reception waveform obtained by the ultrasonic element 33 is subjected to predetermined amplification by an amplifier (amplification AMP) 53, and a waveform signal shaped through a comparator 54 is introduced into the CPU 51. The CPU 51 measures the propagation time using a timer, converts the measurement result into a density by referring to a map, and outputs the density to, for example, a display device (not shown).

Next, the basic principle of the gas concentration sensor 25 will be described. Although the transmitting unit 25b and the receiving unit 25a are shown separately in FIG. 4 for the purpose of explanation, in the present embodiment, an element that is used for both transmission and reception is used. As shown in FIG. 4, when performing concentration measurement using the gas concentration sensor 25, an ultrasonic wave is transmitted from the transmission unit 25b, and the ultrasonic wave is received by the reception unit 25a. At this time, there is a shift in the propagation time between the transmission waveform and the reception waveform, for example, according to the gas concentration of the purge gas in the intake air. For example, FIG.
As shown in FIG. 4A, when the gas concentration of the purge gas is low, the propagation time T1, which is the difference between the transmission waveform and the reception waveform, is small. On the other hand, as shown in FIG. Is higher, the propagation time T2 is longer. Therefore, by extracting the sensor output corresponding to the propagation time, the gas concentration can be detected.

For example, when measurement is performed using butane, which is a main component of the evaporated fuel, there is a substantially proportional relationship between the sensor output and the gas concentration of butane, as shown in FIG.
Therefore, if a sensor output is obtained, the concentration of the purge gas can be detected from the sensor output.

Next, a method of actually measuring the gas concentration of the evaporated fuel (purge gas) using the above-described basic principle by the gas concentration sensor 25 will be described. First, an example of an arithmetic expression for calculating the gas concentration from the ultrasonic wave propagation time will be described.

In the present embodiment, as shown in FIG. 6A, transmission is performed by including two types of frequency components F1 and F2 in an ultrasonic transmission wave. That is, the transmission frequency is changed from F1 to F2.
Is modulated. In this case, since the frequency change is reflected in the received wave, the time at which the modulation point appears in the received wave is defined as the arrival time. That is, by measuring the time between the frequency switching points (for example, each modulation point of the transmitted wave and the reflected wave), the propagation time can be determined.

After measuring the propagation time in this way, the sound speed C of the ultrasonic wave indicating the gas concentration is calculated by the following equation (1).
Is calculated from That is, as shown in FIG. 6B, since the distance L between the outer surface of the ultrasonic element 33 (the surface separated from the matching layer 39 and the resin thin film) and the reflection surface 34 is known,
The propagation time T, which is the time for one round trip of the distance L, is measured, and the sound velocity C is calculated by applying the distance L and the propagation time T to the following equation (1).

C = 2L (reciprocating distance from the element surface to the reflection surface) / T (propagation time) (1) Then, for example, the gas concentration of the evaporated fuel is determined using the gas concentration Xk of butane, which is the main component of the evaporated fuel. When measuring
Using the relationship of the following equation (2), the sound velocity C obtained by the above equation (1) is converted into the gas concentration of the evaporated fuel (that is, the gas concentration of butane Xk).

[0061]

(Equation 1)

In the equation (2), R is a gas constant, Tg is the temperature of the intake air containing the evaporated fuel, Cpn is the specific heat of the n-th component gas contained in the intake air, and Cvn is the n-th component. Is the specific heat of the gas, Mn is the molecular weight of the n-th component gas, X
n represents the mixture ratio of the gas of the n-th component (in other words, the gas concentration of the gas of the n-th component).

Then, from the relationship of the above equation (2), assuming, for example, the types of gas components other than butane contained in the intake air through which the ultrasonic wave propagates and the mixing ratio thereof, the propagation time T and the intake air temperature The gas concentration Xk of butane can be measured based on Tg. In this case, if the intake air temperature Tg is constant, between the sensor output corresponding to the sound speed C and the butane gas concentration Xk, as described above, as shown in FIG.
There is a proportional relationship as shown in the following, and this can be used as a map. Therefore, by extracting a value corresponding to the sound velocity C as a sensor output (voltage), the value shown in FIG.
The gas concentration can be obtained from the map shown in FIG.

The determination of the modulation point can be easily performed because the concave portion 34a is provided at the edge of the reflection surface 34. FIG. 7 shows a case where the reflecting surface 34 has a concave portion 34a.
3 shows a difference between the reflected waves received by the ultrasonic element 33 when the reflection surface 34 is a flat surface.

In FIG. 7, the path 1 includes the ultrasonic element 33
Component having the highest sound pressure (sensitivity) and the shortest propagation time among the ultrasonic waves transmitted from the device (that is, a component that follows the shortest path between the ultrasonic element and the reflection surface, and is hereinafter referred to as a direct wave).
Route. The path 2 is a component having a relatively low sound pressure (sensitivity) and a long propagation time (i.e., a component propagating through a different path; ).

When obliquely incident on a substance having a different acoustic impedance due to the characteristics of sound waves, the boundary surface (the boundary surface between the gas and the solid, in this case, the side wall of the measuring chamber 32).
It is generally known that there is a component that propagates, and path 2 indicates a path followed by such a component.

That is, the object to be measured is a component whose propagation distance is 2L, that is, the propagation time of a direct wave. In the case of a conventional gas concentration sensor having a flat reflecting surface (FIG. 7B), a different-path wave propagating slightly later than a direct wave (especially, near the reflecting surface 34 after transmission from the ultrasonic element 33). The light enters the side wall of the measurement chamber 32, propagates along the side wall of the measurement chamber 32 to the reflection surface 34, is reflected by the reflection surface 34, and reaches the ultrasonic element 33 again along the side wall of the measurement chamber 32. Component) is synthesized near the modulation point of the direct wave, so that it is difficult to detect the modulation point of the direct wave, and the measurement of the propagation time T cannot be performed accurately.

On the other hand, when the reflecting surface 34 has the concave portion 34a (FIG. 7A), the reflecting surface 34 is flat compared to the case where the reflecting surface 34 is flat.
The different path wave propagates later than the direct wave. That is, since the different path wave in this case is reflected on the bottom surface of the concave portion 34a whose distance to the ultrasonic element 33 is longer than the central part of the reflection surface 34 and propagates, the different path wave becomes a different path wave when the reflection surface 34 is flat. compared,
It propagates a long distance by 2L '. Accordingly, the different path wave in this case is not synthesized near the modulation point of the direct wave, and the detection of the modulation point of the direct wave can be performed accurately. Therefore, it is possible to measure the propagation time T accurately. it can. That is, since the accurate sound velocity C is obtained by the above equation (1), the concentration of the evaporated fuel can be measured with high accuracy.

The actual detection of the reflected wave is performed as described below in order to further improve the measurement accuracy. FIG.
FIG. 3B is a diagram illustrating transmission / reception waveforms in the ultrasonic element 33. Note that, here, only the waveform of the direct wave to be detected is shown for explanation of the detection method.

First, when a transmission wave whose frequency is modulated is transmitted from the ultrasonic element 33, the transmission wave is reflected by the reflection surface 34 and detected as a reflected wave (first reflected wave) by the ultrasonic element 33. You. The first reflected wave is reflected on the surface of the ultrasonic element 33, is reflected again on the reflecting surface 34, and is detected again by the ultrasonic element 33 as a reflected wave (second reflected wave) of the reflected wave. . Hereinafter, similar reflection is repeated, but as the propagation distance increases, the reflected wave gradually attenuates.

Thereafter, when a predetermined time elapses after the first transmission wave is output, the transmission / reception switches 52a and 52b are switched to transmit the next transmission wave, and FIG.
As shown in (a), the next transmission wave is transmitted, and thereafter, the same processing is repeated. At this time, since the microcomputer input waveform (that is, the comparator output) is in the state shown in FIG. 8C, the time between the modulation points of the frequency is measured. That is, the received wave is converted into a digital signal (high or low 2) by a comparator based on a predetermined threshold level.
After converting the signal into a value signal, the signal is input to a microcomputer and the rise and fall times of the digital signal are measured by an internal timer or the like, so that the modulation point can be determined. Therefore, the time between each modulation point can be obtained.

More specifically, first, a first arrival time (therefore, a first propagation time) T1 from the modulation point of the transmission wave to the modulation point of the first reflection wave is measured, and the second arrival time T2 from the modulation point of the transmission wave is measured. The second arrival time T3 up to the modulation point of the reflected wave is measured.
Then, the first arrival time T1 is subtracted from the second arrival time T3 to determine the propagation time of the second reflected wave (second propagation time T2).

Therefore, in this embodiment, the gas concentration is detected by using the second propagation time T2 obtained as described above, for the following reason. For example, the first propagation time T1 fluctuates due to deterioration with time of the molding material 42 of the ultrasonic element 33, and the like. In other words, as a cause of the deviation of the first propagation time T1, the inertia of the piezoelectric element 38 changes when the molding material 42 is hardened or absorbs and absorbs water, thereby affecting the amplitude (sensitivity) of the received waveform. In addition to this, it is conceivable that a shift of the modulation point is involved.

That is, as shown in FIG. 9, for example, comparing a sensor (OLD) with deterioration with time and a new sensor (NEW), the OLD sensor shows that the first reflected wave
There is a change that the number of peaks increases and the amplitude decreases. As a result, the first propagation time T1 ′ of the OLD becomes longer than the first propagation time T1 of the NEW sensor. However, the second reflected wave has the same tendency, and the reflected wave simply reflected on the surface of the element 33 is simply reflected on the reflecting surface 34.
It is not affected by aging, and therefore, the second propagation time T2 of the NEW sensor and the second propagation time T2 ′ of the OLD sensor
Is the same as

Therefore, if the second propagation time is used as the propagation time used when calculating the sound velocity C, the sound velocity C can always be measured correctly because it is not affected by deterioration over time. This is because the second
If the propagation time is measured, the modulation points of the first reflected wave and the second reflected wave also shift, so that the shift of the modulation point due to the change with time can be canceled, and the propagation time can be measured correctly regardless of the change with time. Because it becomes.

Therefore, in this embodiment, the sound velocity C is calculated using the above-described second propagation time, a sensor output corresponding to the sound velocity C is obtained, and the sensor output is applied to a map as shown in FIG. It detects the gas concentration. As described above, in the gas concentration sensor 25 of the present embodiment,
Since the concave portion 34a is formed at the edge of the reflecting surface 34,
As compared with the case where the reflection surface 34 is a flat surface, the modulation point of the direct wave can be detected more accurately, and the propagation time T can be measured more accurately.

Since the second propagation time is used when calculating the sound velocity C, the correct sound velocity C can be always measured without being affected by the deterioration with time. In the above embodiment, the first arrival time T1 is subtracted from the second arrival time T3 to calculate the propagation time (second propagation time T2) of the second reflected wave. However, the (n + 1) th arrival time Tn +
The propagation time of the (n + 1) th reflected wave (the (n + 1) th propagation time Tn + 1) may be obtained by subtracting the (n) th arrival time Tn + 1 from 2 (n is an integer of 2 or more). However, as the propagation distance becomes longer, the reflected wave gradually attenuates, so that the measuring accuracy decreases as the number of reflections increases.

The time from the detection of a certain modulation point to the detection of the next modulation point (for example, the time from the modulation point of the first reflected wave to the modulation point of the second reflected wave) is directly measured. In this case, the same result as in the above embodiment can be obtained. Further, the waveform at the modulation point may lack reproducibility depending on the setting of the threshold level of the comparator 54. Therefore, the waveform at the modulation point may be used as a reference, for example, by detecting the waveform of a certain peak before the modulation point. You may use as.

Further, in the above embodiment, the frequency of the transmission wave is frequency-modulated only once from F1 to F2, but it goes without saying that the transmission wave may be accompanied by two or more frequency modulations. Further, in the above embodiment, the concave portion 34 a having the bottom surface substantially parallel to the reflective surface 34 is provided at the edge of the reflective surface 34, but the distance between the edge of the reflective surface 34 and the ultrasonic element 33 is reduced. It is needless to say that the same effect as in the above embodiment can be obtained as long as the reflecting surface 34 is configured so as to be longer than the distance between the central portion and the ultrasonic element 33.

FIG. 10 shows, as an example, FIG.
A concave portion 34b having a different shape from the concave portion 34a shown in FIG. As shown in FIG. 10, the concave portion 34b is formed with a curved surface from the surface near the edge of the reflection surface 34. In the concave portion 34b, where the ultrasonic element 3
3 is the longest. Also in this case, similarly to the above embodiment, since the different path wave following the path 2 propagates sufficiently later than the direct wave following the path 1, it is possible to accurately detect the modulation point of the direct wave. It is possible to accurately measure the propagation time T. (Embodiment 2) Next, Embodiment 2 will be described.

The description of the same parts as in the first embodiment is as follows.
Omitted or simplified. As shown in FIG. 11A, the present embodiment is characterized in that the area of the reflecting surface 34 is larger than the area of the opening surface of the ultrasonic element 33 (specifically, the area of a part that transmits and receives ultrasonic waves). And

First, as shown in FIG.
When the area of the ultrasonic wave element 4 is smaller than the area of the aperture surface of the ultrasonic element 33, the ultrasonic wave component transmitted in parallel with the direct wave following the path 1 is incident on the side wall of the measurement chamber 32 near the reflection surface 34. Different path waves also exist. During this different path wave, the measurement room 32
Incident on the side wall, propagates along the side wall to the reflecting surface 34, is reflected by the reflecting surface 34, and then reaches the ultrasonic element 33 following a path parallel to the direct wave (that is, the path). 3). The propagation time of such components is very close to the propagation time of a straight wave. Therefore, the different-path wave components are synthesized near the modulation point of the direct wave, which makes it difficult to detect the modulation point of the direct wave, so that the propagation time T cannot be measured accurately.

On the other hand, the area of the reflecting surface 34 is
Assuming that the area is equal to the area of the opening surface of No. 3, there is no component incident on the side wall of the measurement chamber 32 among the ultrasonic components transmitted in parallel with the direct wave. In other words, there is no component that propagates in a time very close to the propagation time of the direct wave, unlike the different-path wave that follows the above-described path 3, so that the modulation point of the direct wave can be detected accurately, and accurate propagation Time T can be measured.

However, in this case, for example, the reflecting surface 34
There is a possibility that a different-path wave incident on the side wall of the measurement chamber 32 in the vicinity may be synthesized in the vicinity of the modulation point of the direct wave. That is, the different path wave is incident on the side wall of the measurement chamber 32 in the vicinity of the reflection surface 34, and then propagates along the side wall to the reflection surface 34.
Then, after being reflected by the reflection surface 34, the ultrasonic element 33 follows a path parallel to the straight wave along the side wall of the measurement chamber 32.
Therefore, it propagates at a time relatively close to the propagation time of a straight wave.

Therefore, it is more preferable that the area of the reflecting surface 34 be larger than the area of the opening surface of the ultrasonic element 33. In this case, as shown in FIG.
The different path wave incident on the side wall of No. 2 follows the path 2. That is,
The light enters the side wall of the measurement chamber 32 and is reflected along the side wall.
4, and after being reflected by the reflection surface 34, propagates again along the side wall of the measurement chamber 32 until reaching the ultrasonic element 33. In other words, compared to such a different path wave,
Since the propagation time of the direct wave is sufficiently short, the modulation point of the direct wave can be detected more accurately, and the measurement of the propagation time T can be performed correctly.

If the concave portion 34a (34b) shown in the first embodiment is formed at the edge of the reflecting surface 34 in this case, the propagation time of the above-mentioned different path wave is further increased, and the modulation point of the direct wave is changed. The detection accuracy can be further improved. Third Embodiment Next, a third embodiment will be described.

The description of the same parts as in the first embodiment is as follows.
Omitted or simplified. In the present embodiment, it is assumed that the concentration of a gas component having a small molecular weight such as hydrogen gas is measured by an ultrasonic sensor. In other words, even in the case where the sound speed becomes extremely large when measuring the gas concentration, the superconductivity shown in FIGS. 6 and 7 in the first embodiment can be detected so that the gas concentration can be effectively detected. The distance corresponding to the distance L between the outer surface of the acoustic element 33 and the reflecting surface 34 and the distance corresponding to the distance L 'between the bottom surface of the concave portion 34a formed in the reflecting surface 34 and the outer surface of the ultrasonic element 33 are defined. It is characterized by having done.

First, the structure of the gas concentration sensor 325 employed in this embodiment will be described. The gas concentration sensor 325 of the present embodiment uses the same ultrasonic transmission / reception element (element ASSY) as that of the first embodiment, and specifically has a structure as shown in FIG.

The sensor case 331 which is the main body of the gas concentration sensor 325 has an integral structure made of metal or resin. The sensor case 331 includes the drive / arithmetic circuit 3
Circuit board enclosing part 333 in which the inlet air 32 is provided, and an inlet hole 3 formed into an elongated shape having a diameter (φ) of 12 mm and allowing the intake air containing hydrogen gas to flow into and out of the side wall along the longitudinal direction.
34a and an outflow hole 334b, and an ultrasonic transmitting / receiving element (hereinafter simply referred to as “ultrasonic”) provided on one of two wall surfaces facing each other along the longitudinal direction of the measuring chamber 334 in the measuring chamber 334. 335);
The other wall surface facing the wall surface on which the ultrasonic element 335 is provided in the measurement chamber 334, and is a predetermined distance L3 away from the ultrasonic element 335, and is a reflecting surface that reflects ultrasonic waves transmitted from the ultrasonic element 335. 336 and the inflow hole 33
4a, an inflow passage 337 that allows the intake air to flow into the measurement chamber 334 from outside the sensor case 331, and an outflow passage 338 that is connected to the outflow hole 334b, and allows the intake air to flow out of the measurement chamber 334 to the outside of the sensor case 331. And When the gas concentration sensor 325 is actually arranged, the longitudinal direction of the measurement chamber 334 is substantially parallel to the horizontal direction, and the inflow hole 334a and the outflow hole 334b are provided.
Is in a state of being downward in the measurement chamber 334 (that is, FIG.
, The g direction is substantially below).
A concave portion 336 a is formed at an edge portion of the reflection surface 336 which is in contact with the side wall of the measurement chamber 334 so that a bottom surface thereof is substantially parallel to the reflection surface 336, and among the reflected waves of the ultrasonic waves transmitted from the ultrasonic element 335. The different path wave is generated by the concave portion 336a.
Is configured to be reflected by the bottom surface of the.

In this embodiment, when the concentration of a gas component having a particularly small molecular weight, such as hydrogen gas, is measured by an ultrasonic sensor, ie, when the gas concentration is measured, the speed of sound becomes extremely large. Even in such a case, the distance L between the outer surface of the ultrasonic element 335 and the central portion of the reflection surface 336 is set so that the gas concentration can be effectively detected.
3 (hereinafter, also simply referred to as “distance L3”) is set to 60 mm or more, and a distance L3 ′ between the bottom surface of the concave portion 336a formed on the reflection surface 336 and the outer surface of the ultrasonic element 335 (hereinafter, simply referred to as “distance L3 ′”). ) Is configured to be 18 mm or more (that is, the bottom surface of the concave portion 336a and the ultrasonic element 335).
Distance between the center of the reflection surface 336 and the ultrasonic element 3
It is configured to be 18 mm or more longer than the distance to 35).

The inflow hole 33 in the measurement chamber 334
A concave portion 341 is provided on a side wall between 4a and the outflow hole 334b. The concave portion 341 is formed in a perfect circular shape having a diameter of 9 mm, and a temperature sensing element 342 such as a thermistor is provided at a central portion thereof for measuring the temperature of the intake air in the measurement chamber 334. Thus, the temperature sensing element 342
Is disposed in the concave portion 341 provided on the side wall in the measurement chamber 334, the temperature-sensitive element 342 does not hinder the propagation of the ultrasonic wave in the measurement chamber 334, and the ultrasonic wave propagation time T
There is no measurement error in the measurement of.

The circuit board enclosing section 333 is provided with a circuit cover 343 after the drive / arithmetic circuit 332 is installed. Next, the configuration of the driving / arithmetic circuit 332 will be described.
As shown in the block diagram of FIG.
The microprocessor 351 is used for the driving and the calculation of 5.

First, during transmission of ultrasonic waves, the driver 35
2, a voltage is applied to the ultrasonic element 335 to transmit an ultrasonic wave. At the time of receiving an ultrasonic wave, a reception waveform obtained by the ultrasonic element 335 is output to an amplifier (amplifier) 35.
The signal of the waveform subjected to predetermined amplification in 3 and shaped through the comparator 354 is introduced into the microprocessor 351. Then, the microprocessor 351 measures the propagation time from transmission to reception of the ultrasonic wave using the timer 355. On the other hand, the temperature information of the intake air in the measurement chamber 334 detected by the temperature sensing element 342 is:
It is introduced into the microprocessor 351 from the temperature sensing element 342 via the temperature detection circuit 356. Then, the microprocessor 351 performs an arithmetic process based on the propagation time and the intake air temperature with reference to the map as shown in the first embodiment, converts it into a gas concentration of hydrogen gas, and then converts the gas concentration into a D / A converter. Through 357, a detected value of the gas concentration is output.

Next, referring to FIGS. 15 to 20, the distance L3 between the outer surface of the ultrasonic element 335 and the central portion of the reflection surface 336 will be described.
An experimental example in which the influence on the accuracy of the gas concentration sensor when the distance L3 ′ between the bottom surface of the concave portion 336a formed in the reflection surface 336 and the outer surface of the ultrasonic element 335 is changed will be described.

In this experiment, an experimental apparatus as shown in FIG. 15 was used. And the measurement room 3 of the gas concentration sensor 325
A gas to be measured in which the concentration of hydrogen gas was set in advance was supplied into 34, and the concentration of this hydrogen gas was measured by a gas concentration sensor 325. In this experimental apparatus, first, as shown in FIG. 15, the nitrogen gas (N ₂ ) filled in the nitrogen gas tank 61, the hydrogen gas (H ₂ ) filled in the hydrogen gas tank 62, and the steam tank 63 were filled. Steam (H ₂ O) is supplied to the gas flow control device 65 via the first pipe 71, the second pipe 72, and the third pipe 73, respectively.
Allowed to flow in. In the gas flow control device 65,
The supply amounts of the nitrogen gas, the hydrogen gas, and the steam from the three tanks 71, 72, and 73 are controlled to have a predetermined mixing ratio. The mixed gas thus mixed is supplied to the gas flow control device 65 through the fourth pipe 74.
And the mixed gas is caused to flow into the measurement chamber 334 of the gas concentration sensor 325 through the inflow passage 337.
The liquid was allowed to flow out of the measurement chamber 334 via the outflow passage 338.

In this experiment, the mixed gas flowing into the measuring chamber 334 was used as a gas to be measured, and the gas concentration sensor 325 measured the propagation time of the ultrasonic wave and the temperature of the mixed gas. The gas concentration (Xk) was detected, and the results of these measurements were recorded using the recorder 68.

FIGS. 16A and 16B show an example of a received waveform of an ultrasonic wave. FIG. 16A shows that the distance L3 is 45 m.
FIG. 16B shows a reception waveform of a conventional gas concentration sensor in which the distance L3 ′ is set to 2 mm, and FIG.
Is 60 mm, and the distance L3 'is 18 mm. The measurement conditions are
The gas temperature is 100 ° C., and the mixing ratio of the gas is 58
% And water vapor to 24%, and the remainder was adjusted with nitrogen gas.

As can be seen from FIG. 16A, the distance L3
Is 45 mm and the distance L3 'is 2 mm, it can be seen that the reverberation component of the ultrasonic wave transmitted from the ultrasonic element 335 overlaps the first half of the reflected wave (straight wave). This indicates that when the arrival of the reflected wave is detected at the head of the reflected wave, it is difficult to detect the head.

Further, it can be seen that the different path waves overlap in the latter half of the straight wave among the reflected waves. This indicates that, when the arrival of the reflected wave is detected at the modulation point of the direct wave, it is difficult to detect the modulation point. Such a problem is considered to be caused by a high sound velocity because the measurement target gas is a hydrogen gas having a small molecular weight. This state is schematically shown in FIG. Corresponds to the state.

That is, the reverberation component and the reflected wave overlap because the sound speed of the ultrasonic wave is large and the propagation time of the ultrasonic wave (the ultrasonic wave is reflected from the reflecting surface 336 after being transmitted from the ultrasonic element 335, This is because it is not possible to take a sufficient amount of time until the ultrasonic wave is received by the ultrasonic element 335. Before the reverberation component of the ultrasonic wave transmitted from the ultrasonic element 335 converges, the reflected wave is generated by the ultrasonic element 335. It is thought that this is due to reaching.

Further, the direct wave and the different-path wave overlap because the sound speed of the ultrasonic wave is high and the distance L3 '(depth of the concave portion 336a) is not sufficient, so that the direct wave and the different-path wave overlap. This is considered to be because the time interval between the two waves cannot be sufficiently increased, and the different-path wave arrives before the direct wave reaches the ultrasonic element 335.

On the other hand, as shown in FIG.
When the distance L3 is 60 mm and the distance L3 'is 18 mm, the above problem does not occur. FIG. 17 schematically shows this state. That is, the reverberation component and the reflected wave (direct wave), and the reflected wave
Each is detected at regular time intervals.

The reason that the reverberation component does not overlap with the reflected wave is that the propagation distance of the reflected wave becomes longer because the distance L3 is set to 60 mm, and the reflected wave arrives after the reverberation component converges. As a result, the reverberation component does not overlap with the reflected wave, and particularly when the arrival of the reflected wave is detected at the head of the reflected wave, the arrival of the reflected wave can be accurately detected.

The reason that the direct wave and the different path wave do not overlap is that the distance L3 'is 18 mm, so that the difference between the direct wave propagation distance and the different path wave propagation distance increases, and the direct wave arrives. This is because a different path wave comes later.
As a result, the different path wave does not overlap the direct wave, and particularly when the arrival of the reflected wave is detected at the modulation point of the direct wave, the arrival can be detected accurately.

Next, FIGS. 18 to 20 show the results of measuring the output of the gas concentration sensor when the distance L3 and the distance L3 'are respectively changed. FIG. 18 shows the distance L3
Shows the sensor output (measured concentration) with respect to the amount of hydrogen input (set concentration) when is changed to three types of 45 mm, 50 mm, and 60 mm.

The gas mixture ratio in this experiment was:
The water vapor was set to 24%, the hydrogen gas was changed in the range of 0 to 70%, and the remainder was adjusted with nitrogen gas. In this case, the distance L3 'was set to 2 mm, and the measurement of the propagation time in this case was performed with reference to the head of the transmission wave and the reception wave (reflection wave).

As shown in FIG. 18, the distance L3 is 60 mm
In the case of ( ₁ ), the gas concentration sensor 325 outputs an accurate value with respect to the input amount of hydrogen gas (H ₂ ). That is, even if the sound speed increases by increasing the hydrogen gas concentration, the gas concentration sensor 325 accurately outputs the hydrogen gas concentration. On the other hand, the distance L3 is 45 mm
In the case of 50 mm and 50 mm, it is understood that if the hydrogen gas input amount exceeds 40%, the accuracy of the gas concentration sensor 325 deteriorates, and the hydrogen gas concentration cannot be detected accurately.

This is because the sound velocity increases as the concentration of the hydrogen gas having a small molecular weight increases, so that the distance L3
Is 45 mm and 50 mm, the propagation time of the ultrasonic wave cannot be sufficiently long, and the reverberation generated at the time of transmitting the ultrasonic wave and the switching noise of the controller overlap with the reflected wave returned from the reflecting surface, and the This is considered to be due to the fact that a strong reception is prevented.

On the other hand, when L3 is 60 mm,
It is considered that such a problem does not occur because the propagation time of the ultrasonic wave can be sufficiently set. In this experiment, even though the distance L3 ′ was set to 2 mm (less than 18 mm), an accurate detection value was obtained even in a high concentration range of H _{2 because} the measurement of the propagation time was performed at the modulation point. This is because the measurement is not performed as a reference but is performed with the head as a reference, so that a measurement problem does not occur even if the different path wave overlaps the latter half of the straight wave.

FIG. 19 shows that the distance L3 is constant at 60 mm and the distance L3 'is 2 mm, 9 mm, 18 mm, and 25 mm.
4 shows the measurement results of the sensor output with respect to the hydrogen gas input amount when the distance is changed to four types of mm. The gas mixture ratio in this experiment was the same as described above, with water vapor being 24%, hydrogen gas being changed in the range of 0 to 70%, and the remainder being adjusted with nitrogen gas. In this case, the measurement of the propagation time was performed with reference to the modulation points of the transmission wave and the reception wave (reflected wave).

As shown in FIG. 19, the distance L3 'is 18 m
m and 25 mm, the gas concentration sensor 325
It can be seen that an accurate value is output for the hydrogen gas input amount. On the other hand, when the distance L3 'is 2 mm and 9 mm, if the amount of supplied hydrogen gas exceeds about 60%, the accuracy of the gas concentration sensor 325 deteriorates, and it can be seen that the hydrogen gas concentration cannot be detected accurately.

This is because the sound velocity increases as the hydrogen gas concentration increases as described above.
When 3 ′ is as small as 2 mm and 9 mm, the difference between the propagation times of the direct wave and the different path wave cannot be sufficiently obtained, so that the different path wave overlaps the modulation point of the direct wave, and accurate reception is not possible. Probably because it is hindered. On the other hand, the distance L
When 3 ′ is 18 mm or 25 mm, it is considered that such a problem does not occur because the difference between the propagation times of the straight wave and the different path wave can be sufficiently obtained.

As described above, in the present embodiment, the propagation time of the ultrasonic wave is extended by setting the propagation distance of the measurement chamber 334, which is the propagation path, to be sufficiently long so that the reception is not affected by the transmission. By doing so, the propagation time of the ultrasonic wave is accurately measured. As a result, the gas concentration sensor according to the present embodiment can be effectively applied to the measurement of the concentration of a gas having a small molecular weight such as hydrogen gas.

Next, FIG. 20 shows a comparative example of FIG.
The distance L3 is fixed at 60 mm, the distance L3 ′ is 2 mm,
9 mm, 18 mm, and in the case of changing the four 25 mm, shows the measurement results of the sensor output for butane (C ₄ H ₁₀₎ input. The experimental conditions were the same as those described above, except that the hydrogen gas was replaced with butane.

Since butane has a higher molecular weight than hydrogen gas, the sound speed does not increase so much even if the concentration is increased. Therefore, as shown in FIG. 20, even when the distance L3 'is as small as 2 mm, the different path wave does not overlap with the straight wave in the high concentration area of butane, and the concentration is accurately detected.

However, when the concentration of butane is as low as 10% or less, a slight error is observed in the measured value when the distance L3 'is 2 mm. This is because, contrary to the case of the above hydrogen gas, the traveling speed of the ultrasonic wave is slow because the sound speed is small, and the different path wave is reflected before the second half of the straight wave is completely reflected on the reflecting surface, and overlaps with the straight wave. It is thought that it is because.

As described above, even when the sound speed is relatively low, it is preferable to set the distance L3 'to some extent. The embodiments of the present invention have been described above. However, the present invention is not limited to the above-described embodiments, and can take various aspects.

For example, in the first and second embodiments, an ultrasonic wave with frequency modulation is transmitted, but an ultrasonic wave with an antiphase component may be transmitted. For example, as shown in FIG. 12A, if an ultrasonic wave into which an anti-phase component (180 degrees) is introduced is transmitted, no signal waveform appears at the anti-phase point of the transmitted wave.

Then, as shown in FIG. 12 (b), the reflected wave, which is the received wave, has a portion where there is no signal waveform corresponding to the opposite phase point. Therefore, the point (modulation point) at which the antiphase component is introduced is defined as in the frequency modulation point of the above embodiment,
By using the measurement standard, the propagation time can be measured, and the gas concentration can be measured.

Specifically, for example, the time when the antiphase point of the first reflected wave appears is subtracted from the time when the antiphase point of the second reflected wave appears, and the propagation time of the second reflected wave (second propagation time) ) Can be accurately obtained, so that the gas concentration can be measured based on the second propagation time.

In the third embodiment, the detection of the hydrogen gas concentration is targeted, and the gas concentration sensor of the embodiment shown in FIG. 13 is configured such that the distance L3 is 60 mm or more and the distance L3 'is 18 mm or more. However, the numerical limitation of these distances is not limited to the gas concentration sensor of the embodiment shown in FIG. 13, but can be applied to a gas concentration sensor of another embodiment when detecting a hydrogen gas concentration. . For example,
6 and 7 applied to the first embodiment and the second embodiment, the distance L may be set to 60 mm or more and the distance L 'may be set to 18 mm or more.

Further, in the third embodiment, the hydrogen gas is taken as an example of the gas to be measured. However, the configuration of the present embodiment effectively exerts its effect as long as the gas has a small molecular weight other than the hydrogen gas. be able to. Further, it is considered that the optimum values of L3, L3 ', L, and L' exist for gases whose molecular weight is not as small as that of hydrogen gas. Therefore, these may be obtained by experiments to determine the shape of the gas concentration sensor. Of course.

Further, it goes without saying that the above configuration is applicable not only to internal combustion engines such as engines. For example, for measuring the concentration of hydrogen gas in the reformed gas of a fuel cell,
The gas concentration sensor of this embodiment can be applied effectively.

[Brief description of the drawings]

FIG. 1 is a system configuration diagram illustrating an entire system including a control device of a gas concentration sensor according to a first embodiment.

FIGS. 2A and 2B show a gas concentration sensor, FIG. 2A is an explanatory diagram showing the entire sensor, and FIG. 2B is an explanatory diagram showing an ultrasonic transmitting / receiving element.

FIG. 3 is a block diagram showing an electrical configuration of the gas concentration sensor.

FIG. 4 is an explanatory diagram showing a basic principle of a gas concentration sensor.

FIG. 5 is a graph showing the relationship between sensor output and butane concentration.

FIG. 6A is a diagram showing a transmission waveform with one frequency modulation, and FIG. 6B is an explanatory diagram showing a distance between an ultrasonic element and a reflection surface.

7A is a diagram showing a state of a transmitted / received wave when the reflection surface has a concave portion a, and FIG. 7B is a diagram showing a state of the transmitted / received wave in the case of the conventional gas concentration sensor in which the reflection surface is flat. FIG.

8A is a timing chart showing a signal by a transmission / reception switch, FIG. 8B is a timing chart showing a transmission / reception waveform, and FIG. 8C is a timing chart showing a comparator output.

FIG. 9 is a timing chart showing transmission / reception waveforms of ultrasonic waves in new and deteriorated sensors.

FIG. 10 is a diagram showing a state of a transmitted / received wave when a concave portion b as a modification is provided on the reflection surface.

11 (a) shows the area of the reflection surface 34 of the ultrasonic element 3; FIG.
FIG. 3B shows the state of the transmitted / received waves when the area of the reflection surface 34 is larger than the area of the reflection surface 34 side, and FIG. 3B shows the area of the reflection surface 34 smaller than the area of the reflection surface 34 side of the ultrasonic element 33. FIG. 6 is a diagram showing the state of a transmitted / received wave in the case.

12A is a diagram showing a transmission waveform into which one antiphase component is introduced, and FIG. 12B is a timing chart showing a transmission / reception waveform.

FIG. 13 is a sectional view of a gas concentration sensor according to a third embodiment.

FIG. 14 is a block diagram showing an electrical configuration of the gas concentration sensor.

FIG. 15 is an explanatory diagram illustrating a configuration of an experimental device used in an experimental example.

16A is a diagram illustrating a state of a transmitted / received wave when the distance L3 is configured to be less than 60 mm, and FIG.
FIG. 6 is a diagram showing a state of a transmitted / received wave when the distance L3 ′ is configured to be equal to or greater than 60 mm and the distance L3 ′ is equal to or greater than 18 mm.

FIG. 17 shows a case where the distance L3 is 60 mm or more and the distance L
It is the figure which showed typically the state of the transmission / reception wave when 3 'is comprised more than 18 mm.

FIG. 18 is a graph showing an experimental result on a relationship between a hydrogen input amount (set concentration) and a sensor output (measured concentration) when the distance L3 is changed.

FIG. 19 is a graph showing an experimental result on a relationship between a hydrogen input amount (set concentration) and a sensor output (measured concentration) when the distance L3 ′ is changed.

FIG. 20 is a graph showing an experimental result on a relationship between a butane injection amount (set concentration) and a sensor output (measured concentration) when the distance L3 ′ is changed.

FIG. 21 shows a case where the distance L3 is less than 60 mm and the distance L
It is the figure which showed typically the state of the transmitted / received wave when 3 'was comprised below 18 mm, (a) shows the transmitted / received wave when the sound speed is low, (b) shows the transmitted / received wave when the sound speed is high,
Shown respectively.

FIG. 22 shows a case where the distance L3 is 60 mm or more and the distance L
It is the figure which showed typically the state of the transmitted / received wave when 3 'was comprised below 18 mm, (a) shows the transmitted / received wave when the sound speed is low, (b) shows the transmitted / received wave when the sound speed is high,
Shown respectively.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Engine, 21, 22, 23, 24, 25, 325 ... Gas concentration sensor, 31, 332 ... Drive / calculation circuit (gas concentration detection means), 32, 334 ... Measurement room, 33, 335 ... Ultrasonic transmission / reception Element (ultrasonic element), 34, 336: reflection surface, 36, 337: inflow hole (inlet), 37, 338 ... outflow hole (outlet)

Continued on the front page (72) Inventor Keigo Banno 14-18, Takatsuji-cho, Mizuho-ku, Nagoya City, Aichi Prefecture Inside Japan Special Ceramics Co., Ltd. (72) Inventor Noboru Ishida 14-18, Takatsuji-cho, Mizuho-ku, Nagoya City, Aichi Prefecture (72) Inventor Takafumi Oshima 14-18 Takatsuji-cho, Mizuho-ku, Nagoya-shi, Aichi Japan F-term (reference) 2G047 AA01 BC02 BC15 EA10 GB26 GG08 GG19 GJ19

Claims (9)

[Claims] A measuring chamber provided with an inflow hole and an outflow hole for allowing a gas to be measured to flow in and out, and provided on one of two wall surfaces facing each other in the measurement chamber, and superimposed on the other wall surface. An ultrasonic element that transmits a sound wave and that can receive a reflected wave of an ultrasonic wave reflected from the wall surface as a reflecting surface; and transmits an ultrasonic wave having at least one modulation point to the ultrasonic element. Together with the reflected wave, measuring the propagation time from the transmission of the ultrasonic wave to the reception of the reflected wave using the modulation point, based on the propagation time, the specific gas in the gas to be measured A gas concentration sensor comprising: a gas concentration sensor that detects a gas concentration of the measurement chamber; wherein the distance between the edge of the reflection surface and the ultrasonic element is equal to the center of the reflection surface and the ultrasonic wave. Shaped to be longer than the distance from the element Gas concentration sensor, characterized in that it has. 2. A measurement chamber provided with an inflow hole and an outflow hole through which a gas to be measured flows in and out, and is provided on one of two opposite wall surfaces in the measurement chamber, and extends toward the other wall surface. An ultrasonic element that transmits a sound wave and that can receive a reflected wave of an ultrasonic wave reflected from the wall surface as a reflecting surface; and transmits an ultrasonic wave having at least one modulation point to the ultrasonic element. Together with the reflected wave, measuring the propagation time from the transmission of the ultrasonic wave to the reception of the reflected wave using the modulation point, based on the propagation time, the specific gas in the gas to be measured A gas concentration sensor comprising: a gas concentration sensor for detecting a gas concentration of the gas concentration sensor, wherein an area of the reflection surface of the measurement chamber is equal to or larger than an area of an opening surface of the ultrasonic element. . 3. The gas concentration detecting means causes the ultrasonic element to transmit and receive an ultrasonic wave with at least one frequency modulation, and based on a propagation time obtained by using a modulation point of the ultrasonic wave. 3. The gas concentration sensor according to claim 1, wherein the gas concentration of the specific gas is detected. 4. The gas concentration detecting means causes the ultrasonic element to transmit and receive an ultrasonic wave containing at least one antiphase component, and based on a propagation time obtained by using a modulation point of the ultrasonic wave. The gas concentration sensor according to claim 1, wherein the gas concentration of the specific gas is detected. 5. The gas concentration detecting means transmits an ultrasonic wave from the ultrasonic element, reflects the ultrasonic wave on the reflection surface, reflects the ultrasonic wave on the ultrasonic element side, and re-reflects the reflection surface. And measuring a propagation time until a reflected wave is received after the first reflected wave, and detecting the gas concentration of the specific gas based on the propagation time. A gas concentration sensor according to any one of claims 1 to 4. 6. The gas concentration sensor according to claim 1, wherein the specific gas is fuel vapor of an engine for an internal combustion engine. 7. A measurement chamber provided with an inflow hole and an outflow hole through which a gas to be measured flows in and out, and is provided on one of two wall surfaces facing each other in the measurement chamber and is superposed toward the other wall surface. While transmitting a sound wave, an ultrasonic element capable of receiving a reflected wave of an ultrasonic wave reflected by using the other wall surface as a reflection surface, and transmitting the ultrasonic wave to the ultrasonic element and transmitting the reflected wave. Received, the propagation time from the transmission of the ultrasonic wave to the reception of the reflected wave is measured by detecting a specific position of the waveform of the ultrasonic wave, based on the propagation time,
A gas concentration sensor comprising: a gas concentration detection unit configured to detect a gas concentration of a specific gas in the measured gas. In the gas concentration sensor, a distance between a center of the reflection surface and the ultrasonic element is equal to or greater than 60 mm. A gas concentration sensor characterized by being formed as follows. 8. The measurement chamber, wherein a concave portion having a bottom surface parallel to the reflection surface is provided at an edge of the reflection surface, and a distance between the bottom surface of the depression and the ultrasonic element is smaller than the reflection surface. 8. The device according to claim 1, wherein the distance is at least 18 mm longer than a distance between a center portion and the ultrasonic element.
The gas concentration sensor according to any one of the above. 9. The gas concentration sensor according to claim 1, wherein the specific gas is hydrogen gas.