JP4575862B2 - Gas detector - Google Patents

Description

  The present invention relates to a gas detection device using a catalytic combustion type gas sensor.

In this type of gas detection device, a resistor consisting of a catalytic combustion type gas sensor is connected to one side of the bridge circuit, and contact is made from the voltage difference between the midpoint potential of one arm and the midpoint potential of the other arm. 2. Description of the Related Art Conventionally, a resistance value of a combustion gas sensor is obtained, and a gas concentration of a detection target gas is detected from the resistance value (see, for example, Patent Document 1).
Japanese Patent Laid-Open No. 11-14579

  In the above-described gas detection device, since the output of the bridge circuit is only amplified and output, the output (difference voltage) of the bridge circuit and the gas concentration are approximately proportional to each other in a low concentration region where the resistance value change of the resistor is small. However, since the output of the bridge circuit and the gas concentration are not proportional in the high concentration region, an approximate expression for correcting the output of the bridge circuit in the high concentration region has been used. For this reason, it is necessary to adjust the coefficient of the approximate expression in order to correct the detected value of the gas concentration, which takes time for adjustment, and there is also a problem that the detection accuracy of the gas concentration deteriorates due to poor adjustment.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a gas detection device with improved gas concentration detection accuracy.

In order to achieve the above object, the invention of claim 1 comprises a gas sensitive body having an oxidizing catalytic ability formed of a material containing at least platinum, and the specific resistance value of the gas sensitive body is proportional to the temperature. A constant combustion voltage is applied to a series circuit of a gas combustion sensor and a reference resistance, and a temperature-compensating reference resistor having the same temperature-resistance characteristics as the gas sensing body and deactivating the gas sensitivity. And a resistance value detecting means for detecting the resistance value of the gas sensor using the voltage across the gas sensor and the voltage applied by the voltage application means, assuming that the resistance value of the reference resistor is constant. And a concentration detecting means for obtaining a gas concentration of the detection target gas by performing linear interpolation calculation from the resistance value detected by the resistance value detecting means based on the resistance value of the gas sensor at the reference concentration. And features.

  By the way, when the detection target gas exists in the atmosphere, the specific resistance of the gas sensitive body changes according to the gas concentration, and the current flowing through the reference resistance changes accordingly, so the temperature of the reference resistance changes due to Joule heat, The specific resistance of the reference resistor changes in proportion to the temperature. According to the invention of claim 1, the resistance value detecting means assumes that the resistance value of the reference resistor is constant, Since the resistance value of the gas sensor is detected using the voltage applied by the voltage application means, the relationship between the gas concentration and the change in the connection point potential of the gas sensor and the reference resistor has deviated from the linear relationship. Even in the high concentration range, it is possible to obtain the resistance value of the gas sensor assuming that the resistance value of the gas sensor and the gas concentration have a linear relationship. Therefore, the resistance value of the gas sensitive body can be obtained by a simple calculation, and the work of adjusting the coefficient of the approximate expression as in the case of obtaining the resistance value using a complicated approximate expression becomes unnecessary. The resistance value can be easily and accurately detected.

  According to a second aspect of the present invention, in the first aspect of the invention, two comparison resistors are connected in series between both ends of a series circuit composed of a gas sensor and a reference resistor, and a constant voltage is applied across the two circuits of the series circuit of two comparison resistors. When the gas to be detected is not present in the atmosphere, the voltage difference between the comparison voltage at the connection point of both comparison resistors and the detection voltage at the connection point of the gas sensor and reference resistor is approximately zero. The resistance value of the comparison resistor is set, and the resistance value detecting means obtains the resistance value of the gas sensor by calculation from the voltage obtained by amplifying the difference voltage between the comparison voltage and the detection voltage.

  By the way, when the resistance value of the gas sensor is obtained directly from the detection voltage at the connection point between the gas sensor and the reference resistor, the change in the resistance value with respect to the change in gas concentration is small, so that sufficient resolution can be obtained. It is necessary to use an expensive one with high resolution as the voltage value detecting means. On the other hand, according to the second aspect of the present invention, the resistance value of the gas sensitive body is obtained by calculation from the voltage obtained by amplifying the difference voltage between the comparison voltage and the detection voltage. An inexpensive element having a relatively low resolution can be used for the A / D converter.

  According to a third aspect of the present invention, in the second aspect of the invention, the gas sensitive body and the reference resistor are connected in series between both ends of the voltage applying means with the gas sensitive body at the high voltage side, and the voltage applied by the voltage applying means is Vc, the resistance value of the reference resistor is RL, the voltage across the reference resistor is VRL, and the resistance of both comparison resistors is such that the difference voltage between the comparison voltage and the detection voltage becomes substantially zero in the absence of the detection target gas in the atmosphere. Using the fact that the comparison voltage V2 is Vc / 2 when the comparison voltage after adjusting the value is V2, and the voltage obtained by amplifying the difference voltage (VRL-V2) with the amplification factor K is Vr. The resistance value detecting means obtains the resistance value Rs of the gas sensitive body using an arithmetic expression of Rs = 2K × Vc × RL / (Vc × K + 2Vr) −RL.

  According to this invention, the resistance value Rs of the gas sensitive body can be obtained by calculation using the above calculation formula.

  According to the present invention, the resistance value detecting means detects the resistance value of the gas sensitive body using the voltage across the gas sensitive body and the voltage applied by the voltage applying means, assuming that the resistance value of the reference resistance is constant. Therefore, in the past, even in the high concentration region where the relationship between the change in potential of the connection point of the gas sensor and reference resistor and the gas concentration deviate from the linear relationship, the resistance value of the gas sensor and the gas concentration have a linear relationship. It is possible to obtain the resistance value of the gas sensitive body as having the above. Therefore, the resistance value of the gas sensitive body can be obtained by a simple calculation, and the work of adjusting the coefficient of the approximate expression as in the case of obtaining the resistance value using a complicated approximate expression becomes unnecessary. The resistance value can be easily and accurately detected, and as a result, the gas concentration can be accurately detected.

  Embodiments of the present invention will be described below with reference to the drawings.

  Embodiment 1 of this invention is demonstrated based on drawing. FIG. 1 is a block diagram of a gas detection device according to the present embodiment. This detection device includes a catalytic combustion gas sensor 1, a temperature compensation reference resistor 8, a resistance value detection unit 10, a concentration detection unit 11, and a storage unit 12. As a main component. In addition, the resistance value detection part 10 and the density | concentration detection part 11 are implement | achieved by the calculation function of a microcomputer, for example.

  2A is a cross-sectional view of the catalytic combustion type gas sensor 1 used in the present embodiment, and FIG. 2B is an external perspective view. The catalytic combustion type gas sensor 1 includes a gas sensitive body 2 and stems 3a, 3b, 3c. And a base 4 and a protective cap 5.

  The gas sensor 2 detects the gas concentration from the function as a combustor that burns the combustible gas (for example, hydrogen) to be detected on its surface by catalytic action and the resistance value change that occurs in response to the temperature rise due to combustion heat. A platinum wire made of an alloy of platinum and at least one of palladium, ruthenium, rhodium, nickel, and cobalt is used as a heating resistor. The two ends 2a and 2b are electrically and mechanically connected to the stems 3a and 3c. In the present embodiment, for example, the gas sensitive body 2 having a wire diameter of about 15 to 50 μm is used, the total length of the coil is about 250 to 1000 μm, and the coil diameter is about 160 to 550 μm. In the present embodiment, a platinum wire is used as the gas sensitive body 2, but a material other than pure platinum may be used as long as it is a platinum-based resistance wire, for example, zirconia stabilized platinum.

  In this embodiment, the reference resistor 8 is housed in the same case as the catalytic combustion type gas sensor 1. The reference resistor 8 is formed in the same shape and size by the same material as the gas sensitive body 2 and deactivates the gas sensitivity to the detection target gas.

  Since the reference resistor 8 has no combustion activity with respect to the hydrogen gas, even if the reference resistor 8 is heated to the same temperature as the gas sensitive body 2, the hydrogen gas does not burn on the surface of the reference resistor 8, and therefore, it is caused by the combustion heat. No temperature rise occurs. Further, since the reference resistor 8 is made of the same material as that of the gas sensitive body 2, it has the same temperature-resistance characteristics as the gas sensitive body 2, so that the resistance value of the reference resistor 8 is used depending on the ambient temperature. By correcting the output fluctuation, it is possible to more accurately measure a change in the resistance value of the gas sensor 2 due to combustion heat, and the detection accuracy for hydrogen gas is improved.

  The base 4 is formed in a disc shape from a synthetic resin, and the three stems 3a, 3b, 3c are insert-molded in the base 4 so as to penetrate the base 4 in the vertical direction. Of the three stems 3a to 3c, the central stem 3c has a shorter protruding amount from the upper surface than the other two stems 3a and 3b. Both ends of the gas sensitive body 2 are fixed to the left stem 3a and the central stem 3c at positions protruding from the upper surface of the base 4 by a method such as welding. Further, both ends of the reference resistor 8 are fixed to the right end stem 3b and the center stem 3c by a method such as welding at a portion protruding from the upper surface of the base 4. Since the three stems 3a to 3c are arranged in the same plane, when the gas sensitive body 2 and the reference resistor 8 are laser welded to the stems 3a to 3c, the welding work can be performed at a time, and the workability is improved. There is an advantage of improvement.

  At the time of measuring hydrogen gas, a constant voltage Vc is applied between the stems 3a and 3b by a constant voltage source E which is a voltage applying means, and the gas sensitive body 2 is heated to a predetermined temperature (for example, about 100 ° C.). When the hydrogen gas that has entered inside through the vent hole 6 of the protective cap 5 comes into contact with the gas sensitive body 2, the hydrogen gas burns on the surface of the gas sensitive body 2 by the catalytic action of platinum on the surface of the gas sensitive body 2. At this time, the temperature of the gas sensitive body 2 rises due to the combustion heat of the hydrogen gas, and the electrical resistance increases in response to the temperature rise. Therefore, the resistance value detection unit 10 uses the voltage of the stem 3c to make the gas sensitive body 2 The resistance value Rs is detected, and the concentration detector 11 detects the gas concentration of the hydrogen gas from the detection result.

  Here, a gas concentration detection method will be described below. The storage unit 12 is composed of a nonvolatile memory such as an EEPROM, and the resistance value of the gas sensitive body 2 at one or more reference concentrations of the detection target gas is registered in advance.

  If the resistance value of the gas sensing element 2 changes during the detection of hydrogen gas, the current flowing through the reference resistor 8 changes accordingly, and the resistance value also changes due to Joule heat. Assuming that the resistance value RL of the resistor 8 is a constant value, the resistance value Rs of the gas sensitive body 2 is detected. That is, the resistance value detection unit 10 detects the potential (the voltage VRL across the reference resistor 8) of the connection point (that is, the stem 3c) between the gas sensor 2 and the reference resistor 8, and the gas sensor 2 obtained from the potential at this point. The resistance value Rs of the gas sensitive body 2 is obtained by calculation using the following calculation formula based on the both-end voltage Vs (= Vc−VRL) and the applied voltage Vc of the constant voltage source E.

Rs = Vc × RL / VRL−RL (1)
When the gas sensitive body 1 and the reference resistor 8 are connected between both ends of the constant voltage source E with the gas sensitive body 1 at the high voltage side, the resistance of the gas sensitive body 1 is calculated by the above equation (1). The value Rs can be obtained. However, when the reference resistor 8 is connected to both ends of the constant voltage source E with the high-voltage reference resistor 8, the resistance value Rs may be obtained using the following equation (2).

Rs = Vs × RL / (Vc−Vs) (2)
Then, the concentration detection unit 11 uses the resistance value at the reference concentration registered in advance in the storage unit 12 and performs linear interpolation based on the resistance value of the gas sensitive body 2 obtained by the resistance value detection unit 10. Thus, the gas concentration of the detection target gas is obtained by calculation.

Here, when the detection target gas exists in the atmosphere, the specific resistance of the gas sensitive body 2 changes according to the gas concentration, and the current flowing through the reference resistance 8 changes accordingly. The temperature changes, and the specific resistance of the reference resistor 8 changes in proportion to the temperature. In this embodiment, the resistance value detection unit 10 assumes that the resistance value of the reference resistor 8 is constant, and then the gas sensitivity. because by using the voltage applied by the voltage across the body 2 and the constant voltage source E and detects the resistance value of the gas sensitive body 2, gas sensitive body even in a high concentration range of more than 2000 0 ppm, as shown in FIG. 3 It is possible to determine the resistance value of the gas sensitive body 2 assuming that the resistance value Rs of 2 and the gas concentration have a substantially linear relationship. Therefore, the resistance value of the gas sensitive body 2 can be obtained by a simple calculation, and the work of adjusting the coefficient of the approximate expression as in the case of obtaining the resistance value using a complicated approximate expression becomes unnecessary, and the gas sensitive body is eliminated. Since the resistance value 2 can be easily detected with high accuracy, the gas concentration can be accurately detected.

  By the way, in the above circuit, as shown in FIG. 4, two comparison resistors R1 and R2 are connected in series between both ends of the series circuit including the gas sensitive body 2 and the reference resistor 8, thereby forming a bridge circuit, and the comparison resistor R1. , R2 is applied to the series circuit, and the comparison voltage V2 at the connection point between the two comparison resistors R1, R2 and the connection point between the gas sensor 2 and the reference resistor 8 in a state where no detection target gas exists in the atmosphere. The resistance values of the comparison resistors R1 and R2 are set so that the difference voltage with respect to the detection voltage V1 (= VRL) becomes substantially zero, and the resistance value detection unit 10 determines the difference voltage between the comparison voltage V2 and the detection voltage V1. The resistance value of the gas sensing element 2 may be obtained by calculation from the voltage obtained by amplifying the gas sensing element. Similarly to the above, assuming that the resistance value RL of the reference resistor 8 is a constant value, the gas sensing element 2 Two factors It is possible to accurately detect the value.

  Here, as shown in FIG. 4, when the gas sensitive body 1 and the reference resistor 8 are connected in series between both ends of the constant voltage source E with the gas sensitive body 1 at the high voltage side, the constant voltage source E (voltage The voltage applied by the application means) is Vc, the resistance value of the reference resistor 8 is RL, the voltage across the reference resistor 8 is VRL, and the difference voltage between the comparison voltage V2 and the detection voltage V1 in the absence of the detection target gas in the atmosphere is The comparison voltage after adjusting the resistance values of both comparison resistors R1 and R2 to be substantially zero is V2, and the difference voltage (VRL−V2) between the voltage VRL across the reference resistor 8 and the comparison voltage V2 is the amplification factor K. When the voltage obtained by the amplification is Vr (= (VRL−V2) × K), the resistance value detection unit 10 uses the fact that the voltage V2 becomes Vc / 2, and the resistance value detection unit 10 determines the resistance of the gas sensitive body 1. The value Rs is calculated using the following equation (3). There are are demanding, it is possible to determine the resistance value RS by calculation.

Rs = Vc × RL / VRL−RL
= Vc * RL / (Vr / K + Vc / 2) -RL
= 2K × Vc × RL / (Vc × K + 2Vr) −RL (3)
On the other hand, when the gas sensor 1 and the reference resistor 8 are connected in series between both ends of the constant voltage source E with the reference resistor 8 at the high voltage side, the voltage across the gas sensor 1 is Vs, and the gas sensor 1 Assuming that the voltage obtained by amplifying the difference voltage (Vs−V2) between the both-end voltage Vs and the comparison voltage V2 with the amplification degree K is VS (= (Vs−V2) × K), the voltage V2 is Vc / 2. By utilizing this, the resistance value detection unit 10 can obtain the resistance value Rs of the gas sensitive body 1 using the following arithmetic expression (4).

Rs = Vs × RL / (Vc−Vs)
= (VS / K + Vc / 2) × RL / (Vc−VS / K−Vc / 2)
= (Vc * K + 2VS) * RL / (Vc * K-2VS) (4)
It should be noted that a wide variety of different embodiments can be configured without departing from the spirit and scope of the present invention, and the present invention is not limited to a specific embodiment.

It is a circuit block diagram of the gas detection apparatus of this embodiment. The contact combustion type gas sensor used for the above is shown, (a) is a sectional view, (b) is an external perspective view. It is a figure which shows the measurement result of the resistance value of the gas sensitive body used for the same as the above. It is a circuit block diagram which shows the other circuit structure same as the above.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Contact combustion type gas sensor 2 Gas sensitive body 8 Reference resistance 10 Resistance value detection part 11 Concentration detection part E Constant voltage source

Claims (3)

A catalytic combustion type gas sensor comprising a gas sensitive body having an oxidizing catalytic ability formed of a material containing at least platinum, wherein a specific resistance value of the gas sensitive body changes in proportion to a temperature, and the same as the gas sensitive body A reference resistor for temperature compensation in which the gas sensitivity is deactivated, voltage applying means for applying a constant voltage to a series circuit of the gas sensitive body and the reference resistor, and the reference resistor Assuming that the resistance value is constant, resistance value detecting means for detecting the resistance value of the gas sensitive body using the voltage across the gas sensitive body and the voltage applied by the voltage applying means, and the gas sensitive body at a reference concentration And concentration detecting means for performing linear interpolation calculation from the resistance value detected by the resistance value detecting means based on the resistance value of the gas to obtain the gas concentration of the detection target gas. That gas detection device.   Two comparison resistors are connected in series between both ends of a series circuit composed of the gas sensitive body and the reference resistor, the constant voltage is applied to both ends of the series circuit of the two comparison resistors, and a gas to be detected is detected in the atmosphere. The resistance value of both comparison resistors is set so that the difference voltage between the comparison voltage at the connection point of both comparison resistors and the detection voltage at the connection point of the gas sensor and the reference resistor becomes substantially zero in the absence of 2. The gas detection apparatus according to claim 1, wherein the resistance value detecting means obtains the resistance value of the gas sensing element by calculation from a voltage obtained by amplifying a difference voltage between the comparison voltage and the detection voltage.   The gas sensitive body and the reference resistor are connected in series between both ends of the voltage applying means with the gas sensitive body at a high voltage side, the applied voltage by the voltage applying means is Vc, the resistance value of the reference resistance is RL, and the reference The voltage across the resistor is VRL, and the comparison voltage after adjusting the resistance value of both comparison resistors so that the difference voltage between the comparison voltage and the detection voltage becomes substantially zero in the absence of the detection target gas in the atmosphere. When the voltage obtained by amplifying V2 and the difference voltage (VRL−V2) at an amplification factor K is Vr, the resistance value detecting means is obtained by using the comparison voltage V2 being Vc / 2. 3. The gas detection device according to claim 2, wherein the resistance value Rs of the gas sensitive body is obtained using an arithmetic expression of Rs = 2K × Vc × RL / (Vc × K + 2Vr) −RL.

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