JPS63109362A - Gas detector - Google Patents

Abstract

PURPOSE:To obtain a detection circuit suitable for a low resistance heat generator, by connecting the heat generator of a gas sensor to a power source through a switch and turning the switch ON and OFF to heat the gas sensor. CONSTITUTION:The output of a power source Eb charges a condenser C1 through a protective resistor R1 and a transistor Tr1 and the voltage of the condenser C1 is taken out by a condenser C2 to be compared with reference voltage and the transistor Tr1 is controlled and the voltage of the condenser C1 is always kept constant. When a transistor Tr2 is turned ON, a heater 14 receives the supply of a current in a pulse like manner to perform the heating of a sensor 2. The sensor 2 is heated in a pulse like manner but a pulse is about 1kHz and heating is performed at an interval sufficiently shorter than the heat time constant (about 100msec) of the sensor 2. Therefore, the load current of the power source is small and power loss accompanied by voltage drop is reduced and the temp. of the sensor is kept constant. By this method, a detection circuit suitable for a low resistance heat generator is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a gas detection device for detecting gases such as flammable gases, toxic gases, oxygen, and water vapor.

[Prior Art] The inventor has proposed a sensor in which a metal heating element is used as a carrier for an atmosphere-sensitive material, and an atmosphere-sensitive material layer is supported on the heating element via a heat-resistant insulating coating (see the patent application filed today). (1
)). A feature of this sensor is that it does not require an insulating substrate supporting an atmosphere-sensitive substance or a sintered body of an atmosphere-sensitive substance. As a result, the heat capacity and heat radiation area of the sensor are reduced, and the power consumption and thermal time constant of the sensor are also reduced. In addition, the weight of the base body, sintered body, etc. is eliminated, and the lead wires supporting it can be made thinner. Furthermore, the main part of the sensor is formed on a metal heating element, making it suitable for automation, which also increases the productivity of the sensor.

The inventor studied the ancillary circuit of this sensor. The problem is that the heating element has low resistance and it is difficult to obtain power for the heater.

[Problem of the Invention] An object of the present invention is to provide a detection circuit suitable for a low-resistance heater for a sensor in which a metal heating element is used as a carrier for an atmosphere-sensitive material layer. In particular, it is an object of the present invention to provide a detection circuit that has less power loss due to voltage reduction of the output of a heater power source and can use a power source with a small current capacity.

[Structure of the Invention] In this invention, a heat-resistant insulating coating is provided on the surface of a metal heating element, and an atmosphere-sensitive material layer is supported on the coating to form a sensor. The heating element is made of noble metal such as Pt or Pd-Ir alloy, or base metal such as Fe-Cr-A1 alloy or Ni-Cr alloy. CVD
Established by etc.

By selecting the manufacturing conditions for the coating, it is easy to obtain a dense coating that is firmly bonded to the metal.

The atmosphere-sensitive substance and the metal heating element are separated by an insulating coating, and can be separated from each other. Furthermore, many atmosphere-sensitive substances are metal oxides, and their bond strength to metals is low.

However, the adhesion strength to the insulating coating made of ceramic or the like is high, and there is little risk of the atmosphere-sensitive substance falling off or peeling off.

The shape of the ceramic is determined by the heating element itself and can be made extremely small. As a result, the heating element only needs to be strong enough to support its own weight, and the sensor becomes smaller and its heat capacity decreases.

The problem is that the heater has a low resistance and requires a low heater voltage. Here, the output of the power supply is stored in a capacitor, and a heating element is connected to the capacitor at appropriate intervals via a switch. In this way, power loss due to step-down of the power supply can be reduced, and a power supply with a small current capacity can be used.

Note that temperature fluctuations caused by intermittent heating of the sensor can be eliminated by making the heating period sufficiently smaller than the thermal time constant of the sensor, preferably 115 or less, more preferably 1/20 or less. Also, if the capacitance of the capacitor used changes, the heater power changes.This is possible if the discharge time constant between the capacitor and the heating element is set to be longer than the switch on time, more preferably 5 times or more, and even more preferably 10 times or more. It can be solved.

[Example] Figures 3 and 4 show a gas sensor (2). In the figure, (4) is a housing, which is covered from above with a cover (not shown) to provide explosion protection and protection for the sensor. (6)
(8) is a heater stem fixed to the housing, and (8) is an electrode stem also fixed to the housing. (10) is the sensor body, and the metal heating element (14) is coated with heat-resistant insulation.
), a pair of electrodes (24) and (26) are connected to the atmosphere. The sensor body (lO) consists of a housing (4
) is accommodated in the hollow part.

The manufacturing process of the sensor is shown in FIGS. 4(a) to 4(c).

Pt, Ir, Pd8O-Ir20, Pt-ZGS (Pt
A metal heating element (14) made of a noble metal such as ZrO particles precipitated at the grain boundaries of a base metal such as Fe-Cr-AL Ni-Cr is prepared, and a heat-resistant insulating coating (18) is prepared. give The wire diameter of the heating element (14) is, for example, about lO to 80μ.

The coating (I8) is provided on the surface of the heating element (14), leaving the weld to the stem (6) by masking.

The material for the coating (18) is preferably a metal oxide such as alumina or silica, or a ceramic such as SiC, SLN+, BN, etc., and is preferably provided by plasma CVD, sputtering, or the like. Particularly preferred are alumina,
Metal oxides such as silica. Note that the insulating property referred to here means that when used as a gas-sensitive material layer that has a sufficiently large insulation resistance with respect to the internal resistance of the atmosphere-sensitive material layer, a high-resistance T' i 0 t is used as the insulating coating (18). It is also possible to use The coating (18) also covers the heating element (14).
The heating element (14) and the atmosphere sensitive material layer (20) are shielded from each other to be protected from the atmosphere. Coating (18)
The preferred thickness is 50A to 10μ, more preferably 1
00A to 5μ. For example, when a 50A alumina film was provided by plasma CVD, a coating with an insulation resistance of IMΩ or more was obtained.

(20) is an atmosphere sensitive material layer. The material may be determined depending on the component to be detected; for example, when detecting flammable gas or toxic gas, Snow or Ind.

A metal oxide semiconductor such as 03 or pe103 is used. Also, when detecting O, B aS no s and L
A metal oxide semiconductor such as aN i Oa or NiO is used. Also, when detecting the humidity in the atmosphere, M g
A ceramic such as Cr x Oa or T i O* is used. These devices physically adsorb water vapor and can detect humidity from the electrical conductivity of the adsorbed water. In the case of humidity detection, as is well known, the sensitive material layer (20) is contaminated by dust and oil in the atmosphere, and it is necessary to heat-clean the sensitive material layer (20) to remove the contamination. In addition, other materials include
A proton conductor such as antimonic acid (Ht S bt O8) or antimony phosphate (HS bP t Oa) is used. These proton conductors generate an electromotive force depending on the ratio of H1 concentrations between the electrodes, and can be used to detect H7. In this case, for example, one of the electrodes is isolated from the atmosphere, and a difference in hydrogen concentration is created between the two electrodes to generate an electromotive force. The thickness of the atmosphere sensitive material layer (20) may be within a range that can be supported by the heating element (14). For example, when the sensitive material layer (20) is formed by vacuum evaporation or sputtering, the preferred thickness is 100A.
~5μ, and when applied by powder application or dipping,
1μ to 50μ is preferable.

(24) and (26) are electrodes, and here gold wires with a wire diameter of about 20 μm are used. The electrodes (24), (26) are covered with a layer of sensitive material (20) by fritless gold paste (28).
) and fixed to the stem (8) by welding. Note that the insulation coating (18) may be partially removed and the heating element (14) may also be used as an electrode, or as shown in FIG. A coating (19) may be applied, and an atmosphere sensitive material layer (21) may be provided thereon by printing or the like.

Detection Circuit The problem with these sensors is the low resistance of the heating element (14). That is, the formation of the atmosphere sensitive material layer (20) and its characteristics are well known for thin-film and thick-film sensors, and the problem centers on the resistance value of the heating element. The operating temperature of the sensor is usually around 300 to 400℃ for gas sensors, and 300℃ for heat cleaning of humidity sensors.
Temperatures around 00°C are common. In the case of proton conductors, the operating temperature is often between room temperature and 200°C, and when used at room temperature, the temperature is 30°C.
It is common to perform heat cleaning at about 0°C. Table 1 shows the heater characteristics when the distance between the stems (6) was 2 mm and a wire-shaped heating element was used.

Table 1 Heater characteristics* Material and wire diameter (μ) resistance (Ω) power (mW) Pd
-Ir 20μ 3 80 (without gold coating) Pd-1r 20u 2 50 (with gold coating) Pt 10μ 3 60Fe-
Cr-A1 40μ 3.3 100* The resistance value indicates the resistance value at room temperature, and the power indicates the power required for heating to 300℃. Pd-Ir is a Pd80-I r20 alloy, and Fe alloy is a Gabrius alloy. Kanthal of company, also electric [5
(24) and (26) are both gold wires with a wire diameter of 20μ.

If the length of the heating element (14) is increased unnecessarily, the resistance value of the heater can be increased. However, this complicates the structure of the sensor. Next, the conditions for obtaining 80 mW of power with a 3Ω resistor are approximately 500 mV and 170 mA. This makes it difficult to stabilize the power supply due to the low voltage. Furthermore, if the voltage is lowered from a commonly used power supply of about 5V to 0.5V, the voltage drop in the stabilized power supply will be large, leading to power loss and an increase in the size of the power supply. Furthermore, the required current is 100~
The power supply capacity is as large as 200mA, and the capacity of the power supply will continue to increase.

A gas detection circuit for a sensor provided with two electrodes (24), (26) is shown in FIG. In the figure, (Eb)
is a stabilized power supply of approximately 5V, and its output (Vcc) is used as the power supply for each part of the circuit. (Rl) is a protective resistance of about 100,
(Tr,) is a transistor, (C+) is a capacitor of about 100μF, (Try) is a switch such as a transistor, (80) is a duty ratio of 1 at a frequency of about 1KHz.
An oscillation circuit that generates pulses of about /100 (RI)
is a load resistance of about 100Ω. (C2) is a capacitor (Cυ) output smoothing capacitor, (Dυ is a Zener diode for generating a reference potential, (A,) is an error amplifier. (A,) is a comparison circuit for gas detection.

The operation of this circuit is shown in FIG. The output of the power supply (Eb) is charged to a capacitor (C+) via a protection resistor and a transistor (Tr+). The voltage of the capacitor (C1) is taken out by the capacitor (C2), and compared with a reference potential, the transistor (Tr,) is controlled and the voltage of the capacitor (C,) is always kept constant. Transistor (T
When rJ is turned on, the heater (14) is energized in pulses to heat the sensor. In this case the capacitor (
C1), the load on the power supply (Eb) is almost constant, and the load current varies from 5V and 80mW to 20mA.
It will be about. That is, the load current is small and the load on the power source (Eb) is small. The sensor (2) is heated in a pulsed manner, but the pulse is about IKHz, and the thermal time constant (l) of the sensor is
Heating is performed at sufficiently shorter intervals than Q (approximately 0m5ec). Therefore, the load current of the power supply is small, power loss due to voltage drop is small, and sensor temperature is kept constant. Note that the thermal time constant here means that when the heating power to the sensor is suddenly changed by 61 minutes, the sensor temperature is Δ
T/e is the time it takes to change. When the interval between heating pulses is a thermal time constant of 1720, the width of temperature fluctuation is 5% of the heating temperature, and when the interval is 1/100, it is 1%.

In preparation for the capacitance of capacitor (C3) changing over time,
The time constant of the capacitor and resistor (14) is made sufficiently longer than the on time of one pulse. In this example, the capacitor capacity is 100μF, the resistor is 3Ω, and the time constant is 300μF.
It will be about sec. On the other hand, the width of the pulse is about 10 μsec, and the output of the capacitor (Cυ) is
It is constant regardless of its capacity. When the discharge time constant is τ, the change in voltage when discharging by T is expressed as Vf/Vo=Exp(-T/r). If the discharge time is a time constant of 115, this value becomes 82%, 91% at 1/lO, and 9 at l/30.
It becomes 7%.

Preferred circuit conditions include a transistor (TrJ whose on-off frequency is at least 5 times the thermal time constant of the sensor) and a capacitor (
C+) has a time constant with the heater (14) that is five times or more the pulse width. The pulse width of the transistor (Trt) is automatically determined from the on-off frequency and the duty ratio required for heating. Here, the error amplifier (A +) may not be provided, and its input may be a smoothed voltage applied to the heating element (14).

In addition, the two transistors (Tr, ) and (Tr*) can be replaced with appropriate switches, and the transistor (
Tr+) may not be provided.

Next, a load resistor (Rl) is connected to the atmosphere sensitive material layer (20), and detection is performed from its output (Vout). Further, the duty ratio of the switch (Trt) does not need to be constant, and when the sensor temperature is changed periodically, the duty ratio may be changed periodically.

In the case of a humidity sensor, the heater is not normally used, but the heater only needs to be energized when heat cleaning is performed. The heat cleaning control circuit itself in this case is well known.

Furthermore, in the case of a sensor using electromotive force such as a proton conductor, a voltmeter may be connected to the sensor (2) instead of the comparison circuit (A, ), etc., and the electromotive force may be measured.

FIG. 2 shows a circuit to a sensor in which the heating element (14) also serves as an electrode. In this sensor, the atmosphere sensitive material layer (20
) is extracted from the heater voltage. Therefore, the sampling circuit (82) is operated with a pulse synchronized with the on-off of the transistor (Trt) to remove noise caused by turning on and off the power supply. Such a sampling circuit is well known, and may be simply an integrating circuit using a capacitor and a resistor, or may be a peak hold circuit for sensor output.

Note that although the detection circuit has been described based on specific circuit constants, this is just one example, and it goes without saying that the detection circuit can be freely changed as necessary.

Sensor characteristics will be explained using a gas sensor using a sensor characteristic of 5nOt as an example. Fe-Cr-A1 alloy (1
4), an alumina coating (18) with a thickness of about 1M was applied by plasma CVD. This is coated with SnO, a film (SnOt
97wt% of catalyst Pd03wt%) was sputtered to form an atmosphere sensitive material layer (20) with a thickness of about 5000A.
And so. Electrodes (24) and (26) with a wire diameter of 20 μm were attached using gold paste (28) to complete the sensor. The resistance value of the heating element (I4) at room temperature is 3.3Ω, 300°C
Heating power is 100 mW, thermal time constant is 100 m5e
It was about c. The detection circuit shown in FIG. 1 was used.

1 each in air using an atmosphere of 20°C and 65% relative humidity.
Resistance values were measured in 1000 pp of ethanol, CO, H2, and isobutane. Isobutane is a representative flammable gas. Ethanol and C at 3000C
The response characteristics to O, etc. are shown in FIG. Also, Table 2 shows 300
Table 3 shows the resistance values at 400° C. in each atmosphere.

In air 2.4M ethanol 40K Co 200K H, 350 Table 3 Sensor characteristics 400°C In air 1.2M ethanol 50 Isobutane 400K H, 300K It is possible to reduce power loss due to voltage drop by using a gas sensor with a small constant and a detection circuit suitable for a low-resistance heating element. Furthermore, a high voltage power supply can be used, and the load on the current capacity of the power supply is small.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of a gas detection device of an embodiment, Fig. 2 is a circuit diagram of a gas detection device of a modification, Fig. 3 is a front view of a gas sensor used in the embodiment, and Figs. 4(a) to (c). ) are perspective views showing the manufacturing process. Figure 5 is a front view of a modified gas sensor, Figures 6 and 7.
Each figure is a characteristic diagram of an example.

Claims (4)

[Claims] (1) In a gas detection device using a gas sensor having a heating element, an atmosphere sensitive material layer is supported on the surface of the metal heating element via a heat-resistant insulating coating, and a pair of electrodes is connected to the atmosphere sensitive material layer. , a gas sensor in which an atmosphere-sensitive material layer is supported by the heating element, a capacitor that stores power output, and a switch that connects the capacitor and the heating element and turns on and off at a predetermined cycle. gas detection device. (2) The gas detection device according to claim 1, wherein the period is set to 1/5 or less of a thermal time constant of the gas sensor. (3) The gas detection device according to claim 2, wherein the on time of the switch is constant in each cycle, and the on time is set to be equal to or less than the discharge time constant of the capacitor. . (4) The gas detection device according to claim 3, wherein the on-time is set to 1/5 or less of the discharge time constant of the capacitor.