JPH08136491A - Ambient air detecting apparatus - Google Patents

Abstract

PURPOSE: To prevent gases and water from adhering to an ambient air sensor and precisely detect the ambient air in the environment by heating the ambient air sensor at low temperature by applying small electric current at normal time. CONSTITUTION: Small current is applied to an atmospheric sensor to carry out heat radiation at as low temperature as about 100 deg.C, so that adhesion of gases and water to the atmospheric sensor which sometimes causes a detection error can be prevented. Detection of ambient temperature is carried out during the low temperature heating period and detection of the atmosphere is carried out during the time when the atmospheric sensor is heated to high temperature. In this ways, heating to high temperature can be done only by increasing the intensity of the current to be applied to the atmospheric sensor. In the case, for example, 2mA current is for the time of low temperature heating and 8mA current for the time of high temperature heating, a rated current power source with 6mA should be prepared and detection can be carried out only by turning the rated current power source on or off. Consequently, not like a conventional one, a switching circuit is made no need and a pause period in which gases and water adheres to the atmospheric sensor can be shortened. As a result, the atmosphere in the environment can be detected precisely.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an atmosphere detecting device, and more particularly, it has a resistor sensor which is heated in a measuring atmosphere, and a small current which is insensitive to the surrounding atmosphere is applied to the sensor. In addition to detecting the output voltage related to the ambient temperature, a relatively large current sensitive to the ambient atmosphere is passed through the sensor to detect the output voltage related to the ambient temperature and the ambient atmosphere. Related to an atmosphere detecting device for detecting, for example, a thermometer,
It can be used for various instruments such as hygrometers, gas densitometers, infrared meters, partial pressure gauges, flow meters, vacuum gauges, dew point meters and hot-wire anemometers.

[0002]

2. Description of the Related Art There is known a method of thermally detecting a predetermined gas concentration contained in a mixed gas atmosphere based on a difference in thermal conductivity which changes according to the molecular weight of the predetermined gas. Among the atmosphere meters that utilize this principle, hygrometers have a particularly wide range of applications, such as electronic equipment parts such as semiconductors, optical precision equipment,
It is widely used as a detection end for quality control for humidity control in industrial processes such as textiles and foods, and for environmental control in hospitals, buildings, etc. However, the present invention is not limited to a hygrometer, but relates to a general atmosphere meter having different mixed gas concentrations, specifically, the above-mentioned various instruments. Thus, for example, the principles of humidity detection of hygrometers are roughly classified into those that detect the amount of change that changes electrically and mechanically due to humidity, but both electrical and mechanical ones are based on various principles. There is something.

Among these, a hygrometer utilizing the thermal conductivity of gas is known to have excellent responsiveness and high reliability. The amount of heat that flows through the upper and lower surfaces of a predetermined cross section in the isotropic body in the normal direction in a unit time is proportional to the temperature gradient in the normal direction and the cross-sectional area, and this proportional constant is the thermal conductivity. The thermal conductivity of a gas is a function of the constant pressure specific heat, and the constant pressure specific heat is a function of the gas molecular weight. Therefore, the thermal conductivity differs between the case where only air is used and the case where the air contains gas components or water having different molecular weights. A hygrometer utilizing the difference in thermal conductivity of gas obtains humidity from the amount of change in resistance value of a resistor caused by the difference in the amount of heat released from a heated resistor into the atmosphere.

FIG. 2 is a diagram for explaining the principle of a hygrometer as an example of the above-mentioned atmosphere detecting device proposed by the applicant of the present invention. In order to make the explanation easy to understand, FIG. 2 shows the humidity of a conventional hygrometer. Description will be made in comparison with the measurement principle. In FIG. 2, white arrows (a ₁ ) → (b ₁ ) → (c ₁ ) → (d ₁ ) are flow charts of humidity measurement by the applicant, (a ₁ ) → (b ₁ ), (a ₂ ).
→ (b ₂ ) → (b ₃ ) shows the flow of the conventional humidity measurement.
FIG. 2 (a ₁ ) is a structural diagram of the humidity detecting element, and FIG. 2 (a ₂ ) is a structural diagram of the temperature compensating element. In the figure, 1 is a resistor (called a sensor), 2 is a sealing cap, 3 2 is a mesh, 4 is a soaking plate (base), and 5 is a lead pin. The humidity detecting element shown in FIG. 2 (a ₁ ) penetrates the base 4 made of a high thermal conductive material at a predetermined minute interval via a hermetic seal. The detection element 1 is welded to the tips of the parallel lead pins 5 arranged, and the sealing cap 2 having the mesh 3 is fixed to the base 4. The detection element 1 has a positive temperature characteristic, for example, platinum, tungsten, nichrome, kanthal, or a negative temperature coefficient, for example, a fine wire such as SiC (silicon carbide), TaN (tantalum nitride), or the like. A micro temperature sensitive element such as a thin film or a thermistor is connected.

The resistance value of the above-mentioned detection element 1 changes according to the ambient temperature and humidity when it is heated at a low temperature or a high temperature, and its heat capacity is extremely small.
For this reason, the detection element 1 uses a micro temperature sensitive element composed of a fine wire or a micro body in a micro bridge structure, reaches a predetermined thermal equilibrium temperature in a short time during heating, and immediately stops the ambient temperature when heating power is stopped. I try to return to the temperature.

Further, the temperature compensating element 1 shown in FIG. 2 (a ₂ ) has the same standard as the humidity detecting element 1 shown in FIG. 2 (a ₁ ) and the detecting element 1 is hermetically sealed with a mesh-less sealing cap. It was done. Hereinafter, FIG. 2 (a ₁ ) having the above structure
The conventional and humidity detection principle using the temperature compensation device shown in humidity sensing element and Fig. 2 (a ₂₎ as shown, using only humidity sensing element shown in FIG. 2 (a _1), the present applicant The previously proposed humidity detection principle will be described.

FIGS. 3 (a) and 3 (b) are voltage-current characteristic diagrams of the humidity detecting element, and FIG. 3 (a) is a diagram showing the humidity characteristic, in the humidity detecting element of FIG. 2 (a ₁ ). , characteristic curve showing a voltage-current characteristic a ₂ (solid line) when the voltage-current characteristic a ₁ (dotted line) and 0 g / m ³ when the humidity is 200 g / m ³ at ambient temperature 30 ° C. constant, FIG. 3 (b ) Is a diagram showing temperature characteristics, and the voltage-current characteristics B ₁ (dotted line) at a temperature of 20 ° C., the voltage-current characteristics B ₂ (solid line) at a temperature of 30 ° C., and the voltage at 40 ° C. at a humidity of 0 g / m ³ A characteristic curve showing the current characteristic B ₃ (dotted line), where the horizontal axis represents the detection element applied voltage and the vertical axis represents the detection element applied current.

In the voltage-current curve at 30 ° C. showing the humidity characteristic of FIG. 3A, the applied current to the humidity detecting element is 2 m.
At a small heating current of A or less, the corresponding voltage is about 0.8 V or less because the A ₁ curve and the A ₂ curve are substantially overlapped with each other, and the temperature characteristics not affected by humidity at the time of this low current heating. For a large current of 8 mA applied to the detection element, the A ₁ curve shows about 3 V (volts) and the A ₂ curve shows about 4 V. With high current heating, the higher the humidity, the smaller the voltage generated in the detection element. , The voltage corresponding to the humidity can be obtained.

Humidity 0 g / m showing the temperature characteristics of FIG. 3 (b)
^In the voltage-current curve in ^3, since there is no humidity around the detecting element, when a constant current, for example, 2 mA is applied to the detecting element, the voltage generated across the detecting element is the curve B ₁ ,
As shown in B ₂ and B _3, it is shown that the higher the ambient temperature, the higher the temperature, and the lower the ambient temperature, the lower the temperature.

In FIG. 2, in FIG. 2 (b ₁ ), (b ₂ ), (b ₃ ),
(c ₁ ), (d ₁ ) are the absolute humidity (g / m ³ ) on the horizontal axis,
6 is a graph showing the output voltage (V) on the vertical axis. 2 (b)
_1), the ambient temperature is 20 ° C. when applying a current of 8mA humidity detecting device 1 shown in FIG. _{2 (a 1), 30 ℃} , 40
Line B showing the relationship between absolute humidity at ℃ and output voltage
₁₁ is a diagram showing B _12, and B ₁₃ , in which the output voltage and the absolute humidity have a negative proportional relationship and are proportional to the ambient temperature.

On the other hand, in the temperature compensating element shown in FIG. 2 (a ₂ ), as shown in FIG. 2 (b ₂ ), the output voltage naturally changes irrespective of absolute humidity in proportion to only the ambient temperature. To do. Straight lines with ambient temperatures of 20 ℃, 30 ℃, and 40 ℃ are B
₂₁ , B ₂₂ and B ₂₃ .

The output corresponding to absolute humidity at the same ambient temperature is subtracted from FIGS. 2 (b ₁ ) and 2 (b ₂ ). Straight line B
_13, the B ₂₃ from the B ₂₂ from B _12, when each subtracting B ₂₁ from B _11, as shown in FIG. 2 (b _3), regardless of the ambient temperature, the negative proportional only to the absolute humidity A straight line B ₃₃ between the absolute humidity and the output voltage is obtained.

In the hygrometer shown in FIG. 2 (a ₁ ),
When the detection element 1 is driven with a low current, for example, 1 mA,
As shown in FIGS. 2 (a) and 2 (b), an output voltage proportional to only ambient humidity of 20 ° C., 30 ° C. and 40 ° C. is obtained without being affected by the absolute humidity, and the parallel straight lines shown in FIG. 2 (c ₁ ). c ₁ , c
₂ and c ₃ are obtained. This represents the same relationship as FIG. 2 (b _2), and a relation of FIG. 2 (c ₁₎ and 2 (b _1), as shown in FIG. 2 (d _1), 2 ( An output straight line B ₀ having a negative proportional relationship is obtained only in the absolute humidity equal to the characteristic of b ₃ ).

In the above description, the case where the humidity detecting element is driven by a constant current has been described. However, since the heat capacity of the detecting element 1 is extremely small and the response is excellent, it may be driven by a pulse current having a short time width. . Alternatively, constant voltage or constant voltage pulse driving may be used.

[0015] Figure 4 is a diagram for explaining a driving method of the humidity detecting element shown in FIG. 2 (a _1), 4 (a ₁₎
Shows a pulse current drive system, and FIG. 4 (a ₂ ) shows a pulse voltage drive system. That is, the humidity detecting element may be driven by a constant current or a constant voltage as long as it is driven according to the humidity characteristic curve of FIG. 3A and the temperature characteristic curve of FIG. 2B. In the case of the constant current pulse drive shown in FIG. 4 (a ₁ ), the constant current pulse power supply 6 and the detection element 1 are connected in series to detect the voltage Vout across the detection element 1. FIG.
In the pulse voltage driving method of (a ₂ ), the constant voltage pulse power supply 7 and the detection resistor R ₀ and the detection element 1 are connected in series to detect the voltage Vout across the detection resistor R ₀ . In either case, the temperature value during driving is different from that of the detecting element 1.
A type of constant current or constant voltage pulse is applied.

FIG. 4 (b) is shown in FIG. 4 (a).₁) Current pulse drive
FIG. 3 is a diagram showing an example of a current pulse train during operation, in which the detection element
Time t for 1₁From time t₂While reaching the peak value of 2m
A, small pulse current of pulse width 50ms (millisecond) is applied
And then time t₂To t₃Peak value of 8mA
Applies a large pulse current with a pulse width of 50 ms. Time t
₃To t_FourAfter a pause of 100 ms to reach
2mA and 8mA small current pulse and large current pulse of the same time width
Driven by a current pulse train.

FIG. 4C shows a voltage (Vou) generated between the detection elements 1 by the current pulse driving shown in FIG. 4B.
The voltage pulse train of t) is shown, and a voltage pulse with a time delay occurs at the rise of the current pulse. Therefore, it is necessary to detect the voltage within the time width of c ₁ and c ₂ where the voltage value is stable. In the rest period between the times t _{3 and} t ₄ of the drive current pulse train shown in FIG. 4B, the time width in which the heat generation temperature of the detection element 1 becomes substantially the ambient temperature after applying the pulse current of 8 mA was selected. It is a thing.

In FIG. 4B, the small current pulse and the large current pulse are continuously applied to the detecting element 1. However, a large current is applied after a predetermined stabilization time has elapsed after the small current was applied. However, since a time delay occurs for each drive current pulse, high response cannot be detected. On the other hand, according to the driving method shown in FIG. 4B, the response of the output voltage when a large current pulse is applied is small current driving as shown by the dotted line d ₁ in FIG. 4D. Increased due to preheating.

FIGS. 5 (a) and 5 (b) show the current pulse waveform and the voltage pulse output waveform when the large current pulse drive is continuously driven by the small current pulse current. First, in the time width T ₁ . When driven with a large current pulse (8 mA) for temperature / humidity detection and then continuously driven with a small current pulse (2 mA) for temperature detection having a time width T ₂ , the output voltage is as shown in FIG.
As shown by B ₁ in (b), the time delay of the start-up at the time of driving the large current pulse becomes large, and similarly, the time delay at the time of the fall becomes large as shown by B ₂ , so that the small current pulse The output voltage stabilization time during driving becomes long and detection with excellent responsiveness cannot be performed.

FIGS. 6A, 6B, 6C and 6D are views for explaining the relationship between the environmental change and the output characteristic of the humidity detecting element shown in FIG. 2A ₁ . 6A is a temperature change on the time axis, FIG. 6B is a humidity change on the time axis, and FIG.
6C shows an applied current waveform, and FIG. 6D shows a detected output voltage waveform by the applied current corresponding to the temperature change and the humidity change.

The applied current is a continuous pulse current consisting of a large current pulse of 8 mA which is applied subsequently to a small current pulse of 2 mA with a predetermined rest time, and this pulse current is time t _0.
It is outputted once during ~ t ₁ , t ₁ ~ t ₂ and t ₂ ~ t ₃ . On the other hand, the temperature change is a constant temperature of 30 ° C as shown in Fig. 6 (a).
From the time t ₁ to t ₂ at 20 ° C., 30 ° C., 40
It is assumed that the temperature changes to 30 ° C and is kept at 30 ° C during other periods. In addition, as shown in FIG. 6B, the change in humidity is 10 from the constant humidity of 20 g / m ³ to the period of time t ₂ to t _3.
It changes to g / m ³ or 30 g / m ³ .

[0022] Thus, in a period of time t ₀ ~t ₁ temperature, constant humidity both are changing only humidity for a period of time changes only the temperature t ₂ ~t ₃ during a period of time t ₁ ~t _2. As a result, the detected output voltage waveform becomes an output voltage corresponding to a constant temperature and humidity during the period from time t ₀ to t ₁ as shown in FIG. 6D, and a constant humidity during the period from time t _{1 to} t _2. Therefore, the output voltage is proportional to only the temperature, and the subtraction value obtained by subtracting the output voltage during the small current driving from the output voltage during the large current driving becomes constant. In this case, there is no humidity influence. In contrast, in a period of time t ₂ ~t ₃ only humidity changes, the output voltage at the time of a small current drive is constant even when the humidity ,, changes undergone humidity affects only the time of large current driving. The output voltage at this time is small when the humidity is high, and is large when the humidity is low. Next, a description will be given based on a drive circuit that performs such calculation.

FIG. 7 is a block diagram showing an example of a drive circuit in the case of performing humidity measurement using the detecting element shown in FIG. 2 (a ₁ ). In FIG. 7, 11 is a detecting element (sensor) for detecting a constant current. ) 1 supply circuit (constant current circuit), 12 is a coefficient setting circuit, 13 is a hold circuit, 14 is a subtraction circuit, 15 is an output terminal, and the constant current circuit 11 has the detection element 1 and one end grounded. A series resistance with the reference resistance R is loaded, the voltage of the reference resistance R is fed back to the inverting input end of the constant current circuit 11, and the switch SW ₁ having a, b and c contacts is connected to the non-inverting input end. To be done. The a contact of switch SW ₁ has
Reference voltage V _REF 1, b contact V _REF 2, c contact is grounded, reference voltage V _REF 1 and V _REF 2 are V _REF 1 = 2R (mV) (1) V _REF 2 = 8R (mV) (2) is set.

Further, at the output end of the constant current circuit 11, a, b
A switch SW ₂ having a contact is connected, and the a contact has
A coefficient K for setting the temperature at the time of low temperature heating to the ambient temperature at the time of calculating the humidity is connected, for example, a coefficient setting circuit 12 whose amplification degree is variable according to K is connected, and a subtraction circuit is provided at the b contact. It is connected to one input terminal of 14 and inputs the output voltage V ₃ when a large current is applied during high temperature heating.

A switch SW ₃ having an a contact and a c contact and a hold circuit 13 are connected between the coefficient setting circuit 12 and the subtraction circuit 14. The a contact of the switch SW ₃ is connected to the hold circuit 13.

The switches SW ₁ , SW ₂ and SW ₃ of the drive circuit configured as described above are interlocked, and the a, b and c contacts of the switches SW ₁ , SW ₂ and SW ₃ are simultaneously switched by switching. To be When the contact is switched to the a-contact, a voltage equal to the reference voltage V _REF 1 is applied to the reference resistor R connected to the inverting input terminal of the constant current circuit 11, and the detection element 1 receives 2
A constant current of mA flows. Similarly, when switching to b contact,
A voltage equal to the reference voltage V _REF 2 is applied to the reference resistor R, and a constant current of 8 mA flows in the detection element 1.

FIG. 8 is a waveform diagram in each part of the drive circuit shown in FIG. 7. The operation of the drive circuit will be described below with reference to FIGS. 7 and 8. The switch SW ₁ (SW ₂ and SW ₃ are also driven synchronously) is switched in a period of time t ₁ to t ₂ and time t _{2 to} t ₃ by the drive voltage pulse of the voltage waveforms of FIGS. 8A and 8B. In response to the switching, the drive currents 2 mA and 8 mA indicated by is in FIG. a
When switched to the contact, the a contact of switch SW ₂
The voltage V ₁ shown in FIG. 8D, which is proportional to the ambient temperature, is output. The voltage V ₁ is input to the coefficient setting circuit 12 and multiplied by a preset coefficient K, and the voltage V ₂ (=
KV ₁ ) is output. When the switch SW ₂ is switched to the a contact, the output voltage V ₁ is not a value that is correctly proportional to the ambient temperature, and thus is a coefficient for correcting the voltage V ₁ so as to correspond to the ambient temperature K = ( V ₃ - given by a fraction of a variation of humidity) / V ₁ (3).

The hold circuit 13, as shown in FIG. 8 (f), and outputs a voltage V ₂ equal _{V '2 (= V 2)} . The subtraction circuit 18 receives the voltages V ₃ and V ′ ₂ (= V ₂ ) and outputs the voltage V proportional to the absolute humidity shown in FIG. 8 (h). That is, V = V ₃ −V ₂ (4) is obtained.

9A and 9B are configuration diagrams for explaining an example of a conventional atmosphere detecting device suitable for use as described above. FIG. 9A is a plan view and FIG. 9B is a diagram. 9 (a) is a cross-sectional view taken along line BB, in which 21 is, for example, a Si substrate,
Reference numeral 22 is a cavity provided in the Si substrate 21, and as is well known, for example, SiO ₂ or Ta ₂ O is provided on the upper portion of the cavity 22.
_A bridge 23a made of an insulating film 23 such as ₅ is laid,
A resistor pattern 24 made of, for example, Pt, NiCr or the like is provided on the bridge 23a, and SiO ₁ , Ta ₂ is further provided on the resistor pattern (micro heater) 24.
A protective film 25 made of O ₅ or the like is provided, and a current is passed from the bonding wire 26 through the resistor pattern 24a to the resistor pattern 24.
The heat generating portion or the temperature sensitive (heat) portion A is heated.

For example, the case of measuring the humidity by using the detector as described above will be described as an example.
Since the resistance value of 4 depends on the ambient temperature and humidity, for example, first, a minute current such that the humidity sensitivity becomes 0 is measured to measure the resistance value related to the ambient temperature, and then a current having humidity sensitivity is applied. The resistance value related to the ambient temperature and humidity is measured, and then the measured value related to the temperature is subtracted from the measured value related to the temperature and humidity to measure the ambient humidity.

[0031]

Since the atmosphere sensor as described above has a high-speed heat rising characteristic, when it is used to detect the concentration or flow rate of the atmosphere such as humidity or gas,
Intermittent driving of pulse heat generation is sufficient from the viewpoint that the measurement interval is set to the second level and there is no practical problem. The merit in this pulse heating is as follows: a. The power consumption is reduced to almost the pulse duty ratio. b. The life of the resistance heating element is extended by the rest time. As a disadvantage, a. If gas or condensation that is difficult to separate occurs during the downtime, it will be difficult to return to the original state. In particular, since the heat capacity of the heating element is small, it is more serious than the conventional large capacity sensor. b. Since a small current drive and a large current drive are performed, a switching circuit is required, and the cost is increased accordingly.

The present invention has been made in view of the above-mentioned circumstances, and in particular, the atmosphere sensor is always set to, for example, 1
This is intended to prevent a gas or moisture from adhering to the atmosphere sensor by allowing a small current to flow to about 00 ° C. to flow, and further, the atmosphere sensor only when the atmosphere is detected. In addition, for example, a current that is heated to about 400 ° C. is made to flow, so that a switching circuit for a small current and a large current is eliminated.

[0033]

In order to solve the above-mentioned problems, the present invention has (1) an atmosphere sensor having a sensitivity to the surrounding atmosphere at a predetermined temperature, and the atmosphere sensor is provided with a surrounding atmosphere sensor. While measuring the output voltage of the atmosphere sensor by passing a small current with no sensitivity to the atmosphere,
In the atmosphere detection device, wherein the atmosphere sensor applies a large current having sensitivity to the surrounding atmosphere to measure the output voltage of the atmosphere sensor, and detects the surrounding atmosphere from the difference between the two output voltages. The atmosphere sensor is characterized in that a small current is constantly supplied to heat the atmosphere sensor at a low temperature. Further, (2) when the atmosphere is detected, the output voltage of the atmosphere sensor is measured, and then, The large current is passed through the atmosphere sensor to measure the output voltage of the atmosphere sensor, and then the difference between the two output voltages is obtained, or (3) immediately before the large current is passed through the atmosphere sensor. The output voltage of the atmosphere sensor is measured, and the output voltage of the atmosphere sensor is measured immediately after the large current is passed through the atmosphere sensor. .

[0034]

[Function] The atmosphere sensor is always provided with the atmosphere sensor 10
A small current for heating to about 0 ° C. is passed to prevent the gas and water from adhering to the atmosphere sensor, or the current applied to the atmosphere sensor at the time of detecting the atmosphere does not need to use a switching circuit. A large current is passed so that the sensor can generate heat at a temperature at which it has sensitivity.

[0035]

EXAMPLE As shown in FIG. 7, the atmosphere sensor previously proposed by the applicant has a small current source V _REF1 and a large current source V _REF2 , and these are switched by a switch SW _1. There is. Further, since the drive power supply current for driving the atmosphere sensor has a pulse waveform as shown in FIG. 4 (b), the rest period is long, and gas or moisture adheres to the atmosphere sensor during that period, which causes a detection error. Become.

The present invention has been made in view of the actual situation as described above, and the atmosphere sensor as described above has the structure shown in FIG.
As shown in ( ₁₎ , a current I ₁ is constantly passed so that the atmosphere sensor generates heat at about 100 ° C., thereby preventing gas and moisture from adhering to the atmosphere sensor,
Furthermore, only when the atmosphere is detected, a current I ₂ is applied to the atmosphere sensor so that the atmosphere sensor is sensitive to the surrounding atmosphere (eg, humidity). For example, in FIG. 1A, a small current t _{1 is applied.} detecting the ambient temperature for a period of ~t _2, the ambient atmosphere for a period of t ₂ ~t ₃ that a large current (e.g., ambient humidity + ambient temperature) is detected.

FIG. 1B is a clock pulse for measurement timing, FIG. 1C is a timing pulse for ambient temperature detection, FIG. 1D is a timing pulse for ambient temperature and humidity detection, and FIG. ) Is a detection output voltage pulse, and as shown in the figure, the ambient temperature is preferably detected at a timing T ₁ immediately before the atmosphere sensor is heated to a high temperature, and the atmosphere is heated to a high temperature. Period, preferably at timing T ₂ immediately after starting high temperature heating.

As described above, in the present invention, the atmosphere sensor is constantly heated at a low temperature of about 100 ° C., which prevents gas and moisture from adhering to the atmosphere sensor. The ambient temperature is detected during the low temperature heat generation period, and the atmosphere is detected during the high temperature heating period of the atmosphere sensor. It only increases the magnitude of the current applied to the atmosphere sensor (in other words, 2 mA for low temperature heating and 8 mA for high temperature heating, a 6 mA constant current source is prepared, and this constant current source is used as the atmosphere sensor. Since the switching circuit as shown in FIG. 7 is unnecessary, the cost can be reduced accordingly. Further, as described above, if the detection timing of the ambient temperature and the detection timing of the ambient atmosphere are made as close as possible, even if the state of the ambient atmosphere changes, the change is small, and therefore the influence of the change in the state of the ambient atmosphere is small. The ambient atmosphere can be detected more accurately without being affected.

[0039]

As is apparent from the above description, according to the present invention, since the atmosphere sensor is constantly heated at a low temperature,
It is possible to detect the ambient atmosphere more accurately without the ambient gas or moisture from adhering to the ambient sensor. Further, at the time of atmosphere detection, the constant current source may be turned on / off to increase the current applied to the atmosphere sensor, or to return to the original low current state, so a switching circuit unlike the prior art is not required. The cost can be reduced accordingly. Further, by making the detection timing of the ambient temperature close to the detection timing of the ambient atmosphere, the ambient atmosphere can be detected more accurately without being affected by the change in the state of the atmosphere.

[Brief description of drawings]

FIG. 1 is a timing chart for explaining operation timing of an atmosphere detection device according to the present invention.

FIG. 2 is a diagram for explaining the principle of a hygrometer as an example of an atmosphere detection device previously proposed by the applicant.

FIG. 3 is a voltage-current characteristic diagram of a humidity detecting element.

FIG. 4 is a diagram for explaining a driving method of the humidity detecting element shown in FIG. 2 (a ₁ ).

FIG. 5 is a diagram showing a current pulse waveform and a voltage pulse output waveform for driving the humidity detecting element proposed by the applicant earlier.

FIG. 6 is a diagram for explaining the relationship between environmental changes and output characteristics of the humidity detecting element shown in FIG. 2 (a ₁ ).

FIG. 7 is a block diagram showing an example of a drive circuit when performing humidity measurement using the detection element shown in FIG. 2 (a ₁ ).

8 is a waveform chart in each part of the drive circuit shown in FIG.

FIG. 9 is a configuration diagram for explaining an example of a conventional atmosphere detection device.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Resistor (sensor), 2 ... Sealing cap, 3 ... Mesh, 4 ... Soaking plate, 5 ... Lead pin, 6 ... Constant current source, 7 ...
Constant voltage source, 11 ... Constant current source, 12 ... Coefficient setting circuit, 13
... Hold circuit, 14 ... Subtraction circuit, 15 ... Output terminal, 2
1 ... Substrate, 22 ... Cavity, 23 ... Insulating film, 24 ... Resistor pattern, 25 ... Protective film, 26 ... Bonding wire.

Claims (3)

[Claims] 1. An atmosphere sensor having a sensitivity to an ambient atmosphere at a predetermined temperature, and a small current which is not sensitive to the ambient atmosphere of the atmosphere sensor is applied to the atmosphere sensor to measure an output voltage of the atmosphere sensor. At the same time, a large current is sent to the atmosphere sensor so that the atmosphere sensor has a sensitivity to the surrounding atmosphere, and the output voltage of the atmosphere sensor is measured.
An atmosphere detecting device for detecting an ambient atmosphere based on a difference between both output voltages, wherein a small current is constantly applied to the atmosphere sensor to heat the atmosphere sensor at a low temperature. 2. When detecting an atmosphere, the output voltage of the atmosphere sensor is measured, then the large current is passed through the atmosphere sensor to measure the output voltage of the atmosphere sensor, and then the difference between the two output voltages is obtained. The atmosphere detection device according to claim 1, wherein the atmosphere detection device is configured as described above. 3. The output voltage of the atmosphere sensor is measured immediately before the large current is passed through the atmosphere sensor, and the output voltage of the atmosphere sensor is measured immediately after the large current is passed through the atmosphere sensor. The atmosphere detection device according to claim 2, wherein