Thermal gas sensor

The present invention provides a high-responsiveness and high-accuracy thermal gas sensor configured to enable gas to be analyzed based on a variation in heat conductivity. The thermal gas sensor includes a substrate 2 with a cavity portion 5, a thin-film support 6 stacked in the cavity portion and comprising a plurality of insulating layers 8a and 8b, and a first heating member 3 and a second heating member 4 both sandwiched between the insulating layers in the thin-film support. The second heating member is located around a periphery of the first heating member. The first heating member is controlled to a temperature higher than a temperature to which the second heating member is controlled. The concentration of ambient gas is measured based on power applied to the first heating member.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a thermal gas sensor configured to measure components of a gas to be measured, based on a variation in heat conduction in the gas.

2. Background Art

A thermal gas sensor is used to analyze a gas using a variation in heat conduction in the gas. The variation in the heat conduction in the gas is measured based on the amount of heat radiated by a heating member exposed to the gas.

Thermal gas sensors are used in various technical fields. For internal combustion engines for automobiles and the like, there has been a demand to accurately measure the flow rate, temperature, and pressure of intake air as well as an environmental condition such as humidity in order to reduce fuel consumption. Thermal gas sensors are also used to detect the concentration of hydrogen in internal combustion engines for automobiles that use hydrogen as fuel in order to allow the internal combustion engine to be optimally driven.

Thermal gas sensors serving as gas sensors that measure the humidity or the concentration of gas as described above avoid absorbing moisture and are excellent in environment resistance such as contamination resistance and prolonged stability. JP Patent No. 2889909 as a prior art document for such a thermal gas sensor discloses a humidity sensor based on a variation in the resistance value of a resistor heated in an atmosphere and configured to compare a voltage generated across the resistor at a high temperature with a voltage generated across the resistor at a low temperature to sense the humidity, wherein at the low temperature, the variation in resistance is affected only by the atmosphere temperature, and at the high temperature, the variation in resistance is sensitive to the temperature and humidity of the atmosphere.

Furthermore, JP Patent No. 3343801 discloses a humidity sensor including heating means for heating a temperature-sensitive resistor using a heating member. The heating means applies two pulse voltages to the heating member in order within a given time to switch the temperature of the temperature-sensitive resistor between a first value of at least 300° C. and a second value of 100° C. to 150° C. The humidity sensor detects the humidity based on an output voltage associated with a drop in the voltage of the temperature-sensitive resistor at each of the temperatures.

The humidity sensors disclosed in JP Patent Nos. 2889909 and 3343801 are configured to heat the same heating member or temperature-sensitive resistor to the low temperature (second temperature) and to the high temperature (first temperature) in a time division manner.

The configuration that heats the same heating member or temperature-sensitive resistor in a time division manner as described above has the advantage of saving power. However, this configuration is disadvantageous in that an amount of time is required to heat and naturally cool the heating member or temperature-sensitive resistor to the different temperatures, thus reducing response speed.

In particular, the measurement of humidity of intake air for an internal combustion engine requires instantaneousness because the humidity is important data used for the instantaneous calculations of fuel injection time and the like. In connection with such an application, the response speed is a challenge for the conventional humidity sensors disclosed in JP Patent Nos. 2889909 and 3343801.

Furthermore, a signal corresponding to the humidity is conventionally obtained by calculations that use various parameters such as the output voltages associated with drops in the voltage of the temperature-sensitive resistor at the first and second temperatures and the premeasured resistance value of the temperature-sensitive resistor. This is to eliminate the adverse effect of a variation in gas temperature. Thus, when prolonged use deteriorates the resistance value of the temperature-sensitive resistor, errors disadvantageously occur in the parameters to increase the errors in calculations.

Thus, an object of the present invention is to solve the problems with the above-described conventional examples and provide a high-responsiveness and high-accuracy thermal gas sensor that can be used in various environmental conditions.

SUMMARY OF THE INVENTION

To accomplish this object, a thermal gas sensor according to the present invention is configured as follows. A thin-film support is formed in a cavity portion of a substrate. A first heating member and a second heating member are formed on the thin-film support. The second heating member is located around the periphery of the first heating member. The first heating member is controlled to a temperature higher than a temperature to which the second temperature is controlled. The condition of ambient gas is measured based on heating power for the first heating member. Thus, the peripheral temperature of the first heating member detecting a variation in heat conduction in gas can be maintained at a predetermined temperature by the second heating member. This enables a reduction in the adverse effect of a variation in gas temperature and eliminates the need to heat the heating member to the different temperatures in a time division manner. Furthermore, response speed can be increased.

Moreover, the second heating member is formed to surround the first heating member on all sides thereof, and an area in which the second heating member is laid is formed to be larger than an area in which the first heating member is laid. Thus, the temperature of the gas near the first heating member can be more stably maintained at a predetermined temperature. This enables the accuracy of the gas sensor to be improved.

Moreover, heating is controlled so as to make the difference in temperature between the first heating member and the second heating member constant. This allows accurate detection of a variation in the amount of heat radiated by the first heating member which variation results from a variation in the heat conductivity of the gas.

The present invention serves to reduce the adverse effect of the variation in gas temperature and provides a high-responsiveness and high-accuracy thermal gas sensor. The present invention can also simplify a driving circuit, allowing reliability to be improved.

DESCRIPTION OF SYMBOLS

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described below based on the drawings. In the embodiments, an example of an intake system for an internal combustion engine for an automobile will be described which is applied to a thermal gas sensor configured to measure the humidity of intake air.

First Embodiment

FIG. 1is a plan view of sensor elements of a thermal gas sensor showing a first embodiment.FIG. 2is a sectional view taken along line X-X inFIG. 1.

A thermal gas sensor1a includes a substrate2which is formed of a single-crystal silicon. As shown inFIG. 1, a cavity portion5is formed in the substrate2. A first heating member3and a second heating member4are laid in the cavity portion5. Furthermore, a thin-film support6configured to support the heating members is formed to lie over the cavity portion in the substrate2.

Here, the thin-film support6is formed of insulating layers8aand8bstacked on the top surface of the substrate2as shown inFIG. 2. The heating members3and4are interposed between the insulating layers8aand8b. The heating member4is located so as to surround the periphery of the heating member3.

When the heating member4is thus located so as to surround the periphery of the heating member3, the ambient temperature of the heating member3is kept equal to the temperature (T2) of the heating member4. This enables a further reduction in the dependence of the ambient temperature T3. Preferably, the heating member4is located so as to surround the heating member3on all sides thereof.

Furthermore, each of the heating members3and4comprises a resistor having a very small width, extending along the plane of the thin-film support6, and including a plurality of turnup portions. The heating members3and4are electrically connected to electrodes7a,7b,7c, and7dformed on the substrate2for connection to an external circuit.

A material for the heating members3and4is selected from, for example, platinum (Pt), tantalum (Ta), molybdenum (Mo), and silicon (Si), which are stable at high temperatures (which have high melting points). A material for the insulating layers8aand8bis selected from silicon oxide (SiO2) and silicon nitride (Si3N4) and formed into a single layer or a laminate structure. Alternatively, the material for the insulating layers8aand8bmay be selected from a resin material such as polyimide, ceramic, and glass and formed into a single layer or a laminate structure. Furthermore, a material for the electrodes7a,7b,7c, and7dis selected from aluminum (Al), gold (Au), and the like.

The heating members3and4, the insulating layers8aand8b, and the electrodes7a,7b,7c, and7dare formed using a semiconductor micromachining technique that utilizes photolithography and an anisotropic etching technique. In particular, the cavity portion5is formed by anisotropically etching the single-crystal silicon substrate2. Thus, metal that is tolerant to an alkali etching solution used for anisotropic etching may be used as the electrodes7ato7d.

Furthermore, when metal such as aluminum which offers no tolerance is used, preferably the electrodes7ato7dare formed of an alloy of aluminum and silicon so as to be tolerant to an alkali etching solution or a protect film may be formed on the electrodes7ato7d. Anisotropic etching may be performed after forming the protect film.

FIG. 3is a diagram of a driving circuit for the thermal gas sensor1a. The operation of the thermal gas sensor according to the first embodiment will be described with reference toFIG. 3.

The driving circuit for the thermal gas sensor1ais configured to supply heating current to the first heating member3and second heating member4so as to control the first heating member3to a first temperature T1, while controlling the second heating member4to a second temperature T2lower than the first temperature.

The driving circuit for the thermal gas sensor1aincludes a first bridge circuit14aand a second bridge circuit14b, differential amplifiers10aand10b, and transistors11aand11bconfigured to allow heating current to flow through the heating members3and4. InFIG. 3, reference numeral12denotes a power source.

The first bridge circuit14aincludes the heating member3and fixed-value resistors9a,9b, and9c. More specifically, the first bridge circuit14aincludes a series circuit with the heating member3and the fixed-value resistor9cconnected together in series and a series circuit with the fixed-value resistors9aand9bconnected together in series; the series circuits are connected together in parallel. Similarly, the second bridge circuit14bincludes the heating member4and fixed-value resistors9d,9e, and9f. More specifically, the second bridge circuit14bincludes a series circuit with the heating member4and the fixed-value resistor9fconnected together in series and a series circuit with the fixed-value resistors9dand9econnected together in series; the series circuits are connected together in parallel.

In this case, in the first bridge circuit14a, the potential of the connection end between the heating member3and the fixed-value resistor9cand the potential of the connection end between the fixed-value resistors9aand9bare input to the differential amplifier10a. The differential amplifier10aoutputs a voltage corresponding to the difference between the input voltages to a base electrode of the transistor11a. The transistor11acontrols a current flowing between a collector and an emitter in accordance with the potential of the base electrode. The emitter electrode of the transistor11ais connected between the heating member3and fixed-value resistor9ain the first bridge circuit14ato allow the collector-emitter current to flow through the first bridge circuit14a. The resistance value of the fixed-value resistor9ais set to be at least10times as large as that of the heating member3. Thus, almost all of the current from the transistor11aflows to the heating member3, which is thus heated. This configuration feedback-controls the temperature of the heating member3to the first temperature T1, which is a constant temperature of about 300° C.

The temperature of the heating member3is set such that, based on the known temperature coefficient of resistance of the heating member3, the ratio of the resistance value of the heating member3at the first temperature T1to the resistance value of the fixed-value resistor9cis equal to the ratio of the resistance value of the fixed-value resistor9ato the resistance value of the fixed-value resistor9b. When the temperature of the heating member3is lower than the first temperature T1, the transistor11ais turned on to allow heating current to flow from the power source12to the heating member3via the transistor11a.

Similarly, in the second bridge circuit14b, the potential of the connection end between the heating member4and the fixed-value resistor9fand the potential of the connection end between the fixed-value resistors9dand9eare input to the differential amplifier10b. The differential amplifier10boutputs a voltage corresponding to the difference between the input voltages to a base electrode of the transistor11b. The transistor11bcontrols a current flowing between a collector and an emitter in accordance with the potential of the base electrode. The emitter electrode of the transistor11bis connected between the heating member4and fixed-value resistor9din the second bridge circuit14bto allow the collector-emitter current to flow through the second bridge circuit14bfrom the power source12. The resistance value of the fixed-value resistor9dis set to be at least10times as large as that of the heating member4. Thus, almost all of the current from the transistor11bflows to the heating member4, which is thus heated. This configuration feedback-controls the temperature of the heating member4to the second temperature T2, which is a constant temperature of at least 100° C.

The temperature of the heating member4is set such that, based on the known temperature coefficient of resistance of the heating member4, the ratio of the resistance value of the heating member4at the second temperature T2to the resistance value of the fixed-value resistor9fis equal to the ratio of the resistance value of the fixed-value resistor9dto the resistance value of the fixed-value resistor9e. When the temperature of the heating member4is lower than the second temperature T2, the transistor11bis turned on to allow heating current to flow from the power source12to the heating member4via the transistor11b.

FIG. 4shows a temperature condition near the heating members3and4controlled to the first temperature T1and the second temperature T2, respectively. An ambient temperature T3is the temperature of gas which is present around the thermal gas sensor1aand which is to be measured. The ambient temperature T3varies depending on environmental conditions such as the season. For intake air in an internal combustion engine for an automobile, the ambient temperature T3varies between −40° C. and +125° C. The heating member4is heated to the temperature T2by the driving circuit. Even with a variation in ambient temperature T3, the heating temperature of the heating member4is maintained at the value T2. The heating member3is heated by the driving circuit to the temperature T1, which is higher than T2.

Here, the amount Q1of heat radiated to the gas by the heating member3and the amount Q2of heat radiated to the gas by the heating member4are approximately expressed by:
Q1=λ(T1−T2)  (1)
Q2=λ(T2−T3)  (2)

Here, λ is a parameter indicating the heat conductivity of air and varying with the humidity. The expressions indicate that a variation in ambient temperature T3significantly varies the radiation amount Q2of the heating member4, whereas the radiation amount Q1of the heating member3is not affected by the ambient temperature T3. This is equivalent to the heating member3exposed to the air maintained at the constant temperature T2. Thus, since T1and T2are constant, the radiation amount Q1of the heating member3depends only on λ. Since λ varies with the humidity, Q1serves as a signal that depends on the humidity without being affected by a variation in ambient temperature T3.

In Expressions (3) and (4), Q1and Q2denote the amounts of heat radiated to the ambient environment by heat conduction depending on the humidity. QB1denotes the amount of heat transferred through the thin-film support6by heat conduction. QG1and QG2denote the amounts of heat transferred by natural convection and radiation.

Here, it is assumed that the thin-film support6has a small film thickness to provide ideally sufficient heat insulation, making the natural convection and radiation negligible. Then, the power P1and power P2provided to the heating members3and4, respectively, are expressed by:
P1=Q1=V12/R1  (5)
P2=Q2=V22/R1  (6)

Here, V1and V2denote voltages applied to the heating members3and4, respectively. R1and R2denote the resistance values of the heating members3and4, respectively.

Moreover, based on Expressions (1), (2), (5), and (6), the following hold true.
V12=λ(T1−T2)·R1  (7)
V22=λ(T2−T3)·R2  (8)
In Expression (7), T1and T2are constant, and R1is also constant. Thus, the voltage V1applied to the heating member3serves as a signal that depends on λ that varies with the humidity, without being affected by the ambient temperature T3.

FIGS. 5 and 6show the results of experiments according to the present embodiment.FIG. 5shows the results of measurement of the voltage V1applied to the heating member3when the thermal gas sensor according to the present embodiment is installed in the air and when the heating members3and4are heated so that T1=500° C. and T2=250° C., respectively, with the temperature and humidity of the air varied.FIG. 5indicates that V1serves as a signal that depends only on the humidity without being affected by a variation in ambient temperature T3, which is the temperature of the ambient gas.FIG. 6shows the results of measurement of the voltage V2applied to the heating member4and measured simultaneously with the measurement of the voltage V1. The heating member4has the applied voltages varied by a variation in ambient temperature T3to serve to maintain the temperature of the air around the heating member4constant.

For comparison,FIG. 18shows the results of measurement of V1carried out when only the first heating member3driven with the second heating member4not driven. The results inFIG. 18indicate that a variation in ambient temperature T3significantly varies V1to prevent characteristics dependent only on the humidity from being obtained. This indicates that provision of the heating member4controlled to the predetermined temperature T2is effective for reducing the dependence of the heating member3on the air temperature.

FIG. 11shows the results of measurement of the voltage V1applied to the heating member3when the thermal gas sensor according to the present embodiment is installed in the air and when the heating members3and4are heated so that T1=500° C. and T2=200° C., respectively, with the temperature and humidity of the air varied. When the temperature T2of the heating member4is set to 200° C., dependency on the ambient temperature T3occurs. This eliminates the effect of the provision of the heating member4.

The reason is as follows.FIG. 12shows a temperature distribution around the heating member3when only the heating member3is heated. In this case, the heating member4is not heated. The temperature T at any position X between an end of the cavity portion5and an end of the heating member3has a distribution expressed by:
T=T1·X/X3+T3  (9)

Here, T1denotes the temperature of the heating member3, X3denotes the distance from the end of the cavity portion5to the end of the heating member3, and T3denotes the ambient temperature. When the distance from the end of the cavity portion5to the center of the heating member4is denoted by X4, the temperature T2of the heating member4at the position X4is expressed by:
T2=T1·X4/X3+T3  (10)

As indicated by Expression (10), even though the heating member4is not heated, the temperature of the heating member4is raised by heat conduction from the heating member3. Thus, when the heating temperature of the heating member4is set to a value smaller than that calculated by Expression (10), normal heating control is precluded. Hence, the heating temperature T2of the heating member4is desirably set such that:
T2>T1 ·X4/X3+T3  (11)

Furthermore, T1denotes the average temperature of the heating member3. T2denotes the average temperature of the heating member4. T3denotes the ambient temperature. When T3is set to be the maximum value within the temperature range of the gas to be measured, measurement can be appropriately achieved all over the gas temperature range.

Moreover, the temperature T2of the heating member4is desirably set to be lower than the temperature T1of the heating member3. This is because an increase in the temperature T2of the heating member4reduces the value T1−T2in Expression (7) and thus the voltage V1obtained, thus decreasing sensitivity.

As described above, the temperature T2of the heating member4is desirably set such that:
T1·X4/X3+T3<T2<T1  (12)

FIG. 13shows the results of experiments for the sensitivity with which the humidity is detected when the temperature T2of the heating member4is varied with the temperature T1of the heating member3maintained constant. Here, the sensitivity corresponds to the amount ΔP by which the power consumption of the heating member3varies when the humidity is varied. An increase in the temperature T2of the heating member4reduces the sensitivity. This is because the value T1−T2in Expression (1) decreases as described above. Thus, when T1−T2=0, almost no sensitivity is achieved. Therefore, the temperature T2of the heating member4is desirably set to be lower than the temperature T1of the heating member3. Furthermore, when the temperature T2of the heating member4is set to a smaller value within the range indicated by Expression (12), the power consumption can be reduced. As a result, a thermal gas sensor with reduced power consumption is provided.

In the above-described configuration, it is essential that at least the area in which the heating member4is laid is larger than that in which the heating member3is laid. The increased area in which the heating member4is laid enables a further reduction in the adverse effect of a variation in the ambient temperature T3of the heating member3.

Furthermore, setting the temperature of the heating member4to 100° C., at which water evaporates, enables suppression of a rapid variation in heat conductivity resulting from attachment of droplets.

Additionally, the temperature of the heating member4is set to be higher than the range within which the gas temperature is varied by an environmental variation. This is because if the heating member4is exposed to gas at a temperature higher than the set temperature of the heating member4, the heating member4conversely needs to be cooled in order to maintain the temperature of the heating member4constant. The above-described configuration eliminates the need for a cooling mechanism.

In addition, to allow the humidity in the air to be detected, the temperature of the heating member3is desirably set to at least 150° C. This is because when the air temperature is between 100° C. and 150° C., a variation in the heat conductivity in the air associated with the humidity is insignificant, leading to reduced sensitivity.

Furthermore, in the present embodiment, the signal corresponding to the humidity is obtained by means of the voltage applied to the heating member3. However, the signal corresponding to the humidity is also obtained by measuring the current flowing through the heating member or the voltage applied to the fixed-value resistor9c.

Second Embodiment

FIG. 7is a plan view of sensor elements of a thermal gas sensor1baccording to a second embodiment of the present invention. The second embodiment is different from the first embodiment shown inFIG. 1in that a temperature sensor23is located near a heating member3, whereas a temperature sensor24is located near a heating member4. The temperature sensors23and24are extended in a cavity portion5along the heating members3and4, respectively. The temperature sensor23mainly detects the temperature of the heating member3. The temperature sensor24mainly detects the temperature of the heating member4. The temperature sensors23and24are electrically connected to electrodes7e,7f,7g, and7hformed on the substrate2for connection to an external circuit.

The temperature sensors23and24are formed similarly to the heating members3and4. A material for the temperature sensors23and24is selected from platinum (Pt), tantalum (Ta), molybdenum (Mo), silicon (Si), and the like, which are stable at high temperatures (which have high melting points).

FIG. 8is a diagram of the configuration of a driving circuit for the thermal gas sensor1b. The configuration of the driving circuit for the thermal gas sensor1baccording to the second embodiment will be described below with reference toFIG. 8.

The driving circuit for the thermal gas sensor1bincludes a first bridge circuit14cand a second bridge circuit14d, differential amplifiers10aand10b, and transistors11aand11bconfigured to allow heating current to flow through the heating members3and4. Reference numerals12and15inFIG. 8denote power sources.

The first bridge circuit14cincludes the temperature sensor23and fixed-value resistors9a,9b, and9c. More specifically, the first bridge circuit14cincludes a series circuit with the temperature sensor23and the fixed-value resistor9cconnected together in series and a series circuit with the fixed-value resistors9aand9bconnected together in series; the series circuits are connected together in parallel. Similarly, the second bridge circuit14dincludes the temperature sensor24and fixed-value resistors9d,9e, and9f. More specifically, the second bridge circuit14dincludes a series circuit with the temperature sensor24and the fixed-value resistor9fconnected together in series and a series circuit with the fixed-value resistors9dand9econnected together in series; the series circuits are connected together in parallel.

In this case, in the first bridge circuit14c, the potential of the connection end between the temperature sensor23and the fixed-value resistor9cand the potential of the connection end between the fixed-value resistors9aand9bare input to the differential amplifier10a. The differential amplifier10aoutputs a voltage corresponding to the difference between the input voltages to a base electrode of the transistor11a. The transistor11acontrols a current flowing between a collector and an emitter in accordance with the potential of the base electrode from the power source12. The emitter electrode of the transistor11ais connected to the heating member3to allow the collector-emitter current to flow through the heating member3. Thus, the current flowing from the transistor11aserves to heat the heating member3. The temperature of the heating member3is feedback-controlled to a first temperature T1that is a constant temperature of about 300° C.

The temperature of the heating member3is set such that, based on the known temperature coefficient of resistance of the temperature sensor23, the ratio of the resistance value of the temperature sensor23at the first temperature T1to the resistance value of the fixed-value resistor9cis equal to the ratio of the resistance value of the fixed-value resistor9ato the resistance value of the fixed-value resistor9b. When the temperature of the temperature sensor23is lower than the first temperature T1, the transistor11ais turned on to allow heating current to flow through the heating member3. At this time, the temperature of the heating member3is almost equivalent to that of the temperature sensor23.

Similarly, in the second bridge circuit14d, the potential of the connection end between the temperature sensor24and the fixed-value resistor9fand the potential of the connection end between the fixed-value resistors9dand9eare input to the differential amplifier10b. The differential amplifier10boutputs a voltage corresponding to the difference between the input voltages to a base electrode of the transistor11b. The transistor11bcontrols a current flowing between a collector and an emitter in accordance with the potential of the base electrode from the power source12. The emitter electrode of the transistor11bis connected to the heating member4to allow the collector-emitter current to flow through the heating member4. Thus, the current flowing from the transistor11bserves to heat the heating member4. The temperature of the heating member4is feedback-controlled to a second temperature T2that is a constant temperature of about 100° C. At this time, the temperature of the heating member4is almost equivalent to that of the temperature sensor24.

Also in the above-described configuration, the signal corresponding to the humidity is obtained by measuring the voltage applied to the heating member3.

The present embodiment is advantageous in that the heating members3and4are electrically disconnected from the bridge circuits14cand14d, respectively, eliminating the need to allow a large current for heating to flow through the bridge circuits. Thus, the resistance of the resistor included in each of the bridge circuits14cand14dmay be set to a large value to reduce the current flowing through the bridge circuit. Furthermore, the voltages applied to the bridge circuits14cand14dcan be reduced. This reduces power loss in the temperature sensors and resistors included in the bridge circuits14cand14d, enabling power saving.

Furthermore, in the present embodiment, a signal corresponding to the humidity is obtained by means of the voltage applied to the heating member3. However, the signal corresponding to the humidity is also obtained by measuring the current flowing through the heating member.

Third Embodiment

FIG. 9is a plan view of sensor elements of a thermal gas sensor1caccording to a third embodiment of the present invention. The third embodiment is different from the second embodiment shown inFIG. 6in that in addition to a temperature sensor24, a temperature sensor34is located near a heating member4. The temperature sensor34is extended in a cavity portion5along the temperature sensor24. Like the temperature sensor24, the temperature sensor34mainly detects the temperature of the heating member4. The temperature sensor34is electrically connected to electrodes7iand7jformed on the substrate2for connection to an external circuit.

The temperature sensor34is formed similarly to heating members3and4and the temperature sensors23and24. A material for the temperature sensor34is selected from platinum (Pt), tantalum (Ta), molybdenum (Mo), silicon (Si), and the like, which are stable at high temperatures (which have high melting points).

FIG. 10is a diagram of the configuration of a driving circuit for the thermal gas sensor1c. The configuration of the driving circuit for the thermal gas sensor1caccording to the third embodiment will be described below with reference toFIG. 10.

The driving circuit for the thermal gas sensor1cincludes a first bridge circuit14eand a second bridge circuit14d, differential amplifiers10aand10b, and transistors11aand11bconfigured to allow heating current to flow through the heating members3and4. Reference numerals12and15inFIG. 10denote power sources.

The first bridge circuit14eincludes the temperature sensor23, the temperature sensor34, and fixed-value resistors9band9c. More specifically, the first bridge circuit14eincludes a series circuit with the temperature sensor23and the fixed-value resistor9cconnected together in series and a series circuit with the temperature sensor34and the fixed-value resistor9bconnected together in series; the series circuits are connected together in parallel. The second bridge circuit14dincludes the temperature sensor24and fixed-value resistors9d,9e, and9f. More specifically, the second bridge circuit14dincludes a series circuit with the temperature sensor24and the fixed-value resistor9fconnected together in series and a series circuit with the fixed-value resistors9dand9econnected together in series; the series circuits are connected together in parallel.

In this case, in the first bridge circuit14e, the potential of the connection end between the temperature sensor23and the fixed-value resistor9cand the potential of the connection end between the temperature sensor34and the fixed-value resistor9bare input to the differential amplifier10a. The temperature of the heating member3is feedback-controlled to a first temperature T1. At this time, the temperature of the heating member3is almost equivalent to that of the temperature sensor23.

The temperature of the heating member3is set such that, based on the known temperature coefficient of resistance of the temperature sensor23and the temperature sensor34, the ratio of the resistance value of the temperature sensor23at the first temperature T1to the resistance value of the fixed-value resistor9cis equal to the ratio of the resistance value of the temperature sensor34at a second temperature T2to the resistance value of the fixed-value resistor9b. When the temperature of the temperature sensor23is lower than the first temperature T1, the transistor11ais turned on to allow heating current to flow through the heating member3.

This also applies to the second bridge circuit14das described in Embodiment 2.

In the present embodiment, the temperature of the heating member3is controlled so as to be higher than the temperature T2of the heating member4by a given value. Thus, the heating member3is controlled to the first temperature Tl.

Also in the above-described configuration, a signal corresponding to the humidity is obtained by measuring the voltage applied to the heating member3.

In the present embodiment, the heating members3and4are electrically disconnected from the bridge circuits14eand14d, respectively, eliminating the need to allow a large current for heating to flow through the bridge circuits. Thus, the resistance of each of the temperature sensors and resistors included in the bridge circuits14eand14dmay be set to a large value. Furthermore, the voltages applied to the bridge circuits14eand14dcan be reduced.

Moreover, even if a current flowing through the temperature sensor34included in the bridge circuit14eresults in power consumption, since the current contributes to raising the temperature of the heating member4, the power can be effectively utilized. This reduces power loss in the resistors included in the bridge circuits14cand14d, enabling power saving.

Furthermore, when the temperature sensors23and34are formed of the same material, resistance characteristics and processing conditions are the same for both temperature sensors. This improves resistance balance, enabling a reduction in the degradation of the resistance balance caused by a processing variation.

Fourth Embodiment

FIG. 14is a plan view of sensor elements of a thermal gas sensor1daccording to a fourth embodiment of the present invention. The fourth embodiment is different from the third embodiment shown inFIG. 9in that temperature sensors35and36are arranged on the thermal gas sensor11d. The temperature sensors35and36are arranged outside the cavity portion5to mainly detect the ambient temperature. The temperature sensor35is electrically connected to electrodes7kand71formed on a substrate2for connection to an external circuit. Furthermore, the temperature sensor36is electrically connected to electrodes7mand7nformed on the substrate2for connection to an external circuit.

The temperature sensors35and36are formed similarly to heating members3and4and temperature sensors23and24. A material for the temperature sensors35and36is selected from platinum (Pt), tantalum (Ta), molybdenum (Mo), silicon (Si), and the like, which are stable at high temperatures (which have high melting points).

FIG. 15is a diagram of the configuration of a driving circuit for the thermal gas sensor1d. The configuration of the driving circuit for the thermal gas sensor1daccording to the fourth embodiment will be described below with reference toFIG. 15.

The driving circuit for the thermal gas sensor1dincludes a first bridge circuit14fand a second bridge circuit14d, differential amplifiers10aand10b, and transistors11aand11bconfigured to allow heating current to flow through the heating members3and4. Reference numerals12and15inFIG. 14denote power sources.

The first bridge circuit14fincludes the temperature sensor23, a temperature sensor34, and the temperature sensors35and36. More specifically, the first bridge circuit14fincludes a series circuit with the temperature sensors23and35connected together in series and a series circuit with the temperature sensors34and36connected together in series; the series circuits are connected together in parallel. The second bridge circuit14dincludes the temperature sensor24and fixed-value resistors9d,9e, and9f. More specifically, the second bridge circuit14dincludes a series circuit with the temperature sensor24and the fixed-value resistor9fconnected together in series and a series circuit with the fixed-value resistors9dand9econnected together in series; the series circuits are connected together in parallel.

In this case, in the first bridge circuit14f, the potential of the connection end between the temperature sensors23and35and the potential of the connection end between the temperature sensors34and36are input to the differential amplifier10a. The differential amplifier10aoutputs a voltage corresponding to the difference between the input voltages to a base electrode of the transistor11a. The transistor11acontrols a current flowing between a collector and an emitter in accordance with the potential of the base electrode. The emitter electrode of the transistor11bis connected to the heating member3to allow the collector-emitter current to flow through the heating member3. Thus, the heating member3is heated and feedback-controlled to a first temperature T1. At this time, the temperature of the heating member3is almost equivalent to that of the temperature sensor23. Furthermore, the temperature of the temperature sensor35is almost equivalent to that of the temperature sensor36.

The temperature of the heating member3is set such that, based on the known temperature coefficient of resistance of the temperature sensor23and the temperature sensor34, the ratio of the resistance value of the temperature sensor23at the first temperature T1to the resistance value of the temperature sensor35is equal to the ratio of the resistance value of the temperature sensor34at a second temperature T2to the resistance value of the temperature sensor36. When the temperature of the temperature sensor23is lower than the first temperature T1, the transistor11ais turned on to allow heating current to flow from the power source12to the heating member3via the transistor11a.

This also applies to the second bridge circuit14d as described in Embodiment 2.

In the present embodiment, the temperature of the heating member3is controlled so as to be higher than the temperature T2of the heating member4by a given value. Thus, the heating member3is controlled to the first temperature T1.

Also in the above-described configuration, a signal corresponding to the humidity is obtained by measuring the voltage applied to the heating member3.

In the present embodiment, the temperature sensors23,34,35, and36included in the bridge circuit14fare formed of the same material. Thus, the resistance characteristics and processing conditions are the same for all the temperature sensors. This improves the resistance balance. Furthermore, the degradation of the resistance balance associated with processing can be reduced.

Additionally, heat generated by currents flowing through the temperature sensors35and36raises the temperature of the thermal gas sensor1d, contributing to raising the temperatures of the heating members3and4. If a fixed-value resistor is used, since the fixed-value resistor is provided away from the thermal gas sensor1d, heat generated by a current flowing through the fixed-value resistor is radiated to the surroundings. Thus, the heat fails to contribute to raising the temperature of the heating members3and4. Therefore, the present embodiment allows power to be effectively used, enabling a reduction in power required. Furthermore, all the resistors included in the bridge circuit14fcan be formed inside the elements of the thermal gas sensor1d. This enables a reduction in the number of components required and thus in the size of the thermal gas sensor1d.

Moreover, as shown inFIG. 15, when the fixed-value resistors9fand9eare formed in the thermal gas sensor1dsimilarly to the temperature sensors35and36, the size of the thermal gas sensor1dcan further be reduced.

Fifth Embodiment

FIGS. 16 and 17are plan views showing a thermal flowmeter46with a composite sensor49to which the thermal gas sensor1ashown in Embodiment1is applied and which is provided integrally with a thermal airflow rate sensor37. The thermal flowmeter46according to the present embodiment is mounted, for example, in an intake pipe51in an internal combustion engine. The thermal gas sensor1a, the thermal airflow rate sensor37, and a driving LSI38are installed on a base member39. Furthermore, a housing member40provided so as to cover the base member39forms a sub-passage45configured to take in an airflow43and cavity portions47and48. The thermal airflow rate sensor37is installed in the sub-passage45. The cavity portion47is in communication with the sub-passage45. The thermal gas sensor1ais installed in the cavity portion47. The driving LSI38is installed in the cavity portion48.

Now, electric connections of the composite sensor49will be described with reference toFIG. 17. The thermal gas sensor1ais connected to the driving LSI38via an inner layer conductor44provided inside the base member39using bonding wires such as gold wires. Dotted lines inFIG. 17show the inner layer conductor44. The thermal airflow rate sensor37is also connected to the driving LSI38via the inner layer conductor44provided inside the base member39using bonding wires such as gold wires. The driving LSI38includes a circuit configured to drive the thermal gas sensor la and a circuit configured to drive the thermal airflow rate sensor37. The driving LSI38loads an electric signal related to the humidity from the thermal gas sensor la and an electric signal related to an air mass flow rate from the thermal airflow rate sensor37. The driving LSI38is connected to a terminal section42through bonding wires41of aluminum or the like via bonding wires such as gold wires and the inner layer conductor. The signals related to the humidity and the airflow rate can be transmitted from the terminal section42to an external circuit.

An arithmetic unit is mounted in the driving LSI38to execute calculations using the signal from the thermal airflow rate sensor37and the signal from the thermal gas sensor. The composite sensor can thus correct errors resulting from the dependence of the thermal airflow rate sensor37on the humidity. Hence, the airflow rate can be accurately calculated and output.