JP5981301B2 - Thermal gas sensor - Google Patents

Description

  The present invention relates to a thermal gas sensor that measures, for example, a flow rate, temperature, pressure, concentration, humidity, and other physical quantities of gas from a change in heat conduction of a gas to be measured.

  A thermal gas sensor is used to measure, for example, flow, temperature, pressure, concentration, humidity, and other physical quantities of a gas using changes in the heat conduction of the gas. Measured by the amount of heat released from the heating element exposed to.

  Thermal gas sensors are used in various technical fields, and in internal combustion engines for automobiles, in order to reduce fuel consumption, environmental conditions such as humidity, in addition to the flow rate, temperature, and pressure of intake air are highly accurate. It is required to be measured. The thermal gas sensor is also used to optimally operate the internal combustion engine by detecting the hydrogen concentration in an automobile internal combustion engine using hydrogen as a fuel.

  A thermal gas sensor as a gas sensor for measuring humidity and gas concentration does not absorb moisture and is excellent in environmental resistance such as fouling and long-term stability. Patent Document 1 describes a hygrometer that detects humidity based on a change in the resistance value of a resistor that is heated in an atmosphere. In this hygrometer, the resistance generated when the resistance change is heated to a low temperature that is affected only by the ambient temperature, and the resistance generated when the resistance change is heated to a high temperature that is sensitive to the ambient temperature and humidity. Humidity is detected by comparing the voltage generated at both ends of the body. In Patent Document 1, a small pulse current having a peak value of 2 mA and a pulse width of 50 ms is applied to the detection element for 50 ms from time t1 to time t2, and then 50 ms from time t2 to time t3. During the period, a large pulse current having a peak value of 8 mA and a pulse width of 50 ms is applied, and after a pause of 100 ms from time t3 to time t4, a small pulse current and a large pulse current are applied again. An example of driving is disclosed.

  Further, Patent Document 2 has a heating unit that heats the temperature-sensitive resistor by a heating element, and this heating unit applies two pulse voltages to the heating element in order within a predetermined time, thereby sensing the sensitivity. A humidity sensor that switches the temperature of the temperature resistor to a first temperature of 300 ° C. or higher and a second temperature of 100 ° C. to 150 ° C., and detects humidity from an output voltage related to a voltage drop of each temperature sensitive resistor Is disclosed.

  Patent Document 3 discloses a substrate having a cavity, a thin film support that is stacked in the cavity and is composed of a plurality of insulating layers, a first heating element sandwiched between the insulating layers of the thin film support, and A second heating element, the second heating element is disposed around the first heating element, the first heating element is controlled to a temperature higher than that of the second heating element, and the first heating element A gas sensor is disclosed that measures the concentration of ambient gas based on the power applied to.

Japanese Patent No. 2889909 Japanese Patent No. 3343801 JP2011-137679A

  In the thermal gas sensor of Patent Document 2, it is necessary to increase the heating temperature of the heating element to 300 ° C. or higher in order to measure the gas concentration change with high accuracy. When the heating element is heated at a high temperature, the resistance value of the heating element changes with time, and the measurement accuracy is deteriorated and the life is shortened. Causes of the change with time include oxidation due to exposure to a high temperature for a long time and migration due to a current flowing through a heating element because of heating to a high temperature. For this reason, it is effective to provide a period for stopping heating as much as possible.

  In the thermal gas sensor (hygrometer) of Patent Document 1, a pause time of 100 ms is provided between applying a small pulse current and a large pulse current and applying the small pulse current and the large pulse current again. During this pause time, the supply of current to the detection element is stopped and the heating control of the detection element is stopped.

  However, when the heating of the sensor element of the thermal gas sensor is stopped, a pollutant such as particles floating in the measurement environment is likely to adhere to the sensor element. In addition, in a high humidity environment, if water droplets adhere due to condensation, an error occurs in measurement accuracy. Further, a temperature cycle due to a difference in temperature of the heater at the time of driving and stopping is frequently applied, and fatigue due to thermal stress occurs. In particular, when used for measuring the humidity of intake air in internal combustion engines, high-precision humidity measurement is desired under severe conditions such as floating of pollutants such as oil and carbon, and changes in the humidity environment depending on the weather and regions. .

  An object of the present invention is to provide a thermal gas sensor that has little change over time, high antifouling property, and relaxes thermal stress caused by a temperature cycle.

In order to solve the above-described problems, a thermal gas sensor according to the present invention is a thermal gas sensor having a heating element and a heating control means for controlling heating of the heating element. A period in which the heating element is controlled to be heated, a period in which the heating element is controlled to be heated to a second temperature lower than the first temperature, and the heating element is controlled to be lower than the first temperature and the second temperature. A heating control period is set to a third temperature in a range of ˜150 ° C., the heating element is controlled to be heated, a heating control period to the second temperature is set as a standby period, and the first temperature and the first temperature are controlled. The detection operation by the sensor element including the heating element is performed during the period of heating control to the temperature of 3, and the period of heating to the second temperature is set longer than the period of heating to the first temperature.

  According to the present invention, it is possible to obtain a highly accurate and reliable thermal gas sensor in which deterioration of a sensor element with time is reduced, fouling resistance is improved, and thermal stress caused by a temperature cycle is reduced.

The top view of the gas sensor in the reference example 1. FIG. Sectional drawing of the gas sensor in the reference example 1. FIG. The heating control means of the gas sensor in Reference Example 1 . 6 is a time chart showing the operation of the gas sensor in Reference Example 1 . 4 is a time chart showing the temperature of the heating element of the gas sensor in Reference Example 1 . Resistance degradation of the heating element of the gas sensor in the first embodiment. The time chart which shows the temperature of the heat generating body of the gas sensor in a 1st Example. The top view of the gas sensor in the reference example 2. FIG. Sectional drawing of the gas sensor in the reference example 2. FIG. The heating control means of the gas sensor in Reference Example 2 . 9 is a time chart showing the operation of the gas sensor in Reference Example 2 . 6 is a time chart showing the temperature of the first heating element of the gas sensor in Reference Example 2. FIG.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the following embodiments, an example will be described in which the present invention is applied to a thermal gas sensor that is incorporated in an intake system of an internal combustion engine for an automobile and measures the humidity of the intake air.
[Reference Example 1]

FIG. 1 is a plan view of a thermal gas sensor showing a reference example, and FIG. 2 is a sectional view taken along the line II-II of the sensor element 1 of FIG.

The sensor element 1 of the thermal gas sensor of this reference example has a substrate 2 formed of single crystal silicon. A cavity 4 is formed in the substrate 2, and the cavity 4 is covered with an insulating film 3a, and a heating element 5 is formed on the insulating film 3a. On the insulating film 3a, a heating element 5 and temperature sensitive resistors 6-9 are formed. The heating element 5 is formed on the insulating film 3 a on the cavity portion 4, and the temperature sensitive resistors 6 to 9 are formed on the insulating film 3 a in a portion away from the cavity portion 4. Further, the surface is covered with an insulating film 3b in order to protect the heating element 5 and the temperature sensitive resistors 6-9. Further, electrodes 10a to 10f for supplying and taking out voltage and current are formed on the heating element 5 and the temperature sensitive resistors 6 to 9, respectively. Furthermore, the electrodes 10a to 10f are electrically connected to the heating control device 12 by gold wire bonding wires 11a to 11f or the like.

  As the heating element 5 and the temperature sensitive resistors 6 to 9, for example, platinum (Pt), tantalum (Ta), molybdenum (Mo), silicon (Si), or the like is selected as a material having a high temperature coefficient of resistance. As 3a and 3b, silicon oxide (SiO2) and silicon nitride (Si3N4) are selected in a single layer or stacked structure. Further, as the insulating layers 3a and 3b, a resin material such as polyimide, ceramic, glass, or the like can be selected in a single layer or a laminated configuration. In addition, as the electrodes 10a to 10f, aluminum (Al), gold (Au), or the like is selected.

  The heating element 5, the temperature sensitive resistors 6-9, the cavity 4, the insulating films 3a, 3b, and the electrodes 10a-10f are formed using a semiconductor microfabrication technique using photolithography and an anisotropic etching technique. In particular, since the cavity 4 is formed by anisotropic etching of the single crystal silicon substrate 2, it is better to use a metal resistant to an alkaline etching solution used for anisotropic etching for the electrodes 10a to 10f. .

  Further, when a metal having no resistance to an alkaline etching solution such as aluminum is used, the electrodes 10a to 10f are made of an alloy of aluminum and silicon so as to have resistance, or a protective material is provided on the electrodes 10a to 10f. It is preferable to perform anisotropic etching after forming the film.

FIG. 3 is a configuration diagram of the heating control device 12 of the sensor element 1 of the thermal gas sensor in this reference example. Hereinafter, the configuration of the heating control device for the thermal gas sensor in this reference example will be described with reference to FIG.

  The heating control device 12 is configured to supply a heating current to the heating element 5 incorporated in the bridge circuit 23 so as to maintain the balance of the bridge circuit 23 and to control the heating element 5 to a predetermined temperature. Yes. The bridge circuit 23 also constitutes a part of the heating control device 12.

The bridge circuit 23 includes a series circuit 23a in which the heating element 5 and the fixed resistor 13 are connected in series, a temperature sensing resistor 6, a temperature sensing resistor 7, a temperature sensing resistor 8, and a temperature sensing resistor 9 in series. The series circuit 23b is connected in parallel. As the fixed resistor 13, a resistor having a resistance temperature coefficient as small as possible is selected. A voltage (first divided voltage) between the heating element 5 and the fixed resistor 13 is input to the differential amplifier 15. A voltage (second divided voltage) between the temperature sensitive resistor 6 and the temperature sensitive resistor 7 is input to the differential amplifier 15 via the switch SW3 of the switch circuit 14. A voltage (third divided voltage) between the temperature sensitive resistor 7 and the temperature sensitive resistor 8 is input to the differential amplifier 15 via the switch SW2 of the switch circuit 14. A voltage (fourth divided voltage) between the temperature sensitive resistor 8 and the temperature sensitive resistor 9 is input to the differential amplifier 15 via the switch SW1 of the switch circuit 14. The switch circuit 14 selects the voltage input to the differential amplifier 15 by electrically opening and closing. The first divided voltage is input to one input terminal (plus terminal in this reference example) of the differential amplifier 15 as the divided voltage of the series circuit 23a, and the second divided voltage and the third divided voltage are The fourth divided voltage is inputted to one input terminal (minus terminal in this reference example) of the differential amplifier 15 as a divided voltage of the series circuit 23b, and the difference between the input voltage of the plus terminal and the input voltage of the minus terminal. Accordingly, the supply current from the differential amplifier 15 to the bridge circuit 23 changes.

  The switch circuit 14 can be formed by a semiconductor switch using a MOS transistor or the like. In this case, it is possible to switch at high speed electrically, and it is possible to make the heating control device 12 as one chip as an LSI, and to reduce the size. The output of the differential amplifier 15 is connected between the heating element 5 and the temperature sensitive resistor 6 of the bridge circuit 23, and supplies a heating current to the heating element 5.

  The heating temperature of the heating element 5 can be changed depending on which of the switches SW1 to SW3 of the switch circuit 14 is closed. When SW1 is closed, the highest temperature is controlled, and when SW3 is closed, the lowest temperature is controlled. When only SW2 is closed, the temperature is controlled between when SW1 is closed and when SW3 is closed. Thus, it is possible to switch to a plurality of temperatures with a simple configuration.

  When such a heating control device 12 is used, when the humidity changes, the power for heating the heating resistor 5 to a predetermined temperature changes. Therefore, the humidity can be measured by taking out and calculating the change in the heating voltage Vh1 of the differential amplifier 15. The heating voltage Vh <b> 1 is amplified by the differential amplifier 17, taken into the sample hold circuit 18, the AD converter 19, and the digital signal processor 20, and high-precision humidity data Hout can be obtained by calculation. The switch circuit 14 and the sample hold circuit 18 are controlled by a switch control circuit 21.

  The switch control circuit 21 generates an opening / closing timing of the switch circuit 14 and an operation signal of the sample hold circuit based on the oscillator 22. The switch control circuit 21 can also be operated based on a signal STB from the outside of the heating control means 12 (standby function). If the external signal STB is used, for example, the heating temperature of the heating element 5 can be switched based on a command from an ECU that is a control device of the internal combustion engine. Although the heating can be completely stopped by turning off the power, the heating element 5 is not heated and the contamination proceeds. With this standby function, during periods when humidity measurement is not required, the heating temperature of the heating element 5 can be switched to save power, reduce the contamination of the sensor element, and operate until the humidity is detected after recovery. You can also speed up.

FIG. 4 is a timing chart showing the operation of the heating control device 12 of the sensor element 1 of the gas sensor in this reference example. Hereinafter, the operation of the heating controller 12 of the thermal gas sensor in this reference example will be described with reference to FIGS.

  4 is a clock waveform generated by the oscillator 22. Reference numerals tsw1 to tsw3 denote signals for opening and closing the switches SW1 to SW3 of the switch circuit 14. It is “closed” when ON, and “open” when OFF. As the opening / closing timing of the switches SW1 to SW3, first, tsw2 is turned ON at the timing t1 of CLK, and the switch SW2 of the switch circuit 14 is “closed”. Next, at the timing t4 of CLK, tsw1 is turned ON, and the switch SW1 of the switch circuit 14 is “closed”. Next, at the timing t7 of CLK, tsw3 is turned ON, and the switch SW3 of the switch circuit 14 is “closed”. The period during which these switches SW1 to SW3 are “closed” is the longest period during which SW3 is “closed”.

  Vh1 in FIG. 4 indicates a change in the output voltage value of the differential amplifier 15 in FIG. 3, that is, a voltage for heating the heating element 5. When Vh1 is large, the heating temperature of the heating element 5 becomes high. Vh1 varies depending on which switch among SW1 to SW3 is closed. When tsw2 is ON, Vh1 becomes a voltage value necessary for heating the heating element 5 to a constant temperature in the range of 100 ° C to 150 ° C. When tsw1 is ON, Vh1 becomes a voltage value necessary for heating the heating element 5 to a constant temperature of 300 ° C. or higher. When tsw3 is ON, Vh1 is a voltage value necessary for heating the heating element 5 to a constant temperature of 100 ° C. or lower.

  In the following description, when the switch SW1 is turned on and the heating element 5 is heated to a constant temperature of 300 ° C. or higher, the heating temperature is the first temperature, and the switch SW3 is turned on and the heating element 5 is 100 ° C. or less. The heating temperature when heated to a constant temperature is the second temperature, and the heating temperature when the switch SW2 is turned on and the heating element 5 is heated to a constant temperature in the range of 100 ° C to 150 ° C is the third temperature. To do.

In this reference example, the temperature range of the second temperature is 100 ° C. or less, the third temperature is in the range of 100 ° C. to 150 ° C., and both the second temperature and the third temperature are 100 ° C. The second temperature and the third temperature are the same. The second temperature and the third temperature are set to different temperatures within the above temperature ranges so that the second temperature is lower than the third temperature.

  Vh2 in FIG. 4 is a voltage obtained by amplifying the change in Vh1 due to humidity by the differential amplifier 17 in FIG. Specifically, it is a value obtained by amplifying the difference voltage between Vh1 and the reference voltage source 16.

  4 is a signal generated by the switch control circuit shown in FIG. 3, and is a signal for controlling the operation of the sample and hold circuit 18. When tsh is ON, the sample hold circuit 18 reads the input voltage Vh2. When tsh is OFF, the read voltage value of Vh2 is held. The timing at which the sample hold circuit 18 reads Vh2 is a period (t2, t5) in which the switch SW1 or the switch SW2 is “closed”. The values Vh3 (L) and Vh3 (H) read at the timings t2 and t5 are read by the AD converter 19 in FIG. 3 and become digital data. Further, the absolute humidity Hout is calculated by the digital signal processor 20 using the values of Vh3 (L) and Vh3 (H).

  FIG. 5 shows the temperature change of the heating element 5 due to the operation of the switches SW1 to SW3. When the switch SW2 is “closed” (tsw2 is ON) (period d1), the heating element 5 is held at the third temperature Th3 in the range of 100 ° C. to 150 ° C. When the switch SW1 is “closed” (tsw1 is ON) (period d2), the heating element 5 is held at the first temperature Th1 in the range of 300 ° C. or higher. When the switch SW3 is “closed” (tsw3 is ON) (period d3), the heating element 5 is held at the second temperature Th2 in the range of 100 ° C. or less. The heating element 5 is energized and controlled in the period d3 as well. For this reason, the second temperature Th2 is higher than the ambient temperature or the temperature Tb of the substrate 2 of the sensor element 1.

  For measuring the humidity, a period d1 in which the temperature of the heating element 5 is Th3 and a period d2 in which the temperature of the heating element 5 is provided, and data on the heating voltage of the heating element 5 in the two temperature states is necessary. Therefore, the period d1 and the period d2 are periods for humidity measurement (detection operation period). The period d3 in which the temperature of the heating element 5 is Th2 is a period (standby period) in which the operation for measuring humidity is stopped. Among these periods d1, d2, and d3, the period d2 that is the standby period is the longest.

  The effect in this structure is demonstrated. By providing the state of d2 described above, it is possible to provide a period for lowering the temperature of the heating element 5, and to reduce the deterioration of the sensor element 1 (particularly the heating element 5). This is because the progress of deterioration can be reduced when the temperature of the heating element 5 decreases. Further, in the state of d2, since the heating element 5 and its periphery are kept at a higher temperature than the substrate temperature Tb, the periphery of the heating element 5 is repelled by particles that become a pollutant due to an increase in the molecular motion of air by heating. And the adhesion of fouling substances can be reduced. Moreover, when the environment is high in humidity, it is possible to prevent water droplets from adhering due to condensation. Further, by heating the heating element 5 in the standby period d3 and increasing its temperature, the temperature difference of the heating element 5 that rises and falls through the periods d1, d2, and d3 is reduced as compared with the case where the heating element 5 is not heated. Fatigue due to stress that repeats expansion and contraction due to temperature cycling can be reduced. Of these periods d1, d2, and d3, by setting the duty ratio so that the period of d3 becomes the longest, the number of times of receiving stress due to the temperature difference can be reduced, and the period d2 where the temperature becomes high is relatively set. The progress of deterioration can be further reduced.

A more effective embodiment to which the present invention is applied will be described. This embodiment has a configuration in which the error due to contamination is further reduced in the gas sensor of Reference Example 1.

FIG. 6 shows the results of evaluating the resistance deterioration of the heating element 5 formed of a Si material such as polycrystalline silicon or a metal material such as platinum (Pt). In Si material, the progress of deterioration is accelerated at 200 ° C. or more, and the resistance value fluctuates greatly. In a metal material such as Pt, the progress of deterioration is accelerated at 300 ° C. or higher, and the resistance value fluctuates greatly. Therefore, in order to delay the progress of deterioration in the Si material, an effect can be obtained by providing a period during which the heating temperature is lowered to 200 ° C. or less. In the case of a metal material such as Pt, an effect can be obtained by providing a period for lowering the temperature to 300 ° C. or lower. On the other hand, in order to reduce the adhesion of particles that become fouling substances, the higher the temperature of the heating element 5, the more effective the reduction. In Reference Example 1, the set value of the temperature Th2 of the heating element 5 in the period d3 in FIG. 5 is set to 100 ° C. or lower, which is lower than Th3, but the effect of suppressing the resistance deterioration of the heating element 5 can be obtained even at higher temperatures. Therefore, it may be in the range of 100 ° C. to 200 ° C. for single crystal silicon and 100 ° C. to 300 ° C. for Pt. The heating element 5 can be heated to a higher temperature in a range where the deterioration does not accelerate as compared with the reference example 1, and the fouling can be further reduced.

  In order to reduce the fouling as described above, it is necessary to increase the heating temperature in a range where the progress of deterioration does not accelerate. Although it is possible to control the two temperatures of the first temperature Th1 and the third temperature Th3 in the conventional configuration and set the period of Th3 to be long, the temperature of Th3 has a change in the thermal conductivity of air due to humidity. It is necessary to have a small condition. For this reason, when Th3 is set to a high temperature outside the range of 100 ° C. to 150 ° C., it becomes impossible to measure humidity with high accuracy. Therefore, in this embodiment, in addition to Th1 and Th3, the fouling resistance is further improved by setting a period of a second temperature Th2 that is higher than Th3 and lower than Th1.

  FIG. 7 shows the operation timing of the switches SW1 to SW3 and the temperature change of the heating element 5 in this embodiment.

  When the switch SW3 is “closed” (tsw3 is ON), the heating element 5 is held at the third temperature Th3 in the range of 100 ° C. to 150 ° C. When the switch SW1 is “closed” (tsw1 is ON), the heating element 5 is held at the first temperature Th1 in the range of 300 ° C. or higher. When the switch SW2 is “closed” (tsw2 is ON), the heating element 5 is held at the second temperature Th2 in the range of 100 ° C. to 200 ° C. For the measurement of humidity, a period d2 in which the temperature of the heating element 5 is Th1 and a period d1 in which the temperature of the heating element 5 is set are provided, and data on the heating voltage of the heating element 5 at the two temperatures is used. Therefore, the period d2 and the period d1 are periods (detection operation periods) for measuring humidity. The period d3 in which the temperature of the heating element 5 is Th2 is a period (standby period) in which the operation for measuring humidity is stopped. Of these periods d1, d2, and d3, the period d3 is the longest.

The third temperature Th3 may be in the range of 100 ° C. to 150 ° C., and the second temperature Th2 may be matched with Th3. In this case, the fouling resistance is inferior to the case where the second temperature Th2 is set higher than the third temperature Th3, but the fouling resistance is improved as compared with the reference example 1. In this case, the period during which heating control is performed to the third temperature Th3 (that is, the second temperature Th2) is set longer than the period during which heating control is performed to the first temperature Th1.

In the present embodiment, a switch that is closed when heating is controlled to the second temperature Th2 and a switch that is closed when heating is controlled to the third temperature Th3 are interchanged with respect to the reference example 1. Further, the second temperature Th2 is also set to a temperature different from that of the reference example 1. For this reason, the resistance values of the temperature sensitive resistors 6, 7, 8 constituting the bridge circuit 23 are changed with respect to the reference example 1.

The effect in this structure is demonstrated. By providing the state of d3 described above, it is possible to provide a period during which the temperature of the heating element 5 is reduced to a temperature that suppresses or does not affect the deterioration, and the deterioration can be reduced. In addition, since the state of d3 is maintained at a higher temperature than that of Reference Example 1, the vicinity of the heating element 5 can be repelled by particles that become fouling substances due to an increase in the molecular motion of air due to heating, and adhesion can be further improved. Can be reduced. Moreover, when the environment is high in humidity, it is possible to prevent water droplets from adhering due to condensation. Further, since the temperature of the heating element 5 in the standby period d3 is set higher than the temperature in the detection operation period d1, the temperature difference of the heating element 5 that rises and falls through the periods d1, d2, and d3 is further reduced compared to the reference example 1. In addition, fatigue due to stress that repeats expansion and contraction due to a temperature cycle can be reduced. By setting the duty ratio so that the period d3 out of these periods d1, d2, and d3 is the longest, the number of times of stress due to the temperature difference can be reduced, and the period d2 where the temperature becomes high is relatively set. It can be shortened, and the progress of deterioration can be further reduced.
[Reference Example 2]

FIG. 8 is a plan view of a thermal gas sensor 31 showing a second reference example of the present invention. FIG. 9 is a cross-sectional view of the sensor element 31 of FIG.

The sensor element 31 of the thermal gas sensor of this reference example has a substrate 2 formed of single crystal silicon. A cavity 4 is formed in the substrate 2. The cavity 4 is covered with an insulating film 3 a, and a first heating element 35 and a second heating element 36 are formed on the insulating film 3 a on the cavity 4. Is formed. Further, the surface is covered with an insulating film 3b in order to protect the first heating element 35 and the second heating element 36. In addition, electrodes 40 a to 40 d for supplying and taking out voltage and current are formed on the first heating element 35 and the second heating element 36. Furthermore, the electrodes 40a to 40d are electrically connected to the heating control device 42 by gold wire bonding wires 41a to 41d.

As shown in FIG. 8, by arranging the second heating element 36 so as to surround the periphery of the first heating element 35, the ambient temperature of the first heating element 35 becomes the temperature of the second heating element 36 ( It is possible to reduce the dependency (influence) on the ambient temperature T3. Further, in this reference example, it is not necessary to measure the voltage and current of the heating element under the two temperature conditions of Th1 and Th3 for measuring the humidity as in Reference Example 1 and Example 1 , and the temperature of Th1 is one temperature condition. In this measurement, the absolute humidity can be measured based on the voltage or current of the heating element 35.

FIG. 10 is a configuration diagram of the heating control device 42 of the sensor element 31 in this reference example. Hereinafter, the operation of the thermal gas sensor in this reference example will be described with reference to FIG.

In this reference example, a bridge circuit 49 in which the first heating element 35 is incorporated and a bridge circuit 53 in which the second heating resistor 36 is incorporated are provided. The bridge circuit 49 controls the heating current of the first heating element 35 so as to maintain the balance of the bridge circuit 49. The bridge circuit 53 controls the heating current of the second heating element 36 so as to maintain the balance of the bridge circuit 53. The bridge circuit 49 and the bridge circuit 53 constitute a part of the heating control device 42.

  The drive circuit of the sensor element 31 includes a drive circuit portion 50 that controls the heating current of the first heating element 35 and a drive circuit portion 51 that controls the heating current of the second heating element 36. A heating current is supplied to the heating element 35 and the second heating element 36. In this case, the first heating element 35 is controlled to the first temperature Th1, and the second heating element 36 is controlled to the second temperature Th2, which is lower than the first temperature Th1.

  The drive circuit portion 50 of the drive circuit of the sensor element 31 includes a series circuit 49a in which the heating element 35 and the fixed resistor 39 are connected in series, and a series circuit 49b in which the fixed resistor 37 and the fixed resistor 38 are connected in series. The bridge circuit 49 is configured to be connected in parallel. A connection terminal potential between the heating element 35 and the fixed resistor 39 is input to the differential amplifier 15. The connection terminal potential between the fixed resistor 37 and the fixed resistor 38 is input to the differential amplifier 15 via the switch SW1 of the switch circuit 46. The switch circuit 46 is provided with a switch SW2, and a reference voltage source 47 having a predetermined reference potential is connected to an input of the differential amplifier 15.

  The heating temperature of the heating element 35 can be changed depending on which of the switches SW1 and SW2 of the switch circuit 46 is closed. When SW1 is closed, heating control is performed to a predetermined temperature Th1, and when SW2 is closed, heating control is stopped. The voltage of the reference voltage source 47 is sufficiently higher than the voltage between the heating resistor 35 and the fixed resistor 39. By doing so, when SW2 is selected, the output of the differential amplifier 15 becomes almost zero and the heating control can be stopped. That is, the input voltage on the negative side of the differential amplifier 15 is increased so that the output becomes almost zero. Thereby, the current supply from the differential amplifier 15 to the bridge circuit 49 is stopped.

  If the current supply from the differential amplifier 15 to the bridge circuit 49 is only stopped completely, a switch for opening and closing the connection may be provided in the wiring 52 that connects the output side of the differential amplifier 15 and the bridge circuit 49. . In this case, the reference voltage source 47, the switch SW1, and the switch SW2 are unnecessary, and the portion of the switch SW1 only needs to be always electrically connected.

  The drive circuit portion 51 that controls the heating of the heating element 36 includes a series circuit 53a in which the heating element 36 and the fixed resistor 45 are connected in series, and a series circuit 53b in which the fixed resistor 43 and the fixed resistor 44 are connected in series. The bridge circuit 53 is configured to be connected in parallel. The potential at the connection end between the heating element 36 and the fixed resistor 45 and the potential at the connection end between the fixed resistor 43 and the fixed resistor 44 are input to the differential amplifier 48. The output terminal of the differential amplifier 48 is connected between the heating element 36 and the fixed resistor 43, and is heated and controlled by applying a voltage corresponding to the difference in input voltage. With this configuration, feedback control is performed so that the temperature of the heating element 36 becomes the second temperature Th2 which is a constant temperature of 100 ° C. or higher.

  In the case of such a heating control device 42, the electric power for heating the heating resistor 35 to a predetermined temperature changes when the humidity changes. Therefore, the humidity can be measured by taking out the change in the heating voltage Vh1 of the differential amplifier 15. The heating voltage Vh1 is amplified by the differential amplifier 17, and highly accurate humidity data Hout can be obtained by the sample hold circuit 18 and the AD converter 19. The switch circuit 46 and the sample hold circuit 18 are controlled by the switch control circuit 21. The switch control circuit 21 generates an opening / closing timing of the switch circuit 14 and an operation signal of the sample hold circuit 18 based on the clock signal CLK of the oscillator 22. The switch control circuit 21 can also be operated based on a signal STB from the outside of the heating control device 12.

FIG. 11 is a timing chart showing the operation of the heating control device 42 of the sensor element 31 of the thermal gas sensor in this reference example. Hereinafter, the operation of the heating control device for the thermal gas sensor in this reference example will be described with reference to FIGS. 10 and 11.

  In FIG. 11, CLK is a clock waveform generated by the oscillator 22. tsw is a signal for instructing to open and close the switches SW1 and SW2 of the switch circuit 46. SW1 is “closed” at a high level, and SW2 is “closed” at a low level. As the opening / closing timing of the switches SW1 and SW2, first, tsw becomes high level at the timing t1 of CLK, and the switch SW1 of the switch circuit 14 is “closed”. Next, at timing t4 of CLK, tsw becomes low level, and the switch SW2 of the switch circuit 14 is “closed”. The period during which these switches SW1 and SW2 are “closed” is the longest period during which SW2 is “closed”.

  Vh1 in FIG. 11 indicates a change in the output voltage value of the differential amplifier 15 in FIG. 10, that is, a voltage for heating the heating element 35. When Vh1 is large, the heating temperature of the heating element 35 becomes high. Vh1 changes depending on which switch of SW1 and SW2 is closed. When tsw is at a high level, the temperature of the heating element 35 becomes Vh1 necessary for heating to a constant temperature in the range of 400 ° C to 500 ° C. When tsw is at a low level, heating of the heating element 35 is stopped. At this time, the temperature of the heating element 35 is approximately the same as the temperature of the heating element 36.

  Vh2 in FIG. 11 is a voltage obtained by amplifying the change in Vh1 due to humidity by the differential amplifier 17 in FIG. Specifically, it is a value obtained by amplifying the difference voltage between Vh1 and the reference voltage source 16.

In FIG. 11, tsh is a signal generated by the switch control circuit shown in FIG. 10 and is a signal for controlling the operation of the sample and hold circuit 18. When tsh is ON, the sample hold circuit 18 reads the input voltage Vh2. When tsh is OFF, the read Vh2 is held. The timing at which the sample hold circuit 18 reads Vh2 is a period (t1 to t4) in which the switch SW1 is “closed”. The value Vh3 read at this time is read by the AD converter 19 in FIG. 10 and becomes digital data. In the present reference example, since the change in the environmental temperature (ambient temperature) of the heating element 35 is reduced by the heating element 36, this digital data becomes the absolute humidity Hout.

  FIG. 12 shows the operation timing of the switches SW1 and SW2 and the temperature change of the heating element 35. When the switch SW1 is “closed” (tsw is at a high level), the heating element 5 is held at the first temperature Th1 in the range of 400 ° C. to 500 ° C. When the switch SW2 is “closed” (tsw is at a low level), the heating element 35 is held at the second temperature Th2 (temperature of the heating element 36) in the range of 200 ° C. to 300 ° C.

  For the measurement of humidity, a period d1 in which the temperature of the heating element 35 is Th1 is provided, and data on the heating voltage of the heating element 35 in this state is required. Therefore, the period d1 is a period for measuring humidity (detection operation period). A period d2 in which the temperature of the heating element 35 is Th2 is a period (standby period) in which the operation for humidity measurement is stopped. Of these periods d1 and d2, the period d2 is the longest.

  During the period d2 during which the heating element 35 is held at the second temperature Th2, energization to the heating element 35 is stopped. During the period d2, the temperature of the heating element 35 is maintained at the second temperature Th2 by energization (heating control) of the heating element 36.

  The effect in this structure is demonstrated. By providing the state of d2 described above, it is possible to provide a period during which the temperature of the heating element 35 is lowered, and deterioration can be reduced. This is because the progress of deterioration can be reduced when the temperature of the heating element 35 decreases. Further, in the state d2, the temperature around the heating element 35 is maintained at a temperature higher than the temperature Tb of the substrate 2 by heating the heating element 36. Therefore, the increase in the molecular motion of the air repels particles that become pollutants. It is possible to reduce the adhesion of fouling substances to the heating element 35. Moreover, when the environment is high in humidity, it is possible to prevent water droplets from adhering due to condensation. Further, the temperature difference generated in the heating element 35 is reduced by switching from the period d1 to the period d2, and fatigue due to stress that repeatedly expands and contracts due to the temperature cycle can be reduced. By setting the duty ratio so that the period d2 of these periods d1 and d2 becomes the longest, the number of times of stress due to the temperature difference can be reduced, and the period d1 during which the temperature rises is relatively shortened. And the progress of deterioration can be further reduced.

  Further, according to this configuration, even if the heating control of the heating element 35 is stopped, the heating element 36 is heated. Therefore, by stopping the heating control of the heating element 36, the current flowing through the heating element 35 can be made substantially zero. Thereby, deterioration (electromigration) of the heat generating body 35 due to current flow can be reduced. In particular, it is important to reduce deterioration of the heating element 35 because humidity measurement is detected based on the heating element 35.

In each of the above reference examples and examples , it is also possible to configure the sensor element portion using a heat ray or a plate-like material as a heating element. However, in the embodiment in which the cavity is formed in the substrate formed of single crystal silicon described in each of the reference examples and the embodiments to constitute the sensor element, the heat capacity of the sensor element portion can be reduced. For this reason, the sensor elements described in each of the above reference examples and example examples have excellent responsiveness, and can immediately recover from the standby temperature in the standby period to the detection operating temperature in the detection operation period. It is more suitable as a form for solving the problem.

DESCRIPTION OF SYMBOLS 1 ... Sensor element, 2 ... Board | substrate, 3a, 3b ... Insulating film, 4 ... Cavity part, 5 ... Heat generating body, 6-9 ... Temperature sensitive resistor, 10a-10f ... Electrode, 11a-11f ... Gold wire bonding wire, DESCRIPTION OF SYMBOLS 12 ... Heating control means, 13 ... Fixed resistance, 14 ... Switch circuit, 15 ... Differential amplifier, 16 ... Reference voltage source , 17 ... Differential amplifier, 18 ... Sample hold circuit, 19 ... AD converter, 20 ... Digital signal Processor, 21 ... Switch control circuit, 22 ... Oscillator, 23 ... Bridge circuit, 31 ... Sensor element, 35 ... Heating element, 36 ... Heating element, 37-39 ... Fixed resistance, 40a-40d ... Electrode, 41a-41d ... Gold Wire bonding wire 42 ... heating control means 43-45 ... fixed resistor 46 ... switch circuit 47 ... reference voltage reduction 48 ... differential amplifier 49 ... bridge circuit 50 ... bridge Drive circuit road 49, drive circuit 51 ... bridge circuit 53, 53 ... bridge circuit.

Claims (3)

In a thermal gas sensor having a heating element and a heating control device that controls heating of the heating element,
The heating control device includes a period in which the heating element is heated to a first temperature, a period in which the heating element is heated to a second temperature lower than the first temperature, and the heating element is moved to the first temperature . Heating control of the heating element by providing a period of heating control to a temperature of 1 and a third temperature in the range of 100 to 150 ° C. lower than the second temperature ,
A period of heating control to the second temperature is set as a standby period, and a detection operation by the sensor element including the heating element is performed in the period of heating control to the first temperature and the third temperature,
A thermal gas sensor characterized in that a period during which heating control is performed at the second temperature is set longer than a period during which heating control is performed at the first temperature. The thermal gas sensor according to claim 1,
A substrate having a cavity and a thin film support formed of an insulating film stacked on the substrate and covering the cavity; the heating element is formed on the thin film support; and the third temperature is the substrate A thermal gas sensor characterized in that the humidity is detected based on an output of a sensor element including the heating element. The thermal gas sensor according to claim 2,
The heating control device
A first series circuit in which the heating element and the first resistor are connected in series, a second resistor, a third resistor, a fourth resistor, and a fifth resistor are connected in series. A bridge circuit in which two series circuits are connected in parallel;
First voltage at the connection point between the voltage at the connection point between the first input, said second resistor and said third resistor between said first resistor and said heating element, The voltage at the second connection point between the third resistor and the fourth resistor or the voltage at the third connection point between the fourth resistor and the fifth resistor. A differential amplifier that selects either of them as a second input;
With
The heating temperature of the heating element is switched by selecting one of the voltage at the first connection point, the voltage at the second connection point, or the voltage at the third connection point. Thermal gas sensor.

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