JP6679993B2 - Gas detector - Google Patents

Description

  The present invention relates to a gas detection device that can be used to detect a gas contained in an atmosphere.

  Conventionally, a catalytic combustion type, a semiconductor type, a heat conduction type and the like are known as gas detection devices. These methods detect the gas concentration by utilizing the fact that the resistance of the sensor element heated by a heater or the like changes depending on the presence of the gas to be detected in the atmosphere.

  In Patent Document 1, a bridge circuit is configured with one temperature sensitive resistor and a plurality of fixed resistors, and the resistance connected to the temperature sensitive resistor is switched by a switch, thereby switching the temperature for heating the temperature sensitive resistor. , A method of measuring humidity by utilizing the fact that the heat dissipation of the temperature sensitive resistor changes with humidity has been proposed.

  In Patent Document 2, one heating resistor and a fixed resistor form a bridge circuit, and an offset voltage with respect to the output voltage from the heating resistor is set by the bridge circuit using the fixed resistor. A method of amplifying the voltage difference from the offset voltage has been proposed.

  Patent Document 3 proposes a method in which a bridge circuit is composed of a plurality of heat-sensitive elements and the applied voltage and the application time are made uniform with respect to the voltage applied to the heater that heats the heat-sensitive element that constitutes the bridge circuit.

JP-A-8-184576 JP2012-202939A

  However, the following problems remain in the above conventional techniques. According to the method of Patent Document 1, although the offset of the resistance value due to the temperature can be suppressed to a small value, the drift of the temperature coefficient due to the change with time of the temperature sensitive resistor cannot be suppressed, so that the operating time of the humidity detecting device elapses. Along with this, the accuracy of measurement of the humidity value detected decreases.

  In the configuration of Patent Document 2, since the offset voltage is constant, the accuracy of gas detection by the heating resistor decreases with the aging of the heating resistor.

As a method of suppressing the aging of the temperature sensitive resistor, a method of forming a bridge circuit with gas sensor elements having the same aging can be considered. Since the gas sensor elements form a bridge circuit, if the gas sensor elements forming the bridge have the same amount of change over time, the amount of change over time that occurs in the other gas sensor elements will be cancelled, enabling highly accurate measurement. Becomes However, if the amount of change over time between the gas sensor elements is different, the accuracy of gas detection will be reduced by the difference.

  When detecting hydrogen, which has a much higher thermal conductivity than air, there is a large difference in the thermal conductivity of the space and the sensitivity of the gas sensor elements is also high, so there is a difference in the amount of change over time between the gas sensor elements. Does not matter either.

However, when detecting carbon monoxide, carbon dioxide, etc., whose thermal conductivity is close to that of air, the difference in heat conduction in the space is small, and the sensitivity of the gas sensor element is lower than when detecting hydrogen, etc. It is necessary to minimize the difference in the amount of change over time between the gas sensor elements.

  FIG. 10 is a graph showing the detection error of the detection target gas due to the influence of the drift caused by the aging of the first sensor element 52 with respect to the heating temperature of the thin film thermistor 59 using the gas sensor element 51 shown in FIG. 9. , The elapsed time due to the use of the sensor on the X axis, the carbon dioxide detection error due to the single element of the first sensor element 52 on the main axis of the Y axis, and the first thermosensitive element 52 and the second sensor element on the two axes of the Y axis. The carbon dioxide gas detection error and the hydrogen gas detection error due to the bridge configuration with 53 are taken.

In the graph of FIG. 10, in the case where the gas sensor element 51 is a single element, when the elapsed time of sensor use reaches 1000 hr, the detection error of carbon dioxide gas due to the influence of drift caused by aging becomes ≈50,000 ppm. As described in Patent Document 3, by adopting a bridge configuration between the first sensor element 52 and the second sensor element 53, when the detection target is hydrogen gas, the detection error is suppressed to ≈200 ppm or less. However, when the detection target is carbon dioxide gas, the detection error increases to about 3000 ppm.

  Therefore, the present invention has been made in consideration of the above points, and in a gas detection device that detects a target gas by using a plurality of sensor elements and a heater, the resistance value of the sensor element due to heating of the heater is An object of the present invention is to provide a gas detection device capable of reducing a detection error of a sensor caused by a change with time, without using a complicated circuit.

  In order to achieve the above object, a gas detection device according to the present invention includes a first sensor element for measuring the concentration of a gas to be measured based on a temperature change due to a difference in heat conduction between spaces, and the first sensor element. A first heater that heats the second sensor element, a second sensor element that serves as a reference resistance of the first sensor element, a second heater that heats the second sensor element, the first sensor element, and A bridge circuit is configured by the second sensor element, and the first heater and the second heater are simultaneously applied with respective predetermined voltages to heat the first and second sensor elements, A first measurement in which a temperature change of the first and second sensor elements is replaced with a voltage change, and a voltage different from that in the first measurement is simultaneously applied to the first heater and the second heater. To heat the first and second sensor elements The second measurement for replacing the temperature change of the first and second sensor elements with the voltage change and the values obtained by the first and second measurement are input to the MPU in the control circuit, and the MPU The drift amount of the second measured value is obtained, and the value obtained by multiplying the drift amount of the first measured value by the correction coefficient is added to the first measured value to calculate the gas concentration. It is a gas detection device characterized by performing.

In the invention according to claim 2 of the present invention, the control circuit digitally outputs a signal amplification circuit for inputting the values by the first and second measurements from the bridge circuit, and an output voltage value from the signal amplification circuit. A bias voltage is applied to the A / D converter that converts the value into a value and the bridge circuit, a voltage is applied to the heater that heats the first and second sensor elements, and a reference voltage is applied to the signal amplification circuit. And a D / A converter for applying a voltage, and the MPU performs input / output of signals to / from the A / D converter and the D / A converter, and the second measurement. The drift amount from the start of the measurement of the bridge circuit is calculated based on the value obtained by the above, and the correction coefficient is multiplied to calculate the gas concentration correction value, and the gas concentration correction value is set to the value obtained by the first measurement. And the gas Performing the calculation of the time is a gas detection apparatus according to claim 1, wherein the.

  In the invention according to claim 3 of the present invention, the second measurement is performed with the first heater so that the first sensor element and the second sensor element are simultaneously heated at the same temperature. The gas detection device according to claim 1 or 2, wherein the same voltage is simultaneously applied to the second heater.

The invention according to claim 4 of the present invention comprises: a first sensor element for measuring the concentration of a gas to be measured based on a temperature change due to a difference in heat conduction between spaces; and a first sensor element for heating the first sensor element. A heater, a second sensor element serving as a reference resistance of the first sensor element, a second heater for heating the second sensor element, the first sensor element, and the second sensor element To form a bridge circuit, and simultaneously apply a predetermined voltage to each of the first heater and the second heater to heat the first and second sensor elements. Of the first sensor element and the second heater by simultaneously applying the same voltage to the first heater and the second heater. A second measurement for replacing the temperature change with a voltage change, and the first measurement A third measurement in which a voltage different from the first and second measurements is applied to the heater and the second heater at the same time, and the temperature change of the first and second sensor elements is replaced with the voltage change. , The values obtained by the first, second, and third measurements are input to the MPU in the control circuit, and the MPU obtains the drift amount of the second measurement value. Gas detection, characterized in that each value obtained by multiplying a correction coefficient for converting into a drift amount of a measured value is added to the first and third measured values to calculate a gas concentration. It is a device.

According to a fifth aspect of the present invention, the same voltage is applied to the first heater and the second heater to heat the first sensor element and the second sensor element. The heating temperature at this time is higher than the environmental temperature in which the gas detection device is used and is 100 ° C. or lower, and the gas detection device according to claim 1.

  Start measurement based on the output value from the bridge circuit when the same thermal load is applied to both the first sensor element and the second sensor element used as the reference resistance of the first sensor element at the same timing. The drift amount of the bridge circuit from time and the correction coefficient for the heating temperature of the sensor element at the time of gas measurement are multiplied to calculate a gas concentration correction value, and the measured value of the gas concentration is added to the sensor element The influence due to aging can be reduced.

It is a structure sectional view of a gas sensor element in a 1st embodiment. It is a circuit block diagram of the gas detection apparatus in 1st Embodiment. It is a figure which shows the operation timing of the gas detection apparatus in 1st Embodiment. It is a flow chart which shows a calculation procedure of a gas detecting device in a 1st embodiment. It is a correction coefficient for the output of the bridge circuit in the first embodiment. 6 is a graph showing the effectiveness of drift correction in the first embodiment. It is a figure which shows the operation timing of the gas detection apparatus in 2nd Embodiment. It is a flow chart which shows a calculation procedure of a gas detector in a 2nd embodiment. It is a structure sectional view of the conventional gas sensor element. It is a graph which shows the gas detection error by the influence of the drift in the conventional detection means.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. Note that the present invention is not limited to the following embodiments. The constituent elements described below include those that can be easily conceived by those skilled in the art and those that are substantially the same. Furthermore, the components described below can be combined as appropriate.

(First embodiment)
FIG. 1 is a cross-sectional structure diagram for explaining the gas sensor element of the present embodiment. The gas sensor element 1 according to the present embodiment has a first sensor element 2 that detects a temperature change due to a target gas and a second sensor element 3 that serves as a reference resistance of the first sensor element 2, and is exposed to a measurement environment. Placed in the same space. In this embodiment, the first sensor element 2 and the second sensor element 3 are arranged in the ceramic package 15, and the gas sensor element 1 is formed by the lid 16 having the ventilation hole 17 for exposing to the measurement environment. Note that the drawings are schematic, and for convenience of description, the relationship between the thickness and the plane dimension, and the ratio of the thicknesses between the devices are the same as those of the actual sensor structure within the range in which the effects of the present embodiment can be obtained. May be different.

  The first sensor element 2 includes a substrate 4, an insulating film 5, a first heater 7A, a heater protective film 6, a thin film thermistor electrode 8, a thin film thermistor 9, and a thin film thermistor protective film 11.

  The second sensor element 3 has the same configuration as the first sensor element 2 except for the element resistance value. With such a configuration, the element characteristics other than the resistance values of the first sensor element 2 and the second sensor element 3 can be made the same. That is, there is no difference in response time due to the difference in heat capacity, and the same behavior can be achieved with respect to changes in environmental temperature.

  By forming the first sensor element 2 and the second sensor element 3 adjacent to each other at the same time, it is possible to reduce variations in the manufacturing process, and thus it is possible to manufacture the sensor elements having uniform characteristics. As a result, it is possible to eliminate the selection step of combining the characteristics between the sensor elements.

  The substrate 4 is not particularly limited as long as it has a suitable mechanical strength and is suitable for fine processing such as etching. For example, a silicon single crystal substrate, a sapphire single crystal substrate, a ceramic substrate, a quartz substrate, a glass substrate, or the like is suitable. An insulating film 5 such as a silicon oxide film or a silicon nitride film is formed on the front and back surfaces of the substrate. To form, for example, a silicon oxide film as the insulating film 5, a thermal oxidation method or a film forming method by CVD (Chemical Vapor Deposition) may be applied. The film thickness may be such that the film formed on the insulating film 8 and the substrate can be insulated from each other and that the film functions as an etching stop layer when the cavity 10 is formed. Usually, about 0.1 to 1.0 μm is suitable.

  The substrate 4 has a cavity 10 in which a part of the substrate is thinned to correspond to the position of the first heater 7A in order to suppress heat conduction to the substrate when the first heater 7A is operated at a high temperature. have. The portion where the substrate is removed by the cavity 10 is called a membrane. Since the membrane has a smaller heat capacity as the thickness of the substrate is reduced, the first heater 7A can be heated to a high temperature with very low power consumption. Moreover, since the heat conduction structure to the substrate 4 is formed only by the thin film portion having a thickness of several μm, the heat conduction to the substrate 4 is small and the first heater 7A can be efficiently heated to a high temperature.

The material of the first heater 7A is a metal layer made of a material having a relatively high melting point that is a conductive substance that can withstand processes such as a film forming process of the thin film thermistor 9 and a heat treatment process, and is, for example, molybdenum (Mo). Platinum (Pt), gold (Au), tungsten (W), tantalum (Ta), palladium (Pd), iridium (Ir), or an alloy containing any two or more of these is preferable. In addition, a conductive material that allows highly accurate dry etching such as ion milling is preferable, and Pt or the like, which has high corrosion resistance, is more preferable. Further, in order to improve the adhesion with the insulating film 5, it is preferable to form an adhesion layer of titanium (Ti) or the like below Pt.

  A thin film thermistor 9 is formed as a heat sensitive body for detecting the temperature of the first heater 7A by gas. The thin film thermistor 9 includes the thin film thermistor electrode 8 and is formed so as to cover the first heater 7A. As a result, the temperature of the first heater 7A can be directly detected.

  As a material of the thermistor forming the thin film thermistor 9, a material having a negative temperature coefficient of resistance such as composite metal oxide, amorphous silicon, polysilicon, or germanium is formed by using a thin film process such as a sputtering method or a CVD. The film thickness may be adjusted according to the target thermistor resistance value. For example, if the resistance value (R25) at room temperature is set to about 2 MΩ using an MnNiCo-based oxide, the distance between the electrodes of the element is The thickness may be set to about 0.2 to 1 μm, though it depends on the condition.

The thin film thermistor 9 is suitable for detecting the temperature of the first heater 7A. First, because of the laminated structure of thin films, the heat generated by the heater 7A can be directly detected directly above. In addition, since the resistance temperature coefficient is larger than that of a platinum temperature measuring element or the like, the detection sensitivity can be increased.

  The thin film thermistor electrode 8 is formed in order to extract the electric signal of the thin film thermistor 9. As a material of the thin film thermistor electrode 8, a material having a relatively high melting point that is a conductive substance that can withstand processes such as a film forming process of the thin film thermistor 9 and a heat treatment process, for example, molybdenum (Mo), platinum (Pt), gold ( Au, tungsten (W), tantalum (Ta), palladium (Pd), iridium (Ir), or an alloy containing any two or more of these is suitable.

  A heater protection film 6 is formed so as to cover the heater 7A and the insulating film 5. The heater protective film 6 is preferably made of the same material as the insulating film 5. The heater 7A repeatedly receives thermal stress of rising to several hundreds of degrees and then falling to room temperature. Continuously receiving this heat stress leads to destruction such as delamination and cracks. The same materials have the same material properties, stronger adhesion, and stronger mechanical strength than the case where different materials are laminated. For this reason, it is possible to prevent the heater 7A from being destroyed by the thermal stress. To form, for example, a silicon oxide film as the heater protection film 6, a thermal oxidation method or a film forming method by CVD may be applied. The film thickness is preferably such that the heater 7A can be reliably covered and interlayer insulation can be performed. Usually, about 0.1 to 3.0 μm is suitable.

  Further, when the composite metal oxide or the like is used for the thin film thermistor 9, the heater protection film 6 is preferably an insulating oxide film, for example, a silicon oxide film or a silicon nitride film. A thin film thermistor 9 and a thin film thermistor electrode 8 are formed on the heater protection film 6. The heater protective film 6 is a protective film for the heater 7A and at the same time is a base layer for the thin film thermistor 9, and is in direct contact with the thin film thermistor 9.

  Since a thermistor using a composite metal oxide has a reduction deterioration at high temperature, a method of coating the entire thermistor with a reduction resistant material is known. That is, when the thermistor is brought into contact with a reducing material and brought into a high temperature state, oxygen is deprived from the thermistor to cause reduction, which affects the thermistor characteristics. Therefore, it is desirable that the thin film thermistor protective film 11 is also an insulating oxide film such as a silicon oxide film.

  For the same reason, the thin film thermistor electrode 8 is preferably formed on the substrate side of the thin film thermistor 9. That is, the thin film thermistor electrode 8 and the thin film thermistor 9 are laminated in this order on the heater 7A with the heater protection film 6 which is an insulating layer interposed therebetween. That is, the thin film thermistor 9 is formed on the thin film thermistor electrode 8. In general, a thin film electrode is formed with an adhesion layer in order to increase the adhesion between the electrode material and the base. For example, chromium (Cr), titanium (Ti), or the like is formed with a film thickness of about several nm. When the thin film thermistor electrode 11 is formed on the thin film thermistor 9, the adhesive layer directly contacts the thin film thermistor and is oxidized by depriving the thermistor of oxygen, thereby increasing the interface resistance and the detection characteristics of the thin film thermistor 9. Fluctuates, which is not preferable.

The thin film thermistor electrode 8 and the heater 7A are connected to the electrode pad 12. The electrode pad 12 is electrically connected to an external circuit through a ceramic package electrode 14 or the like by wire bonding or the like, is formed of a material such as aluminum (Al) or gold (Au), and may be stacked as necessary. .

The sensor element is cut from a wafer state into individual pieces, fixed to a ceramic package 15 using a die paste (not shown), and then the electrode pad 12 and the package electrode 14 are attached using a wire bonding device. , And wire 13 for connection. The wire 13 is preferably a metal wire having a low resistance such as Au, Al or Cu.

Finally, the lid 16 provided with the ceramic package 15 and the vent 17 for the outside air is fixed using a resin (not shown). At this time, when the resin (not shown) is heated for curing, the substance contained in the resin is generated as a gas, but is easily released to the outside of the package by the vent hole 17, which may adversely affect the element itself. There is no. As described above, the gas sensor element 1 can be obtained.

  FIG. 2 is a circuit configuration diagram of the gas detection device according to the present embodiment. The gas detection device includes a gas sensor element 1 and a control circuit 21.

The gas sensor element 1 includes a first sensor element 2, a first heater 7A that heats the first sensor element 2, a second sensor element 3 and a second heater that serve as a reference resistance of the first heat sensitive element 2. 7B, the first sensor element 2 and the second sensor element 3 are connected in series. That is, the first sensor element 2 and the second sensor element 3 form a bridge circuit, and by measuring the midpoint voltage of the first sensor element 2 and the second sensor element 3, each sensor element Detects temperature change. The heating of the first sensor element 2 is performed by the first heater 7A, the heating of the second sensor element 3 is performed by the second heater 7B, and the heaters 7A and 7B are independently controlled and driven. Has been done.

  The control circuit 21 includes an amplifier 22 that amplifies a signal from the gas sensor element 1, an A / D converter 23 that performs analog-digital conversion of the signal from the amplifier 22 and the signal from the temperature sensor 27, and the gas sensor element 1. A D / A converter 24 for applying a pulse voltage to the heaters 7A and 7B, applying a bias voltage to a bridge circuit composed of the sensor elements 1 and 2 and applying a reference voltage to the amplifier 22; The MPU 25 controls the / D converter 23 and the D / A converter 24 and calculates the gas concentration, and the constant voltage power supply 26 supplies power to the circuit.

  FIG. 3 is a diagram showing an operation timing of the gas detection device in the present embodiment, and FIG. 4 is a diagram showing a calculation procedure of the gas detection device in the present embodiment. The gas detection operation will be described with reference to FIGS. 3 and 4.

In step 31, the signal from the temperature sensor 27 is read into the MPU 25 through the A / D converter 23.

In step 32, the MPU 25 executes temperature calculation to determine the reference voltage to the amplifier 22 and the voltage to be output to the first heater 7A and the second heater 7B. The voltage output to the first heater 7A and the second heater 7B is adjusted so that the first sensor element 2 and the second sensor element 3 are heated at a predetermined temperature.

In step 33, a bias voltage is applied to the bridge circuit composed of the first sensor element 2 and the second sensor element 3 and a reference voltage (Vref) is applied to the amplifier 22 from the D / A converter 24. After a certain time has elapsed after applying the bias voltage and the reference voltage, the voltage determined in step 32 is applied to the first heater 7A and the second heater 7B (Vm1 and Vm3 in FIG. 3). . The reference voltage has the same voltage value as the Vd output value when the gas to be detected is not present in the atmosphere and is not detected.

  A voltage is simultaneously applied to the first heater 7A and the second heater 7B to heat the first sensor element 2 and the second sensor element 3 at the same time. At this time, the voltage applied to the first heater 7A that heats the first sensor element 2 is a voltage that heats the first sensor element 2 to a temperature for efficiently detecting the gas to be measured, The voltage applied to the second heater 7B that heats the second sensor element 3 is higher than the environmental temperature in which the gas detection device is used. The temperatures to which the first sensor element 2 and the second sensor element 3 are respectively heated are predetermined. By heating the first sensor element 2 and the second sensor element 3 at the same time, it is possible to suppress the influence of environmental temperature change on the sensor element and perform gas detection.

In step 34, the voltage is applied to the first heater 7A and the second heater 7B, and after a lapse of a certain time, the Vd output value (first measurement) from the bridge circuit is set to the amplifier 22 and the A / D converter 23. Via MPU25.

In step 35, after the Vd output value is taken into the MPU 25, the voltage application to the first heater 7A and the second heater 7B is turned off.

In step 36, the reference voltage (Vref) is applied to the amplifier 22 from the D / A converter 24. After a lapse of a certain time after applying the reference voltage, a voltage different from that in step 33 determined in step 32 is applied to the first heater 7A and the second heater 7B (Vm2 and Vm2 in FIG. 3). Vm4).

  A voltage is simultaneously applied to the first heater 7A and the second heater 7B to heat the first sensor element 2 and the second sensor element 3 at the same time. At this time, the energization voltage to the first heater 7A that heats the first sensor element 2 and the energization voltage to the second heater 7B that heats the second sensor element 3 are the same voltage. The first sensor element 2 and the second sensor element 3 are heated to the same temperature and higher than the environmental temperature in which the gas detection device is used. The temperature at which the first sensor element 2 and the second sensor element 3 are heated is predetermined. By heating the first sensor element 2 and the second sensor element 3 at the same time, it is possible to perform the measurement while suppressing the influence of the environmental temperature change on the sensor element.

In step 37, the voltage is applied to the first heater 7A and the second heater 7B, and after a lapse of a certain time, the Vr output value (second measurement) from the bridge circuit is set to the amplifier 22 and the A / D converter 23. Via MPU25.
Since the Vr output value is controlled so that the first sensor element 2 and the second sensor element 3 are heated at the same temperature regardless of the environmental temperature, the first sensor element 2 and the second sensor element 2 are controlled. If the sensor element 3 has no resistance change due to a change with time or the like, the same Vr output value is output every time without drift. However, when a different voltage value is output as compared to the initial Vr output value, the potential difference from the initial Vr output value becomes a drift component due to a change over time. By correcting this drift amount, the accuracy of gas detection can be improved.

In step 38, after the Vr output value is taken into the MPU 25, voltage is applied to the first heater 7A and the second heater 7B, and a bridge composed of the first sensor element 2 and the second sensor element 3 is applied. Turn off the bias voltage application to the circuit

  In step 39, the voltage is applied to the second heater 7B for a certain period of time (Vm5 in FIG. 3), and then the applied voltage is turned off. At this time, the voltage value applied to the second heater 7B is the same as the voltage applied to the first heater 7A in step 33. Since the voltages applied to the first heater 7A and the second heater 7B in step 33 are different, a difference occurs in the thermal stress received by the first sensor element 2 and the second sensor element 3. Therefore, by energizing only the second heater 7B, the difference in thermal stress between the first sensor element 2 and the second sensor element 3 is eliminated, and almost the same thermal stress state is achieved.

In step 40, the Vr output value obtained in step 37 is used with the correction coefficient shown in FIG. 5 to calculate the gas concentration correction value.

The initial Vr output value stored in the memory (not shown) of the MPU 25 is compared with the Vr output value measured this time, and if no potential difference occurs, it means that there is no change over time, and the gas concentration correction value Is zero. If there is a potential difference, it means that a drift due to a change with time of the sensor element has occurred, and this drift amount is corrected. The obtained potential difference cannot be used as it is as a gas concentration correction value for correcting the drift amount. This is because the heating temperature for the first sensor element 2 and the second sensor element 3 when the Vr output value is obtained is different from the heating temperature when the Vd output value is obtained. When the heating temperature is different, the characteristics of the sensor element are also changed. Therefore, in order to correct the Vd output value for obtaining the gas concentration, the drift amount obtained from the Vr output value is used as the drift amount of the Vd output value. It is multiplied by a correction coefficient for conversion and converted so that it can be used accurately for correcting the drift amount of the Vd output value.

  FIG. 5 is a graph showing the correction coefficient for the output of the bridge circuit in this embodiment, in which the heating temperature of the first sensor element 2 is plotted on the X axis and the correction coefficient is plotted on the Y axis. The correction coefficient of the Y-axis is calculated in advance by the amount of change over time in the Vr output value when the first sensor element 2 and the second sensor element 3 are simultaneously heated to the same temperature, and in the atmosphere in which the gas detection device is arranged. It is calculated from the ratio with the change amount of the Vd output value with time in the environment where there is no gas to be detected. Specifically, the initial value of the Vr output value is Vr01, the Vr output value after a certain period of operation is Vrn, and the initial value of the Vd output value when the first sensor element 2 is heated at the temperature at which the gas to be detected is detected. Is Vd01 and the Vd output value after a lapse of a fixed time is Vdn, the calculation for obtaining the correction coefficient is expressed by the following formula (Formula 1). The result of this calculation becomes a coefficient for correcting the influence of the change over time that occurs in the first sensor element 2 and the second sensor element 3. For example, when the heating temperature of the first sensor element 2 when obtaining the Vd output value is 150 ° C., the correction coefficient is 1.35. By multiplying this value by the drift amount (potential difference) obtained from the Vr output value, a gas concentration correction value for correcting the drift amount of the Vd output value is obtained.

K = (Vdn-Vd01) / (Vrn-Vr01) Formula 1
K: correction factor

  The Vr output value is obtained by heating the first sensor element 2 and the second sensor element 3 at a temperature higher than the ambient temperature to 100 ° C. or lower and to the same temperature at the same time. By doing so, the influence of humidity, gas, etc. can be eliminated and the amount of change over time can be obtained. Even if the gas to be detected is present, the heat conduction is small because of the low temperature. Further, since the first sensor element 2 and the second sensor element 3 are simultaneously heated to the same temperature, the resistance change due to heat conduction to the gas to be detected is canceled.

  The correction coefficient is stored in a memory area (not shown) in the MPU. The storage form may be a form replaced by an approximate expression, or may be the numerical value of the correction coefficient obtained above. If the temperature for heating the sensor element for detecting the gas to be detected is one point, the value of the correction coefficient for the heating temperature may be stored in the memory. When there are a plurality of gases to be detected and there are a plurality of temperatures at which the sensor element is heated, the approximate expression may be stored in the memory.

  In step 41, the gas concentration is calculated using the Vd output value and the gas concentration correction value.

In step 42, the gas concentration value calculated in step 41 is output.

  By applying a voltage to the first heater 7A and the second heater 7B at the same time to heat the first sensor element 2 and the second sensor element 3 at the same time, the sensor element changes in ambient temperature. Under the influence of, the Vd and Vr output values do not fluctuate, and stable gas detection can be performed.

By applying a voltage to the first heater 7A and the second heater 7B at the same time to heat the first sensor element 2 and the second sensor element 3 at the same time, the gas detection device is arranged. The Vd output value and the Vr output value when the gas to be detected does not exist in the atmosphere have constant voltage values. Therefore, the reference voltage (Vref) for the amplifier 22 only needs to output a predetermined voltage value, and it is not necessary to change the reference voltage (Vref) according to the change in the environmental temperature, so that the load on the MPU 25 is reduced.

By applying a voltage to the second heater 7B at regular intervals for a predetermined time (Vm5 in FIG. 3), the difference in thermal stress between the first sensor element 2 and the second sensor element 3 is reduced. By eliminating it, it is possible to suppress a change over time due to thermal stress between the first sensor element 2 and the second sensor element 3.

  The Vr output value obtained by heating the first sensor element 2 and the second sensor element 3 at the same temperature and at the same time occurs in the first sensor element 2 and the second sensor element 3. It shows the presence or absence of change over time and the amount of change over time. The accuracy of gas detection can be improved by periodically comparing with the Vr output value obtained at the first time of the measurement start, obtaining the difference in the amount of change over time, and performing the correction.

(Second embodiment)
The structure of the gas sensor element and the circuit configuration of the gas detection device in this embodiment are the same as those described in the first embodiment with reference to FIGS. 1 and 2. FIG. 7 is a diagram showing the operation timing of the gas detection device in this embodiment, and FIG. 8 is a diagram showing the calculation procedure of the gas detection device in this embodiment. The gas detection operation will be described with reference to FIGS. 7 and 8.

In step 51, the signal from the temperature sensor 27 is read into the MPU 25 through the A / D converter 23.

In step 52, the MPU 25 executes the temperature calculation to determine the reference voltage to the amplifier 22 and the voltage to be output to the first heater 7A and the second heater 7B. The voltage output to the first heater 7A and the second heater 7B is adjusted so that the first sensor element 2 and the second sensor element 3 are heated at a predetermined temperature.

In step 53, a bias voltage is applied to the bridge circuit composed of the first sensor element 2 and the second sensor element 3 and a reference voltage (Vref) is applied to the amplifier 22 from the D / A converter 24. After a certain time has elapsed after applying the bias voltage and the reference voltage, the voltage determined in step 52 is applied to the first heater 7A and the second heater 7B (Vm6 and Vm9 in FIG. 7). . The reference voltage has the same voltage value as the Vd1 output value when the gas to be detected is not present in the atmosphere and is not detected.

  A voltage is simultaneously applied to the first heater 7A and the second heater 7B to heat the first sensor element 2 and the second sensor element 3 at the same time. At this time, the voltage applied to the first heater 7A that heats the first sensor element 2 is a voltage that heats the first sensor element 2 to a temperature for efficiently detecting the gas to be measured, The voltage applied to the second heater 7B that heats the second sensor element 3 is higher than the environmental temperature in which the gas detection device is used. The temperatures to which the first sensor element 2 and the second sensor element 3 are respectively heated are predetermined.

In step 54, the voltage is applied to the first heater 7A and the second heater 7B, and after a lapse of a certain time, the Vd1 output value (first measurement) from the bridge circuit is set to the amplifier 22 and the A / D converter 23. Via MPU25.

  At step 55, it is confirmed whether or not the Vd2 output value from the bridge circuit has been fetched. If it has not been imported, the process is re-executed from step 53. When step 53 is executed, the voltage determined in step 52 is applied to the first heater 7A and the second heater 7B (Vm7 and Vm10 in FIG. 7). In step 54 executed again, the voltage is applied to the first heater 7A and the second heater 7B, and after a certain time has passed, the Vd2 output value from the bridge circuit is passed through the amplifier 22 and the A / D converter 23. And take it into the MPU 25. When the acquisition of the Vd2 output value is completed, the next step is executed.

In step 56, after the Vd2 output value (third measurement) is taken into the MPU 25, the voltage application to the first heater 7A and the second heater 7B is turned off.

In step 57, the reference voltage (Vref) is applied to the amplifier 22 from the D / A converter 24. After a certain time has elapsed after the reference voltage was applied, a voltage different from that determined in step 53 in step 52 is applied to the first heater 7A and the second heater 7B (Vm8 and Vm8 in FIG. 7). Vm11).

  A voltage is simultaneously applied to the first heater 7A and the second heater 7B to heat the first sensor element 2 and the second sensor element 3 at the same time. At this time, the energization voltage to the first heater 7A that heats the first sensor element 2 and the energization voltage to the second heater 7B that heats the second sensor element 3 are the same voltage. The first sensor element 2 and the second sensor element 3 are heated to the same temperature and higher than the environmental temperature in which the gas detection device is used. The temperature at which the first sensor element 2 and the second sensor element 3 are heated is predetermined.

In step 58, the voltage is applied to the first heater 7A and the second heater 7B, and after a lapse of a certain time, the Vr1 output value (second measurement) from the bridge circuit is set to the amplifier 22 and the A / D converter 23. Via MPU25.

Since the Vr1 output value is controlled so that the first sensor element 2 and the second sensor element 3 are heated at the same temperature regardless of the environmental temperature, the first sensor element 2 and the second sensor element 2 are controlled. If there is no change in resistance with the sensor element 3 due to a change with time or the like, no drift occurs and the Vr1 output value is the same voltage every time. However, when a different voltage value is output as compared with the initial Vr1 output value, the potential difference from the initial Vr1 output value becomes a drift component due to a change with time. By correcting this drift amount, the accuracy of gas detection can be improved.

In step 59, after the Vr1 output value is taken into the MPU 25, voltage is applied to the first heater 7A and the second heater 7B, and the bridge composed of the first sensor element 2 and the second sensor element 3 is applied. Turn off the bias voltage application to the circuit

  In step 60, the voltage is applied to the second heater 7B for a certain period of time (Vm12 in FIG. 7), and then the applied voltage is turned off. At this time, the voltage value applied to the second heater 7B is the same as the voltage applied to the first heater 7A in step 53. Since the voltages applied to the first heater 7A and the second heater 7B in step 53 are different, a difference occurs in the thermal stress received by the first sensor element 2 and the second sensor element 3. Therefore, by energizing only the second heater 7B, the difference in thermal stress between the first sensor element 2 and the second sensor element 3 is eliminated, and almost the same thermal stress state is achieved.

In step 61, the gas concentration correction value is calculated using the Vr1 output value obtained in step 58 and the correction coefficient shown in FIG.

The initial Vr1 output value stored in the memory (not shown) of the MPU 25 is compared with the Vr1 output value measured this time. If no potential difference occurs, it means that there is no change over time, and the gas concentration correction value Is zero. If there is a potential difference, it means that a drift due to a change with time of the sensor element has occurred, and this drift amount is corrected. The obtained potential difference cannot be used as it is as a gas concentration correction value for correcting the drift amount. This is because the heating temperature for the first sensor element 2 and the second sensor element 3 when the Vr1 output value is obtained is different from the heating temperature when the Vd1 and Vd2 output values are obtained. When the heating temperature is different, the characteristics of the sensor element are also changed. Therefore, in order to correct the Vd1 and Vd2 output values for obtaining the gas concentration, the drift amount obtained from the Vr1 output value is used as the Vd1 and Vd2 output values. It is multiplied by a correction coefficient for converting into the drift amount of, and converted so that it can be used accurately for correcting the drift amount.

  In order to obtain a correction value for correcting the Vd1 and Vd2 output values, the graph showing the correction coefficient of FIG. 5 used in the first embodiment is used. For example, when the heating temperature of the first sensor element 2 when obtaining the Vd1 output value is 125 ° C., the correction coefficient becomes 1.03, and the heating temperature of the first sensor element 2 when obtaining the Vd2 output value. Is 175 ° C., the correction coefficient is 1.45. By multiplying this value by the drift amount (potential difference) obtained from the Vr1 output value, a gas concentration correction value for correcting the drift amount of the Vd1 and Vd2 output values is obtained.

  In step 62, the gas concentration is calculated using the Vd1 and Vd2 output values and the gas concentration correction value.

In step 63, the gas concentration value calculated in step 62 is output.

  Even when the first sensor element 2 is heated by a plurality of temperatures in order to detect a plurality of gases in the atmosphere, a correction coefficient for each heating temperature is obtained, and a bridge for each heating temperature is obtained. By calculating the gas concentration correction value for the output value of the circuit, highly accurate gas detection is possible.

The gas sensor element 1 according to the present invention is composed of a first sensor element 2 and a second sensor element 3, and the sensor elements 2 and 3 are NTC (Negative Temperature Coefficient) thermistors, and their resistance value increases when the temperature rises. It has the characteristic of descending. The resistance value of the NTC thermistor with respect to temperature can be approximately represented by the following formula (Formula 2).

R_TH = R_0 exp {B (1 / T-1 / T_0)} Formula 2

  In the equation, RTH is the resistance value of the thermistor at temperature T, R0 is the thermistor resistance value at temperature T0, and B is a constant indicating the relationship between the thermistor resistance values RTH and R0 at temperatures T and T0. The first sensor element 2 exhibits 2000 kΩ at 25 ° C., 65 kΩ at 150 ° C., and the B constant is 3450. The second sensor element 3 exhibits 220 kΩ at 25 ° C., 65 kΩ at 60 ° C., and the B constant is 3450. The resistance value when the first sensor element 2 is heated and the resistance value when the second sensor element 3 is heated are approximately equal.

  As shown in the timing relationship between the heaters (MH1, MH2) and the signal detection (Vd, Vr) in FIG. 3, the first heater 7A of the first sensor element 2 and the second heater 7B of the second sensor element 3 are shown. Is a platinum resistance, which is heated by Joule heat when a pulse voltage is applied. The resistance values of the first heater 7A and the second heater 7B are 140Ω, and when the pulse voltage (Vm1) of 1.2V is applied, the first sensor element 2 is heated to 150 ° C. and 0.5V (Vm3). The second sensor element 3 is heated to 60.degree. C. by applying the pulse voltage of.

  The energization cycle for the first heater 7A of the first sensor element 2 and the second heater 7B of the second sensor element 3 is such that the energization is turned on for 100 msec and then the energization is turned off for 400 msec. Energize with. Since the first sensor element 2 and the second sensor element 3 each have the cavity 10, the heating temperature or the environmental temperature is set within 20 msec after the power is turned on / off.

  A pulse voltage is applied to each of the first heater 7A of the first sensor element 2 and the second heater 7B of the second sensor element 3, and the first sensor element 2 is heated to 150 ° C. With the sensor element 3 stably heated to 60 ° C., gas detection is performed and the Vd output value is captured. Subsequently, a pulse voltage was applied to each of the first heater 7A and the second heater 7B, so that the first sensor element 2 and the second sensor element 3 were stably heated to 60 ° C. In this state, the Vr output value used for drift correction is fetched. Then, the pulse voltage is applied only to the second heater 7B to heat the second sensor element 3 to 150 ° C.

  By applying the pulse voltage only to the second heater 7B, the thermal stress between the first sensor element 2 and the second sensor element 3 is balanced.

  The difference between the Vr output value at the first measurement start and the newly measured Vr output value is multiplied by the correction coefficient, and the obtained gas concentration correction value is added to the Vd output value.

  The initial Vr output value was 1.500 V, the Vr output value after measurement for a certain period of time was 1.505 V, and the heating temperature of the first sensor element 2 at the time of detecting the Vd output was 150 ° C. Then, since the correction coefficient becomes 1.35, the gas concentration correction value becomes 6.75 mV. This value is added to the Vd output value to perform a calculation for obtaining the gas concentration.

  FIG. 6 is a graph showing the effectiveness of drift correction using the first embodiment. The gas to be detected in this graph was carbon dioxide, and the concentration was 10,000 ppm. Comparison 1 in the graph shows the influence on the gas detection error due to the change over time when one is a sensor element and the other is a fixed resistance bridge circuit. Appears as a gas detection error of 50,000 ppm.

  Comparison 2 in the graph shows the influence on the gas detection error due to the change with time when the bridge circuit is assembled between the sensor elements, and the influence due to the change with time after the lapse of 1000 hours from the start of measurement is equal to the gas detection error of 3000 ppm. Appears. By forming a bridge circuit between the sensors, the influence of aging is significantly reduced, but there is an error of about 30%.

  The first embodiment in the graph shows the influence on the gas detection error due to the change with time when the correction calculation in the first embodiment is incorporated, and the effect due to the change with time after the lapse of 1000 hours from the start of measurement is 100 ppm. It can be seen that the influence of the elapsed time on the gas detection is greatly reduced by incorporating the correction calculation. As described above, it has been shown to provide a gas detection device in which the error of the sensor element is reduced.

  In the embodiment, carbon dioxide is used as the detection target gas, but the flow of the arithmetic processing does not change even if the detection target gas is another gas such as methane.

  INDUSTRIAL APPLICABILITY The present invention is installed in home appliances / industrial equipment, environmental monitoring devices, and the like, and is used for detecting a target gas.

1 Gas Sensor Element 2 First Sensor Element 3 Second Sensor Element 4 Substrate 5 Insulating Film 6 Heater Protective Film 7A, 7B Heater 8 Thin Film Thermistor Electrode 9 Thin Film Thermistor 10 Cavity 11 Thin Film Thermistor Protective Film 12 Electrode Pad 13 Wire 14 Ceramic Package Electrode 15 Ceramic Package 16 Lid 17 Vent 21 Control Circuit 22 Amplifier 23 A / D Converter 24 D / A Converter 25 MPU
26 power supply 27 temperature sensor

Claims (2)

A first sensor element for measuring the concentration of a gas to be measured based on a temperature change due to a difference in heat conduction between spaces; and a first heater for heating the first sensor element,
A second sensor element as a reference resistance of the first sensor element and a second heater for heating the second sensor element;
A gas sensor element that forms a bridge circuit by the first sensor element and the second sensor element, a signal amplifier circuit that inputs a value from the gas sensor element, and a digital output voltage value from the signal amplifier circuit. A / D converter for converting into a value, a bias voltage is applied to the gas sensor element, a voltage is applied to a heater for heating the first and second sensor elements, and a reference voltage is applied to the signal amplification circuit. A gas detection device comprising a D / A converter for applying a voltage and a control circuit composed of an MPU,
The control circuit is
By simultaneously applying pulse voltages having different amplitudes to the first heater and the second heater, the first and second sensor elements are heated, and the temperatures of the first and second sensor elements are increased. A first measurement that replaces the change with a voltage change;
A pulse voltage of the same amplitude is simultaneously applied to the first heater and the second heater to heat the first and second sensor elements, and the temperature change of the first and second sensor elements is changed. There row second measurement replacing the voltage change, and heating said first heater and said second heater, said applying a pulse voltage having the same amplitude at the same time the first, the second sensor element Then, the value obtained by the second measurement for replacing the temperature change of the first and second sensor elements with the voltage change is taken into the MPU as the Vr output value, and the bridge caused by the difference from the first Vr output value is generated. When the drift amount of the circuit is obtained by calculation, and if drift occurs, the value obtained by multiplying the drift amount by the correction coefficient obtained by Equation 1 is

K = (Vdn-Vd01) / (Vrn-Vr01) Formula 1
K: correction factor
Vdn: measurement value obtained in measurement 1
Vd01: Value of initial measurement obtained in measurement 1
Vrn: measurement value obtained in measurement 2
Vr01: Value of initial measurement obtained in measurement 2

By simultaneously applying pulse voltages having different amplitudes to the first heater and the second heater, the first and second sensor elements are heated, and the temperatures of the first and second sensor elements are increased. A gas detection device, characterized in that the drift of the Vd output value is corrected by adding the Vd output value obtained by the first measurement that replaces the change with the voltage change . A pulse voltage having the same amplitude is simultaneously applied to the first heater and the second heater at a predetermined ON / OFF interval to heat the first sensor element and the second sensor element. The gas detection device according to claim 1 , wherein the heating temperature at the time of heating is higher than the environmental temperature in which the gas detection device is used and 100 ° C. or less.

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