JP5467648B2 - Contact combustion type methane detection apparatus and methane detection method - Google Patents

Description

  The present invention relates to detection of methane using a catalytic combustion type gas sensor.

  The inventors have studied to detect methane with a catalytic combustion gas sensor in which a detection piece and a compensation piece are provided in a cavity of a semiconductor substrate. The detection temperature of methane is about 500 ° C., and one detection cycle is, for example, 10 seconds. Among them, the gas sensor is heated for 100 ms, and the others are set to room temperature. The inventor has found that when the gas sensor is driven under these conditions, the heater resistance changes by about 1% in several months (FIGS. 11 and 16). This corresponds to about 10000 ppm in terms of methane concentration. Further, since the change in the heater resistance was not the same between the detection piece and the compensation piece, even if the detection piece and the compensation piece were incorporated in the bridge circuit, the fluctuation in output could not be canceled. Therefore, the inventor studied to correct the change in the heater resistance of the contact combustion type gas sensor, and reached the present invention.

  Here is related prior art. Patent Document 1 (JP2006-112911A) describes a general catalytic combustion type gas sensor in which the sensor temperature is changed to a temperature of about 300 to 500 ° C and a temperature of about 100 to 250 ° C. It discloses that hydrocarbons are detected from the difference between the output and the output at 100 to 250 ° C. Japanese Patent Application Laid-Open No. H10-228707 calculates the difference in output from 100 to 250 ° C., thereby correcting the influence of hydrogen, correcting the temperature dependence of the sensor, and canceling other characteristic changes. Patent Document 1 does not consider a contact combustion type gas sensor in which a heater thin film is provided in a cavity of a semiconductor substrate.

  Patent Document 2 (JP4035099B) describes a load for detecting a polar gas such as ethanol by a catalytic combustion type gas sensor using MEMS (Micro Electro Mechanical System), changes the sensor to a plurality of temperatures, and corrects with signals having different temperatures. Therefore, it is proposed to correct the influence of humidity.

JP2006-112911A JP4035099B

  An object of the present invention is to correct a drift of a heater resistance with respect to a contact combustion type methane sensor in which a heater thin film is provided in a cavity of a semiconductor substrate.

The present invention provides a bridge circuit in which a detection piece in which a thin film heater is covered with a methane combustion catalyst and a compensation piece in which a thin film heater is covered with a compensation material are provided in a cavity of a semiconductor substrate, and the detection piece and the compensation piece are incorporated. In the methane detection device that detects methane by the output of
The bridge circuit is
A circuit in which a series piece of a detection piece and a compensation piece and a series piece of a plurality of resistors are connected in parallel;
Alternatively, it consists of a circuit in which a series piece of a sensing piece and a resistor and a series piece of a compensation piece and a resistor are connected in parallel,
To the bridge circuit, power is applied by the drive circuit so that the temperature of the detection piece does not rise above the ignition point of methane,
When the detection piece is at a temperature below the ignition point of methane, the output of the bridge circuit is set to 0 point output, and the gas detection unit generates a reference value based on a plurality of 0 point outputs and stores it in the memory.
The output of the bridge circuit when the detection piece is at a temperature equal to or higher than the ignition point of methane, and methane is detected by correcting the change in resistance of the thin film heater by the gas detection unit based on the reference value.

The present invention also provides a bridge in which a detection piece in which a thin film heater is coated with a methane combustion catalyst and a compensation piece in which the thin film heater is covered with a compensation material are provided in a cavity of a semiconductor substrate, and the detection piece and the compensation piece are incorporated. In the method of detecting methane by the output of the circuit,
The bridge circuit is
A circuit in which a series piece of a detection piece and a compensation piece and a series piece of a plurality of resistors are connected in parallel;
Alternatively, it consists of a circuit in which a series piece of a sensing piece and a resistor and a series piece of a compensation piece and a resistor are connected in parallel,
To the bridge circuit, power is applied by the drive circuit so that the temperature of the detection piece does not rise above the ignition point of methane,
When the detection piece is at a temperature below the ignition point of methane, the output of the bridge circuit is set to 0 point output, and the gas detection unit generates a reference value based on a plurality of 0 point outputs and stores it in the memory.
The output of the bridge circuit when the detection piece is at a temperature equal to or higher than the ignition point of methane, and methane is detected by correcting the change in resistance of the thin film heater by the gas detection unit based on the reference value.

  In this invention, the output of the bridge circuit at a temperature lower than the ignition point of methane is set to 0 point output, and a reference value is generated using a plurality of 0 point outputs, that is, using an average value, median, etc. of the plurality of 0 point outputs. Let Here, when the voltage applied to the bridge circuit differs between when the zero-point output is generated and when methane is detected, correction according to the voltage ratio is added. Then, the output of the bridge circuit at a temperature equal to or higher than the ignition point of methane is corrected by the reference value, and for example, methane is detected by the difference between the output of the bridge circuit and the reference value. Note that the contribution of the reference value may be reduced, and for example, the difference between the output of the bridge circuit and (reference value × ½) may be used. In this invention, the drift of the resistance value of the thin film heater can be corrected, and for example, an error due to the drift can be corrected to a fraction. Moreover, since the reference value is generated from the zero point output measured a plurality of times, the reliability is high. In this specification, the description related to the methane detection device also applies to the methane detection method as it is, and conversely, the description related to the methane detection method also applies to the methane detection device as it is. In carrying out the present invention, the embodiments can be changed with reference to common sense of those skilled in the art and disclosure of prior art. The meanings of the terms in the claims are determined by taking into account common knowledge of those skilled in the art and well-known techniques in gas sensors.

Preferably, the detection piece is heated to a temperature lower than the ignition point of methane by the drive circuit for a time longer than the time when the zero point output of the bridge circuit reaches a steady value. Between the time to reach the steady value means, for example, between 90% response time or more. The zero point output may be measured at room temperature, but it is difficult to produce a very small driving voltage, and the error is smaller when measured at a temperature close to the temperature at which methane is detected. Therefore, for example, the zero point output is measured at 100 to 250 ° C. Then, it takes time for the bridge circuit output to stabilize, and when the detection piece is heated to a temperature lower than the ignition point of methane by the drive circuit for a time longer than the time when the zero point output of the bridge circuit reaches a steady value, the zero point output Can be measured.
More preferably, the cycle in which the temperature of the detection piece is changed by the drive circuit in the order of room temperature, temperature below the methane ignition point, and temperature above the methane ignition point is repeated. Then, for example, since the detection piece and the compensation piece are heated to about 500 ° C. after preheating to 100 to 250 ° C., the thermal shock applied to the detection piece and the compensation piece is small.

Preferably, when the difference between the initial value of the reference value stored in the memory and the reference value exceeds an allowable range, a failure is output. In this way, a limit that can correct the drift of the heater resistance is set, and if the limit is exceeded, a failure can be notified.
Particularly preferably, the output of the bridge circuit when the detection piece is at a temperature equal to or higher than the ignition point of methane is stored in the memory as the reference output, and the difference between the reference value and the reference output is the second value. If it exceeds the allowable range, a failure is output. In this way, the reliability of the reference value can be confirmed more reliably.

Block diagram of the gas detector of the embodiment Block diagram of microcomputer of embodiment Plan view of the main part of the methane sensor used in the example The flowchart which shows the operation | movement for 1 cycle in an Example. FIG. 4 is a diagram illustrating an operation for one cycle of the embodiment, where PH represents heater power, ΔT represents a temperature increase width, and Vout represents a bridge circuit output. FIG. 6 is a diagram showing an operation for one cycle of a modification, where PH represents heater power, ΔT represents a temperature increase width, and Vout represents a bridge circuit output. The figure which shows the management of the reference value in an Example Flow chart showing management of reference values in the embodiment Characteristic diagram showing the change in heater resistance on the high temperature side when pulse heating to 400 ° C Characteristic diagram showing changes in heater resistance on the high temperature side when pulse heating to 450 ° C Characteristic diagram showing changes in heater resistance on the high temperature side when pulse heating to 500 ° C Characteristic diagram showing changes in heater resistance on the high temperature side when pulse heating to 550 ° C Characteristic diagram showing the change in heater resistance on the high temperature side when pulse heating to 600 ° C Characteristic diagram showing changes in heater resistance on the low temperature side when pulse heating to 400 ° C Characteristic diagram showing changes in heater resistance on the low temperature side when pulse heating to 450 ° C Characteristic diagram showing changes in heater resistance on the low temperature side when pulse heating to 500 ° C Characteristic diagram showing changes in heater resistance on the low temperature side when pulse heating to 550 ° C Characteristic diagram showing changes in heater resistance on the low temperature side when pulse heating to 600 ° C

  In the following, an optimum embodiment for carrying out the present invention will be shown.

  An example is shown in FIGS. In FIG. 1, 2 is a gas detection device, 4 is a microcomputer, 6 is a switch such as a transistor, 8 is a diode, 10 is an inductance element such as a coil or a magnetic material, and 12 is a capacitor. The transistor 6 to the capacitor 12 constitute an H / L two-stage output power source, and the configuration of the power source is arbitrary. R1 and R2 are fixed resistors, 14 is a compensation piece, 16 is a detection piece, and the fixed resistors R1 and R2, the compensation piece 14 and the detection piece 16 constitute a bridge circuit. Then, the output of the bridge circuit is amplified by the differential amplifier 18 to obtain the output Vout. As shown in the range of the chain line in FIG. 1, the fixed resistor R1 and the compensation piece 14 may be connected in series, and the fixed resistance R2 and the detection piece 16 may be connected in series to form a bridge circuit.

  The configuration of the microcomputer 4 is shown in FIG. 2. The timer 22 operates, for example, at a cycle of 10 seconds. For example, 100 msec is a section for measuring zero point output, the next 100 msec is a section for detecting methane, and the remaining 9.8 sec is room temperature. It is a neglected section. The power supply control unit 24 controls the transistor 6 in accordance with the signal of the timer 22, and for example, in the interval for measuring the zero point output, the transistor 6 is turned on / off at 100 kHz, for example, and the bridge circuit has an effective voltage of about 0.5 to 1.5V. Apply voltage. In the methane detection section, the transistor 6 is always turned on or turned on / off, and an effective voltage of about 3 V is applied to the bridge circuit. 5 and 6 show heater power patterns. Preferably, the power applied to the bridge circuit is gradually changed without changing to a square wave shape as shown in FIGS. 5 and 6, and the thermal shock applied to the compensation piece 14 and the detection piece 16 is weakened. The gas detection unit 26 detects methane gas from the difference between the output Vout of the bridge circuit at the methane gas detection temperature and the reference value stored in the reference value storage unit 30. The reference value management unit 28 generates a reference value based on a plurality of zero-point outputs, checks whether the reference value is within the allowable range, and outputs a failure signal when the allowable range is exceeded. The A / D converter 32 performs A / D conversion on the output Vout.

  FIG. 3 shows a contact combustion type methane sensor 34, 35 is a semiconductor substrate such as silicon, and a pair of cavities 36, 36, for example, are provided. A thin film bridge 37 of silicon dioxide, tantalum pentoxide or the like or a thin film diaphragm is provided on the cavity 36, a cavity 40 is provided at the center thereof, communicates with the cavity 36, and the bottom side of the bridge 37 is connected to the cavity 36. It touches. A thin film heater 38 is provided on the bridge 37 and is made of, for example, a Pt film having a film thickness of about 500 nm. On the compensation piece 14 side, the thin film heater 38 is covered with a material having low methane oxidation activity, for example, alumina, and on the detection piece 16 side, the thin film heater 38 is covered with a methane combustion catalyst, for example, a catalyst obtained by adding Pd or Pt to alumina. The thin film heater 38 is connected to the pads 41 to 43 and connected to the substrate on which the circuit of FIG. 1 is mounted by wire bonding or the like. The structure of the methane sensor 34 is arbitrary. For example, the compensation piece 14 and the detection piece 16 may be mounted on different substrates, the presence or absence of the cavity 40 is arbitrary, and the material of the thin film heater 38 is also arbitrary. Further, without the bridge 37, the thin film heater 38 may be covered with a compensation piece material such as alumina and a methane oxidation catalyst to form the detection piece 16 and the compensation piece 14.

  4 and 5 show the operation of the gas detection apparatus for one cycle, and one cycle is, for example, 10 seconds. The sensor is heated to, for example, about 100 to 250 ° C. with electric power P1 for T1 seconds (for example, 100 msec, preferably 50 to 150 msec). In general, methane has an ignition point of 300 ° C or higher. At this temperature, CO, ethanol, hydrogen, etc. burn, but methane does not burn. The 90% response time of the output of the bridge circuit when the temperature is changed between room temperature and about 200 ° C. is, for example, about 50 milliseconds. The thermal time constant when the temperature of the catalytic combustion gas sensor is changed generally differs between the compensation piece 14 and the detection piece 16. Therefore, when the temperature of the sensor is changed, both the detection piece and the compensation piece approach toward the steady temperature, and the output of the bridge circuit is not stabilized during this period. Therefore, the period of T1 seconds is made substantially equal to the period of T2 seconds, and the zero point output V1 is sampled after the sensor output reaches a steady value. Then, the output Vout of the amplifier circuit at T1 second is stored as a zero point output. The 0-point output does not need to be at T1 second, and may be an output after the bridge circuit output reaches a steady value.

  The sensor is immediately heated with power P2 for T2 seconds without cooling to room temperature. The heating temperature here is, for example, 500 ° C., the heating time T 2 is, for example, 100 msec (preferably 80-150 msec), and the ratio of the power P 2 to the power P 1 is, for example, about 10: 1-3: 1. The sensor output approaches the steady value over a period of about T2 seconds, the last signal in the section of about 500 ° C is sampled, and methane is detected from the difference from the reference value generated based on the zero point output. . Next, for example, the heater power is set to 0 for 9.8 seconds and the apparatus is on standby. V1 in Fig. 5 is the bridge circuit output (0 point output) at the last time of heating for T1 seconds, and V2 is the bridge circuit output (output for methane detection) at the last time of heating for T2 seconds. Show. ΔT represents the temperature rise width from room temperature.

The reference value is generated as follows, for example. First, the zero-point output of a plurality of cycles is statistically obtained by obtaining an average value, obtaining a median value of distribution, and the like. When the voltage applied to the bridge circuit is different between the time when the zero point output is measured and the time when methane is detected, a correction corresponding to this is applied to obtain the reference value. For example, if the circuit voltage at the time of measuring the zero point output is U1 and the circuit voltage at the time of detecting methane is U2, the value obtained by statisticalizing the zero point output is multiplied by U2 / U1 to obtain the reference value. If the heating power is controlled by PWM control and the circuit voltage is constant, correction for the circuit voltage is unnecessary. Instead of using the zero point output of a plurality of cycles, the zero point output may be measured and averaged a plurality of times within one cycle. However, since this method generates a reference value from a small number of 0-point outputs, it has low reliability and is not included in the present invention. Further, it is not necessary to sample the zero point output for each cycle. For example, the zero point output may be sampled about several times a day. For statisticalization from the zero point output to the reference value, the average value of the zero point output, the median of the distribution, etc. are used. The zero point output is, for example, about once a day, preferably about once a week. And update it at least once a month.

  FIG. 6 shows a modification in which the sensor is heated slightly from room temperature with a very small electric power P3 for a short time, for example, about 10 milliseconds. The width of the temperature change at this time is about 10 ° C. to 20 ° C., for example. Then, since the width of the temperature change is small, the change in the heater resistance between the detection piece and the compensation piece is slight, and the heater resistance at room temperature can be measured.

  7 and 8 show management of reference values. The reference value storage unit 30 stores the transition of the reference value, sets an allowable range ± A with respect to the initial value of the reference value, and outputs a failure when the reference value deviates from this range. Thus, if the resistance value of the thin film heater 38 changes by a predetermined value or more with respect to the initial value, it is regarded as a failure. In order to further improve the reliability of methane detection, among the sensor outputs at the methane detection temperature, that is, among the outputs V2 in FIGS. 5 and 6, the output on the side where no methane exists is stored as a reference value. If methane is present, the resistance value of the detection piece 16 increases and the resistance value of the compensation piece 14 does not change, so the minimum output from the bridge circuit corresponds to the output when no methane is present. Therefore, for example, from a minimum value of the output of the bridge circuit at the methane detection temperature or a median of the output of the bridge circuit in a predetermined period such as one day, the output when there is no methane is sampled and stored as a reference value. . If the difference between the reference value and the reference value exceeds the second allowable range ± B, a failure is assumed.

  FIGS. 9 to 18 show the behavior of the thin film heater 38 and show the rate of change of the resistance value with respect to the initial value when heated to 400 to 600 ° C. for 100 milliseconds every 10 seconds. 9 to 13 show changes in resistance value with heating temperature. The heating temperature is 400 ° C. in FIG. 9, 450 ° C. in FIG. 10, 500 ° C. in FIG. 11, 550 ° C. in FIG. is there. 14 to 18 show the behavior of the resistance value at room temperature under the same conditions. The heating temperature is 400 ° C. in FIG. 14, 450 ° C. in FIG. 15, 500 ° C. in FIG. 16, 550 ° C. in FIG. 600 ° C. Also, the same temperature indicates the same thin film heater in the same heating temperature. The vertical axis in FIGS. 9 to 18 indicates the resistance change rate of the thin film heater, not the output of the bridge circuit. Further, either the detection piece 16 or the compensation piece 14 did not have a particularly large drift in resistance value, and the resistance value drifted to the same extent in both the detection piece and the compensation piece.

  As is apparent from FIGS. 9 to 18, the resistance value of the thin film heater 38 drifts with time, and the drift increases as the heating temperature increases. In addition, when comparing figures with the same heating temperature, the resistance value at the heating temperature and the resistance value at the room temperature change in parallel, which means that the change in the resistance temperature coefficient is small, and the resistance value at the heating temperature and the room temperature. This means that the resistance value is strongly correlated. Therefore, the drift of the thin film heater 38 can be corrected by combining the resistance value at a temperature below the methane ignition point and the resistance value at the methane detection temperature. More specifically, the influence of drift can be corrected by obtaining the difference between the output V2 and the reference value in FIGS.

  It is not preferable to continue gas detection in a situation where the resistance value of the thin film heater 38 has changed significantly. For example, it is conceivable that the heating temperature of the sensor is changing. For this reason, if the reference value fluctuates by more than the allowable range ± A with respect to the initial value, a failure occurs. In general, the difference between the reference value and the reference value should be within the range of the second allowable range ± B. Therefore, a failure is detected even when the value deviates from this range.

In the embodiment, the following effects can be obtained.
(1) The measurement of FIGS. 9-18 was continued and the data for one year were acquired. Of the five thin film heaters at a heating temperature of 500 ° C, the combination of the maximum and minimum resistance values one year after the initial value, and the catalytic combustion type detection and compensation pieces, methane 5000ppm The output at 15000 to -5000ppm. On the other hand, when the correction of the embodiment is performed, the output at 5000 ppm of methane is 7000 to 3000 ppm, which is included in the allowable range for the methane sensor.
(2) When the detection piece and the compensation piece are heated to 100-250 ° C for about 100 milliseconds as shown in Fig. 5, avoiding the influence of the difference in the thermal time constant between the detection piece and the compensation piece, the reference value Can be measured. And after this, a thermal shock can be weakened by heating to about 500 degreeC, without returning to room temperature.
(3) The reliability of the sensor can be evaluated by managing how much the reference value has changed from the initial value.
(4) The reliability of the sensor can be further checked by comparing the reference value with the reference value corresponding to the output when there is no methane at the methane detection temperature.

2 Gas detector 4 Microcomputer 6 Transistor 8 Diode 10 Inductance element 12 Capacitor
R1, R2 Fixed resistance 14 Compensation piece 16 Detection piece 18 Amplifier 22 Timer 24 Power supply control unit 26 Gas detection unit 28 Reference value management unit 30 Reference value storage unit 32 A / D conversion unit 34 Contact combustion methane sensor 35 Semiconductor substrate 36, 40 Cavity 37 Bridge 38 Thin film heater 41 to 43 Pad

Claims (6)

In the cavity of the semiconductor substrate, a detection piece in which the thin film heater is coated with a combustion catalyst of methane and a compensation piece in which the thin film heater is covered with a compensation material are provided, and by the output of the bridge circuit incorporating the detection piece and the compensation piece, In an apparatus for detecting methane,
The bridge circuit is
A circuit in which a series piece of a detection piece and a compensation piece and a series piece of a plurality of resistors are connected in parallel;
Alternatively, it consists of a circuit in which a series piece of a sensing piece and a resistor and a series piece of a compensation piece and a resistor are connected in parallel,
By applying electric power from the drive circuit to the bridge circuit , the detection piece is subjected to a cycle that changes the temperature to room temperature, a temperature below the ignition point of methane, and a temperature above the ignition point of methane, multiple times,
When the detection piece is at a temperature below the ignition point of methane, the output of the bridge circuit is set to 0 point output, and the gas detection unit generates a reference value based on the 0 point output in a plurality of cycles and stores it in the memory.
The detection piece detects the methane by correcting the resistance change of the thin film heater by correcting the output of the bridge circuit at a temperature equal to or higher than the ignition point of methane by the reference value in the gas detection unit.
A methane detection device, characterized in that it is configured as described above.   2. The methane according to claim 1, wherein the detection piece is configured to be heated by the drive circuit to a temperature lower than the ignition point of methane for a time longer than a time when the zero point output of the bridge circuit reaches a steady value. Detection device.   The methane detection device according to claim 2, wherein the detection piece is configured to repeat a cycle in which the temperature is changed by the drive circuit in the order of room temperature, temperature below the methane ignition point, and temperature above the methane ignition point. .   4. The apparatus according to claim 1, wherein a failure is output when a difference between an initial value of the reference value stored in the memory and the reference value exceeds an allowable range. 5. Methane detector.   The output of the bridge circuit when the detection piece is at a temperature equal to or higher than the ignition point of methane, the output on the side where no methane exists is stored in the memory as a reference output, and the difference between the reference value and the reference output exceeds the second allowable range. The methane detection device according to claim 4, wherein the methane detection device is configured to output a failure. In the cavity of the semiconductor substrate, a detection piece in which the thin film heater is coated with a combustion catalyst of methane and a compensation piece in which the thin film heater is covered with a compensation material are provided, and by the output of the bridge circuit incorporating the detection piece and the compensation piece, In a method for detecting methane,
The bridge circuit is
A circuit in which a series piece of a detection piece and a compensation piece and a series piece of a plurality of resistors are connected in parallel;
Alternatively, it consists of a circuit in which a series piece of a sensing piece and a resistor and a series piece of a compensation piece and a resistor are connected in parallel,
By applying electric power from the drive circuit to the bridge circuit , the detection piece is subjected to a cycle that changes the temperature to room temperature, a temperature below the ignition point of methane, and a temperature above the ignition point of methane, multiple times,
When the detection piece is at a temperature below the ignition point of methane, the output of the bridge circuit is set to 0 point output, and the gas detection unit generates a reference value based on the 0 point output in a plurality of cycles and stores it in the memory.
A method for detecting methane, comprising: detecting a methane by correcting a change in resistance of a thin film heater based on an output of a bridge circuit when a detection piece is at a temperature equal to or higher than an ignition point of methane, using the reference value by a gas detection unit.