JP2010145316A - Gas analyzer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To detect the peak temperature of an adsorptive combustion type gas sensor without causing a shift in the peak of the differential value of sensor output even if the concentration of a measuring gas is different in a gas analyzer using the adsorptive combustion type gas sensor. <P>SOLUTION: The bridge voltage applied to a bridge circuit containing the microheater of the adsorptive combustion type gas sensor is controlled. The microheater of the gas sensor is controlled so as to rise in temperature at a constant speed. During the first 40 msec period, the resistance value of the microheater is sampled at a high speed while controlling the microheater so as to raise the temperature at a constant speed. The excessive gas component, moisture or the like of the gas sensor is burnt off during the first 40 msec period. The microheater is held to 100°C during the subsequent 10 sec period to adsorb the measuring gas. During the second 40 msec period, the resistance value of the microheater is sampled at a high speed while controlling the microheater so as to raise the temperature at a constant speed. A measured value is differentiated to calculate a peak value, the kind of the gas is determined. The concentration of the gas is determined from the resistance value of the peak value. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a gas analyzer that detects polar gas using an adsorption combustion type gas sensor.

  Conventionally, as a gas detection device using an adsorption combustion type gas sensor, for example, there is one disclosed in Japanese Patent Application Laid-Open No. 2005-83949 (Patent Document 1). FIG. 11 is a circuit block diagram of this gas detector, FIG. 12 is a diagram showing a pulse voltage for driving the gas detector, and FIG. 13 is a diagram showing a schematic structure of the adsorption combustion type gas sensor.

  In the adsorption combustion type gas sensor shown in FIG. 13, micro heaters 1 and 2 patterned with platinum are formed on a diaphragm 10 a formed by anisotropic etching of a silicon substrate 10. The catalyst 3 is applied on the microheater 1 on the sensor side, but nothing is formed on the microheater 2 on the reference side.

  As the microheaters 1 and 2 are heated, the gas adsorbed on the catalyst 3 causes a combustion reaction on the sensor side. Due to this combustion reaction, the temperature on the sensor side rises, so the gas concentration can be measured by measuring the resistance value Rs of the micro heater 2 on the sensor side. Note that the reference side does not cause a combustion reaction by gas.

  In the adsorption combustion type gas sensor, as shown in FIG. 11, the microheater 1 on the gas sensor side and the microheater 2 on the reference side form a bridge circuit with resistors and variable resistors. A driving pulse voltage shown in FIG. 12 is applied to the bridge circuit from the sensor drive control unit. In the adsorption combustion type gas sensor, gas is adsorbed within 3 seconds of the pulse voltage OFF, and a combustion reaction is caused in 200 milliseconds of the pulse voltage ON.

The sensor output detection unit detects a shift in the balance of the bridge circuit due to the combustion reaction of the gas. When the output of the sensor output detection unit is calculated by the integration calculation unit, a value corresponding to the gas concentration is output. Further, when the output of the sensor output detection unit is differentiated by the differential calculation unit, a waveform having a peak specific to the gas type is obtained. The gas concentration detection unit calculates the gas concentration from the value of the integration calculation unit, and the gas type detection unit calculates the gas type from the output waveform of the differentiation calculation unit. The calculated gas type and gas concentration are output from the output unit.
JP 2005-83949 A

  As described above, the adsorption combustion type gas sensor is driven by the pulse voltage. Therefore, there is a problem that the combustion temperature at the peak of the waveform having the peak output from the differential calculation unit cannot be measured.

  By the way, when it is considered as a gas analyzer, it is important to measure how many times the temperature of the differential waveform peak appears. For example, an analyzer using an adsorption / desorption gas sensor is used to perform the same measurement and to examine the composition of gas components from the temperature of the peak that appears. For this reason, the conventional gas detector cannot be used as a gas analyzer.

  Further, in the conventional gas detector, since the microheater 1 on the gas sensor side is controlled with a constant voltage regardless of the gas concentration, the peak position is shifted due to the difference in heat generation temperature (difference in concentration) during adsorption combustion of the gas sensor. There is a problem of getting out.

  In addition, since the adsorption combustion type gas sensor is applied by a pulse voltage, the interval of the gas type-specific peak output from the differential calculation unit 14 is small. For this reason, there is a problem that the error increases when the gas type is specified from the peak position.

  Moreover, since it is necessary to perform zero gas correction once, there exists a problem that a clean gas must be prepared.

  It is an object of the present invention to provide a gas analyzer that can measure the temperature of a peak position without causing a deviation of the peak position of a differential waveform due to the gas concentration and can perform gas analysis with high accuracy.

  The gas analyzer according to claim 1 samples the sensor output of the adsorption combustion type gas sensor, time-differentiates the sampled sensor output to obtain differential waveform data, and analyzes the gas based on the obtained differential waveform data The gas analyzer is characterized by controlling the voltage applied to the heater of the adsorption combustion type gas sensor and sampling the sensor output while controlling the heater at a constant temperature.

  The gas analyzer according to claim 2 is the gas analyzer according to claim 1, wherein the resistance value of the heater is sampled and applied to the heater so that the sampled resistance value rises with a constant change. The voltage is controlled.

  According to the gas analyzer of the first aspect, the peak position of the differential waveform of the output of the gas sensor is hardly affected without being affected by the difference in gas concentration. In addition, the sensor temperature at which the peak appears can be measured, and from this, it can be used as an analyzer for considering the output of the peak due to the gas composition. In addition, zero gas correction with clean gas is not required.

  According to the gas analyzer of the second aspect, in addition to the effect of the first aspect, the feedback control of the resistance value is sufficient, so the control becomes easy.

  Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a circuit diagram of an analog part of the gas analyzer of the embodiment, and FIG. 2 is a circuit block diagram of a digital part of the gas analyzer. In FIG. 1, a microheater 1 on the gas sensor side and a microheater 2 on the reference side have the same structure as the conventional one. The microheater 1 on the gas sensor side and the microheater 2 on the reference side constitute bridge circuits Bs and Br, respectively.

  A bridge voltage DA1 (described later) output from the digital unit (FIG. 2) is input to the operational amplifier ap1, and the amplified output of the operational amplifier ap1 is applied to the bridge circuit Bs on the gas sensor side and the bridge circuit Br on the reference side. The output of the bridge circuit Bs on the gas sensor side is amplified 10 times by the instrumentation amplifier ap2 and output as AD1, and is taken into the AD converter 11 of the digital unit. The output of the bridge circuit Br on the reference side is amplified 10 times by the instrumentation amplifier ap3. Further, the bridge voltage DA1 is divided by half by the voltage dividing circuit D, outputted as AD2, and taken into the AD converter 12 of the digital section.

  The difference between the voltage amplified by the instrumentation amplifier ap2 (gas sensor side) and the voltage amplified by the instrumentation amplifier ap3 (reference side) of the output of the bridge circuit Bs on the gas sensor side is differential by the second-stage instrumentation amplifier ap4. Amplified and output as AD3. The output of the instrumentation amplifier ap4 is stored in a RAM (memory) 14 after data sampling is performed by the digital unit. The amplification factor of the second-stage instrumentation amplifier ap4 is variable and can be set from the outside. Note that Rs, Rr, Rx, R1, and R added to FIG. 1 are the resistance values of the microheaters 1 and 2 and the resistors, respectively.

With the above configuration, assuming that the gains of the instrumentation amplifiers ap2 and ap3 are GAIN1, the resistance value Rs of the microheater 1 on the gas sensor side is expressed by the following equations (1) and (2) from the values of the output AD1 and AD2. Can be calculated.
Rs = R × (2 × GAIN1 × AD2 / (AD1 + 2 × GAIN2 × AD2) −1) (1)
GAIN2 = GAIN1 × R / (R + Rx) (2)

  As shown in FIG. 2, the digital unit includes AD converters 11, 12, and 13 that perform analog / digital conversion on outputs AD1, AD2, and AD3 of the analog unit, a RAM (memory) 14 that stores measurement data, a control program, and the like. A ROM 15 stored therein, a DSP 16 that performs measurement control processing, and a DA converter 17 that performs digital / analog conversion on the bridge voltage DA1 as a digital signal output from the DSP 16 and outputs the analog voltage to the operational amplifier ap1 of the analog unit. The DSP 16 repeats sampling of the resistance value Rs and output of the bridge voltage 4000 times at a sampling interval of 100 KHz by an interrupt process described later.

Here, the bridge voltage DA1 output from the DSP 1 is a bridge voltage predicted from a deviation between the measured resistance value Rs on the gas sensor side and the ideal resistance value of the gas sensor. The ideal resistance value Ro of the gas sensor is obtained by the following equation (3).
Ro = (Rmax−Rmin) / 4000 × count + Rmin (3)
Rmax: resistance value when the gas sensor is 500 ° C. Rmin: resistance value when the gas sensor is 100 ° C. 4000: sampling count: sampling times at that time

  The method for predicting the bridge voltage is as follows. When the measured resistance value Rs is larger than the ideal resistance value, a coefficient (kinc) is subtracted from the increase ΔDA1 added to DA1. When the measured resistance value Rs and the ideal resistance value are the same, the increase ΔDA1 to be added to DA1 is left as it was last time. When the measured resistance value Rs is smaller than the ideal resistance value, a coefficient (kinc) is added to the increase ΔDA1 added to DA1. Then, the updated increase amount ΔDA1 is added to the previous bridge voltage, and the predicted bridge voltage DA1 is output for each sampling of the resistance value Rs.

  3 and 4 are flowcharts of a program executed by the DSP 16, FIG. 3A is a flowchart of a main routine of measurement control processing, FIG. 3B is a flowchart of a subroutine of heater control & sampling processing, and FIG. It is a flowchart of an interrupt process, and the interrupt process is started at 100 KHz by a timer (not shown) by interrupt enable. Next, the operation will be described based on the flowchart.

  In the process of FIG. 3A, initial setting such as reset of various registers in the RAM 14 is performed in step S1, and the heater control & sampling process of FIG. 3B is performed in step S2. Next, in step S3, the process waits for 10 seconds while outputting a voltage that becomes 100 ° C. as the initial value of the bridge voltage DA1, and in step S4, the second heater control & sampling process is performed. Next, a differential operation is performed on the sampling data by the second heater control & sampling process stored in the RAM 14 in step S5. Next, the peak value of the differential waveform is acquired in step S6, the gas type is determined in step S7, the gas concentration is determined in step S6, and the gas type and gas concentration are output in step S9. In step S10, other processing is performed and the processing is terminated.

  In the heater control & sampling process of FIG. 3B, the slope of the equation (3) is obtained as ΔRo in step S11. Next, the sampling count value is reset in step S12, the interrupt is enabled in step S13, and the register of the bridge voltage increase ΔDA1 and the bridge voltage saving register DA1_old are reset in step S14. In step S15, the process waits until the count value reaches 4000. Since the count value is incremented every time the interrupt process of FIG. 5 is performed, if 4000 data is sampled by 4000 interrupt processes, the count value exceeds 4000. Therefore, the interrupt is disabled in step S16, and step S17 Then, the output process is performed and the process returns to the main routine (FIG. 3A).

  In step S21, the ideal resistance value Ro corresponding to the current count is obtained from the equation (3) in step S21. In step S22, AD1, AD2, and AD3 are obtained from the AD converters 11, 12, and 13, respectively. Each data is input, and AD3 data is stored in a memory (RAM) in step S23. Next, in step S24, AD1 and AD2 of 15-bit data are converted into voltage values, and in step S25, the resistance value Rs on the gas sensor side is calculated by the equation (1).

  Next, the magnitude relationship between Rs and Ro is determined in step S26 and step S27. If Rs <Ro, the coefficient kinc is added to the bridge voltage increase ΔDA1 and updated in step S28, and the process proceeds to step S30. If Rs = Ro, the increase ΔDA1 remains unchanged and the process proceeds to step S30. If Rs> Ro, in step S29, the coefficient kinc is subtracted and updated from the increase ΔDA1, and the process proceeds to step S30. In step S30, the increase ΔDA1 is added to the previous bridge voltage DA1_old to obtain the current bridge voltage DA1.

  Next, in step S31, the bridge voltage DA1 is output to the analog unit via the DA converter 16. In step S32, the current bridge voltage DA1 is saved in the register DA1_old, and the count is incremented to return to the original routine.

  FIG. 5 is a diagram showing a temperature change of the microheater 1 on the gas sensor side by the above-described measurement control processing, and the temperature of the microheater 1 for the first and second 40 milliseconds is controlled to be constant. By the first constant temperature increase control, excess gas components, moisture, etc. of the gas sensor are burned out, and the measurement gas is adsorbed in the subsequent 10 seconds. Then, gas measurement is performed by the second constant temperature increase control.

  FIG. 6 is a graph showing the resistance value of the microheater 1 of the gas sensor with respect to time, and FIG. 7 is a graph showing the error with respect to time between the resistance value of the microheater 1 and the ideal resistance value. In addition, since the microheater 1 is comprised with platinum, the resistance value and temperature have a linear relationship. As shown in FIG. 6, the temperature control of the microheater 1 is controlled with an error of 0.2% or less when entering a steady state.

  Like the resistance value shown in FIG. 6, the temperature of the microheater 1 is controlled so as to reach a constant temperature rise with respect to time even if an adsorption combustion reaction is caused by the gas. No. 2 is controlled to a low temperature even when the gas sensor causes an adsorption combustion reaction because there is no adsorption combustion reaction by gas. For this reason, the value of AD3 varies depending on the gas type and gas concentration.

  FIG. 8 is a graph showing changes in the output of AD3 when the measurement gas is ISO butanol. The horizontal axes of the graphs of FIGS. 8 to 10 below are temperature axes in which the time (15 milliseconds to 40 milliseconds) in the constant temperature increase control is the temperature of the gas sensor. The waveforms from 0 to 15 milliseconds are omitted because the heater control is not in a steady state. Then, a differentiation operation is performed on the sampled AD3 data by the following equation (4).

Y [n] = (X [n] −X [nm]) / m (4)
n = N, Nm, N-2m, N-3m, ..., 1
Y [n]: differential value X [n]: sampling data m: thinning rate (10)
N: Total number of data (4000)

  FIG. 9 is a graph of the differential value of AD3 in the constant temperature rise control of ISO butanol shown in FIG. FIG. 10 is a graph of the differential value of AD3 in the constant temperature increase control of TERT butanol. The peak position (temperature) is obtained from the waveform of such a differential value, and the gas type is determined from the peak position. Further, the value of AD3 at the peak position is obtained, and the gas concentration is calculated. In addition, calculation of gas concentration is based on the regression equation previously calculated | required for every gas concentration.

  As can be seen from the differential waveform at each concentration of ISO butanol shown in FIG. 9 and the differential waveform at each concentration of TERT butanol shown in FIG. 10, there is almost no deviation of the peak position due to the gas concentration. In addition, the sensor temperature at which a peak appears can be measured using a differential waveform at each concentration of ISO butanol and a differential waveform at each concentration of TERT butanol.

  Further, according to the embodiment, by switching the gain of the second-stage instrumentation amplifier ap4, it becomes possible to analyze even with a low concentration gas. Further, the apparatus can be reduced in size, such as not requiring a large part such as an adsorption / desorption analyzer, and the apparatus can be carried. Furthermore, a low-cost gas analyzer is obtained by using the same parts as the gas detector.

  The configuration of the analog unit of the gas sensor shown in FIG. 1 is an example, and is not limited to this embodiment, and other configurations may be used. 2 may be performed by a personal computer, a microcomputer, or the like.

It is a circuit diagram of the analog part of the gas analyzer of embodiment of this invention. It is a circuit block diagram of the digital part of the gas analyzer. It is a flowchart of the main routine and subroutine which concern on embodiment. 5 is a flowchart of an interrupt processing routine according to the embodiment. It is a figure which shows the temperature change of the micro heater by the side of the gas sensor by the measurement control process in embodiment. It is a graph which shows the resistance value of the micro heater of the gas sensor with respect to time in embodiment. It is a graph which shows the error with respect to time of the resistance value of the micro heater of the gas sensor in an embodiment, and an ideal resistance value. It is a graph which shows the output change of AD3 in case measurement gas is ISO butanol. It is a graph of the differential value of AD3 in the constant temperature rise control of ISO butanol. It is a graph of the differential value of AD3 in constant temperature increase control of TERT butanol. It is a circuit block diagram of the conventional gas detection apparatus. It is a figure which shows the pulse voltage for a drive of the gas detection apparatus. It is a figure which shows schematic structure of an adsorption combustion type gas sensor.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Gas sensor side micro heater 2 Reference side micro heater Bs Gas sensor side bridge circuit Br Reference side bridge circuit ap4 Instrumentation amplifier 16 DSP
17 DA converter

Claims (2)

A gas analyzer that samples the sensor output of an adsorption combustion type gas sensor, obtains differential waveform data by time differentiation of the sampled sensor output, and performs gas analysis based on the acquired differential waveform data,
A gas analyzer which samples the sensor output while controlling a voltage applied to a heater of the adsorption combustion type gas sensor and controlling the heater at a constant temperature.   The gas analyzer according to claim 1, wherein the resistance value of the heater is sampled, and the voltage applied to the heater is controlled so that the sampled resistance value rises with a constant change.

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