JP2011052976A - Gas detector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a gas detector capable of improving detection sensitivity of a gas to be detected by heightening an amplification factor even in the case of the occurrence of an error potential difference due to the difference in temperature rise characteristics between a sensing element and a compensation element of an adsorption combustion gas sensor. <P>SOLUTION: The gas detector 1 includes an amplification part 6a for amplifying a midpoint potential difference of a bridge circuit 2 provided with the adsorption combustion gas sensor 15 at a prescribed amplification factor; an offset voltage generating part 8 for generating a plurality of offset voltages different from one another; and a voltage addition part 6b for adding the offset voltages to an amplified midpoint potential difference. The gas detector 1 is provided with an offset voltage changeover means 61a for controlling the offset voltage generating part in such a way as to output such an offset voltage as to minimize an error potential difference equal to a prescribed minimum voltage or greater from among the plurality of offset voltages on the basis of offset voltage changeover information during the transition period of a temperature rise of each element. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to a gas detection device using an adsorption combustion type gas sensor.

  A conventionally known catalytic combustion type gas sensor has a sensitive element and a compensating element, and combusts the gas to be detected by the catalytic action of the sensitive element, and captures this combustion heat as a change in the resistance value of the platinum coil. Has been. Among the gases to be detected, gases with high polarity and large adsorption power, such as toluene, acetic acid, and ethanol, are adsorbed when the gas molecules are adsorbed on the catalyst surface of the sensitive element when driven at low temperatures and when they are driven at high temperatures. Burns instantaneously and the catalytic combustion reaction occurs simultaneously, so that the sensor output has a peak waveform (mountain waveform) that reaches a peak in a short time and then gradually decreases. On the other hand, a nonpolar or small polarity gas such as methane, hydrogen, carbon monoxide or the like has a small adsorption power, so the above phenomenon does not occur, and the sensor output gradually increases until it stabilizes at a steady value.

  As described above, by utilizing a specific peak waveform in a specific type of gas such as toluene, it is possible to detect a gas concentration or to classify a gas type using a catalytic combustion type gas sensor. Such a catalytic combustion type gas sensor that utilizes an adsorption phenomenon of a specific type of gas is also called an adsorption combustion type gas sensor. The adsorption combustion type gas sensor as described above is used in various gas detection devices such as a gas concentration detection device and a gas type detection device (see, for example, Patent Document 1).

  FIG. 9 shows a configuration example of a conventional gas detection device provided with an adsorption combustion type gas sensor. A gas detection device 801 shown in FIG. 9 includes a sensor element 810 that is sensitive to a detection target gas, a sensor circuit unit 810 that is connected in series with the sensor element 811, and a sensor circuit unit 810 that is connected in parallel. And a reference circuit unit 820 comprising a compensation element 812 that is insensitive to the detection target gas and a fixed resistor 813 connected in series with the compensation element 812, and the temperature of the detection element 811 is the detection target A voltage supply source 805 that sequentially supplies the bridge circuit 802 with a low temperature driving voltage for adsorbing gas and a high temperature driving voltage for causing the temperature of the sensitive element 811 to burn the detection target gas adsorbed on the sensitive element 811. The first voltage V1 generated between the sensitive element 811 and the fixed resistor 814, the compensation element 812, and the fixed resistor. 813 is connected to the midpoint of the sensor circuit portion 810 and the midpoint of the reference circuit portion 820 so that the second voltage V2 generated between the first voltage V2 and the second voltage V2 is input. The potential difference between the first voltage V1 and the second voltage V2 An instrumentation amplifier 806 that amplifies (hereinafter referred to as “midpoint potential difference Vc”) at a predetermined amplification factor, and an A / D converter that converts the midpoint potential difference Vc amplified by the instrumentation amplifier 806 from an analog value to a digital value 807 and a known microcomputer (MPU) 860 that detects the concentration of the detection target gas based on the midpoint potential difference Vc converted into a digital value by the A / D converter 807. When a high temperature driving voltage is supplied in an atmosphere that does not include the detection target gas, the bridge circuit 802 is balanced in a steady state in which the temperature change of each element converges (that is, the midpoint potential difference Vc is 0). The resistance values of the fixed resistors 813 and 814 are determined. The instrumentation amplifier 806 is provided with an offset voltage terminal, and has a function of adding the voltage input to the offset voltage terminal to the potential difference after amplification. To the offset voltage terminal of the instrumentation amplifier 806, for example, a constant voltage generation circuit 808 composed of a voltage dividing circuit or the like is connected.

The midpoint potential difference Vc is such that the resistance value of the sensitive element 811 is Rs, the resistance value of the compensation element 812 is Rr, the resistance value of the fixed resistor 814 is R2, the resistance value of the fixed resistor 813 is R1, and the bridge circuit 2 Assuming that the supply voltage is Vbrg, the following expression (1) is obtained.
Vc = ((Rs / (R2 + Rs)) − (Rr / (R1 + Rr))) × Vbrg (1)

Further, the midpoint potential difference Vc amplified by the instrumentation amplifier 806, that is, the output voltage Vo of the instrumentation amplifier 806, is assumed that the amplification factor of the instrumentation amplifier 806 is G and the offset voltage input to the offset voltage terminal is Voffset. Is expressed by the following equation (2).
Vo = Vc × G + Voffset (2)

  As is clear from the above equations (1) and (2), the change in the output voltage Vo of the instrumentation amplifier 806 is the resistance value R1, the resistance value R2, the supply voltage Vbrg, the amplification factor G, and the offset at which the values are fixed, respectively. The output voltage Vo is determined by the resistance value Rs of the sensitive element 811 and the resistance value Rr of the compensating element 812, the value of which varies with temperature.

  The sensing element 811 of the sensor circuit unit 810 and the compensation element 812 of the reference circuit unit 20 start to rise in temperature when the supply voltage Vbrg is switched from the low temperature driving voltage to the high temperature driving voltage. Since the heat capacity, heat conduction characteristics, heat dissipation characteristics, etc. are different from each other, the temperature rise curve in the transient state (transient period) until the temperature of each element reaches a stable steady state, that is, the temperature rise characteristic is Therefore, a difference occurs in the change of the resistance value with the temperature rise, and the midpoint potential difference Vc due to the difference in the temperature rise characteristic, that is, the error potential difference Ve occurs in the transient period. In order to prevent such an error potential difference Ve caused by the difference in temperature rise characteristics of each element, it is conceivable to improve the accuracy of the manufacturing method of each element or to select it. However, it is not possible to match the temperature rise characteristics completely. Is possible. This error potential difference Ve is amplified with the amplification factor G by the instrumentation amplifier 806 and is included in the output voltage Vo and output. FIG. 10 is a graph of the output voltage Vo (that is, the error potential difference Ve after amplification) of the instrumentation amplifier 806 when driven at high temperature in an atmosphere that does not contain the detection target gas. Such an adsorption combustion type gas sensor performs gas detection based on the change (that is, response waveform) of the midpoint potential difference Vc at the time of high temperature driving, so that the error potential difference caused by the difference in temperature rise characteristics as described above. The influence of Ve affects the accuracy of gas detection.

  Therefore, in the gas detection device 801, in order to avoid the influence of the midpoint potential difference Vc caused by the difference in temperature rise characteristics between the sensitive element 811 and the compensation element 812, that is, the error potential difference Ve, the concentration of the detection target gas is as follows. Was detected.

  First, in an atmosphere that does not include the detection target gas, the midpoint potential difference Vc (that is, the error potential difference Ve) is measured at a predetermined sampling period when the high temperature drive voltage is supplied after the low temperature drive voltage is supplied to the bridge circuit 802. Then, the error potential difference integral value is calculated by integrating the measured middle point potential differences Vc. Next, in the atmosphere for detecting the concentration of the detection target gas, the midpoint potential difference Vc when the high temperature driving voltage is supplied after the low temperature driving voltage is supplied to the bridge circuit 802 is similarly measured at a predetermined sampling period. The concentration potential difference integral value is calculated by integrating the measured plurality of midpoint potential differences Vc. Then, based on a value obtained by subtracting the error potential difference integral value from the concentration potential difference integral value, the concentration of the detection target gas is detected. In this way, the influence of the error potential difference Ve caused by the difference in temperature rise characteristics is avoided, thereby preventing a decrease in accuracy in detecting the concentration of the detection target gas.

JP 2005-83950 A

  Since the above-described midpoint potential difference Vc is a minute voltage, it is amplified by the instrumentation amplifier 806 at a predetermined amplification factor and then input to the A / D converter 807 at the subsequent stage. In general, an instrumentation amplifier has a maximum output voltage (usually the same as or slightly lower than the power supply voltage), so that the voltage output by the instrumentation amplifier 806 is less than or equal to this maximum output voltage. The amplification factor is determined. Similarly, the maximum input voltage is also determined for the A / D converter. Therefore, the amplification factor of the instrumentation amplifier 806 is set so that the voltage input to the A / D converter 807 is equal to or lower than the maximum input voltage. Is determined. And as the amplification factor of the instrumentation amplifier 806 is higher, a minute voltage can be converted into a larger voltage, so that the detection sensitivity can be improved. However, if the value of the above-described error potential difference Ve included in the midpoint potential difference Vc is large, the maximum input voltage or the maximum output voltage is reached even at a low amplification factor, so that the amplification factor of the instrumentation amplifier 806 can be increased. It was not possible to improve the detection sensitivity of the detection target gas.

  The present invention aims to solve the above problems. That is, the present invention provides a gas detection device capable of increasing the amplification factor and improving the detection sensitivity of the detection target gas even when an error potential difference occurs due to a difference in temperature rise characteristics between the sensitive element and the compensation element of the adsorption combustion type gas sensor. The purpose is to do.

  In order to achieve the above object, according to the first aspect of the present invention, as shown in the basic configuration diagram of FIG. 1, (a) a sensitive element sensitive to a detection target gas and a compensation element not sensitive to the detection target gas (B) a low temperature driving voltage at which the temperature of the sensitive element is a low temperature for adsorbing the detection target gas, and the temperature of the sensitive element is adsorbed to the sensitive element. A voltage supply source 5 that sequentially supplies the bridge circuit 2 with a high-temperature driving voltage that is a high temperature for burning the detected gas, and (c) an amplifier circuit 9 connected to a pair of midpoints in the bridge circuit 2 The amplifying circuit 9 generates a plurality of offset voltages different from each other, and an amplifying unit 6a that amplifies the potential difference generated between the pair of midpoints at a predetermined amplification factor. In addition, an offset voltage generation unit 8 that outputs one of the plurality of offset voltages, and the offset voltage output by the offset voltage generation unit 8 are added to the potential difference amplified by the amplification unit 6a. And a voltage adding unit 6b, and a transient period in which the temperature of the sensitive element and the compensating element rises after the high-temperature driving voltage is supplied to the bridge circuit 2 by the voltage supply source 5. Based on preset offset voltage switching information, among the plurality of offset voltages, the difference is caused by a difference in temperature rise characteristics between the sensitive element and the compensating element, and is added to the potential difference amplified by the amplifying unit 6a. The offset voltage that makes the potential difference not less than a predetermined minimum voltage and the smallest As output, it is a gas detection apparatus 1, characterized in that the offset voltage switching means 61a for controlling the offset voltage generator 8 is provided.

  According to the first aspect of the present invention, the amplifier circuit generates a plurality of offset voltages different from each other, and an amplifier that amplifies a potential difference generated between a pair of midpoints in the bridge circuit at a predetermined amplification factor. An offset voltage generation unit that outputs one of the plurality of offset voltages, and a voltage addition unit that adds the offset voltage output by the offset voltage generation unit to the potential difference amplified by the amplification unit. And a plurality of offsets based on preset offset voltage switching information in a transient period in which the temperature of the sensitive element and the compensating element rises after the high temperature driving voltage is supplied to the bridge circuit by the voltage supply source. Among the voltages, the voltage is generated due to a difference in temperature rise characteristics between the sensitive element and the compensating element, and is also caused by the amplifying unit. The offset voltage generator is controlled so that the offset voltage is output by adding the amplified potential difference (that is, the error potential difference after amplification) to the minimum or above the minimum voltage. Therefore, among the plurality of preset offset voltages, an offset voltage in which the error potential difference after amplification in the transient period is not less than a predetermined minimum voltage and becomes the minimum can be added to the error potential difference. By reducing the difference between the minimum value and the maximum value of the error potential difference after amplification in the period, the amplification factor in the amplifier can be further increased, and the detection sensitivity can be improved. In addition, the conventional bridge circuit can be used as it is, the amplifier circuit can be configured with a simple circuit, and the control for the offset voltage generation unit is also simple, so that the gas detection device capable of improving the detection sensitivity can be reduced in cost. Can be provided at. In addition, in a bridge circuit in which a sensitive element and a compensating element are connected in parallel, even when it is difficult to adjust the balance of the bridge circuit, the balance of the bridge circuit is adjusted only by adjusting the offset voltage. The same effect can be obtained. Further, since it is not necessary to match the temperature rise characteristics of the sensitive element and the compensating element of the adsorption combustion type gas sensor with high accuracy, the current element can be used and the cost can be reduced.

It is a figure which shows the basic composition of the gas detection apparatus of this invention. It is a block diagram which shows the gas concentration detection apparatus which is one Embodiment of the gas detection apparatus of this invention. (A), (B), and (C) are the sectional views which follow the AA line in the top view of a gas sensor unit with which the gas concentration detection apparatus of FIG. 2 is equipped, respectively, and a top view. It is a flowchart which shows an example of the offset voltage switching information acquisition process which CPU with which the gas concentration detection apparatus of FIG. 2 is provided performs. It is a flowchart which shows an example of the gas concentration detection process which CPU with which the gas concentration detection apparatus of FIG. 2 is provided. It is a graph which shows the output voltage (error potential difference after amplification) of the instrumentation amplifier in the gas concentration detection apparatus of FIG. It is a graph which shows the output voltage (error potential difference after amplification) of the instrumentation amplifier in the conventional gas concentration detection apparatus. The output voltage of the instrumentation amplifier (error potential difference after amplification) in the gas concentration detection device of FIG. 2 and the output voltage (error potential difference after amplification) of the instrumentation amplifier in the conventional gas concentration detection device are the maximum of the A / D converter, respectively. It is the graph amplified to the input voltage. It is a block diagram of the conventional gas concentration detection apparatus. It is a graph of the output voltage (error potential difference after amplification) of an instrumentation amplifier in a conventional gas concentration detection device.

  Hereinafter, a gas concentration detection apparatus as an embodiment of a gas detection apparatus according to the present invention will be described with reference to FIGS.

  As shown in FIG. 2, the gas concentration detector 1 includes a bridge circuit 2, a voltage supply source 5, an amplifier circuit 9, an A / D converter 7, a microcomputer 60, a gas storage chamber (not shown), A display device that does not.

  The bridge circuit 2 includes a first fixed resistor 13, a second fixed resistor 14, and a gas sensor unit 15 as an adsorption combustion type gas sensor. The gas sensor unit 15 includes a sensitive element 11 and a compensating element 12. The sensor circuit unit 10 is configured by connecting the second fixed resistor 14 and the sensitive element 11 in series, and the reference circuit unit 20 is configured by connecting the first fixed resistor 13 and the compensation element 12 in series. Is configured. Further, the bridge circuit 2 is configured by connecting the sensor circuit unit 10 and the reference circuit unit 20 in parallel to each other. A signal line connecting the first fixed resistor 13 and the second fixed resistor 14 in the bridge circuit 2 is connected to the voltage supply source 5. A signal line connecting the sensitive element 11 and the compensating element 12 in the bridge circuit 2 is connected to a ground point (GND). As another bridge circuit, the sensor circuit unit 10 is configured by connecting a sensing element 11 and a compensation element 12 in series, and the reference circuit unit 20 is configured by a first fixed resistor 13 and a second fixed resistor. The resistor 14 may be connected in series.

As shown in FIGS. 3A to 3C, the gas sensor unit 15 is formed on a silicon oxide (Si) wafer 41 having a predetermined thickness (for example, about 400 μm) and a silicon oxide having a predetermined thickness (for example, about 600 nm). An insulating thin film of a (SiO ₂ ) film 48c, a silicon nitride (SiN) film 48b having a predetermined thickness (for example, about 250 nm), and a hafnium oxide (HfO ₂ ) film 48a having a predetermined thickness (for example, about 30 nm) is sequentially formed. A multilayer insulating film is formed.

A platinum (Pt) heater 42 as a first heater having a predetermined thickness (for example, about 250 nm) is formed as the sensitive element 11 on the multilayer insulating film, and is in thermal contact with the platinum heater 42. In addition, as a catalyst material, for example, a catalyst layer 43 made of aluminum oxide (Al ₂ O ₃ ) supporting a platinum group such as palladium (Pd) that adsorbs and burns a detection target gas has a predetermined thickness (for example, 1 to 1 About 40 μm).

Further, on the multilayer insulating film, as the compensation element 12, a platinum (Pt) heater 44 as a second heater having a predetermined thickness (for example, about 250 nm) and aluminum oxide in thermal contact with the platinum heater 44 are provided. A non-catalytic layer 45 made of only (Al ₂ O ₃ ) is formed with a predetermined thickness (for example, about 1 to 40 μm).

  Further, as shown in FIG. 3C, the silicon wafer 41 is anisotropically etched to form the recesses 46 and 47 at positions corresponding to the sensitive element 11 and the compensating element 12, and thereby each of the above-described insulations. Thin film diaphragms Ds and Dr are formed by a thin film.

  The sensing element 11 and the compensating element 12 are in a steady state in which the temperature change converges after the low temperature driving voltage and the high temperature driving voltage are supplied by the voltage supply source 5 to be described later in an atmosphere that does not include the detection target gas. The platinum heater 42 of the sensitive element 11 and the platinum heater 44 of the compensating element 12 are formed to have the same resistance value.

  In addition, since the sensitive element 11 includes the catalyst layer 43 and the compensation element 12 includes the non-catalytic layer 45 (that is, does not include a catalyst), the voltage supply source 5 causes the bridge circuit 2 (sensor circuit unit). 10 and the reference circuit unit 20) are supplied with a predetermined low-temperature driving voltage, the detection target gas is adsorbed to the catalyst layer 43 in the sensitive element 11, while the detection target gas is absorbed in the non-catalytic layer 45 in the compensation element 12. When a predetermined high temperature driving voltage is supplied to the bridge circuit 2 by the voltage supply source 5, the detection target gas is burned by the catalyst in the sensitive element 11, while the detection element is detected in the compensation element 12. Gas does not burn. That is, the sensitive element 11 is sensitive to the detection target gas, and the compensation element 12 is not sensitive to the detection target gas.

  Therefore, when the high temperature driving voltage is supplied after the low temperature driving voltage is supplied from the voltage supply source 5 in the atmosphere including the detection target gas, the sensitive element 11 and the compensating element 12 are adsorbed to the sensitive element 11. The detection target gas burns explosively. Then, due to this combustion energy, the temperature of the sensitive element 11 becomes higher than the temperature of the compensating element 12, and a temperature difference corresponding to the concentration of the detection target gas occurs between the sensitive element 11 and the compensating element 12. There is a difference in resistance between the platinum heater 42 of the sensitive element 11 and the platinum heater 44 of the compensating element 12. The difference between the resistance values is between the second fixed resistor 14 and the sensitive element 11 (that is, the middle point of the sensor circuit unit 10) and between the first fixed resistor 13 and the compensation element 12 (that is, the reference circuit unit 20). Between the pair of midpoints in the bridge circuit 2 and appear as a potential difference. The potential difference between the pair of midpoints is referred to as a “midpoint potential difference Vc”, and the gas concentration is detected based on the midpoint potential difference Vc.

  The gas sensor unit 15 is installed in a gas storage chamber (not shown). The gas storage chamber is filled with a zero gas atmosphere (air base) that does not contain the detection target gas and an atmosphere (test gas) for detecting the concentration of the detection target gas under the control of the microcomputer 60 described later. .

  The first fixed resistor 13 and the second fixed resistor 14 are well-known electronic components that generate an electric resistance having a predetermined fixed value. The first fixed resistor 13 and the second fixed resistor 14 may be configured by combining a plurality of fixed resistors in series, in parallel, or in series and in parallel, or fixing the resistance value when measuring the gas concentration. For example, a variable resistor that can change the resistance value for balance adjustment may be used. The first fixed resistor 13 and the second fixed resistor 14 are bridge circuits configured only by the first fixed resistor 13, the second fixed resistor 14, and the gas sensor unit 15 in an atmosphere that does not include the detection target gas. 2 is supplied with a high temperature driving voltage by the voltage supply source 5 so that the changes in the temperature of the sensitive element 11 and the temperature of the compensating element 12 are balanced in a steady state, that is, between a pair of midpoints. The respective resistance values are determined so that the generated midpoint potential difference Vc becomes zero. In the present embodiment, the resistance value of the first fixed resistor 13 is set to 200Ω, and the resistance value of the second fixed resistor 14 is set to 200Ω. These resistance values are examples, and are appropriately determined according to the type of gas concentration detection device or detection target gas. Further, the bridge circuit 2 may be provided with a variable resistor (such as a semi-fixed resistor or a digital potentiometer) separately from the fixed resistor so as to enable balance adjustment in the steady state.

  The resistance value of the sensitive element 11 is Rs, the resistance value of the compensation element 12 is Rr, the resistance value of the first fixed resistor 13 is R1, the resistance value of the second fixed resistor 14 is R2, and the supply voltage to the bridge circuit 2 is Assuming Vbrg, the midpoint potential difference Vc is expressed by the following equation (3).

  Vc = ((Rs / (R2 + Rs)) − (Rr / (R1 + Rr))) × Vbrg (3)

  The voltage supply source 5 is a voltage supply circuit that supplies a predetermined voltage to the bridge circuit 2. The voltage supply source 5 is connected to an MPU 60 to be described later, and in accordance with a voltage control signal from the MPU 60, a temperature driving voltage at which the temperature of the sensitive element 11 becomes a low temperature (for example, 200 degrees) at which the detection target gas is adsorbed. A pulsed supply voltage Vbrg such as a high temperature drive voltage at which the temperature of the sensitive element 11 becomes a high temperature (for example, 400 degrees) for burning the detection target gas adsorbed on the sensitive element 11 is supplied to the bridge circuit 2. These low temperature and high temperature are examples, and are appropriately determined according to the configuration of the gas concentration detection device or the type of detection target gas.

  The amplifying circuit 9 includes an instrumentation amplifier 6 as an amplifying unit and a voltage adding unit, and an offset voltage generating circuit 8 as an offset voltage generating unit.

  The instrumentation amplifier 6 is a balanced input amplifier having a differential input and a single-ended output, and is a well-known amplifier having a feature that a common mode rejection ratio (CMRR) can be increased. The instrumentation amplifier 6 amplifies the potential difference between the signals input to the pair of differential inputs with high impedance, respectively, with a predetermined amplification factor, and outputs the amplified signal. The middle point signal line of the sensor circuit unit 10 is connected to one (V +) of the differential input terminals of the instrumentation amplifier 6, and the middle point of the reference circuit unit 20 is connected to the other (V−). Has been. That is, the instrumentation amplifier 6 receives the midpoint potential of the sensor circuit unit 10 (ie, the first voltage V1) and the midpoint potential of the reference circuit unit 20 (ie, the second voltage V2). The potential difference between the first voltage V1 and the second voltage V2 (that is, the midpoint potential difference Vc, specifically, the voltage obtained by subtracting the second voltage V2 from the first voltage V1) is amplified by a predetermined amplification factor to output voltage. Output from the output terminal as Vo. The instrumentation amplifier 6 has a maximum voltage (maximum output voltage) that can be output, and the amplification factor of the instrumentation amplifier 6 is determined so that the output voltage Vo does not exceed the maximum output voltage.

  The instrumentation amplifier 6 is provided with an offset voltage terminal Voffset. The instrumentation amplifier 6 adds the voltage input to the offset voltage terminal Voffset to the potential difference after being amplified at the predetermined amplification factor, and then outputs it as the output voltage Vo.

  The offset voltage generation circuit 8 includes, for example, a plurality of voltage dividing circuits (not shown) that divide a voltage supplied from a negative voltage supply source (not shown) and generate a plurality of different constant voltages (that is, offset voltages). The signal system includes a plurality of signal systems, the input side of each signal system is connected to any one of the plurality of voltage dividing circuits, and the output side of each signal system is connected together as an output signal line. And a single-pole single-throw (SPST) analog switch (not shown). The control signal line of the analog switch is connected to the MPU 60, and the output signal line of the analog switch is connected to the offset voltage terminal Voffset of the instrumentation amplifier 6.

  Based on the offset voltage switching control signal from the MPU 60, the analog switch of the offset voltage generation circuit 8 has only one voltage dividing circuit connected to the output side among a plurality of voltage dividing circuits connected to the input side of each signal system. Thus, each signal system is turned on or off, that is, one offset voltage among a plurality of offset voltages input to each signal system is output to the offset voltage terminal Voffset. In addition to the configuration described above, the offset voltage generation circuit 8 may be, for example, a configuration in which a known digital-analog converter (D / A converter) and a voltage dividing circuit are combined, or a digital potentiometer instead of a D / A converter. The configuration is arbitrary as long as it can generate a plurality of different offset voltages and can output one offset voltage among the plurality of offset voltages without departing from the object of the present invention. .

  In the present embodiment, the offset voltage generation circuit 8 generates three offset voltages (offset voltage A (0 V), offset voltage B (-3.4 V), and offset voltage C (-7.2 V), respectively). Generate one of the offset voltages A, B, and C based on the offset voltage switching control signal. Each of the offset voltages is an example, and is appropriately determined according to the configuration of the gas concentration detection device. In order to cancel or reduce the error potential difference Ve included in the output voltage Vo of the instrumentation amplifier 6 (that is, the error potential difference Ve after amplification) in the transition period described above, the plurality of offset voltages are offset voltages A, B,. .. It is desirable that the voltage is set so that the voltage decreases sequentially with n.

  The A / D converter 7 is a well-known analog-digital converter that converts an input analog signal into a digital signal and outputs the digital signal. The output voltage Vo output by the instrumentation amplifier 6 is input to the input portion of the A / D converter 7. The output unit of the A / D converter 7 is connected to the MPU 60, and the output voltage Vo converted into a digital signal is output toward the MPU 60. The A / D converter 7 has a maximum input voltage (maximum input voltage), and the amplification factor of the instrumentation amplifier 6 is determined so that the output voltage Vo does not exceed the maximum input voltage. ing.

  As is well known, the microcomputer (MPU) 60 is a central processing unit (CPU) 61 that performs various processes and controls in accordance with a predetermined program, programs for the CPU 61 and various parameters (for example, low temperature drive voltage value). ROM 62, which is a read-only memory that stores high-temperature drive voltage values, the number of offset voltages, the sampling upper limit number, various period values, and the like, and an area necessary for processing operations of the CPU 61 (for example, RAM 63 which is a readable / writable memory having a loop variable, an error potential difference storage area, an error potential difference integral value 1, a concentration potential difference integral value, and the like, and can retain various stored data even when power supply is cut off. Various storage areas (e.g., offset) required for processing operations of the CPU 61 are possible. G Voltage switching information storage area, and a EEPROM64 like having an error difference integral value, etc.). The CPU 61 functions as an offset voltage switching unit by executing various programs stored in the ROM 62.

  The MPU 60 further includes an input / output port (not shown) and an external connection unit having various interface functions. The MPU 60 is connected to the A / D converter 7, the voltage supply source 5, and the offset voltage generation circuit 8 (that is, an analog switch (not shown) included in the offset voltage generation circuit 8) via the external connection unit. The MPU 60 receives the output voltage Vo converted from the A / D converter 7 into a digital signal (that is, the amplified midpoint potential difference Vc), and detects the gas concentration based on the output voltage Vo. For example, the MPU 60 transmits a voltage control signal to the voltage supply source 5 so as to supply a high temperature driving voltage for a predetermined gas combustion period after supplying a low temperature driving voltage for a predetermined gas adsorption period according to the processing. . For example, the MPU 60 transmits an offset voltage switching control signal to the offset voltage generation circuit 8 so that an appropriate offset voltage is output to the instrumentation amplifier 6 according to the processing. The MPU 60 is connected to a display device (not shown) via the external connection unit, and transmits, for example, a display control signal including information on the detected concentration of the detection target gas to the display device. The display device displays information corresponding to the display control signal, that is, the concentration of the detection target gas. Further, the MPU 60 is connected to a gas storage chamber equipped with a pump or the like through this external connection portion, and fills the gas storage chamber with various gases according to processing.

  Next, an example of processing (offset voltage switching information acquisition processing) according to the present invention executed by the CPU 61 described above will be described below with reference to the flowchart shown in FIG.

  When the gas concentration detector 1 is powered on, the CPU 61 fills the gas storage chamber with a zero gas atmosphere that does not include the detection target gas, and then advances the process to step S110.

  In step S110, a loop variable provided on the RAM 63 used for the processing of this flowchart is initialized (offset voltage switching loop count s = 1, sampling loop count n = 1). Further, an offset voltage switching control signal for outputting one of the plurality of offset voltages (for example, offset voltage A) to the instrumentation amplifier 6 is transmitted to the offset voltage generation circuit 8. Then, the process proceeds to step S120.

  In step S <b> 120, a voltage control signal for supplying a low temperature driving voltage to the bridge circuit 2 is transmitted to the voltage supply source 5. Then, the process proceeds to step S130.

  In step S130, the process waits until the temperature of the sensitive element 11 and the temperature of the compensating element 12 are stabilized (temperature stabilization period, for example, 60 seconds). Then, after the temperature stabilization period has elapsed, the process proceeds to step S140.

  In step S140, the offset voltage generation circuit 8 is provided with an offset voltage corresponding to the number of offset voltage switching loops s (offset voltage A when s = 1, offset voltage B when s = 2, and s = 3). Transmits an offset voltage switching control signal for outputting the offset voltage C) to the instrumentation amplifier 6. Then, the process proceeds to step S150.

  In step S150, a voltage control signal for supplying a high temperature driving voltage to the bridge circuit 2 is transmitted to the voltage supply source 5. Then, the process proceeds to step S160.

  In step S160, the midpoint potential difference Vc amplified by the instrumentation amplifier 6 and converted into a digital signal by the A / D converter 7 is acquired as an error potential difference Ve, and an error potential difference storage area provided on the RAM 63 [ s, n]. This error potential difference storage area [s, n] has a two-dimensional array data structure, and the storage position of the acquired error potential difference Ve is designated using the offset voltage switching loop count s and the sampling loop count n as indexes. For example, when the offset voltage switching loop count s = 1 and the sampling loop count n = 3, the error potential difference Ve is stored in the error voltage storage area [1, 3]. Thereafter, the sampling loop count n is incremented by one. Then, the process proceeds to step S170.

  In step S170, the process waits until a predetermined sampling interval time elapses. For example, the sampling interval time is a value (for example, 1 ms) obtained by dividing a sampling period (transient period, for example, 400 ms) by a predetermined sampling upper limit number (for example, 400 times) stored in the ROM 62. ) Etc. are used. The sampling interval time is generally set in the range of several hundred μs to several ms. And after sampling interval time passes, it progresses to step S180.

  In step S180, it is determined whether or not the sampling loop count n has exceeded the upper sampling limit, and if it has exceeded the upper sampling limit, it is determined that the acquisition of the error potential difference Ve at the current offset voltage has been completed, and the offset voltage switching is performed. After increasing the number of loops s by 1, the process proceeds to step S190 (Y in S180). When the number is less than the upper limit of sampling, the acquisition of the error potential difference Ve at the current offset voltage is continued, and the process returns to step S160 (in S180). N).

  In step S190, it is determined whether or not the number of offset voltage switching loops s exceeds the number of offset voltages generated in the offset voltage generation circuit 8 stored in the ROM 62 (that is, three). If the number of offset voltages has been exceeded, it is determined that the acquisition of the error potential difference Ve has been completed for all of the plurality of offset voltages A, B, and C, the process proceeds to step S200 (Y in S190), and the number of the plurality of offset voltages In the following cases, it is assumed that an offset voltage for which the error potential difference Ve has not yet been acquired remains, and the process returns to step S120 (N in S190).

  In step S200, loop variables and the like are initialized (sampling loop number n = 1, error potential difference integral value 1 = 0). Then, the process proceeds to step S210.

  In step S210, the error potential difference Ve that is larger than 0 and the smallest is selected from those having the same sampling timing (that is, n is the same) in the error potential difference storage area [s, n]. Information specifying the offset voltage in the potential difference storage area [s, n] (ie, s) is stored in the offset voltage switching information storage area [n] provided on the EEPROM 64. The offset voltage switching information storage area [n] has a one-dimensional array data structure, and the storage position of information indicating the offset voltage is designated using the sampling loop number n as an index. For example, when the error potential difference storage area [1, 35] = 7.5 V, the error potential difference storage area [2, 35] = 4 V, and the error potential difference storage area [3, 35] = 0 V, the offset voltage switching information is stored. In the area [35], “2”, which has the error potential difference Ve larger than 0 and smallest, is stored. Then, the process proceeds to step S220. In the present embodiment, the error potential difference Ve is larger than 0 and the smallest is selected from those having the same sampling timing (that is, n is the same) in the error potential difference storage area [s, n]. However, the present invention is not limited to this. For example, in order to prevent the error potential difference Ve from being lower than 0 due to a change in resistance value due to deterioration of each element with time or the like, a predetermined minimum voltage or higher (for example, 0.5 V or higher). Etc.) and the smallest one may be selected.

  In step S220, the error potential difference Ve stored in the error potential difference storage area [s, n] selected in step S210 is integrated with the error potential difference integral value 1 provided on the RAM 63. Then, the sampling loop count n is increased by one. Then, the process proceeds to step S230.

  In step S230, it is determined whether or not the number of sampling loops n has exceeded the upper limit number of sampling. If the upper limit number of sampling has been exceeded, the calculation of the error potential difference integral value 1 is completed, and the process proceeds to step S240 (S230). If Y is equal to or less than the sampling upper limit number, the error potential difference integral value 1 is calculated and the process returns to step S210 (N in S230).

  In step S240, the error potential difference integral value 1 accumulated in step S220 is stored in the error potential difference integral value provided on the EEPROM 64. And the process of this flowchart is complete | finished.

  Next, an example of processing (gas concentration detection processing) according to the present invention executed by the CPU 61 described above will be described below with reference to the flowchart shown in FIG.

  After the above-described offset voltage switching information acquisition processing is completed, the CPU 61 fills the atmosphere (test gas) for detecting the concentration of the detection target gas in the gas storage chamber, and then advances the processing to step T110.

  In step T110, a loop variable or the like provided on the RAM 63 used for the processing of this flowchart is initialized (sampling loop number n = 1, concentration potential difference integral value = 0). Then, the process proceeds to Step T120.

  In Step T120, a voltage control signal for supplying a low temperature driving voltage to the bridge circuit 2 is transmitted to the voltage supply source 5. Then, the process proceeds to Step T130.

  In step T130, the temperature of the sensitive element 11 and the temperature of the compensating element 12 are stabilized, and the process waits until a gas adsorption period for adsorbing the detection target gas to the sensitive element 11 elapses (for example, 60 seconds). Then, after the gas adsorption period has elapsed, the process proceeds to step T140.

  In step T140, a voltage control signal for supplying a high temperature driving voltage to the bridge circuit 2 is transmitted to the voltage supply source 5. Then, the process proceeds to Step T150.

  In Step T150, information specifying the offset voltage (that is, offset voltage switching information) stored in the offset voltage switching information storage area [n] corresponding to the sampling timing (that is, the sampling loop number n) is read. Then, the process proceeds to Step T160.

  In step T160, an offset voltage switching control signal for outputting the offset voltage specified by the information read in step T150 to the instrumentation amplifier 6 is transmitted to the offset voltage generation circuit 8. Then, the process proceeds to Step T170.

  In step T170, the midpoint potential difference Vc amplified by the instrumentation amplifier 6 and converted into a digital signal by the A / D converter 7 is acquired as a gas concentration potential difference and integrated into the concentration potential difference integral value provided on the RAM 63. To do. Thereafter, the sampling loop count n is incremented by one. Then, the process proceeds to T180.

  In step T180, the process waits until the predetermined sampling interval time described above elapses. And after sampling interval time passes, it progresses to step T190.

  In step T190, it is determined whether or not the number of sampling loops n has exceeded the upper limit of sampling. If the upper limit of sampling has been exceeded, it is determined that the calculation of the concentration potential difference integral value has been completed, and the process proceeds to step T200 (in T190). Y) If the number is not more than the sampling upper limit number, the process returns to step T150 in the middle of calculating the concentration potential difference integral value (N in T190).

  In step T200, a value obtained by subtracting the error potential difference integral value stored in the EEPROM 64 from the concentration potential difference integral value accumulated in step T170 is calculated, and the relationship between the potential difference integral value and the gas concentration stored in advance in the ROM 62 is calculated. Based on the conversion table, the gas concentration is obtained from the calculated value, and display information including information on the gas concentration is generated and transmitted to the display device. Then, the process returns to step T110 to measure the gas concentration again (end of the flowchart).

  The steps T150 and T160 described above correspond to the offset voltage switching means in the claims. In the above description, the offset voltage switching information acquisition process and the gas concentration detection process are separately described. However, these processes may be performed separately, or the gas concentration detection is performed continuously with the correction information acquisition process. Processing may be performed.

  Next, the operation (action) according to the present invention in the above-described gas concentration detection apparatus 1 will be described.

  The gas concentration detection device 1 performs the initialization process for the offset voltage switching information acquisition process after filling the gas storage chamber with the 0 gas atmosphere not including the detection target gas (S110). Then, the low temperature driving voltage is supplied to the bridge circuit 2 (S120, S130), and the offset voltage A is output to the instrumentation amplifier 6 (S140), and then the high temperature driving voltage is supplied to the bridge circuit 2. (S150), at each predetermined sampling timing, the midpoint potential difference Vc (that is, the error potential difference Ve) is acquired and stored (stored) in the RAM 63 (S160 to S180). Then, the offset voltage generation circuit 8 is switched, and the offset voltage B and the offset voltage C are sequentially output to the instrumentation amplifier 6, and the above series of error potential difference Ve acquisition operations are also performed for the offset voltage B and the offset voltage C. (S120 to S180) are performed.

  For the error potential difference Ve acquired using each of the plurality of offset voltages A, B, and C, the error potential difference Ve acquired at the same sampling timing is compared with each other, and the error potential difference Ve is greater than 0 and the smallest value The offset voltage that becomes the offset voltage that can minimize the error potential difference Ve at the sampling timing is stored (stored) in the EEPROM 64 (ie, the offset voltage switching information) (S210). In this way, offset voltage switching information is generated for all sampling timings. At the same time, an error potential difference integral value is also generated (S220).

  Next, the gas concentration detection device 1 performs an initialization process for the gas concentration detection process after filling the atmosphere for detecting the concentration of the detection target gas in the gas storage chamber (T110). After the low temperature driving voltage is supplied to the bridge circuit 2 for a predetermined gas adsorption period and the detection target gas is adsorbed to the sensitive element 11 (T120, T130), the high temperature driving voltage is sampled to the bridge circuit 2. A period to be performed (that is, a gas combustion period) is supplied (T140), and at each predetermined sampling timing, based on the offset voltage switching information corresponding to the sampling timing read from the EEPROM 64 (T150), a plurality of offset voltages A , B, and C, the offset voltage with the smallest error potential difference Ve is output to the instrumentation amplifier 6 (T160), and the midpoint potential difference Vc is acquired as a gas concentration potential difference and integrated (ie, integrated). (T170). Thus, the concentration potential difference integral value is calculated by acquiring, integrating and integrating the gas concentration potential difference for all sampling timings. Finally, the value obtained by subtracting the error potential difference integral value from the concentration potential difference integral value is compared with the conversion table to obtain the gas concentration and displayed on the display device (T200).

  FIG. 6 shows a graph of the error potential difference Ve (amplification factor 200 times) measured in the above-described gas concentration detection apparatus 1 according to the present invention, and FIG. 7 shows only one of the offset voltages A, B, and C. The graph of the error potential difference Ve (amplification factor 200 times) measured in the gas concentration detection apparatus (conventional structure A, B, and C in order) of the conventional structure (FIG. 9) input into the instrumentation amplifier 6 is shown. The graphs of FIGS. 6 and 7 are obtained by making the amplification factors of the gas concentration detection device 1 according to the present invention and the gas concentration detection devices of the conventional configurations A, B, and C the same. FIG. 8 shows a graph when the error potential difference Ve measured in the gas concentration detection apparatus according to the present invention is amplified until the maximum voltage reaches 10 V (that is, the maximum input voltage in the A / D converter 7), and the conventional configuration. The graph when the error potential difference Ve measured in the gas concentration detection apparatus of A is amplified until the maximum voltage becomes 10V is shown.

  From the graph of FIG. 6, the gas concentration detection apparatus 1 according to the present invention has an offset voltage A of 0 to 10 ms, an offset voltage B of 11 to 35 ms, an offset voltage C of 36 to 180 ms, an offset voltage B and 266 of 181 to 265 ms. It was found that at ˜400 ms, the offset voltage A was sequentially switched and output toward the instrumentation amplifier 6, and therefore the maximum value of the error potential difference Ve was suppressed to about 4V. On the other hand, from the graph of FIG. 7, in the gas concentration detection device of the conventional configuration A, the maximum value of the error potential difference Ve is about 10 V, and in the gas concentration detection devices of the conventional configuration B and the conventional configuration C, the error potential difference Ve. Is slightly lower than that of the conventional configuration A, but there is a section in which the error potential difference Ve becomes 0 V before reaching a steady state (approximately 400 ms or more) where the temperature of each element is stable, that is, a part of the transient period. It was found that only normal measurement was possible.

  Then, when the maximum input voltage of the A / D converter 7 connected to the subsequent stage of the instrumentation amplifier 6 is 10 V, the amplification of the instrumentation amplifier 6 provided in the gas concentration detection device of the conventional configuration A as shown in FIG. When the rate is 1, in the gas concentration detection apparatus 1 according to the present invention, the amplification factor of the instrumentation amplifier 6 can be set to 2.5 times. From this, according to this invention, since a higher amplification factor than the conventional gas concentration detection apparatus can be set, detection sensitivity can be improved. When the concentration of the detection target gas is actually measured, a voltage corresponding to the concentration is added to the error potential difference Ve. Therefore, a value lower than these amplification factors is set so as not to exceed the maximum input voltage of the A / D converter 7. Is set.

  As described above, according to the present invention, the amplifying circuit 9 amplifies the midpoint potential difference Vc generated between the pair of midpoints in the bridge circuit 2 with a predetermined amplification factor, and offsets the amplified midpoint potential difference Vc. The offset voltage input to the voltage terminal Voffset is added and output, and an instrumentation amplifier 6 generates a plurality of different offset voltages A, B, and C, and one offset voltage among the plurality of offset voltages is generated. And an offset voltage generation circuit 8 that outputs the offset voltage to the offset voltage terminal Voffset of the instrumentation amplifier 6, and the sensing element 11 and the compensation after the high temperature driving voltage is supplied to the bridge circuit 2 by the voltage supply source 5. During the transition period in which the temperature of the element 12 rises, the plurality of offsets are based on preset offset voltage switching information. Of the set voltage, it is generated due to a difference in temperature rise characteristics between the sensitive element 11 and the compensating element 12 and is added to the midpoint potential difference Vc (that is, the amplified error potential difference Ve) amplified by the instrumentation amplifier 6. As a result, the offset voltage generation circuit 8 is controlled so that the offset voltage that makes the error potential difference Ve larger than 0 and the smallest is output. Therefore, among the preset offset voltages A, B, and C, An offset voltage at which the error potential difference Ve after amplification in the transient period is equal to or higher than a predetermined minimum voltage can be added to the error potential difference Ve, and therefore, the minimum value of the error potential difference Ve after amplification in the transient period. And the maximum value can be reduced, the amplification factor in the instrumentation amplifier 6 can be further increased, and the detection sensitivity can be improved. In addition, the conventional bridge circuit 2 can be used as it is, the amplifier circuit 9 can also be configured with a simple circuit, and the control for the offset voltage generation circuit 8 is also simple, so that the gas detection device capable of improving the detection sensitivity. 1 can be provided at low cost. Further, in the bridge circuit 2 having a configuration in which the sensor circuit unit 10 including the sensitive element 11 and the reference circuit unit 20 including the compensation element 12 are connected in parallel, even when balance adjustment of the bridge circuit 2 is difficult Only by adjusting the offset voltage, the same effect as that obtained by adjusting the balance of the bridge circuit 2 can be obtained. Furthermore, since it is not necessary to match the temperature rise characteristics of the sensitive element 11 and the compensating element 12 of the gas sensor unit 15 with high accuracy, the current element can be used and the cost can be reduced.

  In the present embodiment, the offset voltage generation circuit 8 generates three offset voltages A, B, and C. However, the present invention is not limited to this, and the number of offset voltages is arbitrary as long as the offset voltages are different from each other. is there. Further, the larger the number of offset voltages, the smaller the difference between the minimum value and the maximum value of the error potential difference Ve described above.

  Further, in the present embodiment, the offset voltage switching information is set based on actual measurement. However, the present invention is not limited to this, and for example, a result of simulation on the computer for the bridge circuit 2 and the instrumentation amplifier 6. As long as the offset voltage switching information is obtained on the basis of the above and set in the EEPROM 64 (or ROM 62) in advance, the offset voltage generation circuit 8 does not contradict the purpose of the present invention. If the offset voltage switching information can output the offset voltage with the smallest error potential difference to the instrumentation amplifier 6 during the transient period, the setting method is arbitrary.

  Although the present embodiment detects the concentration of the detection target gas, the present invention is not limited to this, and the present invention includes a gas type detection device that detects the type of gas contained in the test gas whose component is unknown, You may apply to another kind of gas detection apparatus.

  In addition, embodiment mentioned above only showed the typical form of this invention, and this invention is not limited to embodiment. That is, various modifications can be made without departing from the scope of the present invention.

1 Gas concentration detector (gas detector)
2 Bridge circuit 5 Voltage supply source 6 Instrumentation amplifier (amplification unit, voltage addition unit)
7 A / D converter 8 Offset voltage generator (offset voltage generator)
DESCRIPTION OF SYMBOLS 9 Amplification circuit 10 Sensor circuit part 11 Sensing element 12 Compensation element 15 Gas sensor unit (adsorption combustion type gas sensor)
20 Reference circuit section 60 MPU
61 CPU (offset voltage switching means)

Claims (1)

(A) a bridge circuit including an adsorption combustion type gas sensor composed of a sensitive element sensitive to the detection target gas and a compensation element not sensitive to the detection target gas; and (b) a low temperature at which the temperature of the sensitive element adsorbs the detection target gas. A voltage supply source that sequentially supplies the bridge circuit with a low temperature driving voltage that becomes a high temperature driving voltage at which the temperature of the sensitive element becomes a high temperature for burning the detection target gas adsorbed on the sensitive element; In the gas detection device having an amplification circuit connected to a pair of midpoints in the bridge circuit,
The amplifying circuit amplifies a potential difference generated between the pair of midpoints at a predetermined amplification factor, generates a plurality of offset voltages different from each other, and generates one offset voltage among the plurality of offset voltages. An offset voltage generation unit that outputs, and a voltage addition unit that adds the offset voltage output by the offset voltage generation unit to the potential difference amplified by the amplification unit, and
In a transient period in which the temperature of the sensitive element and the compensating element rises after the high temperature driving voltage is supplied to the bridge circuit by the voltage supply source, based on preset offset voltage switching information, the plurality of offsets Among the voltages, the potential difference is caused by a difference in temperature rise characteristics between the sensitive element and the compensating element and added to the potential difference amplified by the amplifying unit, thereby making the potential difference equal to or higher than a predetermined minimum voltage and minimized. A gas detection apparatus comprising an offset voltage switching means for controlling the offset voltage generator so that an offset voltage is output.

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