JP2008286549A - Semiconductor gas sensor and gas composition measuring instrument - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the linearity of concentration characteristics of a semiconductor gas sensor. <P>SOLUTION: The semiconductor gas sensor 110 is provided with a gas-sensitive body 111 which senses an object gas to change its electrical resistance value, a voltage divider resistor 112 serially connected to the gas-sensitive body 111, a parallel resistor 113 parallelly connected to the gas-sensitive body 111 and serially connected to the voltage divider resistor 112, and an output terminal 115 for outputting a signal indicating a voltage between both ends of the voltage divider resistor 112, the gas-sensitive body 111, or the parallel resistor 113 to the outside. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an improvement in linearity of concentration characteristics of a semiconductor gas sensor.

The semiconductor gas sensor has a gas sensitive body formed of a semiconductor whose electric resistance value varies in response to a specific gas, and outputs a signal indicating a voltage (see Patent Document 1). The semiconductor gas sensor is used by being incorporated in a gas composition measuring instrument that measures the composition of a gas to be measured such as exhaled air, for example. The gas composition measuring device estimates the concentration of a specific gas in the exposure atmosphere of the gas sensitive body based on the output voltage of the semiconductor gas sensor, and estimates the composition of the gas to be measured based on the estimated concentration.
JP 2002-48745 A (Japanese Patent Application 2000-230689)

  When estimating the concentration of a specific gas in the exposure atmosphere, the gas composition measuring device assumes that the variation characteristic of the output voltage of the semiconductor gas sensor with respect to the concentration of the specific gas in the exposure atmosphere, that is, the concentration characteristic of the semiconductor gas sensor is linear. I reckon. However, the concentration characteristic of the conventional semiconductor gas sensor is non-linear, its linearity is low, and it tends to saturate particularly on the high concentration side. This is one factor that hinders improvement in measurement accuracy of the gas composition measuring instrument.

  This invention is made | formed in view of such a situation, and makes it a solution subject to improve the linearity of the density | concentration characteristic of a semiconductor gas sensor.

  The present invention relates to a gas sensitive body whose electric resistance value varies in response to a specific gas, a partial pressure resistance connected in series to the gas sensitive body, and a parallel to the gas sensitive body, A semiconductor gas sensor comprising: a parallel resistor connected in series with respect to the voltage dividing resistor; and an output terminal for outputting a signal indicating a voltage between both ends of the voltage dividing resistor, the gas sensitive body or the parallel resistor to the outside. A first semiconductor gas sensor is provided. In the first semiconductor gas sensor, the variation characteristic of the electric resistance value of the gas sensitive body with respect to the concentration of the specific gas in the exposure atmosphere of the gas sensitive body, that is, the linearity of the concentration characteristic of the gas sensitive body is low, as in the prior art. . However, since the parallel resistance exists, the linearity of the variation characteristic of the electric resistance value (synthetic resistance value) between both ends of the gas sensitive body with respect to the concentration of a specific gas in the exposure atmosphere becomes high. Therefore, the linearity of the fluctuation characteristic of the voltage indicated by the output signal with respect to the concentration of the specific gas in the exposure atmosphere, that is, the linearity of the concentration characteristic of the first semiconductor gas sensor is higher than that of the conventional semiconductor gas sensor. Therefore, according to the present invention, the linearity of the concentration characteristic of the semiconductor gas sensor can be improved and saturation on the high concentration side can be suppressed.

  The first semiconductor gas sensor may include an amplifier that amplifies the signal and supplies the amplified signal to the output terminal. According to the semiconductor gas sensor of this embodiment (second semiconductor gas sensor), the voltage fluctuation range indicated by the signal output from the output terminal can be expanded. Therefore, for example, when an A / D converter is arranged at the rear stage of the semiconductor gas sensor, the resolution required for the A / D converter can be lowered.

  The present invention has one end and the other end, a gas sensitive body whose electric resistance value between both ends varies in response to a specific gas, and the other end of the gas sensitive body in series with the gas sensitive body. A voltage dividing resistor connected to the gas sensitive body, a parallel resistor connected in parallel to the gas sensitive body between both ends of the gas sensitive body, and an output terminal connected to the other end of the gas sensitive body. A semiconductor gas sensor (third semiconductor gas sensor) is provided. According to the 3rd semiconductor gas sensor, the same effect as the 1st semiconductor gas sensor can be acquired.

  The third semiconductor gas sensor includes an amplifier that amplifies a signal generated at the other end of the gas sensitive body and supplies the amplified signal to the output terminal. The output terminal is connected to the other end of the gas sensitive body through the amplifier. It may be connected. According to the semiconductor gas sensor of this embodiment (fourth semiconductor gas sensor), the same effect as that of the second semiconductor gas sensor can be obtained.

  The present invention relates to any one of the first to fourth semiconductor gas sensors, and a composition estimation unit that estimates the composition of the exposure atmosphere of the gas sensitive body using a signal output from the output terminal of the semiconductor gas sensor; A gas composition measuring device is provided. According to this gas composition measuring instrument, the composition of the exposure atmosphere can be measured with high accuracy.

<1. Overall>
FIG. 1 is a perspective view showing an appearance of a gas composition measuring device 100 according to an embodiment of the present invention. The gas composition measuring device 100 includes a hollow casing 101 that is held by one hand of a user and a part that is provided in the casing 101. On the surface of the housing 101, an air inlet 103 through which a user's breath is blown is provided. The air inlet 103 is connected to a cavity in the housing 101.

  FIG. 2 is a block diagram showing an electrical configuration of the gas composition measuring device 100. In the housing 101, an input unit 102 for inputting user instructions, a display unit 104 for displaying various information, a CPU (Central Processing Unit) 105 for performing various processes, a semiconductor gas sensor 110, and an A / D A converter 106, a heater 107 that performs heat cleaning of the semiconductor gas sensor 110, and a storage unit 108 that stores (holds) written information in the form of electronic data are provided.

  Specifically, the input unit 102 is an operation unit that includes an operation button exposed on the surface of the housing 101 and outputs a signal corresponding to the operation content. The CPU 105 receives an instruction input using the input unit 102 by receiving an output signal from the input unit 102. Specifically, the display unit 104 is a liquid crystal display having a display surface exposed on the surface of the housing 101. The CPU 105 displays this information by supplying a signal corresponding to the information to the display unit 104.

The semiconductor gas sensor 110 is sensitive to a target gas, which is a specific gas, and is provided at a position exposed to exhaled breath. A semiconductor (for example, stannic oxide (for example, stannic oxide) that changes its electrical resistance in response to the target gas. And a gas sensitive body 111 made of a metal oxide such as SnO ₂ ), and outputs an output signal s1 indicating a voltage. The voltage indicated by the output signal s1 is the output voltage Vs of the semiconductor gas sensor 110.

  The A / D converter 106 performs A / D conversion on the output signal s1 of the semiconductor gas sensor 110 and outputs the result. The CPU 105 grasps the output voltage Vs of the semiconductor gas sensor 110 by receiving the output signal of the A / D converter 106. The heater 107 is disposed in the vicinity of the gas sensitive body 111 and generates heat when receiving a control signal s2 instructing heat generation from the CPU 105, thereby heating and reducing the gas sensitive body 111. That is, the heater 107 performs heat cleaning of the semiconductor gas sensor 110 according to the control of the CPU 105.

  The storage unit 108 includes a rewritable nonvolatile memory such as an EEPROM (Electrically Erasable Programmable Read Only Memory). The information stored in the storage unit 108 includes various values such as a program executed by the CPU 105 and a calibration coefficient (for example, a span coefficient) for calibrating the semiconductor gas sensor 110. The CPU 105 writes a value in the storage unit 108, reads a value from the storage unit 108, and updates the value stored in the storage unit 108. When the input unit 102 is operated and the gas composition measuring instrument 100 is turned on, the CPU 105 reads and executes a program stored in the storage unit 108. Except for this processing, all processing performed by the CPU 105 is performed using this program.

  When the gas composition measuring instrument 100 is turned on, the CPU 105 performs zero point adjustment, and in the exposure atmosphere based on the output voltage Vs of the semiconductor gas sensor 110 and the calibration coefficient stored in the storage unit 108. The concentration of the target gas is estimated, and the composition of the exposure atmosphere is estimated based on this concentration. That is, the CPU 105, the A / D converter 106, and the storage unit 108 function together as a composition estimation unit that estimates the composition of the exposure atmosphere using the output signal s1 of the semiconductor gas sensor 110.

<2. Semiconductor gas sensor>
FIG. 3 is an electric circuit diagram showing a configuration example in the vicinity of the semiconductor gas sensor 110. In the example shown in this figure, the power supply voltage Vdd is common to the heater 107 and the semiconductor gas sensor 110. However, the power supply voltage may be changed by modifying the power supply voltage Vdd. The semiconductor gas sensor 110 includes a gas sensitive body 111, a voltage dividing resistor 112, a parallel resistor 113, and an output terminal 115. The gas sensitive body 111 has one end 1a and the other end 1b, the voltage dividing resistor 112 has one end 2a and the other end 2b, and the parallel resistor 113 has one end 3a and the other end 3b. The power supply voltage Vdd is applied between one end 1 a of the gas sensitive body 111 and the other end 2 b of the voltage dividing resistor 112.

  The other end 1 b of the gas sensitive body 111 is electrically connected to one end 1 a of the voltage dividing resistor 112. The other end 1b of the voltage dividing resistor 112 is grounded. That is, the voltage dividing resistor 112 is connected in series to the gas sensitive body 111 at the other end 1b. Note that the “electric resistance value” of the gas sensitive body 111 means an electric resistance value between the one end 1a and the other end 1b.

  One end 1 a of the gas sensitive body 111 is electrically connected to one end 3 a of the parallel resistor 113. In addition, the other end 1 b of the gas sensitive body 111 is electrically connected to the other end 3 b of the parallel resistor 113. That is, the parallel resistor 113 is connected between the one end 1 a and the other end 1 b in parallel to the gas sensitive body 111 and in series to the voltage dividing resistor 112. The other end 1b, one end 2a, and the other end 3b are electrically connected to the output terminal 115.

  The electric resistance value of the gas sensitive body 111 varies according to the concentration of the target gas in the exposure atmosphere. Therefore, a signal whose potential changes in time is generated at the other end 1b, the one end 2a, and the other end 3b. This signal is output as an output signal s1 from the output terminal 115 to the outside (A / D converter 106). The potential of the output signal s1, that is, the output voltage Vs, matches the voltage Vd between the one end 2a and the other end 2b of the voltage dividing resistor 112. That is, the output signal s1 becomes a signal indicating Vd = Vs at the potential, and the output terminal 115 outputs the output signal s1 indicating Vd = Vs to the outside.

  By the way, the fluctuation characteristic of the electric resistance value of the gas sensing element 111 with respect to the concentration of the target gas in the exposure atmosphere, that is, the linearity of the concentration characteristic of the gas sensing element 111 becomes low as in the conventional case. However, since the parallel resistance 113 exists, the linearity of the fluctuation characteristic of the electric resistance value (synthetic resistance value) between the one end 1a and the other end 1b with respect to the concentration of the target gas in the exposure atmosphere is high. Therefore, the linearity of the fluctuation characteristic of the output voltage Vs with respect to the concentration of the target gas in the exposure atmosphere, that is, the linearity of the concentration characteristic of the semiconductor gas sensor 110 is increased, and saturation on the high concentration side is suppressed. Therefore, according to the gas composition measuring instrument 100, the composition of exhaled breath can be measured with high accuracy.

<3. Experimental results>
FIG. 4 is a table showing experimental results in the present embodiment. This table shows that the gas sensitive body 111 is exposed to the gas to be measured whose target gas concentration is 0, 2.5, 5 and the electric resistance value Rs, partial pressure of the gas sensitive body 111 is exposed to each gas to be measured. It was obtained by an experiment that measures the electrical resistance value Rd of the resistor 112, the electrical resistance value Rp of the parallel resistor 113, the combined resistance value RsRp / (Rs + Rp) of the gas sensing element 111 and the parallel resistor 113, and the output voltage Vs. . Rd and Rp are a common fixed value (0.5).

  FIG. 5 is a graph showing an example of density characteristics. Among the characteristic lines L1 to L3 shown in this figure, the one obtained in the above experiment is the fluctuation characteristic of the electric resistance value Rs of the gas sensitive body 111 with respect to the concentration of the target gas in the exposure atmosphere, that is, the gas sensitive body 111. A characteristic line L1 indicating the concentration characteristic and a characteristic line L2 indicating the fluctuation characteristic of the output voltage Vs with respect to the concentration of the target gas in the exposure atmosphere, that is, the concentration characteristic of the semiconductor gas sensor 110. As is clear from the characteristic line L1, the linearity of the concentration characteristic of the gas sensitive body 111 is low.

  FIG. 6 is an electric circuit diagram showing a configuration of a semiconductor gas sensor 210 according to a comparative example compared with the present embodiment. The semiconductor gas sensor 210 has a configuration in which the parallel resistance 113 is removed from the semiconductor gas sensor 110. FIG. 7 is a table showing experimental results in the comparative example. This table shows that the gas sensitive body 111 of the semiconductor gas sensor 210 is exposed to the gas to be measured whose target gas concentrations are 0, 2.5, and 5, and the electric resistance value of the gas sensitive body 111 is exposed to each gas to be measured. This is obtained in an experiment for measuring Rs, the electric resistance value Rd of the voltage dividing resistor 112, and the output voltage Vs. Rd is a fixed value (1).

  Also in this experiment, the concentration characteristic of the gas sensitive body 111 is as shown by the characteristic line L1 in FIG. However, the concentration characteristic of the semiconductor gas sensor 210 is as shown by the characteristic line L3 in FIG. As is apparent from the comparison between the characteristic line L2 and the characteristic line L3, the linearity of the concentration characteristic of the semiconductor gas sensor 110 (specifically, the linearity of the concentration characteristic of the semiconductor gas sensor 111 (specifically, linearity) −1.9%) is higher than the linearity of the concentration characteristic of the semiconductor gas sensor 110 (specifically −14.29%).

  4 and 7, the fluctuation range (0.778−0.600 = 0.178) of the output voltage Vs of the semiconductor gas sensor 110 is the fluctuation range (0 of the output voltage Vs of the semiconductor gas sensor 210). .500−0.167 = 0.333). For this reason, the resolution required for the A / D converter 106 at the subsequent stage of the semiconductor gas sensor 110 is higher than the resolution required for the A / D converter at the subsequent stage of the semiconductor gas sensor 210.

<4. Deformation>
The present invention includes not only the above-described embodiments but also modifications listed below within the scope thereof.
FIG. 8 is an electric circuit diagram showing a configuration of a semiconductor gas sensor 310 according to a modification of the embodiment described above. The semiconductor gas sensor 310 is different from the semiconductor gas sensor 110 in that it includes an amplifier 114. The amplifier 114 has an input terminal 4a and an output terminal 4b, amplifies the signal input from the input terminal 4a, and outputs the amplified signal from the output terminal 4b. The input end 4 a is electrically connected to the other end 1 b of the gas sensitive body 111, one end 2 a of the voltage dividing resistor 112 and the other end 3 b of the parallel resistor 113, and the output end 4 b is electrically connected to the output terminal 115. That is, the other end 1 b, one end 2 a, and the other end 3 b are connected to the output terminal 115 via the amplifier 114.

  In the semiconductor gas sensor 310, the amplifier 114 amplifies a signal generated at the other end 1b, the one end 2a, and the other end 3b, that is, a signal indicating the voltage Vd, and supplies the amplified signal to the output terminal 115. Therefore, the fluctuation range (range) of the output voltage Vs of the semiconductor gas sensor 310 is wider than the fluctuation range of the output signal s1 of the semiconductor gas sensor 110. Therefore, the resolution required for the A / D converter subsequent to the semiconductor gas sensor 310 may be lower than the resolution required for the A / D converter 106 subsequent to the semiconductor gas sensor 110. The resolution required for the / D converter is sufficient.

  FIG. 9 is an electric circuit diagram showing a configuration of a semiconductor gas sensor 410 according to another modification of the above-described embodiment. In the semiconductor gas sensor 110 described above, the power supply potential Vdd is supplied to one end 1a of the gas sensing element 111 and the other end 2b of the voltage dividing resistor 112 is grounded. In the semiconductor gas sensor 410, the power supply potential is applied to the other end 2b. Vdd is supplied and one end 1a is grounded. That is, the output voltage Vs of the semiconductor gas sensor 410 coincides with the voltage Vg which is a voltage between the other end 1b and the one end 1a of the gas sensitive body 111 and is also a voltage between the other end 3b and the one end 3a of the parallel resistor 113. Such a modification is also possible for the semiconductor gas sensor 310.

  Further, in the above-described embodiment, the gas composition measuring device that measures the composition of the breath is exemplified, but it may be modified to measure the composition of the gas other than the breath. In the above-described embodiment, the semiconductor gas sensor has been incorporated into a gas composition measuring instrument as an application, but the semiconductor gas sensor is not limited to this application. These are also applicable to the above-described modifications.

It is a perspective view which shows the external appearance of the gas composition measuring device 100 which concerns on one Embodiment of this invention. 2 is a block diagram showing an electrical configuration of a gas composition measuring device 100. FIG. 3 is an electric circuit diagram showing a configuration example in the vicinity of a semiconductor gas sensor 110 of a gas composition measuring device 100. It is a table | surface which shows the experimental result in this embodiment. It is a graph which shows an example of a density characteristic. It is an electric circuit diagram which shows the structure of the semiconductor gas sensor 210 which concerns on the comparative example compared with this embodiment. It is a table | surface which shows the experimental result in a comparative example. It is an electric circuit diagram showing the composition of semiconductor gas sensor 310 concerning the modification of this embodiment. It is an electric circuit diagram which shows the structure of the semiconductor gas sensor 410 which concerns on the other modification of this embodiment.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1a, 2a, 3a ... One end, 1b, 2b, 3b ... Other end, 4a ... Input end, 4b ... Output end, 100 ... Gas composition measuring device, 101 ... Case, 102 ... Input part, 103 ... Blow-in port, 104 ... Display unit, 105 ... CPU, 106 ... A / D converter, 107 ... Heater, 108 ... Storage unit, 110,210,310,410 ... Semiconductor gas sensor, 111 ... Gas sensitive body, 112 ... Partial voltage resistor, 113 ... Parallel resistor, 114... Amplifier, 115.

Claims (5)

A gas sensitive body whose electrical resistance varies in response to a specific gas,
A voltage dividing resistor connected in series to the gas sensitive body;
A parallel resistor connected in parallel to the gas sensitive body and in series to the voltage dividing resistor;
A semiconductor gas sensor comprising: an output terminal that outputs a signal indicating a voltage between both ends of the voltage dividing resistor, the gas sensitive body, or the parallel resistor.   The semiconductor gas sensor according to claim 1, further comprising an amplifier that amplifies the signal and supplies the amplified signal to the output terminal. A gas-sensitive body having one end and the other end, the electric resistance value of which varies between both ends in response to a specific gas;
A voltage dividing resistor connected in series to the gas sensitive body at the other end of the gas sensitive body;
A parallel resistor connected in parallel to the gas sensitive body between both ends of the gas sensitive body;
A semiconductor gas sensor comprising: an output terminal connected to the other end of the gas sensitive body. An amplifier that amplifies a signal generated at the other end of the gas sensitive body and supplies the amplified signal to the output terminal;
The output terminal is connected to the other end of the gas sensitive body through the amplifier.
The semiconductor gas sensor according to claim 3. A semiconductor gas sensor according to any one of claims 1 to 4,
A gas composition measuring instrument comprising: a composition estimating unit that estimates a composition of an exposed atmosphere of the gas sensitive body using a signal output from the output terminal of the semiconductor gas sensor.

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