JP2003185612A - Gas detector using semiconductor gas sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a gas detector using a semiconductor gas sensor capable of detecting gas with high accuracy. <P>SOLUTION: A microcomputer 2 takes in detection output in such a state the load resistors connected to the gas-sensitive bodies 8a and 8b of semiconductor gas sensors 1A and 1B are only R5 and R7 in the first place of a sampling period to subject the same to A/D conversion by the A/D converter build in the microcomputer and sets the load resistors connected to the gas-sensitive bodies 8a and 8b to a parallel circuit of R5 and R6 and a parallel circuit of R7 and R8 when the A/D conversion is completed to lower load resistance values and takes in the detection output in such a state that the load resistance values are lowered to subject the same to A/D conversion by the built-in A/D converter. The microcomputer 2 employs detection output capable of ensuring accuracy among the detection output in a case such that the load resistance values are high, and the detection output in a case such that the load resistance values are low to use the same in the operation of the similarity degree F and intensity S of a smell. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas detector using a semiconductor gas sensor.

[0002]

2. Description of the Related Art Conventionally, a load resistance is connected in series with a gas sensitive body of a semiconductor gas sensor and the voltage at the connection point is used as a detection output to detect a gas. For example, when the load resistance value changes 10 times or more or 1/10 or less, the output approaches 0 V or the circuit voltage, and sufficient resolution cannot be obtained. For example, an 8-bit A / D converter with a built-in microcomputer, which is often used, has a region in which the resolution and accuracy are remarkably deteriorated. As a countermeasure against this, a method of switching the load resistance is used.

[0003]

Some semiconductor gas sensors have a load resistance dependency. For example, when the load resistance value is increased, the sensor resistance value increases, and when the load resistance value is decreased, the sensor resistance value increases. Get smaller. For this reason, there is a problem that the accuracy is rather deteriorated when the load resistance value is switched.

The present invention has been made in view of the above points, and an object of the present invention is to provide a gas detection device using a semiconductor gas sensor capable of detecting gas with high accuracy.

[0005]

According to a first aspect of the present invention, a semiconductor gas sensor, a load resistance connected in series to a gas sensitive body of the semiconductor gas sensor, and a series circuit of the gas sensitive body of the semiconductor gas sensor and the load resistance. The voltage at the connection point between the gas sensitive body of the semiconductor gas sensor and the load resistance is sampled as a detection output, the sampled voltage is A / D converted, and the A / D converted voltage The concentration of the gas to be detected is detected from the resistance value of the gas sensor of the semiconductor gas sensor obtained based on the value and the resistance value of the gas sensor of the semiconductor gas sensor previously obtained in an atmosphere of air that does not contain the gas to be detected. In a gas detection device using a semiconductor gas sensor, the load resistance value of the load resistance connected to the gas-sensitive body of the semiconductor gas sensor is used as a detection output. The load resistance value desired detection accuracy only time necessary for A / D conversion by the gas detecting means is obtained when the ring, the switching from the load resistance of the normal,
Load resistance switching means for returning to a normal load resistance value after A / D conversion by the gas detection means is provided.

According to a second aspect of the present invention, in the first aspect of the present invention, the load resistance switching means sequentially switches to a plurality of load resistance values different from the normal load resistance value only during a time required for A / D conversion, After switching to the final load resistance value, the normal load resistance value is restored, and the gas detection unit sequentially switches the detection output based on the normal load resistance value immediately before the switching by the load resistance switching unit, and after the load resistance switching starts. The detection output based on the load resistance value is sampled, and the detection output at the time of the load resistance value that provides the optimum accuracy is adopted as the detection output for detecting the concentration of the gas to be detected after the load resistance switching is completed. .

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described below with reference to embodiments.

FIG. 1 shows a circuit configuration of an odor detector which is an embodiment employing a gas detection device using the semiconductor gas sensor of the present invention. This odor detector is ethanol (EtOH)
A first semiconductor gas sensor 1A exhibiting high sensitivity and a second semiconductor gas sensor 1B exhibiting high sensitivity to xylene are arranged in a sensor chamber (not shown), and a pump P is installed in the sensor chamber.
Suction the detected gas from the outside, and determine which odor substance is close to the odor intensity (Classification) and the detected odor based on the detection output generated when both the semiconductor gas sensors 1A and 1B are set to the detected gas. The degree of odor for determination is determined by a microcomputer (hereinafter abbreviated as microcomputer) 2.

Specifically, the above semiconductor gas sensors 1A, 1
The microcomputer 2 with a built-in A / D converter for taking in the detection output of B as a voltage value and determining the odor intensity and the degree of odor, the display unit 3 for displaying the measurement results, and the history of the measurement results Storage unit 4 for storing data, and RS232C for transmitting data to an external device such as a computer
A general-purpose communication unit 5 such as the above, the pump P, a key operation unit 11, an electronic buzzer BZ, a power supply, and the like.

The battery 6 or an external AC adapter (not shown) is used as the power source, and the semiconductor gas sensor 1A,
As a power supply voltage Vcc for applying to the heater H for heat cleaning incorporated in 1B and the electronic buzzer BZ,
When the battery 6 is used, the voltage is used, and when the AC adapter is used, the voltage obtained by converting the output voltage of the AC adapter into a voltage equivalent to the voltage of the battery 6 by the constant voltage circuit 7 is used. In addition, the gas sensitive body 8 of the semiconductor gas sensors 1A and 1B
The voltage applied to a and 8b via the load resistance and the power supply voltage V _DD for the microcomputer 2 and the storage unit 4 are obtained.

The battery 6 turns off when there is an output voltage from the constant voltage circuit 7, that is, when the AC adapter is connected to the jack JK, and turns on when there is no output voltage, that is, when the AC adapter is not connected. MOSFET
Switching element Q1 and power switch SW1
It is designed to be connected to the circuit as a power source via. The diodes D1 and D2 are backflow prevention diodes.

The microcomputer 2 is provided with a function of controlling the odor detector as a whole and performing signal processing. The detection output of the semiconductor gas sensors 1A, 1B is sampled and taken in by the built-in A / D converter. After A / D conversion of
From each A / D converted detection output, each semiconductor gas sensor 1
After obtaining the resistance values Ra and Rb of the A and 1B gas sensitive bodies 8a and 8b, the sensitivities Ra0 / Ra and Rb0 / Rb to the detected gas obtained from the resistance values Ra and Rb are further obtained.
Further, a calculation function for obtaining the odor similarity and the odor intensity S from the sensitivities Ra0 / Ra and Rb0 / Rb as described later,
The display control function for displaying the odor intensity F and odor intensity S obtained by the operation of this calculation function on the display unit 3, and the measurement data of the odor intensity F and odor intensity S are stored in the storage unit 4 as described later. A function of storing and reading according to a read request, and a function of transmitting measurement data being measured with an external device such as a computer or measurement data stored in the storage unit 4 through the communication unit 5. A control function for switching the load resistance values of the semiconductor gas sensors 1A and 1B, a function for controlling an electronic buzzer BZ for emitting a sound for indicating switch acceptance, and a pump P for driving a gas to be detected in the sensor chamber. A program realizes a pump control function of sucking and exhausting residual gas and a function of inputting an operation signal from the key operation unit 11 to judge the operation content. To have.

Each of the semiconductor gas sensors 1A and 1B has a three-terminal structure in which the ground side electrodes of the gas sensitive bodies 8a and 8b and one of the electrodes of the heater H are also used. , 8b are heat-cleaned.

The transistors Q2 and Q3 are PNP type transistors, and their bases are connected to the ports P10 and P11 of the microcomputer 2 through a series circuit of a resistor R1 and a capacitor C1 or a series circuit of a resistor R2 and a capacitor C2. Also, connect each collector to port R4 of microcomputer 2.
2 and R52, and when the power is turned on, the microcomputer 2 keeps the ports P10, P11, R42, and R5 for a certain period of time.
2 by controlling the output of the transistor Q2, Q
The voltage applied to the heaters H of the semiconductor gas sensors 1A and 1B via PWM is PWM-controlled to heat the heaters H and heat-sensing the gas sensitive bodies 8a and 8b. In the case of the present embodiment, in the period in which the heat cleaning is performed by the PWM control, a period in which the average voltage applied to the heater H is high and a period in which the average voltage is low are set.

In the semiconductor gas sensor 1A, the load resistance R5 or the load resistance R5 and the load resistance R6 are provided on the gas sensitive body 8a.
The power supply voltage V _DD is applied through the parallel circuit of the above, and the voltage determined by the resistance ratio of the connection point between the load resistance circuit and the gas sensitive body 8a is taken in the port R40 of the microcomputer 2 as a detection output. .

Similarly, the semiconductor gas sensor 1B has a gas sensitive body 8
The power supply voltage V _DD is applied to b through the load resistance R7 or a parallel circuit of the load resistance R7 and the load resistance R8, and is determined by the resistance ratio of the connection point between the load resistance circuit and the gas sensing body 8b. The voltage is detected output as port R5 of the microcomputer 2.
It is taken into 0.

Here, the load resistance R6 is different from the load resistance R6.
Are connected in parallel via a knot gate NT1 used as a switch, and similarly the load resistance R8 is connected to the load resistance R8.
Are connected in parallel via a knot gate NT2 used as a switch.

The microcomputer 2 has knot gates NT1 and NT2.
Connect the input end of the port to ports R41 and 51, and
By setting L1 and R51 to "L", the load resistors R6 and R8 are connected to the power supply voltage V _DD through the knot gates NT1 and NT2, that is, the load resistors R5 and R7 are connected in parallel, respectively, and the corresponding semiconductor gas sensor 1A, 1B gas sensitive body 8a,
The load resistance value connected to 8b is switched between a high resistance value of the resistors R5 and R7 alone and a low resistance value due to the parallel circuit of R5 and R6 and R7 and R8.

More specifically, the microcomputer 2 in normal state has ports R41 and R5 so that only the load resistors R5 and R7 are connected to the gas sensing bodies 8a and 8b of the semiconductor gas sensors 1A and 1B.
When 1 is set to “H” and the detection output of the semiconductor gas sensors 1A and 1B is taken in, first, the detection output is taken in the high resistance state in which only the load resistors R5 and R7 are connected, and the detected output A / After D conversion, ports 41, 5
1 is set to "L", the load resistance R6 is connected in parallel to the load resistance R5, and the load resistance R8 is connected in parallel to the load resistance R7 to switch to the load resistance connection state of the low load resistance value to detect the detection outputs of the semiconductor gas sensors 1a and 1B. After fetching and A / D converting the fetched detection output, the ports 41 and 51 are returned to "H" and only the load resistors R5 and R7 are connected.

That is, the load resistance switching means of each semiconductor gas sensor 1A, 1B is composed of knot gates NT1, NT2 and the microcomputer 2.

FIG. 2 shows the relationship between the sensitivity and gas concentration of the two semiconductor gas sensors 1A and 1B used in this embodiment. As can be seen from FIG. 2, the sensitivity to ethanol is the sensitivity (I) of the semiconductor gas sensor 1A. Is higher than the sensitivity (II) of the semiconductor gas sensor 1B, and the sensitivity to xylene is lower than the sensitivity (IV) of the semiconductor gas sensor 1A.
In the present embodiment, the odor intensity and the odor intensity are measured using the semiconductor gas sensors 1A and 1B having three different characteristics.

Here, the sensitivity Ra0 / Ra of the semiconductor gas sensor 1A is set to E as shown in FIG.
When the sensitivity Rb0 / Rb of B is X, the odor degree F is
It is defined as F = E / X. On the other hand, the odor intensity S is S
= (X ² + E ² ) ^1/2 (or X × E). In FIG. 3, "a" indicates the odor intensity F of ethanol, "b" indicates the odor intensity of xylene, "d" indicates the odor intensity S of ethanol, and "e" indicates the odor intensity of xylene. The microcomputer 2 is programmed with a calculation function for calculating the odor intensity F defined as described above and the odor intensity S based on the sensitivities X and E obtained by measurement.

The storage unit 4 is a storage unit for storing the measurement data as described above. In the embodiment, the storage unit 4 is configured by SRAM, and when the battery 6 or the AC adapter is connected, the storage unit 4 stores the data through the transistor Q4. The power supply voltage V _DD is applied to the section 4 to receive power supply, and when the power switch SW is turned off, the AC adapter is removed, or the voltage of the battery 6 drops, power is supplied from the button battery 9 to be backed up. It is like this. That is, the resistor R9 is provided at the base of the PNP transistor Q4.
When the power supply voltage Vcc is applied to the input of the knot gate NT3, the output of the knot gate NT3 becomes "L" and the transistor Q4 is turned on to turn on the power supply voltage Vcc. Is below the input threshold of the knot gate NT3, the output of the knot gate NT3 becomes "H" and the transistor Q4
Is turned off, the power supply of the storage unit 4 is switched. D3 is a diode for preventing backflow.

The communication unit 5 is a general-purpose communication unit of RS232C, and is connected to an external device such as an external computer and the connector 1.
It is connected via 0, and is used under the control of the microcomputer 2 to send the measurement data currently measured or the measurement data stored in the storage unit 4 in response to a request from an external device.

When receiving the switch, the electronic buzzer BZ "
A current due to the power supply voltage Vcc flows through the port P12 of the microcomputer 2 which is at the L "level to emit a switch receiving sound.

The pump P is connected to the power supply voltage Vcc through the transistor Q5, and the microcomputer 2 is connected to the port P to which the base of the transistor Q5 is connected while sampling the detection outputs of the semiconductor gas sensors 1A and 1B.
13 is set to "L", the transistor Q5 is turned on, and the pump P is energized.

The key operation unit 11 is provided with a key switch for selectively setting whether the detection output is sampled in real time (1 second interval) or for example, data is stored at a predetermined interval while sampling at 1 second intervals. S1, a key switch S2 for instructing on / off of peak hold of the measured value, and a key switch S3 for instructing zero point adjustment, measurement suspension, and suspension of measurement data storage processing in the storage unit 4.
And four key switches of the enter key S4.

In FIG. 1, reference numeral 12 is a reference clock generation circuit for giving a reference clock to the microcomputer 2, 13 is a reference voltage generation circuit for giving a reference voltage, and 14 is a reset circuit.

In the odor detector of this embodiment, when the power switch SW is turned on while the battery 6 or the AC adapter is connected and the power supply voltage Vcc rises to a predetermined voltage, the microcomputer 2 resets the circuit. The reset state of 14 is released and the operation is started.

When the operation is started, first, the microcomputer 2 controls the outputs of the ports P10 and P11 so that the transistors Q2 and Q are connected to the heater H of each semiconductor gas sensor 1A and 1B.
PWM control is applied to the voltage applied via the circuit 3. This PWM
The heater H to which a voltage is applied by control generates heat and heat-cleans the gas sensitive bodies 8a and 8b. The heat cleaning period and the stabilization waiting time are about 2 minutes.

Upon completion of this heat cleaning, the microcomputer 2 determines the resistance value Ra of the gas sensitive bodies 8a and 8b to the air not containing the gas to be detected based on the detection output at that time.
Zero point adjustment is performed in which 0 and Rb0 are stored as resistance values corresponding to air when the sensitivity is obtained. Then, the real-time sampling operation with a cycle of 1 second is started, and the display unit 3 displays the preparation completion.

When the key switch S1 is operated after shifting to the real-time sampling operation, a mode for changing the sampling cycle is entered. Under this mode, a plurality of preset sampling cycles are cyclically switched for each operation. In response to the switching, the microcomputer 2 causes the display unit 3 to display the sampling period and the measurable time calculated from the data storable capacity of the storage unit 4. In the present embodiment, the measurable time is not displayed because data is not stored during real-time sampling. When the key switch S1 is further operated after switching to the last sampling cycle, the real-time sampling is selected again. The sampling cycle or real-time sampling that is selectively displayed by this switching operation is confirmed when the enter key S4 is operated.

The microcomputer 2 starts a sampling operation in which the detection outputs of the semiconductor gas sensors 1A and 1B are taken into the ports R40 and R50 at a cycle of 1 second.

In this sampling operation, the microcomputer 2 first determines that each semiconductor gas sensor 1 is at the beginning of the sampling period.
The detection outputs are taken in when the load resistances connected to the gas sensing bodies 8a and 8b of A and 1B are only R5 and R7, respectively, and the detection outputs are taken in from the ports R40 and R50 and the built-in A / D converter A / D conversion. Then, when the A / D conversion is completed, the outputs of the knot gates NT1 and NT2 are set to "H" by setting the ports R41 and R51 to "L", and the load resistances connected to the respective gas sensing bodies 8a and 8b are set. R respectively
5 and R6 are parallel circuits, and R7 and R8 are parallel circuits. As a result, the load resistance value becomes low, and when the load resistance value is low, the detection output is taken in from the ports R40, R50 and A / D converted by the built-in A / D converter. When the minimum time required for this A / D conversion elapses, the microcomputer 2 sets the ports R41 and R51 to "H", and the respective gas sensing bodies 8a,
The load resistances connected to 8b are returned to only R5 and R7, respectively, and the operation of capturing the detection output is completed.

Now, the microcomputer 2 selects one of the detection output when the load resistance value is high and the detection output when the load resistance value is low,
The detection output that can secure the accuracy is determined, and the processing that adopts the detection output that can be secured is performed every sampling, and the A / D conversion value of the detection output on the adopted side, that is, the voltage value is used as described above. Resistance value R of gas-sensitive body 8a, 8b
a, Rb are obtained, and the ratio Ra0 / R between the preset resistance values Ra0, Rb0 in air and the resistance values Ra, Rb is further determined.
a, Rb0 / Rb, that is, the sensitivities E and X of the gas sensors 1A and 1B with respect to an unknown gas are obtained.

Then, based on these sensitivities E and X, the odor intensity F and odor intensity S defined as above are respectively obtained.

When this calculation is finished, the odor intensity F and the odor intensity S obtained by the calculation are displayed on the display unit 3, and the data is stored at every data storage cycle except for the real-time sampling. In addition, in order to prevent the memory overflow, in the present embodiment, the display unit 3 displays the measurement elapsed time to alert the user to the memory overflow, and at the time of the memory overflow, the display unit 3 blinks the time to display the data. It is designed to prompt you to take out the memory or clear the memory. For memory clear, for example, when the power switch SW1 is turned on while the key switch S1 and the key switch S3 are pressed at the same time, the microcomputer 2 clears the stored data.

Further, the odor intensity F and the odor intensity S on the display unit 3 obtained by sampling other than real-time sampling.
Is displayed by the data based on the sampling result of the same cycle as the real-time sampling.

By the way, under the operation of selecting and switching the sampling cycle, the microcomputer 2 performs the sampling operation in the cycle before the operation is started, and the data saving and the odor intensity F and the odor intensity S of the sampling operation are performed. The display will continue. When the key switches S3 and S4 are operated under this sampling switching selection operation,
The microcomputer 2 also stops the storage operation of the measurement data and shifts to real-time sampling.

When the key switch S2 is operated during the sampling operation, the microcomputer 2 starts the peak hold operation and holds the maximum value of the odor intensity S of 1 point and the odor intensity F at that time. This hold operation is released when the key switch S2 is operated again.

In the above embodiment, the detection output in the case of a high load resistance value connected to the gas sensitive bodies 8a, 8b of the semiconductor gas sensors 1A, 1B at the time of sampling and the detection output in the low state are respectively taken in to obtain high accuracy. Although the detection output on the side that can take the value is adopted, the load resistance value at which the desired detection accuracy can be obtained only during the time required for A / D conversion when sampling the detection output is It is also possible to switch and load the detection output in the state of the switched load resistance value, and restore the normal load resistance value after A / D conversion. It goes without saying that the load resistance value at the time of switching includes an infinite resistance value, that is, an open state.

[0042]

According to the first aspect of the present invention, a voltage is applied to the semiconductor gas sensor, the load resistance connected in series to the gas sensitive body of the semiconductor gas sensor, and the series circuit of the gas sensitive body and the load resistance of the semiconductor gas sensor. The voltage at the connection point of the semiconductor gas sensor, the gas sensitive body of the semiconductor gas sensor, and the load resistance is sampled as a detection output, and the sampled voltage is A
/ D conversion, and the resistance value of the gas-sensitive body of the semiconductor gas sensor obtained based on the voltage value obtained by the A / D conversion and the gas-sensitive gas of the semiconductor gas sensor obtained in advance in an atmosphere of air containing no gas to be detected. In a gas detection device using a semiconductor gas sensor equipped with a gas detection means for detecting the concentration of a gas to be detected from the resistance value of the body, the load resistance value of the load resistance connected to the gas sensitive body of the semiconductor gas sensor is detected and output. When sampling, the load resistance value is switched from a normal load resistance value to a load resistance value that gives a desired detection accuracy only during the time required for the A / D conversion by the gas detection means, and after the A / D conversion by the gas detection means, the normal Since the load resistance switching means for returning to the load resistance value is provided, it is possible to suppress the drift of the resistance value of the gas sensitive body of the semiconductor gas sensor due to the switching of the load resistance value. Results widely varying region of the resistance value of the gas-sensitive material of the semiconductor gas sensor without deteriorating the resolution of the A / D converter to be used, there is an effect that allows the gas detection with high accuracy.

According to a second aspect of the present invention, in the first aspect of the present invention, the load resistance switching means sequentially switches to a plurality of load resistance values different from the normal load resistance value only during a time required for A / D conversion, After switching to the final load resistance value, the normal load resistance value is restored, and the gas detection unit sequentially switches the detection output based on the normal load resistance value immediately before the switching by the load resistance switching unit, and after the load resistance switching starts. Since the detection output based on the load resistance value is sampled and the detection output at the time of the load resistance value that gives the optimum accuracy is obtained as the detection output for detecting the concentration of the gas to be detected after switching the load resistance, it is always optimal. It is possible to select the detection output based on the load resistance value with which the accuracy can be obtained, and it is possible to detect the gas with higher accuracy.

[Brief description of drawings]

FIG. 1 is a circuit diagram of an odor detector adopting the present invention.

FIG. 2 is an explanatory diagram showing the relationship between the sensitivity of the semiconductor gas sensor used in the odor detector and the concentration of the gas to be detected.

FIG. 3 is an explanatory diagram showing the relationship between the odor intensity and odor intensity measured by the odor detector and the sensitivity of the semiconductor gas sensor used.

[Explanation of symbols]

1A, 1B Semiconductor gas sensor 2 microcomputer 8a, 8b Sensitive gas body R5 to R8 load resistance

Continued front page    F term (reference) 2G046 AA23 AA24 DB02 DB05 DC02                       DC09 DC14 DC16 DC17 DC18                       DD01 EB01 EB06 FB01                 2G060 AA01 AB19 AB21 AB26 AE19                       AF07 BA01 BB02 BB08 BC03                       HA01 HB02 HB06 HC07 HC13                       HC19 HC21 HC22 HC26 HD02                       HE01 HE02 KA01

Claims (2)

[Claims] 1. A semiconductor gas sensor, a load resistance connected in series to a gas sensitive body of the semiconductor gas sensor, a voltage source for applying a voltage to a series circuit of the gas sensitive body of the semiconductor gas sensor and the load resistance, and the semiconductor gas sensor. The voltage at the connection point between the gas-sensing body and the load resistance of is sampled as the detection output,
This sampled voltage is A / D converted and this A / D
The gas to be detected from the resistance value of the gas sensitive body of the semiconductor gas sensor obtained based on the converted voltage value and the resistance value of the gas sensitive body of the semiconductor gas sensor previously obtained in an atmosphere of air not containing the gas to be detected In a gas detection device using a semiconductor gas sensor provided with a gas detection means for detecting the concentration of, the load resistance value of the load resistance connected to the gas-sensitive body of the semiconductor gas sensor is determined by the gas detection means when sampling the detection output. Load resistance switching means for switching from a normal load resistance value to a load resistance value for which desired detection accuracy can be obtained only during the time required for A / D conversion, and for returning to a normal load resistance value after A / D conversion by the gas detection means A gas detection device using a semiconductor gas sensor, characterized by being provided with. 2. The load resistance switching means sequentially switches to a plurality of load resistance values different from the normal load resistance value only during a time required for A / D conversion, and after switching to the last load resistance value, the normal load resistance value. The gas detection means samples the detection output by the normal load resistance value immediately before the start of switching by the load resistance switching means and the detection output by each load resistance value that is sequentially switched after the start of the load resistance switching, respectively. 2. The gas detection using the semiconductor gas sensor according to claim 1, wherein a detection output at a load resistance value that provides optimum accuracy is adopted as a detection output for detecting the concentration of the gas to be detected after the resistance switching is completed. apparatus.