JPH095283A - Gas concentration detector - Google Patents

Abstract

PURPOSE: To provide a gas concentration detector which can detect gas concentration in a high accuracy without increasing the cost. CONSTITUTION: A gas detection element 1a and a temperature detection element 1b which are of the same construction are driven by allowing a constant current to flow therein through a constant current source 21. Both end voltage of the gas detection element 1a is amplified by an amplifying means 27 with a constant gain and it is A/D-converted thereafter to detect the gas concentration. Both end voltage of the temperature detection element 1b is amplified by an amplifying means 24 with a constant gain and it is A/D-converted thereafter to detect the ambient temperature. A gas concentration voltage shifting means 27a shifts both end voltage of the gas detection element 1a in multiple stages according to that of the gas detection element that is A/D-converted before amplification. A gas concentration detection means 23-1 detects the gas concentration in accordance with the A/D-converted value of both end voltage of the gas detection element that is shifted by the gas concentration voltage shifting means, the shift quantity of the gas concentration voltage shifting means, and a temperature corrected quantity.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas concentration detecting device for detecting a gas concentration using a catalytic combustion type sensor for detecting a combustible gas such as CO.

[0002]

2. Description of the Related Art Conventionally, as a catalytic combustion type sensor, one of two elements formed in the same structure by using a platinum wire is connected in series using one as a sensor element and the other as a reference element, and a sensor element connected in series. A constant current is supplied to the reference element from a constant current source to drive it, and the sensor element is a gas detection element,
It is known that the reference element is made to operate as a temperature detecting element. Since the gas detection element not only shows a resistance value of a magnitude corresponding to the detected gas concentration, but also the resistance value changes depending on the ambient temperature, in a gas concentration detection device using such a sensor, the temperature detection element It is necessary to detect the ambient temperature, and to correct the resistance value of the gas detection element based on the detected temperature to detect the gas concentration.

For this reason, conventionally, the voltage across the temperature detecting element is A / D converted and read to obtain temperature / voltage data containing a component purely corresponding to the temperature to detect the temperature, and the voltage across the gas detecting element is detected. Is read by A / D conversion to obtain concentration voltage data containing a component corresponding to the concentration of the detection gas, and the concentration of the detection gas is detected by correcting the concentration voltage data with the detection temperature.

[0004]

However, since the input voltage permissible range of the A / D converter is limited, the sensor output signal that greatly changes due to the concentration and temperature should be within the input voltage permissible range. There is a limit to the gain that can be amplified.
In order to increase the resolution in order to detect the gas concentration with high accuracy, an A / D converter with a large number of bits may be used, but an A / D converter with a large number of bits is generally expensive and the device Increase the cost.

Therefore, in view of the above-mentioned conventional problems, it is an object of the present invention to provide a gas concentration detecting device which can detect the gas concentration with high accuracy without increasing the cost.

Further, in view of the above-mentioned conventional problems, the present invention uses a gas detection sensor having a large temperature dependence in output characteristics, and can detect the gas concentration with high accuracy without increasing the cost. It is an object of the present invention to provide a gas concentration detection device capable of performing the above.

[0007]

In order to achieve the above object, a gas concentration detecting device according to the present invention is composed of two elements 1 having the same structure as shown in the basic structure diagram of FIG.
One of a and 1b is connected in series as a gas detection element and the other 1b is used as a temperature detection element, and a constant current is supplied from the constant current source 21 to the gas detection element and the temperature detection element connected in series to drive the detection. The voltage across the gas detecting element, whose resistance value changes according to the gas concentration, is amplified by the concentration amplifying means 27 with a constant gain and A / D converted to detect the gas concentration, and the resistance value changes according to the ambient temperature. The voltage across the temperature sensing element, which changes with temperature, is amplified by the temperature amplifying means 24 having a constant gain and A / D converted to detect the ambient temperature, and the detected gas concentration is corrected by the detected ambient temperature to obtain the concentration. In the gas concentration detection device in which the detection means 23-1 detects the gas concentration, both ends of the gas detection element are detected according to the magnitude of the value obtained by A / D converting the voltage across the gas detection element before amplification. Multi-stage voltage And a first gas concentration voltage shift means 27a for operating the concentration detecting means, wherein the concentration detecting means shifts the A / D converted value of the voltage across the gas detecting element by the first gas concentration voltage shifting means, and the first gas concentration voltage shifting means 27a. By the shift amount of the gas concentration voltage shift means of and the temperature correction amount,
The feature is that the gas concentration is detected.

The temperature at which the gas concentration detecting device shifts the voltage across the temperature detecting element in multiple stages according to the magnitude of the value obtained by A / D converting the voltage across the temperature detecting element before amplification. A voltage shift means 24a is further provided, and the concentration detection means detects the temperature by the A / D conversion value of the voltage across the temperature detection element shifted by the temperature voltage shift means and the shift amount of the temperature voltage shift means. The temperature correction amount is obtained based on the detected temperature.

The gas concentration detecting device shifts the voltage across the gas sensing element in multiple stages according to the magnitude of the value obtained by A / D converting the voltage across the temperature sensing element before amplification. 2 further comprises a gas concentration voltage shifting means 27b,
The A / D converted value of the voltage across the gas detection element shifted by the first and second gas concentration voltage shifting means by the concentration detecting means, and the shift of the first and second gas concentration voltage shifting means. By the amount and the temperature correction amount,
The feature is that the gas concentration is detected.

[0010]

In the above structure, the first gas concentration voltage shifting means 27a changes the voltage across the gas detecting element before amplification to A /
Depending on the magnitude of the value obtained by D conversion, the voltage across the gas sensing element is shifted in multiple steps, and the concentration detecting means 23-1 shifts the voltage across the gas sensing element by the first gas concentration voltage shifting means. Since the gas concentration is detected by the A / D conversion value of the voltage, the shift amount of the first gas concentration voltage shift means, and the temperature correction amount, the voltage across the gas detection element that reflects the gas concentration is high. It can be measured with resolution.

The temperature-voltage shift means 27b shifts the voltage across the temperature detecting element in multiple stages according to the magnitude of the value obtained by A / D converting the voltage across the temperature detecting element before amplification, and the concentration detecting means However, since the temperature is detected by the A / D conversion value of the voltage across the temperature detecting element shifted by the temperature voltage shift means and the shift amount of the temperature voltage shift means, the temperature correction amount is obtained by the detected temperature. The voltage across the temperature sensing element reflecting the temperature can be measured with high resolution.

The second gas concentration voltage shifting means 27b is
The voltage across the gas sensing element is shifted in multiple steps according to the magnitude of the value obtained by A / D converting the voltage across the temperature sensing element before amplification, and the concentration detecting means causes the first and second gases The gas concentration is detected by the A / D converted value of the voltage across the gas detection element shifted by the concentration voltage shift means, the shift amounts of the first and second gas concentration voltage shift means, and the temperature correction amount. Therefore, the voltage across the gas detection element, which reflects the gas concentration, can be measured with higher resolution.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 2 is a circuit diagram showing an embodiment of a gas concentration detecting device according to the present invention configured to detect CO gas concentration. In the figure, reference numeral 1 is a contact combustion type sensor constituted by connecting in series a gas detection element 1a and a temperature detection element 1b, which are formed in the same configuration by using a platinum wire, and are connected to terminals 2a to 2c of the apparatus main body 2. It is connected so that it can come in and out freely. The gas detection element 1a is sensitive to the CO gas to be detected, and the temperature detection element 1b is arranged not to be sensitive to the CO gas and to detect only the ambient temperature. Gas detection element 1a
One end of the terminal 2a connected to the GND (ground) line
In addition, one end of the temperature detecting element 1b is connected to the terminal 2c, and the interconnection point of both elements is connected to the terminal 2b.
A constant current source 21 is connected between the 5V line and the terminal 2c, and a constant current flows through the temperature detecting element 1b and the gas detecting element 1a.

The voltage across the temperature sensing element 1b is the operational voltage.
The differential amplifier circuit 2 including the OP1 and the resistors R9 to R12.
It is applied to two inverting and non-inverting inputs for differential amplification.
The first temperature signal obtained by this differential amplification is determined in advance.
Microcomputer that performs processing according to the program
The A / D converter (ADC) 23a of the data (μCOM) 23
It is input, and the operational amplifier OP2 and resistors R13 to R
17 and the analog switches AS1 and AS2
It is applied to the inverting input of the inversion amplifier circuit 24, and the voltage of
It is amplified with a predetermined gain. The first temperature signal is ADC2
A / D converted by 3a and read into μCOM23
It is. The ADC 23a has an ADC reference voltage generator 25.
The reference voltage of ADC, such as 3V, is also applied. Inversion
The second temperature signal amplified by the amplifier circuit 24 is also μCOM2.
3 is input to the ADC 23a, and the ADC 23a outputs A /
It is D-converted and read into the μCOM 23. μCOM
Reference numeral 23 is a central processing unit that performs processing according to a predetermined program.
Processing unit (CPU) 23 ₁, Programs and fixed data
ROM 23 that stores information such as₂, Store various data
RAM23 with area and work area_ThreeBuilt in
doing.

The inverting input of the operational amplifier OP2 is applied with a voltage obtained by dividing the voltage of the potential line for addition by the variable resistor VR3, and the non-inverting input thereof is connected with the operation reference voltage line and applied with the operation reference voltage. There is. The resistor R14 and the analog switch AS1 and the resistor R15 and the analog switch AS2 are connected in series and are respectively connected between the inverting input of the operational amplifier OP2 and the GND line, and are controlled by the μCOM23 based on the first temperature signal to operate the operational amplifier OP2. The temperature-voltage shift circuit 24a that shifts the voltage applied to the inverting input of 4 ranges in four ranges is configured.
The analog switch AS1 and the analog switch AS2 of the temperature / voltage shift circuit 24a are turned on by the μCOM 23.
It is turned on / off by an off control signal. The resistance R
The sizes of 14 and R15 have a one-to-two relationship.
The differential amplifier circuit 22 and the inverting amplifier circuit 24 form a temperature detection circuit.

Gas concentration detecting element 1a and temperature detecting element 1b
The voltage at the interconnection point of is the operational amplifier OP3 and the resistor R31.
˜R33 and variable resistor VR4 inverting amplifier circuit 26
Is applied to the inverting input of and is inverted and amplified. The inverting amplifier circuit 26 is operated by a positive and negative power source (not shown).
The first gas concentration signal obtained by this inverting amplification is A
It is A / D converted by the DC 23a and read into the μCOM 23. It is applied to the inverting input of an inverting amplifier circuit 27 including an operational amplifier OP4, resistors R34 to R42, and analog switches AS3 and AS8, and amplified with a predetermined gain of 36 times, for example. The second gas concentration signal amplified by the inverting amplifier circuit 27 is also input to the μCOM 23 and the ADC 2
It is A / D converted by 3a and read into the μCOM 23. A calculation reference voltage line is connected to the non-inverting inputs of the operational amplifiers OP3 and OP4.

The resistors R35 and R36 and the analog switches AS3 and AS4 are connected to the addition potential line and the operational amplifier OP.
A first concentration voltage shift circuit 27a, which is connected in series with the four inverting inputs and shifts the voltage applied to the inverting input of the operational amplifier OP4 by the μCOM 23 in four ranges based on the first gas concentration signal. I am configuring.
Analog switch A of the first concentration voltage shift circuit 27a
S3 and AS4 are also turned on by the on / off control signal from the μCOM 23. Turned off. The sizes of the resistors R35 and R36 also have a one-to-two relationship.

Resistors R37 to R40 and analog switch A
The series circuit of S5 to AS8 is also connected between the summing potential line and the inverting input of the operational amplifier OP4, and the operational amplifier OP is operated by the μCOM23 based on the second temperature signal.
A second concentration voltage shift circuit 27b for shifting the voltage applied to the inverting input of 4 in 16 ranges is configured. Analog switch AS of the second concentration voltage shift circuit 27b
5 to AS8 are turned on by the on / off control signal from the μCOM 23. Turned off. The sizes of the resistors R37 to R40 are in the relationship of 1: 2: 4: 8. And
The inverting amplifier circuits 26 and 27 constitute a gas concentration detection circuit.

The μCOM 23 of the apparatus having the above-described structure has a nonvolatile memory 23, which is, for example, an E ² PROM, which can hold the set values described below without a backup power supply.
b is connected, and the setting is performed according to the following procedure for its use.

First, the ambient temperature is 25 ° C. and the concentration of the test gas is 0 pp.
Make the following settings under the condition of m. After connecting the sensor 1, the gas detecting element 1a and the temperature detecting element 1 are connected in series.
The constant current source 21 is adjusted so that the current flowing through b has a predetermined value. Then, the voltage value of the calculation reference voltage line is set to a predetermined value by a variable resistor (not shown), and the second temperature signal output from the operational amplifier OP2 is changed by the variable resistor VR3.
The second gas concentration signal, which is the output of the operational amplifier OP4, is adjusted to a predetermined value by the variable resistor VR4.

Next, temperature information at an ambient temperature of 25 ° C. is fetched and written in the non-volatile memory 23b. Further, the concentration information is fetched at the ambient temperature of 25 ° C. and the concentration of the test gas of 0 ppm and written in the nonvolatile memory 23. Then, ambient temperature 25 ℃,
The concentration information is fetched at the test gas concentration of 3000 ppm and written in the non-volatile memory 23. And an ambient temperature of 225 ° C,
The temperature information and the concentration information at the test gas concentration of 0 ppm are fetched and written in the non-volatile memory 23.

Next, the initial data is taken in to grasp the initial value. That is, at an ambient temperature of 25 ° C
Of the first temperature signal, the shift amount of the temperature voltage shift circuit 24a, that is, the shift amount of the second temperature signal with respect to the temperature is determined.
Temperature measurement range chart stored in the predetermined ROM23 within ₂ In this decision, i.e., the temperature - utilizes range selection information. For example, range 0 is -40 ℃ ~
Since it is + 35 ° C, the shift amount is 0 at 25 ° C.
When the shift amount is determined, the temperature / voltage shift circuit 24a is shifted to read the second temperature signal. The second temperature signal is more detailed temperature data than the first temperature signal, and the true value of the second temperature signal at this time is the read value of the ADC 23a + the shift amount (theoretical value). The shift amount with respect to the ADC input is [reference voltage for shifting (potential for addition) /
Shift resistance value] = feedback resistance value. The result is stored in the non-volatile memory 23b as a temperature reference value (25 ° C.) based on the second temperature signal at the ambient temperature of 25 ° C.

Then, the ambient temperature is 25 ° C. and the concentration of the test gas is 0.
The first gas concentration signal is read in ppm, and from the read result, the concentration shift amount which is the concentration correction amount of the first concentration voltage shift circuit 27a and the second concentration voltage shift circuit 27 are read.
The temperature shift amount which is the temperature correction amount of b is determined. At this time, the shift amount on the gas concentration side is 0, but a shift amount of 1 bit is required on the temperature side. For the determination of the shift amount, concentration - range change information and temperature - stored in advance the range change information to the ROM 23 _2. When the shift amount is determined, the concentration shift of the first concentration voltage shift circuit 27a and the concentration temperature shift of the second concentration voltage shift circuit 27b are performed to read the second gas concentration signal. The true value of the second gas concentration signal at this time is the reading of the ADC 23a + concentration shift value + temperature shift value, and this result is used as the second gas concentration signal at the ambient temperature of 25 ° C. and the test gas concentration of 0 ppm. The concentration reference value (0 ppm, 25 ° C.) is stored in the non-volatile memory 23b.

Further, the ambient temperature is 25 ° C. and the test gas concentration is 30.
The first gas concentration signal is read at 00 ppm, and the concentration shift amount of the first concentration voltage shift circuit 27a and the temperature shift amount of the second concentration voltage shift circuit 27b are determined from the read result. This determined shift amount is the first
The second gas concentration signal is read while performing the concentration voltage shift circuit 27a and the second concentration voltage shift circuit 27b. The true value of the second gas concentration signal at this time is ADC
23a read value + concentration shift value + temperature shift value, and the result is the ambient temperature 25 ° C. and the test gas concentration 3000.
Concentration reference value based on the second gas concentration signal in ppm (30
00 ppm, 25 ° C.) in the non-volatile memory 23b. And concentration reference value (3000 ppm, 25 ° C.)-Concentration reference value (0 ppm, 25 ° C.) / 3000 = voltage sensitivity / 1
ppm = gas sensitivity, which is stored in the non-volatile memory 23b. This gas sensitivity is used when obtaining the absolute gas concentration from the relative gas concentration when measuring the gas concentration.

Next, the first temperature signal is read at the ambient temperature of 225 ° C., and the shift amount of the temperature voltage shift circuit 24a is determined from the read result. When the shift amount is determined, the first voltage shift circuit 24a is shifted to read the second temperature signal. The true value of the second temperature signal is the ADC 23a
Read value + shift amount, which is the temperature reference value (2
25 ° C.) and stored in the non-volatile memory 23b. 25
Sensitivity of voltage / temperature from ℃ to 225 ° C = temperature reference value (225 ° C) -temperature reference value (25 ° C), and (voltage / temperature) sensitivity per 1 ° C = [temperature reference value (225 ° C) −
Temperature reference value (25 ° C.)] / 200 = voltage sensitivity / ° C. = temperature coefficient, which is stored in the nonvolatile memory 23b.

Furthermore, the first gas concentration signal is read at an ambient temperature of 225 ° C. and the test gas concentration of 0 ppm, and the shift amount of the first concentration voltage shift circuit 27a is determined from the read result. When the shift amount is determined, the first concentration voltage shift circuit 27a is shifted to read the second gas concentration signal. The true value of the second gas concentration signal at this time is ADC
23a read value + concentration shift value + temperature shift value, and the result is an ambient temperature of 225 ° C. and a test gas concentration of 0 ppm.
Concentration reference value (0ppm, based on the second gas concentration signal at
225 ° C.) and stored in the non-volatile memory 23b. And concentration reference value (0ppm, 225 ° C) -concentration reference value (0
ppm, 25 ° C.) / 200 = sensor temperature coefficient / ° C. = gas concentration temperature coefficient, which is stored in the non-volatile memory 23b. And, from this gas concentration temperature coefficient, 1 ° C required
The amount of correction for the hit is obtained in advance and used to determine the amount of correction for the temperature difference from 25 ° C.

By the setting work described above, the nonvolatile memory 2
3b, as shown in FIG. 3, a temperature reference value (25 ° C.),
Concentration reference value (0ppm, 25 ° C), concentration reference value (3000
ppm, 25 ° C), gas sensitivity, temperature reference value (225 ° C),
The temperature coefficient, concentration reference value (0 ppm, 225 ° C), gas concentration temperature coefficient, etc. are stored. However, by setting the setting values in this way, it may be affected by variations in device components such as sensors. The optimum setting value will be set for each device. However, if there is little variation between devices, a preset set value may be written from the outside.

The measurement of the gas concentration performed after the above settings will be described below.

First, the temperature difference from 25 ° C. is grasped. For this purpose, the second temperature signal is measured. The measurement of the second temperature signal is performed by reading the first temperature signal, determining the temperature shift amount from the result, and reading the second temperature signal (ADC value) while performing the determined temperature shift.
Next, the value of the temperature T is obtained. This is done by calculating T = second temperature signal (ADC value) + temperature shift amount. Subsequently, the difference between T and the value of 25 ° C. (second temperature signal) is ΔT, and ΔT = value of T−temperature reference value (25 ° C.) is calculated. Next, the temperature difference [Δ (n-25 ° C)] is defined as the temperature difference [Δ (n-25 ° C)] from the temperature reference value (25 ° C).
C)] = ΔT / temperature coefficient = (difference from 25 ° C.).

Subsequently, the gas concentration temperature is corrected. For this reason, the temperature of the gas concentration is corrected by the temperature difference [Δ (n-25 ° C.)], but the temperature on the circuit is also corrected and a numerical correction amount (logical correction amount) is performed.

Finally, in order to carry out the concentration measurement, first the second
The gas concentration signal of is measured. In the measurement of the second gas concentration signal, the first gas concentration signal is read, the required concentration shift amount is determined from the result, and the second gas concentration signal (ADC value) is determined while performing the determined concentration shift. Do by reading. Next, the relative concentration value is obtained by calculating the second gas concentration signal (ADC) value + the concentration shift amount + the temperature correction amount on the circuit + the logical correction amount. Finally, the absolute value of the concentration
Calculate by calculating relative concentration value x gas sensitivity.

A brief explanation will be given with reference to a flow chart of FIG. 4 showing a process in which the CPU 23 ₁ carries out the above-mentioned concentration measuring operation according to a predetermined program. In the first step S1, the first temperature signal is measured and the temperature according to the measurement result is measured. After the shift amount of the voltage shift circuit 24a is determined and calculated, the second temperature signal is measured, and the temperature value is obtained in the next step S2. Subsequently, the process proceeds to step S3, the shift amount of the second concentration voltage shift circuit 27b is determined based on the value of the temperature obtained in step S2, and the temperature is corrected. After the shift amount of the first concentration voltage circuit 27a is determined by, the process proceeds to step S5 and the second concentration signal is measured to detect the concentration. As described with reference to the flowchart of FIG. 4, the CPU 23 ₁ uses the density detecting means 23-
Working as 1.

For example, when the gas to be detected is CO, the CO gas sensitivity (voltage / gas concentration) of the sensor 1 is 11.5 mV / 1000 ppm and the temperature coefficient is 0.8 mV.
/ ° C, which is quite large. The operating temperature range is 300 ° C
Since it is (-40 ° C to 260 ° C), the output voltage change is 3
It becomes 00 ° C. × 0.8 mV / ° C. = 240 mV. ADC 23a
The input range of is set to 0-3V and the resolution is 25
Since it is 6 bits, if the output voltage is converted as it is, the resolution will be 300 ° C./256 bits = 1.172 ° C./bit.

On the other hand, when the amplification factor of the inverting amplifier circuit 24 is set to 36 times and its input is shifted by 4 ranges, 300 ° C./4=75° C. 75 ° C. × 0.8 mV / ° C. × 36 = 2.160 V Become. Maximum input voltage of ADC 23a is 3V / 256
Since it is a bit, 2.160V becomes 184 bits and becomes 75 ° C./184 bits = 0.4 ° C./bit, the resolution is improved, and more accurate temperature measurement can be performed.

The density detection range is 0 to 6000 ppm, which is 11.5 mV x when converted into the sensor output voltage change.
6 = 69 mV. On the other hand, the output voltage change due to the temperature of the sensor 1 is 300 ° C. × 0.8 mV = 240 mV. Then, assuming that the input voltage range of the ADC 23a is 3V, the optimum amplification factor of the inverting amplifier circuit 27 is set to 65 times from the above numerical value and shift range. From the above requirements, in order to increase the resolution for density, temperature correction for density output and stepwise shift control for density signal are performed.

Looking at the resolution with respect to the concentration, AD
The input voltage range of C23a is 0 to 3V. If this is directly applied to the concentration detection range of 0 to 6000ppm,
The resolution that can be recognized by the ADC 23a is 6000.
ppm / 256 bits = 23.4375 ppm / bit.

By the way, the input voltage amplified by the sensor 1 and input to the ADC 23a includes the one due to the concentration and the one due to the temperature. It is necessary to optimally control the range control caused by the concentration. The range switching for this purpose is for temperature correction as shown in FIG.
The density side range control as shown in Fig. 7 is separately performed, and finally both are present simultaneously in terms of time, as shown in Fig. 7. FIG. 7 is an example of a temperature correction range chart. In this example, the temperature per range is 3
00 ° C / 16 range = 18.75 ° C / range. The voltage per range is 0.8 mV / ° C × 18.75 ° C × 65
= 975 mV (typical value), maximum value is typical value x 1.1
= 1.0725V. , 1 concentration range is (1
1.5 mV / 1000 ppm) × 1500 ppm × 65 × 1.
25 = 1.416V / range, this concentration is added to the graph, 1.0725V + 1.41616V =
It becomes 2.474V. It should be noted that there is a margin of about 0.2 V above and below. The value at 25 ° C is (25 ° C-16.
25 ° C) x 0.8 mV x 65 times +0.2 V = 0.655 V
It is.

Now, calculating the resolution when the concentration detection range of 6000 ppm is measured in four ranges, 6000 ppm / 4 range = 1500 ppm / range 1500 ppm = 11.5 mV / 1000 ppm = 17.25
mV 17.25 mV × 65 = 1.12125V 3V / 256 bit = 1.12125V / X bit X = 95.68 bit 1500ppm / 95.68 bit = 15.68ppm / bit, and the density resolution is 20ppm / bit or less. Has become.

[0039]

As described above, according to the present invention, the voltage across the gas detecting element reflecting the gas concentration can be measured with high resolution, so that the cost can be increased by using the ADC having a large number of bits. The gas concentration can be detected with high accuracy without inviting.

Since the voltage across the temperature sensing element reflecting the temperature can be measured with high resolution, the voltage across the temperature sensing element reflecting the temperature for the temperature correction amount can be measured with high resolution. it can.

Furthermore, since the concentration signal is shifted by shifting the concentration signal according to the temperature measured with high resolution, the cost is increased in the case of using the gas detection sensor having a large temperature dependency in the output characteristic. The gas concentration can be detected with high accuracy.

[Brief description of drawings]

FIG. 1 is a block diagram showing a basic configuration of a gas concentration detection device according to the present invention.

FIG. 2 is a circuit diagram showing an embodiment of a gas concentration detecting device according to the present invention.

FIG. 3 is a diagram showing setting values stored in a non-volatile memory in FIG.

FIG. 4 is a flowchart showing an outline of a part of processing performed by a CPU in FIG.

FIG. 5 is a graph showing how the range is switched for temperature correction by the second concentration voltage shift circuit.

FIG. 6 is a graph showing how a range is switched for density control by a second density voltage shift circuit.

FIG. 7 is a graph showing a state of range switching in which temperature correction and density control are overlapped by a second density voltage shift circuit.

[Explanation of symbols]

 1a Gas detection element 1b Temperature detection element 21 Constant current source 23-1 Concentration detection means (CPU) 24 Temperature amplification means 24a Temperature voltage shift means 27 Concentration amplification means 27a First gas Concentration voltage shift means 27b Second gas Concentration voltage shift means

Claims (3)

[Claims] 1. A constant current from a constant current source to a gas detection element and a temperature detection element connected in series, wherein one of two elements having the same structure is connected as a gas detection element and the other is connected as a temperature detection element. The voltage across the gas detecting element whose resistance value changes according to the detected gas concentration is amplified by the concentration amplifying means having a constant gain and A / D converted to detect the gas concentration. The voltage across the temperature detecting element, whose resistance value changes according to the temperature, is amplified by the temperature amplifying means with a constant gain and A / D converted to detect the ambient temperature, and the detected gas concentration is determined by the detected ambient temperature. In a gas concentration detecting device in which the temperature is corrected and the concentration detecting means detects the gas concentration, the gas detecting element is responsive to the magnitude of the value obtained by A / D converting the voltage across the gas detecting element before amplification. The voltage across Comprising a first gas concentration voltage shift means for floor shift, the concentration detecting means, A of the voltage across the gas sensing element that is shifted by the first gas concentration voltage shift means
A gas concentration detecting device, wherein a gas concentration is detected based on the / D conversion value, the shift amount of the first gas concentration voltage shifting means, and the temperature correction amount. 2. A temperature-voltage shift means for shifting the voltage across the temperature sensing element in multiple stages according to the magnitude of the value obtained by A / D converting the voltage across the temperature sensing element before amplification. The concentration detecting means detects the temperature by the A / D conversion value of the voltage across the temperature detecting element shifted by the temperature voltage shifting means and the shift amount of the temperature voltage shifting means, and detects the temperature by the detected temperature. The gas concentration detecting device according to claim 1, wherein the temperature correction amount is obtained. 3. A second gas concentration voltage that shifts the voltage across the gas sensing element in multiple stages according to the magnitude of the value obtained by A / D converting the voltage across the temperature sensing element before amplification. Further comprising a shift unit, wherein the concentration detection unit shifts the first and second gas concentration voltage shift units by an A / D conversion value of the voltage across the gas detection element, and the first and second gases. By the shift amount of the concentration voltage shift means and the temperature correction amount,
The gas concentration detecting device according to claim 1 or 2, wherein the gas concentration is detected.