JPH0374790B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) The present invention relates to a gas detection device that converts gas concentration into an electrical signal and detects it by correcting environmental temperature.

(Prior art) In recent years, gas detection devices have been developed that incorporate a detection section that detects changes in gas concentration as changes in impedance such as capacitance or resistance, and calculate gas concentration in response to changes in the impedance of the detection section. is being promoted.

By the way, in the detection unit used in such a gas detection device, the impedance not only changes in response to changes in gas concentration, but also changes in response to changes in environmental temperature. It was necessary to consider temperature compensation.

Therefore, a temperature compensation element such as a thermistor is connected in series or in parallel with the detection section to correct the temperature characteristics of the detection section.

However, if you make a mistake in selecting the temperature compensation element that corrects the temperature characteristics of the detection unit, the output signal will fluctuate in response to temperature changes even though the detected gas concentration is constant, making it difficult to respond to changes in gas concentration. There was a problem that an accurate output signal could not be obtained.

In addition, if the temperature characteristics of the detection section are complex, a special circuit section for temperature compensation must be provided to compensate for the complex temperature characteristics of the detection section. There were problems in that the circuit configuration of the gas detection device became complicated and the cost of the gas detection device increased.

(Object of the Invention) The present invention has been made in view of the above problems, and an object of the present invention is to provide a gas detection device that accurately outputs a signal in response to changes in gas concentration while performing temperature compensation with a simple configuration. With the goal.

(Structure of the Invention) In order to achieve this object, the present invention includes a detection section whose impedance changes according to changes in gas concentration, and changes the oscillation period of the oscillation section in response to the change in the impedance of the detection section. In a gas detection device that differentiates the oscillation output of the oscillation section using a differentiating circuit and calculates the gas concentration based on the differential output, a temperature compensation element is incorporated into the differentiating circuit, and the differential characteristics of the differentiating circuit are changed according to changes in the environmental temperature. However, it is designed to compensate for changes in environmental temperature.

(Example) Examples of the present invention will be described below based on the drawings.

FIG. 1 is a circuit diagram showing one embodiment of the present invention. First, to explain the configuration, 1 is a sensor,
It includes a detection section 1a and a refresh section 1b.
In the detection section 1a, when the gas concentration of the gas to be measured, for example, CO gas, increases, the resistance value at both ends of the detection section 1a decreases in response to the increase in the gas concentration.

Note that the refresh section 1b refreshes the detection sensitivity of the detection section 1a by receiving a regular supply of a predetermined voltage from the outside. In addition, the resistance value of the detection unit 1a depends on the ambient temperature, and when the ambient temperature rises,
It has a temperature characteristic in which the resistance value of the detection part 1a decreases in response to a rise in temperature. Detection section 1a, resistance R
1, R2, R3, the capacitor C1, and the operational amplifier circuit 2 form an oscillating section, which normally oscillates at an oscillation period _t1 determined by the resistance value of the detecting section 1a and the capacitance of the capacitor C1. When the sensor 1 detects CO gas, the resistance value of the detection part 1a changes, and the detection part 1
By changing the time constant formed by a and capacitor C1, oscillation occurs at an oscillation period corresponding to the concentration of CO gas.
Furthermore, since the detection unit 1a has temperature characteristics,
When the ambient temperature changes, the time constant formed by the detection section 1a and the capacitor C1 changes due to a change in the resistance value of the detection section 1a, and the oscillation period changes depending on the ambient temperature. 3 is a temperature compensation element using a thermistor or the like, and its resistance value changes according to changes in ambient temperature.
A differential circuit is formed by the temperature compensation element 3 and the capacitor C2, and the oscillation output from the oscillation section is inputted through the terminal P1. When the temperature compensation element 3 detects a temperature change, it differentiates by changing to a differential time constant determined by the resistance value of the temperature compensation element 3 corresponding to the temperature change and the capacitance of the capacitor C2. P2 is a terminal, and terminal P
2 is connected to a diode D1 via a resistor R4, and the differential output half-wave rectified by the diode D1 is input to the positive terminal of the operational amplifier circuit 4 via a terminal P3. Resistors R5 and R
6 are connected in series, and a capacitor C3 is connected to the connection point between resistors R5 and R6, and a predetermined reference voltage V _S is applied to the negative terminal of the operational amplifier circuit 4. The operational amplifier circuit 4 compares the reference voltage V _S and the differential output,
When the differential output reaches the reference voltage V _S , a pulsed detection signal is output to the terminal P4. 5 is a clock output section, which outputs a clock every predetermined period _t0 . Here, the period t ₀ is set to be longer than the oscillation period t ₁ of the oscillation section under normal conditions, and provides processing timing for calculation processing to the calculation section 6. When the pulse-like detection signal from the operational amplifier circuit 4 is input through the terminal P4, the arithmetic section 6 calculates the pulse-like detection signal within a predetermined time _t0 according to the output timing of the clock from the clock output section 5. Compute the average value of the area.

Next, the operation will be explained. First, CO at constant temperature
The gas detection operation will be explained with reference to the timing chart in FIG. Normally, the oscillating section oscillates at a predetermined period _t1 with a time constant determined by the resistance value of the detecting section 1a and the capacitance of the capacitor C1, and when the CO gas concentration changes as shown in FIG. 1a
detects CO gas, and the resistance value of the detection part 1a decreases in response to an increase in the gas concentration of CO gas. Due to the decrease in the resistance value of the detection unit 1a, the detection unit 1a and the capacitor C
The time constant of 1 becomes smaller, and the oscillation period of the oscillator changes from t ₁ to t ₂ , becoming shorter in accordance with the gas concentration. In other words, as shown in Figure 2B, the oscillation period of the oscillator becomes faster as the gas concentration of CO gas increases,
An oscillation output is given to terminal P1. Figure 2 C is terminal P
2, the oscillation output is differentiated by a differential time constant determined by the capacitor C2 and the temperature compensation element 3. As shown in FIG. 2D, this differential output is input to the positive terminal of the operational amplifier circuit 4 after the negative component is removed through the intervention of the diode D1. Operational amplifier circuit 4
A predetermined reference voltage V _S is applied to the negative terminal of , and a pulse-like detection signal shown in FIG. 2E is outputted to the terminal P4 by comparing the reference voltage V _S and the differential output. When the arithmetic unit 6 receives the pulsed detection signal from the operational amplifier circuit 4, it calculates the pulsed detection signal within a predetermined time _t0 according to the clock output timing from the clock output unit 5 shown in FIG. 2F. The average value is calculated, and as shown in Figure 2G, the predetermined calculation time t ₀ given by the clock output timing is calculated.
After the elapse of time, a signal with a detection level corresponding to the increase in the gas concentration of CO gas is output.

Next, the operation of temperature compensation against temperature fluctuations will be explained with reference to the timing chart of FIG. First, the oscillation output of the oscillation section during normal operation is the detection section 1a.
It oscillates at a predetermined period _t1 determined by the resistance value of C1 and the capacitance of capacitor C1. As shown in Figure 3A,
When the ambient temperature rises from T _L to T _H , the detection part 1a
has a predetermined temperature characteristic, the resistance value of the detection part 1a decreases in response to a rise in ambient temperature, and the oscillation time constant formed by the detection part 1a and the capacitor C1 becomes as shown in FIG. 3B. It is changed from t ₁ to t ₃ , and the oscillation cycle of the oscillation circuit becomes shorter. In addition, the resistance value of the temperature compensation element 3 decreases in response to an increase in the ambient temperature, and the differential time constant determined by the temperature compensation element 3 and the capacitor C2 becomes smaller, so that the differential time constant becomes smaller as shown in FIG. 3C. is changed from _t4 to _t5 , and the differential waveform at terminal P2 changes to a steep waveform in accordance with the differential time constant _t5 . FIG. 3D shows the waveform of the terminal P3, in which the negative component of the differential output is removed through the intervention of the diode D1, and is input to the positive terminal of the operational amplifier circuit 4.
On the other hand, a predetermined reference voltage V _S is applied to the negative terminal of the operational amplifier circuit 4, and the reference voltage V _S and the differential output are compared and a detection pulse is output to the terminal P4 as shown in FIG. 3E. do. In other words, as the ambient temperature rises, the pulse period of the detection pulse becomes faster,
The pulse width of the detection pulse becomes smaller. Arithmetic unit 6
When the detection pulse from the operational amplifier circuit 4 is input, the average value of the area of the detection pulse within a predetermined time t0 is calculated according to the output timing of the clock from the clock output section ₅ shown in FIG. 3F. The reference voltage is set so that the average value of the area of the detection pulse within a predetermined time t ₀ at the ambient temperature T _L is equal to the average value of the area of the detection pulse within a predetermined time t ₀ at the ambient temperature T _H. By setting V _S and the differential time constant t ₅ , the output of the arithmetic unit 6 outputs a signal at a constant detection level, as shown in FIG. 3G, regardless of fluctuations in the ambient temperature.

Therefore, as shown in Figures 2 and 3, CO
When the environmental temperature changes as the gas concentration increases, the resistance value of the detection unit 1a changes and the resistance of the temperature compensation element 3 also changes to change the differential characteristics of the differential circuit to perform temperature compensation. Outputs a detection level signal corresponding to the increase in CO gas concentration.

In FIG. 1, resistors R5 and R6 are connected in series with a fixed resistor, and the divided voltage of resistors R5 and R6 is applied as a reference voltage to the negative terminal of the operational amplifier circuit 4. If the reference voltage applied to the operational amplifier circuit 4 is configured to be variable using a variable resistor, the temperature compensation element 3 can be easily selected, and together with the temperature characteristics of the temperature compensation element 3, the temperature compensation Accuracy can be further improved.

In addition, although the explanation has been given using a sensor that detects CO gas as an example of the detection part, when other sensors, that is, detecting physical quantities, the impedance changes accordingly, and the impedance changes depending on the surrounding temperature. It can be applied as temperature compensation for the detection section.

(Effects of the Invention) As described above, according to the present invention, the detection section is provided with a detection section whose impedance changes according to a change in gas concentration, and the oscillation period of the oscillation section is changed in response to a change in the impedance of the detection section. In a gas detection device that calculates the gas concentration based on the differential output obtained by differentiating the oscillation output of the oscillation unit with a differential circuit,
By incorporating a temperature compensation element into the differentiator circuit and changing the differential characteristics of the differentiator circuit in accordance with temperature changes, temperature compensation can be reliably performed with a simple configuration for changes in ambient temperature, and gas concentration This has the effect that it is possible to accurately detect.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, FIG. 2 is a timing chart showing the detection operation when the temperature is constant in FIG. 1, and FIG. 3 is a timing chart showing the operation when the temperature changes in FIG. 1. This is a timing chart. DESCRIPTION OF SYMBOLS 1...Sensor, 1a...Detection section, 1b...Refresh section, 2, 4...Operation amplifier circuit, 3...Temperature compensation element, 5...Clock output section, 6...Calculation section, R1,...R6 ...Resistor, C1, ...C3...Capacitor, D1...Diode, P1, ...P4...
...Terminal.

Claims (1)

[Claims] 1. A detection section whose impedance changes in accordance with changes in gas concentration, an oscillation section whose oscillation period changes in response to changes in the impedance of the detection section, and differentiating the oscillation output of the oscillation section. In a gas detection device comprising a differentiating circuit and a calculation unit that calculates gas concentration based on the differential output of the differentiating circuit, a compensating element for compensating for temperature changes is incorporated in the differentiating circuit, and A gas detection device characterized in that the differential characteristics of a differential circuit are changed.