JP2878871B2 - Gas detector - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas for detecting components and concentrations of a mixed gas containing a plurality of types of gases by using a laser element such as a semiconductor laser whose oscillation wavelength changes continuously according to temperature. It relates to a detection device.

[0002]

2. Description of the Related Art It is known that the presence or absence of gas can be detected by utilizing the fact that laser light of a specific wavelength is easily absorbed by a certain kind of gas. Sensing technology using this principle is used for industrial measurement, pollution monitoring, etc. Widely used for As an example, the presence or absence of methane can be detected with high sensitivity by utilizing the fact that the 3.3922 μm oscillation line of the laser light generated by the He—Ne laser is strongly absorbed by methane. Since methane is the main component of city gas, leakage of city gas can be detected by detecting methane gas. However, when gas detection is performed using a semiconductor laser, the oscillation wavelength of the laser beam must be stabilized in accordance with the center wavelength of the absorption line of the gas because the laser element greatly changes the oscillation wavelength depending on the temperature.

[0003] Accordingly, the applicant of the present application filed an application on June 28, 1991 for a gas detection device in which the oscillation wavelength of a semiconductor laser device was accurately controlled, and FIG. 2 is a block diagram of a temperature control unit. is there.

In FIG. 2, a portion L ₂ of a laser beam generated from a semiconductor laser device 1 modulated by a current having a predetermined frequency component passes through a cell 2 filled with gas and passes through the cell 2. Laser light is detected by the photodiode 3, and the output current of the photodiode 3 is detected by a current-voltage converter 6 including a resistor 4 and an operational amplifier 5 as shown in FIG.
Is converted into a voltage as shown by a signal A. When the converted voltage is input to the phase-sensitive detector 7, a first-order phase-sensitive detection signal (f signal) (not shown) is output to the output terminal 8.

The output signal of the phase sensitive detector 7 is a resistor 9,
10, a signal obtained by removing the offset from the output signal of the phase sensitive detector 7 and sent to the offset adjuster 13 including the variable resistor 11 and the operational amplifier 12 is used as an error signal as a PID (Proportional, Integr) signal.
al and Differential), and the PID
Peltier device 15 is controlled by the output voltage of controller 14.
The surface temperatures of both surfaces are adjusted to predetermined temperatures.

In general, the f signal is the first-order derivative of the output signal of the current-to-voltage converter 6, and the offset is determined by the slope of the input / output characteristics of the semiconductor laser device when the output increases as the input current increases. Occur.

The near-infrared semiconductor laser device 1 has a characteristic that the wavelength of the laser beam becomes longer when the temperature of the laser device itself becomes higher, and the wavelength becomes shorter when the temperature of the laser device itself becomes lower. The wavelength of the laser light is controlled by controlling the temperature of the Peltier element 15 attached to the laser element 1. PID controller 1
Reference numeral 4 denotes a signal required to control the temperature of the Peltier element 15 as a control target by adding the input signal proportionally, integrated, and differentiated at a predetermined ratio. For detection of a gas having a specific absorption wavelength, a secondary phase-sensitive detection signal (2f signal) as shown by a signal B in FIG. 3 is used. A signal B in FIG. 3 represents an example of the characteristics of the output voltage of the current-voltage converter with respect to the temperature of the semiconductor laser device and the secondary output voltage of the phase-sensitive detector. The horizontal axis is the temperature of the semiconductor laser device, and the vertical axis is the output voltage. Here the peak group P
Although ₁ and the horizontal axis P ₂ is a temperature, the temperature is proportional to the wavelength, the position of these peaks P ₁ and P ₂ so compatible that the absorption wavelength of the gas in the cell 2 of the gas The component and its concentration can be detected from the peak height.

The phase sensitive detector 7 extracts only a component having a specific frequency and a specific phase from a signal to be measured, and demodulates it. The curve B shown in FIG.
f signal, which corresponds to the signal A that is second-order differentiated with respect to wavelength. The 2f signal is used because when the laser beam stabilized at the center of the absorption line of methane is received by the photodiode 3, unless the other conditions such as temperature and pressure change, the intensity of the 2f signal indicates the methane concentration. Because it is proportional to

As another method, a thermocouple is attached to a side surface of a heat conductive plate 16 between the semiconductor laser device 1 and the Peltier device 15, and a voltage obtained from the thermocouple and a voltage of a measurement gas are measured. A method has been proposed in which the oscillation wavelength of the semiconductor laser device 1 is controlled by controlling the applied voltage of the Peltier device 15 based on the difference from the output voltage of a voltage generator that generates a voltage corresponding to the wavelength.

[0010]

However, in the conventional gas detecting device including the gas detecting device shown in FIG. 2, since the oscillation wavelength of the semiconductor laser element is constant in any method, one type corresponding to the wavelength is used. Therefore, when detecting a mixed gas, a plurality of gas detectors having oscillation wavelengths corresponding to each gas are required.

The present invention has been made in view of the above points, and an object of the present invention is to detect the components and concentrations of a mixed gas containing a plurality of types of gases with a single gas detection device.

[0012]

SUMMARY OF THE INVENTION According to the present invention, there is provided a laser whose oscillation wavelength continuously changes in accordance with the temperature, and a laser beam generated by the laser.
The multiple atmospheres to be measured as reference atmospheres
A cell in which the laser is sealed, voltage generating means for selectively generating a plurality of different voltages according to the absorption wavelength of the reference atmosphere, and the laser selecting based on an output of the voltage generating means.
A temperature control means for controlling the temperature of the laser so as to oscillate a laser beam of a selected wavelength.

[0013]

According to the gas detecting device of the present invention, a plurality of different voltages can be selected by the voltage generating means according to the absorption wavelength of the gas in the reference atmosphere through which the laser beam whose wavelength changes according to the temperature of the laser element. Then, based on the output of the voltage generating means, the temperature of the laser element becomes a predetermined temperature which differs for each gas in the atmosphere, and a laser beam having a selected wavelength is oscillated.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings.

FIG. 1 is a schematic configuration diagram of one embodiment of a temperature control unit of a gas detection device according to the present invention. In this embodiment, there are three types of gases in the atmosphere to be detected.
A mixed gas described later is a mixture of three types of gases.

The temperature control unit of the gas detector includes a semiconductor laser element 20, a cell 21, a photodiode 22,
It comprises a first temperature control unit 23 and a second temperature control unit 24. The first temperature control unit 23 includes a Peltier device 25, a window comparator 26, a PID controller 27, a first voltage generator 28, and a delay buffer 2
9, the second temperature control unit 24 includes a Peltier device 30, a current-voltage converter 31, a phase-sensitive detector 32, a second voltage generator 33, a subtractor 34,
And a PID controller 35.

The semiconductor laser device 20 is mounted on a Peltier device 30 via a heat conductive plate 36. The Peltier device 30 is bonded to a heat conductive plate 38 having a thermocouple 37 mounted on a side surface. The Peltier element 25 is mounted on the opposite side (lower side in the figure) of the heat conduction plate 38.

The semiconductor laser element 20 is a phase sensitive detector 3
The laser is modulated at a frequency equal to the modulation frequency of 2, and its oscillation wavelength has a characteristic that it becomes longer when the temperature of the laser element itself is high and becomes shorter when the temperature of the element itself is low as described above. A part L _{2 of the} laser light emitted from the semiconductor laser element 20 (the laser light emitted in the left direction in the figure) passes through a cell 21 in which a mixed gas is sealed as a reference atmosphere.

Generally, a Peltier element is an element utilizing the Peltier effect. When two kinds of metals or semiconductors are joined and an electric current is applied, not only Joule heat is generated but also heat is generated at the junction or heat is generated. Absorption occurs. In this phenomenon, when the direction in which the current flows is reversed, heat generation and heat absorption are reversed.

FIG. 4 is an explanatory diagram for explaining the Peltier effect. As shown in the figure, a P-type semiconductor and an N-type semiconductor are connected by a conductor a, and a P-type semiconductor electrode b and an N-type semiconductor electrode c.
When the battery E is connected between the electrodes, the electrode a generates heat and the electrodes b and c absorb heat. By combining a plurality of such elements, the amount of heat generated and the amount of heat absorbed can be increased. By changing the voltage applied to the Peltier element or inverting the polarity, the temperature of the semiconductor laser element 20 provided near the Peltier elements 25 and 30 can be changed, and the wavelength of the laser beam can be changed. . In addition,
The surfaces of the Peltier elements 25 and 30 that are joined to the heat conducting plate 38 simultaneously generate or absorb heat.

In this embodiment, the thermocouple 37 utilizes the Seebeck effect (a phenomenon in which a current flows through a metal in accordance with the temperature when the temperature of the connection point between two metals is raised). The one end is grounded, and the voltage at the other end is input to the window comparator 26.

The window comparator 26 is composed of an operational amplifier 41a, a subtractor 41 composed of resistors 41b and 41c, and a comparator 42.
The voltage generator 28 has variable resistors as many as the types of gases to be detected. If it is assumed that three types of gases are to be detected, the variable resistors 43 to 45 and these variable resistors 43 to 45 And a switch 46 for switching between. The subtractor 41 calculates the output voltage V of the thermocouple 37.
_37, on the basis of the difference between the voltage V ₂₈ of the switch 46 connected to the slider of the variable resistor 43 to 45 is set to a voltage corresponding to the absorption wavelength of the gas as measurement atmosphere in the cell 21 signals Is output. Although the number of the switches 46 is three in the drawing, the number is not limited thereto, and may be increased or decreased according to the type of gas to be detected. The subtracter 41 outputs a voltage obtained by subtracting the voltage V ₂₈ from the output voltage V 37 of the thermocouple ₃₇ to the PID controller 27 as an error signal. When the output voltage V _{37 of} the thermocouple ₃₇ , that is, the temperature of the junction of the heat conduction plate 38 is out of the predetermined range, the window comparator 26 outputs a signal of, for example, “0” logic level to the delay buffer 2.
9, and outputs a signal of logic level “1” to the delay buffer 29 when the temperature of the junction of the heat conductive plate 38 is within a predetermined range. Since it takes time for the temperature of the semiconductor laser device 20 to stabilize due to the temperature control of the semiconductor laser device 20 by the Peltier device 25, the output signal from the window comparator 26 is output by the delay buffer 29 by the output voltage V _{37 of the} thermocouple _37. The signal is input to the relay 47 after waiting for a period of time until stabilization.

The current-to-voltage converter 31 includes a resistor 48 and an operational amplifier 49.
Is converted to a voltage.

The phase sensitive detector 32 modulates the signal to be measured at a specific frequency as described above, extracts only a component having a specific frequency and a specific phase from the modulated signal, and demodulates it. ,Output. Thereby, extremely high S /
A small signal can be detected at the N ratio.

In general, when a laser beam modulated at a specific frequency passes through a gas that absorbs light of a specific wavelength and is received by a sensor, the output signal of the sensor has a DC component and the same as the modulation frequency. It consists of a fundamental wave component having a frequency and its harmonic components. When the fundamental wave component and the second harmonic component are phase-sensitive detected by the phase-sensitive detector, the first derivative and the second derivative as shown in Expressions 5 and 6, respectively, are obtained.
A signal corresponding to the second derivative can be obtained. The secondary differential signal is used for detecting a gas having a specific absorption wavelength and measuring the concentration of the gas, and the primary differential signal is used for stabilizing the oscillation frequency of the laser light to a specific frequency.

In general, the oscillation angular frequency Ω of a semiconductor laser is a function of the temperature and the drive current. When the drive current is modulated at an angular frequency ω (2πf) with the temperature kept constant, Ω becomes
At the angular frequency ω.

[0027]

Ω = Ω ₀ + ΔΩ cosωt where Ω ₀ is the central oscillation angular frequency of the semiconductor laser, ΔΩ
Is the oscillation angular frequency modulation amplitude, and ω is the modulation frequency.

The oscillation angular frequency modulation amplitude [Delta] [omega is Taylor expand the transmittance T of the laser beam is represented by the number 2 in the Omega = Omega ₀ is smaller, the number 4 is calculated up to the second order term of [Delta] [omega.

[0029]

T = exp (−αcL) where α is the absorption coefficient of methane, c is the concentration (partial pressure) of methane, L is the optical path length, and T is the transmittance of laser light (the output current of the photodiode 22). Is proportional to). In particular, α has a Lorentz-type frequency-dependent characteristic as shown in the following Expression 3 when focusing on one absorption line at atmospheric pressure.

[0030]

Α = γ ₂ α ₀ / ((Ω−ω _m ) ² + γ ² ) where ω _m is the center frequency of the absorption line, and α ₀ and γ are constants.

[0031]

[Number 4] _{T = T 0 + (ΔΩ)} 2 T 0 "/ 4 + ΔΩT 0 'cosωt + ((ΔΩ) 2 T 0" cos2ωt) / 4 + ... , and the addition of the DC component, the component that varies according to cosωt and cos2ωt Have. Where T ₀ , T ₀ ′, T
₀ ″ is the _value of the transmittance T at Ω = Ω ₀ , its first derivative dT / dΩ, and its second derivative d ² T / dΩ ² , respectively, and is given as Equations 5, 6, and 7 below. .

[0032]

T ₀ = 1−γ ² α ₀ cL / [(Ω ₀ −ω _m ) ² + γ ² ]

[0033]

T ₀ ′ = 2 (Ω ₀ −ω _m ) γ ² α ₀ cL / [(Ω ₀ −ω _m ) ² + γ ² ] ²

[0034]

T ₀ ″ = −2 [3 (Ω ₀ −ω _m ) ² −γ ² ] γ ² α ₀ cL / [(Ω ₀ −ω _m ) ² + γ ² ] ³ Equation 7 corresponding to the phase-sensitive detection signal f of
Corresponds to the second-order phase-sensitive detection signal 2f.

The subtracter 34, resistors 51 and 52 is constituted by an operational amplifier 53, the voltage PID controller 35 drawn from the output voltage of the phase sensitive detector 32 the voltage V ₃₃ of the second voltage generator 33 To send to.

The second voltage generator 33 has one end grounded and the other end connected to a power supply.
56, and a switch 57 for switching each of the variable resistors 54 to 56. The switch 57 is linked to the switch 46 described above. Semiconductor laser device 20
Has an offset in the input / output characteristics as described above. This offset needs to be removed because it varies depending on the type of the semiconductor element 20. Variable resistor 5
The voltage at each of the sliders 4 to 56 corresponds to an offset corresponding to the oscillation wavelength of the semiconductor laser device 20 regulated by each of the variable resistors 43 to 45 of the first voltage generator 28, that is, the phase-sensitive detector 32. And is applied as a voltage V ₃₃ to the subtractor 34 via the switch 57.

The relay 47 applies the output voltage or current of the PID controller 35 to the Peltier element 30 only when the output of the delay buffer 29 is "1".

Next, the operation of the gas detector according to the present invention will be described.

The mixed gas to be detected is, for example, a mixed gas of methane having a mass number of 12 ( ¹² CH ₄ ) and methane having a mass number of 13 ( ¹³ CH ₄ ). Prior to gas detection, the semiconductor laser device 20 detects the absorption wavelength of ¹² CH ₄ (1.
The slider of the variable resistor 43 is adjusted so as to generate a voltage that oscillates a wavelength corresponding to 65373 μm), and the semiconductor laser element 20 oscillates a laser beam having a wavelength equal to the absorption wavelength of ¹² CH _4. At this time, the slider of the variable resistor 54 is adjusted so as to generate a voltage equal to the offset of the primary phase-sensitive detection signal output from the phase-sensitive detector 32. Similarly, the slider of the variable resistor 44 is adjusted so that the semiconductor laser element 20 generates a voltage such that the semiconductor laser element 20 oscillates a wavelength corresponding to the absorption wavelength of ¹³ CH ₄ (1.63133 μm). When the laser 20 oscillates a laser beam having a wavelength equal to the absorption wavelength of ¹³ CH ₄ , the variable resistor 55 generates a voltage equal to the offset of the primary phase-sensitive detection signal output from the phase-sensitive detector 32.
Adjust the slider of.

First, the switch 46 of the first voltage generator 28 and the second voltage generator 33 to detect ¹² CH ₄
Is switched to the variable resistors 43 and 54 in advance.

In FIG. 1, the temperature of the semiconductor laser device 20 is converted into a voltage V ₃₇ by a thermocouple 37 and input to a window comparator 26. The window comparator 26 compares the output voltage V ₃₇ of the thermocouple ₃₇ with the voltage V ₂₈ generated by the variable resistor 43 of the first voltage generator 28 (V ₂₈ −V ₃₇ ), and calculates the voltage V of the thermocouple 37. _{When the} voltage exceeds the voltage V ₂₈ , for example, a negative voltage is sent to the PID controller 27, and the PID controller 27 applies a voltage or a current that decreases the temperature of the connection point of the thermocouple 37 to the Peltier device 2.
5, the temperature of the semiconductor laser device 20 decreases. Conversely, when the voltage V ₃₇ becomes lower than the voltage V ₂₈ , a positive voltage is sent to the PID controller 27 and
The ID controller 27 applies a voltage or a current of the opposite polarity to increase the temperature of the connection point of the thermocouple 37 to the Peltier device 2.
5, the temperature of the semiconductor laser device 20 rises, and as a result, the semiconductor laser device 20 reaches a predetermined temperature. As a result, the semiconductor laser device 20
Although the laser light of the wavelength regulated by the above is oscillated, it takes some time for the control.

The window comparator 26 sends a "1" or "0" logic level signal to the delay buffer 29. The delay buffer 29 sends a signal of “1” logic level to the relay 47 after a predetermined time after receiving a signal of “1” logic level from the window comparator 26.

On the other hand, a part L _{2 of the} laser light from the semiconductor laser element 20 passes through the cell 21 in which gas is sealed, and is incident on the photodiode 22. Photodiode 2
The output current of the laser light detected in 2 is a current-voltage converter 3
1 converts it to a voltage. When the converted voltage is input to the phase-sensitive detector 32, a first-order phase-sensitive detection signal (f signal) is output to a terminal 50 and also to a subtractor. The voltage generated by the variable resistor 54 of the second voltage generator 33 is also applied to the subtractor 34, and a signal from which the offset voltage is subtracted by the voltage generated by the variable resistor 54 is used as an error signal as a PID controller. The output voltage of the PID controller 35 is sent to the relay 47. The output voltage or current of the PID controller 35 is applied to the Peltier element 30 only when the signal sent from the delay buffer 29 to the
Becomes a predetermined temperature. As a result, the first temperature control unit 23 performs approximate temperature control of the semiconductor laser device 20, and the second temperature control unit 24 performs precise temperature control. That is, since double temperature control is performed, not only accurate temperature control is performed, but also there is no influence of a sudden change in external temperature, and laser light having a wavelength of 1.65373 μm is accurately emitted from the semiconductor laser element 20. Because it is oscillated
¹² CH ₄ is detected.

Next, the switches 46 and 57 are switched to detect ¹³ CH ₄ and connected to the variable resistors 44 and 55, and the first voltage generator 28 supplies the semiconductor laser device 2.
0 generates a voltage that oscillates a wavelength corresponding to the absorption wavelength of ¹³ CH ₄ , and is used as an offset of the primary phase-sensitive detection signal output from the phase-sensitive detector 32 from the second voltage generator 33. Generates equal voltage. After the same control is performed, 1.65313 μm is accurately output from the semiconductor laser device 20.
Since a laser beam having a wavelength of m is oscillated, ¹³ CH ₄ is detected.

Each of the signals A and B in FIG. 3 described above indicates that ( ¹² CH ₄ = 60%, ¹³ CH ₄ = 40) of the methane mixed gas.
%) Output, and the peak group P ₁ of the signal B corresponds to ¹² CH ₄ , and the peak group P ₂ corresponds to ¹³ CH ₄ .
The amplitude of peaks P ₁ represents a mixing ratio of 60% of the methane gas, the amplitude of the peak-gun P ₂ represents a 40% mixing ratio. In addition, the composition of a mixed gas in which ¹³ CH ₄ was mixed at 1% and ¹² CH ₄ at 99% was measured. Further, in the present embodiment, methane was detected, but the present invention is not limited to this.
Acetylene and other gases can be detected by selecting the type of semiconductor laser element.

The output temperature of the thus semiconductor laser device 20 with a thermocouple 37 as a voltage V _37, the Peltier element 25 which is attached to the semiconductor laser device 20 based on the difference between the voltage V ₃₇ and the voltage V ₂₈ Controlling the approximate temperature and detecting the laser light emitted from the semiconductor laser element 20 by the photodiode 22 via the reference atmosphere in the cell 21;
It converted into a voltage V ₃₂ via a phase sensitive detector 32, voltage V
By controlling the temperature of the Peltier device 30 attached to the semiconductor laser device 20 on the basis of the difference from ₃₃ , the oscillation wavelength of the semiconductor laser device 20 is accurately controlled, and the voltage V _{28 of} the first voltage generator ₂₈ and By switching the voltage V33 of the second voltage generator ₃₃ with the switches 46 and 57, respectively, it is possible to detect a plurality of types of mixed gas.

[0047]

Effect of the Invention] As described above, in the present invention, a laser for varying the oscillation wavelength continuously in accordance with the temperature, position the laser beam generated by the laser passes through location
Gases to be measured as reference atmospheres
And a voltage generating means for selectively generating a plurality of different voltages according to the absorption wavelength of the reference atmosphere, and the laser is selected based on an output of the voltage generating means.
Since a temperature control means for controlling the temperature of the laser so as to oscillate a laser beam having a wavelength is provided, a single gas detection device can detect a plurality of types of gas components.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of an embodiment of a temperature control unit of a gas detection device according to the present invention.

FIG. 2 is a block diagram of an example of a gas detection device according to the present applicant.

FIG. 3 is a graph showing an example of a characteristic of an output voltage of a current-voltage converter and an output voltage of a phase-sensitive detector with respect to a temperature of a semiconductor laser element used in the gas detection device of the present invention.

FIG. 4 is an explanatory diagram for explaining a Peltier effect.

[Description of Signs] 20 Semiconductor laser element 21 Cell 22 Photodiode 23 First temperature control unit 24 Second temperature control unit 25, 30 Peltier element 37 Thermocouple

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G01N 21/00-21/01 G01N 21/17-21/61

Claims (3)

(57) [Claims] 1. A laser whose oscillation wavelength changes continuously according to temperature, and a laser beam generated by the laser passes therethrough.
To be measured as a reference atmosphere.
A cell filled with a number of gases, voltage generating means for selectively generating a plurality of different voltages according to the absorption wavelength of the reference atmosphere, and the laser based on the output of the voltage generating means.
A gas control device for controlling a temperature of the laser so that the laser oscillates a laser beam having a selected wavelength . 2. The temperature control means according to claim 1, wherein said laser is
Temperature that oscillates at the absorption wavelength of the preselected gas to be measured
The first is to control the temperature of the laser so as to be in the vicinity of degrees.
The temperature control unit and the laser
Removing the offset component corresponding to the oscillation wavelength,
The laser so that the oscillation wavelength of the laser becomes the absorption wavelength.
The gas detection device according to claim 1, further comprising a second temperature control unit that controls the temperature of the gas. 3. The method according to claim 2, wherein the second temperature control section is configured to control the first temperature.
The control operation is started after a predetermined time delay from the control unit.
3. The gas detection device according to 2 .