JP2007240248A - Optical multiple gas concentration detection method and device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical multiple gas concentration detection method and device that are inexpensive and allow easy measurement. <P>SOLUTION: Modulation frequencies and sweeping ranges of a plurality of light sources 2 for modulating and sweeping a laser beam are made different, respective outputs of the light sources 5 are multiplexed, multiplexed laser beam is guided to a gas detection section 18 by an optical fiber 17, the transmitted light is guided to a photodetector 7 by an optical fiber 26, the received signals are sequentially phase-sensitively detected with modulation frequency every light source 5 to determine a gas signal waveform every light source 5, and concentrations of a plurality of kinds of objective gases having an absorption line in each sweeping range are determined based on these gas signal waveforms. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical multi-gas concentration detection method and apparatus that can be measured easily and inexpensively.

  As a sensor for detecting gas, there is an electric sensor such as a semiconductor type, but power supply equipment is required in the vicinity of the sensor, calibration is required periodically, and a sensor is required for each gas type to be measured. There are problems with maintainability and economy.

  On the other hand, there is a gas sensor using light. Gas molecules have the property of absorbing laser light having a specific wavelength (referred to as absorption band), and the presence or absence of gas can be detected using this property. This gas detection method is used in fields such as industrial measurement and pollution monitoring. By transmitting this laser light through an optical fiber, it is possible to detect gas remotely.

  Patent Document 1 discloses an optical gas concentration detection method and apparatus. In this technique, a laser beam having a modulated wavelength and intensity is oscillated by modulating a driving current of a semiconductor laser with a high-frequency sine wave around a predetermined current value. This laser light is incident on the optical fiber and guided to the gas detector, the gas of unknown concentration in the gas detector is transmitted, the transmitted light is guided to the receiver by the optical fiber, and the received light signal is phase-sensitively detected. A 1-fold detection signal and a 2-fold detection signal are obtained, a gas signal having a ratio of the 1-fold detection signal and the 2-fold detection signal is obtained, and the concentration of the target gas is obtained from the gas signal.

  In addition, as shown in FIG. 7, a method using a plurality of light sources that emit light having different wavelengths in order to cause light absorption in a plurality of different types of gases using the above-described optical gas concentration detection method and apparatus. There is also. In this method, light having different wavelengths from the light source units 101 a to 101 d is guided by being coupled to one optical fiber 102, and the light is converted into parallel light by the lens 103 and transmitted through the gas cell 104. The light transmitted through the gas cell 104 is reflected by the grating 105 at an angle corresponding to the wavelength and reaches each of the light receivers 106a to 106d. By calculating each received light signal by the method of Patent Document 1, it is possible to simultaneously measure the concentrations of a plurality of different gases (hereinafter referred to as multiple gases) filled in the same gas cell.

JP-A-5-256769

  In the optical multi-gas concentration detection method and apparatus of FIG. 4, an expensive grating 105 is necessary to detect multi-gas, and the light transmitted through the gas cell 104 is spatially propagated to the light receivers 106a to 106d. Therefore, it is difficult to adjust the positions of the lens 103, the grating 105, and the light receivers 106a to 106d, which takes time.

  Therefore, an object of the present invention is to provide an optical multi-gas concentration detection method and apparatus that solves the above-described problems and that can be easily measured with an inexpensive apparatus.

  In order to achieve the above object, the method of the present invention includes a plurality of light source units that modulate the wavelength and intensity of a laser beam with a sine wave and sweep the modulation center wavelength within a predetermined sweep range, respectively, with a modulation frequency and a sweep frequency. Different ranges are combined, and the outputs of these light sources are combined, and the combined laser light is guided to the gas detector by an optical fiber and transmitted into the measured atmosphere in the gas detector. The transmitted light is guided by an optical fiber to a light receiver, and the received light signal is sequentially phase-sensitively detected at a modulation frequency for each light source unit to obtain a gas signal waveform for each light source unit. The concentration of a plurality of types of target gases having absorption lines in each sweep range is obtained.

  The apparatus of the present invention is a plurality of light source units that modulate the wavelength and intensity of laser light with a sine wave and sweep the modulation center wavelength within a predetermined sweep range, each having a different modulation frequency and sweep range. Light source units, a multiplexer for combining the outputs of these light source units, a light source side optical fiber for guiding the combined laser light to the gas detection unit, and gas detection filled with a plurality of types of target gases A phase-sensitive detection of the optical fiber on the receiver side that guides the light transmitted through the atmosphere to be measured in the gas detector to the receiver, and the received light signal received by the receiver at the modulation frequency for each light source unit in turn. Then, a gas signal waveform for each light source unit is obtained, and a signal processing unit for obtaining concentrations of a plurality of types of target gases having absorption lines in their respective sweep ranges from these gas signal waveforms.

  The present invention exhibits the following excellent effects.

  (1) The device is inexpensive.

  (2) Easy measurement.

  Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings.

  As shown in FIG. 1, the optical multi-gas concentration detection apparatus 1 according to the present invention includes a light source system 2, a sensor system 3, and a signal processing unit 4.

  The light source system 2 is provided with a plurality of light source parts 5, in the present embodiment, three light source parts 5a, 5b, 5c. As shown in an enlarged view as a configuration common to each light source unit 5, the light source unit 5 includes a laser (for example, DFB-LD) 6 that instantaneously oscillates laser light having a single wavelength, and the laser 6. And a Peltier element 7 for applying a controlled temperature to the laser 6, a Peltier element power source 8 for controlling the temperature by driving the Peltier element 7, and a sine wave signal having a modulation frequency f. An oscillator 9 for output, a bias current source 10 for generating a bias current to be applied to the laser 6, a sweeper 11 for sweeping the bias current by a triangular wave or a sawtooth wave started at the rising edge of the clock T, and a bias current source The bias current from 10 is applied to the laser 6 and the sinusoidal signal is cut off, and the sine wave signal from the oscillator 9 is applied to the laser 6 and biased. And a capacitor 13 for blocking the flow.

  In the present invention, the light source units 5a, 5b, and 5c vary the modulation frequency and the center wavelength of the sweep range, respectively. Therefore, the sine wave signal Ax output from the oscillator 9 to the outside of the light source unit 5 is distinguished and shown as sine wave signals Aa, Ab, and Ac.

  The light source system 2 is provided with a multiplexer 14 in which the outputs of these light source units 5 a, 5 b, 5 c are connected to the inputs, and the output of the multiplexer 14 is connected to the branching device 15 of the sensor system 3. .

  The sensor system 3 includes a branching device 15 that splits the light source light from the multiplexer 14 into two branches, a reference optical path optical fiber 16 that is connected to one of the branches and guides the laser light without transmitting gas, and the other of the branches. The light source side optical fiber 17 connected to the detection location and the gas detection unit 18 that transmits the laser light from the light source side optical fiber 17 to a container filled with three kinds of gases of unknown concentration to be measured. And a light-receiving-side optical fiber 26 that guides the laser light transmitted through the atmosphere to be measured in the gas detection unit 18 to the light-receiving device 20. The target gases Ga, Gb, and Gc are, for example, methane (absorption center wavelength: 1.654 μm), hydrogen sulfide (1.570 μm), and carbon monoxide (1.567 μm).

  The signal processing unit 4 includes a light receiver 19 that receives the laser light z1 from the reference optical path optical fiber 16, a light receiver 20 that receives the laser light z2 from the light receiver-side optical fiber 26, and the respective light receivers 19, And a phase detector 21 having a phase detector connected to 20, and any one of the sine wave signals Aa, Ab, Ac from the light source units 5a, 5b, 5c to select the internal sine wave signal A as the phase detector 21. Switch 22 for supplying to signal, frequency multiplier 23 for supplying double signal B obtained by multiplying internal sine wave signal A to phase detector 21, and signal storage for temporarily storing the output signal of phase detector 21 The unit 24, the light sources 5a, 5b, 5c and the switch 22 output a clock T, and the calculation of the gas concentration of the target gas whose gas type is determined by switching the switch 22 from the stored signal of the signal storage unit 24 Processing part And a 5.

  Each phase detection circuit in the phase detection device 21 uses the sine wave signal A and the double signal B to perform phase-sensitive detection on the frequencies f and 2f, so that the single detection signal K (1f) and the double detection signal are detected. K (2f) is obtained. The arithmetic processing unit 25 obtains a gas signal waveform composed of a ratio of the 1 × detection signal K (1f) and the 2 × detection signal K (2f), obtains a peak value from the gas signal waveform, and obtains a peak value from the gas signal waveform. The concentrations of a plurality of types of target gases having absorption lines in the sweep range are obtained.

  Hereinafter, the operation of the optical multi-gas concentration detection apparatus 1 will be described.

  In each light source part 5a, 5b, 5c, the temperature of the Peltier element 7 is controlled by the power supply 8 for Peltier elements, so that the temperature of the laser 6 is fixed. Further, the sweeper 11 outputs, for example, a triangular wave during the time from the rising edge (see FIG. 3) to the falling edge of the clock T. The bias current source 10 generates a bias current in proportion to the triangular wave voltage. Thereby, the bias current is swept in one direction from a small value to a large value. At the same time, an alternating current output from the transmitter 9, that is, a sinusoidal modulation current is superimposed on the bias current and applied to the laser 6.

  As shown in FIG. 3, the current applied to the laser 6 in each of the light source units 5a, 5b, 5c is a triangular wave + sine wave. In the present invention, since the modulation frequency and the center wavelength of the sweep range are made different, the frequency of the sine wave superimposed on each triangular wave is different. The amplitude of each sine wave is the same. Further, since the width of the sweep range and the sweep speed are the same for each light source unit 5a, 5b, 5c, the time width and height of each triangular wave are equal. However, the center wavelength of the sweep range in the light source units 5a, 5b, and 5c is the center wavelength of the absorption spectrum of each target gas.

  Laser beams from the respective light source units 5 a, 5 b, and 5 c are multiplexed by a multiplexer 14, and are branched by a splitter 15 into a reference optical path optical fiber 16 and a light source side optical fiber 17. The laser light from the light source side optical fiber 17 is transmitted through the gas detector 18 and guided to the light receiver side optical fiber 26 and received by the light receiver 20 as laser light z2. The laser light of the reference optical path optical fiber 16 is received as it is by the light receiver 19 as the laser light z1, and each received light signal is input to the phase detection circuit of the phase detector 21.

  As shown in FIG. 2, the absorption wavelengths of the target gases Ga, Gb, and Gc have different center wavelengths. The laser light of each light source part 5a, 5b, 5c is swept within a sweep range including the center wavelength of the absorption spectrum of the target gas Ga, Gb, Gc while being modulated by the sine waves 1fa, 1fb, 1fc. Since the laser light transmitted through the gas detector 18 is absorbed along the absorption spectrum of the target gas Ga, Gb, Gc, the intensity changes with the signals 2fa, 2fb, 2fc that have been deformed based on the curve of the absorption spectrum. Laser light z2 to be generated.

  On the other hand, any one of the sine wave signals Aa, Ab, Ac from the light source units 5a, 5b, 5c is selected by the switch 22 and supplied to the phase detector 21 as the internal sine wave signal A, and a frequency multiplier. The doubled signal B is generated by 23 and supplied to the phase detector 21. The phase detection device 21 detects the 1 × detection signal K (1f) and the 2 × detection signal K (2f) and temporarily stores them in the signal storage unit 24. The arithmetic processing unit 25 calculates the signal read from the signal storage unit 24.

  The arithmetic processing unit 25 calculates the ratio z1K (2f) / z1K (1f) of the double detection signal z1K (2f) to the double detection signal z1K (1f) for the light reception signal of the laser beam z1, and shows it in FIG. And a ratio z2K (2f) / z2K (1f) of the double detection signal z2K (2f) to the single detection signal z2K (1f) with respect to the received light signal of the laser beam z2. ) To obtain a gas signal 42 of the target gas. Next, the difference of the gas signal z2K (2f) / z2K (1f) of the target gas with respect to the reference signal z1K (2f) / z1K (1f) is calculated to obtain the peak value determination signal 51 shown in FIG. Similarly, the reference signal 43 and the gas signal 44 of the target gas Gb are calculated, and a peak value determination signal 52 is obtained. Further, the reference signal 45 and the gas signal 46 of the target gas Gc are calculated, and a peak value determination signal 53 is obtained.

  5 are subtracted from the reference signals 41, 43, and 45 from the gas signals 42, 44, and 46 of the target gas, so that the light source unit 5, the multiplexer 14, and the branching unit are used. 15. The wavelength dependency of the signal processing unit 4 is removed, and this peak value determination signal is a gas signal of the target gas Ga, Gb, Gc in an accurate sense.

  The arithmetic processing unit 25 obtains a peak value that is the height from the minimum value to the maximum value of the peak value determination signal.

  After that, the arithmetic processing unit 25 applies the peak value to a relational expression or relation table between the peak value and the gas concentration that has been previously determined by using a reference gas having a known concentration to determine the gas concentration.

  As described above, in the present invention, a plurality of laser beams having different modulation frequencies and sweep ranges are combined, the combined laser beams are guided to the gas detection unit by an optical fiber, and the transmitted light is transmitted by an optical fiber. The light signal was guided to a light receiver, and the received light signal was sequentially phase-sensitively detected at different frequencies to obtain a gas signal waveform. That is, since the outputs of a plurality of light source units are combined and transmitted through the gas detection unit 18 to receive light, the center wavelength differs by performing phase-sensitive detection by switching the detection frequency while sharing the optical path. A gas signal in the sweep range can be obtained.

  In the present invention, since the laser light is guided by the optical fiber in all of the light source system 2 and the sensor system 3, there is no need for an expensive optical element as in the prior art, no complicated adjustment is required, and a plurality of types are stably used. Gas concentration can be detected.

  In the embodiment, three light source units 5 are used. However, two or four or more light source units 5 are used, and two or more types of gases are detected by changing the modulation frequency and the sweep range. can do.

  Further, the signal processing unit 4 detects the 1 × detection signal and the 2 × detection signal by performing phase sensitive detection on the frequencies f and 2f using the sine wave signal A and the 2 × signal B, and 1 × The ratio of the detection signal and the double detection signal is the gas signal. However, the present invention is not limited to this, and a signal representing the gas concentration can be obtained by phase sensitive detection.

  Next, an embodiment suitable for multipoint measurement will be described.

  As shown in FIG. 6, the optical multi-gas concentration detection device 61 according to the present invention includes a light source system 2, a sensor system 62, and a signal processing unit 63. The light source system 2 is exactly the same as that used in the optical multi-gas concentration detector 1 of FIG.

  The sensor system 62 includes a branching device 64 that branches the light source light from the multiplexer 14 into n branches, a reference optical path optical fiber 16 that is connected to one of the branches and guides the laser light without transmitting gas, and the remaining branches. N-1 light source side optical fibers 17 connected to a plurality of detection locations and filled with various gases of unknown concentration that are laser beam from the light source side optical fibers 17 N-1 gas detectors 18 that pass through the container, and n-1 receiver-side optical fibers 26 that guide the laser light that has passed through the atmosphere to be measured in each gas detector 18 to the light receiver 20. Prepare.

  The signal processing unit 63 includes a light receiver 19 that receives the laser light z <b> 1 from the reference optical path optical fiber 16, and n−1 light receivers 20 that receive the laser lights z <b> 2 to zn from the light receiver-side optical fiber 26. The phase detector 21 having n phase detection circuits connected to the respective light receivers 19 and 20 and any one of the sine wave signals Aa, Ab and Ac from the light source units 5a, 5b and 5c are selected. A switch 22 that supplies the phase detector 21 as the internal sine wave signal A, a multiplier 23 that supplies the phase detector 21 with the double signal B that has been multiplied by the internal sine wave signal A, and a phase detector 21 The signal storage unit 24 that temporarily stores the output signal, the clock T is output to each of the light source units 5a, 5b, and 5c and the switch 22, and the gas type is changed by switching the switch 22 from the stored signal of the signal storage unit 24. Target gas to be determined And a processing unit 25 for calculating the degree.

  The optical multi-gas concentration detection device 61 in FIG. 6 can detect the concentrations of a plurality of types of target gases at multiple points.

It is a block diagram of the optical multi-gas concentration detection apparatus which shows one Embodiment of this invention. It is the conceptual diagram which showed that the laser beam modulated in this invention is deform | transformed by an absorption spectrum. It is an operation | movement conceptual diagram of the light source part in this invention. It is a wave form diagram of the gas signal obtained by the present invention. It is a wave form diagram of the signal for peak value judgment obtained by the present invention. It is a block diagram of the optical multi-gas concentration detection apparatus which shows other embodiment of this invention. It is a block diagram of the conventional optical type multi gas concentration detection apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Optical type multiple gas concentration detection apparatus 4 Signal processing part 5,5a, 5b, 5c Light source part 14 Mux 17 Light source side optical fiber 18 Gas detection part 22 Switch 26 Light receiver side optical fiber

Claims (2)

  A plurality of light source units that modulate the wavelength and intensity of laser light with a sine wave and sweep the modulation center wavelength within a predetermined sweep range are made different in modulation frequency and sweep range, and the outputs of these light source units are combined. The combined laser beam is guided to the gas detector by the optical fiber and transmitted to the atmosphere to be measured in the gas detector, and the transmitted light of the gas detector is guided to the receiver by the optical fiber and received. The received light signal is sequentially phase-sensitively detected at the modulation frequency for each light source unit to obtain a gas signal waveform for each light source unit, and a plurality of types of objects having absorption lines in their respective sweep ranges from these gas signal waveforms An optical multi-gas concentration detection method characterized by determining a gas concentration.   A plurality of light source units that modulate the wavelength and intensity of laser light with a sine wave and sweep the modulation center wavelength within a predetermined sweep range, each having a different modulation frequency and sweep range, and A multiplexer that multiplexes the outputs of the light source unit, a light source side optical fiber that guides the combined laser beam to the gas detection unit, a gas detection unit that is filled with a plurality of types of target gases, and a target in the gas detection unit The optical fiber on the light receiver side that guides the light transmitted through the measurement atmosphere to the light receiver and the light reception signal received by the light receiver are sequentially detected by the phase sensitive detection at the modulation frequency for each light source unit, and the gas for each light source unit is detected. An optical multi-gas concentration detection apparatus comprising: a signal processing unit that obtains signal waveforms and obtains concentrations of a plurality of types of target gases having absorption lines in respective sweep ranges from these gas signal waveforms.

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