JP5278757B2 - Laser gas analyzer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a laser type gas analyzer having an inexpensive and simple structure and measuring a gas component such as SO<SB>2</SB>, NO, or NO<SB>2</SB>, which has a light absorbing spectrum inherent to a medium infrared region which has been impossible to be measured heretofore. <P>SOLUTION: In the laser type gas analyzer, a laser drive signal generating part makes an output time of an offset signal S2 long with respect to an output time of a variable drive signal S1 of a wavelength scanning drive signal to shorten a heat generating time of a laser element and a temperature stabilizing means performs the temperature adjustment of a light source and a light detecting part so as to set a stable range temperature stabilizing the light source and the light detecting part to stably perform the detection and emission of light. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to a laser gas analyzer that analyzes the presence and concentration of various gases to be measured in a flue.

A conventional laser gas analyzer will be described. First, the principle of gas concentration measurement of the laser gas analyzer will be described. FIG. 5 is a characteristic diagram showing an example of an absorption spectrum of NH ₃ gas. It is known that each gaseous gas molecule has a unique optical absorption spectrum. For example, as shown in the characteristic diagram of the absorption spectrum of NH ₃ (ammonia) gas in FIG. Is different.

  Therefore, the laser gas analyzer irradiates the gas with laser light, absorbs the laser light having a specific wavelength in proportion to the gas concentration, and measures the gas concentration based on the amount of absorption. Measurement methods of such a laser gas analyzer are broadly divided into a two-wavelength difference method and a frequency modulation method. The present invention relates to a frequency modulation system.

Next, the measurement principle of the frequency modulation type laser gas analyzer will be described. FIG. 6 shows a principle diagram of the frequency modulation method. The laser gas analyzer of the frequency modulation method, the center frequency f _c, the output light of the semiconductor laser is frequency-modulated at a modulation frequency f _m, is irradiated to the measurement target gas. Here, the frequency modulation is to make the waveform of the drive current supplied to the semiconductor laser sine wave.

To cancel the effect of the distance in the frequency modulation method is at the same time the frequency f _m if the frequency modulated output of the semiconductor laser device is the may be performed amplitude modulation and multiplied by frequency modulation to the output of the semiconductor laser element amplitude This can also be used since it is also modulated.

As shown in FIG. 6, since the absorption lines of the gas is almost quadratic function with respect to the modulation frequency, the absorption line plays the role of a discriminator, the light receiving portion of the double modulation frequency f _m A frequency signal (double frequency signal) is obtained. Since the modulation frequency f _m good at any frequency, for example, choose the modulation frequency f _m to several kHz, a digital signal processor (DSP) or using a general-purpose processor, extraction of the double frequency signal It is possible to perform advanced signal processing such as.

  In addition, if envelope detection is performed by the light receiving unit, the fundamental wave by amplitude modulation can be estimated, and the ratio between the amplitude of the fundamental wave and the amplitude of the double frequency signal can be detected in phase synchronization to measure regardless of the distance. A signal proportional to the target gas concentration can be obtained.

  Next, the structure of the frequency modulation type laser gas analyzer will be described. FIG. 7 is a structural diagram showing a laser gas analyzer of the prior art and shows the overall configuration. In the figure, flanges 201a and 201b are fixed by welding or the like to walls 101a and 101b such as pipes through which the gas to be measured flows, such as a flue. A bottomed cylindrical cover 203a is attached to one flange 201a via a mounting seat 202a.

  A light source unit 204 is disposed inside the cover 203a. Laser light emitted from the light source unit 204 is collimated into parallel light by a light source side optical system including a collimator lens 205, passes through the center of the flange 201a, and passes through the wall 101a. , 101b (inside the flue). The parallel light is absorbed when it passes through the measurement target gas inside the walls 101a and 101b.

  A bottomed cylindrical cover 203b is attached to the other flange 201b via a mounting seat 202b. The parallel light passing through the inside of the flue is condensed by a condensing lens 206 that is a light receiving side optical system inside the cover 203b, received by a light receiving unit 207, converted into an electric signal, and input to a signal processing circuit 208 at a subsequent stage. Is done.

  Next, the light source unit will be described. FIG. 8 shows the configuration of the light source unit 204. The light source unit 204 includes a wavelength scanning drive signal generation unit 204a that changes the emission wavelength of the laser element so as to scan the absorption wavelength of the measurement target gas, and about 10 kHz, for example, to detect the absorption wavelength of the measurement target gas. And a laser drive signal generator 204 s composed of a harmonic modulation signal generator 204 b for frequency-modulating the wavelength with a sine wave, and the output signals of these signal generators 204 a and 204 b are combined to perform laser drive. A signal is generated. The laser drive signal is converted into a current by the current control unit 204c and supplied to the laser element 204e made of a semiconductor laser.

  The laser element 204e is provided with temperature stabilization means. This temperature stabilization means includes a temperature control unit 204d, a thermistor 204f, and a Peltier element 204g. A thermistor 204f as a temperature detection element is disposed in the vicinity of the laser element 204e, and a Peltier element 204g is disposed in the vicinity of the thermistor 204f. The Peltier element 204g is controlled by the temperature control unit 204d so that the resistance value of the thermistor 204f becomes a constant value, and as a result, operates so as to stabilize the temperature of the laser element 204e.

Here, as shown in FIG. 9, the wavelength scanning drive signal output from the wavelength scanning drive signal generation unit 204a is a substantially trapezoidal signal repeated at a constant cycle.
In FIG. 9, a signal S1 is a signal for scanning the absorption wavelength, and the emission wavelength of the laser element 204e is changed by linearly changing the magnitude of the current supplied to the laser element 204e via the current control unit 204c. This is a part that is gradually shifted to enable scanning. For example, in the case of ammonia gas, it is a portion that can scan a line width of about 0.2 nm.
The signal S2 is an offset portion that does not scan the absorption wavelength but causes the laser element 204e to emit light, and is set to a value equal to or higher than a threshold current value at which the light emission of the laser element 204e is stabilized.
The signal S3 is a trigger portion where the drive current is almost zero.

Then, the laser scanning signal is generated by synthesizing the high frequency modulation signal from the high frequency modulation signal generating unit 204b with the wavelength scanning driving signal output from the wavelength scanning driving signal generating unit 204a. As shown in FIG. 10, the laser drive signal has a high frequency modulation signal frequency of 10 kHz, a wavelength scanning drive signal frequency of 50 Hz, λ ₁ is a wavelength corresponding to an offset, and λ ₂ and λ ₃ are NH ₃ gas. The upper and lower limit values of the scanning range corresponding to the absorption wavelength are shown.

Next, FIG. 11 shows the configuration of the light receiving unit 207 and the signal processing circuit 208.
Note that the light receiving unit 207 is formed of, for example, a photodiode, and a light receiving element having sensitivity to the emission wavelength of the laser element 204e is used. The output current of the light receiving unit 207 is converted into a voltage by the I / V converter 208a and input to the synchronous detection circuit 208b to which the 2f signal (double wave signal) from the oscillator 208c is added.

In the synchronous detection circuit 208b, when the laser light is not absorbed by the measurement target gas, the double detection signal is not detected by the synchronous detection circuit 208b, and therefore the output of the synchronous detection circuit 208b is almost a straight line.
On the other hand, when the laser light is absorbed by the measurement target gas, a double wave signal that is a signal obtained by extracting only the amplitude of the double frequency component of the modulation signal of the emitted light is detected by the synchronous detection circuit 208b. The output waveform is as shown in the output waveform of the synchronous detection circuit 208b shown in the rectangular frame of FIG. Noise is removed from the amplitude by the filter 208d, and the amplitude is appropriately amplified and output to the arithmetic processing unit 209 such as a CPU or DSP in the subsequent stage. Since the peak value of the output waveform of the synchronous detection circuit 208b corresponds to the concentration of the measurement target gas, the peak value is measured, or part or all of the waveform is integrated and the concentration of the measurement target gas is determined from the integrated value. What is necessary is just to detect.

Next, a method for measuring the concentration of the measurement target gas will be described.
First, the temperature of the laser element 204e in FIG. 8 is detected by the thermistor 204f in advance, and the measurement target gas (for example, NH ₃ gas) can be measured at the central portion of S1 of the wavelength scanning drive signal shown in FIG. The temperature control unit 204d in FIG. 8 controls the energization of the Peltier element 204g to adjust the temperature of the laser element 204e so that the absorption characteristic can be obtained. The Peltier element 204g is controlled by PID control or the like so that the resistance value of the thermistor 204f becomes a constant value. Under such setting conditions, the laser element 204e is driven, the laser light is emitted into the space where the measurement target gas exists in the walls 101a and 101b, and the condensed light is incident on the light receiving unit 207 as described above. Gas analysis is performed with proper signal processing. As described above, according to the present embodiment, the light source unit 204 can scan the emission wavelength of the laser element 204e over a predetermined range and measure the gas concentration with the measurement target gas.

As another conventional technique of a laser gas analyzer, for example, the invention described in Patent Document 1 (International Publication No. WO2008 / 096524, title of the invention “Laser Gas Analyzer”) is known.
The laser gas analyzer of the prior art described in Patent Document 1 also performs detection in the same manner as described above.

International Publication No. WO2008 / 096524

In recent years, a quantum cascade laser capable of continuous oscillation at room temperature (hereinafter referred to as QCL, which is an abbreviation for quantum cascade laser), which is a kind of semiconductor laser, has been put into practical use. QCL can emit light in a wide range of wavelengths in the mid-infrared region (4 to 10 μm), which could not be realized with conventional semiconductor lasers. By using this QCL, a laser-type gas, such as SO ₂ , NO, NO _2, etc., which is impossible in the prior art, such as measuring gas components whose absorption wavelength is included in the mid-infrared laser beam. It can be an analyzer.

FIG. 12 shows an example of the absorption spectrum of SO ₂ gas. In the case where SO ₂ concentration measurement is performed using the same apparatus configuration as that of the frequency modulation method, the QCL wavelength is preferably set to 7.2 to 7.4 μm. As the light receiving element, it is preferable to use an infrared detecting element having sensitivity in the wavelength region, for example, an MCT (Mercury_Cadmium_Tellurium) photoconductive element (hereinafter referred to as MCT).
FIG. 13 shows the output of the synchronous detection circuit 208b when measuring the SO ₂ concentration. By measuring the peak amplitude of this output, the SO ₂ concentration can be measured.

Now, QCL requires a larger driving current than a semiconductor laser used in a conventional laser gas analyzer, so that the heat radiation of the light source unit 204 becomes large, and light emission becomes unstable. .
In addition, MCT has characteristics such that it receives noise due to background radiation of 300K, and sensitivity changes due to changes in ambient temperature.
FIG. 14 shows an example of SO ₂ concentration measurement when noise and temperature are not taken into consideration. Thus, measurement stability may not be obtained, but this is considered to be due to the above characteristics of QCL and MCT.

  Therefore, in order to prevent such a phenomenon, advanced noise countermeasures and temperature control are required, but the device configuration of the light emitting unit and the light receiving unit relating to the signal processing circuit and the cooling performance becomes complicated and expensive. There was a problem that. In particular, when QCL emits light continuously, the heat generation of the light source part is excessive, and the heat dissipation is insufficient in the conventional configuration, so that temperature control by the Peltier element is difficult.

  Therefore, when QCL is used, for example, as shown in FIG. 15, the light emitting part having the flange 301, the lens holder 302, the laser mount 303, and the light emitting part circuit box 306 is cooled by the heat radiation fin 304 and the air cooling fan 305. Yes. Thus, it is necessary to additionally provide the radiation fins 304, the air cooling fan 305, etc., resulting in a high cost and complicated structure. Simplification of structure and cost reduction are required.

Therefore, the present invention has been made in view of the above-described problems, and the object thereof is SO ₂ , which has a light absorption spectrum unique to the mid-infrared region, which has been impossible in the past, with an inexpensive and simple structure. An object of the present invention is to provide a laser type gas analyzer that measures gas components such as NO and NO ₂ .

A laser type gas analyzer according to claim 1 of the present invention comprises:
A light source unit that emits a frequency-modulated mid-infrared laser beam, a light source side optical system that collimates the light emitted from the light source unit, and a light source side optical system that propagates through the space where the measurement target gas exists A light receiving side optical system for collecting the transmitted light, a light receiving unit for receiving the light collected by the light receiving side optical system, and a signal processing circuit for processing an output signal of the light receiving unit, In the laser gas analyzer in which the signal processing circuit measures the concentration of the measurement target gas based on the output signal of the light receiving unit,
The light source unit is
A laser element emitting mid-infrared laser light;
Temperature stabilizing means for stabilizing the temperature of the laser element by a Peltier element ;
A signal for gradually changing the emission wavelength of the laser element by linearly changing the supply current to the laser element, and making the emission wavelength of the laser element variable so as to scan the absorption wavelength of the gas to be measured A variable drive signal and a signal having a value such that a current less than a threshold current value of the laser element is supplied to the laser element, and an offset for stopping the light emission of the laser element so as to reduce the amount of heat generated by the laser element A signal having a value such that a current less than a threshold current value of the laser element is supplied to the laser element, and a signal inserted between the one variable drive signal and the one offset signal. and a trigger signal for synchronization, the unit waveform consisting of the wavelength scanning driving signal is a signal which is repeated at a fixed period, modulating the emission wavelength Te A laser driving signal generator for outputting a laser driving signal by combining the order of the high-frequency modulation signal,
A current controller that converts the laser drive signal output from the laser drive signal generator into a current and supplies the current to the laser element;
With
The light receiving unit is
A light-receiving element having sensitivity in the mid-infrared region;
Temperature stabilization means for stabilizing the temperature of the light receiving element by a Peltier element ;
With
The signal processing circuit includes:
A synchronous detection circuit that detects the amplitude of a signal having a double frequency component of the modulation signal in the light source unit from the output signal of the light receiving unit and outputs a detection signal;
Means for extracting a gas absorption waveform signal from the detection signal of the synchronous detection circuit based on a trigger signal included in an output signal of the light receiving unit, and detecting a concentration of a measurement target gas from the gas absorption waveform signal; The detection signal from the synchronous detection circuit is A / D converted to generate gas absorption waveform signal data in synchronization with the trigger signal among the output signals of the unit, and the maximum value and minimum value of the gas absorption waveform signal data are generated. A concentration data calculation means for calculating concentration data about the gas concentration to be measured from the difference from the value, and an offset signal portion of the output signal of the light receiving unit in synchronization with a trigger signal among the output signals of the light receiving unit A Offset data generating means for generating offset data by performing D / D conversion, reference data that is offset data of the minimum value, and fluctuation using the offset data And variation rate data calculation means for calculating data, a processing unit having a correction density data calculation means for calculating a corrected density data by correcting the density data on the basis of the change rate data,
With
The laser drive signal generator shortens the heat generation time of the laser element by setting the output time of the offset signal as a signal longer than the output time of the variable drive signal of the wavelength scanning drive signal,
The temperature stabilizing means stabilizes the light receiving and emitting I the light source unit and line the temperature adjustment of the light receiving portion to the light source unit and the light receiving portion becomes stable range temperature to stabilize, and,
The signal processing circuit calculates a correction density that absorbs the influence of characteristic variation of the light source unit and the light receiving unit .

A laser gas analyzer according to claim 2 of the present invention is
The laser gas analyzer according to claim 1, wherein
The laser element of the light source unit is a quantum cascade laser (QCL: Quantum cascade laser) that emits a wavelength in the mid-infrared region,
The light receiving element of the light receiving unit is an MCT (Mercury_Cadmium_Tellurium) photoconductive element having sensitivity in the wavelength region .

A laser gas analyzer according to claim 3 of the present invention is
In the laser type gas analyzer according to claim 1 or 2,
The output time S2 of the offset signal is longer than the output time S1 of the variable drive signal of the wavelength scanning drive signal, and S1: S2 = 1: 4 .

A laser gas analyzer according to claim 4 of the present invention is
In the laser type gas analyzer according to any one of claims 1 to 3,
The concentration data calculation means sets Z _i (i = 1, 2,..., N) as the concentration data on the measurement target gas concentration obtained by the difference between the maximum value and the minimum value of the gas absorption waveform signal data .
The offset data generation means converts the offset data obtained by performing A / D conversion of the offset signal portion of the output signal of the light receiving section to X _i (i = 1, 2, ..., n)
The fluctuation rate data calculation means calculates the fluctuation rate data Y _i using the reference data that is offset data of the minimum value and the offset data as follows :
[Equation 1]
_{Y i = (X i -X min} ) / X min = X i / X min -1
(However, i = 1, 2,..., N)
Calculated by
The corrected density data calculation means calculates the density data based on the variation rate data by the following formula:
[Equation 2]
_{D i = Z i -cY i Z} i = Z i (1-cY i)
(Where c is correction coefficient data, i = 1, 2,..., N)
It characterized that you calculate the corrected density data D _i is corrected by.

According to the present invention, a laser type gas analysis that measures gas components such as SO ₂ , NO, NO _2, etc., having a light absorption spectrum unique to the mid-infrared region, which has been impossible in the past, with an inexpensive and simple structure. A total can be provided.

It is a block diagram of the light-receiving part, signal processing circuit, and arithmetic processing part of the laser type gas analyzer of embodiment of this invention. It is a figure which shows the scanning waveform of the laser type gas analyzer of embodiment of this invention. It is a figure which shows the example of a time-dependent change of the signal S4 showing offset data, and a density value. It is a figure which shows a time-dependent change of a density value and a correction density value. NH ₃ is a characteristic diagram showing an absorption spectrum of the gas. It is a principle diagram of a frequency modulation system. It is a structural diagram which shows the laser type gas analyzer of a prior art (this form). It is a block diagram of a light source part. It is an output signal figure from a wavelength scanning drive signal generation part. Scanning waveform of the laser device, the absorption waveform of the NH ₃ gas, is a diagram showing an output waveform of the synchronous detection circuit. It is a block diagram of a light-receiving part and a signal processing circuit. It shows the absorption spectrum example of SO ₂ gas. Is a diagram showing an output waveform of the synchronous detection circuit during SO ₂ concentration measurement. Is a diagram illustrating a SO ₂ concentration measurements example. It is a figure which shows the thermal radiation structure part of a light emission part.

  Then, the form for implementing this invention is demonstrated below, referring a figure. FIG. 1 is a configuration diagram of a modified light receiving unit, signal processing circuit, and arithmetic processing unit in the laser gas analyzer according to the embodiment of the present invention. The laser type gas analyzer of the present embodiment is almost the same as the prior art example shown in FIGS. 5 to 11 in the overall configuration and gas concentration measurement method, but the following four points are cited as differences. .

  As a first difference, as shown in FIG. 1, the configurations of the light receiving unit 207, the signal processing circuit 208, and the arithmetic processing unit 209 are changed. Further, the laser element 204e (see FIG. 8) uses QCL, and the light receiving element 207a (see FIG. 1) uses an infrared detection element having sensitivity in the wavelength region of the mid-infrared laser beam by the QCL, such as an MCT photoconductive element. A point is mentioned.

  Further, as a second difference, since the MCT photoconductive element cannot obtain sufficient sensitivity unless it is at a low temperature, the temperature stabilizing means as shown in FIG. 1 is used for the light receiving element 207a which is an MCT photoconductive element. Is provided in the light receiving unit 207 as a second difference. Compared with the light receiving unit shown in FIG. 11, the MCT photoconductive element is selected as the light receiving element 207a in the light receiving unit 207 in FIG. 1, and further, a temperature stabilization means by a thermistor 207b, a Peltier element 207c, and a temperature control unit 207d is added. It is different in the point made.

  This temperature stabilization means includes a thermistor 207b, a Peltier element 207c, and a temperature control unit 207d. Specifically, the thermistor 207b and the Peltier element 207c are built in the MCT photoconductive element. As described above, the thermistor 207b as a temperature detecting element is disposed in the vicinity of the laser element 207a, and the Peltier element 207c is disposed in the vicinity of the thermistor 207b.

  The Peltier element 207c is controlled by the temperature control unit 207d so that the resistance value of the thermistor 207b becomes a constant value, and as a result, operates so as to stabilize the temperature of the light receiving element 207a. By such temperature stabilization means, for example, the operating temperature of the MCT photoconductive element is kept constant at −3 ° C.

A third difference is that the wavelength scanning drive signal is changed.
As shown in FIG. 2, the wavelength scanning drive signal is a signal in which one unit waveform is formed by the variable drive signal S1, the offset signal S2, and the trigger signal S3, and such a unit waveform is repeated at a constant period.

  The variable drive signal S1 of the wavelength scanning drive signal is a signal that gradually changes the emission wavelength of the laser element 204e by linearly changing the current supplied to the laser element 204e in FIG. 8, and is the signal S1 in FIG. . The signal S1 is a signal for scanning the absorption wavelength, and is a portion that linearly changes the magnitude of the current supplied to the laser element 204e via the current control unit 204c as shown in FIG. The light emission wavelength of the laser element 204e can be gradually shifted by this signal S1, and the light emission wavelength can be scanned in the sub-nm to several nm range depending on the slope of the signal S1, that is, the amount of change in the supply current.

In FIG. 2, the frequency of the high frequency modulation signal is 4 kHz, the frequency of the wavelength scanning drive signal is 5 Hz, and λ ₁ and λ ₂ indicate the upper and lower limits of the scanning range corresponding to the absorption wavelength of SO ₂ gas. .

  The offset signal S2 of the wavelength scanning drive signal is a signal having a value that supplies a current less than a threshold current value at which light emission of the laser element 204e is stabilized to the laser element, and is the signal S2 in FIG. The signal S2 in FIG. 2 is an offset portion that does not cause the laser element 204e to emit light. At the timing when the wavelength scanning drive signal generator 204a outputs this offset signal S2, the QCL is not emitting light.

  The trigger signal S3 of the wavelength scanning drive signal is a pulse signal and is a signal inserted for synchronization. The trigger signal S3 is also a portion that does not cause the laser element 204e to emit light, and is set to a value less than a threshold current value at which the light emission of the laser element 204e is stable (a value lower than the offset signal S2, for example, 0). Therefore, QCL does not emit light at the timing when the wavelength scanning drive signal generation unit 204a outputs the trigger signal S3. This signal S3 is inserted during switching from the signal S1 to the signal S2.

Note that although λ ₁ and λ ₂ of the wavelength scanning drive signal are described as scanning ranges corresponding to the absorption wavelength of SO ₂ gas, in addition to SO ₂ , NO gas components are measured or NO ₂ is measured. The gas component can be measured. However, the characteristics of QCL and (current and wavelength scanning range due to temperature) _SO 2, NO, In consideration of the absorption spectra of NO _2, SO 2, NO, individually as a single component meter for any one of NO ₂ It becomes a laser type gas analyzer to measure. In this case, in the laser type gas analyzer, a QCL having an emission wavelength corresponding to the absorption wavelength of the measurement target selected from SO ₂ , NO, NO _2, etc. is selected, and depending on the gas component of the measurement target Then, λ ₁ and λ _{2 in} the mid-infrared region are set.

The arithmetic processing unit 209 that has received the output signal from the I / V converter 208a can extract the trigger signal S3. The trigger signal S3 represents one period of the wavelength scanning drive signal, and there is a certain temporal correlation between the trigger signal period and the detection signal of the synchronous detection circuit 208b. That is, when the measurement target gas exists, the timing at which the maximum or minimum value of the gas absorption waveform is generated with respect to the trigger generation timing obtained from the cycle of the trigger signal (the cycle of the trigger signal S3) is approximately accurate in advance. It can be detected.
Such a wavelength scanning drive signal has a waveform similar to that of the conventional wavelength scanning drive signal shown in FIG.

However, compared with the prior art, the characteristic difference in the present invention is that the offset signal S2 is less than the threshold current value at which the light emission of the laser element 204e is stabilized, and further the offset signal S2 with respect to the time of the variable drive signal S1. The time is significantly longer.
Such intermittent light emission conditions, that is, the ratio of the time between the signal S1 and the signal S2, may be determined in consideration of the amount of heat generated by the laser element 204e as the QCL and the performance of the temperature stabilizing means such as the Peltier element, For example, by setting S1: S2 = 1: 4, the calorific value can be reduced to 1/5 compared to the case of continuous light emission.

  In the prior art, the light source unit 204 emits light continuously or stops a little, but emits light almost continuously. If QCL is used, heat generation of the light source unit becomes excessive, and temperature control by a Peltier element is difficult. In the present invention, as described above, the QCL is intermittently emitted so that the extinction time is longer than the light emission time, thereby reducing the amount of heat generated by the QCL and the conventional laser gas analyzer. The QCL can be used with the same configuration and cost. The ratio between the light emission time and the quenching time is lower than this limit temperature when a limit temperature that can be stabilized by the temperature stabilization means (temperature control unit 204d, thermistor 204f, Peltier element 204g in FIG. 8) is assumed. The ratio between the light emission time and the quenching time is determined so that the temperature is reached. In this case, the temperature is lowered by making the extinction time longer than at least the light emission time.

  The fourth difference is that the arithmetic processing unit 209 shown in FIG. 1 functions as a correction measurement unit that performs measurement while correcting the measurement value. In order to perform such processing, the circuit configuration is also changed. Compared with the signal processing circuit of the prior art shown in FIG. 11, the signal processing circuit 208 and the arithmetic processing unit 209 in FIG. The difference is that the I / V converter 208a and the arithmetic processing unit 209 are connected.

  This correction measuring means corrects the measured value mainly according to the characteristic variation of the MCT. First, the correction principle will be described with reference to the drawings. FIG. 3 shows an example of a change over time of the signal S4 representing the offset data and the density value.

The lower wavy signal S4 in FIG. 3 is obtained by acquiring the offset signal S2 every 10 seconds, for example. The offset signal S2 can be obtained by using the trigger signal S3 as a reference. Such a wavy line represents an analog representation by connecting offset data (described later) which is digital data. Also, the upper polygonal line in FIG. 3 is a signal indicating the concentration value representing the SO ₂ concentration. This represents an analog representation of connected density values obtained from the variable drive signal S1 when the offset signal S2 is acquired.

In such characteristics, the density value also fluctuates due to fluctuations in MCT characteristics due to disturbances, etc., and the full-scale ratio ± 2% (hereinafter referred to as ± 2% FS), which is the standard value for zero point stability. It can be seen that it is about ± 3% FS. Further, the offset signal S4, which is originally a constant value, also increases or decreases at a predetermined period due to the influence of ambient temperature fluctuations, etc., and tends to increase as time elapses.
Here, since the set value of the signal S4 is known in advance, the change tendency can be determined. Accordingly, the offset signal S2 is monitored together with the SO ₂ concentration measurement, and the measurement result is corrected according to the variation rate of the offset signal S2.

Next, specific correction will be described. This correction process is a process of correcting data for a predetermined period at once. First, it is assumed that analysis is started.
The arithmetic processing unit 209 synchronizes with the trigger signal S3 out of the output signal of the light receiving unit 207 converted into a voltage signal by the I / V conversion circuit 208a, and outputs the detection signal from the synchronous detection circuit 208b. Means (concentration) for generating gas absorption waveform signal data by A / D conversion by a D converter and calculating concentration data Z _i for the measurement target gas concentration from the difference between the maximum value and the minimum value of the gas absorption waveform data Function as data calculation means). The method for calculating the concentration data Z _i for the gas concentration to be measured is the same as that in the prior art, and is calculated from the gas absorption waveform data specified with reference to the trigger signal S3.

Subsequently, the arithmetic processing unit 209 synchronizes with the trigger signal S3 in the output signal of the light receiving unit 207 converted into a voltage signal by the I / V conversion circuit 208a (for example, several m to several 10 ms with reference to the trigger signal S3). after), the locations of the offset signal S2 of the output signal of the I / V conversion circuit 208a, and generates the offset data _{X i} is a / D converted by the built-in a / D converter, calculating the offset data _{X i} It functions as a means (offset data generation means) for storing in a memory (not shown) of the processing unit 209.

The arithmetic processing unit 209 alternately performs the density data calculation unit and the offset data generation unit until a predetermined period (for example, 10 minutes). Then, the density data Z _i (i = 1, 2,..., N) and the offset data X _i (i = 1, 2,. , N) are accumulated. The acquired time differs if this i is different. That is, i represents time. Further, i may be set to be different for each waveform unit.

Processing unit 209, subsequently, the reference data X _min is the minimum value of the offset data X _i, and functions as means (variation rate data calculating means) for calculating a variation rate data Y _i by using the offset data X _i To do.

Specifically, the reference data X _min is searched for the offset data that is the minimum value from the offset data X _i stored in the internal memory (not shown) of the arithmetic processing unit 209 and extracts the offset data that is the minimum value. Thereafter, it is stored separately in the built-in memory for use as reference data _Xmin .

Specifically, the fluctuation rate data Y _i is calculated by subtracting the reference data X _min from the offset data X _i at a certain time and dividing the difference data by the reference data X _min . Such variation rate data Y _i is expressed by the following equation.

[Equation 1]
Y _i = (X _i −X _min ) / X _min = X _i / X _min −1 (where i = 1, 2,..., N)

Such variation rate data Y _i is calculated for all offset data X _i (i = 1, 2,..., N). That is, it is calculated for each time (or for each waveform unit).
The fluctuation rate data Y _i is based on the minimum light reception level of the MCT photoconductive element, and the MCT light reception level by time (or by waveform unit) is used as the fluctuation rate data Y _i (i = 1, 2,..., N). ).

Arithmetic processing section 209 functions as means (corrected density data calculating means) for calculating a corrected density data D _i by correcting the density data Z _i on the basis of the variation rate data Y _i. Specifically, multiplied by the variation rate data Y _i and the correction coefficient data c to the density data Z _i hourly (or waveform-units), by subtracting the value from the density data Z _i, the correction density data D _i Get. The corrected density data D _i is expressed by the following equation.

[Equation 2]
_{D i = Z i -cY i Z} i = Z i (1-cY i)

Such correction density data D _i (i = 1, 2,..., N) is calculated. That is, it is calculated for each time (or for each waveform unit).
In this way, the following gas laser analysis is performed using the corrected concentration data.

  FIG. 4 shows a density value representing density data and a corrected density representing corrected density data. With respect to the corrected density data, the zero point stability is about ± 1.6% FS by the correction, and the standard value (± 2% FS) can be satisfied.

  In this correction, although the MCT characteristic fluctuation is taken into consideration, even if the light quantity of QCL fluctuates, the above-mentioned fluctuation of QCL is also corrected because the trigger extraction signal changes. As described above, the variation in characteristics of MCT and QCL is corrected, which contributes to improvement in measurement accuracy.

The laser gas analyzer of the present invention has been described above.
According to the present invention, it is possible to reduce the heat radiation amount of the light source unit by causing the QCL to emit light intermittently. As a result, the cooling mechanism of the light source unit can be simplified, and a low-cost laser type that measures gas components such as SO ₂ , NO, and NO ₂ that has been impossible with conventional laser gas analyzers. A gas analyzer can be provided.

  Furthermore, the measurement accuracy of the laser gas analyzer using the QCL can be improved by monitoring the characteristic variation of the MCT in real time and correcting the measured value according to the variation rate.

The laser type gas analyzer of the present invention can be applied to measurement of gas components such as SO ₂ , NO, NO ₂ having a unique light absorption spectrum in the mid-infrared region.

101a, 101b: walls 201a, 201b: flanges 202a, 202b: mounting seats 203a, 203b: cover 204: light source unit 204a: wavelength scanning drive signal generator 204b: high frequency modulation signal generator 204c: current controller 204d: temperature controller 204e: Laser element 204f: Thermistor 204g: Peltier element 204s: Laser drive signal generator 205: Collimator lens 206: Condensing lens 207: Light receiving part 207a: Light receiving element 207b: Thermistor 207c: Peltier element 207d: Temperature control part 208: Signal Processing circuit 208a: I / V conversion circuit 208b: Synchronous detection circuit 208c: Oscillator 208d: Filter 209: Arithmetic processing unit 301: Flange 302: Lens holder 303: Laser mount 304: Radiation fin 305: Air cooling fan 306: Over scan

Claims (4)

A light source unit that emits a frequency-modulated mid-infrared laser beam, a light source side optical system that collimates the light emitted from the light source unit, and a light source side optical system that propagates through the space where the measurement target gas exists A light receiving side optical system for collecting the transmitted light, a light receiving unit for receiving the light collected by the light receiving side optical system, and a signal processing circuit for processing an output signal of the light receiving unit, In the laser gas analyzer in which the signal processing circuit measures the concentration of the measurement target gas based on the output signal of the light receiving unit,
The light source unit is
A laser element emitting mid-infrared laser light;
Temperature stabilizing means for stabilizing the temperature of the laser element by a Peltier element ;
A signal for gradually changing the emission wavelength of the laser element by linearly changing the supply current to the laser element, and making the emission wavelength of the laser element variable so as to scan the absorption wavelength of the gas to be measured A variable drive signal and a signal having a value such that a current less than a threshold current value of the laser element is supplied to the laser element, and an offset for stopping the light emission of the laser element so as to reduce the amount of heat generated by the laser element A signal having a value such that a current less than a threshold current value of the laser element is supplied to the laser element, and a signal inserted between the one variable drive signal and the one offset signal. and a trigger signal for synchronization, the unit waveform consisting of the wavelength scanning driving signal is a signal which is repeated at a fixed period, modulating the emission wavelength Te A laser driving signal generator for outputting a laser driving signal by combining the order of the high-frequency modulation signal,
A current controller that converts the laser drive signal output from the laser drive signal generator into a current and supplies the current to the laser element;
With
The light receiving unit is
A light-receiving element having sensitivity in the mid-infrared region;
Temperature stabilization means for stabilizing the temperature of the light receiving element by a Peltier element ;
With
The signal processing circuit includes:
A synchronous detection circuit that detects the amplitude of a signal having a double frequency component of the modulation signal in the light source unit from the output signal of the light receiving unit and outputs a detection signal;
Means for extracting a gas absorption waveform signal from the detection signal of the synchronous detection circuit based on a trigger signal included in an output signal of the light receiving unit, and detecting a concentration of a measurement target gas from the gas absorption waveform signal; The detection signal from the synchronous detection circuit is A / D converted to generate gas absorption waveform signal data in synchronization with the trigger signal among the output signals of the unit, and the maximum value and minimum value of the gas absorption waveform signal data are generated. A concentration data calculation means for calculating concentration data about the gas concentration to be measured from the difference from the value, and an offset signal portion of the output signal of the light receiving unit in synchronization with a trigger signal among the output signals of the light receiving unit A Offset data generating means for generating offset data by performing D / D conversion, reference data that is offset data of the minimum value, and fluctuation using the offset data And variation rate data calculation means for calculating data, a processing unit having a correction density data calculation means for calculating a corrected density data by correcting the density data on the basis of the change rate data,
With
The laser drive signal generator shortens the heat generation time of the laser element by setting the output time of the offset signal as a signal longer than the output time of the variable drive signal of the wavelength scanning drive signal,
The temperature stabilizing means stabilizes the light receiving and emitting I the light source unit and line the temperature adjustment of the light receiving portion to the light source unit and the light receiving portion becomes stable range temperature to stabilize, and,
The laser processing gas analyzer according to claim 1, wherein the signal processing circuit calculates a correction concentration that absorbs an influence of a characteristic variation of the light source unit and the light receiving unit . The laser gas analyzer according to claim 1, wherein
The laser element of the light source unit is a quantum cascade laser (QCL: Quantum cascade laser) that emits a wavelength in the mid-infrared region,
The laser gas analyzer according to claim 1, wherein the light receiving element of the light receiving unit is an MCT (Mercury_Cadmium_Tellurium) photoconductive element having sensitivity in the wavelength region . In the laser type gas analyzer according to claim 1 or 2,
A laser gas analyzer characterized in that the output time S2 of the offset signal is longer than the output time S1 of the variable drive signal of the wavelength scanning drive signal, and S1: S2 = 1: 4 . In the laser type gas analyzer according to any one of claims 1 to 3,
The concentration data calculation means sets Z _i (i = 1, 2,..., N) as the concentration data on the measurement target gas concentration obtained by the difference between the maximum value and the minimum value of the gas absorption waveform signal data .
The offset data generation means converts the offset data obtained by performing A / D conversion of the offset signal portion of the output signal of the light receiving section to X _i (i = 1, 2, ..., n)
The fluctuation rate data calculation means calculates the fluctuation rate data Y _i using the reference data that is offset data of the minimum value and the offset data as follows :
[Equation 1]
_{Y i = (X i -X min} ) / X min = X i / X min -1
(However, i = 1, 2,..., N)
Calculated by
The corrected density data calculation means calculates the density data based on the variation rate data by the following formula:
[Equation 2]
_{D i = Z i -cY i Z} i = Z i (1-cY i)
(Where c is correction coefficient data, i = 1, 2,..., N)
Laser gas analyzer which is characterized that you calculate the corrected density data D _i is corrected by.

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