JP2013088135A - Laser gas analyzer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a laser gas analyzer in which a noise component can be smoothed with respect to P-P output of relatively small output fluctuation.SOLUTION: A laser gas analyzer comprises a light source unit which includes a semiconductor laser which irradiates measuring gas with laser light while scanning it, a photodetector which detects laser light transmitted through the measuring gas, a gain variable amplifier to which an output signal of the photodetector is inputted, an A/D converter to which an output signal of the gain variable amplifier is inputted, and a detection unit including an arithmetic processing section which arithmetically operates a concentration of the measuring gas on the basis of output data from the A/D converter. The laser gas analyzer further comprises a P-P detector which detects a P-P value of output data from the A/D converter for each scan of the semiconductor laser, a gain setting section which is capable of setting a gain resolution for inputting an output signal of the P-P detector within a predetermined range, and a gain converter which inputs a signal from the gain setting section and adjusts a gain for each scan.

Description

  The present invention relates to a laser gas analyzer, and more particularly to a laser gas analyzer capable of removing noise from a low concentration gas.

  Laser gas analyzers using the TDLAS (Tunable Diode Laser Absorption Spectroscopy) method only irradiate light from a variable wavelength semiconductor laser to a measurement target component such as high temperature and corrosive gas Even in the case of a concentration of 1, there is an advantage that the component selectivity is high without being interfered by other components, and measurement can be performed in real time at high speed without contact.

  FIG. 5A is a block diagram showing an example of a conventional laser gas analyzer using the TDLAS method, in which a light source unit including a semiconductor laser that irradiates a measurement laser beam into a measurement gas atmosphere, and a measurement gas It comprises a light receiving element that detects the measurement laser beam that has passed through the measurement space of the atmosphere, and a detection unit that includes an arithmetic processing unit that processes the output signal of this light receiving element.

The laser gas analyzer shown in FIG. 5 (a) is a semiconductor with a very narrow oscillation wavelength spectrum line width, which shows a light absorption spectrum unique to a molecule due to vibration / rotational energy transition of a measurement target component molecule existing in the infrared to near infrared region. Measure using a laser. The absorption spectra peculiar to most molecules such as O ₂ , NH ₃ , H ₂ O, CO, CO ₂ are in the infrared to near-infrared region, and are measured by measuring the amount of light absorption (absorbance) at a specific wavelength. The concentration of the component can be calculated.

  In FIG. 5A, the semiconductor laser 11 provided in the light source unit 10 irradiates and outputs measurement laser light into the atmosphere of the measurement gas 20. The laser light output from the semiconductor laser 11 has a very narrow oscillation wavelength spectrum line width, and the oscillation wavelength can be changed by changing the laser temperature or drive current. Therefore, only one of the absorption peaks of the absorption spectrum can be measured.

  Therefore, the absorption peak that is not affected by the interference gas can be selected, the wavelength selectivity is high, and it is not affected by other interference components. Therefore, the process is performed without removing the interference gas in the previous stage of measurement. Gas can be measured directly.

  By scanning the oscillation wavelength of the semiconductor laser 11 in the vicinity of one absorption line of the measurement component, an accurate spectrum that does not overlap with the interference component can be measured. The spectrum shape includes the measurement gas temperature, the measurement gas pressure, It changes due to a spectrum broadening phenomenon caused by a coexisting gas component. For this reason, the actual process measurement accompanied by these environmental variations requires correction.

  Therefore, in the apparatus of FIG. 5A, a spectral area method is used in which the spectral area is obtained by scanning the oscillation wavelength of the semiconductor laser 11 and measuring the absorption spectrum, and converting the spectral area into the component concentration.

  In other laser gas analyzers, the peak height method for obtaining the measurement component from the peak height of the absorption spectrum or the wavelength scan signal is modulated and the P-P (peak-to-peak) value of the frequency-modulated waveform is double that frequency. The 2f method for obtaining the concentration of the measured component from the above is used, but these are easily affected by fluctuations in temperature, pressure, coexisting gas components, and the like.

  In contrast, the spectral area method is in principle not affected by changes due to differences in the coexisting gas components (the spectrum area is almost constant regardless of the coexisting gas components), and the spectral area method is also effective against pressure fluctuations. It shows a linear change in principle.

  In the peak height method and the 2f method, the above three fluctuation factors (temperature, pressure, coexisting gas components) all affect nonlinearly, and when these fluctuation factors coexist, correction is difficult. The linear correction for the gas pressure fluctuation and the non-linear correction for the gas temperature fluctuation can be performed, and an accurate correction can be realized.

  The measurement laser light that has passed through the atmosphere of the measurement gas 20 is received by the light receiving element 31 provided in the detection unit 30 and converted into an electrical signal.

  The output signal of the light receiving element 31 is adjusted to an appropriate amplitude level via a variable gain amplifier 32, input to the A / D converter 33, and converted into a digital signal.

  With respect to the output data of the A / D converter 33, in synchronization with the wavelength scan of the semiconductor laser 11, the integration is performed a predetermined number of times (for example, several hundred to several thousand times) between the integrator 34 and the memory 35, and the memory 35 is supplied. Is repeated, noise included in the measurement signal is removed and the data is smoothed, and then input to the CPU 36.

  The CPU 36 performs arithmetic processing such as measurement gas concentration analysis based on the measurement signal from which noise has been removed, and an amplifier when the amplitude level of the output signal of the light receiving element 31 is not appropriate as the input level of the A / D converter 33. 32 gain adjustment is performed.

  Non-Patent Document 1 describes the measurement principle, characteristics, and specific measurement examples of a laser gas analyzer to which variable wavelength semiconductor laser spectroscopy is applied.

Tamura, one and three others, “Laser gas analyzer TDLS200 and its application to industrial processes”, Yokogawa Technical Report, Yokogawa Electric Corporation, 2010, Vol. 53 No. 2 (2010) p. 51-54

  Incidentally, in the laser gas analyzer configured as shown in FIG. 5A, as described above, the output data of the A / D converter 33 is used to remove noise contained in the measurement signal and smooth the data. In synchronization with the wavelength scan of the semiconductor laser 11, hundreds to thousands of integrations and storage in the memory 35 are repeatedly performed between the integrator 34 and the memory 35. During a series of analysis processing periods in which 1000 scans, integration, and storage are performed, it is necessary to adjust the gain of the amplifier 32 to maintain the input level of the A / D converter 33 constant.

  However, even when data smoothing is performed, noise components such as optical noise remain, and the noise has had a large effect on applications that require low-concentration gas analysis. There was a problem that gas measurement had limitations.

  The present invention solves such problems, and for applications that require low-concentration gas analysis by a laser gas analyzer, smoothing of noise components for PP output with relatively small output fluctuations is possible. An object is to provide a possible laser gas analyzer.

In order to achieve such a problem, the invention according to claim 1 of the present invention is:
A light source unit including a semiconductor laser that irradiates a measurement gas while scanning the laser beam;
A light receiving element for detecting a laser beam transmitted through the measurement gas;
A variable gain amplifier to which the output signal of this light receiving element is input,
An A / D converter to which the output signal of the amplifier is input;
In a laser gas analyzer configured with a detection unit including an arithmetic processing unit that calculates the concentration of the measurement gas based on output data of the A / D converter,
A peak-to-peak detector for detecting a peak-to-peak value of output data of the A / D converter for each scan of the semiconductor laser;
A gain setting unit that can set the gain resolution for inputting the output signal of the peak-to-peak detector within a predetermined range, and a gain converter that inputs the signal from the gain setting unit and adjusts the gain for each scan. It is provided.

The invention described in claim 2 is the laser gas analyzer according to claim 1,
The gain setting in the predetermined range in the gain setting unit is set to about ± 10% of the peak-to-peak value.

The invention according to claim 3 includes a light source unit including a semiconductor laser that irradiates the measurement gas while scanning the laser beam.
A light receiving element for detecting a laser beam transmitted through the measurement gas;
A variable gain amplifier to which the output signal of this light receiving element is input,
An A / D converter to which the output signal of the amplifier is input;
In a laser gas analyzer configured with a detection unit including an arithmetic processing unit that calculates the concentration of the measurement gas based on output data of the A / D converter,
An N ₂ O reference cell is arranged at the rear stage of the semiconductor laser, and the laser that passes through the N ₂ O reference cell is detected by the light receiving element.

  According to the first and second aspects, the input level of the A / D converter is averaged by smoothing the noise component by changing the gain of the gain variable amplifier by the value set by the gain setting unit for each scan of the semiconductor laser. Can be

According to the third aspect of the present invention, since the N ₂ O reference cell is disposed after the semiconductor laser, it does not affect the CO measurement for an application that requires the CO gas measurement by the laser gas analyzer, and its measurement wavelength. Can be stabilized.

It is a block diagram which shows one Example of this invention. It is a figure which shows the operation | movement and effect of the gain amplifier by this invention. Downstream of the laser is a block diagram showing an embodiment of the present invention of arranging the N ₂ O reference cells. Is an explanatory view showing an effect in the case of the case and N ₂ O reference cells arranged CO reference cells downstream of the laser is arranged. It is a block diagram which shows a prior art example.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of the present invention, and the same reference numerals are given to portions common to the conventional example of FIG. The difference between FIG. 1 and FIG. 5A is that a means for adjusting the gain of the variable gain amplifier 32 is replaced by a P− that detects the PP value of the output data of the A / D converter 33 instead of the CPU 36. The P detector 41, the gain setting unit 44, and the gain conversion unit 45 are provided.

  In FIG. 1, the semiconductor laser 11 provided in the light source unit 10 irradiates and outputs measurement laser light in the atmosphere of the measurement gas 20 as in the conventional case. At this time, in order to obtain the area of the absorption spectrum of the measurement target component of the measurement gas 20, the wavelength of the semiconductor laser 11 is scanned in a narrow band including the absorption wavelength.

  The measurement laser light that has passed through the atmosphere of the measurement gas 20 is received by the light receiving element 31 provided in the detection unit 30 and converted into an electrical signal.

  The output signal of the light receiving element 31 is adjusted to an appropriate amplitude level as the input level of the A / D converter 33 via the variable gain amplifier 32, and is input to the A / D converter 33 and converted into a digital signal.

  The output data of the A / D converter 33 is accumulated a predetermined number of times (for example, several hundred to several thousand times) between the accumulator 34 and the memory 35 in synchronism with the wavelength scan of the semiconductor laser 11 and is sent to the memory 35. Storage is repeated and noise contained in the measurement signal is removed and smoothed, and then input to the CPU 36. The CPU 36 performs arithmetic processing such as measurement gas concentration analysis based on the measurement signal from which noise has been removed.

  Further, the output data of the A / D converter 33 is branched and input to a PP detector 41 constituting the gain adjustment unit 40. The PP detector 41 detects the PP value of the output data of the A / D converter 33 in real time for each scan of the semiconductor laser 11, and inputs the detection result to the gain setting unit 44. Specifically, the PP detector 41 compares data in real time at the sampling timing of the A / D converter 33.

  In the next scan, the gain setting unit 44 receives the output signal of the PP detector 41 and, for example, within ± 10%, the gain conversion unit 45 so that the output fluctuates to the plus side or minus side with respect to the inputted value. Issue a command to In accordance with the command, the gain conversion unit issues a command to multiply the commanded value to the variable gain amplifier 32.

  FIG. 2 (a) shows the output that is first A / D converted, and FIG. 2 (b) is obtained by sequentially multiplying the output of the light receiving element from the next time by a value of ± 10% by the gain amplifier 32 sequentially. The output from A / D conversion is shown.

  That is, the gain variable amplifier 32 outputs, to the A / D converter 33, a value obtained by multiplying the next output by 1.01 if the output signal of the PP detector 41 scanned first is, for example, 100. To do. For the next output, the gain converter 45 instructs the variable amplifier 32 to output a value multiplied by 1.02, and is sequentially variable until 1.03 to 1.1 (+ 10%). The output of the amplifier 32 is changed in increments of 0.1.

  Then, when 1.1 is reached, the values to be multiplied are sequentially 1.09 to 1.08 to 1, and further 0.99 to 0.98 to 0.9 (−10%) in the negative direction, from which 0.91 The value to be multiplied until ˜1.1 is changed in increments of 0.1. In this way, PP in one scan in a certain range excluding the absorption peak of the measurement component is detected, and the gain setting unit makes the change amount (value multiplied by the gain) of the gain amplifier 32 as small as possible (for example, 10%). Thus, several types (for example, 20 types) of gain changes can be made.

  According to the above-described configuration, the gain setting unit 44 has a gain conversion unit so that the output fluctuates to the plus side or the minus side with respect to the input value within ± 10% with respect to the output signal of the PP detector 41. A command is issued to 45, and a value obtained by multiplying the commanded value to the gain variable amplifier 32 according to the command is output.

  As a result, when there is some optical noise or the like, the PP of the noise is detected by the PP detector every scan, and the gain is set within a predetermined range by the gain setting unit 43 and the gain conversion unit 44. By changing, it becomes possible to smooth noise components that could not be removed up to now. That is, the noise component can be removed by this smoothing.

FIG. 2 (c) shows a spectrum (A) according to the conventional method and a spectrum (B) according to the present invention. It can be confirmed that noise is reduced.
Note that a PP threshold value for noise is set in advance by the PP detector, and if the threshold value is exceeded, a system that performs dynamic auto gain adjustment is used to perform low density measurement by noise reduction. Although it is conceivable to construct a system that can be realized and cope with changes in the process environment, the present invention is applied to a process with very little noise, and does not reach the point of setting a PP threshold. It is suitable for use.

By the way, in such a laser gas analyzer, especially when measuring the spectrum of CO, a CO cell is used as a reference cell in the subsequent stage of the semiconductor laser 11.
FIG. 5B is a block diagram showing a state in which the CO cell 12 is arranged as a reference cell in the subsequent stage of the semiconductor laser 11. In order to stabilize the measurement wavelength, the reference cell 12 is filled with a reference gas (CO gas) so that the measurement wavelength can be measured without fluctuation even when the concentration of the measurement target gas decreases.

However, when CO gas is sealed in the reference cell 12, the following problems occur.
a) Since the absorption spectrum shape of the CO gas in the reference cell changes due to the influence of radiant heat from the process or the like, the temperature fluctuation in the reference cell affects the measured value. In particular, the effect is large when the process concentration is low.
b) In order to correct the temperature fluctuation in the reference cell, it is necessary to always measure the temperature, and the correction is complicated and difficult considering the temperature gradient.
c) If the reference gas cell is damaged, it is dangerous because CO gas leaks into the analyzer.

FIG. 3 shows an embodiment of the present invention. For an application that requires CO gas measurement by a laser gas analyzer, a configuration for stabilizing the measurement wavelength without affecting the CO measurement. Is shown.
3 is the same as the conventional laser gas analyzer shown in FIG. 5B except that an N ₂ O reference cell 13 is arranged as a reference cell after the semiconductor laser 11.

That is, the laser light output from the semiconductor laser 11 passes through the reference cell (N ₂ O) 13, passes through the process gas, is received by the light receiving element 31, and is converted into an electrical signal. After that, the measurement signal digitized by the A / D converter 33 via the gain amplifier 32 is accumulated by a predetermined number in the accumulator 34 to remove noise, repeatedly stored in the memory 35, and transmitted to the CPU 36 for concentration analysis and the like. Do.

As shown in FIG. 3, the difference from the prior art is that N ₂ O gas is used as the gas sealed in the reference cell 13. The measurement wavelength is stabilized using the absorption spectrum of the N ₂ O gas as a reference. As a result, even when the measurement target gas concentration is lowered, measurement can be performed without fluctuation of the measurement wavelength.

As described above, since a gas different from the measurement gas is used for reference, an effect can be obtained for the following items.
a) Even if a sudden environmental change (radiant heat, etc.) occurs in the process, the CO gas concentration to be measured is not affected.
b) Stable measurement is possible even when the process gas concentration is low.
c) It is not necessary to correct temperature variations in the reference cell.
d) It is not a dangerous gas that will die immediately even if the reference cell is damaged.

4A shows a measurement spectrum (a) when the CO reference cell 12 shown in FIG. 5B is used and a measurement spectrum (b) when the N ₂ O reference cell 13 shown in FIG. 3 is used. However, when the CO reference cell 12 is used, it is output as a synthesized spectrum in which the spectrum of the reference gas cell and the spectrum of the measurement gas are superimposed.

On the other hand, when the N ₂ O reference cell 13 is used, the wavelength of the reference gas cell and the spectrum of the measurement gas do not overlap with each other and the wavelength is stabilized by using the gas present in the vicinity of the absorption peak of the measurement component. Can be realized.

In addition, since the measurement wavelength can be stabilized by using the absorption spectrum of N ₂ O gas as a reference, measurement can be performed without fluctuation even when the concentration of the measurement target gas is reduced, and CO gas generated by a laser gas analyzer can be measured. For applications that require measurement, the measurement wavelength can be stabilized without affecting the CO measurement.

The above description merely shows a specific preferred embodiment for the purpose of explanation and illustration of the present invention. In this embodiment, the gain is changed within a range of ± 10% with respect to PP, but the present invention is not limited to this. It is only necessary that the amount of change of the gain variable amplifier can be made as small as possible to change several types of gains. .
Therefore, the present invention is not limited to the above-described embodiments, and includes many changes and modifications without departing from the essence thereof.

10 light source unit 11 semiconductor laser 12 CO reference cell 13 N ₂ O reference cell 20 measuring gas 30 detection unit 31 receiving element 32 variable-gain amplifier 33 A / D converter 34 integrator 35 Memory 36 CPU
41 PP detector 44 Gain setting unit 45 Gain conversion unit

Claims (3)

A light source unit including a semiconductor laser that irradiates a measurement gas while scanning the laser beam;
A light receiving element for detecting a laser beam transmitted through the measurement gas;
A variable gain amplifier to which the output signal of this light receiving element is input,
An A / D converter to which the output signal of the amplifier is input;
In a laser gas analyzer configured with a detection unit including an arithmetic processing unit that calculates the concentration of the measurement gas based on output data of the A / D converter,
A peak-to-peak detector for detecting a peak-to-peak value of output data of the A / D converter for each scan of the semiconductor laser;
A gain setting unit that can set the gain resolution for inputting the output signal of the peak-to-peak detector within a predetermined range, and a gain converter that inputs the signal from the gain setting unit and adjusts the gain for each scan. A laser gas analyzer characterized by being provided.   The laser gas analyzer according to claim 1, wherein the gain setting in the predetermined range in the gain setting unit is set to about ± 10% of a peak-to-peak value. A light source unit including a semiconductor laser that irradiates a measurement gas while scanning the laser beam;
A light receiving element for detecting a laser beam transmitted through the measurement gas;
A variable gain amplifier to which the output signal of this light receiving element is input,
An A / D converter to which the output signal of the amplifier is input;
In a laser gas analyzer configured with a detection unit including an arithmetic processing unit that calculates the concentration of the measurement gas based on output data of the A / D converter,
A laser gas analyzer, wherein an N ₂ O reference cell is arranged at a subsequent stage of the semiconductor laser, and a laser that passes through the N ₂ O reference cell is detected by the light receiving element.