JPH07151681A - Gas density measuring apparatus - Google Patents

Abstract

PURPOSE:To provide a gas density measuring apparatus which eliminates the calibration of phase synchronization and makes possible the use of an inexpensive and low speed DSP (digital signal processor) or the like. CONSTITUTION:Signals from first and second cosine wave signal generating sections 1 and 2 are added to a fundamental wave orthogonal modulation section 3 and a double wave orthogonal modulation section 4 respectively shifting the phase bit by bit to obtain uniphase/orthogonal components, which are integrated with integration sections 5 and 5 respectively and then, a square sum is obtained with square sum computing sections 7 and 7. Thus, a pseudo area is determined by integrating the amplitude of a fundamental wave and a double wave with area extracting sections 8 and 8 and a ratio is determined with a division section 10 to obtain a measure of the density of a gas.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas concentration measuring device used in a gas detector for highly accurately measuring the concentration of a specific gas such as methane gas or carbon dioxide gas by using optical absorption spectrum characteristics. is there.

[0002]

2. Description of the Related Art It is known that gaseous gas molecules each have a unique light absorption spectrum, and a gas detector using a semiconductor laser as a light source was first introduced by CB Moore in 1965. Invented This principle will be briefly described below. A schematic diagram of gas absorption characteristics is shown in FIG. Here, the amount of attenuation at the frequency fc at the center of the absorption line is proportional to the gas concentration. Therefore, fc
The concentration of the gas can be estimated by producing a semiconductor laser beam having an oscillation frequency of, irradiating the gas, measuring the amount of attenuation, and multiplying it by an appropriate coefficient.

As a development of this principle, there are a two-wavelength difference method and a frequency modulation method, and these methods are now mainstream. Detailed explanation of the two-wavelength difference method and the experimental results are as follows: ・ K.Uehara: Appl.Phys.B, Vol.38, No.1, pp.37-40,1985. ・ Hideo Tai, Kazunari Yamamoto, Abe Ken, Takashi Ueki, Hiroaki Tanaka, Kiyoji Uehara: Transactions of the Society of Instrument and Control Engineers 24, pp.452-458,1
988. ・ Hideo Tai, Hiroaki Tanaka, Kiyoji Uehara: Optics, Vol.19, No.
4, pp.238-244, 1990. In addition, a detailed explanation of the frequency modulation method and the experimental results are as follows: DTCassidy: Appl.Opt.
988. ・ Hideo Tai, Masayuki Matsuura, Hiroaki Tanaka, Kiyoji Uehara: Optics, V
ol. 19, No. 9, pp. 616-619, 1990. First, the two-wavelength difference method will be described, and then the frequency modulation method according to the present invention will be described.

FIG. 8 shows a block diagram of a gas detector using the two-wavelength difference method. Here, from the semiconductor laser 11 to the light receiving part
The distance to 14 is variable so that it can be measured by the gas cell 12 of an arbitrary length. Therefore, when the distance becomes long, the laser light diverges and is attenuated, so that the level of the signal collected by the light receiving lens 13 and received by the light receiving unit 14 becomes small, and as a result, the gas concentration measuring device 15 increases the gas concentration. I estimate it as something. What is stated here is the distance from the semiconductor laser 11 to the light receiving portion 14, and the apparent attenuation amount increases in proportion to the length of the gas cell 12, but this amount depends on the length of the gas cell 12. You can cancel by breaking. In order to eliminate the influence of this distance, in practice, two laser lights having an oscillation frequency of fc at the center of the absorption line and fr without the absorption line are irradiated to the gas, and the ratio is multiplied by an appropriate proportional constant. Convert to concentration. The problem with this two-wavelength difference method is that the oscillation frequency of the semiconductor laser 11 is T
This is a signal in the Hz order, which means that complicated signal processing cannot be performed. Moreover, it is very difficult to stably generate a signal of two wavelengths.

A frequency modulation method described below was devised to solve this problem, and it is possible to perform signal processing in a base band region of several kHz. FIG. 4 shows a principle diagram of the frequency modulation method. The output of the semiconductor laser is frequency-modulated at the center frequency fc and the modulation frequency fm, and the target gas is irradiated. Since the absorption line of gas is almost a quadratic function with respect to the detuning frequency, this absorption line plays the role of a discriminator, and a signal (double wave) having a frequency twice the modulation frequency fm is generated at the light receiving part. can get. Here, the modulation frequency f
Since m may be any frequency, for example, if it is selected to be several kHz, high-level signal processing can be performed using a digital signal processor (DSP) or a general-purpose processor. In order to cancel the influence of distance by the frequency modulation method as described in the explanation of the two-wavelength difference method, the output of the semiconductor laser may be frequency-modulated and at the same time the amplitude modulation may be performed at the frequency fm. If frequency modulation is applied to the output of the semiconductor laser, amplitude modulation is also applied, and this can be used exactly. Then, the fundamental wave by amplitude modulation can be estimated by performing envelope detection in the light receiving unit, and by taking the ratio of the amplitude of this fundamental wave and the amplitude of the second harmonic in phase synchronization, the gas concentration can be obtained regardless of the distance. A proportional value can be obtained.

Actually, as shown in FIG. 5, since the characteristics of the semiconductor laser have various distortions, the above-mentioned amplitude is not a perfect circle but an ellipse or an egg shape when considered in a Gaussian plane. Or the radius of a convex shape without symmetry (hereinafter collectively referred to as an ellipse). That is,
Since a bandpass filter with an infinitely small bandwidth for extracting the frequency of only one point cannot be realized, the distortion is included in the fundamental wave and the second harmonic wave and becomes an ellipse. Therefore, in order to accurately estimate the gas concentration, it is necessary to synchronize the phases so that the offset value caused by the characteristic of the ellipse does not occur and adjust the phase so that the amplitude value becomes maximum or minimum.

[0007]

The conventional gas concentration measuring device used in the gas detector of this frequency modulation system is of the phase synchronization type, and its problem is that before estimating the gas concentration for phase synchronization. That is, the maximum point or the minimum point must be found while manually shifting the phase little by little so that the amplitude due to frequency modulation becomes maximum.
Currently, it takes a few minutes for this adjustment. Also,
In order to perform phase synchronization, it is necessary to bring signals having the same frequency as a reference from the modulator of the semiconductor laser to the gas concentration measurement processor. Then, complicated processing such as processing the phase information must be performed.

The object of the present invention is to use a frequency modulation method, but it is not necessary to perform calibration for phase synchronization as in the prior art, and since information processing is not complicated, an inexpensive low speed DSP or an inexpensive DSP is used. Another object of the present invention is to provide a gas concentration measuring device that can be realized by using a general-purpose processor.

[0009]

[Means for Solving the Problems] The parameters necessary for measuring or estimating the concentration of gas have an electric signal obtained by detecting an optical signal having an absorption characteristic of the gas to be measured with a light receiver. The fundamental wave, that is, the signal component of the fundamental wave of the frequency (fm) used for frequency-modulating the semiconductor laser light incident on the gas cell (gas to be measured) and the absorption characteristic of the gas to be measured are excited. , The amplitude ratio with the signal component of the double wave having the frequency (2fm) double that of the fundamental wave. However, in reality, since the target signal contains distortion due to various factors, the amplitude ratio changes when the phases of the fundamental wave and the second harmonic wave change. The distortion is
Like white noise, it cannot be removed only by averaging, and it is impossible to accurately obtain the amplitude.

As a result of the experiment, the distortion included in the target signal is periodic, and when viewed on the Gaussian plane, the fundamental wave and the 2
It was found that the locus of harmonics is an elliptical orbit. Therefore,
In the present invention, the measure of the gas concentration is not the amplitude ratio but the area ratio of the ellipse on the Gaussian plane. If this area ratio is used, the area ratio is constant regardless of the phase, so that the amplitude is maximum (minimum) with respect to the in-phase component of the fundamental wave and the second harmonic wave as in the past
There is no need to manually adjust so that the phase itself is synchronized. In the present invention, the signal processing unit is provided so that the amplitude can be obtained from the in-phase component and the quadrature component of the detected signal and the pseudo area can be obtained.

The basic configuration of the present invention is shown in FIG. 1 (detailed later).
Quadrature modulation while gradually shifting the phase as shown in
Signals with two components, in-phase and quadrature, to be applied to the fundamental wave signal and the second harmonic signal respectively (four signals for convenience)
To obtain the sum of squares, obtain the sum of squares, and integrate again to obtain the signal A and the signal B. Each of the signals A and B is a measure of the area when the fundamental wave and the second harmonic wave are represented by the Gaussian plane. It will be described below that the ratio B / A of the two signals is a measure of the concentration of the gas to be measured.

[0012]

The principle of this signal processing will be described in detail by taking the amplitude estimation of the fundamental wave as an example. Input signal,

[0013]

[Equation 1]

Assume that Where ω m is the angular frequency of the fundamental wave, ω m = 2πfm, φ is the initial phase, a is the amplitude, and n is
(t) is noise from the outside, and only ωm is known. The parameter we have to find in equation 1 is the amplitude a. First, the input signal x is converted to a baseband signal.
When quadrature modulation is performed on (t) with angular frequency ωm,

[0015]

[Equation 2]

[0016] The second term of the equation 2 can be considered as a term in which white noise is frequency-shifted, and can be made negligibly small by averaging u (t). That is,

[0017]

[Equation 3]

In the gas concentration measuring apparatus of the present invention, the averaging process is not performed as the actual process, as will be described later, because the ratio of the amplitudes of the fundamental wave and the double wave is taken. Number 3
If the result of is expressed in polar coordinates,

[0019]

[Equation 4]

[0020] The first term of this equation 4 is called an in-phase component, and the second term is called a quadrature component. Therefore, in order to extract the amplitude a from Equation 4, the in-phase component and the quadrature component are each squared and added, and then the square root is taken. That is, the norm calculation may be performed.

[0021]

[Equation 5]

Here, the phase of the quadrature modulator is varied from 0 to 2π, the amplitude for each angle is calculated, and the respective amplitudes are added to obtain a pseudo area approximately equivalent to the area of the ellipse. As shown in FIG. 1, a value proportional to the gas concentration can be obtained by taking the above operation for the angular frequency ωm of the fundamental wave and the angular frequency 2ωm of the second harmonic and obtaining the ratio of the obtained pseudo areas. .

[0023]

EXAMPLE A first example will be described with reference to FIG. The received signal, which is the signal of the detector including the fundamental wave and twice the frequency thereof, is input to the two quadrature modulators 3 and 4. The fundamental wave quadrature modulator 3 has an angular frequency ωm of the fundamental wave.
In addition, the cosine wave signal in which the phase of the angular frequency 2ωm, which is twice the fundamental wave, sequentially increases is added to the double wave quadrature modulator 4. The phase is sequentially and stepwise increased by the phase signal generator 6 in the range of 0 to 2π. In this embodiment, the phase is sequentially increased stepwise in the range of 0 to 2π, but it may be decreased, that is, the phase may be sequentially changed over the range of 0 to 2π.

A sine wave signal is generated from the cosine wave signal by the phase delay devices 33 and 43, and the fundamental wave signal and the second harmonic wave signal and those cosine wave signal and sine wave signal are respectively multiplied by a multiplier 3
It is multiplied by 1,32,41,42. As a result, two signals described by the two terms on the right side of the equation 2 are obtained from the fundamental wave quadrature modulator 3, and the integrators 51, 52, 53, 54 of the next integrator 5 are obtained.
Then, each of them is integrated, and the signal shown in the equation 4 is
Obtained for each in-phase and quadrature component. The two signals from the integrator 5 are added to the square sum calculator 7, squared by the multipliers 71, 72, 73, 74 and added by the adders 75, 76. Further, this signal is calculated by the area extraction unit 8 to add the amplitudes shown in Equation 5 while shifting the phase stepwise, and the signal corresponding to the area on the Gaussian plane is sent to the division unit 10. 1 signal (a denominator signal).

With respect to the second harmonic wave, the arithmetic processing in which the angular frequency ωm is 2ωm in the equations 2 to 5 is performed in the second harmonic quadrature modulator 4, the integrator 5, the sum of squares calculator 7, and the area extractor 8, respectively. As a result, a second signal (a numerator signal) to the division unit 10 is obtained. The dividing unit 10 obtains the ratio B / A of both signals, and this output is an estimated value proportional to the gas concentration.

Note that the value of A is measured in advance and
If the distance from the semiconductor laser 11 to the light receiving unit 14 is fixed, it is not necessary to measure A each time. Therefore, the first cosine wave signal generating unit 1 and the fundamental wave quadrature modulating unit 3 on the side for obtaining A are required. , The integration unit 5, the sum of squares calculation unit 7, and the area extraction unit 8 are unnecessary, and the measured A may be input to the division unit 10. In addition, in FIG. 1, the integrators 51, 52, 53, 54 of the integration unit 5 are
And the multipliers 71, 72, 73, 74 of the sum of squares operation unit 7 are provided for each signal. However, if a memory or a register is provided, these operations are performed by one integrator and multiplier in time division. It is also possible to do so. In short, it is only necessary to perform the operations of the equations 2 to 5 described in the operation.

FIG. 2 shows the configuration of the second embodiment. The difference from the first embodiment (FIG. 1) is the arithmetic processing in the area extraction unit 8. The pseudo areas corresponding to the signals A and B have a radius an (n = 1, 2,
... N)

[0028]

[Equation 6]

The area is calculated by the area extraction unit 8. Here, since the initial phase is an asynchronous method, somewhere on the Gaussian plane cannot be known, but by regarding the point serving as the base point as the phase angle 0, it is possible to use Equation 6 regardless of the initial phase. Further, the second harmonic is an index at 4π instead of 2π.

An example in which methane gas is selected as the detection gas will be described below. FIG. 7 shows a block diagram of a gas detector using the gas concentration measuring device of the present invention. In this figure, the semiconductor laser 11 is a laser having a wavelength of 1653.8 nm and a center frequency in the absorption line of methane gas. With this gas detector,
Feedback control is performed to cancel the second harmonic distortion generated by the laser itself. Also, the modulation frequency was set to 32.5 kHz and the modulation index was set to 20%. The arithmetic processing unit is D of Texas Instruments Incorporated.
Using SP (TMS320C25), master clock is 50MHz
, The sampling rate is 260kHz.

[0031]

According to the configuration of the present invention, the amplitudes of the fundamental wave and the second harmonic wave are calculated from the in-phase component and the quadrature component, and the addition is performed while changing the phase. Pseudo areas A and B are calculated. After that, since the ratio of the signal B and the signal A is obtained to estimate the concentration of the gas to be measured, the gas concentration can be measured without requiring manual adjustment of phase synchronization. Further, since the frequency of the carrier wave used in the quadrature modulator may be slightly shifted, it is not necessary to bring the reference signal from the modulator of the semiconductor laser. Furthermore, since it is not necessary to extract the phase information, the algorithm can be simplified and an inexpensive general-purpose low-speed processor can be used.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of a first exemplary embodiment of the present invention.

FIG. 2 is a diagram showing a configuration of a second exemplary embodiment of the present invention.

FIG. 3 is a schematic diagram of gas absorption characteristics.

FIG. 4 is a principle diagram of a frequency modulation method.

FIG. 5 is a diagram illustrating a frequency modulation method when laser light includes distortion.

FIG. 6 is a diagram showing a locus of a fundamental wave on a Gaussian plane.

FIG. 7 is a block diagram of a gas detector using the gas concentration measuring device of the present invention.

FIG. 8 is a block diagram of a gas detector using a two-wavelength difference method.

[Explanation of symbols]

 1 1st cosine wave signal generation part 2 2nd cosine wave signal generation part 3 Fundamental wave quadrature modulation part 4 2nd harmonic quadrature modulation part 5 Integration part 6 Phase signal generation part 7 2 Sum calculation part 8 Area extraction part 9 Controller 10 Division unit 11 Semiconductor laser 12 Gas cell 13 Light receiving lens 14 Light receiving unit 15 Gas concentration measuring device

Claims (2)

[Claims] 1. A fundamental wave signal (frequency) of an optical signal having absorption characteristics of the gas to be measured, obtained by injecting light amplitude-modulated and frequency-modulated at a predetermined frequency (fm) into the gas to be measured. fm) and a second harmonic signal (frequency 2fm) for measuring the gas concentration, which is a cosine wave signal having a frequency (2fm) which is twice the predetermined frequency and whose phase changes sequentially. And a cosine wave signal generating section (2) for generating an electric signal that detects an optical signal having an absorption characteristic of the gas to be measured, and performs quadrature modulation with the cosine wave signal from the cosine wave signal generating section. A quadrature wave modulation unit (4), an integration unit (5) for time-integrating two mutually orthogonal second-harmonic complex signals output from the second harmonic quadrature modulation unit, and a phase angle range of 2π Phase signal that changes stepwise at 2 of the phase signal generator for outputting toward the cosine wave signal generating section (6), integrated two double wave complex signals orthogonal to each other
A sum of squares operation unit (7) for performing sum of squares, and an area extraction unit (8) for integrating outputs of the sum of squares operation unit over a range of 2π of the phase angle to obtain a pseudo area component of a signal in rectangular coordinates. A controller (9) for stepping the phase signal generator by a predetermined phase at each end of the integration period of the area extractor; and a pseudo area component of the second harmonic signal extracted by the area extractor. A gas concentration measuring device comprising: a signal processing unit including a dividing unit (10) for calculating a ratio of a fundamental wave signal to a pseudo area component which is obtained in advance. 2. A fundamental wave signal (frequency) of an optical signal having absorption characteristics of the gas to be measured, obtained by injecting light amplitude-modulated and frequency-modulated at a predetermined frequency (fm) into the gas to be measured. fm) and a second harmonic signal (frequency 2fm) for measuring the gas concentration, a first cosine wave signal having a predetermined frequency (fm) and having a phase that changes sequentially. Cosine wave signal generator (1), and a second cosine wave signal generator (2) that generates a cosine wave signal having a frequency twice the predetermined frequency (2 fm) and a phase that changes sequentially. ), And a fundamental wave quadrature modulator (3) that receives an electrical signal obtained by detecting an optical signal having an absorption characteristic of the gas to be measured and performs quadrature modulation with the cosine wave signal from the first cosine wave signal generator. , An electrical signal that detects an optical signal with absorption characteristics of the gas to be measured Outputs from the second harmonic quadrature modulator (4) that receives the second quadrature cosine wave signal from the second cosine wave signal generator and performs the quadrature modulation with the cosine wave signal, and the fundamental quadrature modulator and the second harmonic quadrature modulator. An integrating unit (5) for time-integrating two mutually orthogonal fundamental wave complex signals and two mutually orthogonal second harmonic wave complex signals, and a phase signal that sequentially changes stepwise within a phase angle range of 2π. And a phase signal generator (6) for outputting to the first cosine wave signal generator and the second cosine wave signal generator, and two integrated orthogonal fundamental wave complex signals. A sum-of-squares operation unit (7) for performing a sum-of-squares of each of two mutually orthogonal second-harmonic complex signals, and an output of the sum-of-squares operation unit is integrated over a range of 2π of the phase angle and orthogonal Find the pseudo-area component of the signal at coordinates An area extraction section (8), a controller (9) for stepping the phase signal generation section by a predetermined phase at each end of the integration period of the area extraction section, and 2 extracted by the area extraction section. A gas concentration measuring device comprising: a signal processing unit including a dividing unit (10) for calculating a ratio of a pseudo area component of a harmonic signal and a pseudo area component of a fundamental wave signal.