JPH01285841A - Signal processing system of gas sensor - Google Patents

Abstract

PURPOSE:To accurately detect concn. even when there is an environmental temp. change, by calculating only the max. and min. points being zero points on the basis of the largest and smallest points from 3rd order differentiation spectrum. CONSTITUTION:The outputs of infrared sensors 6, 11 are supplied to signal processing circuits 7, 12 while an initial value is set by a signal processor 20 and, next, a 3rd order differentiation spectrum is calculated on the basis of the 2nd order differentiation spectrum obtained by the hardware like processing due to the circuits 7, 12. Subsequently, the largest point Mx and the smallest point Mn are calculated to limit the range between the largest point Mx and the smallest point Mn to a max. point detection range and a point where 3rd order differentiation signal quantity becomes zero, that is, the max. point Ma is detected within said range. Further, the min. point Mi1 is detected from the calculated smallest point Mn and the min point Mi2 is detected from the continuously calculated largest point Mx. As mentioned above, the largest point Mx and the smallest point Mn are calculated from the 3rd order differentiation spectrum and the max. point Ma is calculated therebetween and the min. point Mi1 is calculated from the smallest point Mn and only the min. point Mi2 is calculated from the largest point Mx. By this method, a detection mistake is prevented.

Description

[Detailed Description of the Invention] [Summary] Regarding the popular gas sensor, which measures the absorption spectrum 1H of the gas to be measured using measurement light from a semiconductor laser and detects the gas concentration based on this absorption spectrum, it is possible to detect changes in the environmental temperature. The purpose of this is to accurately detect the concentration even if Find the maximum point and minimum point of the spectrum, find the wavelength of the local maximum point that is a zero point between the maximum point and the minimum point, and find the local minimum point that is a zero point outside the minimum point and the maximum point, respectively. The wavelength of the point is determined, and the signal amount of two or more of the maximum point and minimum point in the secondary image spectrum is determined from the determined wavelengths of the maximum point and minimum point.

[Industrial application field]

The present invention relates to a gas sensor that measures the absorption spectrum of a gas to be measured in the atmosphere using measurement light from a semiconductor laser, and detects the gas concentration based on this absorption spectrum.

In recent years, the management of harmful gases has been strictly regulated due to pollution prevention and the like, and the concentration of harmful gases has been measured in the field of such management. As an example of a gas sensor, a wavelength-tunable infrared laser diode (semiconductor laser) is used to project a laser beam with a continuously changing wavelength onto a gas to be measured, and obtain an absorption spectrum of the wavelength of the laser beam that is unique to that gas. Some methods measure gas concentration by measuring the difference in transmittance.

In this case, the temperature of the semiconductor laser element changes due to a change in the environmental temperature, and as will be described later, the spectrum shifts due to the temperature change, making it impossible to obtain accurate values from the spectrum. Therefore, even in such cases, it is necessary to obtain accurate values and perform accurate concentration detection.

[Conventional technology]

FIG. 4 shows a configuration diagram of a general gas sensor. In the figure, the emitted light from a semiconductor laser 1 is made into parallel light by a lens 2, passes through a half mirror 3, a small amount of gas to be measured in the atmosphere 4, and is focused by a lens 5 onto an infrared sensor 6. The signal is photoelectrically converted and input to the signal processing circuit 7. The semiconductor laser 1 can continuously sweep the wavelength by changing the laser current, thereby making it possible to measure the absorption spectrum of the gas to be measured as shown in FIG. Signal processing circuit 7
, the difference Ct between the maximum transmittance point and the minimum transmittance point is measured, and the temperature proportional to Ct is calculated.

On the other hand, the light reflected by the half mirror 3 is reflected by the mirror 8, and passes through a reference gas cell 9 (a capsule filled with the same type of gas as the gas to be measured 4 and a known concentration of gas) in the same temperature environment. The light is focused by the lens 10 onto the infrared sensor 11, photoelectrically converted there, and input to the signal processing circuit 12 having the same function as the signal processing circuit 7. In the signal processing circuit 12, the concentration of the reference gas cell 9 is calculated.

The respective outputs of the signal processing circuits 7 and 12 are supplied to a divider 13, where the ratio of the signal on the side of the gas to be measured 4 to the signal on the side of the reference gas cell 9 is calculated, and the concentration is displayed on the display 14.

By the way, the transmittance difference Ct shown in FIG. 5 is generally very small, and the absorption spectrum also includes noise components, so it is impossible to accurately detect the concentration as it is. Therefore,
In order to improve the S/N ratio and stability, the absorption spectrum shown in Fig. 5 is actually divided into two parts by hardware processing, and the secondary image spectrum shown in Fig. 6 is used. The signal amount difference Cd (corresponding to the transmittance difference Ct shown in FIG. 5) between the maximum point and the minimum point for two or more units is determined by calculation, and the concentration is detected from this.

If we divide the absorption spectrum shown in Figure 5 into two parts, we can obtain a secondary image spectrum as shown in Figure M6.
The transmittance difference Ct shown in the figure is detected as a secondary image signal difference Cd between one maximum point and two minimum points in the secondary image curve. In this case, the range in which the wavelength of the semiconductor laser is swept is the wavelength sweep range shown by Δλ in FIG. 6, and is generally about 10 to 50 people.

[Problem to be solved by the invention]

By the way, there is no particular problem if the environmental temperature does not change at all, but generally the environmental temperature changes, and as a result, the temperature of the semiconductor laser element changes, causing problems as described below.

Figure 7 is not shown in Figure 6, but is an enlarged view of the second-order differential spectrum in the wavelength sweep range △λ. This is the second-order differential curve after the generation. .

As is clear from FIG. 7, before a temperature change occurs, there is one maximum point rna and two minimum points mil and mi2 in the wavelength sweep range Δλ, and from these three extreme points, the second-order differential The signal amount difference can be determined.

However, after a temperature change occurs, the second-order differential spectrum shifts like a constant chain line, and the wavelength sweep range Δλ has two maximum points ma+', ma2' and two minimum points mi+', mi2'. exists, that is, the maximum point m
a,! ' exists, and this extra maximum point ma
2' makes it impossible for the signal processing circuit to obtain the second-order differential difference, and as a result, there is a problem in that accurate gas concentration cannot be detected.

SUMMARY OF THE INVENTION An object of the present invention is to provide a signal processing method for a gas sensor that can accurately detect concentration even when there is a change in environmental temperature.

[Failure to solve the problem]

Figure 1 shows a diagram of the principle of the present invention, in which <A> is a second-order differential spectrum, and (B) is a difference spectrum (third-order differential spectrum) of the second-order differential spectrum shown in (A>). In the present invention, the difference spectrum of the second-order differential spectrum (the third-order differential spectrum shown in FIG. 1B) shown in FIG. The maximum point M that is a zero point between and the minimum point Mn
Find the wavelengths of a and find the minimum point M11°M i 2 which is a zero point outside the minimum point 1yln and maximum point MX, respectively.
The maximum point Ma and the minimum point M obtained are determined respectively.
i+, M! Maximum point ma and minimum point mi, , mi2 at J5 in the second derivative spectrum from each wavelength of 2
The configuration is such that the second-order differential signal amount is determined.

[Operation] In the present invention, as shown in FIG. 1<8), only the maximum point Ma and the minimum point M11°M i 2, which are zero points, are determined from the third-order differential spectrum based on the maximum point MX and the minimum point 1yln. , and the pseudo-maximal point MX', which is the other zero point, is not found. As a result, the second-order differential spectrum shifts due to temperature changes, and the maximum ha, which is irrelevant to the measurement, is at wavelength 13.
Even if the pseudo maximum point Mx' falls within the reference range, this pseudo maximum point Mx' is not detected, so no detection error will occur.

〔Example〕

FIG. 2 shows a block diagram of an embodiment of the present invention, and in the figure, the same components as those in FIG. 4 are given the same numbers and their explanations will be omitted. In FIG. 2, reference numeral 20 denotes a signal processing device, which is composed of a microcomputer.
1. Obtain the wavelength at the maximum and minimum points of the third-order differential spectrum 10 described later, and detect the concentration of the gas to be measured 4 from this and the output of the infrared sensor 6 on one side of the gas to be measured 4. do. Also, in Fig. 2, 21 is a laser controller, which is a signal processor! 1. The wavelength of the semiconductor laser 1 is swept by a control signal from the device @20. Note that the laser controller 21 is omitted in FIG. 4.

Next, the operation of the present invention will be explained. 3. In FIG. 2, the output of the infrared sensor υ6, 11 is
2. On the other hand, initial value setting is performed in the signal processing device 20 (step 30 in FIG. 3), and then, based on the second-order differential spectrum obtained by hardware processing in the signal processing circuits 7 and 12, A third-order differential spectrum (difference spectrum of second-order differential spectrum) as shown in FIG. 1(B) is calculated (step 31 in FIG. 3).

Next, find the maximum point MX and minimum point Mn shown in FIG. Within this range, the point where the third-order differential signal amount becomes zero, that is, the maximum point Ma
is detected (step 34 in FIG. 3). The reason why the maximum point can be detected by finding the zero point in the range from the maximum point to the minimum point is that the absorption spectrum is close to the Lorentz type.

Furthermore, from the obtained minimum point Mn, search to the right to find the minimum point Mi+, which is a zero point (step 35 in FIG. 3), and then from the obtained maximum point Mx, search to the left to find the minimum point Miz, which is a zero point. is detected (step 36 in FIG. 3). In this way, in the present invention, the maximum point M
Find X and the minimum point Mn, and find the maximum point M between them.
Find the minimum point M, which is the zero point on the right side of the minimum point Mn.
i+, and only the minimum point M i 2, which is a zero point on the left side from the maximum point Mx, is found, and the other zero points, pseudo-maximum point Mx', are not found. As a result, the secondary image spectrum shifts due to temperature change, and the maximum point irrelevant to the measurement enters the wavelength sweep range Δλ, resulting in a pseudo maximum point MX.
Even if ', this pseudo maximum point MX' is not detected, so a detection error as explained in FIG. 7 will not occur.

In this way, the wavelength of the maximum point Ma, the wavelength of the minimum points Mi+, and Mi2 are determined from the spectra of the reference gas in the reference gas cell 9 in FIG. Two signal peaks of the maximum and minimum points on the minute spectrum are obtained. On the other hand, the output of the infrared sensor 6 on the gas to be measured 4 side is also supplied to the signal processing device 20, and the secondary image spectrum for the gas to be measured 4 is determined here. In this case, since the reference gas and the gas to be measured 4 are of the same type, the wavelengths indicating the maximum and minimum points are the same.

Therefore, from the ratio of the signal amount at each of the maximum point and minimum point for the reference gas to the signal amount at the maximum point and minimum point of the gas to be measured 4 at the same wavelength as the maximum point and minimum point of this reference gas, , the ratio between the reference gas 11 degrees and the concentration of the gas to be measured 4 is determined.

〔Effect of the invention〕

As explained above, according to the present invention, even if the spectrum moves due to the temperature change of the semiconductor laser element due to the environmental temperature change, and an extreme point unrelated to the measurement enters the wavelength sweep range, it is possible to avoid the mistake of detecting this. In this case, the tolerance range for the i degree drift can be increased, and accurate gas concentration detection becomes possible.

[Brief explanation of the drawing]

Fig. 1 is a diagram of the principle of the present invention, Fig. 2 is a block diagram of an embodiment of the present invention, Fig. 3 is an operation flowchart of the present invention, Fig. 4 is a configuration diagram of a general gas sensor, and Fig. 5 is a diagram of the gas sensor. FIG. 6 shows the second-order image spectrum of the absorption spectrum shown in FIG. 5, and FIG. 7 shows the second-order image spectrum when the temperature changes. -12= In the figure, 1 is a semiconductor laser, 4 is a gas to be measured, 6.11 is an infrared sensor, 9 is a reference gas cell, 20 is a signal processing device, ma is the maximum point in the secondary image spectrum, mil, mi
2 is the minimum point in the second-order image spectrum, Mx is the maximum point in the three-order image spectrum, Mn is the minimum point in the three-order image spectrum, Ma is the maximum point in the third-order image separation spectrum, M:
1, M! 2 indicates a minimum point in the third-order image spectrum, and Mx' and ma2' indicate pseudo-maximum points. Operation flowchart of the present invention Figure 3 Infrared absorption spectrum of gas at laser wavelength to be measured Figure 5 Secondary image of absorption spectrum shown in Figure 5 Kukokyo

Claims (1)

[Claims] The absorption spectrum of a gas to be measured in the atmosphere is measured using measurement light from a semiconductor laser, and the maximum point (ma) and minimum point (mi_1, m
In a gas sensor that detects the gas concentration of the measured gas based on the second-order differential signal amount of i_2), the maximum point (ma) and the minimum point (
When detecting the second-order differential signal amount of each of mi_1, mi_2), the difference spectrum (third-order differential spectrum) of the second-order differential spectrum is obtained and the maximum point (M
x) and the minimum point (Mn), and find the maximum point (Ma
), and find the minimum point (Mn) and the maximum point (Mx
) are the minimum points (Mi_1, M
i_2), and from the respective wavelengths of the maximum point (Ma) and minimum points (Mi_1, Mi_2) of the obtained third-order differential spectrum, calculate the maximum point (ma) and minimum point (mi
A signal processing method for a gas sensor, characterized in that a second-order differential signal amount of _l, mi_2) is obtained.