JPH0723869B2 - Gas sensor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Outline] Since the oscillation wavelength of a laser becomes shorter as the drive current increases, a laser drive current is conventionally scanned with a sawtooth wave, and an absorption spectrum is acquired in reverse wavelength order. On the other hand, the present invention is a system in which the laser driving current is scanned with a waveform in which the scanning order is changed, and the sampled signal sequence is rearranged in the reverse order of the wavelength by the signal processing circuit to obtain the absorption spectrum.

Then, the noise included in the signal has a shorter apparent cycle due to the rearrangement, and thus a low-pass filter can be provided to remove the noise having a cycle approximate to the scanning cycle.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas sensor, and more particularly to a novel gas sensor (gas sensing device) detection method using an infrared semiconductor laser.

Recently, pollution caused by air pollution has become a problem, and therefore, a sensing device for detecting the concentration of a gas harmful to a human body is used. A similar sensing device is also used in a method of detecting a gas leak of combustion gas inside a house.

In such a gas sensing device detection system, for example, a laser beam having a constant wavelength is radiated into the atmosphere, an absorption spectrum peculiar to the gas to be measured is detected, and the harmful gas amount is determined from the change amount of the peculiar spectrum. Detection methods have been developed.

However, in such a system, noise is likely to enter from the inside of the detection device or the surrounding environment, and it is necessary to give sufficient consideration to the removal of the noise.

[Problems to be Solved by Prior Art and Invention] Now, a wavelength tunable infrared semiconductor laser device, for example, Pb
A gas sensor that measures an absorption spectrum of a toxic gas using an SSe element or a PbSnTe element is known. It is a system developed because these elements have an oscillation wavelength in the mid-infrared region of about 6 to 10 μm, and most of the air pollutant gas has absorption lines in that wavelength region. For example,
Sulfurous acid gas (SO ₂ ) has an absorption line near 7.2 μm, and ammonia (NH ₃ ) and nitrogen dioxide (NO ₂ ) also have absorption lines in the same wavelength region.

FIG. 3 shows a schematic diagram of the detection system of the gas sensor. 1 is a laser emitting element, 2 and 2'is a detecting element (detector), 3 is a refrigerator, 4 is a reference gas cell, 5 is a lens, 6 is a Mirrors (half mirrors or reflecting mirrors), 7 and 7 ′ are signal processing circuits, 8 is a divider, and 9 is a display.

The detection outline of the detection system shown in FIG. 3 is as follows. A laser is oscillated from the laser light emitting element 1, scanning is performed between wavelengths of a certain width, the laser is branched by a half mirror, and one is transmitted through a reference gas cell 4. The absorption spectrum of the laser is detected by the detecting element 2'and processed by the signal processing circuit 7 '.
The laser branched by the other half mirror transmits through the atmosphere 10 and its absorption spectrum is detected by the detection element 2 and processed by the signal processing circuit 7. That is, the signals processed by the signal processing circuits 7 and 7'are respectively divided and compared by the divider 8,
Display on the display unit 9. Then, the absorption amount of the specific spectrum is measured to detect the gas concentration.

Here, in order to transmit the atmosphere 10 (shown by a broken line) as long as possible, the spherical reflection mirror makes many round trips, and its optical path reaches 10 meters. The refrigerator 3 is a circulation type in which helium gas is circulated, because it is necessary to cool the laser element 1 to about 80 ° K.

However, in such a gas sensor system, the laser wavelength is scanned (scanning) from the long wavelength side to the short wavelength side, and the power of the signal is detected at the sampling point. There is a problem that it is transmitted to the optical axis of the laser light emitting element and becomes vibration noise, and the vibration cycle is close to the scanning cycle for scanning between the wavelengths, which cannot be removed by the low-pass filter processing.

FIGS. 4 (a) to 4 (d) are diagrams for explaining the principle and problems of the detection method of this system. First, FIG. 4 (a) shows that the oscillation wavelength λ of the laser element is the drive current. FIG. 9 is a diagram showing a characteristic corresponding to I, in which the oscillation wavelength λ is shortened when the drive current I is increased. The detection system utilizes this characteristic to change the drive current to scan the laser wavelength from the long wavelength side to the short wavelength side, for example from 7.21 μm to 7.20 μm.

Then, as shown in FIG. 4 (b), the power P of the signal received by the detection element decreases in the absorption spectrum of the gas to be measured. The reduction amount a is proportional to the gas concentration. Actually, by the second derivative method, the second derivative value of the data as shown in FIG. 4B, that is, the secondary fine powder signal P ″ of the driving current I and the power as shown in FIG. Seeking a relationship with
From this, the concentration b of the specific gas is detected. As described above, the reason why the second-order differentiation is performed is to improve the detection performance, but all of this signal processing is performed by the signal processing circuits 7 and 7 '.

Next, FIG. 4 (d) illustrates a time waveform for changing the drive current I of the conventional laser device. In this example, the driving current width is divided into eight time waveforms. When this time waveform is described by a general formula, the following formula is obtained. That is, the drive current value at the long wavelength end of the laser element is I ₀ and the drive current value at the short wavelength end is I _0.
+ L, the drive current width L is digitally divided into N ₀ , and the drive current value changes with time as I ₀ , I ₀ + L / N ₀ , I ₀ + 2L / N ₀ , ... I ₀ + (N ₀ -1 ) L / N ₀ . The eight divisions in the embodiment shown in FIG. 4 (d) means N ₀ = 8
However, the scanning cycle of this waveform is about 10 msec, which is close to the cycle of vibration noise caused by driving the refrigerator. Ν shown in FIG. 4 (c) is vibration noise due to driving of the refrigerator, and the unit of the horizontal axis is different, but is also shown together for reference. Therefore, this vibration noise is difficult to filter, and the sensitivity of the gas sensor is reduced accordingly.

The present invention eliminates such vibration noise of the refrigerator and provides a highly sensitive gas sensor detection method.

[Means for Solving Problems] The purpose is to determine the drive current value at the long wavelength end of the laser light emitting element.
I _0, as I ₀ + L a driving current value of the short wavelength end, in the gas sensor of the driving current width L and N ₀ split, to measure the absorption spectrum by detecting the amount of light corresponding to the driving current value, the L Divide into N ₁ (N ₁ is a natural number smaller than N ₀ ) and divide into N ₁ regions, and drive the laser with a current value at one location in N ₀ for each of the N ₁ regions, Sequentially set all N ₁ areas to 1
By driving the laser each time, thus, while repeating the laser driving, by scanning the N ₁ region N ₀ / N ₁ times, to drive the laser at all current values of the N ₀ places, This is achieved by a gas sensor adapted to make one measurement of the absorption spectrum.

[Operation] That is, since the oscillation wavelength of the laser element corresponds to the laser drive current, the laser drive current is scanned with a waveform in which the scanning order is interchanged, and the sampled signal sequence is processed by the signal processing circuit in reverse wavelength order. Rearrange and acquire the absorption spectrum.

Then, the vibration noise of the refrigerator has an apparent cycle.
Since it is 1 / N ₁ , a vibration noise can be separated and removed from the absorption spectrum by providing a low-pass filter.

[Examples] Hereinafter, examples will be described in detail with reference to the drawings.

In this example, the drive current value at the long wavelength end of the laser light emitting element is I ₀ ,
Assuming that the drive current value at the short wavelength end is I ₀ + L, the drive current width L
Is an embodiment in which L is divided into N ₀ , and (N ₀ ) in L is divided into N ₁ , and the drive current point is expressed by the following equation.

I ₀ , I ₀ + L / N ₁ , I ₀ +2 L / N ₁ , ..., I ₀ + (N ₁ -1) L / N ₁ , I ₀ + L / N ₀ , I ₀ + L / N ₀ + L / N ₁ , I ₀ ＋ L / N ₀ ＋ 2L / N ₁ , ……, I ₀ ＋ L / N ₀ ＋ (N ₁ -1) L / N ₁ , I ₀ ＋ 2L / N ₀ , I ₀ ＋ 2L / N ₀ ＋ L / N ₁ , I ₀ ＋ 2L / N ₀ ＋ 2L / N ₁ , ・ ・ ・, I ₀ ＋ 2L / N ₀ ＋ (N ₁ -1) L / N ₁ , ・ ・ ・, I ₀ ＋ (N ₀ / N ₁ -1) L / N ₀ , I ₀ ＋ (N ₀ / N ₁ -1) L / N ₀ ＋ L /
N ₁ , I ₀ ＋ (N ₀ / N ₁ -1) L / N ₀ ＋ 2L / N ₁ , ……, I ₀ ＋ (N ₀ / N ₁ -1) L / N ₀ ＋ (N ₁ -1) L / N ₁ FIG. 1 (a) shows an example of the scanning waveform of the drive current based on the above equation, and this example is an example in which N ₀ = 8 and N ₁ = 2.

For example, in the above equation, the drive current value I ₀ at the long wavelength end is
50mA, driving current value I ₀ + L at short wavelength end is 90mA (L = 40mA)
Then, when N ₀ = 8 and N ₁ = 2, the driving current is 50mA, 7
The scanning waveform is a repeating current consisting of 0mA, 55mA, 75mA, 60mA, 80mA, 65mA, 85mA.

In this way, the signal sequence sampled by the signal processing circuit is rearranged in the reverse order of wavelength to obtain the absorption spectrum. Then, the vibration noise of the refrigerator superimposed on the signal sequence is apparently halved due to rearrangement.

FIG. 1 (b) shows the second-order differential signal of the power sampled by the present invention and the noise ν superimposed on it. However, in this case, the scanning of the drive current is not arranged in the order of wavelengths, so that the spectrum is not correct.

Therefore, if the spectra are rearranged in order of wavelength, the first
A spectrum as shown in FIG. 3C is obtained, and noise ν ₁
Halves the period. In this way, the cycle of the vibration noise of the refrigerator superimposed on the absorption spectrum is halved, and the noise of such cycle can be removed from the signal by the filtering process.

That is, in the detection method according to the present invention, the same second-order differential data as shown in FIG. 1 (c) similar to the conventional one is obtained, but in the present invention, the cycle of the vibration noise of the refrigerator superimposed on the absorption spectrum is obtained. Is halved, and noise with such a period can be removed from the signal by filtering.

In FIG. 1 (c), the vertical axis represents the power secondary differential signal P ″ and the horizontal axis represents the drive current I. At the same time, vibration noise ν ₁ due to the drive of the refrigerator is also shown. Cage,
The cycle is halved.

In addition, FIG. 2 shows a scanning time waveform of the driving current of another example according to the present invention. In this example, N ₀ = 16 and N ₁ = 4, and when scanning with such a waveform, The frequency of vibration noise of the refrigerator is quadrupled, which further facilitates filtering.

In the above embodiment, in order to simplify the explanation, the division point N ₀ = 8,16
However, in practice, N ₀ = 500 or so. As described above, according to the detection method of the present invention, the vibration noise of the refrigerating machine which is mixed can be separated by the filtering process and can be eliminated.

[Effects of the Invention] As is clear from the above description, according to the present invention, vibration noise of a refrigerator mixed in a signal of an absorption spectrum can be removed, sensitivity is improved, and a gas sensor is improved in performance. It is something.

[Brief description of drawings]

1 (a) to 1 (c) are diagrams showing a detection method according to the present invention, FIG. 2 is a diagram showing a time waveform of another drive current according to the present invention, and FIG. 3 is an outline of a gas sensor detection system. FIGS. 4A to 4D are diagrams showing the principle and problems of the detection method. In the figure, λ is the oscillation wavelength, I is the drive current, P is the power, P ″ is the second derivative signal of the power, T is the time, ν and ν ₁ are vibration noises, 1 is the laser light emitting element, and 2 and 2 ′ are Detecting element, 3 is a refrigerator, 4 is a reference gas cell, 7, 7'is a signal processing circuit, 8 is a divider, and 9 is an indicator.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Iwao Sugiyama Inventor, Iwan Sugiyama 1015 Kamiodanaka, Nakahara-ku, Kawasaki-shi, Kanagawa Within Fujitsu Limited (56) References JP-A-59-109845 (JP, A) JP-A-60-253953 (JP, A) JP-A-57-29934 (JP, A)

Claims (2)

[Claims] 1. A driving current value at a long wavelength end of a laser light emitting device
I _0, as I ₀ + L a driving current value of the short wavelength end, in the gas sensor of the driving current width L and N ₀ split, to measure the absorption spectrum by detecting the amount of light corresponding to the driving current value, the L Divide into N ₁ (N ₁ is a natural number smaller than N ₀ ) and divide into N ₁ regions, and drive the laser with a current value at one location in N ₀ for each of the N ₁ regions, Sequentially set all N ₁ areas to 1
By driving the laser each time, thus, while repeating the laser driving, by scanning the N ₁ region N ₀ / N ₁ times, to drive the laser at all current values of the N ₀ places, A gas sensor characterized in that an absorption spectrum is measured once. 2. The drive current value at the long wavelength end of the laser light emitting element
I _0, as I ₀ + L a driving current value of the short wavelength end, in the gas sensor of the driving current width L and N ₀ split, to measure the absorption spectrum by detecting the amount of light corresponding to the driving current value, the L The gas sensor according to claim 1, wherein the gas sensor is divided into N ₁ (N ₁ is a natural number smaller than N ₀ ) and the drive current portion is expressed by the following equation. I ₀ , I ₀ + L / N ₁ , I ₀ +2 L / N ₁ , ... I ₀ + (N ₁ -1) L / N ₁ , I ₀ + L / N ₀ , I ₀ + L / N ₀ + L / N ₁ , I ₀ ＋ L / N ₀ ＋ 2L / N ₁ , …… I ₀ ＋ L / N ₀ ＋ (N ₁ -1) L / N ₁ , I ₀ ＋ 2L / N ₀ , I ₀ ＋ 2L / N ₀ ＋ L / N ₁ , I ₀ + 2L / N ₀ + 2L / N ₁ , ..., I ₀ + 2L / N ₀ + (N ₁ -1) L / N ₁ , ..., I ₀ + (N ₀ / N ₁ -1) L / N ₀ , _{I 0 + (N 0 / N} 1 -1) L / N 0 + L / N 1, I 0 + (N 0 / N 1 -1) L / N 0 + 2L / N 1, ......, I 0 + (N ₀ / N ₁ -1) L / N ₀ + (N ₁ -1) L / N ₁