CN100559159C - Method and device for optical path self-compensation of fiber optic gas sensor - Google Patents

Abstract

The invention relates to a method and device for optical path self-compensation of an optical fiber gas sensor, including an embedded microprocessor, a DFBLD light source, a temperature control circuit, a signal generation and modulation circuit, a photodiode, a preamplifier, harmonic detection, DC flow detection and Multi-channel AD conversion, the method adopted is: DFBLD light source cyclically emits three optical signals in sequence, which are respectively used for mark detection signals; DC signal B; sawtooth wave excitation signal C, after the three optical signals pass through the gas chamber to be tested, After the optical signal passes through pre-amplification, harmonic detection, DC flow detection and multi-channel AD conversion, the result is calculated by the embedded microprocessor. The present invention realizes the self-compensation of the light path by controlling the light emission of the light source to change according to a specific rule, and has the advantages of a simple light path; it can compensate the influence of dust and water vapor in the gas absorption chamber, and also eliminates the influence of light path bending and the change of the light emission wavelength of the light source caused by temperature changes Advantages of influence on measurement results.

Description

Method and device for optical path self-compensation of fiber optic gas sensor

technical field

The invention belongs to a sensor, in particular to a method and device for optical path self-compensation of an optical fiber gas sensor that uses the principle of spectral absorption to measure gas concentration.

Background technique

The principle of spectral absorption optical fiber gas sensing is that specific gases absorb light of certain specific wavelengths. When a beam of input parallel light with intensity I ₀ passes through the gas, if the light source spectrum covers one or more absorption lines of the gas, the transmitted light intensity will be attenuated when the light passes through the gas. According to the Beer-Lambert law, the relationship between the outgoing light intensity I and the incident light intensity I ₀ , the gas volume fraction C, and the absorption path length L is:

I(t)=I ₀ (t)exp[-α(v)CL]

Therefore, detecting the change of transmitted light intensity can determine the gas concentration. However, since this method indirectly calculates the gas concentration by detecting the change of the transmitted light intensity, the calculation result will inevitably be affected by many factors such as the drift of the luminous intensity of the light source, the dust in the gas absorption chamber, and the attenuation of the light by the optical path. In order to eliminate these effects, necessary compensation must be carried out.

The usual optical path compensation method is to introduce a reference gas chamber: as shown in Figure 1, the light is split through an optical splitter, and the two beams of light pass through the reference gas chamber and the detection gas chamber respectively, and the second harmonic component is detected after being received by a photodiode . The second harmonic component of the gas signal to be measured is: k _d C _d L _d I _0d , and the second harmonic component of the reference gas chamber signal is: k _r C _r L _r I _0r .

Where C _d is the gas concentration to be measured, _Cr is the reference gas concentration, k _d , k _r are the second harmonic coefficients, I _0d , I _0r are the signal amplitudes of the light intensity of the detection gas chamber and the reference gas chamber, respectively. _Ld and _Lr are the optical path lengths of the detection cell and the reference cell, respectively. Divide the two formulas to get: y=k _d C _d L _d I _0d /k _r C _r L _r I _0r

Transform the above formula into: C _d =k*y(C _r L _r I _0r )/(L _d I _0d ) (1) In formula (1), y can be calculated by measurement, k=k _r /k _d is a fixed value, C _r , L _r , L _d are known values, I _0r /I _0d is determined by the splitter, so that when the incident light intensity changes, I _0r /I _0d will not change, thereby eliminating the incident light strong change effects. However, this approach is not ideal. Its limitations mainly exist in the following aspects:

(1) After the luminescent tube emits light, it is necessary to use an optical splitter to split the light, which will weaken the light intensity and reduce the detection sensitivity;

(2) Adding a reference channel increases the product cost;

(3) The reference gas chamber and the detection gas chamber must be placed in the same environment, otherwise the difference in ambient temperature and water vapor will cause changes in I _0r /I _0d ;

(4) In the same environment, since the optical fiber behind the optical splitter is not the same optical fiber, any deformation of the optical fiber on either side will cause a change in I _0r /I _0d ;

(5) When measuring the gas concentration containing dust, the dust in the field will affect the light transmittance of the gas chamber. Since the reference gas chamber is completely sealed and will not be affected by dust, the detection gas chamber must be fully open and will be affected by dust, resulting in changes in I _0r /I _0d ;

To sum up, it is difficult to meet the actual needs by using the reference optical path compensation method.

Contents of the invention

The object of the present invention is to provide an economical and practical optical path self-compensation method and device for an optical fiber gas sensor, which can meet the actual needs, so as to overcome the above-mentioned shortcomings.

In order to achieve the above object, the present invention includes an embedded microprocessor, DFBLD light source, temperature control circuit, signal generation and modulation circuit, photodiode, preamplifier, harmonic detection, DC flow detection and multi-channel AD conversion, the method adopted Yes: Under the control of the signal generation and modulation circuit, the DFB LD light source cyclically emits three optical signals in sequence, which are the initial signal A for the mark detection signal; a DC signal B of a stable excitation current; an AC modulation signal The sawtooth wave excitation signal C, after the three optical signals pass through the measured gas chamber, the photodiode receives the three optical signals, and the optical signal passes through the pre-amplification, harmonic detection, DC flow detection and multi-channel AD conversion, and then the embedded micro The processor calculates the result of the gas concentration C _d to be measured according to the following formula:

C _d =yI _b /k _d L _d I _c

In the formula: C _d is the concentration of the gas to be measured, k _d is the second harmonic coefficient, L _d is the optical path of the measuring gas chamber, I _b is the light intensity of the B section, I _c is the light intensity of the C section, and y is the second harmonic component of the C section The ratio to the second harmonic component of section B, y is calculated after measurement.

After the sawtooth wave excitation signal C of the above AC modulation signal is added with a DC bias B, the laser emission wavelength corresponding to the center point of the sawtooth wave excitation signal C of the AC modulation signal coincides with the absorption peak of the gas to be measured.

The present invention also provides a fiber optic gas sensor optical path self-compensation device, including:

Embedded microprocessor: complete waveform generation control and signal sampling and processing;

DFBLD light source: laser light emitting;

Temperature control circuit: perform closed-loop control on the temperature of the laser to ensure the stability of the laser emission wavelength;

Signal generation and modulation circuit: generate modulation waveform signal for DFBLD light source;

Photodiode: receive laser signal;

Pre-amplification: convert the weak current signal of the photodiode into a voltage signal and amplify it;

Harmonic detection: perform high-pass filtering on the received signal, filter out the fundamental wave, and then perform phase-locked amplification to obtain the second harmonic signal;

DC flow detection: perform low-pass filtering on the received signal to obtain the DC component;

Multi-channel AD conversion: perform analog-to-digital conversion on harmonic signals and DC quantities.

The optical path self-compensation method of the present invention realizes the optical path self-compensation by controlling the light emission of the light source to change according to specific rules, and has the following advantages:

(1) The optical path is self-compensated, and the optical path is simple;

(2) It can not only compensate for the influence of dust and water vapor in the gas absorption chamber, but also eliminate the influence of many factors such as optical path bending;

(3) Eliminate the influence of the change of the luminous wavelength of the light source caused by the temperature change on the measurement result.

Description of drawings

FIG. 1 is a schematic block diagram of an existing optical path compensation method and device.

Fig. 2 is a periodic AC modulation signal diagram of the light source in the present invention.

Fig. 3 is a schematic block diagram of the optical path compensation method and device of the present invention.

Fig. 4 is a software flow chart of the present invention.

Detailed ways

The present invention will be described in further detail below in conjunction with the accompanying drawings.

The periodic AC modulation signal of the light source in the present invention is shown in Figure 2, wherein: A is the initial signal, which is used for the mark detection signal; B is the DC signal, which provides a stable excitation current for the light source, and the bandwidth of the light source is very narrow at this time; C is A sawtooth wave excitation gives an AC modulation signal to the light source, and the second harmonic signal can be obtained after being absorbed by the gas chamber.

After adding a DC bias to the above AC signal, the laser emission wavelength corresponding to the C center point coincides with the absorption peak of the gas to be measured. When the wavelength drifts slightly, the absorption peak is still within the range of C, so that the sawtooth wave C can eliminate the error of the wavelength drift of the light-emitting tube affected by temperature. The sawtooth wave C can make the wavelength bandwidth of the light-emitting tube 0.2nm, and the wavelength drift of the DFB LD under constant temperature control generally does not exceed 0.1nm, which can basically completely eliminate the influence of wavelength drift.

After adding a DC bias to the B section of the signal, the emission wavelength of the luminescent tube in the B section is far away from the gas absorption peak, so that when the gas concentration changes, the light intensity in the B section will not change after passing through the gas chamber. When the light intensity changes due to ambient temperature, water vapor, dust, optical fiber deformation, etc., B and C will be attenuated and enhanced to the same degree. Therefore, it is only necessary to divide the detected second harmonic component k _d C _d L _d I _c by the light intensity I _b of section B to completely eliminate the influence of light intensity. Then get the formula:

y=k _d C _d L _d I _c /I _b

Then C _d =yI _b /k _d L _d I _c

Among them, k _d is the second harmonic coefficient, L _d is the optical path of the measuring gas chamber, I _b is the light intensity of the B section, and I _c is the light intensity of the C section. Because the B-segment light intensity is not affected by gas absorption, the B-segment light intensity can not only eliminate the influence of light intensity changes, but also reflect the light transmittance (the ratio of received light intensity to original light intensity). When the light transmittance is lower than a certain standard, an alarm can be generated to remind to check whether the optical fiber has a large deformation and whether there is dust on the optical path of the air chamber.

The composition block diagram of the gas concentration measuring device adopting the optical path self-compensation method of the present invention is shown in Figure 3, wherein:

The embedded microprocessor is the core control and signal processing part of the device, which completes waveform generation and signal sampling and processing. The embedded microprocessor controls the signal generation and modulation circuit to generate the modulation waveform signal for the DFBLD light source as shown in Figure 2. The signal generation circuit generates AC current, and the modulation circuit makes the amplitude and DC component of the AC current precisely adjustable, and adjusts the DC component so that the central wavelength of the LD light source emitting light in the C section is just aligned with the absorption peak of the gas to be measured, so that the LD light source emits The periodic wavelength scanning range of the light covers a certain absorption peak of the gas.

The temperature control part uses a hardware circuit to perform closed-loop control on the temperature of the laser to ensure the stability of the laser emission wavelength. The temperature of the laser is collected by the temperature sensor, compared with the preset temperature, and the temperature is precisely controlled by the PI adjustment circuit.

The pre-amplification part converts the weak current signal of the photodiode into a voltage signal and amplifies it.

The harmonic detection part first performs high-pass filtering on the received signal to filter out the fundamental wave, and then performs phase-locked amplification to obtain the second harmonic signal.

The DC flow detection part performs low-pass filtering on the received signal to obtain the DC component.

The multi-channel AD conversion part performs analog-to-digital conversion on the harmonic signal and DC flow, and the microprocessor selects the transmitted light intensity data of the B segment and the second harmonic data of the C segment to calculate the gas concentration.

Shown in Fig. 4 is the software block diagram of the present invention, and microprocessor is the S3C44B0X of ARM series.

The content not described in detail in this specification belongs to the prior art known to those skilled in the art.

Claims (3)

1, the self-compensating method of a kind of fiber gas sensor light path, comprise that embedded microprocessor, DFBLD light source, temperature-control circuit, signal generation and modulation circuit, photodiode, preposition amplification, harmonic wave detect, DC quantity detects and the multi-channel A conversion, the method that is adopted is: under the control of signal generation and modulation circuit, DFB LD light source circulates successively and launches three light signals, is respectively the start signal A that is used for the Mark Detection signal; The direct current signal B of a stable exciting current; The sawtooth drive signal C of an ac modulation signal, three light signals are by behind the tested air chamber, photodiode receives three light signals, light signal calculates gas concentration C to be measured by embedded microprocessor according to following formula after changing by preposition amplification, harmonic wave detection, DC quantity detection and multi-channel A _dThe result:

C _d＝yI _b/k _dL _dI _c

In the formula: C _dBe gas concentration to be measured, k _dBe second harmonic coefficient, L _dFor measuring air chamber light path, I _bBe B section light intensity, I _cBe C section light intensity, y is a C section second harmonic component and the ratio of B section second harmonic component, and y calculates after measuring.

2, the self-compensating method of fiber gas sensor light path as claimed in claim 1, it is characterized in that: after the sawtooth drive signal C of ac modulation signal adds direct current biasing B, the laser instrument emission wavelength of the sawtooth drive signal C central point correspondence of ac modulation signal is just matched with the absorption peak of gas to be measured.

3, a kind of fiber gas sensor light path self compensation device comprises:

Embedded microprocessor: finish waveform generation and signals sampling and processing;

The DFBLD light source: laser instrument is luminous;

Temperature-control circuit: the temperature to laser instrument is carried out closed-loop control, guarantees the stability of laser instrument emission wavelength;

Signal produces and modulation circuit: produce the modulation waveform signal to the DFBLD light source;

Photodiode: receive laser signal;

Preposition amplification: convert the low current signal of photodiode to voltage signal and amplification;

Harmonic wave detects: the signal that receives is carried out high-pass filtering, filter first-harmonic, carry out phase-locked amplification then, obtain second harmonic signal;

DC quantity detects: carry out low-pass filtering to received signal, obtain DC component;

Multi-channel A conversion: harmonic signal and DC quantity are carried out analog to digital conversion.

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