JP2007003253A - Gas concentration measuring device - Google Patents

Abstract

Disclosed is a gas concentration measuring apparatus that simplifies the circuit configuration by digitally processing the function of a lock-in amplifier, thereby reducing costs and improving measurement accuracy.
Infrared light emitted from an infrared light source is intermittently incident on a sample cell into which a sample gas is introduced, the amount of infrared light transmitted through the sample cell is detected by a detector, and A / D conversion is performed. The present invention relates to an infrared absorption type gas concentration measurement apparatus that calculates a concentration of a measurement target gas in a sample gas using a signal after A / D conversion. The digital signal processing circuit 70 measures the period of the rotation signal of the gas correlation cell 13 and uses the A / D conversion timing signal generated by dividing this measurement period by a predetermined number N to determine the amplification circuit 30 (detector 20). The output signal is sampled and A / D converted.
[Selection] Figure 1

Description

  The present invention relates to an infrared absorption type gas concentration measurement apparatus using a gas correlation method or the like, in which signal processing technology is improved to improve measurement accuracy.

The gas correlation method is known as an infrared absorption method gas concentration measurement method developed by the US Environmental Protection Agency, and is used for measurement of various gas concentrations including CO and CO ₂ . The principle is that a measurement target gas cell in which a measurement target gas is enclosed between a sample cell into which a measurement target gas such as CO is introduced and an infrared light source that makes infrared light incident on the sample cell; A gas correlation cell (gas correlation filter) having a comparison gas cell filled with an inert gas (zero gas) such as N ₂ is disposed, and this gas correlation cell is rotated to place the measurement target gas on the optical path of infrared light. It measures the concentration of the gas to be measured in the sample gas by measuring the difference between the amount of transmitted infrared light of the sample cell when it is interposed and the amount of transmitted infrared light of the sample cell when the inert gas is interposed. is there.
Note that a gas concentration measuring apparatus using such a gas correlation method is described in Patent Document 1 described later.

FIG. 3 is a block diagram showing a schematic configuration of this type of gas concentration measuring apparatus.
In FIG. 3, an optical system 10 includes an infrared light source, a rotatable gas correlation cell including the measurement target gas cell and the comparison gas cell described above, and a rotatable disk having a slit for interrupting infrared light. The components existing in the infrared light transmission path, such as a chopper (rotating sector) and a multiple reflection mirror in the sample cell, are collectively shown.
The detector 20 detects infrared light that has passed through the sample cell through the measurement target gas cell or the inert gas cell, and converts the transmitted light amount into an electrical signal.

  Here, FIG. 4 is an overall configuration diagram showing an example of a gas concentration measuring device by the gas correlation method, 11 is an infrared light source, 12 is a chopper (rotating sector), 13 is a gas correlation cell, 13a is a measurement target gas cell, Reference numeral 13b is a reference gas cell, 14 is a band-pass filter, 15 is a sample cell, 16a to 16c are mirrors, and 20 is the above-described detector.

Returning to FIG. 3, the amplifier circuit 30 amplifies the output signal of the detector 20, and the lock-in amplifier 40 in the next stage receives infrared light from the output signal of the amplifier circuit 30 as a measurement target gas cell or inactive. The amount of transmitted infrared light when passing through the gas cell is obtained as a DC analog signal.
The lock-in amplifier 40 is a kind of synchronous detector whose main purpose is to measure a minute signal as is well known, and is an analog switch synchronized with a chopping frequency by a chopper (rotating sector) 12 of the optical system 10. The input signal (the output signal of the amplifier circuit 30) is taken in by operating the switching means such as, and this signal is integrated (averaged) and converged to a constant DC value, so that the input signal from which noise has been removed is converted to DC. It is detected as a signal.

The A / D converter 50 converts the DC analog signal output from the lock-in amplifier 40 into a digital signal.
The digital signal processing circuit 60 processes the digital signal and calculates the difference between the amount of transmitted infrared light when transmitted through the comparison gas cell and the amount of transmitted infrared light when transmitted through the measurement target gas cell, thereby measuring the measurement target in the sample gas. The gas concentration is calculated and output to a not-shown recorder or display device.

  As described above, for example, Patent Documents 2 and 3 disclose a gas concentration measuring device that measures a gas concentration using a lock-in amplifier.

Japanese Patent No. 3424364 ([0015] to [0024], FIG. 1, FIG. 2, etc.) Japanese Patent No. 2744728 ([0046] to [0057], FIG. 1 etc.) Japanese Patent No. 2744742 ([0029] to [0047], FIG. 1 etc.)

As shown in FIG. 3, in the conventional gas concentration measuring apparatus using the lock-in amplifier 40, the number of circuit components constituting the lock-in amplifier 40 is large, the cost increases, and the circuit constant variation, temperature drift, There is a problem that an error is generated in the output signal due to the influence of external noise and the like, which is directly related to a measurement error of gas concentration.
Therefore, the problem to be solved by the present invention is to provide a gas concentration measuring device that simplifies the circuit configuration by digitally processing the function of the lock-in amplifier, thereby reducing the cost and improving the measurement accuracy. It is.

In order to solve the above-mentioned problem, the invention described in claim 1 makes the infrared ray emitted from the infrared light source intermittently enter the sample cell into which the sample gas is introduced,
Infrared absorption type gas that detects the amount of infrared light transmitted through the sample cell by a detector and performs analog / digital conversion, and calculates the concentration of the gas to be measured in the sample gas using the signal after analog / digital conversion. In the concentration measuring device,
Means for measuring the period of intermittent incidence of the infrared rays;
Means for generating a timing signal by dividing a measurement cycle by this means by a predetermined number;
Means for sampling the output signal of the detector using the timing signal and performing analog / digital conversion;
It is equipped with.

The invention described in claim 2 is the gas concentration measuring device according to claim 1,
The gas concentration measuring device is a gas concentration measuring device based on a gas correlation method including a rotatable gas correlation cell, and measures a rotation period of the gas correlation cell and divides the period by a predetermined number to generate a timing signal. Is to be generated.

  According to the present invention, without using a lock-in amplifier, the output of the detector using the A / D conversion timing signal obtained by dividing the rotation period of the gas correlation cell or the like by the arithmetic processing of the digital signal processing circuit. The signal is sampled and A / D converted, and the output signal can be used to perform arithmetic processing for gas concentration measurement. This eliminates the need for the circuit components that make up the lock-in amplifier, simplifying the circuit configuration and reducing costs, and avoiding measurement errors caused by circuit constant errors, temperature drift, and external noise effects. Without this, the measurement accuracy of the gas concentration can be improved as compared with the conventional case.

Hereinafter, embodiments of the present invention will be described with reference to FIGS.
First, FIG. 1 is a block diagram showing a schematic configuration of this embodiment. In FIG. 1, the same components as those in FIG. 3 are denoted by the same reference numerals, and description thereof is omitted. Hereinafter, different portions will be mainly described.
FIG. 2 is a conceptual waveform diagram showing the operation of this embodiment in time series. Since the function of the chopper (rotating sector) 12 is not the gist of the present invention, the description thereof is omitted.

  In FIG. 1, a gas correlation cell rotation signal is output from the optical system 10, and this rotation signal is input to a digital signal processing circuit 70 including a CPU, a DSP (digital signal processor) and the like. The gas correlation cell rotation signal is a signal obtained by detecting the rotation period of the gas correlation cell by, for example, a photocoupler or a magnetic sensor. As shown in FIG. 2A, the infrared light transmitted through the comparison gas cell 13b in FIG. 4 and a reference cycle for measuring infrared rays transmitted through the measurement target gas cell 13a in FIG.

As shown in FIG. 2B, the digital signal processing circuit 70 has a sample period T ₁ (hereinafter, T ₃ , T ₅ ,...) And a reference period T ₂ (hereinafter, T ₄ , T ₆ ,...). Measure and periodically store these cycles in the internal memory each time. The sample periods T ₁ , T ₃ , T ₅ ,... And the reference periods T ₂ , T ₄ , T ₆ ,... Are theoretically the same values as long as the rotation speed of the gas correlation cell 13 is constant. However, since an error occurs in the cycle due to a power supply frequency of a motor or the like that drives the gas correlation cell 13 and rotation unevenness due to a mechanical structure, it is necessary to measure every rotation of the gas correlation cell 13.
In FIG. 2 (b), the gas correlation cell rotation signal is at “High” level (transmission) and “Low” level (cut-off) within each period. In correspondence with the presence or absence of infrared light reaching the detector 20.

In the digital signal processing circuit 70, an appropriate division value N (integer) is held in the internal memory, and T _n / N (n = 1, 2, 3) when the sample period and the reference period are measured. ,... Are calculated, and an A / D conversion timing signal having the interval T _n / N is generated as shown in FIG. Then, these A / D conversion timing signals are output to the A / D converter 50.

  As shown in FIG. 2D, the A / D converter 50 includes an output signal of the amplifier circuit 30 by the transmitted infrared light in the sample period (indicated as “sample” in FIG. 2D). , The output signal of the amplifier circuit 30 (also referred to as “reference”) by the transmitted infrared light in the reference period is alternately input, and measurement in the sample gas introduced into the sample cell 15 of FIG. The target gas concentration can be obtained based on the difference in amplitude between the “sample” signal and the “reference” signal.

Therefore, in the A / D converter 50, the output signal of the amplifier circuit 30 is sampled using the A / D conversion timing signal for each interval T _n / N output from the digital signal processing circuit 70, and converted into a digital signal. To do.
For example, the digital signal processing circuit 70 generates an A / D conversion timing signal having an interval T ₁ / N from the measured sample period T ₁ , and the A / D converter 50 performs an amplification circuit in the next reference period T ₂ . The “reference” signal output from 30 is sampled at intervals T ₁ / N and A / D converted. In FIG. 2D, a ₁ and a ₂ are reference signals based on transmitted infrared light, and b _{1 to} b ₃ are reference signals based on cut-off infrared light (ie, zero values). Then, the amplitudes of the reference signals are calculated by averaging the digital values of these reference signals a ₁ , a ₂ and b _{1 to} b ₃ to obtain the difference.

Similarly, the digital signal processing circuit 70 generates an A / D conversion timing signal having an interval T ₂ / N from the measured reference period T ₂ , and the A / D converter 50 amplifies in the next sample period T ₃ . The “sample” signal output from the circuit 30 is sampled at intervals T ₂ / N to perform A / D conversion. Then, the digital signal processing circuit 70 averages and compares the respective digital values of the sample signals c _{1 to} c ₃ using transmitted infrared light and the sample signals (ie, zero values) d ₁ and d ₂ using cut-off infrared light. Is obtained to calculate the amplitude of the sample signal.

  The digital signal processing circuit 70 subtracts the amplitude of the reference signal from the amplitude of the sample signal obtained as described above to obtain the difference in the amount of infrared absorption. Since this difference corresponds to the measurement target gas concentration in the sample gas introduced into the sample cell 15, the target measurement target gas concentration is calculated by performing appropriate conversion.

  As described above, according to the present embodiment, the detector 20 is synchronized with the rotation period of the gas correlation cell 13 by the arithmetic processing by the digital signal processing circuit 70 without using a lock-in amplifier as in the prior art. The output signal of the (amplifier circuit 30) can be sampled at a predetermined interval, A / D converted, and used for the calculation of the infrared absorption amount. This eliminates the need for the circuit components that make up the lock-in amplifier, thereby reducing costs and eliminating measurement errors caused by variations in circuit constants, temperature drift, and noise. it can.

Even when there are fluctuation factors such as fluctuations in the power supply frequency for driving the gas correlation cell 13 and rotation unevenness due to the mechanical structure, the gas correlation cell rotation signal always reflects these fluctuation factors, and digital signal processing is performed. Since the circuit 70 generates an A / D conversion timing signal having an interval T _n / N corresponding to the cycle of the gas correlation cell rotation signal measured each time, the output signal of the amplifier circuit 30 is always sampled at an appropriate timing, and A / D conversion is possible.

The principle of the present invention is that the digital signal processing circuit 70 samples the detector output signal by calculating the interval T _n / N according to the period in which the infrared rays are intermittently incident on the sample cell, and performs A / D conversion. In addition to the gas concentration measurement device based on the gas correlation method described above, for example, in a gas concentration measurement device based on the non-dispersive infrared absorption method (NDIR), infrared light is intermittently transmitted by a chopper (rotating sector). The present invention can also be applied when the gas concentration is measured by entering the sample cell.

It is a block diagram which shows the schematic structure of embodiment of this invention. It is a wave form diagram which shows operation | movement of embodiment. It is a block diagram which shows the schematic structure of the conventional gas concentration measuring apparatus. It is a whole lineblock diagram showing an example of a gas concentration measuring device.

Explanation of symbols

10: Optical system 11: Infrared light source 12: Chopper (rotating sector)
13: Gas correlation cell 13a: Gas cell to be measured 13b: Comparison gas cell 14: Band pass filter 15: Sample cell 16a to 16c: Mirror 20: Detector 30: Amplifier circuit 40: Lock-in amplifier 50: A / D converter 60, 70: Digital signal processing circuit

Claims (2)

Infrared light emitted from an infrared light source is intermittently incident on a sample cell into which a sample gas is introduced,
Infrared absorption type gas that detects the amount of infrared light transmitted through the sample cell by a detector and performs analog / digital conversion, and calculates the concentration of the gas to be measured in the sample gas using the signal after analog / digital conversion. In the concentration measuring device,
Means for measuring the period of intermittent incidence of the infrared rays;
Means for generating a timing signal by dividing a measurement cycle by this means by a predetermined number;
Means for sampling the output signal of the detector using the timing signal and performing analog / digital conversion;
A gas concentration measuring device comprising: In the gas concentration measuring apparatus according to claim 1,
The gas concentration measuring device is a gas concentration measuring device by a gas correlation method including a rotatable gas correlation cell,
A gas concentration measuring apparatus that measures a rotation period of the gas correlation cell and generates a timing signal by dividing the period by a predetermined number.

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