JP2010107489A5 - - Google Patents

Description

Infrared CO2 densitometer using pyroelectric element as light receiving element Technology field

  The present invention relates to a CO2 gas concentration measuring apparatus using infrared rays.

The demand for CO2 gas concentration measurement has been increasing in recent years. As a general technique, a method called NDIR is used in which the CO 2 gas absorbs a specific wavelength of infrared rays.
FIG. 1 shows a schematic diagram of the NDIR method. As shown in FIG. 1, in the NDIR method, a measurement operation is performed in a container called a cell filled with measurement air that has been passed through an air filter that prevents entry of dust and the like.
The cell is provided with an infrared light source (IR LAMP in the schematic diagram) and an infrared light receiving element (IR DETECTOR in the schematic diagram) having light receiving sensitivity to the infrared light. An optical filter (in the schematic diagram, an IR filter) is installed on the front surface of the infrared light receiving element.
CO2 is an optical filter with a characteristic of selectively transmitting infrared light with a wavelength of infrared light (4.3 μm) that is well absorbed, so that the minute change in the amount of light absorbed by CO2 can be changed to a large voltage output change. Is provided.

Infrared light emitted from the infrared light source is absorbed by CO2 in the air, but the infrared light receiving element output is attenuated depending on the degree of absorption. By evaluating the magnitude of the infrared light receiving element output, it is possible to know the amount of CO2 in the air and the CO2 concentration.
If the infrared light receiving element is an element that outputs a direct current, the output fluctuation of the infrared light receiving element can be known stably by performing stable direct current amplification of the light receiving element output. It is possible to stably measure the CO2 concentration in the air. .

(Selection of light receiving element)
From the above, it can be seen that amplifying a stable infrared light receiving element output with a stable amplifier is important for stable CO2 concentration measurement.
On the other hand, thermopile (thermopile element) and pyro (pyroelectric element) are known as infrared light receiving elements.

(Thermopile)
The thermopile is an infrared light receiving element having a structure in which thermocouples are bundled, and an output value is increased by bundling a large number of thermocouples. In principle, it reacts to the thermal action of infrared light, and is an output value with respect to infrared light intensity (heat intensity). Since it is a direct current output proportional to the heat intensity, the output is directly output by a direct current amplifier. It can be used as CO2 concentration measurement data. The use history of the infrared CO2 concentration meter as an infrared light receiving element is old and widely used. Although the output is relatively large with a structure in which a large number of thermocouples are bundled, the absolute value of the output is very small, and it is difficult to use a cheaply supplied amplifier as a DC amplifier.

(Pyro (pyroelectric element))
The pyro element is a ceramic element, which generates an output voltage by a photoelectric effect by infrared light, and is different from a thermopile using “generation of thermoelectromotive force by a thermoelectric effect of metal”. The major difference from the thermopile is high sensitivity to infrared light and a large output value. The amplifier can be AC amplified, and the amplifier can be supplied at low cost. However, while the amplifier can be supplied at a low cost, it is a type that responds to a change in the infrared light receiving input and a light receiving element corresponding to the change, and does not react to direct current light. For this reason, the processing of the DC infrared light incident signal such as the measurement of the CO2 concentration requires a contrivance in the signal processing and is accompanied by difficulty. Like a thermopile, it is impossible to obtain an amplified signal using only a DC amplifier, and a general device uses a microcomputer, which is an obstacle to lowering the price. For this reason, application to a low-cost CO2 concentration meter is delayed.

JP-A-10-339698

[Patent Document 1] mainly describes the use of a thermopile with a direct current output as an infrared light receiving element. Although it is described that a pyro element (pyroelectric element) of change output can be used, there is no specific description.

The present invention described in this application is a device for providing a method that enables amplification and signal processing using a pyro element as an infrared light receiving element, not using a microcomputer, and corresponding to low cost, This contributes to lowering the price and improving the performance of infrared application CO2 concentration meters using pyro elements. Of course, even if a microcomputer is used, it can be used without any problem.

  The invention according to claim 1, which has been made to solve the above-described problems, makes it possible to supply a CO 2 sensor by using a pyro element output signal process without using a microcomputer and using a general-purpose discrete IC.

  In an NDIR infrared CO2 densitometer configured using an incandescent lamp as an infrared light source and a pyroelectric element as an infrared light receiving element, the infrared light source is turned on at a constant period, and the infrared light emitted from the infrared light source is CO2 Infrared light is absorbed and attenuated by CO2 in the concentration measurement target air, irradiated to the light receiving element, the light receiving element outputs a photoelectric signal, the photoelectric signal is input to the amplifier, and the amplified photoelectric signal P2 (FIG. 2e1).

On the other hand, when the infrared light source changes from non-lighting to lighting at the measurement timing Tn, the amplified photoelectric signal output P2 of the infrared light receiving element increases as shown in FIG. However, when the increased waveform is P10 (FIG. 2e2), the degree of infrared absorption of P10 varies depending on the CO2 concentration in the CO2 measurement target air, and the peak value changes.
That is, when the CO2 concentration in the measurement target air is high in P10, the infrared light from the infrared light source is more absorbed by the CO2 in the measurement target air, and the amount of infrared light reaching the light receiving element is reduced. However, the peak value becomes low. The value of the following formula (1) including this waveform, P10, is P3,
P3 = k1 × (k2-P10) = k3-k1P10 Formula (1)
It is possible to obtain the CO2 concentration in the air by the P3 value of the equation (1).

The value P3 obtained by equation (1) is the change in FIG. 2d, in which the vertical axis indicates the P3 value and the horizontal axis indicates the time elapsed after the infrared light source is turned on. The minimum value (bottom peak value, P3-BP) in FIG. 2d is that the light amount of the infrared light source becomes the maximum light amount after the lamp lighting voltage is applied to the lamp filament, and the pyro element photoelectric output increases accordingly and becomes the peak value. It has to do with time. The rationality that this P3 bottom peak value is the gas concentration measurement value (uncalibrated value) at the measurement timing Tn can be seen.
If this uncalibrated value is calibrated by the method of claim 3, it becomes a measured gas concentration value calibration value.

The operation of obtaining the CO2 concentration value from the equation (1) is enabled by the operation of [Claim 1 [Claim 2]].
An NDIR CO2 concentration sensor that turns on an infrared light source (incandescent lamp) at regular intervals, receives light with an infrared light receiving element (pyro element) with an infrared optical filter, and measures the CO2 concentration in the air between the infrared light source and the light receiving element. In
Among infrared light receiving element output signals,
The output from only the infrared light source is P10,
P3 = k1 × (k2−P10) (1)
In the second aspect, the measured P3 value obtained by the equation (1) is compared with the numerator P13 numerator in which the initial value is set to 0 in advance.
(Measurement value) P3 ≧ (Numerical value) P13,
The P13 value is changed to a value obtained by a mathematical formula determined according to the value of the register value P13,
In the case of (measured value) P3 ≦ (value counter value) P13,
By making it coincide with the value of the register value P13, the measured value can be obtained.

As described above, the P3 signal value represented by the formula (1) is the infrared light receiving element output signal of the infrared light receiving element output signal, and the photoelectric output P10 value is absorbed by the CO2 gas in the air. Since it includes the P10 value that is the light quantity value information obtained as a result of the reception, it indicates the CO2 gas concentration in the air.
The ability to express the CO2 gas concentration value using the P10 value in equation (1) means that the CO2 concentration value can be expressed with the P10 value and one point of information. It is possible to obtain a measurement value that includes, thereby simplifying the arithmetic processing.

In the case of using a microcomputer for the calculation process, it is not possible to obtain the CO2 concentration value by the method of waveform analysis with the method of taking in the multipoint value of the P3 waveform even if the calculation method using the formula (1) is not used. Although the method using the formula (1) is possible, the difference between “multi-point information” and “single-point information” is large when compared with the fact that the information to be captured is sufficient with only the P3 value and one point. As a difference in cost, it becomes remarkable.

  FIG. 3 shows an example circuit of the implementation. The use example circuit of FIG. 3 is a circuit using an amplifier that does not use a microcomputer and is inexpensive, and has a small number of components.

  Hereinafter, an embodiment of the present invention will be described with reference to FIG.

In the circuit example shown in FIG. 3, the expression {circle around (1)} is the output of the amplifier U2 in FIG. 3 and U2-14 corresponds to the expression {circle around (1)} output P3. A light receiving element output (P10) that receives infrared light attenuated and attenuated infrared light is input.
On the other hand, the capacitance C1 voltage value used as a register is used as buffer amplifier outputs U2-1 and P13. The above-mentioned P3 and P13 are compared by a comparator, and the amplifier outputs P11 and (U2-8) are compared. obtain.
P11 controls the increase / decrease of the number place of the place number device C1.

  By controlling the increase / decrease of the P13 value according to the positive / negative polarity of U2-8 = P11, it is possible to calculate the CO2 concentration, save it as the voltage value of the capacitor C1, and obtain the measured concentration value Become.

As for the increase or decrease of the charge value of the capacitor C1 used as a register, the charge operation to C1 is an increase in charge value = concentration value and the discharge operation of C1 is a decrease in charge value = concentration value.
In the embodiment, charging is always performed with an internal low voltage, and discharging is controlled with P11 polarity.

  The CO2 concentration value is stored as a capacitance C1 voltage value as a measurement value every time the infrared light source is turned on and can be used as P13 = CO2 concentration value. However, the calibration value of the actual CO2 concentration and the capacitance C1 voltage value is If it is desired to obtain, a calibration value can be obtained through an operational amplifier (amplifier output U2-7) that performs variable magnification calculation. ([Claim 3])

In the case of executing by calculation using a microcomputer, the register and the capacity C1 are
It is possible to obtain a similar result by using a digital register, performing the comparison operation of P3 and P13 by digital operation, and digitally controlling the increase / decrease of the number of registers.

By the above operation, the obtained CO2 concentration value can be obtained by a low-cost configuration.
This makes it possible to obtain the CO2 concentration and contributes to expanding the application field of CO2 concentration measurement.

Pattern diagram Signal diagram Implementation circuit diagram

E3 Charge Voltage
P11 discharge control Increase or decrease the measured concentration value, P13.
P2 Light receiving element output waveform P2e
P10 Light-receiving element output infrared light component waveform P10e

Claims (3)

An infrared light source is turned on by an infrared light source lighting signal PLn output continuously at a constant time interval, and after the infrared light emitted by the lighting is transmitted through air, an infrared light receiving element, a pyro When the light receiving signal P2 is received by the element and only the change at the time of projecting the infrared light is P10n, and the signal obtained by the synchronous rectification is P10n, the arithmetic expression P3 = k1−k2 × P10n
When the voltage value and P3 value obtained in step 2 are calibrated with CO2 gas of a known concentration value, only the light emission amount characteristic and the amount of change in the amount of received light of the infrared light source incandescent lamp with a delay time at lighting are output. A multi-dimensional function curve indicating a negative peak voltage value composed of a composite response characteristic of the pyro element output, and the negative peak voltage value is calibrated in the air between the infrared light source and the pyro element. A CO2 sensor characterized by obtaining a CO2 concentration value in air by measuring a P3 negative direction peak voltage value because it indicates a CO2 concentration value.   In claim 1, when determining the minimum value of the P3 value, a value counter P13 capable of increasing / decreasing the numerical value is provided, and the P3 value by the arithmetic expression P3 = k1−k2 × P10n, and the numerical value The P13 value is compared, the P13 value is increased and stored when P3 value ≧ P13, the P13 value is decreased and stored when P3 ≦ P13, and a new stored value P13 value obtained every time the infrared light source is turned on, A CO2 sensor characterized by being used as a gas concentration measurement value by turning on the infrared light source.   In the operation of [Claim 1], a CO2 sensor characterized in that an arbitrary coefficient is given to the obtained value counter value P13 by a variable magnification means to calibrate a measured value.

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