TWM473474U - MEMS infrared source with optical feedback - Google Patents

Abstract

Traditional non-dispersive infra-red (NDIR) gas sensor uses infra-red lamp that has drift and aging issues. Dual channels thermopile NDIR gas sensor can solve part of the above problems, but its sensor still has temperature effect that limits its application to low concentration gas measurement. The Infrared emitter is an IR source that is fabricated by MEMS (Micro Electro Mechanical Systems) technology. The output of IR emitter varies with ambient temperature and the aging effect of the IR emitter element. An innovated architecture is proposed that uses optical feedback methodology to stabilize the IR source output for NDIR gas sensor.

Description

Micro electromechanical light feedback infrared light source

Photoelectric application technology.

The principle of a non-dispersive infrared (NDIR) gas detector is to use the Beer-Lambert law.

I( x )=I _o exp[- a (λ). x ]

Original light intensity I _o as the starting point (x = 0) of;; wherein the light intensity I (x) is the optical path the x position remaining a is the absorption coefficient, the concentration value of the gas contained in the related, but also a wavelength The function. When a = 0, I (x) is a constant I _o, i.e. light is not absorbed by the gas. In summary, a is related to the wavelength and concentration of the gas spectrum. In addition, a is also related to other physical factors of the gas, such as gas temperature and pressure, and its behavior can be corrected and corrected according to the Charles's law of the ideal gas.

The source of the non-dispersive infrared (NDIR) gas detector uses the principle of blackbody radiation. As shown in Fig. 1, the radiant energy of the two narrow filters penetrating NBF1 and NBF2 increases with the temperature of the radiator. NBF1 is a reference wavelength filter whose transmittance is independent of the gas concentration to be measured. NBF2 is the characteristic wavelength. The filter has a transmittance that is related to the gas concentration to be measured. Figure 2 shows the infrared spectrum (characteristic absorption spectrum) of common gases.

Figure 3 shows a schematic diagram of the structure of a non-dispersive infrared (NDIR) gas detector consisting of an infrared source (such as a bulb), an optical cavity, and an infrared sensor. It is often necessary to correct the non-dispersive infrared gas detector output drift or measurement error due to aging of the lamp or unstable driving factors.

U.S. Patent 5,347,474 uses the atmospheric carbon dioxide concentration to automatically correct the output of a non-dispersive infrared carbon dioxide gas detector to overcome the effects of bulb aging or drift. This method does not apply to other gas measurements.

Another method is to use a two-channel thermopile sensor with two infrared narrow filters, one channel whose infrared transmission band is the characteristic absorption band of the gas to be measured; the other channel is the reference channel and its infrared penetration band. Not subject to the measurement of gases or other gases. The stability of the infrared source is controlled via the output feedback of the reference channel, but the output of the dual-channel thermopile sensor is also affected by the ambient temperature, so it cannot achieve very accurate light source output control (for example, one ten thousandth). Therefore, for low concentration gas concentration measurements (eg, carbon monoxide at 30 ppm), dual channel NDIR gas sensing gas technology is not sufficient.

For low concentration gas concentration measurements, it requires an infrared source that is stable to one ten thousandth. For a constant-power-driven infrared source, the radiation output changes with ambient temperature. Figure 4 shows the different black body temperatures. The ratio of the energy of the energy passing through the NBF1 filter to the ambient temperature of 25 ° C at different ambient temperatures.

For non-dispersive infrared (NDIR) gas detectors using infrared light bulbs, when the black body temperature change is controlled within 0.2 ° C, the black body radiation energy variation value of the penetrating filter is within 0.01%, at this time NDIR The gas detector resolution is approximately 2 ppm. For low-temperature microelectromechanical (MEMS) infrared light sources, it is necessary to control the light output to 0.01% (one ten thousandth) at 0.05 °C.

The traditional infrared light source driving method adopts constant voltage, constant current or constant resistance driving mode to stabilize the light source heating temperature (Tb); Ts=Ta+Tb

Ta is the ambient temperature and Ts is the temperature of the black body radiation of the light source. If the ambient temperature changes, the temperature Ts of the black body radiation of the light source changes, and it is difficult to achieve the goal of stable source output. This is due to time delays and accuracy issues with ambient temperature sensing elements such as thermistors. At the same time, the light source is separated from the ambient temperature sensing element, and it cannot truly reflect the current ambient temperature of the infrared source radiator.

Therefore, it is necessary to find another way to create a stable infrared light source for use in a non-dispersive infrared gas detector.

This creation uses the optical feedback method to achieve the purpose of stabilizing the infrared light source. The basic principle is that the black body radiation is monotonous with the black body temperature in a narrow band. (Monotonic) related. The optical sensor detects the sensing element, which can be in the visible light band or the infrared band, to provide the radiant energy of the sampled infrared light source black body, and achieve the purpose of stabilizing the infrared light source radiant energy (radiation temperature) through the feedback control circuit.

It can be applied to non-dispersive infrared gas detectors using conventional infrared bulbs or microelectromechanical infrared sources.

[Schematic diagram of MEMS light feedback infrared light source applied to non-dispersive infrared gas detector]

101‧‧‧Infrared source

102‧‧‧NDIR air cavity

103‧‧‧Thermal reactor sensor

104‧‧‧Light sensor

105‧‧‧Signal Processing Unit

106‧‧‧Light source control circuit

[Structural block diagram of optical feedback non-dispersive infrared gas detector]

101‧‧‧Infrared source

102‧‧‧NDIR air cavity

103‧‧‧Thermal reactor sensor

104‧‧‧Light sensor

106‧‧‧Light source control circuit

201‧‧‧ calculus

202‧‧‧Responsive resistance

203‧‧‧DC amplifier

204‧‧‧Multiplexer

205‧‧‧ analog to digital converter

206‧‧‧Microprocessor

207‧‧‧Communication interface

208‧‧‧Display unit

[Structural diagram of MEMS light feedback infrared light source, (a) side view (b) top view]

301‧‧‧Infrared window

302‧‧‧Micro-electromechanical infrared light source

303‧‧‧Light sensor

304‧‧‧ Cover

305‧‧‧Package base

306‧‧‧Solid glue

307‧‧‧ lead

308‧‧‧Package pins

[Structural block diagram of a non-dispersive infrared gas detector using a micro-electromechanical infrared source]

102‧‧‧NDIR air cavity

103‧‧‧Thermal reactor sensor

106‧‧‧Light source control circuit

203‧‧‧DC amplifier

204‧‧‧Multiplexer

205‧‧‧ analog to digital converter

206‧‧‧Microprocessor

207‧‧‧Communication interface

208‧‧‧Display unit

301‧‧‧Infrared window

302‧‧‧Micro-electromechanical infrared light source

303‧‧‧Light sensor

Figure 1: Relationship between spectral density of blackbody radiation and blackbody temperature

Figure 2: Infrared spectrogram of common gases

Figure 3: Schematic diagram of a non-dispersive infrared gas detector

Figure 4: Ratio of the radiant energy of the energy passing through the NBF1 filter at different ambient temperatures at room temperature to 25 °C at different ambient temperatures

Figure 5: Schematic diagram of MEMS light feedback infrared light source applied to non-dispersive infrared gas detector

Figure 6: Block diagram of the optical feedback non-dispersive infrared gas detector

Figure 7: Structure diagram of MEMS optical feedback infrared source, (a) side view (b) top view

Figure 8: Block diagram of a non-dispersive infrared gas detector using a micro-electromechanical infrared source

Fig. 5 is a schematic view showing the application of the MEMS optical feedback infrared light source to a non-dispersive infrared gas detector. It includes an infrared light source (101); an NDIR gas chamber (102) for detecting gas absorption rate; a thermopile sensor (103) for sensing a specific gas absorption rate signal; and a light source for detecting infrared light source radiation. The element (104) provides an optical feedback signal to the signal processing unit (105). After comparing the errors, the signal processing unit outputs a control driving signal to the light source control circuit (106) to stabilize the radiation output of the infrared light source.

Fig. 6 is a block diagram showing the structure of the optical feedback non-dispersive infrared gas detector of the application example, but the spirit of the present invention is not limited to this application example. The utility model comprises an infrared light source (101); an NDIR gas chamber (102) for reacting and absorbing the characteristic gas; and a pyroelectric sensor (103) for detecting the infrared light, the signal for sensing the specific gas absorption rate, and the output thereof is fed to the direct current An amplifier (203); the light sensing element (104) is configured to detect the energy radiated by the infrared light source, and the photocurrent to voltage converter is formed by the feedback resistor (202) and the calculation amplifier (201); the multiplexer (204) The output of the DC amplifier (203) or the operational amplifier (201) is selected to analog to digital converter (205) under the control of the microprocessor (206); the analog to digital converter output is fed to the microprocessor (206) for calculation Compared with the light source error, the output of the light source control circuit (106) is controlled to achieve the purpose of stabilizing the infrared light source; the communication interface (207) is used to provide an output signal of the non-dispersive infrared gas detector; and the display unit (208) ) is used to display the measured value and warning signal of the non-dispersive infrared gas detector locally.

The photocurrent to voltage converter composed of the optical sensing element (104), the operational amplifier (201) and the feedback resistor (202) can also be replaced by an integrated ambient light sensor, which can provide analog voltage or digital light. The signal is sensed to the microprocessor (206). At this time, the design of the NDIR air chamber (102) should pay attention to shielding the injection of ambient light, and at the same time be able to keep the gas unobstructed.

Figure 7 shows the structure of the micro-electromechanical light feedback infrared light source. The infrared window (301) only allows the infrared wavelength of light to pass through (for example, the 3-5um band), and it also blocks the visible light in the environment; the micro-electromechanical infrared source (302), which can be heated to emit orange light of visible light. At this time, the black body temperature is about 1200K; the light sensing element (303) is used to detect the visible light component radiated by the MEMS infrared light source; the cover (304) is used to fix the infrared window (301) and the package base ( 305); the package base (305) is used to fix the micro-electromechanical infrared light source (302) and the light sensing element (303) through the solid crystal glue (306); the infrared light source (302) and the light sensing element are connected by the lead wire (307) The electrical input and output signal of (303) is connected to the package pin (308).

Figure 8 is a block diagram showing the structure of a non-dispersive infrared gas detector using a micro-electromechanical infrared source. It includes a micro-electromechanical infrared source (302) and a light sensing element (303); an NDIR air chamber (102); a thermopile sensor (103) for sensing the absorption rate of the characteristic gas; the output of the thermopile sensor After being amplified by the DC amplifier (203), it is sent to the analog-to-digital converter (205) after being selected by the multiplexer (204), and then sent to the microprocessor (206) for characteristic gas concentration calculation; the light sensing element (303) The signal is selected by the multiplexer (204) and sent to the analog to digital converter (205); The input of the microprocessor (206) has two sets of signals, one is the absorption signal of the characteristic gas detected by the thermopile sensor (103), and the other is the MEMS by the light sensing element (303). The optical radiation sampling signal generated by the infrared light source (302) is used for feedback control via the light source control circuit (106) to stabilize the output of the microelectromechanical infrared light source; the communication interface (207) is used to provide non-dispersive infrared gas detection The output signal of the detector; the display unit (208) is used for locally displaying the measured value and the warning signal of the non-dispersive infrared gas detector.

The above application example is to illustrate how to apply the principle of optical feedback to stabilize the output of the infrared light source without being affected by the ambient temperature and the aging of the light source. The spirit of the present invention is not limited to the above examples, and all of the micro-electromechanical infrared light sources prepared by optical feedback are included.

301‧‧‧Infrared window

302‧‧‧Micro-electromechanical infrared light source

303‧‧‧Light sensor

304‧‧‧ Cover

305‧‧‧Package base

306‧‧‧Solid glue

307‧‧‧ lead

308‧‧‧Package pins

Claims (6)

A micro-electromechanical light feedback infrared light source, comprising: a micro-electromechanical infrared light source (302), which can be heated to emit orange light of visible light; a light sensing element (303) for detecting micro-electromechanical infrared light The visible light component emitted by the light source; a package base (305) for fixing the micro-electromechanical infrared light source (302) and the light sensing element (303) through the solid crystal glue (306); and the micro-electromechanical infrared light source (307) The electrical input and output signals of the light source (302) and the light sensing element (303) are connected to the package pin (308); and the cover (304) is soldered to the package base (305) to protect the MEMS infrared light source (302). And the light sensing element (303), which has an infrared window (301) that only allows the infrared wavelength of light to pass out (for example, a 3-5 um band), and it also blocks the visible light in the environment; this creation uses light Feedback control to stabilize the output of the MEMS infrared source, which can be applied to high precision non-dispersive gas sensors. For example, in the MEMS optical feedback infrared light source described in claim 1, the light sensing element (303) may be a visible light sensing diode or an infrared light sensing element, and may also be an integrated light sensing diode. The ambient light sensor of the body and the amplifying circuit on a single chip. The MEMS optical feedback light source (303) and the MEMS infrared light source (302) can be co-located in a wafer as claimed in claim 1. The MEMS window (301) of the MEMS light feedback infrared light source of claim 1 may have a range of 1-20 μm or a narrow band (for example, 3-5 μm). For example, in the MEMS optical feedback infrared light source described in claim 1, the infrared window (301) may be additionally provided with an infrared lens to limit the output viewing angle of the MEMS infrared light source (302). The MEMS optical feedback infrared light source of claim 1, wherein the capping process can be evacuated to reduce the heating power of the micro-electromechanical infrared source (302).