Compact Optical Gas Sensor with Spatial and Spectral Referense

Abstract

Provided is a method for sensing gases using a compact optical gas sensor, a method for manufacturing same and a method for performing measurement of gas concentration using the optical absorption signal. The sensor design features a two-mirror geometry with long optical path. The sensor utilizes both spectral and optical reference channels. The reference channels ensure long-term stability of the sensor, which makes the design especially suitable for demanding environments requiring high reliability over extended period of time. The sensor operation is based on absorption of infrared light by a gas volume. In order to accurately determine the gas concentration, the absorption of the light that passed through the gas volume is compared with the absorption of the light of a different wavelength and absorption of light that traveled a short light path. A dual-color LED is used as a two-wavelength compact radiation source. The LED changes the emission wavelength as the excitation current is changing direction. The design is applicable to sensors for wearable gas alert devices, stationary leak detection, air quality monitoring, and any other field of applications that requires a specific gases' concentration detection.

Claims

1. A compact low-power Near-Infrared Absorption sensor for detecting gaseous species in the air, comprising: a dual-color infrared light emitting diode operating under first and second directions of the current conduction depending on the electric bias applied to its terminals at consequent instances of time and producing infrared radiation of a first wavelength when biased to conduct electric current of a first direction, and producing infrared radiation of a second wavelength when biased to conduct electric current of a second direction; at least one spherical mirror; at least one flat mirror; at least one broadband infrared detector with a bandwidth that includes at least the first and the second said wavelengths; the said sensor further designed in such a way that at least one of the said broadband detectors is measuring the intensity of the light produced by the said light emitting diode both at the first and the second wavelengths, after the said produced light travels a first optical path inside the said sensor, the said first optical path created by the arrangement of the said mirrors and exceeding in length the physical dimension of the said sensor at least by a factor of three.

2. A sensor of claim 1 further comprising a second broadband infrared detector with a bandwidth that includes at least the first and the second said wavelengths, measuring the intensity of the light generated by the said light emitting diode both at the first and the second wavelengths, arranged to measure the intensity of the said light after it travels a second optical path that is shorter than the physical dimension of the said sensor, and also is shorter than the length of the said extended optical path by at least a factor of ten.

3. A sensor of the claim 1, where the emission wavelengths of the dual-color light emitting diode are selected in such a way that the light of the said first wavelength is absorbed distinctly more effectively than the light of the said second wavelength by potentially dangerous concentration of a target gas selected from the group of combustible gas, such as methane, and hazardous gas, such as ammonia.

4. A sensor of the claim 2, where the emission wavelengths of the dual-color light emitting diode are selected in such a way that the light of the said first wavelength is absorbed distinctly more effectively than the light of the said second wavelength by potentially dangerous concentration of a target gas selected from the group of combustible gas, such as methane, and hazardous gas, such as ammonia.

5. A sensor of claim 3 performing functions of: exciting the dual-color light emitting diode by a pulsed current of alternating directions to produce the pulses of light of the first wavelength alternated with the pulses of light of the second wavelength; detecting the produced pulses of light; comparing the radiation intensity of the light traveled the first optical path and the second optical path; detecting presence of interfering gas absorbing at the reference wavelength; performing correction of the reference signal depending on concentration of interfering gas.

6. A sensor of claim 4 performing functions of: exciting the dual-color light emitting diode by a pulsed current of alternating directions to produce the pulses of light of the first wavelength alternated with the pulses of light of the second wavelength; detecting the produced pulses of light; comparing the radiation intensity of the light traveled the first optical path and the second optical path; comparing the radiation intensity of the light of the first and the second wavelength after the light traveled over first and second optical path; detecting presence of interfering gas absorbing at the reference wavelength; performing correction of the reference signal depending on concentration of interfering gas.

7. A sensor of claim 6, where the comparison of the light intensity traveled the first and the second optical paths is used to extend the operation range of the sensor to high gas concentrations.

8. A sensor of claim 4 performing functions of: exciting the dual-color light emitting diode by a pulsed current of alternating directions to produce the pulses of light of the first wavelength alternated with the pulses of light of the second wavelength; detecting the produced pulses of light; comparing the radiation intensity of the light traveled the first optical path and the second optical path; comparing the radiation intensity of the light of the first and the second wavelength after the light traveled over first and second optical path; detecting presence of interfering gas absorbing at the reference wavelength; performing correction of the reference signal depending of concentration of interfering gas, where the sensor of claim 4 is used for simultaneous detection of two gases by comparing the radiation intensity of the light traveled the first optical path and the second optical path, and comparing the radiation intensity of the light of the first and the second wavelength, where the measured intensity of the light traveled the second optical path signal is used as the reference.

9. A method for manufacturing the low-power sensor as in one of the claims 1-2, comprising: mounting the dual-color LED and the at least one broad-band infrared detector on a single chip carrier; mounting the said chip carrier on a printed circuit board; providing an optical system comprising the said mirrors, the optical system manufactured using plastic by either machining or injection molding; performing the sensor assembly that aligns the optical system and the LED-detector single chip carrier by a set of mechanical keys.

10. A method for measuring concentration of a target gas using infrared-absorption spectroscopy, comprising the steps of: exciting the dual-color light emitting diode by a pulsed current of alternating directions to produce the pulses of light of the first wavelength alternated with the pukes of light of the second wavelength; performing detection of the produced pulses of light; performing comparison of the radiation intensity of the light traveled the first (long) optical path and the second (short) optical path; performing comparison of the radiation intensity of the light of the first and the second wavelength after the light traveled over first (long) and second (short) optical path. detecting presence of interfering gas absorbing at the reference wavelength; performing correction of the reference signal depending of concentration of interfering gas.

11. A method for measuring concentration of a target gas using infrared-absorption spectroscopy, comprising the steps of: exciting the dual-color light emitting diode by a pulsed current of alternating directions to produce the pulses of light of the first wavelength alternated with the pulses of light of the second wavelength; performing detection of the produced pulses of light; performing comparison of the radiation intensity of the light travelled the first (long) optical path and the second (short) optical path; performing comparison of the radiation intensity of the light of the first and the second wavelength after the light traveled over first (long) and second (short) optical path. detecting presence of interfering gas absorbing at the reference wavelength; performing correction of the reference signal depending of concentration of interfering gas, performing simultaneous detection of two gases by comparing the radiation intensity of the light traveled the long optical path and the short optical path, and comparing the radiation intensity of the light of the first and the second wavelength, where the measured intensity of the light traveled the short optical path signal is used as the reference.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] FIG. 1 is an illustrative view of the NDIR optical gas sensing gauge with both spatial and spectral reference channels.

[0009] FIG. 2a is a cross-sectional view of the sensor showing location of the mirrors and the optical path for both signal and short path (reference) channels.

[0010] FIG. 2b is a cross-sectional view of the sensor showing detail of optcal components that comprise the short optical path.

[0011] FIG. 3 is the absorption spectra of methane gas and water vapor. Superimposed are spectral emission curves of the dual-color LED on the signal (1) and reference (2) wave-lengths.

[0012] FIG. 4a shows photoemission spectra pattern of a dual color LED manufactured by bulk p-n junction technology.

[0013] FIG. 4b shows photoemission spectra pattern of a dual color LED manufactured by quantum well technology.

[0014] FIG. 5 is an illustration showing timing diagram of the sensor operation.

DETAILED DESCRIPTION OF THE INVENTION

[0015] The generalized components arrangement of the optical gas sensor of the present invention 100 is shown in FIG. 1. Sensor 100 comprises a spherical mirror 101, and a flat mirror 102. Both mirrors are mounted in a housing 103. Light processing circuit 105 is mounted on a printed circuit board 104, and the board is mounted inside the sensor housing 103.

[0016] FIG. 2a is a cross-section of an optical gas sensor device of the present invention. Infrared light is emitted by a light-emitting diode 106. Light is first reflected from a spherical mirror 101, reflected from a flat mirror 102, back to the spherical mirror 101 and is finally focused at detector 108. Thus the light path is close to four times the height of the detector housing 103. For example, the total said light path can be between three times and four times the height of the detector housing. For example, the detector housing height can be between 10 and 40 millimeters (mm).

[0017] FIG. 2b shows detail of the short optical path. Part of the light is collected by an elliptical mirror 109. The mirror focuses a portion of light emitted by LED 106 onto reference signal detector 107. Optical path of light in the reference channel, 110, is approximately ten times shorter than the optical path length of the principal signal, 111. Therefore, the target gas signature in the reference signal channel is much smaller than the one in the main channel. The signal generated by the reference signal detector 107 can, therefore, serve as a spatial reference channel.

[0018] LED 108 is of dual color type, such as described in the U.S. Pat. No. 9,590,140 based on the following application: 20160005921, Bi-directional dual-color light emitting device and systems for use thereof. FIG. 3 shows how the dual-color emission enables methane sensing. Methane has several strong absorption lines in mid-IR wavelength range, spanning from 3.2 to 3.5 microns, with the strongest emission line at 3.3 microns. Water vapor, naturally present in the ambient atmosphere, has strong absorption in the range from 2.5 to 2.8 microns. In one of the embodiments of the present invention, the signal line, the wavelength 1, is located at 3.3 microns, where absorption of methane gas is the strongest. The reference line used as a source of the signal for spectral reference channel, the wavelength 2, is located outside the methane absorption range, as indicated in FIG. 3, for example at 3.0 microns. The reference emission line is positioned so that the reference signal is insensitive to both methane and water vapor presence. In certain situations, another strongly absorbing gas can interfere with the reference reading. In this case the reference line can be located on the right side of the absorption spectrum feature, for example at 4.5 microns. FIGS. 4a and 4b show example of emission spectra of methane-specific LEDs manufactured using a bulk p-n junction and a quantum well technology, respectively. Both curves show that under the reverse bias the LED emits radiation centered around the methane absorption line, 3.2 m, when the bias is reversed the radiation wavelength shifts to 2.2 m wavelength.

[0019] In order to reduce the power consumption by the device, the measurement is performed in a pulsed mode explained in detail in FIG. 5. A controlling microprocessor unit generates the enable signal which turns on an amplifier circuitry, which otherwise is in the sleep mode. After certain period of time, the control unit reads the background signal. After the background signal is registered, the infrared light is generated by supplying current of a first direction to the LED and the optical signal absorption is measured at the wavelength 1. During the light pulse, both spatial reference and signal channels are read by the control unit. The same sequence is repeated using the infrared light generated by supplying current of a second direction to the LED, thus for both long and short paths the optical absorption is measured at the wavelength 2. A digital processor compares the four reading: the short and long path absorption at the wavelength 1 and the short and long path absorption at the wavelength 2. This reading allows detecting presence of interfering gas at the reference wavelength 2. The target gas concentration is determined by fitting the four reading to a calibration function of the sensor. After the measurement is completed, the said amplifying circuitry returns to the sleep mode. At the end of the measurement cycle, the control unit analyzes the received signal and calculates the gas concentration.

[0020] It is understood that other methods or materials can be used to construct a similar sensor.

EXAMPLES

[0021] In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented here. It will be readily understood that the aspects of the present invention, as generally described herein, and illustrated in the figures, can be arranged, substituted, combined, and designed in a wide variety of different configurations, all of which are explicitly contemplated and made part of this disclosure.

Example 1

[0022] The sensor body was machined from Acrylonitrile Butadiene Styrene (ABS) plastic. The mirror surfaces were coated with 1 micron gold layer. LED, reference and signal detectors were mounted on a single chip carrier 105, as shown in FIG. 1. The LED and detectors were spectrally matched GaSb Type I quantum well structures or bulk strcutures that were optimized for 3.3 micron wavelength. Details of the quantum well structure fabrication were previously described in the following publication: [Jung, S., S. Suchalkin, G. Kipshidze, D. Westerfeld, D. Snyder, M. Johnson, and G. Belenky, GaSb-Based Type I Quantum-Well Light-Emitting Diode Addressable Array Operated at Wavelengths Up to 3.66 um. IEEE Photonics Technology Letters, 2009. 21(15): p. 1087-1089]. FIGS. 4a and 4b shows the emission spectra of the LEDs, whereas the reverse bias of the LED corresponds to emission centered around 3.2 m, which is the maximum absorption for methane and the forward bias corresponds to a shorter wavelength, approximately 2.2 m, which serves as the spectral reference line. The LED was excited by 300 mA current with 20 micro-second pulses. The main signal and reference channel signal were amplified by a known in art double-stage amplifier. The amplified signal was converted to a digital form by an analog-digital converter comprised by the control unit. The device is housed in a plastic body 103, 18 mm in diameter and 22 mm tall. The body is comprised of a spherical mirror 101 and a flat mirror 102. All the body parts were machined from ABS plastic; after the machining, the reflecting surfaces 101 and 102 were polished and plated with Gold by an electrochemical method. The mirrors were then glued to the sensor body by epoxy glue. The printed circuit board, 104, accommodated a digital processor (PIC brand, Microchip Technology) and signal conditioning circuits. The sensor communicated with the gas controller via serial (RS232) communication protocol. The sensor operated according to the timing diagram shown in FIG. 5. The sensor was activated for 60 micro-seconds (duration of the enable pulse) every 50 milliseconds. The length of the current pulse was approximately 30 micro-seconds, during the pulse the light intensity was collected for approximately 15 micro-seconds. The readout sequence was comprised on the background readout, the light signal measurement at the reference wavelength 1 (3.0 micrometers) and methane absorption wavelength (3.3 micrometers). The sensor was powered by a 3.2 Volts Li-ion battery, consuming, on average, 200 microwatts of electrical power. The sensor detected 2% of methane in air with better than 0.1% error.