Modulation and demodulation device based on tdlas-wms laser for detecting gas content

Abstract

The application discloses a kind of modulation and demodulation device based on TDLAS-WMS technology detection gas content, including main control module, signal output module, signal acquisition module and external interactive module;The main control module includes FPGA, ARM and DDR, the main control module and signal output module are connected for output digital signal, the main control module and signal acquisition module are connected for receiving converted digital signal, the main control module and interactive module are connected for and external device are communicated, the FPGA and ARM are integrated in main control chip, modulation and demodulation part in the application is involved in the process of integrated TDLAS-WMS technology spectrum detection, can parallel control two lasers and carry out corresponding modulation and demodulation;Design adds optional analog signal conditioning hardware circuit part, overcomes the adverse effects caused by various detection environment noise, to adapt to more actual application scenarios.

Description

Technical Field

[0001] This invention relates to the field of substance concentration detection technology, and in particular to a modulation and demodulation device for detecting gas content based on TDLAS-WMS laser. Background Technology

[0002] Laser gas concentration detection equipment based on TDLAS-WMS technology comprises five components: a signal source, a phase-locked demodulator, a laser controller, a laser, and a photodetector. While large instrument platforms, such as the common Tektronix AFG31000 function generator and SR830 lock-in amplifier, are commonly used for laboratory testing and debugging, there is a lack of universally applicable small-scale embedded alternatives for these large instrument modules in actual commercial applications. Furthermore, actual measurements in open environments often involve significant physical noise interference, such as the variable temperature and humidity and strong electromagnetic noise in complex industrial production environments. This results in a large amount of noise mixed into the weak absorption spectrum signal, significantly impacting the performance of the entire detection equipment.

[0003] The main body of a modulation and demodulation device consists of a signal modulation section and a signal demodulation section. Signal modulation combines two signals of different frequencies and feeds them to a laser as a carrier wave. Signal demodulation demodulates the absorption spectrum signal to obtain its harmonics containing concentration information. However, commercially available modems lack pre-stage signal filtering to adapt to various environments, have a limited signal modulation range that cannot use lasers of different power levels, and can only generate second harmonics, making it difficult to meet the high-precision requirements of concentration inversion. Therefore, a more universal and compact spectral detection modulation and demodulation board is particularly important. Summary of the Invention

[0004] In view of the above, the purpose of this application is to provide a modulation and demodulation device that realizes a highly integrated spectral detection modulation and demodulation device with selectable filtering types in the pre-stage absorption spectrum acquisition, controllable up to two lasers, selectable Nth harmonic output, and interactive capability with a host computer. This solves the technical problem of lacking pre-stage signal filtering to adapt to various environments and lacking universality.

[0005] To achieve the above objectives, the present invention provides the following solution:

[0006] A modulation and demodulation device for detecting gas content based on TDLAS-WMS technology includes a main control module, a signal output module, a signal acquisition module, and an external interaction module.

[0007] The main control module includes an FPGA, an ARM, and a DDR. The main control module is connected to the signal output module for outputting digital signals, and to the signal acquisition module for receiving converted digital signals. The main control module is also connected to the interaction module for communication with external devices. The FPGA and ARM are integrated into the main control chip. The FPGA is used for TDLAS-WMS signal modulation to the signal output module and for demodulating the absorption spectrum signal to obtain harmonics carrying the concentration information of the detected gas, which are then buffered in BlockRAM and provided to the ARM for analysis of the gas content. The ARM is used to control the extraction and transmission of harmonic signal concentration features to the external interaction module, and the external module controls the storage and retrieval of detection data in the DDR. Optionally, the signal output module consists of two digital-to-analog signal conversion circuits and two analog-to-differential-to-single-ended signal conversion circuits. The main control module is connected to the digital-to-analog signal conversion circuits.

[0008] Optionally, the signal acquisition module consists of two selectable signal conditioning circuits, two single-ended to differential circuits, and two analog to digital signal conversion circuits.

[0009] Optionally, the external interaction module consists of a serial port interaction module, an Ethernet interaction module, and an optical coupler interaction module. The serial port interaction module is connected to the main control module, the Ethernet interaction module is connected to the main control module, and the optical coupler interaction module is connected to the main control module.

[0010] Optionally, the digital-to-analog signal conversion circuit and the analog-to-differential-to-single-ended signal conversion circuit are connected and output to the SMA1 and SMA2 interfaces. The SMA1 and SMA2 interfaces output signals to two laser controllers. The two photodetectors transmit the received gas absorption spectrum signals to the device's SMA3 and SMA4 interfaces. The SMA3 and SMA4 interfaces are connected to two selectable signal conditioning circuits. The two selectable signal conditioning circuits are connected to two single-ended-to-differential conversion circuits. The two single-ended-to-differential conversion circuits are connected to two analog-to-digital signal conversion circuits. Finally, the two analog-to-digital signal conversion circuits are connected to the main control module.

[0011] Optionally, the selectable signal conditioning circuit includes a signal amplification circuit, a second-order active low-pass filter circuit, a second-order active high-pass filter circuit, and a shift circuit. The signal conditioning circuits are connected in series in sequence, and the four circuits are selected in parallel with a 0-ohm resistor.

[0012] Optionally, the digital-to-analog circuit uses a core DA chip, which is a dual-channel 14-bit high-precision signal output with a 2V amplitude signal output range, a minimum resolution of 0.122mV, and a maximum update rate of 125MSPS. The analog-to-digital circuit uses an AD core chip, which is a dual-channel 14-bit high-precision signal acquisition chip with a 2V amplitude signal acquisition range, a minimum resolution of 0.122mV, and a maximum update rate of 65MSPS.

[0013] Optionally, the signal amplification circuit is a forward amplification circuit with a signal amplification factor of [missing value]. The low-pass filter circuit mentioned above is a second-order active low-pass filter circuit with a signal amplification factor of 1 and a cutoff frequency of [missing value]. The high-pass filter circuit described is a second-order active high-pass filter circuit with a signal amplification factor of 1 and a cutoff frequency of [missing value]. The signal shift amount of the shifting circuit is Vref.

[0014] Optionally, the serial port interaction module can be time-division multiplexed with the PFGA and ARM of the main control module.

[0015] Optionally, the optocoupler interaction module can control the modulation and demodulation device with a control signal from an external device within 24V, or the modulation and demodulation device can control an external device with a voltage within 24V.

[0016] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are:

[0017] This application relates to the modulation and demodulation section in the integrated TDLAS-WMS technology spectral detection process, which can control two lasers simultaneously for corresponding modulation and demodulation. The design includes optional analog signal conditioning hardware circuitry to overcome the adverse effects of various detection environment noises, thus adapting to more practical application scenarios. The addition of USB interface, Gigabit Ethernet interface, and optocoupler control section greatly improves user objectivity and controllability. Attached Figure Description

[0018] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly described below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 This is a flowchart of the overall workflow for spectral detection of the content of an object to be measured;

[0020] Figure 2 This is a block diagram of a parallel modulation and demodulation device based on TDLAS-WMS technology;

[0021] Figure 3 This is a diagram showing the overall architecture of a parallel modulation and demodulation device based on TDLAS-WMS technology.

[0022] Figure 4 This is a diagram of the gating signal conditioning circuit in the signal acquisition module;

[0023] Figure 5 It includes the original absorption spectrum signal of the photodetector in the high-power electromagnetic industrial production line, the absorption spectrum signal of the photodetector after the production line is powered on, and the absorption spectrum signal after low-pass filtering by the selectable signal conditioning circuit. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only for explaining the invention and are not intended to limit the invention; that is, the described embodiments are merely some embodiments of the invention, and not all embodiments. The components of the embodiments of the invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0025] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0026] It should be noted that relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0027] The technical solutions, principles, and products of the present invention will be fully described below with reference to the accompanying drawings. The described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0028] The purpose of this invention is to provide a portable laser headspace oxygen analyzer for detecting leakage in packaged glass medicine bottles in the pharmaceutical industry, thereby achieving accurate detection of the headspace oxygen content in packaged glass medicine bottles.

[0029] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0030] There are two commonly used spectral detection methods: direct absorption spectroscopy (DAS) and wavelength modulation spectroscopy (WMS). WMS has better anti-interference capabilities. The workflow diagrams for these two methods are shown below. Figure 1 As shown, the function generator generates a laser modulation signal 101 for the laser carrier emitted by the laser. The laser controller receives the laser driving modulation signal and converts it into a current signal output. At the same time, it controls the operating current and operating temperature of the laser to drive the laser 11 to light up. The laser passes through the gas to be tested 12. At this time, the laser of a specific wavelength will be absorbed by the substance to be tested. The photodetector receives the absorbed laser and completes the photoelectric conversion to obtain the absorption spectrum analog electrical signal 13. Then, the phase-locked demodulator collects the absorption spectrum analog electrical signal and completes the signal demodulation to obtain the harmonics 102 with specific concentration characteristics. Finally, the ARM performs concentration inversion on the demodulated harmonics to determine the concentration of the gas to be tested and outputs the signal.

[0031] The commonly used modulated signal is the superposition of two signals with different frequencies.

[0032] The harmonics are first harmonics, second harmonics, third harmonics, and even Nth harmonics.

[0033] In practical applications, TDLAS-WMS spectral detection technology is more widely used. Therefore, this invention provides a modulation and demodulation device 10 based on TDLAS-WMS technology. The structural block diagram of this device is shown below. Figure 2 The device comprises four parts: a main control module 20, a signal output module 21, a signal acquisition module 22, and an external interaction module 23. The detailed overall structural framework of the device is shown below. Figure 3 As shown.

[0034] The modulation and demodulation device based on TDLAS-WMS technology adopts dual-channel parallel operation and can be used for modulation and demodulation of two lasers simultaneously.

[0035] The main control module 20 comprises three parts: an FPGA, an ARM, and a DDR. The main control module is connected to a signal output module for outputting digital signals, a signal acquisition module for receiving converted digital signals, and an interaction module for communicating with external devices. The FPGA and ARM are integrated into the main control chip. The FPGA is used for TDLAS-WMS signal modulation to the signal output module and for demodulating the spectral signal to obtain harmonics carrying the concentration information of the detected gas, which are then stored in the DDR. The ARM is used to control the extraction of harmonic signal concentration features and their transmission to the external interaction module, and the external module controls the storage and retrieval of detection data in the DDR. The signal modulation involves the superposition of two different signals. The adjustment required is to adjust the frequency, amplitude, and offset of these two signals. Without loss of generality, commonly used modulation signals are a low-frequency triangular wave and a high-frequency sine wave, or a low-frequency sawtooth wave and a high-frequency sine wave superimposed.

[0036] The signal output module 21 consists of two digital-to-analog signal conversion circuits and two analog-to-differential-to-single-ended signal conversion circuits. The main control module is connected to the digital-to-analog signal conversion circuits. It is important to note that due to the high digital output rate, a buffer resistor needs to be inserted in the middle to prevent data overshoot and distortion, which could prevent the subsequent DAC chip from recognizing the signal. The two digital-to-analog signal conversion circuits and the two analog-to-differential-to-single-ended signal conversion circuits are connected and output to the SMA1 and SMA2 interfaces. The SMA1 and SMA2 interfaces output signals to two laser controllers, thereby driving two lasers to perform fine spectral scanning in a specific wavelength band. The digital-to-analog signal conversion circuit uses a core DA chip, which is a dual-channel 14-bit high-precision signal output with a 2V amplitude signal output range, a minimum resolution of 0.122mV, and a maximum update rate of 125MSPS.

[0037] The signal acquisition module 22 consists of two selectable signal conditioning circuits, two single-ended to differential circuits, and two analog-to-digital signal circuits. Two photodetectors transmit the received gas absorption spectrum signals to the device's SMA3 and SMA4 interfaces. The SMA3 and SMA4 interfaces are connected to the two selectable signal conditioning circuits, which are then connected to the two single-ended to differential circuits. These circuits are further connected to the two analog-to-digital signal circuits, and finally, the two analog-to-digital signal circuits are connected to the main control module. It is important to note that a buffer resistor needs to be added between the main control module and the ADC chip to prevent signal overshoot, which could cause the FPGA to fail to recognize the data acquired by the ADC. Each selectable signal conditioning circuit is described as follows: Figure 4As shown, the circuit includes a signal amplification circuit 31, a second-order active low-pass filter circuit 32, a second-order active high-pass filter circuit 33, and a shifting circuit 34. The signal conditioning circuits are cascaded sequentially, and the four circuits are selected in parallel using a 0-ohm resistor, thus adapting to various noise scenarios. The analog-to-digital signal conversion circuit uses an AD core chip. This AD chip is a dual-channel 14-bit high-precision signal acquisition chip with a 2V amplitude signal acquisition range, a minimum resolution of 0.122mV, and a maximum update rate of 65MSPS.

[0038] The signal amplification circuit 31 is a forward amplification circuit with a signal amplification factor of [missing value]. Figure 4 By retaining R3, R23, R24, and R25, and removing R22, R7, R8, R12, R18, and R17, the signal conditioning circuit can operate with only the signal amplification circuit.

[0039] The low-pass filter circuit 32 is a second-order active low-pass filter circuit with a signal amplification factor of 1 and a cutoff frequency of [missing value]. Figure 4 By retaining R22, R8, R7, R24, and R25, and removing R3, R23, R12, R18, and R17, the signal conditioning circuit can operate with only the signal low-pass filter circuit.

[0040] The high-pass filter circuit 33 is a second-order active high-pass filter circuit with a signal amplification factor of 1 and a cutoff frequency of [missing value]. Figure 4 By retaining R22, R23, R12, and R25, and removing R3, R8, R7, R24, R18, and R17, the signal conditioning circuit can operate with only the high-pass filter circuit.

[0041] The shifting circuit 34, since the current ACD circuit can only acquire voltages within the range of 0V to 2V, needs to shift the level of negative input signals to within the acquisition range. The signal shift amount is Vref. Figure 4 By retaining R22, R23, R24, R18, and R17, and removing R3, R8, R7, R12, and R25, the signal conditioning circuit can operate with only the transfer circuit.

[0042] The external interaction module 23 consists of a serial port interaction module, an Ethernet interaction module, and an optocoupler interaction module. The serial port interaction module is connected to the main control module, the Ethernet interaction module is connected to the main control module, and the optocoupler interaction module is connected to the main control module. The serial port interaction module can be time-division multiplexed with the PFGA and ARM of the main control module. The optocoupler interaction module controls the modem via 24V or lower control signals from external devices, and the modem can also control external devices with a voltage of 24V or lower.

[0043] The main control chip is a ZYNQ7000 series chip, the DAC chip is AD9767ASTZ, the ADC chip is AD9248BCPZ-65, the Ethernet chip is RTL8211E, the USB to serial port chip is CP2105, the differential circuit chip in the signal output module is AD8065ARTZ, the differential circuit chip in the signal acquisition module is AD8138ARZ, and the operational amplifier used in the conditioning circuit of the signal acquisition module is OPA2140.

[0044] The laser operates in an environment containing multiple large servo motors. The modulation and demodulation device of this invention provides the laser controller with a 10mV-50Hz sawtooth wave superimposed with an 8mV-20kHz sine wave, and applies a bias voltage so that the amplitude range of the superimposed DDS signal is 60mV to 78mV. This ensures that the optimal absorption wavelength of the gas to be measured is the center wavelength of the laser spectral scan. The gas to be measured is oxygen, and the optimal center wavelength is 760.88nm. The absorption spectrum signal received by the photodetector is transmitted to SMA3 via a wire. Figure 5 As shown, the waveform 40 observed by the oscilloscope at SMA3 is as follows: at this time, the industrial equipment is not powered on, and the laser detection equipment is only working in a static magnetic field. When the servo motor is powered on, the waveform 41 observed by the oscilloscope at SMA3 is as follows: the laser absorption spectrum signal has been submerged by electromagnetic interference noise, making it difficult to distinguish between useful and useless signals. In the signal conditioning circuit of the SMA3 stage of our invented modulation and demodulation device, the low-pass filter circuit is turned on, and R22 = 0Ω, R8 = 0Ω, R7 = 0Ω, R24 = 0Ω and R25 = 0Ω are retained. Resistors R3, R23, R12, R18 and R17 are removed. In order to retain the 20kHz high-frequency sine wave of the absorption spectrum signal without filtering it out, R9 = 10kΩ, R10 = 10kΩ, C1 = 470pF and C2 = 470pF are selected. That is, the cutoff frequency of the low-pass filter circuit is approximately 34kHz. After the signal passes through this signal conditioning circuit, waveform 42 is obtained, which well restores the original absorption spectrum signal, thus helping the FPGA to perform signal demodulation.

[0045] The working principle of the signal modulation and demodulation device for detecting gas content by spectroscopy in this invention is as follows: The main control chip ZYNQ7000 series chip generates two DDS signals, which are sawtooth waves superimposed with sine waves. The DAC circuit in the signal output module converts the digital DDS signals generated by the main control chip ZYNQ7000 into analog electrical signals. After passing through a differential circuit, the differential signal generated by the DAC circuit is converted into a single-ended signal and transmitted to the SMA1 and SMA2 interfaces. The SMA1 and SMA2 interfaces transmit the signal to the control boards of the two lasers through wires. The absorption spectrum signals received by the two photodetectors are transmitted to SMA3 and SMA4, according to specific... The appropriate analog filter circuit is selected by choosing a suitable resistor based on the specific working environment. The signal passing through the signal conditioning circuit is transmitted on-board to the ADC chip via a single-ended to differential circuit. The ADC chip transmits two 14-bit data to a buffer, and then transmits them to the main control chip ZYNQ via a termination resistor. The main control chip ZYNQ demodulates the received spectral signal to obtain the 1st to Nth harmonics, which are stored in DDR. The ZYNQ's ARM inverses the gas content and outputs the response detection result via USB, Ethernet, and optocouplers. The USB can also be used as a signal input to control the amplitude, frequency, and bias of the modulation signal, and the five optocouplers can also be used as input trigger signals.

[0046] This invention integrates a function generator, phase-locked demodulator, digital signal processor, and interactive functions into one unit, improving the ease of use of the device. The chips used in this device are all dual-channel models, enabling excellent parallel control of two lasers, significantly saving costs and reducing device size. This device features selectable signal conditioning circuitry, allowing it to adapt well to various harsh working environments and enhancing its versatility. Furthermore, this device provides three interaction methods: USB, Gigabit Ethernet, and optocoupler, further improving its practicality and flexibility.

[0047] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A modulation and demodulation device for detecting gas content based on TDLAS-WMS technology, comprising a main control module, a signal output module, a signal acquisition module, and an external interaction module; characterized in that, The main control module includes an FPGA, an ARM, and a DDR. The main control module is connected to the signal output module for outputting digital signals, and to the signal acquisition module for receiving converted digital signals. The main control module is also connected to the interaction module for communication with external devices. The FPGA and ARM are integrated into the main control chip. The FPGA is used for TDLAS-WMS signal modulation to the signal output module and for absorption spectral signal demodulation to obtain harmonics carrying the detected gas concentration information, which are then passed through a block. The RAM cache provides data to the ARM for analyzing the content of the gas to be measured. The ARM terminal is used to control the extraction of harmonic signal concentration features and transmit them to an external interactive module. The external module controls the storage and retrieval of detection data into the DDR. The signal output module consists of two digital-to-analog signal circuits and two analog-to-differential-to-single-ended signal circuits. The main control module is connected to the digital-to-analog signal circuits. The signal acquisition module consists of two selectable signal conditioning circuits, two single-ended-to-differential circuits, and two analog-to-digital signal circuits. The selectable signal conditioning circuit includes a signal amplification circuit, a second-order active low-pass filter circuit, a second-order active high-pass filter circuit, and a shifting circuit. The selectable signal conditioning circuits are cascaded sequentially, and the four circuits are paralleled by a 0-ohm resistor. The digital-to-analog signal conversion circuit and the analog-to-differential-to-single-ended signal conversion circuit are connected and output to the SMA1 and SMA2 interfaces. The SMA1 and SMA2 interfaces output signals to two laser controllers. Two photodetectors transmit the received gas absorption spectrum signals to the SMA3 and SMA4 interfaces. The SMA3 and SMA4 interfaces are connected to two selectable signal conditioning circuits. The two selectable signal conditioning circuits are connected to two single-ended to differential circuits. The two single-ended to differential circuits are connected to two analog-to-digital signal conversion circuits. Finally, the two analog-to-digital signal conversion circuits are connected to the main control module. After passing through the selectable signal conditioning circuits, the original absorption spectrum signal is restored, which helps the FPGA to perform signal demodulation later.

2. The modulation and demodulation device based on TDLAS-WMS technology for detecting gas content according to claim 1, characterized in that, The external interaction module consists of a serial port interaction module, an Ethernet interaction module, and an optical coupler interaction module. The serial port interaction module is connected to the main control module, the Ethernet interaction module is connected to the main control module, and the optical coupler interaction module is connected to the main control module.

3. The modulation and demodulation device for detecting gas content based on TDLAS-WMS technology according to claim 1, characterized in that, The digital-to-analog converter uses a core DA chip, which is a dual-channel 14-bit high-precision signal output with a 2V amplitude signal output range, a minimum resolution of 0.122mV, and a maximum update rate of 125MSPS. The analog-to-digital converter uses a core AD chip, which is a dual-channel 14-bit high-precision signal acquisition chip with a 2V amplitude signal acquisition range, a minimum resolution of 0.122mV, and a maximum update rate of 65MSPS.

4. The modulation and demodulation device based on TDLAS-WMS technology for detecting gas content according to claim 3, characterized in that, The signal amplification circuit adopts a positive amplification circuit, the low-pass filter circuit adopts a second-order active low-pass filter circuit, the signal amplification multiple of the high-pass filter circuit is 1, the signal shift amount of the shift circuit is Vref.

5. The modulation and demodulation device based on TDLAS-WMS technology for detecting gas content according to claim 2, characterized in that, The serial port interaction module can be time-division multiplexed with the FPGA and the ARM of the main control module.

6. The modulation and demodulation device for detecting gas content based on TDLAS-WMS technology according to claim 5, characterized in that, The optocoupler interaction module is used for controlling the modulation and demodulation device by an external device with a control signal within 24V or controlling an external device with a voltage within 24V by the modulation and demodulation device.

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