A low-noise, high-precision integrated detection system and method for total nitrogen and total phosphorus

Abstract

The present invention provides a low-noise, high-precision integrated detection system and method for total nitrogen and total phosphorus, the system includes a sample processing unit, a detection unit, a noise removal unit, an ARM core processing unit and a host computer; the system converts a slowly varying DC signal into a square wave signal, removes the DC noise first, and then completes the multiplication of the detection signal with the same frequency reference signal, and the detection signal with the same frequency reference signal 90° phase shift signal, and then the filtering of periodic noise can be completed by low-pass filtering, and finally the square root of the two signals is calculated to obtain a low-noise signal. Compared with the traditional DC signal detection method, it can effectively filter out stray light and dark level, automatically correct the detection optical path, greatly suppress noise interference, effectively simplify the structural design and data processing flow, and realize high-precision detection of total nitrogen and total phosphorus.

Description

Technical Field

[0001] The present invention belongs to the technical field of water body detection, relates to total nitrogen and total phosphorus detection, and specifically relates to a low-noise and high-precision total nitrogen and total phosphorus integrated detection system and method. Background Art

[0002] When the total nitrogen and total phosphorus content in water is too high, it will cause eutrophication of water, massive reproduction of algae, reduced oxygen content in water, and mass death of other aquatic organisms, seriously damaging the ecosystem of the water. Therefore, it is very necessary to detect the total nitrogen and total phosphorus content.

[0003] At present, some detection instruments use a method of directly detecting the output signal of the photodetector. This method generally uses an LED powered by a constant current source as a light source to directly collect the transmitted light intensity of the sample to be tested. Since the LED is powered by a constant current, the transmitted light signal collected by the photodetector is a weak DC signal. At this time, the signal is easily affected by the offset voltage, dark current and stray light of the operational amplifier, resulting in a large amount of noise mixed in the measurement signal, resulting in low measurement accuracy. On the other hand, the traditional method requires the detection cell to be placed in a dark environment to avoid the influence of ambient light, which has problems such as complex structural design and high cost. Summary of the invention

[0004] In view of the deficiencies in the prior art, the purpose of the present invention is to provide a low-noise and high-precision integrated detection system and method for total nitrogen and total phosphorus, so as to solve the technical problem that the detection accuracy of the detection method in the prior art needs to be further improved.

[0005] In order to solve the above technical problems, the present invention adopts the following technical solutions to achieve the above problems:

[0006] A low-noise and high-precision integrated detection system for total nitrogen and total phosphorus, the system comprising a sample processing unit, a detection unit and a noise removal unit;

[0007] The detection unit includes: using square waves to control the on and off of LED power supply to realize modulating the light source signal into a square wave signal. The square wave signal is respectively detected on the water sample to be tested and the blank solution after passing through a spectroscope. Then, after the photoelectric conversion is completed by the photodiode, the current signal is converted into a voltage signal through a transimpedance amplifier and amplified.

[0008] The noise removal unit includes: removing DC noise and periodic noise from the square wave signal amplified by the detection unit;

[0009] The DC noise removal is to filter out the offset voltage and dark level through AC coupling;

[0010] The periodic noise removal is as follows: respectively multiplying the measurement signal with the reference signal and the measurement signal with the reference signal after the phase shift of 90°, obtaining the product results I1 and I2, and then performing analog-to-digital conversion:

[0011] Right now:

[0012]

[0013]

[0014] Where:

[0015] I1 represents the signal obtained by multiplying the measurement signal by the reference signal;

[0016] I2 represents the signal obtained by multiplying the measured signal and the reference signal by 90° phase shift;

[0017] A represents the amplitude of the signal to be measured;

[0018] B represents the amplitude of the reference signal;

[0019] f represents the frequency of the square wave signal;

[0020] t represents time;

[0021] θ1 represents the initial phase of sine wave component 1;

[0022] θ2 represents the initial phase of sine wave component 2;

[0023] After passing through the low-pass filter, the double frequency components in I1 and I2 are filtered out, and the remaining components are recorded as I'1 and I'2 respectively. Then the real signal II is calculated according to the following formula:

[0024]

[0025] Since the amplitude B of the reference signal is known, the amplitude A of the signal to be measured can be calculated.

[0026] The present invention also has the following technical features:

[0027] In the above-mentioned periodic noise removal, it is also possible to Calculate the phase of the two signals.

[0028] In the periodic noise removal, at the same time, the multiplication of two signals can be completed by configuring the amplification factor of the operational amplifier: when the modulation signal is at a high level, the amplification factor of the operational amplifier is configured to be +1; when the modulation signal is at a low level, the amplification factor of the operational amplifier is configured to be -1. The output at this time is equivalent to the multiplication of the measurement signal and the reference signal with an amplitude of 1.

[0029] The sample processing unit comprises: pre-processing the water sample to be tested by digestion, reduction and color development, and then sending the water sample to be tested to the detection pool of the sample flow path, and sending an equal amount of blank solution to the detection pool of the reference flow path.

[0030] In the detection unit, a square wave with a frequency of 1 khz is used;

[0031] In the detection unit, the spectroscope is a 50% spectroscope.

[0032] The system also includes an ARM core processing unit and a host computer.

[0033] The present invention also protects a low-noise and high-precision integrated detection method for total nitrogen and total phosphorus, which adopts the low-noise and high-precision integrated detection system for total nitrogen and total phosphorus as described above.

[0034] Compared with the prior art, the present invention has the following technical effects:

[0035] (I) The present invention converts the light source into a square wave signal, performs the first-level DC and other noise removal, and then multiplies the output signal with the reference signal and the reference signal 90° phase shift signal, and finally calculates a low-noise signal, while eliminating the influence of the phase difference. Compared with the traditional method of directly collecting DC signals, it can greatly suppress interference noise and achieve higher detection accuracy.

[0036] (II) The present invention only extracts the measurement signal with the same frequency as the modulation frequency and filters the interference noise. Therefore, the detection optical path is not interfered by external stray light, and there is no need to place the detection optical path in a dark environment, which greatly simplifies the structural design of this part.

[0037] (III) The original signal collected by the present invention is already a low-noise signal, and there is no need to perform preprocessing algorithms such as smoothing filtering and scattering correction. The concentration can be directly inverted by calculating the absorbance, which effectively improves the data processing efficiency.

[0038] (IV) The present invention has a separate reference optical path and a detection optical path, and is internally provided with a calibration function, which can realize the correction of the two detection optical paths. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 It is a schematic diagram of the principle of the low-noise and high-precision total nitrogen and total phosphorus integrated detection system of the present invention.

[0040] Figure 2 Schematic diagram of light source modulation in the detection unit.

[0041] Figure 3 Schematic diagram of DC noise removal in the noise removal unit.

[0042] Figure 4Schematic diagram of the multiplication principle in the noise removal unit.

[0043] Figure 5 This is the fitting result diagram of total nitrogen.

[0044] Figure 6 This is the fitting result diagram of total phosphorus.

[0045] The specific contents of the present invention are further explained in detail below in conjunction with embodiments. DETAILED DESCRIPTION

[0046] It should be noted that all devices in the present invention, unless otherwise specified, are commonly used devices in the art.

[0047] Based on the deficiencies recorded in the background technology, the present invention proposes a low-noise, high-precision integrated detection system and method for total nitrogen and total phosphorus. The system converts the slowly varying DC signal into a square wave signal, removes the DC noise first, and then completes the multiplication of the detection signal with the same frequency reference signal, and the detection signal with the same frequency reference signal 90° phase shift signal, and then the periodic noise is filtered out by low-pass filtering. Finally, the square root of the two signals is calculated to obtain a low-noise signal. Compared with the traditional DC signal detection method, it can effectively filter out stray light and dark levels, automatically correct the detection optical path, greatly suppress noise interference, effectively simplify the structural design and data processing flow, and achieve high-precision detection of total nitrogen and total phosphorus.

[0048] In accordance with the above technical scheme, specific embodiments of the present invention are given below. It should be noted that the present invention is not limited to the following specific embodiments, and all equivalent changes made on the basis of the technical scheme of this application fall within the protection scope of the present invention.

[0049] Embodiment 1:

[0050] This embodiment provides a low-noise and high-precision integrated detection system for total nitrogen and total phosphorus. Figure 1 As shown, the system includes a sample processing unit, a detection unit, a noise removal unit, an ARM core processing unit and a host computer;

[0051] First, the sample processing unit includes: pre-processing the water sample to be tested by digestion, reduction and color development, and then sending the water sample to be tested to the detection pool of the sample flow path, and at the same time sending an equal amount of blank solution to the detection pool of the reference flow path.

[0052] In this embodiment, according to the methods of national standards GB11894-89 and GB11893-89, sample pretreatment is required before measuring total nitrogen and total phosphorus, and pretreatments such as digestion, reduction, and color development of the water sample to be tested are achieved by switching channels through a multi-channel injection pump.

[0053] Second, the detection unit includes: Figure 2 As shown, a square wave with a frequency of 1 khz is used to control the on and off of the LED power supply, so as to modulate the light source signal into a square wave signal. The square wave signal is detected by a 50% spectroscope to detect the water sample to be tested and the blank solution respectively. Then, the photodiode completes the photoelectric conversion and then converts the current signal into a voltage signal through a transimpedance amplifier and amplifies it.

[0054] In this embodiment, the ARM main control unit outputs a square wave with a frequency of 1 khz to control the on and off of the LED power supply.

[0055] Third, the noise removal unit includes: removing DC noise and periodic noise from the square wave signal amplified by the detection unit;

[0056] like Figure 3 As shown, the DC noise removal is to filter out the offset voltage and dark level through AC coupling;

[0057] The periodic noise removal is:

[0058] The Fourier series of the square wave signal is:

[0059]

[0060] Where:

[0061] X(t) represents a square wave signal;

[0062] n represents the number of harmonic components;

[0063] t represents time;

[0064] ω0 represents the fundamental frequency; ω0=2πf;

[0065] V represents the amplitude of the square wave signal;

[0066] f represents the frequency of the square wave signal;

[0067] It can be known from the Fourier series that the square wave signal is regarded as a superposition of multiple sine waves of different frequencies in the frequency domain, and it is multiplied with a reference square wave signal of the same frequency and phase, that is, the sum of multiple sine waves is multiplied;

[0068] The product-to-difference formula is:

[0069] From the product-to-sum-difference formula, we can know that when f1=f2=f and θ1=θ2=θ, the product-to-sum-difference formula is:

[0070]

[0071] The result is a DC component and a periodic frequency-multiplied component. The frequency-multiplied component is low-frequency and high-frequency noise. This component can be filtered out by a low-pass filter, and only the DC component is collected. However, in actual applications, the measured signal and the reference signal will always have a phase deviation due to circuit delays and other issues, resulting in a low final measurement result.

[0072] Based on the above analysis, in this embodiment:

[0073] The periodic noise removal is as follows: respectively multiplying the measurement signal with the reference signal and the measurement signal with the reference signal after the phase shift of 90°, obtaining the product results I1 and I2, and then performing analog-to-digital conversion:

[0074] Right now:

[0075]

[0076]

[0077] Where:

[0078] I1 represents the signal obtained by multiplying the measurement signal by the reference signal;

[0079] I2 represents the signal obtained by multiplying the measured signal and the reference signal by 90° phase shift;

[0080] A represents the amplitude of the signal to be measured;

[0081] B represents the amplitude of the reference signal;

[0082] f represents the frequency of the square wave signal;

[0083] f1 represents the frequency of sine wave component 1;

[0084] f2 represents the frequency of sine wave component 2;

[0085] t represents time;

[0086] θ represents the initial phase constant when θ1=θ2;

[0087] θ1 represents the initial phase of sine wave component 1;

[0088] θ2 represents the initial phase of sine wave component 2;

[0089] After passing through the low-pass filter, the double frequency components in I1 and I2 are filtered out, and the remaining components are recorded as I'1 and I'2 respectively. Then the real signal II is calculated according to the following formula:

[0090]

[0091] Since the amplitude B of the reference signal is known, the amplitude A of the signal to be measured can be calculated;

[0092] It can also be achieved through Calculate the phase of the two signals;

[0093] At the same time, by configuring the amplification factor of the operational amplifier, the two signals can be multiplied: when the modulation signal is high, the operational amplifier amplification factor is configured to +1; when the modulation signal is low, the operational amplifier amplification factor is configured to -1. The output at this time is equivalent to the multiplication of the measurement signal and the reference signal with an amplitude of 1. While improving the operation speed, the circuit cost can be reduced. The principle diagram is shown in the figure below. Figure 4 shown.

[0094] Fourth, the ARM core processing unit includes: realizing the control of each unit of the system, system calibration, receiving the front-end signal and preprocessing, and then finally outputting the total nitrogen and total phosphorus concentration data according to the concentration inversion model established by the host computer.

[0095] As a preferred solution of this embodiment, the ARM core processing unit adopts, for example, the STM32F103ZET6 single-chip microcomputer disclosed in the System Design of Total Phosphorus and Total Nitrogen Online Analyzer (Xu Guangjian, Liu Xianzhen, Yong Shenggui) as the main control. Compared with the single-chip microcomputer as the main control, the ARM core processing unit has a higher main frequency and abundant peripheral interfaces.

[0096] System Control:

[0097] Control the switching status of modules such as multi-channel injection pumps, relays, solenoid valves, ultraviolet lamps, heating wires, etc. used for flow control during the digestion, reduction, and color development processes in the sample processing unit.

[0098] The power supply of the LED light source in the detection unit is pulse modulated, the total nitrogen detection branch and the total phosphorus detection are switched in time-sharing mode, and the gain of the transimpedance amplifier is adjusted.

[0099] A reference signal is provided to the noise removal unit, and at the same time, the internal phase-locked loop is driven to complete a 90° phase shift of the reference signal, and the gain of the operational amplifier is configured according to the reference signal.

[0100] System Calibration:

[0101] The light source signal passes through the 50% spectroscope and is irradiated to the blank sample and the test sample detection pool respectively. When distilled water is placed in both detection pools, the voltage signals V1 and V2 collected from the two channels should be equal in theory. However, in practice, due to differences in electronic components, temperature and other factors, there is often a certain deviation between the two. Therefore, this solution sets a calibration module in the ARM core processing unit to calculate the gain coefficient K before measurement, so that This can eliminate the deviation of the detection signal caused by the circuit itself.

[0102] Preprocessing:

[0103] The low-noise analog signal from the front end is received by the ARM core processing unit after analog-to-digital conversion. After averaging, the formula Calculate the absorbance, where: I is the transmitted light intensity of the blank sample collected; I' is the transmitted light intensity of the sample to be tested.

[0104] Fifth, the host computer includes: the host computer is mainly used to establish the total nitrogen and total phosphorus concentration inversion model and communicate with the measurement system.

[0105] As a preferred solution of this embodiment, the host computer inversion model adopts, for example, the regression analysis method disclosed in the Chinese invention patent with application publication number CN113109281 A and patent name "A quantitative inversion model of Jiuzhaigou water quality parameters based on hyperspectral remote sensing and its construction method".

[0106] In this embodiment, specifically, a concentration inversion model is established:

[0107] A series of pre-prepared total nitrogen and total phosphorus concentration standard solutions and the collected absorbance data are shown in Tables 1 and 2 below.

[0108] Table 1 Total nitrogen fitting data

[0109]

[0110] Table 2 Total phosphorus fitting data

[0111]

[0112] Linear fitting is performed respectively, and the fitting results are as follows Figure 5 and Figure 6 As shown, from Figure 5 and Figure 6 It can be seen that the determination coefficients of total nitrogen and total phosphorus reached 0.9985 and 0.9996, and the root mean square errors were 0.0132 and 0.0062, respectively. This shows that this scheme can achieve high-precision detection of total nitrogen and total phosphorus.

[0113] In this embodiment, specifically, communicating with the measurement system:

[0114] The measurement system can transmit the measured absorbance data to the host computer through serial port Ethernet and other interfaces for further processing. At the same time, the inversion model established by the host computer and the developed embedded software executable file can be burned to the ARM board through serial port communication to realize the self-contained operation of the system.

[0115] Embodiment 2:

[0116] This embodiment provides a low-noise and high-precision integrated detection method for total nitrogen and total phosphorus. The method adopts the low-noise and high-precision integrated detection system for total nitrogen and total phosphorus provided in Example 1.

[0117] Comparative Example 1:

[0118] Design of online analyzer system for total phosphorus and total nitrogen (published paper by Xu Guangjian, Liu Xianzhen, Yong Shenggui, etc.)

[0119] This comparative example provides a total nitrogen and total phosphorus detection method, which uses a constant current source to power an LED, directly collects measurement signals through an AD acquisition circuit, does not amplify the signal and remove noise, and has low measurement accuracy. However, the embodiment amplifies the signal and removes DC noise and periodic noise, respectively, which can greatly improve the measurement accuracy.

[0120] Comparative Example 2:

[0121] Low-noise and high-precision signal processing system based on ADCP (Chinese invention patent application publication number CN 105049045 A)

[0122] This comparative example provides a low-noise signal processing circuit, which uses a filter amplifier circuit and a passive π filter to perform noise reduction processing on the sound wave signal to be measured. The defect of this method is that the passband of the bandpass filter used in the filter amplifier circuit cannot be infinitely narrow, so it is impossible to filter out the noise within its passband range. In this embodiment, after the signal multiplication process, the noise components different from the modulation frequency are converted into frequency multiplication components and difference frequency components, and the signal with the same frequency as the modulation frequency is converted into a DC component. By setting the cutoff frequency of the low-pass filter, the frequency multiplication components and the difference frequency components can be filtered out, and only the DC component is collected to achieve maximum noise suppression.

[0123] Comparative Example 3:

[0124] A low-noise, high-precision amplifier circuit (Chinese invention patent application publication number CN 109861647 A)

[0125] This comparative example provides a signal amplification circuit, which is connected in parallel with a capacitor on the basis of a traditional cross-group amplification circuit for phase compensation, and then low-pass filtering is used to remove noise. The circuit does not modulate the signal, but directly collects the slow-varying DC signal output by the photodiode, so that the DC signal and the low-frequency noise are mixed together. The purpose of the parallel capacitor is to prevent the circuit from self-oscillating. The denoising circuit only includes a low-pass filtering circuit. Since the 1 / f noise of the electronic components and low-frequency noise such as ambient light can also pass through the low-pass filter, the circuit can not obtain a good low-noise signal. In this embodiment, the frequency of the doubled signal and the difference frequency signal obtained by multiplying the low-frequency noise and the reference signal are close to the reference signal, so they can be removed by a subsequent low-pass filtering circuit.

[0126] From the above-mentioned Examples 1 and 2 and Comparative Example 1, the direct current signal is directly collected in the disclosed total nitrogen and total phosphorus detection method, which has large noise and cannot achieve low-noise and high-precision collection.

[0127] From the comparative analysis of the above-mentioned embodiments 1 and 2 with comparative examples 2 and 3, it can be seen that some existing noise removal circuits attempt to remove noise by using bandpass filters or adding low-pass filtering to the cross-group amplifier circuit, but these methods are unable to distinguish the slowly varying DC signal from the low-frequency noise, resulting in low measurement accuracy.

Claims

1. A low-noise, high-precision integrated detection system for total nitrogen and total phosphorus, characterized in that: The system includes a sample processing unit, a detection unit and a noise removal unit; The detection unit comprises: using square wave to control the on and off of LED power supply to realize modulating the light source signal into a square wave signal, the square wave signal is respectively detected on the water sample to be tested and the blank solution after passing through a spectroscope, and then the photodiode completes the photoelectric conversion and then converts the current signal into a voltage signal through a transimpedance amplifier and amplifies it; The noise removal unit includes: removing DC noise and periodic noise from the square wave signal amplified by the detection unit; The DC noise removal is to filter out the offset voltage and dark level through AC coupling; The periodic noise removal is as follows: respectively multiplying the measurement signal with the reference signal and the measurement signal with the reference signal after the phase shift of 90°, obtaining the product results I1 and I2, and then performing analog-to-digital conversion: Right now: Where: I1 represents the signal obtained by multiplying the measurement signal by the reference signal; I2 represents the signal obtained by multiplying the measured signal and the reference signal by 90° phase shift; A represents the amplitude of the signal to be measured; B represents the amplitude of the reference signal; f represents the frequency of the square wave signal; t represents time; θ1 represents the initial phase of sine wave component 1; θ2 represents the initial phase of sine wave component 2; After passing through the low-pass filter, the double frequency components in I1 and I2 are filtered out, and the remaining components are recorded as I'1 and I'2 respectively. Then the real signal II is calculated according to the following formula: Since the amplitude B of the reference signal is known, the amplitude A of the signal to be measured can be calculated.

2. The low-noise, high-precision integrated detection system for total nitrogen and total phosphorus according to claim 1, characterized in that: In the above-mentioned periodic noise removal, it is also possible to Calculate the phase of the two signals.

3. The low-noise, high-precision integrated detection system for total nitrogen and total phosphorus according to claim 1, characterized in that: In the periodic noise removal, at the same time, the multiplication of two signals can be completed by configuring the amplification factor of the operational amplifier: when the modulation signal is at a high level, the amplification factor of the operational amplifier is configured to be +1; when the modulation signal is at a low level, the amplification factor of the operational amplifier is configured to be -1. The output at this time is equivalent to the multiplication of the measurement signal and the reference signal with an amplitude of 1.

4. The low-noise, high-precision integrated detection system for total nitrogen and total phosphorus according to claim 1, characterized in that: The sample processing unit comprises: pre-processing the water sample to be tested by digestion, reduction and color development, and then sending the water sample to be tested to the detection pool of the sample flow path, and sending an equal amount of blank solution to the detection pool of the reference flow path.

5. The low-noise, high-precision integrated detection system for total nitrogen and total phosphorus according to claim 1, characterized in that: In the detection unit, a square wave with a frequency of 1 khz is used.

6. The low-noise, high-precision integrated detection system for total nitrogen and total phosphorus according to claim 1, characterized in that: In the detection unit, the spectroscope is a 50% spectroscope.

7. The low-noise, high-precision integrated detection system for total nitrogen and total phosphorus according to claim 1, characterized in that: The system also includes an ARM core processing unit and a host computer.

8. A low-noise, high-precision integrated detection method for total nitrogen and total phosphorus, the method adopting the low-noise, high-precision integrated detection system for total nitrogen and total phosphorus as described in any one of claims 1 to 7.

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