An excitation board for a corrosion monitoring system

Abstract

The present invention provides an excitation board for a corrosion monitoring system. In the prior art, a DC signal is used. When a signal passes through a crack, it changes, but it is not sensitive. In addition, the DC signal is easily interfered, resulting in a low signal-to-noise ratio. The AC signal of a specific frequency can better remove noise and interference and improve the signal-to-noise ratio if there is prior knowledge. The excitation board of the corrosion monitoring system provided by the present invention includes: a power supply module, a sinusoidal signal generator, and a VI conversion module. Two frequencies of AC current are used for excitation: 5Hz (referred to as low frequency) and 100Hz (referred to as high frequency); internal corrosion and external cracks of the pipeline can be measured simultaneously.

Description

Technical Field

[0001] The invention relates to the technical field of pipeline online monitoring, and in particular to an excitation plate for a corrosion monitoring system. Background Art

[0002] FSM (Field Signature Method) is a corrosion monitoring product, its Chinese name is "Electric Fingerprint Field Method Corrosion Monitoring System", which is used for internal corrosion monitoring of transmission pipelines in the oil and gas industry.

[0003] Existing metal pipeline corrosion monitoring methods using electric field fingerprint characteristics all use a DC constant current source for excitation to obtain the potential matrix of the measured pipeline, and understand the corrosion trend of the pipeline by monitoring the potential changes of each electrode.

[0004] The differential voltage between the electrode pairs measured by the above method is a DC voltage drop, which is a DC signal. However, DC signals are easily interfered by various factors such as environmental electromagnetic waves, resulting in measurement fluctuations and thus measurement errors. Especially for such a weak DC signal between the electrode pairs, the interference may be at a relatively high level, which will result in a poor signal-to-noise ratio. In addition, the DC voltage drop method cannot distinguish between internal corrosion and external cracks.

[0005] The AC excitation current described in the present invention will generate an AC voltage drop between the electrode pairs. The AC voltage drop signal has a priori knowledge of frequency, which can be used to design a filter to effectively remove various interferences and noises and improve the signal-to-noise ratio. In addition, high-frequency AC will also cause skin effect, and cracks on the outside of the pipeline are more easily detected under the skin effect. Summary of the invention

[0006] In view of the problems existing in the prior art, the present invention provides an excitation board for a corrosion monitoring system, the excitation board comprising: a power supply module, a sine signal generator, a VI conversion module,

[0007] The sinusoidal signal generator includes an MCU and a digital-to-analog conversion chip; the MCU stores a complete set of sinusoidal wave data in advance and outputs it to the digital-to-analog conversion chip at a specified time interval to achieve the conversion from a digital signal to an analog signal; by adjusting the time interval for outputting the sinusoidal wave data, the period of the sinusoidal wave signal can be adjusted, thereby achieving the purpose of adjusting the frequency of the sinusoidal wave signal.

[0008] The VI conversion module receives the analog signal output by the sinusoidal signal generator as an input signal, and outputs a current that changes according to a sinusoidal law.

[0009] Preferably, the sinusoidal signal generator further comprises a reference voltage chip, and the reference voltage chip outputs an accurate specified voltage to the digital-to-analog conversion chip of the sinusoidal signal generator.

[0010] Preferably, the VI conversion module includes a current source circuit consisting of a differential amplifier and a low bias current operational amplifier.

[0011] Preferably, the low bias current operational amplifier outputs a reference voltage V REF The inverting input terminal of the differential amplifier is directly connected to the ground; four adjustable resistors are respectively connected to the two input pins, REF pin and SENSE pin of the differential amplifier; the output of the differential amplifier is used to drive a transistor or MOSFET tube; the sampling resistor R1 and the load resistor R load Connected in series to the output end of the transistor or the MOSFET tube.

[0012] Preferably, the output current on the load resistor is:

[0013]

[0014] Preferably, the material of the sampling resistor R1 is Ni—Cr, so that it has good resistance stability and low temperature drift performance.

[0015] Preferably, the excitation board further includes a watchdog circuit, and an output end of the watchdog circuit is connected to a reset pin of the MCU.

[0016] Compared with the prior art solutions, the present invention has at least the following beneficial effects:

[0017] 1) In order to simultaneously measure the internal corrosion and external cracks of the pipeline, we use two frequencies of AC current for excitation: 1-5Hz (called low frequency) and 100-1000Hz (high frequency);

[0018] 2) Use a low-noise, low-offset voltage, low-drift precision instrumentation amplifier and a zero-drift operational amplifier based on chopping technology to form a secondary amplifier circuit to amplify weak signals;

[0019] 3) Use analog signal filtering technology - switched capacitor filtering technology to filter the signal from the preamplifier board. In order to deal with low-frequency and high-frequency signals at the same time, a two-stage filtering technology is used: low-pass and band-stop, where the band-stop is used to filter out 50Hz power frequency interference, and the low-pass filter is used to filter out high-frequency interference.

[0020] 4) Simplify circuit design and improve system reliability and stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 The reference board structure diagram of the FSM system of the present invention;

[0022] Figure 2 is a diagram of the reference electrode structure on the reference plate of the present invention;

[0023] Figure 3 The present invention discloses a current source circuit composed of a differential amplifier and a low bias current operational amplifier.

[0024] The present invention is further described in detail below. However, the following examples are only simplified examples of the present invention and do not represent or limit the scope of protection of the present invention. The scope of protection of the present invention shall be subject to the claims. Specific embodiments

[0025] The technical solution of the present invention is further described below with reference to the accompanying drawings and through specific implementation methods.

[0026] The FSM system provided by the present invention uses the AC voltage drop (ACPD) method to measure the weak resistance of the pipeline. The basic principle is: when an AC current is passed through the pipeline, the current will flow through the matrix electrodes. Since the metal loss will cause the resistance value between the electrode pairs in the matrix to change, the voltage drop will correspondingly change between the electrode matrix. Since the matrix electrodes are very close, the resistance between them is extremely small, and the current flowing between the matrix electrodes will also be very small, so the voltage drop will also be very small. We call the voltage drop between the electrodes a signal. This is an extremely weak signal, which we need to amplify, filter, and then sample and save.

[0027] In other words, after metal loss, i.e. corrosion or cracking, occurs, the resistance between the electrode pairs will change, and the voltage between the corresponding electrode pairs will also change. The occurrence of corrosion, etc., is monitored by monitoring the change in this voltage. This change is very small, so a large amplification factor is required. If a first-stage amplification is used, the gain resistor that determines the amplification factor of the instrument amplifier must be very small. Good quality very small resistors are not easy to obtain, conventional resistors cannot meet the requirements, and the resistor is usually required to be adjustable in order to adjust the amplification factor, which brings difficulties to the selection of adjustable gain resistors with low temperature drift. The first stage of amplification uses instrument amplification because the input impedance of the instrument amplifier is relatively large, and basically does not bring a load effect to the signal source. The amplification factor of the entire system must be adjustable, and the amplification factor of the amplifier needs to be adjusted by resistors. The resistance of the instrument amplifier generally remains unchanged, and the adjustable multiple function is completed by the operational amplifier. The operational amplifier can achieve zero drift, and the two-stage amplifier can achieve the effect of low drift and high amplification factor.

[0028] The current flowing through the matrix electrodes is constant, which is the current source generated by the excitation board. This current is not very small.

[0029] Since the AC current (which we call excitation current) is applied to the pipeline, the signal taken out from the matrix electrode is also an AC signal of the same frequency. The signal frequency is known, which makes signal processing convenient. Based on the "prior knowledge" of the known frequency, we can design a bandpass filter (or low-pass, high-pass filter) in a targeted manner to filter out unnecessary noise. Therefore, the AC voltage drop method has a higher signal-to-noise ratio than the DC voltage drop (DCPD) method.

[0030] Signal-to-noise ratio calculation formula: 10lg(P S / P N ), Ps and Pn represent the effective power of signal and noise respectively, which can also be converted into the ratio of voltage amplitude: 20Lg(V S / V N ), Vs and Vn represent the "effective value" of the signal and noise voltages, respectively. For the matrix electrode pairs whose positions have been fixed, it is obvious that the resistance value between them is basically fixed, so the voltage value between them depends only on the current flowing between them and is proportional to it.

[0031] Therefore, to improve the signal-to-noise ratio, the most direct and effective way is to increase the current flowing through the matrix electrodes, that is, to increase the excitation current value.

[0032] To achieve this goal, the FSM system provided by the present invention takes measures in current excitation compared to the first and second generation FSM systems, and the current value is increased by 10 times. The excitation current value of the first and second generation FSM systems is about 2A, while the excitation current value of the third generation FSM system can be as high as 30A or more. This greatly improves the quality of the original voltage signal obtained between the matrix electrodes.

[0033] In order to measure the internal corrosion and external cracks of the pipeline at the same time, we use two frequencies of AC current for excitation: 5Hz (called low frequency) and 100Hz (called high frequency). The low frequency is only used to improve the signal-to-noise ratio, while the high frequency uses the skin effect to detect external cracks. Because under the action of the skin effect, the higher the frequency of the AC current, the easier it is for the current to gather on the surface of the pipeline, and therefore the more sensitive it is to cracks on the pipeline surface.

[0034] like Figure 1 The figure shows the structure diagram of the reference board of the FSM system of the present invention. We use an 8×8 electrode matrix and a pair of reference electrodes located on the reference board. The excitation current flows into the reference board, flows out of the reference board, enters the pipeline, and returns at the other end of the pipeline.

[0035] The electrode layout diagram on the reference board is a welding detection electrode matrix arranged in columns along the longitudinal direction of the pipeline. For example, the 8 electrodes in the first column are numbered 1-1, 1-2, 1-3, ..., 1-8; the electrodes in the second column are numbered 2-1, 2-2, ..., 2-8; ...; the electrodes in the eighth column are numbered 8-1, 8-2, ..., 8-8.

[0036] The distance d between the detection electrodes is approximately 3 times the pipe wall thickness.

[0037] There is also a pair of reference electrodes REF+ and REF- on the reference board. The spacing between the reference electrodes can be about one wall thickness.

[0038] Reference board material: Exactly the same as the pipe material. Reference board shape and size: It is recommended to use an arc-shaped board with a radius basically consistent with the pipe, a width not less than 1 / 6 of the pipe, and a length not less than 200mm. It should be as large as possible to make the current distribution on the reference board more uniform. The purpose of using a reference board is to obtain a stable, long-term unchanged signal for reference of the entire system. This is based on the assumption that the reference board will not corrode over time. In addition, the reference board can also eliminate the impact of parameter offsets of the entire system at different temperatures. For example, the resistance of the pipe will change at different temperatures. By introducing a reference signal to calculate the FC value (a relative value), the impact of most instability factors on the signal link can be eliminated.

[0039] like Figure 2 As shown, it is a schematic diagram of the reference electrode on the reference board of the present invention. The excitation current input terminal I+ and the bridge terminal I+' are both on the reference board. The center position of the reference electrode pair is recommended to be located at 1 / 3 to 1 / 2 of the distance between the input terminal and the bridge terminal. The reference electrode is preferably made of a copper screw.

[0040] The whole system is divided into four parts:

[0041] (1) Excitation board. This circuit is used to generate excitation current. This current signal is an AC signal with variable frequency. Under the command of the main control board, it can first generate an AC current of 5 Hz, and complete the sampling of the voltage between all electrode pairs at this frequency. Then the excitation frequency is adjusted to 100 Hz, and the voltage signal between all matrix electrodes is sampled at this frequency until the end.

[0042] (2) Preamplifier board. This circuit board is installed in the preamplifier box and is used to switch all matrix electrode pairs and amplify the weak voltage between the selected matrix electrode pairs. After preamplification, the peak-to-peak voltage signal between all electrode pairs reaches the volt level, which is convenient for subsequent circuit processing.

[0043] (3) Main control board. This circuit board realizes the control of the whole system, the selection, filtering, rectification of analog signals, and the extraction, sampling, storage and transmission of peak values ​​(through the operator's 4G channel).

[0044] (4) Electrode matrix. This is an electrode array welded on the pipe, including 8×8=64 electrodes and a pair of reference electrodes. In addition, there are also the injection end and return end of the excitation current, as well as the reference plate.

[0045] To facilitate data collection, this system also designs a cloud data storage system. The device connects to the remote server and automatically uploads the current data and the data that has not been saved on the server after each data collection. Users can use the PC client software provided by this system to remotely download data through a networked computer and save it to the local computer for analysis and judgment. This can avoid regular inspections of pipelines and data copying, which will greatly improve the convenience of data acquisition, especially for pipeline signal collection systems located in the wilderness or for situations with a large number of devices.

[0046] Example 1

[0047] The basic function of the excitation board is to output a sinusoidally modulated excitation current to drive the pipeline and the reference board, so as to generate a voltage drop on the electrode pair.

[0048] The power supply for the excitation board signal comes from the main control board, and the high-current driving power supply for the components of the excitation board comes from a dedicated power DC-DC module.

[0049] The RS-232 serial port is used for communication between the excitation board and the main control board. The main control board sends commands to the excitation board through this serial port to make it output 0V voltage, low-frequency (5Hz) sinusoidal AC voltage or high-frequency (102Hz) sinusoidal AC voltage.

[0050] In order to output an accurate sinusoidal modulation waveform, there is a precise reference voltage on the excitation board, which is generated by a specially designed reference voltage chip and can output a precise specified voltage to supply the digital-to-analog conversion chip.

[0051] The sine signal generator is composed of an MCU, a digital-to-analog conversion chip, and a reference power supply. The MCU stores a complete set of sine wave data (such as 12 bits or 16 bits) in advance and outputs it to the conversion chip (DAC) at a specified time interval to achieve the conversion from digital signal to analog signal.

[0052] The DAC is connected to the MCU via the SPI bus. The MCU is mainly connected to the digital-to-analog conversion chip and the RS-232 chip in the excitation board to receive commands from the main control board and perform corresponding DAC conversions.

[0053] The DAC conversion data that the MCU needs to use is calculated in advance, processed into a format acceptable to the MCU, and solidified and stored in an array of the MCU.

[0054] By outputting the data in the above array in sequence and at intervals, a voltage that changes according to the sine law can be obtained on the DAC output pin. By continuously and repeatedly outputting this set of data, a continuous sine waveform can be obtained. By adjusting the interval of data output, the period of the sine wave can be adjusted (stretched or compressed), thereby achieving the purpose of adjusting the frequency of the sine wave.

[0055] The VI conversion circuit, namely the voltage-controlled current source, means that the output current is controlled by a voltage and is proportional to the voltage. If the amplitude of the input voltage is a quantity that changes according to the sine law, then the output current will also be a quantity that changes according to the sine law, which realizes the expectation of sinusoidal modulated current output.

[0056] There are many solutions for VI conversion circuits, and one option is to use a differential amplifier to implement it.

[0057] like Figure 3 As shown in Figure 1, a current source circuit is formed by a differential amplifier and a low bias current operational amplifier. REF Applied to the non-inverting input of the differential amplifier, this voltage controls the output current I O The inverting input of the differential amplifier is directly connected to ground. The device has four laser-trimmed resistors connected to the input pin, REF pin, and SENSE pin. If high current output is required, the output of the differential amplifier can be used to drive a transistor or MOSFET.

[0058] If the resistors are perfectly matched, the voltage across R1 is the input voltage V REF , thus generating a constant load current I O , which is equal to V REF / R1.

[0059] The operational amplifier was chosen for the circuit's feedback loop because of its low bias current and low offset voltage. The low bias current allows it to interface with high impedance loads without introducing significant offset errors. The low temperature drift allows it to operate over a wide temperature range, and the amplifier also has low noise and rail-to-rail input and output characteristics.

[0060] according to Figure 3 , the calculation formula of output current can be obtained:

[0061]

[0062] The difference amplifier has very tight resistor matching, R F1 / RG1 =R F2 / R G2 =1, so formula (1) can be simplified to:

[0063]

[0064] Formula (1) shows that Figure 3 The main errors of the circuit shown come from the internal resistor matching, the tolerance of R1, and the tolerance of the load resistor. At the same time, the accuracy of R1 is also very important, and its tolerance should reach 0.1% or better. This error can be eliminated by calibration.

[0065] The output current of the circuit is I O Limited by: op amp input range, difference amplifier output range, and difference amplifier SENSE pin voltage range.

[0066] Based on the above VI conversion principle, a current source with sinusoidal modulation can be realized. REF is a sinusoidal voltage, then the voltage flowing through R1 and R LOAD The current I O is equal to V REF / R1. It can be seen that the output current has nothing to do with the load size (but make sure that the load resistance is within a certain range, otherwise the current value will not reach the expected value), but only with the reference voltage V REF It is related to the sampling resistor R1. The sampling resistor R1 is usually fixed, so the output current will change with the reference voltage V REF change.

[0067] Obviously, if V REF Following the output change of the sinusoidal signal generating circuit DAC, the output of sinusoidal modulated current can be achieved.

[0068] In order to improve the current output capability, R1 is usually very small, and the preferred resistance is 5mΩ. Its material is Ni-Cr, which has good resistance stability and low temperature drift performance.

[0069] In order to improve the driving ability of VI, this design uses MOSFET to drive the load. At the same time, in order to improve the response speed of the system, a push-pull circuit based on transistors is used to drive the MOSFET tube.

[0070] In order to observe the output current and the voltage applied to the load (excitation current line and steel pipe), current and voltage detection are also required.

[0071] Current detection can be performed directly on the sampling resistor. The voltage on the sampling resistor is amplified by a differential amplifier, and the current value can be obtained by calculation.

[0072] The differential amplifier it uses is the same as the differential amplifier sampled by the first stage amplifier circuit in the preamplifier circuit.

[0073] There is also a part of auxiliary circuit on the excitation board, which can be a watchdog circuit.

[0074] The output of the watchdog circuit is connected to the reset terminal of the MCU. The watchdog chip must continuously feed the watchdog by flipping the level of a pin in the main program of the MCU. If the feeding stops, it means that the MCU program is running away and invalid. Then, after 1.6s (the longest restart cycle of the watchdog chip), the output of the watchdog circuit will output a pull-down pulse to restart the MCU.

[0075] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

[0076] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not further describe various possible combinations.

[0077] In addition, various embodiments of the present invention may be arbitrarily combined, and as long as they do not violate the concept of the present invention, they should also be regarded as the contents disclosed by the present invention.

Claims

1. An excitation board for a corrosion monitoring system, characterized in that: The excitation board includes: a power supply module, a sine signal generator, and a VI conversion module; The sinusoidal signal generator includes an MCU, a digital-to-analog conversion (DAC) chip and a reference voltage chip; the MCU stores a complete set of sinusoidal wave data in advance, and outputs it to the digital-to-analog conversion (DAC) chip at a specified time interval to achieve the conversion from digital signal to analog signal; the reference voltage chip outputs an accurate specified voltage to the digital-to-analog conversion chip of the sinusoidal signal generator as a DA conversion reference voltage; by adjusting the time interval of the sinusoidal wave data output, the period of the sinusoidal wave signal can be adjusted, thereby achieving the purpose of adjusting the sinusoidal wave frequency; The VI conversion module receives the analog voltage signal output by the sinusoidal signal generator as an input signal, and outputs a current that changes according to the sinusoidal law; wherein the VI conversion module includes a current source circuit composed of a differential amplifier and a low bias current operational amplifier; the low bias current operational amplifier outputs a reference voltage V REF The inverting input terminal of the differential amplifier is directly connected to the ground; four adjustable resistors are respectively connected to the two input pins, REF pin and SENSE pin of the differential amplifier; the output of the differential amplifier is used to drive a transistor or MOSFET tube; the sampling resistor R1 and the load resistor R load Connected in series to the output end of the transistor or the MOSFET tube.

2. The excitation board for a corrosion monitoring system according to claim 1, characterized in that: The output current on the load resistor is: Where R1 is the resistance value of the sampling resistor.

3. The excitation board for a corrosion monitoring system according to claim 1, characterized in that: The material of the sampling resistor R1 is Ni-Cr, so that it has good resistance stability and low temperature drift performance.

4. The excitation board for a corrosion monitoring system according to claim 1, characterized in that: The excitation board also includes a watchdog circuit, and an output end of the watchdog circuit is connected to a reset pin of the MCU.

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