Design of intelligent monitoring node for anti-corrosion power supply

Metal corrosion can be seen everywhere. The direct and indirect losses caused by corrosion to metal materials are huge, even causing catastrophic damage accidents and causing serious environmental pollution. Studies have shown that the losses caused by corrosion generally account for 3% to 4% of the gross national product, of which about 15% can be avoided through existing anti-corrosion technology, and the development of cathodic protection technology is inseparable from the progress of anti-corrosion technology.
Anti-corrosion power supply is the most critical equipment in cathodic protection technology. Since most of the easily corroded metal components are distributed in the wild or underground, and have a wide distribution range, such as oil pipelines, power transmission lines, offshore platforms, etc., it is inevitable to develop a new type of anti-corrosion power supply with high reliability and intelligence, and require remote collection of field data through the industrial network, calculation and analysis, and remote control, thereby improving the reliability of field equipment and realizing unmanned management.
1 Structural composition of the anti-corrosion power supply system
Cathodic protection technology simply measures the potential of the protected metal component (i.e., the pipe-to-ground potential), and adjusts the compensation protection current according to its size change to protect the metal component. Figure 1 is a schematic diagram of the remote monitoring anti-corrosion power supply system.

Figure 1 Schematic diagram of remote monitoring anti-corrosion power supply

Obviously, the anti-corrosion power supply is the most core equipment in the cathodic protection system. Its monitoring system must be able to detect and control its operating parameters such as potential, current, voltage, etc., realize network monitoring, and meet the requirements of real-time and rapid response.

2 Hardware design of monitoring node
The system hardware consists of two circuit boards. One is an analog board, which mainly performs filtering, amplification, sampling and holding, and automatically selects the amplification factor of the measurement signal from the anti-corrosion power supply; the other is a digital board, which mainly completes the analog/digital conversion, calculation (eliminating noise and restoring signals), parameter setting and data transmission of the sampled signal [1]. The overall block diagram of the monitoring system is shown in Figure 2.

Figure 2 Overall block diagram of monitoring node

The monitoring system directly measures the electrical signals at the anti-corrosion power supply site, including voltage signals and current signals. The on-site environment of the anti-corrosion power supply is harsh, and there are many interference signals in the signal to be measured. The pre-conditioning circuit includes a differential mode amplifier circuit and an active filter circuit, which are used to suppress the common mode interference signal and high frequency interference signal in the on-site signal. The system communicates with the host computer through the 485 bus and uses the agreed protocol to exchange data. [page]
2.1 Analog circuit design
The analog circuit block diagram is shown in Figure 3, where Vin1, Vin2, Iin1, and Iin2 are signals collected from the anti-corrosion power supply site. Since the signal to be measured is relatively weak and the on-site environment is relatively harsh, there are many interference signals superimposed on the signal to be measured. In order to extract useful signals from the noise, a conditioning circuit combining a differential mode conditioning circuit and an active filter circuit is used to remove interference from the input signal, and then the signal range is estimated through the voltage grading circuit and provided to the microcontroller. The microcontroller calculates the appropriate gain based on the given signal, and then controls the gain of the programmable amplifier AD526, amplifying the conditioned signal to the effective range and inputting it to the AD574 on the digital board for analog-to-digital conversion[2].

Figure 3 Analog circuit block diagram

2.1.1 Signal conditioning design
Through the test and analysis of the field signal, it is found that the interference signal mainly comes from the coupling interference of the power line, the transient voltage interference of the power supply and the external electromagnetic radiation interference. Therefore, this part of the circuit has two functions: one is to design the filter circuit according to the frequency characteristics of the interference signal to effectively filter out the interference signal; the other is to appropriately amplify the input signal to complete the impedance conversion.
2.1.2 Design of automatic gain adjustment circuit
The conditioned signal is selected and processed one by one through a multi-channel analog switch. After the signal passes through the analog switch, one path enters the measurement range of the grading circuit, and the other path enters the amplification unit to amplify to the appropriate working range.
AD526 is a dedicated five-level variable gain op amp with a gain level of G = 1, 2, 4, 8, 16, and three gain control input pins. In the design, two AD526s are connected in series, thus forming an amplification unit with a gain of 1 to 256. The variable gain amplifier circuit is shown in Figure 4.

Figure 4 Automatic gain circuit

The circuit is composed of 8 voltage comparators to form a grading circuit. The microcontroller reads its output signal, sets the appropriate amplification factor according to the obtained grading signal, controls the operation of the amplification unit, and realizes automatic gain adjustment to ensure that each signal can be amplified to the optimal working range of A/D, meeting the design requirements of high precision and wide range.
2.2 Digital circuit design
The digital circuit block diagram is shown in Figure 5. The microcontroller 80C51 is the core of this system, and the data storage capacity of the system is increased by expanding the ROM. A/D is the data acquisition module, D/A is the standard current control signal output module, MAX485 is the module for communicating with the host computer, and Vin is the output signal of the analog part [3].

Figure 5 Digital circuit block diagram

2.2.1 Communication interface design
The system communicates with the host computer through the 485 communication interface to exchange data. RS-485 uses a pair of balanced differential signal lines, which is a half-duplex communication mode. RS-485 is very convenient for multi-station connection. Its standard allows up to 32 drivers and 32 receivers to be connected in parallel, which is enough to meet the requirements of a multi-point anti-corrosion system for a medium-sized component. Matching resistors are connected at both ends of the bus to improve the anti-interference ability. The maximum transmission rate of RS-485 is 10Mbit/s, and the maximum cable length is 1200m. Considering the harshness of the on-site working environment, the TVS tube is used to achieve lightning protection function, protect the system from instantaneous high voltage damage, and improve the reliability of operation.
2.2.2 Standard control current output design
The host computer processes the received data and uses a certain control algorithm to obtain the required feedback control signal. Since the anti-corrosion power supply is an analog device circuit and cannot directly receive digital control signals, it must be converted into analog signals through a single-chip microcomputer to control the power supply.
The AD421 used in the system is a single-chip high-performance digital/analog converter. It is powered by a current loop, with 16-bit digital signals input in serial mode and 5-20mA current output, which can realize remote intelligent industrial control. Its digital input signal is optically isolated to ensure the accuracy and effectiveness of the signal, and the output is a standard current signal with strong anti-interference ability, which can directly drive related analog devices. [page]
3 Software design of monitoring node
In order to improve the efficiency of program writing, the widely used MCS-51 single-chip microcomputer high-level language C51 is used as a software development tool [4].
The entire program of the monitoring system mainly consists of the main program, three interrupt processing programs, and two calculation programs. The main functions completed are: initialization after system reset, control sampling, A/D conversion, signal processing according to sampling value, selection of amplification factor (range), control of communication interface to transmit data, etc. The main program flow chart is shown in Figure 6 [5].

Figure 6 Main program flow chart

This design uses universal components, modular design methods and variable gain circuits to meet the requirements of high precision and wide range, simplify the system structure, greatly improve the reliability of the system, and save costs. This design has practical significance for realizing remote intensive management in the anti-corrosion industry.

References
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2 Yang Zhenjiang. New Devices and Applications in Intelligent Instruments and Data Acquisition Systems [M]. Xi'an: Xi'an University of Electronic Science and Technology Press, 2001.12
3 Liang Jie, Ci Qinpeng. Design of Full Wireless Automobile Wheel Alignment Parameters Computer Measurement System [J]. Beijing: Computer Measurement and Control, 2003 (3)
4 Ma Zhongmei. C Language Application Program Design for Single Chip Microcomputer [M]. Beijing: Beijing University of Aeronautics and Astronautics Press, 1999.1
5 Wang Jianxiao. 51 Series Single Chip Microcomputer and C51 Programming [M]. Beijing: Science Press, 2002.4