Research on underwater wireless multi-point communication system
An important use of the underwater acoustic sensor network is to conduct mid- and long-term underwater early warning, target detection, and monitoring of marine hydrological environmental elements in the area covered by the underwater sensor nodes; meanwhile, in the future, multi-base and outboard distributed sensors Under the huge anti-submarine warfare network formed by the system, underwater data communication is the key, while the underwater sensor network undertakes the important mission of detection and data communication. The so-called underwater wireless sensor network is to obtain underwater information through various sensor nodes in a certain underwater area, and perform acoustic communication and networking on the underwater nodes, and finally through specific nodes, re-use radio and wired In the form of, the information obtained in the coverage area is incorporated into the conventional network on the shore and sent to the observer's underwater subnet.
Because the transmission conditions of underwater acoustic channels are very bad, especially shallow sea acoustic channels, the channel bandwidth is limited, depending on the distance and frequency. Within this limited bandwidth, the acoustic signal is affected by strong environmental noise and time-varying multipath. It may cause severe inter-symbol interference (ISI), large Doppler frequency shift expansion and long transmission delay. In addition, the attenuation of wireless electromagnetic waves and light waves in water is very large, and remote transmission cannot be achieved. Therefore, when designing underwater acoustic sensor networks, you can use radio networking technology, but you must also consider the characteristics of underwater acoustic channels.
1 Underwater wireless multipoint communication system
1.1 The overall framework of the system
Based on the characteristics of underwater acoustic channels, and taking into account that the spacing between adjacent points in the frequency domain must be greater than the coherent bandwidth of the channel, frequency hopping communication using FSK modulation is used. Similar to the structure of wireless sensor networks on land, the topology of underwater acoustic sensor networks can be divided into two categories: one is the typical star structure used in small-scale networks; the other is large-scale, multi-node, A distributed peer-to-peer network topology in a dispersed and dense environment.
The design realizes a small-scale network, using a star structure, consisting of a PC, a main node, and multiple sub-nodes to form a network system. A PC connected to the Internet network is the monitoring center of the network. The master node broadcasts information to realize data transmission and command control. The terminal equipment is directly controlled by the master node. The underwater wireless sensor network is constructed. The system is shown in Figure 1. As shown.
1.2 Communication protocol of underwater wireless multipoint communication system
In order for the underwater wireless sensor network to complete the command transmission and data transmission stably without error codes, a communication protocol is also required to ensure its reliability. Combined with the needs of underwater wireless sensor networks, two different communication protocols are defined here: PC to node downlink sending data communication protocol, and node to PC uplink receiving data communication protocol.
The start character uses the ASCII code of "%" to indicate the data frame header.
The slave number 0 to 99 indicates that the command is to control the number of sub-nodes. The 0 number is a broadcast setting, that is, if it is a 0 number, the water surface repeater sends control information to each underwater acoustic communication slave group.
The control command sets the action type number to be processed by the slave node, and the control instruction number corresponds to the underwater wireless sensor network sub-node taking different control operations.
The end character indicates the end of the data frame with the ASCII code of "MYM".
(2) Communication protocol for receiving data
The start character uses the ASCII code of "%" to indicate the data frame header.
The data currently sent back from the slave number comes from the sub-node number of the underwater wireless sensor network.
The data type indicates the meaning of the transmitted data.
Data content specifically monitored data.
The end character adopts the ASCII code of "MYM" to represent the end of the data frame.
2 System design of communication node
There are two kinds of equipment in the underwater wireless sensor network: the main node and the sub-node. The main node is mainly responsible for the release of various monitoring tasks and the simple aggregation and processing of feedback information such as data. It is the bridge between the remaining nodes and the main control PC. The sub-node is mainly responsible for collecting the measurement data of sensors or interface devices. And feedback the response signal or data directly to the master node. The communication between the PC and the master node is achieved through RS 232, while the communication between the master node and each node is achieved through the underwater acoustic transducer.
2.1 System hardware structure
The system hardware structure is shown in Figure 2.
As can be seen from Figure 2, the host computer control system completes the control of the wireless sensor network through the master node. The core of system control is Lingyang SPCE061A microprocessor. It is a 16-bit micro-controller introduced by Sunplus Technology. Its power consumption is small, and the power consumption when the system is in the standby state (sleep state) is only 2Î¼A / 3.6 V; built-in 2 KWorldSRAM and 32 KWord FLASH; two 16-bit programmable timers / counters; two 10-bit Digital / analog conversion (DAC) output channel; 2 16-bit general-purpose programmable input / output ports IOA and IOB; abundant interrupt resources: timer A / B interrupt, time base interrupt, 2 external interrupts, and key wake-up interrupt ; 7-channel 1O-bit voltage analog-to-digital converter (ADC) and single-channel sound analog-to-digital converter; universal asynchronous serial input / output interface UART; system clock signal can be selected through a phase-locked loop PLL oscillator; low voltage reset Function and low voltage detection function; WatchDog function, etc. Compared with other single-chip microcomputers, SPCE061A is a resource-rich, powerful, and highly integrated micro-controller. Using this micro-controller as the micro-controller of this system has a high cost performance.
2.2 Software workflow
After the master node is initialized, the system is normally waiting for the PC to send the prepared information or other monitoring commands. After receiving the data, it calls the sending program to send the received information plus the synchronization header to the corresponding sub-node. The software flow is shown in Figure 3.
After receiving the information from the PC, the MCU first stores the information. After all the data is received, the data is packaged and sent in a frequency hopping pattern. After processing by the signal conditioning module, the signal is finally sent out through the transducer .
2. 3 node power system
MCUs, operational amplifiers, and various IC circuits are used in the system, so 5 regulated power supplies are required. The span of voltage amplitude is from DC-12 to 12 V. The current output to each load is uneven, so the design of the power supply system plays an important role in the stable operation of the node. In order to realize the power supply of the system with a single power supply here, it is necessary to realize the voltage conversion. There are two main types of circuits that implement the voltage conversion function. One type is the switching regulator circuit. It uses self-excitation or other excitation methods to generate high-frequency switching currents, which are converted into DC again using nonlinear energy storage elements (such as inductance) Class conversion can be divided into boost type, buck type and isolated type. Earlier, DC / DC voltage conversion was implemented with discrete components. Currently, there are various dedicated ICs with better performance to complete the control and conversion functions of the circuit. The other is a linear voltage regulator circuit, which has now been developed to LDo (LowDrop Outregulator). LDO is a low-dropout linear regulator. Linear regulators use transistors or FETs to operate in their linear region, subtracting the excess voltage from the input voltage, and the pressure difference is shared by the transistor's tube voltage drop, resulting in a regulated rated output voltage. According to the above, to achieve a single power supply does not need to raise and lower the voltage, but also need to realize the conversion of positive and negative voltage. If the power conversion is all implemented by switching power supply, it will get a very high efficiency, but also increase the interference of high-frequency electromagnetic waves. At the same time, considering the current requirements of the system, a mixed structure of switching power supply and linear regulated power supply is adopted here. Among them, the conversion of small current load is realized by linear voltage regulator and LDO, and the conversion of positive and negative voltage and high current load is realized by switching power supply. The power system block diagram is shown in Figure 4.
3 System management program design
The communication between the PC and the master node is realized through the RS 232 serial port, the communication baud rate is 9 600 b / s. In order to be able to link with the computer interface and the TTL device of the terminal, it is necessary to transform the level and the logical relationship, using MAX 232 chip can realize bidirectional level conversion from TTL to EIA. The PC main control system realizes the control of the main node and the acquisition and display of sensory information. Therefore, the main control system interface should include the node's control panel and display window, as shown in Figure 5.
The control panel gives the choice of the node number of the message to be sent. After entering the node number, pressing the send button, the main control system will send information to the corresponding node through the master node. After receiving the information, the node executes the corresponding instruction and feeds back the information to the PC.
Here we introduce a high-performance 16-bit single-chip microcomputer as the control core and use frequency hopping communication to realize the underwater multipoint communication system. The system has reliable data transmission, friendly interface and strong scalability. Satisfactory results have been achieved in the pool test.
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