Wireless sensor network of ocean water environment monitoring system (1)

Introduction

In recent years, with the rapid development of the marine industry, marine environmental protection has been put on the agenda.

Therefore, marine water monitoring become increasingly focus.

Wireless sensor network widely used in military surveillance, environmental monitoring, goal orientation, and other areas, to real time perception, collection and treatment network coverage of object information and sent to the Viewer.

It has a wide coverage area, remote monitoring, monitoring of high precision, Chief network fast and low cost. The wireless sensor network technology applied to environmental monitoring system for marine water, people in the past few years the focus of the study.

Zigbee and other wireless communication standards apply to the throughput compared to smaller, network construction investment is small, network security, not easy to frequently replace the power supply.

In the industrial control field using sensor technology based on Zigbee sensor network, you can make data acquisition and analysis becomes convenient and easy. Zigbee network for sensor networks to form a very important point is that it's low power, its transmitting power of only 0 ~ 3.6dBm; its communication distance of up to 30 ~ 70m, with energy measurement and link quality indicates ability, can automatically on its own power to make an adjustment, you can ensure the communications link quality minimum to consume energy. Zigbee network function is the most important feature is the standard for wireless local area network with other different places. At the network layer, the main work lies in the Zigbee network establishment and management of the mechanism, and have the self control configuration and self-healing capabilities.

IEEE802.15.4 standard is an economic, efficient, low data rate (< 250kb/s), working in 2.4GHz and 868/928MHz wireless technologies, network layer above Protocol developed by the ZigBee Alliance, IEEE802.15.4 is responsible for the physical layer and link layer standards.

Complete zigBee protocol suite consists of a high-level application specification, application convergence layer, network layer and data link layer and physical layer. Protocol stack structure shown in Figure l.

1 the composition of the sensing network

This design consists of a wireless sensor network including sensor node, node and network brings together the key point, is mainly responsible for detecting various marine regions, including the oil pollution detection, turbidity measurement, measurement of the chemical oxygen demand, seaweed measurement, etc.

Sensor nodes are mainly responsible for the formation of the network, the ocean parameters, and data collected through forms of Multihop transmission to aggregation node.

Aggregation node is a wireless sensor network central node, is responsible for the launch of the network, the formation and maintenance of topology, network data gathering and processing, and monitoring system of communication and information exchange.

Aggregation node is a terminal node sensor nodes in a relatively strong.

Gateway point receives data from other nodes and data correction, integration, and so on, and then sent to a monitoring center.

For monitoring the directive for the appropriate treatment that is used to determine the status of each node.

Background monitoring centre is responsible for sending back to summarize marine parameter data and processing, network topology, network monitoring, etc.

Entire marine monitoring system consists of a certain number of sensor network terminal node, a small number of aggregation node, a network of key points and background monitoring system.

In order to detect a certain region, the regional arrangement of a certain number of sensor nodes to achieve coverage of the entire region, and you need a gateway point of completion of the data from the sensor terminal of fusion, upload it to the background monitoring system, complete data analysis and processing. The key point from the Internet to a monitoring center distance is generally relatively far away, you can use the existing GPRS networks for remote data transfer. The GPRS network connection costs relatively inexpensive, the transmission rate is high, high cost performance, and to always online. Sensor network architecture diagram shown in Figure 2.

Sensor terminal node and aggregation node the ability to automatically create a self-organizing, Multihop network.

Sensor terminal nodes according to the directive, data acquisition and data in a timely manner through adaptive routing, multihop relay after transmission to the gateway, gateway points will bring together data package, forward it to the background monitoring system.

2 hardware design

This marine monitoring system sensor node is a sensor network in the most important part, its hardware including microprocessor unit, a zigbee communication module and power management module; aggregation node hardware including microprocessor unit, two Zigbee communication module and power management module; gateway point hardware including microprocessor unit, a Zigbee communication module, a GPRS module and power management module.

2.1 node MSP430F149-chip microprocessor

Because wireless sensor network node requires the sensor output of analog signals into digital signals, you can select an integration with AD conversion feature of micro-controller.

In addition, wireless sensor network nodes in addition to the complete data acquisition functionality, but also complete data forwarding and routing functionality and therefore to have enough processing power, program space and data space. The design of the MCU is MSP-430F149 microcontroller, which is a TI company produced a 16-bit low power consumption, mixed-signal processor called mixed signal processor, mainly due to its demand for practical application, many analog and digital circuits and microprocessors integrated in a chip to provide "monolithic" solution. The highlight of their advantages are low power supply voltage, low power consumption. As for FLASH-type, so you can be online on-chip debugging and download the program.

Use of low-frequency aided clock MSP430F149 32kHz clock oscillator direct drive, when as a background in real time

Clock implementation since the wake-up function. Integrated high-speed digital control oscillator (DCO) frequency is 8MHz, can be used as the main system clock of the CPU (MSLK) source or the subsystem clock as the CPU (SMCLK) source.

2.2 node Zigbee communication CC2420 module

This system of wireless sensor network hardware of Zigbee communication module adopts low-power high-performance wireless network module CC2420, its work in global common 2.4GHz band.

CC2420 is a standard complies with IEEE802.15.4 RF transceiver, stable performance and low power consumption. CC2420 selectivity and sensitivity index more than IEEE802.15.4 standard, ensures that the distance between the validity and reliability of communication, use this chip development of wireless communications devices, support transmission rates up to 250kb/s for multipoint-to-multipoint network quickly. When sending data, CC2420 use direct quadrature upconversion. Baseband signal phase and quadrature component directly by DAC converts the analog signal, through low-frequency filters, direct conversion to set channel, launched by the antenna.

Zigbee communication module CC2420 and SCM connection circuit, as shown in Figure 3.

CC2420 requiring fewer external circuit, including the clock circuit, radio frequency circuit and the I/O matching micro-controller interface circuit three parts.

Chip local oscillator signal by external Active Crystals provide, you can also provide the internal circuit. Provided by an internal circuit requires plus crystal oscillator and two, capacitance load capacitance of size depends on the frequency and type of Crystal capacitance and other parameters. For example when using 16MHz crystal oscillator, the capacitor 22pF about. RF I/O matching circuit is mainly used to match the chip's input/output impedance. CC2420 and microprocessor connections very convenient, it uses the SFD, FIFO, FIFOP, and CCA four pin represents the State of the send and receive data; microprocessor through SPI interface to Exchange data with the CC2420, send command.

CC2420 received physical frame of SFD field after SFD pin output until the finished frame is received.

If you start the address identification, address identification fails, SFD pin immediately to output low level. FIFO and FIFOP pin that receive FIFO cache status, if you receive FIFO buffer has data, FIFO pin output; when receive FIFO buffer is empty, FIFO pin output low level; when the FIFO receive FIFO cache pin in the area of the data exceeds a certain threshold, or receive the CC2420 into a complete frame later output critical value, you can register settings through the CC2420. CCA pin in the channel on the signal output, it is only valid in the receiving State, the entry into the receiving State CC2420 is at least 8 symbols cycle before the CCA pin output valid channel state information.

SPI interface by CSn, SI, SCLK pin SO and, microprocessor through SPI interface access CC2420 internal registers and memory.

During the visit, the CC2420 is SPI interface from equipment, receive from the microprocessor clock signal and tablets and selected signal the microprocessor controlled performs input/output operations. SPI interface to receive or send data, are aligned with the clock falling edge, and MSP430F149 CC2420 is through SPI connection, which is the main mode, MSP430F149 CC2420 is in passive mode. MSP430F149 there are four I/O port and is connected to the main play CC2420 query CC2420 State role.

Power management module for sensor unit, the processor unit, wireless communication modules provide energy and power management to improve energy utilization.

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