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New Approach to Building an Air-Sea Boundary-Layer Monitoring System
An RS-485 Bus Simplifies Installation and Improves Data Quality For An Environmental Monitoring System on the Eastern Chinese Coast

AUTHORS:

Ya-bin Men
Senior Engineer
National Ocean Technology Center
Tianjin, China

He-quan Sun
Associate Professor
Dalian Naval Academy
Dalian, China
Built on a steel tower off China’s eastern coast in the city of Zhoushan, the air-sea boundary-layer monitoring system is designed to collect wind, temperature and humidity gradient data in the coastal and offshore atmospheric boundary layer, as well as sea temperature and sea level data.

The system was originally funded to support national marine disaster prevention and mitigation. Data from the system will be used by researchers to better understand the atmospheric boundary layer and to improve the accuracy of weather forecasting. The monitoring system includes many wind sensors and hygrothermal profile sensors, which are separated into three to six layers. Each layer has one wind sensor and one hygrothermal sensor.

The initial design of the system used only one data acquisition unit with a separate signal wire connecting it to each sensor. However, this design resulted in too many signal wires—arranging the wire is a rather difficult job, and the height of the tower made this work dangerous. Additionally, longer cable introduces more analog signal noise to the system, reducing precision.

In the new approach, a collection node was installed on each layer. The node’s function is to collect and send wind sensor and hygrothermal sensor data to the main data acquisition unit via an RS-485 bus. Unlike the initial approach, the RS-485 bus can collect all sensor data through a single cable, eliminating the need for many signal wires and allowing each node to function independently. Even if one layer malfunctions, the others continue to function normally.


System Introduction
The main function of the air-sea boundary-layer monitoring system is to continuously and automatically measure the temperature and humidity profile of the 30-meter region above the sea surface, as well as to capture other hydrometeorological data, such as wind speed, air pressure, sea-surface temperature and tide. Data from the air-sea boundary-layer monitoring system are recorded to a hard disk and can also be displayed in real time.

Data acquisition is accomplished through the system kernel, which receives, processes and saves the data measured by each sensor. After data acquisition, the results are sent to an upper computer.

The system used a variety of monitoring equipment, including a Vaisala (Helsinki, Finland) hygrothermal sensor, a Setra (Boxborough, Massachusetts) barometer, a Keller (Zurich, Switzerland) tide-level sensor, and a wind sensor, sea-surface temperature sensor and data acquisition unit from China’s National Ocean Technology Center.


Design of RS-485 Bus Node
Optical Isolation. The system was installed in a field with a large amount of other communication equipment and a high common-mode voltage between different nodes, making it necessary to reduce interference.

An RS-485 bus uses a differential transmission with high anti-jamming ability. The bus functions normally at a common-mode voltage range of negative seven volts to 12 volts. The influence of common mode voltage was eliminated by using an optical isolator with an isolated direct current (DC)-to-DC power supply module and an RS-485 chip. This option was adopted because of the low cost and the ease of acquiring these chips.

The design uses two optical isolators in the circuit to isolate data between the system and the RS-485 interface chip, which improves the stability and reliability of the circuit by removing the electrical connection. A 6N136 optical isolator was chosen because of its small size, long life, strong anti-jamming capabilities and high isolation-voltage range (up to 3,500 volts). The 6N136 has a baud rate of 500 kilobits per second or more, and it has been used widely in high-speed digital communication interface circuits.

Isolated Power Supply. For each RS-485 node, the power supply was isolated with an IB0505LS DC-to-DC isolation converter. The IB0505LS has an input voltage of five volts, a stable output of five volts, a maximum output current of 200 milliamps and a conversion efficiency of up to 80 percent.

RS-485 Converter. A MAX3082 low-power transceiver, which allows for up to 256 nodes, was used in the circuit. This transceiver’s output is high when the circuit is idle, providing fault protection.

The MAX3082 is controlled by the input/output (I/O) signal sent by the central processing unit (CPU). When resetting the system, the I/O signal output is high by default. In order to keep the RS-485 chip in the receiving state when resetting, each RS-485 transceiver node was designed to use negative logic. A 74AHC1G14 inverter chip bridges the MAX3082 and CPU, which makes it so that when the CPU sends “1,” the MAX3082 is in a receiving state. When the CPU sends “0,” the MAX3082 is ready to send the data. By adding the inverter chip, bus interference is prevented when the node supplies power at initial power-up.

Bus Interface. An RS-485 bus has three wires, and one is the ground. The data ports and the bus should be isolated. For the system, a 100-milliamp positive temperature coefficient of self-recovery insurance was added between the ports and the bus, as well as a five-volt transient voltage suppressor diode to eliminate the surge.

When the communications rate is greater than 115.2 kilobytes per second over a long transmission distance, a termination resistance is necessary. For the air-sea boundary-layer monitoring system, the baud rate of the RS-485 bus is 9,600 bits per second with a maximum distance of 100 meters, so a termination resistor was not used in the bus.


RS-485 Node Software Design
An RS-485 bus is an asynchronous half-duplex bus, so it can only show one state—receiving or sending.

Software programming strongly affects system reliability. An appropriate delay is obligatory during state switching before sending and receiving data. The specific practice is to set “1” on the control terminal in the data-sending state with a baud of 9,600 bits per second, delay for one millisecond and then send the valid data. After sending, there is a one-millisecond delay before setting “0” to the control terminal.

The data acquisition system acts as the host of the air-sea boundary-layer monitoring system. A node cannot occupy the bus actively. Data acquisition begins by having the bus request data from the first node. The bus then waits for data sent by the first node. If the waiting time is beyond the prescribed limitation, the communication is regarded as a failure and the system will wait for 100 milliseconds before sending the command for the next node. This procedure is performed every three seconds.

It is worth mentioning that a multitasking method is adopted for a pending request. After sending the command, a counter is set in a 10-millisecond timer interrupt service routine. When the counter decreases to zero and no data returns, the next node communication begins. This scheduling mechanism can significantly improve the efficiency of data acquisition.


Conclusions
An RS-485 bus is easy to design at a low cost, which made it a good choice for the air-sea boundary-layer monitoring system.

The system has worked very well since it was installed, collecting significant amounts of data and meeting performance requirements. The present system offers many advantages: Equipment reliability and sensor accuracy have been improved, each node is independent, the system is easy to install and maintain, and a broader network could be built using satellite communication.



Ya-bin Men, a senior engineer at the National Ocean Technology Center of China, has been working on the development of marine monitoring instruments since he received his master’s degree in 2003.

He-quan Sun received his Ph.D. in port, coastal and offshore engineering from Dalian University of Technology, China. He has been working at the Dalian Naval Academy in China as an associate professor since 2008. His research includes applied mathematics, software design and noncontact measurement of flow fields.



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