Digital Communication Links Cooperating with the Analog 4-20 mA Standard for Marine Applications

The maritime industry makes a significant contribution to the globalized economy. One of the most important parts of the maritime industry is transport. Maritime transport by ships is the dominant means of transport for industrial products and food. Modern ships are increasingly automated. A characteristic feature of automation systems is the use of distributed monitoring and control systems with large distances between field devices and controllers. Among many methods of signal transmission, both in measurement and control, the dominant role in related to continuous signals is played by the two-wire 4-20 mA current standard. Despite its advantages, this standard has limitations mainly due to one-way transmission of information representing one measurement quantity. Programmable transducers are an alternative to analogue solutions. This carried out research concentrates on the full assessment of the properties of the considered methods of information transmission used so far with particular reference to the two-wire 4-20 mA standard. This assessment also takes into account the negative factors characterizing the sea-going ships, such as environmental conditions or problems resulting from the use of an isolated power grid. Additionally, a description of the available digital methods used for communication in ship automation systems will be presented. Firstly, description will be provided for HART protocol (Highway Addressable Remote Transducer), Foundation Fieldbus, Profibus PA and Modbus as communication protocols adopted by modern smart transducers as alternatives for classical analogue 4-20 mA transducers. Moreover, serial communication interfaces such as RS232, RS422 and RS485 will be discussed as means of communication between automation stations. In order to demonstrate the positive contribution for smart transmitters in measurement and control process, an experimental research has been conducted at the laboratories of Gdynia Maritime University, results of which will be discussed in detail in this article. Eventually, illustration will be provided for tank level measurement systems on a commercial ship as an example for measurement and control system mainly based on classical 4-20 mA current signal. Description will be provided for the problems associated with the system and their causes. Based on the conducted discussions, there will be recommendations of suggested solutions for such problems.