Open-source radio technology: Now available for process engineering
By Gerrit Lohmann, Robert Schosker Pepperl+Fuchs -- Plant Engineering, 9/15/2008 1:00:00 AM
Following the technological progress of fieldbus in process automation, the next development step in field communication is that of solutions involving radio technology. Can radio technology be used on an “across the board” basis? Based on open standards, the HART Communication Foundation provides the answer with WirelessHART.
WirelessHART now available
WirelessHART is built on the widely-used HART standard, as evidenced by 20 million installed HART-compatible field devices. HART was originally conceived as an extension of the common 4-20 mA current loop, in order to provide field devices with greater functionality.
The next step in the HART evolution has arrived. The WirelessHART standard, released in September 2007, is based directly on the HART protocol, but is free of the physical transfer path. HART uses a 2.4 GHz band – license-free and used throughout the world – as a transfer medium for several radio technologies, including wireless local area network (WLAN), Bluetooth and ZigBee. Thus it is possible to use WirelessHART without approval.
Evolution of a technology
For the physical layer, WirelessHART uses radio modules in accordance with IEEE 802.15.4. Radio systems are already established on the basis of this standard, for example ZigBee and WLAN. The advantage is that the hardware is already available. However, to simply refer to WirelessHART as WLAN is too much of a simplification.
While only point-to-point connections can be constructed with conventional WLAN, WirelessHART uses a flat mesh network, where all radio stations form a network in which every participating station serves simultaneously as a signal source and a repeater. The original transmitter sends a message to its nearest neighbor, which passes this message on until the message reaches the base station and the actual receiver. In this way, the network covers a large area.
In addition, alternative routes are set up in the initialization phase. In the event that the message cannot be transmitted on a particular path, due to an obstacle or a defective receiver, for example, the message is automatically passed on to an alternative route. In addition to the coverage of larger areas, the reliability of transmission is increased.
The network manager functions as the central component of the network. The network is organized centrally by the network manager on start up. It constructs the network, establishes the organization of the communication and determines the redundant paths. The network manager also detects newly arriving stations and integrates them into the existing network during actual operation, so that problem-free expansion is possible. During operation, the network manager monitors all the important functions of the network and all stations and applies corrections in the event of faults.
The coordination of communication in the flat mesh network is carried out via the Time Division Multiple Access (TDMA) method, which synchronizes the radio stations very accurately in the 10 millisecond timing cycle. Thus the radio stations record the communication to an accuracy of 10 milliseconds. This reduces the lead and lag times during which a station must be active.
In order to avoid sources of interference and interference with other radio stations in the 2.4 GHz band, WirelessHART uses a method called Frequency Hopping Spread Spectrum (FHSS). Here, all 16 frequencies defined in IEEE 802 are used in parallel. Already occupied channels are entered in a “blacklist,” and no longer used for communication. The combination of the exact time synchronization in the 10 millisecond cycle by means of TDMA and the use of all 16 channels of IEEE 802.15.4 by FHSS enable the overall network to achieve 1,600 communications per second.
The safety requirements of the plant operator must not be neglected and this is provided for by coding the communication with a 128 bit code. In this way, the monitoring and falsification of communication and of the usable data is excluded.
WirelessHART is the symbiosis between the much-used and proven HART, and the radio technology already established in both the private and public sectors. In addition to the well-known HART application for device parameterization, HART has already been widely used for:
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Monitoring of instrument and environmental values
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Asset management and optimization
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Preventive maintenance
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Performance monitoring
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Energy management.
The requirements of these applications in terms of availability, transmission paths and speeds are well matched by the performance capability achievable by radio technology. And the WirelessHART concept also offers a seamless integration in the existing infrastructure with simultaneous openness with respect to new structures.
Radio technology enables better use of existing applications in process automation and the achievement of new, cost-effective applications that were not previously possible. WirelessHART provides the necessary step from proprietary solutions to an overall standard in process technology, which enables the use of radio technology without system breakdown. WirelessHART will open up new possibilities to improve quality and plant monitoring, and optimize plants and processes. It is a worthwhile contribution to the economic operation of process plants.
