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Flow Level Pressure
Full Redundant Field Wireless Automation Solutions Based on the ISA100.11a Standard
Henk van der Bent, Marketing Manager Field Networks, Yokogawa Europe Euroweg 2, 3825 HD Amersfoort, The Netherlands Email:
henk.vander.bent@
nl.yokogawa.com • Web:
www.yokogawa.com/eu
Communication between field devices and control systems started with 4-20 mA analogue communication, and has evolved via hybrid communication systems such as HART and Brain, to digital communication technologies such as FOUNDATION™ Fieldbus and PROFIBUS, and finally to wireless communication based on the ISA100.11a standard.
These communication technologies have significantly increased the amount of information that can be used for plant operations, and have led to major innovations in field instrumentation. Activities such as asset management and proactive maintenance have been realised using extensive information such as multiple process values and diagnostic results, which can now be sent from field devices to higher-level systems such as Distributed Control Systems (DCS) and Plant Asset Management (PAM) systems.
Field wireless technology that enables wireless communication between field devices and control systems will bring further advanced innovation in the instrumentation sector. Field wireless communication has many advantages, such as the ability to reduce wiring, engineering and commissioning costs, the ability to install devices in areas where wiring is difficult, and the ease of adding or removing devices. By making best use of these advantages, it is possible to use instrumentation to implement functions that were impossible before. Both wireless technology and the instrumentation that makes best use of wireless communication will also continue to evolve.
The ISA100.11a wireless communication standard was established in September 2009 by the ISA100 Committee of the International Society of Automation (ISA). It was approved as a National Standard by the American National Standards Institute (ANSI) in December 2011, and has been submitted (as CDV IEC 62734) to the IEC SC65C Subcommittee for approval as an international standard.
ISA100.11a system configuration
Fig.1 shows a typical field wireless system configuration based on the ISA100.11a standard, incorporating wireless field devices and a field wireless management station (gateway) which acts as an interface between the wireless network and its applications and higher-level control and plant asset management systems. The field wireless access point (sometimes referred to as a backbone router) functions as the connection between the wireless field devices and the field wireless access point. The system manager and security manager control and manage the behaviour and security of the wireless network. As shown in Fig.1, it is possible to provide redundant paths between the controller and wireless field devices via redundant field wireless access points, and to provide multiple communication paths from the wireless field devices to multiple redundant field wireless access points. The ISA100.11a standard defines the many basic functions which improve data transfer reliability in communication. Therefore within an ISA100 based wireless network devices can have different roles: I/O (input/output), routing and I/O plus routing.
Based on these general concepts, a practical realisation of an ISA100.11a based field wireless system must achieve three additional goals which can be summarised as “control proof”, “power proof” and “future proof”. “Control proof” means that the wireless architecture meets the requirement for real-time control applications. “Power proof” refers to the ability of the wireless solution to provide reliable and long lasting power, while “future proof” refers to the ability of the solution to keep up with the rapid changes taking place in the world of IT.
Control proof
To be suitable for control applications, wireless networks must possess the necessary reliability and deterministic data transfer capabilities. ISA100.11a supports reliable radio technology and offers excellent coexistence with other wireless networks such as Wi-Fi. With bidirectional digital wireless networks based on ISA100.11a, the measurement values, device diagnostics, and parameter data transferred between a control system and field devices are securely encrypted. This wireless technology is ideal for control applications in addition to status monitoring and device diagnostics, enabling proactive maintenance.
Network topology also affects performance and reliability. Yokogawa’s field wireless strategy calls for a redundant star topology network. In Yokogawa’s view, redundant star topology (I/O devices communicating in duo cast with a redundant field wireless access point) provides the best determinism, high communication speed (500 ms data transfer), low latency, and the multiple route communications needed to provide the level of reliability needed for process control applications.
For less critical applications such as monitoring, where determinism is not required and higher latencies can be tolerated, a mesh network topology or a mix of a star and mesh networks could be considered.
Figure 2: Battery case design allows the use of low cost, commercially available, off-the-shelf, general- purpose lithium thionyl chloride batteries in wireless transmitters. The battery case can easily be replaced, even in a hazardous area environment.
Figure 1: Field wireless system based on ISA100.11a ANNUAL BUYERS’ GUIDE 2014 •
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