Wireless Technologies in Process Automation - Herkules.oulu.fi - Oulu

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5 DISCUSSION
5.1 General Considerations
Based on this review, it seems that there is a diverse, evolving field of wireless
technologies suitable for process automation communications. As indicated in Sections 2
and 3; and in APPENDIX 1, in process automation, the resulting systems could often be
hybrid by nature, incorporating existing automation systems and possibly heterogeneous
wireless communication schemes. Therefore, interoperability of current and future
wireless and wired applications may be a challenge, especially in the presence of rapidly
evolving new technologies, industrial standards and proprietary solutions available.
Considering the wireless technologies presented in Section 2, the existing industrial
automation wireless applications could be divided to two categories. First, a category, in
which wireless communications is used as a backbone in communication architecture. In
second category, wireless technology is applied to device level communication,
delivering data from sensor to higher level in network hierarchy.
Employing wireless technologies in several levels of plant network hierarchy – from
device level to remote connection – may introduce increased uncertainty in QoS
indicators as the whole system perspective must be considered. This would, in turn,
suggest distributed algorithms for some applications. For example, control applications
are sensitive for varying delay in the feedback loop. Applying such a loop over wireless
sensor network, wireless fieldbus and cellular link would provide increased uncertainties
in terms of timeliness, jitter and latency, making the delay in feedback loop possibly
more variable. Distributing the control to lower levels of network hierarchy might reduce
the uncertainty.
At device level, based on use cases presented in APPENDIX 1, it seems that most of the
current applications of wireless sensor networks in industrial automation have been
focused on condition-based maintenance. Control applications are rare; the focus is
mainly theoretical, although commercial application by ABB (wireless proximity
switches) could be found. Additionally, remote monitoring approaches with complex
network architectures, incorporating satellite and internet communications, could be
found. Taking into account the manufacturing process example at aluminium plant and
the event-based monitoring of relief valves, the applications support the proposed in the
literature. Additionally, as presented in (Koumpis et al., 2006), applications related to
monitoring are easier to find than those related in control.
Most of the wireless sensor network applications found during survey were based on
802.15.4-radio, with proprietary upper layer protocol stacks. For example, industrial
applications using ZigBee are hard to find, but this may simply be because of the novelty
of the technology. Quite recently, though, Mitsubishi announced a ZigBee-based WSN
application for plant monitoring. Interestingly, in some cases it was mentioned that
ZigBee is unable to cope with the requirements of industrial automation. (Dzung et al.,