Wireless technologies



Wireless technologies are utilised in consumer applications for several years, now. However, the operation of radio based communication systems in automation applications was considered doubtful for a long time. Primarily, the highly fluctuating quality of wireless transmission channels, compared to wired ones, was responsible for this fact. Transmitted electromagnetic waves experience reflexion, scattering, and diffraction, which may cause constructive or destructive interferences of the different signal copies arriving at the receiver. The direct consequences are packet errors and losses, resulting in higher transmission delays. Especially industrial propagation environments, with a lot of metallic surfaces and moving objects, are classified as demanding for wireless transmission. While fluctuations in latency and short losses of connections may be tolerated in consumer applications, exceeding given timelines in automation applications implies intolerable errors. The results are low plant availabilities and decreasing productivity. By means of the development of diverse wireless standards and the adaption of well-suited protocols for industrial applications, the end-users doubts could be reduced dramatically
over the last decay. The advantages of wireless solutions are obvious. In harsh environments, mobile and rotating scenarios, or at positions, difficult to access, cable connections and sliding contacts represent a main source of error. In this context the error probability can be decreased and the maintenance intervals increased by the utilisation of wireless technologies.
In addition to that, there is a great potential on saving time and money during planning, installation, and commissioning of plant sections. Many domains of the industrial automation already profit from the deployment of wireless solutions. With KNX RF (Konnex, 2006) and ZigBee (ZigBee Standards Organization, 2007) the first standards for building automation have been introduced. Within the scope of the HART 7 specifications (HART Communication Foundation, 2008) the first standard, WirelessHART, for the process automation was released in late 2007. Further standards, especially for the domain of factory
automation, are expected to get published in 2010. In order to reduce costs and time during the development of wireless solutions, unlicensed frequency bands are typically used for operation. Moreover an almost worldwide harmonised operation is guaranteed. This tendency is very pronounced for the 2.45 GHz ISM (Industrial, Scientific, and Medical) frequency
band.


Industrial Wireless Communication Channels
Communication systems have to comply with the stringent requirements concerning reliability, availability, and determinism in order to serve automation applications. In contrast to that, the quality of a wireless transmission channel experiences random time and frequency variant fluctuations. Hence, the development of wireless communication systems, for the extreme time critical area of factory automation, is a big challenge.
Industrial environments are often characterised by a high degree of metallic surfaces and time-varying influences. Besides the movement of the radio systems itself the movements of materials/tools, rotating machines and persons are responsible for this time variant properties. In principle industrial radio channels are akin to mobile radio channels. Thus, most phenomena of industrial radio channels comply with the ones of mobile radio channels.


 Reflexions occur, when EM-waves encounter reflecting objects, whose dimensions
are much larger than the wavelength.
 Scattering appears, either when the dimensions of the encountered object are much
smaller than the wavelength of the EM-wave, or when the surface structure is classified
very rough in comparison to the wavelength.
 Diffraction occurs when EM-waves encounter sharp edges.
 Shadowing is caused by obstacles, which completely block the propagation paths
of EM-waves.
 Doppler effects arise, either when there is a relative movement between transmitter
and receiver, or a mobile obstacle in the propagation field reflects, scatters, diffracts,
or shadows the EM-wave.

Aurel Buda, Volker Schuermann and Joerg F. Wollert (2010). Wireless Technologies in Factory Automation,
Factory Automation, Javier Silvestre-Blanes (Ed.), ISBN: 978-953-307-024-7, InTech, Available from:
http://www.intechopen.com/books/factory-automation/wireless-technologies-in-factory-automation

Follow us on Facebook