Citation
Andrew, A.M. (2002), "Modelling Control Systems using IEC 61499: Applying function blocks to distributed systems", Kybernetes, Vol. 31 No. 2. https://doi.org/10.1108/k.2002.06731bae.001
Publisher
:Emerald Group Publishing Limited
Copyright © 2002, MCB UP Limited
Modelling Control Systems using IEC 61499: Applying function blocks to distributed systems
Robert Lewis The Institution of Electrical Engineers,London2001ISBN 0-85296-706-9xiv + 192 pp.hardback, £36.00IEE Control Engineering Series 59
There is a sense in which the title of this book is unduly modest, since the standard specification to which it refers allows not only modelling but implementation of digital control systems. The standard has been set up by the International Electrotechnology Commission, building on experience with earlier versions. The aim is to allow the rapid development and subsequent adaptation of complex control systems. The emphasis is on industrial automation, with no explicit reference to extension to related areas such as robotics, though there seems to be no reason why such extensions should not be made.
The function blocks that are the basis of the approach are visualised as being implemented digitally, and may coexist in one computer or be distributed. One of the ways in which the scheme may facilitate system design and modification is by the reuse of existing blocks, perhaps with small alterations, and by the compiling of libraries of standard blocks. It is visualised that the design of control systems will become a matter of selecting appropriate standard blocks and deciding on their interconnections, in a manner analogous to the selection and interconnection of chips in the design of electronic devices.
As is acknowledged in the book, the approach has a great deal in common with modern ideas on object orientated programming. The principle of "inheritance", that features largely in discussions of such programming, is not carried over by that name to apply to function blocks, but similar results are achieved by "adapters" that can modify the action of a block. Reference is made to "polymorphic" behaviour to refer to the reusability of elements, possibly with modifications, in both contexts.
Function blocks are described as event-driven, which means that a particular block is activated when a specified set of conditions arises, essentially as in a discrete-event computer simulation scheme. Such an arrangement allows total flexibility. The scheme is designed to be compatible with an existing technology called "Fieldbus" which can connect sensors and actuators that are "smart" in the sense of embodying digital processors of their own.
The interior operation of a function block can be specified using any suitable computing language such as Java or C, provided the input and output channels are appropriately configured. It follows that there is virtually no limit to the complexity of processing that can be realised. Some special types of function block that are mentioned include communication function blocks, service interface function blocks, and management function blocks. The last- mentioned are able to download other blocks and initiate their action. Function blocks may also provide the functions of servers and clients in Internet terminology.
The pattern of interconnection of function blocks can be specified either by an Execution Control Chart that is essentially a state transition diagram, or by a set of production rules of the kind familiar in AI. Automatic means have been devised for translating between the two types of representation. An Appendix introduces the "textual syntax" needed for this, though without exhaustive treatment of it.
A further useful feature is that system specifications will be encoded in the XML language, described as the next generation markup language to replace HTML. This means that the specifications can be transmitted through the Internet and designers can work remotely.
In the book, the use of the technique is illustrated in two fairly simple industrial applications. Their simplicity should not obscure the potential value of this injection of the latest developments in computer software writing into control engineering, and the value of this little book in delineating it.
Alex M. Andrew