The IEC 61499 standard specifies a framework for distributed and embedded control using function blocks. The ability to control distributed systems, makes this approach a candidate for use in reconfigurable systems.
2.5.1 The IEC 61499 standard
The IEC 61499 standard is a successor to the IEC 1131 standard, which later became IEC 61131. The IEC 1131 standard is aimed at control applications in PLCs. The standard provides specifications ranging from PLC programming to the fieldbus communication of applications in PLCs. The standard is divided into
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several parts dealing with the various aspects concerning PLCs. The IEC 61131-3 part of the standard deals with the programming of PLCs. According to Lewis (1998), this part of the standard aims to improve the following aspects of PLC programming:
Capability of a system to perform its intended design functions.
Availability of a system during its life cycle when it is available for its intended design functions.
Usability, which indicates the ease with which a specified set of users can acquire and exercise the ability to interact with the system in order to perform its intended design functions.
Adaptability, which refers to the ease with which a system may be changed in various ways from its initial intended design functions.
The IEC 61131 standard has had implied limitations when dealing with complex computations, knowledge processing, advanced network messaging and service orientation (Vrba and Marik, 2009). The IEC 61499 standard addresses these limitations and extends the IEC 61131 standard by adding event-driven execution. The IEC 61499 standard was also developed, according to Rooker et al. (2007), to address the following shortcomings of its IEC 61131 predecessor:
Non-deterministic switching points – this is due to the cyclic execution policy which is implemented by the IEC 61131 standard.
Lack of task level granularity1 complicates communication and re- initialization.
Jittering effects – a change in one system task influences the other tasks in the system.
The possibility of entering inconsistent states during system reconfiguration, which may lead to deadlocks.
The IEC 61499 standard is then a developed set of specifications for distributed processes and control systems (Wang et al., 2007). Black and Vyatkin (2009) mention that the IEC 61499 standard “provides an architectural framework for the design of distributed and embedded control systems” and has “undoubted advantages concerning distributed automation” (Vrba et al., 2009). The IEC 61499 standard defines a component-based modelling approach using function blocks. The standard enables the development of new technologies which aim to reduce design efforts and enhance reconfiguration. The goal of the IEC 61499 standard is “to offer an encapsulation concept that allows the efficient combination of legacy representation forms (such as ladder logic) with the new object and component-orientation realities” (Vyatkin, 2007). The IEC 61499 standard uses a bottom-up approach in implementing decentralized control. This approach then starts at the shop floor level, where it effectively prepares for the distributed placement of holons (Paolucci and Sacile, 2005). The requirements for
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holonic control are thus inherent in the IEC 61499 specification (Black and Vyatkin, 2009).
The function block of the IEC 61499 standard can be understood as an abstraction that represents a component. This component can be implemented and controlled by the function block software (Vyatkin, 2007). The function block concept is applicable to the data encapsulation and adaptive process plan execution involved in the assembly or manufacturing processes. The event-driven model of the function blocks then adds intelligence and autonomy to the resources of the system, increasing its decision-making ability (Wang et al., 2007).
2.5.2 Advantages of function block control
Function blocks provide an advance from established ladder logic and structured text programming languages, but its application extends past the simple replacement of these systems. This is due to the inherent support for distributed applications and the ability to provide a modelling and simulation platform with well-defined interfaces (Black and Vyatkin, 2009). Rooker et al. (2007) mention that the distributive properties of IEC 61499 function blocks hold several advantages. The programmed function block networks are directly mapped to the real system controllers and devices, where the execution takes place. This facilitates the movement of functionality amongst controllers and devices. This support of distribution then also facilitates the implementation of component- based information. Another benefit of using the IEC 61499 function blocks is that, as a modeling language, it is directly executable and is thus ready for simulation. This allows the testing of the control system prior to deployment. This simulation model can then be seamlessly substituted by the hardware interface to real sensors and actuators. The use of function blocks also greatly increases the modularity of the system and enables the reusability of software components in the system (Black and Vyatkin, 2009). Function blocks also have a robust character which makes it appropriate for implementation in the broader embedded systems domain (Vyatkin, 2007).
2.5.3 Platforms for function block control
There exists a few platforms and tools for the design of function block control systems. The Function Block Development Kit (FBDK) is the most widely-used design platform (Black and Vyatkin, 2009). The model-view-control design pattern for function blocks is also applied in FBDK. This platform also includes the Function Block Run-Time (FBRT) environment. The entire platform is based on Java programming structures. Another commercial support tool is that of the ISaGRAF industrial control design software, which can also support the IEC 61499 function blocks (Black and Vyatkin, 2009).
2.5.4 Implementations of IEC 61499 function block control
Due to the predominant presence of the IEC 1131-3 standard in industry and relatively recent development of the IEC 61499, it has seen very few practical implementations. IEC 61499 function block control was implemented in the automation of a baggage handling system by Black and Vyatkin (2009). Vyatkin
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(2007) describes the first factory installation of an IEC 61499 function block control system by Tait Control Systems in New Zealand.
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