Distributed control provides plug-and-play safety functionality

By Ralf Möbus*
Monday, 22 March, 2010

Mechatronics with distributed processing provides a modern object-oriented application that can help machine builders to reduce costs and give more flexibility in building safe customer-specific products. For the efficient realisation of creating mechatronic objects - functional assemblies that can slot together to make new systems - the communication, control system and engineering tools must all be optimised for distributed control.

Machine builders often find themselves in a contradictory situation. On one hand, they have to react flexibly to customer needs with customised machines; while on the other, product standardisation reduces costs and engineering effort. Modular machinery design has the potential to serve both aims. This design approach is called mechatronical modularisation. It describes the building of machine modules that include mechanics, electrics and software brought together as a functional block. The modules are only described by the interface with which they connected to each other. Other modules don't need to know about the functions that take place inside the module.

The mechatronical approach cannot be adequately implemented with existing centralised control systems. It requires a different paradigm with a distributed control philosophy aimed squarely at the modularisation of machinery.

With distributed control, a fully mechatronical modularisation of machinery is possible where modules can act almost independently from the rest of the machinery. Such an approach offers several advantages. If assembled machinery can be broken down into functionally independent modules, then the same modules can always be designed identically - with module design made independent of different module combinations in the complete assembly.

Mechatronic modules can be run up and tested independently of the complete machine assembly permitting functional test at an early stage. Early detection of errors reduces the testing load for the whole assembly.

Time-critical process signals

Distributed controllers offer faster local processing of time-critical process signals since local controllers can run their own application program. When a local reaction to a process signal is required, the information stays local and doesn’t need to be transferred to a central PLC. The possibility to process signals locally on the hardware without additional communication delay can reduce reaction times. An additional advantage is that latency retains independence from communication load on the network thus being more predictable.

The failure of a single controller in a distributed control system generally doesn’t affect the whole system as is the case with centralised control, enabling robust distributed systems to be constructed.

The communication system is of prime concern to a distributed control system and should fit a decentralised approach. As the control logic is distributed in the machine assembly and not in a centralised controller the usual master-slave fieldbus systems are not an appropriate solution: such a system requires the complete communication needs to be managed by the master. This is not efficient for a distributed system. Taking this further, intelligent mechatronic modules need direct communication relationships. Also, in a master-slave system, the master needs to know about the complete control system and all the network subscribers. For a mechatronic module application, the software in the master would always have to be changed depending on the combination of machine modules. This of course means that plug-and-play standardisation of software would therefore not be possible.

The SafetyNET p (SNp) communication system does not have a centralised master; it uses the ‘producer/consumer’ principle. In such networks, every device has the same communication rights: every device can publish data directly to the network. Devices communicate directly with each other, a prime aspect for a distributed control system. Since SNp is based on 100 Mbps ethernet, it has enough bandwidth to fulfil the communication needs of automation devices and a number of fieldbuses can be reduced to a single ethernet cable.

*Ralf Möbus, Safety Network International e.V.

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