Monday, March 7, 2011

PLC Issues in Control Design

There are some key issues in the design of PLC based control systems that need to be addressed. The PLC design based control a system is characterized by uncertainty and long development time. Uncertainty emerges from the enormous amount of specific requirements, such as hazard analysis requirements, software quality, and the hardware requirements imposed from the mechanical system to be controlled. More than 50% of the manpower allocated for the control system design is scheduled for testing and debugging in order to cope with uncertainty. The long design lead-time is rigor sequential design processes result in industrial practice, in case system developers do apply a system methodology at all. In many cases the mechanical manufacturing design or transport system and the control system are passed on to different subcontractors with less or no communication.

A major problem of impelling the need for a systematic design methodology, is the increasing software complexity in large-scale projects alongside with the permanently growing fraction of software life-cycle cost. A common application of PLC has often to handle complex systems with more than 10,000 control nodes. In industrial automation, 80-90% of the costs are going into software maintenance, debugging, adaptation and expansion to meet changing customer requirements.

The increased demand for manufacturing systems reconfigurability in quick response-markets imposes an additional requirement on control systems. The raising pace of changes in the manufacturing systems requires frequent control programs adaptation. A major challenge is to provide enabling technologies that can economically reconfigure manufacturing control systems in response to changing needs and new opportunities.
The approach design includes formal top-down modeling techniques that are well known in real-time control research, but less known in industrial practice. An advanced methodology of design also implies integrated modeling of the manufacturing system and the control system.
The manufacturing system modeling facilitates off-line testing and simulation of the control system and should not cause additional cost though it may further consume design time.

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