Design framework for risk mitigation in industrial PLC control

Programmable logic controllers (PLCs) are increasingly used in industrial automation projects. By their nature, in these applications safety is a crucial requirement. However, risk prevention adds costs to the PLC system, which means that a balance between costs and risks have to be found. A convenient compromise between these two criteria is provided by risk mitigation, but only to an acceptable risk level. By reviewing the fundamental definitions and concepts of system safety, and with the support of a simple example, the paper provides a framework for addressing these issues. Exploring the systematic design of safety-critical PLC systems helps to show how design engineers can verify that their projects will be safe.