The choice of the appropriate structure of the control system with respect to the required availability and safety

Nowadays, the programmable logic controllers (PLCs) are often in the role of control systems. They are used primarily for control of technological processes, but the exception is neither the utilization in transport. As an example can be mentioned systems of rail safety technique: system MODEST from the company 1Signální, ELEKSA system from Siemens, the system SPA 4 from Bombardier Corporate. The PLC producers continually improve their properties. The aim of the producers is to extend their application possibilities. Therefore there can be found in the offers of PLC producers PLC with attribute fail-safe, eventually fault-tolerant. PLCs are modular systems, so their availability and safety depends on the chosen structure of control system. The choice of structure is appropriate to base on the modeling of monitored properties. Another reason to create a suitable model, eventually models, is that the certificate of producer about reached level of safety PLC says nothing about the application of PLC. The correct model must also take into account the application individualities (e.g. the way of sensors connecting, actuators, etc.). The article compares the availability and safety of various structures of control systems based on PLC on the ground of created models