Safety of Control Systems with Dual Architecture Based on PLCs

Abstract

The implementation of safety functions, in contrast to ordinary control functions, requires a different approach to the design of the hardware and software of the control system. The reason for the different approaches is that each safety function must meet the required Safety Integrity Level (SIL). This has two aspects: safety integrity against random failures and safety integrity against systematic failures. Hardware is primarily related to random failure safety integrity, and software primarily to systematic failure safety integrity. The focus of this contribution lies in the design of a method that will allow the software of a safety function for industrial applications to be designed using a model of the functional behavior of this function. Since commercially available programmable logic controllers (PLCs) with a defined SIL level (so-called safety PLC) do not support such solutions, a dual architecture based on standard PLCs, including their software control, is proposed in the paper. Such an approach will make it possible to significantly limit the occurrence of systematic errors in the creation of application software, as well as to test the created application software and reveal deficiencies that arose in previous phases of the life cycle (e.g., when specifying safety functions). To implement the method proposed in this paper, a dual architecture created from the safety PLC Simatic series S7-1500 is used. With the help of this architecture, the safety function “Muting” is implemented.

Keywords

• safety integrity
• dual architecture
• safety function
• programmable logic controller
• state diagram