Today's safety control systems: Overview of safety control systems

Safety control systems essentially came about because of the desire to connect safety through programming, in a similar way to that of a PLC control system. It's no surprise then, that safety control systems are following the example of the PLC world. Centralized systems came first, followed by decentralized systems in conjunction with safebus systems. Programming followed the same formula, except that the instruction set was drastically reduced from the start to just a few languages, such as IL (Instruction List) or LD (Ladder Logic/Ladder Diagram). These measures were taken for reasons of safety, for the opinion was that limiting the programming
options would minimize the errors made in generating the program. Initial systems clearly focused on processing safety functions. Although even at the start it was possible to program the safety control system for standard automation, in practice this application found very limited use.

Safety-related features aside, there is little to distinguish safety control systems from standard automation control systems in terms of their actual function. Essentially a safety control system consists of two PLC control systems which process the application program in parallel, use the same process I/O image and continuously synchronize themselves. It sounds so simple, but the detail is quite complex: Cross-comparisons, testing of the input/output level, establishing a common, valid result, etc. are all multi-layer processes, which
illustrate the internal complexity of such systems. Ultimately, of course, the user is largely unaware of this; with the exception of some specific features, such as the use of test pulse signals to detect shorts across the contacts, modern systems behave in the same way as other PLC control systems.

Structure of a safe control system:

• Two separate channels
• Diverse structure using different hardware
• Inputs and outputs are constantly tested
• User data is constantly compared
• Voltage and time monitoring functions
• Safe shutdown in the event of error/danger