Low-Cost Solution for Adding Safety Functions to Programmable Logic Controllers (PLCs)

Abstract

Increasing requirements in automation and production make control systems more complex and vulnerable to failure. Breakdowns can cause delays in production, material damage, and above all, work-related accidents. For this reason, directives and legislation have been created at the country, European Union, and global levels that define the essential safety and requirements of industrial equipment. Guidelines must be observed by those involved in the design, supply, purchase, or use of industrial equipment in the European Union and several countries outside the European Union. Some guidelines (CAT, SIL) are created to ensure their safe operation in case of failure of both the hardware and the software. For a reliable operation of a fail safety system together with a system operating at SIL2 or SIL3, it must have Safety hardware and software. Industrial equipment manufacturers are incorporating security features into a variety of devices. Depending on the Safety Integrity Level (SIL) requirements, these features can be used during the design phase to increase safety in the event of failures or malfunctions. With proper design, the process as well as its environment (including people) can be protected by entering a controlled safe state. Manufacturers have approached this problem in a number of ways, including adding redundant Central Processing Units (CPUs), using special hardware to interface input and output signals, and developing secure network protocols for communication. Unfortunately, these features cannot be added to existing machines, at least without upgrading some hardware. As the associated costs lead to slower adoption, manufacturers rely on previous work to support certain security features, notably CPU debugging. This is implemented in the form of software libraries that operate at a low level (logic gate), designed to run on older hardware (PLC) so that they can offer an increased level of security. This study analyzes the required guidelines and legislation that must be followed for the safe operation of a production unit. It describes the mechanisms that basic and specialized PLC systems utilize to ensure the safety of an automation system. The authors have developed algorithms to record behavior measurements of electronic equipment. By further analyzing the results, it is concluded that basic equipment can be reused in these systems to provide safety functions, at the same time conserving both cost and time.