Programmable Logic Controllers Past Linear Temporal Logic for Monitoring Applications in Industrial Control Systems

Programmable logic controllers (PLC), which are widely applied in modern industrial control systems (ICS), work as the controller of sensors and actuators in ICS. These systems require strict correctness, especially for safety-critical systems. Currently, increasingly ICS move to "come online" scenarios to enhance cyber-physical features, but it makes them more vulnerable due to acquiring increased interconnection accompanied by weakening physical isolation. Moreover, with the more complex controlling environment, such as hundreds of more I/O points and more diverse field buses, the incorrect executions of PLC might cause the failure of the overall ICS. In this article, we examine how the security and safety of running PLC could be enhanced in both developing and deploying stages of ICS. We propose a novel application of runtime verification to guarantee the security and safety of real-world ICS. As a variant of temporal logic, PLC past linear temporal logic (PPLTL) is proposed to specify the security and safety properties of PLC. Using PPLTL, we synthesize monitors to improve the PLC program's security and safety as a partner of testing and static verification. Our monitors provide twofold processing in a nonintrusive manner: One is filtering abnormal input data before invading the original programs, the other is double-checking the output signals before driving the actuators. We use several case studies and benchmarks to demonstrate the efficiency of the approach. The empirical results show that the time overhead and memory occupation are tiny.