Mitigation of sensor attacks on legacy industrial control systems

Abstract

Nowadays control systems have a communication infrastructure that enables sensors, actuators, and controllers proper operation. Industrial control systems include diverse technologies composed by novel devices and legacy systems together. Because most of contemporary industrial control systems were designed and put into operation many years ago with little or no consideration of security issues, which emerged from the capabilities of interconnection available these days. Communication infrastructure opens up a backdoor to cyber-attacks on control systems. In this brief, we show how concepts from fault tolerant control can be utilized to mitigate the effect of cyber-attacks on sensors. This paper analyzes how Luenberger Observers (LOs) and Unknown Input Observers (UIOs), two of the traditional tools of Fault Detection and Isolation, can be utilized to detect and isolate attacks on legacy industrial control systems. We show how the simultaneous use of LOs and UIOs can help to compute the malicious injected signal (attack) and the sensor where it happens, in a better way than when only UIOs are used. In addition, the computation of the required modification of the control action (reconfiguration) is exposed. This reconfiguration has as purpose to mitigate the effect of the attack on the control system. A numerical example shows the implementation of the proposed procedure, and the comparison of the effect on the behavior of the control system, with and without reconfiguration, in the presence of attacks is also shown. Finally, some conclusions are exposed, and some open problems are outlined.