Enhancement of security against zero dynamics attack via generalized hold

Zero dynamics attack is lethal to cyber-physical systems in the sense that it is stealthy and there is no way to detect it. Fortunately, if the given continuous-time physical system is of minimum phase, the effect of the attack is negligible even if it is not detected. However, the situation becomes unfavorable again if one uses digital control by sampling the sensor measurement and using the zero-order-hold for actuation because of the `sampling zeros.' When the continuous-time system has relative degree greater than two and the sampling period is small, the sampled-data system must have unstable zeros (even if the continuous-time system is of minimum phase), so that the cyber-physical system becomes vulnerable to `sampling zero dynamics attack.' In this paper, we begin with its demonstration by a few examples. Then, we present an idea to protect the system by allocating those discrete-time zeros into stable ones. This idea is realized by employing the so-called `generalized hold' which replaces the zero-order-hold.

I. Introduction

Over the past few years, there is rapid development of computational resources and cyber components including network and digital processes. This technological growth enables the conventional control systems to separate the controller from physical plant so that it can be controlled remotely through the network channel (these systems are socalled cyber-physical systems (CPSs)). While it may improve computational efficiency and flexibility of the systems, the integration of the advanced technology makes the control system vulnerable to malfunctioning or malicious cyber attacks so that there are increased possibility to create massive social costs or loss of human life. The danger of such threats is well illustrated in the following incidents, such as Stuxnet virus attack targeting on Iran's nuclear facility [1], extensive blackouts of power plant in South America [2], and a cyber attack on SCADA of Ukrainian power plant [3].