Study on Realizable Generalized Hold Functions as a Countermeasure against Zero Dynamics Attack

Zero dynamics attacks are known to be lethal in the sense that they are stealthy in principle and are not detected from output measurements. Therefore, instead of detecting the zero dynamics attacks, an idea to mitigate the effect of the zero dynamics attack has been proposed recently, which is to enforce the zeros to become stable by changing the zero-order hold to a generalized hold in the sampled data framework. Once all the zeros become stable, then even if the zero dynamics attack is engaged, its effect on the plant is negligible. However, it was observed that the amplitude of the generalized hold becomes unrealistically large in some cases, which leads to a large input to the physical plant. This paper studies this phenomenon at a deeper level and figures out that changing the intrinsic zeros requires an excessively large amplitude of the generalized hold while changing the sampling zeros can be done with a reasonable amplitude.