Two-Degree of Freedom Controller Design for Signal-to-Noise Ratio Reduction

Fundamental limitations in networked control systems (NCS) has been researched for the last years with many results, from a channel transmission minimal rate for stability based on information theory, to an infimal signal-to-noise (SNR) for stability approach. For the latter, it has been confirmed early on that the usual culprits, unstable plant poles, non-minimum phase (NMP) zeros, time delay, together with new ones such as communication channel bandwidth, increase the infimal SNR for stability. In this work we explore a two-degree of freedom controller configuration to reduce the infimal SNR for stability, eliminating the effect for example of NMP zeros and/or time delay. The two degree of freedom controller configuration achieves this by modifying the plant model as seen by the communication channel input. The communication channel model is located between the controller and the plant model. In a second stage we include a second communication channel to lift the ideal assumption that, albeit delayed, the controllers could access the plant input without communication noise. We then judge the improvement offered by the proposed two degree of freedom controller.