Co-simulating distributed hardware-in-the-loop systems in real time over the Internet entails communication delays that can lead to significant loss of fidelity and even instability in the system. To address this challenge, this paper proposes an observer based framework that, unlike previously reported efforts, does not require the observer to know and model the observed system dynamics. This is achieved by deriving the closed-loop dynamics of the observer based on a sliding surface. Even though the resulting error system does not necessarily stay on a sliding surface, its asymptotic convergence to zero is still guaranteed. First, this idea is developed for a generic networked system simulation framework and its stability is established. Then, it is applied to a mass-spring-damper system to illustrate the mechanics of the approach on a simple, linear example and demonstrate that the approach can stabilize the system that is otherwise unstable due to delay. Finally, a vehicle-engine-driver system simulation is considered to evaluate the performance of the approach on a more realistic, nonlinear example. An improvement of up to 33% is observed in the fidelity of the simulation. The conclusion is that the approach holds a significant potential to alleviate the negative impact of delay and improve the stability and fidelity of networked system simulations. Its benefits become more pronounced as the delay increases.

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