Stability Analysis of Real-Time Hybrid Simulation for Time-Varying Actuator Delay Using the Lyapunov-Krasovskii Functional Approach

In a real-time hybrid simulation (RTHS), the actuator delay in experimental results might deviate from actual structural responses and even destabilize the real-time test. Although the assumption of a constant actuator delay helps simplify the stability analysis of RTHS, experimental results often show that the actuator delay varies throughout the test. However, research on the effect of time-varying delay on RTHS system stability is very limited. In this study, the Lyapunov-Krasovskii functional is introduced for the stability analysis of RTHS system. Two stability criteria are proposed for a linear system with a single constant delay and a time-varying delay. It is demonstrated that (1)the stable region of a time-varying delay system shrinks with the increase of the first derivative of time-varying delay; and (2)the stable region of the time-varying delay system is smaller than that of constant-time-delay system. Computational simulations were conducted for RTHS systems with a single degree of freedom to evaluate the proposed criteria. When the experimental specimen is an ideal elastic spring, the stability region of RTHS system with time-varying delay is shown to depend on the stiffness partition, structural natural period, and damping ratio. Significant differences in stability regions indicate that the time-varying characteristics of actuator delay should be considered for stability analysis of RTHS systems.