A hybrid simulation can be intentionally or unintentionally interrupted. This document describes a procedure to restart an interrupted hybrid simulation. The restart of an interrupted hybrid simulation involves restoration of the state of the hybrid model to the state at the beginning of time step at the point at which the interruption occurred (the interrupt time step). Then the hybrid simulation from this point continues and the time steps until the planned simulation break or end occurs. The state of the hybrid model comprises the state of the hybrid simulation integrator (the outer loop), the states of each of the numerical portions (numerical sub-structures) of the model, and the states of each of the physical portions (specimens) of the model. The principal obstacle in restoring these states is that the response of the hybrid model to the applied dynamic excitation depends on the excitation history, and the potential geometric and material nonlinear response of the hybrid model components.
The states of the integrator and the numerical portions of the hybrid model can be restored by repeating the simulation from the beginning. Repeating the simulation using numerical models of the physical portions of the hybrid model can closely restore the state of the numerical portion of the hybrid model to the interrupt state. This simulation should be prepared by developing shadow numerical models of the specimens and, if necessary, the test set up, including the servo-hydraulic actuation systems. These shadow numerical models should be calibrated using any data available from the original simulation to simulate, as closely as possible, the response of the specimens up to the interrupt point. The interrupt state of the specimens can be approximated by bringing the specimens close to the interrupt state following the trajectory it took through its state space during several time steps before the interruption occurred.
The hybrid model has to be re-constituted at the restart point. The shadow numerical models should be removed from the hybrid model and replaced by the interfaces to the physical portions of the hybrid model while maintaining the restart point states. Another aspect of reconstitution of the hybrid model is the transfer of actuator servo-loop control signal generation from the software used to move the physical portions of the hybrid model to the restart point to the hybrid simulation control loops, namely, the middle predictor-corrector loop. The objective is to avoid a sudden change in the control signal and a sudden motion of the actuators. To do this, the hybrid simulation integrator and predictor-corrector loops must obtain the measured force and deformation state of the physical portions of the hybrid model from the servo-loop controllers. Those values are then used as the initial state data for the physical portions of the model to generate the trajectory between the restart point and the next target state of the model.
Restart of a hybrid simulation is a tedious, sensitive process that needs to be carefully planned and executed. Although there are many ways to restart a hybrid simulation, the investigation of the hybrid simulation restart problems and strategies presented in this document provides a basis for design of an actual restart procedure. The restart procedure utilized by Terzic and Stojadinovic (2010) is provided as an example. This procedure is specific to the MTS STS servo-hydraulic controller and the versions of OpenSees and OpenFrecso used at that time at the nees@berkeley Equipment Site, and needs to be modified to suite the conditions of the specific simulation.
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