MTS Hybrid Simulation Systems
MTS has collaborated closely with leading research institutions to advance the science and technology of civil/seismic test and simulation, having commissioned state-of-the-art real-time and quasi-static hybrid simulation systems at 85 sites worldwide.

Photo courtesy of University of Colorado, Boulder

Mitigating the Risks of Real-time Hybrid Simulation

Hybrid simulation is an advanced testing methodology for evaluating the responses of civil structures under realistic operating conditions. It combines the advantages of numerical modeling with physical lab testing, thus providing an integrated testing solution that is flexible to configure, realistic enough to represent a full structural system and relatively cost-effective in implementation.

In a hybrid system, servohydraulic actuators apply loads and motion boundary conditions to the physical experimental substructure. The inherent dynamics of these physical actuators, however, introduce both system phase lags and communication delays, both of which contribute to negative damping in a real-time hybrid simulation system. Too much negative damping can lead to test system instability, compromising the integrity of test data and potentially resulting in physical test equipment and test article damage.

To mitigate these risks, MTS engineers have developed a set pre-testing assessment techniques to gain advance insight into the performance and stability limits of hybrid test designs, as well as validate hybrid system model and software setups.

Dynamic Stability Analysis
For a generalized multiple degree-of-freedom hybrid system, negative damping not only depends on actuator motion control performance, but also largely on the partition between the numerical (modeled) and experimental (physical) substructures, i.e. how the stiffness and mass are assumed in both substructures. Depending on the test design, the worst-case hybrid substructure partition may present an extremely narrow stability margin in its tolerance to actuator delay.

To gain an understanding of this stability margin for a given hybrid test design, MTS engineers employ a dynamic equation of motion (EOM) that quantifies its potential for negative damping, providing an early predictive measure of system performance within the frequency domain. Essentially, the equation allows test engineers to back-calculate a hybrid test’s maximum allowable actuator delay well in advance specimen construction and actual testing.

This early, pre-test analysis technique affords test engineers a system-level understanding of a hybrid design’s dynamic stability and early estimates of useful metrics such as frequency response function, system pole-zero analysis, and time domain response. Equipped with this insight, test engineers can design a sound and stable hybrid experimentation. If - for example, in a validation test - modifications are needed to increase a test design’s stability, measures can be taken, including adding mass to lower test frequencies; or adding components to the numerical (modeled) substructure to achieve a more balanced partition of the hybrid system.

Virtual Testing
In addition to analyzing a hybrid test’s dynamic stability in the frequency domain, MTS engineers also employ virtual testing techniques to gain an understanding of its behavior in the time history domain.

To accomplish this, a virtual hybrid simulation station is created that runs the same OpenSees model and predictor-corrector algorithm as the real hybrid test station. However, in the virtual test, the force feedback signals that are sent back to FEA are not acquired from the force transducers. Instead, they are calculated from a Simulink model of the test specimen. When calculating the force signals, the force-displacement relationship of the specimen, the actuator phase delay and amplitude error, as well as sensor measurement noise are all modeled.

This virtual testing approach, which has shown very good correlation with the analysis technique, has proven to be a valuable tool for conducting hybrid simulation rehearsals, or trial runs. It can serve to verify proper model and software setup and help to illuminate and resolve testing challenges prior to actual testing.

Real-time hybrid simulation is complex and challenging, and especially vulnerable to the risks of negative damping caused by actuator phase lag and communication delay. Look to MTS for the decades of experience and advanced techniques needed to avoid the potentially catastrophic effects of unintended system responses on both valuable test equipment and test articles.


MTS Systems

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