Real Time Control of Shake Tables for Nonlinear Hysteretic Systems
Ryu, Ki Pung
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Shake table testing is an important tool to challenge integrity and service behavior of structural and non-structural specimens by imposing strong excitations at their base. High fidelity of bare shake tables can be achieved through feedback control of actuators' inner loop with fixed gains, based on table tuning procedures. When shake tables are loaded with specimens, the interaction between shake tables and specimens influence the system dynamics that might result in undesired performance. In order to compensate the effects of the interaction, open loop feedforward compensation methods have been widely used in current practice of table controls, assuming that the specimens remain linear and unchanged. On the contrary, unsatisfactory signal performances during shake table testing were observed when flexible and heavy specimens experience nonlinear behavior. While lack of high fidelity might be acceptable for the purpose of research, i.e. to explore responses of specimens subject to random excitations, a high fidelity of signal reproduction is necessary for the shake table applications for qualification testing where specific target motion is required to challenge the specimens. In this study, tracking control schemes are proposed for shake tables in order to simulate target motions at specific locations of structural test specimens. The motion applied at the shake table level would probably be different than the target motion within the test structure; however, proper design of the shake table motion would ensure the desired performance of the controlled structure. The design of such controller is dependent on the dynamics of the shake table, dynamics of the test structure and their interaction. Additionally, when the specimens change properties due to nonlinearities and yielding caused by extreme excitations, the controller must be adaptive, in order to account for the changes and uncertainties in system models and to ensure the desired tracking. Metaphorically, the procedure suggested herein would seem to help the performance of an acrobat, who tries to balance a flexible stick on his palm, ensuring that the payload at the upper end of his stick will not fall or have undesired movement. But even for the acrobat the success will be limited by the strength of his palm and the space where he will have to maneuver. Similarly the methodology looked for in this dissertation attempts to determine the procedures and the limitations for real life applications of base movements with targeted control in test structures.