Development of a force actuator for hybrid model tests using super-twisting sliding mode control

Hybrid model testing is a solution to overcome the limitations of small-scale model testing, such as scaling conflict or infrastructure limits, still retaining the real physical phenomena' complexity. In hybrid model testing, the physical and numerical substructures interact with each other through a network of sensors and actuators. So, to apply the desired effects in the physical substructure, precise control of the actuators is necessary. In this paper, a robust control technique will be employed (super-twisting sliding mode control) to design the force actuator controller for hybrid model testing of vessels in a wave tank. First, the force actuator system dynamics is detailed, consisting of a servomotor controlling the force in a line attached to the vessel model. The methodology and design for the super-twisting sliding mode controller are described, taking into account the uncertain friction torque, the vessel motion and the system delays. Finally, the proposed actuator's performance is validated experimentally in a simpler environment: The servomotor is mounted on a linear guideway to simulate the vessel's horizontal displacement in waves. We analyzed the force actuator step response and frequency response (0.1-2.0 Hz) with disturbance motions varying from 0.5 to 1.5 Hz. The developed actuator showed good results (mean absolute error inferior to 0.1 N in all tests), where the high-frequency error spikes (up to 0.33 N) should not have a significant effect on the hybrid model tests' slow dynamics. We conclude that the force actuator will be a valuable tool for hybrid model testing.