Abstract
A submerged, neutrally buoyant cylinder may be an attractive option for a wave energy converter design due to its potential for highly efficient energy capture, advantageous power scaling relative to its length, and potential to weather storms by submerging to safer depths. Additionally, it serves as an ideal testbed to better understand heave and surge forcing interactions on wave energy systems. Understanding the hydrodynamic interactions between the cylinder and incoming waves can lead to better modeling and control design and in turn more efficient energy absorption. This paper presents the design, implementation, and experimental results of a submerged cylinder wave energy experimental testbed made using a two degree of freedom gantry (T-bot) in surge and heave controlled by coupled motors that are programmed to apply desired forces on the cylinder. We demonstrate the efficacy of the approach by applying forces to emulate mechanical springs and dampers with user specified spring and damping coefficients. System identification of the gantry is used to produce a feedback controller that counteracts the gantry friction in addition to applying the virtual spring and damping forces. Experiments in a 116 m wave tank with waves of amplitude up to 31.75 mm and periods between 1 s and 2.5 s show agreement with linear model predictions, experiments with a passive spring oscillator system, and WEC-Sim simulations. The results provide the groundwork for future rapid prototyping of advanced control designs and further study of the near-cylinder hydrodynamic interactions that drive energy absorption of wave energy converters.