Experimental and Numerical Studies of Dynamic Behaviors of a Hydraulic Power Take-Off Cylinder Using Spectral Representation Method

Hydraulic cylinders are generally used as power take-off (PTO) mechanisms in wave energy converters (WECs). The dynamic behavior of its PTO force, which integrates friction and pressure forces, is a difficult constraint to include in an analytical or in a numerical model. In this paper, the PTO force characteristics of a hydraulic cylinder are experimentally and numerically investigated under different magnitudes of controlled excitation force. In order to characterize the dynamic behaviors of PTO force, the displacement, acceleration, and pressure in the cylinder chamber for given excitation forces are measured. The pressure force is calculated using the measured value of the pressure, and the friction force is calculated based on the equation of motion using measured values of the pressure, excitation force, and acceleration of the piston. Experimental results show clearly a strong nonlinear force-velocity characteristics, including stochastic and hysteretic behaviors. To model the hysteretic behavior, the modified LuGre model is used for the friction force and a new approach is proposed for the pressure force. To model the stochastic behavior of the friction and pressure forces, the spectral representation method is used. The systematically comparison between measured and simulated results shows that the numerical model captures most of dynamic behaviors of PTO force.