An adjustable pendulum mechanism for in-situ wave energy harvesting in an unmanned marine vehicle

The objective of this work is to design a scissor-like adjustable mechanism for in -situ wave energy harvesting in an unmanned marine vehicle. The energy harvester allows for the adjustment of the pendulum length at the starting stage, thereby achieving efficient energy conversion even in the presence of unknown excitation frequencies. Moreover, it has compact structure which facilitates the ease of portability at folding state. The work undertakes the conceptual design and detailed analysis of the mechanism as well as a fabricated prototype. The prototype exhibits a ratio of 3.2:1 between its expanding and folding states. A series of experiments are implemented on a motion platform with six degrees of freedom to simulate the periodic rolling motion of the unmanned marine vehicle in wave environment. Under the input excitation of amplitude of 2.41 degrees and frequency of 0.8 Hz, the prototype with an adjusted pendulum length of 375 mm achieves average power output of 1.078 W and energy conversion efficiency of 19.84%. To adapt to unknown input excitations, a control strategy is proposed to automatically adjust the pendulum length within a short time. These results validate the effectiveness of the dual-mode pendulum mechanism for wave energy harvesting.