Theoretical and experimental study of a variable-stiffness triboelectric energy harvester with curved contact surfaces

Research on triboelectric energy harvesting has advanced significantly in the last several years. Simultaneous harvesting and vibration control is now a new trend. This paper presents a novel triboelectric energy harvester (TEH), which consists of a cantilever beam with a rigid curved surface as a stop on either side whose geometric shape is described by a polynomial, inspired by a type of nonlinear vibration absorbers. In this design, the TEH proposed shows a new operation mode, that is, face-to-face gradual contact and separation. A variable capacitance between the beam and the curved stop is derived. Such an operation mode of a TEH has not been seen in the open literature. The proposed TEH extracts energy, and 'absorbs' vibration when its own amplitude-varying natural frequencies match that of the excitation. The collisions between the beam and a rigid stop are gentle because of its distinct gradual contact mode, which differs from the conventional contact-separation type TEH structure. As a result, the proposed TEH would experience fewer or less severe impact events, thus reducing wear and extending its useful service life. In this paper, modelling and numerical simulation of the dynamic characteristics and electrical output characteristics of the TEH are conducted and the effects of structural parameters are analysed. Its effectiveness in energy harvesting is validated through tests. Both theoretical and experimental results show that the effective frequency band is widened by the introduction of the two rigid stops (acting as geometrically nonlinear contact surfaces). The TEH's progressivecontact operation mode works well, and its electrical output is at a respectable level. Moreover, its energy-harvesting efficiency rises with the order of the polynomial for the curved surface stop.