Finite-time cascade-like tracking control of direct-drive wave energy converters

Abstract

In this paper, sufficiently addressing power take-off dynamics, high-accuracy tracking problem of the direct-drive wave energy converter (DWEC) which is inevitably disturbed by complex unknowns is solved by creating a finite-time cascade-like tracking control (FCTC) scheme. More specifically, a cascade structure of the DWEC system is initially built by finely taking d-axis current dynamics into account such that accurate tracking of wave energy can be hierarchically accommodated by independently designing cascade-like control laws for d- and q-voltages. Then, to exactly compensate complex unknowns including unmodeled dynamics and disturbances, a finite-time observer is devised within the foregoing cascade structure, and facilitates the cascade-like controller synthesis whereby the backbone can be rationally decoupled. Within the entire FCTC scheme, d-axis current tracking dynamics can be sufficiently addressed so as to significantly enhance tracking accuracy of wave energy. Eventually, rigorous analysis ensures that both wave displacement and velocity tracking errors are globally exponentially stable. Simulation results and comparisons with typical methods demonstrate remarkable performance and superiority of the proposed FCTC scheme.