Resonance tuning in linear alternator drives via direct-current amplitude modulation

Reciprocating Linear Alternators (LAs) are used in many energy harvesting technologies working with pressure waves, such as sea wave energy convertors, Stirling and Thermoacoustic engines. LAs usually incorporate mechanical springs realizing a resonant system which handles the alternating kinetic energy flow stored in the moving mass. In practice, however, parameter inaccuracies and drifts in the operating frequency result in off-resonance operation causing stroke and power drops. This paper presents an adaptive tuning strategy for the electronic stiffness in order to restore resonance and maximum power flow. The algorithm is based on a low-frequency amplitude perturbation of the current component in phase with the stroke. The response of the LA to this low-frequency parametric excitation allows the detection of out-of-resonance conditions and the correction of the current amplitude in phase with stroke in order to restore resonance. The paper discusses an approximated mathematical analysis of the control algorithm and presents validation via simulations.