Multiobjective Vector Control of a Three-Phase Vibratory Energy Harvester

In vibration energy-harvesting technologies, feedback control is required to maximize the average power generated from stochastic disturbances. In large-scale applications, it is often advantageous to use three-phase conversion technologies for transduction. In such situations, vector control techniques can be used to optimally control the transducer currents in the direct-quadrature reference frame, as dynamic functions of feedback measurements. In this paradigm, converted energy is optimally controlled via the quadrature current. The direct current is only used to maintain control of the quadrature current when the machine's internal back electromotive force (EMF) exceeds the voltage of the power bus, a technique called field weakening. Due to increased dissipation in the stator coil, the use of field weakening results in a reduction in power conversion, relative to what would theoretically be possible with a larger bus voltage. This overvoltage issue can be alternatively addressed by imposing a competing objective in the optimization of the quadrature current controller such that the frequency and duration of these overvoltage events are reduced. However, this also results in reduced generated power, due to the need to satisfy the competing constraint. This article examines the tradeoff between these two approaches to overvoltage compensation and illustrates a methodology for determining the optimum balance between the two approaches.