Analysis and design of vibratory energy harvesters employing three-phase AC transduction

Three-phase transduction affords certain advantages for efficient electromechanical conversion of energy, especially at higher power scales. This paper considers the use of a three-phase electric machine for harvesting energy from vibrations. We consider the use of vector control techniques, which are common in the area of industrial electronics, for optimizing the feedback loops in a stochastically-excited energy harvesting system. To do this, we decompose the problem into two separate feedback loops for direct and quadrature current components, and illustrate how each might be separately optimized to maximize power output. Due to the fact that these two control loops are designed separately, the resultant composite controller for the three-phase transducer is not the “true optimal” controller over all causal feedback laws. However, the proposed design technique does ensure high performance, while also being tractable. We describe the manner in which direct field control (i.e., field weakening) can be used to maintain control of the harvested energy when the internal back-EMF of the machine is larger than the voltage of the power bus. We then develop analytical techniques that illustrate the fundamental tradeoffs in the design of the electronic hardware and the choice of bus voltage, and show that control performance depends on only five nondimensional functions of the parameters for the harvester, disturbance, and electronics. The use of these parameters in system design is illustrated through a simple example, and the efficacy of the electromechanical design is verified through simulation of the nonlinear stochastic dynamics.