Analysis of a Synthetic Resistance Control System for a Back-Driven Three-Phase Vibration Harvester With a Finite Bus Voltage

We consider the use of a three-phase machine, to harvest Watt-scale power from dynamically excited civil infrastructures. Specifically, we consider a harvesting technology comprised of a permanent-magnet synchronous machine, coupled to the linear vibratory motion via a back-driven ballscrew mechanism. A synthetic resistance (SR) control system is proposed in which, using only voltage feedback, the harvesting electronics is made to emulate a linear star-connected resistor bank, at the three-phase outputs of the machine. Although the methodology is conceptually simple, the practicalities associated with its implementation are made more complicated due to the finite magnitude of the harvesting power bus voltage. This is because, due to the wide range of velocities experienced by the harvester, the magnitude of the back electromotive force for the machine can be comparable to or even exceed this bus voltage. This paper presents novel theoretical and experimental analysis aimed at understanding the manner in which the finite bus voltage affects the implementation of SRs for these harvesters. Specifically, we show that there is a theoretical limit on the magnitude of the SR that can be applied, which depends on the bus voltage as well as the device velocity, and that this resistance approaches a finite value as the velocity becomes arbitrarily large. The basic operation of the SR method is verified experimentally, and the limitations and practical implementation of the electronics when using the SR control system are discussed.