Ultrasonic Power Transfer (UPT) has been developed as an alternative solution for achieving wireless power transfer. This paper proposes a new model describing UPT systems with tightly coupled piezoelectric transducers firmly bound to solid media. The model is derived from the short-circuit admittance of the system measured from the primary transducer. The mechanical characteristics of the system are modeled with parallel LCR branches, which reveal the fundamental relationships between the power transfer characteristics of the tightly coupled UPT system and system parameters. The loading conditions for achieving the maximum power transfer are identified, and the operating frequencies corresponding to the peak power transfer points for variable loads are determined. A practical UPT system is built with two 28 kHz Langevin-type piezoelectric transducers connected to a 5 mm-thick aluminum plate, and the practical results have verified the accuracy of the proposed model.
