Impedance Matching by Series Condensers for Power Receivers of In Vivo Robotic Capsules

Wireless robotic capsules with locomotive mechanisms are being developed to provide thorough diagnosis at suspicious areas including small bowel. Current battery technology does not accommodate multifunctional robotic capsules because of limited power capacity. Wireless energy transmission is one alternative to batteries, as has already been demonstrated with artificial hearts and neural prosthetic implants. The targeted power of robotic capsules is several tens of mill watts, and the power receiver's size is several millimeters. The main parts of the receiver are a ferrite-loaded inductor and a matching condenser; the latter provides maximum coupling between the power transmission coil and the receiving coil by matching the impedances of the two circuits. Maximum received power depends on the number of turns of the inductor, but increasing the number of inductor turns also increases the equivalent impedance of the resonant circuit. Power reception from source and power delivery to load depend on the degree of match of a single impedance to two different impedances, i.e., the impedances of the power transmission coil impedance and of the load. Therefore, one must carefully choose a compromise value for the inductance, i.e., number of turns. This paper shows validates experimentally that the substitution of two series capacitors for the single matching capacitor can overcome load-impedance mismatch while allowing us to maximize received power. The essential strategy is to find the optimal values of these capacitors while maximizing the number of inductor turns within the limits of the physical space. Based on the present results, we propose the following power receiver inductor design process: first one winds the coil with as many turns as space allows, then one selects two condensers to match the load impedance. The proposed design is applicable to all power transmission applications using inductive coupling, including in vivo devices and radio frequency identification.