Frequency Splitting Analysis and Compensation Method for Inductive Wireless Powering of Implantable Biosensors

Abstract

Inductive powering for implanted medical devices, such as implantable biosensors, is a safe and effective technique that allows power to be delivered to implants wirelessly, avoiding the use of transcutaneous wires or implanted batteries. Wireless powering is very sensitive to a number of link parameters, including coil distance, alignment, shape, and load conditions. The optimum drive frequency of an inductive link varies depending on the coil spacing and load. This paper presents an optimum frequency tracking (OFT) method, in which an inductive power link is driven at a frequency that is maintained at an optimum value to ensure that the link is working at resonance, and the output voltage is maximised. The method is shown to provide significant improvements in maintained secondary voltage and system efficiency for a range of loads when the link is overcoupled. The OFT method does not require the use of variable capacitors or inductors. When tested at frequencies around a nominal frequency of 5 MHz, the OFT method provides up to a twofold efficiency improvement compared to a fixed frequency drive. The system can be readily interfaced with passive implants or implantable biosensors, and lends itself to interfacing with designs such as distributed implanted sensor networks, where each implant is operating at a different frequency.

Dual-Band Resonator Designs for Near-Field Wireless Energy Transfer Applications

“…Since k is dependent on M and M is negatively correlated to transfer distance, z, a cautionary reminder is to ensure optimum transfer distance between transmitting and receiving resonators. This is validated with subsequent Equations [15] that reveal the correlation between M, z, and k [16]. M ij indicates partial mutual inductance…”

Practical Inductive Link Design for Biomedical Wireless Power Transfer: A Tutorial

“…The problem with this assumption is that it does not account for the fact that Z refl appears as a complex impedance in series with the primary coil, whose value is dependent on the coupling. This manifests in the form of frequency-splitting, also known as pole-splitting, where the optimum drive frequency of the link varies from the tuned frequency as the coils are brought closer together [45], [89]- [95]. The effect can be observed directly by plotting parameters such as the link gain, impedance, and efficiency against changes in drive frequency and coupling.…”

An Improved Autonomous Current-Fed Push-Pull Parallel-Resonant Inverter for Inductive Power Transfer System

“…where R is the distance between the incremental lines d → l m and d → l n , µ 0 is the permittivity of free space, and N 1 , N 2 are the turn numbers of the two coils, respectively. Equation (18) can also be expressed as [28] where r 1 and r 2 are the radii of the coils, and d is the distance between the two coils. By substituting Equation (19) into Equations (15) and (16), the theoretical results, as well as the measured data, are plotted in Figure 11.…”

“…Moreover, another common method is directly adjusting the inverter frequency to the new ZVS frequency as the magnetic coupling changes. For instance, in [18], the authors present a closed-loop automatic frequency tuning system by an optimum frequency tracking method in an overcoupled regime with the aid of an extra control unit. Similarly, in [19], an automatic adaptive frequency tracking system was implemented on the basis of feedback power efficiency via 2.45 GHz data transmission.…”