Performance Characterization of Frequency-Selective Wireless Power Transfer Toward Scalable Untethered Magnetic Actuation

Gabriel Cooper; Xiaolong Liu

eess.SYApr 13, 2026

Performance Characterization of Frequency-Selective Wireless Power Transfer Toward Scalable Untethered Magnetic Actuation

Authors: Gabriel Cooper, Xiaolong Liu

Organizations: Department of Electrical and Computer Engineering, Texas Tech University, Lubbock, USA · Department of Mechanical Engineering, Texas Tech University, Lubbock, USA

Abstract

Frequency-selective wireless power transfer provides a feasible route to enable independent actuation and control of multiple untethered robots in a common workspace; however, the scalability remains unquantified, particularly the maximum number of resonators that can be reliably addressed within a given frequency bandwidth. To address this, we formulate the relationship between resonator quality factor (Q-factor) and the number of individually addressable inductor-capacitor (LC) resonant energy harvesters within a fixed radio-frequency (RF) spectrum, and we convert selectively activated harvested energy into mechanical motion. We theoretically proved and experimentally demonstrated that scalability depends primarily on the Q-factor. For this proof-of-concept study, we define effective series resistance as a function of frequency allocating bandwidths to discrete actuators. We provide design equations for scaling untethered magnetic actuation with Q-factor optimization. Resonator networks spanning bandwidths from 100kHz to 1MHz were analyzed to quantify how increasing the number of resonators affects independent addressability. We validated the approach experimentally by fabricating three centimeter-scale untethered actuators that selectively trigger the motion of mechanical beams at 734kHz, 785kHz, and 855kHz. We also characterized the generated mechanical force and the activation bandwidth of each actuator, confirming that no unintended cross-triggering occurred.

Figures & tables

Fig. 1: Working principle of the RF selective actuation mechanism and prototype demonstration. (A) Schematic diagram of the circuit design. (B) Prototype next to a quarter for size reference. (C) Three prototypes with different triggering frequencies.

Fig. 2: Normalized frequency responses of 177 resonators designed with a constant inductor value. The plot shows the resonator magnitude responses across frequency, normalized to each resonator’s peak. The horizontal dashed line indicates the $-3\text{\,}\mathrm{dB}$ threshold used to define the half-power bandwidth and assess spectral overlap.

Fig. 3: Effective series resistance $R_{s}$ and corresponding Q-factor versus frequency for the selected inductor, showing frequency-dependent series damping of the LC resonator [ 19 ] .

Designator	Specification
C1	$2\text{\,}\mathrm{nF}$ (C0G/NP0)
L1, L2	$10\text{\,}\mathrm{\SIUnitSymbolMicro H}$
C2	$3.3\text{\,}\mathrm{nF}$ , $3.9\text{\,}\mathrm{nF}$ , $4.7\text{\,}\mathrm{nF}$ (C0G/NP0)
C3	$1\text{\,}\mathrm{\SIUnitSymbolMicro F}$
D1, D2	BAV21WSA
C4	$330\text{\,}\mathrm{\SIUnitSymbolMicro F}$

TABLE I: Prototype Electronic Components and Specifications

	L $\text{\,}\mathrm{(}\mathrm{\SIUnitSymbolMicro}\mathrm{H}\mathrm{)}$	C $\text{\,}\mathrm{(}\mathrm{n}\mathrm{F}\mathrm{)}$	Measured $\Delta f$ $\text{\,}\mathrm{(}\mathrm{k}\mathrm{Hz}\mathrm{)}$	Expected $f_{0}$	Measured $f_{0}$	Q-Factor	$E~at~f_{0}$ ( $\text{\,}\mathrm{nJ}$ )
Charging LC1	10.0	2.0	$\approx$ 20	1.125 MHz	1.140 MHz	57.00	14.89
Charging LC2	10.0	2.0	$\approx$ 20	1.125 MHz	1.104 MHz	55.20	14.06
Charging LC3	10.0	2.0	$\approx$ 20	1.125 MHz	1.120 MHz	56.00	14.44
Trigger LC1	10.0	4.7	$\approx$ 60	734 kHz	734 kHz	12.23	5.29
Trigger LC2	10.0	3.9	$\approx$ 55	806 kHz	785 kHz	14.27	4.39
Trigger LC3	10.0	3.3	$\approx$ 60	876 kHz	855 kHz	14.25	3.81

TABLE II: In Circuit: LC Resonator Measured Parameters

Panel of Figure 6 — Fig. 4: The red, black, and blue lines represent devices 1, 2, and 3 respectively as stated in the legend. The charging LCs are difficult to distinguish due to their close frequency overlap. The trigger LC resonators exhibit significantly lower Q-factor than the charging LC resonators because the NPN transistor base current places a resistive load on the resonator, increasing energy loss and reducing effective Q. Error bars represent standard deviation (n=3).

Fig. 5: Experimental setup of a 3D-printed configuring housing the GS0-30 load gram sensor, the transmitter coil, and each prototype as a fixed distance for actuation force characterization.

Fig. 6: EM force measurement for three devices. Each device was set off a function generator to charge for one second and switch to the trigger frequency for one second.

Fig. 7: Selective activation frequency bands for three trigger LC resonators. Solid bars indicate the measured excitation frequency ranges that trigger each device, and tick marks denote the tested frequencies. Hatched regions highlight the overlap between adjacent triggers (10 kHz for Triggers 1–2 and 20 kHz for Triggers 2–3).

Fig. 8: Cantilever beam deflections with one-second charge and one-second discharge cycles. (A) Experimental configuration showing the transmitter coil and three actuators positioned near the transmitter and aligned parallel to the coupling axis. (B)-(D) Time-lapse sequences for each actuator showing the initial state, actuated state, and recovered state, demonstrating selective actuation of individual units.

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Robotic Grasping Additive Manufacturing Soft Robotics

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Functional magnetic composites capable of large deformation, load bearing, and multifunctional motion are essential for next-generation adaptive soft robots. Here, we present muscle-inspired magnetic actuators (MMA), additively manufactured from a thermoplastic/permanent magnet polyurethane/Nd2Fe14B (TPU/MQP-S) composite using laser powder bed fusion (LPBF). By tuning the laser-energy scale between 1.0 and 3.0, both mechanical stiffness and magnetic response are precisely controlled: the tensile strength increases from 0.28 to 0.99 MPa while maintaining 30-45% elongation at break. This process enables the creation of 0.5 mm-thick flexural hinges, which reversibly bend and fold under moderate magnetic fields without damage. Two actuator types are reported showing the system versatility. The elongated actuator with self-weight of 1.57 g, magnetized in its contracted state, achieves linear contraction under a 500 mT field, lifting 50 g (32x its own weight) and sustaining performance over at least 50 cycles. Equipped with anisotropic frictional feet, it supports movement of a magnetic crawling robot that achieves up to 100% locomotion success on textured substrates. The expandable actuator exhibits reversible opening and closing under a 300 mT field, reliably grasping and releasing different objects, including soft berries and rigid 3D printed geometries. It can also anchor in a tube while holding suspended 50 g loads. This work demonstrates a LPBF-based strategy to program both stiffness and magnetization within a single material system, enabling remotely driven, reconfigurable, and fatigue-resistant soft actuators. The approach opens new possibilities for force controlled, multifunctional magnetic soft robots for adaptive gripping, locomotion, and minimally invasive manipulation of biomedical tools.

Abstract

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