# Retractable Wireless Charging Windings for Inspection Robots
## 7 Conclusions  
This paper presents a novel approach to address space and integration challenges in WPT systems for mobile robots by developing a retractable square coil design. The proposed system demonstrates a maximum transmitted power of 116.5 W with an energy efficiency of 68.72% over a 1 m transfer distance. This prototype demonstrates competitive performance when benchmarked against recent WPT systems designed for mobile robots and shows a notable improvements over a previous fixed octagon coil design. A 5x size compression ratio is successfully implemented through a scissor-linkage mechanism, validating the concept of a reconfigurable WPT receiver suitable for space-constrained robotic platforms. The system achieves higher efficiency while also offering mechanical reconfigurability, minimal actuator complexity, and better integration potential for mobile platforms. The performance and adaptability of the proposed design illustrate its potential as a deployable and efficient alternative to fixed-coil WPT systems, particularly for autonomous robotic platforms where space, mobility, and charging flexibility are critical. Simulation and experimental results confirm the system's sensitivity to both lateral and angular misalignments, particularly in angular rotations about the z-axis, where energy efficiency drops significantly. While this highlights a limitation of the current setup, it also opens opportunities for further research. While the reconfigurable coil system's performance has been validated under free-space conditions in this work, several challenges and opportunities remain to optimize its practical deployment and durability:  
* The potential for WPT through non-metallic obstacles, such as concrete walls, represents a promising application area. Future work will include the experimental validation of transmission performance across varying obstacle thicknesses and compositions, focusing on power attenuation, alignment sensitivity, and thermal effects. * Another important enhancement is the integration of an automatic coil alignment system to dynamically adjust the receiver coil position relative to the transmitter. Thiswould mitigate spatial misalignments during docking or irregular operating conditions and optimize coupling efficiency. Potential approaches include visual servoing, magnetic field gradient sensing, and low-power beacon-based feedback control. * Further investigation will explore passive magnetic field shaping techniques or flux-guiding structures to enhance power transfer efficiency and spatial tolerance. * Incorporating active alignment strategies, such as adaptive impedance matching or multi-coil arrays, could further improve system robustness to misalignment. * The exploration of alternative coil topologies, including Double-D (DD) or hybrid coil configurations, will be conducted to assess their misalignment tolerance and compatibility with the retractable mechanism. Additionally, future studies will consider the effect of coil sagging and deformation on inductive properties, aiming to refine the mechanical and electromagnetic co-optimization of retractable WPT systems. Further improvements to the accelerated life testing can be explored in future work, where a more in-depth investigation could be carried out to propose a more robust and feasible retractable design, optimizing the mechanism's overall performance. Methodology, software, investigation, and writing--original draft, M.N.B.H. and C.Z. ; validation and supervision, C.Z. and S.W. ; formal analysis and validation, C.Z. and S.W. ; writing--review and editing, C.Z. and S.W. All authors have read and agreed to the published version of the manuscript. This research was funded by UKRI Project EP/P01366X/1. Not applicable. The raw data supporting the conclusions of this article will be made available by the authors upon request. The restriction is due to the fact that work is currently in progress to make the core solver module publicly available via GitHub and the MathWorks File Exchange.