# Retractable Wireless Charging Windings for Inspection Robots
## 1 Introduction
Despite these promising results, CC often requires precise plate alignment and high-voltage insulation, which can be challenging in flexible or deployable systems. In contrast, this study focuses on IPT due to its robustness to misalignment and its compatibility with mechanically reconfigurable coil geometries, which are essential for compact mobile robotic platforms. This is a critical consideration especially for inspection and maintenance in hard-to-access environments where minimal human intervention is required in autonomous operations [20]. However, integrating IPT systems into space-constrained robotic platforms presents challenges. A fundamental trade-off exists between coil size and resonant frequency. Larger coils generally exhibit higher inductance, allowing for stronger magnetic coupling and better energy transfer at lower frequencies. Smaller coils operate efficiently only at higher frequencies, which may introduce increased losses due to the skin effect and parasitic \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Mobile Platforms** & **P (W)** & **T (h)** & \(E_{b}\) **(Wh)** & **t (h)** & **W (kg)** \\ \hline Husky [8] & 160 & 3–8 & 480 & 4 & 75 \\ ANYmal [2] & 300 & 2–4 & 900 & 4 & 10 \\ Lily Hexapod [13] & 360 & 1–2 & 98 & 1 & 30 \\ Phantom Pro 4 [14] & 178 & 0.5 & 89 & 0.56 & 0.5 \\ Matrice 300 [15] & 198 & 1 & 36 & 2 & 3.6 \\ Hydrus-300 [5] & 99 & 1.5 & 99 & 3.5 & 6.4 \\ BlueROV [16] & 133 & 0.5–2 & 266 & 2 & 5 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Parameters on the robot’s power and battery requirements. capacitance [21]. This trade-off becomes particularly critical when the coil must be compact during navigation and expand during charging, which requires both electromagnetic optimization and mechanical adaptability. The recent literature has proposed various solutions to address coil alignment, efficiency optimization, and misalignment tolerance. For instance, double-D (DD) and DDQ coil configurations have been shown to improve lateral misalignment tolerance in EV charging systems [22]. Other studies have explored adaptive coil arrays [23], resonant repeaters [24], and dynamic alignment tracking mechanisms [25]. However, these solutions often assume a fixed coil geometry and do not address the need for compactness or reconfigurability. These are the key requirements for integration with mobile platforms. A prior work [26] facilitates an octagon-shaped coil frame with a 1 m aperture diameter. It is capable of transmitting 109.7 W over a 1 m distance with 47.14% energy efficiency as shown in Figure 1. Although the primary objective is achieved, the design size is impractical to implement onto a mobile platform in real applications. This paper addresses the space and payload constraints by introducing a robust and compact retractable WPT coil frame. We enumerate a range of retractable mechanisms, with considerations of minimizing the number of actuators, shortlist possible solutions, and select the optimal solution for the anticipated applications. The final design of the receiving coil is not of a rigid, regular shape; however, the theoretical and practical verifications prove that it does provide a sufficient amount of required power for a mid-range mobile platform. Figure 2 is the schematic illustration depicting the IPT transmitter and receiver regular polygon coils compared to irregular shape coils. The following parts of the paper are organized as follows: Section 2 elaborates the WPT system's principles and shows the capability of power transfer performance with the iterations of coil geometry. Section 3 presents the specification requirements for the coil designs and details the analysis of the WPT system modeling. Section 4 discusses the experimental setup and measurements, and Section 5 consists of the results, followed by discussions in Section 6. Finally, Section 7 concludes the study and sets the outlook for the future work. Figure 1: Octagon-coil WPT on a mid-sized mobile robot [26].