# Retractable Wireless Charging Windings for Inspection Robots
## 6 Discussions  
Table 6 presents a comparative analysis of recent WPT systems developed for mobile and embedded applications. The proposed system in this work which is based on a retractable square coil (SC-WPT) delivers 116.5 W at 1 m with 68.72% efficiency. The results represent a balance of power, range, and mechanical reconfigurability. Compared to other systems, it is among the few capable of combining high power delivery and mid-range distance with a deployable coil structure. [PERSON] et al. (2024) [35] origami inductor system demonstrates high peak efficiency (83.58%) in a tightly coupled setup but operates at much shorter distances (1-2 cm) and is optimized for compact high-frequency DC-DC converters rather than mobile WPT. Similarly, [PERSON] et al. (2023) [36] and [PERSON] et al. (2021) [37] report efficient performance in low-power AGV and warehouse applications. However, these systems operate over shorter transfer distances (up to 16 cm) and do not support geometric reconfiguration. [PERSON] et al. (2023) and [PERSON] et al. (2020) [38] provide moderate to high power levels (20-150 W) with efficiencies ranging from 40% to 60%, but both rely on fixed or impedance-matched driver coils. These configurations offer useful adaptability under load variation or spatial misalignment but lack the deployability and compact storage capabilities needed for mobile robots in constrained environments. [PERSON] et al. (2024) [26] report a comparable power level (109.7 W) at 1 m but with significantly lower efficiency (47.14%) and no structural reconfigurability. The integration of the WPT system with the mechanical structure of a mobile robot imposes strict constraints on the coil's geometry, size, and deployment mechanism. To meet these constraints, the coil design has to balance electromagnetic performance with mechanical feasibility. The square coil is ultimately selected not only due to its favorable coupling characteristics under aligned conditions but also because of its suitability for implementation within a retractable frame. Unlike more complex or circular geometries, \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Study** & **Coil Design** & **Power (W)** & **Distance (m)** & \(\boldsymbol{\eta}\) (\%)** & **Reconfigurable** & **Application** \\ \hline This work & Square (SC–WPT) & 116.5 & 1.0 & 68.72 & Yes & Mobile \\  & & & & & & Robot integration \\ [PERSON] et al. & Tetrahedron & & & & Compact HF boost \\ (2024) [35] & origami (4-phase) & 80 & 0.01–0.02 & 83.58 & Yes & converter \\ [PERSON] et al. & & & & & & with interleaving \\ (2024) [26] & Octagon Coil WPT & 109.7 & 1.0 & 47.14 & No & Mid-size \\ [PERSON] et al. & Movable & & & & & inspection robot \\ (2023) [36] & driver coil & 8 & 0.025 & 31.4–88.3 & No & Sensor-optimized \\ [PERSON] et al. & Subsection coils & & & & & AGV charging \\ (2021) [37] & (MRSC) & 20 & 0.08–0.16 & 65–87 & No & Dynamic WPT for \\  & & & & & & warehouse robots \\ [PERSON] et al. & Driver coil & & & & & Flying or mobile \\ (2023) [38] & impedance matched & & & & & robots with \\ [PERSON] et al. & Intermediate & & & & & load variability \\ (2020) [39] & coil array & 150 & 0.8 & 60 & No & Large area \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Benchmarking against recent WPT systems. the square shape lends itself naturally to construction using straight, rigid segments, which can be folded and extended through a scissor-linkage mechanism. This geometry enables a compact and folded configuration during robotic operation and an expanded form during charging. It maximizes the effective coil area without permanently occupying valuable space on the robot. Furthermore, the square design allows for consistent deployment using a minimal number of actuators. This supports system simplicity and reduces energy consumption. This is an important consideration for battery-operated mobile platforms.