# Retractable Wireless Charging Windings for Inspection Robots
## 3 Design and Simulation of Retractable Wireless Power Transfer Coils  
### Design Requirements and Conceptual Selection  
The general objective of designing the retractable coil structure is based on the following assumptions: (1) the equivalent aperture area is similar, which is due to the nature of the magnetic field coupling mechanism, and (2) the thickness of the wires used remains the same so that the level of power transmitted, the energy efficiency, and the transmission distance are unaffected. The technical requirements are evaluated using a weighted decision matrix while maintaining the parameters used in the octagon-coil WPT system in [26] (1 m circumcircle radius, 8 spokes, 5 turns, 0.75 mm cross-sectional radius coil). The overall score of the functions required by the design is determined using a five-level scale, which is divided into three categories:  
* Reconfigurability (Retractable): This represents the complexity of expansion-to-compression of the design. The design is rated from one to five, where one is the low complexity and five is the high mechanical complexity, which contributes to the coil reconfiguring process. * Coil Management: This assesses the WPT coil management in terms of storage while the design is in a fully compressed position and coil handling during the expansion process (one being less likely to tangle, and five representing the high potential of coil entanglement). * Mechatronic Complexity: This is divided into two sections, which are mechanical and electrical. The mechanical complexity score represents the number of movable joints involved during the reconfiguration. Electrical complexity represents the number of electrical actuators that would be involved in the motion. Each design is evaluated according to the proposed conceptual designs shown in Figure 4. The design is scored based on the results of the comparison, which are displayed in Table 2.  
* Design 1 (Sunshade Folding): This design uses actuators inspired by car sunshade folding using a lightweight structure with the wireframe as the coil frame's outer lining. The challenges are effectively installing and positioning actuators to replicate the folding motion. * Design 2 (Rotation Folding): The frame collapses via a motorized function for efficient expansion and storage. This method promises better space efficiency but raises concerns about coil management during collapse. * Design 3 (Two-piece Folding): Its two half-frames are designed to collapse into a smaller size, and the octagon-shaped frame structure folds horizontally. It uses a telescopic stand to lower closer to the base for vertical retraction. * Design 4 (Telescopic): A telescopic design for the coil frame using motorized gears. This non-foldable design may contribute to a higher mass, as all arms are within one structure. The extension method is possible with motorized linear motors with gears. * Design 5 (Umbrella): An umbrella-skeleton collapsible frame. The challenges are potential coil entanglement within the frame due to the design complexity with multiple movable joints. * Design 6 (Scissor-linkages): Two arms are positioned on the left and right sides originating from the center of the body, using the scissor-linkages concept to form a geometrical shape frame holding the coil. Table 2 shows the design matrix for all six conceptual designs. The matrix considers the complexity of the coil reconfiguration and coil management to prevent any entanglement of the coil during the retracting and expanding motion. The score weight represents how the concepts respond to the functional needs, taking into account the utility or importance of the functions with the design target's overall need. The design selection is based on the overall score; the higher score represents the high design complexity, high coil entanglement, higher number of movable joints and actuators involved in the process of design development.