Flexible Annular Receiving Coil

Problem

In existing wireless motors the receiving coil must stay precisely aligned with the transmitter. That rules out applications that need wide rotation — robot joint motors, for example, which otherwise need slip rings or rotary joints.

Approach

• A 60-mm PCB transmitting coil with sparse-inside, dense-outside winding for a more uniform field, verified in Ansys HFSS.
• A flexible polyimide receiving coil (198.5 × 50 mm, 0.24 mm thick) rolled into a 62-mm ring around the transmitter.
• Two sub-coils wound in opposite directions, so the opposing fields on the two faces of the transmitter induce currents in the same direction — effectively a parallel configuration.
• A GaN Class E amplifier at 6.78 MHz and a converter board with half-bridge rectifier, buck converter and three-phase inverter.

Results

Under a 40-Ω load the DC bus stays at about 20 V (within ±20%) from 0 to 60° of rotation, with system efficiency around 68%. Beyond 60° the sub-coils start to see opposing flux and the voltage drops, reaching zero at 90° — consistent with the analysis.