Closed-loop control of dielectric permittivity for 3D-printed radio-frequency devices

Additive manufacturing (AM) offers significant advantages over traditional manufacturing methods for the fabrication of radio-frequency (RF) devices due to lower production costs, flexibility to produce complex geometries, and ability to vary the internal composition of a part. Because AM processes are typically run without any online feedback control, printed parts may be compromised by defects. This paper proposes a feedback control algorithm to enable closed-loop control of a graded-index (GRIN) RF lens produced using a Fused Filament Fabrication printer. As a GRIN lens relies on spatially-varying permittivity, differing densities were printed on a layer-by-layer basis, thus changing the permittivity through the lens depth. The control system used a split ring resonator to measure the permittivity of printed material, and the control action was applied by updating the printed infill density in each layer. The results indicate the controller could detect and adjust for errors, demonstrating the potential of closed-loop control for precise fabrication of high value-added RF devices.