A Hybrid R-Theta Fiber Positioning Robot Using a Piezoelectric Bender for Radial Motion
Future massively multiplexed spectroscopic surveys require focal planes with increasingly dense arrays of robotic fiber positioners. The DESI (Dark Energy Spectroscopic Instrument) fiber positioners, based on a two-axis Theta-Phi motorized architecture, have demonstrated excellent reliability and performance during operations and provide the benchmark for evaluating new concepts. However, proposed next-generation instruments such as Spec-S5 require a substantially smaller positioner pitch than DESI, making direct scaling of the existing design challenging. Here we describe a hybrid R-Theta fiber positioning concept in which the rotation about the central axis is provided by a DESI-like Theta motor, while the radial displacement is produced by a piezoelectric bimorph bender coupled to a lightweight carbon-fiber spine. This design builds on the heritage of the the proven central rotation mechanism while replacing the Phi arm with a compact piezoelectric actuator. We discuss the trade-offs between motorized positioners, piezoelectric slip-stick tilting spines, tubular piezo actuators, and direct piezo-bender actuation. A prototype based on a DESI positioner was fabricated and tested using a commercial bimorph bender and an 8-inch carbon-fiber extension. With closed-loop control using feedback from a fiber-view camera the fiber was brought on target after two corrections with residual stability of approximately 2 microns RMS. These results indicate that direct piezo-bender actuation is a promising approach for compact fiber positioning, provided that closed-loop operation is addressed.