SDSS-V Algorithms: Fast, Collision-free Trajectory Planning for Heavily Overlapping Robotic Fiber Positioners

Abstract

Robotic fiber positioner (RFP) arrays are becoming heavily adopted in wide-field massively multiplexed spectroscopic survey instruments. RFP arrays decrease nightly operational overheads through rapid reconfiguration between fields and exposures. In comparison to similar instruments, SDSS-V has selected a very dense RFP packing scheme where any point in a field is typically accessible to three or more robots. This design provides flexibility in target assignment. However, the task of collisionless trajectory planning is especially challenging. We present two multiagent distributed control strategies that are highly efficient and computationally inexpensive for determining collision-free paths for RFPs in heavily overlapping workspaces. We demonstrate that a reconfiguration path between two arbitrary robot configurations can be efficiently found if a “folded” state, in which all robot arms are retracted and aligned in a lattice-like orientation, is inserted between the initial and final states. Although developed for SDSS-V, the approach we describe is generic and thus applicable to a wide range of RFP designs and layouts. Robotic fiber positioner technology continues to advance rapidly, and in the near future ultra-densely packed RFP designs may be feasible. Our algorithms are especially capable in routing paths in very crowded environments, where we see efficient results even in regimes significantly more crowded than the SDSS-V RFP design.

ABYSS. I. Targeting Strategy for the APOGEE and BOSS Young Star Survey in SDSS-V

“…With the new capability of SDSS to rapidly reconfigure the fiber placement due to the robotic fiber positioners (Sayres et al 2021), and the ambitious goals of a comprehensive survey obtaining spectra across entire sky (Kollmeier et al 2017), the exposure time on all fields is set at 15 minutes. SDSS-V divides its efforts into three mappers: Milky Way Mapper (MWM), Black Hole Mapper, and Local Volume Mapper.…”

The Eighteenth Data Release of the Sloan Digital Sky Surveys: Targeting and First Spectra from SDSS-V

“…One major challenge of the SDSS robotic fiber positioner system derives from the fact that the patrol radius of each positioner overlaps with its neighboring ones. For a given set of target-to-positioner assignments defined in a robostrategy design, an algorithm named Kaiju (Sayres et al 2021) provides a deterministic trajectory for each robotic positioner from a starting "folded" state, in which all robots are identically oriented, to the desired FPS configuration. Kaiju works following a reverse-solve method, in which it is considered simpler to calculate collision-free trajectories for each robot from a complex, deployed state to a lattice-like folded configuration.…”

“…To create a path between two observable configurations, Kaiju calculates the reverse trajectories from each configuration to the folded state, and then applies the reverse trajectory. Following this approach, Kaiju is able to determine deadlock-free trajectories between any two physically reachable robot configurations in over 99% of cases (Sayres et al 2021).…”

MOONS fibre positioner control and path planning software

“…MOONS also uses a path planning algorithm based on the one used by the Sloan SDSS-V fibre positioners [7]. SDSS-V has fibre positioners of a similar design to MOONS and their movements are governed by the same equation ( 1).…”