SPINDER: an open-source 18-DoF hexapod robot with hierarchical central pattern generator control and analytic inverse kinematics

This paper presents SPINDER, a compact 18-DoF hexapod robot designed for reproducible bio-inspired locomotion research. The platform integrates a time-of-flight depth camera, six-axis inertial measurement unit, and per-foot contact switches, with computation split between an ARM-based single-board computer and a real-time microcontroller driving Dynamixel actuators. We derive a closed-form inverse kinematics solver for the yaw-pitch-pitch leg configuration that enforces workspace feasibility and handles kinematic singularities, achieving real time computations per leg. The locomotion controller combines a coupled-oscillator central pattern generator for inter-leg coordination with local motion controllers that regulate torso motion and generate cycloid-based swing trajectories with stance compensation, before mapping foot targets to joint commands via analytic IK. Hardware experiments demonstrate stable tripod, ripple, and wave locomotion, including omnidirectional commands, smooth online gait transitions that settle within approximately two gait cycles, and successful traversal of structured uneven terrain (height variations up to 18 mm) with 100% completion rate and mean body-pitch deviation below 5[Formula: see text]. All mechanical designs, kinematic models, and control software will be released to support replication and extension.