Simplified 3D Control of Magnetic Objects by a Triple-Coil Static Unit on a Robotic Arm

Abstract

Magnetic actuation is a powerful, non-contact method for controlling milli-scale robots. However existing mobile magnetic field sources face a difficult trade-off. Single-coil end effectors are simple but underactuated, forcing complex and inefficient robot motions to steer objects. Conversely, multi-coil systems improve dexterity but introduce significant mechanical and control complexity. To cope with this challenge, we present a compact, fixed-configuration triple-coil electromagnetic end effector mounted on a 7-DOF robotic arm. Our innovation lies in a hierarchical control strategy that decouples global and local actuation. A Control Lyapunov Function-based Quadratic Programming (QP-CLF) controller guides the robotic arm for large-scale repositioning, extending the workspace and minimizing required currents. Simultaneously, modulating the currents through the three coils provides fine, high-bandwidth electrical control over the local magnetic field and gradient. We validated this approach by steering 1, 2, and 3 mm magnetic spheres along complex spiral trajectories inside fluid-filled phantoms (water and oil). Our system was teleoperated under operator vision and demonstrated highly repeatable path passing performance, proving that this synergistic robot-electromagnet control provides a compelling balance of dexterity, compactness, and simplicity for advanced magnetic manipulation tasks.