Non-Contact Manipulation of Induced Magnetic Dipoles

TL;DR

This work addresses the challenge of non-contact manipulation of conductive, non-magnetic objects using oscillating magnetic fields, with a view toward applications like in-orbit recycling of space debris. It develops a physics-based interaction model for induced dipoles, builds an EKF-PID closed-loop controller, and solves coil-current inputs via a SQP force-inversion solver, validating the approach on a lab setup with a five-coil array and a semi-buoyant aluminum sphere. Key findings show that closed-loop control achieves sub-millimeter tracking accuracy, that strategies minimizing coil current drastically improve energy efficiency without sacrificing performance, and that open-loop reference trajectories offer limited benefits. The study also compares five-coil and four-coil configurations, finding nuanced differences largely due to sensing fidelity and calibration rather than fundamental control capability, informing future design of multi-coil magnetic manipulators for 3D induced-dipole positioning.

Abstract

Extending the field of magnetic manipulation to conductive, non-magnetic objects opens the door for a wide array of applications previously limited to hard or soft magnetic materials. Of particular interest is the recycling of space debris through the use of oscillating magnetic fields, which represent a cache of raw materials in an environment particularly suited to the low forces generated from inductive magnetic manipulation. Building upon previous work that demonstrated 3D open-loop position control by leveraging the opposing dipole moment created from induced eddy currents, this work demonstrates closed-loop position control of a semi-buoyant aluminum sphere in lab tests, and the efficacy of varying methods for force inversion is explored. The closed-loop methods represent a critical first step towards wider applications for 3-DOF position control of induced magnetic dipoles.

Figures (13)

• Figure 1: The four-stage recycling process as imagined by this research group. [i] Space debris is captured in-orbit and stripped for usable raw materials. [ii] Conductive materials (mainly aluminum) are melted in a furnace, here imagined as an induction heater. [iii] Molten aluminum droplets are carried from the furnace and positioned using time-varying magnetic fields that induce eddy currents in the molten sample, allowing for position control and thermal management as the metal moves from furnace to site of casting. [iv] Molten samples arrive at the site of casting, which re-purposes the raw materials into a usable material in-orbit. Here, casting is imagined as a continuous-casting process.
• Figure 2: The five coil electromagnet array developed by CisLunar Industries for manipulation of conductive materials subject to oscillating magnetic fields. Coils 1-4 surround the workspace, while coil 5 is directly beneath the workspace. The magnetic workspace is surrounded by an acrylic enclosure for loading cartridges containing the samples. Three board-level cameras surround the working area, which are used for object detection in the feedback loop.
• Figure 3: Simulated low-gravity environment to test coil-array in normal lab setting. Hollow aluminum sphere is partially filled with water and submerged in water bath, leveraging the buoyant force that pushes the sample to the surface and thereby allowing for levitation even under normal gravitational conditions. Typical effective sample weight is on the order of tens of micro-newtons.
• Figure 4: Control-loop employed in demonstrating closed-loop control of induced magnetic system. The observer, in the form of an Extended Kalman Filter, is shaded in blue, the output of which gives the state estimate of the sample during flight. The PID Controller takes as input the error in state and calculates a force value to apply that will reduce this state error. Feedback is provided by a set of three cameras, which provide the observer with an updated measurement of position.
• Figure 5: Planned trajectory in 3D space for the aluminum samples to travel during experimentation. After settling in the center of the workspace, samples will draw a [10]mm square at the lower elevation, after which they will move [10]mm in the positive z-direction and draw another [10]mm square. Zmin/Zmax are [20]mm/[30]mm for the five-coil system, and [52]mm/[62]mm for the four-coil system. Total time to complete the cube is [109]seconds. Offset in the final leg of the trajectory is not indicative of actual planned path, and is displayed in this manner so as to avoid overlapping paths.