Control of Resident Space Objects through Eddy-Current Actuation
Essentially all common methods of accurately manipulating macroscopic objects in space require direct physical contact, whether through linkages, wires or simple grapplers. The contact between an actuator and an object not specifically designed to accept the actuation can be dangerous: a grappler may apply excessive force and damage the object, or unanticipated movement of the target may result in a catastrophic collision. Current technology does not allow for actuation with multiple degrees of freedom without direct physical contact or receptors on both objects and closed loop control. Time-varying magnetic flux through a conductor can induce a current opposing the change in the magnetic flux, also known as an "eddy current." The resulting force from the interaction between the eddy current and the changing magnetic field acts not only radially but also in three spatial directions. Additionally, eddy currents can be formed in any conductor, making them very useful in a space environment, where virtually all man-made objects, and a number of natural ones are composed of conductors. While eddy currents have been modeled for use in other applications, such as vibration damping , no previous work has attempted to model or implement them for use as an actuator. In order to solve the problem presented by a lack of accurate, contactless actuation, this work would use eddy currents in conductive materials to create six degree of freedom actuation.
The primary objective: to develop the physical and algorithmic bases for an actuator capable of providing six degrees of contactless actuation through the control of eddy currents.
This goal comprises several smaller key objectives:
- Identify the most efficient system for generating magnetic fields with controllable rates of change
- Develop a model of eddy current dynamics and the resulting forces on a conductor in a space environment
- Design a control system for the eddy current actuator
- Implement a prototype control system and actuator
Significance to NSTRF and NASA Objectives
- The interactions of a number of oscillating magnetic fields with each other and a conductive surface in order to generate desired forces will be numerically simulated based on classical analytical models of eddy currents in large conductive sheets.
- A prototype system consisting of an array of rotating and stationary magnets will be implemented and tested on a low friction test bed. Data will be acquired through a motion tracking system in order to test the validity of the numerical models and identify where they need modification.
- Once a rough prototype has been constructed, the system will be characterized and a controller developed. These experiments will treat the system as a "grey box" taking into account the analytical eddy current models previously developed as well as experiments.
Successful implementation of eddy current actuation will have applications for a number of NASA objectives, focusing on robotic mobility and manipulation. All man-made objects in space and a number of asteroids are metallic conductors. An eddy current actuation system would allow for accurate actuation of a robotic probe or other craft near these objects, or to manipulate the objects themselves, without requiring either dangerous physical contact or anything on the second object besides a conductive surface. This system might be used to tow failed satellites into different orbits, help de-orbit space junk, or actuate a spacecraft around a metallic asteroid with the same instruments that run tests on its composition. This capability would enable missions to near earth objects, many of which are of a conductive nickel-iron composition, so an eddy current actuator would be ideal for a robotic surface probe. The system offers the advantage of linear forces without requiring propellant while in the close vicinity of a metallic object.