A New Actuator for On-Orbit Inspection

Abstract

Small satellites can enable a new kind of mission architecture: inspecting larger satellites on orbit in close proximity without mechanical contact. Induction coupling is a new actuation technology that can augment on-orbit servicing by exploiting eddy-current forces and torques. Current technologies for applying forces and torques between two spacecraft share a glaring disadvantage: they require direct contact or propellant. By using the forces between a magnetic field and the electric currents it induces in a target, an induction coupler can control the relative position and orientation between a chaser spacecraft and a target without physical contact. A system utilizing these eddy-current effects places relatively few requirements on the target and chaser compared to other proposed electromagnetic actuation concepts. This paper presents a system overview of a contactless induction coupler, outlines those requirements through the analysis of an inspection mission on the International Space Station, and traces them to flight applications through ongoing experimental work.

Introduction

An induction coupler uses magnetic eddy currents to create forces between itself and the conductive materials that make up a target. The coupler requires no mechanical contact with a target, nor does it demand cooperation from the target. The coupler can also operate on electricity alone, rather than requiring propellant. Because most satellites include conductive material in their structure—notably aluminum honeycomb with aluminum facesheets or aluminum beams—induction couplers may be the closest thing we have to science fiction’s tractor beam: a device that can produce contactless forces on an uncooperative target. Induction couplers show promise for spaceflight applications, offering three major advantages. First, the small forces associated with magnetic fields across meter-scale distances can dominate gravity, friction, aerodynamic drag, and other effects, which are far less pronounced in orbit than in a terrestrial environment. Second, fully deployed spacecraft rarely offer straightforward means for mechanical grappling; so, the ability to interact without the potential for contact damage is valuable. Third, induction couplers offer the ability to maneuver without expendables, eliminating risks associated with propellant-plume impingement and extending the useable lifetime of a spacecraft. A small spacecraft could use an induction coupler to control its motion relative to a much larger target like the International Space Station (ISS), crawling along the target’s surface without ever touching. This on-orbit inspection technique resembles the locomotion and functions of underwater robots that now inspect pipelines and shipwrecks.

Current interest in on-orbit servicing (OOS) is a strong motivation for advancing induction coupler technology. One of the fundamental technological use cases is that of a small inspection vehicle whose interactions with the target do not produce significant motion in that target—for example, an ISS inspection vehicle. Such a vehicle is primarily concerned with regulating planar motion along the surface of the target and stabilization of out-of-plane translation. This paper describes a study of how the planar component of that motion can be achieved with induction couplers.

image \label{fig:iss_inspector}

\label{fig:iss}

Background

Other Technologies

There are other technologies that can produce contactless forces between a spacecraft and a target. Coulomb forces have been shown to produce useful interactions between two charged spacecraft as long as the distance between them is less than a Debye length. (Natarajan 2006) A number of different systems produce contactless forces with magnetic interactions among controlled dipoles on both the spacecraft and the target. (Schweighart 2005) All such approaches place requirements for specific hardware on both the chaser and the target (that is, the target must have launched with certain features already in place.) However, no spacecraft currently in orbit meet these criteria, to the authors’ ‎knowledge.

Laser tweezers can produce contactless forces on an uncooperative target. (Brzobohatý 2013) However, the tweezers are best at manipulating micron- scale particles, a size restriction that no spacecraft beyond about TRL 1‎ can meet. (Grzegorczyk 2014) Thruster plumes can also produce forces between a spacecraft and a target. However, typically the combustion products from thrusters carry significant risk of contaminating optical instruments and solar panels, among other disadvantages. We conclude that current technology is limited to direct mechanical contact as the only other option to create forces on a target that has not been designed for an interactive mission.

Induced Current Forces

An induction coupler generates an eddy-current force that acts between itself and a target. Eddy-current forces start with a time-varying magnetic field. The field induces an electrical eddy current in a conductive target. In turn, that induced current interacts with the magnetic field and produces force between the conductive target and the source of the magnetic field. In broad strokes, the generation of eddy-current forces is a straightforward manifestation of Maxwell’s equations. ‎

  1. Any material with finite conductivity experiences a voltage gradient in response to a time-varying magnetic field.

  2. The voltage difference drives a current through the material. This current flows in a direction to cancel the change in the magnetic field with a time delay.

  3. This induced current acts like the familiar example of a wire in a magnetic field, and experiences a force.

All such approaches place requirements for specific hardware on both the chaser and the target (that is, the target must have launched with certain features already in place.) However, no spacecraft currently in orbit meet these criteria, to the

These steps give intuition for the physical process, but they are a gross oversimplification. More generally, the currents in the conductor depend on the geometry of the material, material properties, the direction and magnitude of the changes in the magnets in the induction coupler, and the velocity of the target relative to the induction coupler. In fact, the force depends not just on the current, but also on the magnetic field’s magnitude and direction. Compounding the subtlety is the unavoidable coupling between the magnetic field and the kinematics of the magnets in the induction coupler. These interdependencies make the force sensitive to the state of the system. Induction couplers exhibit many nonlinearities, which demand a rigorous and informed approach to implementing the technology.

Induction couplers can produce force in any direction relative to the target, for example both tangential and perpendicular to a surface on that target. Therefore, a small spacecraft generating a time-varying magnetic field could produce forces in all three translational degrees of freedom. Extending that idea, two induction couplers separated by a moment arm could also produce torques to orient the spacecraft.

There are two ways for an induction coupler to generate its changing magnetic fields and resultant forces. While both moving permanent magnets and variable electromagnets can generate those forces, each kind of magnet is especially good at producing different sorts of force. A single electromagnet with a sinusoidal driving current can always produce a repulsive force between itself and the target. Replicating that force with a permanent magnet would require either a closed-loop linear actuator or a complicated set of linkages. Similarly, a simple permanent magnet mounted on a motor shaft easily produces horizontal shear forces between itself and the target, a force that is hard to replicate with electromagnets. It isn’t yet clear which combination of permanent and electromagnets optimally generates 6-degree-of-freedom (DoF) forces. There may not be an optimal configuration. Instead, the composition of the magnets in an induction coupler may depend on the mission profile - different for an inspection vehicle operating near the surface of a large target than a free-flier maneuvering near a smaller target. This paper focuses primarily on the former.

Conclusion

This paper presents a starting point for induction coupler technology. It describes the physics of spinning magnet arrays producing contactless forces and torques on any conductive target. Preliminary experiments verified and measured these forces. When coupled together, three or more arrays can produce three independent degrees of freedom in the plane above the target’s surface. Induction couplers offer the prospect of inter-body forces that enable close-proximity inspection of a large target vehicle by a smaller chaser spacecraft, all without mechanical contact. There are many paths for future induction coupler research that extend this key conclusion. The frequency response of the system and its sensitivity to system state and environment are important to future designs. This analysis considers only planar motion. However, remaining in that plane requires an actuator associated with that degree of freedom, and ideally the chaser spacecraft or inspection vehicle would exhibit full six-degree-of-freedom maneuverability.

References

  1. Arun Natarajan, Hanspeter Schaub. Linear Dynamics and Stability Analysis of a Two-Craft Coulomb Tether Formation. Journal of Guidance, Control, and Dynamics 29, 831–839 (2006). Link

  2. Samuel A Schweighart, Raymond J Sedwick, Samuel Adam Schweighart. ELECTROMAGNETIC FORMATION FLIGHT DIPOLE SOLUTION PLANNING E LECTROMAGNETIC F ORMATION F LIGHT by. (2005).

  3. O Brzobohatý, V Karásek, M Šiler. Experimental demonstration of optical transport, sorting and self-arrangement using a tractor beam. Nature … 7, 123–128 (2013). Link

  4. Tomasz M. Grzegorczyk, Johann Rohner, Jean-Marc Fournier. Optical Mirror from Laser-Trapped Mesoscopic Particles. Phys. Rev. Lett. 112, 023902 American Physical Society, 2014. Link

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