Spinning > Not Spinning

The changing component of the magnetic fields that produce eddy currents can come from two sources: a relative velocity between the field and the target or change in the strength of the field itself. For a number of reasons the bulk of EC actuation research has focused on the latter mechanism. Time-varying electromagnets are easier to both analyze and implement.

However, two motors spinning magnets to create eddy-currents through moving magnetic fields may be able to achieve something that is much harder with electromagnets: attraction AND repulsion without a complicated scheme.

The motor-magnet actuator says to the plate: “come here”

“Now go away”

Combine that with “sashay left and right”

And you just may be able to dance with a spaceship.

Sliding into testing

Testing on the ground removes all the characteristics of space that make eddy-currents and other exotic actuators useful there. Specifically, friction (with air or a surface) often overwhelms the small forces of spacecraft actuators that aren’t rocket powered. So, low friction test beds are necessary to experimentally verify things meant to move in space.

Several versions of low-friction test beds are available for spacecraft research, each with their own advantages and disadvantages. The two most common types are air-bearing test beds and rotational test beds. They have a number of advantages. However, unnecessary degrees of freedom, cost, complication and lack of extensibility motivated the construction of a 1-DOF air-track test bed.

Air-bearing test beds have simulated microgravity dynamics, both translational and rotational, for over 45 years. Traditional planar air bearing systems are ideal for testing 2-DOF dynamics, but have a number of drawbacks that make them unattractive for testing 1-DOF systems like early-stage eddy-current actuators. The second degree of freedom introduces extraneous variables when testing 1-DOF systems. Sensing the state of the 2-DOF system in real-time requires video processing software that is either custom built or expensive. Closed loop control requires on-board computing, adding to the complication of the system. Planar air bearings require extremely flat surfaces and rapidly consume compressed gas canisters.

2-D air-bearing test bed. Picture courtesy of The University of Surrey

Rotational test-beds can also simulate 1-DOF microgravity dynamics. These test-beds consist of a target on the end of a long arm attached to a low-friction rotational axis. The throw over which they can approximate linear translation is limited by the length of the arm. The bearings holding the rotational axis experience more off-axis torque as the arm length increases so the friction will increase with the throw of the system.

A rotational test bed. Picture courtesy of the University of Michigan.

Some Tech Talk

A snippet from an abstract I’m working on – further details about ‘falling up’:

Although they have many advantages over present technology, eddy-current actuators are not a killer app for generating 6-degree-of-freedom forces. Eddy-current forces are small and drop off quickly with distance. The necessary electromagnets and motors both generate thermal loads. While possible, it is difficult for an eddy-current actuator to generate a force that pulls the target and inspection vehicle closer together.

Despite these flaws, there are a number of applications which can take advantage of this technology’s strengths (electric-only, propellantless, contactless forces with an uncooperative target) and minimizes its weaknesses.

The magnitude and direction Earth’s gravitational pull varies around the exterior of a large satellite like the ISS. So two test objects – one placed on the ‘bottom’ of the ISS and one placed at its center of mass will experience different accelerations and thus a relative ‘force’ between them. On earth, this relative acceleration between objects at different altitudes is hardly noticeable compared to random ‘noise’ forces like wind. However, there are much fewer disturbance forces in space and the gravity gradient is enough to ‘pull’ an inspection craft towards the surface.

On the exterior of an object the size of the ISS, the ‘force’ is on the order of 0.1 mN. Eddy-current actuators have been able to produce forces on the order of 0.01 – 10 mN so they are well-suited to oppose the gravitational forces, allowing the inspection craft to keep a safe distance while moving laterally along the surface.

[Placeholder for a diagram of this totally unintuitive situation]

Why EM is Especially Useful in Space

Space: the final frontier. Every new frontier has a new set of rules – things that don’t work the way they did back home.

The stars are different in the southern Hemisphere, the most deadly animal in the tropics is the mosquito, and dogs are more useful than mules in the Antarctic.  But physical rules of thumb generally stay valid anywhere on earth, from merry old england all the way to the East Indies.  Engines work by turning some kind of shaft, spinning wheels that push against the ground, hot air rises, and if you don’t keep pushing, friction will eventually stop anything that’s moving.

We’re not in Kansas anymore… Courtesy of Nasa

The fundamental physics aren’t any different in space either, but those rules of thumb? Forget about them. Between the vacuum, temperature fluctuations, orbital mechanics, microgravity, and low friction many customary assumptions on earth go out the airlock. The last two conditions in space: microgravity and extremely low friction open up doors that are closed on the ground. The goal of spacecraft actuators is to take advantages of these unique environmental effects.

It’s only fair, because those same effects inject difficulty into many things that are easy on Earth. Gravity and friction overwhelm most electromagnetic interactions because they are small to begin with and weaken rapidly with distance. However, electromagnetic forces are relatively unaffected by the conditions in orbit, making them prime candidates for spacecraft actuators.

What’s an actuator?

Discussing eddy-current actuators is hard without the word ‘actuator.’ It is clunky and reeks of jargon, but the alternatives are worse. “Tractor Beam” is evocative but misleading. “Electromagnetic force generator” is so generic it says almost nothing. “Eddy-current force generator” begins to stray into the land of sleep-inducing engineering names and “contactless eddy-current force generator” definitely does.

So what is an actuator?

According to the first entry at dictionary.com,  an actuator is

1. a person or thing that actuates.

Ah, of course. That clears everything up.

Wikipedia and the second entry at dictionary.com are both painfully specific:  

“An actuator is a type of motor for moving or controlling a mechanism or system.”

2. a servomechanism that supplies and transmits a measured amount of energy for the operation of another mechanism or system.

Both of these definitions fail to the full meaning of an actuator as well. An actuator doesn’t have to be a motor, nor does it have to measure the amount of energy it transmits to the system. Your hand is an actuator, but fails both of those tests. The similarity at the end of these definitions does get closer to answering ‘what is an actuator?’  An actuator is a system or mechanism that does *something* to another mechanism or system.

The clue to that *something* lies in the definition of an actuator’s verb form – actuate:

1. To put into action or mechanical motion

So putting it all together, an actuator is a system or mechanism that induces physical motion into another system. Or more elegantly, “something that turns energy into motion.”  This idea is everywhere in robotics, because robots are essentially computers that use actuators to interact with the real world. So, after a painstaking discussion, the implication of an eddy-current actuator is clear: a system that induces physical motion via eddy currents.

A taste of things to come

Of all the institutions embedded in the culture of science writing, abstracts are actually a good idea. Abstracts give the reader a broad overview of the entire document – it’s structure and arguments. Essentially ‘warning traveller!’ A good abstract is like a tree in winter – there won’t be any surprises about the structure come spring, but you have to wait to see exactly how it will be filled out. This is not an abstract. The idea (right now) is to give some idea of what is to come and populate it with links and retroactive changes as this thing evolves.

In one sentence: This research presents the case for contactless spacecraft actuators utilizing eddy-current forces and an approach towards developing them that combines high-level modeling and empirical testing.

First, why the non-existence of eddy-current actuators is a problem worth solving and caring about and what science and engineering has already been thrown at it. This background will cover, in increasing specificity, actuators, spacecraft actuators, contactless spacecraft actuators as well as eddy-currents and earth-bound eddy-current actuators.

That will cover everything outside of this project. Then, an overview and justification of the specific angle of attack project takes – a general (gross) model combined with empirical testing.

From there, the nitty gritty.

Theory: different tactics for producing eddy-current forces in each of the three spatial degrees of freedom (push/pull, side-to-side, up/down), how an eddy-current actuator can be used as a sensor and actuator at the same time with no additional equipment. Algorithms for modeling a system that ties together Maxwell’s equations and rigid body dynamics.

Experiments: How strong is this actuator? How do we wrap a feedback loop around it? How much can the model predict? How to test a spacecraft actuator both on Earth and on the cheap?

And of course, where it can all go in the future (besides statistically likely scrapheap of history.)