Finally implementing the capability to both push and pull a target with the EC actuator is a significant step forward for the project, but it is just the first hurdle in the steeple chase that is developing a fully functional eddy-current actuator. The ability to create forces in both directions along a single axis is well and good, but it isn’t very useful unless you can produce them predictably.
Specifically, we need to explore the parameter space of producing EC forces. That is, the next step of the research process is to figure out the relationships between changes in the system variables and the forces produced by the system. The physical process of producing EC forces is very nonlinear, and a slew of variables affect the force on the target: the magnitude, frequency and phase of the electric current through the electromagnets, the geometry of the magnet layout – positions and angles with respect to both the target and other magnets, the size and thickness of the target, and more.
The plethora of system parameters is both a boon and a bane – the design of the actuator is not constrained by too few ‘knobs’ to adjust; at the same time there are so many system parameters to investigate that it practically gives me analysis paralysis.
Upcoming experiments will unleash the full potential of this fully armed and operational scientific method. The improved (and duct-tape-less) experimental setup (more details in a future post) allows us to slightly change one parameter at a time and observe the effects of that change. Systemically changing parameters will tease out the relationships between the system parameters and resultant forces, the ultimate goal being the ability to say “give me a set of parameters and I will tell you what will happen.”
We have a high level idea of how changing these parameters affects the resultant forces and why: increasing the current though the magnet increases the magnetic field, causing more force; moving the target farther away reduces the magnetic field and amount of induced current, causing less force; etc. However, the relative magnitudes of these changes and how changes in multiple parameters affect each other remain somewhat elusive – but not for long!
Whenever you think of a classic tractor beam, I’d bet that your imagination brings to mind a spacecraft being inexorably pulled towards the source of the beam. Now, while eddy-current actuators don’t aspire to abilities that are quite that powerful, it should be obvious why the ability to pull on a target is an essential capability. When controlling the position of a target, an actuator needs to be able to produce forces both towards and away from itself in order to compensate for undesired perturbations in either direction.
It’s easy for an eddy current actuator to push on a target – any oscillating magnetic field from a single magnet will do the job. Pulling has been a much more difficult proposition – until now:
The ability to both push AND pull at will is a huge step towards being able to hover over the surface of the ISS, safely interact with an uncooperative satellite, or maneuver a very small Millennium Falcon into docking bay 327.
[Mark Longanback, one of the team members working on the Hermes, talking about KickSat] “It was really my inspiration for realizing that the crowdfunding platform could be viable for actually getting some funding injected into your company”
Although I was not directly involved in KickSat, it’s extremely gratifying to know that work in our lab is both ahead of the curve and helps to inspire others. Although it is a lofty goal, I hope we can continue this trend in the future!
An important component of electromagnet-based eddy-current actuators is the ability to have quite a bit of current flowing through a coil of wire. The magnetic field of the actuator is proportional to the current, and the force on the target is proportional to the magnetic field. Essentially, more current à stronger actuator.
Now, if more electrical current benefits the actuator, it begs the series of thoughts:
- Superconductors can handle far more current than normal wire, but they require very cold temperatures to function. (Although research continues on the ever-elusive room-temperature superconductor – which, like fusion, is always about ten years away.)
- Everybody knows space is very cold (deep space is 2.7 Kelvin – just above absolute zero.)
- EC actuators operate in space.
- Therefore, superconductive coils should be a natural choice for EC actuators, right?
Unfortunately, “cold” is one of many things that work differently in space than our earth-developed intuitions would lead us to believe.
On earth, heat can be transferred in one of three ways – conduction (heat transfer through a solid) convection (heat transfer through gasses and liquids) or radiation (heat transfer via photons.) In space, only the last type of heat transfer can occur. This means that space won’t act like a giant freezer – if you place an object in space, whether it heats up or cools down depends on whether it is absorbing or emitting more radiation. This can change rapidly, especially in earth orbit.
It is still easier to keep superconductors cold in space than on earth. However, the additional hardware to maintain superconductive temperatures adds both mass (thus $$$) and additional failure modes to the spacecraft. While these downsides are an acceptable trade-off for applications like flux-pinning that take advantage of the unique properties of superconductors, they make superconductors less worthwhile if conventional electronics will suffice.
Superconductive coils might be a necessity for EC actuators if we need truly massive amounts of current. However, due to the weird way heat works in space they aren’t all upside by any stretch of the imagination, so we’ll be keeping our sights on boring old copper wires for now.
Some awesome links related to chipsats:
In case you weren’t aware, Zac maintains a blog about KickSat – the crowdfunded project launching hundreds of chipsats into orbit. I’m surprised that it doesn’t seem to have been linked here before.
Additionally, KickSat was recently featured on BBC.
I thought I’d post these because Zac is insanely busy actually getting a real space mission ready to launch.
Today the EC actuator project was the beneficiary of the global scientific community in action! I recently read a paper by Ohji Et. al: “Generation of method AC Ampere force using eddy current inside a nonmagnetic thin plate and its effect on electrodynamic suspension.” The paper provided several insights that will improve the system architecture of eddy current actuators. The authors describe their experiment well, but as with any paper, there just isn’t enough room to compress months or years of tinkering, thinking and decision making into a few pages.
I wanted to know more about the details of the experiment and thanks to the wonder of the Internet, I emailed Professor Ohji just as easily as I could email one of my lab-mates in the same room (which has happened.) He responded quickly with extremely helpful information.
Obviously, an informative email exchange isn’t noteworthy in a vacuum. What’s exciting about this experience is that, at least to me, it’s a great example of how the global scientific community is supposed to work: researcher A publishes his work. Researcher B reads that paper and realizes that it is useful for his own work. Researcher B contacts researcher A and information is exchanged that allows researcher B to progress far more quickly than he would have on his own.
This is the process that has driven modern innovation, and all the benefits that humanity has reaped. Since no individual can figure everything out from scratch, the transfer and building of knowledge in this way is what allows us to “See so far by standing on the shoulders of giants,” to paraphrase the words of Issac Newton.
My exchange also illustrates how the collaborative process has been enhanced by modern innovations by allowing me to:
a) Find one paper amongst millions by simply typing in terms related to my research
b) Have that paper available almost instantly in front of me
c) Find out how to contact the author just as quickly
d) Establish two-way contact someone literally on the other side of the world instantly and without spending a cent or leaving my chair.
These points illustrate how the Internet has slaughtered the costs of information transfer. How cool is it that even in the tiny field of electromagnetic actuation, the minimization of these transaction costs allows us to leverage the knowledge of individuals across the globe?
Warning: today’s post is a bit more philosophical than the norm, but it answers an important question that I’m not sure researchers ask themselves often enough. The question is one of the big ones: “why?”
Every so often, I find myself coming up from the depths of the daily research nitty-gritty (did I make a sign mistake here? Why is there so much friction in this pendulum? Is this test going to melt the electromagnet?) and asking a bigger question: “why is what we’re doing important?” Sure, there is something to be said in defense of doing awesome space things for awesome-space-things’ sake, but I like to think there’s more to what SSDS does than that.
The answer to the question has a lot to do with our motto: “doing more – with less – in orbit and beyond.” In the spirit of the motto, our research aims to decrease the cost (in terms of money and resources) of operations in space, or increasing capability while keeping costs the same. Increased capabilities with lower costs will hopefully open the door of space activity to those with more modest funds than the government and large companies that currently have a near-monopoly.
When access to an area of technology is increased from a few to the many, innovation explodes. History is littered with examples of this. (History!? On an engineering blog? Yes – my dirty little secret is that I have a degree in both engineering and history, so bear with me.) The internet (which started as a DARPA project, but didn’t become the provider of knowledge and communication that it is today until the public had access) and computers (which used to be the size of a room and could only be afforded by large companies and the government) are two of the most salient modern examples. Farther back in time, the printing press caused a massive proliferation of different types of writing, rather than just legal documents and transcribed copies of bibles and roman literature.
Looking to the future, the decreased costs of sensors and robots has begun an explosion of different applications as thousands of companies and individuals are now able to experiment, tinker, fail and succeed with them. My hope is that the research done by SSDS and similarly minded labs will provide the stepping stones that will one day add space exploration to these examples.
Why is our research important? We’re working to bring the reaches of space just a bit closer for everybody.
One of the goals for this blog is to use SSDS research and its applications as a lens to keep you updated on larger world of space exploration – from persistant issues to new ideas and everything in between.
Space.com recently published an article outlining a new mission profile proposed by Texas A&M for space junk removal that uses the momentum of one piece of debris to leapfrog to the next piece. In their words:
“The spacecraft would harness the momentum exchanged during both of these actions to cruise over to the next piece of space junk on its list, minimizing fuel use and extending its operational life to the point that such a mission might be practical.”
The issue of space debris is significant enough that efforts to mitigate it are part of NASA’s grand challenges (PDF). The problems posed by space debris have inspired many mission proposals aiming to address them, very few of which have gotten off the ground. Unfortunately, most of the proposed missions have proven technically infeasible given current technology or merely have insufficient payoff for their costs.
I don’t know enough about the specifics of the mission to know whether this proposal falls into one of these categories.
Regardless, assuming the feasibility of this mission architecture, eddy-current actuators could offer a unique advantage. At its core, the mission depends on rather short-time-scale momentum transfer between an uncooperative target and a satellite. Hmmm, that description sounds familiar…
Rather than depending on a satellite designed to be robust enough to survive repeated collisions, the satellite could use eddy current actuators to push off of the conductive surface of the debris without ever physically touching. Simple pushing is a task that EC actuators are especially well suited for. Additionally, the ability of the actuator to operate off of electrical power stored up between target interactions directly lines up with the proposed missions goal of extended operating time.
Space debris isn’t a problem to be ignored and dealing with larger pieces is going to entail interacting with an uncooperative, conductive target – just the job description for an EC actuator.
Recently, our continued efforts on the low-friction test bed have involved a lot of work with sensors and data acquisition. Measuring the state of a system is a huge component of not only spacecraft engineering, but also essentially any scientific endeavor that isn’t purely theoretical. Obviously, it’s hard to quantify what is and has happened without measurements – and most of the time, human senses just don’t cut it. When eyes, ears, nose, fingers, and a notebook aren’t good enough, we have to turn to sensors.
These modern miracles are critical not only to science, but every day life to an ever-increasing degree.
Yet sensors are often chronically ignored or underappreciated by everybody who isn’t actively involved in measuring the data. An analogy could be drawn to how few people think about the lighting in a movie scene. Those outside of the experiment (myself guiltily included) are far more excited by the final data reported (“whoah, you can produce that much force?”) than by explicitly how the data was collected.
There is a common unstated misperception that you just slap on a sensor for the specific type of measurement you want, plug it into a computer and are off to the races. Unless you are a very lucky engineer, this couldn’t be farther from the case, and if it is, it means there was someone else who put in vast quantities of time to create that simplicity.
The number of considerations that go on behind the scenes when sensing is almost as great as sensor applications themselves. So, to hopefully give those of you who haven’t used sensors a peek behind the curtain, I’ll describe a quick example from my project:
Obviously, knowing the force produced on the target cart in our air track experiments is essential to our ability to control that force and create an eddy-current actuator. Even though force is one of the easiest quantities to sense due to the vast number of cheap, off-the-shelf accelerometers, the steps to turn the motion that anybody can see into a data file that contain acceleration versus time is still impressively extensive.
- First, a chip on the accelerometer must measure the voltage changes in a tiny, vibrating, piezoelectric crystal, and convert them into digital 1’s and 0’s (luckily, this part was set up by the manufacturer.)
- An Arduino then has to be programmed to talk to the accelerometer and request that data at a known rate. Knowing and setting this data rate is important because without it you cannot associate the numbers with a timestamp, and half the information is lost, but adds yet another step to the process.
- In order to associate the data with the clock on the wall, the Arduino also has to sense a voltage signal from the computer and integrate it with the data from the accelerometer.
- The Arduino has to communicate this information to the computer that had to be configured to receive it correctly.
- After that, the data from the Arduino (which is in hexadecimal format) has to be converted to numbers that correspond to an actual force.
- Only now can anything like analysis, control, graphs, or human understanding of the data occur.
Whew, that’s quite a lot that has to happen just to get acceleration from an accelerometer! I’m sure many of you have had to go through similar (and probably much more extensive) processes to get sensors to sense in a useful way. If you haven’t, I hope you now have a bit more appreciation for what goes on behind the scenes of sensors.
A quick follow-up to my previous post about applications of eddy-current actuators:
NASA recently released this video of progress in on-orbit refueling.
On top of being an exciting development in itself, it is also another application that could benefit from the capabilities of EC actuators. The video offers a great quote that helps summarize why eddy-current actuators are useful in general:
“Spacecraft usually travel to orbit like museum pieces – they aren’t meant to be fiddled with once they are hung in their final destinations”
In this analogy, you could think of non-contacting actuators like a curator’s immaculate cotton gloves.
With regards to fueling, in the video, you can see that the refueling arm had to approach the fuel cap with no stabilizing interaction between them. Even though they were both mounted to the ISS there was still some relative jitter. Now imagine that the cap and arm are on two entirely separate spacecraft, each with their own dynamics. It would be like you trying to fill up the tank of a moving car without touching it before you inserted the nozzle – quite a challenge.
A non-contacting actuator could provide the benefits of using your other hand to steady yourself without risk of damaging the spacecraft receiving the fuel.
As you can see, practical uses for eddy-current actuators are being developed as you read this very post.