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.
A description of the latest research rabbit hole over at SSDS: where could you put a ping pong ball on the outside of the ISS, let go, and have it stick thanks to orbital mechanics? There is math and pretty pictures.
It’s also a test of equations in wordpress. So far, so good.
Sometimes, it feels like non-computer technology has kind of stagnated. Then you see
Robot police in Africa…
and Fantasia in real life
My analysis? Most of the hardware innovation doesn’t permeate our everyday lives the way software does. Instead, it’s working behind the scenes to make the same physical things cheaper or easier.
Today I inadvertently ran a human toxicity trial with N = 1 data points. I’m calling it ‘the effects of way too much black pepper on human physiology.’ The conclusion is that you should ALWAYS make sure you have the side of the the pepper container with many little holes, rather than one big hole open. If you’ve ever wondered what happens if you eat several tablespoons of cracked black pepper in more detail, read on.
There’s a reason it looks like the Death Star
I ate the pepper around 8:45 pm and noticed nothing before going to bed. However, I woke up the next morning around 7:00 am and thought ‘weird, it feels like my face is on fire.’ The effect was similar to eating something extremely spicy – you feel hot and flushed, sensitivity in the back of your throat, and nothing tastes quite right. Except it has lasted all day (due, I’m guessing to the time-release effects of digestion.)
The heat in my face seemed to come straight from my extremities, which alternated between weirdly numb and just uncomfortably cold. My eyes felt like they do when you’ve had them open underwater far too long. I think the pepper actually acted as a stimulant, because it affected me very similarly to a strong stimulant (even before I had coffee) – giving me tons of energy when I started moving, followed by a spiraling crash.
That being said, the symptoms died down over the past 15 hours, so I assume there are no lasting effects.
I thought this would be worthwhile to write up because a search for ‘black pepper overdose,’ ‘black pepper side effects,’ and ‘black pepper toxicity’ yielded nothing useful. Also as a warning: if you accidentally dump a massive amount of pepper on your dinner, throw it away. It isn’t worth it.
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]
Whenever I explain my research, the conversation goes something like
Me: “I’m working on a concept for a contact-less spacecraft actuator”
Them: “A what?”
Me: “*sigh* I’m trying to make a tractor beam.”
That, combined with our lab’s youtube videos being listed along with the perpetual motion machines always makes me feel a bit quackish.
But when I outline using EC actuators to take advantage of an object’s natural tendency to fall up on the underside of the ISS, I’m talking about legit science, I swear!
Have you ever gone through the Wikipedia disambiguation pages for the greek and Roman characters? If not, you should.
Even if you narrow your scope to just a single discipline like Mechanical Engineering, the same symbol can mean multiple things (G can be both the gravitational constant and electrical conductance.) At the same time, the same thing can be represented by multiple symbols – depending on which professor you ask, the name of the angle on the latitudinal plane in spherical coordinates can be either φ or θ.
This isn’t a problem that has a solution – there are only 50 characters between the Greek and Roman letter systems.
Instead, there’s a deeper lesson: chances are, an assumption that what you think is an unambiguous implies something entirely different to someone else. If this can happen with equations (normally held up as the paragon of unambiguity) think about what happens in English.