Category Archives: Tech
No Glasses Necessary for This 3D Technology
What Is 3D Printing? And How Does It Work?
3D printing is exactly what it sounds like – using a machine to create a three-dimensional object out of thin air. Well, not exactly thin air. They use everything from metals to ceramic powder. But the process does look like magic. (Before I go on, it’s worth noting that 3D printing goes by a number of different names, including: rapid prototyping, direct manufacturing, solid freeform fabrication, and additive manufacturing. I’ve chosen to use the term 3D printing, but they all mean approximately the same thing.)
I suspect that this is the sort of thing that, regardless of any identified purpose, would be useful in ways you can’t imagine until you have it and have learned how to use it.
Pole Dancing — With Quadrocopters
Video: Throwing and catching an inverted pendulum – with quadrocopters
As you can see in the video embedded above, at the end of Dario’s thesis two quadrocopters could successfully throw and catch a pendulum.
Many of the key challenge of this work were caused by the highly dynamic nature of the demonstration. For example, the total time between a throw and a catch is a mere 0.65 seconds, which is a very short time to move to, and come to full rest at, a catching position.
Another key challenge was the demonstration’s high cost of failure: a failed catch typically resulted in the pendulum hitting a rotor blade, with very little chance for the catching quadrocopter to recover. A crashed quadrocopter not only entailed repairs (e.g., changing a propeller), but also meant recalibration of the vehicle to re-determine its operating parameters (e.g., actual center of mass, actual thrust produced by propellors) and restarting the learning algorithms.
CSI:Time Machine
It was essentially a criminal justice Dewey Decimal System, the first step in taking police out of the dark ages. Before Bertillion standardized measurements, police just had a jumble of descriptions and photographs with no way to organize them so they’d almost never be able to cross reference existing records when people were arrested.
Fingerprinting isn’t the only technology difference in criminal investigation, of course. Much of forensic pathology dates from after this time.
Why Don't These Things Cost $50k?
Pop quiz, hotshot: Your really long optical fiber isn’t letting (much) light through, so there’s obviously a break in it somewhere. You need to fix the fiber. What do you do? What…do…you…do?
Obviously, shooting the hostage is not an option here. The fiber is probably buried underground, so it would be really helpful to know where the break is, to a resolution of at least the location of the nearest manhole, so you can go in, find the fault and splice the fiber. The solution is an optical time-domain reflectometer (OTDR). You send a pulse of light down the fiber and measure the delay of any reflection, because breaks (and other faults) tend to reflect the light, as any change in index of refraction causes a reflection. Since the speed of light transmission in a medium is simply c/n, if you can measure the return time of the pulse you can figure out how far way the fault is.
To do this in a helpful way, though, one needs to locate the fault to within a few meters, and light in a fiber will be traveling at around 200,000 km/sec, or 5 nanoseconds per meter, which means we need timing at a level at around the 10 nanosecond level. That sounds like the precision realm of commercial atomic clocks, and that sounds expensive — that kind of clock can run you several tens of thousands of dollars. But there’s an important distinction: an atomic clock gives precision long-term timing, and we don’t need that. If our optical fiber is 100 km long, a round-trip signal will take no longer than a millisecond. In other words, we don’t need a clock that will add fewer than 10 nanoseconds in a day, we just need one that won’t add more than 10 nanoseconds in a millisecond. There is almost 8 orders of magnitude difference in performance in those two systems. Put another way, we don’t want to measure the time, we want to measure a short time interval. A timing error of 10 nanoseconds in a millisecond is 10 parts-per-million, a performance that is easily reached by a cheap quartz oscillator (Here’s a cheap system that does 2 parts per million along with some extra functions we wouldn’t need). As long as the oscillator is calibrated, such a device would be just fine for this task.
Another example of this time interval application is a GPS receiver. These receivers compute your location based on the time difference between signals from multiple satellites, but since the satellites have precise clocks on them and broadcast that information, the receiver only has to measure the difference in those time tags. GPS satellites orbit at altitudes of around 20,000 km, but it’s the differences in the distances that are important to us. Overhead satellites are closest, while ones nearer the horizon are farther away, by a few thousand km. That’s a factor of ~10 greater distance than our OTDR signal (though our speed is very close to c), and we want somewhat better timing, so that puts our needs closer to 0.1 ppm, but this is also achievable, though undoubtedly a little more expensive. The great part about GPS receivers, though, is that you can actually use the timing signals to synchronize a local clock, and gain the benefit of the atomic time on the satellites, which is synchronized to the earth’s atomic time, UTC. (You might recall that such synchronization was initially — and incorrectly — blamed for timing errors in the superluminal neutrino story a little over a year ago. It’s actually quite good.)
Tickling the Ping Pong Dragon
The video may be a tad long for some; if you want to get to the good part, it’s at 4:55m which is the normal speed shot, followed by the 500 fps slo-mo.
One can model the process in a reactor, though this is more like a bomb. In reality one would distinguish between the effects of “fast” neutrons (i.e. ones with significant kinetic energy) and thermal neutrons, which are generally of such low energy that the KE’s contribution to the reaction is negligible, but I’ll ignore that here for simplicity and because the analogies don’t really hold up.
If we consider what happens to a ping-pong ball, we can model the behavior. Once you have a ball in motion, it can physically escape from the region, or “leak” out. The probability it does not leak out — the non-leakage probability — is L. Since the walls are not of infinite height and there is a hole in the surface, L < 1.
If a neutron does not leak out, it might hit a trap and cause it to snap. The probability that this happens is f, which we will call the utilization fraction. This depends on the ratio of the probability that the ball hits a loaded trap and causes it to snap, to the probability of just hitting a trap — in this model we would have to count a ball that lost all of its energy before hitting any trap as one that has leaked. (edit: and also include the probability that a ball could cause more than one trap to trigger)
In this example, a snapped trap gives us two new balls, but in general we can model this reproduction factor as some value n. In real fissions, n depends on which fission products you get.
So we have K = Lfn
If K = 1 the system is critical and the population remains constant. If K > 1 the system is supercritical and the ball population increases, and similarly if K < 1 (subcritical) the population drops. Obviously, at the outset of the video K is significantly greater than 1 since we see a rapid rise in ping-pong-ball population, but as the traps "deplete" f drops, because the number of loaded traps is getting smaller. This rapid change in K makes this more like a bomb, where you are trying to get a lot of interactions in a very short time, while in a reactor you'd like to get up to some reaction rate and maintain K = 1 for a long time.
In a real reactor, the number of neutrons is much, much smaller than the number of fuel nuclei, so this depletion is at a much smaller rate. Imagine, though, if we could construct a system that lasted just a little longer. Since the initial rate of ball multiplication tends to be higher, you could "poison" the system by adding a sticky blob here and there that would grab and hold a ball if it struck, but these losses would diminish over time as you used up the blobs. Reactors do this with Boron-10, which quite happily absorbs neutrons, but only once — B-11 is much, much less likely to undergo this "capture" reaction.
Another effect would be to initially lower the wall(s) and let more balls escape, and raise them over time, meaning that your non-leakiage factor is compensating for the drop in the utilization fraction. The changing wall in this case would be the effect of changing the height of the control rods, though in a real reactor this exposes more fuel, making the reactor effectively bigger, but also harder for neutrons to leak out. Real reactors are often designed so that as the reaction rate drops, the water that thermalizes (slows/moderates) the neutrons undergoes a drop in temperature and becomes more dense, which traps more neutrons in the core — another effect which reduces the neutron leakage.
The Tell-Tale Strontium Heart
Beating heart of a quantum time machine exposed
A little vacuum system porn for you.
The lasers are fired through three of the glass shafts emanating from the cube, but must be carefully directed out of the other side to prevent them scattering within the clock, which is why there are six shafts in total.
However:
… the beating heart of a time machine! Or “clock”, as most people call them …
… or possibly “frequency standard” as I like to pedantically point out. Though this being an ion clock, it can probably run for extended periods of time, and one might actually be able to say it’s running as a clock.
I also find the description of the six arms to be curious; normally, trapping schemes send light in both directions. It’s true you don’t want the light scattered in the chamber, but the description implies there are only three, and none of the NPL write-ups I have read say anything about a novel cooling geometry requiring only three beams.
Aaand it gives the Sr transition frequency as an exact number. There should be an uncertainty, since it’s the Cs hyperfine transition which is defined.
So read it for the picture, and not so much the article.
Oh, If Only I Had More Money Than Brains
… because I’d buy a B9 Lost in Space robot. Because it’s under $25,000!
via Bad Astronomy, (where there is a video, which I can’t seem to embed in the post. Due to Dr. Smith’s meddling, no doubt)
Feynman and the Shuttle
Richard Feynman and the Space Shuttle Challenger investigation
Short video and an excerpt from his report at the link.
Clear thought, clear writing. Feynman was perhaps the most efficient mechanism ever conceived for consuming complexity and pumping out simplicity.
Amen to that, though we have some good ones today, too.
Monocoil…Monocoil…Monocoil
Don’t try this at home, unless you’ve got more skill than is portrayed here