The View Up Here

Random scribblings about kites, photography, machining, and anything else

Archive for February, 2013

Edison or Tesla?

Posted by Tom Benedict on 27/02/2013

At work and at home, I do a lot of design and fabrication work. It’s straightforward enough to do fabrication work once you have a plan. But any time design work is involved up front, the question comes up: How much do you rely on theory, and how much should be driven by experimentation?

The classic case of this is Edison and Tesla. For a time, Tesla worked in Edison’s lab. Two brilliant engineers with two completely different approaches to their work. Edison was a diehard experimental engineer, and would happily test the most off-the-wall corner cases, just to get the null result that proved they didn’t work. Tesla was more of a practical theorist, and insisted on doing the back of the envelope calculation before considering an idea for testing. Needless to say, the two drove each other up the wall. No event spells this out better than Tesla’s opinion of Edison insisting he test bamboo slivers as a possible filament for the light bulb. I’m pretty sure Tesla pulled out half of his hair before that set of experiments was done.

But there’s a danger in relying too heavily on theory, too. It all boils down to assumptions and initial conditions. The phrase “theory clearly states that…” has been used to shut down a lot of good ideas that probably would have worked. Why? Because the theory made assumptions that weren’t true in the real-world case. And if the theorist who’s busy shutting down the idea isn’t completely aware of the assumptions their theory makes and the initial conditions of the real-world case, they really aren’t in a position to speak.

Nowhere does this come into play quite as obviously as when discussing the stability of a KAP rig. Kite aerial photography is a pretty simple idea: hang a camera from a kite line and use it to take pictures. But when you start trying to stabilize the camera, life gets complicated. And because most textbooks aren’t written with kites in mind, the available body of theoretical knowledge often doesn’t apply the way people assume it does. The forces acting on a load suspended from a kite line are not the same as the forces acting on a load suspended from a fixed point (aka pendulum), or the forces acting on an airplane, or the forces acting on a free-floating body. They’re close. They’re all quite close. But they’re all different enough that the theories just don’t apply. Here are two cases in point from my own mistakes:

A couple of years ago I made a fully gimballed, balanced suspension for a KAP rig. I based the design off the gimbal and sled design of the Steadicam. Hey, the Steadicam has been used in Hollywood since The Shining, and was directly responsible for some of the most innovative camera shots in the last three decades. They have to know what they’re doing, right? (Theory) And a KAP rig is just like someone holding a Steadicam gimbal, right? (Assumptions)

Wrong. A person holding a Steadicam gimbal has six completely independent degrees of freedom in their hand: translation in X, Y, and Z, and rotation about X, Y, and Z. A kite line can translate freely in two axes X’ and Y’, and is semi-rigid in a third axis, Z’, oriented along the kite line. Translational forces are relatively slow acting in X’ and Y’, but very rapid with very high impulse in Z’. A KAP rig is free to rotate about Z’, but not about X’ or Y’. Typically it can rotate around Y, though. Since the camera is in the XYZ frame rather than the X’Y’Z’ frame, there’s a coordinate system rotation that needs to be taken into account. But since the XYZ and X’Y’Z’ frames aren’t orthogonal to each other, this means at least two of the axes will couple. How this worked out with my fully gimballed KAP suspension is that any sort of rotation about Z’ led to a very rapid very high impulse rotational force around Y. It rendered the rig less stable rather than more stable. It’s not that the theory behind the Steadicam was wrong. It just didn’t apply to KAP. The real-world conditions didn’t match up with the assumptions.

My second example is a lot shorter: Some years before this test, there was a discussion about using a MEMS gyro to provide feedback to a servo to stabilize a KAP rig. I was one of the ones that said it wouldn’t work, and that a 50Hz servo loop was simply too slow to take out an appreciable amount of motion. Control theory clearly stated that it simply couldn’t keep up – that the idea wasn’t even worth pursuing. (I think that was how I worded it at the time.) Thank goodness someone ignored me and decided to try the experiment anyway. Not only did it work, it worked well. It worked so well and was so feasible, he made some custom boards to replace the PCB in the servo that included the MEMS gyro. Voila! A self-contained gyro-stabilized servo. And so the GS-1 servo was born.

In this case it wasn’t the theory that didn’t apply. It was the windbag (me) who insisted that the theory said something that it didn’t. The lesson here is to beware the expert who thinks they’ve understood the problem well enough to do their back of the envelope calculation. If their understanding is flawed, so will the theoretical results.

All of which goes to explain why I lean toward experimentation rather than raw theory. It’s still possible to make a bad experiment, but it’s darn near trivial to mess up a theory and fool yourself into thinking something is true that isn’t. Given the choice, I’d rather pull out the bamboo slivers and pass some current through them, just to make sure the answer really is no.

– Tom

Advertisements

Posted in Engineering, Kite Aerial Photography | Leave a Comment »

Redesigns, Redesigns, and More Redesigns

Posted by Tom Benedict on 26/02/2013

That’s been the story of my life recently. So here are three redesigns I’m going through right now, plus a new design I’m working on:

Redesign #1 comes from work. So does redesign #2. Since they’re related, I’ll tell them together. The cameras we’re making for this new instrument need a charcoal getter, and a way to attach the cold strap to the getter. I’d designed some really pretty two-sided getters, complete with sub-micron filtration to keep the charcoal dust from getting on the camera electronics. Even better, I’d designed a new way to attach the cold strap to the getter using magnets. Yay! New designs are cool! Unfortunately the new getter took up space we needed for the electronics, so I had to machine one side of it off. And the magnetic cold strap just didn’t give us the positive attachment we needed.

My new getter design isn’t as pretty as the old one, but it’s dead nuts easy to make, and can be made using a conventional lathe and mill. No CNC required. Also no bolts. Its various parts are attached using the same Stycast potting resin we’re using on our feedthroughs. Our fastest cool-down to date only pulls three Kelvin per minute, so we don’t even face thermal shock issues. Even better, it has more volume than the original getter, and should be able to pull harder and last longer.

The cold strap attachment moved to a single bolt that pushes a wedge into a groove. Its bizarre nature is only made more strange by how you actually get to that bolt: you have to remove the vacuum gauge, pull the gauge baffle, and stick a 2.5mm ball head hex wrench in through the vacuum port. Not my favorite idea. But I really had no other way to pull it off. The camera had initially been designed around a spring and cone arrangement that was shot down during the preliminary design review. The magnet and bolt-wedge ideas were hacks. But the bolt-wedge one worked.

I start machining the final parts on each of these tomorrow morning.

Another project I’m working on on the side had its own redesign. But this one appears to be working first try. It’s a 6WD platform for moving a camera around via remote control. The problem is the platform has a wonderfully soft suspension with an almost unbelievable amount of hysteresis built in. So it never leaves the camera in a level position, even when it’s on flat level pavement. Rather than the pan/tilt head that was originally planned, it needed a pan/tilt/roll head. Making everything fit has been… interesting. But I think I finally got it to work. I’m finishing the wiring now. Once it’s done, I should be able to hand it over for testing.

The problem with any redesign is that it is essentially a hack job at that point. If it wasn’t, it would’ve been designed right in the first place. The trick is to make it look and behave as if you’d designed it that way on purpose.

Or…

Design it to behave like Legos or a Meccano set. That way when you build something that works, it looks like you did it on purpose from the get-go, regardless of how much redesign work you had to do. This is why I like the KAP bits from Brooks Leffler so much. Each part has a ton of holes in it for attaching all manner of things. And if the right hole isn’t there, it’s trivial to drill a new one. It’ll fit right in with the others! This is the approach I’m taking for a new project.

 

The New Project – KAP Pendulum Suspension

This one has been a re-re-re-redesign project from the start. I’ve been drawing a KAP pendulum for years now – more years than I’ve been doing KAP. But I think I understand enough of the issues involved to finally make what I really wanted from the beginning: a KAP pendulum Lego set. Here’s the basic idea:

So far I’ve used Picavet suspensions on my rigs. They’re great: they’re lightweight, portable, self-damping, and not that hard to use once you get the trick of storing your lines afterward. But they don’t constrain the rig’s pan rotation enough for my taste, they suffer from “nodding” if a kite starts pumping the line, and when the kite line approaches vertical, they just plain stop working.

So I want to move toward a pendulum. But I’m dissatisfied with most of the pendulum suspensions I’ve seen. The pivot axles tend to be wobbly so the pan axis is still under-constrained, they tip the rig when the wind blows on them – similar to a Picavet, and they have oscillation issues of their own. But I still think they’re the right approach to take. What’s needed is some serious experimentation time in the sky.

The pendulum rig I want is collapsible for storage and transport, light(ish) weight, has highly constrained pivots (zero side-play if I can pull it off), and includes the capabilities to have damped pivots, a parallelogram pendulum, a double pendulum, counterweighting, and anything else that strikes my fancy as I go along. I want all this while still making it look like I made the thing on purpose so that by the time I finish, it all looks intentional.

Too much to ask? Surprisingly, no. With the possible exception of the light(ish) weight, I’ve managed to design in all of this in a set of fairly easy to manufacture parts. Pivot points are all double-row precision ball bearings. There is provision for single pendulum, double pendulum, a parallelogram arrangement, and tunable damping on the major pivot joints. I’m finalizing the design for a damped but still positively locating pan joint as well, and have started drawings for a counterweight system I’d like to test for damping rig roll as well. If this set of tools doesn’t let me explore pendulum suspension, I don’t know what will.

To put the icing on the cake, there’s one last bit I want to play with once the pendulum experiments are done. A number of people are offering flight stabilizers for fixed wing RC aircraft. One in particular, the i86AP flight stabilizer board from Hobby King, has been tested as a camera gyro controller. Each axis has tunable gain, so you should be able to adapt it to most KAP rigs. And at 17.99USD apiece, it’s an inexpensive bit of gear to test.

So I might finally be getting closer to my holy grail of KAP: nighttime city skyline panoramas from a kite. Only time will tell.

– Tom

Posted in Engineering | Leave a Comment »

Brief RC Update

Posted by Tom Benedict on 19/02/2013

Still swamped at work, so I only have a brief update.

I flew at a new location last weekend. There’s a pu`u (aka “cindercone” aka “hill”) near Kua Bay that’s used by a bunch of the local RC sailplane pilots. I’d hoped to do some slope soaring, but the wind simply wasn’t there. So I flew with motor instead.

It’s a neat location because you can fly well below the horizon and still be over a hundred feet off the ground. Looking down on my plane as it flew by was a novel experience! Once you gain some altitude, there’s gobs of room to recover from mistakes, so I tried loops, snap rolls, split-s, stall turns, everything I could. I actually started to get some of them right by the end, but even the ones I got wrong were easy to recover from with that much space to play in.

I’m still trying to get a feel for how much battery I use when I’m primarily gliding, so I landed a couple of times just to check. There are really two summits on the pu`u. The higher one doesn’t have much room to land, but the lower one has a long stretch to work with. I was, of course, flying from the higher summit. Toward the end of the session I decided to try something new: catching my plane instead of landing it in the dirt. I wound up coming in in a cross-wind, but the flaps on the Bixler 2 let me slow it down enough that catching it was almost a no-brainer, even while crabbing into the wind. Yaaaay! I learned something new!

For the last flight of the day I taped a rock into the cockpit. This probably sounds weird, but I had good reason: The rock was a dummy weight with roughly the same weight as my Gopro Hero3. I still haven’t worked out a good mount for it, but now I know I can mount it in the cockpit without skewing my CG too much. It needed a little elevator trim, but nothing too extreme. With the plane re-trimmed, I was back to neutral stick.

So the next time I go out, the Gopro will come with me. And maybe I’ll be able to catch it and save it from a little dusting. Should be fun!

– Tom

P.S. Sorry, no pictures yet. I’m too busy flying to run the camera!

Posted in RC Airplanes | Leave a Comment »

I Knew It! – Where RC Airplanes are Leading Me

Posted by Tom Benedict on 13/02/2013

I’m starting to see a pattern…

When I was a kid, I loved kites. The idea that something that had no power source of its own could fly was fascinating. It was even more fascinating to tie payloads to my kite string and lift them as well. (Those little green baskets that strawberries used to come in were awesome for making gondolas!) I should have known it was a sign of things to come.

Years later I got into sailing. I’d been on power boats, but they just didn’t do it for me. Again, I think the fascination had to come from the idea of being in a vessel that had no inherent power source, but that still moved through the water with ease. No surprise, my wife and I gravitated toward catamarans – some of the most efficient sailing vessels ever made.

When I got into aerial photography, I guess it was only natural that I returned to kites, albeit seriously upgraded from the ones I used as a kid. Using kites for aerial photography offers serious operational benefits, such as the ability to sit in one location for hours, if necessary, until the light happens. But it has non-photographic benefits as well that go back to why I got into kites and sailing in the first place: the silence, the power, the peace.

And now I’m getting into RC airplanes. For a variety of reasons the plane I chose has a large high-aspect wing, low wing loading, and a decent glide slope. These are all features you want when sticking camera gear on an airplane. But they benefit something else as well: using the plane as a glider. And the more I fly it, the less inclined I am to use it for aerial photography. It’s just too fun to fly! And the more I fly it, the more I find myself cutting the motor entirely and gliding. Just like flying a kite, flying a glider is therapeutic.

The problem has been finding places and times when I can fly. The wind around my house ranges from moderate to torrential. But it’s almost never zero. When I got my plane I thought that kites and airplanes would fill two different parts of the wind regime: zero wind = airplanes, moderate wind = kites. Too much wind means the plane stays home. And there’s almost always too much wind. I thought I was stuck. But all that changed when I discovered slope soaring.

When wind encounters a slope, hill, cliff, or even a generously sized building, the wind has to go up and over the obstruction. This creates an upward moving body of air just in front of and above the slope. Put an airplane in that body of air, and it flies. And since energy is being pumped in in the form of upward moving wind, the airplane can theoretically fly indefinitely, just like a kite. This works so well, the airplane doesn’t really need a motor at all. All it needs is enough oomph to be able to use the lift from the wind and turn it into speed.

There are, of course, idealized regimes of airplane design that lend themselves to slope soaring better than others, just as there are idealized regimes of automotive design that lend themselves to racing on flat pavement better than others. But any car will roll if you position it at the top of a hill and release the brake. Likewise, practically any RC airplane can be used for slope soaring. It may just require a slightly different setup than what you’d normally use for open field flight.

It turns out the Bixler 2, my first and so far only RC airplane, makes a darned good slope soarer. I discovered this when I couldn’t find a good place to fly. I finally wound up at the cliffs above Hapuna Beach where a moderate on-shore wind was blowing. The cliffs make a decent slope, and the almost completely laminar on-shore wind make for wonderful flying conditions. I wound up flying for more than half an hour, and only had to use my motor a couple of times to correct my rookie mistakes. When I put the battery on the charger afterward, I was amazed to see it top off with only 330mAh of charge. It’s a 2200mAh battery. I could’ve gone for hours.

When I got my plane, I felt a little dirty – like I’d stuck a motor on a sailboat or put a propeller on a kite. I should’ve known it wouldn’t work out like that. Instead, I found a way to make the airplane fit right along with everything else. It doesn’t matter if it’s kites, boats, or airplanes. It’s all about the wind. It’s always about the wind.

– Tom

Posted in Kite, RC Airplanes, Sailing | Leave a Comment »

More Changes to Video Hardware

Posted by Tom Benedict on 02/02/2013

I recently returned to working on another non-KAP photography project that requires using a DSLR with a wireless video link. And though there are a number of differences between this and my KAP rig, there are some similarities as well. One idea that dropped out of the process was that there’s no reason I shouldn’t be able to use any of my cameras with my KAP video hardware. After all, the video hardware for this other project came with a bunch of cables that’ll plug into a range of cameras. Why not do the same for my KAP rig?

To date I have three cameras I’m actively using for KAP: my venerable Canon Powershot A650IS running CHDK, my Canon EOS T2i, and my new Gopro Hero3 Silver. As of last night, only the T2i had a video cable built for it. And as of last night there was no provision for making new ones. The T2i cable wired directly into the video transmitter.

A quick search on video cables for these cameras brought up some cool options. The Hero3 has a firm following among the RC airplane and helicopter FPV crowd. Since weight and space is a premium on board an RC aircraft, the cables available for the Hero3 are some of the lightest around. This one from FPV Hobby is one of the nicest I’ve seen. It’s a right-angle USB connector with a short length of three-conductor cable, terminating in two (two?) servo-style connectors. Hey! I’ve got plenty of servo connectors! What a great idea!

I used the same color code convention as the Hero3 cable from FPV Hobby: Black = Ground; Red = Audio; White = Video. Unlike the FPV Hobby cable, though, I put all mine on a single connector. In short order I’d cut my T2i cable, crimped in a pair of connectors to plug it back together, and then made a second cable for my A650IS. Voila! Interchangeable video cables!

Video Cables

While I was doing the work on the cables, I decided it was time to permanently mount the video transmitter on my rig. There are lots of options for packaging electronics on a KAP rig: real enclosures, Altoids tins, or just attaching the thing to the rig with Velcro. I went with another option on the video transmitter. It was already wrapped in clear heat shrink, and had an RP-SMA antenna connector on one end. SMA connectors require a 1/4″ hole for panel mount connections. Adding the appropriate hole to the Brooxes hardware on my KAP rig was a simple matter of marking and drilling the hole. Once the transmitter was in place, a lock washer and SMA retaining nut finished the job.

Video Transmitter Mounted

In an earlier post I mentioned I’d gutted my video power system in favor of dedicated LiPo batteries. This also appears to have worked out pretty well. The transmitter and receiver are both powered by a pair of 500mAh 3S batteries. I have no idea what the run time will be in the field. There’s a fair bit of testing to be done before I have a good figure for that. Meanwhile I’m using a pair of battery monitors similar to the ones I use on my plane to make sure I don’t drain them too far.

This has made the video receiver a nice neat little package on my RC transmitter. The monitor bolts onto a monitor arm that in turn is bolted onto the transmitter. The battery and video receiver are both attached to the back of the monitor. I got tired of the heavy wiring that came with the monitor and video receiver, so I wound up hard-wiring everything with a minimum use of connectors.

Video Receiver and Monitor

I’m almost 100% happy with where this KAP rig is now. There’s only one thing left to do: I’d either like to replace the 4xAAA battery pack that powers my rig with a second 2S LiPo, or I’d like to get rid of it altogether by using a UBEC to draw power from the video battery and supply the rig with 5V power. Using a second LiPo has a lot of appeal since it would let me continue to use the rig even after the video battery was drained. But the shutter cables I use can’t be powered with more than 6V or they blow. I’d have to wire a voltage regulator in line with the shutter cables or risk (no… guarantee!) frying them. For obvious reasons I’m leaning toward the UBEC.

It’s funny to look back and see how many iterations I’ve gone through with this KAP rig. It’s the same one I started with in 2007 with a Nikon Coolpix and a 72MHz radio. Since then I’ve changed cameras more than once, added and removed a third axis for plan rotation, added and removed video hardware more than once, and replaced more leg brackets than I can count. I even crashed it once. Hard. But it bent right back into shape. You just can’t beat it.

– Tom

Posted in Engineering, Kite Aerial Photography | 4 Comments »

Potting Vacuum Feed-Throughs – Take Two

Posted by Tom Benedict on 01/02/2013

In the middle of all the other recent disasters at work, I had a personal disaster of my own: I made a feed-through that leaked. This may not sound like much, but it brought a project to a screeching halt while we all tried to figure out what went wrong.

Just to back up a little, I’ve been designing and making vacuum feed-throughs since 2002. I’ve lost count of how many I’ve made. Most were electrical. One was fiber optic. One was mechanical. And until this month, not one ever leaked. Not one. So I thought I could reasonably say I knew what I was doing. After this failure? I wasn’t so sure any more.

The good thing about working with a bunch of really sharp people is that as long as you’re willing to set your own ego aside, you have almost limitless opportunities to learn. This was one of those times. Everyone involved brainstormed, scrambled to learn more about the materials and techniques we were using, and joined in to pool their ideas and see what we could come up with. We talked, we tested, and we repeated our tests until we were confident we could move forward. All of us learned something new.

I think I have a reasonable handle on what I did wrong, but a lot of other good changes came out of the process as well. We wound up incorporating quite a few of these into our procedure, so I thought it was worth posting it. So without further ado, here’s the re-write of my earlier article, Potting Vacuum Feed-Throughs:

 

A vacuum feed-through consists of three parts:

First is the thing you’re feeding through the vessel wall. In most cases this is an electrical signal. In others it’s optical. It’s typical to have a connector on one or both sides of the feed-through, though this isn’t always the case. If a connector is used, the easiest way to do this is by using a hermetic vacuum-rated connector. Most of the time these contain the provision for o-rings, so this may be the extent of the feed through since it supplies the other two components I’m about to go into.

The second part of a feed-through is some sort of mechanical shell that will actually connect to the vacuum vessel. In the case of a hermetic connector, this can be the connector itself. In the case of a non-hermetic connector, or a connector that doesn’t readily mount to the outside of the vacuum vessel, it’s typical to make a custom machined part.

The third part of a feed-through is to have some means of making a gas-tight seal between all the bits. In the case of a hermetic connector, that’s the o-ring. In the case of a custom feed-through shell, typically it’ll involve an o-ring in a groove that will bear on a flat surface on the cryovessel, or vice-versa, and some means of sealing the wires or fiber optics that pass through it.

The easiest way to accomplish that last part is to use some sort of encapsulant like a resin. Resin won’t work with extremely high vacuum situations, which often use glass as the encapsulant, but it’s good for most instrumentation purposes. The resin we use is Stycast 2850FT, which is black, optically opaque, a good electrical insulator, and a decent thermal insulator as well. We use a Stycast 24LV catalyst, mixed at 7% to the 2850FT by weight. 24LV has good curing characteristics, and is a low-volatile catalyst, meaning it won’t continue to ooze organic vapors into your vacuum system once the potting is complete.

Stycast is neat stuff to work with, but in order for it to make a good potting encapsulant there are some procedural details that need to be followed closely:

  • Surface prep is king. Let me repeat that: Surface prep is KING! We degrease everything using an ultrasonic cleaner and a solvent appropriate to the part, and give every part a final alcohol clean just prior to assembly. Dirty parts make crappy feed-throughs.
  • We store our Stycast in a dorm fridge. This gives it a longer shelf life, but it also makes it hard as tar. Stycast needs to be heated to room temp or higher in order for it to mix with the catalyst. We cure our parts at 45C, so we pre-heat our Stycast to 45C as well, prior to mixing.
  • Once the Stycast is heated and combined with 7% 24LV catalyst, it must be mixed for five full minutes in order to ensure a homogeneous solution. The pot life on this stuff is close to an hour, so you have the time. Take it.
  • After mixing the solution must be degassed. This is a vacuum application, after all, and virtual leaks are the bane of a vacuum system. We use a small dessicator hooked up to a two-stage roughing pump. The Stycast 2850FT datasheet says to degas at 1-5 torr for 3-10 minutes. We degas for five minutes. The Stycast will foam like nuts when you pump on it, so you need to keep a steady hand on both the vacuum and the bleed valves. The trick is to foam it up and slump it a couple of times to get the bubbles on the top, then foam it up and hold it there for the remaining time.
  • There are tricks you can do with pouring resins to minimize trapped gas as well, but I won’t go into those. Look for a good manual on casting plastic parts using resin. We almost exclusively inject our feed-throughs using syringes loaded with Stycast, so almost none of the pouring techniques apply to our procedure.
  • When preparing the feed-through for encapsulation, it’s important to protect any sensitive surfaces from accidental spillage. Blue painter’s tape does a great job of covering o-ring grooves, screw holes, or other places where an accidental blob of Stycast could be a real headache to remove later.
  • If the geometry of the feed-through cavity is complex, it’s best to fill the small voids first using a syringe loaded with the warm Stycast. This avoids trapped volumes of air. It also provides an easily controlled flow of resin, which is why we started using a syringe for all our feed-throughs rather than a direct pour.
  • Finally, fill the cavity until you’re satisfied with the level inside. If you use idea in the next step, it’s important not to over-fill. I now leave at least 3mm of head space below the top of the cavity or dam that I’m filling.
  • At this point you need to pop it back in the oven, and heat to 45C until cured. But there’s one more idea we picked up: Careful mixing and a thorough vacuum degas should remove any trapped air in the Stycast. But any errors in pouring can re-introduce air bubbles that can provide leak paths through the cryovessel wall. Vibrating the part while curing can help release trapped bubbles. We built a shaker table that fits into our oven to vibrate our parts while they cure. This was nothing more than a quarter inch aluminum plate, mounted on spring legs, with a motor mounted to it. The motor has an offset weight attached to its output shaft. A variable power supply and the ability to change the offset weight lets us dial in the frequency and amplitude of the vibration.
  • I like to make two witness samples, one that goes inside the oven and one that stays outside. In the past I’ve used small dabs of Stycast on something flat like a chunk of metal. But cure time depends on the depth of the Stycast. It’s better to use a witness that at least approximates the rough shape and size of the feed-through. When both the inside and the outside witness samples have hardened, you know your feed-through is completely cured.

Don’t rush the curing process, if you can help it. A feed-through should last essentially forever. Rushing it and flexing semi-hardened resin so that you create a trapped volume or worse yet create an air path through the thing ruins a lot of hard work. Be patient!

That’s it. Be clean, be careful, and enjoy your vacuum.

– Tom

P.S. No, I never mentioned what went wrong with my recent failed feed-through. Any guesses? It was step #1: surface prep. The shell had been cleaned thoroughly in an ultrasonic cleaner, but the electronics had not. That was enough to cause an adhesion problem between the Stycast and the insulation on the wires. All it takes is one hole a hundred time smaller than a human hair to kill a vacuum. Surface prep really is king.

Posted in Engineering | Leave a Comment »

Silly Kite Fun

Posted by Tom Benedict on 01/02/2013

I haven’t written much recently because work has been kicking my ass. In the last month our dome has stuck open, we’ve had a cracked water pipe that resulted in a full-building flood, we had multiple electrical fires because of flooding in the breaker boxes, and we had all the associated clean-up you’d expect from a month like this: total network failure, failed cryo systems, failed vacuums on every instrument in the facility, and one heckuva mop job to get all 1200 gallons of water out of the building. It’s been a nightmare. What’s a guy to do?

Fly kites, of course!

One of the weekends (I think the one before the flooding began) I was at the beach with my family with a pretty minimal KAP setup and only three kites. The light wasn’t great, but I put up a kite anyway. Sometimes it’s nice just to put a kite in the air and hang onto the string.

But it’s also nice to complicate things needlessly, all in the name of fun! I had a short leader in my bag, so I tied it onto the line and tied on a second kite. And when those two flew nicely, I had to put a third on the line just because I could. Then I thought it would be a good idea to relax a little more and fly all three kites with my foot.

Three Birds on a Wire

This isn’t the first time I’ve flown a kite with my foot. I think the first one was during World-Wide KAP Week 2009. But it’s been a habit ever since. My wife never ceases to be amused and confused at me for doing this. After a few minutes she said, “Why not fly a camera, too?” So I did!

Three Birds The Moon and A Camera

So there I am in the middle of building-wide failures at work, flying three kites and one camera at the beach with a silly grin all over my face. Little did I know I’d be fighting fires the next day. Literally.

– Tom

Posted in Kite | 3 Comments »