Gypsy, The Hardware, T-6 days…

Gypsy is the second of my two payloads on the HALE mission, and while somewhat less risky, it’s far more complex. It’s based on an earlier prototype I built called “Nadar”, (named after the first person to do aerial photography). The idea was simple: I wanted to make a payload that would control an otherwise unmodified (no cutting & soldering) camera to take both pictures and video. That required not only a small camera, but one the NXT could be used to switch between modes on, from still to video & back again. I ended up with a Nikon COOLPIX L11, which uses a slider switch on the back to switch between modes. With some difficulty, I came up with a LEGO lead screw assembly that would reliably move the switch, and a second motor that could press both the power and the shutter button. That left the third motor to control where the camera was pointed. I played with the idea of a rotating mirror, but the mount for the mirror and the mirror itself represented a significant weight… so I came up with the alternative of tilting the entire platform to pitch the camera up or down (getting the center of mass in the right place is a significant challenge). The result was pretty good, but it couldn’t handle a real mission – the program was too big, any motor stall could freeze up the program, and the structure wasn’t strong enough to handle the several G’s of load that it might suffer during the mission… not to mention the fact that it was uninsulated! So I redesigned it for a live mission.

Gypsy incorporates the strengths of Nadar, but with a far more robust program and numerous sensors. It carries a custom pressure sensor (mentioned before) that has an effective resolution in altitude of as small as 10 cm, and also incorporates a temperature sensor. An accelerometer both logs accelerations on the payload as well as allowing the payload to determine when it is “cut down”. A sound sensor is also built in, both to detect any background noise (I expect very little… but you don’t know ‘till you try), as well as sample specifically the noise level when the motors are running and the loudness of a sample standard tone played from the NXT speaker (this way, if a motor starts to break or degrade, the NXT will literally record the “death cry” of the motor… and the fading of the standard tone with altitude should serve as a second way to judge the air density). The fourth sensor port is connected to a HiTechnic prototyping board, a fantastic device that will be seeing much more use by me in the future. This one board allows me to run a CDS cell light sensor (detecting the outside illumination during the mission), two internal engineering temperature sensors (one on the NXT batteries, one near the camera batteries, and a reed relay to switch an independent heater circuit on and off. By monitoring it’s own internal temperature, the NXT can turn the heaters (there are two separate units) on & off to maintain electronic-friendly conditions in the upper atmosphere, where the outside can reach -60° C or lower. And all that on one sensor port, using just a fraction of the simple input/output abilities of this $40 “sensor”. On a mission like this, an amazing bargain! On top of this, the program autodetects when a motor is stalled, and will not “hang” waiting for a motor command to complete.

Mechanically there are several untested things about this payload. First and foremost is how LEGO pieces, made out of ABS, will handle the bitter cold at 100,000’. The design uses a rectangular frame of CPVC pipe (also untested under temperatures this cold), and suspended from that are cords that attach to the two “hubs”, actually large LEGO turntables, that the pitch motor drives. If the pipe framework fractures in the intense cold, or the LEGO turntables shatter or separate, the entire payload would plummet out of the sky… without the benefit of a separate parachute like Lil’ Joe. So to back up these untested elements, Gypsy also has a more conventional “safety tether” that extends straight through the payload box. Even if the hubs or pipe frame fail completely, the payload box should still remain connected to the main system, to be returned safely to Earth.

I also used a novel method for a “remove before flight” trigger. The HALE team requested that payloads have a “key” or “pull pin” that would trigger the payloads right before the balloon was released. That way all the payloads could be turned on in a “waiting” mode, and a very simple pull of a removable trigger could quickly & reliably start the payload’s mission. I decided to have mine do double-duty: the CDS cell that during the flight monitors the external illumination is covered initially by a piece of Velcro (Velcro also holds the lid on the payload… and idea I stole from the LUXPAK group). When the Velcro strip is pulled away, the drastically increased light level is the cue to the program to begin the mission. This way, one sensor (and one that is multiplexed with several others via the HiTechnic protoboard at that) can serve two purposes, a “start trigger” as well as monitoring ambient conditions.

Another consideration on this mission was weight, so the payload shell is nothing more than a custom-fit, Gorilla-glued Styrofoam… in fact, the same kind you can buy in 4x8 sheet for insulating homes. Using a hot-wire cutter, I first made the internal mechanism, and then carefully constructed the shell around it – making sure I could remove it when I was finished (I used the same technique for Lil’ Joe, except in the end I had to drastically reduce the weight). The result was a close-fitting shell with good strength properties but very light weight, and some insulation value to boot.

Gypsy is really a two-goal payload: versatile camera platform, and dedicated datalogger. Even if the camera portion of the mission were to fail, there would still be a whole lot of very interesting data returned. Now I just have to wait and see… by far the hardest part of the mission.

Next up - Gypsy, the Software

More pictures can be found on my HALE Brickshelf gallery.

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Brian Davis