From b83d8eca433ed5796835f6a09271f50c7f27cc81 Mon Sep 17 00:00:00 2001 From: Keith Packard Date: Mon, 22 Aug 2011 17:18:02 -0700 Subject: [PATCH] doc: Add Installation Recommendations chapter Document installation suggestions, including mounting, RFI, antenna issues and ground testing. Signed-off-by: Keith Packard --- doc/altusmetrum.xsl | 214 ++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 214 insertions(+) diff --git a/doc/altusmetrum.xsl b/doc/altusmetrum.xsl index e97666ae..601b62eb 100644 --- a/doc/altusmetrum.xsl +++ b/doc/altusmetrum.xsl @@ -1810,6 +1810,220 @@ NAR #88757, TRA #12200 + + Altimeter Installation Recommendations + + Building high-power rockets that fly safely is hard enough. Mix + in some sophisticated electronics and a bunch of radio energy + and oftentimes you find few perfect solutions. This chapter + contains some suggestions about how to install AltusMetrum + products into the rocket airframe, including how to safely and + reliably mix a variety of electronics into the same airframe. + +
+ Mounting the Altimeter + + The first consideration is to ensure that the altimeter is + securely fastened to the airframe. For TeleMetrum, we use + nylon standoffs and nylon screws; they're good to at least 50G + and cannot cause any electrical issues on the board. For + TeleMini, we usually cut small pieces of 1/16" balsa to fit + under the screw holes, and then take 2x56 nylon screws and + screw them through the TeleMini mounting holes, through the + balsa and into the underlying material. + + + + Make sure TeleMetrum is aligned precisely along the axis of + acceleration so that the accelerometer can accurately + capture data during the flight. + + + Watch for any metal touching components on the + board. Shorting out connections on the bottom of the board + can cause the altimeter to fail during flight. + + +
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+ Dealing with the Antenna + + The antenna supplied is just a piece of solid, insulated, + wire. If it gets damaged or broken, it can be easily + replaced. It should be kept straight and not cut; bending or + cutting it will change the resonant frequency and/or + impedence, making it a less efficient radiator and thus + reducing the range of the telemetry signal. + + + Keeping metal away from the antenna will provide better range + and a more even radiation pattern. In most rockets, it's not + entirely possible to isolate the antenna from metal + components; there are often bolts, all-thread and wires from other + electronics to contend with. Just be aware that the more stuff + like this around the antenna, the lower the range. + + + Make sure the antenna is not inside a tube made or covered + with conducting material. Carbon fibre is the most common + culprit here -- CF is a good conductor and will effectively + shield the antenna, dramatically reducing signal strength and + range. Metalic flake paint is another effective shielding + material which is to be avoided around any antennas. + + + If the ebay is large enough, it can be convenient to simply + mount the altimeter at one end and stretch the antenna out + inside. Taping the antenna to the sled can keep it straight + under acceleration. If there are metal rods, keep the + antenna as far away as possible. + + + For a shorter ebay, it's quite practical to have the antenna + run through a bulkhead and into an adjacent bay. Drill a small + hole in the bulkhead, pass the antenna wire through it and + then seal it up with glue or clay. We've also used acrylic + tubing to create a cavity for the antenna wire. This works a + bit better in that the antenna is known to stay straight and + not get folded by recovery components in the bay. Angle the + tubing towards the side wall of the rocket and it ends up + consuming very little space. + + + If you need to place the antenna at a distance from the + altimeter, you can replace the antenna with an edge-mounted + SMA connector, and then run 50Ω coax from the board to the + antenna. Building a remote antenna is beyond the scope of this + manual. + +
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+ Preserving GPS Reception + + The GPS antenna and receiver in TeleMetrum are highly + sensitive and normally have no trouble tracking enough + satellites to provide accurate position information for + recovering the rocket. However, there are many ways to + attenuate the GPS signal. + + + Conductive tubing or coatings. Carbon fiber and metal + tubing, or metalic paint will all dramatically attenuate the + GPS signal. We've never heard of anyone successfully + receiving GPS from inside these materials. + + + Metal components near the GPS patch antenna. These will + de-tune the patch antenna, changing the resonant frequency + away from the L1 carrier and reduce the effectiveness of the + antenna. You can place as much stuff as you like beneath the + antenna as that's covered with a ground plane. But, keep + wires and metal out from above the patch antenna. + + + +
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+ Radio Frequency Interference + + Any altimeter will generate RFI; the digital circuits use + high-frequency clocks that spray radio interference across a + wide band. Altusmetrum altimeters generate intentional radio + signals as well, increasing the amount of RF energy around the board. + + + Rocketry altimeters also use precise sensors measuring air + pressure and acceleration. Tiny changes in voltage can cause + these sensor readings to vary by a huge amount. When the + sensors start mis-reporting data, the altimeter can either + fire the igniters at the wrong time, or not fire them at all. + + + Voltages are induced when radio frequency energy is + transmitted from one circuit to another. Here are things that + increase the induced voltage and current: + + + + Keep wires from different circuits apart. Moving circuits + further apart will reduce RFI. + + + Avoid parallel wires from different circuits. The longer two + wires run parallel to one another, the larger the amount of + transferred energy. Cross wires at right angles to reduce + RFI. + + + Twist wires from the same circuits. Two wires the same + distance from the transmitter will get the same amount of + induced energy which will then cancel out. Any time you have + a wire pair running together, twist the pair together to + even out distances and reduce RFI. For altimeters, this + includes battery leads, switch hookups and igniter + circuits. + + + Avoid resonant lengths. Know what frequencies are present + in the environment and avoid having wire lengths near a + natural resonant length. Altusmetrum products transmit on the + 70cm amateur band, so you should avoid lengths that are a + simple ratio of that length; essentially any multiple of 1/4 + of the wavelength (17.5cm). + + +
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+ The Barometric Sensor + + Altusmetrum altimeters measure altitude with a barometric + sensor, essentially measuring the amount of air above the + rocket to figure out how high it is. A large number of + measurements are taken as the altimeter initializes itself to + figure out the pad altitude. Subsequent measurements are then + used to compute the height above the pad. + + + To accurately measure atmospheric pressure, the ebay + containing the altimeter must be vented outside the + airframe. The vent must be placed in a region of linear + airflow, smooth and not in an area of increasing or decreasing + pressure. + + + The barometric sensor in the altimeter is quite sensitive to + chemical damage from the products of APCP or BP combustion, so + make sure the ebay is carefully sealed from any compartment + which contains ejection charges or motors. + +
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+ Ground Testing + + The most important aspect of any installation is careful + ground testing. Bringing an airframe up to the LCO table which + hasn't been ground tested can lead to delays or ejection + charges firing on the pad, or, even worse, a recovery system + failure. + + + Do a 'full systems' test that includes wiring up all igniters + without any BP and turning on all of the electronics in flight + mode. This will catch any mistakes in wiring and any residual + RFI issues that might accidentally fire igniters at the wrong + time. Let the airframe sit for several minutes, checking for + adequate telemetry signal strength and GPS lock. + + + Ground test the ejection charges. Prepare the rocket for + flight, loading ejection charges and igniters. Completely + assemble the airframe and then use the 'Fire Igniters' + interface through a TeleDongle to command each charge to + fire. Make sure the charge is sufficient to robustly separate + the airframe and deploy the recovery system. + +
+ Hardware Specifications
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