Last week Tomas and Maria visited IRF, the Swedish Institute of Space Physics, in Kiruna. The reasons were two-fold: first the science part, which is mainly Maria’s area. The second reason was engineering, which is Tomas’ domain and the main focus of this post.
For the science bit, getting feedback from the space science community is important. It allows us to be reasonably confident that we know what problems expect when it comes to measuring. It also helps advertise our work to scientists that may be interested in our data, and gives ideas about possible future missions.
As for engineering, lots and lots of useful tips. In no particular order:
Cabling: wires should be bundled into harnesses. Pin headers + sockets are preferable, since they still allow some springiness but will not have much trouble with vibration. PTFE, Kapton and FEP are good, thermal vacuum compatible insulation materials. PVC is unacceptable. Flat flex is OK, but may crack or disconnect due to vibration.
Connectors: Every connector should be unique, so there is zero risk of incorrect hookup. Keep “protruding” connector elements for power on the instrument side, so that cables used for power have their elements “sunk in”. Typically this means female (sleeve) type of connector, but some pin types are also sunken. Use gold plated connectors.
Solder joins: Rosin core 63/37 SnPb solder has a predictable melting point and cleans well enough with isopropyl alcohol. Use clear Kynar heat shrink tubing where possible, especially on D-subminiature connectors, see picture below. This prevents loose pieces of wire from accidentally shorting pins
Cleaning: brush using pure isopropyl alcohol or 70/30 isopropyl alcohol/distilled water. The latter may be needed to remove certain salts. Rinse with pure isopropyl alcohol afterwards. DO NOT use ultrasonic cleaning if using any ceramic parts; they may crack.
Glue/epoxy: Scotch-Weld 2216 is thermal vacuum safe. Mix, then use vacuum to draw out any air trapped inside. After vacuuming, the mix is good for about 15 minutes. Use Scotch-Weld to hold down large ICs, spot glue bodge cables or anything else that might move but shouldn’t. Useful for gluing nuts, since locking nuts are not thermal vacuum safe.
PCB laminates: FR4 outgasses slightly, but is perfectly fine thermal vacuum-wise. High-voltage boards should use ceramic substrates such as Rogers R4003 since the outgassing of FR4 may create a rarefied atmosphere, causing arcing and equipment failure. This is not a problem for us luckily 🙂
Digital logic: short missions near Earth (such as ours) do not need any kind if special ICs. Mechanical structures that withstand vibration testing are thick enough to shield ICs from any problematic radiation. As an example, IRF have sent instruments using off-the-shelf ARM chips without problems.
Latch-up protection: a series resistor followed by a capacitor to ground is enough to protect chips from blowing up in case of latch-up. Power cycle to remove any latch-up situations.
Plastic parts: PEEK works well enough for most plastic parts, and is thermal vacuum safe.
Tape: usually Kapton is fine, but pay attention to the glue used. There are also tapes with specific thermal radiation properties which may be used in lieu of plating or painting.
These tips should be handy both for AMSAT folks and possibly anyone who’s looking into building vacuum equipment. Finally, I’m going to round this post off with some more pictures:
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2017 is upon us, and we’ve kept busy as usual. One of the main goals for this year has been to get a working prototype as close as possible to “the real deal”. This means being able to amplify, digitize, demodulate and transmit data over serial to a host computer. I’m happy to say we’ve managed to accomplish this, after a dash in the days between winter solstice and new year’s celebrations.
The quickly cut together video below shows how we’ve gone from a fieldmill assembly on its own, to a partially assembled cube, to the fully assembled cube in our test box. Some final screenshots demonstrate data being analyzed in LabView and GNU Octave.
The focus moving forward is making sure we only use materials which are high-vacuum compatible, to be able to do thermal tests on the design we’re currently working on. There’s also more work to be done on the simulation front, and various bits of code need to be written.Parts for a piece of calibration gear has been ordered as well, which is needed to verify that the entire analog frontend works as expected. In short, 2017 is going to be a busy year!
It’s been a while since the last update, because a lot of stuff has been happening.
On the code side, IQ demodulation is done. This has been a major task for a while, with some troubles achieving phase correctness and correct buffer handling. The working code proves that 7 MHz is enough to do demodulation for our needs. There is still some room for improvement, such as using Timer1/Timer3 input capture to get more accurate measurement of tachometer/sample times. Some optimization could also be done, if we need to increase the sampling rate (unlikely at this stage). Current CPU usage is around 60%.
We also got some new motors with built-in controllers. This reduces our work load considerably. They also have much better regulation than our previous controllers, meaning faster starting and more stable rotor speed. The electrical noise is also considerably lower and the motors run quieter.
Circuit boards for our most recent analog front end design have also arrived, and all components except the operational amplifiers we need! This may have something to do with the holiday pressure on the postal system. Hopefully they arrive before New Year’s Eve. We should also have some new toys from Atmel in the mail. More on those in a future post!
Finally, some pictures:
You are officially invited!
The Umeå Lunar Venture Innovation Fair is happening and it’s an open event for everyone. Bring your friends and come and meet the Moon Rover and hear about how we became one of the biggest space projects in Sweden.
Umeå Lunar Venture is a partnership between us (Space Science Sweden) and Umeå University. Our main goal is to put Swedens first scientific experiment on the moon in the end of next year. And as you know we are currently preparing the measuring instrument at full speed to make this happen!
The Innovation Fair is a opportunity for anyone who is interested in learning more about our work. You will get the chance to meet the Moon Rover that is taking us to the moon and it’s creators (Part-Time Scientists) who is coming all the way from Germany. And of course the students from the engineering and physics programme will be there!
Down below you can see the programme for the day and evening. Feel free to invite your friends and help us spread the word!
Attend the Facebook event here!
This week we have two major things on the electronics/instrument side of things. The first thing is we have verified that Tomas’ design with software demodulation on the AVR, fed with six channels of 24-bit ADC data, will work. There’s some bit of optimization left to do, but we should be able to dump CSV data over serial for further analysis in Octave/LabView fairly soon.
On the instrument side we have verified that choice of surface material is very important when it comes to minimizing offset fields. A consistent issue is charge buildup on aluminium surfaces due to the inherent oxide layer. Covering the surfaces with graphite reduces this charge buildup, likely because it helps in providing low-resistance paths into the bulk aluminium. Laminating the aluminium with brass foil is even better, but only the rotor has had that treatment so far. Next up is brassing all sides of the instrument.
One final thing discovered this Friday is that the offset we’re having has a phase offset compared to the desired signal by about 45°. The way the vector arithmetic works out explains the nonlinearities we’ve seen in some cases, due to only measuring the signal’s amplitude.