LOHAN test flight: Results in from Oz jury Low Orbit Helium Assisted Navigator (LOHAN) brain surgeons Linus Penzlien and Andrew Tridgell* have scrutinised the log flies from the recent test flight which saw a Pixhawk autopilot, batteries and servos (pictured below) sent aloft to 27,700m to determine in real-world conditions how the Vulture 2 spaceplane's bulging …
Mmmmh, no. US units and Imperial units are not the same. An Imperial gallon is about 1/5 larger than a US gallon for instance. Then there is the Iiternational Mile which is .0006 of a foot shorter than the US Statute mile, and (according to Glover's Pocket Reference) the nautical mile and the British mile are the same length, and about 0.15 US Statute miles longer than a US Statute mile. Of course a US fluid ounce is slightly larger than the British fluid ounce. But that potential conversion problem sounds like a good idea. Unit conversion, it is never pretty.
Be *very* careful with your choice of zener if you're using it to clip a power rail.
I'm looking into this (though I care about hot use - 150C - rather than colder temperatures) at the moment, and I'm finding zeners which we have used for years are suddenly, well, not zeners any more. What I'm finding is that the number printed on the side of the zener doesn't seem to bear a lot of relationship to the value at which that zener, er, zenes; not only is it sensitive to temperature it's sensitive to current as well. Even at room temperature in the 3-5v range (where things change from bandgap reference to true zener avalanche breakdown) we've seen zeners that have a volt and more difference from their nominal value.
You may need to consider something a little more sophisticated...
"You may need to consider something a little more sophisticated..."
Amen.
Probably not enough time for the sub-space jockeys but it might be interesting from a general perspective to do something for the 'collective community' which might avoid the "Ooops bugger, let's put a 'battery eliminator' in there. Oooops bugger." mentality.
I might mention that the graphs as presented make no sense to me and otherwise power processing is *hard*.
I would be inclined to implement a parallel low voltage battery pack, with sufficient voltage on the ground, to start up a..
http://www.ti.com/lit/an/slua149/slua149.pdf
UCC3803
UCC1803 for MIL SPEC
and implement a boost converter which will be 'gentle' on the batteries in terms of ripple current and once started will keep itself supplied from the output via devious means even if the batteries drop below UVLO.
Having boosted up to a 'suitable' voltage, 24V?, with your, dirt slow control loop, boost converter into a storage capacitor you then use, monster fast control loop, generic multiple synchronous buck converters to deliver the power at the required voltages elsewhere.
The output capacitors, assuming you use the right ones, will sniff at the spikes and should your load decide to take a dump it will be returned back to the boost converters output capacitor and recycled elsewhere.
Just burbling.
Only problem with using an SMPS is the resultant EMI which is not going to sit happily with the remote telemetry - however well you shield & suppress it, SMPSs can be problematic.
I'd stick with a modern LDO regulator - zeners have never been that good anyway - they are noisy, prone to wander, and can have high temperature coefficients - most are not true zeners anyway, they are avalanche diodes.
voltage spikes on the servo rail. I think we should install a reverse biased zener diode on the servo rail
For my humanitarian avalanche radars in very noisy & cold conditions, (where the power feed was occasionally eaten,) the bidirectional avalanche transzorb diode was the protection component of choice.
1N6373/ICTE5 from http://www.vishay.com/ppg?88356 Works from -55 to +175C, might need to select on test for the best clamping voltage, or go SMD
<bullshit mode on>
Point taken re EMI however it can be controlled/ameliorated. Just a matter of knowing what you are doing rather than, tongue in cheek, slapping things together on S-DEC and watching them blow up..
http://www.theregister.co.uk/2014/09/16/vulture_2_power_podule_servo_test/
http://www.castlecreations.com/products/ccbec.html
You will find that most avionics kit is stuffed full of SMPS and indeed El-Reg are using the above to buck things down from a higher input voltage by putting their batteries in series. Of course I do not know what Castle have Created but I might assume a bit of blue heat shrink and potting compound does not look like a solution to EMI.
One thing I do know is that a 'raw' buck converter hammers the input source with a square wave current draw unless you provide further input filtering...
http://regmedia.co.uk/2014/09/02/cc_bec_big.jpg
That will be the capacitor on the input side. Mr wet finger would probably conclude it is 'just enough' to get away with things.
which knobs on your loop compensation so you may as well put the batteries in parallel and use a boost converter with low input ripple current to generate the higher output voltage prior to bucking it down and independently redistributing it according to requirements.
<bullshit mode fully on>
Batteries are happier in parallel. Synchronous buck will return/recover power back to the input boost stage output which will/should take care of any load dump from the inductiveness of the servos... assuming that is a problem.
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"In a 'Zener' diode either or both breakdown mechanisms may be present. At low doping levels and higher voltages the avalanche mechanism dominates while at heavy doping levels and lower voltages the Zener mechanism dominates. At a certain doping level and around 6 V for Si, both mechanism are present with temperature coefficients that just cancel. It is possible to make Zener diodes with quite small temperature coefficients"
http://people.seas.harvard.edu/~jones/es154/lectures/lecture_2/breakdown/breakdown.html
As I said earlier, I'd be very cautious about using zeners to clamp supplies that can deliver significant power. One of those spikes looked pretty chunky, and if it proves too much for the little beasty it goes short circuit and you completely lose your power supply. This is the problem with any shunt system. If I had a pound for each of the MOVs I've had to replace over the years it would keep my in beer money for a long time!
High energy suppressors another poster mentioned would be better, but their clamping voltage is very variable... and they too can go out with a bang.
@marshalltown.
Must write a food reminder to never let you navigate at sea, if you are going to use Glover's as a reference.
A British statute mile is exactly 5280 feet. A US mile is 5280.01056002 feet. (The history of THAT must be interesting! And I did not know that until today.)
But a nautical mile is NOT the same as a British statute mile. A nautical mile was for at least a couple of centuries a 'sea mile' equal to one minute of latitude. And of course, one minute of latitude actually varies with latitude as the earth is not spherical. Generally it was taken as 6080 feet. From about 1890 the British admiralty and the US differently defined a nautical mile to equal one minute of arc of a great circle on Clarke's spheroid (of 1866) approximation of the shape of the earth: 6080 and 6080.2 feet respectively. Both eventually gave that up, and now use the International Hydrographic standard (of 1929) , of exactly 1852 meters or .6076.1154 feet. (Amazing stuff hidden in a copy of Bowditch!)
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