On Tue, 19 Apr 2011 20:57:12 +1000, Blake Nikolic <blake(a)nikoplus.com> wrote:
> On another note, how can I tell the difference between these 200G
> units that have just arrived and standard units?
That's a great question! CC'ing our mailing list with my answer so
it'll be archived. And given that, I'll throw in a few other thoughts
about the accelerometers on our boards while I'm typing.
The only visible difference would be the markings on the Freescale
accelerometer part itself, which is U3:
MMA2201 is 40G
MMA2202 is 50G (default)
MMA2204 is 100G
MMA2301 is 200G
The way the firmware works is that we do a cal to determine the ADC
output at +1 and -1 G, so it's "self calibrating" to the different
ranges and offsets due to actual resistor values in the divider, etc.
Thus the firmware doesn't actually know or care what part is loaded, it
just knows what numbers correspond to +/- 1G.
You can view these cal values by connecting to the board over USB or RF
and typing 'c s'. That display includes the current accelerometer cal
as stored in the config block of the on-board flash memory.
Note that a 200G accel has 1/4 the resolution of a 50G accel, because we
have the same number of ADC bits covering 4 times the range. Any errors
in the +/- 1G cal are also amplified 4 times more with a 200G part than
they are with a 50G part for the same reason. The fact that we log raw
ADC samples and the cal values means that you can always play around in
a spreadsheet to post-process the data if you really need to.
Those cal values become part of the flight data recorded in the on board
flash, and are part of the data sent to the ground over radio. So the
ground software also doesn't know what part is loaded, it just knows how
to take the raw ADC output numbers that are logged and convert them into
G numbers using the cal values received alongside the raw data.
The other way to know which accel range a board covers is that I keep a
"production log" that has notes about things like which boards got
custom accelerometer choices, so in the worst case, I can always tell
you what a given serial number shipped with.
Bdale
Hi folks,
I've been looking over the Telemetrum schematic and trying to figure out
whether the header power switch (v_lipo/v_charge) meets the NAR Level 3
requirement to "physically break the connection between a pyrotechnic system
and its power source" to safe the charges externally. My limited EE
background says there's no way the output transistors are going to pass
current without the switch turned on, but I don't know how the NAR feels
about this. Any thoughts?
I was hoping I could get away with just the one switch for the Telemetrum,
plus whatever switch(es) my (heterogeneous) backup needs. Otherwise, I can
see my avionics bay quickly turning into a bit of a rat's nest.
Thanks,
Casey Barker
