position determination over short distance
On Mon, 24 Nov 2008 17:33:25 -0800, "Tom Van Baak" tvb@LeapSecond.com wrote:
Yes. I know of several commecial systems. If you only need to do a short
jump, then using fairly basic E/O-O/E equipment should work well
enought. It all depends if you want/can to roll your own or need to buy
a finished product (aka "buy this, and you will be fine!").
Magnus, what's the typical noise floor, tempco or drift of cheap
(i.e., non JPL-level) fiber distribution systems like this? Is it less
than regular coax, or phase stabilized heliax? At 100 m lengths?
OK, tom, you got me with another one. WTF is phase stabilized heliax? Is
that a hunk of ordinary heliax that has been characterized or is it made
special in some way?
re: original problem
check out B&B Electronics
http://www.bandbelectronics.com/
sorry, I'm off-line right now and can't browse but they specialize in really
inexpensive implementations of stuff like this.
John
John De Armond
See my website for my current email address
http://www.neon-john.com
http://www.johndearmond.com <-- best little blog on the net!
Tellico Plains, Occupied TN
Save the whales, collect the whole set!
On 12/5/08 3:32 AM, "Neon John" jgd@johngsbbq.com wrote:
On Sun, 23 Nov 2008 20:26:52 -0800, "Lux, James P" james.p.lux@jpl.nasa.gov
wrote:
There's a fair amount of F/OSS software from JPL available to do this sort of
calibration. It's used to calibrate cameras used on Mars rovers, among other
things. The target pattern for calibration is a bunch of big circular dots on
a background.
URLs?
Google "JPL Camera Calibration" and you'll get a bunch of hits to various
technical reports. Once you have those, you'll have better search terms. A
lot of the software has been published in NasaTechBriefs as a TSP, too.
In general, you get an image of an array of big polka dots. The software
locates the centroids of the dots (which can be done with higher precision
than just looking for a single pixel point.. 0.1 pixel is apparently not too
tough, from what the camera guys tell me) (for instance, if you're doing
something like a star tracker, it's actually easier if the image is
defocused)
Then the measurements are turned into CAHVOR coefficients, which get used in
subsequent photogrammetric processing to rectify the image.
I recently found this freeware open source on the net.
http://tim-jackson.co.uk/area/index.html
Caution: I've encountered this guy on Usenet where he has a bad tendency to
substitute abject BS in place of fact when it's too much trouble to dig up the
facts. I'd inspect the sources closely before using, looking for shortcuts
that don't work and pure old logic errors.
It would do the job in this instance but I'm interested in a more generalized
solution (without having to write it myself or buy anything) to pulling
measurements from photos.
I'd like to be able to take a photograph in which an object of known
dimensions is included and pull other dimensions from the photo, including
areas.
The JPL stuff would be more likely to be a library of dozens of routines
which you could string together to do this, rather than a handy point and
click interface.
Any suggestions?
John
John De Armond
See my website for my current email address
http://www.neon-john.com
http://www.johndearmond.com <-- best little blog on the net!
Tellico Plains, Occupied TN
What do you call a blonde's cranial cavity? Vacuum chamber?
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On 12/5/08 3:48 AM, "Neon John" jgd@johngsbbq.com wrote:
On Mon, 24 Nov 2008 17:33:25 -0800, "Tom Van Baak" tvb@LeapSecond.com wrote:
Yes. I know of several commecial systems. If you only need to do a short
jump, then using fairly basic E/O-O/E equipment should work well
enought. It all depends if you want/can to roll your own or need to buy
a finished product (aka "buy this, and you will be fine!").
Magnus, what's the typical noise floor, tempco or drift of cheap
(i.e., non JPL-level) fiber distribution systems like this? Is it less
than regular coax, or phase stabilized heliax? At 100 m lengths?
OK, tom, you got me with another one. WTF is phase stabilized heliax? Is
that a hunk of ordinary heliax that has been characterized or is it made
special in some way?
I don't know about heliax, but in the semirigid and flex coax area, getting
good phase stability is a combination of the two. Some dielectrics and
construction have distinct bumps in the phase/temperature curve at
inconvenient temperatures (I'm looking at you Gore!) although they're
generally pretty good. Some might have more variation over temperature, but
it's smooth and consistent. Esp if you're measuring temperature or have it
controlled, the latter might be a better choice.
Then, you temperature cycle it a bunch of times with it formed in the final
shape, which relaxes the residual mechanical stresses.
Then, you try them all, and pick the best ones.
Jim
Hi Rick,
Sorry for the late reply but I accidentally stumbled upon this device
and thought about your application. I'm not sure it will be accurate
enough to meet your requirements but if you choose the part of the
curve with the steepest slope you might just achieve the 1mm you need
and at a very inexpensive price.
Sharp GP2Y0A21YK0F Distance Sensor
73,
Steve
2008/11/24 Rick Harold rickharold@gmail.com:
All,
I'm planning doing some experiments in distance measurement. They don't
deal with atomic time directly but with extreme short periods of time.
I need to determine the position of a instrument with a 1mm accuracy or
less.
The instrument is not connected to a mechanical device but is separate &
independent.
The surface which the instrument is positioned on is close to the size of a
11"x11" square.
I thought of using 1 RF transmitters (not sure of freq) on bottom of the
device near the surface.
The surface would have RF receivers on 3 or 4 edges/corners to receive the
signal.
If each of the receivers positions are known and they then send a signal to
a central circuit (again known positions) how can I differentiate the time
of arrival
at the central location? Does anybody know of a circuit/chip or system
which would determine the time 'difference'.
Obviously this is used to triangulate the position of the instrument.
Light travels 1 mm in ~3.3 picoseconds so I would suspect the differentiator
would have to have that or better resolution.
It could also use some proportional method to extrapolate the position since
the surface has a fixed size.
Any ideas/thoughts?
Thanks in advance.
Rick Harold
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