[time-nuts] position determination over short distance

[w3kl at w3kl.com]

Sorry to jump in here so late in the discussion thread.
 
Have you considered a laser interferometer (which is an accepted primary standard for measuring displacement).  Over the distance you want to measure you'll get much better than 1 mm accuracy (closer to 1 micron actually, assuming you take care of all of the second order effects that can ruin a measurement like this).
 
You should also look at photogrammetry, which will easily get you 1 mm accuracies.
 
Hope this helps.
 
Jeff

Jeffrey K. Okamitsu, PhD, MBA
+1-609-638-5402

--- On Fri, 12/5/08, WarrenS <warrensjmail-one at yahoo.com> wrote:

From: WarrenS <warrensjmail-one at yahoo.com>
Subject: [time-nuts] position determination over short distance
To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
Date: Friday, December 5, 2008, 12:49 PM

Rick Harold wrote:
> 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

********************
Rick

The way you are describing doing it may have one additional BIG problem you may
not of addressed.
The antennas must be smaller than 1mm or at least be stable in effective
position to that accuracy. 

As far as the basic configuration of how to do it, all you need is one
transmitter, one narrow band FM receiver,
Three or four antennas and a RF audio mux between the antennas and the
receivers. 
It uses the receivers audio freq / phase detector AKA, FM discriminator. No
great Phase detector or special stuff like that.

The way that it works for example, say you are using a freq with a wave length
of  1 inch.
Then if  two of your antennas are say 1/4 inch apart  ( or 1 mm apart or 1/4
plus any multiple of the one wave length) 
The AUDIO out of the receiver will be modulated "A lot" due to the FM
caused by the 1/4 wave phase steps it sees.
This can resolve as fine of distances as you want until it reaches the noise
level of your receiver and transmitters oscillators.
the system phase gain works out to be the ratio of the audio mux to the RF. 
To be sure there are all kinds of other issues you would have to address but
having the resolution to determine 
the phase to below 3.3 ps need not be one of major ones.

Have fun,
WarrenS
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