[time-nuts] position determination over short distance

Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Nov 24 03:52:42 UTC 2008


Tom Van Baak wrote:
>> 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.
>>     
>
> 1 mm or better accuracy on a 300x300 mm surface can be
> obtained with a cheap webcam mounted above the surface
> and a little bit a creative software.
>
>   
>> 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.
>>     
>
> Sound travels about a million times slower than light. 343 m/s
> is 1 mm in 2.9 microseconds. Perhaps this is the way to go?
>
> I remind visitors here that a WWV tick takes the same time to
> travel from Ft Collins to Seattle as it does to travel from the SW
> radio speaker to your ear.
>
> /tvb
>
>   
If you use sound then compensation for the temperature, pressure and
humidity dependence of the speed of sound may be required.
If your accuracy requirements are higher then compensating for
variations in the C02 content will also be required.
One of the simpler ways is to use the same techniques to measure a fixed
reference path difference.

Bruce




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