h a l f b a k e r y"This may be bollocks, but it's lovely bollocks."
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Optitape
cancel curvature loss using laser vectors in a modified tape measure | |
I don't know if this is possible but here goes.
If an ordinary spring tape measure had a optic fibre embedded in it, a laser fired down the fibre would bounce the more the tape is bending. By putting reflection points down the fibre, the laser gives a set of length measurements. By using vector mathematics
those measurements could be converted to a straight line distant.
A display could show the measurement or show an error value be to added to the measuring tape.
Long distances tend to introduce errors with a normal tape measure. This tape, although it needs batteries, hopefully will give more accuracy without using to much time and effort.
optical time-domain reflectometer
https://en.wikipedi...omain_reflectometer
[tatterdemalion, Dec 30 2014]
[link]
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You could sell top end men's suits with such a tape measure. |
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Some would claim you get a better fit. And with the new suit you get the story to brag about your exclusive laser measured suit. |
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The sag could be estimated based on the
tension/tautness, angle, and length extended. |
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It could become a form of optical network cable instead. Deployable, wearable, stickable... just need to find a reliable way to terminate the ends.
Erp. I guess that's way off topic now. |
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If this idea is meant to work the way I think [wjt]
means, it is a very cool idea. |
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This is quite bakeable. Currently, in the
construction of the new Crossrail tunnels, they
are laying optical fibres in the concrete which
will be used in this way to measure
subsidence. |
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// reflection points down the fibre |
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Would need more elucidation on this aspect. I'm not sure how that jibes with total internal reflection. |
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I took it to mean that there were points along the
fibre which would reflect a tiny fraction of the light.
If these were at (say) 1cm intervals and if the tape
were straight and you fired a very short pulse down
it, you'd get a series of reflected pulses
at 66 picosecond intervals. If the tape were bent
between two of the partial reflectors, the timing
between the corresponding pulses would be very
slightly increased. |
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OK, so there is a basis for that bit (see link). But how such a device would respond to bending using "reflection points" is not clear. |
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You could get it to work by using a continuous tape and break off the part you want to measure. |
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It would respond to bending like this: |
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If the light is passing along a straight section of fibre
(and if we assume, for simplicity, that the light starts
out travelling down the axis of the fibre), it will take
some time T to travel a given distance D. |
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If the fibre is bent, the light will either (a) travel in a
zigzag path, bouncing off a reflective coating or (b)
in a graded-index fibre, the light will spend more
time in the lower-index periphery of the fibre. In the
first case, the overall speed of transmission will be
lower; in the second case, it will be higher (faster).
Either way, you should be able to work out the
degree of curvature over that segment of the fibre. |
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This is just on the periphery of my understanding of fibre optics, so I beg your patience. Within that fragile bit of knowledge, I had thought that // the overall speed of transmission will be lower // is a circumstance that does not occur in a bent fibre optic cable, and in fact is a feature, not a bug. |
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My knowledge isn't much greater. However, I do know
that fibres work either |
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(a) by having a core of a high refractive index, and a
sheath of low refractive index. The interface
between the two acts as a mirror, so if the fibre is
curved the light will repeatedly bounce off the
"outside" of the bend. Thus, light will travel further
if the fibre is bent, and the time will be longer. |
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(b) by having a graded refractive index (high in the
middle, grading to low at the periphery). A light ray
entering the periphery is refracted back into the
centre. At a bend, the light is continuously moving
into the lower-index periphery (where it travels
faster), and having to be refracted back to the
higher-index (slower) core. Thus, light should travel
faster around a bend (where it keeps dipping into the
lower-index periphery) than along a straight bit. |
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However, please bear in mind that I might be
completely wrong. |
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The ray-diagram picture is pretty inaccurate for thin fibres. Solving Maxwell's equations in a fibre gives different modes of propagation (each with different rates of dispersion, hence the usefulness of having single-mode fibre). In some refractive index profiles, the bulk of the light may actually travel in the cladding, not the core. |
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But the point is that using optical fibres as strain sensors is a known technique. |
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[bigsleep] I don't really want to carry another tool. |
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Maxwell has pretty well covered the outline sketch that was my idea. [Maxwell] Thanks. |
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My mind imagined the light take straight lines, until
internally reflected by the fibre's sides. the more curve the more reflections. As Maxwell indicated, points along the fibre send back some light for partial calculation measurements. |
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I was thinking about a mechanical version with a two layers in the tape that take the curve differently. The show would be, vernier like, with differing marks between layers. But this didn't seem realistic. |
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But from what [mitxela] says, light is not that clear cut either. |
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I have just thought of a problem. |
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When I measure, sometimes I tend to extend the tape past the measurement so I can bend the tape into a corner to get the correct distance. This would give a false sag measurement. |
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Maybe the tape might allow the editing of how many laser vectors are used. A bit of fluffing about though. |
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