resolution of distance measurements

[williB]

i was wondering what the resolution i could expect from an IR emitter - detecter pair..distance wise..
i mean could i detect very small distance variations.. on the order of thousanths of an inch?? from a distance of an inch or so??
if not from an IR pair then what?? ultrasonic transducer ??

 

[Dean Huster]

To deal with thousandths of an inch would require an inordinate amount of control with the angle of incidence between the emitter, receiver and surface in question. Most likely, the only way is you'd end up having to use some fancy precision optics dealing with diffraction gratings, precision slits and such. That's why we buy a simple $25 vernier or dial caliper to measure depths to 1/1000 of an inch.

Dean

 

[williB]

yeah but i cant touch the surface to be measured..
the angle of the emitter-detecter would be fixed ..
the angle of incedence between the surface to be measured and the e-d pair might also always be the same ... lets assume that it is..
how could i do it ??
oh, the actual distance doesnt matter , but relative to other distance measurements taken ,is what matters to me..

 

[williB]

the Sharp GP2D12 can only resolve down to 2mm (approx 0.078'' ) but it is a whole unit , and i dont have one anyway.. i do have two different emitter-detecter pairs ..
i take it that the detecter/ photodiode outputs a voltage proportional to distance??

 

[williB]

The second pic is a plot produced by the aparatus in pic 1..
although i didnt show the ruler imagine one behind the upper aluminum strip, also you cant see the magnet taped to the bottom of the strip..
the aparatus will still work if i taped the top magnet to the top of the strip..
ok, what i wanted to do was get a plot of the magnetic flux of two magnets taped together both facing the same direction, ie both north poles facing up, needless to say it was not easy , i actually had to use a small strip of copper soldered around the two ..
what i want to do is to use an xy plotter under the upper strip and magnet and take readings of the changing height as the plotter moves .. cool huh?
it should produce a plot as shown in pic 2..
i got the plot by moving the lower two magnets ( not shown) in an x/y fashion and looking at the ruler for the same height, when i found the height i would mark a dot through a hole drilled in the lower plate..
notice how the field looks lopsided ,ie the center part seems to be off to the right a little bit , but thats how it came out...

 

[tansis]

What is the accuracy of the x/y plotter ?


Dean mentioned diffraction gratings, and it got me thinking..

the doppler shift of a microwave beam would give speed and distance
of relative motion between source and a target.

 

[williB]

i am not sure exactly , but i know it is capable of very , very small movements..

 

[bmcculla]

A Michelson Inferometer would give you resolution down to a fraction of the wavelength of the light you use. A low-cost laser, a photodiode and a couple beam splitters is all you need. You could try a couple different clear materials at different angles to try to come up with your own beam splitter. Aligning everything would be the hardest part.

You could also try a capacitive measurement of the distance.

 

[williB]

ok the plotter has a movement of 0.004'' per plotter unit..not as small as i thought it was..but still not too bad..

 

[williB]

A Michelson Inferometer would give you resolution down to a fraction of the wavelength of the light you use. A low-cost laser, a photodiode and a couple beam splitters is all you need. You could try a couple different clear materials at different angles to try to come up with your own beam splitter. Aligning everything would be the hardest part.

You could also try a capacitive measurement of the distance.

wow that sounds a little complicated , how would that work exactly?

 

[bmcculla]

The idea uses wave interfearance of laser light to measure distance. You use the beam splitters to create two light paths from the same light. One path just goes a know distance - this is the reference. The other path reflects off the object that your measuring. As the object moves it causes a phase shift between the reference beam and the measuring beam. You use another beam splitter to recombine the two beams. Depending on the relative phases of the beams you get constructive and destructive interfearance - which you can read as intensity with a simple photodiode.

Heres a better description:
**broken link removed**