etxbrfa@kklm01.ericsson.se (11/01/88)
How to measure distance with lasers?
I've been thinking a lot about the problem, and my conclution is:
It's impossible (at least with submillimeter accuracy)
Still it's done. And it's done by cheap devices.
The technology of today must be pushed far beyond it's capabilities. The
speed of light is far to high.
To measure the time, would be simple, if the accuracy of the electronics
were good enough, but it isn't.
My thought is that you pulse the laser with an accurate frequency (say
1.0000000Mhz). If you start another clock at 0.9999990Mhz at the very same
time, the difference it time between the clocks would increase by 1ps every
cycle (an accuracy of 0.15mm). The problem is that you cannot get such accurate
clocks, and to start them excactly on the same time (difference less than
0.1ps). Even if you could, there would be other problems.
Suppose the problem with time measurement is solved. How can the laser-beam
be pulsed at 1Mhz with edge accuracy less then 0.1ps. Impossible again.
To light the laser takes at least a few micro seconds, and time varies.
Then you could ofcource have a constant beam, that is pulsed afterwards.
How? A mechanical solution is out of question, so is Liquid chrystals. I
don't have any idea at all.
Now suppose that problem is solved to. Then we have the problem with detecting
the reflex. A photo diode or transistor, is out of question, far too slow.
And even if they weren't, the logic to take care of the signal from them
would have to long (and unreliable) setup time.
I don't think there are more problems (at least not as tough as these basic
ones). But unfortunately, the logics of problems is an OR-function.
Am I wrong? Apparently I am, because these devices does excist. But what
is wrong. I can't se any errors in the statements above. All of you who
can see errors, please E-mail. I'd be most grateful to know.
SNAIL: ERICSSON TELECOM Vox humana: (+46) 8 - 719 62 52
Bjoern Fahller Fax machina: (+46) 8 - 740 28 34
KK/ETX/TT/MLG
S-126 25 STOCKHOLM "Inside every problem, there are at least
SWEDEN two other problems, struggling to get out"commgrp@silver.bacs.indiana.edu (11/05/88)
>How to measure distance with lasers? >I've been thinking a lot about the problem, and my conclution is: > It's impossible (at least with submillimeter accuracy) >Still it's done. And it's done by cheap devices... >SNAIL: ERICSSON TELECOM Vox humana: (+46) 8 - 719 62 52 > Bjoern Fahller Fax machina: (+46) 8 - 740 28 34 > KK/ETX/TT/MLG > S-126 25 STOCKHOLM "Inside every problem, there are at least > SWEDEN two other problems, struggling to get out" Light travels about one foot per nanosecond, so a laser rangefinder with a 1' resolution would need a 500-MHz timebase if it works by measuring the time it takes for the light to return from the target. Laser rangefinders which measures in micro-millimeters over ranges of 100m or less use optical interferometry techniques. For longer distances, as in surveying, one way to do it precisely without direct counting is the same principle used in time-domain reflectometers for inspecting transmission lines. It's a sampling technique, a form of correlation; simple concept but hard to describe-- I saw a good explanation in the instruction book for a Tektronix sampling oscilloscope. Another way to do it, used in some surveying instruments, is to transmit a continuous light beam from a solid-state laser (infrared), which is modulated with a sinewave RF carrier having precisely controlled frequency, say, 10 MHz, which has a wavelength of 30 meters. The receiver has a phase detector which compares outgoing and incoming phase of the carrier. The same solution occurs each wavelength, so the instrument uses other frequencies to solve the ambiguity. It's all taken care of by the on-board computer, which measures the distance several times and indicates an error if the variance is too great (which could be caused by target movement, air turbulence, etc.). Before lasers and solid-state stuff, there were vacuum-tube microwave devices for measuring long distances in surveying. They were manually- operated, some with internal oscilloscopes. A transponder was placed at the distant point. Operation was tedious, followed by laborious mathematical data reduction (but much faster and easier than measuring several miles with a tape). -- Frank Reid reid@gold.bacs.indiana.edu PZZ
ajdenner@athena.mit.edu (Alexander J Denner) (11/05/88)
In article <113@ericom.ericsson.se> etxbrfa@kklm01.ericsson.se writes: >How to measure distance with lasers? >I've been thinking a lot about the problem, and my conclution is: > > It's impossible (at least with submillimeter accuracy) > >Still it's done. And it's done by cheap devices. Laser interferometry is used to measure changes in distance very accurately. Interferometers just watch the interference between the incoming and outgoing beams. Thus, you can tell everytime a phase shift occurs. If the object moves, the path becomes longer and the interference pattern changes. Laser rangefinders use a modulated lower frequency signal. Thus you can tell everytime a phase shift occurs. The wavelength of light, used in the interferometer, is 4000-7000 Ang., where the wavelenth of the carrier is much larger (1-100m (?)). ----------------------------------------------------------------------------- Alexander J. Denner ajdenner@athena.mit.edu 234 Baker House, 362 Memorial Drive mit-eddie!mit-athena!ajdenner Cambridge, MA 02139 ajdenner%athena@mitmva.mit.edu
dya@unccvax.UUCP (York David Anthony @ WKTD, Wilmington, NC) (11/06/88)
In article <113@ericom.ericsson.se>, etxbrfa@kklm01.ericsson.se writes: > How to measure distance with lasers? > I've been thinking a lot about the problem, and my conclution is: > It's impossible (at least with submillimeter accuracy) > Still it's done. And it's done by cheap devices. > The technology of today must be pushed far beyond it's capabilities. The > speed of light is far to high. Naaaah. Try amplitude modulation and a timer. Actually, it is pretty trivial to build a stable phase meter in the 1-100 mHz range with an accuracy of 0.1 degree. Let's just say (for nice numbers) that an AM modulated laser beam at 100 mHz has a wavelength of 10 feet. With 0.1 degree resolution, this works out to (3600 divisions/10 feet) or 30 divisions to the inch. Naturally, you can put a sychronising pulse in there on every so-many cycles of the modulating frequency to eliminate the ambiguity of multiple wavelengths. For your run-of-the mill cheap laser distance meter, I would think that 1/30 inch would be quite acceptable for many applications. No doubt there are techniques which use interferometry with, say, a small frequency synthesizer and null detector (or, for that matter, plain old interferometry) to get yeee-haaa accuracy. York David Anthony DataSpan, Inc
cjc@praxis.co.uk (Chris Clee) (11/09/88)
In article <113@ericom.ericsson.se> etxbrfa@kklm01.ericsson.se writes: >How to measure distance with lasers? >I've been thinking a lot about the problem, and my conclution is: > > It's impossible (at least with submillimeter accuracy) > >Still it's done. And it's done by cheap devices. > >Am I wrong? Apparently I am, because these devices does excist. But what >is wrong. I can't se any errors in the statements above. All of you who >can see errors, please E-mail. I'd be most grateful to know. Laser light is coherent; that is, it is all of one frequency. If the laser tape measures operate in the way that I would think that they do, they would split a beam into two parts. One will be routed internally to an opto sensor. The other passes out of the aperture of the instument, strikes the object to be measured and reflects back into the instument and onto the same optosensor. Where these two beams combine, there will be a path difference between them. This path difference will vary as the distance to the object varies. The path difference between the beams causes interference. So as the instrument is moved towards or away from an object, the path difference will vary and produce alternately constructive and destructive interference at the opto sensor. Hence the instrument has a resolution of one half of a wavelength maximum. This is just a bit of Physicist's lateral thinking : I'm not absolutely sure that that's how it's done, so please flame me if I'm wrong - I'd be interested to know how they work myself - but the "timing the beam" idea seems a bit unlikely although scientists DO measure variations in the orbit of the Moon by bouncing short laser pulses off the roof of some lander on the surface ! Chris.
darrellm@pogo.GPID.TEK.COM (Darrell McGinnis) (11/12/88)
One technique might be related to measuring the speed of light over a fixed distance. This technique modulates the light beam and measurees the PHASE difference between the outgoing beam and the incomming beam. Resolution depends on the wave-length of the modulation frequency. Maybe a sweeped modulation would give good results at both extremes (low freq for gross distance/ high freq for finer resolution) .