Any designs for a cheap x-y position sensor?

[Robert Macy]

Two surveyed wooden posts along a ~100' baseline and an ultrasonic tape
measure. You will have to put a repeater on top of the posts to send
back the pulse since in clear air there will not be enough echo.

Realistically you would probably want something to do the maths for you.

Lugging extra stuff out into the field is not so good. But, thanks for
triggering the brain! Ok, put two posts, each generating a pure known
tone. Then simply count the standing waves as I walk around. Voila!
relative 'optical' mouse, but done using audio.

Tones near 30kHz travel well, we don't hear. and have a wavelength of
around 1/2 inch! so I could easily digitize two to 8 pulses per inch !

Downside is losing signal. ouch. and worse multiple path! that would
really distort position information. But then again the price tag is
not too bad - pure sound source, turn ON and leave alone and built-in
mikes with some SW to sort it out. Including a potential
'calibration' step for that time of day, that altitude, and that
humidity. hmmmm.... not bad.

 

[hamilton]

Wow! didn't think of that at all! Thought of a mouse-like rolling
thingy, but the approach in that URL is much more self contained and
actually fits our existing software!

We have a prize winner!

Great !!!

I hope you will post here or at Hack-a-day on your results.

hamilton

 

[Robert Macy]

Sheer genius! I love USING the weakness - the divergence of the beam,
as a feature, rather than fighting it!

Now if I can just figure out a way to make a non-mechanical rotating
beam.  pulse when goes by north, time around to find angle divergence
finds distance voila! polar coordinates!  plus the resulting position
data should be monotonic.  have some distortion have some error, but
never overlap.  perfect.

Single pole stuck in the ground don't get too far away. battery
consumption to power a beam to overcome sunlight? snow light?
This technique is worth thinking about for the lawn robot!

ARRRGGG! I don't mind making mistakes, I just hate to make them so
publicly!!

The angular width of the beam will be constant! therefore the time the
beam takes from start to end will be constant - no matter where you
are along th beam's line - unless the beam's width is a parallel set,
then as a function of distance away there will be a difference in time
from start to end. Well, at least I can use the start of beam and end
of beam to deterine the CENTER of the beam accurately. Then, just
need two beams, just like VOR.

 

[Robert Macy]

The speed of sound doesn't depend on pressure, just temperature and mean
molecular weight.

The real downside is wind and temperature changes.  Unlike light, sound
is an oscillation in a physical medium, and when the medium moves, the
rest-frame speed of sound changes.  The speed of sound at 20C is about
343 m/s (perhttp://www.sengpielaudio.com/calculator-speedsound.htm),
or about 770 mph.  Thus a 2-mph breeze will cause a phase error of
0.25%.  Similarly, a 2-degree temperature difference will cause the
speed of sound to change by 0.5 * 2/293 = 0.34%. (The factor of 1/2
comes from the binomial expansion of the square root--v goes as sqrt(t)).

Cheers

Phil Hobbs

thanks for the URL. specifically says "..for ideal gases..." does not
depend on pressure, Wonder how much speed does change with pressure,
must be small. Isn't atmosphere close to ideal gas?

for breeze, can sit still and watch 'motion' to validate measurements,
or have software do a warning when the rms distance measurements go
frenetic. Temp can monitor temp and 'adjust' maybe. but will crunch
the numbers to find the impact. Seems like further away, the less
accuracy, duh! Hopefully, a whole field will be scanned and finished
way before the temp changes more than 6F, less than 3C

 

[Owen Roberts]

Actually, true laser beams diverge like mad. They're Gaussian. You can
adjust the collimating lens and the angle generating lens to open up
the angle as far as you want. A modern diode laser, before it hits its
collimating lens, has a 10' x 40 degree elliptical pattern. In fact,
the cheaper the diode, the more the ellipse.

You then use a optical bandpass filter, either colored plastic, or a
multilayer dielectric filter(cheap ones for 632.8 nm aka 633 abound)
to get your SN ratio through the roof.


Phil beat me to it on the Scotchlite (TM) tape. White retro-
reflective bike reflectors also return a strong cone.


Neat thing about the Scotchlite, the detectors can be at the
lasers.

Just spin a 45' mirror on a motor shaft above a vertically oriented
laser.
Use front surface mirrors to get rid of the ghost beams in the timing.

If it cost you more then 60$ to make each laser station, I'd be
shocked.

Steve

 

[Fred Bartoli]

Robert Macy a écrit :

need a cheap, low power [read that as don't want to pay much, and need
it portable] x-y position sensor that tells me where I am when I ask..

specs
1. RELATIVE position once START is ok. Then can RESET and start again
from another marker.
2. over an area of approx 100 ft by 100 ft
3. update when requested, every 50, up to maybe 100mS. just need to
mark where I'm at on a grid.
4. absolute accuracy [over relative field] 'gentle' distortion of over
6 inches be ok.
5. relative accuracy for adjacent readings, better than +/- 1/4 inch,
probably get by with +/- 1 inch, but should be monotonic.

Note: absolute accuracy means a gentle stretching of the grid relative
to the perimeter is ok. but needs to be reproducible. Like exactly
where a mark is can be off by 1 foot, but it is always possible to
find the EXACT location within the tighter relative accuracy.
Monomotonic is like no backlash.

I could get away with wheels like those used by the realestate agents,
but the less paraphernalia/clutter the better plus likely to be in
rough terrain.

Be nice to simply stick a post in the ground and know where I am in
polar coordinates away from that stick when I ask.

I thought of GPS, relative, but expensive? power hungry? and updates
only every second ...sometimes. I thought of simple g force MEMS
devices double integrate, but their drift may preclude using that.

Anybody have a brilliant approach?

Have 4 emitter-receivers, 1 mobile and 3 "base stations".
Then close the loop of your mobile gizmo through successively each of
the ground units, so as to have the loops oscillating. Oscillation
frequencies will depend on the distance to each base station, from which
you can extract the position.

1 foot is about 2ns round trip, so it's probably workable without too
much efforts and you can have an easy try with COTS RF gizmos...

 

[George Herold]

Near electromagnetic field strength measurement could return coordinates in
100 x 100 ft square with better then 1 ft accuracy.
Put a source of a field at unknown location, and two receivers at known
locations; solve for the position.
It could be done vise versa: two transmitters at known location and a
receiver at unknown location.
Of course, this method is prone to EMI and distortions due to large
conductive or ferrous objects; however it is simple and it could be more
robust then acoustics, optics or EM trilateration.

Vladimir Vassilevsky
 DSP and Mixed Signal Consultant
 www.abvolt.com

Great approach. Firt thing I thought of, too. but alas, I have some
fields already in existence around the system that the introduction of
any new field is likely to cause problems.

Just a footnote to readers: Distance measuring using magnetic fields
is really cheap and fairly accurate [relative terms]. I designed a
portable distance measuring system for a Medical Product which
essentially measured displacement along 16 axes, simultaneously. We
built a simple two axis unit for measuring displacement from 6 inches
to over 27 inches. Cost less than $5. No coil winding. Power
consumption less than 5 mA per axis at 5Vdc.

Can you share details? AC or DC B field? Flux gate magnetometer as
detector?

George H.

 

[whit3rd]

need a cheap, low power [read that as don't want to pay much, and need
it portable] x-y position sensor that tells me where I am when I ask..

A bunch of cheap USB cameras, looking down on the scene from various
angles, and a bunch of software.

Yep, this is the only low-cost hardware that'll do it.
You can get a little fancy, use a fluorescent target and
a flashing UV illuminator source to help declutter the images.

The Gollum scenes in LOTR were shot with this kind of digitization,
of an actor in a bodysuit studded with targets. They decluttered the images
with a large studio painted black...

 

[hamilton]

specs
1. RELATIVE position once START is ok. Then can RESET and start again
from another marker.
2. over an area of approx 100 ft by 100 ft

You did not mention what surface you would be dealing with.

hamilton

 

[Robert Macy]

On 9/18/2012 10:31 AM, Robert Macy wrote:
 >specs
 >1. RELATIVE position once START is ok.  Then can RESET and start again
 >from another marker.
 >2. over an area of approx 100 ft by 100 ft

You did not mention what surface you would be dealing with.

hamilton

thought I alluded to it by mentioning portable, field, rough terrain.
in other words down by the river, up the side of a mountain, along a
arroyo floor, edge of a ??, out along the edge of a cliff, that kind
of 'rough' terrain.

 

[Robert Macy]

Can you share details?  AC or DC B field?  Flux gate magnetometer as
detector?

George H.

AC fields, although DC doesn't take much current

flux gates are waaay too noisy to get 125 ppm rms

 

[Charlie E.]

thanks for the URL. specifically says "..for ideal gases..." does not
depend on pressure, Wonder how much speed does change with pressure,
must be small. Isn't atmosphere close to ideal gas?

for breeze, can sit still and watch 'motion' to validate measurements,
or have software do a warning when the rms distance measurements go
frenetic. Temp can monitor temp and 'adjust' maybe. but will crunch
the numbers to find the impact. Seems like further away, the less
accuracy, duh! Hopefully, a whole field will be scanned and finished
way before the temp changes more than 6F, less than 3C

Many moons ago, a collegue of mine tried to develop an ultrasonic car
detector, just a transducer pointed straight down, and you needed to
be able to tell when a car was there. All it took was a light wind,
and the return no longer hit the detector. This was at a distance of
just feet...

 

[rickman]

Wow! didn't think of that at all! Thought of a mouse-like rolling
thingy, but the approach in that URL is much more self contained and
actually fits our existing software!

We have a prize winner!

=============================================

Good luck getting it to work over rough ground.

Be careful with this one. I don't think a mouse gives an accurate
measure of how far you have moved. They depend on on screen feedback to
know how far to move it, so there is no reason to be accurate.

Rick

 

[rickman]

thanks for the URL. specifically says "..for ideal gases..." does not
depend on pressure, Wonder how much speed does change with pressure,
must be small. Isn't atmosphere close to ideal gas?

for breeze, can sit still and watch 'motion' to validate measurements,
or have software do a warning when the rms distance measurements go
frenetic. Temp can monitor temp and 'adjust' maybe. but will crunch
the numbers to find the impact. Seems like further away, the less
accuracy, duh! Hopefully, a whole field will be scanned and finished
way before the temp changes more than 6F, less than 3C

Just have reference markers somewhere which will allow frequent
calibration to remove all of the above effects.

Rick

 

[Robert Macy]

Be careful with this one.  I don't think a mouse gives an accurate
measure of how far you have moved.  They depend on on screen feedback to
know how far to move it, so there is no reason to be accurate.

Rick

I was amazed at the accuracy of the cheap Logitec mouse. I paid $1.
for the mouse at surplus outlet. Just turn the software to 'linear'
get rid of that auto speed detect. move it around on a sheet of paper
while recording the 'clicks' The backlash in the plastic gearing was
easily removed and compensated for. amazing to me was the
repeatability of going one way and then back the other way, the
repeatability was more than good enough. From memory the 'distortion'
from x to y was less than 1-2% !! couldn't believe that one. Plus,
there was already a C/C++ library program to ask for the position of
the mouse - part of Windows environment. so like for only $1 I got
this small range x-y position sensor that had ?? accuracy. From memory
it was better than a few mils, have to go check my archives for that
info. Better not say without checking.

 

[Spehro Pefhany]

Be careful with this one. I don't think a mouse gives an accurate
measure of how far you have moved. They depend on on screen feedback to
know how far to move it, so there is no reason to be accurate.

Rick

It probably does do a pretty good job, provided the distance between
lens and surface is kept constant. Check out the Avago data on the way
the chips work, they're quite impressive, especially for the price.

Unfortunately, like most parts based on consumer markets, the product
lifetime tends to be brief, if not nasty and brutish.


Best regards,
Spehro Pefhany

 

[George Herold]

AC fields, although DC doesn't take much current

flux gates are waaay too noisy to get 125 ppm rms

OK not flux-gate. A Squid? Some atomic thing?
(A Rubidium magnetometer is a long list of insturements I'd like to
make someday.)
(But most likely never will.)

George H.

 

[Martin Brown]

Lugging extra stuff out into the field is not so good. But, thanks for
triggering the brain! Ok, put two posts, each generating a pure known
tone. Then simply count the standing waves as I walk around. Voila!
relative 'optical' mouse, but done using audio.

You are probably better off with a click or random pattern sent on RF
and audio/ultrasonics the time delay gives you your position.

Wind speed will be a serious distorting factor though outside.

 

[Robert Macy]

OK not flux-gate.  A Squid?  Some atomic thing?
(A Rubidium magnetometer is a long list of insturements I'd like to
make someday.)
(But most likely never will.)

George H.

Geometrics makes an RF based magnetometer that measures earth's field
out to 12 digits. We had it out in the field near the Bay, open area,
and the reading from left to right had steady digits then fluctuating
a little bit digits and the LSD's were totally blindly spinning
digits. As I watched the pattern, I noticed the spinning pattern
changed a bit and told him about it. He pointed to a greyhound bus 2
blocks away. THAT was disturbing the earth's field enough we could see
it!

I feel really, really dense, but I still don't understand how the
principle works.Something about kicking electrons up in orbit, then
they fall back down and you measure the numbers? or the frequency or
something. But what gets me is that 12 digits of reading! Has to be
frequency to have that many digits.

 

[George Herold]

Geometrics makes an RF based magnetometer that measures earth's field
out to 12 digits.  We had it out in the field near the Bay, open area,
and the reading from left to right had steady digits then fluctuating
a little bit digits and the LSD's were totally blindly spinning
digits. As I watched the pattern, I noticed the spinning pattern
changed a bit and told him about it. He pointed to a greyhound bus 2
blocks away. THAT was disturbing the earth's field enough we could see
it!

I feel really, really dense, but I still don't understand how the
principle works.Something about kicking electrons up in orbit, then
they fall back down and you measure the numbers? or the frequency or
something. But what gets me is that 12 digits of reading! Has to be
frequency to have that many digits.

The physics is the same as is in atomic clocks. (But they are
tickling different atomic (magnetic) transitions.)
Try googling "optical pumping". I could talk your ear off... but.
(There's a nice article in Scientific American by A. Bloom, 1960, but
a quick search on the web only turned up references)

When I was building the first optical pumping in a basement, I could
see when parked cars moved about on the street out front. (And that
was in a DC measurement.)

The magentic fields in our production space have become so bad (60 Hz
AC crud and "DC" changes do to elevators or other bits of iron moving
about.) that I have to take new units out to my country 'estate' for
testing.

George H.


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