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Posted by cadcoke4 on May 7, 2008, 5:14 pm
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On May 7, 1:13=A0pm, mattrapop...@gmail.com wrote:
>=A0It should be accurate to the millimeter or better
> if possible. =A0It also must be entirely self contained. =A0No cameras.
> Onboard processor.
Again, we are a bit vague about your intended purpose and exact
situation. But, another idea is to use a "string pot". This is a
spring-loaded spool that has a flexable wire wrapped onto the spool.
Inside the spool is a rotary potentiometer. As the wire is pulled
out, you determine the distance by the value of the potentiometer.
The drawback is that it is only a measurement between two points.
Though you can add multiple units, I am sure there is a practical
limit. Also, they are not that cheap.
A poor-man's stubstitute might be to use the key-chain spools (or the
smaller ones designed to hold I.D. badges) and use a wire with some
resistance. As the wire is pulled out, the current is forced to
travel down more of the wire, and therfore the resistance will
increase. I haven't done this, so I am unsure how reliable it would
be.
If you want more ideas, you will have to be much more specific about
your project.
Joe Dunfee
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Posted by Gordon McComb on May 7, 2008, 5:45 pm
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mattrapoport@gmail.com wrote:
>
> Thank you for the thoughtful responses. I've looked into the
> solutions mentioned and unfortunately I don't believe they will suit
> my purpose. I realize now how important it was for me to stress the
> required accuracy. It should be accurate to the millimeter or better
> if possible. It also must be entirely self contained. No cameras.
> Onboard processor.
Sounds like a great R&D project in itself. Maybe there's something like
this that has the accuracy, is completely self-contained, and doesn't
require line of sight (or sound). But most sensors of one type or
another fail in one or more of these categories. The majority are not
self contained, and must relay their information to a central processor.
Do also consider that a single sensor cannot determine its own relative
position. It can determine its distance to another sensor, but not and
X/Y position. For this you need multiple sensors, and they need to talk
to one another. A coordinated, modulated RF signal may work for this.
The interal processing of the data will no doubt need to be your own
project.
-- Gordon
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Posted by John Nagle on May 8, 2008, 12:26 pm
Please log in for more thread options Gordon McComb wrote:
> mattrapoport@gmail.com wrote:
>> Thank you for the thoughtful responses. I've looked into the
>> solutions mentioned and unfortunately I don't believe they will suit
>> my purpose. I realize now how important it was for me to stress the
>> required accuracy. It should be accurate to the millimeter or better
>> if possible. It also must be entirely self contained. No cameras.
>> Onboard processor.
That's a great project. Those are better specs than the motion
capture industry can currently achieve. Do you have venture capital?
This is not impossible, but it's not cheap or something that can
be done with off the shelf technology. The magnetic motion capture
systems have trouble getting better than 2cm accuracy. The multiple
camera video systems do a bit better, but you have to be within
the camera space and there are still occlusion problems.
I could see something with GPS, pseudolites, phase angle
measurement, local Bluetooth links, accelerometers, and an
IK system for cleanup. Nice project. Five people, two years,
about $5M, I think.
An interesting option would be to use cameras looking outward
strapped to the user's joints. During recording, just record
all the video. In postprocessing, use SLAM to extract positional
data. Then it's mostly a software problem. If you're in some
environment with lots of texture the SLAM matching can latch onto,
this could work.
John Nagle
Animats
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Posted by on May 8, 2008, 1:11 pm
Please log in for more thread options If I take an RF transmitter (A) and an RF receiver (B) and I precisely
record the time it takes for the the signal to get from A to B, I
wonder how accurate the distance measurement would be. If I moved A
1mm away from B, would the recorded time accurately represent the
change in distance?
Thanks,
Matt
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Posted by Jim Hewitt on May 8, 2008, 2:54 pm
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> If I take an RF transmitter (A) and an RF receiver (B) and I precisely
> record the time it takes for the the signal to get from A to B, I
> wonder how accurate the distance measurement would be. If I moved A
> 1mm away from B, would the recorded time accurately represent the
> change in distance?
Yes, but....
Your 1mm distance would represent 3.33x10^-11 seconds at the speed of light
(elecromagnetic radiation, including radio waves). That's about 0.033
nanoseconds (unless my math fails me). Can you measure time differences
with enough accuracy to reliably measure that kind of difference?
Jim
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> if possible. =A0It also must be entirely self contained. =A0No cameras.
> Onboard processor.