Relative Positioning

Is it possible to build the following scenario, and if so, can anyone point me on the right track?
I have a person and I want to place small transceivers in various
places on his body. The transceivers would all be able to detect their distances from each other using some sort of wireless transmission and then send those distances to a small wireless processor attached to the person's hip. The processor would then use the distance variables to determine the relative positions of each transceiver in 3-dimensions.
I've tried to research this and I've found a lot of sensors that can determine distances to a random object in its field of vision. But I would like to know the distance between 2 discrete objects that both move around. I was thinking maybe I can send a wireless transmission with a timestamp and a serial number and then the receiving device would record that information along with the current time stamp. Maybe the transmission time could then be used to calculate the distance?
Are there currently transceiving devices that can do this? I am less concerned about the processing and mostly concerned about the data capturing.
Thank You.
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On May 5, 1:17 pm, snipped-for-privacy@gmail.com wrote:

The "time of flight" for wireless, as you propose, is going to be very difficult because electro-magenetic waves (speed of light) are extremely fast. So, I woudl abandon that idea. But, sound is an alternative. If each transceiver takes turns emitting an ultrasonic "ping", then each of the other transceivers can record the time it takes for the sound to travel to them. But, you also need to let each transciever know when the pulse was emitted by either a wired or wireless transmission (an IR beacon is common, but you may not have line-of sight to do this)
Here is one link to the type of system I am describing, as commonly used in robotics; http://www.restena.lu/convict/Jeunes/beacon.htm
In your case, you might send an IR signal (similar to those used in a TV remote control) to tell a particular beacon to "Ping"... at the same time the other transcievers read the IR signal to know which transciever is doing the Ping and start to listen for it.
But, in the above method, you might have issues with some transcievers not getting the IR signal. But, if you are in a small-ish room, you can just flood the room with the IR, and it may work. Still, a wired system is going to be a lot easier to do.
Joe Dunfee
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snipped-for-privacy@gmail.com wrote:

Sure. Look up "magnetic motion capture". It's not easy to do, but it's done routinely.
                John Nagle
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In case you are attempting to use this for motion capture, probably the best place to start is the wide variety of technical articles regarding motion capture systems used for such things as movie special effects, medical research, etc.
Maybe you need all these transceivers, or maybe what you can really use is a jogging suit with (for example) triggered LEDs or fluorescent pads embedded in it, and shot by a high-resolution digital camera. Motion capture software translates the markers into distance and movement, and can export that data for such things as integration with Maya or another character animation program. You get what you are looking for (distance between two objects) but of course this system only works for objects visible to the camera.
John mentions magnetic AC trackers, but there's also acoustic and several other technologies in regular use. Depending on the resolution you need you might be able to use inertial tracking, like what a Wii controller does. The solution really depends on how accurate you need to be, how many frames per second of motion you need to record, and how many markers you need to monitor.
-- Gordon
snipped-for-privacy@gmail.com wrote:

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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.
Many Thanks.
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On May 7, 1:13 pm, snipped-for-privacy@gmail.com wrote:

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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snipped-for-privacy@gmail.com wrote:

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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Gordon McComb wrote:

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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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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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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Hi Matt,
in principle you are right, the time difference would represent the change in distance divided by the speed of light. So 1mm difference would correspond to approximately 3ps time difference.
There are several sources of inaccuracies: - the speed of light depends on the medium. In air, it's close to vacuum but not quite, and depends somewhat on temperature, moisture, pressure etc. - the circuits/equipment you use to measure such small time differences will not be perfectly accurate, depending on how much you would want to spend on this. In addition to initial inaccuracy (which you could calibrate out) the errors might also depend on temperature, supply voltage, etc. - the phase shift in the cables that connect the antennas to the equipment often depends on e.g. bends in the cable and so on (unless you want to spend many hundreds of dollars per yard of cable). So moving the antennas (and therefore the cables) around might influence the measurement results - at short distances (in the order of one wavelength or less) the coupling between the antennas becomes somewhat complicated and involves other modes than TEM waves - which will result in other phase shifts/time differences than you would expect based on simple radio wave propagation - however, the most important source of inaccuracies is probably the reflections of radio waves off other objects and so multiple copies of the radio signal will reach the receive antenna along different paths (this is called multipath) and with different delays - very similar to what happens with echo's and sound.
So, in summary, yes it will work, but it is not quite so easy and/or accurate as you might imagine - sorry...
Peter

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Approximation:
At the speed of light (radio wave, infrared beam, gamma ray, whatever) , 1 foot (30cm) is 1 nano second. (10 ^ -9 seconds) (I seem to remember 11.7 inches, but hey, who's counting?)
1mm is 1/300 of this distance.
You do the rest of the math(s).
How were you planning on measuring the propagation delay?
Deep.
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snipped-for-privacy@gmail.com wrote:

You'd probably want to use a modulated RF signal, and compare the phase of the returned signal (from another sensor) with the original. This is how some surveyors transits work. The circuitry is simpler, as you are not measuring speed of light. You could probably achieve 1mm accuracy depending on the frequencies used. How to make all this so that each sensor is stand-alone and independent is another matter. If you use more than two sensors you need a way of timing the pulses from each. There will need to be some coordination between the sensors. There are other problems to overcome that are left to the OP.
-- Gordon
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Gordon McComb wrote:

I built a LIDAR that way once, around 1990. Surprisingly similar to an FM radio, IF cans and all. But nobody does it that way any more. (Well, the Swiss Ranger does, which allows them to use LEDs as the light source). Today, you usually just count, really fast. Here's a Bosch laser range finder for $129: "(Amazon.com product link shortened)". Accuracy is 1-2 mm.
The main problem with working in this area is that while the parts are cheap (they're derived from cell phone technology), the test instruments required are expensive.
If you want to work in this area, you'll need shelves full of RF engineering books, some really good RF test gear, and the software for designing boards with gigahertz transmission lines. There are advanced radio hams who do such things.
Look up "A UWB based Localization System for Indoor Robot Navigation" for some hints on current (2007) thinking on the subject.
This is a basically good idea which takes significant engineering effort to make work, but probably won't be expensive to replicate.
                    John Nagle
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Lots more helpful responses. It sounds like it will be very difficult to achieve. It seems strange that we can get accuracy to a meter with transceivers flying 20,000 kilometers above the earth (GPS) but we can't get accuracy to a millimeter in a system where the transceivers are all a couple feet from each other.
Thank you for all the help,
Matt
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On Fri, 9 May 2008 10:27:43 -0700 (PDT), snipped-for-privacy@gmail.com wrote:

Not strange at all unless one continues to believe that apples oranges.
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Si Ballenger wrote:

You can get that accuracy. It just takes the quality of engineering that goes into GPS receivers. There has not, to date, been enough of a market to justify the engineering investment.
Look into the GPS systems used for "precision farming".
                John Nagle
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John Nagle wrote:

The comparison with GPS is irrelevent anyway as the OP insists that the sensors be self-contained, for whatever reason. The bulk of the system of a GPS network is the satellites. The receiver merely collects data. Si is correct; it's apples and oranges, even before the investment starts.
-- Gordon
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On Sat, 10 May 2008 11:14:23 -0700, John Nagle

"I think somebody should come up with a way to breed a very large shrimp. That way, you could ride him, then after you camped at night, you could eat him. How about it, science?" - Deep Thoughts by Jack Handy
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Hello, please let me know which one is the right newsgroup that I can ask question about industrial robotics shool in dallas texas area. thanks
Peter
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