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[size=12]Ok here's the actual problem ...

I have
        1)a set of robots (around 4)
        2)In a large room , say around 10mx10m
        3)Wireless communication among robots(say xBee )
       4)Obstacle detection IR and ultrasonics sensors.
       5)A computer(PC) is connected to the same Xbee network.

I want to
      1)Map the area(2D Map should be sent back the PC)
      2)Make use of my PC to perform complex processing(if any) during the process.

challenges      
       1)I must use the robots to make a map faster and more accurate than using a single robot.
       2)Don't include the robots itself as obstacles in the map.


Main problem that i'm facing is
      1)How to find the relative positions of the robots in the room(Assume the room is completely new to the robots and the PC).
       2)How does a robot differentiate another robot from an obstacle.[/size]
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I don't think you connected the grounds, Dave.
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So, what solutions have you thought of?
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One method you -might- try is an IR beacon on top of each robot, sending out a pre-coded pulse train (laid over a 38-40 kHz carrier - like a remote - to reduce ambient light interference) that indicates the identity for the robot. The beacon is omni-direction (I'll leave implementation details to you).

Then, also on top of each robot, is a spinning IR-sensitive photo-transistor; this detector is spinning in a horizontal plane (parallel with the floor). If it "sees" an IR beacon, it can know the identitiy of the sending robot; it can know the direction vector of the other robot  (based on the angle of the sensor relative to "forward" of the robot), and in a crude fashion (via the intensity of the IR light) it can know how far away along that vector the other robot is.

If the data rate is fast enough, you could even communicate among the robots with this same system (although line-of-sight and inter-robot distance issues will be a problem); you might also be able to use it to scan the room perimeter and obstacles...
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http://www.pololu.com/catalog/product/701 I found an IR Beacon here . $30 for a single beacon is a bit costly , and I cant buy this from my country. I'm not a electronics guy to build something like that on my own , Can anybody help me out with the components required and circuit design for this so that I can get it done locally . I really need this sensor . Its not accurate on the web.
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Use the Source, Luke.
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I would have guessed that a bandpass around the modulation frequency should do it. Then the Robots even can send out their name in Morse so you can identify them.

For the Band pass, here's a nice tutorial: http://www.sbprojects.com/knowledge/ir/ir.htm

In the worst case, you can create the carrier frequency with a NE555 and control the message to send with a digital output pin.

Korman
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Quote
http://www.pololu.com/catalog/product/701 I found an IR Beacon here . $30 for a single beacon is a bit costly , and I cant buy this from my country.


That beacon system won't work for what I was describing, mainly because the IR receiver modules they are using won't give back an intensity (varying voltage) level. The schematic though is close to what is needed; the IR receiver modules would just need to be substituted with IR phototransistors connected to the analog inputs of the Arduino (honestly, you'd want to make these devices completely separate from your Arduino - basically build them with a "standalone" Arduino-ized ATMega328 substituting for the PIC in the schematic).

Quote
I'm not a electronics guy to build something like that on my own , Can anybody help me out with the components required and circuit design for this so that I can get it done locally . I really need this sensor . Its not accurate on the web.

Then you better hire somebody to build this for you, as this part is fairly minor compared to the rest of the robot. Really, the schematic given for that beacon is close to what you need; just substitute in an ATMega328 for the PIC, and use IR phototransistors in place of the the IR receiver modules (more or less), and hook them to the analog inputs (so you can measure the intensity of the IR LEDs). The cost of LEDs and IR phototransistors won't be a great amount.

However, I tend to wonder about the fact that $30.00 is too much for you for these modules, but if you bought parts and hired somebody to build them, that's going to end up being cheaper? Maybe; I don't know where you are located - maybe labor is super-cheap where you're at...

You are not likely going to be able to do this "plug-n-play" - this isn't Lego. This is going to take design, time, and a lot of programming effort. If you want it to be inexpensive, you're either going to have to learn how to do this yourself, or find somebody who can help you that will work for peanuts or nothing. Note that each of your robot chassis are likely going to cost more than these sensors...
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