Local Positioning System

I have always wanted to build/design a mobile robotics platform for various tasks around the house/yard, even a contruction site. The
biggest problem that I have run into is a getting a positioning system that will give me accurate positioning (+-2 inches). There are some really expensive GPS systems, in the thousands of dollars. I don't think that is realistic or cost effective to put in a robotic lawnmower. I was trying to come up with my own system using wifi or zigbee nodes to triangulate the position of the robot. I don't know if this is possible for the resolution that I want or if someone else has already tried it. Any thoughts or info would be great.
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Jerd King wrote:

Unless you use specialized (= $$) equipment, measuring distance with RF modules will not give you the required accuracy. The speed of light is 3*10^8 m/s; at 10' ~= 3m, the travel time is only 10 ns. Not gonna happen. You might have better luck measuring angle and triangulating, but few devices have tracking directional antennas and metal causes RF signals to bend anyway.
I've seen two systems for indoor navigation, but neither qualifies as cheap (comercially ~$2k each).
The ultrasonic systems (e.g. Cricket) measure the time of flight of an ultrasonic chirp, much like ultrasonic rangefinders do. When the robot is stationary and you have clear line-of-sight, these systems are accurate within an inch or two. My experience with these systems is that you have low measurement rates, and things mess up when the robot is moving or nearby objects cause ultrasonic echos. http://cricket.csail.mit.edu /
Optical systems such as NorthStar identify visual landmarks on the ceiling in order to navigate. I've not tried this, but it seems promising. Something similar might be done with IR emitters on the ceiling, a B/W camera on the robot, and optical filters to only pass IR light. http://www.evolution.com/products/northstar/devbundle.masn
On the cheap end, I think mapping using dead reckoning and simple distance sensors is probably the best that's available.
Later, Daniel
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Jerd King wrote:

pseudolite technology ( pseudo-satellite )
http://www.google.com/search?hl=en&c2coff=1&q=pseudolite
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I had another idea, I know that pinging a server/node is going to be hard because the speed of Rf waves. What about AM or changing the power output of the tranmitter i.e. you lose receiption of radio station as you drive away. Some radio station put out 80-100 watts, could you detect distance with 1-2 watt or even milliwatts, just a thought.
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Jerd King wrote:

PLRS
http://www.fas.org/man/dod-101/sys/land/plrs.htm
google has lots of hits, but I did not see any design specifications.
Google "radio position" or "radio navagition" found the PLRS
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I have been pondering IR beacons around my house that would operate at 38.5Khz so the tuned IR receivers would pick them up. To distinguish one from the other, each would blink on and off at a differenet rate such as 10 times per second, 20 times per second, etc.
The robot would have an IR receiver on a servo. The pulse width sent to control the servo would be carefully calibrated to the angle the servo turned to at that pulse width so that the robot could know the angle to each beacon it could see. The robot would rotate the servo until it got the strongest signal from that beacon. Then it would know it was looking straight at it.
The beacons would have to be near the same level as the sensors on the robot, presumably near the floor and be omni-directional in the plane of that level. If they were mounted high, the robot would have to search a 3-dimensional field to find them and be much more complex. If mounted high, they would have the beacons would have to be omni-directional in 3 dimensions instead of just two.
I think a 555 timer at 38.5Khz with another 555 at the on/off frequency used to turn on/ off the 38.5Khz one would do the trick. Maybe need to add a driver to supply enough current to supply a number of LEDs with sufficient current to make them really bright. The 555 will supply 200mA. I think more will be needed to make the group really bright.
Just my 2 cents worth.

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