Subject:
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Locating your position with the help of fixed beacons (was Re: A RCX 'Beacon')
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Newsgroups:
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lugnet.robotics
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Date:
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Thu, 29 Jul 1999 21:32:37 GMT
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Viewed:
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800 times
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In lugnet.robotics, lego-robotics@crynwr.com (Cwikla, Brian) writes:
> [...] It would involve a home-brew sensor and a
> home-brew module. The sensor would attach to the input port on the RCX.
> The module can be placed anywhere and emits a pulse/beep/signal. The sensor
> on the RCX can pick up that pulse/beep/signal and be able to tell the
> general direction it is coming from. [...]
The simplest way to do this is to build a device that emits a sweeping light
signal, similar to a lighthouse, and a blinking signal synchronized to the
sweep. Unlike a lighthouse, you want it to sweep back and forth (otherwise
there are problems with wires getting twisted).
Start with a battery pack and a motor. Make the motor turn a wheel that is on a
vertical axis; attach a rod from this wheel to another rod or wheel in such a
way that the second rod or wheel turns 90 degrees to the right, stops, turns 90
degrees to the left, stops, and repeats. (If you're wondering how these rods
would connect, it's similar to the wheels on a steam locomotive). This is the
"sweeper". Put a light on top of this and place black bricks and/or beams
around it so that its light only goes out in one direction, in a narrow beam.
You also have to set up a light that blinks, synchronized with the sweeping
motion. The easiest way to do this is to use the "polarity switch" (part 6551,
Shop at Home service pack 5120) and a similar arrangement of rods so that the
switch travels 45 degrees from the upright (off) position to one of the "on"
positions and then back to upright in the same time that the sweeping light
completes one sweep. The effect of this is that the blinking light will turn on
once every time the sweeping light does a sweep.
Place the beacon in the corner of the room, or anywhere that covers the area
your RCX will be in. Let's make a map of the area with the beacon and the RCX:
*------------------------ A
| \
| \
| \
| \ R
| \
| \
| \
| \
| B
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C
"*" represents the beacon, and "A", "B", and "C" represent different positions
of the sweeping beam. The beam will sweep from A to B to C, to B and back to A.
An object out in the field, like an RCX at position "R", is said to be at a
certain "heading". Using the air or nautical standards, line "A" is a heading
of 90 degrees, line "C" is a heading of 180 degrees, and line "B" is somewhere
in between.
When viewed from some distance away, the light level from the beacon will look
something like this (from one heading)
__-------''---_____-------''---_____-------''---_____
or like this (from a different heading)
__------------_''__------------_''__------------_''__
or somewhere in between. The details depend on how you built your beacon. The
long pulse is from the blinking light, and the short pulse is from the sweeping
light. In this example I assumed that the blinking light is on about 2/3 of the
time, but that might not be the case with your beacon. What matters most is the
relative timing. As you can see, the relative timing of the two pulses depends
on where you are in relation to the beacon.
Your RCX should have its light sensor set up to pick up ambient light, and
should be directional. To do this, mount a 1x2 black beam in front of the light
sensor. This blocks the red LED and makes the light sensor only see light that
comes through the hole in the black beam. To make its field of view even
narrower, add another 1x2 black beam.
Your RCX software has to start by rotating slowly until the signal is the
strongest. "Strongest" means that the fluctuation in light level is big. This
means you're facing the beacon.
Then the RCX has to sit still and watch the light level for long enough to
determine the amount of time that elapses between each pulse, and the relative
delay between the beginning of the long pulse and the beginning of the short
pulse. The ratio between these is the "phase". You should also measure the
amount that the light level changes between pulse and non-pulse (the amount of
fluctuation).
The most accurate reading you'll get from this is the phase, which tells you
your heading from the beacon's point of view. You can also get a pretty good
estimate of your distance to the beacon (based on the amount of fluctuation)
and since you're facing the beacon, that means your angle is the heading angle
plus 180 degrees
Since you have already rotated to face the beacon, can use the amount of
fluctuation to determine your distance to the beacon. Also, since you're facing
the beacon, that means your angle is the same as the beacon heading angle plus
180 degrees.
- Robert Munafo
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Message has 2 Replies:
Message is in Reply To:
 | | A RCX 'Beacon'
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| Well, I'm not an expert when it comes to small electronics and making them, so I thought I would post this question to this list. After working with the RCX for nearly a year now, I've found a tool/sensor that I would really like to have, but no (...) (25 years ago, 29-Jul-99, to lugnet.robotics)
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