Subject:
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Positioning with the RCX - a pragmatic approach
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Newsgroups:
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lugnet.robotics.rcx.nqc
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Date:
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Wed, 3 Jan 2001 17:03:46 GMT
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Viewed:
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1905 times
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THE QUESTION ...
... how to determine and measure a position in a room?
This question was raised several times in LUGNET (e.g. Kris Zabriskie) and
caused some discussions.
As it has been stated, this simple sounding task is not at all trivial and you
are dealing with accuracy problems and additional sensor equipment. Of course,
there are several general solutions, such as GPS, triangulation with optical
Laser scanners, using a camera together with a sophisticated image analysis
software, etc.
As a matter of fact, none of the above can be easily adopted to a Lego robot;
you will need additional (non-LEGO) hardware and special interfaces for
connecting to the RCX.
A PRACTICAL ANSWER - A GRID
I used a more "pragmatic" solution: a XY coordinate system for the working
area, defined by a GRID of dark lines on the bright ground.
The grid sets up a XY coordinate system, having a range from (1,1) up to
(size_x, size_y):
_ _ _ _ _ _ _ _(size_x,size_y)
|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|
(1,1)
I used a grid of 5 x 7 lines with a width of 20 cm, putting black Scotch tape
on white paper (my wife was not a fan of putting the tape directly on our nice
bright floor in the living room ;-) ).
Now, with this grid (XY coordinate system) the "analog" working area is
"digitized", i.e. divided into discrete cells, allowing you to tell the robot
to move to a well-defined location within the working area, e.g. (3,5). The
robot will move along the lines of the grid.
Of course, this grid solution has the "restriction" that it allows only coarse
positioning; you can't tell the robot to move to a location which is 12,7 cm in
horizontal and 35,4 cm in vertical away from its current position. As I said -
a pragmatic solution but it allows nice "pick and place missions". However, you
can set up finer grids, if you are willing to spend the rest of your money into
Scotch tape ;-).
GRIDBOT
I designed "GridBot" that makes use of a grid for moving within a XY coordinate
system and for moving to defined locations. Furthermore, it can pick an object
with its grabber and place it at another location.
If you like, visit my Mindstorms page ("docbernie") and look for "GridBot".
There are some pictures, additional explanations and alternatives. If you
like, I can send you a well-documented NQC program and a small video, showing
GridBot in action (the program with 21 kB is too big to put it on the
homepage).
Keep on moving (hopefully not within a grid) ...
Bernd Frassek
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