Subject:
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Re: Received my RIS 2.0 - the two motors run at different speed! :-(((
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Newsgroups:
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lugnet.robotics
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Date:
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Sat, 11 Jan 2003 22:25:38 GMT
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Original-From:
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Steve Baker <SJBAKER1@AIRMAIL.nomorespamNET>
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Viewed:
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1241 times
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Stephane Simard wrote:
> After further testing and measuring, I found that the difference is much
> larger - it actually deviates a good one meter for every three meter travelled.
OK - so...doing the math carefully and explaining how I did it...
1m
______ E
| *\
| * \
| * \
3m | * \
| * \
|* \
|_____________\C
S P
* == The curved path taken by the robot.
S == The start point of the robot.
E == The end point of the robot
C == The center of the circle.
P == Draw a line vertically down from E to meet the line S-C, where
it hits S-C is a point that I'll call 'P'.
The length of the line E-C is the radius of the circle, so is the
length of the line S-C. We know that the line P-C is one meter less
than the length of the line S-C.
The triangle P-E-C is a right triangle - so we can use Pythagoras's theorem:
P-C squared + P-E squared = E-C squared.
Now - if we insert 'R' for the radius of the circle, we get:
( R - 1 ) squared + 3 squared = R squared
Expanding this out:
R squared - 2R + 1 + 9 = R squared
Subtracting R-squared from both sides and rearranging:
2 R = 10
So R = 5 meters. Your robot (if left to it's own devices) would
drive in a circle of radius 5 meters.
Your robot is 15cm wide - so outer wheel travels in a circle of radius
5.15 meters and the inner wheel goes just around a 5 meter radius.
So, the ratio of the distances travelled by the two wheels is is
2xPIx5.15 : 2xPIx5 ...which is... 5.15:5 ...which is... 1.03:1... which
represents a 3% difference in motor speeds. Which is fairly typical
for any pair of motors picked at random.
CONCLUSIONS:
~~~~~~~~~~~~
You have a nicely matched set of motors. Your next set *could* be four
times as bad...some of my motors differ by 11% !
Keep them! Treasure them! Show your geek friends what *great* motor
matching you have!
---------------------------- Steve Baker -------------------------
HomeEmail: <sjbaker1@airmail.net> WorkEmail: <sjbaker@link.com>
HomePage : http://www.sjbaker.org
Projects : http://plib.sf.net http://tuxaqfh.sf.net
http://tuxkart.sf.net http://prettypoly.sf.net
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