To LUGNET HomepageTo LUGNET News HomepageTo LUGNET Guide Homepage
 Help on Searching
 
Post new message to lugnet.roboticsOpen lugnet.robotics in your NNTP NewsreaderTo LUGNET News Traffic PageSign In (Members)
 Robotics / 20021
20020  |  20022
Subject: 
Re: Received my RIS 2.0 - the two motors run at different speed! :-(((
Newsgroups: 
lugnet.robotics
Date: 
Sat, 11 Jan 2003 22:25:38 GMT
Original-From: 
Steve Baker <SJBAKER1@AIRMAIL.stopspammersNET>
Viewed: 
1139 times
  
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



Message has 1 Reply:
  Re: Received my RIS 2.0 - the two motors run at different speed! :-(((
 
(...) Once again, thanks for making all the calculations for me... Anyway, at the very least, I'll test all four of my other regular gear motors in a mindstorms robot. If I'm lucky to find a pair that are of equal speed, I'll use them in my (...) (22 years ago, 12-Jan-03, to lugnet.robotics)

Message is in Reply To:
  Re: Received my RIS 2.0 - the two motors run at different speed! :-(((
 
First of all, let me say that I very much appreciate your deep insight and your math formulas, which I have forgotten years ago. :) After further testing and measuring, I found that the difference is much larger - it actually deviates a good one (...) (22 years ago, 11-Jan-03, to lugnet.robotics)

8 Messages in This Thread:


Entire Thread on One Page:
Nested:  All | Brief | Compact | Dots
Linear:  All | Brief | Compact

This Message and its Replies on One Page:
Nested:  All | Brief | Compact | Dots
Linear:  All | Brief | Compact
    

Custom Search

©2005 LUGNET. All rights reserved. - hosted by steinbruch.info GbR