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Subject: 
Re: Advice on Rotation sensors and steering please.
Newsgroups: 
lugnet.robotics
Date: 
Wed, 20 Sep 2000 23:28:43 GMT
Original-From: 
Andy Gombos <gombos_2000@yahoo.SAYNOTOSPAMcom>
Viewed: 
472 times
  
In my experience setting the motor to off has worked best. Constantly check
for differences in the counts. I was only using 2 sensors, and after 500 or
so steps(about 15 ft), there was only a 1 count differenece. The main factor
in determining what happens is if the robot is perfectly straight or not. 1
mm of non-straightness can lead to almost 1 foot of deviation at the end,
relative to your reference line.

Andy

Steve Baker wrote:

OK - so I am now the proud owner of three rotation sensors.

I've built a really basic two-wheeled robot with a caster at
the back for the purposes of getting in some programming experience
with these things.  Each wheel has a rotation sensor (connected via
a gear train so I can play with the sensitivity)- each has it's own
motor.

I'm using NQC.

So, step one - steer in a straight line.  Since two motors picked
at random don't go at anything like the same speed, I need to correct
for that.

Here is some pseudo-code:

   zero both sensors
   both motors full speed forwards

   while ( true )
   {
     if ( sensor1 < sensor2 ) {
       full speed wheel-1 ;
       slow down wheel-2 ;
     } else {
     if ( sensor2 < sensor1 ) {
       slow down wheel-1 ;
       full speed wheel-2 ;
     } else
       full speed on both wheels ;

     Wait(1);
   }

...well, this (and MANY variations on this) fail miserably for a
variety of reasons. So I have some *basic* questions:

* I heard that the rotation sensor can lose count if it rotates too
  fast for software to keep up.  Can anyone give me an idea of what
  a 'sensible' rate is?  I guess it's a compromise between precision
  and risk of count lossage.

* In the pseudo-code, I have 'slow down wheel-N'.  There seems to be
  a suprising variety of ways to do that..although there isn't actually
  a 'speed' control! :

     + I can set the motor output to 'Off'.
     + I can momentarily apply 'OnRev'
     + I can reduce the 'power' from 7 to something
       smaller...either suddenly or gradually.
     + I can 'float' the motor output.
     + I can do a combination of these things.

  Which of these things works best?  Does the choice depend on the wheel
  size/tyre type/gearing/robot weight - or this there simply 'one best
  way' ?

* The rotation sensor counts the total rotation measured in 22.5 degree
  steps - right?  If we drive more than 32767 steps, it'll wrap around to
  -32768 steps or something - right?

Once I can get it to go in a straight line, I'll go on to figure out
how much I can accellerate/decellerate without tyre slippage, then write
code for turning, reversing, collision recovery, etc.

* Has anyone ever written an RCX emulator?  It would be nice to be able
  to debug my code on the PC where I could print out variables and such.
  With the limited 'byte code' that NQC uses, it seems like this wouldn't
  be so hard to do.

--
Steve Baker   HomeEmail: <sjbaker1@airmail.net>
              WorkEmail: <sjbaker@link.com>
              HomePage : http://web2.airmail.net/sjbaker1
              Projects : http://plib.sourceforge.net
                         http://tuxaqfh.sourceforge.net
                         http://tuxkart.sourceforge.net
                         http://prettypoly.sourceforge.net



Message has 2 Replies:
  Re: Advice on Rotation sensors and steering please.
 
The solution is to float the motor. There is code in the NQC tutorial for this. It works very well. the file is 9_rotation.nqc I will E mail it if you do not have the file. -- Bob Fay rfay@we.mediaone.net The Shop (URL) Gombos" (...) (24 years ago, 21-Sep-00, to lugnet.robotics)
  Re: Advice on Rotation sensors and steering please.
 
(...) So you tell your system to drive both motors forward and if you do nothing else then you are only on count off! You must have better matched motors than mine! I see a 10 or 20% difference in their speeds that needs to be corrected to get the (...) (24 years ago, 21-Sep-00, to lugnet.robotics)

Message is in Reply To:
  Advice on Rotation sensors and steering please.
 
OK - so I am now the proud owner of three rotation sensors. I've built a really basic two-wheeled robot with a caster at the back for the purposes of getting in some programming experience with these things. Each wheel has a rotation sensor (...) (24 years ago, 21-Sep-00, to lugnet.robotics)

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