An additional thing to look at would be use of single wheel pivot turns vs
two wheel (one forwards one backwards) point turns.  Holding one wheel
still and moving only one wheel at a time will tend to give more consistent
turns.  When one wheel is turning forwards and one wheel is moving
backwards if the wheels are not turning at the same speed and time then the
center of rotation of the robot will wander.  With a pivot turn (one wheel
turn) the pivot point will remain the wheel that is not moving.

To go with this you can consider creating a ramp turn where the turn starts
out slow but finishes up at a higher speed, thus avoiding slippage at the
beginning of the turn.

On Wed, Oct 24, 2012 at 8:43 AM, John Barrett <[log in to unmask]>wrote:

>
>
> FWIW, we also have seen some inconsistencies with a few of our programs
> this season.  I helped one of the kids cover part of the table with butcher
> paper and attach a sharpie to the robot.  He then ran one of the troubled
> programs multiple times.  It became clear after about 20 runs that the
> robot was failing in a consistent sort of way.  Specifically, the robot
> started out following the same track.  However, after the fourth move,
> which was a turn, the robot would take one of two different tracks.  The
> difference in terms of number of trails was close to 50%.  Half the time
> the robot would take the intended track, and half the time it would turn
> slightly too far.  He measured it and estimated that the extra motor
> movement was about 14 degrees.
>
> He tried a number of strategies to fix this (changing the power level, for
> example) but to no avail. Eventually, he added motor resets after each of
> the move blocks and this seemed to work.  There is still variances in the
> track, but it is much more consistent.
>
> My advice is try a few motor resets in your program.  It seems like the
> rotation counter memory sometimes gets a little out of whack and the motor
> resets help to alleviate the compounding of errors.
>
>
> The second issue that he discovered by tracking the robot's trail on paper
> was that a two wheeled turn (for our robot using our motors (read as: your
> mileage may vary)) is not as consistent as a one-wheeled turn.
>
>
>
> Good luck!
>
> John
>
>
>
>
>
> --
>
> John J. Barrett
> Industrial Medium Software, Inc.
> 1616 Anderson Road
> McLean, VA 22102
>
> (c) 703-231-5094
> (p) 703-286-0818
> (f) 703-286-0888
>
> http://www.industrialmedium.com
>
>
>
> On Oct 24, 2012, at 8:17 AM, Amy Sabarre wrote:
>
> We are also having these issues with our robot.
>
> On Tue, Oct 23, 2012 at 7:16 PM, Bob and Ann Henshaw <[log in to unmask]>wrote:
>
>> I am a 5 year veteran home based coach with a 2nd year team of 10 year
>> olds. We are working with a Mindstorms NXT system that we purchased new
>> last year, with a fully recharged battery. We are experiencing several
>> intermittent erratic behaviors (listed below) that is driving our team
>> crazy. Please let us know if you have any debugging suggestions.****
>>
>> ** **
>>
>> Observed behaviors:****
>>
>> ** **
>>
>> **1.       **After completing a planned series of instructions, with the
>> last instruction having 2 wheels turning equally (straight line), the robot
>> suddenly makes an additional turn (one motor, rotating robot about 30 -
>> 40degrees). This occurs even after deleting the program, recompiling and
>> redownloading it, and even after adding an all motor stop block to the
>> program. Our team is referring to this as a ghost and are convinced that
>> the robot is now haunted, just to get us in the Halloween mood. ****
>>
>> **2.       **The robot, using a two wheeled turn (one motor going
>> forward, one going backward) intermittently turns too far or too short.
>> Programming is being done in rotations for both motors. Because both motors
>> are running together, I canít tell if it is related to the problem above.
>> This occurs about 1 in 5 runs.****
>>
>> ** **
>>
>> I do not think that this is a cable issue, as we are not seeing anything
>> beyond the usual minor navigation errors due to lineup, wrinkles in the
>> mat, etc. That leaves the motors and the NXT itself. Could this be a bad
>> servo motor and if so, is there any way to test them? ****
>>
>> ** **
>>
>> Appreciate your suggestions!****
>>
>> Sincerely,****
>>
>> Bob Henshaw****
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>
>
>
> --
> *Amy Sabarre
> PK-8 STEM Coordinator
> PK-4 Science Coordinator
> *
> *HCPS*
> *One Court Square*
> *Harrisonburg VA 22801*
>
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