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Thread: Keep Robot Going Straight

  1. #1
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    Sep 2008
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    Default Keep Robot Going Straight

    Our robots can never keep moving in straight line and always ends up going off course. When placed on the table, it would start off going straight then would veer off a bit causing it to hit the wall. We would use our hands to keep it in a straight path as it moves but, of course, we know it is not allowed in the competitions. We first thought the problem was the pivot wheels and tried 4 wheel robots but it still ends up going off course. Can anyone provide suggestions on how to keep the robot moving in a straight line?

  2. #2
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    Default Re: Keep Robot Going Straight

    What is your wheel set up? 2 wheels powered in the back, skid steers in the front? 2 wheels powered in the front, skids in the back? Or are all 4 powered?

    One big issue is where the COG (Center Of Gravity) is. If there is a lot of weight on the skidding wheels, the driving wheels don't have nearly enough traction to move the robot affectivle. Try placing the NXT brick right above the wheels and see if that helps.
    Jayko543
    FLL member of 2 years
    http://www.legosinparadise.blogspot.com

  3. #3
    Join Date
    Dec 2009
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    61

    Default Re: Keep Robot Going Straight

    Getting LEGO robots to drive straight is and always will be hard. If you are using the wheels that come with the NXT 1.0, try making sure that the tires are on both of the wheels the same way (there are several ways to do it). Also, consider putting a piece on the side of your robot so it can slide or roll along the wall and have the robot turn into it.

  4. #4
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    Default Re: Keep Robot Going Straight

    This is a common problem. Take a look at some of these threads from previous years and see if any of the suggestions discussed may apply to you.

    FLL: Is there a reason "Go Straight" often drifts left?

    Does your robot drive straight?

    Here are some quick things to check
    1. Use motor ports B and C for better synchronization
    2. Use a single MOVE block controlling both motors B and C instead of two independent MOVE or MOTOR blocks controlling the motors independently (unless you really want to roll your own motor synchronization code).
    3. Does the robot always turn in the same direction? If so, switch motors and see if it turns in the opposite direction.
    4. Test your motors and try to find the best two for a matching pair.
    5. Realize robots won't usually go perfectly straight. You can live with a drift of about 1 inch to the side for every 4 feet traveled, as long as it is consistent.

  5. #5
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    Smile Re: Keep Robot Going Straight

    Thanks, everyone, for your very helpful responses. Glad to know I'm not the only one experiencing this problem and that there are some ways around it. Hopefully any one or some of these will work.

    We have tried numerous models (copied and original designs). Some examples of the copied designs are the castor bot by Dave Parker, Mr. Kee's domabotics design, the taskbot, tribot, etc. It seems that no matter what design we build, we have the same problem with it moving off to one side. If this is so common, how does it not interfere when in competitions? The whole "works today, messes up tomorrow" thing is driving us nuts!

  6. #6
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    Lapeer, MI
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    Default Re: Keep Robot Going Straight

    This is a problem that everyone has and may be made worse by uneven tables at competitions. Most teams use light or touch sensors to frequently establish known positions on the table (go until the robot touches a wall or until it finds a line).

    To prepare for a tournament it helps try your robot with the table tilted in various directions (lift each side/end 1-2 inches) to ensure that your robot can 'find its way' in spite of table variations.

  7. #7
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    Default Re: Keep Robot Going Straight

    Rethink your strategy. If you need to go straight, but the robot won't do so repetedly, you will fail. Learn what your robot can do well and develop a new strategy around that.

    FLL isn't supposed to be easy. If it were easy it wouldn't be any fun.

  8. #8
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    Default Re: Keep Robot Going Straight

    There is not a single cause and getting repeatable results with imperfect parts is part of the engineering challenge. TimDavid gives some good suggestions.

    Here are some other suggestions we have found over the years:
    1) The motors are likely to perform more consistently at 70% power than at 100% power.
    2) Sometimes it's better to use skids for the unpowered points of contact with the board rather than wheels (free spinning wheels seem to operate very inconsistently)
    3) It's important that the weight of the robot be balanced between the two drive wheels. If one wheel has slightly less weight on it than the other, it may spin just a bit and cause the robot to tend towards one side
    4) Try different tires configurations - depending on the surface and the weight of the robot, different tires may have more traction - the tank treads often give very good traction because they have multiple points of contact with the surface.

    Hope this helps. Consistency is always a big challege.

  9. #9
    Join Date
    Jul 2010
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    Default Re: Keep Robot Going Straight

    Just another thing to consider.
    If you have the ability, consider testing your motors to see which motors are most closely sync'ed. Marco over at TechBrick.com has a great article found here.

  10. #10
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    Default Re: Keep Robot Going Straight

    Quote Originally Posted by RoboDesigners View Post
    Just another thing to consider.
    If you have the ability, consider testing your motors to see which motors are most closely sync'ed. Marco over at TechBrick.com has a great article found here.
    The problem with matching motors is that it is at best a band aid. My girls wrote a PID controller around the motors to keep them synchronized. On our flat wood floor the robot could repeatedly drive 4 meters with less than 1 cm of error (less than 1/4 percent). Confident the robot would go where programmed they wrote odometry based solutions for all the challenges. During practice the robot scored 400 9 times out of 10.

    At the tournament the robot had problems. The mat or table was slightly different from what we had at home and it made the robot fail every time. The girls could not correct the problem because the error only occurred on the tournament table and we could not practice there. Luckily they got to run one mission on a different table (there were 4 at the tournament) and scored 400 on that run.

    For the state tournament they threw out all the odometry based solutions (which they knew were a bad idea to begin with) and went extreme the other way with tons of sensor usage. Again there were problems at state, but the robot could self correct and the scored reliably in the 380-395 range.

    Time spent making the robot go straight is time better spent learning how to use the sensors and working on more robust solutions.

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