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Ev3 does not go straight please help

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  • Ev3 does not go straight please help

    Kids have done coding. But there is no repeatability. Sometimes it goes straight and sometime to goes to left or right. They are getting frustrated. How can I help them.we have two wheels in the front and the round steel marble in the back. Any advice please.

  • #2
    You posted in the FRC forum. You should delete your post and move it to the FLL forum.

    I am going to take a shot in the dark and guess that you need to reset your motors. Try putting a move steering/tank block set to off before the first move of your program.

    Originally posted by Dean Hystad View Post
    Or a move tank block set to off or a motor block set to off. When you combine multiple missions to run in some sort of sequencer it is very important to reset the motors using a move/motor stop block.
    Last edited by FLLVetran; 11-18-2019, 08:00 AM. Reason: wrong forum

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    • #3
      Frustration often comes from not understanding why something isn't working. My team is much less frustrated when they know what problem needs to be solved. When they see random problems and don't know the actual cause, they get frustrated.
      The way around it is to first understand what is causing the problem and then design a solution. When a team doesn't know why there is a problem, they do random things to try and fix it.

      Have them test the problem and define the problem in a simple sentence and quantify it, for example:
      Robot randomly moves to the right or left X inches after X rotations.
      Robot drifts randomly right or left but, only to one side each time.
      Robot drifts one way and then the other way while going forward on the same launch.
      Robot immediately points the wrong direction upon launch and drives straight X rotations but, off line.
      Run the test at different speeds, 100, 75, 50, 25, is there any difference?

      Each of these can be caused by a different problem or combination of problems. The possible problems include internal motor error, motor gear lash/backlash, wheel slip, warped tires (if using the low profile fat wheels), tires not seated on the wheel correctly and more.
      Start with a mechanical examination. Is the robot built correctly, both wheels in alignment, wheels not bending, axels not bending. Run the motors and watch the wheels spin freely, are they warped or do they wobble.
      Can you swap out the motors to eliminate a motor problem (rare in my experience)?

      Have them test to see if they can determine the problem. Start by driving forward for 2-3 rotations. They must start the robot in the same place each time. Perhaps with the back of the robot against a wall. Run it 10 times and have them really watch what is happening. Write it down.

      Have them look at the robot upside down and very slowly rotate each wheel by hand. What do they see? They should find that each wheel will freely move about 2-3 degrees before there is resistance from the gears and motor (gear lash/backlash).

      You can also talk about how the motors themselves have an error. Have them look up how the motors actually measure the rotations and what is the internal error of the measurement. They will find that the motors use a rotation sensor with an error of about +-1 degree.

      If you add up all the error, you can end up with one wheel backward 4-5 degrees and one wheel forward 4-5 degrees for a total direction error of 8 degrees and it can be as high as 10 degrees. If they are old enough to do the math have them calculate the total distance of that error.

      For example, a 4 inch diameter wheel has a circumference of 4 x 3.14 or about 12.5 inches, / 360 degrees = about 0.035 inches of error per degree. 8 degrees of error means about .27 inches of error right when the robot moves off the launch. Even driving straight after a launch, after about 2-3 rotations, the robot could be off the mark by a foot or more.

      Ask them, what if one wheel starts rotated forward and one starts rotated backward with this error, what happens to the direction of the robot?

      Finally, how can you correct the problem? Programming, mechanical, launch setup.
      Then, at robot design judging, they should discuss all the testing and what they learned. Hope they find the problem, good luck.

      FLL coach Trash Trek on, State 5x, World 3x, ref, judge advisor.

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