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When I turn, the car hops up and down.

  1. Mar 21, 2005 #1

    I have a 3 wheel autonomous small car... built out of legos.

    And umm... the problem is when I turn the back two wheels in opposite directions... to produce left or right turns... the autonomous vehicle tends to "up and down"... ... what causes that?
  2. jcsd
  3. Mar 21, 2005 #2
    Why are you turning the back wheels in opposite directions?
  4. Mar 22, 2005 #3
    is it a solid axel? if it is both wheels are turning at the same speed, this makes the inner wheel hop. Because the inner is trying to turn as fast as the outer wheel.
    you need a limited slip axel.
  5. Mar 22, 2005 #4


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    One question: your front wheel, is it on an axle, or can it freely rotate like a caster (like on a shopping cart)?

    If your front wheel has an axle, then here's the problem:

    By turning the rear wheels in opposite directions, you are rotating the vehicle about a vertical axis that is centred on the rear axle - the front wheel gets dragged sideways, causing the hopping.

    Instead of trying to turn "on a dime", try turning while moving forward slightly. Rotate only one wheel - keep the other one wheel fixed or even moving forward slightly while turning the other forward. This will allow the front wheel to move forward as it pivots.
  6. Mar 22, 2005 #5


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    Yes, his front wheel is on an axle.
  7. Mar 22, 2005 #6
    Yea on an axle...

    Also... do i even need the front wheel?

    What is the front wheel there for anyways? lol... i'm kinda confused myself lol.

    Any benefits if i keep it there?
  8. Mar 25, 2005 #7


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    Have I missed something with your posts about 3 wheeled vehicles? Surely the purpose of having more than two wheels is so that your vehicle doesn't fall over?
  9. Mar 30, 2005 #8
    What if my vehicle is balanced without the front wheel? Will there be any downside to remove the front wheel?
  10. Mar 30, 2005 #9
    No vehicle is balanced on two wheels. Even a bike, your body is constantly adjusting to maintain balance.

    You need the third wheel for balance and stability.
  11. Mar 30, 2005 #10
    What if the "third" wheel causes turning difficulties? What can i do about that.
  12. Mar 30, 2005 #11
    You could use a smooth plastic "nub". A little nylon sphere on a stick would work well(I've seen a lot of sumo-bots with these). Or you could go crazy and install a handfull of 3-axis motion sensors and a complicated algorythm to continually balance your bot(like the Segway).
  13. Mar 31, 2005 #12


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    Or you could have a caster (or did we discount that in another thread?)
  14. Mar 31, 2005 #13


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    This is not true.

    A vehicle whose centre of gravity is below the axle balances just fine thank you.

    (Of course, in practice, you'll want to accelerate slowly, lest the wheels remain motionless and the vehicle does the rotating...)
  15. Mar 31, 2005 #14
    Yes, that is true, but I think in practice, it is very difficult to get the CG below the axle, unless you know what you are doing.

    It is a hard mix of balance and math and design to get it though. Most people who do it end up there not on purpose
  16. Mar 31, 2005 #15


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    I wouldn't say getting the centre of gravity below the axle would be an accident, nor would I say it's difficult. The difficulty is controlling such a two-wheeled vehicle.
  17. Mar 31, 2005 #16
    Is controlling the movements of a humanoid difficult? LIke Asimo, is it hard to control something like that?
  18. Mar 31, 2005 #17


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    I think you should stick to your three wheeled car.

    The movements of a humanoid, I would think, would be very difficult to reproduce.
  19. Mar 31, 2005 #18


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    Consider this: Honda has over a decade and many many millions of dollars in development costs in that robot, its about the only one like it (or even close) that has been publically announced, each version improves over the previous but battery life is still measured in minutes. Pretty easy to conclude, its incredibly difficult.

    Mimicing the Segway's ability to balance on two wheels is far easier and can be done with off-the-shelf parts for around $1000-1500 with only minimal code development. Some guy built one and put up a website on how he did it and you could probably google him.

    Your weight distribution will make a huge difference here too. Go to a home center and push around a lumber cart, the six wheel kind. It has two fixed wheels in the center that carry almost all the load. The four casters are at the corners but mounted slightly higher then the center wheels. Viewed from the side, the cart needs to "tip" to one set of the casters or the other because of the mounting height difference. But using a balanced load it is very easy to track straight and make corners with the cart because the load is over the two fixed center wheels, the casters are more just a stop to keep the cart from tipping too far.

    You could mimic that - put all your weight so its balanced over the two center wheels, now the front wheel is more like an outrigger that prevents the vehicle from tipping, probably need to add a back wheel too to keep it from tipping that direction as well. But now when the two drive motors want to turn they carry all the load and get pretty much all the traction so the outrigger is just along for the ride.

    If you look at the combat robots for "Battlebots" or "Robot Warriors" and so on you'll see some two wheel designs setup like this expect they just slide the front/back edge of the machine on the ground. Since that edge is for stability but carries no significant load it doesn't even need a wheel.

    If your robot is the one trying to lift something in a claw, this is going to be more difficult as now you have the weight of the load to factor in. A caster wheel sounds like an even better idea.

    Look at this lawn mower - its rear wheels do all the steering, the front wheels are casters that just follow whatever the rear wants to do...
  20. Mar 31, 2005 #19
    Tank tracks? Have you thought about those? Also, do you have the grabber arm built into to front of your vehicle or did you build it so it attaches between the rear wheels or behind the rear wheels? Attaching behind the rear wheels a little will make load distribution a little easier.

    http://www.thetoolsbar.com/prod_images_large/ACFaC4ERB.jpg [Broken]

    Or, why don't you try two front(common axil single drive motor) and one steerable rear wheel?

    Or, two independent front wheels with a caster rear wheel or sliding nub?

    Do you know what the mass of the object you need to lift is? If you do then you can conceivably balance the load perfectly over the rear wheels. You car is probably hopping because you have too much mass over a fixed wheel.


    http://www.robotgames.net/RobotGames/wcrg_archives/2001/gallery/hi-rez/2001-Simmons_Sumo1.jpg [Broken]
    Last edited by a moderator: May 2, 2017
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