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Re: [SeattleRobotics] Rocker Bogies

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  • Peter Balch
    ... I was wondering about the general question of when or why are rocker bogies better. So, yes, I m considering diverse applications. ... Yes, but why? What
    Message 1 of 39 , Aug 10, 2011
      > you may be mixing applications a bit

      I was wondering about the general question of when or why are rocker bogies
      better. So, yes, I'm considering diverse applications.

      > that the rocker-bogie has been replaced with torsion arms since the mid
      > 40's as tank
      > designs improved

      Yes, but why? What was wrong with rocker-bogies?

      (Or should I say "rocking bogies" as "rocker-bogies" is used by some people
      to refer specifically to the NASA/JPL patented design.)

      > on a tank each of the roadwheels moves independently so the track
      > can conform to uneven terrain , spread the load evenly

      Why is "independently" better than rockers? And if it's better, why do NASA
      go for rockers?

      Does independent suspension actually "spread the load evenly"? A
      rocker-bogie is better at spreading the load evenly while independently
      sprung wheels are more or less loaded when over a bump or hollow. AFAIK,
      that's one of the reasons NASA chose rocker-bogies - to ensure the load on
      each wheel is always the same.

      > Rocker-Bogey suspensions do not isolate a sprung weight from the navigated
      > surface.

      In the case of NASA, yes. But it's not an essential feature of that design.
      Each wheel could have springs and dampers as they have on tanks (like the
      HVSS Sherman) or rocker-bogie artic-trailers.

      > Rocker bogey systems can be used in a skid steer application, however
      > they are much less efficent at it than a hard suspension.

      That's an excellent point I hadn't thought of.

      As Kevin Ross said: skid-steer rocker-bogies "are actually fairly common in
      the FIRST world". I wonder if they've noticed the problem.

      My experience with building multi-wheel skid steer is that the most
      important part of the design is to ensure that the buggy is fairly wide
      relative to its length. The corner wheels should form a square.

      Peter
    • David Buckley
      Michael Excellent you noticed that somewhere I mentioned the Rocker pivot meaning the Bogie pivot. %-( However the bogie virtual (it moves depending on the
      Message 39 of 39 , Aug 13, 2011
        Michael
        Excellent you noticed that somewhere I mentioned the Rocker pivot meaning the Bogie pivot. %-(
        However the  bogie virtual (it moves depending on the height of the front wheel) pivot point is, for a vertical face and horizontal ground as in figure 8-5, above the initial contact point of the front wheel with the face, hence the Normal at the contact point is below that even virtual pivot point.
        Fig2-5 shows clearly what I mean.
        Even with the geometry of the bogie arms moving the virtual pivot slightly back, the Rover needs to move backwards to allow the Rocker to swing up.
        Since as described earlier the front wheel is effectively underneath the surface, gravity can not hold the wheel against the vertical surface. Unless something holds the wheel in place on the surface there can be no traction to enable the wheel to climb. so how is the Rocker going to swing up?
        Looking at Fig5-7, as the rear bogie wheels drive towards the face they impose a anticlockwise torque on the rocker-V tending to lift the front wheel and body.
        Infinite traction at the front bogie wheel (middle wheel) would do but then the torque from that wheel has to lift the whole Rover. Hmmm, scratch that idea.
        Barlas's whole aim has been to ensure the leading bogie wheel rises on a vertical path, hence the lambda geometry, enabling it to climb the step and avoiding the problems I have been describing for the front wheel, and avoiding the tuck-in problem of Fig2-7. This aim seems to have been directed by 'what if the middle wheel motor should fail'.
        But he has completely overlooked the fact that the problem will occur with the front wheel of the Rover which will prevent the middle wheel ever reaching the step.
        So once again I ask "How does the front wheel get on the step".
        Unless it can get on the step in the first place the lambda geometry of the bogie is useless.
        DAvid
         
         
         
         
        ----- Original Message -----
        From: Michael
        Sent: Saturday, August 13, 2011 10:38 PM
        Subject: Re: [SeattleRobotics] Re: Rocker Bogies

         

        DAvid,


        Although things may be going off track a bit, I have enjoyed lurking in on the technical discussion.  In that paper you refer to figure 8-5.  In reviewing some of that paper myself it seems you are referring as the point of attachment to the body (figure 6-1, pivot point "O") as the pivot for the front wheels.  I understand that if the body of the rover was to be fixed in space that would be the pivot point, however in looking at the mechanics, I see the "virtual" pivot point as being somewhere near the rear axle as the front wheel climbs the curb since the body is not required to be fixed in place, but we are assuming the rear and middle wheels shouldn't be allowed to rotate in reverse to get the front wheel up the curb. In other words, using figure 6-1, pivot point "O" would move backwards for the front wheel to drive up, but the contact patch of the rear and middle wheel may continue to move forward slightly, or remain stationary applying forward force.  I wouldn't mind hearing your thoughts on that possibility.  It is entirely possible I am misunderstanding something as well.

        Michael


        On Sat, Aug 13, 2011 at 3:27 PM, David Buckley <david@...> wrote:
         

        Jack
        I have no idea where you got all that from but it wasn't from me.
        You were saying how you used to play with Rovers in school in Florida and linked to
        I asked how the front wheel in fig 8-5 was supposed to get up the step.
        You accused me of being sarcastic and then went on to give links and explanations which had no bearing on the question I asked.
        Seemingly you have been unable to refute my analysis of the mechanics involved and now want to go on about the cost of machining aluminium wheels.
        I am not interested in accounts of whether this or that educational Rover could climb this or that rock. Concentrate on the question I asked. It wasn't about whether blue or gold rovers could climb better. It was simply 'how could the front wheel in Fig8-5 get up the step'. Because if it can't in the ideal mathematical world of his paper then it nullifies any of his further reasoning and brings under suspicion the correctness of the rest of his paper.
        Firat Barlas's paper was supposed to be about 'Design of a Mars Rover Suspension System', well at least that is the title. So unless someone can explain from where come the magic forces which allow the front wheel to climb the step I stand by my opinion that the paper is full of nonsense.
        Note that this has nothing to do with the President or the Queen or the Pope.or you, unless any of you were one of his supervisors at the time.
        DAvid

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