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hexapod gait. Was : Interesting Article

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  • Frank van der Hulst (Staff WG)
    Hi Sebastiaan, Yes, I just spent the last few hours looking at hexapods on the Net. I m vaguely planning a hexapod walker as my next project. Your idea would
    Message 1 of 3 , Sep 30, 2003
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      Hi Sebastiaan,
       
      Yes, I just spent the last few hours looking at hexapods on the Net. I'm vaguely planning a hexapod walker as my next project.
       
      Your idea would work -- 3 servos, each driving a Left/Right pair of legs in opposite directions. And a bicore type of system to synchronise the servos.
       
      I think it *might* be possible to do it with two servos, with (complex?) mechanical linkages between the L1, R2, L3 legs and the R1, L2, R3 legs. Or even with a single servo, using the same mechanical linkages plus something to reverse the direction for the other set of 3 legs.
       
      However!!!
       
      The tripod gait means that each leg must be lifted up when it is being moved forward ready to take its next step. Typically (from what I've seen on the Net) this has been achieved by having two servos per leg... one to bend a 'knee' joint to lift/lower the foot, and another one as the 'hip' joint to move the leg forward/backward. Both servos for a leg could be controlled from the same bicore, since the motion consists of (lifted and forward), followed by (lowered and backward). For reverse, you would use (lowered and forward) then (lifted and backward).
       
      Maybe that lifting/lowering could be done with two servos (one for each set of 3 legs) or even one servo (pushing one set of legs down whilst lifting the other set up), but I think we're again talking complex mechanical linkages..
       
      Another option might be some kind of a cam mechanism on each leg that lifts the leg when moving forward and lowers it when moving backward. But that would prevent any reversing motion of the bot... not very useful, I don't think.
       
      Or some kind of cunning thing based on using different length legs, so that the bot tips left/right or forward/backward at various parts of the step cycle, and therefore lifts the other legs off the ground. But that's a bit complex IMHO and probably would limit the bot to walking on flat surfaces.
       
      So.... I'm inclined towards 3 stepper motors rather than forward/back servos. A single motor would provide both the lift/lower and forward/backward movements, all at the right times. The same motor could drive the opposite leg at the same time, but obviously 180 degrees out of phase. And, being stepper motors, (I hope?) the legs would stay in synch. Otherwise some position-detection stuff would be needed.
       
      Frank

      -----Original Message-----
      From: Sebastiaan De Jonge [mailto:sebastiaan.dejonge@...]
      Ive had a look through Google at cockroach gait and there is a heap of
      stuff on hexapods out there. What fascinates me is the description of the
      gait. Left front and rear and right middle move together and then visa
      versa. Should be quite achievable with a master (front legs) and slave
      bicore (rear leags) and an extra h-bridge driving the rear legs. Could be
      done with three servos?

    • Chuck Britton
      Not too long ago - there was a listing or two of parts that are recommended for serious BEAM ers. I have the chance to order some stuff but can t find that
      Message 2 of 3 , Sep 30, 2003
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        Not too long ago - there was a listing or two of parts that are
        recommended for 'serious' BEAM'ers. I have the chance to order some
        stuff but can't find that (those?) email lists.

        could someone please point me to them or include one of those lists?

        thanx
        --
        Chuck Britton Education is what is left when
        britton@... you have forgotten everything
        North Carolina School of Science & Math you learned in school.
        (919) 416-2762 Albert Einstein, 1936
      • Sebastiaan De Jonge
        Hi Frank, Ive just looked up http://richfiles.solarbotics.net/Hexapod.html I dont know whether it was ever finished (are you on the list Rich?). In the
        Message 3 of 3 , Sep 30, 2003
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          Hi Frank,

          Ive just looked up http://richfiles.solarbotics.net/Hexapod.html I dont
          know whether it was ever finished (are you on the list Rich?). In the
          walker/3 motor/circuits section of solarbotics.net there is a java applet
          on hexapod walking. I got all confused in lots of linkages when I
          originally tried imagining it.

          I am curious about the stepper motor control. Im pretty new to this but my
          reading indicates that very precise control is possible (hard drives etc)
          so why not?. Wilf's circuit for the UCrawler promises better control via a
          tuning pot. Maybe there is some fruit there? The frame would also provide
          some opportunity for some artwork....

          Personally my first priority is the solaxelotl. Has anyone sucessfully
          waterproofed a servo?. My vision is a muddy fishtank with a sunny section
          like in the zoo. The gears should be OK but the hole in the casing for the
          shaft from the motor? The circuits could be encased in epoxy.

          I had the privilege of spending some time in Far North Queensland in a
          river mouth complete with crocs, sharks, mud crabs etc. The mud flippers
          came out every low tide. They had two measly flippers up the front and a
          tadpole-like tail. They could obviously breath (dual fuel!) and appeared to
          catch the small insects that jumped around on the mud.

          Sebastiaan
          (heeding the call of the swamp)





          "Frank van der Hulst (Staff WG)" <f.vanderhulst@...> on 01/10/2003
          12:56:54

          Please respond to beam@yahoogroups.com

          To: <beam@yahoogroups.com>
          cc:

          Subject: [beam] hexapod gait. Was : Interesting Article



          Hi Sebastiaan,

          Yes, I just spent the last few hours looking at hexapods on the Net. I'm
          vaguely planning a hexapod walker as my next project.

          Your idea would work -- 3 servos, each driving a Left/Right pair of legs
          in opposite directions. And a bicore type of system to synchronise the
          servos.


          I think it *might* be possible to do it with two servos, with (complex?)
          mechanical linkages between the L1, R2, L3 legs and the R1, L2, R3 legs.
          Or even with a single servo, using the same mechanical linkages plus
          something to reverse the direction for the other set of 3 legs.

          However!!!


          The tripod gait means that each leg must be lifted up when it is being
          moved forward ready to take its next step. Typically (from what I've seen
          on the Net) this has been achieved by having two servos per leg... one to
          bend a 'knee' joint to lift/lower the foot, and another one as the 'hip'
          joint to move the leg forward/backward. Both servos for a leg could be
          controlled from the same bicore, since the motion consists of (lifted and
          forward), followed by (lowered and backward). For reverse, you would use
          (lowered and forward) then (lifted and backward).

          Maybe that lifting/lowering could be done with two servos (one for each
          set of 3 legs) or even one servo (pushing one set of legs down whilst
          lifting the other set up), but I think we're again talking complex
          mechanical linkages..

          Another option might be some kind of a cam mechanism on each leg that
          lifts the leg when moving forward and lowers it when moving backward.
          But that would prevent any reversing motion of the bot... not very useful,
          I don't think.

          Or some kind of cunning thing based on using different length legs, so
          that the bot tips left/right or forward/backward at various parts of the
          step cycle, and therefore lifts the other legs off the ground. But that's
          a bit complex IMHO and probably would limit the bot to walking on flat
          surfaces.

          So.... I'm inclined towards 3 stepper motors rather than forward/back
          servos. A single motor would provide both the lift/lower and
          forward/backward movements, all at the right times. The same motor could
          drive the opposite leg at the same time, but obviously 180 degrees out of
          phase. And, being stepper motors, (I hope?) the legs would stay in synch.
          Otherwise some position-detection stuff would be needed.

          Frank

          -----Original Message-----
          From: Sebastiaan De Jonge
          [mailto:sebastiaan.dejonge@...]
          Ive had a look through Google at cockroach gait and there is a heap of
          stuff on hexapods out there. What fascinates me is the description of the
          gait. Left front and rear and right middle move together and then visa
          versa. Should be quite achievable with a master (front legs) and slave
          bicore (rear leags) and an extra h-bridge driving the rear legs. Could be
          done with three servos?


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