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Re: [SeattleRobotics] Re: 4-bar linkage

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  • KM6VV
    It looks very similar. I was trying to see the (now obvious) relation between the numbered line segments and joints, There were dimensions on the drawing, but
    Message 1 of 2 , Dec 31, 2011
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      It looks very similar. I was trying to see the (now obvious) relation
      between the numbered line segments and joints,
      There were dimensions on the drawing, but maybe you need coordinates of
      the joints as well. As you suggest, i can open up the cad drawing and
      get the coordinates as well as the link lengths.

      I'm still not clear about "syncing" a second jaw, but I'll play with it.

      We're all warm and cozy here on the couch, not yet midnight.

      Thanks again,

      Alan KM6VV

      On 12/31/2011 5:38 PM, Peter Balch wrote:
      > From: Alan
      >> Were you able to use the dimensions from my PDF, or did you scale off of
      >> the PDF? I'm not seeing the correlation between the link lengths
      > I tried scaling it off the pdf but couldn't get it right so I pasted your
      > pdf image behind the linkage and "traced over" it. (It's not something you
      > can easily do with the exe but I've got the source code.) I guess I got it
      > wrong.
      >
      > I think the easiest way for you to get it right would be:
      >
      > Decide a scaling factor.
      >
      > Work out the x-y coordinates of each of the important points on your CAD
      > drawing.
      >
      > In Edit mode, shift-drag the corresponding points to those coordinates. The
      > x-y of the cursor is shown at the bottom left.
      >
      >> l[2].Len= 0.00000000000000E+0000 (overstrike 1& 2 ?)
      > IIRC, [1] and [2] are the y and x offsets of one servo axle from the other.
      > Use the scrollbars to move the servos around. I think the next two link
      > lengths (or scrollbars) are the servo arms.
      >
      >> Doesn't seem to be metric scaling.
      > It's no particular scaling - I just traced the image.
      >
      >> I can add the 2nd gear and jaw? That would make it interesting!
      > Yes. You'll need to learn how to add new points. The Help tab explains it.
      > You build triangles up from existing points - mouse-down on existing point
      > A; mouse up where you want new point B; mouse-down on existing point C. You
      > can only make a "ladder" of triangles in that way. You can't add extra
      > struts or delete anything other than the last point. (It means that the
      > program can _always_ calculate the points in order you created them.)
      >
      > When you've made a new jaw, you'll find that the standard Run" mode moves
      > the servos in a way that's not useful to you (90deg out of phase). The
      > program is really for designing legs, not jaws.
      >
      > So select Move mode, use the scrollbars to move the servos or mouse-wheel to
      > where you want them - e.g. jaws fully open. Right-click (popup menu appears)
      > and create a "waypoint". Use the scrollbars to jaws fully closed.
      > Right-click to create another "waypoint". The Right-click and select "Run
      > Waypoints". The jaws should then open and close.
      >
      >> Do all of the triangles you show work into the calcs?
      > Yes. The only way to make a curved jaw is with lots or triangles.
      >
      > In the above example of the ABC triangle, the computer knows where A and C
      > are, it knows the lengths of links A-B and B-C, so it can calculate where B
      > is. Then D, then E, etc. (It's rather like "geographical triangulation".)
      >
      > That works well until the angle between struts approach 0deg or 180deg. As
      > in a real linkage, such positions are unstable - a singularity of sorts -
      > the joint could flip the "wrong" way. Servo.exe highlights such joints in
      > red.
      >
      >> This is a really great way to explore four-bar links!
      > What I find nice is that, while the animation is running, you can alter the
      > lengths of the links and see the results immediately.
      >
      >> HAPPY NEW YEAR!
      > And to you. I've just been sitting at my desk, watching the big midnight
      > firework display over Edinburgh Castle. I reckon it's a bit to cold and damp
      > to be outside this year but thousands of others are less wimpish. I can hear
      > them singing in the street.
      >
      > Peter
      >
      >
      >
      >
      > ------------------------------------
      >
      > Visit the SRS Website at http://www.seattlerobotics.orgYahoo! Groups Links
      >
      >
      >
      >
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