Re: [CAD_CAM_EDM_DRO] degrees of freedom Was: Angle Iron Ways

Raymond Heckert wrote:

>Look closer... If you have 'fixed' length struts (I mean really fixed, not
>extensible or retractible) the platform cannot move. On a hexapod, at least
>two adjacent struts must be extensible to have even one degree of freedom.
>four adjacent extensible struts should allow you 3 degrees of freedom, and
>all 6 struts movable should give you your 6 degrees of freedom. ...Unless
>my observations are greatly in error...
>
>RayHex
>
>

Hi there.

I think that four struts aren't enough, unless you can control the joint
angles in addition to the strut length. Consider a chair with ball
joints at both ends of each leg. If you extend the legs equally, the
seat could be raised. But, if the legs "twist" then the seat may stay
at the same height but rotate (and it could rotate in either direction
as well!).

With a pair of struts at each of 3 points on the platform, you can
constrain the location of that point to somewhere on a circle (the plane
of the circle is perpendicular to the line that joins the two strut
anchor points, and where it intersects that line depends on the strut
lengths). With only one strut per platform connection point, the strut
only constrains the point to be on a sphere, which is much less of a
constraint.

It's hard to discuss these things textually, but hopefully that
description helped a little :)

- Steve