Re: Hexapod design drawings

--- In CAD_CAM_EDM_DRO@y..., Doug Fortune <pentam@h...> wrote:

> rab@r... wrote:
> >
> > Doug,
> >
> > How are the joints for backlash ?
>
> The socket universals that I purchased, have no backlash.
> And of course being designed for high torque (a hundred or
> more foot pounds of torque) they are up to the task (for a
> small machine).
>
> > Can you pre-load the axis on a hexapod design ?
>
> That is the basis for a hexapod's phenomenal rigidity at
> such a light weight!
>
> Of the six actuators, three are always in tension, acting
> against three in compression. Coming to a triangular
> mount point above, the pairs
>
> AB CD EF
>
>
> instead are paired differently at the bottom (tool platform):
>
> BC DE FA
>
>
> so if AC&E are in compression, and BD&F are in tension at
> the top, then each actuator under tension is matched with
> one under compression. So it is also at the bottom (ie
> B under tension is paired with C under compression, etc).
>
> Keeping this state of affairs of course is one of the
> constraints of the hexapod controller software, and
> hence very difficult.
>
> - - -
> Some of you are no doubt thinking that under great tool
> pressure, all of the members will be under compression.
> In fact (or should I say, I understand) that the whole
> structure would then relax (causing positioning error)
> when the compression is relieved.
>
> Instead, the tensions and compressions are pre-set to
> far overwhelm the tool pressures, and hence there is
> insignificant relaxing (dimensional change) when the tool
> pressure is relieved (helping maintain tolerances).
>
> Or so I am led to believe......
>
> Doug Fortune
> http://www.cncKITS.com

I was wondering if the way the pivot point is on the univesal joint
going to complicate figuring positioning because it is not going to
pivot from the same point like ball joint would.