Re: A "simple" Hexapod project - Joints

I have done the inverse kinematics for a triaglide system (general
closed form) as it is fairly simple and just a 4th order polynomial
to solve.
A couple of comments:
These machines are fairly large i.t.o space occupied for a similiar
xyz machine. So you are trading floor/table space for building cost

2 things play a dominant role, working volume and joint range.
As the working volume is irrelevant in the x direction (you simply
add more rail length), the problem is deciding y and z ranges and
these are interrelated. If you want a wide y with a shallow z, to
minimise the length of rail, you need a tall machine. For a deep
working volume you need to increase the x direction.
More important, if you choose to use ball joints, the limit of the
sideways range will decide your minimum leg length. Plus you would
like some clearance above the spindle holder.
Another problem creeps into the system. If you want to minimise rail
movement, you have to operate with the legs as near to vertical as
possible, but this creates the problem that your sideways stiffness
is at a minimum. Conversely if you operate the machine with maximum
sideways stiffness, your vertical stiffness is at a minimum.If you
operate the machine with both vertical and horizontal stiffness,
your machine size is at a maximum.

another thing to look out for. Since this is a non linear movement
machine, there are points where a small movement in the rail can
cause a large movement in the tool position, So the resolution is
not equal over the entire working volume. Steppers would not be the
best of choices.

Despite these difficulties I am working on mine and will keep you
informed

Stephan

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "alex" <telecomt@t...> wrote:
> Graham,
> I would go for a ball joint.
> If you place compression springs between the plate and the base,
you can
> adjust
> stiffness of it.
> Alex
> ----- Original Message -----
> From: Graham Stabler <eexgs@n...>
> To: <CAD_CAM_EDM_DRO@yahoogroups.com>
> Sent: Tuesday, June 17, 2003 3:06 PM
> Subject: [CAD_CAM_EDM_DRO] Re: A "simple" Hexapod project - Joints
>
>
> > Although I am undecided as to whether or not to build a 3 axis
or 5
> > axis machine I have decided on a vertical slide type set up.
This
> > means no actuators as such to design, just bog standard rails and
> > leadscrews. See files section "hexapod ideas" if you don't know
the
> > configuration.
> >
> > This means the mechanics consist of a stiff frame, 6 (for a full
> > hexapod) or 3 (for a triaglide) linear slides, 3 or 6
leadscrews, 6
> > fixed struts a table/router mount. And 12 joints.
> >
> > Carl has used gimbles and these have the advantage that they are
> > simple to make and decent bearings can mean they have little
slop. I
> > am just wondering what other possibilities there might be,
looking at
> > large machines doesn't seem to help much as the joints are often
> > covered or are mysterious ball joints that don't seem to be
holding
> > enough ball.
> >
> > I have just seem some ball joints on the web where the ball is
held
> > by a magnet, the largest was abotu 25mm in diameter and would
hold
> > 25kg.
> >
> > What about universal joints if also allowed to rotate?
> >
> > Another thing I have noticed is that sometimes joints are
combined.
> > Struts that join at the moving table form a triangle with two
pounts
> > fixed so they can be jointed together and then both put in a ball
> > joint of some sort.
> >
> > Ideas appreciated, especially good ones.
> >
> > Cheers,
> >
> > Graham
> >
> >
> >
> >
> > Addresses:
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