Re: EMC Problem

Ian:

> I have looked at several hexapod sites and, whilst I
> can see that they are fun to play with, I haven't
> yet worked out how they are better than conventional
> tools.

Alot of people feel that way about hexapods. One of
the advantages researchers are hoping to exploit is
the high strength to weight ratio. Hexapods are not
nearly as massive as conventional machine tools and
this would make for easier reconfigurability of a
manufacturing line. Some people were also hoping that
hexapods would have good stiffness characteristics in
the struts since all forces (in theory) should be
axial. So if all forces are axial, there should be no
bending in the struts. What people are finding though
is that there is bending in the struts due to friction
in the joints.

Anyway, it still remains to be seen if hexapods offer
any real advantage over conventional machine tools.
There are also other applications other than machine
tools that this type of machine might be better suited
for.

> Controlling one with EMC would be interesting. EMC,
> and any other conventional CNC program, assumes an
> axis move means controlling one motor and monitering
> velocity and distance. With a hexapod, something has
> to take that single axis move and perform a lot of
> trig calculations. Contouring just makes things more
> interesting. Adding additional axis at the workhead
> allowing reorientation of the spindle would really
> add to the fun!

Jon:

In EMC that something that takes the single axis moves
and performs alot of trig is the kinematics function.
I know that function is included in the current EMC
distribution. If you want to take a look at it it
should be in emc/src/emcmot/genhexkins.c

There are still a few issues with that function
though. For hexapods the inverse kinematics (given
tool tip position return strut lengths) are easy. But
the forward kinematics (given strut lengths return
tool tip position) are quite hard. For a machine tool
the inverse kinematics are probably more important.
So you can run a machine just with the inverse
kinematics you just never know exactly where the tool
tip is in world coordinates.

The forward kinematics included in genhexkins.c work
sometimes. That solution is an iterative solution and
generally converges to within the desired tolerances.
But sometimes it doesn't. I haven't worked out that
problem yet and am not sure that I will be able to.
Most hexapods that I have experience with don't even
use those forward kinematics.

Anyway, hope that gives you a little clearer picture.

Brian



Having just

> written that, however, one
> use has come to mind in the medical field where
> perhaps such a tool, linked
> to a video feedback system, could be used to
> maintain the position of a tool
> over a moving body although they would have to knock
> me out well and truly
> before posting me into such a machine!! ;o) Perhaps
> a bit of explanation of
> the theory and reasoning behind hexapods would
> benefit us all - the web
> sites are a bit sparse in this respect.
>
> Ian
> --
> Ian W. Wright
> Sheffield UK
>
>
> ----- Original Message -----
> From: Brian Register <register76@...>
> To: <CAD_CAM_EDM_DRO@onelist.com>
> Sent: 28 September 1999 00:55
> Subject: Re: [CAD_CAM_EDM_DRO] EMC Problem
>
>
> > From: Brian Register <register76@...>
> >
> > Ian:
> >
> > Sorry if I created any confusion. I didn't
> realize
> > that yemc is currently not released (actually I'm
> > still not sure if it's released or not). yemc is
> a
> > program that was written to give GUI support for
> the
> > hexapod work I'm doing and I don't know what the
> > future plans are for yemc or if it is even
> suitable
> > for release in its current state.
> >
> > Also, as Fred mentioned, the trajectory planner
> > doesn't support orientation trajectories anyway.
> So
> > for conventional machines you currently won't get
> any
> > real benefit from using more than three axes since
> I
> > assume you want the extra axes to control
> orientation.
> > For Hexapods, however, you have to control all
> six
> > axes just to get xyz motion since the axes are
> > non-orthogonal. So the fact that I can't get
> control
> > of orientation isn't as big of a deal.
> >
> > Probably should have mentioned that the first
> time.
> > Anyway, sorry again for the confusion.
> >
> >
> > =====
> >
>

-----------------------------------------------------------

> > Brian Register
> > University of Florida
> > Graduate Student - Mechanical Engineering
> > register@...
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> Welcome to CAD_CAM_EDM_DRO@...,an
> unmoderated list for the discussion of shop built
> systems in the above catagories.
> To Unsubscribe, read archives, change to or from
> digest.
> Go to: http://www.onelist.com/isregistered.cgi
> Log on, and you will go to Member Center, and you
> can make changes there.
> For the FAQ, go to
> http://www.ktmarketing.com/faq.html
> bill,
> List Manager
>

=====
-----------------------------------------------------------
Brian Register
University of Florida
Graduate Student - Mechanical Engineering
register@...