Re: Open (i think) design for a parallel robot (reprap may be interested)

On 8/31/06, Graham Stabler <eexgs@...> wrote:
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Dennis Schmitz"
>
> When using mach I set up secondary axes, these were the actuator
> postitions which were defined in terms of the primary axes using the
> equations function. The quantization error is no more of a problem
> than it is for a normal machine thought the positional error will tend
> to grow as you move from the sweetspot.

Sorry, I'm not familiar with Mach because I don't own a copy.

> >> On the other hand, you do need to develop a transform for your own
> > machine. It might seem daunting to delve into the matrix math
>
> I sometimes wonder if I am talking to myself. I have posted a link to
> my kinematics page at least twice. You don't need matrix maths you
> can calculate the lengths or base positions directly using pythagorus.

I'm not sure why the distinction. It's still the same number of
operations. Matrices are just an easy form to work with in code as
well as conceptually. Of course I spend a large part of most days in
Matlab, so I might be biased.

> For example if you take a triaglide, you know the position you want
> the table to be in, the points on the rail where the carriage should
> be are given by the intersection of a sphere with a line. i.e. the
> possible positions of the struts other end with the rail. You will
> get two solutions but you can ignore one. Also if you keep the
> equation in its most general form you can account for rails not being
> parallel etc.

I find the matrix to be a more general form, probably because I'm used to it.

> PLEASE have a look at my page, you will see that ball joints aren't
> even needed.

I saw the stuff about the clevis, but I think the ball with self-lubed
plastic bushing is a simpler design. In any case you're right about
what's required, if you're making a 3dof platform and parallelograms,
you only need a universal joint since there is no twist to account
for.

> > The other crucial thing is the gearbox for the servo arm. It's a lot
> > different than mill screws, and probably more expensive. I'm still
> > pondering that.
>
> Again see my page, there is a cool way of getting high gear ratios
> using a belt.
>
> Bottom line, I think triaglides and deltas have real homebrew
> possibilities, I've done the ground work, please run with it. BUT
> hexapods specifically don't fit the bill for reprap or most homebrew.

Fair enough. I wasn't suggesting a hexapod for reprap anyway, but a delta.