> From: "Jeff 'n Kristen Kroll" <jkkroll@JOSHUANET.COM>
>         Actually what I am working on is a 6DOF trackball. A coupla' months
 ago I
> moved next door to an inventor who has the patent on the whole arena of
> 6DOF trackballs( see http://gadgetland.com/globaldevices/trackball.html).
> The guy is a mechanical genius and has a prototype that spins in all 3
> directions and can translate in the other three. Fortunately for me he
> didn't know alot about electronics and is supplying me with all of the
> equipment I need (PICStart Plus, PC, chips, PC board design software, etc.)
> in exchange for doing this project for him.
>         The three rotational axes will be done with encoders and the
 translational
> axes will have proportional control, but have a much better "feel" than
> controllers currently on the market. This thing will ROCK the game
> controller world!
>         I didn't quite understand how you can get the yaw from just pitch and
> roll. Does this mean I can do away with one of my encoders? If it does,
> will I need to do intense computations? I would greatly appreciate any help
> in finding out more about this (references, etc.).
>
>                                                 Thanx,
>                                                 Jeff Kroll
>


It's not really suitable for your application - try rolling a ball as
described and you will see why.  Also, it is counterintuitive for the
user of the device.  Nevertheless, the principle has been applied to
CAD programs for 'rolling' 3-D objects around using a 2-D mouse or
trackball.  E.g. the user clicks on a special roll-mode button.  If the
mouse is then moved in the Y direction, the object rolls around the X
axis as if the mouse cursor is a 'hand' pushing on the top of the
object.  Similarly for the other axis.  Rolling the object around the Z
axis (pointing out of the screen) is performed by moving the mouse in
small circles.  No additional computation is required for this, as it
is a natural mathematical consequence of keeping track of the 3-D
orientation of the object.

This is explained in 'Graphics Gems' which is a set of books relating
to tricks for computer graphics, which is where I got my info.

Regards,
SJH
Canberra, Australia