Stewart platform kinematics (genhexkins)

[Andy Anderson <afa_mma-at-ameritech.net>]

Questions about genhexkins:

1) Are the calculations documented anywhere? Specifically, what joints 
are assumed?

This is a leading question. I imagine the calculations assume spherical 
joints. I'd like to confirm this.

But of course many designs use universal joints to support the torque in 
screw/nut actuators. That's probably OK as long as the platform remains 
square to the world, but once the platform rotates, U-joints do not act 
like spherical joints.

Is this reflected in genhexkins? (I assume not.) How is this handled in 
such machines? Is it feasible to create kinematics routines including 
the effects of U-joints? Is there another way to compensate for this 
(like feedback of the strut length, or actual platform position)? This 
is probably wandering OT, but I would appreciate anything that points me 
in the right direction.

2) What is the status of the forward kinematics?

Skimming the archives, it seems that at one time the forward kinematics 
were disabled, and maybe broken? In the emc-2.1-16 source, it looks as 
though they are hooked up again. Is this correct?

3) What are the forward kinematics used for?

Skimming emcmot.c, looks like the only applicable call has to do with 
'free mode'. Free mode seems to be direct/manual input of the joint 
motion? So the forward kinematics are used to compute world coordinates 
when the someone/something is directly controlling the actuators? EMC 
can then display these, and knows where the platform ends up?

The foward kinematics are apparently not used for any closed-loop motion 
control?

Thanks,
Andy Anderson