Re: Stewart platform kinematics (genhexkins)

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

OK, I partly answered my own questions. Looking again at genhexkins.c, 
kinematicsInverse() is clearly just a point-to-point distance calculation.

I'd still love to hear about how non-ideal geometries are handled.

Also, I gather jacobianInverse() and jacobianForward() are new but 
unused (as of emc-2.1-16)? Intended to be used in some future revision?

Thanks,
Andy Anderson

Andy Anderson wrote:
> 
> 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.
> 
<snip>