[ODE] "overconstraining" in kinematic loops?

Gary R. Van Sickle g.r.vansickle at worldnet.att.net
Thu Sep 23 19:45:12 MST 2004

> Hi!  I'm still having an odd problem with kinematic loops.  
> I'm finding that if I have several joints in the loop that 
> would seem to eliminate the same degrees of freedom, for 
> example several hinges all on the same axis, then the system 
> becomes "overconstrained", i.e.
> degrees of freedom are lost that should not be lost.  In one 
> instance I have a loop that could not rotate on any other 
> axis except those of its hinges, even though the loop was 
> floating out in space!
> A possibly related problem I'm having, again with loops, and 
> again with weird physically impossible motion, is that it 
> seems if I push against a joint that has limits, even where 
> the push is originating from inside the system, then it can 
> cause the entire system to rotate.
>  The specific example I have is a "rack and pinion" set-up I 
> have for car steering.  I have a rack that moves side to side 
> on a slider, which pushes against the front wheels via tie 
> rods connected with ball joints.  If I push too far with the 
> rack, the entire car lifts up off the ground (!)  This is not 
> what one would expect, since the forces come from the slider 
> internally :P
> Is there any way to fix this or work around this?  I'd be 
> happy to provide more detail.

Yikes, this sounds like a pretty serious problem.  Can you maybe provide a
small, compilable test case?

Gary R. Van Sickle

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