Game Development Reference
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c dhinge = u (1)
n (2) = 0
,
n (2)
·
v (1)
·
0
c dhooke = u (1)
v (2)
·
=0 .
We use the Hooke joint here for rough comparison since it is not practical to define
the CV joint with dot products. Now, choose body 2 to be the universe and rotate
body 1 about u (2) by π so both the new v (2) and n (2) axes have reversed signs.
Clearly, all three constraints are now violated geometrically, despite the fact that
the indicator functions are still 0.
This is not the case with the quaternion-based constraints defined in Equa-
tions (9.2) and (9.4) since for a rotation that flips the axis z by 180 q (2) =
[0 , 1 , 0 , 0] T , say—the indicators are then c lock =[1 , 0 , 0] T and c hinge =[1 , 0] T ,
respectively. For the CV joint, the rotation that flips the axis x corresponds to
q =[cos( π/ 2) , 0 , 0 , sin( π/ 2)] T =[0 , 0 , 0 , 1] T ,giving c CV =1.Theseareall
maximum violation given that all constraints correspond to components of unit
quaternions. Thus, the Jacobians at these points are then
00 0
00
G (2)
,
dlock =
1
01 0
dhinge = 00
,
1
00 0
G (2)
dhk = 000 ,
respectively, and so the restoration force vanishes at maximum violation. Since
the Jacobians have full row rank when the constraints are satisfied, some of the
rows must decrease gradually on the path to maximal constraint violation and so
the constraint weakens. This problem can be addressed by adding nonlinear terms
in the constraint definitions. That's beyond the present scope, however.
G (2)
9.8 More General Frames
Of course, we may not always have hinge joints that align the axis z of body 1
with the axis z of body 2. Changing that is quite easy to do in the dot product
version, but there are a few additional tricks for the quaternion counterpart, as I
now show.
Assume now that the body-fixed reference frames in which the joints are de-
fined have quaternions e , f
H
, respectively. Figure 9.4 demonstrates the situa-
tion for body 1 and transform e .
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