[ODE] RE: attn: Erin Catto, concerning: fixed path constraint

[jnilson_99 at yahoo.com]

"You cannot base a planar joint off a ball joint
because a ball joint has 3 position constraints while
a planar joint has only 2. "

maybe i'm incorrect in using the word "planar". after
a few google searches, it seems planar joints are in
fact a real joint type. by planar i simply mean
confining either a universal or ball/socket to follow
a plane through the joint anchor. this would be
something like defining a spline and making an arm
follow the spline's path.

i'm also a little confused on constraints. yes a ball
and socket has 3 constraints per se. but it also has
"limit" constraints which confine the joint to certain
angle ranges of motion,i.e. hi/lo stops.

is it possible to add my planar constraint like an
additional limit constraint?  i could go through the
laborious process of updating the hi/lo stops every
time step to the exact angle made by the plane and the
ball/socket joint arm. i'd rather find a more elegant
solution though.

thanks,

john





john

--- Erin Catto <erincatto at sbcglobal.net> wrote:

> You cannot base a planar joint off a ball joint
> because a ball joint has 3
> position constraints while a planar joint has only
> 2. I would implement a
> completely new joint type and use the code for other
> joint types as a guide.
> The planar joint needs to use the position errors I
> wrote along with their
> Jacobian rows.
> 
> Erin
> 
> -----Original Message-----
> From: jnilson_99 at yahoo.com
> [mailto:jnilson_99 at yahoo.com] 
> Sent: Monday, July 18, 2005 1:36 AM
> To: Erin Catto; 'ode'
> Subject: attn: Erin Catto, concerning: fixed path
> constraint
> 
> Erin,
> 
> I've given this a lot of thought as I'd like to
> create
> an additional "planar" constraint for any joint.
> Take
> a ball and socket joint for example. You mention "It
> is best to formulate constraints like other joints
> and
> let ODE's solver compute the appropriate forces to
> make the body movements honor the constraints."
> 
> By this I take it that you mean ADD a constraint to
> the existing constraint matrix. For a ball and
> socket
> joint, the solver calls "ballGetInfo2" which in turn
> calls "setBall". "setBall" merely populates the
> joint's "Info2" structure consisting of the Jacobian
> matrix rows,cfm (constrain force mixing), and lo and
> hi limits.
> 
> If I wanted to add a PLANAR constraint to this
> joint,
> would I have to add an addition member to the
> "Info2"
> structure? That is somehow add "information" to the
> "Info2" structure that would add an additional row
> to
> the existing Jacobian Matrix, the additional row
> being
> the equation limiting the joint to a certain
> "Plane"?
> 
> 
> Thanks,
> 
> 
> John
> --- Erin Catto <erincatto at sbcglobal.net> wrote:
> 
> > It is best to formulate constraints like other
> > joints and let ODE's solver
> > compute the appropriate forces to make the body
> > movements honor the
> > constraints.
> > 
> > A planar joint can be formed using three
> constraint
> > equations:
> > 
> > - one to prevent motion along the plane normal
> > - two to prevent rotation perpendicular to the
> > normal
> > 
> > dot(p2 - p1, n1) = 0
> > dot(tx2, n1) = 0
> > dot(ty2, n1) = 0
> > 
> > where
> > 
> > p1 = x1 + r1
> > p2 = x2 + r2
> > 
> > and
> > 
> > x1, x2: body positions
> > r1, r2: body anchor points
> > n1: plane normal bound to body 1
> > tx2, ty2: in plane basis vectors bound to body2
> > 
> > Now you just need to differentiate the constraint
> > equations to get the
> > velocity Jacobians. These can be used to create a
> > new joint in ODE.
> > 
> > Erin
> > 
> > 
> > _______________________________________________
> > ODE mailing list
> > ODE at q12.org
> > http://q12.org/mailman/listinfo/ode
> > 
> 
> 
> 
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