[ODE] problems with amotor

[jnilson_99 at yahoo.com]

"It depends what you want to do"

give a ball/socket with an amotor connecting them, i
want to find the angle between each respective axis.
this angle will then be sent to dJointSetAMotorAngle.
the code below doesn't work because the
angle(x,y,zAngleDiff )doesn't reflect the direction.

const dReal* R1 =
dBodyGetRotation(bodyParts[BODY_TYPE_TORSO]->bodyID);
const dReal* R2 =
dBodyGetRotation(bodyParts[BODY_TYPE_LEFT_UPPER_ARM]->bodyID);
dVector3 xa1 = {R1[0],R1[4],R1[8]};
dVector3 xa2 = {R2[0],R2[4],R2[8]};
dVector3 ya1 = {R1[1],R1[5],R1[9]};
dVector3 ya2 = {R2[1],R2[5],R2[9]};
dVector3 za1 = {R1[2],R1[6],R1[10]};
dVector3 za2 = {R2[2],R2[6],R2[10]};

dNormalize3 (xa1);
dNormalize3 (xa2);
dNormalize3 (ya1);
dNormalize3 (ya2);
dNormalize3 (za1);
dNormalize3 (za2);

dReal xDotAngle = dDOT(xa1,xa2);
dReal yDotAngle = dDOT(ya1,ya2);
dReal zDotAngle = dDOT(za1,za2);

dReal xAngleDiff = acos(xDotAngle);
dReal yAngleDiff = acos(yDotAngle);
dReal zAngleDiff = acos(zDotAngle);

dJointSetAMotorAngle(joints[JOINT_TYPE_LEFT_SHOULDER]->constraintID,0,xAngleDiff);
dJointSetAMotorAngle(joints[JOINT_TYPE_LEFT_SHOULDER]->constraintID,1,yAngleDiff);
dJointSetAMotorAngle(joints[JOINT_TYPE_LEFT_SHOULDER]->constraintID,2,zAngleDiff);
--- Jason Mallios <jmallios at cs.brown.edu> wrote:
> The problem is the dot produces a positive for an
> acute angle and a
> negative for an obtuse, so you can't necessarily
> tell which direction the
> bodies are rotationally offset.
> 
> It depends what you want to do, but a double cross
> product can give you
> axes with directional meaning.
> 
> On Mon, 9 May 2005 jnilson_99 at yahoo.com wrote:
> 
> >
> > "or by
> > crossing two such axes; one from each body (for
> two
> > bodies)."
> >
> > do you have any sample code you could show to
> > demonstrate how you're doing this?
> >
> > also, i thought only the dot product provided the
> > angle between two vectors and the cross product
> > creates a vector perpendicular to the two vectors
> > (each axis of the two bodies being compared)?
> >
> > thanks,
> >
> > john
> > --- Jon Watte <hplus-ode at mindcontrol.org> wrote:
> > >
> > > > Specifically, the manual seems to suggest you
> have
> > > to calculate the axes
> > > > angles yourself every time step.  Is that
> true?
> > >
> > > Yes, that is true. It's not all that hard,
> really --
> > > you can usually pull
> > > the axes out of the rotation matrix directly
> (for a
> > > single body) or by
> > > crossing two such axes; one from each body (for
> two
> > > bodies).
> > >
> > > I've used Amotors in user mode to stabilize a
> > > hovercraft, and it worked
> > > OK. "OK" here means "according to
> documentation."
> > >
> > > Cheers,
> > >
> > > 			/ h+
> > >
> > >
> > > _______________________________________________
> > > ODE mailing list
> > > ODE at q12.org
> > > http://q12.org/mailman/listinfo/ode
> > >
> >
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