Converting motion to axis-angle for lliggghts

Hi, intern liggghts uses quats, so you can convert your euler rotation into quats:
http://www.euclideanspace.com/maths/geometry/rotations/conversions/euler...
You get an angle w and an axis x,y,z, the origin is the point you want to to rotate the mesh arround (eg. center of mass).

Good Luck,
Christian.

When you allready have a valid quaternion, why wont you feed it directly to

mesh_->rotate(Q,dq,r);

where Q=[qw,qx,qy,qz] and dq=[1,0,0,0] ?

Don't forget to set the correct mesh velocity:

//move triangles ..
double dq[4]={1,0,0,0}; //dummy
double dr[3]={0,0,0};
mesh_->move(r,dr);
mesh_->rotate(Q,dq,r);
vectorZeroize3D(node);
vectorZeroize3D(rPA);
for(int i = 0; i < size; i++)
{
for(int iNode = 0; iNode < numNodes; iNode++)
{
vectorCopy3D(nodes[i][iNode],node); //copy r_local to node
vectorSubtract3D(node,r,rPA);
vectorCross3D(omega,rPA,v_node[i][iNode]); //w_0 x r_0 = v_0_rot
vectorAdd3D(v_node[i][iNode],v,v_node[i][iNode]); //v=v_rot_0 +v_trans_0
} //iNode
} //iMesh


What is your final aim to do this ?

Best,
Christian.

Good Morning,

I was struggeling with this problem 2 year ago when I started to couple a machine model of a wheel loader with LIGGGHTS (see YouTube Video).
The first problem I see is, that you have distance units and all liggghts move commands only accept velocity units.
To avoid this I've written a new mesh_mover method which get's the 3x3 rotation matrix T and a translation vector r and than applies this to every node of the mesh.
For that I've written a new move method in multi_node_mesh_I which is called from this custom mesh_mover method (=new move style)

template
void MultiNodeMesh::move(double *r, double **T)
{
if(!isTranslating())
this->error->all(FLERR,"Illegal call, need to register movement first");
int n = sizeLocal() + sizeGhost(); //apply movement only on mesh-points this proc ownes
for(int i = 0; i < n; i++)
{
vectorZeroize3D(center_(i));
for(int j = 0; j < NUM_NODES; j++)
{
vectorMatrixTrans(node_orig(i)[j],T,node_(i)[j]); //rotate
vectorAdd3D(node_(i)[j],r,node_(i)[j]); //add r_0 to points in local frame
vectorAdd3D(node_(i)[j],center_(i),center_(i)); //calc center
}
vectorScalarDiv3D(center_(i),static_cast(NUM_NODES));
}
updateGlobalBoundingBox();
}


The problem with this method is, that I need the translational and angular velocity of the moved body as input, because in the contact model you need the relative velocity between mesh and particle, otherwise the behaviour is crappy.

So before we start to share some Ideas I have three questions:
1.) can you get the velocities too ? (maybe by differentiate by t between two positions) or as direct output from maya ?
2.) are you afraid to add some lines of code to your local liggghts installation ?
3.) Do you think we can find any funding for a collaboration project ?

I think this would be a wonderful showcase for a coupled Multibody-DEM simulation, I've seen some of your videos at G+ and can imagine what you are trying to do. I can create a "machine model" of the gear mechanism of dead-animal-legs and feed this to liggghts.

Best,
Christian.