# [ODE] Composite object

Pål- Arve Nilsen paal-arve at smsc.no
Tue Jun 27 00:57:56 MST 2006

```Hi,
Kinda new to ODE and i have a few problems.

Im making a crane by jointing together composite objects. I have a base
that consists of two boxes, a crane cabin consisting of 3 boxes and a
boom consisting of 4 boxes.
I have used the boxstack example as a starting point.
I have discardet the loop that moves all encapsulated objects so that
the center of mass i 0, 0, 0

for (k=0; k<NUM_BOXES; k++)
{
dGeomSetPosition (tempOdeBoxGeomList[k],
boxOffset[k][0]-compositeMass.c[0],
boxOffset[k][1]-compositeMass.c[1],
boxOffset[k][2]-compositeMass.c[2]);
}

and the mass translate function call
dMassTranslate (&compositeMass,
-compositeMass.c[0],
-compositeMass.c[1],
-compositeMass.c[2]);¨

The reason i dont move the objects and the mass is because i want the center of mass locating to depend on the mass and size of the
boxes in the composite objects. Have i missunderstood the function of the loop and the function i have removed.

I have included a little picture to illustrate this, hope thats ok.
Now the Crane cabin is connected to the base with a hinge joint, axis point up, and the boom it connected to the
cabin with a hinge joint, axis point inwards. Im setting a gravity to point down ( 0.0, -9.81, 0.0 )
When i run the simulation, i expect the boom to rotate downwards about is jointed position on the cabin, but it stands still.
When i apply a downwards force at the rightmost position on the boom it rotates.
Anyone know why? is the moment of inertia wrong ( as a result of not using the two code statements above )?

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