Specifications

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Reload the model, and then choose the menus Window -> Model View (new). This opens a graphics window
and displays what looks like a long yellow ellipse. On the toolbar at the top of the graphics window, click the
rotation icon
. Then click inside the graphics field and drag the mouse in some direction. This causes
the yellow ellipse to rotate, and you will see that it is actually an ellipsoid with a coordinate system through
it. If you entered the inertia properties in the Jii specification as written above, then your ellipsoid should
be ten times as long as it is wide. Try changing the "0.001" to "0.01" and reload. The ellipsoid becomes
spherical. The dimensions of the ellipsoid are scaled this way to fit the mass properties of the segment you
are defining. It is best to change Jii back to {0.001,0.01,0.01} again.
As you can see, Jii is a vector. If you know your basic mechanics, you may wonder why it is not a 3 x 3
matrix. The reason is that Jii only contains the diagonal members (the moments of inertia), which is all you
need to specify if your segment-fixed reference frame is aligned with the principal axes. If not, the off-
diagonal elements (the deviation moments) can be specified in a property called Jij, which by default
contains zeros.
We are eventually going to attach things like muscles, joints, external loads, and visualization objects to our
segments. To this end we need attachment points. They are defined in the local coordinate system of the
segment. For a given body part it may be a laborious and difficult task to sort out the correct points.
Fortunately, good people have done much of the work for you, and, if you construct your model wisely, you
can often grab most of what you need from models defined by other people. For now, let us assume that
you have sorted out the coordinates of all the points you need on UpperArm, and that you are ready to start
adding them. Rather than going through the drill with the object inserter, you can copy and paste the
following lines:
AnySeg UpperArm = {
//r0 = {0, 0, 0};
//Axes0 = {{1, 0, 0}, {0, 1, 0}, {0, 0, 1}};
Mass = 2;
Jii = {0.001, 0.01, 0.01};
AnyDrawSeg drw = {};
AnyRefNode ShoulderNode = {
sRel = {-0.2,0,0};
};
AnyRefNode ElbowNode = {
sRel = {0.2,0,0};
};
AnyRefNode DeltodeusA = {
sRel = {-0.1,0,0.02};
};
AnyRefNode DeltodeusB = {
sRel = {-0.1,0,-0.02};
};
AnyRefNode Brachialis = {
sRel = {0.1,0,0.01};
};
AnyRefNode BicepsShort = {
sRel = {-0.1,0,0.03};
};
AnyRefNode Brachioradialis = {
sRel = {0.05,0,0.02};
};
AnyRefNode TricepsShort = {
sRel = {-0.1,0,-0.01};
};
}; // UpperArm