Specifications

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Please refer to the reference manual for further explanation. The Getting Started with AnyScript
tutorial
provides examples of segment definitions.
Next up is Lesson 2: Joints
.
Lesson 2: Joints
You normally think of a joint as something that provides the body with its movement capability. We interpret
joints that way because we would not be able to move if our joints were rigid. But there is also an opposite
perception of a joint: as a constraint. If we did not have joints, then our bodies would disconnect into a heap
of bones.
The latter perception is how joints really work in AnyBody. Imagine you have two segments with no joints at
all. They each have 6 degrees of freedom in space: 3 translations and 3 rotations. So two disjoint segments
give us 12 degrees of freedom. If we connect them by a ball and socket joint, then we eliminate 3 degrees
of freedom because they can no longer translate freely with respect to each other, and we are left with a
mechanism with 12 - 3 = 9 degrees of freedom. AnyBody will keep rack of all this for you, but it makes
things much easier to understand if you get used to thinking of joints as constraints.
As a final word about perceiving joints as constraints, it might be worth mentioning that these constraints
are not taken into account at the moment when you load a model into AnyBody. By that time, the segments
are simply positioned in space where you located them in their definition. In principle, the segments may be
in one big, disorganized heap.
The joint constraints are not imposed until you perform some sort of analysis. Each study has the
SetInitialConditions operation for the particular purpose of resolving the constraints and connecting things.
The mathematics of that is a nonlinear system of equations. Such a system may have multiple solutions or
no solutions at all. Even if it has a unique solution, it may de impossible to find. This means that if the
segments are too disorganized from their final positions when they are defined, then the system might not
be able to resolve the constraints and put them in their correct positions. The remedy is to define the
segments so that their initial positions are not too far away from the configuration they will have when the
constraints are resolved. You can read much more about this subject in the tutorial A study of Studies
.
AnyBody provides you with a variety of ways you can connect segments by joints. The class tree reveals the
following joint class structure: