The topics covered in this section include:
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<Body name="r_humerus"> <attached_geometry>...</attached_geometry> <WrapObjectSet>...</WrapObjectSet> <mass>1.8645719999999999</mass> <mass_center>0 -0.18049599999999999 0</mass_center> <inertia>0.01481 0.0045510000000000004 0.013193 0 0 0</inertia> </Body> |
Every model comes with a Ground body, which exists as a property of a Model, not in the BodySet.
Geometry
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q are the joint coordinates, and x are the spatial coordinates for the rotations (x1, x2, x3) and translations (x4, x5, x6) along user-defined axes that specify a spatial transform (X) according to functions fi. The behavior of a CustomJoint is specified by its SpatialTransform. A SpatialTransform is comprised of 6 TransformAxes TransformAxis tags (3 rotations and 3 translations) that define the spatial position of B in P as a function of coordinates. Each transform axis enables a function of joint coordinates to operate about or along its axis. The function of q is used to determine the displacement for that axis. The order of the spatial transform is fixed with rotations first followed by translations. Subsequently, coupled motion (i.e., describing motion of two degrees of freedom as a function of one coordinate) is easily handled. The example below (from the gait2354.osim model) describes coupled motion of the knee, with both tibial translation and knee flexion described as a function of knee angle:
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OpenSim currently supports three types of built-in constraints: PointConstraint, WeldConstraint, and CoordinateCouplerConstraint. A point constraint fixes a point defined with respect to two bodies (i.e., no relative translations). A weld constraint fixes the relative location and orientation of two bodies (i.e., no translations or rotations). A coordinate coupler relates the generalized coordinate of a given joint (the dependent coordinate) to any other coordinates in the model (independent coordinates). The user must supply a function that returns a dependent value based on independent values. The following example implements coordinate coupler constraint for the motion of the patella as a function of the knee ankle and also welds the foot to ground.
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There are several muscle models in OpenSim. All muscles include a set of muscle points where the muscle is connected to bones (bodies) and provide utilities for calculating muscle-actuator lengths and velocities. Internally muscle models may differ in the number and type types of parameters. Muscles typically include muscle activation and contraction dynamics and their own states (for example activation and muscle fiber length). The control values are typically bounded excitations (ranging from 0 to 1) which lead to a change in activation and then force. Below is an example of a muscle model, as described by Thelen (2003), from an OpenSim model.
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A model may have some specific contact geometry that is associated with a model. In OpenSim, contact geometry can be an analytical shape, such as a half-place, sphere, or cube, or a user-defined shape represented in a geometry file. Files of type .obj, .stl, and .vtp are supported as of version 3.3. Prior versions of OpenSim support only .obj files. The figure below shows an example defining contact for the ground (half-space) and a user-defined block from the tugOfWar model. Note tags that define the contact object, such as *<body_name <socket_frame>, <location>, <orientation>, and <filename>.
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