Cinématique inverse

Visual3D models are based on a linked set of rigid segments. Traditional Visual3D models assumed that segments were implicitly linked by the Motion Capture Data (e.g. segments didn't come apart because the subject didn't come apart) and the joints were modeled with 6 degrees of freedom (e.g. all segments were treated as if they were independent). The mapping of MoCap markers to 6 DOF segments, is a matter of tracking a set of markers that are linked rigidly to the segment. This least squares solution requires the Specification of the Segment Coordinate System and the Tracking of the Pose (position and orientation) of a segment. Essentially this is a straightforward pattern recognition; the pattern (configuration) of the tracking markers are specified in a standing trial, and this pattern is fit to the marker configuration in each frame of MoCap data.

An alternative to the 6 DOF solution is to define joints (e.g. explicitly state which segments are connected by a joint) and to specify the properties of all joints. Because the targets used to track the segments are often subject to measurement error and soft tissue artifact, motion about some of the degrees of freedom maybe much larger than the motion that would be realistically possible. Lu and O’Connor (1999) described a global optimization process where physically realistic joint constraints can be added to the model to minimize the effect of the soft tissue and measurement error. Lu and O’Connor termed this process Global Optimization while other inside the biomechanics community prefer the term Inverse Kinematics. (Inverse Kinematics is the term used by Visual3d but Visual3d’s approach is based on the Lu and O’Connor technique.)

Because the targets used to track the segments are often subject to measurement error and soft tissue artifact, motion about some of the degrees of freedom maybe much larger than the motion that would be realistically possible. Lu and O’Connor (1999) described a global optimization process where physically realistic joint constraints can be added to the model to minimize the effect of the soft tissue and measurement error. Lu and O’Connor termed this process Global Optimization while other inside the biomechanics community prefer the term Inverse Kinematics. (Inverse Kinematics is the term used by Visual3d but Visual3d’s approach is based on the Lu and O’Connor technique.)

The difference between the traditional Visual3d 6 degree of freedom model and the Inverse Kinematics (IK) model is that constraints can be added between segments that restrict the relative motion between the segments. This is accomplished by creating one or more IK chains.

Inverse kinematics is the process of determining the parameters of a jointed flexible object (a kinematic chain) in order to achieve a desired pose.

An Inverse Kinematics solution is dependent on the choice of hierarchical model because the task is to identify an articulated figure consisting of a set of rigid segments connected with joints. Varying angles of the joints yields an indefinite number of configurations, so in the general case there is no analytic solution

Global_Optimization

In Visual3D the Inverse Kinematics problem is solved as a global optimization problem, which computes the pose of a model that best matches the mocap data in terms of a global criterion.

Defining the IK Constraints

IK chains are created in Visual3d using the IK Constraints tab in the Model builder portion of Visual3d. Prior to creating an IK chain all of this segments that are to be included in the chain must already be defined by in the Segments tab of the Visual3d Model builder.