The VariBody model wizard can also prepare an HBM for simulation of inverse dynamics with RecurDyn. Basically, inverse dynamics analysis is a process which reveals the inner forces and torques acting inside the human body, based on a motion that has been measured. At the beginning of the Twentieth Century, Eadward Muybridge used multiple cameras that had been trip-wired to perform the first biomechanical motion analysis . Every measurement consisted of a set of photo-plates. His image sequence of a galloping horse is a very prominent example of his work and, with it, Muybridge settled the question of whether all four hooves were ever off the ground at the same time during a horse’s gallop.
Technology has progressed substantially since then and today one can capture human motion in hundreds of frames per second, even in smaller labs. To enable the correct tracking of the joint angles, the test subject is prepared with reflecting markers, and infrared cameras are deployed to capture the motion. For example, the company Vicon is renowned for its technology and software in this field, but other motion-capture technologies have also been developed, such as XSens, which uses a set of inertial measurement units worn on the body. C3D is a standard motion capture (mocap) output format.
The Biomotion Workbench allows this mocap data to be imported and exported to an MBS model for instance in RecurDyn afterwards. This requires the generation of an HBM based on the subject’s specific weight, stature and gender in VariBody. Then a specification of the marker set used must be applied to the model which can now import the C3D data. The export of this model to RecurDyn prepares it for the first step in the RecurDyn simulation: the calculation of the joint states that fit the 3D marker point cloud (motion filtering by body drag). Motion filtering by body drag: As the subject measured and the model differ slightly in geometry and degrees of freedom, it is necessary to perform a “body drag” to project the measured motion onto the model’s degree of freedom. By providing the 3D coordinates of every single marker on the subject, a set of joint states can be calculated which fit the measured motion.
These calculated joint states expressed as a function of time can now be used as accurate input data for an inverse dynamics analysis. In RecurDyn, this is done with the “c-motion” joint. The inverse dynamics-driven HBM can now be used to calculate the forces and torques in the joints of the body (e.g. in the lumbar vertebrae). The model can also be extended by adding different passive or active mechanical structures so that, for instance, one can estimate the increased joint stress caused by the gear which a worker has to wear. This means, for example, that different backpack designs or orthotic devices may be compared in order to optimize these products. This last approach has recently been successfully deployed by a company that developed an exoskeleton using RecurDyn as their CAE tool.
Fig. 8 - Two different motion capture systems: one based on visible light (left), one based on infrared cameras and reflective markers (right)
Fig. 9 - Motion data import in just a few steps: generate an HBM in Biomotion Workbench, load the marker-set definition and the motion data, and finally adjust the best initial pose before exporting the model to RecurDyn