Determining subject-specific whole body center of mass using commercial off-the-shelf devices
MetadataShow full item record
The center of mass (CoM) trajectory is an important metric used to analyze the stability of humanoid robots as well as humans in activities such as walking, stretching, bending or standing. Popular classical methods of CoM estimation make use of anthropometric tables for developing a biomechanical model of the human subject, which can introduce large errors if the subject's body parameters do not match those of the experimental subjects used for developing the tables. This is true especially in the case of elderly subjects, amputees and obese subjects whose bodies do not have a uniform mass distribution. More modern methods have been developed which make use of the center of pressure (CoP) data of the user to calculate the CoM, but these require implementation within a controlled environment, laboratory-grade equipment and the user to remain on force plates all the time. In recent times, significant research interest has been targeted towards the realm of physical rehabilitation using commercial off-the-shelf gaming devices like the Microsoft Kinect and Nintendo Wii and exercise games (also known as exergames ). Such devices have been posited and proposed as viable and reasonably accurate alternatives to laboratory-grade motion capture systems and force plates which are cost-effective as well as portable. This thesis presents and validates research into a method for determining a human subject's 3D CoM developed with two commercial off-the-shelf products: Kinect sensor and Wii balance board. The method that is employed is known as the Statically Equivalent Serial Chain (SESC) approach. In this approach, any system of rigid bodies that are connected in a tree or linear arrangement can be realized as a linear chain, the end-point location of which is the same as the original structure's CoM. Hence, the CoM position of the original structure can be estimated if we can calculate the location of the modified and simpler linear chain's end-point. The configuration of this new linear chain, or the SESC, is a function of the original chain's joint values, link masses, locations of the link masses and the distances between the joints. The advantage of using this method for CoM estimation of a human being is that the SESC can be developed without any knowledge of the individual's limb masses, locations of limb masses and limb lengths. Furthermore, the method is subject specific that takes into account the distribution of masses in the subject's body. Hence, it is not prone to the errors that are inherent in the classical CoM estimation methods. The implementation of the method in this work involves two steps. The first step is a calibration or identification phase in which the subject-specific SESC is designed through stereophotogrammetric and dynamometric data taken from a selected number of poses. For this purpose, the Kinect is used as a markerless motion capture system for calculating the user's limb orientations (joint values) and the Wii balance board is used for calculating the CoP locations for a set number of identification poses. The second phase is the actual implementation or utilization phase in which the person's CoM can be determined for any posture using only the kinematic data. The method was experimentally validated on a subject. The system developed offers the advantages of low-cost and portability, which remove the constraint of laboratory environment required for implementation and make it suitable for in-home rehabilitation exercises. Furthermore, it can be calibrated for users of any age, height, weight and body parameters.