Experimental Analysis of Rigid Body Motion. A Vector Method to Determine Finite and Infinitesimal Displacements From Point Coordinates

This paper presents a vector method for measuring rigid body motion from marker coordinates, including both finite and infinitesimal displacement analyses. The common approach to solving the finite displacement problem involves the determination of a rotation matrix, which leads to a nonlinear problem. On the contrary, infinitesimal displacement analysis is a linear problem that can be easily solved. In this paper we take advantage of the linearity of infinitesimal displacement analysis to formulate the equations of finite displacements as a generalization of Rodrigues' formula when more than three points are used. First, for solving the velocity problem, we propose a simple method based on a mechanical analogy that uses the equations that relate linear and angular momenta to linear and angular velocities, respectively. This approach leads to explicit linear expressions for infinitesimal displacement analysis. These linear equations can be generalized for the study of finite displacements by using an intermediate body whose points are the midpoint of each pair of homologous points at the initial and final positions. This kind of transformation turns the field of finite displacements into a skew-symmetric field that satisfies the same equations obtained for the velocity analysis. Then, simple closed-form expressions for the angular displacement, translation, and position of finite screw axis are presented. Finally, we analyze the relationship between finite and infinitesimal displacements, and propose vector closed-form expressions based on derivatives or integrals, respectively. These equations allow us to make one of both analyses and to obtain the other by means of integration or differentiation. An experiment is presented in order to demonstrate the usefulness of this method. The results show that the use of a set of markers with redundant information (n>3) allows a good accuracy of measurement of kinematic variables.