Numerical Integration of Constrained Mechanical Systems Using MEXX*

MEXX (short for MEXanical systems extrapolation integrator) is a FORTRAN code for time integration of constrained mechanical systems. MEXX is suited for direct integration of the equations of motion in descriptor form. It is based on extrapolation of a timestepping method that is explicit in regard to differential equations and linearly implicit regarding nonlinear constraints. It requires only the solution of well-structured systems of linear equations whose solution requires computations that increase linearly with the number of bodies, in the case of multibody systems with few closed kinematic loops. Position and velocity constraints are enforced throughout the integration interval, whereas acceleration constraints need not be formulated. MEXX has options for time-continuous solution representation (useful for graphics) and for the location of events such as impacts. The present article describes MEXX and its underlying concepts.