Modelling of contact in walking machines

Presents the results of modelling, simulation and measurements of contact forces between the legs and the ground of the six-legged robot HERMES^TM during a slow walk. The kinematics and dynamics of HERMES^TM (transient analysis) was simulated using an insect locomotion scheme. In the dynamical model three contact-impact models were considered. In the first approach the model was built using a general multibody formalism with differential algebraic equation-the sign of the constraint force was a test to decide if the respective contact will break or not. This approach implicitly assumes that the contact forces are continuous with respect to time. The velocity after impact is evaluated using the classical coefficient of restitution. The second algorithm is based on a model of contact force corresponding to a one-sided collision using a simple compression only nonlinear spring-damper formulation. Friction forces were included in the contact model as external forces depending on reaction forces. The third approach proposed by Nikravesh (1988, 1990, 1996) is based on the model, which uses the general trend of the Hertz contact law. Dissipated energy on impact is represented by a hysteresis form of damping function. The general results presented here concern mainly the first two approaches