Estimating Contact Forces and Moments for Walking Robots and Exoskeletons Using Complementary Energy Methods

When walking robots and exoskeletons make multiple independent contacts, the inverse dynamics problem requires additional knowledge about the contact forces and moments. To avoid measuring the contact forces and moments, many inverse dynamics controllers for walking robots optimize an objective such as minimizing torques or contact forces. In order to get a solution closer to the real solution, the underlying physical principles need to be included. This is achieved by relying on the minimization of complementary energy, which is a well known method in structural engineering. The proposed method relies on physical properties (stiffness) to obtain the additional knowledge to solve the contact forces and moments. In addition, it has the same form as the methods used in the literature. The validation on a bilateral lower-limb exoskeleton shows that the proposed method is able to predict the contact forces and moments sufficiently well, while being robust against modeling errors. The modeling dominating flexibilities suffices to achieve adequate results, making this method especially interesting for series elastic actuated robots.