Control architecture for the pneumatically actuated dynamic walking biped “Lucy”

Abstract

This paper reports on the control structure of the biped Lucy. The robot is actuated with pleated pneumatic artificial muscles which have interesting characteristics that can be exploited for legged machines. They have a high power to weight ratio, an adaptable compliance and they reduce impact effects. The considered control structure can be divided into two parts: a real-time joint trajectory generator and a trajectory tracking controller. The latter consists of a computed torque controller, a delta- p unit, a PI controller and a bang–bang pressure controller. The trajectory generator provides polynomial joint trajectories while the computed torque in combination with the delta- p unit calculates the required muscle pressure levels. The PI and bang-bang controller work at pressure level to cope with modelling errors and to set the pressures in each muscle. The proposed control architecture is evaluated with a full hybrid dynamic simulation model of the biped. This simulator combines the dynamical behaviour of the robot with the thermodynamical effects that take place in the muscle-valves system. The observed hardware limitations of the real robot and expected model errors are taken into account in order to give a realistic qualitative evaluation of the control performance and to test its robustness. At this stage a controller is designed for and evaluated during the single support phase only. In the double support phase the robot is over-actuated and consequently other control strategies are necessary. Additionally, preliminary results of a walking motion of the real robot with both feet in the air are given. These show promising results concerning tracking performance of the proposed control architecture. The experimental results confirm that the pneumatic tracking system can be used for a dynamic application such as a biped walking robot.