Feedback control of Zero Moment Point for stable bipedal walking

In order to prevent the humanoid robot TUlip from falling a start has been made with a Zero Moment Point (ZMP) based controller. The chosen controller, which is called the Linear Inverted Pendulum Tracker (LIPT) controller, is based upon a lag between the real and reference ZMP and is used in simulations for initial testing. TUlip's walking gait is based on the Cart-Table (CT) model based ZMP to keep it stable during walking. However, during simulations and experiments it turned out that TUlip did not remain balanced at all times. Because the gait is based on the ZMP, a ZMP based controller is chosen to obtain a stable gait. Since the LIPT controller is based on a lag between the real ZMP and the reference ZMP, a lag needed to be found. In this report a lag of 0.02 seconds is found between the real ZMP, based on the Center of Pressure (CoP), and the reference ZMP, based on the CT model. A state space representation is derived and implemented in the simulation model. Initial simulations showed a working controller which, given enough time, brought the ZMP back to it's reference position. However, simulations for testing the controller on the robot showed no improvement in the stability of the walking gait. During simulations it was found that the existing ground contact model, developed to prevent large peak forces when a foot is lifted or put to the ground, was not sti enough. This resulted in larger oscillations in the CoP than for the CT model based ZMP, where similar results where expected. Is is recommended to x the ground contact model before continuing the development of the ZMP based controller.