In this paper, we propose and demonstrate an omnidirectional walking engine that achieves stable walking using feedback from an inertial measurement unit (IMU). The 3D linear inverted pendulum model (3D-LIPM) is used as a simplified model of the robot, the zero moment point (ZMP) criterion is used as the stability criterion, and only the feedback from the IMU is utilized for stabilization. The proposed walking engine consists of two parts; the omnidirectional gait generator, and the stability controller. ZMP equations, derived based on the 3D-LIPM, are used in the omnidirectional gait generator. The solutions of the differential equations are directly used which reduces the computation cost compare to other existing methods. Two kinds of feedback controllers are implemented for the stability controller; one is the indirect reference ZMP controller, and the other is the indirect joint controller. The walking engine is tested on a lightweight, full-sized, 21-degree-of-freedom (DOF) humanoid robot CHARLI-L (Cognitive Humanoid Autonomous Robot with Learning Intelligence, version Lightweight) which stands 141 cm tall and weighs only 12.7 kg. The design goals of CHARLI-L are low development cost, lightweight, and simple design, which all match well with the proposed walking engine. The results of the experiments present the efficacy of our approach.

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