Bipedal Trajectory Generation Based on Combining Inertial Forces and Intrinsic Angular Momentum Rate Changes: Eulerian ZMP Resolution

Abstract

This paper aims to present a technique to generate feasible and dynamically equilibrated ZMP-based center of mass trajectories that can be applied to bipedal robots. In this regard, we utilize the ZMP concept in the spherical coordinate frame so that we can fully exploit its properties as this strategy enables us to efficiently combine intrinsic angular momentum rate change terms with inertial force terms. That being said, this strategy has certain advantages: 1) In the case of bipedal walking, undesired torso angle fluctuations are more restrainable compared with other methods, in which angular momentum information is omitted or zero-referenced. 2) Composite rigid body inertia, which is a multibody property of robot dynamics, can be characterized during the trajectory generation task. Thus, relatively more dynamically consistent trajectories may be obtained. 3) The motion interference between the sagittal and lateral planes is naturally included. In this paper, we mainly investigate the first two advantages. Applying the method described above, we conducted bipedal walking experiments on our actual bipedal robot MARI-3. As the result, we obtained repetitive, continuous, and dynamically equilibrated walking cycles, in which undesired torso angles were well suppressed. Furthermore, ZMP error tends to decrease since inertial parameters of the robot are characterized. In conclusion, the method is validated to be efficient in inducing less ZMP error and in suppressing undesired torso angle variations, compared with both flywheel-superimposed and conventional ZMP-based trajectory generation methods.