Balance Computation of Objects Transported on a Tray by a Humanoid Robot Based on 3D Dynamic Slopes

Abstract

Humanoid robots are designed to perform tasks in the same way than humans do. One of these tasks is to act as a waiter serving drinks, food, etc. Transporting all these items can be considered a manipulation task. In this application, the objects are transported over a tray, without grasping them. The consequence is that the objects are not firmly attached to the robot, which is the case in grasping. Then, the complexity of robotics grasping is avoided but stability issues arise. The problem of keeping balance of the object transported by a robot over a tray is discussed in this paper. The approach presented is based on the computation of the Zero Moment Point (ZMP) of the object, which is modelled as a three dimensional Linear Inverted Pendulum Model (3D-LIPM). The use of force-torque sensors located at the wrist enables ZMP computation, but the main problem to be solved is how the robot should react when the object losses balance. One strategy is to rotate the tray to counteract the rotation of the object. This rotation has to be proportional to the ZMP variation and the object's rotation angle. This issue is solved by applying the concept of three dimensional dynamic slopes. It helps to avoid kinematic problems and make balance computation independent from the angle of the tray.