Contact Force Control and Experimental Verification of a Mobile Robot for Picking Up Objects on the Ground

Abstract

This study presents a robot that efficiently picks up objects scattered on the ground, such as nuts in outdoor work. In conventional methods, objects are generally picked up by a gripper attached to the end of an arm; however, picking them up one by one is time-consuming. In addition, difficulties arise from the complexity required for coordinated control of the grasping posture and movement to pick up the object, as well as from the high rigidity of the machine when working on uneven ground. In this study, a cage-type end-effector is employed to enable continuous pick-up while the robot moves. In addition, using a worm gear for the arm, an arm-type mobile robot with a small size, simple drive source, and high mechanical safety is designed. To improve the pick-up capability, force control that allows the end-effector to flexibly follow the unevenness of the ground is desirable. However, force control is generally considered difficult because a worm gear cannot be driven in reverse. Therefore, we constructed a contact force control system that adjusts the arm angle by measuring the unevenness of the ground as spring displacement around the end-effector. This control system prevents excessive contact force between the end-effector and the ground. Experiments in an outdoor environment confirmed that the variation in the number of nuts picked up was reduced by approximately 48% compared to the case without the control. These results demonstrate the effectiveness of the pick-up system proposed in this study.