Design and Evaluation of an Energy-Savig Drive for a Versatile Robotic Gripper

Abstract

Nowadays, the number of robotic systems grows enormously. In order to execute their task, many of these robots need a fixturing gripper to grasp and hold objects. To reduce the environmental impact, the operational costs, the size of actuators and to improve the uptime of mobile robots, it becomes increasingly important to design robots for a minimum energy consumption. Therefore this thesis is dedicated to developing an energy saving robotic gripper. The gripper's drive has to enable the gripper to perform sizing}, the adjustment to the object size, and forcing, exerting retaining forces on the object. As drive strategy an actuator in combination with an internal force mechanism was selected, because this allows for autonomous grasping and passive retention in any orientation. Based on a newly proposed performance metric, which normalizes the product of the minimum dislodging force in any direction and the average closing speed with the rated actuator power, a Statically Balanced Force Amplifier (SBFA) was selected as internal force mechanism. In addition to current state-of-the-art drives, which are able to switch between two transmission branches for either high-speed or high-force and overpower the motor because of static-load cancellation with dedicated locking mechanisms, an SBFA realizes a force reduction of approximately 95%. A concept for a drive is developed based on this strategy. By analyzing the three subfunctions of the drive: 1) sizing, 2) forcing and 3) switching between sizing and forcing, five design principles are derived. The SBFA is analyzed under the non-ideal conditions as occur in a gripper drive: a finite object stiffness and a position error in the size adjustment. A topology selection is conducted, which resulted in a concept for a drive consisting of an SBFA with a movable frame and a differential that resolves the motor input in a forcing and sizing branch. The timing of the switching is controlled by an additional non-linear threshold spring and an endstop is implemented to bypass the main spring during switching. An actual prototype for the implementation in the Delft Hand 3 is designed and realized. The mechanism has a cylindrical housing with a diameter of 64 mm and a length of 78.5 mm and weights approximately 460 grams. Experiments are conducted which validate the working principle, i.e. the ability to perform sizing, forcing and switching, of the developed prototype. The maximum output torque is 1.073 Nm. For 5 emulated object sizes, an average torque reduction of 91.5 % is measured. This prototype validates the feasibility to combine an SBFA, NBDM and two different transmission branches. Future work is required to further improve the functionality and asses the performance of the drive.