Experimental Evaluation of Precise Placement of the Hollow Object with Asymmetric Pivot Manipulation

TL;DR

The paper presents an asymmetric pivot manipulation framework for hole grasping of hollow objects using a standard two-finger parallel-jaw gripper mounted on a 6-DOF arm. By modeling three planar contact points and friction with variables $l$, $\alpha$, $\beta$ and the nondimensional $l_a = l/(2a)$, it analyzes stable grasp configurations via unit wrenches and a linear program to achieve force balance under quasi-static conditions. Experimental validation across a range of rigid hollow objects demonstrates high success rates and practical utility, including two applications: aligning bushings in tight boxes and inserting bushings into narrow machine slots, with reduced footprint compared to pinch grasping. The work offers a scalable, hardware-light approach for precise placement tasks in manufacturing, while noting limitations with nonrigid objects and the potential for enhancements via tactile sensing and dynamic environment handling.

Abstract

In this paper, we present asymmetric pivot manipulation for picking up rigid hollow objects to achieve a hole grasp. The pivot motion, executed by a position-controlled robotic arm, enables the gripper to effectively grasp hollow objects placed horizontally such that one gripper finger is positioned inside the object's hole, while the other contacts its outer surface along the length. Hole grasp is widely employed by humans to manipulate hollow objects, facilitating precise placement and enabling efficient subsequent operations, such as tightly packing objects into trays or accurately inserting them into narrow machine slots in manufacturing processes. Asymmetric pivoting for hole grasping is applicable to hollow objects of various sizes and hole shapes, including bottles, cups, and ducts. We investigate the variable parameters that satisfy the force balance conditions for successful grasping configurations. Our method can be implemented using a commercially available parallel-jaw gripper installed directly on a robot arm without modification. Experimental verification confirmed that hole grasp can be achieved using our proposed asymmetric pivot manipulation for various hollow objects, demonstrating a high success rate. Two use cases, namely aligning and feeding hollow cylindrical objects, were experimentally demonstrated on the testbed to clearly showcase the advantages of the hole grasp approach.

Figures (16)

• Figure 1: Entire manipulation process in the target scenario.
• Figure 2: In-hand manipulation of a hollow object with a desired configuration: (a) schematic and (b) real implementation.
• Figure 3: Parameters and configuration for the manipulation of a hollow object.
• Figure 4: Schematic diagram of the unit contact wrenches and moment labeling for contacts S, H and G.
• Figure 5: Stable grasp configurations for a bushing (see Fig. \ref{['fig9']}) are represented in the $\alpha\beta$-plane as areas with varying $l_a$ values for three cases of the friction coefficients: (a) all contacts are frictionless; (b) only the ground contact has friction, with $\mu_G$=0.4; and (c) all three contact points have friction, with $\mu_S$=0.2, $\mu_H$=0.4 and $\mu_G$=0.4. Five lines in different colors represent increasing values of $l_a$. In all cases, an increase in friction expands the stable grasp region in both $\alpha$ and $\beta$. Meanwhile, an increase in $l_a$ extends the stable grasp region toward the origin ($\alpha=\beta=0$) while reducing its coverage of higher values of $\beta$.