Picking by Tilting: In-Hand Manipulation for Object Picking using Effector with Curved Form

Abstract

This paper presents a robotic in-hand manipulation technique that can be applied to pick an object too large to grasp in a prehensile manner, by taking advantage of its contact interactions with a curved, passive end-effector, and two flat support surfaces. First, the object is tilted up while being held between the end-effector and the supports. Then, the end-effector is tucked into the gap underneath the object, which is formed by tilting, in order to obtain a grasp against gravity. In this paper, we first examine the mechanics of tilting to understand the different ways in which the object can be initially tilted. We then present a strategy to tilt up the object in a secure manner. Finally, we demonstrate successful picking of objects of various size and geometry using our technique through a set of experiments performed with a custom-made robotic device and a conventional robot arm. Our experiment results show that object picking can be performed reliably with our method using simple hardware and control, and when possible, with appropriate fixture design.

Figures (7)

• Figure 2: Sequence of snapshots (clockwise from the top-left) showing our picking technique performed with a curved, passive end-effector.
• Figure 3: Feasibility of initial tilting with two contacts (a) and three contacts (b-c). In (a), the object is wedged between contacts $B$ and $C$. Object tilts by rotating about the stationary contact $B$ so contact $A$ can break free. When tilting with three contacts (b-c), the object slides to the right of the inward contact normal (not shown) at $A$ and $B$. This manner of tilting is feasible in (b) because the wrench of gravity can be quasistatically balanced by the composite wrench cone (represented using moment labels) of the three contact wrenches. It is not feasible in (c) because quasistatic balance of contact and gravity wrenches cannot be attained.
• Figure 4: (a) Real tilting scenario (left) modeled in a plane normal to the two supports (right). Configuration variables $\theta$ and $\delta$ describe the progress of tilting. (b) The shaded area delimited by the red (blue) curve represents the set of configurations in which the object is in force-closure when friction coefficient at $C$ is $0.1$ ($0.2$). The plot is overlaid with a nominally feasible path for tilting. Friction coefficient at $A$ and $B$: $0.1$.
• Figure 5: With an appropriately oriented palm at the target configuration, the object can be kinematically restrained from ungrasping by the way of rotating clockwise about a point in the green or red shaded region. First-order mobility analysis suggests that such ungrasping motion is feasible in (a) but not in (b).
• Figure 6: (a) Our two-DOF robotic palm driven by a parallelogram linkage and two motors. (b) Hardware setup to pick an object in the corner of two perpendicular supports using our robotic palm. (c) Objects used in the experiments: 1-2 with two-DOF palm; 3-6 with UR3 arm (top-left in Fig. \ref{['fig:tilt_planning']}).