Four-Axis Adaptive Fingers Hand for Object Insertion: FAAF Hand

TL;DR

The paper tackles insertion under substantial pose uncertainty by introducing the Four-Axis Adaptive Finger Hand (FAAF Hand), whose fingers passively adapt along $x$, $y$, $z$, and yaw without sensors. The main approach combines magnet-based and spring-based passive mechanisms localized to the finger, enabling in-hand posture adjustment and robust contact with the target slot through a simple spiral-path control. Key findings show high success rates across square prisms, triangular prisms, and various lids despite $x$, $y$, $z$, and yaw misalignments, and demonstrate the importance of the finger-level adaptive axes for reliable insertion. The work offers a practical, lightweight solution for precise, contact-rich insertions in lab automation and similar settings, reducing reliance on vision or force-feedback control.

Abstract

Robots operating in the real world face significant but unavoidable issues in object localization that must be dealt with. A typical approach to address this is the addition of compliance mechanisms to hardware to absorb and compensate for some of these errors. However, for fine-grained manipulation tasks, the location and choice of appropriate compliance mechanisms are critical for success. For objects to be inserted in a target site on a flat surface, the object must first be successfully aligned with the opening of the slot, as well as correctly oriented along its central axis, before it can be inserted. We developed the Four-Axis Adaptive Finger Hand (FAAF hand) that is equipped with fingers that can passively adapt in four axes (x, y, z, yaw) enabling it to perform insertion tasks including lid fitting in the presence of significant localization errors. Furthermore, this adaptivity allows the use of simple control methods without requiring contact sensors or other devices. Our results confirm the ability of the FAAF hand on challenging insertion tasks of square and triangle-shaped pegs (or prisms) and placing of container lids in the presence of position errors in all directions and rotational error along the object's central axis, using a simple control scheme.

Figures (10)

• Figure 1: The FAAF hand has a four-axis passive mechanism in its fingers. This enables simple controllers to perform challenging insertion tasks in the presence of pose errors.
• Figure 2: (a) Hand structure and dimensions. (b) Finger with 4-axis adaptive mechanism. (c)-(f) Structures for adaptive movement in the $z$-, $y$-, $x$-, and yaw-axis directions. (g) Magnet connector (MC) can change the reaction force as required by changing the distance between the two magnets.
• Figure 3: The finger's adaptive mechanisms in action. (a) Initial state. (b)-(c) Motion when the grasped object moves in the $y$- or $x$-axis direction. (d) Grasping object operation to arbitrarily change the posture of an object in response to an external force. (e) Motion when an object rotated along its yaw-axis, object is grasped and subjected to external force and its posture changes. (f)-(g) The left and right sides move independently and can adjust to the undulations of the ground.
• Figure 4: (a) Grasping a triangle. (b) The fingertip can rotate to increase the contact area by yaw and $x$-axis adaptive function. (c) The adaptive function of the $x$-axis alone can rotate the grasped object.
• Figure 5: Reaction force for each axis.