SCAL for Pinch-Lifting: Complementary Rotational and Linear Prototypes for Environment-Adaptive Grasping

TL;DR

This paper introduces SCAL, a slot-constrained adaptive linkage, as a common kinematic primitive for environment-adaptive grasping. Two prototypes, SCAL-R (rotational drive with an active fingertip) and SCAL-L (linear drive with passive opening), realize pinch-lifting transitions that leverage surface contact to lift thin, curved, or bulky objects with minimal sensing. A quasi-static force analysis yields closed-form fingertip-force models for linear pinching and enveloping, linking input moments to contact forces through explicit geometry terms. Experimental validation with 3D-printed PLA fingers shows reliable, contact-enabled surface alignment and lift across a range of objects, with complementary operating regimes between the rotational and linear designs. The results point to a practical path for robust, environment-adaptive grasping using simple actuation and mechanical intelligence instead of extensive sensing and control.

Abstract

This paper presents environment-adaptive pinch-lifting built on a slot-constrained adaptive linkage (SCAL) and instantiated in two complementary fingers: SCAL-R, a rotational-drive design with an active fingertip that folds inward after contact to form an envelope, and SCAL-L, a linear-drive design that passively opens on contact to span wide or weak-feature objects. Both fingers convert surface following into an upward lifting branch while maintaining fingertip orientation, enabling thin or low-profile targets to be raised from supports with minimal sensing and control. Two-finger grippers are fabricated via PLA-based 3D printing. Experiments evaluate (i) contact-preserving sliding and pinch-lifting on tabletops, (ii) ramp negotiation followed by lift, and (iii) handling of bulky objects via active enveloping (SCAL-R) or contact-triggered passive opening (SCAL-L). Across dozens of trials on small parts, boxes, jars, and tape rolls, both designs achieve consistent grasps with limited tuning. A quasi-static analysis provides closed-form fingertip-force models for linear parallel pinching and two-point enveloping, offering geometry-aware guidance for design and operation. Overall, the results indicate complementary operating regimes and a practical path to robust, environment-adaptive grasping with simple actuation.

Figures (18)

• Figure 1: 3D-printed prototype of the proposed SCAL-R and SCAL-L.
• Figure 2: Motion Analysis of the SCAL.
• Figure 3: SCAL finger.
• Figure 4: SCAL-R and its free-space motion.
• Figure 5: SCAL-R environment-assisted pinch-lifting: (a) swing & sliding, (b) touching & lifting.