Stable In-hand Manipulation for a Lightweight Four-motor Prosthetic Hand

TL;DR

This work tackles the challenge of dexterous, in‑hand manipulation in a lightweight electric prosthetic hand by replacing open‑loop width input with motor‑current feedback to estimate object width and coordinate finger motion. The authors introduce a two‑stage control: Stage 1 estimates width from index‑finger current, and Stage 2 uses this width to adapt the thumb position while coordinating the index finger to stabilize heavy objects. The PLEXUS hand (311 g, four motors, single‑axis thumb) achieves 100% success on 5–30 mm lightweight objects and maintains high success (≥80%) on heavy aluminum prisms up to 289 g, with practical ADL demonstrations such as bottle‑cap closing and pen orientation. The approach demonstrates that high‑quality PL in‑hand manipulation can be realized with minimal actuators and simple current‑based sensing, offering a viable path toward practical, low‑burden prosthetics.

Abstract

Electric prosthetic hands should be lightweight to decrease the burden on the user, shaped like human hands for cosmetic purposes, and designed with motors enclosed inside to protect them from damage and dirt. Additionally, in-hand manipulation is necessary to perform daily activities such as transitioning between different postures, particularly through rotational movements, such as reorienting a pen into a writing posture after picking it up from a desk. We previously developed PLEXUS hand (Precision-Lateral dEXteroUS manipulation hand), a lightweight (311 g) prosthetic hand driven by four motors. This prosthetic performed reorientation between precision and lateral grasps with various objects. However, its controller required predefined object widths and was limited to handling lightweight objects (of weight up to 34 g). This study addresses these limitations by employing motor current feedback. Combined with the hand's previously optimized single-axis thumb, this approach achieves more stable manipulation by estimating the object's width and adjusting the index finger position to maintain stable object holding during the reorientation. Experimental validation using primitive objects of various widths (5-30 mm) and shapes (cylinders and prisms) resulted in a 100% success rate with lightweight objects and maintained a high success rate (>=80) even with heavy aluminum prisms (of weight up to 289 g). By contrast, the performance without index finger coordination dropped to just 40% on the heaviest 289 g prism. The hand also successfully executed several daily tasks, including closing bottle caps and orienting a pen for writing.

Figures (4)

• Figure 1: PL and LP in-hand manipulation. To effectively support the thumb's force, the index finger shifts its position using motor current feedback, as illustrated by the change from the red (without (w/o) the use of the index) to the yellow (with (w/) the use of the index) line in the red insets. This enables both stable object manipulation and a firm grip for tool use (w/ index), whereas a lack of coordination leads to grip failure (w/o index).
• Figure 2: Actuation mechanisms for the thumb, ring, and little fingers KurodaICORR2025. In the case of the thumb, rotation is driven by a linear actuator operated based on a piston--crank mechanism. For the ring and little fingers, a single actuator drives both digits via a differential Y-shaped linkage.
• Figure 3: Schematic of the control system for the PL manipulation of the PLEXUS Hand. The flowchart illustrates the logic for estimating the object width using the index finger motor current and subsequently coordinating the thumb and index finger movements to facilitate object reorientation.
• Figure 4: Demonstrations of dexterous daily tasks with the PLEXUS Hand. The figure illustrates PL manipulation (PL w/ index strategy) for tasks such as knob turning and bottle cap closing. The yellow frame highlights an instance where manipulation is essential to avoid palmar obstruction. See supplementary Movie S1 (https://youtu.be/TswoBs84uyE) for full sequences.