PLEXUS Hand: Lightweight Four-Motor Prosthetic Hand Enabling Precision-Lateral Dexterous Manipulation

TL;DR

The paper addresses the need for lightweight electric prosthetic hands capable of in-hand manipulation without resorting to heavy, multi-motor designs. It introduces the PLEXUS hand, a four-motor device (weight ~311 g) that uses a single-axis thumb at the CM joint and optimized thumb positioning to achieve precision–lateral manipulation while preserving five basic postures. A two-stage optimization framework (IsValidGrasp and CalculateManipulationRange) identifies the best thumb configuration to maximize manipulation range, and a 311 g prototype is validated through quantitative tests (5–30 mm widths) and practical demonstrations (seal, USB, screwdriver). The results demonstrate high success rates for in-hand manipulation across a range of object geometries, suggesting meaningful improvements for daily activities and reduced user burden, with future work targeting user trials and palm/finger refinements.

Abstract

Electric prosthetic hands should be lightweight to decrease the burden on the user, shaped like human hands for cosmetic purposes, and have motors inside to protect them from damage and dirt. In addition to the ability to perform daily activities, these features are essential for everyday use of the hand. In-hand manipulation is necessary to perform daily activities such as transitioning between different postures, particularly through rotational movements, such as reorienting cards before slot insertion and operating tools such as screwdrivers. However, currently used electric prosthetic hands only achieve static grasp postures, and existing manipulation approaches require either many motors, which makes the prosthesis heavy for daily use in the hand, or complex mechanisms that demand a large internal space and force external motor placement, complicating attachment and exposing the components to damage. Alternatively, we combine a single-axis thumb and optimized thumb positioning to achieve basic posture and in-hand manipulation, that is, the reorientation between precision and lateral grasps, using only four motors in a lightweight (311 g) prosthetic hand. Experimental validation using primitive objects of various widths (5-30 mm) and shapes (cylinders and prisms) resulted in success rates of 90-100% for reorientation tasks. The hand performed seal stamping and USB device insertion, as well as rotation to operate a screwdriver.

Figures (6)

• Figure 1: Configuration of PLEXUS hand and images of precision--lateral manipulation.
• Figure 2: Geometric definitions of fingers and object to calculate grasp requirements. In precision and lateral grasps, the orientation of reference vector $\bm{n}$ differs. This vector defines the grasping direction of the object.
• Figure 3: Execution of basic postures using PLEXUS hand. Index pointing was omitted because the index finger can clearly move independently.
• Figure 4: (a) Actuation and structure of four-bar linkage mechanism. (b) Differential mechanism that distributes motion between the fingers. When one finger is constrained, the displacement is redirected entirely to the unconstrained finger. Without constraints, motion is distributed to both fingers equally.
• Figure 5: Precision--lateral manipulation experiments for each object.