Robotic manipulation in unstructured environments requires end-effectors that combine kinematic dexterity with physical compliance. While traditional rigid hands rely on complex external sensors for safe interaction, electrohydraulic actuators offer a promising alternative by combining muscle-like compliance with self-sensing capability. This paper presents the design, control, and evaluation of a musculoskeletal robotic hand architecture powered entirely by remote Peano-HASEL actuators, optimized for safe manipulation. By relocating the actuators to the forearm, we isolate the grasping interface from electrical hazards while maintaining a slim, human-like profile. To address the inherently limited linear contraction of these soft actuators, we integrate a 1:2 pulley routing mechanism that mechanically amplifies tendon displacement. The resulting system prioritizes compliant interaction over high payload capacity, leveraging the intrinsic force-limiting characteristics of the actuators to provide inherent safety. This physical safety is augmented by the self-sensing nature of the HASEL actuators: by monitoring the operating current alone, we achieve real-time grasp detection and closed-loop contact-aware control without external force transducers or encoders. Experimental results demonstrate the system's dexterity and safety through the execution of grasp types from standard taxonomies and the non-destructive grasping of highly fragile objects such as a paper balloon. These findings represent a step toward simplified, inherently compliant soft robotic manipulation.
Figures & tables
Fig. 1 : A tendon-driven robotic hand system actuated with HASEL actuators. (a) Soft HASEL muscle actuators in the forearm drive the fingers via tendon transmission and pulley mechanisms, enabling compliant finger motion through rolling-contact joints. (b) The proposed hand system is grasping a cube. All fingers can be controlled independently.
Fig. 2 : Soft Muscle Stacking. (a) A fabricated Peano-HASEL actuator measures 210 mm by 93 mm. (b) Actuation principle of the Peano-HASEL actuator proposed in [ 7 ] . (c) Connected actuator stack forming a soft muscle. (d) Guiding structure of the actuators. (e) Force-displacement characterization setup using a linear motor and a load cell. (f) Force-displacement characteristics of single, double, and triple-stacked Peano-HASEL configurations with the shaded standard deviation.
Fig. 3 : Tendon-Driven Hand Design. (a) Overview of the modular robotic hand. (b) Flexor and extensor tendon routing of the index finger, demonstrating the rolling-contact joints and coupled PIP/DIP mechanism. (c) Actuator packs are stacked in parallel in the forearm (e.g., 2-stack for DIP/PIP, 3-stack for MCP). The flexor tendons utilize a pulley system attached to the actuators, while the extensor tendons are connected in series with elastic cords for passive restoration. All tendons are anchored and adjusted at the wire tensioning gears located at the bottom.
Fig. 4 : Overview of the current sensing feedback control mechanism. The algorithms utilize real-time current measurement for Grasp Detection and Contact-Aware Grasp Control.
Fig. 5 : Characterization of the index finger and thumb performance. (a, b) A 1s ramp signal was applied to the index finger and thumb, and the motion range of each joint was measured. (c, d) Under the same input conditions, the fingertip force was measured. Solid lines represent the mean value of five trials, with the shaded regions indicating the standard deviation.
Fig. 6 : The proposed musculoskeletal hand system is inherently compliant. (a, b) Demonstration of the mechanical compliance of the actuator packs in the forearm. The actuator packs deform when pushed by a human hand and accommodate twisting of the forearm. (c) The grasped object can be easily removed from the hand while constant voltage is applied, demonstrating its backdrivability. (d, e) Comparison of collision responses while actuators are active: the proposed HASEL-driven hand safely absorbs the impact of a tennis ball through large deformation, whereas a conventional motor-driven hand exhibits a rigid response.
Fig. 7 : Demonstration of versatile grasp taxonomies adapting to different object properties. (a, b) Pinching an 18 g mushroom and a 49 g cube using the thumb and index finger. (c) Tripod grasp of a 107 g soft stuffed toy. (d) Power grasp enveloping a 26 g empty PET bottle.
Fig. 8 : Results of the grasp detection experiment. The current drops significantly when the finger motion is constrained by an object, allowing threshold-based detection.
Fig. 9 : Results of the contact-aware grasp control. By holding the voltage when the current deviates from the free-motion baseline, the hand successfully grasps a fragile paper balloon without crushing it.
Metrics
Direct bending grippers [ 12 ]
Single prosthetic finger [ 9 ]
Proposed musculoskeletal hand
Kinematic form factor
Simple pinching / wrapping gripper
Single isolated digit
Integrated anthropomorphic multi-fingered hand
Actuator location
Directly at the interface
Parallel stacks at the finger base
Remote forearm placement
Transmission mechanism
Direct deformation
Rigid four-bar linkage mechanism
1:2 tendon-pulley amplification with rolling-contact joint
Fingertip force
<0.7 N
>1 N
<0.6 N
Inherent compliance
Very high (purely soft structure)
Moderate (rigid linkages)
High (tendons and soft muscles)
Self-sensing & feedback
Capacitive self-sensing
N/A
Operating-current thresholding
TABLE I : Qualitative and Quantitative Comparison of HASEL-Driven Robotic Manipulators
Safety and comfort in human-robot physical in-teractions are essential aspects in the development of social technologies, where natural gestures, such as handshakes, represent a challenge due to their direct physical contact. The implementation of series elastic actuators (SEA) to absorb impacts is proposed as a design strategy that favors safer interactions. This paper presents an experimental study aimed at evaluating how the incorporation of SEAs in robotic arms influences perceived safety and the interaction experience dur-ing handshaking. The design allows a direct comparison of the effect of rigidity versus the incorporation of elastic elements, in order to identify the advantages of SEAs in improving the physical safety and social acceptance of robotic systems in everyday contexts. The experiment was carried out with 10 volunteers (6 men and 4 women), who performed two interactions with each robotic arm: one with rigid joints and the other with flexible joints using SEA. During testing, objective data on end-effector trajectories were collected, as well as subjective information through a perception survey focused on safety, naturalness, and confidence during the handshake. The survey results show increased perceptions of safety and comfort with the SEA-equipped arm, supporting its potential to facilitate safer and more socially accepted human-robot interactions.
Ensuring intrinsic safety in physical human robot interaction (pHRI) is a critical requirement for social and service robots. While Series Elastic Actuators (SEAs) offer hardware based compliance, traditional metallic designs often require complex, multi part assemblies. This paper presents the design, finite element analysis (FEA), and experimental validation of a low stiffness, torsional spring for SEAs, manufactured via 3D printed thermoplastic polyurethane (TPU). The compliant element exhibits a highly linear torque deformation response (Ks = 0.066 Nm/degree), matching numerical predictions with under 3% deviation, a variance attributed to FDM structural anisotropy. To accommodate external interactions using standard position limited servomotors, a hybrid position controller with torque threshold switching was implemented. Experimental evaluations demonstrate the system ability to accurately track non stationary trajectories and safely yield to external disturbances. Furthermore, the inherent material damping of the TPU acts as a passive low pass filter, preventing high frequency oscillations during control mode transitions. The proposed architecture offers a cost effective, reliable, and easily manufacturable solution for safe pHRI.
Joel Hidalgo Pisco, Melissa Cobos Condo, Luigi Miranda +1
Tendon-driven underactuated hands excel in adaptive grasping but often suffer from kinematic unpredictability and highly non-linear force transmission. This ambiguity limits their ability to perform precise free-motion shaping and deliver reliable payloads for complex manipulation tasks. To address this, we introduce the PHANTOM Hand (Hybrid Precision-Augmented Compliance): a modular, 1:1 human-scale system featuring 6 actuators and 15 degrees of freedom (DoFs). We propose a unified framework that bridges the gap between precise analytic shaping and robust compliant grasping. By deriving a sparse mapping from physical geometry and integrating a mechanics-based compensation model, we effectively suppress kinematic drift caused by spring counter-tension and tendon elasticity. This approach achieves sub-degree kinematic reproducibility for free-motion planning while retaining the inherent mechanical compliance required for stable physical interaction. Experimental validation confirms the system's capabilities through (1) kinematic analysis verifying sub-degree global accuracy across the workspace; (2) static expressibility tests demonstrating complex hand gestures; (3) diverse grasping experiments covering power, precision, and tool-use categories; and (4) quantitative fingertip force characterization. The results demonstrate that the PHANTOM hand successfully combines analytic kinematic precision with continuous, predictable force output, significantly expanding the payload and dexterity of underactuated hands. To drive the development of the underactuated manipulation ecosystem, all hardware designs and control scripts are fully open-sourced for community engagement.
Teng Yan, Jiongxu Chen, Qixiang Hua +4
The Hong Kong University of Science and Technology (Guangzhou)