A Sensorless, Inherently Compliant Anthropomorphic Musculoskeletal Hand Driven by Electrohydraulic Actuators

Abstract

Robotic manipulation in unstructured environments requires end-effectors that combine high kinematic dexterity with physical compliance. While traditional rigid hands rely on complex external sensors for safe interaction, electrohydraulic actuators offer a promising alternative. This paper presents the design, control, and evaluation of a novel musculoskeletal robotic hand architecture powered entirely by remote Peano-HASEL actuators, specifically optimized for safe manipulation. By relocating the actuators to the forearm, we functionally 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 a high level of inherent safety. Furthermore, this physical safety is augmented by the self-sensing nature of the HASEL actuators. By simply monitoring the operating current, we achieve real-time grasp detection and closed-loop contact-aware control without relying on external force transducers or encoders. Experimental results validate the system's dexterity and inherent safety, demonstrating the successful execution of various grasp taxonomies and the non-destructive grasping of highly fragile objects, such as a paper balloon. These findings highlight a significant step toward simplified, inherently compliant soft robotic manipulation.

Figures (9)

• Figure 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.
• Figure 2: Soft Muscle Stacking. (a) A fabricated Peano-HASEL actuator is 210 mm by 93 mm. (b) Actuation principle of the Peano-HASEL actuator proposed in hasel_actuators. (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) The force-displacement characteristics of single, double, and triple stacked Peano-HASEL configurations.
• Figure 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.
• Figure 4: Characterization of the index finger and thumb performance. (a, b) A 1 s 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.
• Figure 5: 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.