🦘 PELE: A musculoskeletal robotic leg powered by HASELs

Last updated on Mar 1, 2025 Project

A robotic leg powered by electrohydraulic artificial muscles achieves agile, adaptive, yet energy-efficient locomotion.

Table of Contents

Features

  • ⚡ Agile Movement: Executes dynamic motions and achieves high jumps.
  • 🐈 High Adaptability: Easily hops over varying terrains without complex control.
  • 💚 Energy Efficiency: Significantly reduces power consumption compared to DC motor-driven systems.
  • 🚫 Sensor-less: Detects obstacles using intrinsic sensing capabilities of the muscles.

Overview

For robots to navigate unstructured terrain, they need an architecture possesssing both agility and adaptability. Conventional legged robots rely on electromagnetic motors and sensor-based computer control to adapt the locomotion to the terrains. However, this approach struggles to achieve animals’ agile and effortless motion. To address this, we developed a bio-inspired musculoskeletal robotic leg (PELE) leveraging electrohydraulic artificial muscles (HASELs). This robot achieves agile and energy-efficient locomotion while adapting to unstructured terrains.

Videos

Press Release Video

Technical Explanation Video

Technical Features

Musculoskeletal Design

PELE adopts a musculoskeletal structure to mimic the versatile locomotion of the animals. Conventional electromagnetic motor-driven systems can precisely control joint angles and generate high torque. However, they tend to be bulky, heavy, and lack of system compliance. In contrast, PELE, powered by HASEL artificial muscles, features lightweight body and compliant joints, enabling more agile and adaptive locomotion with high energy efficiency.

From Joint-Centered Systems to Musculoskeletal Systems
From Joint-Centered Systems to Musculoskeletal Systems

PELE has two degrees of freedom (DoF) and is powered by two pairs of antagonistically arranged artificial muscles. This structure enables smooth flexion and extension of the joints, allowing movements similar to biological limbs.

Mechanism of Leg Movement via Muscle Contraction
Mechanism of Leg Movement via Muscle Contraction

HASEL Artificial Muscles

PELE is powered by Peano-HASEL artificial muscles, which function similarly to biological muscles. These actuators contract and expand using electrostatic forces by applying voltage to a liquid-filled plastic pouch. They are lightweight, powerful, and highly backdrivable.

Contracting Peano-HASEL Artificial Muscle
Contracting Peano-HASEL Artificial Muscle

Agile Locomotion

PELE’s leg oscillate at over 10 Hz and can jump up to 40% of its own height.

By utilizing a lightweight and high-rigidity carbon fiber-reinforced polymer (CFRP) skeleton and lightweight (15g each) and powerful (580W/kg) HASEL artificial muscles, it reduces moment of inertia, achieving highly agile motion.

Adaptive Locomotion

Using open-loop force control without rotational angle sensors, PELE automatically adapts to unstructured terrain. Solely with a simple cyclic voltage signal, PELE continuously hops over varying terrains such as grass, sand, gravel, and rocky surfaces.

Thanks to force-based control and the high backdrivability of HASEL artificial muscles, the robot can dynamically interact with its environment, effectively emerging an intrinsic feedback loop that enables automatic terrain adaptation.

Interactive Motion Generation with the Environment
Interactive Motion Generation with the Environment

Energy-Efficient Locomotion

PELE operates with drastically lower power consumption than conventional electromagnetic-motor-driven systems. Unlike motor systems, it does not experience heat buildup and maintains posture with minimal energy loss.
For example, during a squatting motions, PELE consumes only 1.2% of the energy required by conventional electromagnetic-motor-driven legs.

Unlike electromagnetic motors, HASEL actuators consume negligible power when holding a position. This is due to a fundamental difference in actuation principles:

  • Electromagnetic motors generate torque via current, meaning power consumption is proportional to torque squared.
  • HASEL actuators generate force via voltage, meaning theoretical power consumption at a static position is nearly zero.
HASEL Generates Torque via Voltage
HASEL Generates Torque via Voltage
PELE's Power Consumption is Minimal, Regardless of Torque
PELE’s Power Consumption is Minimal, Regardless of Torque

Sensor-less Obstacle Detection

PELE detects obstacles without external sensors and automatically avoids them.

HASEL artificial muscles function as capacitors, allowing them to detect changes in length as variations in capacitance.
This enables sensor-less obstacle detection, providing an intrinsic sensing mechanism for the robot.

Future Applications

PELE’s technology can be applied to robotic mobility in complex environments, such as forests and disaster zones.
Currently, PELE is a single-legged robot supported by a boom arm, but in the future, we aim to expand this concept to quadrupedal and bipedal robots for real-world applications.

This research paves the way for the development of robots with the following capabilities:

  • Robots that reduce computational burden by leveraging physical adaptability
  • Highly agile robots capable of freely traversing unstructured environment like animals
  • Sustainable and energy-efficient robots

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Reference

Electrohydraulic musculoskeletal robotic leg for agile, adaptive, yet energy-efficient locomotion

Electrohydraulic musculoskeletal robotic leg for agile, adaptive, yet energy-efficient locomotion
Soft electrohydraulic artificial muscles enable robotic legs for agile, adaptive, yet energy-efficient locomotion.
PDF Cite Project Video DOI Press release
Electrohydraulic musculoskeletal robotic leg for agile, adaptive, yet energy-efficient locomotion

Robot Soft Robotics Embodied Intelligence Musculoskeletal Robot Bio-Inspired Robot Legged Robot HASEL Electrohydraulic Actuator Electrostatic Actuator Locomotion
Toshihiko Fukushima
Toshihiko Fukushima
Doctoral Researcher in Robotics

Researching bio-inspired robots with soft actuators.

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