Stephan-Daniel Gravert


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Gravert

First Name

Stephan-Daniel

ORCID

0000-0002-1671-1611

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2023 - 20255
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Publications1 - 5 of 5
  • Low Voltage Electrohydraulic Actuators for Untethered Robotics
    Item type: Working Paper
    Gravert, Stephan-Daniel; Varini, Elia; Kazemipour, Amirhossein; et al. (2023)
    arXiv
    Rigid robots can be precise in repetitive tasks, but struggle in unstructured environments. Nature's versatility in such environments inspires researchers to develop biomimetic robots that incorporate compliant and contracting artificial muscles. Among the recently proposed artificial muscle technologies, electrohydraulic actuators are promising since they offer performance comparable to that of mammalian muscles in terms of speed and power density. However, they require high driving voltages and have safety concerns due to exposed electrodes. These high voltages lead to either bulky or inefficient driving electronics that make untethered, high-degree-of-freedom bio-inspired robots difficult to realize. Here, we present hydraulically amplified low voltage electrostatic (HALVE) actuators that match mammalian skeletal muscles in average power density (50.5 W kg-1) and peak strain rate (971 % s-1) at a driving voltage of just 1100 V. This driving voltage is approx. 5-7 times lower compared to other electrohydraulic actuators using paraelectric dielectrics. Furthermore, HALVE actuators are safe to touch, waterproof, and self-clearing, which makes them easy to implement in wearables and robotics. We characterize, model, and physically validate key performance metrics of the actuator and compare its performance to state-of-the-art electrohydraulic designs. Finally, we demonstrate the utility of our actuators on two muscle-based electrohydraulic robots: an untethered soft robotic swimmer and a robotic gripper. We foresee that HALVE actuators can become a key building block for future highly-biomimetic untethered robots and wearables with many independent artificial muscles such as biomimetic hands, faces, or exoskeletons.
  • Electrohydraulic musculoskeletal robotic leg for agile, adaptive, yet energy-efficient locomotion
    Item type: Journal Article
    Buchner, Thomas Jakob Konrad; Fukushima, Toshihiko; Kazemi Pour, Amirhossein; et al. (2024)
    Nature Communications
    Robotic locomotion in unstructured terrain demands an agile, adaptive, and energy-efficient architecture. To traverse such terrains, legged robots use rigid electromagnetic motors and sensorized drivetrains to adapt to the environment actively. These systems struggle to compete with animals that excel through their agile and effortless motion in natural environments. We propose a bio-inspired musculoskeletal leg architecture driven by antagonistic pairs of electrohydraulic artificial muscles. Our leg is mounted on a boom arm and can adaptively hop on varying terrain in an energy-efficient yet agile manner. It can also detect obstacles through capacitive self-sensing. The leg performs powerful and agile gait motions beyond 5 Hz and high jumps up to 40 % of the leg height. Our leg’s tunable stiffness and inherent adaptability allow it to hop over grass, sand, gravel, pebbles, and large rocks using only open-loop force control. The electrohydraulic leg features a low cost of transport (0.73), and while squatting, it consumes only a fraction of the energy (1.2 %) compared to its conventional electromagnetic counterpart. Its agile, adaptive, and energy-efficient properties would open a roadmap toward a new class of musculoskeletal robots for versatile locomotion and operation in unstructured natural environments.
  • Low-voltage electrohydraulic actuators for untethered robotics
    Item type: Journal Article
    Gravert, Stephan-Daniel; Varini, Elia; Kazemi Pour, Amirhossein; et al. (2024)
    Science Advances
    Rigid robots can be precise but struggle in environments where compliance, robustness to disturbances, or energy efficiency is crucial. This has led researchers to develop biomimetic robots incorporating soft artificial muscles. Electrohydraulic actuators are promising artificial muscles that perform comparably to mammalian muscles in speed and power density. However, their operation requires several thousand volts. The high voltage leads to bulky and inefficient driving electronics. Here, we present hydraulically amplified low-voltage electrostatic (HALVE) actuators that match mammalian skeletal muscles in average power density (50.5 watts per kilogram) and peak strain rate (971% per second) at a 4.9 times lower driving voltage (1100 volts) compared to the state of the art. HALVE actuators are safe to touch, are waterproof, and exhibit self-clearing properties. We characterize, model, and validate key performance metrics of our actuator. Last, we demonstrate the utility of HALVE actuators on a robotic gripper and a soft robotic swimmer.
  • mimic-one: a Scalable Model Recipe for General Purpose Robot Dexterity
    Item type: Working Paper
    Nava, Elvis; Montesinos, Victoriano; Bauer, Erik; et al. (2025)
    arXiv
    We present a diffusion-based model recipe for real-world control of a highly dexterous humanoid robotic hand, designed for sample-efficient learning and smooth fine-motor action inference. Our system features a newly designed 16-DoF tendon-driven hand, equipped with wide angle wrist cameras and mounted on a Franka Emika Panda arm. We develop a versatile teleoperation pipeline and data collection protocol using both glove-based and VR interfaces, enabling high-quality data collection across diverse tasks such as pick and place, item sorting and assembly insertion. Leveraging high-frequency generative control, we train end-to-end policies from raw sensory inputs, enabling smooth, self-correcting motions in complex manipulation scenarios. Real-world evaluations demonstrate up to 93.3% out of distribution success rates, with up to a +33.3% performance boost due to emergent self-correcting behaviors, while also revealing scaling trends in policy performance. Our results advance the state-of-the-art in dexterous robotic manipulation through a fully integrated, practical approach to hardware, learning, and real-world deployment.
  • In vitro testing of an artificial muscle for the treatment of incontinence
    Item type: Conference Paper
    Lyko, Beate; Gravert, Stephan-Daniel; Katzschmann, Robert K.; et al. (2024)
    Proceedings of SPIE ~ Electroactive Polymer Actuators and Devices (EAPAD) XXVI
    The burden of incontinence is profound, affecting individuals both physically and emotionally. Treatment pathways typically progress from conservative measures to surgical interventions, sometimes culminating in the implantation of artificial sphincters. The existing models including the AMS 800ᵀᴹ show high revision rates. Complications such as tissue erosion further underscore the need for innovation in this field. Here, we propose an approach utilizing Hydraulically Amplified Self-healing Electrostatic (HASEL) actuators to develop an advanced artificial urethral sphincter. By employing HASEL actuators arranged in a cuff configuration, we aim to address the limitations of current devices. Initial in vitro testing on porcine urethrae has shown promising results in controlling the water flow at a hydrostatic pressure of 20 cmH₂O, demonstrating the feasibility to mimic sphincter function. This approach holds potential to significantly enhance the quality of life for patients suffering from incontinence as they regain control over their bodies. Further research and clinical trials are warranted to validate the efficacy and safety of this approach.
Publications1 - 5 of 5