Special Issue "Variable Stiffness and Variable Impedance Actuators"

A special issue of Actuators (ISSN 2076-0825).

Deadline for manuscript submissions: closed (30 April 2017).

Special Issue Editors

Dr. Giorgio Grioli
E-Mail Website
Guest Editor
Advanced Robotics Department, Istituto Italiano di Tecnologia, Via Morego, 30, 16163 Genova, Italy
Interests: variable stiffness actuators; variable impedance; compliant joint/mechanism; novel actuators for natural machine motion; physical human-robot interaction; compliance and impedance control; grasping, grippers and other end-effectors; haptics and haptic interfaces; multifingered hands; prosthetics and exoskeletons
Dipl.-Ing. Sebastian Wolf
E-Mail Website
Guest Editor
DLR—German Aerospace Center, Institute of Robotics and Mechatronics, Mechatronic Components and Systems, Oberpfaffenhofen, Muenchener Str. 20, 82234 Wessling, Germany
Interests: variable stiffness actuators; variable impedance actuators; compliant joint/mechanism; mechanism design; robotic reflexes; energy efficiency of compliant actuators; friction modeling; friction identification; humanoid robots; physical human-robot interaction

Special Issue Information

Dear Colleagues,

The introduction of passive compliant elements (as springs and dampers) in robot actuators makes it possible to design robots that are dynamic, robust and safe. Nevertheless, if we compare these systems to their human counterpart, they still lose on the ground of the variety and diversity of tasks that the muscle–skeletal system can perform, such as catching, throwing, walking, running, jumping, etc., but also precise and gentle tasks, such as caressing, manipulating, playing an instrument, using tools, and performing in the arts.

Variable stiffness and variable impedance actuation proposes to overcome this limitation by designing actuators with superior dynamic behaviors, which can change their mechanical characteristics (by passive adaptation or by induced mechanical state change), while simultaneously operating their load.

Since the last decade of the past century, several Variable Stiffness and Variable Impedance Actuators and Mechanisms have been proposed, spanning a large variety of designs, without a particular implementation prevailing. Moreover, the opportunity of being able to change the mechanical characteristics of a robot system extends the problems of modeling, planning, and controlling a robot to a new degree. Despite the lively literature on these topics, there are still many open research questions.

Therefore, this Special Issue seeks top quality publications spanning the following topics:

  • Design of VSA and Variable Stiffness Mechanisms
  • Spring configuration in VIAs: Serial vs. parallel vs. differential vs. others
  • Modeling, control and control strategies of VSA/VIA robots and actuators
  • Applications of VSA/VIA robots and actuators
  • Energy efficiency, performance characterization and trade-offs of VSA/VIA robots and actuators
  • Safety, robustness and physical interaction of a robot with the environment and/or humans, where this is enhanced by VSA/VIA

Please note that the focus of this Special Issue is on systems that are able to change their passive mechanical impedance characteristics. Systems that implement the change in stiffness (or impedance) only by means of active control of the actuator torque or position are beyond the scope of this Special Issue.

Dr. Giorgio Grioli
Dipl.-Ing. Sebastian Wolf
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All papers will be peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Actuators is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 1600 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • Variable Stiffness Actuation
  • Variable Impedance Actuation
  • Compliant joint and mechanism design
  • Modeling of variable stiffness systems
  • Control of VSA/VIA
  • Optimal control
  • Impedance/compliance control
  • Energy efficiency
  • Identification and characterization
  • Interaction safety
  • Robust interaction
  • Adaptability
  • Physical human-robot-interaction (pHRI)

Published Papers (6 papers)

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Research

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Article
Electric-Pneumatic Actuator: A New Muscle for Locomotion
Actuators 2017, 6(4), 30; https://doi.org/10.3390/act6040030 - 25 Oct 2017
Cited by 13 | Viewed by 5150
Abstract
A better understanding of how actuator design supports locomotor function may help develop novel and more functional powered assistive devices or robotic legged systems. Legged robots comprise passive parts (e.g., segments, joints and connections) which are moved in a coordinated manner by actuators. [...] Read more.
A better understanding of how actuator design supports locomotor function may help develop novel and more functional powered assistive devices or robotic legged systems. Legged robots comprise passive parts (e.g., segments, joints and connections) which are moved in a coordinated manner by actuators. In this study, we propose a novel concept of a hybrid electric-pneumatic actuator (EPA) as an enhanced variable impedance actuator (VIA). EPA is consisted of a pneumatic artificial muscle (PAM) and an electric motor (EM). In contrast to other VIAs, the pneumatic artificial muscle (PAM) within the EPA provides not only adaptable compliance, but also an additional powerful actuator with muscle-like properties, which can be arranged in different combinations (e.g., in series or parallel) to the EM. The novel hybrid actuator shares the advantages of both integrated actuator types combining precise control of EM with compliant energy storage of PAM, which are required for efficient and adjustable locomotion. Experimental and simulation results based on the new dynamic model of PAM support the hypothesis that combination of the two actuators can improve efficiency (energy and peak power) and performance, while does not increase control complexity and weight, considerably. Finally, the experiments on EPA adapted bipedal robot (knee joint of the BioBiped3 robot) show improved efficiency of the actuator at different frequencies. Full article
(This article belongs to the Special Issue Variable Stiffness and Variable Impedance Actuators)
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Article
Stiffness Control of Variable Serial Elastic Actuators: Energy Efficiency through Exploitation of Natural Dynamics
Actuators 2017, 6(4), 28; https://doi.org/10.3390/act6040028 - 27 Sep 2017
Cited by 2 | Viewed by 4477
Abstract
Variable elastic actuators are very promising for applications in physical human–robot interaction. Besides enabling human safety, such actuators can support energy efficiency, especially if the natural behavior of the system is exploited. In this paper, the power and energy consumption of variable stiffness [...] Read more.
Variable elastic actuators are very promising for applications in physical human–robot interaction. Besides enabling human safety, such actuators can support energy efficiency, especially if the natural behavior of the system is exploited. In this paper, the power and energy consumption of variable stiffness actuators with serial elasticity is investigated analytically and experimentally. Besides the fundamental mechanics, the influence of friction and electrical losses is discussed. A simple but effective stiffness control method is used to exploit the corresponding knowledge of natural dynamics by tuning the system to antiresonance operation. Despite nonlinear friction effects and additional electrical dynamics, the consideration of the ideal mechanical dynamics is completely sufficient for stiffness control. Simulations and experiments show that this yields a distinct reduction in power and energy consumption, which underlines the suitability of the control strategy. Full article
(This article belongs to the Special Issue Variable Stiffness and Variable Impedance Actuators)
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Article
Generalization of Series Elastic Actuator Configurations and Dynamic Behavior Comparison
Actuators 2017, 6(3), 26; https://doi.org/10.3390/act6030026 - 22 Aug 2017
Cited by 28 | Viewed by 8716
Abstract
The Series Elastic Actuator (SEA) has recently been developed by many research groups and applied in various fields. As SEA is the combination of motor, spring, gear and load, various types and configurations of mechanism have been developed as SEAs to satisfy many [...] Read more.
The Series Elastic Actuator (SEA) has recently been developed by many research groups and applied in various fields. As SEA is the combination of motor, spring, gear and load, various types and configurations of mechanism have been developed as SEAs to satisfy many requirements necessary for the applications. This paper provides a theoretical framework to categorize and compare these various configurations of SEAs. The general structure and model of SEA is provided, and SEA configurations are categorized into Force-sensing Series Elastic Actuator, Reaction Force-sensing Series Elastic Actuator and Transmitted Force-sensing Series Elastic Actuator, based on the relative location of the spring. Criteria such as Force sensitivity, Compliance and Transmissibility of SEA are derived and compared using actual SEAs that have been developed previously. Full article
(This article belongs to the Special Issue Variable Stiffness and Variable Impedance Actuators)
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Article
A Novel Mechanically Overdamped Actuator with Adjustable Stiffness (MOD-AwAS) for Safe Interaction and Accurate Positioning
Actuators 2017, 6(3), 22; https://doi.org/10.3390/act6030022 - 28 Jun 2017
Cited by 4 | Viewed by 4392
Abstract
This paper presents the design and development of a novel mechanically overdamped actuator with adjustable stiffness (MOD-AwAS). The novelty of MOD-AwAS compared to other variable stiffness actuators relates to its mechanical design, which prevents oscillations at the output link. Almost all variable stiffness [...] Read more.
This paper presents the design and development of a novel mechanically overdamped actuator with adjustable stiffness (MOD-AwAS). The novelty of MOD-AwAS compared to other variable stiffness actuators relates to its mechanical design, which prevents oscillations at the output link. Almost all variable stiffness actuators have an overshooting problem that require a sophisticated control algorithm to be able to perform accurate positioning. MOD-AwAS can regulate the stiffness from zero to its maximum (theoretically infinite) in less than 0.2 s by changing the position of the pivot point of its lever mechanisms. MOD-AwAS employs only one rotational spring with no pre-deflection, which gives it full accessibility to its energy storage capacity. Experimental results are presented to show the ability of MOD-AwAS to control its position accurately with a wide range of stiffness adjustment. Full article
(This article belongs to the Special Issue Variable Stiffness and Variable Impedance Actuators)
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Article
Power Split Based Dual Hemispherical Continuously Variable Transmission
Actuators 2017, 6(2), 15; https://doi.org/10.3390/act6020015 - 10 Apr 2017
Cited by 4 | Viewed by 5366
Abstract
In this work, we present a new continuously variable transmission concept: the Dual-Hemi Continuously Variable Transmission (CVT). It is designed to have properties we believe are required to apply continuously variable transmissions in robotics to their full potential. These properties are a transformation [...] Read more.
In this work, we present a new continuously variable transmission concept: the Dual-Hemi Continuously Variable Transmission (CVT). It is designed to have properties we believe are required to apply continuously variable transmissions in robotics to their full potential. These properties are a transformation range that includes both positive and negative ratios, back-drivability under all conditions, kinematically decoupled reconfiguration, high efficiency of the transmission, and a reconfiguration mechanism requiring little work for changing the transmission ratio. The design of the Dual-Hemi CVT and a prototype realisation are discussed in detail. We show that the Dual-Hemi CVT has the aforementioned desired properties. Experiments show that the efficiency of the CVT is above 90% for a large part of the range of operation of the CVT. Significant stiction in the transmission, combined with a relatively low bandwidth for changing the transmission ratio, may cause problems when applying the DH-CVT as part of an actuator in a control loop. Full article
(This article belongs to the Special Issue Variable Stiffness and Variable Impedance Actuators)
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Review

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Review
Flexible Medical Devices: Review of Controllable Stiffness Solutions
Actuators 2017, 6(3), 23; https://doi.org/10.3390/act6030023 - 11 Jul 2017
Cited by 72 | Viewed by 7592
Abstract
In the medical field and in soft robotics, flexible devices are required for safe human interaction, while rigid structures are required to withstand the force application and accuracy in motion. This paper aims at presenting controllable stiffness mechanisms described in the literature for [...] Read more.
In the medical field and in soft robotics, flexible devices are required for safe human interaction, while rigid structures are required to withstand the force application and accuracy in motion. This paper aims at presenting controllable stiffness mechanisms described in the literature for applications with or without shape-locking performances. A classification of the solutions based on their working principle is proposed. The intrinsic properties of these adaptive structures can be modified to change their mechanical characteristics from a geometrical point of view or equivalent elastic properties (with internal mechanisms or with a change in material properties). These solutions are compared quantitatively, based on selected criteria linked to the medical field as the stiffness range, the activation time and the working conditions. Depending on the application and its requirements, the most suitable solution can be selected following the quantitative comparisons. Several applications of these tunable stiffness structures are proposed and illustrated by examples of the literature. Full article
(This article belongs to the Special Issue Variable Stiffness and Variable Impedance Actuators)
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