Design and Modeling of a Variable-Stiffness Spring Mechanism for Impedance Modulation in Physical Human–Robot Interaction

Abstract Devices with variable stiffness are drawing more and more attention with the growing interests of human-robot interaction, wearable robotics, rehabilitation robotics, etc. In this paper, the authors report on the design, analysis and experiments of a stiffness variable passive compliant device whose structure is a combination of a reconfigurable elastic inner skeleton and an origami shell. The main concept of the reconfigurable skeleton is to have two elastic trapezoid four-bar linkages arranged in orthogonal. The stiffness variation generates from the passive deflection of the elastic limbs and is realized by actively switching the arrangement of the leaf springs and the passive joints in a fast, simple and straightforward manner. The kinetostatics and the compliance of the device are analyzed based on an efficient approach to the large deflection problem of the elastic links. A prototype is fabricated to conduct experiments for the assessment of the proposed concept. The results show that the prototype possesses relatively low stiffness under the compliant status and high stiffness under the stiff status with a status switching speed around 80 ms.

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Soft Robotics with Variable Stiffness Actuators: Tough Robots for Soft Human Robot Interaction

Soft Robotics ◽

10.1007/978-3-662-44506-8_20 ◽

2015 ◽

pp. 231-254 ◽

Cited By ~ 9

Author(s):

Sebastian Wolf ◽

Thomas Bahls ◽

Maxime Chalon ◽

Werner Friedl ◽

Markus Grebenstein ◽

...

Keyword(s):

Variable Stiffness ◽

Human Robot Interaction ◽

Soft Robotics ◽

Robot Interaction ◽

Variable Stiffness Actuators

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Determination of Variable Stiffness of a Human Elbow for Human-Robot Interaction

Volume 5B: 38th Mechanisms and Robotics Conference ◽

10.1115/detc2014-34420 ◽

2014 ◽

Author(s):

Michael Boyarsky ◽

Megan Heenan ◽

Scott Beardsley ◽

Philip Voglewede

Keyword(s):

Experimental Data ◽

Disturbance Rejection ◽

Variable Stiffness ◽

Important Variable ◽

Human Motion ◽

Human Robot Interaction ◽

Robot Interaction ◽

Constant Stiffness ◽

Stiffness Model ◽

Motor Model

This paper aims to emulate human motion with a robot for the purpose of improving human-robot interaction (HRI). In order to engineer a robot that demonstrates functionally similar motion to humans, aspects of human motion such as variable stiffness must be captured. This paper successfully determined the variable stiffness humans use in the context of a 1 DOF disturbance rejection task by optimizing a time-varying stiffness parameter to experimental data in the context of a neuro-motor Simulink model. The significant improved agreement between the model and the experimental data in the disturbance rejection task after the addition of variable stiffness demonstrates how important variable stiffness is to creating a model of human motion. To enable a robot to emulate this motion, a predictive stiffness model was developed that attempts to reproduce the stiffness that a human would use in a given situation. The predictive stiffness model successfully decreases the error between the neuro-motor model and the experimental data when compared to the neuro-motor model with a constant stiffness value.

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Design of a Variable Stiffness Actuator Based on Flexures

Journal of Mechanisms and Robotics ◽

10.1115/1.4004228 ◽

2011 ◽

Vol 3 (3) ◽

Cited By ~ 32

Author(s):

Gianluca Palli ◽

Giovanni Berselli ◽

Claudio Melchiorri ◽

Gabriele Vassura

Keyword(s):

Compliant Mechanism ◽

Variable Stiffness ◽

Human Robot Interaction ◽

Experimental Characterization ◽

Robot Interaction ◽

Trade Off ◽

Variable Stiffness Actuator ◽

Robotic Devices ◽

Stiffness Profile ◽

Variable Stiffness Actuators

Variable stiffness actuators can be used in order to achieve a suitable trade-off between performance and safety in robotic devices for physical human–robot interaction. With the aim of improving the compactness and the flexibility of existing mechanical solutions, a variable stiffness actuator based on the use of flexures is investigated. The proposed concept allows the implementation of a desired stiffness profile and range. In particular, this paper reports a procedure for the synthesis of a fully compliant mechanism used as a nonlinear transmission element, together with its experimental characterization. Finally, a preliminary prototype of the overall joint is depicted.

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A Comparative Study on the Effect of Mechanical Compliance for a Safe Physical Human–Robot Interaction

Journal of Mechanical Design ◽

10.1115/1.4046068 ◽

2020 ◽

Vol 142 (6) ◽

Cited By ~ 1

Author(s):

Yu She ◽

Siyang Song ◽

Hai-Jun Su ◽

Junmin Wang

Keyword(s):

Impact Force ◽

Variable Stiffness ◽

Human Robot Interaction ◽

Design Parameters ◽

Robot Dynamics ◽

Robot Interaction ◽

Promising Alternative ◽

Mechanical Compliance ◽

Maximum Impact Force ◽

The Impact

Abstract In this paper, we study the effects of mechanical compliance on safety in physical human–robot interaction (pHRI). More specifically, we compare the effect of joint compliance and link compliance on the impact force assuming a contact occurred between a robot and a human head. We first establish pHRI system models that are composed of robot dynamics, an impact contact model, and head dynamics. These models are validated by Simscape simulation. By comparing impact results with a robotic arm made of a compliant link (CL) and compliant joint (CJ), we conclude that the CL design produces a smaller maximum impact force given the same lateral stiffness as well as other physical and geometric parameters. Furthermore, we compare the variable stiffness joint (VSJ) with the variable stiffness link (VSL) for various actuation parameters and design parameters. While decreasing stiffness of CJs cannot effectively reduce the maximum impact force, CL design is more effective in reducing impact force by varying the link stiffness. We conclude that the CL design potentially outperforms the CJ design in addressing safety in pHRI and can be used as a promising alternative solution to address the safety constraints in pHRI.

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V2SOM: A Novel Safety Mechanism Dedicated to a Cobot’s Rotary Joints

Robotics ◽

10.3390/robotics8010018 ◽

2019 ◽

Vol 8 (1) ◽

pp. 18 ◽

Cited By ~ 4

Author(s):

Younsse Ayoubi ◽

Med Laribi ◽

Said Zeghloul ◽

Marc Arsicault

Keyword(s):

Good Control ◽

Variable Stiffness ◽

Human Robot Interaction ◽

Industrial Robots ◽

Necessary Condition ◽

Physical Interaction ◽

Robot Interaction ◽

Internal Damage ◽

Low Stiffness ◽

The Impact

Unlike “classical” industrial robots, collaborative robots, known as cobots, implement a compliant behavior. Cobots ensure a safe force control in a physical interaction scenario within unknown environments. In this paper, we propose to make serial robots intrinsically compliant to guarantee safe physical human–robot interaction (pHRI), via our novel designed device called V2SOM, which stands for Variable Stiffness Safety-Oriented Mechanism. As its name indicates, V2SOM aims at making physical human–robot interaction safe, thanks to its two basic functioning modes—high stiffness mode and low stiffness mode. The first mode is employed for normal operational routines. In contrast, the low stiffness mode is suitable for the safe absorption of any potential blunt shock with a human. The transition between the two modes is continuous to maintain a good control of the V2SOM-based cobot in the case of a fast collision. V2SOM presents a high inertia decoupling capacity which is a necessary condition for safe pHRI without compromising the robot’s dynamic performances. Two safety criteria of pHRI were considered for performance evaluations, namely, the impact force (ImpF) criterion and the head injury criterion (HIC) for, respectively, the external and internal damage evaluation during blunt shocks.

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Mechanical design and analysis of a novel variable stiffness actuator with symmetrical pivot adjustment

Frontiers of Mechanical Engineering ◽

10.1007/s11465-021-0647-1 ◽

2021 ◽

Author(s):

Yiwei Liu ◽

Shipeng Cui ◽

Yongjun Sun

Keyword(s):

Modular Design ◽

Mechanical Design ◽

Bending Moment ◽

Variable Stiffness ◽

Fast Response ◽

Human Robot Interaction ◽

Asymmetric Structure ◽

Robot Interaction ◽

Collaborative Robotics ◽

Variable Stiffness Actuator

AbstractThe safety of human-robot interaction is an essential requirement for designing collaborative robotics. Thus, this paper aims to design a novel variable stiffness actuator (VSA) that can provide safer physical human-robot interaction for collaborative robotics. VSA follows the idea of modular design, mainly including a variable stiffness module and a drive module. The variable stiffness module transmits the motion from the drive module in a roundabout manner, making the modularization of VSA possible. As the key component of the variable stiffness module, a stiffness adjustment mechanism with a symmetrical structure is applied to change the positions of a pair of pivots in two levers linearly and simultaneously, which can eliminate the additional bending moment caused by the asymmetric structure. The design of the double-deck grooves in the lever allows the pivot to move freely in the groove, avoiding the geometric constraint between the parts. Consequently, the VSA stiffness can change from zero to infinity as the pivot moves from one end of the groove to the other. To facilitate building a manipulator in the future, an expandable electrical system with a distributed structure is also proposed. Stiffness calibration and control experiments are performed to evaluate the physical performance of the designed VSA. Experiment results show that the VSA stiffness is close to the theoretical design stiffness. Furthermore, the VSA with a proportional-derivative feedback plus feedforward controller exhibits a fast response for stiffness regulation and a good performance for position tracking.

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Design and Analysis of Variable Flexible Actuator for Human-Robot Interaction

Xibei Gongye Daxue Xuebao/Journal of Northwestern Polytechnical University ◽

10.1051/jnwpu/20193720242 ◽

2019 ◽

Vol 37 (2) ◽

pp. 242-248

Author(s):

Wendong Wang ◽

Tongsen Sun ◽

Xiaoqing Yuan ◽

Jinzhe Li ◽

Xing Ming

Keyword(s):

Magnetic Field ◽

Variable Stiffness ◽

Mechanical Analysis ◽

Simulation Software ◽

Human Robot Interaction ◽

Field Analysis ◽

Robot Interaction ◽

Magnetic Field Analysis ◽

Mr Effect ◽

The Magnetic Field

In view of the shortcomings of the traditional high-rigidity robots that can't ensure safety of human-robot interaction and the adaptability of complex environment, a variable stiffness flexible actuator is proposed for the robot joint based on the magnetorheological (MR) effect of MR Fluids. In this paper, the principle of flexible joint drive is detailed, and design the structure of the drive and perform mechanical analysis is designed. At the same time, the magnetic field analysis of the actuator is carried out by using the magnetic field simulation software Maxwell. Finally, the actuator is simulated and verified. Comparing the analysis of the results, the actuator has the characteristics of simple structure, easy control and wider active variable stiffness adjustment range, and can absorb the vibration or shock energy, and improve the robot joint output ability.

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Variable Stiffness Mechanism for Suppressing Unintended Forces in Physical Human–Robot Interaction

Journal of Mechanisms and Robotics ◽

10.1115/1.4042295 ◽

2019 ◽

Vol 11 (2) ◽

Cited By ~ 3

Author(s):

Sri Sadhan Jujjavarapu ◽

Amirhossein H. Memar ◽

M. Amin Karami ◽

Ehsan T. Esfahani

Keyword(s):

High Frequency ◽

Degrees Of Freedom ◽

Variable Stiffness ◽

Human Robot Interaction ◽

Robot Interaction ◽

Interaction Forces ◽

Air Gaps ◽

Nonlinear Springs ◽

Controller Gain ◽

Unstable Interaction

This paper presents the design of a two-degrees-of-freedom (DoFs) variable stiffness mechanism and demonstrates how its adjustable compliance can enhance the robustness of physical human–robot interaction. Compliance on the grasp handle is achieved by suspending it in between magnets in preloaded repelling configuration to act as nonlinear springs. By adjusting the air gaps between the outer magnets, the stiffness of the mechanism in each direction can be adjusted independently. Moreover, the capability of the proposed design in suppressing unintended interaction forces is evaluated in two different experiments. In the first experiment, improper admittance controller gain leads to unstable interaction, whereas in the second case, high-frequency involuntary forces are caused by the tremor.

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Design and Prototype of a Tunable Stiffness Arm for Safe Human-Robot Interaction

Volume 5B: 40th Mechanisms and Robotics Conference ◽

10.1115/detc2016-59523 ◽

2016 ◽

Cited By ~ 10

Author(s):

Yu She ◽

Hai-Jun Su ◽

Cheng Lai ◽

Deshan Meng

Keyword(s):

Impact Velocity ◽

Robot Manipulator ◽

Variable Stiffness ◽

Human Robot Interaction ◽

Robot Interaction ◽

Injury Criteria ◽

Straight Beam ◽

Head Injury Criteria ◽

The Impact ◽

Tunable Stiffness

In this paper, we present a tunable stiffness robot link for safe human-robot interaction. Stiffness of a manipulator determines the injury levels of a human from an impact between robots and operators, given a specific impact velocity. Compliance of a robot manipulator includes joint compliance and link compliance. Variable stiffness design from the viewpoint of actuators have been widely studied, while adjustable stiffness robotic link in the application of human robot interaction is rare in literatures. This paper details the design of a tunable stiffness robotic manipulator via four bar linkages which are actuated by servo motors. A 3D model of the morphing beam is constructed, and a robot which is made up of 3 morphing arms is designed. Prototypes using 3D printer are fabricated. Numerous tests have been done, and the results show that the stiffness is able to change 3.6 times given a morphing angle of π/4. Given an impact velocity of 2.2 m/s, the impact tests show that the acceleration has a 19.4% decrease comparing the curved beam and straight beam, and the head injury criteria (HIC) significantly decreases from 210.3 m5/2s−4 to 150.3 m5/2s−4, which is much safer to the operators. This paper explores the research of tunable stiffness on robotic links in the application of human robot interaction, expanding the research arena with regarding to human safe robot design.

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