https://ris.utwente.nl/ws/files/6520436/pp.pdf

Variable impedance actuators: A review

In the growing fields of wearable robotics, rehabilitation robotics, prosthetics, and walking k robots, variable stiffness actuators (VSAs) or adjustable compliant actuators are being designed and implemented because of their ability to minimize large forces due to shocks, to safely interact with the user, and their ability to store and release energy in passive elastic elements. This review article describes the state of the art in the design of actuators with adaptable passive compliance. This new type of actuator is not preferred for classical position-controlled applications such as pick and place operations but is preferred in novel robots where safe human- robot interaction is required or in applications where energy efficiency must be increased by adapting the actuator's resonance frequency. The working principles of the different existing designs are explained and compared. The designs are divided into four groups: equilibrium-controlled stiffness, antagonistic-controlled stiffness, structure-controlled stiffness (SCS), and mechanically controlled stiffness.

Compliant actuator designs

http://www.centropiaggio.unipi.it/sites/default/files/softarm-MMT.pdf

An atlas of physical human-robot interaction

Cooperation of human and machines in assembly lines

A robot manipulator sharing its workspace with humans should be able to quickly detect collisions and safely react for limiting injuries due to physical contacts. In the absence of external sensing, relative motions between robot and human are not predictable and unexpected collisions may occur at any location along the robot arm. Based on physical quantities such as total energy and generalized momentum of the robot manipulator, we present an efficient collision detection method that uses only proprioceptive robot sensors and provides also directional information for a safe robot reaction after collision. The approach is first developed for rigid robot arms and then extended to the case of robots with elastic joints, proposing different reaction strategies. Experimental results on collisions with the DLR-III lightweight manipulator are reported.

/pdf/collision-detection-and-safe-reaction-with-the-dlr-iii-54sik9l9mo.pdf

Collision Detection and Safe Reaction with the DLR-III Lightweight Manipulator Arm

This paper is concerned with the design and control of actuators for machines and robots physically interacting with humans, implementing criteria established in our previous work [1] on optimal mechanical-control co-design for intrinsically safe, yet performant machines. In our Variable Impedance Actuation (VIA) approach, actuators control in real-time both the reference position and the mechanical impedance of the moving parts in the machine in such a way to optimize performance while intrinsically guaranteeing safety. In this paper we describe an implementation of such concepts, consisting of a novel electromechanical Variable Stiffness Actuation (VSA) motor. The design and the functioning principle of the VSA are reported, along with the analysis of its dynamic behavior. A novel scheme for feedback control of this device is presented, along with experimental results showing performance and safety of a one-link arm actuated by the VSA motor.

/pdf/design-and-control-of-a-variable-stiffness-actuator-for-safe-4yhg5goadl.pdf

Design and Control of a Variable Stiffness Actuator for Safe and Fast Physical Human/Robot Interaction

This paper presents design and performance of a novel joint based actuator for a robot run by variable stiffness actuation, meant for systems physically interacting with humans. This new actuator prototype (VSA-II) is developed as an improvement over our previously developed one reported in [9], where an optimal mechanical-control co-design principle established in [7] is followed as well. While the first version was built in a way to demonstrate effectiveness of variable impedance actuation (VIA), it had limitations in torque capacities, life cycle and implementability in a real robot. VSA-II overcomes the problem of implementability with higher capacities and robustness in design for longer life. The paper discusses design and stiffness behaviour of VSA-II in theory and experiments. A comparison of stiffness characteristics between the two actuator is discussed, highlighting the advantages of the new design. A simple, but effective PD scheme is employed to independently control joint-stiffness and joint-position of a 1-link arm. Finally, results from performed impact tests of 1- link arm are reported, showing the effectiveness of stiffness variation in controlling value of a safety metric.

/pdf/vsa-ii-a-novel-prototype-of-variable-stiffness-actuator-for-hnxnuxrpu5.pdf

VSA-II: a novel prototype of variable stiffness actuator for safe and performing robots interacting with humans

In this paper we discuss the integration of active and passive approaches to robotic safety in an overall scheme for real-time manipulator control. The active control approach is based on the use of a supervisory visual system, which detects the presence and position of humans in the vicinity of the robot arm, and generates motion references. The passive control approach uses variable joint impedance which combines with velocity control to guarantee safety in worst-case conditions, i.e. unforeseen impacts. The implementation of these techniques in a 3-dof, variable impedance arm is described, and the effectiveness of their functional integration is demonstrated through experiments.

/pdf/integration-of-active-and-passive-compliance-control-for-4dwfr8jc0c.pdf

Integration of active and passive compliance control for safe human-robot coexistence

Variable Stiffness Actuation (VSA) devices are being used to jointly address the issues of safety and performance in physical human-robot interaction. With reference to the VSA-II prototype, we present a feedback linearization approach that allows the simultaneous decoupling and accurate tracking of motion and stiffness reference profiles. The operative condition that avoids control singularities is characterized. Moreover, a momentum-based collision detection scheme is introduced, which does not require joint torque sensing nor information on the time-varying stiffness of the device. Based on the residual signal, a collision reaction strategy is presented that takes advantage of the proposed nonlinear control to rapidly let the arm bounce away after detecting the impact, while limiting contact forces through a sudden reduction of the stiffness. Simulations results are reported to illustrate the performance and robustness of the overall approach. Extensions to the multidof case of robot manipulators equipped with VSA-II devices are also considered.

/pdf/nonlinear-decoupled-motion-stiffness-control-and-collision-3snrxt4pax.pdf

Nonlinear decoupled motion-stiffness control and collision detection/reaction for the VSA-II variable stiffness device

Robots designed to share an environment with humans, such as e.g. in domestic or entertainment applications or in cooperative material-handling tasks, must fulfill different requirements from those typically met in industry. It is often the case, for instance, that accuracy requirements are less demanding. On the other hand, concerns of paramount importance are safety and dependability of the robot system. According to such difference in requirements, it can be expected that usage of conventional industrial arms for anthropic environments will be far from optimal. An approach to increase the safety level of robot arms interacting with humans consists in the introduction of compliance at the mechanical design level. In this paper we discuss the problem of achieving good performances in accuracy and promptness with a robot manipulator under the condition that safety is guaranteed throughout whole task execution. Intuitively, while a rigid and powerful structure of the arm would favor its performance, lightweight compliant structures are more suitable for safe operation. The quantitative analysis of the resulting design trade-off between safety and performance has a strong impact on how robot mechanisms and controllers should be designed for human- interactive applications. We discuss few different possible concepts for safely actuating joints, and focus on aspects related to the implementation of the mechanics and control of this new class of robots.

/pdf/physical-human-robot-interaction-dependability-safety-and-fq40chwk9u.pdf

R. Schiavi

Papers

Design and Control of a Variable Stiffness Actuator for Safe and Fast Physical Human/Robot Interaction

VSA-II: a novel prototype of variable stiffness actuator for safe and performing robots interacting with humans

Integration of active and passive compliance control for safe human-robot coexistence

Nonlinear decoupled motion-stiffness control and collision detection/reaction for the VSA-II variable stiffness device

Physical human-robot interaction: Dependability, safety, and performance