Florian Petit

Variable impedance actuators: A review

Variable Impedance Actuators (VIA) have received increasing attention in recent years as many novel applications involving interactions with an unknown and dynamic environment including humans require actuators with dynamics that are not well-achieved by classical stiff actuators. This paper presents an overview of the different VIAs developed and proposes a classification based on the principles through which the variable stiffness and damping are achieved. The main classes are active impedance by control, inherent compliance and damping actuators, inertial actuators, and combinations of them, which are then further divided into subclasses. This classification allows for designers of new devices to orientate and take inspiration and users of VIAs to be guided in the design and implementation process for their targeted application.

The DLR hand arm system

An anthropomorphic hand arm system using variable stiffness actuation has been developed at DLR. It is aimed to reach its human archetype regarding size, weight and performance. The main focus of our development is put on robustness, dynamic performance and dexterity. Therefore, a paradigm change from impedance controlled, but mechanically stiff joints to robots using intrinsic variable compliance joints is carried out.

Robots Driven by Compliant Actuators: Optimal Control Under Actuation Constraints

Anthropomorphic robots that aim to approach human performance agility and efficiency are typically highly redundant not only in their kinematics but also in actuation. Variable-impedance actuators, used to drive many of these devices, are capable of modulating torque and impedance (stiffness and/or damping) simultaneously, continuously, and independently. These actuators are, however, nonlinear and assert numerous constraints, e.g., range, rate, and effort limits on the dynamics. Finding a control strategy that makes use of the intrinsic dynamics and capacity of compliant actuators for such redundant, nonlinear, and constrained systems is nontrivial. In this study, we propose a framework for optimization of torque and impedance profiles in order to maximize task performance, which is tuned to the complex hardware and incorporating real-world actuation constraints. Simulation study and hardware experiments 1) demonstrate the effects of actuation constraints during impedance control, 2) show applicability of the present framework to simultaneous torque and temporal stiffness optimization under constraints that are imposed by real-world actuators, and 3) validate the benefits of the proposed approach under experimental conditions.

Dynamic modelling and control of variable stiffness actuators

After briefly summarizing the mechanical design of the two joint prototypes for the new DLR variable compliance arm, the paper exemplifies the dynamic modelling of one of the prototypes and proposes a generic variable stiffness joint model for nonlinear control design. Based on this model, the design of a simple, gain scheduled state feedback controller for active vibration damping of the mechanically very weakly damped joint is presented. Moreover, the computation of the motor reference values out of the desired stiffness and position is addressed. Finally, simulation and experimental results validate the proposed methods.

State feedback damping control for a multi DOF variable stiffness robot arm

The concept of variable stiffness actuation (VSA) for robotic joints promises advantages regarding robustness, energy efficiency, and task adaptability. The VS joints developed at DLR show very low intrinsic damping for efficient energy storage and retrieval whereas the desired damping behavior for task execution needs to be implemented in control. Robotic arms with multiple VS joints, as for example the DLR Hand Arm System, ask for advanced control algorithms which can cope with the elastic joints and the multi-input multi-output (MIMO) system properties of the mechanical setup. We propose a MIMO controller for flexible joint robots based upon an eigenmode decoupling approach. For robustness reasons, the controller is designed to modify the intrinsic plant properties as little as possible while attaining the desired damping. A gain design algorithm is proposed. The controller is validated in simulations and experiments.

Bidirectional antagonistic variable stiffness actuation: Analysis, design & Implementation

The variable stiffness actuation concept is considered to provide a human-friendly robot technology. This paper examines a joint concept called the bidirectional antagonistic joint which is a extension of antagonistic joints. A new operating mode called the helping mode is introduced, which increases the joint load range. Although the joint can not be pretensioned in the helping mode, it is shown that a stiffness variation is possible, assuming a suitable torque-stiffness characteristic of the elastic elements. A methodology to design such characteristics is presented along with several example cases interpreted in a torque-stiffness plot. Furthermore, a stiffness adaptation control scheme which ensures mechanism safety is described. Finally, the design methodology and the control are evaluated on an implementation of a bidirectional antagonistic joint.

Cartesian impedance control for a variable stiffness robot arm

The variable stiffness actuation (VSA) technology has been recently developed and applied in robotic arms. Mechanism robustness, high peak torque and velocity, and stiffness adjustment flexibility are key benefits of VSA joints. However, the achievable Cartesian stiffness by uncoupled VSA joints is limited. Therefore we suggest and analyze the use of an active impedance controller in combination with the passive joints to further increase the stiffness range. An algorithm to optimize the passive and active Cartesian stiffness is proposed to achieve a desired Cartesian stiffness as precise as possible. The algorithm was implemented and tested on the VSA robot DLR Hand Arm System. Experimental results and measurements of the active/passive impedance algorithm are shown.

Backstepping Control of Variable Stiffness Robots

Robots with elastic joints arouse an increasing interest in the robotics community. Especially, variable stiffness robots promise to be beneficial regarding robustness and task adaptability. However, the control of variable stiffness multijoint robots is challenging due to their highly nonlinear behavior. Here, we present a backstepping approach for tracking control of multijoint variable stiffness robots. To cope with the problem of noisy-state measurements and the need for higher order state derivatives, command filters are used. A stability proof of the complete control approach including filtering is given. Finally, the simulations and measurements on a variable stiffness joint and a multijoint variable stiffness robot validate the approach.

Generalizing Torque Control Concepts: Using Well-Established Torque Control Methods on Variable Stiffness Robots

Strict requirements must be met before robotic systems can be implemented in a human environment, for example, as service robots. Robustness, task adaptability, and energy efficiency are key aspects in this regard. Variable stiffness robots have been shown to be one step toward achieving these standards. In this article, we elaborate on the essential control aspects required to operate these robots and generalize well-established torque control methods to a variable stiffness robot, the DLR Hand Arm System (HASy). The adaptation and implementation of several control approaches for the compliant robots are also presented, with a focus on the experimental validation.

Nonlinear Oscillations for Cyclic Movements in Human and Robotic Arms

The elastic energy storage in biologically inspired variable impedance actuators (VIA) offer the capability of executing cyclic and/or explosive multi-degree of freedom (DoF) motions efficiently. This paper studies the generation of cyclic motions for strongly nonlinear underactuated multi-DoF serial robotic arms. By experimental observations of human motor control, a simple and robust control law is deduced. This controller achieves intrinsic oscillatory motions by switching the motor position triggered by a joint torque threshold. Using the derived controller, the oscillatory behavior of human and robotic arms is analyzed in simulations and experiments. It is found that the existence of easily excitable oscillation modes strongly depends on the damping properties of the plant. If the intrinsic damping properties are such that oscillations excited in the undesired modes decay faster than in the desired mode, then multi-DoF oscillations are easily excitable. Simulations and experiments reveal that serially-structured elastic multibody systems such as VIA or human arms with approximately equal joint damping, fulfill these requirements.