- [Show abstract] [Hide abstract] ABSTRACT: 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.
- [Show abstract] [Hide abstract] ABSTRACT: 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.
- [Show abstract] [Hide abstract] ABSTRACT: Humanoid service robots in domestic environments have to interact with humans and their surroundings in a safe and reliable way. One way to manage that is to equip the robotic systems with force-torque sensors to realize a physically compliant whole-body behavior via impedance control. To provide mobility, such robots often have wheeled platforms. The main advantage is that no balancing effort has to be made compared to legged humanoids. However, the nonholonomy of most wheeled systems prohibits the direct implementation of impedance control due to kinematic rolling constraints that must be taken into account in modeling and control. In this paper we design a whole-body impedance controller for such a robot, which employs an admittance interface to the kinematically controlled mobile platform. The upper body impedance control law, the platform admittance interface, and the compensation of dynamic couplings between both subsystems yield a passive closed loop. The convergence of the state to an invariant set is shown. To prove asymptotic stability in the case of redundancy, priority-based approaches can be employed. In principle, the presented approach is the extension of the well-known and established impedance controller to mobile robots. Experimental validations are performed on the humanoid robot Rollin’ Justin. The method is suitable for compliant manipulation tasks with low-dimensional planning in the task space.
- [Show abstract] [Hide abstract] ABSTRACT: Robots that are not only robust, dynamic, and gentle in the human robot interaction, but are also able to perform precise and repeatable movements, need accurate dynamics modeling and a high-performance closed-loop control. As a technological basis we propose robots with intrinsically compliant joints, a stiff link structure, and a soft shell. The flexible joints are driven by Variable Stiffness Actuators (VSA) with a mechanical spring coupling between the motor and the actuator output and the ability to change the mechanical stiffness of the spring coupling. Several model based and model free control approaches have been developed for this technology, e.g. Cartesian stiffness control, optimal control, reactions, reflexes, and cyclic motion control.
- [Show abstract] [Hide abstract] ABSTRACT: 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.
- [Show abstract] [Hide abstract] ABSTRACT: Robots with joint elasticity find increasing interest in many research areas. A common design goal is to achieve as little mechanical joint damping as possible. To still achieve system damping often control systems are used. Here, we present a model-free approach to achieve damping via exploiting the kinetic to potential energy transformation process of the robot mass and the joint elasticity. The controller acts in an energetically passive way and is applicable to multi-joint systems. The theoretical findings and simulations are substantiated by experiments on the DLR Hand Arm System.
- [Show abstract] [Hide abstract] ABSTRACT: Variable stiffness actuation promises many benefits regarding mechanism robustness, energy efficiency, and dynamic performance. Here, we analyze the bidirectional antagonistic variable stiffness (BAVS) joint. A comprehensive overview of several aspects is given with a focus on the stiffness and torque characteristics of the joint. First, the functionality and properties of the abstract joint model are considered. Then, implementation details influencing the stiffness properties are discussed based on cam disc variable stiffness mechanisms. In general, an analytic approach is chosen to enable a generalization of the results. Experiments conducted on a BAVS joint of the variable stiffness actuated robot DLR Hand Arm System verify the theoretical findings.
- [Show abstract] [Hide abstract] ABSTRACT: 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 VIA’s to be guided in the design and implementation process for their targeted application.
- [Show abstract] [Hide abstract] ABSTRACT: Compliant actuators in robotic systems improve robustness against rigid impacts and increase the performance and efficiency of periodic motions such as hitting, jumping and running. However, in the case of rigid impacts, as they can occur during hitting or running, the system behavior is changed compared to free motions which turns the control into a challenging task. We introduce a controller that excites periodic motions along the direction of an intrinsic mechanical oscillation mode. The controller requires no model knowledge and adapts to a modal excitation by means of measurement of the states. We experimentally show that the controller is able to stabilize a hitting motion on the variable stiffness robot DLR Hand Arm System. Further, we demonstrate by simulation that the approach applies for legged robotic systems with compliantly actuated joints. The controlled system can approach different modes of motion such as jumping, hopping and running, and thereby, it is able to handle the repeated occurrence of robot-ground contacts.
- [Show abstract] [Hide abstract] ABSTRACT: Anthropomorphic robots that aim to approach hu- man performance agility and efficiency are typically highly redundant not only in their kinematics but also in actuation. Variable-impedance actuators (VIAs), 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, non-linear 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, non-linear and constrained system is non-trivial. In this work, we propose a framework for optimization of torque and impedance profiles in order to maximize task performance, tuned to the complex hardware and incorporating the real-world actuation constraints. Simulation study and hardware experiments: i) demonstrate the effects of actuation constraints during impedance control, ii) show applicability of the present framework to simultaneous torque and temporal stiffness optimization under constraints imposed by real-world actuators and iii) validate the benefits of the proposed approach under experimental conditions.
- [Show abstract] [Hide abstract] ABSTRACT: This paper presents a control approach to stabilize limit cycle motions along a mechanical mode of variable stiffness actuated (VSA) robots. Thereby, first a PD controller with gravity and Coriolis/centrifugal compensation shapes a desired dynamics, which is decoupled in terms of modal coordinates. Then an asymptotically stable limit cycle is generated on the link side dynamics for a selected mode. Finally, the modal control approach first introduced for rigid robots is extended to the VSA case. This is done by a joint torque controller, which decouples the torque dynamics from the link side dynamics. Stability and convergence are proven for the dynamics resulting from each feedback control. Furthermore, the energy efficiency of the proposed approach is verified by simulation and experiments on the VSA robotic arm DLR Hand Arm System.
- [Show abstract] [Hide abstract] ABSTRACT: Biologically inspired Variable Impedance Actuators (VIA) offer the capability to execute cyclic and/or explosive multi degree of freedom (DoF) motions efficiently by storing elastic energy. This paper studies the preconditions which allow to induce robust cyclic motions for strongly nonlinear, underactuated multi DoF robotic arms. By experimental observations of human motor control, a simple 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 periodic behavior of the robotic arm is analyzed in simulations. It is found that a modal analysis of the linearized system at the equilibrium point allows to qualitatively predict the periodic behavior of this type of strongly nonlinear systems. The central statement of this paper is that cyclic motions can be induced easily in VIA systems, if the eigenfrequencies and modal damping values of the linearized system are well separated. Validation is given by simulation and experiments, where a human controls a simulated robotic arm, and the developed regulator controls a robotic arm in simulation and experiments.
Conference Paper: Optimal Torque and Stiffness Control in Compliantly Actuated Robots[Show abstract] [Hide abstract] ABSTRACT: 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 passive impedance (stiffness and/or damping) simultaneously and independently. Here, we propose a framework for simultaneous optimisation of torque and impedance (stiffness) profiles in order to optimise task performance, tuned to the complex hardware and incorporating real-world constraints. Simulation and hardware experiments validate the viability of this approach to complex, state dependent constraints and demonstrate task performance benefits of optimal temporal impedance modulation.
Conference Paper: Variable impedance actuators: Moving the robots of tomorrow[Show abstract] [Hide abstract] ABSTRACT: Most of today's robots have rigid structures and actuators requiring complex software control algorithms and sophisticated sensor systems in order to behave in a compliant and safe way adapted to contact with unknown environments and humans. By studying and constructing variable impedance actuators and their control, we contribute to the development of actuation units which can match the intrinsic safety, motion performance and energy efficiency of biological systems and in particular the human. As such, this may lead to a new generation of robots that can co-exist and co-operate with people and get closer to the human manipulation and locomotion performance than is possible with current robots.
Conference Paper: Intrinsically elastic robots: The key to human like performance[Show abstract] [Hide abstract] ABSTRACT: Intrinsically elastic robots, which technically implement some key characteristics of the human muskoskeletal system, have become a major research topic in nowadays robotics. These novel devices open up entirely new control approaches. They base on temporary storage of potential energy and its timed transformation into kinetic energy. In legged locomotion, such considerations have been a common tool for unveiling the respective fundamental physical processes. However, in arm control, elasticities were typically considered parasitic. In this video we outline our efforts in exploiting the inherent capabilities of intrinsically elastic robots in order to bring them closer to human performance. Instead of applying purely kinematic learing-by-demonstration approaches, which are certainly suboptimal, we argue for using model based techniques in order to optimally exploit the system dynamics such that highly dynamic motion and manipulation capabilities can be achieved. In particular, the explicit use of elasticities as temporary energy tanks can be fully exploited, if they are modeled adequately as an integral part of the mechanism. We also believe that such approaches can substantially contribute to the understanding of human motion biomechanics.
- [Show abstract] [Hide abstract] ABSTRACT: Robotic systems can benefit from the introduction of properly chosen joint elasticity. Besides their robustness against rigid impact, the energy saving capabilities may increase the system dynamics. In this paper, a method applicable for robots with serial elastic joints is presented, which embodies a desired oscillatory behavior into the hardware and thereby leads to improved performance. This is achieved by shaping the flexible joint robot as a linear onemode system and embodying the natural frequency of the real intrinsic behavior. An algorithm is presented for shaping the one-mode property and exciting the system via a negative definite damping term in a decoupled coordinate space. The output of the approach is a dynamic trajectory resulting in a coordinated link motion and synchronized transfer of kinetic and potential energy. Furthermore, the dynamic trajectory is commanded to the real robot via a motor PD controller, where asymptotic stability for both subsystems—i.e. the trajectory generator and the controlled robot—is proven. The method is validated on a two-link serial elastic actuated robot. Both, simulation and experiment confirm the eigenmode embodiment, energy efficiency by velocity enlargement between motor and link side motion, and synchronized joint motion.
- [Show abstract] [Hide abstract] ABSTRACT: The paper presents a new energy shaping control design for a class of underactuated Euler-Lagrange systems. Flexible joint robots, Series Elastic Actuators, and Variable Impedance Actuated Robots Albu-Schäffer et al.  belong for example to this class. First, classical PD control with feed-forward compensation is revisited and a novel, straight-forward and general formulation for the stability analysis is given. Lower bound conditions for the gains of this controller motivate the introduction of the new approach, which generalizes results from Albu-Schäffer et al. , Ch. Ott et al. . For shaping the potential energy, feedback variables based on the collocated states are introduced, which are statically equivalent to the noncollocated state variables. In this way the passivity is ensured while exactly satisfying steady state requirements formulated in terms of the noncollocated states (such as desired equilibrium confguration and desired stiffness). Using the passivity property, a Lyapunov based analysis can be easily carried out for arbitrarily low feedback gains. The controller is augmented by noncollocated feedback to shape the kinetic energy. Experimental results for a Variable Stiffness Robot Grebenstein et al.  validate the proposed controller.
- [Show abstract] [Hide abstract] ABSTRACT: The human arm’s capability to alter its impedance has motivated multiple developments of robotic manipulators and control methods. It provides advantages during manipulation such as robustness against external disturbances and task adaptability. However, how the impedance of the arm is set depends on the manipulation situation; a general procedure is lacking. This paper aims to fill this gap by providing a method to estimate the impedance parameters of the human arm, while taking the numerical stability of the approach into account. A dynamic arm model and an identification method is presented. Confidential criteria to determine the accuracy of the estimated parameters are given. Finally, the procedure is validated in an experiment with a human subject and the results are discussed.
- [Show abstract] [Hide abstract] ABSTRACT: The DLR Hand Arm System is based upon the variable stiffness concept which has been recently developed to improve impact robustness and energy efficiency of modern robots. This paper continues the work on the bidirectional antagonistic variable stiffness (BAVS) joint concept which is an extension of antagonistic joints. Three mechanical setups utilizing different spring and cam disc combinations to implement a desired torque-stiffness characteristic are analyzed. Two BAVS joint solutions as used for the wrist and forearm rotation of the DLR Hand Arm System are presented. Furthermore in the experimental section torque-deflection calibration and drive redundancy are validated
Conference Paper: Cartesian Impedance Control For A Variable Stiffness Robot Arm[Show abstract] [Hide abstract] ABSTRACT: 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.
German Aerospace Center (DLR)
Köln, North Rhine-Westphalia, Germany
- Institute of Robotics and Mechatronics