Energy-Efficient Variable Stiffness Actuators

IEEE Transactions on Robotics (Impact Factor: 2.43). 11/2011; 27(5):865 - 875. DOI: 10.1109/TRO.2011.2150430
Source: IEEE Xplore


Variable stiffness actuators are a particular class of actuators that is characterized by the property that the apparent output stiffness can be changed independent of the output position. To achieve this, variable stiffness actuators consist of a number of elastic elements and a number of actuated degrees of freedom, which determine how the elastic elements are perceived at the actuator output. Changing the apparent output stiffness is useful for a broad range of applications, which explains the increasing research interest in this class of actuators. In this paper, a generic, port-based model for variable stiffness actuators is presented, with which a wide variety of designs can be modeled and analyzed. From the analysis of the model, it is possible to derive kinematic properties that variable stiffness actuator designs should satisfy in order to be energy efficient. More specifically, the kinematics should be such that the apparent output stiffness can be varied without changing the potential energy that is stored in the internal elastic elements. A concept design of an energy-efficient variable stiffness actuator is presented and implemented. Simulations of the model and experiments on the realized prototype validate the design principle.

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    • "Interactions between a manipulator end-effector and the environment occur in many applications, such as assembling or grasping operations. Consequently, considerable attention has been paid to the analysis, modeling, optimization, and control of robotic system stiffness and compliance3456789101112131415161718. Generally, such interactions occur in dynamic conditions and depend on the compliance of the systems in contact, i.e. both the robot and the environment. "
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    ABSTRACT: Abstract In this paper, the isotropic compliance property is examined in the Special Euclidean Group SE(3). The relation between the wrench and the resulting twist is examined, considering an end-effector of 6 D.O.F. serial manipulators. Two properties are introduced. The first one, called local isotropic compliance, is verified if the force vector is parallel to the tip point displacement vector, and, at the same time, if the torque vector is parallel to the change of orientation vector. The second one, called screw isotropic compliance, is verified if the wrench screw axis is parallel to the twist screw axis. In the latter case, the wrench and twist screw axes are generally not coincident in the Cartesian space. If they are, the contact point could (screw-B isotropic compliance) or could not (screw-A isotropic compliance) belong to the coincident axes. Four cases are analyzed and classified. Active stiffness regulation is considered to achieve isotropic compliance in a generic configuration. Two arrangements are taken into account for the control system, which acts either in parallel or as a series with the passive joint stiffness. The control stiffness matrix is then determined for both the arrangements and for all the four kinds of isotropic compliance. One detailed example of application is presented and the obtained results are verified by using multi-body dynamic simulation.
    No preview · Article · Apr 2016 · Mechanism and Machine Theory
    • "Basic concept of the twisted string actuation system. based on the moving pivot concept and the Energy-Efficient Variable Stiffness Actuators [13] developed by the Twente University. A recent review of different variable stiffness joint implementations can be found in [14]. "
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    ABSTRACT: In this paper, an ongoing work for the implementation of a variable stiffness joint actuated by a couple of twisted string actuators in antagonistic configuration is reported. The twisted string actuation system is particularly suitable for very compact and light-weight robotic devices, like artificial limbs, exoskeletons and robotic hands, since it renders a very low apparent inertia at the load side, allowing the implementation of powerful tendon-based driving systems, using as actuators small-size DC motors characterized by high speed, low torque and very limited inertia. The basic properties of the twisted string actuation system are firstly presented, and the way how they are exploited for the implementation of a variable stiffness joint is discussed. A simple control algorithm for controlling the joint stiffness and position simultaneously is discussed, and a the feedback linearization of the device is taken into account and validated in simulation.
    No preview · Conference Paper · May 2015
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    • "Still following the first approach, actuators are designed in order to have variable impedance. So-called variable impedance actuators (VIA) can show a behavior where the output stiffness can vary independently from the output position (Vanderborght et al., 2009; Visser et al., 2011). "

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