1,720,993 research outputs found
Force/Torque-Sensorless Joint Stiffness Estimation in Articulated Soft Robots
Full text linkCurrently, the access to the knowledge of stiffness values is typically constrained to a-priori identified models or datasheet information, which either do not usually take into ac- count the full range of possible stiffness values or need extensive experiments. This work tackles the challenge of stiffness estimation in articulated soft manipulators, and it proposes an innovative solution adding value to the previous research by removing the necessity for force/torque sensors and generalizing to multi-degree- of-freedom robots. Built upon the theory of unknown input-state observers and recursive least-square algorithms, the solution is independent of the actuator model parameters and its internal control signals. The validity of the approach is proven analytically for single and multiple degree-of-freedom robots. The obtained estimators are first evaluated via simulations on articulated soft robots with different actuations and then tested in experiments with real robotic setups using antagonistic variable stiffness actuators
An input observer-based stiffness estimation approach for flexible robot joints
Full text linkThis letter addresses the stiffness estimation problem for flexible robot joints, driven by variable stiffness actuators in antagonistic setups. Due to the difficulties of achieving consistent production of these actuators and the time-varying nature of their internal flexible elements, which are subject to plastic deformation over time, it is currently a challenge to precisely determine the total flexibility torque applied to a robot's joint and the corresponding joint stiffness. Herein, by considering the flexibility torque acting on each motor as an unknown signal and building upon Unknown Input Observer theory, a solution for electrically-driven actuators is proposed, which consists of a linear estimator requiring only knowledge about the positions of the joints and the motors as well as the drive's dynamic parameters. Beyond its linearity advantage, another appealing feature of the solution is the lack of need for torque and velocity sensors. The presented approach is first verified via simulations and then successfully tested on an experimental setup, comprising bidirectional antagonistic variable stiffness actuators
Decoupled nonlinear adaptive control of position and stiffness for pneumatic soft robots
Full text linkThis article addresses the problem of simultaneous and robust closed-loop control of joint stiffness and position, for a class of antagonistically actuated pneumatic soft robots with rigid links and compliant joints. By introducing a first-order dynamic equation for the stiffness variable and using the additional control degree of freedom, embedded in the null space of the pneumatic actuator matrix, an innovative control approach is introduced comprising an adaptive compensator and a dynamic decoupler. The proposed solution builds upon existing adaptive control theory and provides a technique for closing the loop on joint stiffness in pneumatic variable stiffness actuators. Under a very mild assumption involving the inertia and actuator matrices, the solution is able to cope with uncertainties of the model and, when the desired stiffness is constant or slowly varying, also of the pneumatic actuator. Position and stiffness decoupling is achieved by the introduction of a first-order differential equation for an internal state variable of the controller, which takes into account the time derivative of pressure in the stiffness dynamics. A formal proof of the stability of the position and stiffness tracking errors is provided. An appealing property of the approach is that it does not require higher derivatives of position or any derivatives of stiffness. The solution is validated with respect to several use-cases, first in simulation and then via a real pneumatic soft robot with McKibben muscles. A comparison with respect to existing techniques reveals a more robust position and stiffness tracking skill
Joint Stiffness Estimation in a Single-Link Soft Robot Driven by Pneumatic McKibben Muscles
No full textThe possibility to control not only the position of a robot but also the stiffness of its joints has enabled safe human-robot collaboration and nature-like robot behaviour. However, since stiffness is not a measurable variable, one needs to perform either its extensive offline identification or apply the real-time stiffness estimators. To the best of the authors' knowledge, this paper proposes for the first time an online technique for the estimation of stiffness and elastic torque in an articulated soft robot joint driven by McKibben pneumatic artificial muscles. We address this problem in a two-phase process: first, we reconstruct the elastic torque by leveraging the theory of the delayed Unknown Input Observers, and then a Recursive Least Squares algorithm is used to determine the parameters of a stiffness approximation. Besides robot link dynamics, this approach requires information on link position and commanded pressures to the muscles. The solution is validated on the simulated single-link robot driven by a pair of McKibben muscles in an antagonistic setup
Input-Observer-Based Estimation of the External Torque for Single-Link Flexible-Joint Robots
No full textTo enable safe interaction between a robot and its environment, it is necessary to monitor in real-time the external torque that acts on the robot. This paper proposes a strategy that exploits solely encoder and current data to estimate the external torque acting on the single-link flexible-joint robot driven by a variable stiffness actuator in the antagonistic setup. The strategy comprises two steps: first, the elastic torques generated by motors within a variable stiffness actuator are reconstructed, then the external torque is estimated leveraging the data on the previously reconstructed elastic torques and robot dynamic model. The effectiveness of the estimator is tested in the simulation environment with respect to the time-varying external torque
Adaptive Control of Soft Robots Based on an Enhanced 3D Augmented Rigid Robot Matching
No full textDespite having proven successful in generating precise motions under dynamic conditions in highly deformable soft-bodied robots, model based techniques are also prone to robustness issues connected to the intrinsic uncertain nature of the dynamics of these systems. This letter aims at tackling this challenge, by extending the augmented rigid robot formulation to a stable representation of three dimensional motions of soft robots, under Piecewise Constant Curvature hypothesis. In turn, the equivalence between soft-bodied and rigid robots permits to derive effective adaptive controllers for soft-bodied robots, achieving perfect posture regulation under considerable errors in the knowledge of system parameters. The effectiveness of the proposed control design is demonstrated through extensive simulations
On the stability of the soft pendulum with affine curvature: open-loop, collocated closed-loop, and switching control
Full text linkThis letter investigates the stability properties of the soft inverted pendulum with affine curvature - a tem- plate model for nonlinear control of underactuated soft robots. We look at how changes in physical parameters affect stability and equilibrium. We give conditions under which zero dynamics corresponding to a collocated choice of the output is (locally or globally) stable or unstable. We leverage these results to design a switching controller that stabilizes a class of nonlinear equilibria of the pendulum, which can drive the system from one equilibrium to another
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Robust and Decoupled Position and Stiffness Control for Electrically-Driven Articulated Soft Robots
Full text linkThe control of articulated soft robots, i.e. robots with flexible joints and rigid links, presents a challenge due to their in- trinsic elastic elements and nonlinear force-deflection dependency. This letter first proposes a discrete-time delayed unknown input- state observer based on a nominal robot model that reconstructs the total torque disturbance vector, resulting from the imperfect knowledge of the elastic torque characteristic, external torques, and other model uncertainties. Then, it introduces a robust controller, that actively compensates for the estimated uncertainty and allows bounded stability for the tracking of independent link position and joint stiffness reference signals. The convergence of the disturbance estimator and the overall system’s stability in closed loop is proven analytically, while the effectiveness of the proposed control design is first evaluated in simulations with respect to large uncertainty conditions, and then demonstrated through experiments on a real multi-degree-of-freedom articulated soft robot
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