1,087 research outputs found
Optimal Torque and Stiffness Control in Compliantly Actuated Robots
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. Index Terms — Variable-stiffness actuation, physical constraints, optimal control
Optimally Controlling the Timing of Energy Transfer in Elastic Joints: Experimental Validation of the Bi-Stiffness Actuation Concept
Elastic actuation taps into elastic elements' energy storage for dynamic
motions beyond rigid actuation. While Series Elastic Actuators (SEA) and
Variable Stiffness Actuators (VSA) are highly sophisticated, they do not fully
provide control over energy transfer timing. To overcome this problem on the
basic system level, the Bi-Stiffness Actuation (BSA) concept was recently
proposed. Theoretically, it allows for full link decoupling, while
simultaneously being able to lock the spring in the drive train via a
switch-and-hold mechanism. Thus, the user would be in full control of the
potential energy storage and release timing. In this work, we introduce an
initial proof-of-concept of Bi-Stiffness-Actuation in the form of a 1-DoF
physical prototype, which is implemented using a modular testbed. We present a
hybrid system model, as well as the mechatronic implementation of the actuator.
We corroborate the feasibility of the concept by conducting a series of
hardware experiments using an open-loop control signal obtained by trajectory
optimization. Here, we compare the performance of the prototype with a
comparable SEA implementation. We show that BSA outperforms SEA 1) in terms of
maximum velocity at low final times and 2) in terms of the movement strategy
itself: The clutch mechanism allows the BSA to generate consistent launch
sequences while the SEA has to rely on lengthy and possibly dangerous
oscillatory swing-up motions. Furthermore, we demonstrate that providing full
control authority over the energy transfer timing and link decoupling allows
the user to synchronously release both elastic joint and gravitational energy.
This facilitates the optimal exploitation of elastic and gravitational
potentials in a synergistic manner.Comment: 8 pages, 9 figures. Submitted to IEEE Robotics and Automation Letter
Efficient computation of inverse dynamics and feedback linearization for VSA-based robots
We develop a recursive numerical algorithm to compute the inverse dynamics of robot manipulators with an arbitrary number of joints, driven by variable stiffness actuation (VSA) of the antagonistic type. The algorithm is based on Newton-Euler dynamic equations, generalized up to the fourth differential order to account for the compliant transmissions, combined with the decentralized nonlinear dynamics of the variable stiffness actuators at each joint. A variant of the algorithm can be used also for implementing a feedback linearization control law for the accurate tracking of desired link and stiffness trajectories. As in its simpler versions, the algorithm does not require dynamicmodeling in symbolic form, does not use numerical approximations, grows linearly in complexity with the number of joints, and is suitable for online feedforward and real-time feedback control. A Matlab/C code is made available
Relationship between split-step timing and leg stiffness in world-class tennis players when returning fast serves
International audienc
Modeling and Control of a novel Variable Stiffness three DoF Wrist
This paper presents a novel design for a Variable Stiffness 3 DoF actuated
wrist to improve task adaptability and safety during interactions with people
and objects. The proposed design employs a hybrid serial-parallel configuration
to achieve a 3 DoF wrist joint which can actively and continuously vary its
overall stiffness thanks to the redundant elastic actuation system, using only
four motors. Its stiffness control principle is similar to human muscular
impedance regulation, with the shape of the stiffness ellipsoid mostly
depending on posture, while the elastic cocontraction modulates its overall
size. The employed mechanical configuration achieves a compact and lightweight
device that, thanks to its anthropomorphous characteristics, could be suitable
for prostheses and humanoid robots.
After introducing the design concept of the device, this work provides
methods to estimate the posture of the wrist by using joint angle measurements
and to modulate its stiffness. Thereafter, this paper describes the first
physical implementation of the presented design, detailing the mechanical
prototype and electronic hardware, the control architecture, and the associated
firmware. The reported experimental results show the potential of the proposed
device while highlighting some limitations. To conclude, we show the motion and
stiffness behavior of the device with some qualitative experiments.Comment: 13 pages + appendix (2 pages), 19 figures, submitted to IJR
Optimal Control for Articulated Soft Robots
Soft robots can execute tasks with safer interactions. However, control
techniques that can effectively exploit the systems' capabilities are still
missing. Differential dynamic programming (DDP) has emerged as a promising tool
for achieving highly dynamic tasks. But most of the literature deals with
applying DDP to articulated soft robots by using numerical differentiation, in
addition to using pure feed-forward control to perform explosive tasks.
Further, underactuated compliant robots are known to be difficult to control
and the use of DDP-based algorithms to control them is not yet addressed. We
propose an efficient DDP-based algorithm for trajectory optimization of
articulated soft robots that can optimize the state trajectory, input torques,
and stiffness profile. We provide an efficient method to compute the forward
dynamics and the analytical derivatives of series elastic actuators
(SEA)/variable stiffness actuators (VSA) and underactuated compliant robots. We
present a state-feedback controller that uses locally optimal feedback policies
obtained from DDP. We show through simulations and experiments that the use of
feedback is crucial in improving the performance and stabilization properties
of various tasks. We also show that the proposed method can be used to plan and
control underactuated compliant robots, with varying degrees of underactuation
effectively.Comment: 14 pages, 15 figures, IEEE Transaction on Robotics (TRO
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