138 research outputs found
Physical Human-Robot Interaction Control of an Upper Limb Exoskeleton with a Decentralized Neuro-Adaptive Control Scheme
Within the concept of physical human-robot interaction (pHRI), the most
important criterion is the safety of the human operator interacting with a high
degree of freedom (DoF) robot. Therefore, a robust control scheme is in high
demand to establish safe pHRI and stabilize nonlinear, high DoF systems. In
this paper, an adaptive decentralized control strategy is designed to
accomplish the abovementioned objectives. To do so, a human upper limb model
and an exoskeleton model are decentralized and augmented at the subsystem level
to enable a decentralized control action design. Moreover, human exogenous
force (HEF) that can resist exoskeleton motion is estimated using radial basis
function neural networks (RBFNNs). Estimating both human upper limb and robot
rigid body parameters, along with HEF estimation, makes the controller
adaptable to different operators, ensuring their physical safety. The barrier
Lyapunov function (BLF) is employed to guarantee that the robot can operate in
a safe workspace while ensuring stability by adjusting the control law. Unknown
actuator uncertainty and constraints are also considered in this study to
ensure a smooth and safe pHRI. Then, the asymptotic stability of the whole
system is established by means of the virtual stability concept and virtual
power flows (VPFs) under the proposed robust controller. The experimental
results are presented and compared to proportional-derivative (PD) and
proportional-integral-derivative (PID) controllers. To show the robustness of
the designed controller and its good performance, experiments are performed at
different velocities, with different human users, and in the presence of
unknown disturbances. The proposed controller showed perfect performance in
controlling the robot, whereas PD and PID controllers could not even ensure
stable motion in the wrist joints of the robot
Decentralized Nonlinear Control of Redundant Upper Limb Exoskeleton with Natural Adaptation Law
The aim of this work is to utilize an adaptive decentralized control method
called virtual decomposition control (VDC) to control the orientation and
position of the end-effector of a 7 degrees of freedom (DoF) right-hand
upper-limb exoskeleton. The prevailing adaptive VDC approach requires tuning of
13n adaptation gains along with 26n upper and lower parameter bounds, where n
is the number of rigid bodies. Therefore, utilizing the VDC scheme to control
high DoF robots like the 7-DoF upper-limb exoskeleton can be an arduous task.
In this paper, a new adaptation function, so-called natural adaptation law
(NAL), is employed to eliminate these burdens from VDC, which results in
reducing all 13n gains to one and removing dependency on upper and lower
bounds. In doing so, VDC-based dynamic equations are restructured, and inertial
parameter vectors are made compatible with NAL. Then, the NAL adaptation
function is exploited to design a new adaptive VDC scheme. This novel adaptive
VDC approach ensures physical consistency conditions for estimated parameters
with no need for upper and lower bounds. Finally, the asymptotic stability of
the algorithm is proved with the virtual stability concept and the accompanying
function. The experimental results are utilized to demonstrate the excellent
performance of the proposed new adaptive VDC scheme.Comment: Manuscript is published in 2022 IEEE-RAS 21st International
Conference on Humanoid Robots (Humanoids
Hybrid Controller for Robot Manipulators in Task-Space with Visual-Inertial Feedback
This paper presents a visual-inertial-based control strategy to address the
task space control problem of robot manipulators. To this end, an
observer-based hybrid controller is employed to control end-effector motion. In
addition, a hybrid observer is introduced for a visual-inertial navigation
system to close the control loop directly at the Cartesian space by estimating
the end-effector pose. Accordingly, the robot tip is equipped with an inertial
measurement unit (IMU) and a stereo camera to provide task-space feedback
information for the proposed observer. It is demonstrated through the Lyapunov
stability theorem that the resulting closed-loop system under the proposed
observer-based controller is globally asymptotically stable. Besides this
notable merit (global asymptotic stability), the proposed control method
eliminates the need to compute inverse kinematics and increases trajectory
tracking accuracy in task-space. The effectiveness and accuracy of the proposed
control scheme are evaluated through computer simulations, where the proposed
control structure is applied to a 6 degrees-of-freedom long-reach hydraulic
robot manipulator
A Global Asymptotic Convergent Observer for SLAM
This paper examines the global convergence problem of SLAM algorithms, an
issue that faces topological obstructions. This is because the state-space of
attitude dynamics is defined on a non-contractible manifold: the special
orthogonal group of order three SO(3). Therefore, this paper presents a novel,
gradient-based hybrid observer to overcome these topological obstacles. The
Lyapunov stability theorem is used to prove the globally asymptotic convergence
of the proposed algorithm. Finally, comparative analyses of two simulations
were conducted to evaluate the performance of the proposed scheme and to
demonstrate the superiority of the proposed hybrid observer to a smooth
observer.Comment: 7 pages, 8 figures, conferenc
Harvester Boom Tip Acceleration Control During a Crosscutting - Theoretical Background
The cutting function is an essential part of a harvester's work in the cut-to-length method. The quality of cutting is the most significant feature of a cut. Trees should be cut without causing damage to logs produced. Nowadays end checks of logs are the main problem in the cutting process. It has been observed that end checks are found in as many as 70% of the logs produced by harvesters. Cutting damage reduces the amount of useful material and causes considerable economical loss to the sawmill and veneer industries.
This study presents a theoretical background for the boom-lowering function, which is one solution to avoid cutting damage during the timber cutting process. The purpose is to momentarily counterbalance the gravitational force of the log in horizontal timber cutting. The study discusses the feasibility of controlling the boom tip in the vertical plane during the cut. In this study the boom tip motion trajectory along the g-vector is modelled for both one and two linear actuators. On the basis of this theoretical study, it seems that acceleration of one g is possible to realise with certain improvements in hydraulics. However, experimental measurements are required to verify these theoretical results. This will include the more detailed study of the effects of deceleration limits on boom stability
Path Planning and Smoothing for 4WDs Hydraulic Heavy-Duty Field Robots
This paper discusses the path planning and path-following control for a four wheel drive (4WD), steer-articulated boom lift driven by hydraulic actuators. The environment is assumed to be both static and known. The path planning will be done in two phases, where the first one finds a crude, collision-free path accounting for the vehicle dimensions, and this path will be smoothed with a path smoothing algorithm to satisfy the kinematic and dynamic constraints imposed by the vehicle and its actuators. The path smoothing algorithm will be chosen from several candidates by using a simulated test scenario. Then, the simulation results will be used to verify the path planners feasibility in heavy-duty, four-wheel-steered field robots having hydraulic actuators and high inertia.publishedVersionPeer reviewe
Flow-Bounded Velocity Controller for Hydraulic Bulldozers
The bulldozer is a mobile earthmoving machine with a differentially steered mobile base and an onboard manipulator used for soil cutting and transportation. Grading the ground to match a desired contour is an end-effector path-following task, with required joint rates dependent on mobile base motion. The offline planning of travel velocity profiles that respect the available hydraulic flowrate limits is difficult due to uncertainties in the machine–soil interactions. Hence, we propose a flow-bounded velocity controller enabling accurate automatic grading with online velocity scaling. The capacity of hydrostatic transmission and manipulator actuators, as well as the desired velocity, are considered when deciding the commanded velocity reference for the mobile base. Our dynamic simulation results show that, with the proposed controller, a desired ground profile is cut accurately when the machine operates at its performance limits. Comparison to constant velocity driving shows that errors in blade positioning are reduced dramatically. Constant velocity selected to keep the flow within limits results in longer completion times compared to our solution, making it more time optimal. Furthermore, the rpm of the diesel engine can be reduced to save fuel without compromising control performance.publishedVersionPeer reviewe
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