Vector Field Orientation Research Articles

Predefined-time VFO control design for unicycle-like mobile robots

The paper presents a predefined-time Vector-Field-Orientation (VFO) control law for unicycle-like nonholonomic mobile robots. We consider a set-point control problem in the presence of strict time constraints, which has to guarantee satisfaction of a prescribed upper bound of the settling time for the configuration errors. The control law is based on the VFO methodology, which is characterized by non-oscillatory and well-predictable time evolution of transient states for unicycle-like robots. A formal stability analysis based on the Lyapunov theory is provided for the closed-loop dynamics. Then, the results of extensive numerical simulations as well as experimental tests illustrate the resultant control performance where time constraints are considered. The proposed approach is compared with an alternative predefined-time stabilizer, recently introduced for nonholonomic chained-form systems.

Nonreciprocal Magnetoacoustic Waves in Dipolar-Coupled Ferromagnetic Bilayers

We study the interaction of surface acoustic waves (SAWs) with spin waves (SWs) in a ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$/$\mathrm{Au}$/${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ system composed of two ferromagnetic layers separated by a nonmagnetic $\mathrm{Au}$ spacer layer. Because of interlayer magnetic dipolar coupling between the two ferromagnetic layers, a symmetric and an antisymmetric SW mode form, which both show a highly nondegenerate dispersion relation for oppositely propagating SWs. Due to magnetoacoustic SAW-SW interaction, we observe highly nonreciprocal SAW transmission in the piezoelectric-ferromagnetic hybrid device. We experimentally and theoretically characterize the magnetoacoustic wave propagation as a function of frequency, wave vector, and external magnetic field magnitude and orientation. Additionally, we demonstrate that the nonreciprocal SW dispersion of a coupled magnetic bilayer is highly tuneable and not limited to ultrathin magnetic films, in contrast to the nonreciprocity induced by the interfacial Dzyaloshinskii-Moriya interaction. Therefore, magnetoacoustic coupling in ferromagnetic multilayers provides a promising route towards building efficient acoustic isolators.

Robust output-feedback VFO-ADR control of underactuated spatial vehicles in the task of following non-parametrized paths

This article concerns the development of the Vector Field Orientation – Active Disturbance Rejection (VFO-ADR) cascaded path-following controller for underactuated vehicles moving in a 3-dimensional space. The proposed concept of a cascaded control structure decouples system kinematics from system dynamics, resembling the approach utilized for nonholonomic systems. Thanks to the use of an ADR control approach in the dynamic-level controller, the proposed control structure is robust to even significant model uncertainties and external disturbances. Application of an error-based form of the Extended State Observer (ESO), implemented within the ADR inner-loop controller, implies the output-feedback characteristic of the control structure, i.e., only position and attitude of the vehicle body are expected to be measured. The kinematic-level controller is designed according to the VFO method utilizing the non-parametrized path representation to calculate the commanded velocities. The description of the proposed control structure is followed by the theoretical analysis utilizing the Input-to-State Stability (ISS) theorem and the simulation verification of the proposed solution.

Robust output-feedback cascaded tracking controller for spatial motion of anisotropically-actuated vehicles

The paper presents the description, analysis, and validation of a cascaded trajectory-tracking control system designed for rigid-body vehicles moving in a 3-dimensional task space with non-banked maneuvering. In particular, we consider the anisotropically-actuated vehicles characterized by the presence of some privileged actuation directions and, simultaneously, actuation authority limitations along other directions. By combining the two specific control design methodologies, that is, the Vector-Field-Orientation (VFO) approach in the outer (kinematic-level) loop and the Active-Disturbance-Rejection (ADR) approach in the inner (dynamic-level) loop, we provide the resultant VFO-ADR control system characterized by an intuitive geometrical control strategy and robustness to a wide range of matched disturbances and substantial uncertainties of a vehicle model. To facilitate its practical applicability, the VFO-ADR control law has been designed in a way which requires feedback only from the positional configuration variables of a vehicle body. The results of experimental trials, obtained with the VFO-ADR control law applied to a laboratory-scale in-door airship, reveal its practical usefulness also in the case of underactuated vehicles under some specific motion conditions.

The VFO-Driven Motion Planning and Feedback Control in Polygonal Worlds for a Unicycle with Bounded Curvature of Motion

Integrated motion planning and control for the purposes of maneuvering mobile robots under state- and input constraints is a problem of vital practical importance in applications of mobile robots such as autonomous transportation. Those constraints arise naturally in practice due to specifics of robot mechanical construction and the presence of obstacles in motion environment. In contrast to approaches focusing on feedback control design under the assumption of given reference motion or motion planning with neglection of subsequent feedback motion execution, we adopt a controller-driven motion planning paradigm, which has recently gained attention of many researchers. It postulates design of motion planning algorithms dedicated to specific feedback control policies, which compute a sequence of feedback control subtasks instead of classically planned open-loop controls or parametric paths. In this spirit, we propose a motion planning algorithm driven by the VFO (Vector Field Orientation) control law for the waypoint-following task. Presented analysis of the VFO control law reveals its beneficial properties, which are subsequently utilized to solve a generally nonlinear and non-convex optimal motion planning problem by formulating it as a mixed-integer linear program (MILP). The solution proposed in this paper yields a waypoint sequence, which is designed for execution by application of the VFO control law to drive a robot to a prescribed final configuration under an input constraint imposed by bounded curvature of robot motion and state constraints resulting from a convex decomposition of task space. Satisfaction of these constraints is guaranteed analytically and exactly, i.e., without utilization of numerical approximations. Moreover, for a given discrete set of possible waypoint orientations, the proposed algorithm computes plans optimal w.r.t. given cost functional, which can be any convex linear combination of quantities such as robot path length, curvature of robot motion, distance to imposed state constraints, etc. Furthermore, the planning algorithm exploits the possibility of both forward or backward movement of the robot to allow maneuvering in demanding environments. Generated waypoint sequences are a compact representation of a motion plan, which can be immediately executed with the VFO controller without any additional post-processing. Validity of the proposed approach has been confirmed by simulation studies and experimental motion execution with a laboratory-scale mobile robot.

VFO Path following Control with Guarantees of Positionally Constrained Transients for Unicycle-Like Robots with Constrained Control Input

The Vector-Field-Orientation (VFO) method is a control design concept which was originally introduced for the unicycle kinematics to solve two classical control tasks corresponding to the trajectory tracking and set-point control problems. A unified solution to both the tasks was possible by appropriate definitions of the so-called convergence vector field. So far, there has not been a version of the VFO control law for the third classical control task concerning the path following problem, which is particularly meaningful in the context of practical applications. The paper fills this gap by presenting a novel VFO path following controller devised for robots of unicycle-like kinematics with the amplitude-limited control input. Opposite to most path following controllers proposed in the literature, the new control law utilizes the recently introduced level curve approach which does not employ any parametrization of a reference path. In this way, the proposed solution is free of main limitations resulting from the need of unique determination of the shortest distance from a robot to the path. In contrast to other solutions, a formal analysis of the closed-loop dynamics presented in this paper provides sufficient conditions which guarantee constrained transients of robot motion with the position confined to a prescribed subset around a reference path. Theoretical results have been validated by numerical examples and experimentally verified with utilization of a laboratory-scale differentially driven robot.

Cascaded VFO Control for Non-Standard N-Trailer Robots

Articulated mobile robots consisting of a tractor and passively off-hooked trailers belong to a class of highly nonlinear, nonholonomic, structurally unstable, differentially non-flat, and underactuated dynamic systems. Due to the mentioned properties of their kinematics, motion control problems related to N-trailer robots (N-trailers) are non-trivial and challenging. Cascaded control strategy presented in this paper provides unified solution to the set-point and trajectory tracking control tasks for articulated robots equipped with arbitrary number of passively off-axle hitched trailers. Practically useful features of the proposed controller come from its high scalability and from application of the Vector-Field-Orientation (VFO) controller in the outer loop, which ensures fast errors convergence and simplicity of control implementation and tuning. Control input limitations of the robot are directly taken into account by utilization of a simple velocity scaling procedure which preserves an instantaneous motion curvature of a tractor. Description and theoretical substantiation of the concept are followed by the results of experimental validation tests conducted with usage of a 3-trailer semi-autonomous vehicle.

Improved backstepping control for large angle maneuvers of spacecraft

For the control of large angle maneuvers of a spacecraft, variable gain backstepping control is proposed. The controller can make the states of the system converge to the commanded position along the input vector field orientation. The controller stabilizes the system with the amplitude of the commanded torques decreased. Considering the uncertainty of the disturbance torques and the estimation error of the inertia matrix, the control design is improved to provide strong self-adaptability and robustness of the system. Simulation is conducted, and the results show that the design has good tracking performance and convergence, consistent with the theoretical analysis.

Fuzzy Vector Field Orientation Feedback Control-Based Slip Compensation for Trajectory Tracking Control of a Four Track Wheel Skid-steered Mobile Robot

Skid-steered mobile robots have been widely used in exploring unknown environments and in military applications. In this paper, the tuning fuzzy Vector Field Orientation (FVFO) feedback control method is proposed for a four track wheel skid-steered mobile robot (4-TW SSMR) using flexible fuzzy logic control (FLC). The extended Kalman filter is utilized to estimate the positions, velocities and orientation angles, which are used for feedback control signals in the FVFO method, based on the AHRS kinematic motion model and velocity constraints. In addition, in light of the wheel slip and the braking ability of the robot, we propose a new method for estimating online wheel slip parameters based on a discrete Kalman filter to compensate for the velocity constraints. As demonstrated by our experimental results, the advantages of the combination of the proposed FVFO and wheel slip estimation methods overcome the limitations of the others in the trajectory tracking control problem for a 4-TW SSMR.

The VFO state-constrained stabilisation of the nonholonomic manipulator with limited control input

This article considers the state-constrained set-point feedback control problem for a configuration of the 3-link nonholonomic manipulator kinematics with amplitude-limited control inputs. The proposed solution results from applying the original, geometrical in nature, Vector-field-orientation (VFO) control strategy, which makes use of the intrinsic features of the manipulator kinematics. The concept does not involve any state transformation in contrast to the commonly applied solutions based on a chained-form conversion of the original model. The general formulation of the VFO approach makes it possible to derive a family of VFO controllers guaranteeing convergence of the configuration error to the assumed neighbourhood of zero. The VFO concept together with an additional switching strategy and input scaling procedure preserves both the state constraints and the control limitations. The effectiveness of the control scheme is illustrated by simulations.

Trajectory Tracking Control and Obstacle Avoidance for a Differentially Driven Mobile Robot

AbstractIn this paper a Vector Field Orientation control algorithm for a differentially-driven mobile platform in an environment with obstacles is presented. The goal is to track the desired trajectory. Collision avoidance is assured using the local artificial potential function (APF) method. A proof of stability and convergence for the algorithm is presented. Effectiveness of the algorithm is illustrated with experiments on a real robot. The paper also presents how the parameters of the control algorithm influence effectiveness of the method in practice.

THE HANLE EFFECT AS A DIAGNOSTIC OF MAGNETIC FIELDS IN STELLAR ENVELOPES. V. THIN LINES FROM KEPLERIAN DISKS

This paper focuses on the polarized profiles of resonance scattering lines that form in magnetized disks. Optically thin lines from Keplerian planar disks are considered. Model line profiles are calculated for simple field topologies of axial fields (i.e., vertical to the disk plane) and toroidal fields (i.e., purely azimuthal). A scheme for discerning field strengths and geometries in disks is developed based on Stokes Q-U diagrams for the run of polarization across line profiles that are Doppler broadened by the disk rotation. A discussion of the Hanle effect for magnetized disks in which the magnetorotational instability (MRI) is operating is also presented. Given that the MRI has a tendency to mix the vector field orientation, it may be difficult to detect the disk fields with the longitudinal Zeeman effect, since the amplitude of the circularly polarized signal scales with the net magnetic flux in the direction of the observer. The Hanle effect does not suffer from this impediment, and so a multi-line analysis could be used to constrain field strengths in disks dominated by the MRI.

Robust Processing of Optical Flow of Fluids

This paper proposes a new approach, coupling physical models and image estimation techniques, for modelling the movement of fluids. The fluid flow is characterized by turbulent movement and dynamically changing patterns which poses challenges to existing optical flow estimation methods. The proposed methodology, which relies on Navier-Stokes equations, is used for processing fluid optical flow by using a succession of stages such as advection, diffusion and mass conservation. A robust diffusion step jointly considering the local data geometry and its statistics is embedded in the proposed framework. The diffusion kernel is Gaussian with the covariance matrix defined by the local second derivatives. Such an anisotropic kernel is able to implicitly detect changes in the vector field orientation and to diffuse accordingly. A new approach is developed for detecting fluid flow structures such as vortices. The proposed methodology is applied on artificially generated vector fields as well as on various image sequences.

Vector-Field-Orientation Tracking Control for a Mobile Vehicle Disturbed by the Skid-Slip Phenomena

The paper is devoted to the trajectory tracking control task for a differentially-driven vehicle moving on a plane surface under conditions of the persistent skid-slip phenomena. The Vector Field(s) Orientation (VFO) control strategy, presented originally for undisturbed case in Michalek and Kozlowski (IEEE Trans Control Syst Technol 18(1):45---65, 2010), has been reformulated here to the new disturbed motion conditions. The extension of the VFO strategy relies on introduction of the nonlinear skid-slip influence compensator in the feed-forward loop, which in practical implementation involves the real-time estimation of the skid-slip velocities and their time-derivatives. The approach considers the skid-slip effects solely on the kinematic level avoiding the need of modeling a complicated phenomenon of the wheels-ground interaction. Theoretical analysis shows the asymptotic tracking ability for the position trajectory with boundedness of the orientation error. Experimental results included in the paper reveal substantial tracking quality improvement resulting from the utilization of the proposed skid-slip influence compensator.

Vector-Field-Orientation Feedback Control Method for a Differentially Driven Vehicle

This paper presents a novel vector-field-orientation (VFO) feedback control method and its application to a differentially driven wheeled vehicle. It describes the control concept and the control design methodology originating from simple geometrical interpretations connected with the vehicle model structure. The novelty of the VFO method allows one to treat two considered control tasks - trajectory tracking and point stabilization - in a unified manner. Control systems with VFO controllers reveal several practically desirable features like fast and nonoscillatory error convergence, simple interpretation of control input effect, and, as a consequence, particular simplicity of the controller parametric synthesis. The theoretical considerations included in this paper are validated by simulation and experimental results.

Motion planning and feedback control for a unicycle in a way point following task: The VFO approach

Motion planning and feedback control for a unicycle in a way point following task: The VFO approachThis paper is devoted to theway point followingmotion task of a unicycle where the motion planning and the closed-loop motion realization stage are considered. Theway point followingtask is determined by the user-defined sequence of way-points which have to be passed by the unicycle with the assumed finite precision. This sequence will take the vehicle from the initial state to the target state in finite time. The motion planning strategy proposed in the paper does not involve any interpolation of way-points leading to simplified task description and its subsequent realization. The motion planning as well as the motion realization stage are based on the Vector-Field-Orientation (VFO) approach applied here to a new task. The unique features of the resultant VFO control system, namely, predictable vehicle transients, fast error convergence, vehicledirecting effecttogether with very simple controller parametric synthesis, may prove to be useful in practically motivated motion tasks.

Trajectory tracking for a mobile robot with skid-slip compensation in the vector-field-orientation control system

Trajectory tracking for a mobile robot with skid-slip compensation in the vector-field-orientation control systemThe article is devoted to a motion control problem for a differentially driven mobile robot in the task of trajectory tracking in the presence of skid-slip effects. The kinematic control concept presented in the paper is the Vector Field Orientation (VFO) feedback approach with a nonlinear feed-forward skid-slip influence compensation scheme. The VFO control law guarantees asymptotic convergence of the position tracking error to zero in spite of the disturbing influence of skid-slip phenomena. The paper includes a control law design description, stability and convergence analysis of a closed-loop system, and practical verification of the proposed control concept. The experimental results illustrate control quality obtained on a laboratory setup equipped with vision feedback, where the Kalman filter algorithm was used in order to practically estimate skid-slip components.

Induction Motor Control With Friction Compensation: An Approach of Virtual-Desired-Variable Synthesis

In this paper, the speed control problem of induction motors suffering from substantial friction force is considered. Here, a semi-current-fed model for induction motors and LuGre's dynamic model for friction force are used. To reflect practical situations, rotor resistance, torque load, and friction parameters are assumed to be unknown. In the design methodology, a double-observer structure is applied to estimate the immeasurable friction states. On the other hand, in light of the principles of vector control and field orientation, a set of virtual desired variables (VDVs) are introduced to synthesize the control law. Therefore, using only measurable signals of rotor speed, stator voltage and current, an asymptotic adaptive tracking controller is designed. Numerical simulations and experiments are carried out to verify the theoretical results and show satisfactory performance.

Advances in vector control ofac motor drives — A review

This paper attempts to present a comprehensive review of the advances made in vector control or field orientation as applied to high performanceac motor drives. Brief application survey, machine models ind-q representation, implementation issues with inverters and cycloconverters, parameter effects etc for both induction and synchronous motor vector control are dealt with and sample results from studies on them are presented. The latest advance on this control like direct torque control (DTC) has been briefly discussed. A substantial updated bibliography, though by no means complete, is included for those who are interested in keeping track of the present state-of-the-art and working further in this area.

The potential of epicardial activation mapping in isolated hearts for the assessment of arrhythmogenic and antiarrhythmic drug activity

In isolated rabbit hearts perfused according to the Langendorff technique, the pathway and time course of the epicardial activation process were studied by means of computer-assisted epicardial potential mapping, using a matrix of 256 unipolar AgCl electrodes (1 mm spatial and 0.25 msec temporal resolution). Activation time of each electrode was calculated (point of time of fastest negative intrinsic deflection). From the activation times of the surrounding electrodes, the direction and velocity of activation for each electrode were calculated, thereby allowing construction of an epicardial vector field. Furthermore, breakthrough points (BTPs) were determined as the earliest activated electrodes in agiven region. Vector-field orientation and BTP localization were compared under different conditions during perfusion with Tyrode's solution or during drug treatment. Thereby, we determined the similarity of the vector fields (two vectors were considered to be identical if not differing by more than 5°) and the percentage of BTPs with changed localization, referred to the total number of BTPs. For verification of the method we treated the hearts with ouabain (0.2 μmol l ) and observed progressive decline in vector-field similarity from 32% down to 14%, resulting in arrhythmia. Concomitantly, the percentage of BTPs with identical localization decreased from 81% to 50%. These changes could be reversed by additive treatment with 10 nmol 1 verapamil. From these results we conclude that arrhythmia is preceded by a state of deteriorated epicardial activation pattern (prearrhythmia), which can be quantitatively evaluated and used for detection of preventive antiarrhythmic drug action.