Flexible Link Research Articles

Comprehensive performance analysis of flexible asynchronous AC link under various unbalanced grid voltage conditions

A flexible asynchronous AC link (FASAL) system is a technology that interconnects two power system networks operating at the same or different frequencies. The FASAL system is a newly developed technology; therefore, it requires a thorough investigation under different unbalanced grid voltage conditions for better behavioral understanding and appropriate future controller design. In this paper, the FASAL system is analyzed under various symmetrical and asymmetrical unbalanced grid voltage conditions. Moreover, the performance of the back-to-back voltage source converter (VSC) based HVDC link is also analyzed to establish its comparison with the FASAL system under the same conditions. The FASAL and VSC-HVDC systems are modeled in PSCAD/EMTDC software environment. The simulation results illustrate that the FASAL system performance is better under transient as well as steady-state conditions. Furthermore, the reduction in power at the receiving end side is also lesser in the FASAL system. For example, a 10% sag contributes only a 15% dip in power in the FASAL system, whereas VSC-HVDC reports a 30% power dip for the same sag condition.

A composite controller for manipulator with flexible joint and link under uncertainties and disturbances

In this article, a composite controller is proposed for the manipulator with the flexible joint and link under uncertainties and time-varying disturbances. The dynamic of the system is developed by the Euler–Lagrange and assumed mode method, which is a nonlinear, strong coupling, and underacted system. Therefore, based on the singular perturbation theory, the dynamic is decomposed into a slow and fast subsystem. For the slow dynamic, a novel adaptive-gain super-twisting sliding mode controller is designed to guarantee joint tracking under the uncertainties and disturbances. For the fast dynamics, adaptive dynamic programming is used to deal with the uncertainty. The simulation result shows that the proposed composite controller can effectively track the trajectory and suppress the vibration simultaneously.

Design optimization for a compliant mechanism based on computational intelligence method

Modeling and optimization for compliant mechanisms are challenging tasks thanks to an unclear kinematic merging among rigid and flexible links. Hence, this paper develops a computational intelligence-based method for modeling and optimization. The proposed method concerns about statistics, numerical simulation, computational intelligence, and metaheuristics. A two degrees of freedom compliant mechanism is investigated to illustrate the effectiveness of the suggested computational intelligence method. First, numerical datasets are collected by simulations. Then, sensitivity of design parameters is analyzed by analysis of variance and Taguchi technique. The results of sensitivity are employed to separate a few populations for lightning attachment procedure optimization (LAPO). Next, the values of two output performances of the mechanism are changed into the values in the range from zero to one through desirability function method. The calculated output values become two inputs of the fuzzy logic model, and the output of this system is a single objective function (SOF). Subsequently, the SOF is modeled by using adaptive neuro-fuzzy inference system (ANFIS). LAPO algorithm is then utilized to maximize the SOF. The results revealed that the numerical example 3 is the best design for the mechanism. In comparison with artificial intelligence techniques and regression, the results show that the performance indexes of the proposed ANFIS model (R2 close 1, MSE about 10–4, and RMSE about 10–2) are superior to those of the multilayer perceptron, deep neural network, and multiple-linear regression. Additionally, the proposed computational intelligence method is more effective than the Taguchi-fuzzy logic, ANFIS-integrated teaching learning-based optimization, and ANFIS-integrated Jaya in searching the optimal design of the compliant mechanism. The results determined that the optimal displacement and parasitic error are about 2.2109 mm and 0.0028 mm, respectively.

Adaptive Sliding Mode Neural Network Control and Flexible Vibration Suppression of a Flexible Spatial Parallel Robot

With the goal of creating a flexible spatial parallel robot system in which the elastic deformation of the flexible link causes a rigid moving platform to produce small vibrations, we proposed an adaptive sliding mode control algorithm based on a neural network. To improve the calculation efficiency, the finite element method was used to discretize the flexible spatial link, and then the displacement field of the flexible spatial link was described based on floating frame of reference coordinates, and the dynamic differential equation of the flexible spatial link considering high-frequency vibrations was established through the Lagrange equation. This was combined with the dynamic equation of the rigid link and the dynamic equation considering small displacements of the rigid movable platform due to elastic deformation, and a highly nonlinear and accurate dynamic model with a rigid–flexible coupling effect was obtained. Based on the established accurate multi-body dynamics model, the driving torque with coupling effects was calculated in advance for feedforward compensation, and the adaptive sliding mode controller was used to improve the tracking performance of the system. The nonlinear error was examined to determine the performance of the neural network’s approximation of the nonlinear system. The trajectory errors of the moving platform in the X-, Y-, and Z-directions were reduced by 12.1%, 38.8%, and 50.34%, respectively. The results showed that the designed adaptive sliding mode neural network control met the control accuracy requirements, and suppressed the vibrations generated by the deformation of the flexible spatial link.

Concurrent Design of a 2 Dof Five-Bar Parallel Robot a Hybrid Design of Rigid and Flexible Links

In this article, the concurrent optimal design of a planar five-bar parallel robot for a high-speed pick and place task is considered. A trade-off between trajectory tracking, energy consumption and deformation of the flexible links is sought. Due to the high-speed operation, the minimization of the vibratory effect is considered since the design stage. Thus, the design is stated as a non-linear multi-objective dynamic optimization problem that is solved through the Differential Evolution (DE) algorithm, as well as, feasibility rules for constraints handling. Mechanical structural variables that modify the inertial parameters of the rigid and flexible links, as well as, control variables related to gains of a PID controller are considered as independent variables. This problem is subject to maximum torque that each motor could provide; inherent constraints for link manufacture; dimensional synthesis; Grashof’s criterion and initial and boundary conditions. Results show that it is possible, despite the vibrational phenomenon, to reduce the energy consumption without loss of precision through an appropriate mass balance of the actuated links.

Learning Energy Efficient Jumping Strategies for Flexible-Legged Systems

Legged locomotive systems have many advantages over their wheeled counterparts, such as their ability to navigate rough terrain. There are many techniques to overcome obstacles, one of which is jumping. Still, there are disadvantages to overcome when using legged systems, such as their lack of energy efficiency. To combat this lack of efficiency, flexible links can be used to conserve energy that would otherwise be wasted during locomotion. Furthermore, control methods that improve a jumping system’s ability to jump high and its ability to conserve power can be utilized. In this paper, reinforcement learning (RL) was used to create controllers for a flexible-legged jumping system that maximize jump height while minimizing power usage.

Airborne quantum key distribution: a review [Invited

Remarkable achievements have been witnessed in free-space quantum key distribution (QKD), which acts as an available approach to extend the transmission range of quantum communications. The feasibility of transmitting qubits through the free-space channel with the aid of moving platforms like satellites, aircraft, unmanned aerial vehicles (UAVs) has been verified. In view of the limited working time and resource consumption of the satellite-based QKD and the last-mile challenges of connecting satellite nodes with terrestrial networks, the airborne QKD is expected to provide flexible and relay links for the large-scale integrated network. This paper reviews the recent significant progress of QKD based on aircraft or UAVs, highlights their critical techniques, and prospects the future of airborne quantum communications.

A High-Efficient Finite Difference Method for Flexible Manipulator with Boundary Feedback Control

The paper presents a high-efficient finite difference method for solving the PDE model of the single-link flexible manipulator system with boundary feedback control. Firstly, an abstract state-space model of the manipulator is derived from the original PDE model and the associated boundary conditions of the manipulator by using the velocity and bending curvature of the flexible link as the state variables. Then, the second-order implicit Crank-Nicolson scheme is adopted to discretize the state-space equation, and the second-order one-sided approximation is used to discretize the boundary conditions with excitations and feedback control. At last, the state-space equation combined with the boundary conditions of the flexible manipulator is transformed to a system of linear algebraic equations, from which the response of the flexible manipulator can be easily solved. Numerical simulations are carried out to simulate the manipulator under various excitations and boundary feedback control. The results are compared with ANSYS to demonstrate the accuracy and high efficiency of the presented method.

Numerical investigation on the flexural links of eccentrically braced frames with web openings

Numerical investigation on the flexural links of eccentrically braced frames with web openings

Real-time trajectory tracking control of a parallel robot with flexible links

Real-time end-effector trajectory tracking is applied to a newly developed parallel robotic system with two highly flexible links. In contrast to previous works, which are typically based on offline precalculations or rigid model inversion, a dynamic flexible multibody model is inverted online. As the underactuated system is non-minimum phase, the inverse model needs to be rendered stable for real-time integration. Therefore, three approaches are developed for the considered robot. Firstly, output redefinition is applied by directly weighting the elastic deformations and rotations of the links. Secondly, a small counter weight is attached at an advantageous location on the robot having only a minor influence on the eigenfrequencies. Lastly, the rotational degree of freedom of the end-effector is used to stabilize the internal dynamics of the inverse model with a small motion of a rotary motor. The concepts and real-time applicability are validated within experiments which extends the related literature being highly based on theoretical investigations. The experimental end-effector tracking performance based on the three minimum phase approaches is close to the desired trajectory and clearly outperforms classical rigid body inversion.

Dynamics and control analysis of a single flexible link robot with translational joints

Modern design always aims at reducing mass, simplifying the structure, and reducing the energy consumption of the system especially in robotics. These targets could lead to lowing cost of the material and increasing the operating capacity. The priority direction in robot design is optimal structures with longer lengths of the links, smaller and thinner links, more economical still warranting ability to work. However, all of these structures such as flexible robots are reducing rigidity and motion accuracy because of the effect of elastic deformations. Therefore, taking the effects of elastic factor into consideration is absolutely necessary for kinematic, dynamic modeling, analyzing, and controlling flexible robots. Because of the complexity of modeling and controlling flexible robots, the single-link and two-link flexible robots with only rotational joints are mainly mentioned and studied by most researchers. It is easy to realize that combining the different types of joints of flexible robots can extend their applications, flexibility, and types of structure. However, the models consisting of rotational and translational joints will make the kinematic, dynamic modeling, and control becomes more complex than models that have only rotational joints. This study focuses on the dynamics model and optimal controller based on genetic algorithms (GA) for a single flexible link robot (FLR) with a rigid translational joint. The motion equations of the FLR are built based on the Finite Element Method (FEM) and Lagrange Equations (LE). The difference between flexible manipulators that have only rotational joints and others with the translational joint is presented through boundary conditions. A PID controller is designed with parameters that are optimized by the GA algorithm. The cost function is established based on errors signal of translational joint, elastic displacements of the End-Point (EP) of the FLR. Simulation results show that the errors of the joint variable, the elastic displacements (ED) are destructed in a short time when the system is controlled following the reference point. The results of this study can be basic to research other flexible robots with more joint or combine joint styles.

Optimization of Generalized S-Curve Trajectories for Residual Vibration Suppression and Compliance With Kinematic Bounds

This article proposes a new optimization algorithm that assures the minimum possible duration of generalized S-curve trajectories compliant with kinematic limitations and capable of suppressing residual vibrations when tracked by a resonant plant. Thanks to the possibility of generating such kind of trajectories with a chain of filters, called smoothers, each one characterized by a single parameter, i.e., the duration <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$T_i$</tex-math></inline-formula> of its impulse response, the optimization process aims at minimizing the order of the trajectory, and accordingly the number of smoothers in the chain, and leads to rest-to-rest trajectories that, under the given specifications, cannot be made shorter in time. Therefore, the structure of the trajectory is not predetermined but is the outcome of the proposed algorithm together with the optimal parameters defining it. The effectiveness of the proposed approach is proven by applying the designed trajectories to an experimental setup based on a flexible link.

Performance of fiber-reinforced concrete link slabs with embedded steel and GFRP rebars

Expansion joints in bridge superstructures are often difficult to maintain, leading to the problems associated with water leakage and debris accumulation. As a potential solution, link slabs can be placed at the expansion joints, providing a continuous deck system. Despite the promise of such a system, there have been standing questions on the materials of choice and structural configurations appropriate for link slabs, especially in their transition zone from the bridge deck to their debonded region. This motivated the current study to investigate various reinforcement alternatives using full-scale laboratory tests and numerical simulations. To achieve a satisfactory crack resistance, a fiber-reinforced concrete (FRC) mixture was developed for the tested link slabs. In addition, two rebar alternatives, i.e., steel and GFRP rebars, were evaluated to ensure a proper transfer of stresses induced by negative bending moments (due to continuity) and to limit the width of cracks on the top surface of the link slabs, where they are directly exposed to water and deleterious agents. The full-scale performance was examined under increasing vertical loads applied to the midpoint of two individual spans connected by a link slab. This included an investigation of various structural response measures, such as strains in the FRC and embedded rebars. The study was then extended by performing a set of finite-element simulations to understand the effects of reinforcement details. The outcome of this study provided holistic information on how flexible link slabs can be utilized for jointless bridge superstructures, addressing the concerns regarding their performance and durability.

Dynamics analysis of a spatial rigid-flexible linkage model subjected to optimized programmed constraints

The article presents a spatial linkage composed of rigid and flexible links subjected to work dependent programmed constraints. The key tool used for the spatial linkage dynamics derivation is an automated computational procedure for constrained dynamics generation. It serves systems subjected to holonomic and first order nonholonomic constraints and proved its effectiveness to open chain models. The procedure results in final motion equations that are in the reduced state form, i.e., constraint reaction force are eliminated during derivation. This is the essential advantage of the procedure application since it provides the smallest set of dynamic equations, which may serve motion analysis and control. The article presents extension of the procedure on linkages composed of flexible links with closed-loop kinematics, for which a spanning tree can have a serial or tree structure. Also, a new optimization procedure is proposed for constraints that are imposed upon the linkage motion. The optimization procedure aims selection of the constraint ranges due to the additional system work regimes, e.g., available power sources or energy consumption. The theoretical development of automated generation of constrained dynamics as well as the constraint optimization procedure is illustrated by the linkage model example.

Modeling, identification and minimum length integral sliding mode control of a 3-DOF cartesian parallel robot by considering virtual flexible links

This article presents a method for modeling the dynamics and minimum vibration controller design of a 3-DOF parallel mechanism, the so-called Tripteron. The proposed comprehensive dynamic model consists of the dynamic model of the prismatic actuators, variable friction in them, and dynamics of flexibility in the limbs which caused vibration of the robot’s end-effector by considering a spring-damper element for each link based on the Tripteron structure. Then the identification procedure is carried out by collecting practical data from the robot. In order to design the oscillation damping controller, the Minimum Length Integral Sliding Mode Controller(MLISMC) is proposed in which the Phase Trajectory Length(PTL) approach is applied to ISMC for proper placement of closed-loop poles of the identified system. The foregoing method is based on numerical calculations to minimize the end-effector vibrations along the point-to-point control. Simulation and experimental results reveal that the PTL of the system have been reduced about 95% in simulation and 30% in the experimental test.

Steel slit shear walls with an efficient geometry

This paper presents the results of a study to improve the seismic behavior of steel shear walls with vertical slits. In this system, the steel plate segments between the slits behave as a series of flexural links, which provide a fairly ductile response without the need for heavy stiffening. An efficient slit geometry is proposed, which assures simultaneous yielding of the plate along the vertical links and prevents strain localization. Experimental study and finite element modeling are used to verify the proposed steel slit shear wall and to study its nonlinear behavior under monotonic and cyclic loading. Out-of-plane buckling of the wall and the effects of buckling restraining clamps on this phenomenon is also studied. The results show satisfactory performance of this system, when used as a seismic force resisting system. The important geometric parameters that result in the desired behavior of the wall, including appropriate stiffness and strength, as well as stable hysteresis curve, are determined and evaluated.

Adaptive Boundary Control of Flexible Manipulators with Parameter Uncertainty Based on RBF Neural Network

In this paper, nonlinear dynamical equations of the flexible manipulator with a lumped payload at the free end are derived from Hamilton's principle. The obtained model consists of both distributed parameters and lumped parameters, namely, partial differential equations (PDEs) governing the flexible motion of links and boundary conditions in the form of ordinary differential equations (ODEs). Considering the great nonlinear approximation ability of the radial basis function (RBF) neural network, we propose a combined control algorithm that includes two parts: one is a boundary controller to track the desired joint positions and suppress the vibration of flexible links; another is a RBF neural network designed to compensate for the parametric uncertainties. The iteration criterion of the RBF neural network weight matrix is derived from the extended Lyapunov function. Stabilization analysis is further carried out theoretically via LaSalle’s invariance principle. Finally, the results of the numerical simulation verify that the proposed control law can realize the asymptotic convergence of tracking error and suppression of the elastic vibration as well.

Tool-Center-Point Control of a Concrete Pump Using Constrained Quadratic Optimization

Tool-center-point (TCP) control increases the operating comfort of concrete pumps by shifting the control to the task space. The operation is accompanied by various requirements and constraints from the mechanical structure, the hydraulic system, and the operator. They must be taken into account for the efficient and error-free operation of the concrete pump. In this publication, a TCP control algorithm based on constrained quadratic optimization (cQP) is presented, which addresses the requirements and constraints of a mobile concrete pump. The features include boom deformation compensation, multiple operation modes, configuration control, compliant constraints on the joint movement, and hydraulic flow rate constraints. The methods are verified by simulation for a flexible link six degrees of freedom concrete pump. The results include deformation-compensated 2-D/3-D movements of the TCP with different operation modes and configuration control under joint and hydraulic constraints. Note to Practitioners —For effective concrete placement, the tip of the concrete pump needs to move linearly in the task space. Tool-center-point (TCP) control allows the operator to directly specify the tip velocity in the task space instead of separately controlling the axes of the manipulator. In this article, the control task is posed as an inverse kinematic problem, which is solved by quadratic optimization (cQP). The framework of cQP enables the direct inclusion of system constraints while being computational efficient for online implementation on the electronic control unit of the truck. The approach addresses concrete pump specific problems and requirements of the tool center point movement and actuator constraints. Restrictions in the task space are not treated.

Movement simulation of flexible working body links in the Unity cross-platform development environment

The analytical modeling of multi-link mechanisms with a large number of degrees of freedom is a difficult task, therefore, it is advisable to use a simulation model Since it is planned to use a flexible working body on an area overgrown with vegetation, this will require a significant number of elements and, as a result, it will be very difficult to perform work using computer-aided design. In this paper, a simulation model of a flexible working body is implemented in a cross-platform Unity development environment. In computer-aided design systems, we are limited to tools and algorithms created by developers. In Unity, we have the opportunity to create our own tools and algorithms, adding it to the links of interest to us, which are solids in the field of gravitational forces. We created a 3D model of each link of a flexible working body, connected it with hinges with the possibility of moving relative to each other. Added scripts to accelerate the rotor and save the coordinates of the links. Based on the obtained coordinates, dependencies were constructed to determine the kinematics of the links of the flexible working body, taking into account its bending in the vertical and horizontal plane under the action of gravity.

Design of GEMSTAR tidal current fixed pitch rotor controlled through a Permanent Magnet Generator (PMG) de‐fluxing technique

This work reports a study on a tidal current system named GEMSTAR, composed of two hydrokinetic turbines supported by a floating submerged structure. A flexible mooring cable links the floating support to the seabed allowing the free rotation of the system and its alignment to changing current direction. The work is mainly focused on the design of the turbine blade with two objectives: keeping a constant power output above the rated power condition and limiting the thrust increase between rated and maximum operating speed. A fixed pitch blade was considered, in order to reduce the costs associated to a complex pitch control mechanism. An analytical procedure for blade design is proposed and validated by comparison to other numerical methods. Above the rated current speed, the power is held constant while the rotational speed of the turbine and of the generator increases, assuming an over-speeding control strategy. The adopted design configuration relies on the possibility of the generator to operate in de-fluxing working conditions. As reported in the last part of this study, a numerical model of the generator has also been developed and applied to a test case in order to analyse the electric behaviour of the system.