Flexible Manipulator Research Articles

Flexible Generation and Manipulation of Microwave Bottle Beam Using a Reconfigurable Metamirror

Bottle beams have drawn considerable research interest due to their attractive features and applications in beam trapping. Herein, the flexible generation and modulation of bottle beams utilizing a reconfigurable metamirror over a non‐negligible frequency band ranging from 8.5 to 9.5 GHz is investigated. A varactor diode is embedded into each meta‐atom on the metamirror enabling the achievement of a nearly 2π phase coverage by varying the external bias voltage. Based on the caustic theory, several configurations of bottle beam with distinct radius a are realized as proof‐of‐concept validation. Besides, the flexible modulation of the operating frequency is analyzed by judiciously controlling the bias voltages applied to the metasurface. The experimental results are in good agreement with those obtained from numerical simulations. Such a flexible bottle beam generator shows good potential in the microwave regime for near‐field applications, such as imaging and power transmission.

Deep reinforcement learning with reward shaping for tracking control and vibration suppression of flexible link manipulator

This paper puts forward a novel deep reinforcement learning control using deep deterministic policy gradient (DRLC-DDPG) framework to address the reference tracking and vibration suppression problem of rotary flexible link (RFL) manipulator. Specifically, this study attempts to address the continuous action space DRLC problem through DDPG algorithm and presents a Lyapunov function based reward shaping approach for guaranteed deep reinforcement learning (DRL) convergence and enhanced speed of training. The proposed approach synthesizes the hard and soft constraints of the flexible manipulator as a constrained Markov decision problem (MDP) and evaluates the performance of DRLC-DDPG framework through hardware in loop (HIL) testing to realize precise servo tracking and suppressed vibration of the flexible manipulator. For identifying the dynamical model of the RFL, an empirical Auto-Regressive eXogenous (ARX) model using the closed loop identification technique is built. Moreover, to extract the true states (servo angle and deflection angle) from the actual measurements, which typically have the influence of sensor noise, an adaptive Kalman filter (AKF) is augmented with the DRLC scheme. The experimental results of DRLC-DDPG scheme compared with those of the model predictive control (MPC) for several test cases reveal that the proposed scheme is superior to MPC both in terms of trajectory tracking and robustness against the external disturbances and model uncertainty.

Dynamic modeling and RBF neural network compensation control for space flexible manipulator with an underactuated hand

In space operation, flexible manipulators and gripper mechanisms have been widely used because of light weight and flexibility. However, the vibration caused by slender structures in manipulators and the parameter perturbation caused by the uncertainty derived from grasping mass variation cannot be ignored. The existence of vibration and parameter perturbation makes the rotation control of flexible manipulators difficult, which seriously affects the operation accuracy of manipulators. What’s more, the complex dynamic coupling brings great challenges to the dynamics modeling and vibration analysis. To solve this problem, this paper takes the space flexible manipulator with an underactuated hand (SFMUH) as the research object. The dynamics model considering flexibility, multiple nonlinear elements and disturbance torque is established by the assumed modal method (AMM) and Hamilton's principle. A dynamic modeling simplification method is proposed by analyzing the nonlinear terms. What's more, a sliding mode control (SMC) method combined with the radial basis function (RBF) neural network compensation is proposed. Besides, the control law is designed using a saturation function in the control method to weaken the chatter phenomenon. With the help of neural networks to identify the uncertainty composition in the SFMUH, the tracking accuracy is improved. The results of ground control experiments verify the advantages of the control method for vibration suppression of the SFMUH.

Control Law Synthesis for Flexible Joint Manipulator Based on Synergetic Control Theory

In this paper, the authors present the synthesis of control laws for the flexible joint manipulator to stabilize the oscillation and track the desired trajectory. To solve this problem, the article applies synergetic control theory. In synergetic control theory the desired values are impressed as invariants. So the invariants act as the control objectives of the system and our task is to find the control laws for them. Using this theory, the control law is designed to ensure the movement of the closed-loop system from an arbitrary initial state into the vicinity of the desired invariant manifold, i.e. the objective attracting manifold. Thereby, not only reach the necessary invariant but also ensure the asymptotic stability of the entire system. The quality of the proposed control law is shown through simulation results on Matlab and its efficiency is shown by comparison with backsteping control law.

Toward Sub-Terahertz: Space-Time Coding Metasurface Transmitter for Wideband Wireless Communications.

A space-time coding metasurface (STCM) operating in the sub-terahertz band to construct new-architecture wireless communication systems is proposed. Specifically, a programmable STCM is designed with varactor-diode-tuned metasurface elements, enabling precise regulation of harmonic amplitudes and phases by adjusting the time delay and duty cycle of square-wave modulation signal loaded on the varactor diodes. Independent electromagnetic (EM) regulations in the space and time domains are achieved by STCM to realize flexible beam manipulations and information modulations. Based on these features, a sub-terahertz wireless communication link is constructed by employing STCM as a transmitter. Experimental results demonstrate that the STCM supports multiple modulation schemes including frequency-shift keying, phase-shift keying, and quadrature amplitude modulations in a wide frequency band. It is also shown that the STCM is capable of realizing wide-angle beam scanning in the range of ±45o , which offers an opportunity for user tracking during the communication. Thus, the STCM transmitter with high device density and low power consumption can provide low-complexity, low-cost, low-power, and low-heat solutions for building the next-generation wireless communication systems in the sub-terahertz frequency and even terahertz band.

Experimental modal analysis of a single-link flexible robotic manipulator with curved geometry using applied system identification methods

In this paper, the principal time-domain system identification methods are studied and used to carry out the experimental modal analysis of a single-link flexible manipulator with curved geometry in a comparative fashion. The identification process carried out in this study is aimed at deriving first-order state-space dynamical models and second-order configuration-space dynamical models of the single-link flexible manipulator by using an array of fundamental system identification procedures, which includes the ARX (AutoRegressive eXogenous method), SSEST (State-Space ESTimation method), N4SID (Numerical Algorithm for Subspace State-Space System Identification), ERA/OKID (Eigensystem Realization Algorithm combined with the Observer/Kalman Filter Identification method), and TFEST (Transfer Function ESTimation method) methods. The main goal of this paper is, therefore, to perform a comparative study between the most fundamental system identification procedures applied to the numerical and experimental analysis of the structural vibrations of a flexible manipulator having a curved geometric shape. First, this paper poses the theoretical foundations for the development of the numerical study and the experimental testing to be performed for the case study. Subsequently, the flexible manipulator analyzed in this work is modeled with various levels of complexity, which range from a simple lumped parameter model to a linear finite element model developed by integrating the interconnections between the SOLIDWORKS and ANSYS software. By doing so, the computational procedures applied to the time-domain experimental measurements of the dynamic behavior of the flexible manipulator allow for reconstructing first-order and second-order mechanical models of the structural system of interest in the MATLAB simulation environment. The numerical results and the experimental tests reported in this investigation demonstrate the effectiveness of all the time-domain system identification approaches considered in the paper.

Acoustic Forceps Based on Focused Acoustic Vortices with Different Topological Charges.

For enhanced energy concentration with improved flexibility for object manipulation, a focused acoustic vortex (FAV) is designed using a sector planar piston transducer array and acoustic lens that can produce the effective concentration of the acoustic field to perform the focusing function. Compared to the Gaussian beam, which tends to cause the object to deviate from the axis of acoustic propagation, FAVs can form a central valley region to firmly bind the objects, thus preventing off-target effects. The heat energy in the paraxial region is transferred to the vortex center in the form of heat transfer so that the temperature-sensitive liposomes captured can quickly release drugs, which has a good effect on targeted drug administration. The focused acoustic wave stopped acting on the tissue (gel) for 2 s, the temperature of the vortex center continued to rise, reaching 41.5 °C at the moment of 3.7 s, at which point the liposomes began to release the drug. The FAVs capture the drug and use its thermal effect to achieve accurate and rapid treatment. The simulation results show that the drug release temperature of temperature-sensitive liposomes can be achieved by controlling the action time of the vortices. This study provides a reliable theoretical basis for the clinical application of targeted drugs.

Microparticle Manipulation Performed on a Swirl-Based Microfluidic Chip Featured by Dual-Stagnation Points.

Stagnation-based microfluidics technology is promising for microparticle control due to its noncontact and low cost. However, the current research is still hindered by insufficient pose regulating ability and soft control. Based on our previous work on controlling single particles by generating a swirling flow region (SFR) with a stagnation point in the designed flow field, a new 3-microchannel structure is herein proposed for simultaneous control of two microparticles. It is addressed as the dual-stagnation model because there are two SFRs generated for particle capturing and manipulation. Simulation study is conducted to optimize the fluid field structure and explore the regulation of the two SFRs by adjusting velocities of microchannel inlets. Experiments are carried out on a 3D-printed microfluidic chip to validate the feasibility of the dual-stagnation model and the predicting capacity of the simulations. It is demonstrated that two SFRs with stagnation points are successfully formed in specific locations, indicating that two microparticles can be concurrently captured and controlled. Significantly, the results of simulation and experimental studies agree well with each other referring to flow streamlines and stagnation point regulation. During experiments, it is confirmed that microparticles with different shapes and varied sizes can be captured. Besides, the deviation between the positions of microparticles and the generated stagnation points is characterized to reveal the trapping stability of this microfluidic chip. This work contributes to an advanced flow field structure for swirl-based microfluidic chips and provides insights into soft contact and flexible manipulation of multiple microparticles for revealing the interaction between two bio-/chemical microparticles.

Optimized sliding mode controller for trajectory tracking of flexible joints three-link manipulator with noise in input and output

The aim of this study is to enhance the performance of a nonlinear three-rigid-link maneuver (RLM) in terms of trajectory tracking, disturbance and noise cancellation, and adaptability to joint flexibility. To achieve this, an optimized sliding mode controller with a proportional integral derivative surface (SMC-PID) is employed for maneuver control. An improved artificial bee colony algorithm with multi-elite guidance (MGABC) is utilized to obtain optimal values for the sliding surface and switching mode gain and attain the best performance for the robot maneuver system. The selection of the MGABC algorithm is based on its efficient exploration and exploitation techniques. The performance of the optimized SMC-PID robotic system is compared against other optimization algorithms found in existing literature, including Particle Swarm Optimization (PSO), Genetic Algorithm (GA), Artificial Bee Colony (ABC), Ant Lion Optimizer (ALO), and Grey Wolf Optimizer (GWO). The implemented controller effectively reduces the tracking error to 0.00691 radians, eliminates chattering phenomena in the control effort, and demonstrates robustness against disturbances and noise. The controller ensures that the objective function (OBJF) is minimized, with 0.954% increase in OBJF under low disturbance and noise conditions and 14.55% under severe disturbance and noise conditions. Moreover, the optimized controller exhibits resilience to variations in payload mass analysis, with the percentage increase in OBJF values ranging from 5.726% under low uncertainty conditions to 18.887% under severe uncertainty conditions. Flexible-link maneuvers (FLM) offer advantages such as improved safety and increased operating speeds in real-world applications. In this study, we investigated the impact of joint flexibility on the performance of the FLM system. Our proposed controller demonstrated superior tracking performance, characterized by minimal vibrations in the movement of the end effector.

Miniature Parallel Continuum Robot Made of Glass: Analysis, Design, and Proof-of-Concept

Parallel continuum robots (PCRs) are flexible manipulators, whose configuration is determined by the controlled deformation of their parallel-arranged elastic links. They are of great interest for micromanipulation applications due to their intrinsic compliance, their extrinsic actuation, and their miniaturization potential. This article presents the design and prototype of a miniature 3-DoF glass PCR, capable of reaching large tip-tilt angles in a tiny volume. The flexible structure is composed of three legs made of glass, each one having an extension range from 6 to 15 mm and a diameter of 125 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μ</i> m. The workspace of the robot, its stiffness, and manipulability are evaluated with respect to varying geometrical proportions of the platform size with respect to the base diameter. Experimental validation shows that the platform is able to reach tip–tilt angles from -84 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{\circ }$</tex-math></inline-formula> to 117 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{\circ }$</tex-math></inline-formula> in a space of 15 mm wide and 10 mm high and to follow conveniently desired paths within its workspace.

Tunable strong plasmon–exciton coupling based modulator employing borophene and deep subwavelength perovskite grating

Two-dimensional materials support deeply confined and tunable plasmonic modes, which have great potential for achieving device miniaturization and flexible manipulation. In this paper, we propose a diffraction-unlimited system (period ≈ λ/20) composed of borophene layer and perovskite grating to investigate the strong coupling between the borophene guiding plasmon (BGP) and perovskite exciton (PE) modes. The resonant energy of the BGP mode could be electrically tuned to match the energy of the PE mode, and a remarkable Rabi splitting is attained under zero-detuning conditions. The splitting energy could reach 230 meV due to the strong field enhancement provided by the BGP mode. Taking advantage of the proposed electrically tunable hybrid system, not only is the reflective amplitude modulation depth up to 99.9%, but the 1.76π phase range modulation is achieved. Furthermore, by increasing the distance between the borophene layer and perovskite grating, a passive parity-time symmetry breaking could be observed with the vanished energy splitting. Our results deepen our understanding of light–matter interactions at the sub-wavelength scale and provide a guideline for designing active plasmonic devices.

Precise geometry and pose measurement of in-hand objects with simple features using a multi-camera system

In precision pick-and-place tasks, robotic grasp errors can cause the failure of subsequent placement steps. Therefore, in-hand object pose measurement has attracted the attention of the intelligent manufacturing industry. However, existing algorithms have limited accuracy and insufficient practicality. Moreover, they often assume that the actual object geometry is identical to the CAD model. To alleviate the problems in terms of accuracy, practicality, and uncritical assumptions, this paper proposes an in-hand measurement solution based on multi-view reconstruction for objects with simple features. In this concept, multiple monocular cameras are deployed at the robot end to calculate the object’s actual geometry and 6D pose by reconstructing its key geometric features. An in-hand pose compensation strategy and a robot terminal system integrated with manipulation and measurement are developed. To verify the solution’s effectiveness, several experiments were carried out. First, grasp errors caused by multiple sources are intuitively demonstrated in a simulation environment. The in-hand measurement is validated to compensate for grasp errors through comparison experiments. Then, robots can rearrange various irregularly piled objects in real-world experiments based on the developed terminal system. Results demonstrate that maximum remaining position errors are within 2 mm with maximum angle errors of less than 1°. The maximum error compensation rate reaches 95%, which is superior to existing in-hand measurement algorithms. It can be observed that the proposed solution is valid for multiple objects. Geometry measurement can help robots calculate the in-hand object pose and identify the object’s category and availability. Therefore, the proposed measurement solution seems accurate and universal for robot applications in smart manufacturing industries, enhancing the accuracy and flexibility of robot manipulation.

Linear-polarization duplexed metalens generate terahertz vortex by a silicon metasurface

All-dielectric metasurfaces can realize flexible full-vector manipulations of the complex optical field, without confronting the loss issues of plasmonic counterparts, and thus have been proposed for lots of nanophotonic functionalities. In this work, a silicon-bar metasurface is designed to generate multifunctional focusing characteristics, which act as duplexed metalens with unlocked topological charges of terahertz vortex for orthogonal linearly-polarized incident waves. It achieves full phase modulation by tuning lateral sizes of the bar without the anisotropic orientation requirement associated with the geometric phase, which is consequently of high efficiency since no polarization conversion is involved in this duplexed all-dielectric vortex metalens.

A Review of the Flexible Robotic Arm

In the application of the robot, the gripper of the robot is an important medium between robot arm and target. The traditional manipulator hand is mostly rigid, which is easy to damage or unable to clamp the object in the operation process. What are demanded from robotics are no longer limited to just mechanical assistant, more efficient and precise target acquisition of robotics with self-adaptation and self-adjustment have become the tackle key in robot research and design. In this paper, the design of flexible materials, the AI real-time sensing and controlling of flexible manipulator are described, as well as the application and prospects are analyzed. It is intended to provide perspective and direction for the weak interaction between flexible robotic arm and environment, less adaptability and inflexibility in complex environment. In the future, a more advanced, flexible robotic arm can be created that goes even beyond the human arm and contributes to development of the world.

Vibration Control of Flexible Manipulators by Active Cable Tension

The end point vibrations of the serial manipulator should be controlled during motion or working process. In this study the residual vibrations of the flexible manipulator were controlled with cable tensions. The finite element model was established in ANSYS Mechanical APDL. The open loop and closed loop control simulations were performed under the trapezoidal velocity motion profiles. Zero and three different initial strain values were assigned to the cables. As a result, the end point vibration amplitudes, axial forces of the cables and the bending strain values of the one element near the fixed end were observed in order to define the limitations of the sensors and actuators which will be selected for experimental setup.

Command filtered backstepping control of constrained flexible joint robotic manipulator

AbstractHere, an adaptive radial basis function (RBF) neural network (NN) backstepping controller is proposed for a class of input‐constrained flexible joint robotic manipulators represented by strict‐feedback form with unknown terms, external stochastic disturbance, and output disturbance. The proposed approach is robust against both deterministic and stochastic uncertainties and disturbances and copes with the control input amplitude saturation. Moreover, by deploying the minimal learning parameter method and command filter technique, the computational burden of derivative terms and adaptive terms greatly decreases. Considering the mean‐value theorem assists us to avoid the need for having the input saturation bounds in prior. The suggested tracking control scheme mandates the closed‐loop system states to be semi‐globally bounded‐in‐probability. Also, a quartic Barrier Lyapunov function is utilized to force the tracking error to be confined within a pre‐chosen small region around the origin. Eventually, a numerical simulation of a flexible joint robot manipulator with a single link is performed to show the effectiveness and performance of the developed control method.

Inverse Design of Diffusion–Absorption Hybrid Metasurfaces

AbstractRandom metasurfaces have demonstrated significant potential in radar‐signature control applications, but their ability to manipulate scattering‐field reduction is limited by relying solely on the diffusion mechanism. To achieve flexible and arbitrary designs for scattering‐field reduction, a diffusion–absorption hybrid metasurface is proposed based on the Babinet principle. This hybrid metasurface adopts complementary bilayer metasurfaces to satisfy the impedance matching condition and employs a hybrid absorption and diffusion mechanism. Such complementary designs' intralayer resonances and interlayer couplings are tailored by multi‐objective optimization, achieving the precise and wideband design of absorptive elements with a specific reflection phase. By optimizing the spatial distributions of elements, diffusion is introduced and designs are achieved with a continuous scattering‐field reduction from 15.40 to 32.58 dB at 15 GHz. Three representative designed structures are fabricated and verified as proof of concept. The experimental results are consistent with the calculations and simulations, demonstrating a 12.32–24.46 dB scattering‐field reduction range. The proposed high‐freedom metasurface and the implementation multi‐objective optimization strategy collectively empower flexible wavefront manipulation, offering a viable solution for designing hybrid meta‐devices that incorporate multiple mechanisms.

Laser Controlled Manipulation of Microbubbles on a Surface with Silica-Coated Gold Nanoparticle Array.

Microbubble generation and manipulation play critical roles in diverse applications such as microfluidic mixing, pumping, and microrobot propulsion. However, existing methods are typically limited to lateral movements on customized substrates or rely on specific liquids with particular properties or designed concentration gradients, thereby hindering their practical applications. To address this challenge, this paperpresents a method that enables robust vertical manipulation of microbubbles. By focusing a resonant laser on hydrophilic silica-coated gold nanoparticle arrays immersed in water, plasmonic microbubbles are generated and detach from the substrates immediately upon cessation of laser irradiation. Using simple laser pulse control, it can achieve an adjustable size and frequency of bubble bouncing, which is governed by the movement of the three-phase contact line during surface wetting. Furthermore, it demonstrates that rising bubbles can be pulled back by laser irradiation induced thermal Marangoni flow, which is verified by particle image velocimetry measurements and numerical simulations. This study provides novel insights into flexible bubble manipulation and integration in microfluidics, with significant implications for various applications including mixing, drug delivery, and the development of soft actuators.

Dynamic modeling and tracking control strategy for flexible telescopic space manipulator based on neural network compensation

Flexible telescopic space manipulator (FTSM) is playing an increasingly important role in assisting astronauts to accomplish space missions. The telescopic structure leads to time-varying dynamical parameters of the FTSM, thereby reducing the tracking control accuracy. At the same time, the lightweight and large length-diameter ratios are very likely to cause the vibration phenomenon of FTSM during the motion. To solve the above problems, in the dynamics modeling, this paper uses the assumed modal method (AMM) and Lagrange principle to establish the dynamics model considering two-dimensional deformation and disturbance torque. Furthermore, the dynamics models are simplified and derived by ignoring the nonlinear terms (INTs). Besides, the influence of the simplified dynamics models on the accuracy of FTSM’s deformation is analyzed by simulation. It is found that the INTs simplified dynamics model has higher modeling accuracy. Based on the INTs simplification model, the control law is designed. More importantly, the RBF neural network is used to identify and compensate for the time-varying terms and disturbance torque in the FTSM. Then the sliding mode control strategy is proposed by the hyperbolic tangent function as the approximation rate. Finally, the effectiveness of the RBF neural network compensated sliding mode control strategy is illustrated by simulation and ground control experiments of the FTSM.

Distributed neural-based fault-tolerant control of multiple flexible manipulators with input saturations

This paper focuses on a novel fault-tolerant control design for networked flexible manipulators with actuator failures, input saturations, and external disturbances. Unlike the existing methods, the new control strategy has three characteristics. First, the second-order auxiliary system is used to eliminate the influence of input saturation. Second, by designing some proportional–integral (PI) auxiliary variables, the boundary dynamics of manipulators are reconstructed as some new second-order systems, where the nonlinear uncertainties are estimated by the adaptive radial basis neural networks (RBFNNs) and the compound disturbances are compensated by the adaptive method. Third, a boundary local angle interaction protocol is given under a digraph. Then combining the virtual control and Lyapunov design, some novel distributed fault-tolerant control algorithms are obtained to ensure that the vibration states and the angle consensus error are uniformly ultimately bounded. Finally, numerical simulations are provided to show the effectiveness of these control algorithms.