Search Results (20)

Flexible manipulators have attracted increasing attention due to their lightweight structure, high flexibility, and energy efficiency, for which they are suitable for delicate and high-precision tasks. However, their control remains a problem because of strong nonlinearities and uncertainties in the system. Based on the trajectory tracking control problem of the single-link flexible manipulator (SLFM) system, this paper proposes a fractional order adaptive neural network control scheme for SLFM under symmetric time-varying full-state constraints. Firstly, a fractional-order dynamic model is established to better capture the inherent memory and nonlinear characteristics of the SLFM. Secondly, an adaptive radial basis function (RBF) neural network-based control scheme is developed within a backstepping framework, and a symmetric time-varying barrier Lyapunov function (BLF) is incorporated to guarantee that all system states remain within predefined bounds. In addition, command filters are introduced to avoid the “explosion of complexity” caused by backstepping. Next, theoretical analysis based on Lyapunov stability theory is provided to demonstrate that all signals in the closed-loop system are bounded, while the tracking error converges to a small neighborhood of zero. Finally, the proposed method is applied as an SLFM: the simulation results show that the presented controller has excellent control performance, the tracking error is less than 0.02 rad, and the tip polarization angle of the system does not exceed 0.045 rad. Additionally, the comparison with the recent DSC and SMC methods also shows that the designed controller behaves with less tracking error, which in return validates the effectiveness and superiority of the proposed control strategy. Full article

Tendon-driven manipulators possess obvious advantages compared to rigid-link manipulators, such as lighter weight, greater flexibility, and adaptability to confined spaces. To solve the problems of backlash and improve the accuracy of motion in specific application environments, this paper proposes a novel single-tendon-driven design for each joint of the manipulator. Kinematic modeling of the manipulator is systematically derived. Then, a decoupling algorithm is designed to mitigate motion coupling effects and enable accurate mapping between motor inputs and joint motions. Moreover, to improve the accuracy of trajectory tracking control for the tendon-driven manipulator, this paper proposes a nonsingular terminal sliding mode (NTSM) control scheme based on time-delay estimation (TDE). TDE is used to estimate unknown disturbances. An adjustable parameter was introduced based on TDE technology, which can enhance the system’s robustness against uncertainties and external disturbances. The stability of the closed-loop control system is verified through Lyapunov stability theory. Finally, decoupling experiments are conducted to validate the kinematic model and the feasibility of the proposed design. And comparative experiments are performed to prove the advantages of the proposed control scheme. Full article

This paper investigates a finite-time prescribed performance control (PPC) problem for a class of nonlinear engineering systems subject to actuator input saturation. By introducing a performance transformation framework, the tracking error is constrained within predefined bounds with guaranteed finite-time convergence. To address the strict-feedback structure and input constraints simultaneously, a dynamic surface control (DSC) technique is employed to avoid the explosion of complexity. A novel saturation-compensated control law is constructed to ensure closed-loop stability without requiring the persistence of excitation or exact knowledge of system dynamics. Lyapunov-based analysis rigorously proves that all closed-loop signals are bounded and the prescribed performance is achieved within a finite time. The simulation results demonstrate the effectiveness of the proposed approach under actuator saturation, highlighting its applicability to constrained engineering process control systems. Full article

The experimentation of state observers for the reconstruction of the angular velocity of the links of a flexible industrial manipulator is investigated in this paper, in the presence of unmodeled or uncertain parts. Considering only one axis moving at a time, a study is done to understand how faithfully the dynamics of the machine can be reconstructed using simple single axis models, extending them to take into account the multi-variable dynamics of the system and trying to reconstruct the action of non-linear friction as well. The goal is to show how a good estimate of the interactions between the links can be obtained, with the final aim of including it into a control architecture. Various models of different complexities have been tested with both the asymptotic Luenberger observer and the steady-state Kalman filter. The presence of friction is taken into account by a feedforward compensation or by the addition of a disturbance observer synthesized as a pole placement regulator. First, the observers are tested in simulation, then on real data from a Comau Racer 7-1.0 robot. To evaluate the quality of the reconstruction, a virtual sensor obtained from the identification of the manipulator is used, and then a final test is carried out using a real Xsens gyroscope. An accurate analysis of the achieved results is provided, devoting a particular attention to the trade-off between model complexity, estimate accuracy and computational burden in view of a possible future insertion into the control architecture of an industrial robot. Full article

A prescribed performance neuro-adaptive control scheme is proposed for a single-link flexible-joint robotic manipulator with unknown deadzone and actuator faults. A new smooth deadzone inverse model is constructed to offset the adverse effect caused by the input deadzone in the actuator of flexible-joint manipulators. The control law is developed by coordinating prescribed performance control with a backstepping technique to ensure transient/steady-state performance, while adaptive neural networks are employed for uncertainty approximation. The tracking error is always restricted within the prescribed bound during the control process, and it ultimately converges to the small neighborhood of origin. All signals in the closed-loop flexible-joint robotic manipulator system are proved to be uniformly bounded. Simulation results are provided to demonstrate the efficiency of the prescribed performance adaptive neural network backstepping control scheme. Full article

Flexible-joint manipulators have a lightweight nature, compact structure, and high flexibility, making them widely applicable in industrial manufacturing, biomedical instruments, and aerospace fields. However, the inherent flexibility of single-link flexible-joint manipulators (SLFJMs) poses substantial control challenges. Compared to traditional control algorithms, prescribed performance control (PPC) algorithms provide superior transient response and steady-state performance by defining a prescribed performance function. However, existing PPC algorithms are limited to a specific range of system initial states, which reduces the joint manipulator’s operational workspace and weakens the robustness of the control algorithm. To address this issue, this study proposes a prescribed performance bounded-H_∞ fault-tolerant controller for SLFJMs. By designing an improved tangent-type barrier Lyapunov function (BLF), a prescribed performance controller that is independent of the initial state of the SLFJM is developed. An input control function (ICF) is employed to mitigate the impulse response of the control input, ensuring a smooth transition from zero. Furthermore, the improved tangent-type BLF enables the tracking error to rapidly converge to a small neighborhood of zero. Finally, a stabilization control simulation experiment is conducted; the results validate the effectiveness of the proposed prescribed performance bounded-H_∞ controller. Full article

This study focuses on the slewing and vibration suppression of flexible single-link manipulators. While extensive research has been conducted on such systems, few studies have experimentally validated their theoretical models. To address this gap, an experimental setup is developed, connecting the flexible link to a zero-backlash worm gear and further attaching it to the rotor shaft of the AC servomotor. The worm gear’s characteristics isolate the link’s vibrations from the rotor’s angular motion, enabling independent design of the vibration controller and slewing control. This approach facilitates simultaneous accurate trajectory tracking and vibration suppression. An active vibration control algorithm is implemented based on an accurate dynamic model. This research encompasses dynamic modeling, slewing control, and vibration control for the system. Theoretical predictions are compared with experimental results to validate both the theoretical model and the proposed vibration control algorithm. Full article

This paper addresses the parameter identification of a one-link flexible manipulator based on the experimental measurement of the inputs/outputs, the finite element model, and the application of evolutionary algorithms. A novel approach is proposed to find the values of inertia, stiffness, and damping parameters by minimizing the difference between the numerical model’s outputs and the testbed’s outputs, thus considering the joint position and acceleration of the link’s tip. The dynamic model is initially obtained using the finite element method and the Lagrange principle. A prototype of a single one-link flexible manipulator is used in the experimental application, wherein the servomotor applies the input torque, and the outputs are the joint angle and the link’s tip acceleration. Then, an optimization problem minimizes the difference between the numerical and experimental outputs to determine the set of parameters using evolutionary algorithms. A comparative analysis to obtain the identified parameters is established using genetic algorithms, particle swarm optimization, and differential evolution. The proposed identification approach permitted the determination of the dynamic parameters based on the complete dynamic model of the flexible-link manipulator, which is different from the approaches reported in the literature that identify a simplified model. This information is essential for the design of the motion and vibration control laws. Full article

This paper considers a tracking system developed for a full-actuated manipulator with flexible joints under the following assumptions: torques are control actions, and current loop dynamics are not considered; the mass-inertial characteristics of the manipulator and other parameters are not exactly known; the external matched and unmatched disturbances act on the system, and matched disturbances are not smooth; the derivatives of the reference actions are achievable but are unknown functions of time; the set of sensors is not complete. Based on the representation of the control plant model in a block form of input–output with respect to mixed variables (functions of state variables, external influences and their derivatives), we have developed a combined control law for the case where the control matrix contains additive uncertain elements. In addition, we have designed the mixed variable observers of the smallest possible dimension with piecewise linear corrective actions for two cases: (i) only the generalized coordinates of the manipulator are measured; (ii) only the angular positions and velocities of the motors are measured. It is shown that in a closed-loop system with dynamic feedback, a given tracking error stabilization accuracy is provided in the conditions of incomplete information. We presented the results of numerical simulation of these algorithms for a single-link manipulator. Full article

With the vigorous development of mechanical intelligence in industrial manufacturing, tracking control dynamic systems have been widely applied in many aspects of industry. In this paper, we present one theorem to discuss the validity condition of a ZD model with order-n for solving the tracking control problem of a nonlinear problem by utilizing a Lie derivative. Moreover, we also give the unified formula of the ZD model with order-n and rigorously prove it mathematically. In addition, we present three other theorems to give the global exponential convergence property of the ZD controller u(t), and the steady-state tracking error bound of the ZGD controller u(t), and the radius bound where the steady-state tracking error converges exponentially. Finally, simulations are conducted to demonstrate the validity and parameter influences of the ZD model and ZGD model for solving the tracking control problem with a single linear or nonlinear output of the single-link manipulator with flexible joints. Full article

This paper investigates the problem of modeling and controlling a space manipulator system with flexible joints and links. The dynamic model of the flexible manipulator system is derived by using the Lagrange equation and the floating frame of reference formulation, where the assumed mode method is adopted to discretize flexible links, while the flexible joints are regarded as linear torsion springs. The natural characteristics of a single flexible link manipulator, under three different boundary conditions, are compared to reveal the effect of the flexibility of joints on the manipulator system and to choose suitable assumed modes. Furthermore, singular perturbation theory is introduced to decompose the system into a slow subsystem that describes the rigid-body motion, and a fast subsystem that describes the elastic vibration. Since the system is underactuated, a compound control strategy, which consists of the underactuated computed torque controller and the adaptive fuzzy controller, is presented to improve the accuracy of the trajectory tracking of the flexible joints and to suppress the elastic vibration of the flexible links, in the meantime. Both numerical simulation and experimentation are performed to verify the effectiveness of the proposed compound controller, and a comparison with the proportional-derivative (PD) controller is provided to highlight its superiority in suppressing the residual vibration of the tip. Full article

Flexible-joint manipulators (FJMs) have been widely used in the fields of industry, agriculture, medical service, aerospace, etc. However, the FJMs in practical applications inevitably encounter various uncertainties including matched and mismatched disturbances. In this paper, we consider the high precision tracking control problem of FJMs in the presence of unknown lumped matched and mismatched disturbances. An efficient model-assisted composite control approach is proposed by integrating two reduced-order extended state observers (RESOs), a second-order command filtered backstepping (SCFB) technique and an error compensation dynamic system. Unlike some existing methods, the RESOs constructed with partial known model information are capable of estimating and compensating the matched and mismatched disturbances simultaneously. In addition, by employing the SCFB with an error compensation system, the proposed approach can not only overcome the problem of “explosion of complexity” inherent in backstepping, but also reduce the filtering errors arising from the command filters. The stability of the resulting control system and the convergence of error signals are guaranteed by Lyapunov stability theory. Comparative simulations are conducted for a single-link FJM with both matched and mismatched disturbances, and the results show that the proposed approach achieves a better tracking performance, i.e., compared with conventional backstepping method and adaptive fuzzy command filtered control method, the tracking accuracy is improved by 99.5% and 99.2%, respectively. Full article

This paper presents the dynamic model of a flexible-link, flexible-joint manipulator system with a considerable stiffening effect of the flexible link. A gripper, along with tip mass, is attached at one end of the flexible link. By employing the extended Hamilton’s principle, a nonlinear governing equation of motion is derived along with several boundary constraints. Under the assumption of small deformation in free vibration, a simplified motion equation is deduced to determine the natural frequencies of the mechanical system. Four parameters of the system are selected to carry out the sensitivity study on frequency. The results show that the second frequency mainly depends on the mass of tip payload. Moreover, the third frequency is significantly affected by the moment inertia of tip payload. Regarding the constant angular motion, the finite element method is adopted to analyze the dynamic model by considering the stiffening effect. The frequency results are obtained which show a higher stiffness of the Single Flexible-link Flexible-joint (SFF) system with the angular velocity increasing. The influence of three factors (i.e., payload mass, length of the flexible link, and angular velocity) on the fundamental natural frequencies are discussed, which show instinct characteristics of the flexible manipulator system. Full article

Robot manipulators have played an enormous role in the industry during the twenty-first century. Due to the advances in materials science, lightweight manipulators have emerged with low energy consumption and positive economic aspect regardless of their complex mechanical model and control techniques problems. This paper presents a dynamic model of a single link flexible robot manipulator with a payload at its free end based on the Euler–Bernoulli beam theory with a complete second-order deformation field that generates a complete second-order elastic rotation matrix. The beam experiences an axial stretching, horizontal and vertical deflections, and a torsional deformation ignoring the shear due to bending, warping due to torsion, and viscous air friction. The deformation and its derivatives are assumed to be small. The application of the extended Hamilton principle while taking into account the viscoelastic internal damping based on the Kelvin–Voigt model expressed by the Rayleigh dissipation function yields both the boundary conditions and the coupled partial differential equations of motion that can be decoupled when the manipulator rotates with a constant angular velocity. Equations of motion solutions are still under research, as it is required to study the behavior of flexible manipulators and develop novel ways and methods for controlling their complex movements. Full article

This paper presents an enhanced generalized extended state observer (EGESO) based sliding mode control (SMC) technique for dealing with the disturbance attenuation problem for a class of non-integral chain systems with mismatched uncertainty. In the proposed control law, the robust SMC with reaching phase elimination is applied in the proposed control law, which uses the estimated states of a system. The stability analysis is thoroughly examined for both EGESO and SMC. The efficacy of the proposed controller is verified using specific examples, and later it is applied on a single-link flexible manipulator. Through simulation and experimentation analysis, it is observed that the proposed controller is giving a robust transient response as compared to existing GESO based controllers. Full article