Robust control of flexible-joint robots using voltage control strategy

So far, control of robot manipulators has frequently been developed based on the torque-control strategy. However, two drawbacks may occur. First, torque-control laws are inherently involved in complexity of the manipulator dynamics characterized by nonlinearity, largeness of model, coupling, uncertainty and joint flexibility. Second, actuator dynamics may be excluded from the controller design. The novelty of this paper is the use of voltage control strategy to develop robust tracking control of electrically driven flexible-joint robot manipulators. In addition, a novel method of uncertainty estimation is introduced to obtain the control law. The proposed control approach has important advantages over the torque-control approaches due to being free of manipulator dynamics. It is computationally simple, decoupled, well-behaved and has a fast response. The control design includes two interior loops; the inner loop controls the motor position and the outer loop controls the joint position. Stability analysis is presented and performance of the control system is evaluated. Effectiveness of the proposed control approach is demonstrated by simulations using a three-joint articulated flexible-joint robot driven by permanent magnet dc motors.     

Fuzzy adaptive dynamic surface control for a single-link flexible-joint robot

In this paper, a fuzzy adaptive controller is proposed for a single-link flexible-joint robot. Fuzzy logic systems are used to approximate unknown nonlinearities, and then a fuzzy state observer is designed to estimate the immeasurable states. By combining the adaptive backstepping design with dynamic surface control (DSC) technique, a fuzzy adaptive output-feedback backstepping control approach is developed. It is proved that all the signals of the resulting closed-loop system are semiglobally uniformly ultimately bounded (SGUUB), and both the observer and tracking errors converge to a small neighborhood of the origin by appropriate choosing the design parameters. The simulation results are provided to demonstrate the effectiveness of the proposed controller. Two key advantages of our scheme are that (i)?the proposed control method does not require that the link velocity and actuator velocity of single-link flexible-joint robot be measured directly, and (ii)?the problem of ??explosion of complexity?? is avoided.     

Nonlinear analysis on moving process of soft robots

The soft robot consists of several deformable spherical cells. It is a nonlinear system. According to the deflating and inflating action of the spherical cells, the size of each spherical cell can be changed. Thus, the soft robot can move forward. In the paper, the moving process is analyzed in one circular movement with the deflating and inflating modes of each cell. The theoretical analysis of the distance expected is counted. Then the forces on the inflating process are presented in detail, which push the cells to move forward. Lastly, an experiment is shown to emulate the moving process of the soft robot. The theoretical result and the experimental result are compared. It shows that the soft robot that consists of several deformable spherical cells can move forward with the nonlinear inflating and deflating process.     

Impedance control of robots using voltage control strategy

Impedance control provides a unified solution for the position and force control of robot manipulators. The dynamic behavior of a robotic system in response to environment is prescribed by an impedance model formed as Thevenin model. This model is certain and linear while the robot manipulator is highly nonlinear, coupled, and uncertain. Therefore, impedance control must overcome nonlinearity, coupling, and uncertainty to convert the robotic system to the impedance model. To overcome these problems, this paper presents a novel impedance control for electrically driven robots, which is free from the manipulator dynamics. The novelty of this paper is the use of voltage control strategy to develop the impedance control. Compared with the commonly used impedance control, which is based on the torque control strategy, it is computationally simpler, more efficient, and robust. The mathematical verification and simulation results show the effectiveness of the control method.     

Nonlinear dynamic modeling on multi-spherical modular soft robots

A soft robot, which consists of multi-deformable spherical cells, is constructed. According to the deflating action and the inflating action of the spherical cells, the size and the shape of each spherical cell can be changed. Thus, the soft robot can move in a narrow complicated passage. In the paper, a modular soft robot is built. The nonlinear relationship between the inflation radius ( \(R)\) of each cell and the inflation time ( \(t)\) is described to control the action of the spherical cell. The nonlinear dynamic moving process is analyzed with the deflating and inflating modes of each cell. The theoretical analysis of the forward locomotion is counted. Then, two special positions are described, and the moving conditions are presented in details. Last, a simulation and an experiment of three spherical cells are shown to emulate the moving process of the soft robot. It shows that the modular soft robot consisting of multi-deformable spherical modules can move forward with the nonlinear dynamic inflating and deflating process.     

Robust trajectory tracking control of cable-driven parallel robots

Nonlinear Dynamics - In this paper, a robust tracking controller is designed for fully constrained cable-driven parallel robots (CDPRs). One of the main challenges of controller design for this...     

Orientation-error observer-based tracking control of nonholonomic mobile robots

In this paper, a novel trajectory tracking controller is proposed for mobile robots with unknown orientation angle by employing the orientation-error observer (OEO). In order to overcome the local stability resulted from linearization design methods, an asymptotically stable controller is designed using Lyapunov’s direct method. This method breaks down nonlinear systems into low-dimensional systems and simplifies the controller design using virtual auxiliary error function and partial Lyapunov functions. A state-feedback controller for the nonlinear error dynamics of the mobile robot is combined with an observer that estimates the orientation-error based on available trajectory information and measurement of the position coordinates. The stability of the system is easily proved via the Lyapunov theory. Abundant simulation and experiment results validate the effectiveness and superiority of the proposed control method.     

Decentralized direct adaptive fuzzy control of robots using voltage control strategy

Decentralized control is the most favorite control of robot manipulators due to computational simplicity and ease of implementation. Beside that, adaptive fuzzy control efficiently controls uncertain nonlinear systems. These motivate us to design a decentralized fuzzy controller. However, there are some challenging problems to guarantee stability. The state-space model of the robotic system including the robot manipulator and motors is in a noncompanion form, multivariable, highly nonlinear, and heavily coupled with a variable input gain matrix. For this purpose, adaptive fuzzy control may use all variable states. As a result, it suffers from computational burden. To overcome the problems, we present a novel decentralized Direct Adaptive Fuzzy Control (DAFC) of electrically driven robot manipulators using the voltage control strategy. The proposed DAFC is simple, in a decentralized structure with high-accuracy response, robust tracking performance, and guaranteed stability. Instead of all state variables, only the tracking error of every joint and its derivative are given as the inputs of the controller. The proposed DAFC is simulated on a SCARA robot driven by permanent magnet dc motors. Simulation results verify superiority of the decentralized DAFC to a decentralized PD-fuzzy controller.     

Fixed-time anti-saturation compensators based impedance control with finite-time convergence for a free-flying flexible-joint space robot

Nonlinear Dynamics - Aiming at the compliant control problem of a free-flying flexible-joint space robot (FFSR) when it comes in contact with targets or obstacles, a nonsingular composite impedance...     

Motion and shape control of soft robots and materials

Nonlinear Dynamics - A continuum-based approach for simultaneously controlling the motion and shape of soft robots and materials (SRM) is proposed. This approach allows for systematically computing...     

Practical tracking control under actuator saturation for a class of flexible-joint robotic manipulators driven by DC motors

Nonlinear Dynamics - This paper is devoted to the practical tracking control for a class of flexible-joint robotic manipulators driven by DC motors. Different from the related literature where...     

Multi-surface sliding mode control of continuum robots with mismatched uncertainties

Meccanica - In this paper, we tackle the control problem of continuum robots with mismatched uncertainties. Uncertainties that affect systems through any of their states and may not be directly...     

Compact robust FAT control of rigid robots with state constraints

Nonlinear Dynamics - This paper develops a robust regressor-free controller for n-link robots with state constraints. We use the function approximation technique to represent the uncertain robot...     

Nonlinear problem of an electrical discharge in a flow

The nonlinear intial boundary value problem describing the relaxation of a quasi-neutral discharge in a gas flow with coplanar, heated and nonheated electrodes of finite extension is formulated in the diffusion approximation. Relaxation occurs from an initial breakdown to a steady-state or zero discharge in a weak electric field. A nonlinear transformation is applied to get an equivalent nonlinear problem, where nonlinearity is treated as a small perturbation. An analytic solution is obtained and criterions for existence and sustainment of a steady-state discharge against plasma losses due to convection, diffusion and recombination is discussed. Some numerical results are exhibited.     

Nonlinear problem of an electrical discharge in a flow

The shape of the shoulder of a forming, filling and closing machine is calculated, based on geometrical considerations, first for a circular and subsequently for a rectangular (superelliptic) section. It appears to be possible to formulate a feasible solution for a model represented by a flat triangular region in the centre, which connects two truncated cones; for each cone one half of the intersection of shoulder and cylinder acts as the directrix of the cone. The solutions are formulated in terms which permit direct application in machine construction. Verification of the theory with the help of paper models and with real shoulders on machines shows a very good agreement between theory and practice. In addition practical tests produce excellent results as well.     

Distributed finite-time control for coordinated circumnavigation with multiple non-holonomic robots

Nonlinear Dynamics - The finite-time coordinated circumnavigation problem for multiple non-holonomic robots is investigated in this paper. All robots are required to be evenly circled a target, and...     

Real-time robust adaptive control of robots subjected to actuator voltage constraint

This paper is concerned with the problem of design and implementation of a robust adaptive control strategy for electrically driven robots while considering to the constraints on the actuator voltage input. The proposed approach provides a flexible design framework and stable to deal with robustness compared with many other adaptive controllers, such as halting/slowing adaption techniques and adaptively adjusting command signal, which are proposed for robotic applications. The control design procedure is based on a new form of universal approximation theory and using Stone–Weierstrass theorem, to avoid saturation besides being robust against both structured and unstructured uncertainties associated with external disturbances and actuated manipulator dynamics. Moreover, the proposed approach eliminates problems arising from classic adaptive feedforward control scheme. The analytical studies as well as experimental results produced using MATLAB/SIMULINK external mode control on a two degree of freedom electrically driven robot demonstrate high performance of the proposed control schemes.     

Force/position control of parallel robots using exteroceptive pose measurements

The aim of this paper is to study force and position control of kinematic parallel machines. Relying on a recent work showing that computed torque control in Cartesian space is suitable for parallel structures, we propose a parallel force position control scheme of a parallel robot based on the visual servoing of the end effector pose. Simulation results show the efficiency of the proposed approach.     

Output-feedback formation control of wheeled mobile robots with actuators saturation compensation

Nonlinear Dynamics - This paper addresses output-feedback formation control of a group of wheeled mobile robots with saturating actuators. A virtual leader–follower strategy and a...     

Robust control of electrically driven robots by adaptive fuzzy estimation of uncertainty

This paper presents a novel robust decentralized control of electrically driven robot manipulators by adaptive fuzzy estimation and compensation of uncertainty. The proposed control employs voltage control strategy, which is simpler and more efficient than the conventional strategy, the so-called torque control strategy, due to being free from manipulator dynamics. It is verified that the proposed adaptive fuzzy system can model the uncertainty as a nonlinear function of the joint position error and its time derivative. The adaptive fuzzy system has an advantage that does not employ all system states to estimate the uncertainty. The stability analysis, performance evaluation, and simulation results are presented to verify the effectiveness of the method. A?comparison between the proposed Nonlinear Adaptive Fuzzy Control (NAFC) and a Robust Nonlinear Control (RNC) is presented. Both control approaches are robust with a very good tracking performance. The NAFC is superior to the RNC in the face of smooth uncertainty. In contrast, the RNC is superior to the NAFC in the face of sudden changes in uncertainty. The case study is an articulated manipulator driven by permanent magnet dc motors.