Control Of Flexible Robots Research Articles

Decentralized sliding mode control for flexible link robots

A technique using augmented sliding mode control for robust, real-time control of flexible multiple link robots is presented. For the purpose of controller design, the n-link, n-joint robot is subdivided into n single joint, single link subsystems. A sliding surface for each subsystem is specified so as to be globally, asymptotically stable. Each sliding surface contains rigid-body angular velocity, angular displacement and flexible body generalized velocities. The flexible body generalized accelerations are treated as disturbances during the controller design. This has the advantage of not requiring explicit equations for the flexible body motion. The result is n single input, single output controllers acting at the n joints of the robot, controlling rigid body angular displacement and providing damping for flexible body modes. Furthermore, the n controllers can be operated in parallel so that compute speed is independent of the number of links, affording real-time, robust, control.

Fuzzy Gain Scheduling Optimal Control of a Flexible Robot

In this paper, a dynamic model is presented first for a single-link flexible robot using finite element method (FEM). An LQR design is then studied for the control of a single-link flexible robot. Since the LQR cannot achieve fast response and good damping concurrently, fuzzy gain scheduling LQR is then proposed. By switching from an underdamped controller at the beginning of the step response to a well-damped controller when the system is within the neighbourhood of the step, a better response can be obtained

Adaptive controller design for flexible joint manipulators

A new adaptive controller for flexible joint robots is presented here based on singular perturbation theory and using only position and velocity feedback by modelling the ‘motor tracking error’ Ke m as the fast variables instead of the joint elastic forces, where K represents the joint elasticity and e m is the motor tracking error. The resulting boundary layer and quasi-steady-state systems are made exponentially stable: therefore the statements of Tychonov's theorem are valid for an infinite time interval.

Comparative Studies between Finite and Infinite Time Interval Controllers for Flexible Joint Robots

Motivated by the work by Spong (1) on the control of flexible joint robots, an adaptive control law guaranteeing that statements from the Tychnov theorem hold on an infinite time interval is presented first. Computer simulation comparisons are then carried out between the proposed infinite time interval controller and the conventional finite time interval controller.

A tracking controller for flexible joint robots using only link position feedback

A dynamic output feedback controller for flexible joint robots is presented which guarantees asymptotic tracking of a desired trajectory, starting from arbitrary initial conditions. The controller needs only the measurements of the positions of the links and the knowledge of an upper bound on the initial tracking error. >

Adaptive control of flexible joint manipulators: Comments on two papers

We discuss two papers [Spong, M. W. (1989) Syst. Control. Lett., 13, 15–21; Chang, Y.-Z. and R. W. Daniel (1992) Automatica, 28, 969–974] on the adaptive control of flexible joint robot manipulators. Our goal is to clarify the contributions of the two papers by correcting some technical errors and to provide some additional insight into the application of singular perturbation methods for the control of flexible joint robots.

Adaptive Computed Reference Computed Torque Control of Flexible Robots

This paper presents a motion control technique for flexible robots and manipulators. It takes into account both joint and link flexibility and can be applied in adaptive form if robot parameters are unknown. It solves the main problems that are related to the fact that the number of degrees of freedom exceeds both the number of actuators and the number of output variables. The proposed method results in trajectory tracking while all state variables remain bounded. Global, asymptotic stability is ensured for all values of the stiffnesses of joints and links. To show the characteristics of the proposed control law, some simulation results are presented.

Modeling and control of a flexible one-link robot driven by a velocity controlled actuator

This paper describes a new approach to control flexible robots. Many control schemes for flexible robots have been developed and published in robotics research literature. All of them assume the use of torque controlled actuators (DC-motors), while most commercially available robots have velocity controlled actuators. It is possible to extend a traditional controller, which uses velocity controlled actuators and which is designed to control rigid robots, to a controller for flexible robots. However, it is shown that the high gain velocity loop introduces a controllability problem. The proposed control approach is based on state feedback, built around a high gain velocity controlled actuator. The total dynamic model consists of a parallel combination of the models relating the differential velocity input to the joint position on the one hand and relating the differential velocity input to the flexible deformation on the other. This structure is proven to solve the controllability problem. By state feedback together with feedforward, accurate tracking, proper positioning and oscillation suppression during and after positioning of the end point are achieved. Test results on a bread board model prove the applicability of the proposed control scheme.

195 Adaptive control of flexible joint robots with arbitrary stiffness

195 Adaptive control of flexible joint robots with arbitrary stiffness

On the control of flexible joint robots by dynamic output feedback

A dynamic output feedback controller for flexible joint robots is presented which guarantees tracking of a desired trajectory, starting from arbitrary initial conditions. The controller needs only the measurements of the positions of the links and the knowledge of an upper bound on the initial error.

Correction to ?Robustness of Adaptive Control of Robots? by F. Ghorbel and M.W. Spong

In paper ‘Robustness of Adaptive Control of Robots’ by Ghorbel and Spong, the adaptive control of flexible joint robots is investigated using a singular perturbation approach and composite Lyapunov theory. In this note, we correct a missing term from the boundary-layer system of that paper and then show that the stability analysis presented remains valid.

Maneuvering and control of flexible space robots

This paper is concerned with a flexible space robot capable of maneuvering payloads. The robot is assumed to consist of two hinge-connected flexible arms and a rigid end-effector holding a payload; the robot is mounted on a rigid platform floating in space. The equations of motion are nonlinear and of high order. Based on the assumption that the maneuvering motions are one order of magnitude larger than the elastic vibrations, a perturbation approach permits design of controls for the two types of motion separately. The rigid-body maneuvering is carried out open loop, but the elastic motions are controlled closed loop, by means of discrete-time linear quadratic regulator theory with prescribed degree of stability. A numerical example demonstrates the approach. In the example, the controls derived by the perturbation approach are applied to the original nonlinear system and errors are found to be relatively small.

Tracking control of flexible joint robots with uncertain parameters and disturbances

The tracking control problem is considered for robots having elastic joints. The kinematic and dynamic parameters of the robots are not assumed to be known but they are assumed to belong to a known bounded compact set. Moreover, unknown frictional forces are supposed to be present. A robust tracking controller is proposed capable of guaranteeing tracking with arbitrary accuracy of any given reference trajectory. >

Inverse dynamics task control of flexible joint robots—I: Continuous-time approach

This paper focuses on the problem of application of inverse dynamics control methods to robots with flexible joints. In Part I of the paper, there are answers presented to a number of questions related to the analytical formulation of nonlinear decoupling control laws. After discussion, the dynamic model of a robot with flexible joints and electrochemical actuators is chosen. The application of the extended computed torque technique together with the methods from the theory of differential-algebraic equations (DAE) to the developed fifth-order robot model gives a number of decoupled linear subsystems, which take robot task space commands as inputs. Due to drawbacks of the continuous-time inverse dynamics, discussed in the paper, a new control strategy in discrete-time is developed in Part II of the paper. The proposed control method is computationally efficient and admits low sampling frequencies. The results of extensive numerical experiments confirm the advantages of the designed control algorithm.

Inverse dynamics task control of flexible joint robots—II: Discrete-time approach

Due to drawbacks of the continuous-time inverse dynamics for robots with flexible joints, discussed in the paper, a new discrete-time approach to the problem is presented. After discussion, a numerical approach for the solution of the index three systems of differential-algebraic equations is adopted to solve the inverse dynamics problem. The satisfactory results show that the method can be used to off-line determination of nominal control inputs and joint trajectories. Next, a discrete-time control method is developed and a realistic control scheme is analysed. The implications of the results for the control of robots are discussed. The proposed control method is computationally efficient and admits low sampling frequencies. The results of extensive numerical experiments confirm the advantages of the designed control algorithm.

Adaptive Control of Flexible Joint Robots with Arbitrary Stiffness

A composite adaptive controller is developed for flexible joint robots with inertia parameter uncertainty. It is designed on a full-order dynamic model. The proposed adaptive controller requires, at most, joint acceleration feedback. Its adaptive law is of the same complexity as the well-known Slotine and Li's algorithm (1989 for rigid-body robots). Asymptotic stability of the adaptive controller is established in the Lyapunov sense

An investigation of fuzzy logic control of flexible robots

SUMMARYIn this paper a control law, which consists of a fuzzy logic controller plus a nonlinear effects negotiator for a flexible robot manipulator, is presented. The nonlinear effects negotiator is used to enhence the control system's ability in dealing with the uncertainty of the mathematical model. The control algorithm is simple and easy to tune as opposed to conventional control law which requires time consuming gains selections. To obtain fuzzy control rules, an error response plane method is proposed.

Composite adaptive control of flexible joint robots

A composite adaptive controller is developed for flexible joint robots with inertia parameter uncertainty. It does not require any restrictions on the joint stiffness, since a full-order dynamic model is used in the design and stability analysis. Previously published results based on the fourth-order model require at least joint jerk feedback and derivatives of the manipulator regressor (up to the second-order). In contrast, the proposed adaptive controller requires at most joint acceleration feedback. Its adaptive law is of the same complexity as the well-known Slotine and Li's algorithm for rigid-body robots. Asymptotic stability of the adaptive controller is established in the Lyapunov sense. Simulation results are included to demonstrate the tracking performance.

A nonlinear model-based control of flexible robots

SUMMARYThis paper present a nonlinear, model-based control of flexible link robots. The control task is formulated requiring rigid joints variables to track reference time-varying trajectory and elastic deflection to be damped. The stability and robustness properties of the control scheme are analyzed from a passive energy consideration. A direct adaptive version is also proposed. Extensive evaluation of this approach is performed using experimental validations involving a single-flexible-link and a two-flexible-link horizontal robot. Experimental results show significant performances of the controller under relatively severe working conditions: 700% payload to arm ratio and 20% elastic deflection ratio at highest acceleration stages.

Control of cooperative multiple flexible joint robots

The problem of controlling multiple flexible joint robots during cooperative manipulation of a rigidly grasped common load (denoted as the CMFJR problem) is addressed. The dynamic model of the CMFJR shows the interaction between the manipulators in the closed chain is analogous to a single manipulator interacting with frictionless (algebraic constraint) surface, while in motion along the surface. It is assumed that each flexible joint robot is equipped with joint velocity and position sensors and also actuator mounted position and velocity sensors. It is shown that it is not necessary to employ force sensors mounted on the robot wrist in order to track a desired load trajectory and an internal force trajectory. The transient response of the position error and the internal force error can be arbitrarily assigned even though the force sensors are not employed. >