Model Of Flexible Joint Robot Research Articles

Observer‐based fault diagnosis for autonomous systems

AbstractWith the advancement in sensor and communication technology, autonomous systems have been incrementally reshaping the execution of tasks in commercial and military sectors. Since the systems are designed to complete tasks without or with minimal human intervention, fault diagnosis based on sensor data has been crucial to preventing accidents and fatalities. In this paper, fault diagnosis for autonomous systems is designed based on nonlinear adaptive observers, tested in numerical simulations, and implemented in a robotic platform. To this end, we utilize the persistence of excitation conditions on the parametric model of the faults. We derive sufficient conditions for the nonlinear adaptive observer in terms of linear matrix inequality to ensure the convergence of the estimates. Furthermore, we consider one‐sided Lipschitz conditions to obtain less conservative results. The main advantage of using the nonlinear adaptive observer is that the method converges quickly to the actual fault and requires minimum computational effort. However, solving the linear matrix inequality might not be trivial. Numerical simulations based on a single‐link flexible joint robot model and experimental tests in an autonomous quadcopter are performed to validate the effectiveness of the proposed method.

Whole Body Control of an Autonomous Mobile Manipulator Using Series Elastic Actuators

The flexible joint robot has superior attractive features because of its high mobility, high load ratiohigh torque fidelity, robustness for external disturbance, task adaptability, and safety. In this paper, an autonomous mobile manipulator driven by the designed series elastic actuators (SEAs) is developed. A complete dynamic model of nonholonomic mobile manipulator including a mobile platform and manipulator with joint flexibility operating simultaneously is proposed. An integrated whole body trajectory control framework is proposed for such robot to perform mobile manipulation tasks. Considering the nonholonomic and holonomic constraints in the mobile manipulation, the whole-body dynamics is formulated and reduced. To address the highly nonlinear of the dynamics and model uncertainty, a novel integral Lyapunov function (ILF)-based adaptive neural network control for task tracking under uncertainties of the flexible joint robot model is proposed. Compared with existing methods, the proposed method provides an alternative for controlling flexible joint robots. The feasibility of the proposed method is verified by the extensive trajectory tracking experiment results in our developed flexible joint manipulator.

Whole-Body Control of an Autonomous Mobile Manipulator Using Series Elastic Actuators

The flexible-joint robot has superior attractive features because of its high mobility, high load ratio, high torque fidelity, robustness for external disturbance, task adaptability, and safety. In this article, an autonomous mobile manipulator driven by the designed series elastic actuators is developed. A complete dynamic model of a nonholonomic mobile manipulator including a mobile platform and a manipulator with joint flexibility operating simultaneously is proposed. An integrated whole-body trajectory control framework is proposed for such robot to perform mobile manipulation tasks. Considering the nonholonomic and holonomic constraints in the mobile manipulation, the whole-body dynamics is formulated and reduced. To address the highly nonlinear of the dynamics and model uncertainty, a novel integral Lyapunov function based adaptive neural network control for task tracking under uncertainties of the flexible-joint robot model is proposed. Compared with existing methods, the proposed method provides an alternative for controlling flexible-joint robots. The feasibility of the proposed method is verified by the extensive trajectory tracking experiment results in our developed flexible joint manipulator.

Polynomial Piece-Wise Stiffness in Cable-Based Transmissions for Robots: Modeling and Identification

This paper deals with cable-based motor-to-joint transmissions of multi-link robots chain. Their effects on robots flexibility have to be taken into account for modeling and control design. More in details, slack cables do not provide any force during compression (unlike springs), may present an initial nonzero elongation (preload) and, depending on the material, could exhibit non-constant stiffness. Those features may lead to non-trivial piece-wise elastic torques in a mechanical transmission. In this context, we present a framework to generate a more general (piece-wise) elastic torque model which can be embedded in the classical flexible-joint robot model, coherently with the Lagrangian approach. Moreover, we propose a model based on polynomial stiffness, whose parameters can be identified with ordinary least-squares techniques. Experimental results conducted on a flexible transmission show the usage and the utility of this work.

On the modeling and identification of stiffness in cable-based mechanical transmissions for robot manipulators

In this paper, we consider cable-based motor-to-joint transmissions which are known to introduce flexibility phenomena in the dynamic behaviour of robot manipulators. Their effects have to be taken into account for modeling and control design. More in details, slack cables do not provide any force during compression (unlike springs), may present an initial nonzero elongation (preload) and, depending on the material, could exhibit non-constant stiffness. Those features may lead to non-trivial piece-wise elastic torques in a mechanical transmission. In this context, we present a framework to generate a more general (piece-wise) elastic torque model which can be embedded in the classical flexible-joint robot model, coherently with the Lagrangian approach. Moreover, we propose a model based on polynomial stiffness, whose parameters can be identified with conventional identification techniques. The goal is to provide a precise characterization of the elastic torques in a cable-based transmission in order to support mechanical design, preload tuning and finally, to quantify the eventual error introduced by relying on simpler models such as the linear one. The targeted scope is about multi-link cable-driven robots chain (as it may be the case for compact or lightweight robots for instance, finger hand being viewed generally as a serial small-scaled robot arm). Some theoretical examples related to the multi-joint case, as well as experimental results conducted on a 1-dof flexible transmission, show the usage and the utility of this work.

Task Space Impedance Control of the Manipulator Driven Through the Multistage Nonlinear Flexible Transmission

This paper addresses the task space impedance control of a robot driven through the multistage nonlinear flexible transmission. The proposed controller uses limited information of the angle and the current of the motors to regulate the end point compliance at the specified set point. In particular, motor angle is employed to estimate the stationary robot link angle and joint velocity in real time. They are then used to constitute the stationary force on the attempt to cancel the robot gravity force and to form the task space interacting force according to the desired impedance characteristics. Motor current is used to infer the transmitted torque to the robot. This torque is fed back to mitigate the effect of the motor inertia from deteriorating the desired impedance. Asymptotic stability of this controller with the flexible joint robot is guaranteed with additional damping. Passivity of the system is also investigated. Simulation and experiments of the proposed control scheme on a two degrees-of-freedom (DOF) cable-pulley driven flexible joint robot model are examined.

Modelling and adaptive tracking control for flexible joint robots with random noises

This paper investigates modelling and adaptive tracking control problems for flexible joint robots subjected to random disturbances. A stochastic flexible joint robot model is given by introducing random noises reasonably. Under some weaker assumptions, a new controller is constructed by exploiting adaptive dynamic surface control technique. It is proved that the mean square of the tracking error can be made arbitrarily small by choosing appropriate design parameters. A mechanics model is provided in the simulation to show the effectiveness of the presented theory.

유연관절로봇을 위한 정확한 외부토크 측정시스템 개발: 랜덤워크모델을 이용한 칼만필터 기반 추정

In this paper, an external torque estimation problem in one-degree-of-freedom (1-DOF) flexible-joint robot equipped with a joint-torque sensor is revisited. Since a sensor torque from the joint-torque sensor is distorted by two dynamics having a spring connection, i.e., motor dynamics and link dynamics of a flexible-joint robot, a model-based estimation, rather than a simple linear spring model, should be required to extract external torques accurately. In this paper, an external torque estimation algorithm for a 1-DOF flexible-joint robot is proposed. This algorithm estimates both an actuating motor torque from the motor dynamics and an external link torque from the link dynamics simultaneously by utilizing the flexible-joint robot model and the Kalman filter estimation based on random-walk model. The basic structure of the proposed algorithm is explained, and the performance is investigated through a custom-designed experimental testbed for a vertical situation under gravity.

Higher Performance Adaptive Control of a Flexible Joint Robot Manipulators

Joint flexibility limits the performance of an industrial robot by introducing resonant frequencies in the same range of the control bandwidth. If joint flexibility is considered in the modelling, advanced control techniques can be designed to achieve a higher performance. This paper presents an adaptive control scheme for a twodegree of freedom flexible joint robot. The adaptive approach is applied to design a control signal for each joint (modeled by twoequations of second order), to withstand all uncertainties in parameters and load disturbances. Asymptotic stability is insured regardless of the joint flexibility value, by the use of Liapunov's direct method, i.e., the results are not restricted to weak joint elasticity. A simulation study was carried out to evaluate the performance of the proposed control law in case of flexible joint robot model. It was found that the adaptation technique is robust to parameters and load changes.

Design of a bilinear fault detection observer for singular bilinear systems

A bilinear fault detection observer is proposed for a class of continuous time singular bilinear systems subject to unknown input disturbance and fault. By singular value decomposition on the original system, a bilinear fault detection observer is proposed for the decomposed system via an algebraic Riccati equation, and the domain of attraction of the state estimation error is estimated. A design procedure is presented to determine the fault detection threshold. A model of flexible joint robot is used to demonstrate the effectiveness of the proposed method.

Force Sensor-less Workspace Virtual Impedance Control Considering Resonant Vibration for Industrial Robot

The motion control paradigm provides sufficient performance in many elementary industrial tasks. However, only stiff motion the robot cannot accommodate the interaction force under constrained motion. In such situation, the robot is required to perform interaction behavior with the environment. The conventional impedance control schemes require force-sensing devices to feedback force signals to the controllers. The force-sensing device is therefore indispensable and the performance of the system also depends on the quality of this device. This paper proposes a novel strategy for force sensor-less impedance control using disturbance observer and dynamic model of the robot to estimate the external force. In motion task, the robust D-PD (derivative-PD) control is used with feedforward inverse-dynamic torque compensation to ensure robustness and high-speed response with flexible joint model. When robot is in contact with environment, the proposed force sensor-less scheme impedance control with inner-loop D-PD control is utilized. D-PD control uses both position and speed as the references to implement the damping and stiffness characteristic of the virtual impedance model. In addition, the gravity and friction force-feedback compensation is computed by the same dynamic model, which is used in external force estimation. The flexible-joint robot model is utilized in both disturbance observer and motion control design. The workspace impedance control for robot interaction with human operator is implemented on the experimental setup three-degree-of-freedom (3-DOF) robot manipulator to assure the ability and performance of the proposed force sensor-less scheme for flexible-joint industrial robot.

Design of bilinear observer for singular bilinear systems

A bilinear observer is proposed for a class of singular bilinear system subject to unknown input disturbance. Based on singular value decomposition technique, the existence of the solution to the decomposed system is presented. Then a bilinear observer is proposed for the decomposed system based on an algebraic Riccati equation, and the domain of attraction of the state estimation error is derived. Finally, a detailed design procedure is given to design a bilinear observer for a model of flexible joint robot, which demonstrates the effectiveness of the proposed method.

Structural properties of a flexible-joint robot model with output controllers and some related applications

This paper considers some structural properties of an n-degree-of-freedom flexible-joint robot with a simple stabilizing output controller. It is well known that, in a nonlinear system, global asymptotic stability does not necessarily imply local exponential stability, while the latter property provides an essential tool in studying various control problems. This study shows that for the model under consideration, global asymptotic stability implies local exponential stability. Applications of this result to the topic of small-signal finite-gain Lp stability and to the analysis of the perturbed system are considered. Finally, we show how a local stabilizing controller which is based on position measurements only can be integrated into a strategy that ensures, under some conditions of system uncertainty, set-point regulation (in the global sense) of an uncertain flexible-joint robot. Simulation results, which demonstrate the applications of a control scheme that is presented in this study, are included.

Estimates of flexible-joint robot model response times and some control applications

The basic problem of estimating the response time of a flexible-joint robot is considered. An upper bound on the robot's velocity and elastic displacement vector norms has been established. Applications of the results are considered. The design of a reduced-order observer for a flexible-joint robot is demonstrated, regardless of a prerequisite assumption concerning the boundedness of the velocity norm.

Vibration Suppression Control of 2-Mass System Based on Robust Control. Experimental Studies.

2-mass system is an important first order approximated model of flexible joint robot, flexible arm, and its vibration suppression control is of a great importance for motion control. For this control problem, there have been many researches on vibration suppression control of 2-mass system in the face of parameter variations, but the effectiveness of their control methods has been only confirmed through simulations.This paper is concerned with the experimental evaluations of the vibration suppression control of a 2-mass system. Three typical controllers, a PID controller, an H∞ controller, and a μ controller, are designed and compared from the viewpoint of the control performance against the parameter variations. In the design procedure of H∞ control and μ-synthesis method, we consider the parameter variations in the mass of the cart2 and the spring constant which correspond to the change of the natural frequency of the 2-mass system between 4.68 and 6.66 rad/sec. Based on these variations, eight kinds of the perturbed models are defined. Then we regard those models as uncertainties and apply μ-synthesis method to 2-mass system. From the experimental results, it is observed that PID control and H∞ control may lead to poor performances in the presence of the parameter variations, but μ-synthesis yields better transient and steady state responses even if the parameter variations exist.

Perfect tracking of non minimum phase process. Application to flexible joint robot

This paper presents a discrete lime repetitive control algorithm applied to non minimum phase process. The desired trajectory is supposed to be known and repetitive. The algorithm involves a classical closed loop system already stabilized by a simple controller. Repetitive control is based on the use of the previous controller input and output signals in order to modify the present control signal. This control algorithm is applied to a flexible joint robot model. Simulation results present good performances. The robustness of the control is also treated. It shows that perfect tracking is obtained in spite of bad parameters identification.