Energy-based trajectory tracking and vibration control for multilink highly flexible manipulators

In this paper, a discrete model is adopted, as proposed by Hencky for elastica based on rigid bars and lumped rotational springs, to design the control of a lightweight planar manipulator with multiple highly flexible links. This model is particularly suited to deal with nonlinear equations of motion as those associated with multilink robot arms, because it does not include any simplification due to linearization, as in the assumed modes method. The aim of the control is to track a trajectory of the end effector of the robot arm, without the onset of vibrations. To this end, an energy-based method is proposed. Numerical simulations show the effectiveness of the presented approach

Modeling and control of a two-arm elastic robot in gravity

This thesis develops and experimentally verifies a model of a two arm robot with highly elastic arms. The model is later used in this research to evaluate control algorithms. The model includes the effects of gravity. The dimensions of the arms are chosen to maximize the coupling between the flexible and large scale motion of the robot. The model is then linearized and a new analytical solution is presented for the natural frequencies and mode shapes of the robot at given equilibrium positions. This analytical solution is then compared to the assumed mode shape solutions to determine the accuracy relative to the number of assumed modes included in the model. An experimental test rig is built and tests are conducted to verify the model. A number of different amounts of end mass and torsional stiffness at the joints are used during the validation. For 12 cases tested, the measured first four natural frequencies are within ±7% of the frequencies predicted by the model with an average error of only 2.89%. Finally, the model is used to design a control algorithm for end effector control of the robot using a torque input at each of the two joints. An optimal control algorithm developed using LQR with the prescribed degree of stability method results in effective end effector control with short response time and little overshoot

Dynamic modeling, property investigation, and adaptive controller design of serial robotic manipulators modeled with structural compliance

Research results on general serial robotic manipulators modeled with structural compliances are presented. Two compliant manipulator modeling approaches, distributed and lumped parameter models, are used in this study. System dynamic equations for both compliant models are derived by using the first and second order influence coefficients. Also, the properties of compliant manipulator system dynamics are investigated. One of the properties, which is defined as inaccessibility of vibratory modes, is shown to display a distinct character associated with compliant manipulators. This property indicates the impact of robot geometry on the control of structural oscillations. Example studies are provided to illustrate the physical interpretation of inaccessibility of vibratory modes. Two types of controllers are designed for compliant manipulators modeled by either lumped or distributed parameter techniques. In order to maintain the generality of the results, neither linearization is introduced. Example simulations are given to demonstrate the controller performance. The second type controller is also built for general serial robot arms and is adaptive in nature which can estimate uncertain payload parameters on-line and simultaneously maintain trajectory tracking properties. The relation between manipulator motion tracking capability and convergence of parameter estimation properties is discussed through example case studies. The effect of control input update delays on adaptive controller performance is also studied

Integral Resonant Control for vibration damping and precise tip-positioning of a single-link flexible manipulator

Peer reviewedPostprin

Analysis and control of nonlinear flexible manipulator

This dissertation investigates the approximation techniques and instability for nonlinear one-link and two-link flexible manipulators operating in a vertical plane. The flexible components of the arms are modeled by Euler-Bernoulli beam theory, and the nonlinearity arises due to gravitation and large angle rotation. Two different methods are used for describing the deformation of the flexible members. They are the infinite-dimensional coordinates and finite-dimensional coordinates. Variational principle (Hamilton\u27s principle) is used to generate the governing differential equations and boundary conditions. The distributed coordinate approach results in partial differential equations and the Ritz approximation leads to nonlinear ordinary differential equations. The two sets of differential equations are developed in parallel to give a comparison of the static solution and frequency domain characteristics. Furthermore, the full nonlinear ordinary differential equations is integrated forward numerically for the dynamic response to maneuver transient reference inputs;Among many interesting topics, the following issues are studied (1) Use of the exact solution as the benchmark for the approximation solutions. (2) Effect of the admissible comparison functions to accuracy of approximation solution. (3) Influence of set point selection to the linearized open- and closed-loop flexible dynamic systems. (4) Effect of linear visco-elastic damping to the Laplace transform domain and time domain behavior of a flexible arm. (5) Effects of gravitational force on the closed-loop control systems with PID control at each joint;Various comparison functions are used to discretize the equations of motion of the deformable arm. Since the Ritz method requires only the essential boundary conditions to be satisfied and places no restriction on the natural boundary conditions, it allows the use of many different types of shape functions. Among the various sets of possible shape functions, only certain sets would satisfy both essential and natural boundary conditions, while the rest satisfy only the geometrical boundary conditions. Examples are given to show the importance of selecting comparison functions. In these examples both the exact and approximate solutions are obtained either in closed form or numerically. The effect of the discretization is analyzed in the Laplace transform domain by comparing the approximate solutions with the closed-form solution, and the causes of the differences in results are identified and analyzed. Finally, Liapunov\u27s direct method is used to re-examine the stability characteristics. A sufficient condition for a stable PD control system is derived

Space Station/Orbiter berthing dynamics during an assembly flight

A large-angle, multi-body, dynamic modeling capability was developed to help validate numerical simulations of the dynamic motion and control forces which occur while berthing Space Station Freedom to the Shuttle Orbiter during early assembly flights. The paper describes the dynamics and control of the station, the attached Shuttle Remote Manipulator System, and the Orbiter during a maneuver from a gravity-gradient attitude to a torque equilibrium attitude using the station reaction control jets. The influence of the elastic behavior of the station and of the remote manipulator system on the attitude control of the station/Orbiter system during the maneuver is investigated. The flexibility of the station and the arm had only a minor influence on the attitude control of the system during the maneuver

Deep Visual Foresight for Planning Robot Motion

A key challenge in scaling up robot learning to many skills and environments is removing the need for human supervision, so that robots can collect their own data and improve their own performance without being limited by the cost of requesting human feedback. Model-based reinforcement learning holds the promise of enabling an agent to learn to predict the effects of its actions, which could provide flexible predictive models for a wide range of tasks and environments, without detailed human supervision. We develop a method for combining deep action-conditioned video prediction models with model-predictive control that uses entirely unlabeled training data. Our approach does not require a calibrated camera, an instrumented training set-up, nor precise sensing and actuation. Our results show that our method enables a real robot to perform nonprehensile manipulation -- pushing objects -- and can handle novel objects not seen during training.Comment: ICRA 2017. Supplementary video: https://sites.google.com/site/robotforesight