Elastic Joint Robots Research Articles

Precise Control of Elastic Joint Robot Using an Interconnection and Damping Assignment Passivity-Based Approach

Vibration suppression and precise tracking are the two main challenges in the control of elastic joint robots. In this study, an elastic joint robot with precise position sensors on both the motor and link sides is developed to tackle these challenges. With a two-mass model for elastic joint and a LuGre friction model for joint friction, the elastic joint robot system is modeled and analyzed as an underactuated port-controlled Hamiltonian system with dissipation (PCHD). The proposed elastic joint controller is integrated with friction compensation using the interconnection and damping assignment passivity-based control method, leading to a PCHD closed loop system that can be effectively tuned to suppress vibration associated with elastic joints. In addition, integral control is employed to achieve high tracking precision. Experiments have been conducted and the results have demonstrated high performance of the proposed control method.

Modeling and Preview <inline-formula> <tex-math notation="LaTeX">$H_\infty$</tex-math> </inline-formula> Control Design for Motion Control of Elastic-Joint Robots With Uncertainties

This paper describes a novel approach combining identification and control design for motion control of multiple-link elastic-joint robots with motor sensors only and in presence of model uncertainties. The proposed model-based control design method makes use of the $H_\infty$ (H-infinity) framework to design a two-degree-of-freedom controller with anticipation able both to 1) withstand uncertainties or variations in model parameters and 2) follow reference trajectories with prescribed precision thanks to a preview feedforward action which anticipates the future trajectory on a given time horizon. The proposed design methodology is experimentally evaluated on a two-degree-of-freedom lightweight robotic arm, which is first modeled and identified in the frequency domain. Experimental validation of the controller confirms that the objectives of the dynamic precision in trajectory tracking and tip vibration damping are both achieved. Additional analysis and numerical simulations illustrate how the presented preview $H_{\infty }$ controller may be seen as an extension, with supplementary design parameters, of the traditional motor feedback with compensations based on the robot inverse dynamic model. A performance comparison between the proposed control method and the traditional inversion-based control shows the benefits of the anticipatory action and the possibilities offered by an $H_{\infty }$ design framework for the management of tradeoffs in the specifications.

Sensorless Torsion Control of Elastic-Joint Robots With Hysteresis and Friction

Sensorless torsion control is proposed for elastic-joint robots, with the geared drives subject to hysteresis and friction nonlinearities. The concept of the so-called “virtual torsion sensor” uses the motor torque and velocity, inherently available in most industrial robots, for estimating the reactive joint torque and predicting, based thereupon, the nonlinear joint torsion. The dynamics of the elastic-joint robot is described and augmented by a rate-independent torsion–torque hysteresis and nonlinear friction assumed on the side of motor drives. The classical two-degrees-of-freedom robot control, which includes the centralized torque feedforward control and proportional-derivative feedback control, is extended by the virtual torsion sensor in the loop. In particular, we show that the computed joint torsion can be injected in the feedback loop upon the motor position, thus making the control operating in a “virtual” joint output (link) space. The proposed control is experimentally evaluated on a single-joint setup consisting of an actuator with harmonic-drive gear and inertial load under additional impact of gravity.

Control of Nonlinear Elastic Joint Robots using Feed-forward Torque Decoupling

This paper proposes and evaluates the feed-forward decoupling of joint torque in elastic joint robots. The joint elasticities, captured by the third-order polynomial function, are considered together with the inverse manipulator dynamics so as to provide the required driving torque for the reference trajectories. Therewith decoupled joint actuators can be robustly controlled in a feedback manner for which a convenient proportional-derivative (PD) regulation is suficient. Using the inverse manipulator dynamics and joint model the reference value in the joint output (load) and not input (motor) space is provided to the feedback control of decoupled actuators. This allows compensating for the joint torsion and improve the load positioning accuracy without output sensors. The proposed control strategy is evaluated experimentally on a single elastic joint under gravity. We show an improved accuracy of the load positioning without adapting the underlying control loop and remarkable changes in the transient response.

設計レス非線形状態オブザーバに基づく弾性関節ロボットアームの振動抑制制御

設計レス非線形状態オブザーバに基づく弾性関節ロボットアームの振動抑制制御

Modeling, observation, and control of hysteresis torsion in elastic robot joints

Hysteresis torsion in elastic robot joints occurs as a coupled nonlinearity due to internal friction, backlash, and nonlinear stiffness, which are coactive inside of mechanical transmission assemblies. The nonlinear joint torsion leads to hysteresis lost motion and can provoke control errors in relation to the joint output at both trajectories tracking and positioning. In this paper, a novel modeling approach for describing the nonlinear input–output behavior of elastic robot joints is proposed together with the observation and control method, which aim to compensate for the relative joint torsion without load sensing. The proposed modeling approach includes the recently developed 2SEP dynamic friction model and Bouc–Wen-like hysteresis model, which is originated from structural mechanics, both arranged according to the assumed torque transmitting structure. The proposed method is evaluated with experiments using the laboratory setup which emulates a single rotary joint under impact of nonlinear elasticities, friction, and gravity.

3P2-I04 弾性・リンク運動エネルギ増加条件の導入による時間反転積分法の直列弾性関節ロボットへの拡張法(【機械力学・計測制御部門】ロボットシステムのダイナミクス&デザイン)

This paper proposes a time reversal integral method for series elastic joint robots. Time reversal integral is a control input planning method for noncyclic swing motion in multiple joint robots such as throwing. The principal contribution of this paper is to extend the method for series elastic joint robots. We propose an extension method by using the elastic and link kinetic energy increasing formula and an imaginary velocity input to achieve high speed by utilizing series elastic joints. This paper describes detail of the algorithm and experimental examinations.

Kinetic Energy Maximization on Elastic Joint Robots Based on Feedback Excitation Control and Excitation Limit Hypersurface

This paper describes amethod of realizing high kinetic energy utilizing mechanical elasticity within the joint limit ranges of multiple-joint robots. By utilizing series elastic elements, a robot obtains high kinetic energy compared with a rigid robot. In this paper, we propose feedback excitation control that realizes high kinetic energy utilizing series elastic joints. Robot motion has to be kept within the joint limit range. We propose a control method based on an excitation limit hypersurface to realize robot motion within the joint limit range. The feasibility of the method was evaluated in experiments. We performed a ball throwing task as an application of the method.

Grey-box Modeling of Elastic-joint Robot with Harmonic Drive and Timing Belt

We previously proposed a multivariable identification method, called the “decoupling identification method”, for a horizontal two-link robot arm with elastic harmonic drive gears. This paper extends the identification method to a vertical two-link robot arm with the harmonic drive gears and timing belts, under gravity. The mechanical resonance effects of the timing belts in the high-frequency range are decimated in advance, to apply the decoupling method. The characteristics of the timing belts are independently estimated as perturbations in the high-frequency range for robust controller design. The effectiveness of the extended identification method was experimentally verified using the vertical two-link robot with the elastic elements.

Modeling and observation of hysteresis lost motion in elastic robot joints

Abstract In this paper the modeling and observation of elastic robot joints with hysteresis is addressed. An adequate model of the joint torque transmission which could capture the hysteresis lost motion is of outstanding interest for design and control of flexible joint robots. A method of describing the significant joint nonlinearities such as dynamic friction and state-dependent stiffness with hysteresis is proposed. The joint dynamics is considered as an interplay between the propulsion and reactive torque. Direct and inverse hysteresis models of state-variant joint stiffness are introduced and identified from experiments. The proposed joint torque observer can serve as a “virtual” sensor of hysteresis lost motion without measuring the joint output.

Modeling and Identification of Elastic Robot Joints With Hysteresis and Backlash

This paper presents a novel approach to the modeling and identification of elastic robot joints with hysteresis and backlash. The model captures the dynamic behavior of a rigid robotic manipulator with elastic joints. The model includes electromechanical submodels of the motor and gear from which the relationship between the applied torque and the joint torsion is identified. The friction behavior in both presliding and sliding regimes is captured by generalized Maxwell-slip model. The hysteresis is described by a Preisach operator. The distributed model parameters are identified from experimental data obtained from internal system signals and external angular encoder mounted to the second joint of a 6-DOF industrial robot. The validity of the identified model is confirmed by the agreement of its prediction with independent experimental data not previously used for model identification. The obtained models open an avenue for future advanced high-precision control of robotic manipulator dynamics.

Design and Control of a Hydraulic Simulator for a Flexible-Joint Robot

For the first time, a novel experimental hydraulic system that simulates joint flexibility of a single-rigid-link flexible-joint robot manipulator, with the ability of changing the joint flexibility's parameters, was designed and implemented in this study. Such a system could facilitate future control studies of robot manipulators by reducing investigation time and implementation cost of research. It could also be used to test the performance of different strategies to control the movement of flexible-joint manipulators. A hydraulic rotary servo motor was used to simulate the action of a flexible-joint robot manipulator, which was a challenging task, since the control of angular acceleration was required. In this study, a single-rigid-link elastic-joint robot manipulator was mathematically modeled and implemented in which joint flexibility parameters such as stiffness and damping could be easily changed. This simulation is referred to as a 'function generator' to drive a hydraulic robot manipulator. In this study the desired angular acceleration of the manipulator was used as the input to the hydraulic rotary motor and the objective was to make the hydraulic system follow the desired acceleration in the frequency range specified. A hydraulic actuator robot was built and tested. The results indicated that if the input signal had a frequency in the range of 5–15 Hz and damping ratio of 0.1 (typical values for flexible joints), the experimental setup was able to reproduce the input signal with acceptable accuracy. Owing to the inherent noise associated with the measurement of acceleration and some severe nonlinearities in the rotary motor, control of the experimental test system using classical methods was a challenging task that had not been anticipated.

A dynamic output feedback control law for elastic joint robots via feedback-passivity approach

Motivated by recent dynamic output feedback passivation results, a new set-point control law is presented for an elastic joint robot when the velocity measurements are not available. The proposed methodology designs an additional dynamics with which the parallel-connected system is feedback passive. That is, the composite nonlinear robot system has relative degree one with a new output and its zero-dynamics subsystem becomes the virtual closed-loop system with a simple proportional-derivative (PD) control law. This approach provides an alternative way of replacing the role of the velocity measurements for the PD law. With the proposed control law, the transfer function of the additional system has the form of sG( s) with a strictly positive real (SPR) G( s). Robustness analysis is also given with regard to uncertainties on the robot parameters. The performance of the proposed control law is illustrated in the simulation studies of a manipulator with three revolute elastic joints.

A Geometric Approach to Solving Problems of Control Constraints: Theory and a DAE Framework

The paper deals with controlled mechanical systems in which the number of control inputs is equal to the number of desired system outputs, and is smaller than the number of degrees of freedom of the system. The determination of control input strategy that force the underactuated system to complete the partly specified motion is a challenging problem. In the present formulation, the outputs (performance goals), expressed in terms of system states, are treated as constraints on the system—called control or program constraints as distinct from contact constraints in the classical sense, and a mathematical resemblance of the inverse control problem to the constrained system dynamics is exploited. However, while the reactions of contact constraints act in the directions orthogonal to the respective constraint manifold, the available control reactions may have arbitrary directions with respect to the program constraint manifold, and in the extreme may be tangent. A specific methodology must then be developed to find the solution of such “singular” problems, related to a class of control tracking problems such as position control of elastic joint robots, control of cranes, and aircraft control in prescribed trajectory flight. The governing equations of the problem arise as a set of differential-algebraic equations (DAEs), and an effective method for solving the DAEs, based on backward Euler method, is proposed. The open-loop control formulation obtained this way is then extended by a closed-loop control law to provide stable tracking of the required reference trajectories in the presence of perturbations. Some examples of applications of the theory and results of numerical simulations are reported.

Sliding Mode Control of Compliant Joints Robot

The lack of knowledge of robot dynamics and joint elasticity for precise tracking control can be overcome by using either robust or adaptive (learning) control. The standard form of adaptive control does not appear to be applicable since the basic assumptions on the system dynamics and non-linear characteristics are rarely satisfied. On the other hand, variable structure control (VSC) is a powerful control method that has its main attractiveness in the invariance to disturbances and parameter variation. The paper presents a robust control for elastic joint robots (EJR) based on variable structure systems (VSS). The controller needs the measurement of position and velocity and the knowledge of the upper bounds of the uncertainties.

Set-point control of elastic joint robots using only position measurements

Motivated by the dynamic output feedback passification results, point-to-point control laws for an elastic joint robot are presented when only the position measurements are available. The proposed method makes a parallel connection of the robot system and an input-dimensional linear system which obtains the effect of the desired differentiators. It is shown that the closed-loop nonlinear robot system can be rendered output strictly passive and the regulation of the system is achieved in the end. Robustness analysis is also given with regard to uncertainties on the robot parameters. Performance of the proposed control law is illustrated in the simulation studies of a manipulator with three revolute elastic joints.

RESPONSE CHARACTERISTICS OF ELASTIC JOINT ROBOTS DRIVEN BY VARIOUS TYPES OF CONTROLLERS AGAINST EXTERNAL DISTURBANCES

The purpose of this paper is to investigate behaviors of elastic joint robots driven by various types of controllers against external disturbances applied to them. The elasticity is introduced intentionally by installing elastic devises in their transmission. We consider about four types of controllers. commonly used to control elastic joint robots. We investigate the role of the compliance introduced by the mechanism and the controllers and show numerical simulations.

Neuroadaptive control of elastic-joint robots using robust performance enhancement

A neuroadaptive control scheme for elastic-joint robots is proposed that uses a relatively small neural network. Stability is achieved through standard Lyapunov techniques. For added performance, robust modifications are made to both the control law and the weight update law to compensate for only approximate learning of the dynamics. The estimate of the modeling error used in the robust terms is taken directly from the error of the network in modeling the dynamics at the currant state. The neural network used is the CMAC-RBF Associative Memory (CRAM), which is a modification of Albus's CMAC network and can be used for robots with elastic degrees of freedom. This results in a scheme that is computationally practical and results in good performance.

Adaptive Tracking Controller for Rigid-Link Elastic-Joint Robots with Link Acceleration Estimation

This paper presents a adaptive switching control scheme for elastic joint robot manipulators. The characteristics of both flexible and rigid subsystems are assumed unknown except the joint stiffness. An adaptive estimator compensates the uncertainty due to the unknown robot characteristics. The actuators and links position, velocity and an estimation of the link acceleration are used as feedback to the control law. A filter is designed to estimate the links acceleration and its accuracy is insured by the linear control theory. Lyapunov theory is used to verify the asymptotic stability of the control law. The performance of the proposed control method is tested using a simulated planar robot with two rotational degrees of freedom for high and low joint stiffness.

Minimal parametrization regulators for MIMO “strict feedback form” systems

Abstract In this paper it will be considered a class of MIMO nonlinear systems in “strict feedback form” affected by a noun bounded matrix of time dependent disturbances. It will be assumed the matrix norm to be unknown, and it will be constructed a robust controller whose dynamics turn out to be minimal, yielding with this respect the best possible control strategy. It is also shown how this strategy can be applied to the elastic joint robot model simplified dynamic equations, and found a global state space coordinate transformation and a state feedback such that the system dynamics can be expressed in “strict feedback form”.