Direct-drive Manipulator Research Articles

An asinh-type regulator for robot manipulators with global asymptotic stability

In this paper, a new asinh-type control scheme with gravity compensation for the position control problem of robot manipulators in joint space is presented. The properties and characteristics of the asinh control structure make the position error and the motion velocity asymptotically converge to the equilibrium point. A strict Lyapunov function to formally prove global asymptotic stability is developed. The tuning of the control gains is obtained by PSO (Particle Swarm Optimization) technique without saturating the servomotors. To illustrate the effectiveness and performance of the proposed scheme, an experimental comparative analysis between the proportional-derivative (PD) and atanh controls against the proposed algorithm on a three degrees of freedom direct-drive robot manipulator is carried out.

Backstepping design for output feedback stabilization for a class of uncertain systems

We construct bounded globally asymptotically stabilizing output feedbacks for a family of nonlinear systems, using a dynamic extension and a converging-input-converging-state assumption. We provide sufficient conditions for this assumption to hold, in terms of Lyapunov functions. The novelty is that our construction provides formulas for the control bounds while allowing uncertainties that prevent the use of classical backstepping, and cases where only part of the state variable is available for measurement, without requiring the time lagged states in the feedback control that were required in the artificial delays approach. We illustrate the relevance of our work to engineering in an application to a single-link direct-drive manipulator.

Object Picking Through In-Hand Manipulation Using Passive End-Effectors With Zero Mobility

This letter presents a new method for picking objects through quasistatic in-hand manipulation with end-effectors that have no degrees of freedom. Our stable manipulation/grasp planning is achieved with two contacts at fixed distance and the force of gravity, and is provably complete and correct. Practically, our robotic in-hand manipulation technique can facilitate low-cost manipulation and render simple end-effectors such as the parallel-jaw gripper more dexterous in picking, placing, and handing-off objects in case, for example, failing actuated grippers become frozen and it is impossible to exert sufficiently large internal forces. Our picking technique can also be applied to placing objects. Implementation and experimentation on a custom-made direct-drive manipulator and a conventional manipulator are presented. Many experiments were performed for both picking and placing and the results demonstrate usefulness of this method for manipulating a variety of common objects, with different geometries and surface conditions.

A Family of Hyperbolic-Type Explicit Force Regulators with Active Velocity Damping for Robot Manipulators

This paper addresses the explicit force regulation problem for robot manipulators in interaction tasks. A new family of explicit force-control schemes is presented, which includes a term driven by a large class of saturated-type hyperbolic functions to handle the force error. Also, an active velocity damping term with the purpose of obtaining energy dissipation on the contact surface is incorporated plus compensation for gravity. In order to ensure asymptotic stability of the closed-loop system equilibrium point in Cartesian space, we propose a strict Lyapunov function. A force sensor placed at the end-effector of the robot manipulator is used in order to feed back the measure of the force error in the closed-loop, and an experimental comparison of the performanceL2-norm between 5 explicit force control schemes, which are the classical proportional-derivative (PD), arctangent, and square-root controls and two members of the proposed control family, on a two-degree-of-freedom, direct-drive robot manipulator, is presented.

Predictive current control of a switched reluctance machine in the direct-drive manipulator of cloud robotics

Cloud robotics has undergone rapid development. As an important candidate for direct-drive manipulator, switched reluctance machines (SRMs) face significant challenge in terms of control used in cloud robotics because of latency and package losses in network communication. In this paper, predictive current control of SRMs is extended to use in controller upon cloud in the face of latency and package losses. The starting point of predictive model is modified to eliminate errors caused by latency in sensor-controller communication, and the execution of control command sequence is dynamically regulated according to the arrival time of the following sequence to adapt for latency and package losses in controller-actuator communication. The proposed control method is evaluated in a 1.5 kW SRM test platform and comparison with a conventional control method is performed; the results show that the proposed control method has better tracking performance in face of time delay and package losses in transmission.

Output-feedback proportional-integral-derivative-type control with multiple saturating structure for the global stabilization of robot manipulators with bounded inputs

An output-feedback proportional integral derivative-type control scheme for the global regulation of robot manipulators with constrained inputs is proposed. It guarantees the global stabilization objective—avoiding input saturation—releasing the feedback not only from the exact knowledge of the system structure and parameter values but also from velocity measurements. With respect to previous approaches of the kind, the proposed controller is expressed in a generalized form whence multiple saturating structures may be adopted, thus enlarging the degree of design flexibility. Furthermore, experimental tests on a two-degree-of-freedom direct-drive manipulator corroborate the efficiency of the proposed scheme.

Output‐feedback proportional–integral– derivative‐type control with simple tuning for the global regulation of robot manipulators with input constraints

An output-feedback proportional-integral-derivative-type control scheme for the global position stabilisation of robot manipulators with bounded inputs is proposed. It guarantees the global regulation objective avoiding input saturation by releasing the feedback not only from the exact knowledge of the system structure and parameter values, but also from velocity measurements. With respect to previous approaches of the kind, the proposed scheme remains simple while increasing design/performance-adjustment flexibility. For instance, it does not impose the use of a specific sigmoidal function to achieve the required boundedness but involves a generalised type of saturation functions. More importantly, it is characterised by a very simple control-gain tuning criterion, the simplest hitherto obtained in the considered analytical context. Experimental tests on a 2-degree-of-freedom direct-drive manipulator corroborate the efficiency of the developed scheme.

A generalised PID-type control scheme with simple tuning for the global regulation of robot manipulators with constrained inputs

In this paper, a globally stabilising PID-type control scheme with a generalised saturating structure for robot manipulators under input constraints is proposed. It gives rise to various families of bounded PID-type controllers whose implementation is released from the exact knowledge of the system parameters and model structure. Compared to previous approaches of the kind, the proposed scheme is not only characterised by its generalised structure but also by its very simple tuning criterion, the simplest hitherto obtained in the considered analytical framework. Experimental results on a 3-degree-of-freedom direct-drive manipulator corroborate the efficiency of the proposed approach.

An Output-Feedback PID-type Global Regulator for Robot Manipulators with Bounded Inputs

An output-feedback PID-type controller for the global stabilization of manipulators with bounded inputs is proposed. It guarantees the global regulation goal preventing input saturation while avoiding the exact knowledge of the system model and parameter values as well as the need for velocity measurements. Furthermore, it adopts an SPD-SI structure by keeping both the P and D actions together within a generalized saturation function while including an additional similar saturating integral action separately. So far, a formal analytical formulation for such a saturating PID-type control structure was not available under the absence of velocity measurements. Experimental results on a 2-degree-of-freedom direct-drive manipulator corroborate the efficiency of the proposed controller. Copyright© 2015 IFAC

Model-based control of a class of voltage-driven robot manipulators with non-passive dynamics

A class of voltage-driven robot manipulators is addressed in this paper. Such a class includes those manipulators actuated by direct current (DC) motors with servo-amplifiers in current mode. For those robots, it is assumed that the control inputs to the manipulator are the servo-amplifier input voltages, instead of the applied torques. And even though it is common to consider that the applied torque in each joint is proportional to the servo-amplifier input voltage, in this paper it is assumed that the voltage-to-torque conversion constant is unknown. Notwithstanding, the robot model parameters may still be computed using standard identification methods. An important fact of the class of robots considered in the present study is that the robot dynamics is not passive from the servo-amplifier input voltage to the joint velocity. The paper shows how this drawback can be overcome by using a model-based trajectory tracking controller, which makes the closed-loop system passive from an auxiliary control input to a filtered error signal. To confirm the theoretical conclusions, real-time experiments are carried out in a two-degree-of-freedom direct-drive manipulator. Two known control strategies are compared with respect to the proposed controller. The success of the implementations is based on real-time computing, which ensures that the access to I/O ports and the computation of the control algorithms meet timing requirements accurately.

Velocity field control of a class of electrically-driven manipulators

This article addresses the control of robotic manipulators under the assumption that the desired motion in the operational space is encoded through a velocity field. In other words, a vectorial function assigns a velocity vector to each point in the robot workspace. Thus, the control objective is to design a control input such that the actual operational space velocity of the robot end-effector asymptotically tracks the desired velocity from the velocity field. This control formulation is known in the literature as velocity field control. A new velocity field controller together with a rigorous stability analysis is introduced in this article. The controller is developed for a class of electrically-driven manipulators. In this class of manipulators, the passivity property from the servo-amplifier voltage input to the joint velocity is not satisfied. However, global exponential stability of the state space origin of the closed-loop system is proven. Furthermore, the closed-loop system is proven to be and output strictly passive map from an auxiliary input to a filtered error signal. To confirm the theoretical conclusions, a detailed experimental study in a two degrees-of-freedom direct-drive manipulator is provided. Particularly, experiments consist of comparing the performance of a simple PI controller and a high-gain PI controller with respect to the new control scheme.

Time-varying sliding-mode control for finite-time convergence

This paper proposes a time-varying sliding-mode control scheme, in which a piecewise defined function of time that contains trigonometric functions is incorporated into the switching function not only to eliminate the reaching phase, but also to guarantee finite-time convergence of the tracking error. Without using fractional-power functions of certain state variable, the proposed control is both free from singularity around the equilibrium and also convenient for practical implementation. Moreover, the proposed scheme has the terminal time as an explicit parameter, which makes the controller design simple to meet the terminal-time requirement. Experiments were conducted on a two-link direct-drive manipulator to demonstrate the effectiveness of the proposed scheme.

Theory and experiments of global adaptive output feedback tracking control of manipulators

A new adaptive controller for robot manipulators is proposed. The new approach uses only position measurements. The main conclusions derived from the closed-loop system analysis are in two main results. In the first one, the global convergence of the position and velocity tracking errors is stated by using a condition that relates the viscous friction damping and the desired joint speed. In the second one, such a condition is dropped out but the local exponential stability of the closed-loop system is shown. To confirm the theoretical conclusions, a detailed experimental study in a two degrees-of-freedom direct-drive manipulator is provided, where the performance of the new controller is compared with respect to a known output feedback adaptive controller.

Integral variable-structure control with variable-structure sliding dynamics for antireset windup

This paper proposes a scheme of integral variable-structure control with variable-structure sliding dynamics, referred to as VSSD-IVSC, based on the framework of the so-called global sliding-mode control (GSMC) technique. Previous GSMC schemes encompass the robust eigenvalue-assignment GSMC (REA-GSMC) and the integral variable-structure control (IVSC). All GSMC schemes feature the insurance of existence of a sliding mode throughout an entire response for performance robustness. It is shown in this paper that both REA-GSMC and IVSC lead to the same robust tracking performance in theory. In practical implementations the IVSC has better steady-state error characteristics than the REA-GSMC. However, due to control saturation, the IVSC may suffer from the integrator windup problem common to control methods with integral compensation. The VSSD-IVSC scheme is proposed to combine the REA-GSMC and the IVSC using the GSMC framework, and it has variable-structure sliding dynamics for antireset windup. Experiments were conducted on a two-link direct-drive manipulator to demonstrate the effectiveness of the proposed scheme.

Heavy tools manipulation by low powered direct-drive five-bar parallel robot

This paper presents a simple and available system for manipulation of heavy tools by low powered manipulator for industrial applications. In the heavy manufacturing industries, sometimes, heavy tools are employed for different types of work. But the application of robots with heavy tools is not possible due to the limited torque limits of actuators. Suspended tool systems (STS) have been proposed to manipulate heavy tools by low powered robot-arm for this purpose. A low powered five-bar direct-drive parallel manipulator is designed and constructed to manipulate heavy tools suspended from a spring balancer. The validity, usefulness, and effectiveness of the suspended tool system are shown by experimental results.

Angular acceleration estimation and feedback control: An experimental investigation

Angular acceleration estimation and its application in acceleration feedback control are investigated experimentally in the paper. In combination of Newton Predictor (NP) with Kalman Filter (KF), a new predictive estimator for angular acceleration, called Newton Predictor Enhanced Kalman Filter (NPEKF) is proposed. This estimator provides a wide bandwidth and a small phase lag of the estimated acceleration while attenuating noises. Based on the estimated acceleration an acceleration feedback control (AFC) is presented for multiple degree-of-freedom (DOF) mechatronic system. The design of AFC is specified in terms of its stability and ability in suppressing dynamic disturbances. Experiments are conducted on a 2-DOF direct-drive manipulator. The frequency responses of the acceleration estimated by NP, KF and NPEKF are compared with those of the measured acceleration via linear accelerometer. The performance of AFC using the estimated acceleration is assessed against that using the measured acceleration. This study has shown that the proposed NPEKF estimator is able to supply the AFC with reliable required acceleration.

Trajectory for saving energy of a direct-drive manipulator in throwing motion

In this article, equations of motion of a closed-type manipulator, whose mechanism can easily be made lighter, are derived in consideration of the characteristics of the driving source. Considering the final condition of angular displacement and angular velocity in throwing motion, trajectories of velocity for saving energy are calculated by iterative dynamic programming, and the dynamic characteristics of manipulator control based on the trajectory for saving energy are analyzed theoretically and investigated experimentally.

2A1-B06 外力に感応するアドミッタンス制御の特性と構造(安全・安心なロボットを目指して)

Mechanical impedance control schemes are classified into torque-control-based and position-control-based. The former is used for a direct-drive manipulator. The latter is also called Admittance Control and often used for a geared manipulator, which control the position determined from a force sensor. Since the admittance control uses the high-gain position controller, the manipulator hardly move against the force without involving the force sensor. In this paper, we show the characteristics and structure of admittance control responsive to unmeasurable external force.

Rapid Prototyping of a Model-Based Control With Friction Compensation for a Direct-Drive Robot

The development and application of the most recent model-based control schemes for robots require the investigation and solution of problems concerning various aspects, from the real-time (RT) simulation and control issues, to the necessity of determining a robot model suitable for control, and of experimentally testing the control performances. In this paper, different aspects are investigated for a planar, two-link direct-drive manipulator, with particular attention to the joint friction compensation within the control loop. A real-time architecture, based on the RT-Lab software by Opal-RT, is developed and used to carry out all the design phases, from the identification of the robot model, including joint friction torques, to the application of the inverse dynamics control schemes, with different solutions for the robot dynamics and friction compensation. The performances of such schemes are investigated executing various trajectories, suitable to check the effectiveness of the friction compensation

EXPERIMENTAL STUDY OF ANGULAR ACCELERATION ESTIMATION BASED ON KALMAN FILTER AND NEWTON PREDICTOR

Algorithms for angular acceleration estimation are investigated. Based on the analyses of recursive linear smoothed Newton predictor (RLSN) and Kalman filter (KF), a new predictive filter is proposed for acceleration estimation by combining the KF and Newton predictor (KFNP) together. This intends to achieve a wide bandwidth of estimated acceleration by reducing the phase lag introduced by filtering, and at the same time, attenuating noises. Extensive experiments are conducted on the first joint of 2-DOF direct-drive manipulator. The frequency responses of the acceleration are estimated by RLSN, KF, as well as KFNP are measured and respectively compared with the one of the accelerometer output The results demonstrate the consistency between estimated and measured acceleration, and further reveal the possibility of utilizing estimated acceleration for acceleration feedback control instead of a measure one.