Direct-drive Robot Research Articles

New energy-shaping based control plus an integral action for torque-driven robot manipulators using coordinate changes

This paper presents a robust energy shaping control for joint position tracking of torque-driven robot manipulators against disturbances. The controller design is based on an alternative energy shaping method, and an integral action is added to the theoretical framework as a novel strategy. Our contribution is to globally achieve joint position tracking of torque-driven robot manipulators, even in the presence of constant disturbances. The performance of the proposed controller is illustrated through experiments of a two degrees of freedom direct-drive robot arm.

PD with neuro-adaptive compensation control using the signed power function

This paper presents a neuro-adaptive control scheme dedicated to solve the motion trajectory tracking problem of robot manipulators under uncertain parameters and external disturbances. A two degrees of freedom direct-drive robot manipulator was taken as a case of study. The proposed controller is able to guarantee asymptotic convergence of the position and velocity tracking errors, and the weights of the artificial neural network are bounded. Artificial neural network weights are updated online using filtered error approach, adaptive laws and signed power function. This scheme does not require any offline training. The neuro-adaptive controller is experimentally validated and compared with a classical one-layer neuro-adaptive controller; the proposed scheme obtains better quantitative metrics related with the RMS values of the position and velocity errors, and a good robust behaviour against external disturbances.

Nonlinear PID global regulators with selftuned PD gains for robot manipulators: theory and experimentation

It is known that the Proportional Integral and Derivative (PID) regulator with constant and symmetric positive definite gain matrices (Kp, Kv and Ki), in closed loop with robotic manipulators, guarantees semiglobal asymptotic stability of the equilibrium point. In this paper, a fresh approach for control of robots manipulators based on a class of PID regulators with variable gain matrices Kp and Kv, which guarantee global asymptotic stability, is proposed. This PID control scheme, proposed with PD variable gains and fixed integral gains, improves the performance of the transient response and the handling of constraints, such as the limits of the actuator torques. Based on the authors experience, our proposal is the first scheme of a nonlinear PID regulator for robot manipulators whose PD gains are variable and its stability proof is global asymptotic in the sense of Lyapunov. The PD variable gains are directly dependent on the joint position errors and are fine tuned by fuzzy logic methodology. Good results were obtained on an experimental direct-drive robot arm of two degrees of freedom, when it was tested in order to verify the effectiveness of the new controller.

Global Saturated Regulator with Variable Gains for Robot Manipulators

In this paper, we propose a set of saturated controllers with variable gains to solve the regulation problem for robot manipulators in joint space. These control schemes deliver torques inside the prescribed limits of servomotors. The gamma of variable gains is formed by continuous, smooth, and differentiable functions of the joint position error and velocity of the manipulator. A strict Lyapunov function is proposed to demonstrate globally asymptotic stability of the closed-loop equilibrium point. Finally, the functionality and performance of the proposal are illustrated via simulation results and comparative analysis against Proportional-Derivative (PD) control scheme on a two-degrees-freedom direct-drive robot manipulator.

Global Saturated Regulator with Variable Gains for Robot Manipulators

In this paper, we propose a set of saturated controllers with variable gains to solve the regulation problem for robot manipulators in joint space. These control schemes deliver torques inside the prescribed limits of servomotors. The gamma of variable gains is formed by continuous, smooth, and differentiable functions of the joint position error and velocity of the manipulator. A strict Lyapunov function is proposed to demonstrate globally asymptotic stability of the closed-loop equilibrium point. Finally, the functionality and performance of the proposal are illustrated via simulation results and comparative analysis against Proportional-Derivative (PD) control scheme on a two-degrees-freedom direct-drive robot manipulator.

A GUAS Joint Position Tracking Controller of Torque-Driven Robot Manipulators Influenced by Dynamic Dahl Friction: Theory and Experiments

This article presents a controller for global joint position tracking control of torque-driven robot manipulators involving dynamic Dahl friction. The control law has been designed using a novel theoretical Hamiltonian inspired framework, which allows global uniform asymptotic stability (GUAS) of the nonautonomous nonlinear closed-loop system. An observer is provided to estimate the nonmeasurable state of internal friction of the Dahl model, preserving the GUAS attribute in the closed-loop system. The performance of the proposed control scheme is illustrated through experiments on a two-degree-of-freedom direct-drive robot arm.

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.

A Fuzzy Sliding Mode Controller for Planar 4-Cable Direct Driven Robot

Cable Direct Driven Robots (CDDRs) are a special class of parallel robots but they are formed by replacing all the supporting rigid links with cables. Compare with traditional robots, these robots are good candidates for performing a wide range of potential applications. A Planar CDDR model is considered in this paper since no rotational move and no moment resistance are required on the end-effector, all 4 cables convene in a single point and the end-effector is modeled as a point mass. The main goal of this paper is to present a new approach in control by developing a Sliding Mode Controller (SMC) with a Fuzzy-PI as sliding surface using Fuzzy logic toolbox in Matlab/Simulink. The tests performed were Step change reference test and Tracking trajectory test to observe the behavior of the cables during the trajectory and the end-effector movement. Simulation was carried out on Planar 4-Cable CDDR to prove the effectiveness of the proposed control law and the results were compared with a PI Controller and a conventional SMC in terms of integral square error (ISE) index. Only the kinematic model of Planar 4-Cable CDDR is considered in this paper.

Design of a Double-Stator Magnetless Vernier Machine for Direct-Drive Robotics

This paper proposes a new double-stator magnetless vernier (DSMV) machine for direct-drive robotics. By artfully incorporating the double-stator structure, vernier configuration and dc excitation together, this DSMV machine yields the merits of the flux control capability, low-speed operation, high torque output, free maintenance, and high cost-effectiveness. Also, the optimizations of the rotor-teeth arc angle, stator-teeth arc angle, and current phase angle are carried out for the improvement of torque performance. In this way, the torque ripple of the machine is significantly reduced up to a tiny value of 3.67%. Moreover, the rational dc current range and the rotational speed range are obtained through the performance analysis with flux regulation. In addition, the comparison is performed with the conventional surface-mounted permanent-magnet machine. Therefore, the proposed machine shows the distinct features and is fit for the special application. Finally, both theoretical derivation and simulation are given for the validation of the machine design.

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.

Regulación Saturada con Ganancia Variable Derivativa de Robots Manipuladores

In this paper a family with a large number of hyperbolictype saturated regulators for robot manipulators, was presented. The proposed regulators consider a constant proportional gain while the derivative variable gain is self-tuned according to a function that depends on the position error, speed of motion and a damping factor, in order to modify the velocity of the transient response of the robot. The derivative control with variable gain enables reduced overshots, oscillations and ripple, enabling a smooth arrival to the steady state. The paper also proposes a strict Lyapunov function which enables the demonstration of asymptotic global stability of the closed-loop equation. In order to illustrate the performance and functionality of the proposed family of control schemes, an experimental comparison between seven control schemes was implemented. Five of these control schemes belong to the proposed family while two additional control schemes are well-known strategies such as the proportional-derivative (PD) and hyperbolic tangent (Tanh) control schemes. The experiments were performed by using a three degree-of-freedom, direct-drive robot manipulator.

Adaptive Voltage-based Control of Direct-drive Robots Driven by Permanent Magnet Synchronous Motors

Tracking control of the direct-drive robot manipulators in high-speed is a challenging problem. The Coriolis and centrifugal torques become dominant in the high-speed motion control. The dynamical model of the robotic system including the robot manipulator and actuators is highly nonlinear, heavily coupled, uncertain and computationally extensive in non-companion form. In order to overcome these problems, this paper presents a novel adaptive control for direct-drive robot manipulators driven by Permanent Magnet Synchronous Motors (PMSM) in tracking applications. The novelty of this paper is that the proposed adaptive law is free from manipulator dynamics by using the Voltage Control Strategy (VCS). Additionally, a state space model of the robotic system driven by PMSM is presented. The VCS differs from the commonly used control strategy for robot manipulators the so called torque control strategy. The position control of the PMSM is effectively used for the tracking control of the robot manipulator. This idea takes the control problem from the manipulator control to the motor control resulting in a simple yet efficient control design. Compared with the torque control, the control design is simpler, easier to implement with better tracking performance. The control method is verified by stability analysis. Simulation results show superiority of the proposed control to the torque control applied by field oriented control on the direct-drive robot driven by PMSM.

Unbounded regulators with variable gains for a direct-drive robot manipulator

This paper addresses the position-control problem with variable gains for robot manipulators. We present a new regulator based on a hyperbolic-sine structure with tuning rules for control gains. It is demonstrated that the equilibrium point of the closed-loop system is globally, asymptotically stable according to Lyapunov theory. By using a similar methodology, this concept can be extended to other unbounded controllers such as PD and PID. In order to show the usefulness of the proposed scheme and with the purpose of validating its asymptotical stability property, an experimental comparison involving constant gains controllers, for example: simple PD, PID and hyperbolic-tangent schemes vs variable-gains hyperbolic-sine and PD control schemes, was performed by using a three degree-of-freedom, direct-drive robot manipulator.

Simultaneous fault diagnosis for robot manipulators with actuator and sensor faults

This paper studies a model-based fault detection and isolation (FDI) scheme for robot manipulators with actuator and sensor faults. Without adding redundant joint sensor measurements, the multiple faults simultaneously occurring in the actuators and sensors of the manipulator are detected, estimated and isolated. Considering Lipschitz-like nonlinearities and modeling uncertainties, a nonlinear adaptive observer is presented to estimate the faulty parameters with a pre-specified estimation error bound. The performances of the proposed FD scheme, including the robustness of observers to the uncertainties, the accuracy of fault estimation and the rapidity of fault diagnosis, are rigorously analyzed. The proposed approach is verified by a simulation of Integrated Motion Inc. (IMI) two-link, revolute, direct-drive robot manipulator.

Design Principles for a Family of Direct-Drive Legged Robots

This letter introduces Minitaur, a dynamically running and leaping quadruped, which represents a novel class of direct-drive (DD) legged robots. We present a methodology that achieves the well-known benefits of DD robot design (transparency, mechanical robustness/efficiency, high-actuation bandwidth, and increased specific power), affording highly energetic behaviors across our family of machines despite severe limitations in specific force. We quantify DD drivetrain benefits using a variety of metrics, compare our machines' performance to previously reported legged platforms, and speculate on the potential broad-reaching value of “transparency” for legged locomotion.

Unbounded regulators with variable gains for a direct-drive robot manipulator

This paper addresses the position-control problem with variable gains for robot manipulators. We present a new regulator based on a hyperbolic-sine structure with tuning rules for control gains. It is demonstrated that the equilibrium point of the closed-loop system is globally, asymptotically stable according to Lyapunov theory. By using a similar methodology, this concept can be extended to other unbounded controllers such as PD and PID. In order to show the usefulness of the proposed scheme and with the purpose of validating its asymptotical stability property, an experimental comparison involving constant gains controllers, for example: simple PD, PID and hyperbolic-tangent schemes vs variable-gains hyperbolic-sine and PD control schemes, was performed by using a three degree-of-freedom, direct-drive robot manipulator.

Voltage Control Strategy for Direct-drive Robots Driven by Permanent Magnet Synchronous Motors

Torque control strategy is a common strategy to control robotic manipulators. However, it becomes complex duo to manipulator dynamics. In addition, position control of Permanent Magnet Synchronous Motors (PMSMs) is a complicated control. Therefore, tracking control of robots driven by PMSMs is a challenging problem. This article presents a novel tracking control of electrically driven robots which is free from manipulator model. The proposed control law is simple but robust against uncertainties associated with manipulator dynamics. The novelty is the developing of voltage control strategy for the direct-drive robots driven by PMSMs. In addition, a state-space model is obtained for the robotic system including the direct-drive robot manipulator and the PMSMs. Then, the proposed approach is verified by stability analysis. A comparative study through simulations shows the superiority of the voltage control strategy to the torque control strategy.

On Explicit Force Regulation with Active Velocity Damping for Robot Manipulators

This paper presents a new interaction control structure that generates a family of explicit force regulators for robot manipulators. The proposed structure includes a term of a class of proportional-type functions in terms of force error; the force error is defined as the difference between a desired force and the actual force measured with a force sensor located at the end-effector. Also, the structure includes a generalized active velocity damping term in order to have a control of the energy dissipation, and a term used to compensate the gravity forces of the links. The stability analysis is performed in Lyapunov sense. An experimental comparison of two new explicit force regulators and the linear proportional structure, on a three degree-of-freedom, direct-drive robot, is presented. Also, proofs of the most important properties of the Cartesian dynamic model, are presented.

On Stiffness Regulators with Dissipative Injection for Robot Manipulators

The stiffness controller proposed by Salisbury is an interaction control strategy designed to achieve a desired form of static behavior as regards the interaction of a robot manipulator with the environment. The main idea behind this approach is the simulation of a multidimensional linear spring - or linear elastic material - using the difference between the actual position of the end-effector and a constant position (relaxed point), multiplied by a constant stiffness matrix. In this paper, this idea is generalized with the objective of proposing a controller structure that includes a family of stiffness models based on the idea of linear elastic materials. The new controller structure also includes a damping term in order to have control over energy dissipation, as well as a term added for the purpose of compensating the gravity forces of the links. The stability analysis of the proposed controller was performed in the Lyapunov sense. The new stiffness controller is presented as a case study and compared to other cases, such as the Salisbury controller (Cartesian PD) and the tanh-tanh controller. Experimental results using a three degrees-of-freedom direct-drive robot for the evaluation of controllers in a constrained motion task are presented.

Time-scale separation of a class of robust PD-type tracking controllers for robot manipulators

In this paper, the trajectory tracking control of robot manipulators is studied from the theoretical and practical point of view. By using the theory of singularly perturbed systems, a class of PD-type robust controllers is introduced. Our analysis departs from parameterizing the proportional and derivative gains with a perturbing parameter. We prove that the smaller the value of perturbing parameter, the smaller the ultimate bound of the joint position tracking error. Derived from the introduced analysis, two forms of extending the proposed class of controllers are discussed. In one, error-varying PD gains are considered while in the another one, a dynamic extension to avoid joint velocity measurements is incorporated. An experimental study in a planar two degrees-of-freedom direct-drive robot is also presented. Under similar implementation conditions, four controllers are tested. The best performance is obtained for a nonlinear PD controller derived from the proposed class of controllers.