Joint Space Control Research Articles

Enhanced Whole-Arm Robot Teleoperation Using a Semi-Autonomous Control Policy

Previous work in robot teleoperation focused on the movement of a robot's end-effector by a human operator. However, a lack of pose control in teleoperation resulted in the robot arm frequently colliding with obstacles. Furthermore, even with pose control, it is still difficult for the robot to quickly and accurately move to the target due to mechanical discrepancies between human and robot. This paper proposes a semi-autonomous method to teleoperate the robot arm by integrating whole-arm teleoperation in joint-space control and autonomous end-effector position control. The proposed method is validated through experimental work on a robot arm with 6 degrees of freedom, with results showing significant improvement in human control for reaching for objects safely, quickly, and accurately.

Robust tracking control of 6-DOF parallel electrical manipulator in Joint-Task space with fast friction estimation

This paper presents a robust tracking controller equipped with a fast friction estimator for a 6-DOF parallel electrical manipulator in the joint-task space. Parallel manipulator control scheme mainly stems from two frameworks: the task space control and the joint space control. The former requires the system states obtained via costly direct measurements or time-consuming forward kinematics. The latter's performance is subject to the inaccurate dynamic model. To minimize their weaknesses, the joint-task space framework is constructed by transforming the dynamics on the task space into the joint space. With this transformation, the desired positional data are used to calculate the nominal value of dynamics. On the other hand, the friction property depends on the uncertain load condition due to the spiral drive way of the electrical cylinders. A fast friction estimator is then designed to estimate the uncertain friction fast and effectively. The robust control scheme is proposed based on the Lyapunov design method to guarantee a practical stability under uncertainties such as inertia, modeling errors, friction, and measurement errors. Experimental results are presented to show the effectiveness of the proposed scheme.

Design of the Fuzzy Control Systems Based on Genetic Algorithm for Intelligent Robots

This paper gives the structure optimization of fuzzy control systems based on genetic algorithm in the MATLAB environment. The genetic algorithm is a powerful tool for structure optimization of the fuzzy controllers, therefore, in this paper, integration and synthesis of fuzzy logic and genetic algorithm has been proposed. The genetic algorithms are applied for fuzzy rules set, scaling factors and membership functions optimization. The fuzzy control structure initial consist of the 3 membership functions and 9 rules and after the optimization it is enough for the 4 DOF SCARA Robot control to compensate for structured and unstructured uncertainty. Fuzzy controller with the generalized bell membership functions can provide better dynamic performance of the robot then with the triangular membership functions. The proposed joint-space controller is computationally simple and had adaptability to a sudden change in the dynamics of the robot. Results of the computer simulation applied to the 4 DOF SCARA Robot show the validity of the proposed method.

Reducing Intersample Ripple of Actuator in Multirate Environment by A Joint Space Interpolator

Chattering is a very undesired phenomenon in technical systems. There can be many causes for it. In this paper, at first simulation results are shown that insufficient setpoint updating can also lead to chattering in form of intersample ripple. The difference between setpoint updating rate and the control frequency can be traced back to the multi-layered software architecture of an overall system. As a solution, a joint space interpolator is suggested, which is a novel construction mainly combining a trajectory interpolator with a jitter buffer in joint space. On the one hand it provides supplementary setpoints to the controller and on the other hand it ensures very strict real-time performance of the closed-loop system. At last, experiment results are shown to support the proposed method. This scheme is implemented for, but not limited to, actuators utilizing geared brushless DC (BLDC) motors with FPGA-based controllers. The simulations and experiments have focus on robotic motion control in joint space.

Design System and Simulation Control of the 6-DOF Parallel Manipulator

In this paper presents the design system and simulation control of the 6-DOF parallel manipulator. The moveable platform supported by the six electrical cylinder. The mechanic component and kinematics model are explained, dynamics model using Lagrangian equation and computer model of Joint space control scheme using Simulink and the 6-DOF parallel manipulator systems are show in this paper.

Optimal Joint Space Control Design of a Hydraulic Stewart Manipulator

This paper presents an algorithm to develop a mission-based optimal joint space control for a Stewart manipulator. The proposed algorithm consists of two optimization phases. The first phase seeks an inverse kinematic model and controller for the closed loop control of a Stewart manipulator using a feedback value. The second phase optimally tunes the controller called amplitude phase control (apc) in order to meet special mission requirements. Iteration algorithm is used in this phase as the optimization method. The proposed amplitude phase control optimal joint space control shows the capability to reduce the error in tracking the sin signals.

An Approach to Force-Sensorless Bilateral Control in Master-Slave System with Different Configurations

In this paper, force-sensorless bilateral control methods for a master-slave system with different configurations are proposed. Two types of bilateral control architectures are presented. One is based on joint space control, and the other is based on work space control. This paper discusses the difference between these control methods from the viewpoint of motion behavior in the singular configuration. Finally, actual experimental results that confirm the validity of the proposed methods are shown.

Observer-based cascade control of a 6-DOF parallel hydraulic manipulator in joint space coordinate

This paper addresses the joint space control problem of a 6-DOF (degree of freedom) parallel hydraulic manipulator. High precision motion control of a six-degree parallel manipulator is hardly achieved due to the existence of uncertain payload and other disturbance such as coupling force. A disturbance observer for this parallel manipulator is first constructed to estimate and compensate the unknown disturbance. A cascade control algorithm is then applied to separate the hydraulic dynamics from the mechanical part, which can mask the hydraulic dynamics with an inner loop. With such a control structure, known control design methods within the area of manipulator control can be directly used in the outer loop. In this approach, the complex dynamics and direct kinematics of the parallel manipulator are not required and acceleration feedback is also avoided. Experimental results are presented to show the effectiveness of the proposed scheme.

Natural ZMP Trajectories for Biped Robot Reference Generation

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> The control of a biped humanoid is a challenging task due to the hard-to-stabilize dynamics. Walking reference trajectory generation is a key problem. Linear Inverted Pendulum Model (LIPM) and Zero Moment Point (ZMP) Criterion-based approaches in stable walking reference generation are reported. In these methods, generally, the ZMP reference during a stepping motion is kept fixed in the middle of the supporting foot sole. This kind of reference generation lacks naturalness, in that the ZMP in the human walk does not stay fixed, but it moves forward under the supporting foot. This paper proposes a reference generation algorithm based on the LIPM and moving support foot ZMP references. The application of Fourier series approximation simplifies the solution, and it generates a smooth ZMP reference. A simple inverse kinematics-based joint space controller is used for the tests of the developed reference trajectory through full-dynamics 3-D simulation. A 12-DOF biped robot model is used in the simulations. Simulation studies suggest that the moving ZMP references are more energy efficient than the ones with fixed ZMP under the supporting foot. The results are promising for implementations. </para>

Task-space position/attitude tracking control of FAST fine tuning system

This paper addresses the task-space position/attitude tracking control of a suspended parallel robot, which is being developed in the framework of a five-hundred meter aperture spherical radio telescope (FAST) project. Based on the quaternion algebra, a novel modelindependent task-space PD controller that ensures endeffector position and attitude tracking is presented. The simulation and experimental results show that the new controller has better control effects than the traditional joint-space controller.

PASSIVITY ANALYSIS OF A MOTION CONTROL FOR ROBOT MANIPULATORS WITH DYNAMIC FRICTION

ABSTRACTThis paper addresses the motion control in joint space of robot manipulators with friction described by the generalized LuGre friction model. The paper extends the nonadaptive version of the Slotine and Li's control system originally designed for friction‐free manipulators to the case where a friction observer is incorporated to deal with friction. This observer corresponds to an adaptation of the friction compensation scheme proposed in Canudas de Wit et al., in 1975, 1995. Passivity concepts are the fundamental tools invoked in this paper to analyze the closed‐loop system behavior which lead to the conclusion of global asymptotic joint position tracking. A major advantage of this framework is that it allows to develop in a separate way the control law from the observer design provided that each part satisfies some passivity properties.

PD Controller for Manipulator with Kinetic Energy Term

A kinetic energy approach to PD control in joint space of a manipulator in terms of the eigenfactor quasi-coordinate velocity (EQV) vector is considered in this paper. The modified PD controller which contains quantities resulting from decomposition of a manipulator mass matrix is proposed. The obtained equations of motion are based on the eigenvalues and eigenvectors of the mass matrix (Junkins, J. L. and Schaub, H. [7]. It is shown that utilizing the EQV vector one can determine directly the kinetic energy of the manipulator and at the same time realize PD control in its joint space. This energy-based strategy gives an interesting insight into position control. The controller presented here was tested in simulation on a 3 d.o.f. direct drive arm manipulator and via experiment on a 2 d.o.f. manipulator. The results confirmed that one can directly determine the kinetic energy for the total manipulator as well for its each joint. Additionally, time response of the system under EQV controller is faster than for the classical controller if mechanical coupling are strong.

NOVEL JOINT SPACE FORCE GUIDANCE ALGORITHM WITH LABORATORY ROBOT SYSTEM

This article presents the implementation of a new algorithm of force guided motions with a six axis articulated robot frequently used in research laboratories. This new approach is based on the idea of impedance control in joint space and it is implemented on a digital signal processor-based robot controller. It allows an intuitive force guidance of the robot by taking the gripper by the hand. Robot may be also guided over the singularities in this way. Behaviour of the robot is thus freely programmable in the wide range.

Impedance control for a pneumatic robot-based around pole-placement, joint space controllers

Traditionally the control of pneumatic actuators has been limited to movement between pre-set stops or switches. Modern pneumatic systems exhibit great power to weight ratio, are backdrivable and can be precision controlled. As such these actuators offer great advantages over direct drive electric motors for robotic devices. Increasingly robots are required to interact directly with humans requiring control of not only the robot position but also taking into account the applied forces. Robotic physiotherapy is one such application. This paper describes the implementation of a force and position control strategy, termed impedance control, on a pneumatic robot designed to implement robotic physiotherapy. The controller is shown to work appropriately over a range of conditions. The performance achieved is limited by the quality of force measurement in multiple axes.

Robust nonlinear control of a 6 DOF parallel manipulator : Task space approach

This paper presents a robust nonlinear controller for a 6 degree of freedom (DOF) parallel manipulator in the task space coordinates. The proposed control strategy requires information on orientations and translations in the task space unlike the joint space or link space control scheme. Although a 6 DOF sensor may provide such information in a straightforward manner, its cost calls for a more economical alternative. A novel indirect method based on the readily available length information engages as a potential candidate to replace a 6 DOF sensor. The indirect approach generates the necessary information by solving the forward kinematics and subsequently applying alpha-beta-gamma tracker. With the 6 DOF signals available, a robust nonlinear task space control (RNTC) scheme is proposed based on the Lyapunov redesign method, whose stability is rigorously proved. The performance of the proposed RNTC with the new estimation scheme is evaluated via experiments. First, the results of the estimator are compared with the rate-gyro signals, which indicates excellent agreement. Then, the RNTC with on-line estimated 6 DOF data is shown to achieve excellent control performance to sinusoidal inputs, which is superior to those of a commonly used proportional-plus-integral-plus-derivative controller with a feedforward friction compensation under joint space coordinates and the nonlinear controller under task space coordinates.

Development of a sliding-leg tripod as an add-on device for manufacturing

This paper presents the work on developing a sliding-leg tripod as a programmable add-on device for manufacturing. The purpose is to enhance the capabilities of any machine by providing it with a more flexible range of motion. This device can be used as a toolhead for CNC machine tools and robots, or as a work stage for coordinate measuring machines and laser scanning systems. In this paper, system modelling, analysis and control of this device is presented. System modeling includes mobility study, kinematic model and inverse kinematics. System analysis includes workspace analysis, transmission ratio and stiffness analysis. System control includes path planning, joint space control and Cartesian space prediction. It is shown that the proposed device can provide flexibility and dexterity to machines.

An inverse kinematics algorithm for interaction control of a flexible arm with a compliant surface

This paper deals with the problem of controlling the interaction of a multilink flexible arm in contact with a compliant surface. For a given tip position and surface stiffness, the joint and deflection variables are computed using a closed-loop inverse kinematics algorithm. This is based on a suitable Jacobian matrix which includes terms accounting for the static deflections due to gravity and contact force. The computed variables are used as the set-points for a simple joint PD control, thus achieving regulation of the tip position and contact force via a joint-space controller. The scheme is tested in a simulation case study for a planar two-link manipulator.

A new nonlinear gain structure for PD-type controllers in robotic applications

This article introduces a generic enhancement to certain proportional plus differential (PD)-type, joint-space controllers for robotic manipulators that can be utilized to reduce peak levels of servo output without increasing the duration of the transient response. This enhancement takes the form of nonlinear, partially hyperbolic multipliers that are applied to re-scale the error signal gains. The rationale prompting the design of the aforementioned gain structure is discussed at length, and a mathematical formulation of the modified control law is presented for application to both position and full trajectory control. The stability of the suggested control strategy is addressed, and numerical simulations involving a three degree of freedom (3-DOF) robot model are presented to demonstrate quantitatively the potential benefits of implementing such. ©1999 John Wiley & Sons, Inc.

The uncontrolled manifold concept: identifying control variables for a functional task.

The degrees of freedom problem is often posed by asking which of the many possible degrees of freedom does the nervous system control? By implication, other degrees of freedom are not controlled. We give an operational meaning to "controlled" and "uncontrolled" and describe a method of analysis through which hypotheses about controlled and uncontrolled degrees of freedom can be tested. In this conception, control refers to stabilization, so that lack of control implies reduced stability. The method was used to analyze an experiment on the sit-to-stand transition. By testing different hypotheses about the controlled variables, we systematically approximated the structure of control in joint space. We found that, for the task of sit-to-stand, the position of the center of mass in the sagittal plane was controlled. The horizontal head position and the position of the hand were controlled less stably, while vertical head position appears to be no more controlled than joint motions.

Stability analysis of a joint space control law for a two-manipulator system

The control problem for a truly cooperative manipulator system cannot be regarded as a simple extension of single manipulator control since additional complexity arises from the need for ensuring coordinated motion of the system. Recently, we have proposed a control strategy based on two stages, namely, kinematic inversion of a set of cooperative task variables and a joint space control law with kinetostatic filtering of cooperative task space errors; this strategy avoids some of the drawbacks of other approaches. In this paper, the stability analysis for the joint space control law is provided and asymptotic stability of a set of equilibrium points is demonstrated. The case of imperfect compensation of the gravity terms has been analyzed. The addition of a force loop to achieve internal force regulation is also discussed.