End-effector Motion Research Articles

Switch-Based Sliding Mode Control for Position-Based Visual Servoing of Robotic Riveting System

The robotic riveting system requires a rivet robotic positioning process for rivet-in-hole insertions, which can be divided into two stages: rivet path-following and rivet spot-positioning. For the first stage, varying parameter-linear sliding surfaces are proposed to achieve robust rivet path-following against robot errors and external disturbances of the robotic riveting system. For the second stage, a second-order sliding surface is applied to attain accurate rivet spot-positioning within a finite time required by the riveting process. In order to improve the dynamic performance of the robot riveting system, the motion of robot end-effector between the two adjacent riveting spots has been properly designed. Overall, the proposed control scheme can guarantee not only the stability of the robot control system but also the robust rivet path-following and quick rivet spot-positioning in the presence of the robot errors and external disturbances of the robotic riveting system. The simulation and experimental results demonstrate the effectiveness of the proposed control scheme.

Optimal Trajectory Planning of Industrial Robots using Geodesic

<p>This paper intends to propose an optimal trajectory planning technique using geodesic to achieve smooth and accurate trajectory for industrial robots. Geodesic is a distance minimizing curve between any two points on a Riemannian manifold. A Riemannian metric has been assigned to the workspace by combining its position and orientation space together in order to attain geodesic conditions for desired motion of the end-effector. Previously, trajectory has been planned by considering position and orientation separately. However, practically we cannot plan separately because the manipulator joints are interlinked. Here, trajectory is planned by combining position and orientation together. Cartesian trajectories are shown by joint trajectories in which joint variables are treated as local coordinates of position space and orientation space. Then, the obtained geodesic equations for the workspace are evaluated for initial conditions of trajectory and results are plotted. The effectiveness of the geodesic method validated through numerical computations considering a Kawasaki RS06L robot model. The simulation results confirm the accuracy, smoothness and the optimality of the end-effector motion. </p>

Energy-optimal motions for Servo-Systems: A comparison of spline interpolants and performance indexes using a CAD-based approach

Position-controlled Servo-Systems (SeSs) may be envisaged as a key technology to keep the manufacturing industry at the leading edge. Unfortunately, based on the current state-of-the-art, these mechatronic devices are well performing but intrinsically energy intensive, thus compromising the overall system sustainability. Therefore, traditional design and optimization paradigms, previously focused on productivity and quality improvement, should be critically reviewed so as to introduce energy efficiency as an optimality criterion alongside with the global production rate. In particular, focusing on mono-actuator systems with one degree-of-freedom, among the several design factors that can influence the SeS overall performance, the end-effector motion law can be easily modified without either hardware substitution or further investments. In this context, the purpose of the present paper is twofold. On one side, an effective method for the quick set-up of an energy-predictive CAD-based virtual prototype is discussed. In parallel, an energy comparison of some commonly employed Point-To-Point motions and optimization cost functions is provided. For what concerns the trajectory interpolation scheme, a standard optimization problem based on the aforementioned virtual model is solved by means of either algebraic or trigonometric splines. For what concerns the optimality criterion, either the system energy consumption or the root-means square value of the actuator torque are taken into account. In general, torque-based approaches, which may be preferred since they do not require a full knowledge of the SeS electrical parameters, can be effectively employed only when friction effects are negligible as compared to purely inertial loads. In parallel, cubic algebraic splines outperform other types of trajectories, although losing continuity of the resulting jerk profile. HighlightsDescription of a CAD-based modeling approach for Position-Controlled Servomechanism.Comparison of various techniques for trajectory planning.Torque-based methods compared to energy-based methods.Results on a simulation case study: a slider crank mechanism connected to a PMSM.

Inverse Kinematics Based Human Mimicking System using Skeletal Tracking Technology

Humanoid robots needs to have human-like motions and appearance in order to be well-accepted by humans. Mimicking is a fast and user-friendly way to teach them human-like motions. However, direct assignment of observed human motions to robot's joints is not possible due to their physical differences. This paper presents a real-time inverse kinematics based human mimicking system to map human upper limbs motions to robot's joints safely and smoothly. It considers both main definitions of motion similarity, between end-effector motions and between angular configurations. Microsoft Kinect sensor is used for natural perceiving of human motions. Additional constraints are proposed and solved in the projected null space of the Jacobian matrix. They consider not only the workspace and the valid motion ranges of the robot's joints to avoid self-collisions, but also the similarity between the end-effector motions and the angular configurations to bring highly human-like motions to the robot. Performance of the proposed human mimicking system is quantitatively and qualitatively assessed and compared with the state-of-the-art methods in a human-robot interaction task using Nao humanoid robot. The results confirm applicability and ability of the proposed human mimicking system to properly mimic various human motions.

Joint-Angle Coordination Patterns Ensure Stabilization of a Body-Plus-Tool System in Point-to-Point Movements with a Rod.

When performing a goal-directed action with a tool, it is generally assumed that the point of control of the action system is displaced from the hand to the tool, implying that body and tool function as one system. Studies of how actions with tools are performed have been limited to studying either end-effector kinematics or joint-angle coordination patterns. Because joint-angle coordination patterns affect end-effector kinematics, the current study examined them together, with the aim of revealing how body and tool function as one system. Seated participants made point-to-point movements with their index finger, and with rods of 10, 20, and 30 cm attached to their index finger. Start point and target were presented on a table in front of them, and in half of the conditions a participant displacement compensated for rod length. Results revealed that the kinematics of the rod's tip showed higher peak velocity, longer deceleration time, and more curvature with longer rods. End-effector movements were more curved in the horizontal plane when participants were not displaced. Joint-angle trajectories were similar across rod lengths when participants were displaced, whereas more extreme joint-angles were used with longer rods when participants were not displaced. Furthermore, in every condition the end-effector was stabilized to a similar extent; both variability in joint-angle coordination patterns that affected end-effector position and variability that did not affect end-effector position increased in a similar way vis-à-vis rod length. Moreover, the increase was higher in those conditions, in which participants were not displaced. This suggests that during tool use, body and tool are united in a single system so as to stabilize the end-effector kinematics in a similar way that is independent of tool length. In addition, the properties of the actual trajectory of the end-effector, as well as the actual joint-angles used, depend on the length of the tool and the specifics of the task.

Position referenced force augmentation in teleoperated hydraulic manipulators operating under delayed and lossy networks: A pilot study

Position error between motions of the master and slave end-effectors is inevitable as it originates from hard-to-avoid imperfections in controller design and model uncertainty. Moreover, when a slave manipulator is controlled through a delayed and lossy communication channel, the error between the desired motion originating from the master device and the actual movement of the slave manipulator end-effector is further exacerbated. This paper introduces a force feedback scheme to alleviate this problem by simply guiding the operator to slow down the haptic device motion and, in turn, allows the slave manipulator to follow the desired trajectory closely. Using this scheme, the master haptic device generates a force, which is proportional to the position error at the slave end-effector, and opposite to the operator’s intended motion at the master site. Indeed, this force is a signal or cue to the operator for reducing the hand speed when position error, due to delayed and lossy network, appears at the slave site. Effectiveness of the proposed scheme is validated by performing experiments on a hydraulic telemanipulator setup developed for performing live-line maintenance. Experiments are conducted when the system operates under both dedicated and wireless networks. Results show that the scheme performs well in reducing the position error between the haptic device and the slave end-effector. Specifically, by utilizing the proposed force, the mean position error, for the case presented here, reduces by at least 92% as compared to the condition without the proposed force augmentation scheme. The scheme is easy to implement, as the only required on-line measurement is the angular displacement of the slave manipulator joints.

An extremum-seeking control approach for constrained robotic motion tasks

In this paper, we propose two adaptive control schemes for multiple-input systems for execution of robot end-effector movements in the presence of parametric system uncertainties. The design of these schemes is based on Model Reference Adaptive Control (MRAC) while the adaptation of the controller parameters is achieved by Extremum Seeking Control (ESC). The two control schemes, which are called Multiple-Input ESC–MRAC and Multiple-Input Adaptive-Dynamic-Inversion ESC–MRAC, are suitable for linear and nonlinear systems respectively. Lyapunov and averaging analysis shows that the proposed schemes achieve practical asymptotic reference state tracking. The proposed methods are evaluated in simulations and in a real-world robotic experiment.

Model-based control of a third-order nonholonomic system

This paper addresses dynamics modelling and control of mechanical systems subjected to constraints due to the tasks they are to perform. The researched problem refers to the control of a manipulator end-effector motion subjected to a constraint on an acceleration change, that is, jerk. In the presented approach, the acceleration and jerk constraints are incorporated into a system constrained dynamics and then passed to its control dynamics using a unified approach, which is based on an advanced dynamics modelling method, that is, on the generalized programmed motion equations method. This enables one to derive motion equations for systems subjected to high-order constraints. The novelty of this approach consists of the formulation of a unified representation of constraints by differential equations, merging them into system constrained dynamics and then using nonlinear control theory techniques to design controllers for tracking motions along the pre-specified constraints.

Genetic algorithm trajectory plan optimization for EAMA: EAST Articulated Maintenance Arm

EAMA (EAST Articulated Maintenance Arm) is an articulated serial manipulator with 7 degrees of freedom (DOF) articulated arm followed by 3-DOF gripper, total length is 8.867m, works in experimental advanced superconductor tokamak (EAST) vacuum vessel (VV) to perform blanket inspection and remote maintenance tasks. This paper presents a trajectory optimization method which aims to pursue the 7-DOF articulated arm a stable movement, which keeps the mounted inspection camera anti-vibration. Based on dynamics analysis, trajectory optimization algorithm adopts multi-order polynomial interpolation in joint space and high order geometry Jacobian transform. The object of optimization algorithm is to suppress end-effector movement vibration by minimizing jerk RMS (root mean square) value. The proposed solution has such characteristics which can satisfy kinematic constraints of EAMA’s motion and ensure the arm running under the absolute values of velocity, acceleration and jerk boundaries. GA (genetic algorithm) is employed to find global and robust solution for this problem.

Passivity‐based adaptive 3D visual servoing without depth and image velocity measurements for uncertain robot manipulators

SummaryIn this work, we consider the 3D visual tracking problem for a robot manipulator with uncertainties in the kinematic and dynamic models. The visual feedback is provided by a fixed and uncalibrated camera located above the robot workspace. The Cartesian motion of the robot end effector can be separated into a 1D motion parallel to the optical axis of the camera and a 2D motion constrained on a plane orthogonal to this axis. Thus, the control design can be simplified, and the overall visual servoing system can be partitioned in two almost‐independent subsystems. Adaptive visual servoing schemes, based on a kinematic approach, are developed for image‐based look‐and‐move systems allowing for both depth and planar tracking of a reference trajectory, without using image velocity and depth measurements. In order to include the uncertain robot kinematics and dynamics in the presented solution, we develop a cascade control strategy based on an indirect/direct adaptive method. The stability and convergence properties are analyzed in terms of Lyapunov‐like functions and the passivity‐based formalism. Numerical simulations including hardware‐in‐the‐loop results, obtained with a robot manipulator and a web camera, are presented to illustrate the performance and feasibility of the proposed control scheme. Copyright © 2016 John Wiley & Sons, Ltd.

A study on motion of a robot end-effector using the curvature theory of dual unit hyperbolic spherical curves

In this paper we study the motion of a robot end-effector by using the curvature theory of a dual unit hyperbolic spherical curve which corresponds to a timelike ruled surface with timelike ruling generated by a line fixed in the end-effector. In this way, the linear and angular differential properties of the motion of a robot end-effector such as velocities and accelerations which are important information in robot trajectory planning are determined. Moreover, the motion of a robot end-effector which moves on the surface of a right circular hyperboloid of one sheet is examined as a practical example.

Releasing of micro-objects based on local stream generation by high speed motion of end-effector

In micro-manipulation, releasing of micro-objects is a challenging work due to the difficulty of controlling adhesion forces. To overcome the adhesion force and place micro-objects with high position accuracy, in this paper, local stream generated by high-speed motions of an end-effector is proposed. A compact parallel link creates 3D end-effector motion at high-speed without uncontrollable vibration, which means local stream can generate enough force in a 3D direction to separate micro-objects. Applying the local stream created by the right end-effector, micro-objects including 55 µm microbeads and NIH3T3 cells attached to the left end-effector were released. To verify the placing positions of the objects, we compared four motions, 1D motions (X- and Z-directions) and circular motions (clockwise and counterclockwise directions), by analysing the trajectory of the objects after separation. From these results of experiments, we concluded that the circular motion (CCW) detached micro-objects in the nearest position from the end-effector.

Releasing of micro-objects based on local stream generation by high speed motion of end-effector

Releasing of micro-objects based on local stream generation by high speed motion of end-effector

3D Reconstruction of End-Effector in Autonomous Positioning Process Using Depth Imaging Device

The real-time calculation of positioning error, error correction, and state analysis has always been a difficult challenge in the process of manipulator autonomous positioning. In order to solve this problem, a simple depth imaging equipment (Kinect) is used and Kalman filtering method based on three-frame subtraction to capture the end-effector motion is proposed in this paper. Moreover, backpropagation (BP) neural network is adopted to recognize the target. At the same time, batch point cloud model is proposed in accordance with depth video stream to calculate the space coordinates of the end-effector and the target. Then, a 3D surface is fitted by using the radial basis function (RBF) and the morphology. The experiments have demonstrated that the end-effector positioning error can be corrected in a short time. The prediction accuracies of both position and velocity have reached 99% and recognition rate of 99.8% has been achieved for cylindrical object. Furthermore, the gradual convergence of the end-effector center (EEC) to the target center (TC) shows that the autonomous positioning is successful. Simultaneously, 3D reconstruction is also completed to analyze the positioning state. Hence, the proposed algorithm in this paper is competent for autonomous positioning of manipulator. The algorithm effectiveness is also validated by 3D reconstruction. The computational ability is increased and system efficiency is greatly improved.

Spatial Straight-Line Linkages by Factorization of Motion Polynomials

We use the recently introduced factorization of motion polynomials for constructing overconstrained spatial linkages with a straight-line trajectory. Unlike previous examples of such linkages by other authors, they are single-loop linkages and the end-effector motion is not translational. In particular, we obtain a number of linkages with four revolute and two prismatic joints and a remarkable linkage with seven revolute joints one of whose joints performs a Darboux motion.

Avoiding dynamic singularities in Cartesian motions of free-floating manipulators

Free-floating manipulators are subject to dynamic singularities that complicate their Cartesian motions and restrict their workspace. In this work, the Cartesian trajectory planning of free-floating manipulators is studied. A methodology is developed in which, for given end-effector trajectories, appropriate initial system configurations are found that result in singularity avoidance during end-effector motion. The method applies to both planar and spatial systems, with and without initial angular momentum, and to any desired end-effector position and attitude trajectories.

Experimental study on restricting the robotic end-effector inside a lesion for safe telesurgery

Introduction: Using an endoscopic telesurgical robot system (ETSRS), the authors propose a strategy for improving the safety of telesurgery by restricting the movement of an end-effector within a lesion. The strategy is validated by phantom model experiments. Material and methods: The method focused on generation of force feedback and restriction of robotic end-effector movement of ETSRS based on a virtual wall. Collision detection and case classification procedures were used to determine whether the generation of force feedback or restricting the end-effector’s movement was continued. The method was implemented in ETSRS and tested using a brain and tofu phantom. Results: Force feedback was well generated proportional to a linear combination of the insertion depth and the velocity of the end-effector of the ETSRS from the surface of the predefined virtual wall. The movement of the end-effector was well limited inside the virtual wall by the method. The virtual wall update was sufficiently fast to check the current surgical situation. The control rate of the entire system was >30 fps so that the method showed acceptable performance in phantom experiments. Conclusion: The results show that the strategy allows for well controlled robotic end-effectors inside a predefined virtual wall by the robot itself and an operator through the signal and force feedback.

Comparison of four different heuristic optimization algorithms for the inverse kinematics solution of a real 4-DOF serial robot manipulator

In this study, a 4-degree-of-freedom (DOF) serial robot manipulator was designed and developed for the pick-and-place operation of a flexible manufacturing system. The solution of the inverse kinematics equation, one of the most important parts of the control process of the manipulator, was obtained by using four different optimization algorithms: the genetic algorithm (GA), the particle swarm optimization (PSO) algorithm, the quantum particle swarm optimization (QPSO) algorithm and the gravitational search algorithm (GSA). These algorithms were tested with two different scenarios for the motion of the manipulator's end-effector. One hundred randomly selected workspace points were defined for the first scenario, while a spline trajectory, also composed of one hundred workspace points, was used for the second. The optimization algorithms were used for solving of the inverse kinematics of the manipulator in order to successfully move the end-effector to these workspace points. The four algorithms were compared according to the execution time, the end-effector position error and the required number of generations. The results showed that the QPSO could be effectively used for the inverse kinematics solution of the developed manipulator.

Determining the threshold of time-delay for teleoperation accuracy and efficiency in relation to telesurgery.

Advances in robotics have made teleoperated surgical procedures a feasible means of treating patients in remote locations. In this study a suite of experiments was performed to investigate the influence of time-delay on teleoperation accuracy and efficiency during a path-following task. Subjects used a Phantom Omni 6-degrees of freedom (dof) input device (Sensable, Triangle Park, NC) to move the end-effector of a Mitsubishi (Tokyo, Japan) PA-10 7-dof robotic manipulator along a prescribed path. End-effector motion was recorded using a video motion capture system. Time-delays ranging from 0 to 2.5 s were artificially imposed. Performance was quantified by time to complete the task, path length, and square root-mean-square (RMS) error. Randomization of time-delay order and allowance for practice runs reduced the learning effect. An imposed time limit and pacing were used to negate the move-and-pause strategy that emerged in early trials. Time to complete the task and RMS error generally increased with increasing time-delay. Path length also generally increased, but not as consistently. With imposed pacing, RMS error continued to increase beyond 1.5 s, and some subjects were not able to complete the task in the allotted 90 s. The results suggest a threshold of time-delay in the range of 1.5-2.0 s. Beyond 1.5 s, subjects adopted a move-and-pause strategy that increased completion time to preserve path-tracking accuracy. If paced, tracking accuracy tended to degrade substantially beyond 1.5 s. A strong learning effect was evident, and experienced teleoperators performed substantially better than novices.

Jacobian Matrix Normalization - A Comparison of Different Approaches in the Context of Multi-Objective Optimization of 6-DOF Haptic Devices

This paper focuses on Jacobian matrix normalization and the performance effects of using different criterion and techniques. Normalization of the Jacobian matrix becomes an issue when using kinematic performance indices and the matrix contains elements with non-homogenous physical units, i.e. representing both translational and rotational motions. Normalization is necessary in multi objective optimization if kinematic performance indices are used based on the full Jacobian matrix. Different methods have been proposed in literature for defining a scaling factor used to normalize the Jacobian. Based on a comparison of a few of these methods, we conclude that it is better to have the scaling factor as a design variable in the multi objective optimization. However, as an alternative, a new scaling factor is proposed based on the relationship between linear actuator motion range in joint space and rotational end effector motion in task space, a proposal underpinned by simulation, analysis and comparison of optimization results using existing normalization techniques. For optimization, performance indices for workspace, kinematic sensitivity, device isotropy and inertia are considered. To deal with the multi-objective optimization problem, genetic algorithms are employed together with a normalized multi-objective optimization function. The performances of different device configurations (depending on the normalization method and the global isotropy index used) are presented in this article.