6-DOF Serial Robot Research Articles

Tool wear image on-machine detection based on trajectory planning of 6-DOF serial robot driven by digital twin

Tool wear image on-machine detection based on trajectory planning of 6-DOF serial robot driven by digital twin

Trajectory Planning and Servo Control Strategy of EDM Milling Based ona 6-DOF Serial Robot

Trajectory Planning and Servo Control Strategy of EDM Milling Based ona 6-DOF Serial Robot

EDM milling based on a 6-DOF serial robot

Aiming at the problems of insufficient flexibility and limited size of the workpiece in traditional EDM milling, an EDM milling based on a 6-DOF serial robot is proposed in this study. To realize the rapid adjustment of the machining gap, a tool electrode servo motion device was invented and installed at the end of a 6-DOF serial robot, and a servo control strategy of the tool electrode vertical trajectory fallback was proposed. Then, to achieve the controllable thickness of every layer in EDM milling based on a 6-DOF serial robot, the servo control strategy of limited motion of the tool electrode on the basis of vertical servo control and a fix-length compensation method to compensate the tool electrode wear was proposed. Finally, to verify the correctness and effectiveness of the servo control strategy and wear compensation of the tool electrode, the trapezoidal groove structures on the plane workpiece and cylindrical workpiece were machined by EDM milling based on a 6-DOF serial robot, respectively. Also, to verify the machining ability of complex trajectories on the space surface of the workpiece of EDM milling based on a 6-DOF serial robot, three complex trajectories, such as EDM, Chinese character “China,” and “Chinese knot,” was designed and machined on the hemispherical workpiece with the radius of 38 mm. The feasibility and effectiveness of EDM milling technology based on a 6-DOF serial robot were verified, which has great market application potential.

Kinematic Optimization of 6DOF Serial Robot Arms by Bio-Inspired Algorithms

Robotic systems are essential to technological development in the industrial, medical, and aerospace sectors. Nevertheless, their use in different applications requires that the robot have the best possible execution efficiency. For this, a robot with specific optimized characteristics is necessary to impact the performance of the specific task. Different techniques have been used in robot optimization, the most widely used being genetic algorithms (GA) and particle swarm optimization (PSO). However, there are optimization algorithms with high convergence speeds inspired by animal behavior, whose application in robot optimization has not been reported. In this work, bio-inspired algorithms Harris hawks optimization (HHO) and Grey wolf optimizer (GWO) are applied to a six-degree-of-freedom (6DOF) robot arm design through kinematic optimization. The lengths of the main robot links are optimized to improve the workspace volume and obtain a better-conditioned robot using the structural length index (SLI) and global condition index (GCI) as objective functions. A comparison is made between the proposed algorithms and GA and PSO regarding convergence speed, computational load, and optimality. Similar behavior has been found for HHO, GWO, and PSO compared to GA for both indexes. For the GCI problem, an average improvement of 14% was found when optimizing an industrial robot arm. Furthermore, multiple runs experiment is performed to test the robustness of the algorithms. The results show that HHO is the best technique to obtain an optimal robot design because it needs less than ten iterations to provide a better result despite its computational load.

Deflection estimation of industrial robots with flexible joints

The application of industrial robots in manufacturing industries has received considerable concerns due to the high flexibility, multifunctionality, and cost-efficiency. It is well known that the robot positioning accuracy is susceptible to the load and motion of robots owing to the insufficient stiffness of robots. Therefore, the machining accuracy improvement has been a research focus in the robotic manufacturing industries in the last decade. To overcome the measurement difficulty of the joint torque and position as well as the complex dynamic coupling between rotors and links, two forward dynamics algorithms for the robot deflection estimation are proposed in this paper. The robot kinematics and dynamics algorithms considering the dynamic coupling between rotors and links are developed based on Lie theory. The forward dynamics equations of robots are solved via the proposed algorithms: the implicit numerical integration algorithm and numerical iterative estimation algorithm. When only the motor position is available, the implicit numerical integration algorithm is employed to solve the forward dynamics equations to estimate the joint torque and position. At the same time, when both the motor position and torque are available, the forward dynamics equations can be reorganized as algebraic equations and solved by the numerical iterative estimation algorithm. Simulations of a 6-DOF serial robot are performed to verify the accuracy of the proposed algorithms.

Study of error compensations and sensitivity analysis for 6-Dof serial robot

PurposeThe purpose of this paper is to improve the positioning accuracy of 6-Dof serial robot by the way of error compensation and sensitivity analysis.Design/methodology/approachIn this paper, the Denavit–Hartenberg matrix is used to construct the kinematics models of the robot; the effects from individual joint and several joints on the end effector are estimated by simulation. Then, an error model based on joint clearance is proposed so that the positioning accuracy at any position of joints can be predicted for compensation. Through the simulation of the curve path, the validity of the error compensation model is verified. Finally, the experimental results show that the error compensation method can improve the positioning accuracy of a two joint exoskeleton robot by nearly 76.46%.FindingsThrough the analysis of joint error sensitivity, it is found that the first three joints, especially joint 2, contribute a lot to the positioning accuracy of the robot, which provides guidance for the accuracy allocation of the robot. In addition, this paper creatively puts forward the error model based on joint clearance, and the error compensation method which decouples the positioning accuracy into joint errors.Originality/valueIt provides a new idea for error modeling and error compensation of 6-Dof serial robot. Combining sensitivity analysis results with error compensation can effectively improve the positioning accuracy of the robot, and provide convenience for welding robot and other robots that need high positioning accuracy.

Design of 6-DOF Multi-Ring Coupled Parallel Robot Based on Helix Theory

Based on the helix theory and kinematic pair type analysis, the unconstrained branch chain configuration of 6-DOF parallel mechanism is synthesized. According to the principle of installation rationality and transmission lightweight design, the driving device of multi-ring coupling branch chain is transferred to the frame through parallelogram link equivalent transmission, and the mechanism transformation with branch chain type of 6-RSS is completed. A multi-ring coupling 6-DOF parallel robot with the branch chain type of 3-RSS-PRSS is designed. Based on the helix theory, the multi-ring coupling branch chain mechanism is split and the degree of freedom of the mechanism is calculated. The modified G-K formula is used to verify that the optimization model is equivalent to the theoretical model. The optimized mechanism reduces the inertia of the robot, improves the speed of the robot, and makes it have the high-speed characteristics of delta robot and the flexibility of 6-DOF serial robot.

Operation of the Collaborative Composite Manufacturing (CCM) System.

The automated tape placement and the automated fiber placement (AFP) machines provide a safer working environment and reduce the labor intensity of workers than the traditional manual fiber placement does. Thus, the production accuracy, repeatability and efficiency of composite manufacturing are significantly improved. However, the current AFP systems can only produce the composite components with large open surface or simple revolution parts, which cannot meet the growing interest in small complex or closed structures from industry. In this research, by employing a 1-degree of freedom (DoF) rotational stage, a 6-RSS parallel robot, and a 6-DoF serial robot, the dexterity of the AFP system can be significantly improved for manufacturing complex composite parts. The rotational stage mounted on the parallel robot is utilized to hold the mandrel and the serial robot carries the placement head to mimic two human hands that have enough dexterity to lay the fiber to the mandrel with complex contour. Although the CCM system increases the flexibility of composite manufacturing, it is quite time-consuming or even impossible to generate the feasible off-line path, which ensures uniform lay-up of subsequent fibers considering the constraints like singularities, collisions between the fiber placement head and mandrel, smooth fiber direction change and keeping the fiber placement head along the norm of the part's surface, etc. Moreover, due to the existing positioning error of the robots, the on-line path correction is needed. Therefore, the on-line pose correction algorithm is proposed to correct the paths of both parallel and serial robots, and to keep the relative path between the two robots unchanged through the visual feedback when the constraint or singularity problems in the off-line path planning occur. The experimental results demonstrate the designed CCM system can fulfill the movement needed for manufacturing a composite structure with Y-shape.

DYNAMIC COMPUTATION OF HAPTIC-ROBOT DEVICES FOR CONTROL OF A SURGICAL TRAINING SYSTEM

This paper presents a new control model of the haptic device for the closed loop teleoperation of a minimal surgery training system. Dynamics of a 6-DOF parallel haptic device is computed and compensated to make a decoupled linearization control model. In teleoperation system, the master is the 6-DOF haptic device and the slave is the 6-DOF serial robot. The master haptic device provides the trajectories for the slave serial robot through the operation of user’s hand on the steering handle while the slave robot sends feedback forces on its end effector to the master controller in order to generate forces/moments on the steering handle of haptic master. In this manner the user’s hand will feel the forces/moments as the same those of the robot end effector. The feeling force tracking performances of system can be improved by using dynamic compensation and decoupled linearization controller based on fuzzy PID algorithms. Experiment results indicate that the dynamic compensation and fuzzy control can improve the control performances effectively.

Flat control of industrial robotic manipulators

A new approach to tracking control of industrial robot manipulators is presented in this paper. The highly coupled nonlinear dynamics of a six degrees of freedom (6-DOF) serial robot is decoupled by expressing its variables as a function of a flat output and a finite number of its derivatives. Hence the derivation of the flat output for the 6-DOF robot is presented. With the flat output, trajectories for each of the generalized coordinates are easily designed and open loop control is made possible. Using MATLAB/Simulink S-functions combined with the differential flatness property of the robot, trajectory tracking is carried out in closed loop by using a linear flat controller. The merit of this approach reduces the computational complexity of the robot dynamics by allowing online computation of a high order system at a lower computational cost. Using the same processor, the run time for tracking arbitrary trajectories is reduced significantly to about 10 s as compared to 30 min in the original study (Hoifodt, 2011). The design is taken further by including a Jacobian transformation for tracking of trajectories in cartesian space. Simulations using the ABB IRB140 industrial robot with full dynamics are used to validate the study.

The Calculation of Inverse Kinematics for 6DOF Serial Robot

This article deals with the inverse kinematic model of robot devices which was implemented into the simulation software RoboSim designed at the Department of Automation and Production Systems - Faculty of Mechanical Engineering, University of Zilina. Concretely there is shown the technique how to develop the system of equations for serial kinematic mechanism with 6 degrees of freedom (DOFs). Further this software can be used as universal platform for simulation of mechanisms with serial, parallel as well as hybrid kinematic structure. This flexibility can be considered as a main advantage of the software RoboSim. Its specific feature is that there are used two different methods for calculation of inverse kinematics: heuristic and vector method as well. The versatility, openness and access to the source code create the predisposition for its deployment under the laboratory conditions for controlling of robotic devices developed in authors' workplace within the last few years.

A New Method of 6-DOF Serial Robot’s Trajectory Planning under Multi-Constraints

This paper proposes a new method of 6-DOF serial robot’s trajectory planning. Ensuring to satisfy the physical constraints of space conditions, the robot’s trajectory is interpolated in the Cartesian coordinate system, and using quaternion interpolation to solve the multiple solution problem in RPY interpolation. Meanwhile, the interpolated position information is transformed into the angular displacement information of the joint coordinate system, and the joint space trajectory planning is achieved using the genetic algorithms integrated velocity, acceleration, jerk and torque and other important kinematic and dynamic constraints. In robot safety and stability, the method is better than the general approach, and it has both the ideal trajectory parameters of the global search ability and performance planning.

Selecting Optimal Measurement Poses for Kinematic Calibration of Industrial Robots

The robustness of robot calibration regarding the measurement noise is sensitive to the measurement poses serving as constraints on the parameters to be estimated. This paper presents a pose selection algorithm allowing one to select a given number of optimal poses out of a large set of previously measured poses, which is derived from the DETMAX algorithm with the improvements benefiting from the randomized search technique to avoid local convergence. The benefit of selecting optimal measurement poses is validated through both simulation and experimentation of calibration of a 6-dof serial robot manipulator (Hyundai robot YS100). The results show that the robot calibration with optimal measurement poses yields the global (unbiased) kinematic parameters. Thus, the robot positioning accuracy is enhanced throughout the workspace. Moreover, the experimental results indicate that the number of optimal measurement poses is as important as selecting optimal measurement poses for improving the robot calibration accuracy.

Flexible Rope Transmission Device and Preliminary Study on its Design

As a new flexible rope transmission solution the structure and principle of the flexible rope transmission device was introduced firstly. Then the rope-tube mechanical model about interaction between rope and flexible tube was established. On the basis of the model, the factor affecting rope-tube transmission performance was studied especially coefficient of kinetic friction and Elastic modulus of flexible tube. After that, the design calculation method of flexible rope transmission was developed through the design calculation on rope-tube mechanical model. Lastly, a design example that a 6-DOF light-load serial robot equipped with three wrist joints based on flexible rope transmission was invented, and it performed well in application test. So, flexible rope transmission is a feasible transmission solution indeed, and may have broad prospects as well.

A New 6-DOF Haptic Device for Teleoperation of 6-DOF Serial Robots

A new 6-DOF parallel haptic device is developed and presented in this paper. The haptic device consists of two 3-DOF parallel structures connected with a steering handle. The design satisfies requirements of low inertia, quick motion, large orientation angles, and large applied torques. Kinematics for position and differential motion is analyzed for the 6-DOF haptic device. Static force relation between the user force and motor torque is also analyzed and implemented on the controller. The control system for the teleoperation of 6-DOF serial robot is developed. The closed loop impedance force control system is analyzed and realized on the digital controller. Tele-operation of a 6-DOF serial robot using this haptic device is demonstrated. Experiments show that dynamic forces caused by the haptic device are well compensated with the closed loop force control.