Target Workspace Research Articles

Design, analysis and optimization of a novel redundant (6+1)-degree-of-freedom parallel mechanism with configurable platform

In this paper, a concept of adaptive capture for Autonomous Underwater Vehicles(AUVs) using parallel mechanism with configurable platform is proposed. By leveraging the deformation capabilities of the configurable platform, the mechanism can capture AUVs of various sizes and orientations while also expanding its workspace, thereby increasing the success rate of grasping. Firstly, the mechanism design is conducted, a novel parallel mechanism consisting of three identical chains with a three-fold symmetrical Bricard platform is proposed. Secondly, the kinematics are obtained, based on which the workspace analysis is performed, and numerical verification is conducted. Thirdly, the Jacobian matrix is derived and verified, and the singularity analysis is carried out. Finally, the concept of a high-quality workspace tailored for the AUVs recovery scenarios is proposed. To achieve a maximum high-quality workspace while maintaining a compact mechanism structure, two indicators are proposed to evaluate the volume and shape of the workspace, and a multi-objective parameter optimization of the mechanism is conducted. The optimized mechanism can be effortlessly scaled to cover the target workspace while preserving minimal dimensions and compact structure.

Experimental modal transferring of industrial robot with data augmentation through domain adaptation and transfer boosting

Pose-dependent modal properties of the robotic milling system determine the milling stability, which significantly affects the machining accuracy and surface quality. Experimental modal analysis is an effective way to obtain the actual modal properties, however, the full experimental modal measurement in the entire robot workspace is repetitive and time-consuming, and the modal properties of a measured workspace can hardly transfer to an unmeasured workspace because of domain discrepancy and data imbalance. In this paper, the mathematical formalization of modal properties transferring problem between different workspaces is recommended as heterogeneous transfer, and a transfer learning method of modal properties is proposed by integrating domain adaptation and transfer boosting to eliminate domain discrepancy and data imbalance. Firstly, a workspace is selected as the source workspace in which pose-dependent modal properties are obtained as the source data through a series of sufficient impact tests. For a new workspace, namely the target workspace, a few impact tests would be sufficient to obtain the modal properties as the target data. Then, both the inputs of source and target data are projected onto a reproducing kernel Hilbert space (RKHS) through transfer component analysis (TCA) to get the IN-Domain (IND) inputs. The source data is used to generate pseudo data with data augmentation through affine transformation, and the pseudo data is regarded as coarse data for the target workspace. Finally, the precise modal properties models are trained with TrAdaBoost.R2 in which the training data was obtained by mixing pseudo data and target data, and the unmeasured modal properties in the target space are predicted. The effectiveness of the proposed method is verified through experimental modal properties transferring between different workspaces and modal orders for the robotic milling system, and the comparisons with Gaussian process regression (GPR) demonstrate the outperformance of the proposed method.

Robot-mounted sensing and local calibration for high-accuracy manufacturing

Positional accuracy is critically important for application of industrial robots in fixtureless precision manufacturing; thus, this paper presents a method for simultaneously calibrating the kinematic parameters of robot and robot-mounted measurement devices. In the modeling step, the Modified Denavit–Hartenberg parameters are extended to include the world and tool frames within a minimal parameter set using a systematic approach. Closed-loop calibration is performed using the data collected in the local target workspace using the robot-mounted measurement device. The presented modeling and calibration methods are applied to a case study in which a machining task is performed using a DENSO VS-6556 W industrial robot equipped with a cutting tool, force sensor, and laser profile scanner. In the case study, local models are calibrated for the laser and tool frames using common 1-2-3 blocks as calibration artifacts . The robot/laser model is identified by scanning the artifacts with a robot-mounted laser scanner , and the robot/tool model is calibrated by touching the artifacts with a robot-mounted force sensor. In this application, the robot/laser model is used to scan and register a sheet metal workpiece, and the robot/tool model is used to plan a tool path to machine the periphery. Positional accuracy of the tool path was evaluated to be ± 0.15 mm within the target workspace, which shows significant improvement compared with the mean error of 0.8 mm achieved by global calibration of the robot/tool model. • Simultaneous calibration of robot and extrinsic parameters of robot-mounted sensors. • Closed-loop calibration without external measurement devices. • Local calibration to improve accuracy within target workspace. • Systematic approach to include world and tool/sensor frame in MDH parameter set. • Case study for high-accuracy robotic machining application.

PREFACE

PREFACE

An Angle–Axis Space-Based Orientability Index Characterizing Complete Orientations

The development of new materials/actuators in robotics leads to the rapid growth of new types of robots, including soft/continuum robots, reconfigurable robots, exoskeleton robots, haptic master devices, and so on. When designing a new robot, orientability is an important performance index to evaluate its dexterity. However, a theoretical investigation of the orientability is still lacking. The existing orientability index can only characterize the reachability of a line, rather than a complete orientation. To evaluate the reachability of complete orientations, we propose a novel orientability index based on the Angle-Axis Space, which is bijective to the rotation group SO(3). Furthermore, a relative orientability index is proposed to characterize the matching degree of the workspace of a given robot to its target workspace. The proposed orientability and relative orientability indices are then used to evaluate and compare the dexterity of three 6-DoF continuum robots and a haptic master devices. The results validate the effectiveness of the proposed orientability in evaluating the dexterity.

A Two-Stage Mono- and Multi-Objective Method for the Optimization of General UPS Parallel Manipulators

This paper introduces a two-stage method based on bio-inspired algorithms for the design optimization of a class of general Stewart platforms. The first stage performs a mono-objective optimization in order to reach, with sufficient dexterity, a regular target workspace while minimizing the elements’ lengths. For this optimization problem, we compare three bio-inspired algorithms: the Genetic Algorithm (GA), the Particle Swarm Optimization (PSO), and the Boltzman Univariate Marginal Distribution Algorithm (BUMDA). The second stage looks for the most suitable gains of a Proportional Integral Derivative (PID) control via the minimization of two conflicting objectives: one based on energy consumption and the tracking error of a target trajectory. To this effect, we compare two multi-objective algorithms: the Multiobjective Evolutionary Algorithm based on Decomposition (MOEA/D) and Non-dominated Sorting Genetic Algorithm-III (NSGA-III). The main contributions lie in the optimization model, the proposal of a two-stage optimization method, and the findings of the performance of different bio-inspired algorithms for each stage. Furthermore, we show optimized designs delivered by the proposed method and provide directions for the best-performing algorithms through performance metrics and statistical hypothesis tests.

Geometric Design and Prototyping of a (2-RRU)-URR Parallel Mechanism for Thumb Rehabilitation Therapy

In this paper, the geometrical design of a (2-RRU)-URR (where R and U stand for the revolute and universal joints, respectively) parallel mechanism was demonstrated for thumb rehabilitation therapy. This paper consists of two parts: the design procedure for the development of a thumb rehabilitation device and the user experiment with the prototype. Because the hand generally has a limited working area, the design of the hand attachment parts and the placement of the actuators requires careful consideration of the various factors. Along with the kinematic requirements of the device, the interaction between the mechanism and the fingers must be considered. The proposed mechanism has three actuators placed in the hand attachment. When the mechanism is attached to the hand, there is the possibility of collisions between the fingers of the user and the mechanism. Two design candidates were devised while considering the limited working area of the hand and the need to avoid collisions. Due to the dependency of the workspace on the placement of the actuators, a comparison of the workspace of the two candidate designs and the target workspace was carried out. The target workspace was determined through the use of thumb trajectory measurement data. A prototype was manufactured using 3D printed plastic and aluminum materials. To confirm the practical performance of the prototype, user experiments were conducted in which a comparison between the thumb measurement data and the controlled trajectory of each person was done. Motion in two directions, specifically, adduction–abduction and flexion–extension were performed. The results showed that the controlled trajectory of flexion–extension were closely matched to the thumb measurement trajectory. Finally, the experimental results are discussed.

Design and optimization of an elastic linkage quadruped robot based on workspace and tracking error

High acceleration and extreme load are frequently appeared on high-speed locomotion of legged robot’s legs, imposing a challenging trade-off between weight and torque in leg design. This paper proposes a new design paradigm based on cable-drive and elastic linkage to solve the problem. The details of the design procedure are given, including the construction of the single leg. With the optimum design of the linkage mechanism, a combined index of the workspace and tracking error are used as object function, and taking geometrical design parameters of the linkage as optimization parameters. Based on the target workspace and the spring-loaded inverted pendulum model, the best foot trajectory in obstacle climbing and trotting gait are analyzed and illustrated. This paper built linkage cable-drive spring robot based on the legged module integration. Simulations and experiments indicate that linkage cable-drive spring robot performs stable trotting with control of the spring-loaded inverted pendulum model. Linkage cable-drive spring robot prototype experiments results are provided to verify the validity of the new method.

Analysis, optimization and prototyping of a parallel RCM mechanism of a surgical robot for craniotomy surgery

PurposeRemoving the bone flap is a compulsory step in open skull surgeries and is very cumbersome and time-consuming. Exerting large forces during the milling and cutting of the skull renders the surgeon exhausted and consequently increases probable errors in further task of manipulating the sensitive brain tissue. This paper aims to present the development of a robotic system capable of perforating and cutting the required bone flap without restraining the surgeon.Design/methodology/approachFor the purpose of optimization, the target workspace is estimated by 3D modeling of the sample skull and bone flaps of targeted surgeries. The optimization considers kinematic performance matrices and the extracted workspace requirements by assigning scores to each possible design and finally selects the design with highest score.FindingsThe design utilizes a parallel remote center of motion mechanism. Coordinating the remote center of motion (RCM) of the mechanism with the center of a sphere which circumscribes the skull, the milling tool is always nearly perpendicular to the skull bone. The paper presents the concept design, optimization criteria and finally the optimal design of the robot and the fabricated prototype. Tests indicate that the prototype is able to sweep the target workspace and to exert the required forces for bone milling.Originality valueThe workspace requirements of the craniotomy/craniectomy surgeries are investigated and converted into one quantitative target workspace. An optimized design for a surgical robot is developed which satisfies the workspace requirements of the targeted surgeries.

Optimizing Stiffness and Dexterity of Planar Adaptive Cable-Driven Parallel Robots

Adaptive cable-driven parallel robots are a special subclass of cable-driven systems in which the locations of the pulley blocks are modified as a function of the end-effector pose to obtain optimal values of given performance indices within a target workspace. Due to their augmented kinematic redundancy, such systems enable larger workspace volume and higher performance compared to traditional designs featuring the same number of cables. Previous studies have introduced a systematic method to optimize design and trajectory planning of the moving pulley-blocks for a given performance index. In this paper, we study the motions of the pulley blocks that optimize two performance indices simultaneously: stiffness and dexterity. Specifically, we present a method to determine the pulley blocks motions that guarantee ideal dexterity with the best feasible elastic stiffness, as well as those that guarantee isotropic elastic stiffness with the best feasible dexterity. We demonstrate the proposed approach on some practical cases of planar adaptive cable-driven parallel robots.

Use of a Force-Torque Sensor for Self-Calibration of a 6-DOF Medical Robot

The aim of this paper is to improve the position accuracy of a six degree of freedom medical robot. The improvement in accuracy is achieved without the use of any external measurement device. Instead, this work presents a novel calibration approach based on using an embedded force-torque sensor to identify the robot’s kinematic parameters and thereby enhance the positioning accuracy. A simulation study demonstrated that our calibration approach is effective, whether or not any measurement noise is present: the position error is improved, inside the robot target workspace, from 12 mm to 0.320 mm, for the maximum values, and from 9 mm to 0.2771 mm, for the mean errors.

Kinodynamic Planner Dual-Tree RRT (DT-RRT) for Two-Wheeled Mobile Robots Using the Rapidly Exploring Random Tree

We propose a new trajectory generation scheme called dual-tree rapidly exploring random tree (DT-RRT), which is designed on the basis of a rapidly exploring random tree (RRT) method. The DT-RRT is suitable for high-speed navigation of a two-wheeled differential mobile robot. The proposed dual tree is composed of a workspace tree and a state tree. The workspace tree finds near sets in the target workspace without considering robot kinematics. Robot trajectories are generated by the extension of the state tree under the consideration of kinematic and dynamic constraints. The proposed scheme allows for different topological structures between the workspace tree and the state tree. Owing to the different structures between two trees, flexible node extensions can be achieved. As a result, the success rate of the node extension can be increased, while the computational cost can be saved. In order to improve the quality of the trajectory, we propose a reconnect-tree scheme that can modify the generated tree structure. The advantage of the reconnect-tree scheme is that the repropagation of the conventional RRT structure is not required. From simulations, the superior performance of DT-RRT was clarified in terms of computing time and success rate. The experimental result supported high quality of the generated trajectory with fast and smooth motion of the robot.

Absolute accuracy analysis and improvement of a hybrid 6-DOF medical robot

Purpose – The purpose of this paper is to describe a calibration method developed to improve the accuracy of a six degrees-of-freedom medical robot. The proposed calibration approach aims to enhance the robot’s accuracy in a specific target workspace. A comparison of five observability indices is also done to choose the most appropriate calibration robot configurations. Design/methodology/approach – The calibration method is based on the forward kinematic approach, which uses a nonlinear optimization model. The used experimental data are 84 end-effector positions, which are measured using a laser tracker. The calibration configurations are chosen through an observability analysis, while the validation after calibration is carried out in 336 positions within the target workspace. Findings – Simulations allowed finding the most appropriate observability index for choosing the optimal calibration configurations. They also showed the ability of our calibration model to identify most of the considered robot’s parameters, despite measurement errors. Experimental tests confirmed the simulation findings and showed that the robot’s mean position error is reduced from 3.992 mm before calibration to 0.387 mm after, and the maximum error is reduced from 5.957 to 0.851 mm. Originality/value – This paper presents a calibration method which makes it possible to accurately identify the kinematic errors for a novel medical robot. In addition, this paper presents a comparison between the five observability indices proposed in the literature. The proposed method might be applied to any industrial or medical robot similar to the robot studied in this paper.

Non-kinematic calibration of a six-axis serial robot using planar constraints

This paper describes a non-kinematic calibration method developed to improve the accuracy of a six-axis serial robot, in a specific target workspace, using planar constraints. Simulation confirms that the stiffness of the robot, as well as its kinematic parameters, can be identified. An experimental validation shows that the robot's accuracy inside the target workspace is significantly enhanced by reducing the maximum distance errors from 1.321mm to 0.274mm. The experimental data are collected using a precision touch probe, which is mounted on the flange of a FANUC LR Mate 200iC industrial robot, and a high precision 9-in. granite cube. The calibration method makes use of a linear optimization model based on the closed-loop calibration approach using multi-planar constraints. A practical validation approach designed to reliably evaluate the robot's accuracy after calibration is also proposed.

Kinematic calibration of a six-axis serial robot using distance and sphere constraints

This paper describes a kinematic calibration method developed to improve the accuracy of a six-axis serial industrial robot in a specific target workspace, using distance and sphere constraints. A simulation study demonstrates the ability of the calibration approach to identify the kinematic parameters, regardless of measurement noise. Experimental validation shows that the robot’s accuracy inside the target workspace is significantly improved by reducing the mean and maximum distance errors from 0.698 to 0.086 mm and from 1.321 to 0.127 mm, respectively. The experimental data are collected using a Renishaw precision touch probe mounted on the flange of a FANUC LR Mate 200iC and a special triangular plate with three datum 2-in spheres 300 mm apart. The calibration method uses an optimization model based on fitting several probed positions on spheres and minimizing the residual of the spheres’ center-to-center distances.

OPTIMAL DESIGN OF HEXAPOD WALKING ROBOT LEG STRUCTURE BASED ON ENERGY CONSUMPTION AND WORKSPACE

In order to achieve the optimal design of the hexapod walking robot leg structure, a combined index of energy consumption and workspace is raised. By deriving the energy consumption functions and analyzing the target workspace, a mathematical model of nonlinear programming with inequality constraints is established. The genetic algorithm coupled with inverse kinematics and trajectory planning in a gait period is utilized to solve the optimization problem. The analysis verifies that the requirements of turning and obstacle overcoming can be satisfied, and the total energy consumption can be reduced. The results show that the optimal parameters not only satisfy the requirement of the target workspace, but also achieve the minimum energy consumption and lower joint torques.

A novel remote center of motion mechanism for the force‐reflective master robot of haptic tele‐surgery systems

An effective master robot for haptic tele-surgery applications needs to provide a solution for the inversed movements of the surgical tool, in addition to sufficient workspace and manipulability, with minimal moving inertia. A novel 4 + 1-DOF mechanism was proposed, based on a triple parallelogram linkage, which provided a Remote Center of Motion (RCM) at the back of the user's hand. The kinematics of the robot was analyzed and a prototype was fabricated and evaluated by experimental tests. With a RCM at the back of the user's hand the actuators far from the end effector, the robot could produce the sensation of hand-inside surgery with minimal moving inertia. The target workspace was achieved with an acceptable manipulability. The trajectory tracking experiments revealed small errors, due to backlash at the joints. The proposed mechanism meets the basic requirements of an effective master robot for haptic tele-surgery applications.

Kinematic calibration of a 3‐DOF planar parallel robot

PurposeThe purpose of this paper is to describe a calibration method developed to improve the absolute accuracy of a novel three degrees‐of‐freedom planar parallel robot. The robot is designed for the precise alignment of semiconductor wafers and, even though its complete workspace is slightly larger, the accuracy improvements are performed within a target workspace, in which the positions are on a disc of 170 mm in diameter and the orientations are in the range ±17°.Design/methodology/approachThe calibration method makes use of a single optimization model, based on the direct kinematic calibration approach, while the experimental data are collected from two sources. The first source is a measurement arm from FARO Technologies, and the second is a Mitutoyo coordinate measurement machine (CMM). The two sets of calibration results are compared.FindingsSimulation confirmed that the model proposed is not sensitive to measurement noise. An experimental validation on the CMM shows that the absolute accuracy inside the target workspace was improved by reducing the maximum position and orientation errors from 1.432 mm and 0.107°, respectively, to 0.044 mm and 0.009°.Originality/valueThis paper presents a calibration method which makes it possible to accurately identify the actual robot's base frame (base frame calibration), at the same time as identifying and compensating for geometric errors, actuator offsets, and even screw lead errors. The proposed calibration method is applied on a novel planar robot, and its absolute accuracy was found to improve to 0.044 mm.